Method for producing bio-based liquefied petroleum gas (LPG)

Abstract

A method is provided for synthesizing bio-based LPG from a renewable source using a bio-based syngas feedstock in a two-stage reaction system comprising an oxygenated compound synthesis reaction zone and an oxygenated compound conversion reaction zone configured to generate and convert methanol intermediates to reduce CO2 selectivity. The method includes recovering LPG from the total reaction zone effluent or from a purge stream separated from the total reaction zone effluent for LPG recovery.

Description

[Technical Field]

[0001] This application claims the benefit of priority to U.S. Patent Application Serial No. 18 / 431,474, filed February 2, 2024, the contents of which are incorporated herein by reference.

[0002] This invention relates to a two-stage synthesis of liquefied petroleum gas (LPG) from a bio-based source. [Previous Technology]

[0003] Climate change is a real and growing problem. Carbon dioxide emissions from the combustion of fossil fuels are the primary driver. To address this issue, various government regulations will soon require the use of renewable fuels as components of conventional fuels, including propane and LPG. For example, the European Renewable Energy Policy Framework has just raised the EU's target share of renewable energy to 40% by 2030.

[0004] California Air Resources Board regulations require transportation fuel producers and importers to meet specified fuel average carbon intensity requirements. Low-carbon fuel standards specify fuels including natural gas, electricity, hydrogen, gasoline blended with at least 10% corn-derived ethanol, biomass-based diesel, and propane.

[0005] Landlocked African countries face different problems. Propane and LPG are widely used as cooking fuels, but they must be imported and transported by land. This significantly increases their cost. Producing propane and LPG near inland markets would save costs, and, when produced from renewable resources, would also address climate change issues.

[0006] Therefore, the demand for propane and LPG (bio-based propane and bio-based LPG) made from non-fossil and / or renewable resources is real, current and global.

[0007] Several other issues exist regarding the sequence of synthesis / conversion reactions using syngas as a reactant. First, a significant amount of hydrogen is present in the exit gas from the reactor. While recycling has been proposed, conventional recycling processes involve recompressing the recycled hydrogen at substantial energy costs. Second, in conventional processing, over 10% of the carbon introduced as a reactant is produced as carbon dioxide. This represents a waste of valuable carbon monoxide resources from biosyngas. Therefore, a series of processing steps from bio-based sources is still needed to produce LPG, resulting in high LPG yields with low losses of syngas components (H2, CO, and CO2). [Summary of the Invention]

[0008] In one aspect, the present invention describes various embodiments of systems and methods for converting bio-based synthesis gas, comprising CO and H2, into LPG. One source of bio-based synthesis gas is a light hydrocarbon gas, primarily methane, produced and recovered from one or more biomass sources.

[0009] In another aspect, the present invention provides an improved method for converting syngas into hydrocarbons higher than C3 or C3+ hydrocarbons, primarily LPG, which has multiple uses as a biofuel power source and heat source.

[0010] In another aspect, the present invention provides an improved method for converting greenhouse gases (primarily methane) into bio-based fuels that can be used as heat and power sources for motor vehicles, commercial and residential use, with reduced and, in some cases, minimized environmental impact.

[0011] In another aspect, the present invention provides a method for converting hydrocarbon gases generated from agricultural and urban sources (including wastewater and sewage treatment and solid waste disposal sites) into fuels with low environmental impact.

[0012] In another aspect, the present invention provides an improved two-stage method for producing LPG from a synthesis process while recovering and efficiently recycling unreacted syngas components. The improved recycling process increases the recovery rate of valuable reaction byproducts and unreacted gaseous reactor feed components, while reducing, and in some cases minimizing, the loss of syngas carbon, represented by product CO2, into the atmosphere.

[0013] In another aspect, the present invention provides a method for efficiently recovering LPG from a mixture of gaseous reaction products, while recycling unreacted reaction product components with minimal pressure loss through a recovery process.

[0014] The present invention relates to a method for producing bio-based LPG, the method comprising (a) reacting a blend of bio-based syngas comprising CO, CO2 and H2 in an oxygenated syngas synthesis reaction zone comprising an oxygenated syngas synthesis catalyst to form a first effluent comprising oxygenated compounds and unreacted bio-based syngas, wherein the oxygenated compounds in the first effluent comprise at least 50 mol% methanol; (b) reacting at least a portion of the first effluent in an oxygenated syngas conversion reaction zone comprising an oxygenated syngas conversion catalyst to form a second effluent comprising C2-hydrocarbons, bio-based LPG and C5+ hydrocarbons, wherein the bio-based LPG comprises 0-10%, 0.5-10%, 1-7%, 1-5% or 2-5% olefins; (c) separating at least a portion of the hydrocarbons comprising C2-hydrocarbons, bio-based LPG and C5+ hydrocarbons from the second effluent to form a recycled effluent; and (d) blending at least a portion of the recycled effluent with fresh bio-based syngas to form the blended bio-based syngas.

[0015] A liquid absorption zone is provided to remove at least a portion of the LPG from the reaction zone effluent while returning unreacted syngas components to the recycle stream with minimal pressure drop. The liquid absorption process can process the entire dehydrated gaseous effluent stream. Alternatively, a purge stream comprising approximately 10% to approximately 90%, approximately 25% to approximately 75%, or approximately 30% to approximately 50% of the dehydrated gaseous effluent is contacted with an absorption solvent in the absorption zone to produce an LPG-rich liquid. If the rate is too low, LPG accumulates in the recycle gas to a level that lowers the partial pressure of the syngas and reduces the conversion of the syngas to methanol (and the overall reaction). If the rate is too high, energy costs are too high, and too much CO2, CO, and H2 are lost. The optimal range depends on the composition of the dehydrated gaseous effluent. According to the present invention, it has been found that contacting the effluent with a liquid absorbent, rather than treating the entire effluent stream, is beneficial to the process by reducing CO2 loss without affecting overall LPG recovery.

[0016] A solid adsorption zone is provided for removing light C2- hydrocarbons from the recycle stream before using the recycle stream in LPG synthesis. The solid adsorption zone is selectively used to remove C2- hydrocarbons from the recycle stream, as well as a portion of the CO and CO2 contained in the recycle stream. Most of the H2 in the recycle stream is not removed in the solid adsorption step but is returned to the recycle stream to form an H2-rich stream, which is then recycled to the LPG synthesis reaction. Therefore, the method includes adsorbing at least a portion of the C2- hydrocarbons, CO, and CO2 from the first recycle stream onto a solid adsorbent and returning the unadsorbed H2 to the first recycle stream to form a hydrogen-rich second recycle stream. In one aspect, the entire recycle stream can be treated in the solid adsorption zone. In another aspect, the method includes contacting a second vent stream comprising about 5% to about 50%, about 10% to about 40%, or about 15% to about 30% of the first recirculation stream, and adsorbing C2- hydrocarbons, CO, and CO2 from the second vent stream onto the solid adsorbent. If the amount of the first recirculation stream is too low, excess C2- accumulates in the recirculated gas and reduces the conversion rate. If the amount of the first recirculation stream is too high, excessive CO and CO2 are lost.

[0017] In another aspect, the present invention provides a method for producing bio-based LPG, the method comprising: (a) removing an aqueous product from a second effluent and producing a third effluent; (b) removing C3+ hydrocarbons from the third effluent by contacting at least a portion of the third effluent with a liquid absorbent solvent in an absorption zone to absorb C3+ hydrocarbons from the third effluent and producing a hydrocarbon-enriched fraction and a hydrocarbon-depleted fourth effluent, and recovering the bio-based LPG fraction; (c) removing light gases including C2- hydrocarbons, CO, and CO2 from the fourth effluent by contacting at least a portion of the fourth effluent with a solid adsorbent to adsorb at least a portion of the light gases, desorbing the adsorbed gases and producing a second light gas stream, and returning unadsorbed H2 to the fourth effluent to form the recycled effluent.

[0018] The most common practical method for removing water is through simple cooling and condensation. As an alternative, absorption can be considered to remove trace amounts of water. Sponge oil systems are one of the most commonly used methods for removing C3+ products. As an alternative, solid adsorption, such as silica gel, is used to remove C3+ products. Distillation is another alternative, but it requires expensive low-temperature separation. Removal of C2-hydrocarbons is optimal via solid adsorbents (PSA). A less attractive variant of this process is TSA (temperature swing adsorption).

[0019] The method optionally further includes (a) removing aqueous products from the second effluent and generating a third effluent; (b) removing C3+ hydrocarbons from a third vent stream comprising about 10% to about 90% of the third effluent by contacting at least a portion of the third vent stream with a liquid absorbent solvent in an absorption zone to absorb C3+ hydrocarbons from the third vent stream and generate hydrocarbon-rich and hydrocarbon-poor fractions, and recovering the bio-based LPG fraction; and (c) removing light gases comprising C2- hydrocarbons, CO, and CO2 from a fourth vent stream comprising about 5% to about 50% of the hydrocarbon-poor fraction by contacting the fourth vent stream with a solid adsorbent to adsorb at least a portion of the light gases, desorbing the adsorbed light gases and generating a second light gas stream, and returning unadsorbed H2 to the fourth effluent to form the recycled effluent.

[0020] In fact, the embodiments disclosed in this invention enable the production of LPG from bio-based sources in a recycling process with high LPG yield and low loss of syngas components H2, CO and CO2.

Implementation Method

[0026] As used herein, “C2-hydrocarbon” refers to methane and ethane, alone or in combination. Similarly, “C2+hydrocarbon” refers to a hydrocarbon consisting of two or more carbon atoms (e.g., ethane, propane, etc.). Similarly, “C5+hydrocarbon” refers to a hydrocarbon consisting of five or more carbon atoms (pentane, hexane, etc.).

[0027] As used herein, the term "LPG" refers to liquefied petroleum gas and compositions comprising hydrocarbon gases such as propane, propylene, butene, isobutane, or n-butane. However, depending on the market to which LPG is targeted, the term may refer to slightly different compositions. For example, European LPG is a mixture comprising propane and optionally light hydrocarbons such as n-butane and isobutane. It is synonymous with AutoGas. Smaller amounts of ethane and C5+ may be present. In the United States, LPG primarily refers to propane. Bio-LPG (BioLPG) is LPG made from biogas.

[0028] As used herein, unless otherwise stated, the terms “LPG” and “bioLPG” are used interchangeably. According to the invention, compositions of any LPG produced as described herein can be tailored by distillate fractionation; and the methods of the invention are applicable to the production of LPG from a range of compositions. Therefore, as used herein, the term “LPG” refers to a mixture of propane and butane (n-butane and / or isobutane) in any compositional ratio.

[0029] As used herein, “H2”, “CO”, “CO2”, “MeOH” and “DME” have conventional names and refer to molecular hydrogen, carbon monoxide, carbon dioxide, methanol and dimethyl ether.

[0030] As used herein, “bio-based” materials refer to materials derived from one or more natural resources that will be replenished within a limited timeframe on a human timescale through natural regeneration or other regeneration processes to replace those depleted by use and consumption.

[0031] As used herein, “biogas” means gaseous material comprising methane containing carbon and / or hydrogen derived from bio-based resources. Biogas recovered from biomass processing may also include CO2. In one aspect, biogas comprises methane and CO2 in a molar ratio ranging from 80:20 to 20:80, 70:30 to 30:70, 60:40 to 40:60, or 50:50.

[0032] As used herein, “biomass” means solid or liquid material of biological origin, or derived from municipal solid or liquid waste, agricultural solid and liquid waste, forestry products, or any other natural product or waste, such as seaweed or marine plants, including specialty agricultural products made from gasified materials, many of which ultimately originate from materials of biological origin.

[0033] As used herein, unless otherwise stated, the terms “dehydrated gaseous effluent” and “third effluent” are synonymous terms.

[0034] As used herein, “syngas” or alternatively “synthesis gas” refers to a mixture of H2 and CO in various ratios. Syngas typically contains low concentrations of one or more of CO2, CH4, and H2O.

[0035] As used herein, “oxygenate” refers to a hydrocarbon containing oxygen. Examples include alcohols such as methanol (MeOH) and dimethyl ether (DME). Bio-oxygenates, bio-methanol, and bio-dimethyl ether are these materials derived from biosynthetic gas.

[0036] As used herein, bio-based CO means CO that contains carbon derived from renewable sources, biological sources or carbon capture processes involving the capture of CO and CO2 from the atmosphere, flue gas, etc.

[0037] As used herein, the terms “reaction zone temperature” and “catalyst temperature” refer to the average catalyst bed temperature during the catalytic reaction process. In one aspect, catalyst temperature is the numerical average of the temperature at which the catalyst bed operates at the feed inlet and the temperature at which the catalyst bed operates at the product outlet.

[0038] As used herein, the term "molecular sieve" is a crystalline material having pores of molecular dimensions that allow molecules smaller than a certain size to pass through. It is commonly used as a commercial adsorbent and catalyst. Exemplary molecular sieves include phosphate molecular sieves (including silicon, aluminum, phosphorus, and oxygen); and zeolites (including silicon, aluminum, and oxygen). Non-limiting examples of zeolite molecular sieves include β-zeolite, γ-zeolite, SSZ-13, or ZSM-5. In the context of catalyst particles, "non-zeolite" refers to a catalyst that does not contain zeolite or phosphate molecular sieves. In the context of a catalyst bed, "non-zeolite" refers to a bed of catalyst particles that does not contain zeolite or phosphate molecular sieves.

[0039] The gaseous feed to the reforming reaction zone includes methane, and in some cases, the amount of C2+ higher hydrocarbons recovered from the biogas source is reduced. The biogas generated for use according to the invention comprises biomethane or methane derived in whole or in part from bio-based resources. Exemplary sources of bio-based methane include: (i) methane obtained from the anaerobic bacterial digestion of agricultural waste, municipal biological waste, or from wastewater treatment; (ii) gaseous products of biomass conversion (e.g., composting, biomass gasification, pyrolysis, or hydropyrolysis, such as in the case of supercritical water gasification of biomass); (iii) landfill gas; or (iv) gaseous products of the electrochemical reduction of carbon dioxide. Carbon from bio-based carbon sources is referred to as "bio-based carbon".

[0040] In some aspects, hydrogen can be added to the process, for example, to adjust the H2 / (CO+CO2) content of the syngas feed. Suitable hydrogen sources may include petroleum processing. Alternatively, bio-based hydrogen may be derived from renewable sources, from biological sources, from the electrolysis of water using solar, wind, wave, or other renewable energy sources, or from naturally occurring geological hydrogen (often referred to as natural hydrogen, golden hydrogen, or white hydrogen) or from nuclear-powered water electrolysis (often referred to as powdered hydrogen). Generally, bio-based hydrogen is not formed from the reaction of carbon compounds through steam reforming of methane.

[0041] Biogas may also contain CO2, CO, ethane, water vapor, and nitrogen, depending on the specific process used to generate it. Raw (untreated) biogas may proceed to a reforming process without further treatment. Alternatively, some or all of the non-methane components may be removed, and the treated biogas proceeds to a reformer to be converted into biosynthetic gas. Water present in the raw biogas may be condensed and removed from the biogas, for example, using a water knockout pot for two-phase separation. Non-hydrocarbon compounds present in the raw biogas (such as sulfur-, nitrogen-, or acid-containing compounds) are removed to low levels, typically down to ppm levels, for example, using one or more of aqueous scrubbing, alkanolamine absorption, molecular sieve adsorption, selective catalytic oxidation, and hydrodesulfurization.

[0042] Carbon dioxide can be removed from the raw biogas in conjunction with sulfur removal. Additional CO2 can be removed by membrane separation, cryogenic distillation, or aqueous absorption, which involves contacting the biogas with water or a caustic alkali solution to dissolve CO2, separating the water / CO2 mixture, removing CO2 from the mixture by increasing the temperature and / or decreasing the pressure of the mixture, and recycling the water. CO2 can also be partially removed into condensed water and removed from the biogas by aqueous absorption. In some embodiments, at least a portion of one or more of CO2, CO, and water vapor can be retained in the treated biogas feed to maintain the desired H2 / (CO+CO2) ratio of the biosynthetic gas leaving the reformer.

[0043] Carbon dioxide and / or water may also be added from external sources to the biogas feed to the reformer to control the H2 / (CO+CO2) ratio in the biosynthetic gas produced in the reformer. In an embodiment, the carbon in the added carbon dioxide is derived from a bio-based resource, and the addition of carbon from the bio-based resource is controlled to maintain the biogas carbon content at, for example, at least about 70% by weight of bio-based carbon not derived from petroleum.

[0044] Before feeding the biogas into the reforming reaction zone, a recycled gas including H2, CO, CO2 and optionally methane and trace amounts of C2+ hydrocarbons may be added to the biogas.

[0045] Biosynthetic gas including H2 and CO can be produced by contacting biogas including biomethane with an oxidizing gas selected from O2, CO2 and H2O or combinations thereof in a reforming reaction zone under reforming reaction conditions to produce biosynthetic gas including H2 and CO.

[0046] Biosyngas can be produced by biomass gasification, which involves contacting biomass with some combination of air, oxygen, and / or steam at elevated temperatures. Fluidized beds, fixed beds, or indirectly heated gasifiers can be used. Changing the input steam-to-oxygen ratio is one way to adjust the H2 / (CO+CO2) ratio to meet syngas requirements. In some operations, the gasifier temperature can be from about 1,000ºC to about 1,300ºC or higher.

[0047] On the other hand, syngas (or alternatively biosyngas) can be generated in a methane reformer involving steam reforming, autothermal reforming, or partial oxidation to convert methane into hydrogen and carbon oxide gases. Both fired and electric biogas reformers can be used. Reforming conditions include pressures between about 200 psi and about 600 psi (14-40 bar) and outlet temperatures between about 815ºC and about 925ºC. Because the catalyst is sensitive to sulfur, the sulfur content of the biogas must be reduced to less than 10 ppm, preferably less than 1 ppm.

[0048] In steam methane reforming, the reaction between steam and methane is as follows: CH4 + H2O → CO + 3H2

[0049] Dry reforming proceeds in the absence of steam as follows: CH4 + CO2 → 2CO + 2H2

[0050] The reactants in the reforming reaction zone can also be converted via a water gas shift (WGS) reaction on a metal reforming catalyst (e.g., a shaped nickel alumina catalyst): CO + H2O ⇌ CO2 + H2

[0051] Therefore, the amounts of CO2 and H2O added to the feed to the methane reforming reaction zone are adjusted to control the H2 / (CO+CO2) ratio of the syngas generated during reforming. The composition of the syngas generated in the reforming reaction zone, and especially the H2 / (CO+CO2) ratio in the syngas, can be controlled to effectively convert the syngas downstream to LPG. When this ratio is too low, the CO conversion rate decreases. When it is too high, a large amount of H2 must be recycled. The synthesis of oxygen-containing compounds in the synthesis reaction zone is typically carried out with an H2 / (CO+CO2) molar ratio in the feed to the synthesis reaction zone in the range of 1 to 4 (e.g., in the range of 2 to 3). When the feed contains less than or equal to 1 mol% CO2, the H2 / (CO+CO2) ratio can be in the range of 2.25 to 2.45. When the feed contains more than 1 mol% CO2, the H2 / (CO+CO2) ratio can be in the range of 2.2 to 2.5.

[0052] To manage the control of environmental emissions from this process, additional CO2 may be added to the gaseous feed to the reforming reaction zone. In embodiments, the CO2 used in the reformer is recovered from the biogas generation reactor, from the recycling of unreacted products of the process, or from both. In particular, the biogas generated from biomass includes CO2, which can be removed from the biogas as a pure CO2 product, making it highly suitable for inclusion in the blended syngas feed into the synthesis reaction zone. Thus, in some cases, the syngas feed may further include CO2, for example, in an amount of at least about 5 mol-% (e.g., about 4-50 mol%, or about 7-25 mol%, or about 8-10 mol%).

[0053] Controlling the amount of water supplied to the synthesis reaction zone can also affect the synthesis reaction in the synthesis reaction zone. In particular, adding steam to the reaction zone, and the steam reacting with CO generated in the reforming step in the synthesis reaction zone via WGS, increases the H2 / (CO+CO2) ratio in the reaction zone. Similarly, increasing the CO2 introduced into the synthesis reaction zone decreases the H2 / (CO+CO2) ratio via RWGS (reverse WGS).

[0054] Methane reforming for syngas production is typically carried out with methane-rich feedstocks that contain little or no C2+ components. Processes using biogas feedstocks containing excess C2+ hydrocarbons may include a pre-reformer for converting the C2+ hydrocarbons into methane. In embodiments, the primary sources of C2+ components in the biogas feedstock are recycled LPG fractionators and / or hydrogen from PSA modules, and a suitable pre-reformer may be included to process one or both of these streams. Suitable pre-reformation systems are known and readily available.

[0055] The biosynthetic gas supplied to the LPG synthesis reaction zone includes the biosynthetic gas generated in the reforming reaction zone. Other suitable sources of biosynthetic gas include one or more recycle streams generated in the process. Additional CO and / or H2, some or all of which may be bio-based CO and / or bio-based H2, may be supplied from external sources.

[0056] The method according to the invention relates at least in part to the production of LPG-enriched gaseous effluent via a catalytic synthesis process. In an embodiment, the method comprises reacting a blend of biosynthetic gas, comprising at least a portion of fresh biosynthetic gas and a recycle stream, over an LPG synthesis catalyst in at least one synthesis reaction zone at an LPG synthesis reaction temperature to produce the LPG-enriched gaseous effluent.

[0057] The synthesis catalyst includes at least one oxygen-containing compound synthesis catalyst for converting biosynthetic gas into oxygen-containing compounds (such as methanol), and an oxygen-containing compound conversion catalyst for converting oxygen-containing compounds into hydrocarbons (including LPG).

[0058] In an embodiment, bio-LPG can be synthesized from biosynthetic gas in a two-stage synthesis process, including reacting the biosynthetic gas on an oxygenated compound synthesis catalyst, and reacting a gaseous effluent from the oxygenated compound synthesis reaction zone on an oxygenated compound conversion catalyst under oxygenated compound conversion reaction conditions to produce an LPG-rich gaseous effluent. The two-stage catalyst system can be part of a multi-stage catalyst system, wherein the two stages of the system are separate, or, if in a single reaction vessel, separated by a pacer element such as a heat exchange element.

[0059] The effluent containing oxygen-containing compounds from the oxygen-containing compound synthesis reaction zone can be heated by heat exchange, and the heated effluent is passed to the oxygen-containing compound conversion reaction zone to be converted into hydrocarbons, including LPG.

[0060] In an embodiment, an oxygen-containing compound synthesis reaction zone can be configured and operated to produce a MeOH-rich effluent stream. Under synthesis reaction conditions, the oxygen-containing compound synthesis reaction is carried out by contacting synthesis gas (e.g., biosynthesis gas) with a non-zeolite oxygen-containing compound synthesis catalyst. The synthesis reaction conditions include a first reaction zone temperature between about 200ºC and about 400ºC, or between about 220ºC and about 350ºC, or between about 240ºC and about 280ºC. The inlet pressure is between about 250 psi and about 1500 psi, or between about 400 psi and about 800 psi, or between about 600 psi and about 750 psi. The oxygen-containing compound synthesis catalyst comprises one or more oxygen-containing compound synthesis active metals selected from the group consisting of Cu, Zn, Zr, Al, Pt, Pd, and Cr. CuZnAlOx (with a Cu / Zn / Al molar ratio of approximately 6:3:1) and ZnCrAlOx (with a Zn / Cr / Al molar ratio of approximately 1:1:2) are two suitable examples of catalysts for the synthesis of oxygen-containing compounds. To promote the generation of MeOH in the reaction zone for the synthesis of oxygen-containing compounds, the reaction zone contains less than 5 wt% of a molecular sieve or zeolite catalyst.

[0061] When using an oxygen-containing compound synthesis catalyst having little or no molecular sieve component, more than 50 mol%, or more than 75 mol%, or more than 90 mol%, or even more than 95 mol% of the oxygen-containing compound in the effluent of the oxygen-containing compound synthesis reactor is MeOH. In embodiments, the synthesis reaction zone substantially does not contain molecular sieve or zeolite components. In one aspect, "substantially does not contain molecular sieve or zeolite components" is understood to mean that there is insufficient molecular sieve or zeolite component in the catalyst to have a measurable effect on the performance of the catalyst, and more particularly on the formation of dehydrated oxygen-containing compounds such as DME. In one aspect, using the combination of catalysts described herein for LPG production, the total CO conversion is from about 25% to about 45%.

[0062] Bio-based MeOH is an important commodity used in a variety of applications. Therefore, a portion, and in some cases a large portion, of the MeOH generated in the oxygen-containing compound synthesis reaction zone can be removed from the process, and only a portion of the synthesized MeOH is passed to the second conversion reaction zone to be converted into LPG.

[0063] All or part of the gaseous effluent, including MeOH, from the synthesis reaction zone is directed to the conversion reaction zone. When available, additional MeOH from external sources, including additional bio-based MeOH, can be added to the feed to the oxygenated compound conversion reaction zone. Similarly, ethanol, ethylene, and propylene from microbial fermentation of biomass substrates can further promote the production of bio-based LPG.

[0064] Alternatively, some oxygen-containing compounds present in the synthesis effluent may be removed from the effluent and purified for other uses before the remainder of the synthesis effluent is directed to the conversion reaction zone.

[0065] Before the syngas effluent is fed into the conversion reaction zone, the effluent may be preheated to match the operating temperature of the conversion reaction zone, including, for example, by heating the syngas effluent to a temperature of about 280ºC to about 500ºC, or about 300ºC to about 475ºC, and then feeding it into the conversion reaction zone.

[0066] Although the gaseous reaction stream flowing to the conversion reaction zone includes varying amounts of MeOH, it also includes unreacted syngas components H2, CO and CO2, hydrocarbons, and byproduct water. A portion of the hydrocarbons in the reaction stream is supplied in the recirculation stream. Most of the components in the effluent from the synthesis reaction zone, except for H2 and MeOH, are inert under the conditions of oxygen-containing compound conversion reactions. Any water, hydrocarbons, CO, and CO2 present in the effluent will undergo minimal (if any) reactions that alter the properties of the inert material as it passes through the conversion reaction zone. Therefore, the entire effluent from the synthesis reaction zone can be routed to the conversion reaction zone to convert oxygen-containing compounds in the effluent into hydrocarbons, including LPG.

[0067] In some applications of the method of the present invention, a portion of the inert material included in the effluent is removed before the remaining effluent is passed to the conversion reaction zone. For example, the presence of one or more inert components can alter the concentration of the reaction when passed to the synthesis reaction zone, and thereby reduce the reaction rate of the synthesis reaction.

[0068] In an embodiment, the method includes converting oxygen-containing compounds formed in the oxygen-containing compound synthesis reaction zone into paraffinic hydrocarbons, including C3 and C4 alkanes. The conversion reaction includes dehydration of the oxygen-containing compounds and saturation of olefins formed during dehydration, while limiting the water-gas shift reaction that converts available carbon in the reaction mixture into CO2.

[0069] In one aspect, the oxygen-containing compound conversion catalyst comprises a molecular sieve or a zeolite. Non-limiting molecular sieves suitable for carrying out oxygen-containing compound conversion in the conversion reaction zone to produce LPG-rich gaseous effluents include SSZ-13, SAPO-18, SAPO-34, β-zeolite, ZSM-5, and γ-zeolite. In embodiments, the oxygen-containing compound conversion catalyst converts all oxygen-containing compounds such that they are at low or undetectable levels in the effluent. This simplifies the recovery of desired LPG products.

[0070] On the other hand, the molecular sieve or zeolite component of the catalyst in the conversion reaction zone is characterized by a SiO2:Al2O3 ratio ranging from 10 to 90. The conversion catalyst can be composited in a particulate alumina matrix and used as spheres or extrusions in the reaction zone, the parts having a cross-sectional diameter of 1 / 32 inch to 1 / 4 inch. The extrusions can be shaped into a tri-lobed form or similar form to better access the interior portion of the extrusion while maintaining mechanical strength.

[0071] In another aspect, the oxygen-containing conversion catalyst contains very little (if any) metallic material that is active in the catalytic water-gas shift reaction. Metals that contribute to the water-gas shift activity of the conversion catalyst include Fe, Cu, Zn, Pt, and Pd. The oxygen-containing conversion catalyst in the method of the present invention contains less than 5 wt% of these metals, or less than 1 wt% of these metals, alone or in combination. In embodiments, the oxygen-containing conversion catalyst substantially does not contain water-gas shift active metal components. In one aspect, "substantially does not contain water-gas shift active metal components" is understood to mean that there is insufficient metal component in the catalyst to have a measurable effect on the performance of the catalyst, and more particularly on the WGS activity of the catalyst.

[0072] Oxygen-containing compound conversion reactions are typically carried out at temperatures of about 280ºC to about 500ºC or about 300ºC to about 475ºC. On the other hand, the temperature of the gaseous feed entering the oxygen-containing compound conversion zone is at least 50ºC higher than the temperature of the gaseous feed entering the oxygen-containing compound synthesis zone. The pressures in both reaction zones can be the same, allowing for some pressure drop between the reactors. Therefore, the oxygen-containing compound conversion reaction zone can be operated at pressures of about 250 psi to about 1500 psi, or about 400 psi to about 800 psi, or even about 600 psi to about 750 psi.

[0073] The gaseous effluent from the conversion reactor includes H2O, H2, CO, CO2, LPG, inerts (such as N2), and C2- and C5+ hydrocarbons. The separation and recovery of high-purity LPG involves a separation sequence including one or more separation steps. Liquid absorbents, such as those used in sponge oil absorption processes, can be used to recover most (if not all) of the LPG contained in the effluent. Solid absorbents, such as those used in pressure swing adsorption (PSA) processes, can be used to remove C2- hydrocarbons from the recirculated stream generated from liquid absorption processes. Examples of liquid absorption processes include, but are not limited to, sponge oil absorption processes.

[0074] Sponge oil absorption is a well-established commercial process that removes relatively heavier gaseous hydrocarbons from lighter gaseous hydrocarbons in a gas mixture by contacting the gas mixture with a hydrocarbon liquid (lean liquid) at elevated pressure and relatively low temperature in an absorption zone. The heavier gaseous hydrocarbons are preferentially absorbed in the hydrocarbon liquid. The liquid hydrocarbon containing dissolved heavier gaseous hydrocarbons is called the rich liquid. The rich liquid is then processed in a desorption zone at a temperature higher than that in the absorption zone and a pressure lower than that in the absorption zone. This desorbs the adsorbed heavier gaseous hydrocarbons from the rich liquid, forming a lean liquid, which is then recycled back to the absorption zone. A key characteristic of the hydrocarbon liquid is that it should be fluid under absorption zone conditions and should not have significant partial volatilization under desorption zone conditions. Various hydrocarbon liquids can be used, including kerosene, diesel, jet fuel, heavy naphtha, n-hexadecane, and light cycle oil. Non-limiting examples are US2930752A, US3477946A, or US7107788B2, the contents of which are incorporated herein by reference.

[0075] Pressure swing adsorption (PSA) is a commercial process for separating hydrogen and hydrocarbon gases. In this application context, the adsorption unit should preferably adsorb hydrocarbons (methane, ethane, propane, butane, C5+) without adsorbing large amounts of hydrogen. In this way, hydrocarbons are removed from the recirculated gas stream without a significant reduction in pressure of the hydrogen-rich recirculated gas stream. Maintaining the pressure of the hydrogen-rich recirculated gas minimizes the recompression cost of returning the vapor to the inlet of the bio-oxygenated compound synthesis process. The adsorbed hydrocarbons are desorbed and sent to a deethanizer. C3+ hydrocarbons are recovered as products, and methane and ethane are used as fuel or feed into a biogas reformer. The adsorber will use molecular sieves, typically 5A molecular sieves or carbon molecular sieves. The adsorption unit will optionally include a dehydrator upstream of the adsorption unit. Two types of dehydrators exist: glycol dehydrators and molecular sieve dehydrators.

[0076] Depropanizers, depropanizers, and debutanizers are used in gas processing plants to separate individual hydrocarbons. These are continuously operating distillation columns in which the hydrocarbons mentioned are removed as an overhead stream. The overhead stream in each of these processes will require partial condensation to provide reflux to the column. For this condensation to occur, it is preferable to operate these columns at sufficient pressure so that condensation can be carried out using cooling water instead of a refrigerant.

[0077] In an embodiment, the method of the present invention includes removing liquid water from an LPG-rich gaseous effluent and recovering the dehydrated gaseous effluent. The gaseous reactor effluent typically contains 10 mol%-30 mol% water vapor, which can be at least partially removed by cooling the effluent below the water condensation temperature without condensing the hydrocarbons in the effluent. Therefore, one of the separation steps may involve adequately cooling the effluent stream to condense the liquid aqueous phase and removing the aqueous phase from the dehydrated gaseous effluent in a two-phase separator (commonly referred to as a "water separator") for disposal, recycling to the method of the present invention, or other uses. A portion of the CO2 in the gaseous effluent stream may also be absorbed in the aqueous liquid phase and removed from the gaseous stream.

[0078] Alternatively, the effluent leaving the conversion reaction zone can be sufficiently cooled to condense a portion of the hydrocarbon products in the effluent. The cooled effluent mixture can then be separated in a three-phase separator, from which a liquid aqueous phase, a liquid hydrocarbon phase, and a dehydrated gaseous phase comprising H2O, CO, CO2, and the remaining hydrocarbons can be recovered. The liquid aqueous phase from the three-phase separator, comprising a portion of the CO2 present in the gaseous effluent, can be prepared for disposal, recycling to the method of the present invention, or for other uses. LPG in the liquid hydrocarbon phase is typically separated from the remaining hydrocarbons in the liquid phase by fractionation. The dehydrated gaseous phase comprises CO, CO2 and H2, LPG, C2- hydrocarbons (e.g., methane and ethane), and C5+ hydrocarbons (primarily pentane with reduced amounts of higher hydrocarbons).

[0079] In an embodiment, the method includes contacting at least a portion of the dehydrated gaseous effluent with a liquid absorbent solvent in an absorption zone, and recovering the LPG-rich liquid and a recycle stream with reduced LPG content. The method further includes contacting at least a portion of the dehydrated gaseous effluent with a liquid absorbent solvent in the absorption zone at a relatively high pressure and / or a relatively low temperature, recovering the LPG-rich liquid and the recycle stream with reduced LPG content, and desorbing the absorbed hydrocarbons from the LPG-rich liquid at a relatively low pressure and / or a relatively high temperature. The desorbed hydrocarbons may be fractionated to recover at least a light gas fraction including CO, CO2, H2, C2- hydrocarbons, and an LPG fraction; and the recovered light gas fraction is passed to a reforming reaction zone. An exemplary solvent absorption process is referred to as "sponge oil absorption".

[0080] In one embodiment, the method includes contacting at least a portion of the dehydrated gaseous effluent with a liquid absorbent solvent in an absorbent zone at a pressure of about 250 psi to about 1500 psi, wherein unadsorbed components are returned to the recirculation stream at the same pressure range. Therefore, the recirculation stream leaving the absorbent zone requires minimal compression when it returns to the reforming reaction zone.

[0081] In one aspect, the solvent absorption process removes a significant amount of C3+ hydrocarbons (e.g., propane, butane, pentane) from the dehydrated gaseous effluent. In some cases, a smaller fraction of CO2 may also be removed, with little or no removal of H2, CO, and methane. The removed CO2 may be recycled to one or more reaction zones, discharged, or captured for other uses and other disposal options. The solvent substrate containing dissolved hydrocarbons is recycled to a region with relatively low pressure and / or high temperature, where the absorbed hydrocarbons are removed as a gaseous component for further downstream separation to recover high-purity LPG. The lean-hydrocarbon organic solvent substrate is then recycled back to the absorption step. The organic solvent substrate that can be used for solvent absorption may be selected based on aromaticity. When low aromaticity is required, nC16 alkanes may be selected. For higher aromaticity, kerosene will be an optional solvent. For relatively high aromaticity, light cycle oil will be an optional solvent. All are readily available from standard refinery operations.

[0082] In an embodiment, the method includes separating the dehydrated gaseous effluent into a venting stream and a recirculation stream; contacting the venting stream with a liquid absorbent solvent in an absorption zone and recovering the LPG-rich liquid and the unabsorbed gaseous stream with a reduced LPG content; heating the LPG-rich liquid to evaporate at least a portion of the LPG contained therein; recovering LPG; returning the unabsorbed gaseous stream to the recirculation stream; and directing the recirculation stream to an oxygen-containing compound synthesis reaction zone.

[0083] In some embodiments, the entire dehydrated gas phase is contacted via a solvent absorption separation step. In other embodiments, only a portion of the dehydrated gas phase is treated by removing a purge stream from the dehydrated gas phase to contact with a liquid absorbent solvent. The C3+ hydrocarbons and CO2 in this purge stream are absorbed by a circulating organic solvent and recovered as desired products (e.g., LPG), as byproducts (e.g., C5+ hydrocarbons), or as components returned for recycling (e.g., CO2, CO, and H2). In embodiments, the purge stream treated in the organic solvent absorber represents 10% to 90% of the dehydrated gaseous effluent, or 20% to 80%, or 30% to 70%, or even 40% to 60% of the dehydrated gaseous effluent.

[0084] In one aspect, removing the purge stream from the dehydrated gas phase to separate hydrocarbons has the effect of increasing the amount of residual LPG in the dehydrated gas phase while reducing CO2 loss in the absorber. However, it has been surprisingly found that removing LPG from only a portion of the purge stream rather than from the entire gas phase has several unexpected benefits. In one aspect, separating only the purge stream reduces CO2 loss throughout the process, allowing more recycled CO2 to be used for controlling reforming and oxygen-containing compound synthesis reactions, and reducing CO2 loss to the atmosphere. Furthermore, although removing only a portion of the LPG in the gas phase leads to an increase in the LPG content in the loop, the net effect on the reaction zone size and the synthesis and conversion reactions is minimal. In addition, we have found that the recycled LPG is largely inert for both the synthesis and conversion reactions, so that the LPG neither changes during the reaction nor contributes to the reaction in any substantial way. Furthermore, using an organic solvent absorber to contact only the purge stream of the dehydrated effluent allows for the use of smaller compressors and absorbers, thereby reducing equipment and operating costs. Therefore, although a portion of the LPG generated in the recycling conversion reaction leads to an increase in reactor size to accommodate the additional flow, the increase in size was found to be minimal. In fact, passing only a portion of the dehydrated gas phase through the solvent absorption process during each cycle improved the hydrocarbon removal efficiency in the absorption process while reducing the required size of the absorption process.

[0085] The organic solvent absorption separation described above can only remove a portion of the C2-hydrocarbon byproducts, primarily methane and ethane (if any). To avoid the accumulation of byproducts in the gaseous effluent, a solid adsorption process (also known as "pressure swing adsorption" or PSA process) can be implemented to remove a significant portion of these C2-hydrocarbon byproducts from the recycle stream. Therefore, the method may include the following steps: adsorbing C2-hydrocarbons, CO, and CO2 onto a solid adsorbent at an adsorption pressure above 400 psi; separating a light fraction, including the adsorbed C2-hydrocarbons, CO, and CO2, from the solid adsorbent at a pressure at least 25 psi below the adsorption pressure; recycling the light fraction; and returning unadsorbed H2 to the recycle stream at a pressure above 400 psi. For example, the recycle stream can be used as a blending component for biogas feed to a reforming reaction zone, as a blending component for syngas feed to a first reaction zone, or as fuel for internal or external applications.

[0086] Hydrogen is a valuable component of the recirculation stream, and the separation alternative was chosen to retain most (if not all) of the hydrogen in the reaction and recirculation loops. Small amounts (e.g., less than 10 mol% or less than 1 mol%) of hydrogen are removed from the dehydrated gaseous effluent in a liquid absorbent solvent. In the solid adsorption process, most components in the recirculation stream that come into contact with the adsorbent are adsorbed by the solid adsorbent and removed from the recirculation stream. The exception is H2; more than 80 wt% of hydrogen in the purge stream is returned to the recirculation stream. Furthermore, since the solid adsorbent essentially only rejects H2, the unadsorbed components returned to the recirculation stream include more than 90 mol% of H2.

[0087] For the process of producing LPG in which the PSA process is the only separation process for recovering LPG from the reaction effluent, there may be a silica bed for removing C3+ hydrocarbons and an activated carbon bed for removing C2- hydrocarbons prior to it. Alternatively, the PSA module may operate together with the solvent absorption process before or after solvent absorption. In an embodiment, the method includes contacting at least a portion of the recirculation stream with a solid adsorbent to adsorb at least a portion of the unreacted biosynthetic gas components (including CO and CO2), and at least a portion of the C2- hydrocarbons contained therein, and returning the unadsorbed H2 to the recirculation stream. Typical adsorbents that can be used include one or more of zeolite molecular sieves, activated carbon, silica, alumina, and synthetic resins.

[0088] In one aspect, the method includes contacting the entire recirculated stream from the hydrocarbon liquid absorption process step with a solid adsorbent. In another aspect, the method includes contacting a purge stream comprising a portion (e.g., about 5% to about 50%, or about 10% to about 40%) of the recirculated stream from the solvent absorption zone with the solid adsorbent and adsorbing at least a portion of the C2-hydrocarbons, CO, and CO2 contained therein.

[0089] Except for H2, most components in the purge stream are adsorbed by the solid adsorbent and removed from the purge stream. Therefore, more than 80% by weight of hydrogen in the purge stream can be returned to the recirculation stream. Furthermore, H2 can account for more than 90 mol% of the unadsorbed components returned to the recirculation stream. Thus, the purge stream is removed from the recirculation stream at a pressure of approximately 250 psi to approximately 1500 psi, and the unadsorbed components are returned to the recirculation stream within the same pressure range. At least a portion of the CO, CO2, methane, and ethane adsorbed in the PSA separation is recovered as low-pressure gases. These low-pressure gases can be compressed and sent to the reforming reaction zone.

[0090] In another aspect, the method includes treating a first vent separated from a dehydrated gaseous effluent in a liquid absorption zone, and treating a second vent separated from an unabsorbed component of the first vent in a solid adsorption zone.

[0091] The method for producing bio-LPG products is configured and operated to produce LPG (propane and / or butane in varying proportions) as the main hydrocarbon product. Other hydrocarbons that may be produced include one or more of methane, ethane, and C5+ hydrocarbons (primarily pentane). Separating the hydrocarbon product into purified component products typically involves fractionation. The fractionation series may consist of multiple distillation columns connected in series, including a deethanizer, a depropanizer, a debutanizer, and a butane splitter. The overhead product of the deethanizer is methane and ethane, and one or more of CO, CO2, and H2, depending on the process conditions. The bottoms of the deethanizer may be fed to the depropanizer. The overhead product of the depropanizer is propane, and the bottoms are fed to the debutanizer. The overhead product of the debutanizer is a mixture of n-butane and isobutane, and the bottoms are a C5+ gasoline mixture.

[0092] The operation and catalyst selection of the oxygen-containing compound conversion reaction zone can facilitate a simpler fractionation series. For example, using a molecular sieve catalyst in the oxygen-containing compound conversion reaction zone can limit the relative amount of C5+ hydrocarbons present in the hydrocarbon product stream. Using small-pore molecular sieves, such as SSZ-13, can further limit the production of C5+ hydrocarbons. Under these and similar conditions, the hydrocarbon fractionation series can be limited to a single deethanizer column, where the gaseous overhead is recycled to the reforming reaction zone, and the deethanizer bottoms are recovered as LPG for use as fuel, such as Autogas, a widely recognized transport "green fuel" for reducing CO2 emissions.

[0093] The technical advantages of this invention include reduced water formation in the presence of a methanol synthesis catalyst. Less water means reduced catalyst contamination (longer lifespan) and lower CO2 yield. The procedure of this invention involves careful management of water formation. In the water-gas shift reaction (WGS), water reacts with CO (valuable) to form CO2 (a potential greenhouse gas) in the presence of a metal catalyst. The syngas-to-methanol reaction involves water as a reaction intermediate, but its net formation is not large, and the net WGS reaction is negligible. However, the molecular sieve in the first stage (according to the prior art) reacts with methanol to form dimethyl ether and water, which has a detrimental WGS effect. Therefore, there is no molecular sieve in the first stage. The second stage converts methanol to hydrocarbons, with water as a byproduct. If a metal catalyst (as in the prior art) is not used in the second stage, water does not participate in WGS, and CO is retained for recycling after the second stage. The WGS reaction can also be carried out via a non-catalytic mechanism involving only gaseous substances. However, the amount of CO converted by this reaction is much less than that would be produced if a catalyst were present.

[0094] Exemplary embodiments of the method utilizing recycling in the production of LPG products can be understood through the following description and with reference to the accompanying drawings 1-5. These figures present an illustration of the method involving certain operating principles. For ease of explanation and understanding, these figures provide a simplified overview, and the elements depicted are not necessarily drawn to scale. Valves, instruments, and other devices and systems that are not essential for understanding various aspects of the invention are not shown. It will be readily apparent to those skilled in the art that the method for producing LPG via the reaction disclosed herein can have alternative configurations and elements controlled by specific operational objectives, but these alternatives are still within the scope of the invention.

[0095] Referring to Figure 4, biogas 1 can be generated from bio-based sources, including gaseous products such as anaerobic bacterial digestion, composting, biomass gasification, pyrolysis or hydropyrolysis, landfill gas, or electrochemical reduction of carbon dioxide. Bio-based biogas 1 can be combined with at least a portion of a first light fraction 58 comprising unreacted H2, CO, CO2, and C2-hydrocarbons from separation zone 40 and / or a second light fraction 62 from solid adsorbent separation zone 50, and the mixture 2 is passed to reforming reaction zone 4 to be converted into biosynthetic gas 6.

[0096] Contaminants in the biogas, including sulfur compounds and / or CO2 exceeding the requirements of downstream processing, can be removed from the biogas via stream 7. Sulfur in the biogas can be removed to low levels, typically down to ppm levels, for example, using one or more of aqueous scrubbing, alkanolamine absorption, molecular sieve adsorption, selective catalytic oxidation, and hydrodesulfurization. Excess CO2 can be removed from the biogas in conjunction with sulfur removal. Additional CO2 can be removed by membrane separation, cryogenic distillation, or aqueous absorption, which involves contacting the biogas with water to dissolve CO2, separating the water / CO2 mixture, removing CO2 from the mixture by increasing the temperature and / or decreasing the pressure of the mixture, and recirculating the water. CO2 can also be partially removed from the biogas by aqueous absorption into condensed water. In some aspects, CO2 can also be removed from the product syngas 6 via stream 3 to control the CO2 / CO ratio.

[0097] In the reforming reaction zone 4, methane-containing biogas 2 is converted into biosynthetic gas 6 comprising CO, CO2, H2O, and H2 at a pressure of approximately 200 psi to approximately 600 psi (14-40 bar), with an outlet temperature ranging from 815 to 925ºC. The reforming reaction can be carried out on a shaped nickel-alumina catalyst.

[0098] The H2 / (CO+CO2) ratio in the biosyngas product 6 leaving the reformer is customized to meet the requirements of downstream processing. Therefore, the composition of the biogas feed to the reformer, including the amount of CO2 and H2O included in the biogas feed, can be modified to achieve the desired H2 / (CO+CO2) composition of the biosyngas product 6 using reforming and / or water-gas shift reaction.

[0099] A bio-based syngas 8, comprising fresh biosyngas 6 and hydrogen-rich recycle gas 12 and having an H2 / (CO+CO2) molar ratio between 2 and 3, is introduced into a synthesis reaction zone 10 for reacting the bio-based syngas 8 over a non-zeolite methanol synthesis catalyst (i.e., without molecular sieve components) in the synthesis reaction zone 10, which contains an oxygen-containing compound synthesis catalyst, to form a first effluent 14 rich in MeOH, thereby synthesizing a gaseous oxygen-containing compound including methanol.

[0100] Synthesis reaction zone 10 can be operated at a temperature of about 220ºC to about 350ºC and a pressure of about 250 psi to about 1500 psi. The first effluent from synthesis reaction zone 10, bringing the reaction to the reaction zone temperature, can be heated in heating zone 16 to a temperature of about 280ºC to about 500ºC or about 300ºC to about 475ºC, and the heated effluent 18 is directed to hydrocarbon conversion zone 20, where oxygen-containing compounds in the effluent are converted into hydrocarbons, including LPG. The hydrocarbon conversion zone is operated at a pressure of about 250 psi to about 1500 psi. In an embodiment, the conversion zone is operated at a pressure of about 700 psi to about 1500 psi. Operating conditions for reducing the olefin content in the effluent from the conversion zone may include a pressure of 750 psi to 950 psi, such that the bio-based LPG contains 0.5-10%, 1-7%, 1-5%, or 2-5% olefins.

[0101] The pressures in the synthesis reaction zone and the hydrocarbon conversion zone should be identical, with a small allowable pressure drop between the reactors (less than 50 psig). Examples of pressure ranges for both are 250-1500, 725-1000, or 750-950. If the pressure is too low, the syngas conversion rate will be low, and the overall reaction rate will also be low. If the pressure is too high, the cost of the process will increase.

[0102] Exemplary temperature ranges for the synthesis reaction zone are 200-400, 220-350, and 240-280-250-270. If the temperature is too low, the reaction rate will be too slow. If the temperature is too high, the catalyst will be contaminated quickly, and the yield of methane byproducts will increase.

[0103] Exemplary temperature ranges for the hydrocarbon reaction zone are 280-500, 300-475, and 310-425. If the temperature is too low, oxygen-containing compounds will be present in the effluent, which will complicate product recovery. If the temperature is too high, the catalyst will be contaminated, and the yield of methane byproducts will increase.

[0104] The LPG-rich second effluent 22 comprises LPG, byproducts C2- and C5+ hydrocarbons, water, and unreacted gases H2, CO, and CO2. Product recovery and unreacted gas recycling occur in a series of liquid and gaseous processing and separation processes. The second effluent 22, exiting the conversion reaction zone 20 as heated vapor, is cooled to condense at least a portion of the water vapor contained in the effluent.

[0105] The cooler 24 through which the effluent 22 passes operates under conditions such that at least a portion of the water vapor contained in the effluent is condensed into an aqueous phase 28 and removed as an aqueous product 28 in the separation tank 26 for disposal, recycling to the method of the present invention, or other uses. In one aspect, prior to separation, a portion of the CO2 in the second effluent is absorbed into the aqueous phase 28.

[0106] The LPG-rich third effluent 32 after water separation is passed to the solvent absorption zone 30 to remove C3+ hydrocarbons from the third effluent 32 by contacting the third effluent with the liquid absorption solvent in the absorption zone 30, absorbing the C3+ hydrocarbons from the third effluent, and producing a hydrocarbon-rich liquid absorbent and a hydrocarbon-poor fourth effluent 68, and recovering the bio-based LPG fraction 72.

[0107] The solvent absorption zone includes a contact zone 30A maintained at a relatively low absorbent temperature and / or a relatively high absorbent pressure, and a regeneration zone 30B maintained at a relatively high regeneration temperature and / or a relatively low regeneration pressure. In one aspect, the method includes absorbing at least a portion of the hydrocarbons in the third effluent into a liquid absorbent solvent in the absorption zone at a temperature below 50ºC and at a pressure of about 700 psi to about 1500 psi.

[0108] The hydrocarbons in the third effluent 32 entering the absorption zone are absorbed by the lean absorbent in the contact zone 30A. The resulting hydrocarbon-rich liquid absorbent is then passed to the regeneration zone 30B, where the hydrocarbons are desorbed and passed to the separation zone 40 to recover at least the first light fraction 58 containing C2- hydrocarbons, the bio-based LPG fraction 72, and the C5+ fraction 66 (if any).

[0109] A liquid absorbent is a liquid-phase material that remains liquid at the operating temperature of a solvent absorbent unit. In one aspect, the liquid absorbent is a hydrocarbon liquid in which C3+ hydrocarbons in the gaseous effluent are readily soluble. The liquid absorbent has a normal boiling point greater than 100ºC. In one aspect, the liquid absorbent is selected from the group consisting of nC16 paraffinic hydrocarbons, kerosine, and light cycle oil.

[0110] After separating the hydrocarbons from the liquid absorbent, the separated hydrocarbons are fractionated, and the bio-based LPG fraction is recovered. Separation zone 40 represents one or more fractionation steps, typically in a separate fractionator, for recovering purified LPG 72 and separating C2-hydrocarbons 58. CO and CO2 removed from the dehydrated gaseous effluent are separated in the first light fraction 58. When the hydrocarbon product stream 38 contains a significant amount of C5+ hydrocarbons, they can be separated from the LPG products in a deethaner column (not shown). In some cases, the overhead C2-hydrocarbon fraction 58 also contains unreacted syngas components, primarily CO and CO2, along with some H2. The overhead C2-hydrocarbon fraction 58 containing unreacted syngas components can be blended with the biogas stream 1 and directed to the reforming reaction zone 4. Alternatively, the overhead C2-stream can be used as fuel for internal or external applications.

[0111] After solvent absorption, the fourth effluent 68 includes unreacted gases CO, CO2, and H2, as well as unabsorbed C2-vapor phase hydrocarbons. The vent 52 separates a portion of the fourth effluent to further remove C2- hydrocarbons contained in the effluent and control the amount of hydrogen recycled to the synthesis reaction zone 10.

[0112] A solid-phase adsorbent separation zone 50 is provided for removing light gases, including C2-hydrocarbons, CO, and CO2, from the fourth effluent by: contacting at least a portion 52 of the fourth effluent with the solid adsorbent 50 to adsorb at least a portion of the light gases, desorbing the adsorbed gases, generating a second light gas stream 62, and returning unadsorbed H2 54 to the fourth effluent to form a recycle effluent 12. The solid adsorbent is selected to effectively remove C2-hydrocarbons from the vent stream.

[0113] Pressure Swing Adsorption (PSA) modules are suitable for removing hydrocarbons, CO, and CO2 from a purge stream by adsorption onto a selective adsorbent material (e.g., zeolite or activated carbon), while rejecting hydrogen. Using a PSA module involves adsorbing at least a portion 52 of C2-hydrocarbons, CO, and CO2 from a fourth effluent stream 68 onto a solid adsorbent at an adsorption pressure above 400 psi, and separating a second light fraction 62, comprising the adsorbed C2-hydrocarbons, CO, and CO2, from the solid adsorbent at a pressure at least 25 psi below the adsorption pressure. Unadsorbed H2 54 is returned to the first recirculation stream 68 at a pressure above 400 psi.

[0114] Recycling the unreacted syngas component involves minimal pressure drop, requiring only a small amount of recompression via compressor 56. Therefore, the method includes increasing the pressure of at least a portion of the hydrogen-rich recycle effluent 12 to the range of 5 psi to 50 psi and blending the pressurized hydrogen-rich recycle stream with bio-based syngas.

[0115] During operation in separation zone 40, at least one of PSA exhaust gas 62 and hydrocarbons 38 from absorption zone 30 is fed into separation zone 40 for LPG recovery. C2- hydrocarbons, CO, and CO2 are recovered as stream 58, which can be fed into reforming reaction zone 4. LPG produced in this manner contains only trace amounts of C5+ hydrocarbons, making it meet the standards for use as a European AutoGas fuel.

[0116] Separation zone 40, shown as a single container in FIG. 4, may include two or more fractionators, each fractionator separating different components of feed stream 66 by boiling point range. In embodiments, one of the fractionators may be described by art terms as a "deethanize," "depropanizer," or "debutanizer."

[0117] The selection of units for recovering LPG from the process and for recycling most of the unreacted reactants while minimizing CO2 loss in the system is based in part on the objective of operating the recycling system to minimize the recompression of the recycled components. Therefore, hydrocarbon products in the third effluent 32 are absorbed into the solvent and removed from the recycling stream, while unreacted gaseous components (H2, CO, and CO2) are retained by the absorbed solvent and returned to the recycling stream 68 with minimal pressure drop.

[0118] Similarly, the recycled components removed by the PSA are removed under high pressure and recovered under low pressure. The hydrogen 54 trapped by the PSA is returned to the recycle stream under high pressure, requiring minimal compression to compensate for the pressure loss through the recycle system.

[0119] Figure 5 illustrates one embodiment, including separating a vent from a third effluent for processing in a hydrocarbon solvent absorption process.

[0120] The LPG-rich gaseous (i.e., second) effluent 22 is directed to the separator 26 for separating the aqueous product 28 from the dehydrated gaseous (i.e., third) effluent 32.

[0121] The third effluent 32 includes LPG, C2- and C5+ hydrocarbon byproducts, and unconverted syngas components H2, CO, and CO2. According to the embodiment illustrated in Figure 5, the third effluent 32 is divided into a first vent stream 132 and a recycle stream 134. In this embodiment, the first vent stream 132 accounts for approximately 10% to approximately 90% of the third effluent 32.

[0122] The first vent is contacted with the liquid absorbent solvent in the hydrocarbon solvent absorption zone 130. The liquid absorbent solvent circulating in zone 130 is adapted to absorb at least a portion of the hydrocarbons from the first vent at the absorption temperature and absorption pressure, producing an LPG-rich liquid and an LPG-depleted gaseous fraction 138 comprising C2-hydrogens, H2, CO, and CO2. The LPG-rich gaseous fraction 136 is recovered from the LPG-rich liquid at a temperature above the absorption temperature and / or a pressure below the absorption pressure. The LPG-rich gaseous fraction 136 can be further fractionated to recover bio-based LPG.

[0123] A solid adsorption zone 150, such as a PSA separator, is provided to remove at least a portion of the C2-hydrogens (i.e., the second light fraction 62) present in the lean LPG gaseous stream 138. The C2-hydrogens 62 are valuable as feedstock for the reforming reaction zone or as fuel for internal and / or external applications. In this case, removing the C2-hydrogens reduces the amount of C2-hydrogens flowing to the catalytic reaction zone. In one aspect, the entire lean LPG gaseous stream 138 is directed to the solid adsorption zone.

[0124] In another aspect, the method includes (a) removing aqueous products from the second effluent 22 and producing a third effluent 32; (b) removing C3+ hydrocarbons from a third vent stream 132, which comprises about 10% to about 90% of the third effluent, by contacting at least a portion of the third vent stream with a liquid absorbent solvent in an absorption zone, absorbing C3+ hydrocarbons from the third vent stream, and producing a hydrocarbon-rich fraction 136 and a hydrocarbon-lean fraction 138, and recovering a bio-based LPG fraction; and (c) removing light gases, including C2- hydrocarbons, CO, and CO2, from a fourth vent stream, which comprises about 5% to about 50% of the hydrocarbon-lean fraction 138, by contacting the fourth vent stream with a solid adsorbent to adsorb at least a portion of the light gases, desorbing the adsorbed light gases, and producing a second light gas stream 62, and returning unadsorbed H2 140 to the fourth effluent 138; and (d) blending the resulting fourth effluent into a recirculation stream 134.

[0125] Most components in the purge stream are adsorbed by the solid adsorbent and removed from the purge stream. The exception is H2, which is essentially not removed. Therefore, more than 80 wt% of hydrogen in the purge stream is returned to the recirculation stream 134. And, since the solid adsorbent essentially only rejects H2, the unadsorbed components returned to the recirculation stream include more than 90 mol% of H2. Similarly, the purge stream 142 is removed from the recirculation stream at pressures from 250 psi to 1500 psi, and the unadsorbed components are returned to the recirculation stream within the same pressure range.

[0126] As described above, the PSA exhaust gas (i.e., the second light fraction) 62 removed from the second vent 142 and the C2-hydrocarbon fraction (reference numeral 58 in FIG. 4) separated from the LPG-rich gaseous fraction 136 contain C2-hydrocarbons as well as H2, CO, and CO2. Therefore, the PSA exhaust gas 62 and the C2-hydrocarbon fraction 58, as separate streams or as a blend of the two streams, can be used as a blending component for the syngas feedstock to the reforming reaction zone 4, or as fuel for internal or external applications.

[0127] Example

[0128] The following non-limiting embodiments illustrate the content and technical solutions of the present invention, but do not limit the scope of the present invention.

[0129] Example 1

[0130] The method of this invention was modeled using AspenTech software to evaluate the loss of syngas components H2, CO, and CO2 from the method for various recycling recovery options. In each case, the reaction conditions and process flow remained constant, except for the composition of the recycle stream. The data are summarized in Table 1.

[0131] Four methods were evaluated:

[0132] Run #1: The entire dehydrated gaseous effluent is contacted in the liquid absorption solvent zone to remove hydrocarbons. The resulting lean hydrocarbon recirculation stream is then contacted with a solid adsorbent to remove CO, CO2, and remaining hydrocarbons.

[0133] Run #2: The entire dehydrated gaseous effluent is contacted in the liquid absorption solvent zone to remove hydrocarbons. 10% of the resulting lean hydrocarbon recirculation stream is then removed from the recirculation stream as a release.

[0134] Run #3: The entire dehydrated gaseous effluent is contacted in the liquid absorption solvent zone to remove hydrocarbons. 10% of the resulting lean hydrocarbon recycle stream is then contacted with a solid adsorbent to remove CO, CO2, and the remaining hydrocarbons.

[0135] Run #4: 40% of the dehydrated gaseous effluent is contacted in the liquid absorption solvent zone to remove hydrocarbons. 25% of the lean hydrocarbon recirculation stream obtained from the absorption solvent zone is then contacted with a solid adsorbent to remove CO, CO2, and remaining hydrocarbons.

[0136] The data list in Table 1 shows the loss % for each syngas component for each run, based on the total effluent flow rate leaving the conversion reaction zone.

[0137] Table 1. Gas loss, % of flow to water separation Run #1 Run #2 Run #3 Run #4 H2 12% 10% 1% 1% CO 100% 12% 12% 11% CO2 100% 31% 31% 17%

[0138] The data in Table 1 illustrate that by purging only a portion of the recycle stream, rather than the entire recycle stream, the loss of unreacted syngas components in the recycle is reduced. PSA treatment of the purged stream and return of unadsorbed H2 has the additional benefit of retaining CO, CO2, and H2 in the recycle stream. Surprisingly, optimal results regarding gas loss were achieved when only a portion of the gaseous effluent was treated with a liquid absorption solvent, and only a portion of the lean hydrocarbon recycle stream from the liquid absorption was treated with a PSA process.

[0139] Example 2

[0140] In one aspect, methods for producing bio-based LPG involve operating under conditions that significantly reduce the amount of CO2 produced by the method. Therefore, reducing the reaction selectivity for CO2 formation is desirable. One reaction mechanism for CO2 production involves a water-gas shift reaction. Water vapor added to the feed in the oxygenated compound synthesis stage, or water produced by the reaction occurring in the oxygenated compound synthesis reaction, readily reacts with CO in the reaction stage to form CO2, rather than CO being hydrogenated into the desired LPG product.

[0141] Run #5-7 was evaluated.

[0142] Operation #5 involves converting CO in syngas into LPG in a single-stage reaction zone containing an oxygen-containing compound synthesis catalyst and an oxygen-containing compound conversion catalyst as a combined catalyst. During the reaction under this reaction scheme, water is generated via a water-gas shift reaction catalyzed by the metal component of the methanol synthesis catalyst. In this configuration, CO is converted into oxygen-free hydrocarbons, and one mole of CO converted forms one mole of water. The water formed by the reaction promotes the sintering of the catalyst, and the metal on the catalyst leads to the formation of CO2 via the water-gas shift reaction.

[0143] Operation #6 involves a two-stage reaction zone configuration. In the first stage, methanol synthesis catalyst and methanol dehydration catalyst are used to produce DME in the first-stage effluent. The DME synthesized in the first stage is converted to LPG over a zeolite catalyst in the second stage. This two-stage configuration improves the per-pass conversion of carbon monoxide, but for every mole of CO converted to DME, ½ mole of water is formed in the first reactor.

[0144] Run #7 illustrates a method of the present invention. Run #7 involves a two-stage reaction zone configuration in which a methanol synthesis catalyst is used in a first stage and a methanol conversion catalyst is used in a second stage. The first stage does not contain a molecular sieve component, and the product of the first-stage reactor is almost entirely methanol. Furthermore, there is no significant water formation per mole of carbon monoxide converted. The second stage contains a zeolite catalyst that does not contain any metal components with water-gas shift activity. In this configuration, CO is converted to hydrocarbons without exceeding the water formation that occurs as a short-lived intermediate in methanol synthesis. It was found that this intermediate water has little or no effect on catalyst sintering or CO loss via the water-gas shift reaction.

[0145] The data listed in Table 2 illustrate the superior performance of the method of the present invention regarding water formation during the reaction in the oxygen-containing compound synthesis stage.

[0146] Table 2: Water formation in the oxygen-containing compound synthesis reaction zone Reactor configuration catalyst The number of mol of H2O formed per mol of CO conversion Phase 1 Phase Two Run #5 single phase Cu / ZnO / Al2O3+ β-zeolite 1 Run #6 Two-stage Cu / ZnO / Al2O3+ ZSM-5 SSZ-13 0.5 Run #7 Two-stage Cu / ZN / Al2O3, without molecular sieve components SSZ-13 does not contain WGS active metal components. There is very little or no excess H2O produced.

[0147] As shown in Table 1, contacting 100% of the gaseous effluent with both sponge oil absorption and PSA adsorption resulted in the complete loss of CO and CO2. Contacting 100% of the gaseous effluent with sponge oil absorption and then removing 10% of the gaseous recirculation reduced H2 loss to 10%, CO loss to 12%, and CO2 loss to 31%. With 10% of the recirculated stream being directed to PSA adsorption, H2 loss was reduced to 1%. The lowest levels of H2, CO, and CO2 losses were observed when the sponge oil absorption treatment was applied to 40% of the gaseous effluent purge stream, followed by PSA adsorption treatment on 25% of the resulting recirculated stream. This data clearly illustrates the benefits of using sponge oil treatment and PSA treatment with gaseous recirculation for recovering syngas components from the recirculated gas. The data also illustrate the additional benefits of using these two treatment steps to treat only a portion of the gaseous recirculation. [Simplified Explanation of the Diagram]

[0021] Figure 1 illustrates an embodiment of the present invention describing a method for producing bio-based LPG.

[0022] Figure 2 illustrates another embodiment of the present invention describing a method for producing bio-based LPG.

[0023] Figure 3 illustrates yet another embodiment of the present invention describing a method for producing bio-based LPG.

[0024] Figure 4 illustrates a system and method for synthesizing bio-based LPG from syngas derived from bio-based sources.

[0025] Figure 5 further illustrates an embodiment of the present invention for recovering LPG and other reaction products and byproducts from gaseous reaction effluents.

Claims

1. A method for producing bio-based liquefied petroleum gas (LPG), comprising the following steps: Step 1) reacting a blend of bio-based syngas comprising CO, CO2, and H2 in an oxygenated compound synthesis reaction zone containing an oxygenated compound synthesis catalyst to form a first effluent comprising oxygenated compounds and unreacted bio-based syngas, wherein the oxygenated compounds in the first effluent comprise at least 50 mol% methanol; Step 2) reacting at least a portion of the first effluent in an oxygenated compound conversion reaction zone containing an oxygenated compound conversion catalyst to form a second effluent comprising C2-hydrocarbons, bio-based LPG, and C5+ hydrocarbons; Step 3) separating at least a portion of the hydrocarbons comprising C2-hydrocarbons, bio-based LPG, and C5+ hydrocarbons from the second effluent to form a recycle effluent; Step 4) blending at least a portion of the recycle effluent with fresh bio-based syngas to form the blended bio-based syngas; Step 5) removing aqueous products from the second effluent to produce a third effluent; and Step 6) C3+ hydrocarbons are removed from the third effluent by contacting at least a portion of the third effluent with a liquid absorbent solvent in an absorption zone to absorb the C3+ hydrocarbons from the third effluent, producing a hydrocarbon-rich fraction and a hydrocarbon-poor fourth effluent, and recovering the bio-based LPG fraction.

2. The method as described in claim 1, wherein, The bio-based syngas comprising CO, CO2 and H2 described in step 1) is prepared by contacting a biogas comprising biomethane with an oxidizing gas selected from O2, CO2 and H2O or combinations thereof in a reforming reaction zone under reforming reaction conditions.

3. The method as described in claim 1, wherein, The oxygen-containing compound synthesis catalyst includes one or more active metals selected from the group consisting of Cu, Zn, Zr, Al, Pt, Pd, Rh, Ru and Cr, wherein the oxygen-containing compound synthesis catalyst does not contain molecular sieve components.

4. The method as described in claim 1, wherein, The oxygen-containing compound conversion catalyst contains less than 1% by weight of water-gas shift (WGS) active metal component.

5. The method as described in claim 4, wherein, The oxygen-containing compound conversion catalyst contains virtually no WGS active metal components.

6. The method as described in claim 3, wherein, The oxygen-containing compound conversion catalyst includes zeolites having a SiO2 / Al2O3 molar ratio of less than 90.

7. The method as described in claim 3, wherein, The oxygen-containing compound conversion catalyst includes zeolites having a SiO2 / Al2O3 molar ratio of less than 30.

8. The method as described in claim 1, wherein: 1) The bio-based synthesis gas in the oxygenated compound synthesis reaction zone in step 1) is reacted at a reaction temperature of about 220ºC to about 350ºC and a pressure of about 700 psi to about 1500 psi; and 2) A portion of the first effluent in the oxygenated compound conversion reaction zone in step 2) is reacted at a reaction temperature of about 280ºC to about 500ºC and a pressure of about 700 psi to about 1500 psi.

9. The method as described in claim 8, wherein, The reaction temperature in the oxygen-containing compound conversion reaction zone is at least 25°C higher than the reaction temperature in the oxygen-containing compound synthesis reaction zone.

10. The method as described in claim 8, wherein, A portion of the first effluent is reacted in the oxygen-containing compound conversion reaction zone at a pressure of approximately 750 psi to approximately 1500 psi.

11. The method as described in claim 1, further comprising: Step 7) Remove light gases including C2-hydrocarbons, CO and CO2 from the fourth effluent by contacting at least a portion of the fourth effluent with a solid adsorbent to adsorb at least a portion of the light gases, desorbing the adsorbed gases and generating a second light gas stream, and returning unadsorbed H2 to the fourth effluent to form the recycle effluent.

12. The method as described in claim 1, further comprising separating the hydrocarbon from the liquid absorption solvent; fractionating the hydrocarbon and recovering the bio-based LPG fraction.

13. The method as described in claim 1, wherein, In step 6), at least a portion of the hydrocarbons in the third effluent are absorbed into the liquid absorption solvent in the absorption zone at a temperature below 50ºC and a pressure of about 700 psi to about 1500 psi.

14. The method as described in claim 1, wherein, The liquid absorption solvent is selected from nC16 alkanes, kerosene, and light circulating oil.

15. The method as described in claim 11, further comprising: Step 8) At an adsorption pressure higher than 400 psi, the light gas from the fourth effluent is adsorbed onto the solid adsorbent; Step 9) Separate the adsorbed C2-hydrocarbons, CO, and CO2 from the solid adsorbent at a pressure at least 25 psi below the adsorption pressure; and Step 10) Return the unadsorbed H2 to the fourth effluent at a pressure above 400 psi.

16. The method as described in claim 15, further comprising, after step 9), directing at least a portion of the adsorbed C2-hydrocarbons, CO and CO2 to the reforming reaction zone.

17. The method as described in claim 11, further comprising contacting a first vent stream comprising about 10% to about 90% of the third effluent with the liquid absorbent solvent, absorbing C3+ hydrocarbons from the first vent stream, and at least recovering the bio-based LPG fraction.

18. The method as described in claim 11, further comprising contacting a second effluent comprising about 5% to about 50% of the fourth effluent with the solid adsorbent to adsorb C2-hydrocarbons, CO and CO2 from the second effluent, and returning unadsorbed H2 to the recycled effluent.

19. The method as described in claim 1, wherein, C3+ hydrocarbons account for 10% to 90% of the third effluent; and C2- hydrocarbons, CO, and CO2 account for about 5% to about 50% of the lean hydrocarbon fraction.

20. The method as described in claim 19, further comprising guiding the second light gas stream to a reforming reaction zone to produce a recyclable blend of bio-based syngas.

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