WO2015137910A1 - Membrane-based gas separation processes to produce synthesis gas with a high co content - Google Patents

Membrane-based gas separation processes to produce synthesis gas with a high co content Download PDF

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WO2015137910A1
WO2015137910A1 PCT/US2014/022523 US2014022523W WO2015137910A1 WO 2015137910 A1 WO2015137910 A1 WO 2015137910A1 US 2014022523 W US2014022523 W US 2014022523W WO 2015137910 A1 WO2015137910 A1 WO 2015137910A1
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stream
hydrogen
membrane
carbon dioxide
feed
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PCT/US2014/022523
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French (fr)
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Nicholas P. Wynn
Douglas Gottschlich
Alvin Ng
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Membrane Technology And Research, Inc.
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Priority to PCT/US2014/022523 priority Critical patent/WO2015137910A1/en
Publication of WO2015137910A1 publication Critical patent/WO2015137910A1/en

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    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/02Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen
    • C01B3/32Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air
    • C01B3/34Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents
    • C01B3/38Production of hydrogen or of gaseous mixtures containing a substantial proportion of hydrogen by reaction of gaseous or liquid organic compounds with gasifying agents, e.g. water, carbon dioxide, air by reaction of hydrocarbons with gasifying agents using catalysts
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B3/00Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
    • C01B3/50Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification
    • C01B3/501Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification by diffusion
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/02Processes for making hydrogen or synthesis gas
    • C01B2203/0205Processes for making hydrogen or synthesis gas containing a reforming step
    • C01B2203/0227Processes for making hydrogen or synthesis gas containing a reforming step containing a catalytic reforming step
    • C01B2203/0233Processes for making hydrogen or synthesis gas containing a reforming step containing a catalytic reforming step the reforming step being a steam reforming step
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0405Purification by membrane separation
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/04Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
    • C01B2203/0465Composition of the impurity
    • C01B2203/0475Composition of the impurity the impurity being carbon dioxide
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/12Feeding the process for making hydrogen or synthesis gas
    • C01B2203/1205Composition of the feed
    • C01B2203/1211Organic compounds or organic mixtures used in the process for making hydrogen or synthesis gas
    • C01B2203/1235Hydrocarbons
    • C01B2203/1241Natural gas or methane
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/14Details of the flowsheet
    • C01B2203/146At least two purification steps in series
    • CCHEMISTRY; METALLURGY
    • C01INORGANIC CHEMISTRY
    • C01BNON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
    • C01B2203/00Integrated processes for the production of hydrogen or synthesis gas
    • C01B2203/14Details of the flowsheet
    • C01B2203/148Details of the flowsheet involving a recycle stream to the feed of the process for making hydrogen or synthesis gas

Definitions

  • the invention relates to membrane-based gas separation processes for the production of synthesis gas ("syngas”) with a high yield of carbon monoxide from a light hydrocarbon feedstock. Carbon dioxide recovered from one or more membrane separation steps is recycled within the process.
  • syngas synthesis gas
  • Synthesis gas or syngas - a mixture of carbon m onoxide, carbon dioxide, and hydrogen - is used as a feedstock for making diverse hydrocarbon products, including methanol and synthetic fuels and lubricant oils.
  • Syngas can be produced by steam methane reforming (SMR). At low to moderate pressures and at high temperatures, methane reacts with steam on a nickel catalyst according to the following reforming reactions:
  • reformers are run at elevated pressure, typically 20-30 bar, to facilitate both heating of the reactant gases and heat recovery' from the product gases, which is necessitated by the highly endothermic nature of the reactions.
  • syngas by gasification and oxidation reactions, in which oxygen or air is mixed with a gas, liquid or solid hydrocarbon feed at sub-stoichiometric ratios. If the hydrocarbon is methane, for example, the following reactions occur:
  • nCO + (2n+l )3 ⁇ 4 + nH 2 0.
  • the invention is a process for producing syngas with a high content of carbon monoxide, reflected in a high CO:C0 2 ratio.
  • the process involves integrating membrane-based gas separation and steam methane reforming.
  • the membrane-based gas separation step uses a membrane that exhibits selectivity for carbon dioxide over hydrogen, and for carbon dioxide over carbon monoxide.
  • This step accepts raw syngas from a steam reformer and produces a carbon dioxide enriched stream that is returned to the reformer as part of the feed to the reforming reactions.
  • the residue from the membrane separation step is a syngas product with an elevated CO content compared with what could be produced, under like reforming conditions, from the same amount of hydrocarbon feed absent the membrane separation.
  • the return of C ⁇ 3 ⁇ 4 suppresses the COs-producing reactions and promotes the CO-producing reactions in the reformer, thereby increasing the CO yield and the ratio of CO to C0 in the syngas product.
  • the process reduces, and in some cases can obviate entirely, the need for downstream CO? removal technology, enabling the syngas product to be sent directly as feedstock to a gas-to-liquids process or other use.
  • the process of the invention comprises the following steps:
  • the steam methane reforming step or steps (a) are carried out in a steam reformer train,
  • the reformer train includes one or more individual reactors carrying out at least reaction (1 ) and optionally any of the other reactions discussed above, particularly (2) or (3).
  • reforming reactions are typically carried out at a pressure of a few tens of bar, such as 20 or 30 bar.
  • the raw syngas may be passed to the membrane feed side without adjusting the pressure.
  • the driving force for operating the membrane separation step may be increased by compressing the raw syngas.
  • the permeate stream should be recom pressed before being sent to the reformer.
  • any membranes able to provide adequate separation of carbon dioxide from hydrogen and carbon monoxide may be used.
  • the membranes are polymeric membranes that offer a selectivity in favor of carbon dioxide over hydrogen of at least about 5.
  • the syngas product has a high carbon monoxide content compared with steam methane reforming processes earned out under otherwise similar conditions that are not integrated with membrane-based gas separation.
  • the carbon monoxide content of the product syngas stream (after water removal) is at least about 15%.
  • the ratio of carbon monoxide to carbon dioxide is also higher than in comparable non-membrane-integrated processes, and is preferably at least about 3:1. Likewise, especially if a hydrogen -selective membrane separation step is included, as discussed below, the hydrogen:CO ratio may be lower than in comparable non-membrane-integrated processes, and is preferably below 3:1.
  • the product syngas from the integrated process may be used as desired. The processes of the invention are believed to be particularly beneficial in preparing syngas for use in gas-to-liquids processes, and more specifically as feedstock for Fischer-Tropsch synthesis of synthetic gasolines and other fuels and lubricants.
  • membranes that are selective in favor of carbon dioxide over hydrogen use membranes that are selective in favor of carbon dioxide over hydrogen. It is also possible to include membranes that are selective to hydrogen over carbon dioxide. This reduces the amount of hydrogen that is recycled to the refonner train with the recovered carbon dioxide, thereby enabling more effective C0 2 recycling, as well as reducing the ratio of hydrogen to carbon monoxide in the treated syngas.
  • An embodiment that incorporates both a carbon-dioxide-selective membrane separation step and a hydrogen-selective membrane separation step comprises:
  • the permeate from the first membrane unit it is generally preferred to recompress the permeate from the first membrane unit to a pressure similar to the refonner operating pressure before feeding the first permeate to the second, hydrogen-selective membrane step.
  • the residue from the hydrogen-selective membrane separation step may be introduced to the reformer feed without the need for further compression, in the alternative, the raw syngas from the reformer may be compressed upstream of the first membrane separation step.
  • the permeate side of the first membrane step may conveniently be maintained at about refonner pressure, again enabling the residue stream from the second membrane separation step to be returned to the reformer without additional recompression.
  • any membranes able to provide adequate separation of hydrogen from carbon dioxide and carbon monoxide may be used for the second membrane separation step.
  • the membranes are polymeric membranes that offer a selectivity in favor of hydrogen over carbon dioxide of at least about 5.
  • Preferences for the membranes of the first step are as already described.
  • Embodiments incorporating both carbon dioxide selective and hydrogen selective membranes are especially beneficial in providing a product syngas with a low hydrogen: CO ratio. Ratios below 3: 1 , and in favorable cases below 2.5:1, such as 2.3: 1 , 2.2: 1 or even 2.1 :1 can be achieved.
  • the syngas can be used in any desired fashion, but is especially useful as a feedstock to a GTL process.
  • a third membrane separation step using a hydrogen -selective membrane may be carried out on the raw syngas before it is passed to the carbo -dio ide selective membrane unit.
  • the invention integrates three operations: membrane-based gas separation, steam methane reforming and gas-to-liquids conversion of the product syngas.
  • a membrane that exhibits selectivity in favor of hydrogen over carbon dioxide is used, and the carbon- dioxide-enriched residue stream from the membrane separation step(s), in which the ratio of hydrogen to CO has been reduced, is passed to a Fischer- Tropsch (FT) process or the like, in the FT process, hydrogen and carbon monoxide are consumed, generally according to the overall scheme of reaction (7), to yield a liquid hydrocarbon product.
  • FT Fischer- Tropsch
  • the off-gas or tail gas from the FT steps is rich in carbon dioxide and is returned to the steam reformer to suppress further C0 2 production and enhance CO yield as in the embodiments already discussed.
  • Such an embodiment comprises the following steps:
  • the preferences for operating conditions, membrane selectivity, and so on are as expressed for the previous embodiments described above.
  • Water is commonly removed by cooling and condensing before the syngas is used in the reaction step (f).
  • the pressure of the syngas stream may be adjusted, such as by compression before the membrane separation step or by reducing the pressure after the membrane separation step.
  • the membrane separation step is perform ed on the tail gas from the hydrocarbon forming reactor.
  • the following steps are included:
  • the ratio of hydrogen to CO in the syngas that is fed to the hydrocarbon producing reactor is typically less than 3: 1 , and preferably is less than about 2.3: 1.
  • FIG. 1 is a schematic drawing of a basic embodiment of the process of the invention in which a membrane gas separation step, using membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, is integrated with a steam methane reforming step.
  • FIG. 2 is a schematic drawing of an embodiment of the process of the invention includin two membrane gas separation steps, one using membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, the other using membranes selective in favor of hydrogen over carbon dioxide, integrated with a steam methane reforming step.
  • Figure 3 is a schematic drawing of an embodiment of the process of the invention including three membrane separation steps, one using membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, the other two using membranes selective in favor of hydrogen over carbon dioxide, integrated with a steam methane reforming step.
  • FIG 4 is a schematic drawing of an embodiment of the process of the invention in which the feed to the membrane separation step is compressed prior to entering the membrane separation unit.
  • FIG. 5 is a schematic drawing of a first, embodiment of the process of the invention in which three unit operations: membrane-based gas separation, steam methane reforming, and reaction of CO and hydrogen to form a hydrocarbon product, are integrated.
  • Figure 6 is a schematic drawing of a second embodiment of the process of the invention in which three unit operations: membrane-based gas separation, steam methane reforming, and reaction of CO and hydrogen to form a hydrocarbon product, are integrated.
  • Figure 7 is a graph showing carbon monoxide yield in terms of mole-per-mole conversion of methane to CO as a function of the refonner outlet temperature, for refonners operating at 25 bar and 5 bar, without an integrated membrane separation step.
  • Figure 8 is a graph showing carbon monoxide yield in terms of mole-per-mole conversion of methane to CO as a function of the reformer outlet temperature, for reformers operating at 25 bar and 5 bar without and with an integrated membrane separation step.
  • reformer' , "reformer train”, “steam reformer” and “steam methane reformer” as used herein refer to any equipment or train of equipment that produces syngas from a starting feedstock that includes at least methane and steam.
  • a basic embodiment of the invention that comprises two integrated steps - a membrane-based gas separation step and a steam methane reforming step - is shown in Figure
  • a gaseous hydrocarbon feed stream, 102 is combined with stream, 1 15, (which is discussed below) and passed as feed stream, 103, to the reformer 104.
  • streams 102 and 1 15 may be introduced separately into the reformer.
  • the reforming step or steps comprise reactions of the type discussed in the Background section above, and are carried out under any convenient reforming conditions and in a reformer train including one or more individual reactors, as is well known in the art.
  • the train may contain upfront equipment to purify the gas feedstock, boilers or other steam generators, heat exchangers, condensers and the like.
  • the reactions in the reformer train or steps, 104 produce a raw syngas, stream 105, which comprises hydrogen, carbon monoxide, carbon dioxide, water and methane or other unreacted hydrocarbon feed.
  • the raw syngas will usually be at high temperature, generally 800°C or above, and at elevated pressure, up to about 50 bar, such as 15, 20, 25 or 30 bar.
  • the water content is usually high, typically as much as about 30%.
  • the raw gas is usually cooled, step 106, and passed through a separator, 107, where condensed water, stream 108, is removed.
  • the dried raw syngas, stream 109 is sent as a feed strea to membrane step or unit, 1 10, containing membranes, 11 1, that axe selectively permeable to carbon dioxide over hydrogen.
  • the membranes typically have a selectivity for carbon dioxide over hydrogen of at least 5, and preferably at least about 15.
  • the carbon dioxide permeance of the membrane is typically at least 200 gpu and, preferably, at least 400 gpu.
  • the membrane may be made from inorganic or polymeric materials, and may take the form of a homogeneous film, an integral asymmetric membrane, a multilayer composite membrane, a membrane incorporating a gel or liquid layer or particulates, or any other form known in the art.
  • Representative preferred membranes have a selective layer based on a poiyether.
  • Such materials are described, for example, in two publications by Lin et a!., "Materials selection guidelines for membranes that remove CO ? , from gas mixtures” (J. Mol. Struct., 739, 57-75, 2005) and “Plasticization-Enhaneed Hydrogen Purification Using Polymeric Membranes” ⁇ Science, 311 , 639-642, 2006).
  • a specific preferred material for the selective layer is Pebax®, a polya nide-poly ether block copolymer material described in detail in U.S. Patent No. 4,963,165.
  • Pebax® a polya nide-poly ether block copolymer material described in detail in U.S. Patent No. 4,963,165.
  • the membranes may be manufactured as flat sheets or as fibers and housed in any convenient module form, including spiral-wound modules, plate-and-frame modules, and potted hollow-fiber modules.
  • the making of all these types of membranes and modules is well-known in the art. in general, we prefer to use flat-sheet membranes and spiral-wound modules.
  • Membrane unit 1 10 may contain a single membrane module or bank of membrane modules or an array of modules.
  • a single unit or stage containing one or a bank of membrane modules is adequate for many applications. If either the residue or permeate stream, or both, requires further carbon dioxide removal, it may be passed to a second bank of membrane modules for a second processing step.
  • Such multi-stage or multi-step processes, and variants thereof * will be familiar to those of skill in the art, who will appreciate that the membrane separation step may be configured in many possible ways, including single-stage, multistage, muitistep, or more complicated arrays of two or more units, in serial or cascade arrangements.
  • the membrane separation step can be operated by any mechanism that provides a driving force for transmembrane permeation. Most commonly, this driving force is provided by maintaining a pressure difference between the feed and permeate sides, or by sweeping the permeate side continuously with a gas that dilutes the permeating species, both of which techniques are well known in the membrane separation arts.
  • Figure 1 shows a simple pressure-driven case.
  • the feed side of the membrane is typically maintained at a pressure within the range of about 5 bar to 50 bar, and more preferably within the range of about 15 bar to 30 bar.
  • the permeate side is typically maintained at a pressure of about 10% to 30% of the feed side pressure, that is, about 1 - 0 bar.
  • syngas stream 1 9 flows across the feed side of the membranes 1 1 1 , A residue stream, 1 12, that is depleted in carbon dioxide relative to stream 109, is withdrawn from the feed side of the membrane.
  • This stream forms the syngas product of the integrated process, and has a relatively high CO content compared with steam methane reforming performed without integrated membrane gas separation.
  • the syngas product stream 1 12 has a CO:C0 2 ratio of at least about 3:1, preferably at least about 5: 1 and most preferably at least about 6:1.
  • carbon monoxide concentration the CO content is typically at least about 12%, preferably at least about 15% and most preferably at least about 20%.
  • the permeate stream, 113 is enriched in carbon dioxide compared with the membrane feed, and is recompressed in compression step or steps, 114, preferably to a pressure consistent with the reforming operations, and returned to the reformer as stream 1 15. Compression may be carried out in one or multiple steps as convenient.
  • Figure 1 indicates that stream 115 is mixed with stream 102 before passing into the reformer train, as would be suitable if stream 1 15 were to be directed to the primar reforming reactor.
  • stream 115 may be returned to another reactor, such as a secondary reforming reactor.
  • the choice of where to return stream 1 15 can be readily made by the skilled person, depending on the particular balance of reactions that are being carried out.
  • the effect of returning the carbon-dioxide-rich stream 115 to the reformer is to suppress the C0 2 -producing reactions and promotes the CO-producing reactions in the reformer, thereby increasing the CO yield and the ratio of CO to C0 2 in the syngas product.
  • increasing the CO yield also tends to decrease the hydrogen: CO ratio in the syngas.
  • the processes of the invention are believed to be particularly beneficial in preparing syngas for use in gas-to-liquids processes, especially as feedstock for Fischer-Tropsch processes. In such gas-to-liquids conversions, it.
  • a syngas with a low hydrogen:CO ratio preferably below about 2.5: 1 or 2.3:1 , and most preferably closer to about 2.2:1 or 2.1 :1.
  • a membrane separation step using membranes that are selective to hydrogen over carbon dioxide. This reduces the amount of hydrogen that is recycled to the reformer with the recovered carbon dioxide, enabling more effective C0 2 recycling as well as reducing the 3 ⁇ 4:CO ratio in the product.
  • FIG. 2 Such an embodiment is shown in Figure 2, in which like elements are labeled as in Figure 1 and are as described above.
  • the process differs from the scheme shown in Figure 1 , however, in that strea 1 15 no longer passes to the reformer but instead is directed to a second membrane separation step or unit, 116, which uses membranes, 1 17, that are selective in favor of hydrogen over carbon dioxide and carbon monoxide.
  • the membranes should preferably exhibit a selectivity for hydrogen over carbon dioxide of at least about 5, and more preferably at least about 10, a selectivity for hydrogen over carbon monoxide of at least about 10, and more preferably at least about 20, and a hydrogen permeance of preferably at least about 100 gpu, more preferably at least about 200 gpu.
  • any membranes with suitable performance properties may be used in the second membrane separation step.
  • the membrane may be made from inorganic or polymeric materials, and may take the form of a homogeneous film, an integral asymmetric membrane, a multilayer composite membrane, a membrane incorporating a gel or liquid layer or particulates, or any other form known in the art.
  • Representative materials for the selective layer of the second membrane include polymers, such as polyimides, polyamides, polyurethanes, polyureas, polybenzimidazoles, and polybenzoxazoles; metals, such as palladium; zeolites; and carbon, by way of example and not by way of limitation.
  • suitable polymeric membrane materials include polybenzimidazole ( ⁇ ) based membranes, such as taught by . O'Brien et al. in "Fabrication and Scale-Up of PBI-based Membrane System for Pre-Combustion Capture of Carbon Dioxide” (DOE NETL Project Fact Sheet 2009 ⁇ and po!yimide-based membranes, such as taught by B. T.
  • COi-enriched stream 1 15 passes across membranes 1 17 under conditions that provide a driving force for transmembrane permeation.
  • this driving force is assumed to be provided by maintaining a pressure difference between the feed and permeate sides of the membrane unit, as described above.
  • Hydrogen-enriched permeate stream, 1 19 is withdrawn from the process, and may be sent for recovery of purified hydrogen, for use as fuel, or for any other desired use.
  • Hydrogen-depleted residue stream, 1 1 8, is returned to the reforming steps.
  • the product syngas stream, 1 12, generated by the process of Figure 2 is typically characterized by both a high CO:C0 2 ratio, such as at least 3:1, 5: 1 or 6: 1 , and a low hydrogen.'CO ratio, such as below 3 : 1 , 2.5: 1 , 2.3 : 1 or 2.2: 1 .
  • FIG. 3 An alternative embodiment suitable especially for achieving low hydrogenrCO ratios is shown in Figure 3, with like elements again labeled as in the previous figures.
  • the process differs from the scheme of Figure 2 in that a third membrane separation step, 120, is included.
  • This step uses hydrogen-selective membranes, 121 , and treats stream 1 09 before it is passed to membrane separation step 1 10.
  • the considerations as to membrane types, operating conditions and preferred performance for membranes 121 are the same as for membranes 1 17.
  • the same membranes, such as inorganic membranes or polyimide membranes are used in both steps 120 and 1 16.
  • stream 109 in unit or step 120 produces a hydrogen-depleted residue stream, 123, which is passed as the feed stream to the carbon-dioxide-selective step 110, and a hydrogen-rich permeate, stream 122, that is discharged from the process, and which may be directed to hydrogen purification, fuel or any other desired destination.
  • FIG 1-3 illustrate embodiments in which the feed stream, 109, to membrane separation steps 1 10 and 120 is maintained at about the same pressure as the raw gas from the reformer, a lower pressure is maintained on the permeate side(s), and permeate stream, 1 13, is recom ressed to a pressure suitable for reintroduction to the reforming steps. Such embodiments are preferred when the reformer operating pressure is above about 10 bar.
  • Another option is to compress stream 109 to a pressure higher than the reformer operating pressure, and to maintain the permeate side of membrane step or unit 10 at about the same pressure as the reformer, so that the permeate can be circulated to the reformer without the need for recompression.
  • Such embodiments can be particularly useful if the raw syngas is at comparatively low pressure, by which we mean below about 10 bar, such as 5 bar,
  • Figure 4 shows such an embodiment as it relates to a process scheme similar to that of Figure 2, in that the penneate, 1 13, from unit 110 is treated in unit 116 to remove excess hydrogen before being circulated back to the reforming steps, it will be apparent to those of skill in the art that arrangements in which membrane feed compression is used could also be readily applied to process schemes similar to those of Figures 1 and 3.
  • compression step, 124 is used to compress raw syngas stream 105/109.
  • the cooling and water knock-out steps are not shown in this figure, but typically cooling and resultant water condensation would take place both before and after step 124.
  • Step 124 can be used to compress the raw syngas to any desired pressure, although generally the gas will be compressed to a few tens of bar, such as 15 bar, 20 bar, 25 bar or 30 bar.
  • the compressed stream, 125 is then passed as feed to membrane separation step or unit 1 10.
  • the invention integrates membrane-based gas separation with gas-io-liquids processes, such as the Fischer-Tropsch (FT) process.
  • FT Fischer-Tropsch
  • the overall objective of gas-to- liquids technology is to convert methane or natural gas to heavier hydrocarbons (usually fractions that are easily transportable liquids at ambient temperature and pressure), that can be used as fuels, solvents, lubricants or the like.
  • Methane is first broken down with steam or oxygen to form hydrogen and CO, then the CO and hydrogen are reacted to form hydrocarbons of the desired weight, either directly or via an interm ediate synthesis.
  • nCO + (2n+l )H 2 C R 3 ⁇ 4 n+2 + n3 ⁇ 40.
  • Integrating a membrane separation step provides ratio adjustment of the syngas and enables very high overall conversion of CO to liquid hydrocarbons.
  • the membrane separation step can be integrated with the other steps either upstream or downstream of the hydrocarbon synthesis reactor(s).
  • FIG. 5 An embodiment in which the three operations are integrated with the membrane separation step on the inlet side of the hydrocarbon reactor is shown in Figure 5.
  • steam, 201 is fed into a steam methane reforming step, 204.
  • Methane or natural gas, stream 202 is combined with stream, 212, discussed below, and passed as feed stream, 203, to steam methane reformer 204, where reforming reactions take place as discussed with respect to the earlier embodiments.
  • the reactions form a raw syngas, stream 205. This stream, typically after cooling and water removal, as discussed previously, is passed as a feed sixeam to membrane separation step or unit, 206.
  • the membranes, 207 are selective in favor of hydrogen over carbon monoxide and carbon dioxide, preferably with a hydrogen/carbon dioxide selectivity of at least about 5, more preferably at least about 10, and a selectivity for hydrogen over carbon monoxide of at least about 10, more preferably at least about 20, Choices and operating conditions for the membranes and membrane unit are as discussed above with respect to the hydrogen-selective gas separation steps.
  • a permeate stream, 208, enriched in hydrogen compared with stream 205, is withdrawn as a hydrogen purge stream, and can be sent for fuel or other use.
  • This stream typically contains at least about 90% hydrogen.
  • Residue stream, 209, in which the hydrogenrCO ratio is preferably below about 2.3:1 , and most preferably in the range 2.05-2.3, is passed to the hydrocarbon product synthesis and fractionation/piirification train, shown together simply as element 210.
  • This train will usually include at least one reaction step, typically carried out in a fixed bed, fluidized bed or slurry reactor.
  • Such reactors and operation thereof are well known in the art and are described in detail in, for example, S.T. Sie and R. Krishna, "Fundamentals and selection of advanced Fischer- Tropsch reactors", Applied Catalysis A: General, Vol. 186 (1999) pages 55-70, and in A. P. Steynberg, "Chapter 2 ⁇ - Fischer- ropsch Reactors", Studies in Surface Science and Catalysis , Vol. 152 (2004), pages 64-195, both of which are incorporated herein by reference.
  • the raw product from the reactor usually contains a mix of paraffmic hydrocarbons of different weights, and can be passed to one or more fractionation steps to separate desired products from unreacted feedstock, light hydrocarbon byproducts, particularly methane, inerts and the like.
  • the desired produces are withdrawn as stream 211.
  • the remaining lights, inerts and unreacted hydrogen and carbon monoxide form at least one tax! gas stream, 212. Because excess hydrogen has been removed in stream 208, this tail gas is very lean in hydrogen compared with con ventional tail gas streams.
  • the stream is rich in methane, formed as a byproduct during GTL synthesis, and in carbon dioxide, as well as containing unreacted CO, and is returned to the steam reforming steps.
  • a second option is to position the hydrogen-selective membrane step(s) in the tail gas return line, as shown in Figure 6.
  • stream 205 usually after water removal, is passed to the liquids formation and purification steps, 230, without hydrogen removal.
  • the tail gas stream, 212 forms the feed stream to membrane separation unit or steps, 213.
  • membranes, 214 with hydrogen-selective properties and configurations as discussed above, are used.
  • the step produces a hydrogen-rich permeate purge stream, 215, and an adjusted tail gas residue stream, 216, that is returned to the reforming steps. Preferences for overall operating conditions for the reforming, hydrocarbon synthesis, and membrane separation steps are as already discussed with respect to Figure 5.
  • the process controls the amount of unwanted hydrogen that is returned in the process loop, while recapturing most of the carbon species. Recycling of methane reduces the need to drive the reforming reactions with a excessively high temperature, since unreacted methane is not lost to the process.
  • a base calculation was performed to model the output from a conventional steam reforming process without an integrated membrane separation step. This process is not in accordance with the invention, but serves as a comparative basis for the other calculations.
  • the resul ts of the calculation are shown in Table 1 .
  • cooled syngas product stream 109 has a CO:C0 2 rat o of only 1.7 and an 3 ⁇ 4:CQ ratio of 5.3.
  • Example 2 Steam reforming with one carbon-dioxide selective membrane separation step [0101 j A calculation was performed according to the process schematic of Figure 1 , in which a membrane gas separation step, using membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, is integrated with a steam methane reforming step. The results of the calculation are given in Table 2,
  • the process produces a recycle stream 118 containing 50 mol% CO_> .
  • the syngas product stream 1 12 has a CO:C0 2 ratio of 8.4 and an H 2 :CQ ratio of 2.5.
  • Example 4 Steam reforming with three membrane steps, as in Figure 3
  • a third hydrogen-selective membrane step is added in front of the two membrane steps of Example 3. Instead of going directly to carbon -dioxide-selective step 110, raw syngas 109 is first subjected to membrane separation step 120, producing a hydrogen-depleted reside stream 123. Residue stream 123 is passed as the feed to carbon-dioxide-selective step 1 10.
  • the syngas product stream 112 in this case has a CO:C0 2 ratio of 6.5 and an H 2 :CO ratio of 2.8.

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Abstract

A process for producing syngas with a high content of carbon monoxide, reflected in a high CO:CO2 ratio. The process involves integrating membrane-based gas separation and steam methane reforming.

Description

MEMBRANE-BASED GAS SEPARATION PROCESSES TO PRODUCE SYNTHESIS
GAS WITH A HIGH CO CONTENT
FIELD OF THE INVENTION
[0001] The invention relates to membrane-based gas separation processes for the production of synthesis gas ("syngas") with a high yield of carbon monoxide from a light hydrocarbon feedstock. Carbon dioxide recovered from one or more membrane separation steps is recycled within the process.
BACKGROUND OF THE INVENTION
[0002] Synthesis gas or syngas - a mixture of carbon m onoxide, carbon dioxide, and hydrogen - is used as a feedstock for making diverse hydrocarbon products, including methanol and synthetic fuels and lubricant oils.
[0003] Syngas can be produced by steam methane reforming (SMR). At low to moderate pressures and at high temperatures, methane reacts with steam on a nickel catalyst according to the following reforming reactions:
(1) CH4 + H20 -* CO + 3 H2
(2) CO + H20 — > C02 + H2 , and the reverse reaction
(3) ¾ + C02 --> ¾0 + CO .
Overall, these reactions are highly endoihermic, and maintaining reaction temperature by external heating is a critical part of the process.
[0004] If the syngas is to be used as a hydrocarbon manufacturing feedstock, it is usually desirable to maximize CO production and minimize C02 production, because CO is a valuable reagent, whereas C02 is not. Because reaction (3) is favored by high temperature, the reformer is generally run at as high a temperature as practicable, even as high as 900°C for example. [00Θ5] High methane conversion is also favored by low operating pressure, because the reforming reactions increase the volume of gas. By way of illustration, Figure 7 shows the dependence of CO yield (moles CO generated per mole of methane feed) on reformer temperature for reformer pressures of 5 bar and 25 bar, if the steam to methane ratio in the reformer feed is 3 : 1. At a reformer pressure of 25 bar and temperature of 850°C, about 50% of the methane feed is converted to CO, 28% is converted to CO?, and the remaining 22% passes through as unreacted methane. By contrast, at a pressure of 5 bar, the same level of 50% CO yield can be reached at a temperature of only 740°C. If the temperature were maintained at 85()°C, the yield at 5 bar would be above 60%.
[0006] Despite the ability to achieve better yield at low pressure, reformers are run at elevated pressure, typically 20-30 bar, to facilitate both heating of the reactant gases and heat recovery' from the product gases, which is necessitated by the highly endothermic nature of the reactions.
[0007] The above reactions relate to methane conversion, if ethane or higher paraffins are constituents of the reformer feed, then steam reforming will generate 2 + 1/h moles of hydrogen for every mole of carbon monoxide and 3 + 1/n moles of hydrogen for every mole of carbon dioxide for each constituent hydrocarbon represented by the generic fonnula
[0008] Another option is to make syngas by gasification and oxidation reactions, in which oxygen or air is mixed with a gas, liquid or solid hydrocarbon feed at sub-stoichiometric ratios. If the hydrocarbon is methane, for example, the following reactions occur:
(4) CH4 + Ια 2 → CO + 2 H2
(5) CH4 + 02 -→ C02 + 2 H2
Figure imgf000003_0001
[0009] These reactions are exothermic. In some situations, both oxygen (or air) and steam are fed to the syngas production process. The presence of steam in the feed stream suppresses the hydrogen consuming reaction (6). if the mix of steam reforming and oxidation results in a balance of endothennic and exothermic reactions, the process is termed autothermal reforming,
[0010] Whatever reaction or combination of reactions are used, designing an SMR unit to operate at very high temperatures incurs high capital costs. Metallurg cal requirements are stringent, heat transfer areas are high and designs are complex to accommodate thermal expansion, in practice, even large scale reformers are restricted to operating temperatures of 850-900°€, Autothermal reforming minimizes the heat transfer area and thereby facilitates operational temperatures up to 1,000°C. However, unless nitrogen can be accommodated in the product gas, autothermal reforming requires large quantities of oxygen, substantially increasing operating costs.
{0011 ) Despite efforts to balance the various parameters and considerations discussed above to achieve high CO yield, it remains the case that raw syngas often contains a substantial CO? content. For example, after water removal, the concentration of C02 in the gas ma)' be as high as Ϊ 0%, 15% or ev en higher. Expressing the CO production in terms of molar proportions of CO and C02 produced, the CO:C02 ratio may often be as low as about 2: 1 or even lower. Before the syngas is used as a feedstock, it may be necessary to remove some or most of the carbon dioxide content, thereby necessitating the use of costly, complex or inconvenient treatment steps.
[0012] In addition to maximizing the CO:C02 ratio, it is often desirable to adjust the CO to hydrogen ratio. One of the most important uses for syngas is in GTL (gas-to-liquids) syntheses of liquid hydrocarbons, including synthetic gasoline, other fuels and lubricant oils. In the Fischer-Tropsch process, for example, CX) and hydrogen are combined in a number of reactions, according to the overall stoichiometry:
(7) nCO + (2n+l )¾ = + nH20.
[0013] In the event that, a C8 product is being manufactured, for example, the above stoichiometry requires a hydrogen:CO ratio of 17/8, i.e. 2.125, for the process feed. In contrast, a methane- fed reformer usually yields a syngas with at least a 3:1 hydrogen:CO ratio. When the objective is to generate a suitable feed for Fischer-Tropsch reactions, therefore, it is desirable to reduce the hydrogemCO ratio to below 3:1, more preferably to below 2.5: 1. and most preferably to below about 2.3:1.
[0014] Besides managing the temperature and pressure operating conditions carefully, CO production from a given s upply of natural gas or methane can be increased by adding C02 from an outside source to the reformer feed. This method, known as "dry reforming", has been used in natural gas steam methane reforming. Although this technique results in more carbon monoxide in the product syngas, it does not increase the yield of CO from the hydrocarbon fed to the reformer.
[0015] it is also known to remove C02 from the raw syngas itself by absorption/stripping, then return the recovered C02 to the reformer feed. However, as well as high energy consumption, absorption/desorption processes are complicated to control and use multiple pieces of equipment, including absorber and stripping columns, heat exchangers, pumps, valves and extensive instrumentation. Furthermore, the absorbent introduces an additional fluid into the processing system, and this fluid may have toxic or other undesirable characteristics, requiring costly or inconvenient treatment or disposal.
[0016] Despite all the options described above for maximizing generation of CO from hydrocarbon feedstocks, there remains a need for a process that can provide good CO yield without requiring excessively high reforming temperatures, low operating pressures, the addition of oxygen, or the importation of C02 from an external source. It would also be very desirable to avoid the complexity and operational drawbacks of absorption/desorption. This need is particularly pronounced at a small scale of operation, where the above drawbacks have a proportionately greater impact on the process productivity and economics.
[0017] There also remains a related need for processes in which a low hydrogen: carbon monoxide ratio, preferably below 3:1, can be achieved.
SUMMARY OF THE INVENTION
(0018] The invention is a process for producing syngas with a high content of carbon monoxide, reflected in a high CO:C02 ratio. The process involves integrating membrane-based gas separation and steam methane reforming.
[0019] The membrane-based gas separation step uses a membrane that exhibits selectivity for carbon dioxide over hydrogen, and for carbon dioxide over carbon monoxide. This step accepts raw syngas from a steam reformer and produces a carbon dioxide enriched stream that is returned to the reformer as part of the feed to the reforming reactions. The residue from the membrane separation step is a syngas product with an elevated CO content compared with what could be produced, under like reforming conditions, from the same amount of hydrocarbon feed absent the membrane separation. The return of C<¾ suppresses the COs-producing reactions and promotes the CO-producing reactions in the reformer, thereby increasing the CO yield and the ratio of CO to C0 in the syngas product.
[0020] The process reduces, and in some cases can obviate entirely, the need for downstream CO? removal technology, enabling the syngas product to be sent directly as feedstock to a gas-to-liquids process or other use.
[0021] In a basic embodiment, the process of the invention comprises the following steps:
(a) reacting a gas mixture comprising steam and methane in a refonner train to form a raw syngas stream comprising hydrogen, carbon monoxide and carbon dioxide;
(b) providing a membrane having a feed side and a permeate side, wherein the membrane exhibits a selectivity to carbon dioxide over hydrogen;
(c) passing at least a portion of the raw syngas stream as a feed stream across the feed side;
(d) withdrawing from the feed side a syngas product stream depleted in carbon dioxide compared with the feed stream, the syngas product stream containing carbon monoxide and carbon dioxide in a molar ratio of at least about 3 rl ;
(e) withdrawing from the permeate side a permeate stream enriched in carbon dioxide compared with the feed stream;
(f) returning at least a portion of the permeate stream as a feedstock to the reformer train. [0022] The steam methane reforming step or steps (a) are carried out in a steam reformer train, The reformer train includes one or more individual reactors carrying out at least reaction (1 ) and optionally any of the other reactions discussed above, particularly (2) or (3).
(0023] The various reactions produce a raw syngas stream at high temperature. Before the raw syngas stream is passed to the membrane separation steps, therefore, it is usually cooled, such as to below !20°C or 100°C, This will both condense water from the stream and enable polymeric membranes that could not tolerate high temperatures to be used in the membrane separation step,
[0024] Also as mentioned above, reforming reactions are typically carried out at a pressure of a few tens of bar, such as 20 or 30 bar. The raw syngas may be passed to the membrane feed side without adjusting the pressure. Alternatively, the driving force for operating the membrane separation step may be increased by compressing the raw syngas.
[0025] If the membrane separation step is operated with the permeate side at a lo wer pressure than the operating pressure of the reformer, the permeate stream should be recom pressed before being sent to the reformer.
[00261 Any membranes able to provide adequate separation of carbon dioxide from hydrogen and carbon monoxide may be used. Preferably, the membranes are polymeric membranes that offer a selectivity in favor of carbon dioxide over hydrogen of at least about 5.
[0027] The syngas product has a high carbon monoxide content compared with steam methane reforming processes earned out under otherwise similar conditions that are not integrated with membrane-based gas separation. Preferably, the carbon monoxide content of the product syngas stream (after water removal) is at least about 15%.
[0028] The ratio of carbon monoxide to carbon dioxide is also higher than in comparable non-membrane-integrated processes, and is preferably at least about 3:1. Likewise, especially if a hydrogen -selective membrane separation step is included, as discussed below, the hydrogen:CO ratio may be lower than in comparable non-membrane-integrated processes, and is preferably below 3:1. [0029] The product syngas from the integrated process may be used as desired. The processes of the invention are believed to be particularly beneficial in preparing syngas for use in gas-to-liquids processes, and more specifically as feedstock for Fischer-Tropsch synthesis of synthetic gasolines and other fuels and lubricants.
[0030] The embodiments described above use membranes that are selective in favor of carbon dioxide over hydrogen. It is also possible to include membranes that are selective to hydrogen over carbon dioxide. This reduces the amount of hydrogen that is recycled to the refonner train with the recovered carbon dioxide, thereby enabling more effective C02 recycling, as well as reducing the ratio of hydrogen to carbon monoxide in the treated syngas.
10031 ] An embodiment that incorporates both a carbon-dioxide-selective membrane separation step and a hydrogen-selective membrane separation step comprises:
(a) reacting a gas mixture comprising steam and methane in a reformer train to form a raw syngas stream comprising hydrogen, carbon monoxide and carbon dioxide;
(b) providing a first membrane unit containing a first membrane having a first feed side and a first permeate side, wherein the first membrane exhibits a selectivity to carbon dioxide over hydrogen;
(c) passing at least a portion of the raw syngas stream as a first feed stream across the first feed side;
(d) withdrawing from the first feed side a syngas product stream depleted in carbon dioxide compared with the first feed stream, the syngas product stream containing carbon monoxide and carbon dioxide in a molar ratio of at least about 3:1, and containing hydrogen and carbon monoxide in a molar ratio less than about 3:1 ;
(e) withdrawing from the first permeate side a first permeate stream enriched in carbon dioxide compared with the feed stream;
(f) providing a second membrane unit containing a second membrane having a second feed side and a second permeate side, wherein the second membrane exhibits a selectivity to hydrogen over carbon dioxide;
(g) passing at least a portion of the first permeate stream as a second feed stream across the second feed side; (h) withdrawing from the second feed side a second residue stream depleted in hydrogen compared with the second feed stream;
(i) withdrawing from the second permeate side a second permeate stream enriched in hydrogen compared with the second feed stream;
(j) passing at least a portion of the second residue stream as a feedstock to the refonner train,
[0032] Just as with the embodiments using only the carbon dioxide selective membrane, it is preferred to cool the raw syngas and to condense water from the stream before the first membrane separation step.
[0033] In this embodiment, it is generally preferred to recompress the permeate from the first membrane unit to a pressure similar to the refonner operating pressure before feeding the first permeate to the second, hydrogen-selective membrane step. In this case, the residue from the hydrogen-selective membrane separation step may be introduced to the reformer feed without the need for further compression, in the alternative, the raw syngas from the reformer may be compressed upstream of the first membrane separation step. In this case, the permeate side of the first membrane step may conveniently be maintained at about refonner pressure, again enabling the residue stream from the second membrane separation step to be returned to the reformer without additional recompression.
[0034] Any membranes able to provide adequate separation of hydrogen from carbon dioxide and carbon monoxide may be used for the second membrane separation step. Preferably, the membranes are polymeric membranes that offer a selectivity in favor of hydrogen over carbon dioxide of at least about 5. Preferences for the membranes of the first step are as already described. f 0035 j Embodiments incorporating both carbon dioxide selective and hydrogen selective membranes are especially beneficial in providing a product syngas with a low hydrogen: CO ratio. Ratios below 3: 1 , and in favorable cases below 2.5:1, such as 2.3: 1 , 2.2: 1 or even 2.1 :1 can be achieved. As with the previous embodiments, the syngas can be used in any desired fashion, but is especially useful as a feedstock to a GTL process. [0036] Optionally, if further hydrogen removal is desired, a third membrane separation step using a hydrogen -selective membrane may be carried out on the raw syngas before it is passed to the carbo -dio ide selective membrane unit.
[0037] in another aspect, the invention integrates three operations: membrane-based gas separation, steam methane reforming and gas-to-liquids conversion of the product syngas. In one such embodi ent, a membrane that exhibits selectivity in favor of hydrogen over carbon dioxide is used, and the carbon- dioxide-enriched residue stream from the membrane separation step(s), in which the ratio of hydrogen to CO has been reduced, is passed to a Fischer- Tropsch (FT) process or the like, in the FT process, hydrogen and carbon monoxide are consumed, generally according to the overall scheme of reaction (7), to yield a liquid hydrocarbon product.
[0038| The off-gas or tail gas from the FT steps is rich in carbon dioxide and is returned to the steam reformer to suppress further C02 production and enhance CO yield as in the embodiments already discussed. Such an embodiment comprises the following steps:
(a) reacting a gas mixture comprising steam and methane in a reformer train to form a raw syngas stream comprising hydrogen, carbon monoxide and carbon dioxide;
(b) providing a membrane unit containing a membrane having a feed side and a permeate side, wherein the membrane exhibits a selectivity to hydrogen over carbon dioxide;
(c) passing at least, a portion of the raw syngas stream as a feed stream across the feed side;
(d) withdrawing from the feed side an adjusted syngas stream depleted in hydrogen compared with the feed stream;
(e) withdrawing from the permeate side a permeate stream enriched in hydrogen compared with the feed stream;
(f) passing at least a porti on of the adjusted syngas stream to a reactor and there reacting carbon monoxide and hydrogen to produce a raw hydrocarbon product in the C4-C15 range;
(g) separating the raw hydrocarbon product, thereby producing a hydrocarbon product stream and a tail gas stream comprising carbon dioxide; (h) passing at least a portion of the tail gas stream as a feedstock to the reformer train.
[0039] In this embodiment, the preferences for operating conditions, membrane selectivity, and so on are as expressed for the previous embodiments described above. Water is commonly removed by cooling and condensing before the syngas is used in the reaction step (f). Depending on the preferred operating pressures for the steam methane reformer and the GTL reactor, the pressure of the syngas stream may be adjusted, such as by compression before the membrane separation step or by reducing the pressure after the membrane separation step.
[0040] in a second embodiment incorporating membrane-based gas separation, steam methane reforming and hydrogen+CO reaction steps, the membrane separation step is perform ed on the tail gas from the hydrocarbon forming reactor. In such as embodiment, the following steps are included:
(a) reacting a gas mixture comprising steam and methane in a reformer train to fonn a syngas stream comprising hydrogen, carbon monoxide and carbon dioxide;
(b) passing at least a portion of the syngas stream to a reactor and there reacting carbon monoxide and hydrogen to produce a raw hydrocarbon product in the C4-C15 range;
(c) separating the raw hydrocarbon product, thereby producing a hydrocarbon product stream and a tail gas stream comprising carbon dioxide;
(d) providing a membrane unit containing a membrane having a feed side and a permeate side, wherein the membrane exhibits a selectivity to hydrogen over carbon dioxide;
(e) passing at least a portion of the tail gas stream as a feed stream across the feed side;
(f) withdrawing from the permeate side a permeate stream enriched in hydrogen compared with the feed stream;
(g) withdrawing from the feed side a recycle stream enriched in carbon dioxide and depleted in hydrogen compared with the feed stream;
(h) passing at least a portion of the recycle stream as a feedstock to the reformer train,
[00411 In the aspects in which the invention involves three integrated steps, a reforming step, a membrane gas separation step and a reaction step producing a hydrocarbon product, the ratio of hydrogen to CO in the syngas that is fed to the hydrocarbon producing reactor is typically less than 3: 1 , and preferably is less than about 2.3: 1. BRIEF DESCRIPTION OF THE DRAWINGS
[0042] Figure 1 is a schematic drawing of a basic embodiment of the process of the invention in which a membrane gas separation step, using membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, is integrated with a steam methane reforming step.
[0G43J Figure 2 is a schematic drawing of an embodiment of the process of the invention includin two membrane gas separation steps, one using membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, the other using membranes selective in favor of hydrogen over carbon dioxide, integrated with a steam methane reforming step.
[0044] Figure 3 is a schematic drawing of an embodiment of the process of the invention including three membrane separation steps, one using membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, the other two using membranes selective in favor of hydrogen over carbon dioxide, integrated with a steam methane reforming step.
[[0045] Figure 4 is a schematic drawing of an embodiment of the process of the invention in which the feed to the membrane separation step is compressed prior to entering the membrane separation unit.
[[0046] Figure 5 is a schematic drawing of a first, embodiment of the process of the invention in which three unit operations: membrane-based gas separation, steam methane reforming, and reaction of CO and hydrogen to form a hydrocarbon product, are integrated.
[00471 Figure 6 is a schematic drawing of a second embodiment of the process of the invention in which three unit operations: membrane-based gas separation, steam methane reforming, and reaction of CO and hydrogen to form a hydrocarbon product, are integrated. [0048] Figure 7 is a graph showing carbon monoxide yield in terms of mole-per-mole conversion of methane to CO as a function of the refonner outlet temperature, for refonners operating at 25 bar and 5 bar, without an integrated membrane separation step. f0049| Figure 8 is a graph showing carbon monoxide yield in terms of mole-per-mole conversion of methane to CO as a function of the reformer outlet temperature, for reformers operating at 25 bar and 5 bar without and with an integrated membrane separation step.
DETAILED DESCRIPTION OF THE INVENTION
[0050] The terms "natural gas1' and "methane*' are used interchangeably herein.
[0051] The terms "reformer'", "reformer train", "steam reformer" and "steam methane reformer" as used herein refer to any equipment or train of equipment that produces syngas from a starting feedstock that includes at least methane and steam.
[0052] Gas percentages and ratios given herein are molar unless stated otherwise.
[0053] Pressures as given herein are in bar absolute unless stated otherwise,
[0054] A basic embodiment of the invention that comprises two integrated steps - a membrane-based gas separation step and a steam methane reforming step - is shown in Figure
[0055] It will be appreciated by those of skill in the art that Figure 1 and the other figures showing process schemes herein are very simple block diagrams, intended to make clear the key unit operations of the processes of the invention, and that actual process trains may include additional steps of a standard type, such as heating, chilling, compressing, condensing, pumping, monitoring of pressures, temperatures, flows, and the like, it will also be appreciated by those of skill in the art that the unit operations may themselves be performed as multiple steps or in a train of multiple pieces of equipment. [0056] Referring to Figure 1 , steam, 101 , is fed into a steam methane reforming step, 104. A gaseous hydrocarbon feed stream, 102, usually but not necessarily comprising methane, is combined with stream, 1 15, (which is discussed below) and passed as feed stream, 103, to the reformer 104. In the alternative, streams 102 and 1 15 may be introduced separately into the reformer.
[0057] The reforming step or steps comprise reactions of the type discussed in the Background section above, and are carried out under any convenient reforming conditions and in a reformer train including one or more individual reactors, as is well known in the art. The train may contain upfront equipment to purify the gas feedstock, boilers or other steam generators, heat exchangers, condensers and the like.
[0058] The reactions in the reformer train or steps, 104, produce a raw syngas, stream 105, which comprises hydrogen, carbon monoxide, carbon dioxide, water and methane or other unreacted hydrocarbon feed. Based on the reforming conditions, the raw syngas will usually be at high temperature, generally 800°C or above, and at elevated pressure, up to about 50 bar, such as 15, 20, 25 or 30 bar. The water content is usually high, typically as much as about 30%. 0059] As in conventional syngas production, the raw gas is usually cooled, step 106, and passed through a separator, 107, where condensed water, stream 108, is removed. The dried raw syngas, stream 109, is sent as a feed strea to membrane step or unit, 1 10, containing membranes, 11 1, that axe selectively permeable to carbon dioxide over hydrogen. In particular, the membranes typically have a selectivity for carbon dioxide over hydrogen of at least 5, and preferably at least about 15. The carbon dioxide permeance of the membrane is typically at least 200 gpu and, preferably, at least 400 gpu.
[0060] Any membrane with suitable performance properties may be used in the membrane separation step. The membrane may be made from inorganic or polymeric materials, and may take the form of a homogeneous film, an integral asymmetric membrane, a multilayer composite membrane, a membrane incorporating a gel or liquid layer or particulates, or any other form known in the art.
[0061 ] Representative preferred membranes have a selective layer based on a poiyether. Such materials are described, for example, in two publications by Lin et a!., "Materials selection guidelines for membranes that remove CO?, from gas mixtures" (J. Mol. Struct., 739, 57-75, 2005) and "Plasticization-Enhaneed Hydrogen Purification Using Polymeric Membranes" {Science, 311 , 639-642, 2006).
[0062] A specific preferred material for the selective layer is Pebax®, a polya nide-poly ether block copolymer material described in detail in U.S. Patent No. 4,963,165. We have found that membranes using Pebax® as the selective polymer can maintain a selectivity of 9, 10, or greater under process conditions.
[0063| The membranes may be manufactured as flat sheets or as fibers and housed in any convenient module form, including spiral-wound modules, plate-and-frame modules, and potted hollow-fiber modules. The making of all these types of membranes and modules is well-known in the art. in general, we prefer to use flat-sheet membranes and spiral-wound modules.
[0064] Membrane unit 1 10 may contain a single membrane module or bank of membrane modules or an array of modules. A single unit or stage containing one or a bank of membrane modules is adequate for many applications. If either the residue or permeate stream, or both, requires further carbon dioxide removal, it may be passed to a second bank of membrane modules for a second processing step. Such multi-stage or multi-step processes, and variants thereof* will be familiar to those of skill in the art, who will appreciate that the membrane separation step may be configured in many possible ways, including single-stage, multistage, muitistep, or more complicated arrays of two or more units, in serial or cascade arrangements.
[0065] The membrane separation step can be operated by any mechanism that provides a driving force for transmembrane permeation. Most commonly, this driving force is provided by maintaining a pressure difference between the feed and permeate sides, or by sweeping the permeate side continuously with a gas that dilutes the permeating species, both of which techniques are well known in the membrane separation arts. Figure 1 shows a simple pressure-driven case. The feed side of the membrane is typically maintained at a pressure within the range of about 5 bar to 50 bar, and more preferably within the range of about 15 bar to 30 bar. The permeate side is typically maintained at a pressure of about 10% to 30% of the feed side pressure, that is, about 1 - 0 bar.
|0 66] Returning to Figure 1, syngas stream 1 9 flows across the feed side of the membranes 1 1 1 , A residue stream, 1 12, that is depleted in carbon dioxide relative to stream 109, is withdrawn from the feed side of the membrane. This stream forms the syngas product of the integrated process, and has a relatively high CO content compared with steam methane reforming performed without integrated membrane gas separation. Expressed as a ratio, the syngas product stream 1 12 has a CO:C02 ratio of at least about 3:1, preferably at least about 5: 1 and most preferably at least about 6:1. Expressed as carbon monoxide concentration, the CO content is typically at least about 12%, preferably at least about 15% and most preferably at least about 20%.
[0067] The permeate stream, 113, is enriched in carbon dioxide compared with the membrane feed, and is recompressed in compression step or steps, 114, preferably to a pressure consistent with the reforming operations, and returned to the reformer as stream 1 15. Compression may be carried out in one or multiple steps as convenient.
[0068] Figure 1 indicates that stream 115 is mixed with stream 102 before passing into the reformer train, as would be suitable if stream 1 15 were to be directed to the primar reforming reactor. In the alternative, stream 115 may be returned to another reactor, such as a secondary reforming reactor. The choice of where to return stream 1 15 can be readily made by the skilled person, depending on the particular balance of reactions that are being carried out.
[0069] The effect of returning the carbon-dioxide-rich stream 115 to the reformer is to suppress the C02-producing reactions and promotes the CO-producing reactions in the reformer, thereby increasing the CO yield and the ratio of CO to C02 in the syngas product. [0070] increasing the CO yield also tends to decrease the hydrogen: CO ratio in the syngas. As mentioned in the Summary section above, the processes of the invention are believed to be particularly beneficial in preparing syngas for use in gas-to-liquids processes, especially as feedstock for Fischer-Tropsch processes. In such gas-to-liquids conversions, it. is desirable to use as feedstock a syngas with a low hydrogen:CO ratio, preferably below about 2.5: 1 or 2.3:1 , and most preferably closer to about 2.2:1 or 2.1 :1. To facilitate obtaining a low hydrogen:CO ratio, it can be helpful to include a membrane separation step using membranes that are selective to hydrogen over carbon dioxide. This reduces the amount of hydrogen that is recycled to the reformer with the recovered carbon dioxide, enabling more effective C02 recycling as well as reducing the ¾:CO ratio in the product.
[0071] Such an embodiment is shown in Figure 2, in which like elements are labeled as in Figure 1 and are as described above. The process differs from the scheme shown in Figure 1 , however, in that strea 1 15 no longer passes to the reformer but instead is directed to a second membrane separation step or unit, 116, which uses membranes, 1 17, that are selective in favor of hydrogen over carbon dioxide and carbon monoxide. The membranes should preferably exhibit a selectivity for hydrogen over carbon dioxide of at least about 5, and more preferably at least about 10, a selectivity for hydrogen over carbon monoxide of at least about 10, and more preferably at least about 20, and a hydrogen permeance of preferably at least about 100 gpu, more preferably at least about 200 gpu.
[0072] As with the membranes selected for the first membrane separation step, any membranes with suitable performance properties may be used in the second membrane separation step. Once again, the membrane may be made from inorganic or polymeric materials, and may take the form of a homogeneous film, an integral asymmetric membrane, a multilayer composite membrane, a membrane incorporating a gel or liquid layer or particulates, or any other form known in the art.
[0073] Representative materials for the selective layer of the second membrane include polymers, such as polyimides, polyamides, polyurethanes, polyureas, polybenzimidazoles, and polybenzoxazoles; metals, such as palladium; zeolites; and carbon, by way of example and not by way of limitation. [0074] Specific representative examples of suitable polymeric membrane materials include polybenzimidazole (ΡΒΪ) based membranes, such as taught by . O'Brien et al. in "Fabrication and Scale-Up of PBI-based Membrane System for Pre-Combustion Capture of Carbon Dioxide" (DOE NETL Project Fact Sheet 2009} and po!yimide-based membranes, such as taught by B. T. Low et al. in "Simultaneous Occurrence of Chemical Grafting, Cross-linking, and Etching on the Surface of Polyimide Membranes and Their Impact on H2 CO2 Separation" (Macromolecules, Vol. 41 , No. 4, pp. 1297 - 1309, 2008).
[0075] Other choices with respect to the membrane and module format are generally as discussed above with respect to the first process embodiment.
[0076] Returning to Figure 2, compressed, COi-enriched stream 1 15 passes across membranes 1 17 under conditions that provide a driving force for transmembrane permeation. In the figure, this driving force is assumed to be provided by maintaining a pressure difference between the feed and permeate sides of the membrane unit, as described above. Hydrogen-enriched permeate stream, 1 19, is withdrawn from the process, and may be sent for recovery of purified hydrogen, for use as fuel, or for any other desired use. Hydrogen-depleted residue stream, 1 1 8, is returned to the reforming steps.
[0077] The product syngas stream, 1 12, generated by the process of Figure 2, is typically characterized by both a high CO:C02 ratio, such as at least 3:1, 5: 1 or 6: 1 , and a low hydrogen.'CO ratio, such as below 3 : 1 , 2.5: 1 , 2.3 : 1 or 2.2: 1 .
[0078] An alternative embodiment suitable especially for achieving low hydrogenrCO ratios is shown in Figure 3, with like elements again labeled as in the previous figures. Referring to Figure 3, the process differs from the scheme of Figure 2 in that a third membrane separation step, 120, is included. This step uses hydrogen-selective membranes, 121 , and treats stream 1 09 before it is passed to membrane separation step 1 10. The considerations as to membrane types, operating conditions and preferred performance for membranes 121 are the same as for membranes 1 17. Typically, but not necessarily, the same membranes, such as inorganic membranes or polyimide membranes, are used in both steps 120 and 1 16. [0079J Separation of stream 109 in unit or step 120 produces a hydrogen-depleted residue stream, 123, which is passed as the feed stream to the carbon-dioxide-selective step 110, and a hydrogen-rich permeate, stream 122, that is discharged from the process, and which may be directed to hydrogen purification, fuel or any other desired destination.
[0080] Figure 1-3 illustrate embodiments in which the feed stream, 109, to membrane separation steps 1 10 and 120 is maintained at about the same pressure as the raw gas from the reformer, a lower pressure is maintained on the permeate side(s), and permeate stream, 1 13, is recom ressed to a pressure suitable for reintroduction to the reforming steps. Such embodiments are preferred when the reformer operating pressure is above about 10 bar.
[0081] Another option is to compress stream 109 to a pressure higher than the reformer operating pressure, and to maintain the permeate side of membrane step or unit 10 at about the same pressure as the reformer, so that the permeate can be circulated to the reformer without the need for recompression. Such embodiments can be particularly useful if the raw syngas is at comparatively low pressure, by which we mean below about 10 bar, such as 5 bar,
10082] Figure 4 shows such an embodiment as it relates to a process scheme similar to that of Figure 2, in that the penneate, 1 13, from unit 110 is treated in unit 116 to remove excess hydrogen before being circulated back to the reforming steps, it will be apparent to those of skill in the art that arrangements in which membrane feed compression is used could also be readily applied to process schemes similar to those of Figures 1 and 3.
[0083] Referring to Figure 4, compression step, 124, is used to compress raw syngas stream 105/109. For simplicity, the cooling and water knock-out steps are not shown in this figure, but typically cooling and resultant water condensation would take place both before and after step 124. Step 124 can be used to compress the raw syngas to any desired pressure, although generally the gas will be compressed to a few tens of bar, such as 15 bar, 20 bar, 25 bar or 30 bar. The compressed stream, 125, is then passed as feed to membrane separation step or unit 1 10. [0084] in another aspect, the invention integrates membrane-based gas separation with gas-io-liquids processes, such as the Fischer-Tropsch (FT) process. The overall objective of gas-to- liquids technology is to convert methane or natural gas to heavier hydrocarbons (usually fractions that are easily transportable liquids at ambient temperature and pressure), that can be used as fuels, solvents, lubricants or the like. Methane is first broken down with steam or oxygen to form hydrogen and CO, then the CO and hydrogen are reacted to form hydrocarbons of the desired weight, either directly or via an interm ediate synthesis.
[0085] In the Fischer-Tropsch process, for example, hydrogen and carbon monoxide are consumed, generally according to the overall scheme of reaction (7), repeated below, to yield a liquid hydrocarbon product or products of a desired weight.
(7) nCO + (2n+l )H2 = CR¾n+2 + n¾0.
[0086J Since the goal is to make mostly liquid products, the ideal stoichiometric ratios of hydrogen and carbon monoxide needed for the above synthesis will vary from an extreme of about 2.25, for a C4 product, to another extreme of about 2.08 for a C12 or 2.06 for a C15 product. The steam methane reforming reactions, however, tend to produce a syngas with a hydrogenrCO ratio of about 3 or more.
[0087] Integrating a membrane separation step provides ratio adjustment of the syngas and enables very high overall conversion of CO to liquid hydrocarbons. The membrane separation step can be integrated with the other steps either upstream or downstream of the hydrocarbon synthesis reactor(s).
[0088] An embodiment in which the three operations are integrated with the membrane separation step on the inlet side of the hydrocarbon reactor is shown in Figure 5. Referring to Figure 5, steam, 201 , is fed into a steam methane reforming step, 204. Methane or natural gas, stream 202, is combined with stream, 212, discussed below, and passed as feed stream, 203, to steam methane reformer 204, where reforming reactions take place as discussed with respect to the earlier embodiments. [0089] The reactions form a raw syngas, stream 205. This stream, typically after cooling and water removal, as discussed previously, is passed as a feed sixeam to membrane separation step or unit, 206. in this embodiment, the membranes, 207, are selective in favor of hydrogen over carbon monoxide and carbon dioxide, preferably with a hydrogen/carbon dioxide selectivity of at least about 5, more preferably at least about 10, and a selectivity for hydrogen over carbon monoxide of at least about 10, more preferably at least about 20, Choices and operating conditions for the membranes and membrane unit are as discussed above with respect to the hydrogen-selective gas separation steps.
[0090] A permeate stream, 208, enriched in hydrogen compared with stream 205, is withdrawn as a hydrogen purge stream, and can be sent for fuel or other use. This stream typically contains at least about 90% hydrogen. Residue stream, 209, in which the hydrogenrCO ratio is preferably below about 2.3:1 , and most preferably in the range 2.05-2.3, is passed to the hydrocarbon product synthesis and fractionation/piirification train, shown together simply as element 210.
[0091] This train will usually include at least one reaction step, typically carried out in a fixed bed, fluidized bed or slurry reactor. Such reactors and operation thereof are well known in the art and are described in detail in, for example, S.T. Sie and R. Krishna, "Fundamentals and selection of advanced Fischer- Tropsch reactors", Applied Catalysis A: General, Vol. 186 (1999) pages 55-70, and in A. P. Steynberg, "Chapter 2- Fischer- ropsch Reactors", Studies in Surface Science and Catalysis , Vol. 152 (2004), pages 64-195, both of which are incorporated herein by reference.
[0092] The raw product from the reactor usually contains a mix of paraffmic hydrocarbons of different weights, and can be passed to one or more fractionation steps to separate desired products from unreacted feedstock, light hydrocarbon byproducts, particularly methane, inerts and the like. The desired produces ) are withdrawn as stream 211.
[0093] The remaining lights, inerts and unreacted hydrogen and carbon monoxide form at least one tax! gas stream, 212. Because excess hydrogen has been removed in stream 208, this tail gas is very lean in hydrogen compared with con ventional tail gas streams. The stream is rich in methane, formed as a byproduct during GTL synthesis, and in carbon dioxide, as well as containing unreacted CO, and is returned to the steam reforming steps.
[0094] in this way, no carbon species are lost from the overall integrated process except for the very small amounts of carbon monoxide and dioxide contained in the hydrogen purge stream, and these usually represent no more than about 5% of the purge stream. As a result, the integrated process achieves high overall conversion of CO to products, such as 80%, 90% or above.
[0095] A second option is to position the hydrogen-selective membrane step(s) in the tail gas return line, as shown in Figure 6. Referring to this figure, in which like elements are labeled as in Figure 5, stream 205, usually after water removal, is passed to the liquids formation and purification steps, 230, without hydrogen removal. The tail gas stream, 212, forms the feed stream to membrane separation unit or steps, 213. For this step, membranes, 214, with hydrogen-selective properties and configurations as discussed above, are used. The step produces a hydrogen-rich permeate purge stream, 215, and an adjusted tail gas residue stream, 216, that is returned to the reforming steps. Preferences for overall operating conditions for the reforming, hydrocarbon synthesis, and membrane separation steps are as already discussed with respect to Figure 5.
[0096] Once again, the process controls the amount of unwanted hydrogen that is returned in the process loop, while recapturing most of the carbon species. Recycling of methane reduces the need to drive the reforming reactions with a excessively high temperature, since unreacted methane is not lost to the process.
[0097] The invention is now further described by the following examples, which are intended to be illustrative of the invention, but are not intended to limit the scope or underlying principles in any way. EXAMPLES
[0098] Calculation methodology: The computer calculations in all of the following Examples were performed using a modeling program, ChemCad 6.6 (CnemStations, Inc., Houston, TX) which was modified with differential element subroutines for the membrane separation steps (as applicable). For all calculations, the carbon-dioxide-selective membrane was assumed to have€(½/¾ selectivity of 10, and CO2/CO selectivity of 30, and the hydrogen-selective membrane was assumed to have H2/C02 selectivity of 15, and H2/CO selectivity of 30. The membrane separation steps were sized in each case to achieve a concentration of 3 mol% carbon dioxide in the product syngas.
Example 1. No membrane integration - not in accordance with the invention
[0099] A base calculation was performed to model the output from a conventional steam reforming process without an integrated membrane separation step. This process is not in accordance with the invention, but serves as a comparative basis for the other calculations. The resul ts of the calculation are shown in Table 1 .
Table 1
Figure imgf000024_0001
[0100] According to the calculation, cooled syngas product stream 109 has a CO:C02 rat o of only 1.7 and an ¾:CQ ratio of 5.3.
Example 2. Steam reforming with one carbon-dioxide selective membrane separation step [0101 j A calculation was performed according to the process schematic of Figure 1 , in which a membrane gas separation step, using membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, is integrated with a steam methane reforming step. The results of the calculation are given in Table 2,
Table 2
Figure imgf000025_0001
[0102] Returning carbon dioxide to the reformer suppresses carbon dioxide production and increases CO yield, having a favorable effect on the both the CO:C02 and H2:CO ratios in the syngas product. The permeate/recycle stream, 113, contains 14.5 mol% carbon dioxide. The process can produce a syngas product, stream 1 12, containing only 3 mol% carbon dioxide, compared with almost 8 mo!% in Example 1. The syngas product stream has a CO:C02 ratio of 6.2, compared with only 1.7 in Example 1, and an H2:CO ratio of 3.6, compared with 5.3 in Example 1. Example 3. Steam reforming with two membrane steps, as in Figure 2
[01031 A calculation was performed according to the process scheme of Figure 2, with two membrane gas separation steps, one using membranes selective in favor of carbon dioxide over CO and hydrogen, the other using membranes selective in favor of hydrogen over carbon dioxide, integrated with a steam methane reforming step. The carbon -dioxide selective membrane step is as described in Example 2, but the permeate 113 from that membrane is sent to second step with a hydrogen-selective membrane before being recycled to the reformer. The results of the calculation are given in Table 3.
Table 3
Figure imgf000026_0001
[0104] The process produces a recycle stream 118 containing 50 mol% CO_>. The syngas product stream 1 12 has a CO:C02 ratio of 8.4 and an H2:CQ ratio of 2.5. Example 4, Steam reforming with three membrane steps, as in Figure 3
[0105] A calculation was performed according to the process scheme of Figure 3, which uses three membrane gas separation steps, One of the steps uses membranes selective in favor of carbon dioxide over carbon monoxide and hydrogen, the other two steps use membranes selective in favor of hydrogen over carbon dioxide, integrated with a steam methane reforming step.
[01061 A third hydrogen-selective membrane step is added in front of the two membrane steps of Example 3. instead of going directly to carbon -dioxide-selective step 110, raw syngas 109 is first subjected to membrane separation step 120, producing a hydrogen-depleted reside stream 123. Residue stream 123 is passed as the feed to carbon-dioxide-selective step 1 10.
[0107] Results of the calculation are given in Table 4. As can be seen, recycle stream 118 contains 50 mol% C02 as in Example 2, but the syngas product stream 1 12 has a CO:C0 ratio of 9.6 and an H2:CQ ratio of 2.1.
Table 4
Figure imgf000028_0001
Example 5. Steam reforming with compressed membrane feed and two membrane steps, as in Figure 4
[0108] A calculation was performed according to the process scheme of Figure 4, in which the feed to the membrane separation step is compressed prior to entering the membrane separation unit. In this case, it was assumed that the reformer operates at 5 bar, and the membrane feed, stream 125, is compressed to 25 bar. The permeate side of membrane step or unit 1 10 was assumed to be maintained at 5 bar so that the permeate can be circulated to the reformer without the need for recompression. The results of the calculation are shown in Table 5. Table 5
Figure imgf000029_0001
[0109] As can be seen, the syngas product stream 112 in this case has a CO:C02 ratio of 6.5 and an H2:CO ratio of 2.8.
Example 6, Comparison of CO yield with and without integrated membrane separation steps.
[0110] A series of modeling calculations was performed in the same manner as for Examples 1 and 2 to determine the carbon monoxide yield from a methane feedstock on a mole/mole conversion basis. The reforming temperature was varied from 650°C to 850°C. The calculations were performed at reformer pressures of 5 bar and 25 bar. {0111] The first set of calculations assumed no membrane separation step. The results of these calculations are shown in Figure 7, where percentage methane conversion to CO is plotted as a function of temperature. As can be seen, at a typical operating condition of 850°C arid 25 bar, a CO yield of about 50% can be obtained.
[0112] The second set of calculations assumed a configuration like that of Example 2 and Figure 1, using a carbon-dioxide-seiective membrane. The results of these calculations are shown in Figure 8. As can be seen, for all sets of operating conditions, integration of membrane separation with steam methane reforming provides a process that outperforms the conventional reforming process.

Claims

We claim:
1. A process for the production of syngas comprising the following steps:
(a) reacting a gas mixture comprising sleam and methane in a reformer train to form a raw syngas stream comprising hydrogen, carbon monoxide and carbon dioxide;
(b) providing a membrane having a feed side and a permeate side, wherein the membrane exhibits a selectivity to carbon dioxide over hydrogen;
(c) passing at least a portion of the raw syngas stream as a feed stream across the feed side;
(d) withdrawing from the feed side a syngas product stream depleted in carbon dioxide compared with the feed stream, the syngas product stream containing carbon monoxide and carbon dioxide in a CO:C02 molar ratio of at least about 3: 1 ;
(e) withdrawing from the permeate side a permeate stream enriched in carbon dioxide compared with the feed stream;
(f) returning at least a portion of the penneate stream as a feedstock to the reformer train.
2. The process of claim 1 , further comprising cooling the raw syngas stream before carrying out step (c).
3. The process of claim 1 , further comprising compressing the portion of the permeate stream before carrying out step (f).
4. The process of claim I, further comprising compressing the raw syngas stream before carrying out step (c).
5. The process of claim 1 , wherein the membrane exhibits a selectivity to carbon dioxide over hydrogen of at least about 5.
6. The process of claim 1 , wherein the syngas product stream is passed as a feedstock stream to a Fiseher-Tropscb process.
7. The process of claim 1 , wherein the CO:CO? molar ratio is at least about 5: 1.
8. The process of claim 1 , wherein the CO:C02 molar ratio is at least about 6:1.
9. A process for the production of syngas comprising the following steps:
(a) reacting a gas mixture comprising steam and methane in a reformer train to form a raw syngas stream comprising hydrogen, carbon monoxide and carbon dioxide;
(b) providing a first membrane unit containing a first membrane having a first feed side and a first permeate side, wherein the first membrane exhibits a selectivity to carbon dioxide over hydrogen;
(c) passing at leas a portion of the raw syngas stream as a fi rst feed stream across the first feed side;
(d) withdrawing from the first feed side a syngas product stream depleted in carbon dioxide compared with the first feed stream, the syngas product stream containing carbon monoxide and carbon dioxide in a CO:CO> molar ratio of at least about 3: 1 , and containing hydrogen and carbon monoxide in an H2:CO molar ratio less than about 3:1 ;
(e) withdrawing from the first permeate side a first permeate stream enriched in carbon dioxide compared with the feed strea ;
(f) providing a second membrane unit containing a second membrane having a second feed side and a second permeate side, wherein the second membrane exhibits a selectivity to hydrogen over carbon dioxide:
(g) passing at least a portion of the first permeate stream as a second feed stream across the second feed side;
(h) withdrawing from the second feed side a second residue stream depleted in hydrogen compared with the second feed stream;
(i) withdrawing from the second permeate side a second permeate stream enriched in hydrogen compared with the second feed stream;
(j) passing at least a portion of the second residue stream as a feedstock to the reformer train.
10. The process of claim 9. further comprising cooling the raw syngas stream before carrying out step (c).
1 1. The process of claim 9, further comprising compressing the portion the first permeate stream before carrying out step (g).
12. The process of claim 9, further comprising compressing the raw syngas stream before carrying out step (c).
13. The process of claim 9, wherein the first membrane exhibits a selectivity to carbon dioxide over hydrogen of at least about 5.
14. The process of claim 9, wherein the second membrane exhibits a selectivity to hydrogen over carbon dioxide of at least about 5.
15. The process of claim 9, wherein a hydrogen -enriched stream is removed from the raw syngas stream betwee steps (a) and (c) by treating the raw syngas stream in a preliminary membrane unit containing a third membrane that exhibits a selectivity to hydrogen over carbon dioxide.
16. The process of claim 9, wherein the syngas product stream is passed as a feedstock stream to a Fischer-Tropsch process.
17. The process of claim 9, wherein the CO:C02 molar ratio is at least about 5:1 ,
18. The process of claim 9, wherein the CO:C02 molar ratio is at least about 6:1 .
19. The process of claim 9, wherein the ¾:CO molar ratio is less than about 2.5: 1.
20. The process of claim 9, wherein the ¾:CO molar ratio is less than about 2.3: 1.
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Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4963165A (en) 1987-04-27 1990-10-16 Membrane Technology & Research, Inc. Composite membrane, method of preparation and use
US6214066B1 (en) * 1997-06-06 2001-04-10 Air Products And Chemicals, Inc. Synthesis gas production by ion transport membranes
US20050220703A1 (en) * 2004-03-30 2005-10-06 Japan Oil, Gas And Metals National Corporation Process for producing synthesis gas for the fischer-tropsch synthesis and producing apparatus thereof
US20100129284A1 (en) * 2007-03-29 2010-05-27 Nippon Oil Corporation Method and apparatus for producing hydrogen and recovering carbon dioxide
US20120291484A1 (en) * 2011-05-18 2012-11-22 Air Liquide Large Industries U.S. Lp Process For The Production Of Hydrogen And Carbon Dioxide

Patent Citations (5)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4963165A (en) 1987-04-27 1990-10-16 Membrane Technology & Research, Inc. Composite membrane, method of preparation and use
US6214066B1 (en) * 1997-06-06 2001-04-10 Air Products And Chemicals, Inc. Synthesis gas production by ion transport membranes
US20050220703A1 (en) * 2004-03-30 2005-10-06 Japan Oil, Gas And Metals National Corporation Process for producing synthesis gas for the fischer-tropsch synthesis and producing apparatus thereof
US20100129284A1 (en) * 2007-03-29 2010-05-27 Nippon Oil Corporation Method and apparatus for producing hydrogen and recovering carbon dioxide
US20120291484A1 (en) * 2011-05-18 2012-11-22 Air Liquide Large Industries U.S. Lp Process For The Production Of Hydrogen And Carbon Dioxide

Non-Patent Citations (6)

* Cited by examiner, † Cited by third party
Title
"Plasticization-Enhanced Hydrogen Purification Using Polymeric Membranes", SCIENCE, vol. 311, 2006, pages 639 - 642
A. P. STEYNBERG: "Studies in Surface Science and Catalysis", vol. 152, 2004, article "Fischer-Tropsch Reactors", pages: 64 - 195
B. T. LOW ET AL.: "Simultaneous Occurrence of Chemical Grafting, Cross-linking, and Etching on the Surface ofPolyimide Membranes and Their Impact on H /CO Separation", MACROMOLECULES, vol. 41, no. 4, 2008, pages 1297 - 1309
K. O'BRIEN ET AL.: "Fabrication and Scale-Up of PBI-based Membrane System for Pre-Combustion Capture of Carbon Dioxide", DOE NETL PROJECT FACT SHEET, 2009
LIN ET AL.: "Materials selection guidelines for membranes that remove CO2 from gas mixtures", J. MO/. STRUCT., vol. 739, 2005, pages 57 - 75
S.T. SIE; R. KRISHNA: "Fundamentals and selection of advanced Fischer-Tropsch reactors", APPLIED CATALYSIS A: GENERAL, vol. 186, 1999, pages 55 - 70, XP004271925, DOI: doi:10.1016/S0926-860X(99)00164-7

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