Process and Apparatus for Producing Synthesis Gas

BACKGROUND

[0001] The present invention relates to a process and apparatus for producing synthesis gas by steam/hydrocarbon reforming.

[0002] Hydrogen production by steam/hydrocarbon reforming, also called steam- methane reforming or SMR, is well-known. The steam/hydrocarbon reforming process is an energy intensive process.

[0003] Because steam/hydrocarbon reforming is an energy intensive process, industry desires to improve the thermal efficiency of steam/hydrocarbon reforming processes.

[0004] To improve the thermal efficiency of the synthesis gas production process, industry has decreased the amount of excess steam used in the process. The reduction of excess steam increases the propensity for carbon formation particularly when the feed to the reformer contains higher hydrocarbons, e.g. C2-C12 hydrocarbons. Industry desires to avoid carbon deposition on the reforming catalyst.

[0005] Potassium-promoted reforming catalyst has been used to reduce carbon formation and thereby avoid carbon deposition on the reforming catalyst.

BRIEF SUMMARY

[0006] The present disclosure relates to a process and apparatus for producing synthesis gas.

[0007] There are several aspects of the disclosure as outlined below.

[0008] Aspect #1. A process for producing synthesis gas, the process comprising: contacting a reformer feed gas comprising steam and at least one C1 to C12 hydrocarbon with a potassium-promoted reforming catalyst under reaction conditions sufficient to form synthesis gas thereby reacting the reformer feed gas to produce the synthesis gas, wherein the synthesis gas contains gaseous potassium hydroxide; and

contacting the synthesis gas containing the gaseous potassium hydroxide with a packing comprising one or more metai oxides under reaction conditions sufficient to react the gaseous potassium hydroxide with the one or more metai oxides thereby reacting the gaseous potassium hydroxide with the one or more metal oxides and forming a potassium hydroxide-depleted synthesis gas, wherein the one or more metal oxides have a greater chemical affinity for the gaseous potassium hydroxide than does alumina, and wherein the packing comprises 1 to 100 weight % of the one or more metal oxides.

[0009] Aspect #2. A process as defined in aspect #1 further comprising recovering heat from the potassium hydroxide-depleted synthesis gas thereby cooling the potassium hydroxide-depleted synthesis gas to a temperature below 325°C wherein the potassium hydroxide-depleted synthesis gas has a partial pressure of steam greater than 377 kPa.

[0010] Aspect #3. The process as defined in aspect #2 further comprising contacting the cooled potassium hydroxide-depleted synthesis gas with a shift catalyst under reaction conditions sufficient to form additional hydrogen by the shift reaction.

[0011] Aspect #4. The process as defined in any one of aspects #1 to #3 wherein the one or more metal oxides comprise at least one metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W.

[0012] Aspect #5. The process as defined in any one of aspects #1 to #4 wherein the reaction conditions sufficient to form synthesis gas include a temperature ranging from 500°C to 1000°C and a pressure ranging from 2 to 50 atmospheres.

[0013] Aspect #6. The process as defined in any one of aspects #1 to #5 wherein the reaction conditions sufficient to react the potassium hydroxide with the one or more metal oxides include a temperature ranging from 500°C to 1000°C and a pressure ranging from 2 to 50 atmospheres.

[0014] Aspect # 7. The process as defined in any one of aspects #1 to #6 wherein the one or more metal oxides comprise at least one metal selected from the group consisting of Ti, Zr, Hf, V, Nb, and Ta. [0015] Aspect #8. The process as defined in any one of aspects #1 to #7 wherein the packing supports a catalytic material.

[0016] Aspect #9. The process as defined in any one of aspects #1 to #7 wherein the packing supports a catalytic material selected from the group consisting of Ni, Pd, Pt, Fe, Ru, Os, Co, Rh, and Ir.

[0017] Aspect #10. An apparatus for producing synthesis gas by the process as defined in any one of aspects #1 to #9, the apparatus comprising:

a reformer comprising a plurality of tubular reactors, the plurality of tubular reactors containing the potassium-promoted reforming catalyst; and

the packing comprising the one or more metal oxides;

wherein the packing is contained in one or more of (i) the plurality of tubular reactors downstream of at least a portion of the potassium-promoted reforming catalyst, (ii) a vessel in downstream fluid flow communication of the reformer and in upstream fluid flow communication of a heat transfer device, and (iii) an inlet portion of the heat transfer device for receiving the synthesis gas containing the gaseous potassium hydroxide.

[0018] Aspect #1 1. The apparatus as defined in aspect #10 further comprising the heat transfer device for recovering heat from the potassium hydroxide-depleted synthesis gas.

[0019] Aspect #12. The apparatus as defined in aspect #10 wherein the packing is contained in the plurality of tubular reactors downstream of at least a portion of the potassium-promoted reforming catalyst.

[0020] Aspect #13. The apparatus as defined in aspect #12 wherein the packing supports a catalytic material suitable for reforming reactions.

[0021] Aspect #14. The apparatus as defined in aspect #13 wherein the catalytic material is selected from the group consisting of Ni, Pd, Pt, Fe, Ru, Os, Co, Rh, and Ir.

[0022] Aspect #15. The apparatus as defined in aspect #10 wherein the packing is contained in the vessel in downstream fluid flow communication of the reformer and in upstream fluid flow communication of the heat transfer device.

[0023] Aspect #16. The apparatus as defined in aspect #10 wherein the packing is contained in the inlet portion of the heat transfer device. [0024] Aspect #17. The apparatus as defined in any one of aspects #10 to #16 wherein the one or more metal oxides in the packing comprise at ieast one metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W. BR!EF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS

[0025] FiG. 1 is a schematic of an apparatus for producing synthesis gas showing the packing comprising one or more metal oxides contained in the of tubular reactors.

[0026] FiG. 2 is a schematic of an apparatus for producing synthesis gas showing the packing comprising one or more metal oxides contained in a vessel.

[0027] FiG. 3 is a schematic of an apparatus for producing synthesis gas showing the packing comprising one or more metal oxides contained in a heat transfer device.

DETAILED DESCRIPTION

[0028] The articles "a" and "an" as used herein mean one or more when applied to any feature in embodiments of the present invention described in the specification and claims. The use of "a" and "an" does not limit the meaning to a single feature uniess such a iimit is specifically stated. The article "the" preceding singular or plural nouns or noun phrases denotes a particular specified feature or particular specified features and may have a singular or plural connotation depending upon the context in which it is used. The adjective "any" means one, some, or all indiscriminately of whatever quantity. The term "and/or" placed between a first entity and a second entity means one of (1 ) the first entity, (2) the second entity, and (3) the first entity and the second entity.

[0029] As used herein, "in fluid flow communication" means operatively connected by one or more conduits, manifolds, vaives and the like, for transfer of fluid. A conduit is any pipe, tube, passageway or the like, through which a fluid may be conveyed. An intermediate device, such as a pump, compressor or vessel may be present between a first device in fluid flow communication with a second device unless explicitly stated otherwise.

[0030] Downstream and upstream refer to the intended flow direction of the process fluid transferred. If the intended flow direction of the process fluid is from the first device to the second device, the second device is in downstream fluid flow communication of the first device.

[0031] The present invention relates to a process and apparatus for producing synthesis gas by steam-hydrocarbon reforming. More specifically the present invention relates to a process and apparatus for producing synthesis gas using a potassium- promoted reforming catalyst while avoiding excessive corrosion in any downstream device where heat is recovered from the synthesis gas.

[0032] Applicants have discovered that the use of potassium-promoted reforming catalyst under operating conditions favorable for overall process efficiency may iead to potassium hydroxide in the synthesis gas leaving the reformer, which may lead to a heretofore unknown problem of corrosion of downstream devices such as the waste heat boiler.

[0033] Industry desires to avoid corrosion of process equipment in the synthesis gas production plant.

[0034] The process for producing synthesis gas according this disclosure comprises contacting a reformer feed gas comprising steam and at least one C1 to C12 hydrocarbon with a potassium-promoted reforming catalyst under reaction conditions sufficient to form synthesis gas thereby reacting the reformer feed gas to produce the synthesis gas, wherein the synthesis gas contains gaseous potassium hydroxide.

[0035] As used herein, synthesis gas is any gaseous mixture comprising hydrogen and carbon monoxide. A synthesis gas stream may also comprise steam, carbon dioxide, and/or unconverted methane

[0036] A reformer feed gas is a gaseous mixture comprising steam and at ieast one C1 to C12 hydrocarbon suitable for introducing into a reformer to produce synthesis gas. Typically natural gas and steam are mixed to form a reformer feed gas. The reformer feed gas may also be effluent from a so-called prereformer where the prereformer is adiabatic or convectively heated and wherein the pre-reformer effluent comprises steam and at Ieast one C1 to C12 hydrocarbon.

[0037] As used herein, the term "catalyst" refers to a support, catalytic material, and any other additives which may be present on the support.

[0038] Catalyst supports may be pellets or structured packing, e.g. monolithic structures, or any other catalyst support known in the art. [0039] A reforming catalyst is any catalyst that is suitable for catalyzing the reforming reaction. Reforming catalysts are known in the art.

[0040] A potassium-promoted catalyst contains at least one potassium containing material capable of generating potassium hydroxide under reforming conditions. A potassium-promoted catalyst often comprises a mixture of materials including at least one potassium containing material, designed to control the rate at which potassium hydroxide is generated. Potassium-promoted reforming catalyst is advertised as being suitable for preventing carbon formation when the feed to the reformer contains hydrocarbons heavier than methane and for helping to prevent hot band formation.

[0041] The support for the potassium promoted reforming catalyst may comprise one or more of calcium aluminate, magnesium aluminate, and alumina. The potassium- promoted reforming catalyst may comprise less than 1 weight % of one or more select metal oxides, wherein the select metal oxides in the potassium promoted reforming catalyst are oxides of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W. The inclusion of these select metal oxides would act to prevent the generation of potassium hydroxide under reforming conditions, which is contrary to its function as a potassium-promoted catalyst.

[0042] Reaction conditions sufficient to form synthesis gas may include a temperature ranging from 500°C to 1000°C and a pressure ranging from 2 to 50 atmospheres.

[0043] Appropriate temperatures for reaction are typically provided by combusting a fuel gas in the reformer furnace external to the plurality of catalyst-containing reformer tubes by introducing the fuel gas and an oxidant gas through burners mounted in the ceiling of the reformer furnace to supply energy for reacting steam and at least one C1 to C12 hydrocarbon inside the plurality of catalyst-containing reformer tubes and withdrawing a flue gas from the reformer furnace.

[0044] The process further comprises contacting the synthesis gas containing the gaseous potassium hydroxide with a packing comprising one or more metal oxides under reaction conditions sufficient to react the gaseous potassium hydroxide with the one or more metal oxides thereby reacting the gaseous potassium hydroxide with the one or more metal oxides and forming a potassium hydroxide-depleted synthesis gas, wherein the one or more metal oxides have a greater chemical affinity for the gaseous potassium hydroxide than does alumina, and wherein the packing comprises 1 to 100 weight % of the one or more metal oxides. The potassium hydroxide reacts with the one or more metal oxides to form other metal oxides comprising potassium. [0045] The term "depleted" means having a lesser mole % concentration of the indicated gas than the original stream from which it was formed. "Depleted" does not mean that the stream is completely lacking the indicated gas.

[0046] The reaction conditions sufficient to react the potassium hydroxide with the one or more metal oxides may include a temperature ranging from 500°C to 1000°C and a pressure ranging from 2 to 50 atmospheres.

[0047] As used herein, the generic term "metal oxide" is defined to mean a binary metal oxide of a single metal in an oxide lattice and a mixed metal oxide of two or more metals in an oxide lattice. A packing comprising a metal oxide means that the packing comprises (a) a single binary metal oxide, (b) a single mixed metal oxide, (c) a mixture of two or more binary metal oxides, (d) a mixture of two or more mixed metal oxides, or (e) a mixture of one or more binary metal oxides and one or more mixed metal oxides.

[0048] The one or more metai oxides in the packing may comprise at least one metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W.

[0049] The one or more metal oxides in the packing may comprise at least one metal selected from the group consisting of Ti, Zr, Hf, V, Nb, and Ta.

[0050] The one or more metal oxides in the packing may comprise at least one metal selected from the group consisting of Ti, Zr, Hf, and Ta.

[0051] Chemical affinity is defined relative to the equilibrium concentration of free KOH vapor over alumina under reforming conditions as predicted from the equilibrium constant for the hydrolysis of the potassium aluminate complex to alumina and free KOH. A potassium metal oxide complex which leads to a lower equilibrium

concentration under reforming conditions than that over alumina has a greater chemical affinity for gaseous potassium hydroxide than does alumina.

[0052] lf K ₂ O MO + H ₂ O(10barg) = 2KOH(gas) + MO, where MO is the metal oxide packing, the packing has a greater chemical affinity for potassium hydroxide than alumina if

(K _eq = [K0H(gas)] ² /K ₂ 0 MO) < (K _eq = [KOH(gas)] ² / K ₂ 0 Al ₂ 0 ₃ ).

[0053] Equilibrium constants and concentrations of KOH(gas) may be calculated by commercial software, for example the HSC Chemistry 6 software package and its thermodynamic database from Outo Kumpu Technology, Finland. [0054] The equilibrium constants, equilibrium concentration of KOH(gas) and equilibrium concentration of OH(gas) relative to its concentration over alumina for the above reaction are shown in the following Table.

TABLE 1

[0055] The packing may be in the form of a plurality of pellets, and/or a structured packing, for example, a monolithic support, or other suitable structure known in the art. The packing may optionally comprise one or more catalytic material suitable for catalyzing the reforming reaction, for example Ni, Pd, Pt, Fe, Ru, Os, Co, Rh, and Ir.

[0056] The process may further comprise recovering heat from the potassium hydroxide-depleted synthesis gas thereby cooling the potassium hydroxide-depleted synthesis gas to a temperature below 325°C wherein the potassium hydroxide-depleted synthesis gas has a partial pressure of steam greater than 377 kPa (40psig).

[0057] Heat may be recovered by indirect heat transfer from the potassium hydroxide- depleted synthesis gas. Heat may be transferred to water to form steam in a boiler, often called a waste heat boiler. Heat may be transferred from the potassium hydroxide- depleted synthesis gas to feed gas for a prereformer or reformer. Heat may be transferred from the potassium hydroxide-depleted synthesis gas to a process gas in a convectively heated prereformer.

[0058] The overall efficiency of the process for producing synthesis gas is improved when the heat is recovered from the synthesis gas and cooling the synthesis gas to a temperature below 325°C where the partial pressure of steam is greater than 377 kPa (40psig). Recovering heat with higher process-side (synthesis gas) pressure is desirable because it is cheaper to compress natural gas than hydrogen. However, higher process side pressure means higher process side dew point, which leads to corrosion of the heat transfer device if potassium hydroxide is present in the process gas. One skilled in the art can readily provide process conditions that result in cooling the synthesis gas to a temperature below 325°C and where the partial pressure of steam in the synthesis gas is greater than 377 kPa.

[0059] By depleting the synthesis gas of potassium hydroxide by reacting the potassium hydroxide with the one or more metal oxides, excessive corrosion in the heat exchanger, boiler, or convectively heated prereformer may be avoided.

[0060] The process may further comprise contacting the cooled potassium hydroxide- depleted synthesis gas with a shift catalyst under reaction conditions sufficient to form additional hydrogen by the shift reaction. Shift reactors and suitable shift catalysts are known in the art. The shift catalyst may be an iron-based high temperature shift catalyst, or a copper-based medium temperature shift catalyst, or a copper-based low

temperature shift catalyst. Any suitable shift catalyst may be used. One skilled in the art can readily select a suitable shift catalyst.

[0061] The present invention also relates to an apparatus for producing synthesis gas. The apparatus may be suitable for carrying out the disclosed process. The apparatus will be described with reference to FIGS. 1 to 3.

[0062] The apparatus 1 , 2, 3 for producing synthesis gas comprises reformer 10 comprising a plurality of tubular reactors 20, the plurality of tubular reactors 20 containing potassium-promoted reforming catalyst 22 and packing 24 comprising one or more metal oxides. The tubular reactors are typically called reformer tubes.

[0063] The packing 24 may be contained in one or more of (i) the plurality of tubular reactors 20 downstream of at least a portion of the potassium-promoted reforming catalyst 22, (ii) a vessel 90 in downstream fluid flow communication of the reformer 10 and in upstream fluid flow communication of a heat transfer device 80, and (iii) an inlet portion of the heat transfer device 80 for receiving the synthesis gas containing the gaseous potassium hydroxide.

[0064] The one or more metal oxides in the packing may comprise at least one metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, and W.

[0065] The one or more metal oxides in the packing may comprise at least one metal selected from the group consisting of Ti, Zr, Hf, V, Nb, and Ta. [0066] The one or more metai oxides in the packing may comprise at least one metal selected from the group consisting of Ti, Zr, Hf, and Ta.

[0067] The packing may support a catalytic material suitable for reforming reactions. The catalytic material may be selected from the group consisting of Ni, Pd, Pt, Fe, Ru, Os, Co, Rh, and Ir.

[0068] The apparatus may further comprise heat transfer device 80 for recovering heat from the potassium hydroxide-depleted synthesis gas. The heat transfer device may be a boiler, typically called a waste heat boiler. The heat transfer device may be a convective reactor, for example, a convectively heated pre-reformer. The heat transfer device may be a heat exchanger, for example to heat the feed to an adiabatic pre-reformer.

[0069] FIG 1 shows reformer feed gas 30 comprising steam and at least one C1 to C12 hydrocarbon contacted with potassium-promoted reforming catalyst 22 in the plurality of tubular reactors 20 in reformer 10 to produce synthesis gas containing gaseous potassium hydroxide. The synthesis gas containing gaseous potassium hydroxide is contacted with packing 24 comprising one or more metal oxides in the plurality of tubular reactors 20 to react the gaseous potassium hydroxide with one or more metal oxides to form potassium hydroxide-depleted synthesis gas 70. Heat for the endothermic reforming reaction is provided by combustion fuel in burners 60. The potassium hydroxide-depleted synthesis gas 70 is passed to optional heat transfer device 80 for recovering heat from the potassium hydroxide-depleted synthesis gas and forming a cooled potassium hydroxide-depleted synthesis gas 72.

[0070] FIG 2 shows reformer feed gas 30 comprising steam and at least one C1 to C12 hydrocarbon contacted with potassium-promoted reforming catalyst 22 in the plurality of tubular reactors 20 in reformer 10 to produce synthesis gas containing gaseous potassium hydroxide. The synthesis gas containing gaseous potassium hydroxide is contacted with packing 24 comprising one or more metal oxides in vessel 90 to react the gaseous potassium hydroxide with one or more metal oxides to form potassium hydroxide-depleted synthesis gas 70. Heat for the endothermic reforming reaction is provided by combustion fuel in burners 60. The potassium hydroxide-depleted synthesis gas 70 is passed to optional heat transfer device 80 for recovering heat from the potassium hydroxide-depleted synthesis gas and forming a cooled potassium hydroxide- depleted synthesis gas 72. Vessel 90 is in downstream fluid flow communication of the reformer 10 and in upstream fluid flow communication of heat transfer device 80. [0071] FiG 3 shows reformer feed gas 30 comprising steam and at least one C1 to C12 hydrocarbon contacted with potassium-promoted reforming catalyst 22 in the plurality of tubular reactors 20 in reformer 10 to produce synthesis gas containing gaseous potassium hydroxide. The synthesis gas containing gaseous potassium hydroxide is contacted with packing 24 comprising one or more metal oxides in an inlet portion of heat transfer device 80 for receiving the synthesis gas containing the gaseous potassium hydroxide to form potassium hydroxide-depleted synthesis gas 70. Heat for the endothermic reforming reaction is provided by combustion fuel in burners 60. The potassium hydroxide-depleted synthesis gas 70 is cooled in heat transfer device 80 for recovering heat from the potassium hydroxide-depleted synthesis gas and forming a cooled potassium hydroxide-depleted synthesis gas 72.

[0072] EXAMPLE

[0073] A catalyst support was prepared from a mixture of α-ΑΙ ₂ 0 _3> CaC0 ₃ and Ti0 ₂ . After calcination at 1000°C, XRD showed the substrate contained a-AI ₂ 03, CaTiOs, Ti0 ₂ and CaAI ₂ C>4. The support was impregnated with Ni such that after calcination it contained 17.0 wt% NiO to form sample 1.

[0074] Sample 1 was loaded into a steam aging unit downstream of a potassium- promoted catalyst, marketed as SudChemie G-91 catalyst. Various catalyst

manufacturers market reforming catalyst that uses "controlled release potassium" as part of the catalyst formulation as a coke suppression additive. The test conditions were 850°C, 2.86 MPa (400 psig) steam with a sweep gas of 4 mole % hydrogen, balance nitrogen.

[0075] 54 weight % of the potassium that was released from the G-91 catalyst was reacted in the small bed (1.3 g) of sample 1 catalyst. The reacted sample was analyzed by XRD and detected Ko. ₅ Ti ₄ 0 ₈ .