Patent Application
20220162067
Embodiments of the invention generally relate to a chemical plant including a reforming section for reforming a feed gas comprising hydrocarbons to a synthesis gas and to a process for reforming a feed gas comprising hydrocarbons to a synthesis gas.
Processes based on Autothermal Reforming (ATR) is a route to production of synthesis gas. The main elements of an ATR reactor are a burner, a combustion chamber, and a catalyst bed contained within a refractory lined pressure shell. In an ATR reactor, partial combustion of the hydrocarbon feed by sub-stoichiometric amounts of oxygen is followed by steam reforming of the partially combusted hydrocarbon feed stream in a fixed bed of steam reforming catalyst. Steam reforming also takes place to some extent in the combustion chamber due to the high temperature. The steam reforming reaction is accompanied by the water gas shift reaction. Typically, the gas is at or close to equilibrium at the outlet of the reactor with respect to steam reforming and water gas shift reactions. The temperature of the exit gas is typically in the range between 850° and 1100° C. More details of ATR and a full description can be found in the art such as “Studies in Surface Science and Catalysis, Vol. 152,” Synthesis gas production for FT synthesis”; Chapter 4, p. 258-352, 2004”.
It is also an object of the invention to provide a system for the production of a synthesis gas where the cost for the equipment materials (CAPEX) is reduced compared to known systems for production of similar amounts of synthesis gas using ATRs. Furthermore, it is an object of the invention to provide a process for synthesis gas production using an ATR with a reduced CO2 footprint compared to known processes using ATRs, for similar amounts of synthesis gas produced.
It is an object of the invention to provide a process and system for production of a synthesis gas by use of an ATR wherein the size and duty of the fired heater upstream the ATR is reduced, or ideally removed, compared to known processes and systems using ATRs.
It is a further object of the invention to provide a process and system for production of a synthesis gas by use of an ATR wherein the system can be started up quickly.
An aspect of the invention relates to chemical plant comprising:
When the reforming section of the chemical plant comprises an electrically heated reforming reactor upstream an autothermal reforming reactor, quick and efficient heating of the feed gas takes place in the electrically heated reforming reactor together with steam reforming. The first synthesis gas exiting the electrically heated reforming reactor is heated sufficiently for autothermal reforming to take place in the autothermal reforming reactor. Typically, the temperature of the first synthesis gas exiting the electrically heated reforming reactor has a temperature of at least about 350-400° C., such as above 400° C., preferably above 450° C., more preferably above 500° C., and more preferably above even 600° C. Thereby, it is typically not necessary to heat the first synthesis gas typically between the electrically heated reforming reactor and the autothermal reforming reactor.
A chemical plant with an autothermal reforming reactor typically includes a fired heater unit arranged to preheat the feed gas prior to entering the autothermal reforming reactor. When the chemical plant comprises an electrical reforming reactor upstream the autothermal reforming reactor, the fired heater unit may be reduced considerably in size or it may even be omitted.
When the reforming section comprises an electrically heated reforming reactor upstream an autothermal reforming reactor, the chemical energy of the first synthesis gas inlet into the autothermal reforming reactor may be increased considerably in comparison with a feed gas that has been heated in a fired heater or in another type of heater, such as an electrical heater, and subsequently preformed in an adiabatic prereformer followed by final heating before entering the autothermal reforming reactor. Due to the increased chemical energy, the required amount of oxygen inlet into the autothermal reforming reactor is potentially reduced. Here, the term “chemical energy of a gas” is meant to denote the enthalpy or internal energy of the gas, so that a gas has a higher chemical energy if it has higher enthalpy and/or a higher internal energy state.
It should be noted that part of the feed gas may bypass the electrically heated reforming reactor so that it is sent directly to the autothermal reforming reactor or is mixed with the first synthesis gas upstream the autothermal reforming reactor.
Moreover, the term “steam reforming” or “steam methane reforming reaction” is meant to denote a reforming reaction according to one or more of the following reactions:
CH4+H2O↔CO+3H2 (i)
CH4+2H2O↔CO2+4H2 (ii)
CH4+CO2↔2CO+2H2 (iii)
Reactions (i) and (ii) are steam methane reforming reactions, whilst reaction (iii) is the dry methane reforming reaction.
For higher hydrocarbons, viz. CnHm, where n≥2, m≥4, equation (i) is generalized as:
CnHm+n H2O↔nCO+(n+m/2)H2 (iv)
where n≥2, m≥4.
Typically, steam reforming is accompanied by the water gas shift reaction (v):
CO+H2O↔CO2+H2 (v)
The terms “steam methane reforming” and “steam methane reforming reaction” are meant to cover the reactions (i) and (ii), the terms “steam reforming” and “electrically heated reforming” are meant to cover the reactions (i), (ii) and (iv), whilst the term “methanation” covers the reverse reaction of reaction (i). In most cases, all of these reactions (i)-(v) are at, or close to, equilibrium at the outlet from the reforming reactor. The term “prereforming” is often used to cover the catalytic conversion of higher hydrocarbons according to reaction (iv). Prereforming is typically accompanied by steam reforming of methane and/or methanation (depending upon the gas composition and operating conditions) and the water gas shift reaction. Prereforming is often carried out in adiabatic reactors but may also take place in heated reactors.
Moreover, the terms “autothermal reforming” and “autothermal reforming reactions” also covers combustion and partial combustion of the hydrocarbon feedstock according to reaction (vi) and (vii):
CH4+1/2O2↔CO+2H2 (vi)
CH4+2O2↔CO2+2H2O (vii)
in addition to the steam methane reforming reactions.
The term “synthesis gas” is meant to denote a gas comprising hydrogen, carbon monoxide and also carbon dioxide and small amounts of other gasses, such as argon, nitrogen, methane, etc.
The term “prereforming” is often used to cover the catalytic conversion of higher hydrocarbons, viz. CnHm, where n≥2, m≥4, according to the reaction:
CnHm+n H2O↔nCO+(n+m/2)H2
Prereforming is typically accompanied by steam reforming of methane and/or methanation (depending upon the gas composition and operating conditions) and the water gas shift reaction. Prereforming is often carried out in adiabatic reactors but may also take place in heated reactors. Prereforming typically takes place in a temperature range of ca. 350-550° C. to convert higher hydrocarbons as an initial step in the process. This removes the risk of carbon formation from higher hydrocarbons on catalyst and equipment in the subsequent process steps. Optionally, carbon dioxide or other components may also be mixed with the gas leaving the prereforming step to form the feed gas.
A reforming section with an electrically heated reforming reactor upstream an autothermal reforming reactor is advantageous compared a reforming section with a fired steam methane reforming reactor upstream an autothermal reforming reactor, at least due to the following reasons:
Moreover, a reforming section with an electrically heated reforming reactor upstream an autothermal reforming reactor is advantageous compared a reforming section with a fired heater upstream an autothermal reforming reactor, at least due to the following reasons:
Moreover, the plant of the invention has provided a possibility of carrying out the start-up and increase of load of the autothermal reforming reactor quicker as compared to a system with a fired heater unit and an autothermal reforming reactor.
The combination of an electrically heated reforming reactor and an autothermal reforming reactor has benefits compared to for example a combination of an electrically heated reforming reactor and a fired steam methane reforming reactor or compared to a combination of an electrical heater followed by a fired steam methane reforming reactor. This is due to the possibility of operating at a lower steam-to-carbon ratio in the plant of the present invention as explained in the following.
The typical outlet temperature from an ATR is in the range 850-1100° C. According to the present invention, the second synthesis gas leaving the ATR has a temperature between 850-1100° C., such as between 950-1075° C. It is known in the art that it is possible to operate an ATR at a steam-to-carbon (S/C) ratio less than 2.0, such as less than 1.5, even lower than 1.2, lower than 1.0, lower than 0.8 or even lower than 0.65. A typical fired steam methane reformer operates at an outlet temperature of 800-950° C. At such a temperature level, the minimum possible S/C-ratio in a fired steam methane reformer will be dictated by the risk of carbon formation on the catalyst in the tubes of the fired steam methane reformer. For these reasons, fired steam methane reformers are not operated at S/C-ratios lower than ca. 1.3-1.5.
The present invention is based on the recognition that the optimum temperature of a gas exiting a prereformer to an ATR is 500-700° C., e.g. 650° C. The invention is further based on the recognition that it is possible to operate an electrically heated reforming reactor at any temperature in a range of about 250-1100° C. or even in a broader range and hence at the said optimum temperature, and that therefore it will be highly advantageous to use an electrically heated reforming reactor as a prereformer in combination with an ATR.
The required S/C-ratio to avoid carbon formation on a steam reforming catalyst increases with increasing temperature. As an electrically heated prereformer may be operated at a significantly lower temperature than a fired prereformer, the combination of an electrically heated reforming reactor followed by an autothermal reforming reactor can operate at a significantly lower S/C-ratio than a) a combination of an electrically heated reforming reactor (or an electrical heater) followed by a fired steam methane reformer as well as b) a combination of a fired reforming reactor followed by an autothermal reforming reactor. This means lower steam flows through the process plant and associated reduced costs.
Furthermore, it is possible with the plant of the invention to produce synthesis gas with a reduced H2/CO-ratio as a result of the reduced steam supply to the process This is desirable for various applications for example if the synthesis gas is used for production of synthetic fuels via the Fischer-Tropsch synthesis.
It should be noted that the terms “steam methane reforming” and “steam reforming” are used as synonyms herein. Moreover, the term “feed gas comprising hydrocarbons” is meant to denote a feed gas comprising a hydrocarbon gas with one or more hydrocarbons and possibly other constituents. The feed gas comprising hydrocarbons fed to the reforming sections is a feed gas comprising hydrocarbons and possibly other constituents, such as carbon monoxide, carbon dioxide, and possibly also some nitrogen and argon, mixed with steam and possibly hydrogen. Examples of “a hydrocarbon gas” may be natural gas, town gas, or a mixture of methane and higher hydrocarbons, as well as a prereformed gas.
Typically, the feed gas comprising hydrocarbons let into the reforming section has a predetermined ratio of hydrocarbon gas, steam, and hydrogen. It should be noted, that a feed gas comprising hydrocarbons to which steam, hydrogen and/or other constituents are added is still denoted a feed gas, even though its composition has changed.
The autothermal reforming reactor is arranged to receive the first synthesis gas and to provide the second synthesis gas, viz. an auto-reformed synthesis gas. In addition to the first synthesis gas input into the autothermal reforming reactor, a stream of oxidant gas is inlet. The stream of oxidant gas comprises oxygen and may be e.g. air or oxygen, enriched air, or a mixture of more than 90% oxygen with the balance being e.g nitrogen, steam, CO2, and/or argon.
In an embodiment, the electrically heated reforming reactor comprises:
wherein the electrical power supply is dimensioned to heat at least part of the first catalyst to a temperature of at least 450° C. by passing an electrical current through the electrical heating unit.
In a particular embodiment, the pressure shell has a design pressure of between 5 and 45 bar, preferably between 30 and 45 bar.
A feature of the electrically heated reforming reactor is that the energy is supplied inside the electrically heated reforming reactor, instead of being supplied from an external heat source via heat conduction, convection and radiation, e.g. through catalyst tubes or heater coils. In an electrically heated reforming reactor with an electrical heating unit connected to an electrical power supply via conductors, the heat for the steam reforming reaction is provided by resistance heating. The hottest part of the electrically heated reforming reactor will be within the pressure shell of the electrically heated reforming reactor. Preferably, the electrical power supply and the electrical heating unit within the pressure shell are dimensioned so that at least part of the electrical heating unit reaches a temperature of 450° C.-850° C., such as 500° C.-750° C., and preferably 550° C.-650° C.
The chemical plant of the invention may advantageously comprise one or more compressors and/or pumps upstream the reforming section. The compressors/pumps are arranged to compress the feed to a pressure of between 5 and 90 bar. The constituents of the feed, viz. water/steam, hydrogen and hydrocarbon feed gasses, may be compressed individually and fed individually into the reforming section or to the reforming reactors thereof.
One or more additional feed streams may also be added to the reforming section either upstream the electrically heated reformer, to a location downstream the electrically heated steam reformer and thereby being mixed with the first synthesis gas, or directly to the autothermal reforming reactor. An example is steam added directly to the autothermal reforming reactor or a tail gas from a Fischer-Tropsch unit which for example can be added between the electrically heated reforming reactor and the autothermal reforming reactor or directly to the autothermal reforming reactor. In a Gas-to-Liquids (GTL) plant for production of for example synthetic diesel by Fischer-Tropsch synthesis using natural gas as feedstock, a tail gas is typically recycled from the Fischer-Tropsch synthesis section to the synthesis gas preparation unit. This is done in order to adjust the product composition of the synthesis gas to a desired H2/CO-ratio of about 2.
The first catalyst may be a bed of catalyst particles, e.g. pellets, typically in the form of catalytically active material supported on a high area support with electrically conductive structures embedded in the bed of catalyst particles. Alternative, the first catalyst may be catalytically active material supported on a macroscopic structure, such as a monolith or a metallic tube.
When the electrically heated reforming reactor comprises a heat insulation layer adjacent to at least part of the inside of the pressure shell, appropriate heat and electrical insulation between the electrical heating unit and the pressure shell is obtained. Typically, the heat insulation layer will be present at the majority of the interior surface of the pressure shell to provide thermal insulation between the pressure shell and the electrical heating unit/first catalyst; however, passages in the heat insulation layers are needed in order to provide for connection of conductors between the electrical heating unit and the electrical power supply and to provide for inlets/outlets for gasses into/out of the electrically heated reforming reactor.
The presence of heat insulating layer between the pressure shell and the electrical heating unit assists in avoiding excessive heating of the pressure shell and assists in reducing thermal losses to the surroundings of the electrically heated reforming reactor. The temperatures of the electrical heating unit may reach up to about 850° C., at least at some parts thereof, but by using the heat insulation layer between the electrical heating unit and the pressure shell, the temperature of the pressure shell can be kept at significantly lower temperatures of e.g. 500° C. or even 200° C. This is advantageous since typical construction steel materials are unsuitable or highly expensive for pressure bearing applications at high temperatures, such as 850° C. Moreover, a heat insulating layer between the pressure shell and the electrical heating unit assists in control of the electrical current within the electrically heated reforming reactor, since heat insulation layer is also electrically insulating. The heat insulation layer could be one or more layers of solid material, such as ceramics, inert material, refractory material or a gas barrier or a combination thereof. Thus, it is also conceivable that a purge gas or a confined gas constitutes or forms part of the heat insulation layer.
As the hottest part of the electrically heated reforming reactor during operation is the electrical heating unit, and since a heat insulation layer thermally insulates the pressure shell from the electrically heated reforming reactor, the temperature of the pressure shell can be kept significantly lower than the maximum process temperature. Here, the term “process temperature” is meant to denote the temperature of the gas. This allows for having a relative low design temperature of the pressure shell of e.g. 700° C. or 500° C. or preferably 300° C. or 200° C. of the pressure shell whilst having maximum process temperatures of between 750° and 900°, such as 800°, 850° C. or 900° C.
Another advantage is that the lower design temperature means that in some cases the thickness of the pressure shell can be decreased, thereby saving costs.
It should be noted that the term “heat insulating material” is meant to denote materials having a thermal conductivity of about 10 W·m−1·K−1 or below. Examples of heat insulating materials are ceramics, refractory material, alumina-based materials, zirconia-based materials and similar.
In an embodiment, the electrical heating unit comprises a macroscopic structure of electrically conductive material, where the macroscopic structure supports a ceramic coating and the ceramic coating supports the catalytically active material. Such an electrical heating unit is also referred to herein as a “structured catalyst”. The catalytic conversion in the form of methane steam reforming and gas heating take place simultaneously.
Thus, during operating of the chemical plant, an electrical current is passed through the macroscopic structure and thereby heats the macroscopic structure and the catalytically active material supported thereon. The close proximity between the catalytically active material and the macroscopic structure enables efficient heating of the catalytically active material by solid material heat conduction from the resistance heated macroscopic structure. The amount and composition of the catalytically active material can be tailored to the steam reforming reaction at the given operating conditions. The surface area of the macroscopic structure, the fraction of the macroscopic structure coated with a ceramic coating, the type and structure of the ceramic coating, and the amount and composition of the catalytically active material may be tailored to the steam reforming reaction at the given operating conditions.
In an embodiment, the structured catalyst has at least one electrically insulating part arranged to increase the current path between the conductors to a length larger than the largest dimension of the structured catalyst. The provision of a current path between the conductors larger than the largest dimension of the structured catalyst may be by provision of electrically insulating part(s) positioned between the conductors and preventing the current running through some part of the structured catalyst. Such electrically insulating parts are arranged to increase the current path and thus increase the resistance through the structured catalyst. Hereby, the current path through the structured catalyst can be e.g. more than 50%, 100%, 200%, 1000%, or even 10000% longer than the largest dimension of the structured catalyst.
The term “electrically conductive” is meant to denote materials with an electrical resistivity in the range from: 10−4 to 10−8 Ω·m at 20° C. Thus, materials that are electrically conductive are e.g. metals like copper, silver, aluminum, chromium, iron, nickel, or alloys of metals. Moreover, the term “electrically insulating” is meant to denote materials with an electrical resistivity above 10 Ω·m at 20° C., e.g. in the range from 109 to 1025 Ω·m at 20° C.
As used herein, the term “electrical heating unit comprises a macroscopic catalyst” is not meant to be limited to an electrically heated reforming reactor with a single macroscopic structure. Instead, the term is meant to cover both a macroscopic structure with ceramic coating and catalytically active material supported thereon as well as an array of such macroscopic structures with ceramic coating and catalytically material supported thereon.
The term “macroscopic structure supporting a ceramic coating” is meant to denote that the macroscopic structure is coated by the ceramic coating at, at least, a part of the surface of the macroscopic structure. Thus, the term does not imply that all the surface of the macroscopic structure is coated by the ceramic coating; in particular, at least the parts of the macroscopic structure which are electrically connected to the conductors and thus to the electrical power supply do not have a coating thereon. The coating is a ceramic material with pores in the structure which allows for supporting the catalytically active material of the first catalyst on and inside the coating and has the same function as a catalytic support. Advantageously, the catalytically active material of the first catalyst comprises catalytically active particles having a size in the range from about 5 nm to about 250 nm.
As used herein, the term “macroscopic structure” is meant to denote a structure which is large enough to be visible with the naked eye, without magnifying devices. The dimensions of the macroscopic structure are typically in the range of centimeters or even meters. Dimensions of the macroscopic structure are advantageously made to correspond at least partly to the inner dimensions of the pressure shell, saving room for the heat insulation layer and conductors.
A ceramic coating, with or without catalytically active material, may be added directly to a metal surface by wash coating. The wash coating of a metal surface is a well-known process; a description is given in e.g. Cybulski, A., and Moulijn, J. A., Structured catalysts and reactors, Marcel Dekker, Inc, New York, 1998, Chapter 3, and references herein. The ceramic coating may be added to the surface of the macroscopic structure and subsequently the catalytically active material may be added; alternatively, the ceramic coat comprising the catalytically active material is added to the macroscopic structure.
Preferably, the macroscopic structure has been manufactured by extrusion of a mixture of powdered metallic particles and a binder to an extruded structure and subsequent sintering of the extruded structure, thereby providing a material with a high geometric surface area per volume. A ceramic coating, which may contain the catalytically material, is provided onto the macroscopic structure before a second sintering in an oxidizing atmosphere, in order to form chemical bonds between the ceramic coating and the macroscopic structure. Alternatively, the catalytically active material may be impregnated onto the ceramic coating after the second sintering. When chemical bonds are formed between the ceramic coating and the macroscopic structure, an especially high heat conductivity between the electrically heated macroscopic structure and the catalytically active material supported by the ceramic coating is possible. Due to close proximity between the heat source, viz. the macroscopic structure, and the catalytically active material, the heat transfer is effective, so that the catalytically active material can be very efficiently heated. A compact steam reforming reactor in terms of gas processing per steam reforming reactor volume is thus possible, and therefore the electrically heated reforming reactor housing the macroscopic structure may be compact. The electrically heated reforming reactor of the invention does not need a furnace and this reduces the size of the electrically heated reforming reactor considerably.
According to an embodiment of the system, the macroscopic structure comprises an extruded and sintered monolith or a number of extruded and sintered monoliths electrically connected to each other. According to an embodiment of the system, the macroscopic structure has electrically isolating parts arranged to increase the current path between the conductors to a length larger than the largest dimension of the macroscopic structure.
Preferably, the macroscopic structure comprises Fe, Ni, Cu, Co, Cr, Al, Si or an alloy thereof. Such an alloy may comprise further elements, such as Mn, Y, Zr, C, Co, Mo or combinations thereof. Preferably, the catalytically active material of the first catalyst is particles having a size from 5 nm to 250 nm. The catalytically active material of the first catalyst may e.g. comprise nickel, ruthenium, rhodium, iridium, platinum, cobalt, or a combination thereof. Thus, one possible catalytically active material of the first catalyst is a combination of nickel and rhodium and another combination of nickel and iridium. The ceramic coating may for example be an oxide comprising Al, Zr, Mg, Ce and/or Ca. Exemplary coatings are calcium aluminate or a magnesium aluminum spinel. Such a ceramic coating may comprise further elements, such as La, Y, Ti, K or combinations thereof. Preferably, the conductors are made of different materials than the macroscopic structure. The conductors may for example be of iron, nickel, aluminum, copper, silver, or an alloy thereof. The ceramic coating is an electrically insulating material and will typically have a thickness in the range of around 100 μm, say 10-500 μm. In addition, a fifth catalyst may be placed within the pressure shell and in channels within the macroscopic structure, around the macroscopic structure or upstream and/or downstream the macroscopic structure to support the catalytic function of the macroscopic structure.
In an embodiment, the chemical plant further comprises a prereformer upstream the electrically heated reforming reactor. In the prereformer, the hydrocarbon gas will, together with steam, and potentially also hydrogen and/or other components such as carbon dioxide, undergo prereforming in a temperature range of ca. 350-550° C. to convert higher hydrocarbons as an initial step in the process, normally taking place downstream a desulfurization step. This removes the risk of carbon formation from higher hydrocarbons on catalyst in the subsequent process steps. Optionally, carbon dioxide or other components may also be mixed with the gas leaving the prereforming step to form the feed gas.
In an embodiment, the reforming section furthermore comprises a gas heated steam methane reforming reactor in parallel to the electrically heated reforming reactor and the autothermal reforming reactor, wherein the gas heated steam methane reforming reactor comprises a third catalyst and being operable to receive a second feed gas comprising hydrocarbons and to utilize at least part of the second synthesis gas as heating medium in heat exchange within the gas heated steam methane reforming reactor, the gas heated steam methane reforming reactor being arranged for generating a third synthesis gas.
The gas heated steam methane reforming reactor is operable to utilize at least part of the second synthesis gas as heating media in heat exchange within the gas heated steam methane reforming reactor. The gas heated steam methane reforming reactor is arranged for generating a third synthesis gas over the third catalyst and for outputting the third synthesis gas from the reforming section as at least part of the output synthesis gas output from the reforming section. The overall heat efficiency of the chemical plant is increased by the addition of the gas heated steam methane reforming reactor, since the sensitive heat of the second synthesis gas is used within the gas heated steam methane reforming reactor. Moreover, when the chemical plant includes a gas heated steam methane reforming reactor, the overall output of the chemical plant is increased.
A gas heated steam methane reforming reactor is configured to use a hot gas to supply the heat for the endothermic steam methane reforming reaction by heat exchange, typically over a tube wall. An example of a configuration of a gas heated steam methane reforming reactor has several parallel tubes filled with catalyst which receive the feed gas. In the bottom of the reactor, the product gas from the catalyst filled tubes is mixed with part or all of the hot second synthesis gas from the autothermal reforming reactor to form a mixed gas. The mixed gas carries out heat exchange with the catalyst filled tubes within the gas heated methane reforming reactor and outputs the third synthesis gas. Other configurations of gas heated steam methane reforming reactors are also conceivable.
In an embodiment, the chemical plant further comprises:
In an embodiment, the post processing unit is a post conversion unit having an inlet for allowing addition of gaseous stream comprising heated CO2 to the second and/or third synthesis gas upstream the post conversion unit. The post processing unit houses a fourth catalyst active for catalyzing steam methane reforming, methanation and reverse water gas shift reactions. The post conversion unit is e.g. an adiabatic post conversion unit or a gas heat exchange reactor. The post processed synthesis gas is a synthesis gas with an H2/CO ratio lower than the H2/CO ratio of the output synthesis gas. By adding heated CO2 and carrying out steam methane reforming, methanation and reverse water gas shift reactions in a separate reactor downstream the reforming section, the CO production of the process may be increased and/or the H2/CO ratio may be tailored. The H2/CO ratio of the post processed synthesis gas is e.g. lower than 1.8, lower than 1.5 or even lower than 1.0. The temperature of the heated CO2 added may be e.g. a temperature of about 300° C., 400° C. or even of about 500° C. or above.
In an embodiment, the post processing unit is a water gas shift unit arranged to carry out the water gas shift reaction, thereby providing a post processed synthesis gas. In this embodiment, the post processed synthesis gas, is a water gas shifted synthesis gas, such as a hydrogen rich synthesis gas or a hydrogen gas stream. The water gas shift unit may be a single water gas shift unit, such as a medium temperature water gas shift unit, or a combination of two or more water gas shift units, e.g. a high temperature water gas shift unit and a low temperature water gas shift unit.
In an embodiment, the downstream section comprises gas separation unit(s) arranged to separate a stream of substantially pure CO2, H2, and/or CO from the output synthesis gas inlet to the downstream section, thereby providing a refined synthesis gas. Here, the term “substantially pure” is meant to denote the gas has at least 90% purity, such as 95% and preferably 99% purity.
In an embodiment, the chemical plant further comprises a first separation unit arranged to separate the output synthesis gas or the post processed synthesis gas into a water condensate and an intermediate synthesis gas.
It is an advantage of the invention that the module M of the output synthesis gas and/or the intermediate synthesis gas may be tailored. The module M is the stoichiometric ratio (H2−CO2)/(CO+CO2). The module M may be tailored to between 1.9 and 2.2, such as about 2.0 or 2.1, which is useful in the case where the downstream section comprises a methanol reactor arranged to convert the intermediate synthesis gas to methanol.
In an embodiment, the chemical plant further comprises a downstream section arranged to receive the intermediate synthesis gas and to process the intermediate synthesis gas to a chemical product and an off-gas. The chemical product is e.g. a hydrogen gas, a carbon monoxide gas, higher hydrocarbons, synthetic fuels, methanol, or ammonia.
In an embodiment, the chemical plant further comprises:
In order to ensure that the gas entering the autothermal reforming unit is sufficiently heated, a feed gas would typically be heated within a fired heater. In the chemical plant of the invention, the feed gas is heated and reformed within the electrically heated reforming reactor upstream the autothermal reforming reactor. Therefore, the heating within the fired heater unit may be reduced. In particular, the feed gas reaching the electrically heated reforming reactor may have a temperature of about 200-400° C. and may exit the electrically heated reforming reactor as the first synthesis gas at between 450° and 850° C. When the fired heater is not required to heat the feed gas to more than about 400°, the fired heater may be reduced in size or even omitted.
When the reforming section comprises an electrically heated reforming reactor upstream the autothermal reforming reactor, the fired heater unit upstream the electrically heated reforming reactor may be reduced considerably in size. If the chemical plant comprises a prereformer upstream the reforming section, the fired heater unit is typically positioned upstream the prereformer. A fired heater may also be placed upstream a unit for purifying the feed gas (such as a desulfurization unit). In some cases one fired heater may be used for several pre-heating purposes by using two or more coils or coil sections.
By recycling off-gas from the downstream section back to the fired heater unit, it is rendered possible to maximize the use of hydrocarbons in the feed on the process side and minimize the direct use of such hydrocarbons of the fired heater unit. Thus, the overall consumption of hydrocarbons is minimized for a given output of combined synthesis gas from the reforming section. It is possible to balance the chemical plant so that the operation of the fired heater unit is adjusted to being primarily, or even fully, driven by heat supplied by burning a recycled off-gas. This allows for a minimum use of natural gas imported for being burned off for heat in the chemical plant, which in turn allows for an optimal utilization of feed gasses comprising hydrocarbons to the chemical plant. Typically, a relatively small amount of make-up gas comprising hydrocarbon is also fed to the fired heater unit in order to allow control of the duty of the fired heater unit. The term “duty” is in this context understood as the heat input added to or removed from a unit operation in a chemical plant. Moreover, the term “relative small amount” is meant to denote that less than 20 vol % of the fuel gas burned off in the fired heater unit is recycled off-gas. As an example, between 5 and 20 vol %, e.g. about 10 vol %, of the fuel gas burned off in the fired heater unit is recycled off-gas.
In an embodiment, the downstream section comprises gas separation unit(s) arranged to separate a stream of substantially pure CO2, H2, and/or CO from the intermediate synthesis gas, thereby providing a refined gas.
In an embodiment, the downstream section comprises an ammonia reactor to convert the intermediate synthesis gas or the refined synthesis gas to ammonia, a methanol reactor to convert the intermediate synthesis gas or the refined synthesis gas to methanol, or a Fischer-Tropsch reactor to convert the intermediate synthesis gas or the refined synthesis gas to a mixture of higher hydrocarbons.
Another aspect of the invention relates to a process for producing a chemical product from a feed gas comprising hydrocarbons, in a chemical plant comprising a reforming section. The reforming section comprises an electrically heated reforming reactor housing a first catalyst, and an autothermal reforming reactor downstream the electrically heated reforming reactor, where the autothermal reforming reactor houses a second catalyst. The process comprises the steps of:
It should be noted, that the term “carrying out reforming” in the autothermal reforming reactor is meant to denote the steam methane reforming taking place over the second catalyst. Other reactions, such as partial combustion of the hydrocarbon feed by sub-stoichiometric amounts of oxygen and water gas shift, take place in the autothermal reforming reactor.
Advantages of the process and embodiments thereof correspond to the advantages of the chemical plant and embodiments thereof and will therefore not be described in further detail here.
During operation of the chemical plant 100, a feed gas 21 comprising hydrocarbons undergoes feed purification in a desulfurization unit 101 and becomes a desulfurized feed gas 22. The feed gas 21 comprising hydrocarbons is e.g. natural gas or town gas. The desulfurized feed gas 22 is preheated in a fired heater unit 105 and steam 23 is added, resulting in a gas stream 24. The gas stream 24 is led to a prereforming unit 102 housing steam reforming catalyst. Typically, the prereforming unit 102 is an adiabatic prereforming unit, wherein higher hydrocarbons are reacted so that the prereformed feed gas 25 exiting the prereformer contains no or very small amounts of higher hydrocarbons. The prereformed feed gas 25 is heated in the fired heater unit 105 to a heated prereformed feed gas 25′, which is led to the electrically heated reforming reactor 108
The heated prereformed feed gas 25′ undergoes steam methane reforming in the electrically heated reforming reactor 108, and a first synthesis gas 26 is output from the electrically heated reforming reactor 108. The first synthesis gas 26 is input to the autothermal reforming reactor 109, wherein it undergoes partial combustion together with by sub-stoichiometric amounts of oxygen from the stream 27, followed by steam reforming of the partially combusted hydrocarbon feed gas in a fixed bed of the second catalyst. The second catalyst is a steam methane reforming catalyst. A resulting second synthesis gas 30 is output from the autothermal reforming reactor 109. The second synthesis gas 30 is output from the reforming section as the output synthesis gas.
The electrically heated reforming reactor 108 e.g. comprises a pressure shell housing an electrical heating unit 108′ arranged to heat the first catalyst. The first catalyst is operable to catalyzing steam reforming of the feed gas. The pressure shell has a design pressure of between 5 and 45 bar. A heat insulation layer may be adjacent to at least part of the inside of said pressure shell. At least two conductors are electrically connected to the electrical heating unit and to an electrical power supply 107 is placed outside the pressure shell. The electrical power supply 107 is dimensioned to heat at least part of the first catalyst to a temperature of at least 500° C. by passing an electrical current through the electrical heating unit.
The output synthesis gas 30 is cooled in a heat exchanger 111 to a cooled synthesis gas 30′. The cooled synthesis gas 30′ enters a post processing unit 113, e.g. a water gas shift unit, and a water gas shifted synthesis gas 32 exits the water gas shift unit 113. The water gas shifted synthesis gas 32 is cooled in a second heat exchanger 114 to a cooled water gas shifted synthesis gas 32′, which enters the first separation unit 115. The first separation unit 115 e.g. comprises a flash separation unit. The cooled water gas shifted synthesis gas 32′ thus enters the flash separation unit 115 arranged to separate the cooled water gas shifted synthesis gas 32′ into water 29 and an intermediate synthesis gas 34, viz. a dry synthesis gas. Optionally, the intermediate synthesis gas 34 may enter a PSA unit (not shown in
It should be noted, that the chemical plant 100 typically comprises further equipment, such as compressors, heat exchangers etc.; however, such further equipment is not shown in
During operation of the chemical plant 100, a feed gas 21 comprising hydrocarbons undergoes feed purification in a desulfurization unit 101 and becomes a desulfurized feed gas 22. The feed gas 21 comprising hydrocarbons is e.g. natural gas or town gas. The desulfurized feed gas 22 is preheated in a fired heater unit 105 and steam 23 is added, resulting in a gas stream 24. The gas stream 24 is led to a prereforming unit 102 housing steam reforming catalyst. Typically, the prereforming unit 102 is an adiabatic prereforming unit, wherein higher hydrocarbons are reacted so that the prereformed feed gas 25 exiting the prereformer contains no or very small amounts of higher hydrocarbons. The prereformed feed gas 25 is heated in the fired heater unit 105 to a heated prereformed feed gas 25′, which is led to the electrically heated reforming reactor 108.
A fuel gas 46 comprising hydrocarbons, e.g. natural gas, to is sent to the fired heater unit in order to be burned off to provide heat within the fired heater unit 105. An effluent gas 48 is output from the fired heater unit 105.
A first part 25a of the heated prereformed feed gas 25′ undergoes steam methane reforming in the electrically heated reforming reactor 108, and a first synthesis gas 26 is output from the electrically heated reforming reactor 108. The first synthesis gas 26 is input to the autothermal reforming reactor 109, wherein it undergoes partial combustion together with by sub-stoichiometric amounts of oxygen from the stream 27, followed by steam reforming of the partially combusted hydrocarbon feed gas in a fixed bed of the second catalyst. The second catalyst is a steam reforming catalyst. A resulting second synthesis gas 30 is output from the autothermal reforming reactor 109.
The second synthesis gas 28 is inlet to the gas heated steam methane reforming reactor 112 in order to provide heat for the steam methane reforming reaction of the second part 25b of the feed gas 25 entering the gas heated steam methane reforming reactor 112 from another side. The gas exiting the gas heated steam methane reforming reactor 112 is a third synthesis gas 30. The third synthesis gas 30 is the output synthesis gas output from the reforming section.
The output synthesis gas 30 is cooled in a heat exchanger 111 to a cooled synthesis gas 30′. The cooled synthesis gas 30′ enters a post processing unit 113, e.g. a water gas shift unit, and a water gas shifted synthesis gas 32 exits the water gas shift unit 113. The water gas shifted synthesis gas 32 is cooled in a second heat exchanger 114 to a cooled water gas shifted synthesis gas 32′, which enters the first separation unit 115. The first separation unit 115 e.g. comprises a flash separation unit. The cooled water gas shifted synthesis gas 32′ thus enters the flash separation unit 115 arranged to separate the cooled water gas shifted synthesis gas 32′ into water 29 and an intermediate synthesis gas 34, viz. a dry synthesis gas. Optionally, the intermediate synthesis gas 34 may enter a PSA unit (not shown in
It should be noted, that the chemical plant 100 typically comprises further equipment, such as compressors, heat exchangers etc.; however, such further equipment is not shown in
The chemical plant 300 of
The output synthesis gas 30 exiting from the reforming section 110 is cooled in the heat exchanger 111 to a cooled synthesis gas 30′. The cooled synthesis gas 30′ enters the post processing unit 113, here in the form of a water gas shift unit, and a water gas shifted synthesis gas 32 exits the water gas shift unit 113. The water gas shifted synthesis gas 32 is cooled in a second heat exchanger 114 to a cooled water gas shifted synthesis gas 32′, which enters the first separation unit 115, such as e.g. a flash separation unit 115 arranged to separate the cooled water gas shifted synthesis gas 32′ into a condensate 29 and an intermediate synthesis gas 34, viz. a dry synthesis gas. The intermediate synthesis gas 34 enters the downstream section 116 arranged to process the intermediate synthesis gas 34 to a chemical product 40 and an off-gas 45. The downstream section 116 comprises e.g. an ammonia reactor to convert the intermediate synthesis gas 34 to ammonia, a methanol reactor to convert the intermediate synthesis gas 34 gas to methanol, or a Fischer-Tropsch reactor to convert the intermediate synthesis gas 34 to a mixture of higher hydrocarbons.
The off-gas 45 from the downstream section 116 is recycled as fuel to one or more burners of the fired heater unit 105. The off-gas 45 is combined with a small amount of make-up gas 46 comprising hydrocarbons, e.g. natural gas, to form the fuel gas 47 sent to the one or more burners of the fired heater unit 105. The fired heater unit 105 is arranged to provide heat for preheating the feed gas 21, the desulfurized feed gas 22, and the prereformed feed gas 25. An effluent gas 48 is output from the fired heater unit 105.
A fuel gas 46 comprising hydrocarbons, e.g. natural gas, to is sent to the fired heater unit in order to be burned off to provide heat within the fired heater unit 105. An effluent gas 48 is output from the fired heater unit 105.
A heat exchange fluid 20, such as water, is used for heat exchange in the heat exchanger 111 and a heated heat exchange fluid, such as steam, is exported as stream 20′. A part of the steam is used as addition of steam 23 to the desulfurized feed gas 22.
By recycling off-gas from the downstream section 116 back to the fired heater unit 105, it is rendered possible to maximize the use of hydrocarbons in the feed on the process side and minimize the use of natural gas within the fired heater unit 105. It is possible to balance the chemical plant so that the operation of the fired heater unit 105 is adjusted to being primarily, or even fully, driven by heat supplied by burning a recycled off-gas 45. This allows for a minimum use of natural gas imported for being burned off for heat in the chemical plant 300, which in turn allows for an optimal utilization of feed gasses comprising hydrocarbons to the chemical plant. Typically, a relatively small amount of make-up gas 46 comprising hydrocarbons, e.g. natural gas, is also fed to the fired heater unit 105 in order to allow for control of the duty of the fired heater unit. The term “duty” is in this context understood as the heat input added to or removed from a unit operation in a chemical plant.
The chemical plant 300 of
The output synthesis gas 30 exiting from the reforming section 210 is cooled in the heat exchanger 111 to a cooled synthesis gas 30′. The cooled synthesis gas 30′ enters the post processing unit 113, here in the form of a water gas shift unit, and a water gas shifted synthesis gas 32 exits the water gas shift unit 113. The water gas shifted synthesis gas 32 is cooled in a second heat exchanger 114 to a cooled water gas shifted synthesis gas 32′, which enters the first separation unit 115, such as e.g. a flash separation unit 115 arranged to separate the cooled water gas shifted synthesis gas 32′ into a condensate 29 and an intermediate synthesis gas 34, viz. a dry synthesis gas. The intermediate synthesis gas 34 enters the downstream section 116 arranged to process the intermediate synthesis gas 34 to a chemical product 40 and an off-gas 45. The downstream section 116 comprises e.g. an ammonia reactor to convert the intermediate synthesis gas 34 to ammonia, a methanol reactor to convert the intermediate synthesis gas 34 gas to methanol, or a Fischer-Tropsch reactor to convert the intermediate synthesis gas 34 to a mixture of higher hydrocarbons.
The off-gas 45 from the downstream section 116 is recycled as fuel to one or more burners of the fired heater unit 105. The off-gas 45 is combined with a small amount of make-up gas 46 comprising hydrocarbons, e.g. natural gas, to form the fuel gas 47 sent to the one or more burners of the fired heater unit 105. The fired heater unit 105 is arranged to provide heat for preheating the feed gas 21, the desulfurized feed gas 22, and the prereformed feed gas 25.
A heat exchange fluid 20, such as water, is used for heat exchange in the heat exchanger 111 and a heated heat exchange fluid, such as steam, is exported as stream 20′. A part of the steam is used as addition of steam 23 to the desulfurized feed gas 22.
By recycling off-gas from the downstream section 116 back to the fired heater unit 105, it is rendered possible to maximize the use of hydrocarbons in the feed on the process side and minimize the use of natural gas within the fired heater unit 105. It is possible to balance the chemical plant so that the operation of the fired heater unit 105 is adjusted to being primarily, or even fully, driven by heat supplied by burning a recycled off-gas 45. This allows for a minimum use of natural gas imported for being burned off for heat in the chemical plant 300, which in turn allows for an optimal utilization of feed gasses comprising hydrocarbons to the chemical plant. Typically, a relatively small amount of make-up gas 46 comprising hydrocarbons, e.g. natural gas, is also fed to the fired heater unit 105 in order to allow for control of the duty of the fired heater unit 105. The term “duty” is in this context understood as the heat input added to or removed from a unit operation in a chemical plant.
The embodiments shown in
It should be noted, that the chemical plants shown in
In the classical 2-step reforming methanol layout (2-step-MeOH), the chemical plant comprises a reforming section with a primary reformer in the form of a steam methane reformer (SMR) and a methanol section. Natural gas is used as the principal reformer feed and is prereformed and fed to the primary reformer. Heating of the SMR is provided by natural gas in some part and expanded off-gas from the methanol (MeOH) synthesis which comes in part from the MeOH loop and the MeOH distillation. The partially reformed gas from the primary reformer is sent to a secondary reformer which is supplied with oxygen, typically from an air separation unit (ASU), where oxygen addition is controlled in order to obtain the desired module of the synthesis gas. The synthesis gas from the reforming section is cooled to remove water in the process gas, before it is compressed to 92 barg. The compressor was driven by a steam turbine, operated on superheated steam produced in the waste-heat section of the SMR. The compressed synthesis gas was combined with recycle gas from the methanol loop and sent to the methanol reactor of the methanol section and the resulting product was refined to a final pure methanol product.
In this example, a 2-step reforming methanol layout (2-step-MeOH) as described above is compared with an electrically heated reformer (2-step eSMR-MeOH). Making a 2-step reforming layout with an electrically heated reformer (eSMR) was done by also including a fired heater for feed preheating; the fired heater was fueled partly by expanded off-gas from the methanol section and natural gas. In this specific case, additional burning of natural gas was required to provide sufficient superheated steam to drive the turbine(s).
Comparing consumption figures of the two layouts is shown in Table 1. Using an electrically heated reformer (eSMR) in the 2-step reforming layout in 2-step-eSMR-MeOH was found to have the same feed consumption as the classical 2-step-MeOH layout. However, a large difference is found on the fuel side, where the majority of the natural gas consumption was replaced with electricity. This, obviously, translate into lower CO2 emissions from the 2-step-eSMR-MeOH layout, which is associated with the reduced flue gas emissions.
| Number | Date | Country | Kind |
|---|---|---|---|
| PA 2019 00258 | Feb 2019 | DK | national |
| PA 2019 00873 | Jul 2019 | DK | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2020/055173 | 2/27/2020 | WO | 00 |
