Classifications

• • C—CHEMISTRY; METALLURGY
  • C10—PETROLEUM, GAS OR COKE INDUSTRIES; TECHNICAL GASES CONTAINING CARBON MONOXIDE; FUELS; LUBRICANTS; PEAT
  • C10G—CRACKING HYDROCARBON OILS; PRODUCTION OF LIQUID HYDROCARBON MIXTURES, e.g. BY DESTRUCTIVE HYDROGENATION, OLIGOMERISATION, POLYMERISATION; RECOVERY OF HYDROCARBON OILS FROM OIL-SHALE, OIL-SAND, OR GASES; REFINING MIXTURES MAINLY CONSISTING OF HYDROCARBONS; REFORMING OF NAPHTHA; MINERAL WAXES
  • C10G2/00—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon
  • C10G2/30—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen
  • C10G2/32—Production of liquid hydrocarbon mixtures of undefined composition from oxides of carbon from carbon monoxide with hydrogen with the use of catalysts
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01D—SEPARATION
  • B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
  • B01D53/14—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by absorption
  • B01D53/1456—Removing acid components
  • B01D53/1475—Removing carbon dioxide
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B3/00—Hydrogen; Gaseous mixtures containing hydrogen; Separation of hydrogen from mixtures containing it; Purification of hydrogen
  • C01B3/50—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification
  • C01B3/52—Separation of hydrogen or hydrogen containing gases from gaseous mixtures, e.g. purification by contacting with liquids; Regeneration of used liquids
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
  • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
  • C01B2203/0415—Purification by absorption in liquids
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
  • C01B2203/04—Integrated processes for the production of hydrogen or synthesis gas containing a purification step for the hydrogen or the synthesis gas
  • C01B2203/0465—Composition of the impurity
  • C01B2203/0475—Composition of the impurity the impurity being carbon dioxide
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
  • C01B2203/06—Integration with other chemical processes
  • C01B2203/061—Methanol production
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2203/00—Integrated processes for the production of hydrogen or synthesis gas
  • C01B2203/06—Integration with other chemical processes
  • C01B2203/062—Hydrocarbon production, e.g. Fischer-Tropsch process
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02C—CAPTURE, STORAGE, SEQUESTRATION OR DISPOSAL OF GREENHOUSE GASES [GHG]
  • Y02C20/00—Capture or disposal of greenhouse gases
  • Y02C20/40—Capture or disposal of greenhouse gases of CO2
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
  • Y02P20/00—Technologies relating to chemical industry
  • Y02P20/151—Reduction of greenhouse gas [GHG] emissions, e.g. CO2

Definitions

• This invention relates to a treatment of unreacted synthesis gas (syngas) produced in a gas-to-liquid synthesis and more particularly to a process of contacting unreacted syngas with water to remove CO 2 .
• Natural gas is a primary source of methane which is used to manufacture synthesis gas.
• Synthesis gas (syngas) is primarily a mixture composed of CO and H 2 .
• Techniques are known to convert syngas into useful products such as methanol or into synthetic fuels, lubricants and other hydrocarbonaceous products via Fischer- Tropsch synthesis.
• One method for the preparation of syngas involves catalytically reacting methane and carbon dioxide. While natural gas is a primary source of methane, coal and petroleum have also been utilized to provide hydrocarbon feeds to generate syngas.
• Carbon dioxide is considered by some to be a major factor in global warming. Accordingly, there is an incentive to find means for reducing the production of carbon dioxide and limiting its release into the environment.
• One advantage of the aforementioned process for preparing syngas is that it utilizes carbon dioxide as a reactant.
• GTL gas-to-liquid
• the first involves the conversion of natural gas or coal into syngas by partial oxidation followed by reaction in a Fischer-Tropsch synthesis with further refining of the Fischer-Tropsch products.
• the second technology involves conversion of natural gas or coal into syngas by partial oxidation followed by methanol synthesis, the methanol being subsequently converted into highly aromatic gasoline by the Methanol-To-Gasoline (MTG) process.
• MEG Methanol-To-Gasoline
• the Fischer-Tropsch and MTG processes both have relative merits and disadvantages.
• One advantage of the Fischer-Tropsch process is that the products formed are highly paraffinic. These products have excellent combustion and lubricating properties.
• a disadvantage of the Fischer-Tropsch process is the relatively large amounts of carbon dioxide that are emitted in the facility during the conversion of natural gas into Fischer-Tropsch products.
• the MTG process produces a highly aromatic gasoline and LPG fraction. While the gasoline generally is suitable for use in gasoline engines, durene and other polymethyl aromatics may be present. These materials have high crystallization temperatures and can solidify upon standing.
• the MTG process also suffers from higher capital costs in comparison to the Fischer-Tropsch process and the product cannot be used for lubricants, diesel fuel or jet turbine fuel.
• a typical Fischer-Tropsch process is illustrated in Figure 1.
• a feed of CH 4 , 0 2 and H 2 0 is forwarded via conduit (10) to a syngas generator (15).
• Effluent from the generator containing CO, H 2 and C0 2 is forwarded via conduit (20) to a Fischer-Tropsch reactor (25).
• the products of the reaction are forwarded via conduit (35) to a separation zone (40).
• Hydrocarbonaceous products including C 5 + liquids are recovered and forwarded via conduit (45) to other areas of the facility for further processing into fuels, lubes, etc.
• Gaseous products recovered from the separation zone e.g. tail gas
• a portion of the tail gas is forwarded via conduit (60) for use as a fuel in the facility.
• Another portion of the tail gas is recycled via conduit (50) to be mixed with the feed to the syngas generator (15).
• nCH 2 represents a paraffinic polymer of n CH2 units This formula is exact for mono-olefins and a close approximation for C 5 + paraffins.
• n the average carbon number of the product
• the value of n is determined by the reaction conditions, e.g., temperature, pressure, space rate, catalyst type, and syngas composition.
• the desired net syngas stoichiometry for a Fischer-Tropsch reaction is independent of n, and is approximately 2.0 as determined by the following equation: nCO + 2nH 2 ⁇ nH 2 0 + nCH 2
• nCH 2 represent the major products of a Fischer-Tropsch reaction (olefins and paraffins).
• the ratio of H 2 to CO is 3: 1 which is higher than the 2: 1 ratio desired for the Fischer-Tropsch conversion.
• the ratio of H 2 to CO is 1 : 1 , which is lower than that desired for the Fischer-Tropsch conversion. Also, dry reforming may result in rapid carbon deposition.
• C0 2 is formed in partial oxidation because the reaction is not perfectly selective. Some CH 4 reacts with O 2 to form CO 2 by complete combustion according to the following:
• steam added to the reformer to control coking, or produced in the Fischer-Tropsch reaction can react with CO to form CO 2 by the water gas shift reaction as follows:
• the overall proportion of carbon in the CH 4 that is converted to heavier hydrocarbon products is estimated to be about 68%. The remainder, about 32%, forms significant amounts of C0 2 .
• These estimates of carbon efficiency were provided by Bechtel Corporation for a GTL complex that uses cryogenic air separation, an autothermal reformer, a slurry bed Fischer-Tropsch unit and a hydrocracker for conversion of the heavy wax into products. Details are described in "CO 2 Abatement in GTL Plant: Fischer-Tropsch Synthesis," Report # PH3/15, November 2000, published by the IEA Greenhouse Gas R&D Programme. GTL complexes using alternative technologies would have similar carbon efficiencies and CO 2 emissions.
• syngas conversion processes operate at less than 100% conversion of the CO in the syngas. Typical values are between 40 and 70% per-pass conversion.
• Fischer-Tropsch processes which use 0 2 rather than air, the unreacted syngas is recycled to the Fischer-Tropsch reactor.
• the preferred Fischer-Tropsch process is a slurry bed process.
• the most common catalyst for use in slurry bed units contains cobalt. Cobalt does not promote the water gas shift reaction to a significant extent (or the reverse of this reaction).
• CO 2 for the most part is an inert gas. As CO 2 is recycled to the syngas conversion reactor, its concentration builds up.
• a process scheme which reduces the CO 2 emissions from a Fischer-Tropsch-GTL process while still making the desired product slate would be highly desirable. Reducing the CO 2 emissions also acts to improve the carbon efficiency of the process.
• EPO 516 441 Al discusses several aspects of handling CO 2 in a Fischer-Tropsch-GTL process including: separating CO 2 from the syngas generated in the reformer, and recycling the unreacted tail gas from Fischer-Tropsch to the reformer. As discussed previously, the recycling of CO 2 is to achieve proper H 2 /CO ratio of syngas. This publication notes that: "Separation of carbon dioxide is expensive. It is often carried out by amine stripping. This involves reaction with an amine, followed by boiling, and compression to reach the necessary pressure for recycle back to the reformer.” (page 3, lines 4-6). Likewise: “The cost of removing and recycling the CO 2 is expensive and typically would represent around 30% of the costs associated with producing the syngas within the process.” (page 4, lines 45-46).
• EP 142 887 Bl discusses the benefits of having CO 2 in the feed to the reformer. It also mentions the high costs associated with the separation of the CO 2 from the syngas by amine scrubbing and subsequent compression.
• Neither of these publications disclose the use of an aqueous medium to remove C0 2 from syngas.
• the use of an aqueous medium to remove C0 2 significantly reduces costs associated with typical amine scrubbing, and also provides a source of water for use in the reforming reaction.
• a process comprising the following steps:
• step (e) forwarding at least part of the recovered syngas from step (d) to the syngas conversion reactor;
• step (f) desorbing at least part of the CO 2 from the C0 2 -enriched aqueous stream obtained in step (d) and recovering a CCyrich gas and CO 2 - depleted aqueous stream.
• Deso ⁇ tion can be accomplished, for example, by contacting the C0 2 -enriched aqueous stream with CH 4 in a deso ⁇ tion vessel. Recovered CH 4 can be recycled to the syngas generator. Portions of the recovered C0 2 can be disposed in a marine environment, a terrestrial formation or both.
• Figure 1 is a schematic flow diagram of a conventional Fischer-Tropsch synthesis.
• Figure 2 is a schematic flow diagram of one embodiment of the invention.
• the syngas generator can be a light hydrocarbon reformer or a heavy hydrocarbon reformer. Reforming includes a variety of technologies such as steam reforming, partial oxidation, dry reforming, series reforming, convective reforming, and autothermal reforming. All have in common the production of syngas from methane and an oxidant (steam, oxygen, carbon dioxide, air, enriched air or combinations). The gas product typically contains some carbon dioxide and steam in addition to syngas. Series reforming, convective reforming and autothermal reforming inco ⁇ orate more than one syngas-forming reaction in order to better utilize the heat of reaction. The processes for producing synthesis gas from C,-C 3 alkanes are well known to the art.
• Steam reformation is typically effected by contacting C,-C 3 alkanes with steam, preferably in the presence of a reforming catalyst, at a temperature of about 1300° F (705° C) to about 1675°F (913°C) and pressures from about 10 psia (0.7 bars) to about 500 psia (34 bars).
• Suitable reforming catalysts which can be used include, for example, nickel, palladium, nickel-palladium alloys, and the like. Regardless of the system used to produce syngas it is desirable to remove any sulfur compounds, e.g., hydrogen sulfide and mercaptans, contained in the C,-C 3 alkane feed.
• the process involves converting coal, heavy petroleum stocks such as resid, or combinations thereof, into syngas.
• the temperature in the reaction zone of the syngas generator is about 1800° F-3000 0 F and the pressure is about 1 to 250 atmospheres.
• the atomic ratio of free oxygen in the oxidant to carbon in the feedstock (O/C, atom/atom) is about 0.6 to 1.5, preferably about 0.80 to 1.3.
• the free oxygen-containing gas or oxidant may be air, oxygen-enriched air, i.e., greater than 21 up to 95 mole % O 2 or substantially pure oxygen, i.e., greater than 95 mole % 0 2 .
• the effluent gas stream leaving the partial oxidation gas generator generally has the following composition in mole % depending on the amount and composition of the feed streams: H 2 :8.0 to 60.0; CO:8.0 to 70.0; CO 2 :1.0 to 50.0, H 2 O:2.0 to 75.0, CH 4 :0.0 to 30.0, H 2 S:0.1 to 2.0, COS:0.05 to 1.0, N 2 :0.0 to 80.0, Ar.0.0 to 2.0.
• Particulate matter entrained in the effluent gas stream may comprise generally about 0.5 to 30 wt. % more, particularly about 1 to 10 wt. % of particulate carbon (basis weight of carbon in the feed to the gas generator). Fly ash particulate matter may be present along with the particulate carbon and molten slag. Conventional gas cleaning and/or purification steps may be employed such as that described in U.S. Patent No. 5,423,894.
• the mixture generated in the syngas reactor is then cycled to a syngas converter, preferably a Fischer-Tropsch reactor.
• a syngas converter preferably a Fischer-Tropsch reactor.
• the Fischer- Tropsch synthesis is well-known and widely documented in patent and technical literature. The synthesis converts syngas into a wide variety of hydrocarbonaceous products by contact with a Fischer-Tropsch catalyst under suitable temperatures and pressures.
• the reaction is typically conducted at temperatures ranging from about 300° -700° F, preferably about 400° -550° F, pressures ranging from about 10 to 500 psia, preferably about 30 to 300 psia, and a catalyst space velocities ranging from about 100 to 10,000 cc/g/hr., preferably about 300 to 3,000 cc/g/hr.
• the reaction can be conducted in a variety of reactors such as fixed bed reactors containing one or more catalyst beds, slurry reactors, fluidized bed reactors, or a combination of different types reactors.
• the products may range from C, to C 200 + hydrocarbons with a majority in the range of C 5 -C 100 + .
• the syngas feed to a Fischer Tropsch reactor contains - 1 ess than 1 ppm, preferably less than 100 ppb, most preferably less than 10 ppb sulfur.
• Tail gas recovered from the separation zone contains CO, H 2 , C0 2 , CH 4 and other light hydrocarbons. A portion of the tail gas is drawn off and used as fuel.
• An important feature of the present invention involves contacting at least a portion of the tail gas with an aqueous medium in a scrubber preferably at above atmospheric pressure to remove C0 2 and optionally CH 4 by abso ⁇ tion and admixing the resultant CO 2 -depleted tail gas with the syngas feed to the syngas converter.
• the C0 2 -rich aqueous stream recovered from the scrubber preferably is processed to desorb at least some CO 2 and optionally CH 4
• a CO 2 - enriched gas is recovered and at least a portion thereof is recycled to the syngas generator.
• a C0 2 -depleted aqueous stream is recovered from the desorbing zone and at least a portion is recycled to the scrubber.
• At least a portion of the C0 2 -enriched gas recovered from the desorbing zone may be dissolved in an aqueous phase and disposed in a marine environment, and/or a terrestrial formation.
• At least a portion of C0 2 generated in Fischer- Tropsch GTL process is isolated from the tail gas by scrubbing with an aqueous stream, preferably at a pressure greater than about atmospheric pressure to assist in the removal of C0 2 .
• scrubbing in accordance with the present invention can be performed in any number of locations in a GTL process, preferably, scrubbing is not conducted upon streams that contain significant amounts of liquid hydrocarbons.
• scrubbing to remove CO 2 from streams that also contain significant amounts of liquid hydrocarbons is avoided because the presence of significant amount of liquid hydrocarbons can make separation more difficult.
• Suitable sources include, but are not limited to, sea water, reaction water formed in a Fischer-Tropsch GTL process, spent cooling water from a FischerTropsch GTL facility, river water or other non-potable water sources, and water recovered from crude or gas production.
• the pH of the aqueous stream used in scrubbing is important.
• the aqueous streams exhibit a pH greater than about 6.0, preferably above 7.0 and most preferably, from about 7.5 to about 11.0.
• a high pH aqueous stream facilitates scrubbing of C0 2 .
• the pH of the aqueous stream used for scrubbing C0 2 can be increased using any number of suitable techniques. These include, but are not limited to, adding an alkali such as NaOH or KOH and/or other basic materials, such as ammonia. Use of amines in the aqueous stream should be avoided - 1 ess than 1 wt% is preferred.
• preferred aqueous sources include, but are not limited to, sea water, river water, or other non- potable water sources from the environment
• GTL process water may be a particularly suitable aqueous source because it is abundantly produced during Fischer-Tropsch GTL processing.
• nCO + 2nH 2 ⁇ nH 2 0 + nCH 2 the weight ratio of water to hydrocarbon produced by the reaction is about 1.25.
• a typical Fischer-Tropsch GTL process produces about 25% more water than hydrocarbon, on a weight / basis.
• GTL process water is abundantly produced, a disadvantage of using GTL process water as an aqueous stream, is that GTL process water may contain acidic contaminants, such as, for example, acetic acid and/or other organic acids. Acidic contaminants present in GTL process water can lower the pH, thereby reducing the solubility of CO 2 therein. As a result, if GTL process water is used for scrubbing C0 2 , it is preferable to remove acidic contaminants from the GTL process water before using it as a scrubbing stream.
• the acidic contaminants in GTL process water can be removed using various known methods. Suitable methods for removing acidic component from GTL process water include, but are not limited to, distillation, adso ⁇ tion onto alumina or a basic material, oxidation, combinations thereof and the like.
• an aqueous stream is de-aerated before being used for scrubbing to increase the aqueous stream's capacity to adsorb C0 2 and to minimize the amount of air introduced into the gas stream.
• Suitable processes for de-aeration of aqueous streams are well known in the art and are used, for example, in desalination plants and for preparation of boiler feed water. Such processes are described in detail, for example, in John H. Perry's Chemical Engineering Handbook, 4th Edition, pages 9-51, McGraw Hill Book Company, 1963.
• the aqueous stream may not be as highly selective to scrubbing of C0 2 .
• conditions during disposal of the C0 2 -enriched aqueous phase are such that a high percentage of the C0 2 removed from the C0 2 -containing gas during scrubbing remains dissolved in the aqueous phase.
• Scrubbing of CO 2 from gases using aqueous liquids should be performed at non-extreme pressures to avoid formation of methane, CO 2 and other hydrates.
• the dissolution of C0 2 in water is favored at high pressures.
• gas compression may be desirable, although the typical elevated pressures of a GTL facility will often suffice.
• salinity can also affect the solubility of hydrocarbons in water. For example, a larger "salting-out" effect can occur when using non-ionic hydrocarbons such as, for example, methane. Accordingly, variations in temperature and salinity of an aqueous solution can be used to maximize selectivity for C0 2 removal. In addition, the presence of salt in sea water can create a slight tendency to reduce the temperature at which hydrates will form. Yet another advantage of sea water as an adsorbent is its pH, which is often above 7.0, and thus facilitates the adso ⁇ tion of CO2.
• the composition of a blended syngas to a Fischer- Tropsch unit will change as C0 2 is recycled to the syngas formation reactor.
• Other inert gas components will increase in concentration, while the reactive syngas components will remain essentially constant.
• the blended syngas is the mixture of the fresh syngas from the syngas generator and the syngas that is recycled.
• the following table provides typical ranges of blended syngas compositions for a process that generates syngas from an 0 2 source of about 99.5 percent purity.
• aqueous scrubbing of the syngas from the syngas generator to remove C0 2 also offers the advantage of simultaneously reducing the concentration of the catalytically poisonous nitrogen compounds.
• the level of reduction of HCN should be at least 30%, preferably at least 50%, and most preferably, at least 80%.
• the reduction in the NH3 level should be equivalent to the reduction of HCN.
• Excess CO 2 can be disposed of by injecting it into a marine environment, a terrestrial formation, or both.
• Water is a natural product of the syngas generation and syngas conversion reactions.
• the water from both syngas generation and syngas conversion reactors is separated prior to further processing. If not removed, the water can promote the undesirable reverse water gas shift reaction which converts CO into additional C0 2 .
• the removal of reaction water formed by the syngas generation and syngas conversion reactions will also remove some dissolved C0 2 . Additional water can be added to remove more C0 2 by use of the same separation vessels which remove the reaction waters.
• the deso ⁇ tion of C0 2 from the C0 2 -rich aqueous stream obtained from the scrubber can be achieved either by lowering the pressure (commonly known as pressure swing) or increasing the temperature (commonly known as temperature swing). Since the objective is to use the C0 2 in the syngas formation reactor, which operates at pressures close to the syngas conversion unit, it is preferred not to significantly reduce the pressure to desorb the C0 2 . Rather, it is preferable to desorb the C0 2 by increasing the temperature.
• the most common adsorbents for removal of C0 2 from gas streams are amines. These compounds will decompose if heated to excessive temperatures, and thus they are almost always used in pressure swing adsorption units.
• the pressure of the deso ⁇ tion step should be above atmospheric pressure to minimize C0 2 compression costs.
• the pressure in the deso ⁇ tion stage should not be less than about 100 psi below the outlet pressure of the syngas conversion step.
• the pressure in the adso ⁇ tion scrubber preferably should not be less than about 50 psi below the outlet pressure of the syngas conversion step.
• the temperature of the deso ⁇ tion step preferably is in excess of the temperature of the adso ⁇ tion step.
• the heat needed to desorb the C02 from the C02-rich stream can be provided from other streams in the GTL process including but not limited to streams associated with the syngas formation reactor, syngas conversion reactor, and reactors used to upgrade the product from the syngas conversion reactor.
• the C02-depleted stream needs to be cooled prior to adso ⁇ tion of the C02. This cooling can also come from a variety of sources including but not limited to: river water, air cooling, and gas streams from the air separation unit (N2, 02, etc.), and combinations.
• the aqueous scrubbing concept of this invention has an advantage over the current commercial technology of amine scrubbing because the recovered CO 2 is produced at pressures close to that of the syngas conversion unit. This minimizes the amount of expensive compression needed to dispose of the C0 2 .
• the CO 2 leaving the desorber is not likely to be at sufficient pressure to be directly injected into a marine environment or a terrestrial formation. Most likely it would need additional compression and cooling.
• the energy and cooling for this step can be obtained from other sources in the process including: cooling water, the syngas generator, or gas streams associated with an O 2 purification plant (such as the cold N 2 waste gas) and the like.
• C0 2 There is another option for disposal of C0 2 .
• the C0 2 can be adsorbed into a second aqueous stream, such as sea water, brine, river water, or other non-potable water.
• This second aqueous stream containing dissolved C0 2 can then be disposed in a marine environment or terrestrial formation.
• a two-water system permits the more valuable fresh water used in the initial recovery to be recycled, while the less valuable second aqueous stream is used as a disposal vehicle.
• Disposing of the C0 2 as a dissolved aqueous phase reduces the costs associated with compressing and possibly liquefying the C0 2 so that it can be disposed.
• Compression and liquefaction of a gas use expensive compressors, while compression of a liquid (the C0 2 -containing second water stream) uses inexpensive pumps.
• the choice between the two options (disposal of relatively pure gaseous or liquid C0 2 , or disposal of an aqueous CO 2 mixture) depends on the details of the disposal location.
• An additional advantage of the process of the invention resides in the supply of water to the syngas formation reactor.
• some water is added to a syngas formation reactor in order to control coking, and promote steam reforming.
• Typical values of H 2 0/carbon molar ratios are about 0.1 to 5.0/1.0, preferably 0.2 to 0.8.
• water will also be present in the vapor phase. This water can supplement or supply all of the water needed for the syngas formation reactor.
• Use of water in a temperature swing adso ⁇ tion system will provide a water-rich CO 2 stream and higher temperatures can be selected in the deso ⁇ tion step to provide the needed amount of water.
• makeup water to the scrubbing system can be obtained from boiler feed water, and the ultimate source of the water for the process can be water derived from the syngas conversion process, river water, water from an aquifer, or desalinated sea water.
• the scrubbing aqueous medium may accumulate contaminants from the syngas conversion process. These include hydrocarbonaceous species such as light paraffins, alcohols, and acids such as acetic and formic. For the most part, these contaminants are not a problem, with the exception of the acids.
• the acids will lower the pH of the scrubbing aqueous medium which will decrease the capacity of the system to adsorb C0 2 .
• the aqueous scrubbing medium should be maintained in a relatively pure state with a pH value above 5, preferably above 6, and most preferably above 6.5.
• a basic compound caustic, sodium salt, nitrogen compound, and the like.
• These processing steps can be part of the water recovery systems employed to treat the water produced in the syngas generator or the syngas converter.
• the pH of the aqueous stream needs to be measured on a C0 2 -free sample. This can be done by bubbling N 2 through the sample at atmospheric pressure for 24 hours at 25° C at 10 cm 3 of N 2 per ml of water per minute.
• the scrubbing of CO 2 from gases by an aqueous medium must operate at non-extreme pressures to avoid formation of hydrates of CH 4 , C0 2 , and other hydrates.
• the dissolution of C0 2 in water is favored at higher pressures. It is preferred to operate at as high a pressure as possible (and within economic limits; compression of the gas is not favored).
• Sources of Solubility Data include:
• CH 4 , N 2 , Ar, and other inert gas elements include CH 4 , N 2 , Ar, and other inert gas elements.
• the CH 4 originates from CH 4 ⁇ iip in the syngas generator and CH 4 formation in the Fischer-Tropsch unit.
• the N 2 , Ar, and other inert gas elements come from the air separation unit, and possibly from the original natural gas feedstock.
• C0 2 is the most abundant inert gas component, but when it is removed by the technology of this invention, the other inert gas components will increase in concentration. Thus, some purging of syngas will be necessary and a suitable use for this diluted syngas is as a fuel.
• C0 2 in the recycle gas to the Fischer-Tropsch reactor provides several advantages. It acts as a diluent for the reactive syngas components. It acts to dissipate some of the heat and improves process control. As CO and H2 are consumed in the Fischer-Tropsch reactor, other inert gas components (CH 4 , N 2 and Ar) build up in the recycle gas. They substitute for the extracted C0 2 by improving the management of heat release in the Fischer-Tropsch unit. This means that more of the carbon in the recycle gas can be processed. Eventually, the N 2 and Ar will have to be purged from the system.
• CH 4 , N 2 and Ar inert gas components
• a mixture of CH 4 , 0 2 and H 2 0 is introduced via conduit (10) to a syngas generator (15).
• Product from reactor (15) is conducted via conduit (20) to a syngas converter (25), specifically a Fischer-Tropsch reactor.
• a scrubber (30a) can be located downstream of the syngas generator or upstream of the syngas converter (25) to remove materials such as nitrogen contaminants, particulates, excess C0 2 , unreacted CH 4 , etc. from the products of the syngas generator before circulation to reactor (25).
• the syngas in the Fischer- Tropsch reactor is reacted in the presence of a non-shifting catalyst at a pressure of 300 psig to produce a variety of hydrocarbonaceous productors.
• Effluent from the reactor (25) is circulated via conduit (35) to a separation zone (40). Hydrocarbon products including C 5 + liquids are removed from the separation zone via conduct (45) to be further processed into salable products.
• a tail gas is collected from the separation zone.
• the tail gas has the following composition on a water-free basis:
• the gas is used in two operations, recycle to the Fischer-Tropsch unit and for fuel gas.
• One portion is circulated via conduit (50) to a scrubbing zone (55) and scrubbed with de-aerated water to avoid introduction of contaminants into the Fischer-Tropsch unit.
• the fuel gas is circulated via conduit (60) to a scrubbing zone (65) and scrubbed with water. Both scrubbing operations are done at 20°C.
• the tail gas is scrubbed to remove 50% of the C0 2 and the fuel gas is scrubbed to remove 75%. Contact is counter-current.
• the pressure in the scrubbing zone (55) is generally within 100 psig and preferably within 50 psi of the pressure of the outlet of the separation zone (40).
• a C0 2 -depleted syngas is recovered from scrubbing zone (55). It is recycled via conduit (70) to conduit (20) where it is admixed with fresh syngas and forwarded to reactor (25).
• a C0 2 -enriched aqueous stream (80) is forwarded to deso ⁇ tion zone (75) where C0 2 and optionally CH 4 are removed and recycled (85) to reactor (15).
• C0 2 -depleted aqueous stream (90) is recovered from desorption zone (75) and circulated to scrubbing zone (55).
• a makeup water is added through conduit (100).
• the recovery of Co 2 from deso ⁇ tion zone (75) may be facilitated by injecting a gaseous stream (95), such as methane, into desorption zone (75).
• a gaseous stream (95), such as methane a gaseous stream (95), such as methane, into desorption zone (75).
• Water is formed in both the syngas generator and the syngas conversion reactors. It is separated and removed from the other products of these reactions.
• HCN is extremely soluble in water.
• HCN is more solubile in water than C0 2 , and systems designed to remove C0 2 should simultaneously remove an even greater amount of HCN.
• NH 3 is likewise easily removed by water along with C0 2 .
• Case 1 Scrubbing the tail gas. 50% of the C0 2 was absorbed from the unreacted syngas using water at 20°C and 315 psia. Then, 99.5% of the C0 2 was recovered in a water-rich C0 2 stream and recycled back to the syngas generator. No CH 4 purge is used, so all of the stripping comes from steam generation at 315 psia (217° C).
• Case 2 Scrubbing the tail gas. 50% of the C0 2 was absorbed from the unreacted syngas using water at 20 C and 315 psia. Then, 99.5% of the C0 2 was recovered using CH 4 stripping at 100° C. No reboiler is used, so there should be very little water in the desorbed gas.
• Case 4 Scrubbing the fuel gas. 75 % of the C0 2 was absorbed from the unreacted syngas using sea water at 20° C and 315 psia. The scrubbed gases are used as fuel. Then, 99.5% of the C0 2 was recovered in a water-rich C0 2 stream. The C0 2 -rich, desorbed gases are sequestered. No external purge is used, so all of the stripping comes from steam generation at 315 psia (217° C).
• composition of the scrubbed gas and the desorbed gases for the four cases were calculated by the Kremser-Brown Method with a 100 mol basis of gas to the adsorber.
• the C0 2 -water stream could be fed directly to the syngas generator. This would eliminate the need to have a separate C0 2 gas compressor for the recycle to the reformer.
• the portion of the C0 2 -water stream not used in the syngas generator could be sent to the desorber, where the C0 2 is recovered and disposed.
• the C0 2 content in the blended synthesis drops from about 20% to lower values.
• the C0 2 content will decline to about 15% if a low level of C0 2 is removed in the scrubber (about 25%), to about 10% if a moderate amount of C0 2 is removed (about 50%), and about 8% if a high level of C0 2 is removed (about 75%).

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