Process for the synthesis of methanol

Abstract

The present disclosure relates to a process for the synthesis of methanol with low carbon dioxide emissions. A purge gas stream from the methanol circuit is treated in a purge gas treatment unit by steps of reforming, water gas shift, and carbon dioxide removal to form a hydrogen stream that is used as a fuel, such as a process stream within a heated reforming operation, the methanol circuit, and / or the purge gas treatment unit. The present disclosure also relates to a method of retrofitting a methanol production unit by installing a purge gas treatment unit as described.

Description

Technical Field

[0001] The present invention relates to a method for synthesizing methanol, in particular to a method for synthesizing methanol having the following characteristics: carbon dioxide emissions from the method are low. Background Art

[0002] Methanol synthesis is typically carried out by passing a synthesis gas containing hydrogen and carbon monoxide and / or carbon dioxide through one or more beds of a methanol synthesis catalyst, typically a copper-containing composition, at elevated temperature and pressure in a synthesis reactor. Crude methanol is typically recovered by cooling the product gas stream below its dew point and separating the product as a liquid. The crude methanol is typically purified by distillation. This process typically operates in a loop, so that unreacted gas can be recycled to the synthesis reactor as part of the feed gas via a recycle unit. Fresh synthesis gas (referred to as make-up gas) is added to the recycled unreacted gas to form the feed gas stream. A purge stream is removed from the recycle gas stream to prevent the accumulation of inert gases in the loop.

[0003] There are various methods for recovering hydrogen from the purge gas and reusing it in the methanol synthesis step, especially for hydrogen-deficient synthesis gas. For example, WO 2012 / 069821 A1 describes a method in which a purge stream is removed from the synthesis loop, hydrogen is separated from the purge stream, and the remaining stream is processed in reforming, shift conversion, and hydrogen separation steps. The separated hydrogen is supplied to the synthesis loop.

[0004] Alternatively, instead of processing the tail gas (produced by separating hydrogen from the purge gas) through reforming, shift conversion, and hydrogen separation steps, the tail gas can be used as a fuel that is burned to heat the feed for the process or to generate steam. Because the tail gas contains carbon-containing compounds, such as methane, CO2 emissions are generated during the combustion process.

[0005] Methods for reusing the carbon in the process are known. For example, WO2020 / 249924A1 discloses a methanol synthesis method, which utilizes a reforming unit including a heat exchange reformer and an autothermal reformer in series for synthesis gas production, wherein a hydrogen-rich stream and a carbon-rich stream are separated from a purge gas stream, a portion of the hydrogen-rich stream is fed to a methanol synthesis loop and a portion of the carbon-rich stream is fed to the reforming unit. Alternatively, US2014 / 0058002A1 describes removing a purge stream from a methanol synthesis loop, separating hydrogen from the purge stream, passing the hydrogen-poor purge stream to a reformer, performing a conversion, separating hydrogen from carbon dioxide and supplying the separated hydrogen to a synthesis loop. Summary of the Invention

[0006] Applicants have discovered that to reduce carbon emissions associated with methanol synthesis, at least a portion of the purge gas can be treated in a purge gas processing unit to produce hydrogen, which can then be used as fuel, for example, to heat process streams and / or to raise steam and / or generate electricity.

[0007] Therefore, the present invention provides a method for synthesizing methanol, which comprises the following steps:

[0008] (i) reforming a hydrocarbon feedstock in a reforming section comprising a first autothermal reformer to form a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide;

[0009] (ii) converting the synthesis gas into a methanol product in a methanol loop comprising one or more methanol synthesis reaction units; and

[0010] (iii) recovering a purge gas stream from the methanol loop;

[0011] wherein at least a portion of the purge gas stream is used to generate a purge gas feed stream, the purge gas feed stream being processed in a purge gas processing unit by subjecting the purge gas feed stream to the following operations:

[0012] (iv) performing partial oxidation in a partial oxidation reactor to form a partially oxidized purge gas or performing autothermal reforming in a second autothermal reformer to form a reformed purge gas, followed by:

[0013] (v) subjecting the partially oxidized or reformed purge gas to one or more stages of water gas shift in a water gas shift unit to form a hydrogen rich gas, and

[0014] (vi) removing carbon dioxide from the hydrogen-rich gas in a carbon dioxide removal unit to form a hydrogen stream and a carbon dioxide stream;

[0015] wherein there is no steam methane reformer upstream of the first autothermal reformer; and

[0016] wherein the carbon dioxide stream is recovered and at least a portion of the hydrogen stream is used as fuel.

[0017] Steam methane reformers are well known and typically include a radiant section containing a plurality of catalyst-containing reformer tubes through which a mixture of hydrocarbons and steam passes. The reformer tubes are typically arranged vertically in rows. Fuel and air are fed to a plurality of burners in the walls of the radiant section of the steam methane reformer, which burn the fuel to generate heat for the endothermic steam reforming reaction.

[0018] The partial oxidation reactor or second autothermal reformer, the water-gas shift unit, and the carbon dioxide removal unit can be collectively described as a purge gas processing unit. The use or installation of a purge gas processing unit according to the present invention provides an operator with a means to significantly decarbonize a methanol process by replacing the fuel used for combustion equipment (which is typically a hydrocarbon stream, such as natural gas) with hydrogen fuel generated by the purge gas processing unit. For example, the hydrogen fuel can be used in any fired heater used in the methanol process (e.g., to preheat the feed or generate steam for the process).

[0019] The process can be established in a new methanol production unit, or can be retrofitted into an existing methanol production unit.

[0020] Therefore, the present invention also provides a method for converting a methanol production unit, the methanol production unit initially comprising:

[0021] a reforming section comprising a first autothermal reformer arranged to be fed with a hydrocarbon feedstock; and

[0022] a methanol loop comprising one or more methanol synthesis reaction units, wherein the methanol loop is fed with synthesis gas produced by the first autothermal reformer and produces a methanol product and a purge gas stream;

[0023] wherein there is no steam methane reformer upstream of the first autothermal reformer;

[0024] The method includes installing a purge gas processing unit, the purge gas processing unit comprising:

[0025] a partial oxidation reaction unit or a second autothermal reformer arranged to receive the purge gas stream and produce a partially oxidized purge gas or a reformed purge gas;

[0026] a water-gas shift unit arranged to receive the partially oxidized purge gas or the reformed purge gas and produce a hydrogen-rich stream;

[0027] a carbon dioxide removal unit arranged to receive the hydrogen-rich stream and produce a hydrogen stream;

[0028] The method also includes adapting the methanol production unit to use at least a portion of the hydrogen stream as fuel. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a flow diagram depicting a methanol production unit according to one embodiment of the present invention, the methanol production unit including an autothermal reformer and a purge gas processing unit, wherein the hydrogen product is supplied as fuel to any combustion equipment.

[0030] Figure 2 is a flow chart depicting one embodiment of a purge gas processing unit suitable for use with the present invention.

[0031] It will be understood by those skilled in the art that the drawings are illustrative and that other items of equipment may be required in a commercial plant, such as feedstock cylinders, pumps, vacuum pumps, compressors, gas recycle compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, liquid level controllers, collection tanks, storage tanks, etc. The provision of such auxiliary equipment does not form part of the present invention and is in accordance with conventional chemical engineering practice.

[0032] exist Figure 1 In the invention, a natural gas feed (101) is divided into a first part (103) and a second part (105). The first part is preheated in a fired heater (106) which burns at least a part (104) of a hydrogen stream (157) produced by a purge gas treatment unit. The heated natural gas from the fired heater and an oxygen stream (107) from an air separation unit (not shown) are fed to the burner of a first autothermal reformer (109) where partial combustion occurs to produce a hot gas mixture which is adiabatically reformed in a steam reforming catalyst bed arranged below the burner to produce a synthesis gas mixture comprising hydrogen, carbon monoxide, carbon dioxide and steam. Steam is added upstream of the first autothermal reformer and / or upstream of the fired heater. The synthesis gas (111) is heated at a temperature above 900 o C. and is passed to a heat recovery unit (113) where the synthesis gas is cooled in two or more stages by exchanging heat with process streams in heat exchangers and / or cooling the synthesis gas below the dew point using water and air as coolants to condense steam. In the heat recovery unit, condensate is recovered from the cooled synthesis gas using one or more gas-liquid separators (not shown) to produce a condensate stream (115) which is used as a source of steam for use in the steam reforming stage of the process.

[0033] The separation of the condensate produces a make-up gas (117), which is recovered from the heat recovery unit, mixed with hydrogen produced by the purge gas treatment unit (153) and the hydrogen separation unit (147), and the resulting hydrogen-rich make-up gas is compressed in the synthesis gas compressor (121). The compressed hydrogen-rich make-up gas (123) is then combined with a recycle stream (125) of unreacted gas, and the combined feed gas is fed to the recycle loop compressor (127). The compressed feed gas is preheated in a heat exchanger (129) and fed to a methanol synthesis unit (131), which includes one or more methanol synthesis reaction units containing a methanol synthesis catalyst. The methanol synthesis unit may include one, two or more methanol synthesis reaction units, which may be cooled or uncooled and connected in parallel or in series. The methanol synthesis reaction occurs over the methanol synthesis catalyst to convert hydrogen, carbon monoxide and carbon dioxide into a gaseous methanol product mixture comprising methanol and steam.

[0034] A gaseous methanol product mixture (133) is recovered from the methanol synthesis unit, cooled in a heat exchanger (129), and then cooled in one or more further cooling stages in a heat exchanger (135) to below the dew point at which the methanol and steam condense. The cooled mixture (137) is then fed to a gas-liquid separator (139) which separates a liquid crude methanol stream from unreacted gases. Crude methanol (141) is recovered from the separator and sent for purification to provide a purified methanol product. Unreacted gases (143) are recovered from the separator. A purge gas stream (145) is removed, and the remaining unreacted gases (125) are combined with a hydrogen-rich make-up gas.

[0035] The purge gas stream is fed to a hydrogen separation unit (147) where it is separated into a hydrogen-rich stream (119) and a carbon-rich purge gas stream (149) by passing the purge gas stream through a suitable membrane. The hydrogen-rich gas stream is recovered from the separation unit and mixed with a make-up gas to form a hydrogen-rich make-up gas.

[0036] The carbon-rich purge gas stream is recovered from the separation unit, combined with the second portion of the natural gas to produce a purge gas feed stream (151), and fed to a purge gas processing unit (153). In the purge gas processing unit, reference is made to Figure 2As further described, the combined carbon-rich purge gas stream and natural gas mixture is heated and passed to a purge gas reforming unit where it is autothermally reformed with an oxygen-containing gas (155) fed via a line in a purge gas autothermal reformer, such as air, oxygen-enriched air, or oxygen. The reformed gas undergoes heat recovery and optional condensate separation in a heat recovery unit, followed by water gas shift in a water gas shift unit, and finally CO2 removal in a carbon dioxide removal unit. The CO2 removal unit produces a carbon dioxide stream (156) which is recovered from the purge gas treatment unit, optionally purified, compressed, and sent for storage or sequestration. The removal of CO2 produces a hydrogen stream (157), at least a portion (104) of which is used as fuel in a fired heater (106). Another portion (185) is used for R ratio regulation upstream of the methanol loop.

[0037] exist Figure 2 , one embodiment of a suitable purge gas processing unit is depicted. Optionally, after an adiabatic pre-reforming step, a second portion of the natural gas feed (205) is combined with the carbon-rich gas (249). Other carbon-containing gases, such as waste gas from a discharge vessel and / or distillation overhead, may be compressed and combined with the feed gas. Steam (259) is optionally added, and the resulting mixture (referred to as the purge gas feed stream) is heated in a fired heater (261). The fired heater is heated by combustion of a hydrogen stream (263) with air to produce combustion flue gas (265).

[0038] The heated gas mixture (267) is fed to a purge gas reforming unit comprising a second autothermal reformer (269) where it is partially combusted in a burner with an oxygen-containing gas (255). The oxygen-containing gas fed to the second autothermal reformer may be air and / or a portion of an oxygen-containing gas recovered from an air separation unit used to provide the oxygen-containing gas stream (255). The partially combusted gas is then adiabatically steam reformed in a steam reforming catalyst bed disposed below the burner within the autothermal reformer. The autothermal reforming produces a reformed purge gas (271) comprising hydrogen, carbon monoxide, carbon dioxide and steam which is fed to a heat recovery unit (not shown) to reduce the temperature. In one arrangement, cooling reduces the temperature of the reformed purge gas to between 200°C and 300°C and above the dew point so that the cooled reformed gas can be fed directly to a water gas shift unit (273) after optional steam addition. Less preferably, the reformed purge gas can be cooled to below the dew point, causing the steam to condense, and the condensate recovered using one or more gas-liquid separators and used as the steam source for the process. The dehydrated reformed gas can then be fed to a water-gas shift unit after heating and steam addition.

[0039] The cooled reformed purge gas from the heat recovery unit is passed to a water gas shift unit, desirably comprising an isothermal shift vessel containing an isothermal shift catalyst, wherein the reformed purge gas becomes enriched in hydrogen by the water gas shift reaction to form a hydrogen rich gas stream.

[0040] The hydrogen-rich reformed gas (275) recovered from the water-gas shift unit is then fed to a heat recovery unit (277) which cools the hydrogen-rich gas below its dew point, causing the remaining steam to condense. The heat recovery unit includes one of a plurality of gas-liquid separators which separate condensate (279) which is recovered for use in the process.

[0041] The resulting dehydrated hydrogen-rich gas (281) is fed from the heat recovery unit to a carbon dioxide removal unit (283) operated by means of an amine scrubber which absorbs carbon dioxide from the dehydrated hydrogen-rich gas to produce a hydrogen stream. The hydrogen stream (257) is recovered from the carbon dioxide removal unit and divided between a portion (263) fed to a fired heater in a purge gas treatment unit and a portion (204) used as fuel in one or more combustion devices, including a fired heater upstream of the first autothermal reformer. Another portion of the hydrogen stream (285) can also be used for R ratio regulation upstream of the methanol loop. Regeneration of the amine absorbent in the carbon dioxide removal unit produces a carbon dioxide stream (256) which can be compressed and sent for sequestration.

[0042] In the retrofit of an existing plant with hydrogen recovery, instead of feeding a hydrocarbon-containing stream as fuel to one or more fired heaters, an installed purge gas processing unit is used to produce a recovered carbon dioxide stream and a hydrogen stream that is used as fuel in one or more fired heaters. DETAILED DESCRIPTION

[0043] Any sub-headings are for convenience only and are not intended to limit the invention.

[0044] Feed to the first autothermal reformer

[0045] The hydrocarbon feed can be any gaseous or low-boiling hydrocarbon, such as natural gas, associated gas, LPG, petroleum fractions, diesel, naphtha or their mixture, or hydrocarbon-containing waste gas from a chemical process, such as refinery waste gas or a pre-reformed gas containing methane. The gaseous mixture preferably comprises methane, associated gas or a natural gas containing a considerable proportion of methane (e.g., more than 50% by volume of methane). Natural gas is particularly preferred. The hydrocarbon can be compressed to a pressure generally in the range of 10 bar to 100 bar absolute pressure.

[0046] Before or after compression, the hydrocarbon feedstock may be pretreated to remove impurities such as sulphur compounds, chloride contaminants and / or heavy metal contaminants.The hydrocarbon feedstock may also undergo a pre-reforming step performed upstream of the first autothermal reformer.

[0047] First autothermal reformer

[0048] The hydrocarbon feedstock undergoes steam reforming in a reforming section comprising a first autothermal reformer. The term "first" is used to distinguish the first autothermal reformer from a second autothermal reformer; the latter may be part of a purge gas treatment unit as described below.

[0049] The hydrocarbon feedstock is usually mixed with steam upstream of the first autothermal reformer. Steam introduction can be achieved, for example, by direct injection of steam and / or by contacting the hydrocarbon feedstock with a heated water stream in a saturator to saturate the hydrocarbon feedstock. The amount of steam introduced can be such that the steam to carbon ratio at the inlet of the first autothermal reformer is 0.5 to 3.0, preferably 1.0 to 1.5. For the avoidance of doubt, a steam to carbon ratio of 1.0 means 1.0 mole of steam per gram of hydrocarbon carbon atom in the hydrocarbon feedstock. The amount of steam is preferably minimized because this makes a lower cost, more efficient process.

[0050] The hydrocarbon feedstock containing hydrocarbons and steam is desirably preheated prior to reforming in the first autothermal reformer. If a fired heater is used, it is preferably heated by burning a portion of the hydrogen stream generated in the purge gas processing unit. Desirably, the mixed stream is heated to an inlet temperature in the range of 450°C to 700°C. When no pre-reformer is present upstream of the first autothermal reformer, an inlet temperature in the range of 450°C to 550°C is particularly suitable, while when a pre-reformer is present upstream of the first autothermal reformer, a higher inlet temperature in the range of 550°C to 700°C is particularly suitable.

[0051] Then, the hydrocarbon feedstock / steam gas mixture undergoes reforming in the first autothermal reformer. The autothermal reformer generally includes a burner arranged near the top of the reformer and a combustion zone located below the burner. The hydrocarbon feedstock and oxygen-containing gas are fed into the burner, and the flame generally extends through the combustion zone above a fixed granular steam reforming catalyst bed. In autothermal or secondary reforming, the heat of the endothermic steam reforming reaction is provided by the combustion of hydrocarbons and hydrogen in the feed gas. The hydrocarbon feedstock is generally fed into the top of the autothermal reformer, and the oxygen-containing gas is fed into the burner. Mixing and combustion occur downstream of the burner to produce a heated gas mixture that reaches equilibrium when passing through the steam reforming catalyst. However, some steam can be added to the oxygen-containing gas, preferably without adding steam, so as to achieve a low total steam ratio for the reforming process. The autothermal reforming catalyst is generally nickel supported on a refractory carrier (such as rings or pellets of calcium aluminate cement, aluminum oxide, titanium dioxide, zirconium dioxide, etc.). In a preferred embodiment, the secondary reforming catalyst comprises a layer of Ni and / or Rh catalyst supported on zirconium dioxide, which is more active than conventional Ni supported on alumina, to reduce catalyst support volatilization.

[0052] The oxygen-containing gas used in the first autothermal reformer preferably contains ≥95% O₂, which can be provided by an air separation unit (ASU) or another oxygen source. An O₂ concentration of ≥95% O₂ is preferred instead of air because, if air is used, a larger portion of the circuit must be drawn off as a purge to remove N₂. Preferably, the O₂ content is ≥98% O₂ or ≥99% O₂. The amount of oxygen-containing gas required in the autothermal reformer is determined by the desired composition of the product gas. Generally speaking, increasing the amount of oxygen, and thereby increasing the temperature of the reformed gas leaving the autothermal reformer, results in a lower [H₂] / [CO] ratio and a lower proportion of carbon dioxide. Preferably, the amount of oxygen added is such that the autothermal reformed gas leaves the autothermal reforming catalyst at a temperature in the range of 750°C to 1050°C. The autothermal reformed gas recovered from the autothermal reformer is a synthesis gas comprising hydrogen, carbon monoxide, carbon dioxide, methane, and steam. The amount of methane is affected by the autothermal reformer outlet temperature.

[0053] In some embodiments, the reforming section may include an adiabatic pre-reformer used in conjunction with the first autothermal reformer. In the pre-reforming stage, the hydrocarbons undergo an adiabatic steam reforming step, wherein the hydrocarbon / steam mixture is desirably heated to a temperature in the range of 400°C to 650°C and then adiabatically passed through a suitable catalyst bed, typically one with a high nickel content (e.g., greater than 40% by weight). During this adiabatic reforming step, any hydrocarbons higher than methane react with steam to produce a mixture of methane, carbon oxides, and hydrogen. Using this adiabatic reforming step (often referred to as pre-reforming) is desirable to ensure that the feed to the autothermal reformer does not contain hydrocarbons higher than methane and also contains some hydrogen. This may be desirable in the case of low steam / carbon ratio mixtures to minimize the risk of soot formation in the autothermal reformer.

[0054] In some embodiments, the reforming section may include a gas-heated reformer used in conjunction with a first autothermal reformer. The first autothermal reformer and the gas-heated reformer may be arranged in series or in parallel. When arranged in series, the hydrocarbon feed stream is fed to the catalyst-filled tubes in the gas-heated reformer (tube side), where a steam reforming reaction occurs in the catalyst-filled tubes to produce a partially reformed gas stream. The partially reformed gas stream is fed to the first autothermal reformer, where a further steam reforming reaction occurs to produce a reformed gas stream. The reformed gas stream is fed to the shell of the gas-heated reformer to provide the heat required for steam reforming in the gas-heated reformer. When arranged in parallel, a first portion of the hydrocarbon feed stream is fed to the catalyst-filled tubes in the gas-heated reformer (tube side), where a steam reforming reaction occurs to produce a first partially reformed gas stream. The second portion of the hydrocarbon feed stream is fed to a first autothermal reformer where a steam reforming reaction occurs to produce a second partially reformed gas stream. The first partially reformed gas stream and the second partially reformed gas stream are combined and used to provide the heat required for steam reforming in the gas-heated reformer. Alternatively, the first partially reformed gas stream can be combined with the second portion of the hydrocarbon feed stream fed to the first autothermal reformer.

[0055] The stream leaving the first autothermal reformer (and, if present, the gas heated reformer) is a synthesis gas comprising hydrogen, carbon monoxide, carbon dioxide, and steam. When there is sufficient hydrogen to convert all carbon oxides into methanol, an ideal stoichiometric mixture for methanol synthesis is produced. The methanol synthesis reaction is as follows:

[0056]

[0057]

[0058] The stoichiometric number or R value of the synthesis gas can be calculated from the molar composition of the components as follows: R = ([H2] – [CO2]) / ([CO] + [CO2]). The ideal R value for the methanol synthesis reaction is when R=2. Typically, the synthesis gas recovered from the first autothermal reformer has an R value of about 1.8. H2 is preferably added to the synthesis gas upstream of the methanol loop to adjust its R value. The R value is preferably adjusted to about 2, preferably in the range of 1.95 to 2.05. Preferred hydrogen sources for R value adjustment include a H2-rich stream from a hydrogen separation unit, a H2 stream from outside the process (OSBL) or a portion of the hydrogen stream produced by a purge gas treatment unit. The adjustment of the R value is preferably performed after the heat recovery described below.

[0059] After leaving the first autothermal reformer (and, if present, the gas heated reformer), the synthesis gas is then desirably cooled in one or more heat exchange steps (typically including at least a first steaming stage). Preferably, after such steaming, the reformed gas is cooled by heat exchange with one or more of the following streams: hydrocarbon feedstock, water (including process condensate) used to generate steam (which steam can be used for heating or for the pre-reforming stage), a mixture of hydrocarbons and steam, a pre-reformed gas mixture, and cooling in the distillation of crude methanol.

[0060] Cooling is preferably performed to reduce the temperature of the synthesis gas from the first autothermal reformer to below the dew point, causing steam present in the synthesis gas to condense. The liquid process condensate may be separated from the synthesis gas, which may then be referred to as make-up gas, by conventional gas-liquid separation equipment.

[0061] The make-up gas comprises hydrogen, carbon monoxide, carbon dioxide and a small amount of unreacted methane. The make-up gas can be compressed to the desired methanol synthesis pressure in a synthesis gas compressor to be fed into the methanol loop.

[0062] The methanol loop (described later) produces an unreacted gas mixture that is combined with the make-up gas to form the feed gas mixture. Alternatively, if hydrogen-rich gas is recovered from the purge gas upstream of the purge gas processing unit, this hydrogen-rich gas can also be combined with the make-up gas and the unreacted gas mixture to form the feed gas to the methanol loop. The feed gas can have an R value in the range of 3 to 5 or higher. The addition of hydrogen-rich gas recovered from the recycled purge gas allows the methanol synthesis reactor to operate at its optimal R value of 3 to 5, with the make-up gas having an R value close to the stoichiometric optimum of 2, which maximizes methanol production.

[0063] Methanol loop

[0064] In the method of the present invention, any methanol loop can be used to synthesize methanol. The methanol loop includes one or more methanol synthesis reaction units, such as a first methanol synthesis reaction unit, a second methanol synthesis reaction unit, and optionally a third methanol synthesis reaction unit, each of which includes a methanol synthesis catalyst bed, and these methanol synthesis reaction units are arranged in series and / or in parallel to each produce a product gas stream comprising methanol. Therefore, the methanol loop can include one, two, or more methanol synthesis reaction units, each of which includes a methanol synthesis catalyst bed, and each of which is fed with a feed gas comprising hydrogen and carbon dioxide, and each of which produces a gas mixture comprising methanol. A product gas mixture comprising methanol is recovered from at least one methanol synthesis reaction unit. Methanol is recovered from one or more product gas mixtures. This can be achieved by cooling one or more methanol product gas streams to below the dew point, condensing methanol, and separating the crude liquid methanol product from the unreacted gas.

[0065] Conventional heat exchange and gas-liquid separation equipment can be used. Particularly suitable heat exchange equipment includes a gas-to-gas heat exchanger that cools the methanol product gas stream from the methanol synthesis reactor using the feed gas mixture to the reactor. The methanol product gas streams can be processed separately or combined prior to cooling and / or separation of the crude liquid methanol product.

[0066] The separation of the crude liquid methanol product and one or more methanol product gas streams produces an unreacted gas mixture. A portion of the unreacted gas mixture is returned to one or more methanol synthesis reaction units as a recycle or loop gas stream. The unreacted gas separated from the product gas mixture recovered from a methanol synthesis reaction unit can be returned to the same or different methanol synthesis reaction units. The unreacted gas mixture comprises hydrogen, carbon monoxide and carbon dioxide and can therefore be used to generate additional methanol. The recycle gas stream can be recovered from at least one methanol product gas stream and recycled to at least one methanol synthesis reaction unit. If there is more than one recycle gas stream, these recycle gas streams can be recycled to one or more methanol synthesis reaction units individually, or combined and fed to one or more methanol synthesis reaction units.

[0067] The methanol synthesis reaction unit in the methanol loop can be an uncooled adiabatic reaction unit. Alternatively, the methanol synthesis reaction unit can be cooled by heat exchange with synthesis gas, such as in a quenching reaction unit or in a reaction unit selected from a tube-cooled reformer or a gas-cooled reformer. Alternatively, the methanol synthesis reaction unit can be cooled by low-pressure boiling water, such as in an axial-flow steam lift reformer or a radial-flow steam lift reformer.

[0068] In a process comprising a first methanol synthesis reaction unit and a second methanol synthesis reaction unit, the first methanol synthesis reaction unit is preferably cooled by boiling water, such as in an axial-flow steam lift reformer or a radial-flow steam lift reformer, more preferably in an axial-flow steam lift reformer. The second methanol synthesis reaction unit may be a radial-flow steam lift reformer. This arrangement is particularly useful due to the characteristics and performance of these reaction units with different feed gas mixtures. Alternatively, the second methanol synthesis reaction unit may be cooled by synthesis gas (e.g., a gas containing hydrogen and carbon dioxide). Therefore, the second methanol synthesis reaction unit may be a cooled reaction unit selected from a tube-cooled reformer (TCC) and a gas-cooled reformer (GCC). Tubular-cooled reformers are preferred due to their simpler design. If a third methanol synthesis reaction unit is present, it is preferably cooled by boiling water. The third methanol synthesis reaction unit may then be a steam lift reformer selected from an axial-flow steam lift reformer and a radial-flow steam lift reformer, most preferably an axial-flow steam lift reformer. The first and second methanol synthesis reaction units can be connected in series. In this case, the synthesis gas fed to the second methanol synthesis reaction unit comprises at least a portion of the methanol product gas stream recovered from the first methanol synthesis reaction unit. In this arrangement, the synthesis gas fed to the second methanol synthesis reaction unit preferably comprises the entire methanol product gas stream recovered from the first methanol synthesis reaction unit. Particularly preferred methanol loops are described in US Pat. No. 7,790,775 A, WO 2017 / 121980 A1, and WO 2017 / 121981 A1.

[0069] The methanol synthesis catalysts in each methanol synthesis reaction unit may be the same or different. The methanol synthesis catalyst is preferably a commercially available copper-containing methanol synthesis catalyst. Specifically, the methanol synthesis catalyst is one or more granular copper / zinc oxide / aluminum oxide catalysts, which may contain one or more promoters. Particularly suitable catalysts are Mg-promoted copper / zinc oxide / aluminum oxide catalysts as described in US4788175A and SiO2-doped copper / zinc oxide / aluminum oxide catalysts as described in WO2020 / 212681A1.

[0070] Methanol synthesis can be carried out at a pressure in the range of 10 bar to 120 bar absolute and a temperature in the range of 130 o C to 350 o C range. The pressure at the inlet of the one or more reaction units is preferably 50 to 100 bar absolute, more preferably 70 to 90 bar absolute. The temperature of the synthesis gas at the inlet of the one or more reaction units is preferably in the range of 200°C to 250°C, and the temperature of the synthesis gas at the outlet of the one or more reaction units is preferably in the range of 230°C to 280°C.

[0071] The unreacted gas mixture portion of the recycle gas stream constituting the methanol loop will typically be at a lower pressure than the make-up gas, and therefore preferably, the recycle gas stream is compressed by one or more compressors or circulation devices. At least one compressor is used to circulate the unreacted gas stream. The resulting compressed recycle gas stream can be mixed with the make-up gas to form a feed to one or more methanol synthesis reaction units in the methanol loop.

[0072] In the case of a single methanol synthesis reaction unit, the recycle ratio forming the feed gas mixture to the one or more methanol synthesis reaction units may be in the range of 0.5:1 to 8:1, preferably 2:1 to 6:1, or in the case of a loop with two or more methanol synthesis reaction units arranged in series, the recycle ratio is in the range of 0.5:1 to 5:1, preferably 1:1 to 3:1. By the term "recycle ratio" we mean the molar flow ratio of the recycled unreacted gas stream to the make-up gas that forms the gas mixture fed to the one or more methanol synthesis reaction units.

[0073] The crude liquid methanol recovered from the methanol loop contains water, as well as a small amount of higher alcohols and other impurities. The crude methanol can first be fed into a flash column or a discharge vessel, where the dissolved gases are released and separated from the crude liquid methanol stream. The liquid crude methanol can undergo one or more purification stages, which include one or more, preferably two or three, distillation stages in a methanol purification unit, which includes one, two or more distillation columns. The waste gas can be effectively recovered from at least the top column in the discharge vessel and the distillation unit and fed to a purge gas treatment unit to further reduce carbon dioxide emissions from the method. The degassing stage and the distillation stage can be heated using heat recovered from the process (e.g., in the cooling of the product gas stream) or other sources. Preferably, at least a portion of the crude methanol is purified by distillation to produce a purified methanol product.

[0074] The purified methanol product may undergo further processing, for example to produce derivatives such as dimethyl ether or formaldehyde. Alternatively, the methanol may be used as a fuel.

[0075] Purge gas processing unit

[0076] A portion of the unreacted gas mixture separated from the crude liquid methanol is removed from the loop as a purge gas stream. Preferably, the purge gas stream is continuously removed to prevent unwanted accumulation of inert gases (such as nitrogen, argon and methane) in the synthesis loop. The purge gas stream can be recovered from the separated unreacted gas before or after compression in the circulation device. In processes using steam reforming as a supplementary gas source, the purge gas stream can be hydrogen-rich. The purge gas stream preferably comprises 50% to 90% by volume of hydrogen and one or more of the following: carbon monoxide, carbon dioxide, nitrogen, argon and methane.

[0077] In the present invention, at least a portion of the purge gas stream is subjected to partial oxidation in a partial oxidation reaction unit to form a partially oxidized purge gas or to autothermal reforming in a second autothermal reformer to form a reformed purge gas, followed by one or more stages of water-gas shift in a water-gas shift unit to form a hydrogen-rich gas, and carbon dioxide is removed from the hydrogen-rich gas in a carbon dioxide removal unit to form a hydrogen stream and a carbon dioxide stream. The carbon dioxide stream is recovered, and at least a portion of the hydrogen stream is used as fuel.

[0078] The purge gas stream may contain methanol and therefore, if desired, upstream of the purge gas treatment unit, methanol may be recovered from the purge gas stream optionally using a water wash and the recovered methanol and water sent for purification together with the crude methanol.

[0079] For example, where the synthesis gas has an R-value of 1.95 or higher, at least a portion of the purge gas may be fed directly to the purge gas processing unit.

[0080] Alternatively, the purge gas stream can be separated into a hydrogen-rich gas stream and a carbon-rich purge gas. The carbon-rich purge gas is used to generate the purge gas feed stream. At least a portion of the hydrogen-rich gas stream can be added to the synthesis gas upstream of the methanol loop, for example, to adjust the R value of the synthesis gas from the first autothermal reformer.

[0081] "Carbon-rich purge gas" refers to a gas stream containing a higher proportion of carbon-containing compounds (carbon monoxide, carbon dioxide, and methane) than the parent purge gas. While the individual components may have the same or even lower proportions as those in the parent purge gas, the total amount of all carbon-containing compounds in the carbon-rich purge gas will be higher. Therefore, in this arrangement, the portion of the purge gas fed to the purge gas processing unit is carbon-rich purge gas. Separating hydrogen from the purge gas upstream of the purge gas processing unit also has the advantage of reducing hydrogen losses from the process due to combustion in the purge gas processing unit. Separation of the hydrogen-rich purge gas stream from the carbon-rich purge gas stream can be accomplished using any known separation equipment, such as a hydrogen membrane separator, a pressure swing adsorption unit, a cold box separation system, or any combination of these. Using these techniques, over 50% of the hydrogen present in the purge gas stream can be recovered. When a membrane is used to separate the hydrogen-rich gas, the carbon-rich purge gas stream will be at a pressure that enables it to be fed to the purge gas processing unit for partial oxidation or reforming without further compression, which is highly desirable. In the case where a pressure swing absorption system is used to separate the hydrogen-rich gas, the carbon-rich purge gas can be at a lower pressure and is therefore less preferred. The hydrogen-rich gas stream recovered from the purge gas stream advantageously contains >95% by volume H. The separated hydrogen, in addition to being fed to the methanol loop, can also be used upstream for hydrodesulfurization of the hydrocarbon feedstock and / or for stripping dissolved gases from crude methanol, and / or as fuel.

[0082] The feed to the purge gas treatment unit can be supplemented with a portion of the hydrocarbon feedstock and / or one or more hydrocarbon-containing waste gases. This increases the flexibility of the purge gas treatment unit and is advantageous during process startup. If desired, the hydrocarbon feedstock and / or one or more hydrocarbon-containing waste gases can be treated in a pre-reformer upstream of the partial oxidation reactor or the second autothermal reformer, for example, using an adiabatic pre-reformer as described above.

[0083] If desired, the feed to the purge gas treatment unit may be supplemented with one or more other hydrocarbon-containing waste gas streams.

[0084] Second autothermal reformer or partial oxidation reactor

[0085] The purge gas processing unit includes a partial oxidation reaction device or a second autothermal reformer. Here, "second" is only used to distinguish the autothermal reformer of the purge gas processing unit from the first autothermal reformer described previously.

[0086] The scale of the second autothermal reformer in the purge gas reforming unit can be the same or different from that of the first autothermal reformer, but the design, catalyst, and operation are conveniently the same as described above. If necessary, the portion of the purge gas fed to the second autothermal reformer can be compressed to a pressure in the range of 10 to 50 bar absolute, and if necessary, heated to a temperature of 350°C to 650°C before being fed to the second autothermal reformer. The heating of this portion of the purge gas can be performed by using a fired heater that is fueled by a portion of the hydrogen stream generated by the purge gas processing unit. The oxygen-containing gas fed to the partial oxidation reaction unit or the second autothermal reformer in the purge gas reforming unit can be air, oxygen-enriched air, or oxygen, but is preferably air because the resulting hydrogen stream is used as fuel and the presence of nitrogen can therefore be tolerated. The use of air avoids the need to upgrade or add an additional air separation unit when the existing process is modified with a purge gas processing unit. Steam can be added to the oxygen-containing gas and / or the purge gas portion. The purge gas is autothermally reformed in the autothermal reformer to produce a reformed purge gas. The reformed purge gas will contain hydrogen, steam carbon monoxide, carbon dioxide and possibly small amounts of argon and nitrogen.

[0087] The partial oxidation reaction unit can alternatively be used to convert the portion of the purge gas into a partially oxidized purge gas by partial oxidation using a substoichiometric amount of oxygen. Partial oxidation reaction units or POx reaction units are known and typically include a container to which the feed and oxygen-containing gas are fed via a burner similar to the burner used in an autothermal reformer disposed above the reaction chamber where the partial combustion reaction occurs. Unlike the autothermal reformer, there is no catalyst in the container. The oxygen-containing gas can be the same or different from the oxygen-containing gas used in the first autothermal reformer, but due to the above-mentioned second autothermal reformer, it is convenient to use air. The combustion temperature can be about 1300°C. o C or higher. Steam can be added to the feed and / or oxygen to lower the combustion temperature and reduce soot formation. The idealized formula for this reaction applied to methane in the feed is as follows:

[0088]

[0089] However, yields are less than stoichiometric because part of the feed is completely combusted, and problems with soot formation generally require increased amounts of steam or oxygen, so the purge reforming unit preferably comprises an autothermal reformer rather than a partial oxidation reactor.

[0090] The outlet temperature of the second autothermal reformer or partial oxidation reaction unit can be in the range of 800°C to 1300°C. Therefore, it is desirable to adjust the temperature of the partially oxidized or reformed purge gas upstream of the water gas shift unit. This can be conveniently accomplished by recovering heat in a heat recovery unit, including generating steam in one or more boilers, which can be effectively used for heating or generating electricity using a steam turbine. In some embodiments, the steam generated by heat recovery from the purge gas reforming unit can be used to supplement the steam added to the hydrocarbon feedstock and / or purge gas upstream of the corresponding reforming unit. The additional steam generated in the purge gas processing unit can be used to provide process heating or power for a compressor or for power generation.

[0091] In some embodiments, a gas-heated reformer can be used in conjunction with a partial oxidation reactor or a second autothermal reformer. The partial oxidation reactor or the second autothermal reformer and the gas-heated reformer can be arranged in series or in parallel, similar to that described above for the reforming section.

[0092] Water-gas shift unit

[0093] The partially oxidized or reformed purge gas undergoes one or more stages of water-gas shift in a water-gas shift unit. Steam is required for the water-gas shift reaction. If insufficient steam is present in the partially oxidized or reformed purge gas, steam can be added upstream of the water-gas shift unit, for example, by direct addition.

[0094] The partially oxidized or reformed purge gas can be passed through one or more water-gas shift catalyst beds in one or more shift vessels to produce a hydrogen-rich gas or "shifted" gas. Simultaneously, the water-gas shift unit converts the carbon monoxide in the partially oxidized or reformed purge gas into carbon dioxide. The reaction can be depicted as follows:

[0095]

[0096] The one or more water-gas shift stages may include stages of high-temperature shift, medium-temperature shift, isothermal shift and low-temperature shift.

[0097] High-temperature shift can be operated adiabatically in the shift vessel over a bed of reduced iron catalyst (such as chromium oxide-promoted magnetite) at an inlet temperature in the range of 300°C to 400°C, preferably 320°C to 360°C. Alternatively, a potassium-promoted zinc aluminate catalyst can be used. A single-stage high-temperature shift can be used in the present invention. Alternatively, a combination of high-temperature shift and medium-temperature or low-temperature shift can be used.

[0098] The medium-temperature shift and low-temperature shift stages can be performed using a shift vessel containing a supported copper catalyst, specifically a copper / zinc oxide / aluminum oxide composition. In the low-temperature shift, a gas containing carbon monoxide (preferably ≤6% CO by volume on a dry basis) and steam (at a steam-to-total dry gas molar ratio ranging from 0.3 to 1.5) can be passed over the catalyst in an adiabatic fixed bed, with an outlet temperature ranging from 200°C to 300°C. If additional steam is added, the outlet carbon monoxide content can range from 0.1% to 1.5% by volume on a dry basis, particularly below 0.5%. Alternatively, in the medium-temperature shift, the gas containing carbon monoxide and steam can be fed to the catalyst at an inlet temperature ranging from 200°C to 240°C, but the inlet temperature can be as high as 280°C. The outlet temperature can be as high as 300°C, but can also be as high as 360°C.

[0099] Although one or more adiabatic water gas shift stages, such as a high temperature shift stage, optionally followed by a low temperature shift stage can be employed, the partially oxidized or reformed purge gas can be subjected to an isothermal water gas shift stage in a shift vessel, wherein the catalyst (typically a copper / zinc oxide / aluminum oxide composition) is cooled, optionally followed by one or more adiabatic medium or low temperature water gas shift stages in an uncooled vessel as described above. Although the term "isothermal" is used to describe the cooling of the shift converter, there may be a small increase in gas temperature between the inlet and outlet, such that the temperature of the hydrogen-rich reformed gas stream at the outlet of the isothermal shift converter may be between 1 degree Celsius and 25 degrees Celsius higher than the inlet temperature. The coolant can conveniently be water at a pressure such that partial or complete boiling occurs. The water may be located in a tube surrounded by or surrounding the catalyst. The resulting steam can be used in the process, for example to drive a turbine, for example to obtain electricity, or to provide process steam to supply the process. In some embodiments, the steam generated by the isothermal shift stage can be used to supplement the steam added to the hydrocarbon feed upstream of the first autothermal reformer and / or to the purge gas upstream of the purge gas processing unit. This improves the efficiency of the process and enables the desired steam to carbon ratio to be achieved at low cost.

[0100] After one or more shift stages, the hydrogen-rich reformed gas is desirably cooled to a temperature below the dew point, causing steam to condense. This forms a dehydrated hydrogen-rich gas. The liquid water condensate can then be separated using one or more gas-liquid separators, which may have one or more additional cooling stages between them. Any coolant can be used. Typically, cooling of the hydrogen-rich gas can be provided by boiling water connected to a steam drum under pressure. If desired, cooling can be performed in heat exchange with the process condensate. Thus, a heated water stream can be formed that can be used to supply some or all of the steam required for partial oxidation or reforming. Because the condensate may contain ammonia, methanol, hydrogen cyanide, and CO2, returning the condensate to form steam used in the partial oxidation or reforming stage provides a useful way to return hydrogen and carbon to the process. One or more additional stages of cooling are desired. Cooling can be performed in heat exchange in one or more stages using deionized water, air, or a combination of these substances. In a preferred embodiment, cooling is performed while performing heat exchange with one or more liquids in the CO2 separation unit. Condensate separation can be performed in one, two, or three stages. Any condensate not used to generate steam may be sent as effluent to water treatment.

[0101] Typically, the hydrogen-rich gas stream contains 10% to 30% by volume of carbon dioxide (on a dry basis).In the present invention, carbon dioxide is preferably separated from the hydrogen-rich gas stream in a carbon dioxide removal unit after separation of condensed water.

[0102] Carbon dioxide removal unit

[0103] The carbon dioxide removal unit functions to separate the hydrogen-rich gas stream into a hydrogen stream and a carbon dioxide-containing stream. Any suitable carbon dioxide removal unit suitable for this purpose can be used. For example, the carbon dioxide removal unit can operate by adsorbing carbon dioxide onto a solid adsorbent (such as a molecular sieve) in a pressure swing adsorption (PSA) unit, separating the hydrogen-rich gas using a hydrogen-permeable membrane, or alternatively, by absorbing the carbon dioxide into a liquid in a physical or reactive scrubbing system. Solid adsorbents and membrane systems can be used when the amount of purge gas and / or the purity of the hydrogen stream are not high. However, reactive scrubbing systems, particularly amine scrubbing systems, are preferred for improved carbon dioxide removal. Carbon dioxide can therefore be separated via an acid gas recovery (AGR) process. In an AGR process, a dehydrated, hydrogen-rich reformed gas stream (i.e., dehydrated shifted gas) is contacted with a suitable absorbent liquid stream, such as an amine, for example, monoethanolamine, diethanolamine, methyldiethanolamine, and diglycolamine, particularly a methyldiethanolamine (MDEA) solution, so that the carbon dioxide is absorbed by the liquid, resulting in a loaded absorbent liquid and a gas stream with a reduced carbon dioxide content. The absorbing liquid of load is then regenerated by heating to desorb carbon dioxide and obtain regenerated absorbing liquid, and then the regenerated absorbing liquid is recycled to the carbon dioxide absorption stage. Heating can be suitably provided by steam, hot condensate or another suitable heating medium produced by this process. Alternatively, refrigerated methanol or glycol can be used to capture carbon dioxide in a manner similar to amine. If the carbon dioxide separation step uses a liquid washing step as a single pressure process operation, i.e., adopts substantially the same pressure in the absorption and regeneration steps, then only little recompression will be required to recycle carbon dioxide. The above-mentioned type of carbon dioxide removal unit is commercially available.

[0104] Because the source of the hydrogen-rich gas is the purge gas stream, inerts such as nitrogen and argon may be present. The amine scrubbing carbon dioxide removal unit conveniently leaves these inert gases in the hydrogen stream, so that in this way they can be effectively removed from the process.

[0105] The recovered carbon dioxide is relatively pure and can therefore be compressed and used to make chemicals, purified for use in the food industry, or sent for storage or sequestration, or used in enhanced oil recovery (EOR) processes. Where the CO2 is to be compressed for storage, transport, use in EOR processes, or conversion into other chemical products, the CO2 may first be dried to prevent condensation of liquid water present in trace amounts. For example, the CO2 may be dried to a dew point of ≤10°C by passing it through a bed of a suitable desiccant (such as zeolite) or by contacting it with glycol in a glycol drying unit. o C.

[0106] After separating the carbon dioxide, the method provides a hydrogen stream. When a pure oxygen-containing gas stream (i.e., ≥95% by volume, preferably ≥98% by volume O2) is used in the partial oxidation reactor or the second autothermal reformer in the purge gas reforming unit, the hydrogen stream may comprise 75% to 99% by volume of hydrogen, preferably 90% to 99% by volume of hydrogen, with the balance comprising one or more of methane, carbon monoxide, carbon dioxide, and an inert gas. When air is used in the partial oxidation reactor or the autothermal reformer in the purge gas reforming unit, the hydrogen stream may comprise 50% to 60% by volume of hydrogen, with the balance primarily comprising nitrogen and small amounts of one or more of methane, carbon monoxide, carbon dioxide, and argon. The methane content of the hydrogen stream may range from 0.25% to 7.5% by volume. The carbon monoxide content of the hydrogen stream may range from 0.5% to 7.5% by volume. The carbon dioxide content of the hydrogen stream may range from 0.01% to 2.5% by volume.

[0107] When the purity of the hydrogen stream required for downstream purposes needs to be higher than the purity produced by the carbon dioxide removal unit, the hydrogen stream can be passed to a purification unit to provide a purified hydrogen product stream and an off-gas stream containing carbon compounds. Any suitable purification unit can be used. For example, the purification unit may include a membrane system, a temperature swing adsorption system, or a pressure swing adsorption system. The purification unit is preferably a pressure swing adsorption unit. Such a unit includes a regenerable porous adsorbent material that selectively captures gases other than hydrogen, thereby purifying them. The purification unit produces a pure hydrogen stream with a purity preferably greater than 99.5% by volume, more preferably greater than 99.9% by volume. Such systems are commercially available. The purification unit also produces an off-gas. The off-gas contains carbon compounds and is therefore preferably not used as fuel, but is fed back into the process as feed or fed to the hydrocarbon reforming unit or preferably to the purge gas processing unit.

[0108] The carbon dioxide recovered from the carbon dioxide removal unit may optionally be purified, compressed and sent to storage or sequestration.

[0109] Use of hydrogen flow

[0110] At least a portion of the hydrogen stream, either directly generated by the carbon dioxide separation unit or after further purification in a purification unit, is used as fuel. Preferably, the hydrogen stream is combusted in one or more combustion devices. The hydrogen stream can be combusted as is, or it can be blended with another fuel stream prior to combustion. If desired, a hydrogen fuel burner may be installed.

[0111] In a preferred embodiment, at least a portion of the hydrogen stream is used to heat a process stream and / or for steam generation.

[0112] In a preferred embodiment, at least part of the hydrogen stream is used to adjust the R value of the synthesis gas coming from the first autothermal reformer upstream of the methanol loop.

[0113] In a preferred embodiment, the hydrocarbon feed to the first autothermal reformer is heated using a fired heater fueled by a portion of the hydrogen stream.

[0114] In a preferred embodiment, one or more feeds to the purge gas processing unit are heated using a fired heater combusted with a portion of the hydrogen stream.

[0115] If desired, a portion of the crude hydrogen or a portion of the purified hydrogen can be recycled to the hydrocarbon feed for hydrodesulfurization. If desired, this portion can be compressed before recycling.

[0116] The purge gas processing unit can be fed by more than one methanol synthesis unit. Thus, the purge gas processing unit can be operatively connected to two or more methanol production units, each methanol production unit having an autothermal reformer. Typically, the purge gas streams from each methanol synthesis are combined to form the purge gas feed stream that is fed to the purge gas processing unit.

Claims

1. A method for synthesizing methanol, comprising the following steps: (i) reforming a hydrocarbon feedstock in a reforming section comprising a first autothermal reformer to form a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide; (ii) converting the synthesis gas into a methanol product in a methanol loop comprising one or more methanol synthesis reaction units; as well as (iii) recovering a purge gas stream from the methanol loop; wherein at least a portion of the purge gas stream is used to generate a purge gas feed stream, the purge gas feed stream being processed in a purge gas processing unit by subjecting the purge gas feed stream to the following operations: (iv) performing partial oxidation in a partial oxidation reactor to form a partially oxidized purge gas or performing autothermal reforming in a second autothermal reformer to form a reformed purge gas, followed by: (v) subjecting the partially oxidized or reformed purge gas to one or more stages of water gas shift in a water gas shift unit to form a hydrogen-rich gas, and (vi) removing carbon dioxide from the hydrogen-rich gas in a carbon dioxide removal unit to form a hydrogen stream and a carbon dioxide stream; wherein there is no steam methane reformer upstream of the first autothermal reformer; and wherein the carbon dioxide stream is recovered and at least a portion of the hydrogen stream is used as fuel.

2. The process of claim 1, wherein the hydrocarbon feedstock is natural gas.

3. The method of claim 1 or claim 2, wherein the purge gas stream is separated into a hydrogen-rich gas stream and a carbon-rich purge gas, the carbon-rich purge gas being used to generate the purge gas feed stream.

4. The process according to claim 3, wherein the hydrogen-rich gas stream is added to the synthesis gas upstream of the methanol loop.

5. The method according to any one of claims 1 to 4, wherein the methanol loop comprises one, two or more methanol synthesis reaction units, each methanol synthesis reaction unit containing a methanol synthesis catalyst bed, and wherein the methanol product is recovered from at least one methanol synthesis reaction unit.

6. The process according to any one of claims 1 to 5, wherein the purge gas fed to the purge gas treatment unit is supplemented with a portion of the hydrocarbon feedstock and / or one or more hydrocarbon-containing off-gases.

7. The process according to claim 6, wherein the hydrocarbon feedstock and / or one or more hydrocarbon-containing off-gases are treated in a pre-reformer upstream of the partial oxidation reaction unit or the second autothermal reformer.

8. The process according to any one of claims 1 to 7, wherein the first autothermal reformer uses ≥95 vol. % O2 as oxygen-containing gas.

9. The method according to any one of claims 1 to 8, wherein the partial oxidation reaction device or the second autothermal reformer in the purge gas reforming unit uses air as the oxygen-containing gas.

10. The process according to any one of claims 1 to 9, wherein the hydrocarbon feedstock to the first autothermal reformer is heated using a fired heater combusted by a portion of the hydrogen stream.

11. The process according to any one of claims 1 to 10, wherein at least a portion of the hydrogen gas stream is used for steam generation.

12. The method according to any one of claims 1 to 11, wherein a portion of the hydrogen flow is used to adjust the R value of the synthesis gas from the first autothermal reformer.

13. The process according to any one of claims 1 to 12, wherein one or more feeds to the purge gas treatment unit are heated using a fired heater combusted by a portion of the hydrogen stream.

14. A method for converting a methanol production unit, the methanol production unit initially comprising: a reforming section comprising a first autothermal reformer arranged to be fed with a hydrocarbon feedstock; and a methanol loop comprising one or more methanol synthesis reaction units, wherein the methanol loop is fed with synthesis gas produced by the first autothermal reformer and produces a methanol product and a purge gas stream; wherein there is no steam methane reformer upstream of the first autothermal reformer; The method includes installing a purge gas processing unit, the purge gas processing unit including: a partial oxidation reaction unit or a second autothermal reformer arranged to receive the purge gas stream and produce a partially oxidized purge gas or a reformed purge gas; a water-gas shift unit arranged to receive the partially oxidized purge gas or the reformed purge gas and produce a hydrogen-rich stream; a carbon dioxide removal unit arranged to receive the hydrogen-rich stream and produce a hydrogen stream; The method also includes adapting the methanol production unit to use at least a portion of the hydrogen stream as fuel.

15. The method of claim 14, further comprising installing a hydrogen fuel burner.

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