Patent Application
20100314297
This invention relates to cyclic petroleum refining processes in which a hydrocarbon feedstream is contacted with a solid, particulate contact material in a first step to treat the feedstream after which the solid contact material is separated or disengaged from the treated feedstream and regenerated in a separate regeneration zone before being returned to the first step for contact with additional feedstream. The invention concerns itself particularly with the provision of external process heat to one or both steps of the process using solar and/or nuclear heat sources.
The petroleum refinery is one of the most thoroughly integrated operations devised by the mind of man: apart from contaminants, all that goes into the refinery is either consumed in processing or emerges as useful product. Increasing shortages of crude oil and their accompanying price increases together with the unpredictable political instabilities of many crude-producing regions have resulted in pressures for refineries to increase their overall efficiency by using less crude to fuel process operations, so enabling more to emerge potentially as product. As an approximation, between 20 and 25 percent of the crude intake of a refinery is consumed in fueling refinery operations with the exact figure depending very much on the type of refinery but mainly on the extent of conversion in the refinery. Substitution of fossil fuel process heat by other heat sources is therefore an economically, environmentally and politically attractive option since in a carbon-constrained economy, the potential to dramatically reduce flue gas and extend fossil resource capacity may depend on nontraditional uses of alternative energy. Refining and production operations might make use of nontraditional applications of nuclear and solar energy through their ability to generate heat, electric power and hydrogen.
A significant number of petroleum refining operations are catalytic or otherwise operate in a cyclic manner, using a solid, particulate contact material such as a sorbent or catalyst which is brought into intimate contact with a hydrocarbon feed to effect some chemical change upon it. The best known instance is the fluid catalytic cracking (FCC) process in which a finely-divided, fluidizable catalytic material is brought into contact with a heavy oil feed to effect endothermic cracking reactions and produce hydrocarbons of lower molecular weight, after which the catalytic material is separated in a disengager, stripped of occluded hydrocarbons and then regenerated in an exothermic regeneration step carried out under oxidative conditions. Heat for the endothermic cracking reactions is provided in part by the circulation of hot, regenerated catalyst from the regenerations step but in addition, process heat is applied before the feedstream enters the cracking reactor. Another example of such a cyclic process is continuous catalytic reforming in which a reforming catalyst is circulated between a reactor zone in which highly endothermic reforming reactions (dehydrogenation, dehydrocyclization, cyclization) take place and a regenerator in which the reforming catalyst is oxidatively regenerated and then rejuvenated. The current commercial continuous catalytic reforming processes utilize moving bed catalysts in which the catalytic contact material gravitates downwards in the process vessels in a similar manner to that of the pioneer Thermofor Catalytic Cracking process.
The fluid coking process is a cyclic process in which a heavy, residual oil feed is contacted in a fluid bed coking zone with finely divided particles of petroleum coke to effect further coking (cracking) of the feed with the production of lower molecular weight cracking products and more coke which is laid down on the initial coke particles present in the bed. The coke particles are then taken to a fluid bed combustion zone in which a portion of the coke is burned to generate process heat which is returned to the coking zone by the circulating coke particles.
Another class of processes operated on the continuous cyclic principle are the cyclic sorption processes. Sorption processes using solid sorbents have long been used for purifying gases. The sorbents can be used in a once-through mode and discarded after performing their function or, alternatively, in a cyclic mode in which sorption phases alternate with desorption (regeneration) and possibly other phases such as cooling, heating and activation. The use of such processes for removing sulfur from fuels, including normally liquid hydrocarbons such as gasoline, road diesel, heating oil and kerojet as well as from gases such as natural gas, manufactured gas and syngas has assumed importance in recent years with the stringent regulatory trends severely limiting the sulfur content of various fuels such as the new sulfur limits of 30-50 ppmw for gasoline and diesel which are already effective both in Europe and the U.S.A. or will shortly become so. While conventional technology including extraction processes such as the Merox™ processes, oxidation and hydrodesulfurization have long fulfilled the requirements the future regulatory trans towards even lower sulfur levels may make it uneconomic or, in fact, impossible to use them. With hydrodesulfurization, for example, the hydrogen consumption increases disproportionately with lower product sulfur as does the amount of hydrodesulfurization catalyst: the catalyst volume, for example, must be three times as great to attain a product sulfur level of 50 ppmw and four times as great to reach 30 ppmw, as compared to the historic 100 ppmw requirement, as noted by Inoue et al. Fuel 2000, Vol. 79, No. 7, 843-849. Another difficulty confronting the refiner in fuel oil production is that certain sulfur compounds such as the sterically hindered alkyl-substituted benzothiophenes and dibenzothiophenes are not readily amenable to hydrogenative treatment. Accordingly, alternative technologies have become increasingly attractive.
One non-hydrogenative process that is entering commercial use is the sulfur sorption process, often referred to as sulfur adsorption. This process uses a solid, particulate adsorbent to remove sulfur from the feed in a sorption zone after which the sorbent is separated from the purified feed and regenerated in a separate regeneration zone and then recycled to the sorption zone. A commercially process of this type is the ConocoPhillips S-Zorb™ process in which the sorption is carried out in the presence of hydrogen using a modified zinc oxide sorbent on a carrier. Sulfur from the feed is carried over to hydrogen sulfide which then becomes bound by chemisorption to the zinc oxide as zinc sulfide. In the regenerator, the zinc sulfide is converted back to zinc oxide with an oxygen-containing gas stream at high temperature, typically at temperatures from 475-550° C. under moderate pressure, typically 2-5 barg; the desulfurization is typically conducted at a slightly lower temperature from 400-450° C. and generally at higher pressures from 10-35 barg. The S-Zorb process has been applied both to gasoline desulfurization and is also potentially applicable to diesel desulfurization. The S-Zorb process and the sorbents which may be used in it are described, for example, in the following U.S. Pat. Nos. 6,274,533; 6,649,555; 6,635,795; 6,864,215; 7,182,918; 7,147,769; 2004/0048743; 2004/0120244; 2003/0194356; 2004/0007501, to which reference is made for a description of processes of this type and the sorbents useful in them.
The ConocoPhillips S Zorb SRT™ process, a variant of the S-Zorb process, has been proposed for the removal of sulfur compounds from syngas. Fuel Processing Technology, Vol. 89, Issue 6, June 2008, 589-594. The S Zorb™ sorbents are said to be an outgrowth of Z-Sorb sorbents and technology developed for various desulfurization applications although being more active than Z-Sorb sorbents. Differences between syngas and liquid hydrocarbon desulfurization consist partly in the amount and type of sulfur compounds in the two feeds with the major sulfur compounds in syngas being H2S and COS, while thiophenes and benzothiophenes predominate in gasoline. The general scheme of using a regenerable solid oxide sorbent nevertheless appears to be maintained.
Conceptually, the solid oxide sorption/regeneration processes such as Z-Sorb and S-Zorb are akin to the well-established FCC process used for cracking heavy oils to loser boiling products in that an initial contact between the feed and a finely-divided, particulate solid is made to carry out the desired reactions with the feed, after which the solid contact material is separated from the treated feed and/or the treatment products after which the solid contact material is regenerated and returned to the contact treatment step. In both cases, the initial treatment step is carried out under reducing conditions and the regeneration under oxidative conditions at relatively higher temperatures. For this reason, measures adapted to improve one of these processes may commend themselves also for use in the other.
According to the present invention, heat from solar or nuclear heat sources is applied to provide at least a portion of the heat used in a cyclic process in which a hydrocarbon feedstream is contacted with a solid, finely-divided particulate contact material in a first step to treat the feedstream after which the solid contact material is separated or disengaged from the treated feedstream and regenerated in a separate regeneration zone before being returned to the first step for contact with additional feedstream. The entire cycle may be characterized as including an endothermic step and an exothermic step with heat from the exothermic step being transferred from the exothermic step to the endothermic step by means of the circulating contact material. The nuclear or solar heat is applied to the endothermic step to reduce demands on heat from fossil sources. Generally, the exothermic step will be an oxidative step similar to the oxidative regeneration carried out in the FCC process but may also be a regeneration step applied to a sorbent in a cyclic sorption process as described below.
The present invention therefore comprehends a cyclic petroleum refining process in which a hydrocarbon feedstream is contacted with a solid, particulate contact material in a first step to treat the feedstream after which the solid contact material is separated from the treated feedstream and regenerated in a separate regeneration zone before being returned to the first step for contact with additional feedstream. At least one of the steps is carried out at elevated temperature provided by the application of heat from a nuclear or solar thermal energy source. The use of heat from solar and nuclear heat sources is especially favorable in cases where very high operating temperatures, typically above about 800° C. or higher, e.g. 950 or 1,000° C., are required.
The invention may be effectively applied to a method of regenerating catalysts and sorbents used in processes for removing contaminants from hydrocarbon liquids and gases, e.g. natural gas, as well as from other gases, including syngas. The sulfur sorption process described above is a suitable candidate for the application of heat from nuclear or solar sources. In this case, the thermal energy from the solar and/or nuclear heat source is used to provide process heat for the regeneration step of the process. It is, however, applicable also to other cyclic processes in which fossil fuels have traditionally been used to supply process heat; the manner of its application will be described below. Potential applications of concentrated solar power and nuclear power include, for example, application to the cyclic FCC process which, although distinct chemically, is conceptually related to the sulfur sorption process in terms of the cyclic endo- and exo-thermic reaction steps and materials handling. Application to the continuous catalytic reforming process with its highly endothermic reaction steps which, unlike the cracking step of the FCC process, cannot be effectively supplied with heat from the regeneration steps, is also possible.
The solar or nuclear heat is applied by the use of a heat transfer medium and heat exchange device transferring the heat from the solar or nuclear power source to the process unit in which the process is being operated. The heat transfer medium will be routed from the solar or nuclear source to a heat exchanger providing pre-heat for the process, direct heat to the process environment e.g. by a heating jacket on the reactor used for carrying out the process or by heat transfer coils or tubes inside the reactor. Solar and nuclear heat sources have the capability to generate very high temperatures potentially in excess of 1500° C. and heat of this quality can be used very effectively to provide process heat to the endothermic reaction steps of the cyclic chemical processes, even when transferring heat to reactant gas streams in a heat exchanger. Heat transfer can be effected using transfer media such as liquids, gases, molten salts or molten metals although molten salts and molten metals will often be preferred for their ability to operate at very high temperatures for high energy densities without phase changes; in addition, corrosion problems can be minimized by appropriate choice of medium relative to the metallurgy of the relevant units.
Solar and nuclear thermal energy sources are zero carbon emission sources and by using them hydrocarbon resource utilization for process heat is eliminated. Carbon dioxide evolution associated with burning of a hydrocarbon resource to generate process heat is eliminated. For perspective, each liter of petroleum resid requires approximately 140 liters of natural gas (methane at 15.5° C.) to heat it to 540° C. and hold it at that temperature for six seconds and, as noted above, about 20 to 25 percent of the crude oil input to a refinery is used in processing. The substitution of zero carbon emission sources therefore offers the potential for significant carbon emission reductions in refinery operations where external process hear can be applied and effectively utilized.
As noted above, a number of commercially significant petroleum refining operations are conducted on a continuous cycle with a solid contact material circulating between two zones with different conditions prevailing in each of them. Frequently, as in FCC and the cyclic sulfur sorption process, one zone will operate under endothermic conditions and the other under exothermic conditions; the endothermic step will invariably be a reducing step and the exothermic step an oxidative step. In FCC, for example, the cracking reaction in the riser is endothermic and the oxidative regeneration is exothermic with the heat generated in this zone transferred to the cracking zone in the riser using the regenerated catalyst as the heat transfer medium. When the reducing zone is operated with molecular hydrogen present, as in the sulfur sorption process, strict precautions need to be taken to isolate the zones from one another, for example, by the use of lock hoppers or seals between the zones controlling movement of the contact material.
The present invention enables heat required in endothermic chemical reaction steps to be provided by solar or nuclear heat sources so that carbon emissions for process heat requirements in these steps are either eliminated or substantially reduced. Traditionally, process heat for endothermic reactions has been provided by fossil fuel combustion and in the petroleum refinery, these fossil fuels have conventionally been natural gas, refinery fuel gas, liquid hydrocarbons such as heavy residual oils, or even petroleum coke as in a fluid coker. The temperatures at which refining processes are operated has depended on a number of factors, principally including the thermodynamics and kinetics of the process steps themselves but also upon ancillary factors such as available reactor metallurgies, stability of any catalysts or contact material, reaction by-products, maintenance requirements (e.g. accumulation of reactor deposits) and overall process economics. The economic factors, when all are taken into consideration may be telling: sometimes, for example, a cheaper catalyst is known to be effective for a given reaction but it may require higher temperatures which, if produced by conventional firing with fossil fuels, may be uneconomic at current crude prices and more so if a carbon emissions tax became effective. In any event, the environmental considerations may make it prudent to utilize the less effective materials. The ability of solar and nuclear energy sources to supply high operating temperatures without incurring the concomitant economic and environmental penalties however opens up the possibility of using the more effective materials.
One striking example of the interplay of these considerations is in the cyclic sulfur sorption process described above using solid sulfur sorbents to remove sulfur from hydrocarbon feedstreams. Metal oxides are excellent adsorbents of H2S from synthesis gas streams at high temperature and high pressure (typically 370-450° C., 10-50 bar). The ability to use and regenerate these adsorbents economically at high temperature is a key factor in the successful deployment of solid adsorbent technology.
A number of different potential sorbents for use in this process have been identified and a number of them are shown in Table 1 below, together with the temperatures at which they are operated, either in the desulfurization step or the regeneration step. To take one example, calcium carbonate is both cheap and readily available but it requires a desulfurization temperature of 800-100° C. whereas the zinc-based sorbents can be operated at markedly lower temperatures. Zinc oxide is reported to desulfurize at temperatures as low as 427° C. or, in another case, at 538-650° C. with a regeneration temperature only 50-150° C. above the desulfurization temperature, zinc titanate-copper is reported to function at 425° C. with a regeneration temperature extending up to 650° C. So, on the basis of sorbent cost alone, it would be highly desirable to be able to utilize calcium carbonate as the sorbent but the economic and environmental costs of operating at the required higher temperatures may be effective to preclude this option.
The utilization of solar and nuclear energy to supply high temperature heat in the range of 900 to 1500° C. without incurring additional operating costs makes the use of lower cost alternatives such as calcium carbonate and calcium oxide possible. Although the current capital and operating costs of nuclear energy are higher than of fossil fuel sources and of solar, higher still, the economic picture could alter significantly in the event of a carbon tax. Another advantage of higher temperature operation is that reaction kinetics will proceed faster, thus enabling the sizes of reaction vessels and catalyst volumes to be decreased, affording economies both in capital and operating expenditures.
The use of solar and nuclear heat sources is of primary interest for the regeneration step of the sulfur adsorption process since this is typically carried out at higher temperatures than the initial desulfurization step but since this step is also operated at relatively high temperatures, similar considerations apply: resort may be made to a wider range of sorbents without incurring the economic or environmental penalties of fossil fuel.
Solar thermal energy is provided by the conversion of light to heat energy. This is typically achieved by focusing solar radiation onto a point source using mirrors, and the point source increases in temperature thus generating heat. For commercial applications, multiple mirrors are required to be installed to increase light capture. Once the solar radiation is focused on a point, the heat is transferred to fluid heat transfer medium. Three types of solar thermal device designs have been explored: solar tower, solar trough, and solar reactors.
Solar thermal installations with a tower design use mirrors to focus incoming solar radiation on to a point that is often located on a central tower. Typically, the mirrors in a heliostat system are motorized to follow the sun over the course of the day. At this focal point, a liquid heat transfer medium is heated to the required temperature. Solar trough power plants use curved, trough-shaped mirrors to focus light on to a heat transfer fluid that flows through a tube above them. These trough reflectors tilt throughout the day to track the sun for optimal heating. The heat transfer fluid is heated in the troughs and then flows to a heat exchanger, which is used to produce superheated steam. A modified version of the parabolic trough design, the Fresnel reflector design, is uses a series of flat mirrors with a number of heat transfer receivers. Solar reactors, or Concentrated Solar Power (CSP), are useful for applications such as the present that take advantage of the high-temperature capabilities of tower technology which uses reactors similar to closed volumetric receivers except that a rhodium or another catalyst is dispersed on the surface of the ceramic mesh, directly absorbing the solar energy to produce syngas, hydrogen, and carbon monoxide as disclosed by Moller, S. et al., in 2002: Solar production of syngas for electricity generation: SOLASYS Project Test-Phase, 11th SolarPACES International Symposium on Concentrated Solar Power and Chemical Energy Technologies, Zurich. In its application to the present invention a solar reactor is used for directly heating the heat transfer fluid to high temperatures.
The high temperatures required for the present invention can also be provided by certain nuclear thermal energy sources. While conventional light water reactors are not adequate to supply these high temperatures, high temperature gas-cooled reactors and others have appropriate characteristics. One example is the Toshiba 4S (super safe, small, and simple) nuclear power system is based on a low-pressure, liquid-sodium design which is therefore capable of supplying the required high temperatures. It can be transported in modules and installed in a building measuring 22×16×11 metres and therefore commends itself for appropriate adaptation to refinery usage. High-temperature gas-cooled reactors (HTGRs) which typically use helium as a coolant are another next-generation reactor design that have the potential for driving endothermic chemical reactions, e.g., the regeneration reactions in the sulfur sorption cycle. One factor making HTGRs advantageous for the present application is that in principle the HTGCR can operate at temperatures well above 800° C., a range of refining operations including cracking, reforming and solid contact sulfur sorption as described above. The Siemens PBMR (the pebble bed modular reactor, or PBMR) is an example of a HTGCR which would be particularly useful for these purposes. The pebble bed modular reactor (PBMR) potentially meets US safety standards and includes a required airtight steel-lined reinforced-concrete containment structure. Operation of the PBMR is based on a single helium coolant loop, which exits the reactor core at 900° C. and 70 bar and therefore can be used to heat a heat transfer medium to comparable temperatures for use in refining processes. The PBMR is described in Weil, J., 2001: Pebble-Bed Design Returns, IEEE Spectrum, 38 (11), 37-40.
Heat Transfer from Source to Process Unit
The heat from the solar or nuclear high temperature heat source is applied by the use of a heat transfer medium and heat exchange device transferring the heat from the solar or nuclear power source to the process unit in which the process is being operated. The heat transfer medium will be routed from the solar or nuclear source to a heat exchanger providing pre-heat for the process, direct heat to the process environment e.g. by a heating jacket on the reactor used for carrying out the process or by heat transfer coils or tubes inside the reactor. Heat from solar and nuclear heat sources at temperatures potentially in excess of 1500° C. and heat of this quality can be used very effectively to provide process heat to the endothermic reaction steps of the cyclic chemical processes, even when transferring heat to reactant gas streams in a heat exchanger or through heating jackets or heating coils on the vessel. Heat transfer at the high temperatures contemplated, typically above 800° C. and ideally higher, e.g. 900, 950, 1000° C., even as high as 1500° C., can be effected using transfer media such as liquids, gases, molten salts or molten metals although molten salts and molten metals will often be preferred for their ability to operate at the very high temperatures required for high energy densities without phase changes; in addition, corrosion problems can be minimized by appropriate choice of medium relative to the metallurgy of the relevant units. Molten salt mixtures such as mixtures of nitrate salts, more specifically, a mixture of 60% sodium nitrate and 40% potassium nitrate are suitable but other types and mixtures of molten salts may be used as a heat transfer and a thermal storage medium. Liquid metals such as sodium as well as alloys such as sodium-potassium alloy, bismuth alloys such as Woods metal, (m.p. 70° C.) and alloys of bismuth with metals such as lead, tin, cadmium and indium; the melting point of gallium (30° C.) and its alloys of gallium would, but for the aggressiveness of this metal towards almost all other metals, generally preclude it from consideration. Mercury is excluded for environmental reasons. Hot helium from a HTGCR can be used in a single loop heat exchange circuit from the nuclear reactor to the hydrocarbon process unit since helium is incapable of becoming radioactive and HTGCR reactor design is inherently safe: in the event of a loss of coolant, the temperature in the core will increase until Doppler broadening leads to a breakdown in the fission chain reaction. Outlet temperature and pressure for the helium coolant of the HTGCR are 850° C. and 70 bar, respectively, making it suitable for the present purposes. If required for safety or other reasons, the primary heat exchange fluid can be used to heat a secondary heat exchange fluid in a secondary circuit with this secondary fluid passing to the hydrocarbon process unit.
Solar and nuclear heat may be applied to other petroleum refining processes in a similar manner: heat may be transferred from the primary heat source to the process by means of heat exchangers configured according to the needs of the process. In the case of continuous catalytic reforming processes, for example, the heat transfer fluid would be routed to the feed preheaters and to the interstage heat exchangers. The high temperatures of the heat from nuclear and solar sources make it particularly suitable for this highly endothermic process application. Application to processes such as FCC which operate individually in heat-balanced mode, presents other possibilities, for example, as feed pre-heat to increase the endothermic cracking temperature and permit the use of catalysts optimized for such higher temperatures with shorter contact times and faster feed/catalyst separation. The use of shorter contact times in FCC is desirable to reduce thermal cracking and although higher temperatures will increase thermal cracking thermodynamically and kinetically, the shorter reaction times accompanying these higher temperatures has the potential to achieve a net reduction in undesired cracking reactions as became possible when riser cracking was introduced with high activity zeolite cracking catalysts to reduce feed/catalyst contact times. Modification of the FCCU to accommodate shorter contact times may be required, for example, by the use of downflow FCC reaction zones such as those described in U.S. Pat. No. 4,385,985; U.S. Pat. No. 5,589,139 and WO 2005/080531 or hybrid down/up or up/down flow units as described in U.S. Pat. No. 498,326 and U.S. Pat. No. 5,468,369.
The application of solar and/or nuclear heat to processes such as fluid coking (especially including its variant, the Exxon Flexicoking™ process which is an integrated fluid coking and coke gasification processes), presents another opportunities. In fluid coking, a portion of the coke resulting from the hydrogen rejection in the thermal cracking is combusted in the burner with a stream of fluidizable seed coke particles being returned from the burner to the reactor. Heat at high temperature heat from a solar furnace or nuclear reactor can be used to supplement the heat from the recycled seed coke and in this way the proportion of recycled coke seeds can be reduced to the level required for fluidization and seeding coke formation; the size of the coke particles passing over to the burner can be increased proportionately with the excess coke production taken from the burner as fluid coke product or passed to the gasifier in the Flexicoker.
The use of solar and/or nuclear heat sources to supply heat to heat-balanced or heat-productive processes creates a potential for modifying the thermal configuration of the refinery in a way that the nuclear and/or solar heat is utilized to provide a contribution to the net refinery heat balance equation while retaining the heat from the individual processes. For example, if the use of fossil fuels is reduced with one process, the fuel material may be diverted to other, more valuable, uses and products.
In areas where solar power is not constantly available, fossil fuel can be brought in when necessary and the same can be done when nuclear energy is reduced or cut off, for example, by plant maintenance.
This application relates and claims priority to U.S. Provisional Patent Application No. 61/268,777, filed on Jun. 16, 2009.
| Number | Date | Country | |
|---|---|---|---|
| 61268777 | Jun 2009 | US |
