Patent Grant
8932534
This invention pertains to advanced materials, methods, and apparatus useful in pyrolysis reactors such as may be used for pyrolyzing or cracking hydrocarbons. In one non-limiting example, the invention relates to advanced refractory and ceramic materials, apparatus, and methods suitable for use in cracking hydrocarbon feedstocks in a high-severity, regenerative pyrolysis reactor. Particularly, the invention relates to coking and carbide penetration resistant and carbide corrosion resistant pyrolysis reactor apparatus that are resistant to thermal stress cracking, and pyrolysis methods using the same.
Conventional steam crackers are a common tool for cracking volatile hydrocarbons, such as ethane, propane, naphtha, and gas oil. Other higher severity thermal or pyrolysis reactors are also useful for cracking hydrocarbons and/or executing thermal processes, including some processes conducted at temperatures higher than can suitably be performed in conventional steam crackers. Compared to conventional cracking equipment and processes, higher temperature reactions (e.g., >1500° C.) and processes typically require more complex, costly, and specialized equipment to tolerate the intense heat and physical stress conditions. Economical operation of high severity hydrocarbon cracking processes and equipment requires satisfying numerous competing operational and engineering challenges. Properties such as melt temperature, reaction environment non-inertness, component strength, and toughness limitations commonly define limits for many processes. The high temperatures and process stresses can exceed the long term viability of most conventional apparatus, including conventional refractory ceramics. Although high temperature regenerative pyrolysis reactors are generally known in the art as capable of converting or cracking hydrocarbons, they have not achieved widespread commercial use, due significantly to the fact that they have not been successfully scaled to a commercially economical size or useful life span as compared to less severe alternatives, such as steam cracking.
In high thermal stress applications, many ceramic materials are subject to progressive failure due to onset mechanical and chemical changes within the ceramic matrix. For example, thermal cycles and related stress fluctuations may induce progressive formation of micro-fractures that may continue to grow and disperse until reaching a critical threshold resulting in premature component failure. In addition to physical and thermal performance considerations, component chemical inertness and crystalline stability also are significant considerations. Many prior art ceramic reactor materials that are relatively inert at lower temperatures become susceptible to chemical degradation, ceramic corrosion, and/or crystalline alteration at higher temperatures, in the presence of certain elements such as carbon and oxygen. The increased chemical activity at increased temperature may lead to premature degradation and/or process interference such as by generation of unacceptable levels of contaminants in the process. The presence of carbon from the hydrocarbon feedstock and potential presence of oxygen from within the reaction process, at severe pyrolysis temperatures, present special ceramic-metallurgical crystalline-stability challenges to avoid premature ceramic corrosion. This corrosion can be detrimental in that it may favor initiation and/or sustaining micro-fracture growth and dispersion.
Prior art regenerative pyrolysis reactor systems commonly used alumina as the bed packing material. The commercial embodiments of these reactor systems did not operate at temperatures sufficient to achieve high conversion of methane feed. One reason for this is that pure alumina has a melting point of 2050° C. and practical alumina may have lower melting point due to impurities. Further, maximum practical use temperatures are typically two- to three-hundred degrees lower than the actual melting temperature, which combined with decreases due to impurities, renders alumina unsuitable for many uses in a high temperature (e.g., >1500° C., or >1600° C., or up to 2000° C.) pyrolysis reactor. Although some of the “Wulff” art disclose use of various refractory materials, a commercially useful process for methane cracking or other extreme high-temperature processes has not previously been achieved utilizing such materials. The aforementioned practical obstacles have impeded large scale implementation of the technologies. Materials availability for high temperature, high-stress applications is one of the most critical issues in design and operation of large-scale, commercial, high-productivity, thermal reactors.
A similar, recently recognized problem particular to high temperature hydrocarbon pyrolysis pertains to carburization within the ceramic component, which can produce carbide-oxide conversion chemistry in many ceramic materials, such as some zirconia oxides, that also leads to progressive component degradation, herein considered a type of “ceramic corrosion.” This high temperature hydrocarbon-related corrosion mechanism was not previously identified, understood, or recognized as a concern with high temperature hydrocarbon pyrolysis using ceramics. Carburization is a heat activated process in which a material, such as a ceramic or metal, is heated to temperature below its melting point, in the presence of another material that liberates carbon as it thermally decomposes, such as hydrocarbons. The liberated carbon can permeate the exposed surface and near-surface interior of the ceramic crystal matrix and either remains in spatial regions as coke or at more elevated temperatures react with the ceramic to form ceramic carbides. Such permeation by carbon can over time adversely affect the mechanical and chemical properties of the ceramic material such as are otherwise needed for long-term use in commercial, hydrocarbon pyrolysis reactors. Ceramic component volatility and progressive loss due to the severe temperatures and cyclic temperature swings also may contribute to carburization. Issues include carbon infiltration and coking within the ceramic matrix pores and an associated, undesirable carbide-oxide interaction chemistry resulting in progressive corrosion and degradation of the ceramic matrix, including micro-fractures due to coke expansion. Such problems are of particular interest in high severity pyrolysis of hydrocarbon feedstocks (e.g., >1500° C.).
The pyrolysis art needs a ceramic composition that resists or avoids carbon permeation, carburization, and/or oxide-carbide corrosion, while also providing a mechanism to arrest micro-fracture propagation should such fractures be initiated. The desired materials should concurrently provide and maintain the needed structural integrity, crystalline stability, relatively high heat transfer capability, and chemical inertness required for large scale, commercial applications, particularly with respect to hydrocarbon pyrolysis.
The present invention relates to pyrolysis of feedstocks and in one aspect includes inventive refractory materials and apparatus useful in pyrolyzing hydrocarbon feedstock in a pyrolysis reactor, in some embodiments a regenerative pyrolysis reactor and in other embodiments a reverse flow type regenerative reactor. The inventive material is resistant to carbon deposition, carbide-oxide interactions, and associated ceramic corrosion and degradation. The inventive apparatus may also be resistant to micro-fracture growth and propagation. In one aspect, the inventive apparatus and compositions provide a dispersion of pores within the ceramic material that may intercept propagating micro-fractures.
In another aspect, the dispersion of pores may be of sufficient number and aggregate volume to enable the ceramic matrix material to subtly distort, expand and/or contract, thereby dispersing or redirecting stress concentrations. Additionally, the pores may merely intercept and disperse stress concentrations at the tip of a propagating micro-fracture, preventing further propagation of the intercepted micro-fracture. The disclosed refractory ceramic material resists carbide-oxide interaction corrosion, thereby resisting creation of a matrix crystalline or chemical composition that might favor micro-fracture creation, initiation, and propagation.
In another aspect, the invention includes a carbide-oxide corrosion resistant regenerative pyrolysis reactor apparatus and refractory material useful for pyrolyzing a hydrocarbon feedstock at temperatures of not less than 1200° C. and sometimes of not less than 1500° C., or in some embodiments not less than 2000° C.
In some embodiments, the invention includes a reactor apparatus for pyrolyzing a hydrocarbon feedstock, the apparatus including a reactor component comprising a refractory material in oxide form, the refractory material having a melting point of no less than 2060° C. and that remains in oxide form when exposed to a gas having carbon partial pressure of 10−22 bar and oxygen partial pressure of 10−10 bar, at a temperature of 1200° C.; wherein the refractory material has not less than 4 vol % formed porosity measured at 20° C. (ambient temperature), based upon the bulk volume of the refractory material. An upper limit on porosity will be determined by the necessary physical strength and thermal shock resistance type properties, depending upon the intended application. Typical upper porosity limits may be, for example, not greater than 60 vol %, not more than 50 vol %, or not more than 40 vol % in some embodiments. In many embodiments, the reactor apparatus comprises a regenerative pyrolysis reactor apparatus. In various other embodiments, the regenerative pyrolysis reactor comprises a reverse flow regenerative reactor apparatus. Many reactor embodiments utilize deferred combustion to heat the reactor. In many embodiments, the refractory material has a melting point of not less than 2010° C., and in other embodiments not less than 2160° C.
Other aspects of the invention include a regenerative, refractory corrosion resistant, pyrolysis reactor system for pyrolyzing hydrocarbon feedstock comprising a first reactor and a second reactor in flow communication with the first reactor, at least one of the first and second reactors comprising a refractory material in oxide form, the refractory material having a melting point of not less than 2060° C. and which remains in oxide form when exposed to a gas having a carbon partial pressure of 10−22 bar, a reference oxygen partial pressure of 10−10 bar, and a temperature of 1200° C.; wherein the refractory material has total porosity in the range of from 4 to 60 vol % based upon the volume of the refractory material.
Other aspects include methods for mitigating carbide-oxide ceramic corrosion of a refractory material in the presence of a pyrolyzed hydrocarbon feedstock, comprising the steps of providing in a heated region of a pyrolysis reactor system for pyrolyzing a hydrocarbon feedstock, apparatus comprising a refractory material in oxide form, the refractory material having a melting point of not less than 2060° C. and that remains in oxide form when exposed to a gas having a carbon partial pressure of 10−22 bar, an oxygen partial pressure of 10−10 bar, at a temperature of 1200° C.; wherein the refractory material has not less than 4 vol % formed porosity measured at 20° C., based upon the bulk volume of the refractory material. In other embodiments, the vapor pressure of the refractory material is not greater than 10−7 bar at 2000° C.
In many embodiments, the refractory material has formed porosity of at least 4 vol %, or in other embodiments the material may have total porosity of from 4 to 60 vol %, or in still other embodiments, the formed porosity may range from 5 to 30 vol %. In still other embodiments, the inventive material may include total porosity in the range of from 5 to 35 vol %. In some aspects the refractory material comprises a monomodal or normal grain size distribution, while in other embodiments the refractory material comprises a multimodal grain distribution. The formed porosity is porosity created within the ceramic material that does not include the residual inter-granular spaces provided between the ceramic material grains due to grain packing and crystallization. Formed porosity may be created by any suitable means, such as with a cavitating agent, beads, foreign media particles not part of the chemical structure of the ceramic matrix, by gas or other fluid droplets introduced into the wet ceramic matrix, or by introduction of a sacrificial material such as a polymer (e.g., polymethylmethacrylate, PMMA). The formed porosity may be a substantially empty cavity or may be occupied by a pore-forming agent, such as glass or metal beads. The pore or formed-porosity creating material may be left fully or partially intact within in the sintered ceramic composition or in many embodiments the material may be removed such as by vaporizing the material during sintering, leaving a substantially empty cavity or pore.
Various other aspects of the invention include a method for pyrolyzing a hydrocarbon feedstock using a pyrolysis reactor system comprising the steps of: (a) providing in a heated region of a pyrolysis reactor system for pyrolyzing a hydrocarbon feedstock, apparatus comprising a refractory material in the oxide form, the refractory material having a melting point of no less than 2060° C. and that remains in the oxide form when exposed to a gas having a carbon partial pressure of 10−22 bar, an oxygen partial pressure of 10−10 bar, at a temperature of 1200° C.; (b) heating the heated region to a temperature of no less than 1200° C., or sometimes no less than 1500° C.; (c) introducing a hydrocarbon feedstock into the heated region; and (d) pyrolyzing the hydrocarbon feedstock using heat from the heated region, wherein the refractory material has not less than 4 vol % formed porosity measured at 20° C., based upon the bulk volume of the refractory material. In other embodiments, the method further comprises: (b) heating the heated region to a temperature of not less than 1200° C.; (c) introducing a hydrocarbon feedstock into the heated region; and (d) pyrolyzing the hydrocarbon feedstock using heat from the heated region.
In still other embodiments, the step (a) of providing the refractory material comprises providing a refractory material comprising at least 50 wt % yttria, or at least 80 wt % yttria, or at least 90 wt % yttria, or substantially all yttria, based upon the total weight of the refractory material. Weight percents of yttria include the weight percent of pure yttrium and/or compounds comprising yttrium therein, such as yttria oxide, that otherwise meet the additional requirements of the invention, such as thermodynamic phase stability, and melting temperature. Yttria is merely one exemplary suitable compound.
The present invention relates to advanced refractory type ceramics and uses for the same. In various aspects, the invention includes refractory materials, components, apparatus, and processes having particular application for use with pyrolysis reactors and processes for performing high temperature (e.g., >1200° C., or >1500° C.) chemistry, conversions, cracking, and/or other thermal alteration of process feedstocks such as but not limited to hydrocarbon feeds. The inventive aspects include but are not limited to ceramic refractory components and apparatus using the same that have improved high temperature stability, prolonged component life expectancy, and sustained performance properties that may exceed the life expectancy of one or more of such attributes of previously known ceramics. The term “pyrolysis” as used herein is defined broadly to include substantially any generation, storage, or use of thermal energy (whether produced or used directly such as by flame-fired furnace, or indirectly such as by exothermic reactions, combustion, or heat transfer from a heated media) to cause the molecular or physical conversion, reaction, reforming, degrading, or cracking of a hydrocarbon feedstock into an intermediate or product stream, and may optionally include supplementation by one or more other processes, such as but not limited to catalysis, hydrogenation, esterification, oxidation, reduction, etc., and optionally with diluents, solvents, and/or stripping agents.
The present invention provides ceramic or refractory material compositions that are resistive to carburization and ceramic corrosion problems, while simultaneously providing a porosity system that may disperse stress concentrations and resist propagation of micro-fractures. The inventive materials may also maintain crystalline stability and endure prolonged exposure to high temperatures, substantial temperature swing cycles, and stress cycles due to cyclic flow of feedstocks and reaction materials. The inventive aspects may have particular utility that facilitates enhanced large-scale commercialization of high temperature pyrolysis processes. Exemplary suitable processes and refractory materials may include but are not limited to high-temperature pyrolysis reactors such as but not limited to, reactor for conversion of methane or other hydrocarbon feed, such as hydrocarbon fractions, crude oil, or coal to acetylene or olefins, coal gasification processes, syngas processes, carbon black processes, hydrocarbon steam cracking, hydrocarbon pyrolysis cracking, steam crackers, fired furnaces (direct, indirect, or radiant), etc. Exemplary apparatus may include but are not limited to pyrolysis reactors, reverse flow reactors, and regenerative reactors. Other exemplary reactors may include but are not limited to deferred combustion reactors, gasification reactors (e.g., coal gasification, etc.), syngas reactors, carbon black reactors, and steam cracking reactors and furnaces. Other exemplary inventive components may include but are not limited to reactor components and apparatus that feature engineered or otherwise designed functional forms, shapes, functions, configurations, intricacies, or irregular geometries that benefit from improved mechanical strength and thermal shock resistance at high temperatures (e.g., >1500° C.). Such improvements may also lead to improved processes related thereto.
In one aspect, the present invention provides a ceramic composition that is particularly resistive to carburization, thermodynamic chemical alteration, progressive loss of stabilizer, carbon permeation, and carbide-oxide ceramic corrosion, thereby preserving the crystalline structural under exposure to high temperature and/or highly reactive environments. Exemplary properties benefiting from the corrosion resistance and preserved intra-granular stabilization may include certain performance properties such as but not limited to flexural strength or modulus of rupture (MOR), normalized thermal shock resistance, and chemical stability at high temperature, as compared to such collective properties of previous art. The inventive pore system provided within the materials may afford protection from micro-fracture-induced degradation or failure mechanisms, should such ceramic corrosion still occur over extended time and lead to alterations that may be susceptible to on-set of micro-fracturing. Correspondingly, such improvements may facilitate large scale commercialization of processes and apparatus that were previously, technically and/or economically disadvantaged due to unacceptable component life.
The inventive refractory materials and reactor components include at least 4 vol % “formed” porosity. Porosity determinations are measured under ambient conditions at 20° C. and are based upon the bulk volume of the refractory material. The bulk volume of the refractory material includes only the ceramic body portion of the component or refractory material, including its' associated pore structure and natural fractures, and does not include the volume of component structural features such as flow channels, fluid conduits, formed crevasses, machined void areas, or other void-type component features.
For the present invention, the term “porosity” refers to the volume fraction of total bulk volume of a refractory material that is i) void of matrix material or filled with a fluid but not matrix or solid-filled, or ii) at least partially filled with a solid that is durable (residual) with respect to the matrix material, or iii) both i and ii. Commonly, the terms “porosity” and “pores” are associated only with substantially empty or void space dispersed, isolated, and/or interconnected throughout a solid body, and many embodiments of the present invention will include such spaces within the total porosity fraction. However, some embodiments of the present invention include a volume fraction associated with a dispersion of solid materials that remain distinct or heterogeneous after sintering and annealing with respect to the ceramic matrix material. Such particles that remain mixed within the ceramic matrix after sintering and annealing are referred to herein as durable particles. Even though such durable particle volume is substantially occupied and not truly vacant as traditionally thought of when referring to porosity, such durable component volume is included within the formed porosity value. The durable component may facilitate the same or similar operational features or advantages to the refractory material as facilitated by substantially vacant porosity, and for such reason both values are used to determine total formed porosity. Still other embodiments will include a mix of both void space and the dispersed durable materials. Consequently, as used herein the term “total” porosity includes the aggregate of all void volume fractions and durable component volume fractions within the refractory material bulk volume, and including both residual porosity and formed porosity. Stated differently, total porosity includes the aggregate of all inter-granular (residual) void volume fractions, “formed” void volume fractions, and formed durable component volume fractions within the bulk volume of a refractory material. Total void volume fractions include both inter-granular (“residual”) void volume fractions and formed volume fractions. Residual void volume fractions generally includes that fraction of total porosity existing between grains or crystals (inter-granular) such as the space generally associated with the periphery of the grains and crystal packing, and including micro-fracture volumes. Residual void volume value is determined after sintering, as described in more detail below.
The porosity component of most significance with respect to the present invention is the “formed” void volume fraction or “formed porosity.” Formed porosity refers to that volume fraction of total porosity provided within the refractory material specifically by formulation through mixing of a discontinuous phase of a heterogeneous component into the continuous phase(s) of the ceramic matrix, and does not include the residual (inter-granular) porosity associated with grain spacing, packing and/or micro-fracturing. The distinct or “heterogeneous” component refers to the component (either sacrificial or durable) mixed in the ceramic matrix material that does not form a solution with the ceramic matrix material, that is added at least for the purpose of providing a dispersion of pores/porosity (vacant, filled, or both) throughout the ceramic matrix. Residual porosity refers to the inter-granular porosity and is not to be confused with a durable heterogeneous agent that remains residually within the ceramic matrix after sintering and annealing to create formed porosity and which is recognized as a quantifiable fraction of the formed porosity fraction. The “durable” component is herein so-named in that all or a fraction of such component durably non-sacrificially withstands sintering and annealing, remaining distinct within and from the ceramic matrix. The fraction of total bulk volume provided by the durable component that remains residually within the matrix, as well as the fraction of total bulk volume provided by void space remaining after removal of a sacrificial voiding material, are collectively recognized as the formed porosity or formed volume fraction. The formed porosity value is determined after sintering, as described in more detail below, and may or may not result in vaporization or otherwise loss of the heterogeneous component from within the ceramic matrix during sintering or use, depending upon whether the heterogeneous component is sacrificial or durable. In many embodiments, after sintering and annealing, the formed porosity may comprise from 30% to substantially 100% or even 100% of the total porosity.
By way of illustration, in some embodiments the formed porosity may include volume fractions that are substantially vacant or only partially vacant. For example, a sacrificial voiding agent may be substantially vaporized in full and removed during sintering, while other sacrificial voiding materials may leave a residue within the formed pore space after sintering. In still other embodiments, the formed porosity may include volume fractions that are vacant or substantially vacant as well as volume fractions provided by the durable component (e.g., including but not limited to solid or hollow beads, spheres, ceramic beads, crushed materials, non-volatized materials, etc.). Formed porosity thus constitutes the total volumetric fraction of a dispersion of formed cells of either voids and/or residual durable materials within the refractory material, after sintering.
The various porosity values (e.g., total, formed, inter-granular-residual, durable, etc.) of the refractory material can be determined by any of several optical, spectral, and/or radiological dispersion or direct detection methods, such as but not limited to microscopy methods. Microscopy methods such as optical microscopy (OM) and scanning electron microscopy (SEM) in combination with image analysis software may be used to determine porosity values. A non-limiting example of such image analysis software is Clemex Vision PE™, which is an intuitive software program that empowers to easily develop custom image analysis macros. As an illustrative example, one skilled in the art might use 2-dimensional microscopic OM or SEM images of the refractory material, measuring either or both of formed pores and residual pores. For example, to determine the percent formed pores in an embodiment whereby the formed pores are mostly of a mean diameter of not less than the D50 grain size of the ceramic matrix grains, the scan may measure the aerial fraction of the formed pores and convert data to the percent formed porosity based on the bulk volume of said refractory material.
In many embodiments, the formed porosity is purposefully introduced into the refractory ceramics, such as by methods disclosed herein or by other known methods of mixing materials. The formed porosity may account for the majority of pore space within the refractory materials. Often, the formed pores are larger and sometimes much larger than the size of surrounding ceramic grains and thereby stay in the refractory ceramic body even after subsequent sintering or use at high temperatures for extended period of time. Typically, at least some residual porosity is present in the refractory ceramic body after sintering, though the residual pores are often smaller than the neighboring ceramic grains and a fraction of the remaining residual porosity may further disappear during annealing and subsequent use.
The present invention provides a refractory material suitable for use in a pyrolysis reactor apparatus for pyrolyzing hydrocarbon feedstocks at temperatures in excess of 1500° C. that are resistive to or avoid carburization and ceramic corrosion in active, carbon-rich environments. To achieve the desired function of avoiding the detrimental effects of carburization and carbide-oxide ceramic corrosion, the ceramic compositions according to this invention utilize a thermodynamically stable oxide having an appropriately high melting point, such as for example, an oxide of yttrium (yttria). In some high temperature, carbon-rich pyrolysis environments, a fully stabilized zirconia (e.g., at least 14 wt % or at least 18 wt % or at least 21 wt %, of stabilizer) may not sufficiently achieve the objective of preventing carburization and ceramic corrosion for an acceptable duration of commercial use. A further incremental amount of oxide-stable, refractory material is needed to achieve such objective. The present inventive materials comprise at least 50 wt % of a thermodynamically stable oxide material, (e.g., such as but not limited to yttria), and many embodiments comprise at least 80 wt % of such material, and still other embodiments comprise at least 90 wt % of such material, while still other embodiments are comprised of substantially all, or even consist substantially of such stable oxide material.
The problem process of carbide-oxide corrosion describes effects observed during transition of zirconia compound from an oxide to carbide and then sometimes back to an oxide, with associated undesirable precipitation of carbon and associated undesirable changes in morphological and crystal structure. Such structural changes may also precipitate micro-fractures that lead to structural weakening and provide increased pathways for still further carbon permeation and corrosion. For example, at high temperatures (e.g., >1500° C.) and high carbon partial pressure, zirconium oxide (ZrO2) may be reduced to zirconium carbide (ZrC) when in contact with solid carbon or in carbon-containing gases, through zirconium oxycarbide Zr(CxOy) as an intermediate phase. When ZrC is exposed to moderately high temperatures (>500° C.) in an oxidizing atmosphere that contains oxygen-containing gases, ZrC converts back to ZrO2 through Zr(CxOy) as an intermediate phase. The atomic oxygen displaces carbon in the interstitial vacancies of the ZrC crystal lattice, forming the intermediate, Zr(CxOy). Continuing oxidation of Zr(CxOy) leads to carbon precipitation at the ZrC/ZrO2 interface. Such retention of carbon with the oxidation of ZrC triggers further deposition and buildup of carbon from the process stream, further exacerbating the cyclic oxide-carbide-oxide transition problem and increasing porosity. The altered crystal structure suffers degradation in mechanical strength and thermal shock resistance.
The term oxide, as uses herein without additional specification, is taken to mean one or more elements that are molecularly combined with primarily oxygen, and specifically not primarily combined with carbon. The term carbide, as uses herein without additional specification, is taken to mean one or more elements that are molecularly combined with carbon, but which may also include molecular combination with oxygen. Thus, materials such as zirconium oxycarbide Zr(CxOy) are generally considered to carbides for the purposes of this application. Specifically, oxides, unless indicated otherwise in the present invention are refractory materials that have less than about 10 mole percent carbon, more preferably less than 5 mole percent carbon and even more preferably less than 1 mole percent carbon. Carbides, unless indicated otherwise in the present invention, are refractory materials that have greater than about 10 mole percent carbon.
Further explaining the problem, in the reverse-flow reactor conditions, materials are subjected to myriads of cycles of oxidizing/carburizing gases. Under these conditions, zirconia has been observed to gradually change its morphological appearance leading to “raspberry-like” grain morphology. The raspberry-like grain structure is an observable sign of carbide-oxide ceramic corrosion and can be determined by scanning electron microscopic (SEM) examination of the tested material. The raspberry-like grain structure is believed caused by the cyclic transitions and leads to increased surface area, which in turn leads to still further build-up of carbon, which in turn leads to still further cyclic corrosion transitions. Once the corrosion initiates, the process can accelerate until the effect causes an unacceptable level of material degradation.
After sintering, annealing, or high temperature use, some embodiments may have some residual porosity, while other embodiments may be reformed to a substantially dense matrix having little to no residual porosity such that substantially the only porosity present after sintering is the formed porosity (either void or filled as discussed above). According to the present invention the overall residual porosity of many exemplary refractory materials may be not greater than 15 vol %, or not greater than 10 vol %, often not greater than 7 vol %, other times not greater than 4 vol %, or not greater than 2 vol %, and in some instances not greater than 1 vol %. Residual porosity may not be detrimental to the refractory material so long as it has limited or substantially no effective permeability to carbon that could lead to carbon permeation within the interior of the refractory material. The residual porosity nearer the exposed surface may certainly be susceptible to some carbon permeation and thus potentially some ceramic corrosion. It may be preferable, however, in some embodiments to keep the residual porosity below some effective level, such as but not limited to not greater than 4 vol %, or 2 vol %, or even 1 vol %, to inhibit carbon permeation via residual porosity channels and pores. Those embodiments that have some residual porosity, e.g., greater than 1 vol %, or greater than 2 vol %, or more commonly greater than 4 vol % or even greater than 5 vol %, or even greater than 8 vol %, may become susceptible to carbon permeation and ceramic corrosion. In some embodiments or applications, however, such as where effective permeability is controlled, or where permeation is not a big concern, then substantial residual porosity fractions may be acceptable.
The inventive refractory ceramics can be characterized by total porosity (the aggregate of both formed porosity and residual porosity) in the range of not less than 4 vol %, or in a range of from 4 to 60 vol %, or from 4 to 50 vol %, or 4 to 40 vol %, or 4 to 30 vol %, or 5 to 40 vol %, or 5 to 35 vol %, or 5 to 30 vol %, or 10 to 50 vol %, or 10 to 40 vol %, or 10 to 30 vol %, of the bulk volume of the refractory ceramics.
In another embodiment, the inventive refractory material or apparatus may have formed porosity of not less than 4 vol %, in a range of from 4 to 60 vol %, or from 4 to 50 vol %, or 4 to 40 vol %, or 4 to 30 vol %, or 5 to 40 vol %, or 5 to 35 vol %, or 5 to 30 vol %, or 0 to 50 vol %, or 10 to 40 vol %, or 10 to 30 vol %, of the bulk volume of the refractory ceramics. In other embodiments, the inventive refractory material or apparatus may have formed porosity within a range of from 4 to 50 vol % formed porosity and from substantially 0 to 10 vol % residual porosity, for total porosity in a range of from 4 to 60 vol %. In another non-limiting exemplary form, the refractory ceramics material has formed porosity within a range of from 5 to 30 vol % and from substantially 0 to 10 vol % residual porosity, for total porosity in a range of from 5 to 40 vol %. In another non-limiting exemplary form, the refractory ceramics material has formed porosity within a range of from 5 to 30 vol % and from substantially 0 to 5 vol % residual porosity, for total porosity in a range of from 5 to 35 vol %. In another non-limiting exemplary form, the refractory ceramics material has formed porosity within a range of from 5 to 25 vol % and from substantially 0 to 5 vol % residual porosity, for total porosity in a range of from 5 to 30 vol %. In yet another non-limiting exemplary form, the refractory ceramics material or apparatus has from 10 to 15 vol % formed porosity and from about 0 to 1% residual porosity, with total porosity in a range of from 10 to 16%.
In many aspects of the invention, the pores comprising the formed porosity are not directly connected or in fluid communication with respect to adjacent formed pores, but instead may generally be considered “closed” with respect to adjacent formed pores. In many embodiments, the formed pores are substantially uniformly distributed within the refractory ceramics matrix as discrete pores. In many embodiments, the D50 pore size may be the same or larger than the D50 lower limit of particle mode size of the refractory ceramics. It may be beneficial that the D50 pore size of formed pores is not less than the D50 size of the smallest particle size mode of the refractory ceramics since pores of a size larger than the D50 size of such smallest grain mode size generally do not shrink or disappear during heat treatment (sintering or annealing) or when in use at high temperatures. Pores smaller than the smallest grain sizes tend to disappear during sintering, annealing, and high temperature use. The intended material or apparatus use, application, temperature, and similar parameters may determine a preferred range of ratio of pore size to grain mode size necessary to maintain a desired physical, mechanical, chemical stability, and thermal shock performance.
The D50 grain size of the refractory material may be in a range of from 0.01 μm to 2000 μm. The D50 pore sizes is sometimes preferably not less than the D50 grain size of the smallest grain mode, while in other embodiments, the D50 pore sizes are at least two times, or at least three times, or at least five times, or at least ten times, or at least 20 times, the D50 pore size of the smallest grain mode size. In one non-limiting exemplary form, the D50 pore size of the refractory material may be not less than 1 μm, or not less than 2 μm, or not less than 5 μm when the D50 particle size of the fine grit mode in a multimodal ceramic matrix system is not less than 1 μm. In another non-limiting exemplary form, the D50 formed pore size of the refractory ceramics may be within a range of from 10 μm to 15 μm and the D50 size of the refractory material grains may be within a range of from 3 μm to 5 μm. In yet another non-limiting exemplary form, the D50 formed pore size of the refractory ceramics is within a range of from 15 μm to 30 μm, when the D50 particle size of the fine grit grain mode is within a range of from 5 μm to 10 μm. In yet another non-limiting exemplary form, the D50 formed pore size of the refractory ceramics is within a grain size of from about 50 μm to 100 μm, when the D50 particle size of the fine grit is within a range of from 5 μm to 10 μm. In yet another non-limiting exemplary form, the D50 formed pore size of the refractory ceramic material is from 30 μm to 300 μm while the D50 particle size of the ceramic grains are within a range of from 3 μm to 30 μm. These exemplary ranges of particles and formed pore sizes are merely illustrative and are not an exclusive list of probable size combinations or particle to pore size ratios or ranges.
In many embodiments, it may be beneficial that the formed pores are substantially monosized. The term “monosized” as used herein refers to a modal size distribution of a collection of pores or beads which have a single, discernable maxima on a size distribution curve (weight percent or intensity on the ordinate or Y-axis, and pore (or bead) size on the abscissa or X-axis). Desirably, in the case of monomodal size distributions, at least about 75 vol % and often desirably higher, such as at least 85 vol %, of the pores have a three dimensional size diameter within two standard deviations of the average of the D50 pore size. This can be achieved in one aspect by use of a substantially monosized voiding agent (either sacrificial or durable) such as polymeric micro-beads, having a generally spherical shape and preferably a substantially smooth or regular outer surface during manufacture of the inventive refractory ceramic material. As a non-limiting example, such micro beads may be sized, for example, from a size within a range of from 0.1 μm to 2000 μm, or within a range of from 0.5 μm to 500 μm and can be made by various polymer materials including polyacrylics and polystyrene polymers having varying degrees of cross linking. A non-limiting example of preferred micro beads is made by poly(methyl methacrylate-co-ethylene glycol dimethacrylate). The D50 particle size of such micro beads may be, for example, 8 μm, 20 μm and 50 μm.
In some aspects, it may be beneficial that the D50 pore size is not less than the D50 grain size of the smallest grain mode since pores larger than the surrounding grains tend not to shrink or retract to the point of either disappearing or becoming too small to be of consequence in arresting micro-fracture propagation. It may be desirable that the pores retain at least half and preferably at least 90 vol % and even more preferably substantially all of their original size during subsequent heat treatments or use at high temperatures.
Pore sizes may be monomodal or multimodal. For example, if the ceramic grain structure is comprised of a multimodal grain size distribution, the advantages and benefits of a pore distribution throughout the ceramic matrix may be customized to provide those benefits to each specific grain mode through the use of a multimodal pore distribution. Thereby, smaller pores may be useful to provide micro-fracture propagation resistance to the smaller grain mode, while larger pore sizes may provide similar benefits to larger micro-fractures and grain modes. Provided that the mean size of the smallest pores or pore mode is larger than the lower limit of mean grain mode size of the sintered refractory ceramics, then a multimodal pore size distribution can be employed, e.g., bimodal, trimodal, etc. A bimodal pore size distribution refers to a collection of pores having two clearly discernable maxima on a pore size distribution curve and a multimodal pore size distribution refers to a collection of pores having three or more clearly discernable maxima on a pore size distribution curve. In one non-limiting exemplary form, a refractory ceramic may have a D50 grain size of 15 μm containing bimodal pores having a mode of small pore of 30 μm and larger pore mode with a D50 size of 100 μm. In yet another non-limiting exemplary form, a refractory ceramic may have a D50 grain size of 5 μm contain trimodal pore distribution having D50 mode of small pores of 10 μm in size, intermediate pores with a D50 size of 25 μm and large pores with a D50 size of 50 μm.
In many aspects of the present invention, the pores may be substantially spherical. Spherically shaped pores are particularly beneficial providing flowability during powder processing and green body fabrication and resisting pore collapse during sintering and use at high temperatures. A substantially spherical pore shape may be characterized by an aspect ratio or body factor of not greater than 2.5, or sometimes preferably not more than 2.0, or in other embodiments, not more than 1.5. The term “spherical” as used herein refers to a substantially geometrically symmetrical object where the set of all points in three dimensional space (R3) are at a distance R or relatively near R (within a multiple of not greater than 2.5) from a fixed point of that space, where R is a positive real number called the radius of the sphere. The aspect ratio of spherical shape is the ratio of its longest and shortest ‘axes’ of symmetrical objects that are described by just two measures (e.g. length and diameter). The aspect ratio can be measured by microscopy methods such as optical microscopy (OM) and scanning electron microscopy (SEM) in combination with image analysis software, wherein a two-dimensional shape is projected. In the microscopic images, the ratio of the longer dimension to its shorter dimension is the aspect ratio.
Another aspect of the invention is the micro-morphology of the refractory materials and components. The ceramic or continuous phase grains of the refractory material may be regular or irregular in shape, and many embodiments may be dispersed as, for example but not limited to, spherical, ellipsoidal, polyhedral, distorted spherical, distorted ellipsoidal or polyhedral shaped particles or platelets, or combinations thereof. The pores preferably are uniformly dispersed in the ceramic phase. In one form, at least 50% of the dispersed pores are such that the pore-to-pore spacing averages at least 10 nm. The pore-to-pore spacing may be determined for example by microscopy methods such as OM, SEM, or TEM. The discrete pore-to-pore spacing mitigates collapse of pores into the much larger size. A collapsed large pore may be detrimental to mechanical properties since it can not be uniformly dispersed in the composite microstructure and leads to stress concentrations due to severe local deformation.
In yet another aspect of the invention, an exemplary method for preparing refractory ceramics having uniformly dispersed pores comprises the following operations: a) providing a refractory ceramic composition, wherein the composition comprises one or more of, i) a refractory grade stabilized zirconia having closely packed grains, or ii) advanced dual oxide composites with improved chemical stability, or iii) conventional refractory grade stabilized zirconia with monomodal grit distribution; b) mixing the refractory ceramics composition with polymeric micro beads in the presence of a liquid which is insoluble to the micro beads to form a flowable powder mix; c) placing the flowable powder mix into a die set; d) uniaxially pressing, extruding or casting the die set containing the flowable powder mix at a pressure to form a green body; e) drying the green body to provide enough strength for handling; f) heating the green bodies through a time-temperature profile to effectuate burn-out of the polymeric micro beads and sintering of the green bodies to form a sintered refractory ceramics composition; g) cooling the sintered refractory ceramics composition to form refractory ceramic having uniformly dispersed micro pores.
Controlled heat treatment (e.g., sintering and annealing) may facilitate generation of vacated, uniformly dispersed pores in the microstructure of the refractory material after sacrificial beads are vaporized or burnt out. Controlled heat treatments may include at least two basic steps: polymer burnout and matrix sintering/annealing. Burnout temperature and time depend upon the type of sacrificial voiding agent used in creating the empty pores. For example, for polymer burnout, the produced green bodies are heated at temperatures typically in a range of from 300° C. to 1000° C., for times such as ranging from 10 minutes to 12 hours. For sintering or annealing, the burnt-out bodies are then further heated at temperatures typically in a range of from 1500° C. to 1800° C., for times such as ranging from 10 minutes to 4 hours. In some embodiments, the green body may be sintered at a temperature of not less than 1700° C. for not less than one hour. Both polymer burnout and sintering/annealing steps may be performed in an oxidizing atmosphere, a reducing atmosphere, or in an inert atmosphere, or under vacuum, or combinations thereof. For example, the oxidizing atmosphere may be air or an oxygen enriched atmosphere, the inert atmosphere may be argon, and the reducing atmosphere may be hydrogen.
In some embodiments, the invention may include a refractory material comprising an yttria stabilized or yttria fabricated refractory material (includes but is not limited to Y2O3 and/or another yttrium-containing compound). Such inventive materials may often include at least 21 wt % yttria based upon total weight of the refractory material. However, depending upon the carbon concentration and partial pressure, the useful longevity of such embodiments having an yttria concentration of less than 50 wt % (e.g., 21-49 wt %) may be unacceptably short for some applications, while still other applications may be fine with such composition. Compositions having an yttria composition of at least 50 wt %, based upon the total weight of the refractory material, may be preferred in many applications. In other embodiments, the refractory material may consist essentially of a ceramic material comprising yttria and/or zirconia, notwithstanding the minor presence of additives such as manufacturing and processing additives.
In another aspect, the inventive refractory materials and components may include a stabilized zirconia ceramic material, stabilized primarily by yttria although other stabilizing compounds or elements may also be present in concentrations that are secondary to the yttria concentration. In such stabilized embodiments, yttria may be present in an amount of at least 21 wt % and in many embodiments at least 25 wt %, and in many other embodiments at least 28 wt %, or at least 30 wt % yttria, based upon the total weight of the stabilized zirconia composition present. Such amounts may well exceed the minimum amount needed merely for full stabilization. In many embodiments, at least a portion of the zirconia is fully stabilized and in other embodiments substantially all of the zirconia present is fully stabilized. Generally, fully stabilized zirconia may be preferred, and even an excess amount of yttria may be preferred, in order to avoid detrimental risk of progressive loss of stabilizer and progressive carbide-oxide ceramic corrosion. Additional material pertaining to the zirconia and stabilized zirconia embodiments are found within the patent application, U.S. Ser. No. 12/467,832, filed 18 May, 2009, titled “Pyrolysis Reactor Materials and Methods.” Other embodiments may comprise at least 50 wt % yttria, or at least 60 wt % yttria, or at least 75 wt % yttria, or at least 90 wt % yttria, or consist essentially of yttria. The inventive ceramic materials also include formed pores or porosity.
Formed porosity may be created by any suitable means, such as with a cavitating agent, beads, foreign media particles not part of the chemical structure of the ceramic matrix, by gas or other fluid droplets introduced into the wet ceramic matrix, or by introduction of a sacrificial material such as a polymer (e.g., polymethylmethacrylate, PMMA) or other volatizable material. Formed porosity may be a substantially empty cavity or the cavity may be occupied by a pore-forming agent, such as glass or metal beads, such that each pore is actually a heterogeneous sphere or other particulate feature within the ceramic matrix. The pore or formed-porosity creating material may be left fully or partially intact within in the sintered ceramic composition or in many embodiments the material may be removed such as by vaporizing the material during sintering, leaving a substantially empty cavity or pore. In many embodiments, the formed pores are voided or substantially voided after removal of a sacrificial or volatizable material (e.g., removable, regardless of specific mechanism), such as but not limited to a polymer, gel, or resin material, or other void-forming material that may be vaporized or otherwise removed at a temperature not greater than the sintering or material use temperature. Polymeric materials may be preferred for use in many embodiments, for example but not limited to PMMA, polyethylene, polypropylene, styrene, etc.
In addition to or instead of using a sacrificial polymer or resinous voiding agent to create formed porosity, a non-sacrificial material (e.g., a non-removable or durable), such as hollow or solid grains or particles may be used as a pore forming agent, producing a filled pore as opposed to a vacant pore. Formed porosity includes the combined total of both the formed vacant pore fraction (if any) and a formed durable component pore fraction (if any). Often, the formed porosity is created using at least one of a sacrificial voiding agent, a durable voiding agent, and/or a combination of sacrificial and durable voiding agents.
As a non-limiting example of a durable voiding agent, the hollow grains may comprise thermal spray grade glass or ceramic grains. The hollow-sphere ceramic grains are formed into agglomerated ceramic particles in a spray drier. The agglomerated particles are introduced into a plasma flame or a high velocity oxy fuel (HVOF) flame which can be adjusted so that the particles collected are substantially hollow. Alternatively, the hollow ceramic grains are produced by applying a solid layer to a substantially spherical particle of foamed polymer and pyrolyzing the coated polymer cores. To produce hollow ceramic grains economically, the particles consisting of foamed polymer may be treated in a state of agitation with an aqueous suspension that contains dissolved or suspended binder and metallic and/or ceramic powder particles, whereby a coating is formed on the polymer particles. The coating is then dried and the polymer particles and least part of the binder are subsequently pyrolyzed at temperatures from 400° C. to 1000° C. The resulting ceramic hollow spheres or beads are sintered at temperatures from 1000° C. to 1500° C. Such ceramic hollow grains may be useful to produce the inventive formed refractory material.
Many of the components in the reactor bed made from the refractory material have flow channels for conveying pyrolysis materials or related media within the reactor. Often these channels are conduits, such as but not limited to the conduits within a honeycomb reactor monolith, flow passages, or mixers. For purposes of determining refractory material porosity, the flow channel volume is not included in the determination. The relevant fraction is the fraction in the component walls. For example, honeycomb monoliths may often have 50-60 vol % bulk volume as flow channels, but the channel walls may only have 5% porosity. For the purposes of this invention, the relevant portion is the 5% wall porosity.
In many aspects, it may be preferred that after sintering and annealing, the formed porosity is comprised of substantially “closed” or isolated pores, as compared to “open” or interconnected pores. Open pores are typically within permeable fluid communication with each other. Closed pores typically do not have fluid permeability with each other. Because of the potential ceramic corrosion effects and because hydrocarbon gases that penetrate the refractory material porosity may tend to make coke in that porosity, it may often be preferable that the inventive refractory material include a high percentage of the porosity as substantially closed pores that are not well connected with or permeable to the wall surface.
To achieve such closed pores that are remote to the surface, it may be preferable to utilize a sacrificial polymer that burns out or vaporizes at a temperature that is less than at least one of the sintering or annealing, or use temperature. Thereby, after the vaporized polymer permeates through the residual porosity and formed porosity to the material surface, then sintering and annealing can remove that residual porosity and permeability such that the formed pores that are not at or close to the material surface become isolated or closed with respect to vapor permeation. Thereby, the interior pores should not be susceptible to coking, provided they do not later become susceptible due to micro-fracturing. For such reasons, it may also be useful that the refractory material resists fracture propagation. The inventive material provides such resistance and useful advantages.
In addition to use of sacrificial void forming materials, the above described solid or hollow spheres, or other remaining durable porosity/pore-creating material can also create substantially “closed” porosity or that is in the interior of a well sintered or annealed refractory material that is remote to the wall surface. Porosity created using durable materials that are not removed during sintering and annealing also beneficially produces porosity, albeit that porosity is substantially “filled” and thus not available for permeation by the hydrocarbon vapor and are thus resistant or immune to ceramic corrosion. Such porosity, however, may also be useful at arresting micro-fracture propagation.
In one aspect, the present invention includes a reactor apparatus for pyrolyzing a hydrocarbon feedstock, the apparatus including: a reactor component comprising a refractory material in oxide form, the refractory material having a melting point of not less than 2060° C. and which remains in oxide form when exposed to a gas having carbon partial pressure of 10−22 bar (1×10−22) and oxygen partial pressure of 10−10 bar, measured at a temperature of 1200° C.; wherein said refractory material has not less than 4 vol % formed porosity, measured at 20° C., based upon the bulk volume of said refractory material. The term “remains in the oxide form” as used herein, means that the refractory material has a thermodynamic preference to be in the oxide form (state) at the stated conditions, notwithstanding the possibility that the material may change its state very slowly at the stated conditions. For the present invention, the rate of change of a material's state (e.g. the kinetics of any change of state) is of secondary importance. A primary consideration is the thermodynamic equilibrium condition that will ultimately exist at stated conditions. A material's acceptability is stated in terms of a test that can be evaluated at, for example, 1200° C. However, the materials of the present invention are designed to be used at relatively high temperatures, wherein kinetics will be fast and thermodynamic behavior is of primary importance.
In other many embodiments, the reactor apparatus comprises a regenerative pyrolysis reactor apparatus. In various other embodiments, the regenerative pyrolysis reactor comprises a reverse flow regenerative reactor apparatus. Many reactor embodiments may utilize a deferred combustion process to heat the reactor. In many embodiments, the refractory material has a melting point of not less than 2010° C., and in still other embodiments not less than 2160° C. The gas used to determine the partial pressures of the carbon and oxygen is a reference gas (or gas mixture) that at the stated temperature includes both carbon and oxygen species. Substantially any gas that at such temperature comprises the stated carbon and oxygen partial pressures may be used to determine the oxide stability. In many aspects, the reference gas may be a laboratory gas sample that is specified to have a carbon partial pressure of, for example 10−22 bar, (measured by any test method or known physical property) and an oxygen partial pressure of, for example 10−10 bar, at 1200° C. When a ceramic or refractory material sample is exposed to this reference gas, the sample material remains in oxide form. Materials meeting such requirements, in conjunction with the requisite melting point, may be acceptable according to the present invention, for preventing carburization and/or ceramic corrosion.
In other embodiments, the refractory material has a thermodynamic preference to remain in oxide form when exposed to a gas having a carbon partial pressure of 10−15 bar, an oxygen partial pressure of 10−10 bar, measured at a temperature of 1800° C. In still other embodiments, refractory material remains in oxide form when exposed to a gas having a carbon partial pressure of 10−14 bar and oxygen partial pressure of 10−10 bar, measured at a temperature of 2000° C. In many embodiments, the refractory material remains in oxide form when exposed to a gas having a carbon partial pressure of 10−12 bar and an oxygen partial pressure of 10−10 bar, measured at any and all temperatures over the full range of from 1500° C. to 2000° C.
The refractory material of the present invention overcomes the afore-stated challenges. Unlike ZrO2, it has been learned that more thermodynamically stable oxides can avoid the carbide-oxide phase shift problems. For example, yttrium oxide or yttria (Y2O3) is thermodynamically stable in the presence of carbon-containing gases and does not form yttria carbide (YC). The undesirable compound yttrium carbide (YC) is formed from metallic yttrium (Y), not from Y2O3. Since the stable oxide form, (e.g., Y2O3) is relatively inert (e.g., thermodynamically stable) to carbon or carbon-containing gases as compared to zirconia, the more stable oxide form (e.g., Y2O3) appears to suppress detrimental transition from carbide to oxide. Surprisingly, it has been learned that at sufficient level of the stable ceramic oxide (whether yttria or another suitable ceramic oxide) the carbide-oxide corrosion mechanism can be inhibited or even prevented. In many embodiments, the preferred concentration of stable oxide is determined to be at least 50 wt %, based upon the total weight of the ceramic material. Other suitable and sometimes more preferably levels of the stable oxide form may be at least 70 wt %, 80 wt %, 90 wt %, 95 wt %, 99 wt %, substantially 100%, or 100 wt %, including mixtures of such suitable oxides at such wt % levels. The mechanism by which the stable oxides work is still not wholly understood and is subject to some speculation and uncertainty, but the inventive methods and material formulations have been determined successful at inhibiting or avoiding carburization and the carbide-oxide corrosion mechanism.
In addition high concentrations of suitable oxides, such as yttria, high concentrations of calcia (CaO), magnesia (MgO), and/or ceria (CeO2) might also offer some limited resistance against such corrosion. However, it has also been learned that for many high temperature hydrocarbon applications, calcia and magnesia stabilizers are more volatile than yttria and are therefore potentially of less value as either a stabilizing oxide component than yttria, although the presence of such other stabilizers materials within the composition is not precluded. In many embodiments, the stable oxide refractory materials have a vapor pressure that is less than 10−7 bar at 2000° C. Further, it has also been learned that in addition to thermodynamic stability, suitable oxides should have a melting point of not less than 2060° C. for practical application for long periods of time at pyrolysis temperatures in excess of 1500° C. and up to 2000° C. Suitable oxides may also preferably have melting temperature of not less than 2110° C., or 2160° C., or preferably not less than 2300° C.
In some embodiments, the refractory material may further comprise one or more “secondary oxides” selected from the group consisting of Al, Si, Mg, Ca, Y, Fe, Mn, Ni, Co, Cr, Ti, Hf, V, Nb, Ta, Mo, W, Sc, La, and Ce, and mixtures thereof. The secondary oxides may be merely incidentally present, such as impurities or via contamination or as a result of the sintering, annealing, or other manufacturing process. The secondary oxides may also be purposefully added, such as to improve certain properties or uses, e.g., such as processability during manufacture; or may be generated and deposited as a bi-product from the thermal process and other materials present. The amount of secondary oxides in the stabilized zirconia formed component may typically range from virtually none present, up to 5 wt %, or from 0.001 wt % present up to 10 wt %, or in some embodiments from 0.01 wt % to 5 wt %, or typically in still other embodiments from 0.1 to 3 wt %, based on the weight of the formed stabilized zirconia component.
According to the present inventions, there are two key features of a material that must be satisfied to avoid carburization of ceramic materials during high temperature pyrolysis of hydrocarbon feedstocks. First, the material must have a high melting point. Second, the material must have a thermodynamic preference to remain in the oxide form when exposed to a pyrolysis gas mixture having high carbon activity.
Satisfactory refractory materials for a high-temperature regenerative pyrolysis reactor must have a melting point that is higher than that of pure alumina (2050° C.), which was a preferred material under much of the prior art. Due to impurities and practical use limitations, alumina was not suitable for commercial pyrolysis at temperatures above 1500° C. Satisfactory materials for use in the high temperature zone of the reactor have a melting point no less than 2060° C. More preferred materials have a melting point above 2110° C., and even more preferred to have a melting point above 2160° C.
Another key feature that defines a refractory material's ability to resist carburization and carbide-oxide corrosion is the degree to which the oxide form of the material is stable under the pyrolysis conditions. Materials whose oxide form is stable under all pyrolysis conditions are found to be free of carbide-oxide corrosion. A non-limiting exemplary material is pure Yttria. Materials whose oxide form is stable (“remains in oxide form,” as used or defined herein) under most pyrolysis conditions are found to be resistant to carbide-oxide corrosion. High-yttria zirconias may fall into this category. A non-limiting example is high yttria zirconias, such as greater than 21 wt % yttria, or greater than 28 wt % yttria, or greater than 40 wt % yttria, or greater than 50 wt % yttria, or greater than 70 wt %, or 80 wt %, or 90 wt % or substantially all yttria. Materials that have stable carbide forms under a large portion of the pyrolysis conditions may be subject to unacceptably rapid carburization and carbide-oxide corrosion and thus may not be suitable for use in high-temperature zones of hydrocarbon pyrolysis reactors. For non-limiting example, unstabilized and low-yttria zirconias (e.g., <11.4 wt % yttria) fall into this category. A detailed description of thermodynamic stability follows below.
Phase stability diagrams were generated by calculating the boundaries between ranges of stable solid species. These boundaries are plotted as functions of environmental variables such as the activities or partial pressures of oxygen and carbon species. Here, the term “activity” refers to the effective concentration of a species in a mixture. For carbon, the species C may not normally be thought of or measured as a gas phase species, but its gas phase concentration (or gas-law “activity”) in a hydrocarbon mixture can be calculated in relation to other gas phase (e.g., hydrocarbon) species via thermodynamic equilibrium procedures. This gas phase carbon (as C) activity (or partial pressure) is used herein to define the environment of stability for oxides and carbides of refractory materials. The term “partial pressure” is the effective fraction of the total pressure due to a gaseous species in a gas mixture. Stated differently, in a mixture of gases, a single gas's partial pressure therein is the pressure that such gas would have if it alone occupied the volume. It is noted that partial pressure numerically approximates the activity for a gaseous species. While the actual diagrams were generated using a commercial software known as HSC Chemistry [Version. 5.11, Outokumpu Research Oy, Finland (2002)], the underlying calculations can be done by a person skilled in the art of thermodynamics. This type of calculation involves recording a mass action equation that describes the equilibrium between two solid species plus the environmental species. The activities of these two solid species are then set to one to indicate their co-existence, and the mass action equation is converted into an algebraic equation that mathematically defines the range boundary between these two solid species as a function of environmental variables such as carbon partial pressure or oxygen partial pressure. Similar calculations can then be repeated for all pairs of solid phases to map phase stability diagrams.
As an illustrative example,
Referring to
The boundary conditions for co-existence of both carbide and oxide phases of yttrium are calculated by the following equation.
Y2O3+4C(g)=2YC2+1.5O2(g) [1]
Similarly the boundary conditions for co-existence of both carbide and oxide phases of zirconium are calculated by the following equation.
ZrO2+4C(g)=ZrC4+1.5O2(g) [2]
As an illustrative example, Table A summarizes carbon partial pressure values calculated at temperatures in the range of 1200° C. to 2000° C. and oxygen partial pressure of 10−10 bar, which is typical of a hydrocarbon pyrolysis condition.
For example, at 2000° C. and pO2(g)=1.0×10−10, yttrium carbide (YC2) is stable at carbon partial pressure of greater than 1.69×10−7 and yttrium oxide (Y2O3) is stable at carbon partial pressure of lower than 1.69×10−7. At pC2(g)=1.69×10−7 both YC2 and Y2O3 coexist. Similarly, at 2000° C. and pO2(g)=1.0×10−10 zirconium carbide (ZrC4) is stable at carbon partial pressure of greater than 3.77×10−15 and zirconium oxide (ZrO2) is stable at carbon partial pressure of lower than 3.77×10−15. At pC2(g)=3.77×10−15 both ZrC4 and ZrO2 coexist. Therefore, under the condition of a pyrolysis gas mixture having pO2(g)=1.0×10−10 and pC2(g) in the range of 3.77×10−15 and 1.69×10−7 at 2000° C., both Y2O3 and ZrC4 are stable.
These points are illustrated in
Therefore, suitable refractory materials according to the present inventions include materials in the oxide form and materials which also have a thermodynamic preference to remain in the oxide form when exposed to a pyrolysis gas mixture having carbon partial pressure of greater than 10−22, at conditions of 1200° C. and pO2(g)=10−10. Preferably materials will remain in the oxide form at carbon partial pressure of greater than 10−18 (at conditions of 1200° C. and pO2(g)=10−10), and some materials will remain in oxide form at carbon partial pressure greater than 10−14 (reference conditions of 1200° C. and pO2(g)=10−10). A non-limited example of such refractory material is yttria (yttrium oxide, Y2O3).
It is also preferred that melting point of the refractory material is no less than 2060° C., which is about 10° C. higher than that of pure alumina (2050° C.) which years of commercial experience demonstrated as inadequate for many high temperature processes and applications. A suitable refractory material having a melting point no less than 2060° C. is particularly preferred since it enables to process hydrocarbon feedstocks at much higher temperatures. Thus, a regenerative pyrolysis reactor apparatus for pyrolyzing a hydrocarbon feedstock, the apparatus including a revere flow regenerative pyrolysis reactor is comprised of a material having i) melting point no less than 2060° C. and ii) a thermodynamic preference to be in the oxide form when exposed to a reference gas having a carbon partial pressure of greater than 10−22 bar. In many embodiments, a preferred reactor is a regenerative reactor, and in other embodiments the reactor is a reverse flow reactor, while in still other embodiments the reactor is a deferred combustion reverse flow regenerative pyrolysis reactor.
(Exemplary refractory material, but no formed porosity.) An yttria ceramic composition was prepared by mixing 50 wt % of the first grit Amperit 849.054 Y2O3 powder (H.C. Starck), 30 wt % of the second grit Y2O3 powder (1 μm average particle size, 99.9%, from Alfa Aesar) and 20 wt % of the third grit Grade C Y2O3 powder (H.C. Starck). About 30 wt % additional organic binder was mixed with the ceramic powder to provide green body strength during forming process. The powder was extruded into a honeycomb shape (300 cpsi) in the size of about ¾″ diameter×1.0″ thickness. The extruded honeycomb green body was then sintered at 1600° C. for 4 hours in air and annealed at 1900° C. for 4 hours in argon to form a fired ceramic body. The resultant yttria ceramic body was also comprised of about 15 vol % porosity mainly derived from the first grit Amperit 849.054 Y2O3 powder. This residual porosity provides thermal shock resistance. The produced yttria honeycomb sample was tested in a reverse-flow reactor for about 24 hrs wherein a pyrolysis gas mixture has carbon partial pressure of about 1.0×10−12 bar. The tested sample was microscopically examined by use of scanning electron microscopy (SEM) and revealed no signs of carbide-oxide ceramic corrosion.
(Exemplary refractory material with porosity.) 75 vol % of coarse grit (−105+44 μm particle size range) of stabilized zirconia powder (10˜15% Y2O3, from Alfa Aesar) and 10 vol % of fine grit (1 μm D50 average particle size) of Y2O3 powder (99.9%, from Alfa Aesar) were mixed with 15 vol % polymeric micro beads (15 μm diameter Spheromers® CA15, from Micro beads AS, Norway) by use of HDPE milling jar. The mixture was milled for 10 hours without milling media in a ball mill at 100 rpm. Water was removed from the mixed powders by heating on a hot plate. The dried powder mix was compacted in a 40 mm diameter die in a hydraulic uniaxial press (SPEX 3630 Automated X-press) at 5,000 psi. The resulting green disc pellet was ramped up to 800° C. at 25° C./min in air and held at 800° C. for 2 hrs to decompose polymeric micro beads and then heated to 1600° C. in air and held at 1600° C. for 4 hours. The temperature was then reduced to below 100° C. at −15° C./min. The resultant refractory material comprised: i) 75 vol % coarse partially stabilized zirconia with average grain size of about 50 μm; ii) 10 vol % fine Y2O3-rich oxide with average grain size of about 2 μm; iii) 15 vol % formed porosity comprising uniformly dispersed, substantially monosized pores, with less than 1 vol % residual porosity.
(Exemplary refractory material with porosity.) Fine Y2O3 powder (H.C. Starck, Grade C; D90=2.5 μm, D50=0.9 μm, D10=0.4 μm) was mixed with 15 vol % polymeric micro beads (15 μm diameter Spheromers® CA15, from Micro beads AS, Norway) in water by use of HDPE milling jar. The mixture was milled for 10 hours without milling media in a ball mill at 100 rpm. The water was removed from the mixed powders by heating on a hot plate. The dried powder mix was compacted in a 40 mm diameter die in a hydraulic uniaxial press (SPEX 3630 Automated X-press) at 5,000 psi. The temperature of the resulting green disc pellet was ramped up to 800° C. at 25° C./min in air and held at 800° C. for 2 hrs to decompose and vaporize the polymeric beads and then heated to 1600° C. in air and held at 1600° C. for 4 hours to sinter the disk pellet. The temperature was then reduced to below 100° C. at −15° C./min. The resultant refractory ceramics with uniformly dispersed micro pores comprised: i) 73 vol % yttria matrix with a D50 average grain size of 3 μm; ii) 15 vol % formed porosity with a D50 average pore size of 15 μm; iii) not greater than 2 vol % residual porosity with a D50 average pore size of 0.5 μm.
The terms “grain” and “particle” are used interchangeably herein. The “grains” that are formed and are observable after sintering, are derived from a mixture of particles (some of which might also include several grains, however) that are mixed and combined prior to sintering. Grain size or particle size, refers to the effective diameter or geometric size of individual grains of the matrix comprising a multimodal grain distribution. A grain or particle is the individually definable substantially homogenous units of zirconia or yttria or other granular material that forms the entirety of the ceramic material or component. The grains or particles are sintered and bonded together at grain boundaries to create a formed ceramic component. Dynamic light scattering and laser light diffraction analysis using a unified scatter technique (Microtrac® 3500) can be used to determine average particle size and particle size distribution. Microtrac® instruments can measure particle size ranging from 0.024 to 2800 μm and provide good instrument-to-instrument agreement, sample-to-sample agreement, instrument internal repeatability and particle distribution breadth.
The “D50” or average particle size or pore size (vacated or filled) measured by a laser light diffraction method is one type of average particle size represented as D50 or mean diameter. The D50 average particle size or pore size is a value determined by using a feature size distribution measuring device and represents a particle or pore sample cut diameter which is the 50% volume or weight fraction value after determining the minimum and maximum sizes contributing to the integrated volume of a specific peak of a particle size distribution. Similarly D90, D10, D99 respectively corresponds to the 90, 10 and 99% volume or weight fractions of the particle or pore size distribution. The average (D50) or any other particle size cut value can be determined by microscopy methods such as optical microscopy (OM), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The average particle size values measured by microscopy methods also can be converted to D50 values by methods known in the field. The particle size distribution of the first grains alternatively can be determined by a sieve and mesh classification method as known in the art.
The ceramic material grains or particles can be substantially any shape. In many embodiments, a preferred shape may be those particle shapes that are substantially spherical or more spherical in dimensions than nonspherical. Some non-limiting acceptable examples include spherical, ellipsoidal, polyhedral, distorted spherical, distorted ellipsoidal, distorted polyhedral shaped, angular, rectangular, tetrahedral, quadrilateral, elongated, etc. The shape of smaller grain particles may generally be of less importance than the shape of relatively larger particles. Spherical grains may be particularly beneficial in providing close packing, density, optimum porosity, and flowability during powder processing and fabrication. A preferred spherical shape of the first grain can be characterized by an aspect ratio less of than 2.5, or preferably less than 2.0, or more preferably less than 1.5. Grains with generally smoother surfaces may also be preferred as compared to grains having highly irregular surface shapes. Spherical shape refers to a symmetrical geometrical object where the set of all points in three dimensional space (R3) which are at the distance R from a fixed point of that space, where R is a positive real number called the radius of the sphere. The aspect ratio of a shape is the ratio of its longest axis to its shortest axis. The aspect ratio of symmetrical objects may also be described by the ratio of two measures (e.g. length and diameter). The aspect ratio can be measured by microscopy methods such as optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM), in combination with image analysis software, wherein a two-dimensional shape is projected.
In some embodiments, the inventive refractory material, components, and reactor systems may include ceramic matrix grains having a D50 grain size in the range of from 0.01 μm up to 2000 μm, while in other embodiments, the reactor system may include grain sizes having a D50 grain size in the range of from 0.01 μm to 800 μm, in other embodiments, from 5 μm to 800 μm. The inventive composition might also include other grain sizes, many of which will have a minimum grain size of at least 0.01 μm, or at least 0.125 μm, or at least 0.2 μm, or at least 1 μm, or at least 2 μm, or at least 5 μm, or at least 10 μm. An upper limit for the other grain sizes present may include grain sizes of up to 400 μm, or up to 200 μm, or up to 100 μm, or up to 50 μm, or up to 40 μm, or up to 20 μm, or up to 10 μm, or up to 5 μm, or up to 2 μm, or up to 1 μm. The aforementioned list of grain sizes is merely exemplary and not exhaustive. Other similar ranges of grain sizes are useful for the inventive composition. The size preference may be determined by the particular component being prepared and its intended use, temperature ranges, and stress conditions. For example, dimensionally larger, heftier components can utilize grain sizes within the broader ranges, including the larger ranges, while more intricate or delicate components such as thin-walled honeycomb monoliths or other relatively delicate or high stress component might particularly benefit from the relatively smaller grain size ranges.
In some embodiments, the inventive refractory material, components, and reactor systems may include formed porosity pores (vacated (sacrificial), filled (durable), or partially filled) having a D50 pores in the size range of not less than 0.01 μm up to 4000 μm. In other embodiments, the refractory materials may have formed pores having a D50 pore size in the range of from 0.01 to 2000 μm, and the D50 diameter of said formed pores is within a range of not less than 0.01 μm up to 4000 μm. Still other embodiments may have a formed pores having a D50 pore size of from 0.01 μm up to 2000 μm, while in other embodiments, the reactor system may include pore sizes having a D50 pore size in the range of from 0.01 μm to 800 μm, in other embodiments from 5 μm to 800 μm. The inventive composition might also include other pore sizes, many of which will have a minimum pore size of at least 0.01 μm, or at least 0.125 μm, or at least 0.2 μm, or at least 1 μm, or at least 2 μm, or at least 5 μm, or at least 10 μm. An upper limit for the other pore sizes present may include pore sizes of up to 2000 μm, or up to 1500 μm, or up to 1000 μm, or up to 800 μm, 400 μm, or up to 200 μm, or up to 100 μm, or up to 50 μm, or up to 40 μm, or up to 20 μm, or up to 10 μm, or up to 5 μm, or up to 2 μm, or up to 1 μm. The aforementioned list of pore sizes is merely exemplary and not exhaustive. Other similar ranges of pore sizes are useful for the inventive composition. The size preference may be determined by the particular component being prepared and its intended use, temperature ranges, and stress conditions. For example, dimensionally larger, heftier components can utilize pore sizes within the broader ranges, including the larger ranges, while more intricate or delicate components such as thin-walled honeycomb monoliths or other relatively delicate or high stress component might particularly benefit from the relatively smaller pore size ranges.
As stated previously, in addition to a use of a ceramic composition meeting certain oxide and partial pressure functional limitations as useful at controlling carburization and carbide-oxide ceramic corrosion, it has recently been learned that porosity can also play a significant role in controlling ceramic corrosion. In addition to arresting micro-fracture propagation, the formed porosity may also facilitate small scale matrix flexibility among the ceramic matrix particles while also permitting dissipation of both mechanical and thermal stress concentrations (particularly with respect to arresting crack propagation). Although porosity plays a significant role with respect to the high strength and thermal shock resistance of the inventive ceramic materials, a downside to the porosity is that too much porosity or too much interconnected porosity (open pore system) may permit coking and infiltration of carbon within the ceramic structure, which in turn may lead to localized ceramic corrosion cells. Surprisingly, it has been learned that a balance must be struck between too much porosity and too little. According to the present invention, many suitable refractory materials may include highly dense materials that have less than 5 vol % residual porosity, and in other embodiments less than 2 vol % residual porosity, or less than 1 vol % residual porosity, or even substantially no residual porosity in some embodiments. Such residual-porosity-dense bodies may benefit from improved resistance to carbon permeation or carburization while facilitating addition and use of a beneficial quantity of formed porosity which affords improved benefit of matrix flexibility and strength as compared to residual porosity, and also limited carbon permeation as compared to residual porosity. In many preferred embodiments, the formed pores, whether vacant or filled, have a D50 diameter of not less than the D50 grain size of the refractory material. In many other embodiments, the formed pores have a D50 diameter in a size range of from not less than the D50 grain size of the refractory material up to five times the D50 grain size of the refractory material.
In many embodiments, the refractory material comprises grains of yttria, an yttria stabilized compound (e.g., with a metal such as zirconia, a zirconia stabilized compound, or other suitable ceramic materials). Suitable embodiments include some formed porosity value to take advantage of the previously disclosed mechanical strength and thermal shock resistance benefits of a slightly porous or somewhat porous ceramic body, preferably having substantially uniform sized and evenly distributed pores or durable pore components. In some embodiments, the inventive material or compositions have a minimum total porosity value, such as at least 4 vol %, or in some various embodiments, at least 5 vol %, or at least 8 vol %, or at least 10 vol %, or at least 15 vol %, or at least 20 vol %, or sometimes at least 25 vol %, or even up to 28 vol %, including various combinations of ranges thereof. More importantly however from the standpoint of preventing ceramic corrosion problems, the inventive material or compositions may have a maximum total porosity value, such as up to 10 vol %, or up to 15 vol %, or up to 20 vol %, or up to 25 vol %, or up to 30 vol %, or up to 40 vol %, or up to 50 vol %, or perhaps in some embodiments even up to 60 vol %, based upon the bulk volume of the inventive material. The total porosity range of 4 to 60 vol % as used herein is defined for the limited purpose of preventing carburization and carbide-oxide ceramic corrosion, with the desired, more specific narrower porosity range defined after consideration of other performance properties such as but not limited to strength and thermal shock resistance. For porous embodiments, a suitable total porosity range may be from 4 to 60 vol %, or from 4 to 40 vol %, or from 5 to 35 vol %.
Fully dense or highly dense refractory materials according to the present invention may enjoy the benefit of limiting carbon permeation and resistance thermal distortions, but may over prolonged use be susceptible to ceramic corrosion induced micro-fracturing. The inventive refractory materials including formed pores as taught herein may enjoy ceramic corrosion resistance, as well as resistance to micro-fracture propagation, thereby reducing the depth to which the component may be susceptible to ceramic corrosion. Such embodiments, particularly those in the oxide form, may also enjoy sustained purity and stability to remain in the oxide form. In other embodiments, the prescribed porosity feature may be attributed with providing improved thermal stress crack resistance by inhibiting crack formation and arresting crack propagation, and simultaneously facilitating some elastic deformation of the matrix structure, thereby providing enhanced service life in the stressful, high temperature, cyclic thermal applications. Porosity also provides high surface area for improved heat absorption and dissipation, as compared to typically less-porous, high-density, high-strength ceramics such as common refractory and engineering grade ceramics. Some of the other various functions of the porosity include but are not limited to increased surface area for heat transfer, increased thermal shock resistance, mitigation of crack propagation, improved flexural strength, etc. Selecting the most appropriate density or porosity range depends upon consideration and balancing of these and other various performance objectives, including controlling ceramic corrosion, to ascertain the most desirable porosity for a particular material or component. The porosity of the ceramic matrix of the formed ceramic component is measured at ambient temperature, such as at 20° C., after sintering the green body for at least ten minutes at 1500° C. Preferably, the desired porosity range is retained after prolonged exposure to the operation temperature and after annealing or at operational temperatures above 1500° C., such as up to 1800° C. or even up to 2000° C. Porosity is preferably substantially uniformly dispersed throughout the composition.
It has recently been learned that limiting the maximum total and formed porosity to values that preserves pore isolation (e.g., not more than 60 vol % total or formed porosity) tends to effectively, if not actually limit interconnectivity of the pore spaces with other pore spaces to an extent that inhibits migration of elemental carbon or carbon compounds within the inventive material or components (e.g., limited effective permeability to carbon and carbon compounds). This limited permeability helps mitigate or limit carbon penetration among the pores and matrix of the inventive material during high temperature pyrolysis of hydrocarbon feedstocks, thereby mitigating or limiting the potential for carbide-oxide ceramic corrosion. Carbon may still penetrate the surface pores and perhaps some near-surface pores of the inventive material or components during hydrocarbon pyrolysis, but the limited porosity will constrict carbon from permeating deeper or dissipating throughout the entirety of the material or component, due to lack of effective permeability to carbon or carbon compounds. However, suitable refractory materials in the oxide form should be thermodynamically resistive to alteration to the carbide or other forms, even in the presence of concentrated carbon at pyrolysis conditions. The optimal or acceptable porosity ranges for the inventive material or component depends upon the desired final component performance properties, but is within a range defined by one or more of the minimum porosity values and one or more of the maximum porosity values, or any set of values not expressly enumerated between the minimums and maximums (e.g., between substantially 0 vol % and 28 vol %).
In addition to the desirability of controlling carburization and ceramic corrosion by the presence of the oxide form in the refractory material and such material having a melting point of no less than 2060° C., the overall performance characteristics of the inventive ceramic material may also be attributable at least in part to one or more of various other inventive material characteristics such as but not limited to, the particle size and arrangement, or in some embodiments the multimodal particle sizes and distribution, material selection, degree of stabilization, manufacturing methods and techniques used, actual porosity, and combinations thereof. However, the combination of the porosity and the substantial yttria concentration uniquely preserves an otherwise improved combination of MOR flexural strength, thermal shock resistance, and component life expectancy that was not previously known in the industry. The virtues and functions of these other performance parameters and features have been described in previous patent applications, including U.S. patent application filed Dec. 21, 2006, Ser. No. 11/643,541, entitled “Methane Conversion to Higher Hydrocarbons,” related primarily to methane feedstocks for pyrolysis systems the entirety of each are incorporated herein by reference to the extent not inconsistent with this invention.
The superior thermal shock resistance, relative chemical inertness, preserved crystalline structure, improved flexural strength, and high temperature capability of the inventive compositions, components, and reactors of the present invention provide crystalline stability and structurally soundness under cyclical thermal conditions at temperatures of 1500° C. and higher, such as up to 1700° C., 1800° C., or in some embodiments, up to 2000° C., particularly as compared to prior art refractory and thermal components and reactors. Such attributes and properties may facilitate components and reactors that can replace conventional refractories and also facilitate use of processes in relatively large scale commercial applications that were previously not economical or technically achievable. In particular, the heat stable, formed ceramic components, reactors, and processes may find particular application in refining, petrochemical, chemical processing, and other high temperature thermal applications. It is believed that the improved combination of properties provided according to the present disclosure may facilitate commercial service periods of greater than 1 year, for example even up to about 10 years in some applications.
The particles or grains can be either single crystalline or polycrystalline. Polycrystalline grains are made of many smaller crystallites with varying orientation. Various types of grains can be utilized which include but are not limited to agglomerated and sintered, fused and crushed, and spherodized. In one form, the grains are agglomerated and sintered powder which is produced by spray drying of a suspension consisting of fine powders and organic binder and subsequent sintering. In another form, the grains are fused and crushed, which is produced by fusing in arc furnaces and crushing of the cold block. In still another form of the disclosure, the grains are spherodized, such as by atomization of agglomerates using a plasma flames to fabricate substantially spherical shaped particles.
In one form, the inventive refractory material and components may be prepared by manufacturing techniques such as but not limited to conventional ceramic powder manufacturing and processing techniques, e.g., mixing, milling, pressing or extruding, sintering and cooling, employing as starting materials a suitable ceramic powder and a binder powder in the required volume ratio. The porosity-forming agent (e.g., sacrificial and/or durable agent) is preferably blended into the “wet mixture” at the appropriate place in the manufacturing process to avoid substantial crushing or destroying the shape and grain size of such material. Certain process steps may be controlled or adjusted to obtain the desired porosity range and performance properties, such as by inclusion of various manufacturing, property adjusting, and processing additives and agents. For example, the two or more modes of powders, oxides, preservatives, and/or stabilizers may be milled in a ball mill in the presence of an organic liquid such as ethanol or heptane for a time sufficient to substantially disperse the powders in each other. Thereafter, the porosity forming agent can be added and blended fully using mixing means that do not include grinding or milling. Excessive binder powder and liquids may be removed and the milled powder dried, placed in a die or form, pressed, extruded, formed, caste or otherwise formed into a desired shape. The resulting “green body” is then sintered at temperatures of at least 1500° C. and commonly up to about 1800° C. for at least ten minutes, and often for times typically ranging from about 10 minutes to about two hours and in some applications even up to 4 hours. The sintering operation may be performed in an oxidizing atmosphere, reducing atmosphere, or inert atmosphere, and at ambient pressure or under vacuum. For example, the oxidizing atmosphere could be air or oxygen, the inert atmosphere could be argon, and a reducing atmosphere could be hydrogen. The sintering atmosphere, temperature, and kiln environment may also introduce secondary oxides (as discussed previously herein) into the component, either desirably or undesirably, as a contaminant or desired/permitted constituent of the ceramic component. Thereafter, the sintered body is allowed to cool, typically to ambient conditions. The cooling rate may also be controlled to provide a desired set of crystal sizes and performance properties.
In some embodiments, the present invention includes ceramic compositions having a single mode or “monomodal” grain size distribution, or an essentially non-modally-defined broad grain size distribution, or a substantially single grain size distribution, or combinations thereof. In other embodiments, the present invention includes ceramic compositions having a multimodal (e.g., bimodal, trimodal, etc.) grain distribution, and/or an embodiments lacking in modal definition. However, for purposes of preparing ceramic zirconia compositions, and apparatus that are resistive to oxide-carbide ceramic corrosion, the particular grain structure is less relevant consideration than is the type of stabilizer (e.g., yttria), the minimum concentration thereof, and preferably also the controlled porosity range.
The present invention is not limited to monomodal or multimodal grain distributions, and includes substantially any suitable grain distribution that is resistive to carburization via the methods taught herein, and preferably also including materials that also exhibit other various desirable performance properties, such as but not limited to strength, thermal shock resistance, low vaporization, porosity, etc. In some embodiments, the advantageous physical performance properties and/or characteristics of the material (e.g., flexural strength and thermal shock resistance) may be enhanced or realized in part from close packing of the ceramic grains. For example, in a multimodal embodiments, one mode of a bimodal grain distribution may have a D50 first grain particle size in the range of from 5 to 2000 μm, or from 5 to 800 μm; and the second grain mode of grain distribution, including the preservative component, has a D50 particle size in the range of from at least about 0.01 μm to not larger than one-fourth (¼) of the D50 grain size of the first grain. The second grains are substantially evenly disbursed within the first grains. In other exemplary embodiments, the second grains may have a D50 size value that ranges from 0.01 to 100 μm. In other embodiments, for example, the second mode grains may include a D50 size value that ranges from 0.05 to 44 μm, while in still other embodiments the second mode grains have a D50 size value that ranges from 0.05 to 5 μm. For example, in one embodiment, the first grain mode may have a D50 size that ranges from 20 to 200 μm, while the corresponding second grain mode may range from 0.05 to 5.0 μm. In still other embodiments, the second grain mode may include a D50 average size diameter not greater than one-eight the D50 size of the corresponding first grain mode. In some embodiments the D50 size of the fine mode grains may not exceed one tenth the D50 size of the first mode grains (e.g., not larger than one order of magnitude smaller than the first grain mode), while in other embodiments the D50 size of the second grain mode will generally be less than about two orders of magnitude smaller than the D50 size of the first grain mode (e.g., the second grains are sometimes not larger than about 100 times smaller than the D50 diameter of the first grains).
For still other various exemplary embodiments, the D50 lower limit of the second grain stabilized zirconia may be 0.01 or 0.05 or 0.5 or 1 or 5 μm in diameter. A practical D50 lower limit on the second grain stabilized zirconia grains for many embodiments may be about 0.1 μm. Grains smaller than 0.1 μm may tend to be of limited usefulness in many applications due to the fact that such small grains may not distribute evenly and tend to melt together and combine into sintered grains that are of about the same size as do grains that are at least 0.1 μm. The stabilized zirconia and stabilizer grains that are of at least about 0.1 μm in diameter typically do not change size during or after sintering, whereas the nanoparticles may tend to combine into larger particles. For at least these reasons, the second grain mode of many embodiments of the subject invention might not include nanoparticle D50 size grit, unless such mode is purposefully introduced into and mixed with the coarse and second grain modes as a third or other mode, or as a secondary oxide. Commonly, nanoparticle modes of zirconia or stabilizer may generally only be considered as the second grain mode of the multimode structure when such grains are of sufficient presence to combine with each other to provide a second grain mode after sintering that provides mode grains of at least 0.01 μm after sintering and more preferably at least 0.1 μm after sintering. The D50 upper limit of the second grain mode, including the preservative component particles, may be 500 or 100 or 44 or 20 or 15 or 10 or 5 or 1 μm in diameter. The D50 lower limit of the first grain stabilized zirconia may be 5 or 20 or 25 or 30 or 40 or 100 μm in diameter. The D50 upper limit of the first grain stabilized zirconia may be 800 or 500 or 200 or 100 or 50 μm in diameter. The lower size limitation however, for the second grain mode, including the limitation for the preservative component particles therein, is consistent with size limitations for the other particle size limitations and ranges pertaining to the second grain mode and is a D50 diameter of at least 0.01 μm, and sometimes preferably at least 0.1 μm. “First” and “second” grain sizes referenced above are merely exemplary to a bi-modal embodiment. Numerous modes and non-modally defined grain distributions are also within the scope of the present invention. In these other mode or grain distribution types, suitable grain distribution sizes are within substantially any of the listed grain size ranges discussed with respect to the exemplary “first” and “second” grain embodiments.
In many embodiments, the refractory material comprises a multimodal (e.g., bimodal) grain size distribution including a first grain mode and a second grain mode, the D50 grain size of said first grain mode is not less than three times the D50 grain size of said second grain mode, wherein said formed pores have a D50 diameter in a size range of from not less than the D50 grain size of said second grain mode up to two times the D50 grain size of said first grain mode. In other embodiments, the formed pores have a D50 diameter in a range of from not less than 1.5 times the D50 grain size of the second (smaller) grain mode up four times the D50 grain size of the second grain mode.
A non-limiting example of a multimodal grain distribution may include from 1 to 20 wt % of second grain particles and 80 to 99 wt % of first grain particles. Another non-limiting example is a grain distribution that includes from 1 to 50 wt % of second grain particles and 50 to 99 wt % of first grain particles. Yet another non-limiting example is a grain distribution that includes from 1 to 80 wt % of second grain particles and 20 to 99 wt % of first grain particles. Still another suitable, non-limiting example of a bimodal grain distribution includes 20 to 30 wt % of first grains, such as but not limited to a D50 particle size of 30 μm, and 70 to 80 wt % of second grains, such as but not limited to a D50 particle size of 0.3 μm. Another suitable, non-limiting example of a bimodal grain distribution includes 30 to 40 wt % of first grains, such as but not limited to a D50 particle size of 30 μm, and 60 to 70 wt % of second grains, such as but not limited to a D50 particle size of 0.3 μm. Another suitable, non-limiting example of a bimodal grain distribution includes 50 to 70 wt % of first grains, such as but not limited to a D50 particle size of 30 μm, and 30 to 50 wt % of second grains, such as but not limited to a D50 particle size of 0.3 μm. Another suitable, non-limiting example of a bimodal grain distribution includes 85 to 99 wt % of first grains, such as but not limited to a D50 particle size of 30 μm, and 1 to 15 wt % of second grains, such as but not limited to a D50 particle size of 0.3 μm. Yet another suitable, non-limiting example of a bimodal grain distribution includes 94 to 99 wt % of first grains, such as but not limited to a D50 particle size of 30 μm, and 1 to 6 wt % of second grains, such as but not limited to a D50 particle size of 0.3 μm.
Other key materials properties have been previously identified as having significant importance for some embodiments of the invention, particularly with regard to the high-severity performance of some refractory materials and ceramics in pyrolysis reactors and their corresponding suitability for application in large scale thermal processes; namely, thermal shock resistance and mechanical flexural strength (modulus of rupture, “MOR”). Other properties, such as but not limited to crystalline/chemical stability at high temperature and toughness are also important and must be considered when selecting an appropriate ceramic materials or components for an application. Retention or maintenance of these key and other properties over the useful life of the ceramic component may also be typically desirable. With regard to maintaining longevity of such properties in components, one of the most important factors is the crystalline/chemical stability at high temperatures. The suitably fabricated component must retain its thermal shock resistance and mechanical flexural strength over a suitable life span by not prematurely degrading or undergoing crystalline alteration, such as due to loss of stabilizer and/or due to carbide-oxide corrosion. The instant inventions incorporate particular features that function to maintain or preserve the crystalline stability, thermal shock resistance, and mechanical flexural strength, and possibly other related properties, for extended periods of time as compared to the useful duration of one or more of such properties in materials lacking the inventive preservative and/or ceramic corrosion resistance features.
Regarding the two above-referenced key performance properties, (thermal shock resistance and mechanical flexural strength), thermal shock resistance of a ceramic component can be defined as the maximum change in temperature that the material can withstand without failure or excessive damage. Thermal shock resistance is an evaluated parameter but not a material property. Description of thermal shock resistance may depend upon the type of thermal cycle, component geometry, and strength as well as on material properties or factors. Simplified mathematical expressions relying upon a variety of assumptions can be used to describe material performance under a set of conditions. Alternatively, much more complex analyses may be performed using numerical analysis methods such as finite element and stress-strain analysis. However, for materials performance comparison purposes, a qualitative or direct comparative analysis is also useful and more practical. Thermal shock resistance may be evaluated by means of rapid water quench experiments such as illustrated in ASTM C1525. Thermal shock damage results from buildup of thermal and physical stresses, usually during rapid heating or rapid cooling.
For example, the ASTM C1525 thermal shock resistance test method builds on the experimental principle of rapid quenching of a test specimen (e.g., 1″×1″×⅛″ square, or 2.54 cm×2.54 cm×0.32 cm square) from an elevated temperature (e.g., 1100° C.) into a water bath at room temperature. After water quenching, the specimen is dried and dye-penetrated to investigate both open and closed cracks. For instance, Zyglo® water washable dye penetrants may be used. As the zirconia samples are typically white or yellow, pink dye provides a vivid depiction of cracks and helps differentiate cracks from background or grain boundaries. Methods for determining the cumulative or total crack length per unit area in each specimen are known in the art and may be determined by scanning software electronically aggregating the lengths of all cracks, backed up with visual confirmation by the technician. The electronic scanner resolution or magnification is generally not critical, e.g., from as low as from 50× to as high as 1000×. The tester need only be able to differentiate actual cracks from mere grain boundaries. As with any specified parameter, the value determined must be made over a sufficiently large region to provide a statistically sound representation of the entire sample. The total crack length per unit area may be determined over such area by aggregating and averaging a number of smaller regions that collectively represent a statistically sound region. A whole component may be studied or one or more regions may be evaluated. The studied or relevant region(s) or the whole component may be considered a “component” for test purposes herein.
Utilizing propensity of cracks observed in a test specimen, the thermal shock resistance for a particular region or component may be normalized and qualitatively scored, such as from 1 (the least resistance) to 5 (the most resistance) as summarized hereunder:
The appearance of various degrees of cracking in rapidly quenched zirconia specimens or components and their corresponding qualitative, normalized thermal shock resistance (NTSR) value from 1 to 5 are illustrated in
The inventive refractory materials and components preferably demonstrate a total crack length per unit area after quenching a test specimen of the inventive material from 1100° C. into a water bath at room temperature that is not greater than 5 cm/cm2; that is it preferably has a NTSR of at least 4. Still more preferably, the stabilized refractory materials and components of this invention demonstrate a total crack length per unit area after quenching a test specimen of the stabilized refractory grade zirconia at 1100° C. into a water bath at room temperature that is more preferably not greater than 1 cm/cm2; that is, more preferably has a NTSR of 5. However, for less demanding applications, the inventive components may demonstrate crack lengths in excess of 5 cm/cm2, but preferably not greater than 20 cm/cm2, thus having a corresponding NTSR of 3 or higher. The intended application determines the range of acceptable crack length. Thus, materials according to the present invention include those having a thermal shock resistance rating of 4 or 5, as described herein.
As set forth in ASTM C 1525-04, thermal shock resistance can be assessed by measuring the reduction in flexural strength (MOR) produced by rapid quenching of test specimens heated across a range of temperatures. For purposes of the refractory materials of this invention, regarding quantitative measurement of thermal shock resistance, a critical temperature interval may be determined by a reduction in the mean flexural strength of a determined amount, such as for example, at least 30%. However, the test does not determine thermal stresses developed as a result of a steady state temperature differences within a ceramic body or of thermal expansion mismatch between joined bodies. Further, unless the test is repeated several times, the test is limited in its ability to quantitatively determine the resistance of a ceramic material to repeated or cyclic shocks. Thus, it is preferred that the test is repeated to analyze the effect of cyclic temperature shocks, such as may be experienced in a regenerative reactor.
Another key performance property is flexural strength, which can be measured by 3-point bending tests as illustrated in ASTM F417. The test specimen, a small bar of square cross section, rests on two cylindrical supports in a compression test machine. It is bent by the application of force, at mid-span, to the opposite face of the bar from that resting on the two supports. The bending force is applied by a third cylinder (identical to the other two) at a prescribed constant rate until the specimen breaks. The breaking rod, the dimensions of the specimen, and the test span are used to calculate flexural strength.
As a ceramic material is heated, its density typically increases as a result of residual pore shrinkage due to the sintering effect caused by the heat. Sintering may result in some of the ceramic crystals or components therein melting or undergoing other high temperature fusion or shrinkage, resulting in a slight decrease in bulk volume, but with an increase in component strength. Thus, as a ceramic is heated, its MOR or mechanical flexural strength may typically also correspondingly increase slightly. However, when the hot ceramic is subjected to relatively quick cooling, such as via water quenching, stress fractures may be introduced thereby causing a weakening or reduction in the mechanical flexural strength. The combination of the multimodal grains and the porosity remaining after sintering results in a lattice type structure that provides the improved strength, heat stress dissipation and handling characteristics, and cyclic thermal stress resilience. The ceramic corrosion resistance features prevent degradation of these desirable properties, thereby extending component life.
The MOR and thermal shock property values refer to those property values determined after sintering, unless stated otherwise. ASTM 1505 describes the process for MOR determination. Limited duration exposure of the sintered component to annealing temperatures in excess of 1500° C., such as in excess of 1600° C. or at least 1800° C., may further refine the component properties as described herein. Such further thermal processing or annealing may generally further improve the strength and thermal shock resistance of the inventive components and reactors as compared to such properties after original sintering. After such “annealing” of the sintered component, such as at temperatures in excess of commercial use temperature, such as at an exemplary temperature of at least 1800° C. for two hours, the formed ceramic component according to this invention will demonstrate a retained porosity at ambient temperature in the range of from 5 to 45 vol % based upon the formed volume of the component. Such components also demonstrate a flexural strength (MOR) of at least 6 kpsi, preferably at least 10 kpsi, and provide a thermal shock resistance rating of at least four (4), preferably at least five (5). The MOR flexural strength of the refractory material of this invention and reactor components according to this invention should be greater than or equal to about 6 kpsi (41.3 MPa) after initial sintering to at least 1500° C. and subsequent quenching to ambient temperature. Also, the MOR is preferably greater than or equal to about 6 kpsi (41.3 MPa) when the sintered component is further thermally conditioned, such as by reheating and quenching (e.g., annealed) to operating conditions. For example, the thermal conditioning may entail reheating the component to a temperature in a range such as from 1500° C. to 1800° C. or perhaps even up to 2000° C. Surprisingly, many of the inventive components routinely demonstrate a MOR of at least 6 kpsi (41.3 MPa) after further thermal processing. The combination of a normalized thermal shock resistance rating of 4, with such MOR strength is recognized herein as a minimal MOR and shock resistance properties that are necessary across the required broad reactor temperature spectrum to provide for long-term commercial utilization of high temperature pyrolysis chemistry processes, over a desired life cycle of the reactor component. The corrosion resistance component of this invention functions to prolong these properties of the component within that range of acceptability, correspondingly extending the useful life of the component and process. If desired, the effect of long duration MOR changes may also be evaluated to determine commercial suitability, such as the MOR after, say for example, one month of cyclic processing (annealing). The components and apparatus of the subject invention, however, are expected to provide life duration for the relevant components and apparatus beyond the level that was previously available in the art.
In one aspect, this invention includes a refractory material, such material having application in one aspect for use with a regenerative thermal pyrolysis reactor apparatus, such as for components useful for pyrolyzing a hydrocarbon feedstock (e.g., petroleum liquids, hydrocarbon gas, carbon black feeds, and/or coal). In other aspects, this invention may be utilized for pyrolyzing or otherwise thermally processing various feedstocks other than hydrocarbon feeds at high temperature, such as but not limited to other high temperature chemical processes, reactions, such as but not limited to using various oxidizable, flammable, combustible, or otherwise thermally reactive materials, whether solid, liquid, or gas. Although the inventive materials are useful at high temperatures (>1500° C.), they may also be useful in various lower temperature applications. The term “hydrocarbon feedstock” as used herein is defined broadly to include virtually any hydrocarbonaceous feed and may also include substantially carbonaceous feeds such as graphite or coke. Exemplary hydrocarbon pyrolysis feedstocks that may have particular applicability for use in the present invention typically comprises but are not limited to one or more hydrocarbons such as methane, ethane, propane, butane, naphthas, gas oils, condensates, kerosene, heating oil, diesel, hydrocrackate, Fischer-Tropsch liquids, alcohols, distillate, aromatics, heavy gas oil, steam cracked gas oil and residues, crude oil, crude oil fractions, atmospheric pipestill bottoms, vacuum pipestill streams including bottoms, heavy non-virgin hydrocarbon streams from refineries, vacuum gas oils, low sulfur waxy residue, heavy waxes, coal, graphite, coke, tar, carbon black feeds, atmospheric residue, heavy residue hydrocarbon feeds, and combinations thereof. Undesirable fractions, solids and non-volatiles contained in the feedstreams may optionally be removed by one or more separation techniques, prior to feeding a volatizable fraction into the reactor. Diluents or other additives, such as but not limited to steam, water, methane, and hydrogen, may also be included within the feedstreams.
This invention includes but is not limited to use of components, apparatus, reactors, and methods disclosed in various, previous patent applications, the entirety of each of which are included herein by reference, including (i) U.S. application Ser. No. 60/753,961, filed Dec. 23, 2005, titled “Controlled Combustion for Regenerative Reactors,” (ii) U.S. application Ser. No. 11/639,691, filed Dec. 15, 2006, titled “Controlled Combustion for Regenerative Reactors;” (iii) U.S. application Ser. No. 11/643,541, filed Dec. 21, 2006, titled “Methane Conversion to Higher Hydrocarbons;” (iv) U.S. patent application Ser. No. 12/119,762, filed May 13, 2008, titled “Pyrolysis Reactor Conversion of Hydrocarbon Feedstocks Into Higher Value Hydrocarbons;” and U.S. patent application Ser. No. 12/467,832, filed 18 May, 2009, titled “Pyrolysis Reactor Materials and Methods.” These patent applications teach and disclose various apparatus and methods for pyrolyzing hydrocarbon feeds in reverse flow regenerative pyrolysis reactors, including deferred combustion and controlled heat positioning processes.
The present inventions disclosed herein may be suitable for use with but not limited to refractory applications, and pyrolysis and thermal reactors as disclosed in these previous applications. The inventive components provide the strength, thermal shock resistance, and chemical stability required to enable commercialization of such apparatus and processes to operate at temperatures of at least 1500° C., and even in some embodiments in excess of 1600° C., in still other embodiments in excess of at least 1700° C., and in even other embodiments at temperatures in excess of 2000° C. The inventive materials, components, apparatus, and process enable large-scale, cyclic, high temperature, reactor systems that are useful and operable on a commercially desirable scale and life cycle.
The inventive materials and components may be provided, for example, in one or more pyrolysis reactors, such as but not limited to those reactors having regenerative reactor beds or cores that are useful for carrying out a high temperature chemical reaction. The inventive ceramic components also may be used in construction of one or more reactor embodiments, components, or regions of the reactor system, and may be of substantially any suitable geometry, form or shape, such as but not limited to spheres, beads, honeycomb materials, tubes, pipes, U-tubes, fluid mixers, nozzles, extruded monoliths, bricks, tiles, reactor trays, tray components, and other refractory components that are exposed to high temperature. Such components, as used in a reactor, may by design include flow channels, conduits, or other space which may be regarded or referred to as “void volume” through which fluid passes as part of the function of the component. It is understood that such void volume is not considered to be part of the “porosity” of the ceramic material as described herein. The ceramic porosity specified herein is specifically for the matrix or portion of the component that does not carry flow; often referred to as the wall or the solid portion of the component. Similarly, references to the volume of the material or component are referring to the ceramic matrix volume, including the porosity therein, and not to the primary fluid conducting channels or void volume. The sustained strength and relative inertness properties of the inventive materials may provide for a wider range of component geometries and function than previously available in the art, again also leading to process improvements.
In one aspect, the invention includes a reactor apparatus for pyrolyzing a hydrocarbon feedstock, the apparatus including: a reactor component comprising a refractory material in oxide form, the refractory material having a melting point of no less than 2060° C. and which remains in oxide form when exposed to a gas (e.g., a reference gas mixture containing carbon and oxygen at the stated conditions) having carbon partial pressure of 10−22 bar and oxygen partial pressure of 10−10 bar, measured at a temperature of 1200° C.; wherein the refractory material has not less than 4 vol % formed porosity, measured at 20° C., based upon the bulk volume of the refractory material (but not necessarily the bulk volume of the formed component.) In other embodiments, the typical formed porosity ranges may include but are not limited to from not less than 4 vol % up to a porosity value that retains the desirable feature of maintaining a majority by number of substantially isolated or closed pores. Exemplary maxima for formed porosity may be up to 60 vol %, or up to 50 vol %, or up to 40 vol %, or up to 30 vol %, or up to 20 vol %, or up to 10 vol %. Exemplary maxima for total porosity may be up to 60 vol %, or up to 50 vol %, or up to 40 vol %, or up to 30 vol %, or up to 20 vol %, or up to 10 vol %. Desirable material strength and thermal shock resistance properties should also be considered in selecting the appropriate formed porosity and total porosity maxima. In many embodiments, the refractory material remains in the oxide form when exposed to a gas having carbon partial pressure of 10−22 bar and oxygen partial pressure of 10−10 bar, measured at a temperature of 1200° C. In some embodiments the refractory material has a melting point of not less than 2160° C.
In some preferred embodiments, the refractory material remains in the oxide form when exposed to a gas (e.g., a reference gas mixture) having a carbon partial pressure of 10−15 bar and oxygen partial pressure of 10−10 bar, measured at a temperature of 1800° C. In other sometimes preferred embodiments, the refractory material remains in the oxide form when exposed to a gas having a carbon partial pressure of 10−14 bar and oxygen partial pressure of 10−10 bar, when measured at 2000° C. In still other sometimes preferred embodiments, the refractory material remains in the oxide form when exposed to a gas having a carbon partial pressure of 10−12 bar and oxygen partial pressure of 10−10 bar, when measured at all temperatures over the full range of from 1500° C. to 2000° C.
According to the present invention, in some embodiments the crystalline structure of the refractory material is cubic during heat-up from 1250° C. to 2250° C., while in other embodiments the crystalline structure of the refractory material is cubic during cool-down from 2250° C. to 1250° C. In many preferred embodiments, the vapor pressure of the refractory material is less than 10−7 bar at 2000° C.
In other aspects, the invention includes apparatus fashioned from or including the inventive materials, such apparatus preferably having application with high temperature hydrocarbon pyrolysis systems and methods. In some embodiments, the reactor component includes at least one of flow channels, a reaction fluid mixer, and a reaction heat sink member. In other sometimes preferred embodiments, the reactor component comprises a honeycomb monolith having flow channels within the monolith for conducting at least one of a pyrolysis reactant and a pyrolysis product through the monolith.
Sometimes it may be preferred that the inventive refractory materials meet other preferred application requirements, such as mechanical strength and thermal shock resistance. In some embodiments, the reactor component comprises a thermal shock resistance rating that demonstrates a total crack length per unit area after quenching the reactor component from 1100° C. into a water bath to a temperature of 50° C. (see
In some embodiments, the inventive refractory material may comprise at least 50 wt % yttrium oxide (yttria) based upon the total weight of the refractory material. In other embodiments, the refractory material may comprise at least 70 wt % yttria, at least 75 wt % yttria, at least 80 wt % yttria, at least 90 wt % yttria, at least 95 wt % yttria, at least 99 wt % yttria, or consist substantially of yttria such as substantially 100% yttria. According to many embodiments, the refractory materials do not include ceramics, oxides, (including compounds and elements) that may be toxic, including but not limited to being radioactive, a carcinogen, or other potentially hazardous substance, such as beryllium and thorium. Toxic materials may include materials having an eight hour weight average exposure limit of <2 μg/m3 in air.
In many aspects, the inventive refractory materials comprise a substantially monomodal grain structure, or widely distributed grain structure, while in other embodiments, the inventive materials may comprise multimodal grain distributions. In some embodiments, the refractory materials may include (i) at least 20 wt % of a first grain mode based upon the total weight of the refractory material, the first grain mode having a D50 grain size in the range of from 5 to 2000 μm; and (ii) at least 1 wt % of second grain mode based upon the total weight of the refractory material, the second grain mode having a D50 grain size in the range of from 0.01 μm up to not greater than one-fourth the D50 grain size of the first grain mode.
In many aspects, the inventive materials may include at least one of yttria, an yttrium containing compound, and combinations thereof. In other embodiments, the materials may comprise one or more of yttria, another yttrium containing compound, a zirconium containing compound, and combinations thereof. Although yttria may be used in some embodiments, the inventive refractory materials include other oxide compounds, either with or without the presence of yttria, which meet the melting point and oxide stability requirements of this invention. Some embodiments may comprise refractory materials that are high in purity or neat content, while other embodiments may either intentionally or incidentally include other elements or compounds. For example, some embodiments may further comprises from 0.001 wt % to 5 wt % based upon the weight of the refractory material, of compounds that comprise elements selected from the group consisting of Al, Si, Mg, Ca, Fe, Mn, Ni, Co, Cr, Ti, Hf, V, Nb, Ta, Mo, W, Sc, La, and Ce, and mixtures thereof. In some exemplary pyrolysis reactor applications, the reactor apparatus may comprise a first reactor and a second reactor in flow communication with the first reactor, at least one of the first reactor and the second reactor comprising the refractory material.
In still other aspects, the invention includes a refractory-corrosion-resistant pyrolysis reactor system for pyrolyzing a hydrocarbon feedstock comprising: a first reactor and a second reactor in flow communication with the first reactor, at least one of the first reactor and the second reactor comprising a refractory material in oxide form, having a melting point of no less than 2060° C. and which remains in oxide form when exposed to a gas having a carbon partial pressure of 10−22 bar, an oxygen partial pressure of 10−10 bar, and a temperature of 1200° C.; wherein the refractory material has formed porosity in the range of from 4 to 60 vol % based upon the volume of the refractory material. In some embodiments, the reactor system the matrix or continuous phase ceramic portion of the refractory material may be dense (lacking in porosity) after annealing, while in other embodiments, the material may comprise porosity at 20° C. in the range of from 4 to 28 vol %, or from 4 to 60 vol %, or from 5 to 30 vol %, or from 5 to 35 vol %, based upon the volume of the refractory material. In still other embodiments, the reactor system further comprises: (i) the first reactor further comprises a first channel for conveying a first reactant through the first reactor and a second channel for conveying a second reactant through the first reactor, the first reactant exothermically reacting with the second reactant to generate heat; (ii) the second reactor is heated by the heat to a temperature of not less than 1200° C., or sometimes no less than 1500° C., or sometimes no less than 1700° C., or sometimes no less than 2000° C., for pyrolyzing a hydrocarbon feedstock in the second reactor, wherein the second reactor comprises the inventive refractory material. In some embodiments, the reactor system may comprise a reverse flow regenerative reactor system, and in others a deferred combustion reverse flow regenerative reactor system.
In many embodiments, the reactor system may include a reactant mixer section intermediate the first reactor and the second reactor to combine at least a portion of the first reactant with at least a portion of the second reactant, the reactant mixer section comprising the refractory material. In some embodiments, the mixer's refractory material includes yttria and/or another yttrium containing compound, the refractory material including a grain structure having a D50 grain size in the range of from 0.01 μm to 2000 μm, while still other embodiments may have pores (vacant or filled) with a D50 size of from 0.01 μm to 4000 μm.
In still other aspects, the invention includes a method for mitigating carbide-oxide ceramic corrosion of a refractory material in the presence of a pyrolyzed hydrocarbon feedstock, the method comprising the steps of providing in a heated region of a pyrolysis reactor system for pyrolyzing a hydrocarbon feedstock, apparatus comprising a refractory material in oxide form, having a melting point of no less than 2060° C. and remains in oxide form when exposed to a gas having a carbon partial pressure of 10−22 bar and an oxygen partial pressure of 10−10 bar, measured at a temperature of 1200° C.; wherein the refractory material has no less than 4 vol % formed porosity, measured at 20° C., based upon the bulk volume of the refractory material. In many embodiments, the refractory material has a melting point of at least 2160° C. In some embodiments, the refractory material has a thermodynamic preference to remain in the oxide form when exposed to a gas having a carbon partial pressure of 10−12 bar, an oxygen partial pressure of 10−10 bar, and at a temperature over the full range of from 1500° C. to 2000° C. In still other sometimes preferred embodiments, the crystalline structure of the refractory material may be cubic during heat-up from 1250° C. to 2200° C., while in other embodiments the material may be cubic during such cool down. Although cubic form tends to give the longest life at high temperature and may often be preferred, other oxide structures such as tetragonal sometimes may be acceptable according to the invention. In many other embodiments, the vapor pressure of the refractory material is less than 10−7 bar at 2000° C., or sometimes less than 10−8 bar at 2000° C.
In yet other aspects, the invention includes a method for pyrolyzing a hydrocarbon feedstock using a pyrolysis reactor system comprising the steps of: (a) providing in a heated region of a pyrolysis reactor system for pyrolyzing a hydrocarbon feedstock, apparatus comprising a refractory material in oxide form, the refractory material having a melting point of not less than 2060° C. and that remains in oxide form when exposed to a gas having a carbon partial pressure of 10−22 bar, an oxygen partial pressure of 10−10 bar, at a temperature of 1200° C.; wherein the refractory material has not less than 4 vol % formed porosity, measured at 20° C., based upon the bulk volume of the refractory material. In other embodiments, the method may further comprising the steps of: (b) heating the heated region to a temperature of at least 1200° C., or in some embodiments at least 1500° C., or in some embodiments at least 1700° C.; (c) introducing a hydrocarbon feedstock into the heated region; and (d) pyrolyzing the hydrocarbon feedstock using heat from or within the heated region.
In many embodiments, the pyrolysis reactor apparatus is heated to a temperature of no less than 1200° C., or in other embodiments to no less than 1500° C., or in other embodiments, to no less than 1700° C., and in still other embodiments to no less than 2000° C., while in still other embodiments to a temperature of not greater than 2000° C., and combinations thereof. In other aspects of the invention, the inventive methods include the step of heating the reactor heated region to a temperature of no less than 1200° C., or in other embodiments to no less than 1500° C., or in other embodiments, to no less than 1700° C., in other embodiments to no less than 2000° C., while in still other embodiments to a temperature of not greater than 2000° C., and combinations thereof.
In other embodiments, the method may include the step of heating the heated region to a temperature in a range of from 1500° C. to 2000° C. In many embodiments of the method, the refractory material remains (e.g., has a thermodynamic preference to remain) in the oxide form when exposed to a gas (e.g., a reference gas mixture comprising carbon and oxygen) having a carbon partial pressure of 10−12 bar, an oxygen partial pressure of 10−10 bar, and at a temperature over the full range of from 1500° C. to 2000° C. In some embodiments, the refractory material has porosity of from 2 vol % to 28 vol %. In still other embodiments, the method may include the step of heating the heated region by deferred combustion. Some embodiments of the inventive methods may also further comprise the steps of: (i) flowing at least one reactant in a first direction through the reactor system; (ii) reacting the at least one reactant within the reactor system to heat the heated region; and (iii) flowing a hydrocarbon feedstock through the heated region to pyrolyze at least a portion of the hydrocarbon feedstock and produce a cracked hydrocarbon product.
According to some embodiments, the inventive method comprises providing a refractory material comprising at least 50 wt % yttria based upon the total weight of the refractory material. In other embodiments of the inventive methods, the step of providing the refractory material comprises providing a refractory material comprising at least 80 wt % yttria based upon the total weight of the refractory material. In other embodiments, the step of providing the refractory material comprises providing a refractory material comprising at least 90 wt % yttria based upon the total weight of the refractory material. According to other embodiments of the method, the refractory material comprises a D50 grain size in the range of from 0.01 to 2000 μm. According to still other embodiments of the methods, the vapor pressure of the refractory material is less than 10−7 amt at 2000° C.
The inventive refractory material may be used in a pyrolysis reactor for pyrolyzing a hydrocarbon feedstock at a temperature of no less than 1200° C., or no less than 1500° C., or no less than 1600° C., or no less than 1700° C., or no less than 2000° C., or up to 2000° C., and intermediate range combinations thereof. The refractory material may be used for any of a wide variety of refractory components, such as but not limited to, spheres, beads, honeycomb materials, a tube, a pipe, a U-tube, a fluid mixer, a nozzle, an extruded monolith, bricks, tiles, catalyst tray, reactor tray, tray component, valves, and/or other refractory components. This list is merely exemplary of some common components used with pyrolysis reactors, and such inventive materials may also be useful with other types of pyrolysis reactors, feeds, and processes.
In one aspect, the invention includes a ceramic refractory material, such as may be useful in a pyrolysis reactor for use in pyrolyzing a carbon-containing feedstock, such as a hydrocarbon feedstock. In many embodiments, the reactor will be a regenerative reactor, in other embodiments a reverse flow type of regenerative reactor. A regenerative reactor is substantially any reactor which is cyclically heated and then reduced in temperature, and then reheated to repeat the process. Direction of flow through the reactor is not critical. A reverse flow regenerative reactor is a pyrolysis reactor or reactor system, whereby fluids flow therein for a period of time in one direction through all or selected portions of the reactor and react or are otherwise processed therein. The direction of flow is then reversed and other materials are fed from the opposite direction through the reactor to displace any remaining first materials or reaction products back in the direction opposite from the original flow. The cycle is then repeated. Thereby, the reactor bed or reactor media components are exposed to materials flowing in each direction through the reactor. For example, heat may be produced or added to the reactor by reactants flowing in one direction and that heat may be used to pyrolyze or otherwise facilitate product-generating reactions in the reactor. A substantial part of the heat is then removed during product flow, often in an opposite direction from the flow direction of the original reactants. The pyrolysis reactor system includes one or more hot or heated regions or reaction zones, and preferably a lower temperature quenching zone that serves to absorb heat from the reacted product to quench the reaction process. After cooling the reaction product, the heated quench zone may be cooled by reversing direction of flow through the reactor and feeding new supply of materials through the quench zone to absorb quench zone heat and carry that heat back to the reaction zone where the recovered heat is conserved and reused to pre-heat the reaction zone and reactant materials. After reaction of the pre-heated reactants, the reactor is “regenerated” and ready to pyrolyze the hydrocarbon reactant material (including any diluents or co-feeds) flowing through the reactor system.
At least a portion of the feedstock transferred to or fed into the reactor system is, generally, (i) pyrolyzed (e.g., cracked) in the reaction zone to form the pyrolysis product (e.g., olefins, aromatics, and/or acetylene), and (ii) that cracked reaction product from (i) is quenched in the quenching zone to stop the reaction at the desired pyrolysis product step to thereby yield the pyrolysis product. If the reaction is not timely quenched and stopped, the reaction may continue decomposing the molecules into coke, elemental components, or other less desirable reaction product components.
Separated but simultaneous introduction of two or more reactants into the reactor system, such as through separate flow channels, can facilitate deferred reaction or combustion of the reactants until they are combined with each other, within the desired reactor zone to react with each other within that designated zone. Thereby, a heat bubble may be controllably and repeatedly positioned within the reactor system. In some embodiments, the reverse flow regenerative reactor may be described as comprising two zones or reactors: (1) a heat recuperating (first) zone/reactor, such as for quenching; and (2) a reforming (second) zone/reactor, such as for pyrolysis reaction and reforming. (The first and second reactors need not necessarily be separate components, but instead may be merely different sections of a common reactor or reactor system. The terms first reactor and second reactor are merely used for simplification and to aid in discussion.) In some embodiments, a reactant mixer may be provided intermediate the first and second reactors to assist with mixing and reacting the separately introduced reactants. As a catalyst is preferably not required to facilitate reforming the hydrocarbon vapor to acetylene, in most preferred embodiments no catalyst is present in the reactor beds. However, there may be some applications that benefit from the presence of a catalyst within the reactor system to achieve a certain range of reforming performance and such embodiments are within the scope of the invention.
The requisite high temperature for many pyrolysis reactions may be achieved by creating a high-temperature heat bubble in the middle of the reactor system or within one of the reactors of the reactor system, such as in packed or monolithic bed system. This heat bubble may be created via a two-step process wherein heat is (1) added to the reactor bed via delayed or deferred, in-situ combustion, and then (2) removed from the bed via in-situ endothermic reforming. A key benefit of the invention is the ability to consistently manage and confine the high temperature bubble (e.g., ≧1200° C., or ≧1500° C.) in a reactor region(s) that comprise the inventive refractory material and can tolerate such conditions long term. The inventive apparatus and process enable operation of a substantially continuously operating, large-scale, cyclic, commercial regenerative reactor system.
The inventive material may be provided, for example, in one or more regenerative reactor beds that are useful for carrying out a high temperature chemical reaction. The inventive refractory material may be used in construction of one or more embodiments, components, or regions of the reactor system, and may be of substantially any form or shape, such as but not limited at least one of spheres, beads, honeycomb materials, a tube, pipe, U-tube, fluid mixer, nozzle, extruded monolith, brick, tile, catalyst tray, reactor tray, tray component, valves, and/or other refractory components that are exposed to high temperature. Some embodiments may include the inventive refractory material used in fabrication of at least one of a honeycomb monolith, a reactor bed, a reactor conduit, a reaction fluid mixer, a reactant mixer, checker-brick, and a reactor heat sink member. Additionally, if desired for some embodiments, such as to control costs, a reactor system may also comprise other refractory materials in addition to the inventive materials, in other reactor regions of the reactor that are not exposed to the most severe temperatures, such as glass or ceramic beads or spheres, metal beads or spheres, ceramics (including zirconia), ceramic or metal honeycomb materials, ceramic tubes, extruded monoliths, and the like, provided they are competent to maintain integrity, functionality, and withstand long term exposure to the relevant temperatures for that respective region of the reactor.
The inventive reactor system may utilize inventive material components in all regions of the reactor that may be subjected to the extreme temperatures, such as temperatures in excess of 1200° C., or 1400° C., or 1500° C., or even in excess of 1700° C. In some applications, the inventive material may be useful, for example in applications having temperatures of in excess of 1700° C. and in some other applications the material may be useful at temperatures in excess of 1800° C., such as up to 2000° C. or up to 2200° C.
In one application of the present invention, a first reactant, such as a hydrocarbon fuel, is directed down one or more designated reactor channels or conduits, while a second reactant, such as an oxygen-containing gas, is simultaneously directed down one or more other designated channels through the reactor. During flow in the opposite direction, preferably both of the first and second sets of channels are simultaneously utilized to convey the pyrolyzed and likely expanded product volume through the recuperator reactor. In one embodiment, the channels are included in one or more honeycomb monolith type structures. The term “honeycomb monoliths” is defined broadly to include but not be limited to extruded, ceramic structures as are generally known in the reaction industry, such as in catalytic converters, etc., capable of conveying a fluid through the framework of channels. The term “honeycomb” is also used broadly herein to refer to any framework of channels, regardless of cross-sectional geometry, that includes multiple substantially parallel flow paths or channels and is not intended to limit the structure or shape to any particular geometric shape. The channels each may have generally any cross-sectional shape, although a generally symmetrical cross-sectional shape may be preferred. Each monolith may include a single channel, a few channels, or multiple channels, e.g., tens, hundreds, or even thousands of channels, depending upon the size of the particular monoliths and reactors utilized therein. For example, in one embodiment, the conduits may have a diameter of only a few millimeters, or about one millimeter, or several millimeters and even several centimeters in diameter in some vessels. A reactor may comprise a single, a few, or even numerous monoliths. The monoliths may be further arranged into cells or groups of monoliths, wherein each cell or a group of cells is dedicated to conducting one of the two simultaneously conveyed materials, while another group of cells conveys the other material. A preferred monolith arrangement will provide low pressure loss or drop during reactant or product transference, while providing necessary product contact time and heat transfer during conductance. The arrangement preferably also provides adequate mixing of the conveyed materials after exiting the monoliths, such as in or near the reaction zone. In addition to providing a flow conduit, the channels also facilitate effective material isolation barriers (e.g., function such as conduit walls) to prevent cross flow or mixing between the first and second reactants and maintain a majority of the reactants effectively separated from each other until mixing is permitted. In some preferred embodiments of the present invention, the reactors are comprised of one or more extruded honeycomb monoliths.
In some embodiments, the inventive materials and components preferably provide a conduit packing with an average wetted surface area per unit volume that ranges from about 50 ft−1 to about 3000 ft−1, more preferably from about 100 ft−1 to 2500 ft−1, and still more preferably from about 200 ft−1 to 2000 ft−1, based upon the volume of the first reactor that is used to convey a reactant. Such wetted area values apply to the channels for both of the first and second reactants, with relatively thin walls separating the channels to facilitate good heat transfer between the reactants and the inventive material. The term “thin walls” refers to the distance through which heat must be moved within the solid portions of the component. Thus, for a bed of spherical packing would simply be the sphere diameter. For a reactor bed comprising honeycomb monolith structures, the relevant dimension is simply the wall thickness separating the flow channels. Exemplary wall thickness of some honeycomb monoliths according to the present invention is less than 2.5 mm, frequently less than 1.0 mm, down to a probable minimum wall thickness of not less than around 0.1 mm. These relatively thin walls are enabled by the strength and thermal shock resistance properties of the inventive material, as discussed previously herein and in related patent applications. The durable, stable, corrosion resistant and heat resistant material is ideal at enabling use of thin but strong reactor channel or wall components. The relatively high density also helps mitigate reactant cross-flow through the conduit or channel walls. The relatively high surface area per unit volume values facilitated by the high number of relatively small reactant channel or conduits are likely preferred for many embodiments to aid achieving a relatively quick change in the temperature through the reactor. The quick temperature change is preferred to permit relatively quick and consistent heating and quenching of the reaction to prevent the reaction from continuing and creating coke. The relatively high thermal stability, high thermal shock resistance, and high heat transfer capability of the inventive refractory material also enables these desired quick temperature changes, without experiencing material failure due to thermal shock degradation.
In some embodiments, a reactor will provide media channels and other high temperature-exposed components and packing that includes a high volumetric heat transfer coefficient (e.g., greater than or equal to 0.02 cal/cm3s° C., preferably greater than about 0.05 cal/cm3s° C., and most preferably greater than 0.10 cal/cm3s° C.), with corresponding low resistance to flow (low pressure drop), have operating temperature range consistent with the highest temperatures encountered during regeneration, have high resistance to thermal shock, and have high bulk heat capacity (e.g., at least about 0.10 cal/cm3° C., and preferably greater than about 0.20 cal/cm3° C.).
Some embodiments may use the inventive materials and components for uses other than the described and often preferred honeycomb monoliths, such as whereby the channel conduits/flow paths are substantially linear and tubular. Other alternative embodiments may include more tortuous pathways (e.g. convoluted, complex, winding and/or twisted but not linear or tubular) through a component, than the previously described monoliths, including but not limited to labyrinthine, variegated flow paths, conduits, tubes, slots, and/or a void structure having channels through a portion(s) of the reactor and may include barrier portion, such as along an outer surface of a segment or within sub-segments, having substantially no effective permeability to gases, and/or other means suitable for preventing cross flow between the reactant gases and maintaining the first and second reactant gases substantially separated from each other while axially transiting the reactor. For such embodiments, the complex flow path may create a lengthened effective flow path, increased surface area, and improved heat transfer. Such design may be preferred for reactor embodiments having a relatively short axial length through the reactor. Axially longer reactor lengths may experience increased pressure drops through the reactor. However, for such embodiments, the porous and/or permeable media may include, for example, at least one of a packed bed, an arrangement of tiles, a permeable solid media, a substantially honeycomb-type structure, a fibrous arrangement, and a mesh-type lattice structure. It may often be preferred that the ceramic matrix provides high surface area to facilitate good heat exchange with the reactant and produced gases.
The regenerative pyrolysis reactor system may heat the hydrocarbon feedstock to temperatures in excess of 1200° C., preferably in excess of 1500° C., more preferably in excess of 1700° C. In some reactions, it may even be preferable to heat the feeds for very short time duration, such as less than 0.1 seconds, to a temperature in excess of 1800° C. or even in some instances in excess of 2000° C. An exemplary preferred process may pyrolyze the feed stream within the reactor, such as at temperatures of from 1500° C. to 2000° C., or from 1500° C. to 1900° C., and sometimes preferably from 1600° C. to 1700° C. Exemplary residency times preferably may be short, such as less than 0.1 seconds and preferably less than 5 milliseconds. In some aspects, the conversion or cracking of the separated vapor phase may be performed in the presence of hydrogen, hydride, other hydrocarbons, and/or other diluents or stripping agents. The conversion of the vapor fraction into higher value hydrocarbons such as acetylene typically requires a high reformation temperature, which in the past has been a significant barrier to commercialization and efficiency.
While the present invention has been described and illustrated with respect to certain embodiments, it is to be understood that the invention is not limited to the particulars disclosed and extends to all equivalents within the scope of the claims. Unless otherwise stated, all percentages, parts, ratios, etc. are by weight. Unless otherwise stated, a reference to a compound or component includes the compound or component by itself as well as in combination with other elements, compounds, or components, such as mixtures of compounds. Further, when an amount, concentration, or other value or parameter is given as a list of upper preferable values and lower preferable values, this is to be understood as specifically disclosing all ranges formed from any pair of an upper preferred value and a lower preferred value, regardless of whether ranges are separately disclosed.
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| Number | Date | Country | |
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| 20110120853 A1 | May 2011 | US |
