METHOD FOR SYNTHESIS OF METHANOL WITH OPTIMIZED PREPARATION AND PROCESSING OF SYNTHESIS GAS

Abstract

A process for synthesising methanol is described comprising the steps of (i) reforming a hydrocarbon feedstock in a hydrocarbon reforming unit comprising a fired heater and an autothermal reformer in series to form a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide; (ii) dividing the synthesis gas into a first portion and a second portion, (iii) converting the first portion of synthesis gas into a methanol product in a methanol synthesis unit comprising one or more methanol synthesis reactors; and (iv) recovering a purge gas stream from the methanol synthesis unit, wherein the second portion of synthesis gas is passed to a hydrogen production unit where it is subjected to one or more stages of water gas shift in a water-gas shift unit to form a hydrogen-enriched gas, the a hydrogen-enriched gas is subjected to a step of carbon dioxide removal in a carbon dioxide removal unit to form a hydrogen product stream and a carbon dioxide stream, the carbon dioxide stream is recovered and at least a portion of the hydrogen product stream is fed to the fired heater as a fuel.

Description

[0001] PROCESS FOR THE SYNTHESIS OF METHANOL WITH OPTIMISED SYNGAS PREPARATION

[0002] AND PROCESSING

[0003] This invention relates to a process for synthesising methanol, in particular a process for synthesising methanol with low emissions of carbon dioxide from the process.

[0004] Methanol synthesis is generally performed by passing a synthesis gas comprising hydrogen and carbon monoxide and / or carbon dioxide at an elevated temperature and pressure through one or more beds of a methanol synthesis catalyst, which is often a copper-containing composition, in a synthesis reactor. A crude methanol is generally recovered by cooling the product gas stream to below the dew point and separating off the product as a liquid. The crude methanol is typically purified by distillation. The process is often operated in a loop: thus unreacted gas may be recycled to the synthesis reactor as part of the feed gas via a circulator. Fresh synthesis gas, termed makeup gas, is added to the recycled unreacted gas to form the feed gas stream. A purge stream is taken from the circulating gas stream to avoid the build-up of inert gasses in the loop.

[0005] Various methods exist to recover hydrogen from the purge gas and re-use it in the methanol synthesis step, especially for synthesis gases that are hydrogen deficient. For example, WO2020 / 249923 (A1) discloses (i) forming a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide from a hydrocarbon feedstock in a reforming unit comprising an adiabatic prereformer and autothermal reformer in series; (ii) cooling the synthesis gas in one or more stages of heat exchange, and recovering process condensate from the cooled synthesis gas to form a makeup gas having a stoichiometry value, R, in the range 1 .80 to 1 .95; (iii) passing a feed gas comprising the make-up gas to a methanol synthesis loop; (iv) recovering a product gas mixture containing methanol from the methanol synthesis loop, cooling the product gas mixture to below the dew point to condense crude methanol, and separating the crude methanol from an unreacted gas mixture; and (v) recycling a portion of the unreacted gas mixture to the methanol synthesis loop and recovering a portion of the unreacted gas mixture as a purge gas stream, wherein a hydrogen-rich stream and a carbon-rich stream are separated from the purge gas stream, a portion of the hydrogen-rich stream is fed to the methanol synthesis loop and a portion of the carbon-rich stream is fed to the reforming unit.

[0006] W02023 / 180114 (A1) discloses a process for the co-production of ammonia and methanol with reduced carbon dioxide emission comprising the steps of (a) providing a hydrocarbon feed stock; (b) preheating the hydrocarbon feed stock in a fired heater and / or a reformer waste heat section; (c) steam reforming the preheated hydrocarbon feed stock in at least one steam re- former to a raw synthesis gas comprising hydrogen and carbon oxides where the module M is < 2.05; (d) splitting the raw synthesis gas into a first and second stream; (e) passing the first stream of the raw synthesis gas to water gas shift section comprising one or more shift reactors for generating a shifted synthesis gas; (f) passing the shifted synthesis gas to a carbon dioxide removal section for generating a carbon dioxide depleted synthesis gas; (g) cleaning the carbon depleted synthesis gas in a cleaning unit to a cleaned synthesis gas comprising hydrogen or hydrogen and nitrogen, optionally adding nitrogen to the cleaned synthesis gas to generate an ammonia synthesis gas with a molar ratio of hydrogen to nitrogen of between 2.9 - 3.1 ; (h) converting the ammonia synthesis gas to ammonia; and (i) passing the second stream of the raw synthesis gas to a cooling and water separation section to generate a water depleted synthesis gas; (j) adding a part of the of the carbon depleted synthesis gas from step (f) to the water depleted synthesis gas to generate a methanol synthesis gas with a module M >1 .95; (k) converting the methanol synthesis gas in at least one methanol reactor to methanol and withdrawing a raw methanol product and a purge gas stream containing unconverted methanol synthesis gas; wherein the purge gas stream from step (k) is added to the first stream of the raw synthesis gas upstream to step (e) and / or to the steam reforming in step (c).

[0007] As another example, WO2012 / 069821 (A1) describes removing a purge stream from a methanol synthesis loop, separating hydrogen from the purge stream, passing the purge stream to a reformer, carrying out shift, separating out the hydrogen from carbon dioxide and supplying the separated hydrogen to the synthesis loop.

[0008] Conventionally, the carbon-rich stream, or tail gas, is used as a fuel and combusted to heat feeds for the process, or in the generation of steam. Because the tail gas contains carbon-containing compounds, such as methane, the combustion of the purge gas leads to CO2 emissions from the process.

[0009] The Applicant has found that, a portion of the syngas may be advantageously used as a source of hydrogen to fire the heaters with tail gas recycled to the process. This avoids the need for additional reforming equipment in producing hydrogen for use as fuel and provides a simpler means to manage the purge gas duty.

[0010] Accordingly the invention provides a process for synthesising methanol comprising the steps of (i) reforming a feed gas comprising a hydrocarbon in a hydrocarbon reforming unit comprising a fired heater and an autothermal reformer in series to form a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide; (ii) dividing the synthesis gas into a first portion and a second portion, (iii) converting the first portion of synthesis gas into a methanol product in a methanol synthesis unit comprising one or more methanol synthesis reactors; and (iv) recovering a purge gas stream from the methanol synthesis unit, wherein the second portion of synthesis gas is passed to a hydrogen production unit where it is subjected to one or more stages of water gas shift in a water-gas shift unit to form a hydrogen-enriched gas, the a hydrogen-enriched gas is subjected to a step of carbon dioxide removal in a carbon dioxide removal unit to form a hydrogen product stream and a carbon dioxide stream, the carbon dioxide stream is recovered and at least a portion of the hydrogen product stream is fed to the fired heater as a fuel.

[0011] This process may be established in a new methanol synthesis unit, or an existing methanol synthesis unit may be retrofitted with a hydrogen production unit comprising a water-gas shift unit and a carbon dioxide removal unit and means to recover carbon dioxide and feed a hydrogen stream recovered from the carbon dioxide removal unit to the fired heater as a fuel.

[0012] Accordingly, the invention further provides a method for retrofitting a methanol process comprising a hydrocarbon reforming unit comprising a fired heater and an autothermal reformer in series and a methanol synthesis unit comprising one or more methanol synthesis reactors, wherein the methanol synthesis unit is fed with a synthesis gas from the hydrocarbon reforming unit and generates a methanol product and a purge gas stream; said method comprising (i) installing dividing means downstream of the autothermal reformer and upstream of the methanol synthesis unit to form a first portion of synthesis gas and second portion of synthesis gas, and means to pass the first portion of synthesis gas to the methanol synthesis unit; (ii) installing a hydrogen production unit and means to pass the second portion of synthesis gas to the hydrogen production unit, said hydrogen production unit comprising a water-gas shift unit, a carbon dioxide removal unit and means to feed a hydrogen product stream recovered from the carbon dioxide removal unit to the fired heater as a fuel.

[0013] Together, the water gas shift unit and the carbon dioxide removal unit may be described as a hydrogen production unit because hydrogen is produced in the water-gas shift unit. The use or installation of a hydrogen production unit according to the present invention offer operators a means to significantly decarbonise the methanol process by replacing a carbon containing fuel with a hydrogen fuel for the fired equipment.

[0014] The feed gas comprises a hydrocarbon. The hydrocarbon in the feed gas may be any gaseous or low boiling hydrocarbon, such as natural gas, associated gas, LPG, petroleum distillate, diesel, naphtha or mixtures thereof, or hydrocarbon-containing off-gases from chemical processes, such as a refinery off-gas, or a pre-reformed gas containing methane. The feed gas preferably comprises methane, associated gas or natural gas containing a substantial proportion, e.g. over 50% by volume methane. The feed gas may be compressed to a pressure in the range 10 to 100 bar abs. The pressure of the feed gas may usefully govern the pressure throughout the process. Operating pressure is preferably in the range 15-50 bar abs, more preferably 25-50 bar abs as this provides an enhanced performance from the process. If the feed gas contains sulphur compounds, before or after compression, it is preferably subjected to desulphurisation, e.g. hydrodesulphurisation using Co or Ni catalysts and absorption of hydrogen sulphide using a suitable absorbent, e.g. a zinc oxide bed. To facilitate this hydrogen is preferably added to the feed gas comprising a hydrocarbon. The amount of hydrogen in the resulting mixed gas stream may be in the range 1-20% vol, but is preferably in the range 1-10%, more preferably in the range 1-5%. In a preferred embodiment a portion of the hydrogen product stream recovered from the hydrogen production unit is mixed with the hydrocarbon feed gas. The hydrogen stream may be combined with the hydrocarbon upstream of any hydrodesulphurisation stage.

[0015] If the feed gas contains other contaminants, such as chloride or heavy metal contaminants, these may be removed, prior to reforming, upstream or downstream of any desulphurisation, using conventional adsorbents. Adsorbents suitable for chloride removal are known and include alkalised alumina materials. Similarly, adsorbents for heavy metals such as mercury or arsenic are known and include copper sulphide materials.

[0016] The feed gas desirably comprises steam. The steam to carbon ratio in the feed gas may be in the range of from 0.5:1 to 4:1 , preferably 0.6: 1 to 1 .5: 1 . The steam to carbon ratio is the molar ratio of steam to hydrocarbon carbon in the feed gas.

[0017] The feed gas is pre-heated in the fired heater upstream of the autothermal reformer. However, where the hydrocarbon in the feed gas is a rich natural gas, naphtha or other hydrocarbon-containing feedstock containing hydrocarbons heavier than methane it may be desirable to subject it to a step of adiabatic pre-reforming upstream of the fired heater. Pre-reforming processes are known. In such processes, a hydrocarbon feedstock is mixed with steam, the mixture is then heated, typically to a temperature in the range 400 to 650°C and passed adiabatically through a fixed bed of a suitable particulate steam reforming catalyst, usually a precipitated catalyst having a high nickel content, for example above 40% by weight, expressed as NiO. The heating of the mixture of hydrocarbon feedstock and steam upstream of the pre-reformer may also be performed in the fired heater used to heat the feed gas to the autothermal reformer. During an adiabatic reforming step, any hydrocarbons higher than methane react with steam to give a pre-reformed gas comprising a mixture of methane, carbon oxides and hydrogen. The use of pre-reforming step is desirable to ensure that the feed to the autothermal reformer contains no hydrocarbons higher than methane and also contains a significant amount of hydrogen. This reduces the propensity for cracking and carbon or soot deposition in the autothermal reformer.

[0018] In the present invention, because the second portion of synthesis gas is fed through a carbon dioxide unit that separates CO2 and other gases from the hydrogen product, it is possible to pass a portion of the purge gas stream and / or a portion of the off gas from the hydrogen recovery unit to the feed gas for processing without building up unwanted inerts in the synthesis loop in the methanol synthesis unit. Thus, in some arrangements a portion of the off gas stream is combined with the feed gas and in some arrangements a portion of the purge gas stream is combined with the feed gas.

[0019] The feed gas is pre-heated prior to reforming in the autothermal reformer. This may be achieved by passing the feed though tubes in the fired heater. Desirably, the mixed stream is heated to an inlet temperature in the range 300 to 750°C. Inlet temperatures in the range of 450°C to 550°C are particularly suitable when there is no pre-reformer and higher inlet temperatures in the range of 550 to 650°C are particularly suitable when there is a pre-reformer.

[0020] The reforming unit comprises an autothermal reformer. The autothermal reformer will generally comprise a burner disposed near the top of the reformer, to which is fed the feed gas comprising hydrocarbon and an oxygen-containing gas, a combustion zone beneath the burner through which, typically, a flame extends, above a fixed bed of particulate steam reforming catalyst. In autothermal reforming, the heat for the endothermic steam reforming reactions is provided by combustion of hydrocarbon and hydrogen in the feed gas. The feed gas is typically fed to the top of the autothermal reformer, and the oxygen-containing gas fed to the burner, mixing and combustion occur downstream of the burner generating a heated gas mixture which is brought to equilibrium as it passes through the steam reforming catalyst. If desired, steam may be added to the oxygen containing gas. The autothermal reforming catalyst is usually nickel supported on a refractory support such as rings or pellets of calcium aluminate cement, alumina, titanium dioxide, zirconium dioxide and the like. In a preferred embodiment, the secondary reforming catalyst comprises a layer of a higher activity Ni and / or Rh on zirconium dioxide catalyst over a conventional Ni on alumina catalyst to reduce catalyst support volatilisation.

[0021] The oxygen-containing gas used in the autothermal reformer of the hydrocarbon reforming unit preferably comprises >95% vol. O2, which may be provided by an air separation unit (ASU) or from another oxygen source. Preferably the O2 content is >98% vol or >99% vol. The amount of oxygencontaining gas required in the autothermal reformer is determined by the desired composition of the product gas. In general, increasing the amount of oxygen, thereby increasing the temperature of the reformed gas leaving the autothermal reformer, causes the [H2] I [CO] ratio to decrease and the proportion of carbon dioxide to increase. Preferably, the amount of oxygen added is such that the autothermally reformed gas leaves the autothermal reforming catalyst at a temperature in the range 750-1050°C. The autothermally-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 influenced by the autothermal reformer exit temperature. The hydrocarbon reforming unit produces a synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and steam. The ideal stoichiometric mixture for methanol synthesis arises when there is enough hydrogen to convert all of the carbon oxides into methanol. The methanol synthesis reactions are as follows:

[0022] 2 H2+ CO CH3OH

[0023] The stoichiometry number or R-value of a synthesis gas may be calculated from the molar composition of the components as follows: R = ([H2] - [CO2]) I ([CO] + [CO2]). The ideal R-value for the methanol synthesis reaction is when R = 2. Accordingly, the synthesis gas passed to the methanol synthesis unit preferably has an R-value in the range 1 .95 to 2.05. Autothermal reforming typically produces a synthesis gas having a R value below this range and therefore it is beneficial to add hydrogen to the synthesis gas. This may be achieved as disclosed for example in by adding hydrogen recovered from the purge gas stream. Accordingly, in some arrangements the process comprises (v) separating hydrogen from the purge gas stream in a hydrogen recovery unit to form a separated hydrogen stream and an offgas stream, and combining at least part of the separated hydrogen stream with the first portion of synthesis gas fed to the methanol synthesis unit,

[0024] Alternatively or additionally, a portion of the hydrogen product stream recovered from the hydrogen production unit may be combined with the first portion of the synthesis gas to increase the R-value towards the desired range.

[0025] In the present invention the synthesis gas recovered from the autothermal reformer is divided into first and second portions. The first portion may be between 20 and 90% by volume of the synthesis gas recovered from the autothermal reformer (on a dry gas basis). The first portion is passed to the methanol synthesis unit and the second portion is passed to the hydrogen production unit.

[0026] After leaving the hydrocarbon reforming unit the synthesis gas is then desirably cooled in one or more steps of heat exchange, generally including at least a first stage of steam raising.

[0027] Upstream of the methanol synthesis unit the first portion of synthesis gas is preferably cooled to lower the temperature of the synthesis gas to below the dew point such that steam present in the synthesis gas condenses. The liquid process condensate may be separated using conventional gas-liquid separation techniques to produce a de-watered synthesis gas. Removing steam improves the process efficiency. The de-watered synthesis gas, which may be termed make-up gas, is fed to the methanol synthesis unit. The make-up gas comprises hydrogen, carbon monoxide, carbon dioxide, and small amount of unreacted methane. The make-up gas may be compressed in a synthesis gas compressor to the desired methanol synthesis pressure for feeding to the methanol synthesis unit.

[0028] The methanol synthesis unit operates in a loop from which the purge gas stream is withdrawn. Accordingly, an unreacted gas mixture is combined with the make-up gas to form a feed gas mixture. In addition, hydrogen may be combined with the make-up gas and the unreacted gas mixture to form the feed gas to the methanol synthesis unit. The addition of a hydrogen gas that has been recovered from the purge gas and optionally hydrogen from the hydrogen production unit means that the methanol synthesis reactors can be operated at their optimum R value that will maximise the methanol production.

[0029] Any methanol synthesis unit operating in a synthesis loop may be used in the process of the present invention. The methanol synthesis unit comprises one or more methanol synthesis reactors, for example first, second and optionally third methanol synthesis reactors, each containing a bed of methanol synthesis catalyst, arranged in series and / or parallel that each produce product gas streams containing methanol. The methanol synthesis unit may therefore comprise one, two or more methanol synthesis reactors each containing a bed of methanol synthesis catalyst, and each fed with a feed gas comprising hydrogen and carbon dioxide, each producing a gas mixture containing methanol. A product gas mixture containing methanol is recovered from at least one methanol synthesis reactor. Methanol is recovered from one or more of the product gas mixtures. This may be achieved by cooling one or more of the methanol product gas streams to below the dew point, condensing methanol, and separating a crude liquid methanol product from the unreacted gases.

[0030] Conventional heat exchange and gas-liquid separation equipment may be used. A particularly suitable heat exchange apparatus includes a gas-gas interchanger that uses a feed gas mixture for a methanol synthesis reactor to cool a methanol product gas stream from that reactor. The methanol product gas streams may be treated separately or may be combined before cooling and / or separating the crude liquid methanol product.

[0031] Separation of the crude liquid methanol product from one or more of the methanol product gas streams produces an unreacted gas mixture. A portion of the unreacted gas mixture is returned as a recycle gas stream to one or more of the methanol synthesis reactors. Unreacted gas separated from a product gas mixture recovered from one methanol synthesis reactor may be returned to the same or a different methanol synthesis reactor. The unreacted gas mixture comprises hydrogen, carbon monoxide, and carbon dioxide and so may be used to generate additional methanol. The recycle gas stream may be recovered from at least one of one of the methanol product gas streams and recycled to at least one of the methanol synthesis reactors. If there is more than one recycle gas stream, these may be recycled separately to one or more of the methanol synthesis reactors or combined and fed to one or more of the methanol synthesis reactors.

[0032] The methanol synthesis reactor in the methanol synthesis unit may be an un-cooled adiabatic reactor. Alternatively, the methanol synthesis reactor may be cooled by heat exchange with a synthesis gas, such as in a quench reactor, or a reactor selected from a tube-cooled converter or a gas-cooled converter. Alternatively, the methanol synthesis reactor may be cooled by boiling water under pressure, such as in an axial-flow steam-raising converter, or a radial-flow steam-raising converter.

[0033] In a process comprising first and second methanol synthesis reactors, the first methanol synthesis reactor is preferably cooled by boiling water, such as in an axial-flow steam-raising converter or a radial-flow steam-raising converter, more preferably an axial-flow steam raising converter. The second methanol synthesis reactor may be a radial-flow steam-raising converter. Such arrangements are particularly useful due to the characteristics and performance of these reactors with different feed gas mixtures. Alternatively, the second methanol synthesis reactor may be cooled by a synthesis gas, e.g. a gas comprising hydrogen and carbon dioxide. Accordingly, the second methanol synthesis reactor may be a cooled reactor selected from a tube cooled converter (TCC) and a gas-cooled converter (GCC). A tube-cooled converter is preferred because of its simpler design. The first and second methanol synthesis reactors may be connected in series in which case the synthesis gas fed to the second methanol synthesis reactor comprises at least a portion of a methanol product gas stream recovered from the first methanol synthesis reactor. In such an arrangement, preferably the synthesis gas fed to the second methanol synthesis reactor comprises all of the methanol product gas stream recovered from the first methanol synthesis reactor. Particularly preferred methanol synthesis units are described in US7790775, W02017 / 121980 A1 and WO2017 / 121981 A1.

[0034] The methanol synthesis catalysts in each of the methanol synthesis reactors may be the same or different. The methanol synthesis catalysts are preferably copper-containing methanol synthesis catalysts, which are commercially available. In particular, the methanol synthesis catalysts are one or more particulate copper / zinc oxide / alumina catalysts, which may comprise one or more promoters. Particularly suitable catalysts are Mg-promoted copper / zinc oxide / alumina catalysts as described in US4788175 and SiC>2-doped copper / zinc oxide / alumina catalysts as described in WO2020 / 212681 A1.

[0035] Methanol synthesis may be effected in the one or more methanol synthesis reactors at pressures in the range 10 to 120 bar abs, and temperatures in the range 130°C to 350°C. The pressures at the one or more reactor inlets is preferably 50-100 bar abs, more preferably 70-90 bar abs. The temperature of the synthesis gas at the one or more reactor inlets is preferably in the range 200- 250°C and at the one or more reactor outlets preferably in the range 230-280°C.

[0036] The portion of the unreacted gas mixture making up the recycle gas stream will typically be at a lower pressure than the make-up gas and so preferably the recycle gas stream is compressed by one or more compressors or circulators. At least one compressor is used to circulate the unreacted gas stream. The resulting compressed recycle gas stream may be mixed with make-up gas to form the feed to the one or more methanol synthesis reactors.

[0037] The crude liquid methanol recovered from the methanol synthesis unit contains water, along with small amounts of higher alcohols and other impurities. The crude methanol may first be fed to a flash column or let-down vessel, where dissolved gases are released and separated from the crude liquid methanol stream. The crude liquid methanol may also be subjected to one or more purification stages including one or more, preferably two or three, stages of distillation in a methanol purification unit comprising one, two or more distillation columns. The de-gassing stage and distillation stages may be heated using heat recovered from the process, for example in the cooling of a product gas stream, or by other sources. Preferably at least a portion of the crude methanol is purified by distillation to produce a purified methanol product.

[0038] Dissolved gases released from the flash column or let-down vessel and one or more purification stages may be recycled back to the hydrocarbon reforming unit, methanol synthesis unit or directly to the hydrogen production unit.

[0039] The purified methanol product may be subjected to further processing, for example to produce derivatives such as dimethyl ether or formaldehyde. Alternatively, the methanol may be used as a fuel.

[0040] A portion of the unreacted gas mixture separated from the crude liquid methanol is removed from the loop as the purge gas stream. The purge gas stream is preferably removed continuously to prevent the unwanted build-up of inert gases, such as nitrogen, argon and methane in the synthesis loop. The purge gas stream may be recovered from the separated unreacted gases before or after compression in the circulator. The purge gas stream may comprise 50-90% by volume of hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane.

[0041] In some arrangements, at least a portion and potentially all of the purge gas stream is recycled to the feed gas comprising hydrocarbon that is subjected to autothermal reforming. In this case the inert components are advantageously removed using the hydrogen production unit. However, in preferred arrangements, the purge gas is supplied to a hydrogen recovery unit that separates a hydrogen stream. The separated hydrogen stream may contain at least 80% by volume, preferably at least 90% by volume of H2. The hydrogen recovery units are known and may be selected from a membrane separation unit, a pressure-swing absorption unit or a cryogenic separation unit. The separated hydrogen may be added to the first portion of synthesis gas fed to the methanol synthesis unit to improve the stoichiometry of the synthesis gas. The hydrogen recovery unit also produces an off-gas or carbon-rich tail gas. The off-gas or carbon-rich tail gas may be recycled to the feed gas comprising hydrocarbon that is subjected to autothermal reforming.

[0042] The purge gas stream may contain methanol and so, if desired, upstream of the hydrogen recovery unit, methanol may optionally be recovered from the purge gas stream using a water wash, and the recovered methanol and water sent for purification with the crude methanol.

[0043] If desired, a portion of the purge gas, orwashed purge gas, and / or a portion of the hydrogen product stream recovered from the hydrogen production unit may be used to strip dissolved gases from the crude methanol and the resulting enriched hydrogen fed to the inlet of the compressor feeding the first portion of synthesis gas to the methanol synthesis unit.

[0044] The second portion of synthesis gas is passed to a hydrogen production unit where it is subjected to one or more stages of water gas shift in a water-gas shift unit to form a hydrogen-enriched gas, and a step of carbon dioxide removal from the hydrogen-enriched gas in a carbon dioxide removal unit to form a hydrogen product stream and a carbon dioxide stream. The carbon dioxide stream is recovered and may be stored or used to synthesis additional methanol or other chemical products. At least a portion of the hydrogen product stream is fed to the fired heater as a fuel.

[0045] In addition, if desired a portion of the hydrogen product stream may be combined with the first portion of synthesis gas fed to the methanol synthesis unit.

[0046] The invention may therefore include installation into an existing methanol synthesis unit a hydrogen production unit comprising one or more water-gas shift reaction vessels configured to provide a hydrogen-enriched gas, a carbon dioxide removal unit configured to provide a hydrogen stream and a carbon dioxide stream, and means to convey at least a portion of the hydrogen stream to the fired heater as a fuel.

[0047] The second portion of synthesis gas is subjected to one or more stages of water-gas shift in a water- gas shift unit. Steam is necessary for the water-gas shift reaction. If insufficient steam is present in the synthesis gas, steam may be added upstream of the water gas shift unit, e.g. by direct addition.

[0048] The second portion of the synthesis gas may be passed through one or more beds of water-gas shift catalyst in one or more shift vessels to generate hydrogen and thus produce a hydrogen- enriched, or “shifted”, synthesis gas. At the same time the water gas shift unit converts carbon monoxide in the second portion of the synthesis gas to carbon dioxide. The reaction may be depicted as follows;

[0049] CO + H2O ^ CO2+ H2

[0050] The one or more water-gas shift stages may include stages of high-temperature shift, mediumtemperature shift, isothermal shift and low-temperature shift.

[0051] High-temperature shift may be operated adiabatically in a shift vessel at inlet temperatures in the range 300-400°C, preferably 320-360°C, over a bed of a reduced iron catalyst, such as chromia- promoted magnetite. Alternatively, a potassium promoted zinc-aluminate catalyst may be used. A single stage of high-temperature shift may be used in the present invention. Alternatively, a combination of high-temperature and medium- temperature or low-temperature shift may be used.

[0052] Medium-temperature shift and low-temperature shift stages may be performed using shift vessels containing supported copper-catalysts, particularly copper / zinc oxide / alumina compositions. In low- temperature shift, a gas containing carbon monoxide (preferably < 6% vol CO on a dry basis) and steam (at a steam to total dry gas molar ratio in range 0.3 to 1 .5) may be passed over the catalyst in an adiabatic fixed bed with an outlet temperature in the range 200 to 300°C. The outlet carbon monoxide content may be in the range 0.1 to 1.5%, especially under 0.5% vol on a dry basis if additional steam is added. Alternatively, in medium-temperature shift, the gas containing carbon monoxide and steam may be fed to the catalyst at an inlet temperature in the range 200 to 240°C although the inlet temperature may be as high as 280°C. The outlet temperature may be up to 300°C but may be as high as 360°C.

[0053] Whereas one or more adiabatic water-gas shift stages may be employed, such as a high- temperature shift stage, optionally followed by a low-temperature shift stage, the second portion of synthesis gas may be subjected to a stage of isothermal water-gas shift in a shift vessel in which the catalyst is cooled, optionally followed by one or more adiabatic medium- or low-temperature water-gas shift stages in un-cooled vessels as described above. Whereas the term “isothermal” is used to describe a cooled shift converter, there may be a small increase in temperature of the gas between inlet and outlet, so that the temperature of the hydrogen-enriched reformed gas stream at the exit of the isothermal shift converter may be between 1 and 25 degrees Celsius higher than the inlet temperature. The coolant conveniently may be water under pressure such that partial, or complete, boiling takes place. The water can be in tubes surrounded by catalyst or vice versa. The resulting steam can be used in the process, for example, to drive a turbine, e.g. for electrical power, or to provide process steam for supply to the process. Following the one or more shift stages, the hydrogen-enriched synthesis gas is desirably cooled to a temperature below the dew point so that the steam condenses. This forms a de-watered hydrogen-enriched gas. The liquid water condensate may then be separated using one or more, gas-liquid separators, which may have one or more further cooling stages between them. Any coolant may be used. Typically cooling of the hydrogen-enriched gas may be provided by boiling water under pressure coupled to a steam drum. If desired, cooling may be carried out in heat exchange with the process condensate. As a result, a stream of heated water, which may be used to supply some or all of the steam required for the process may be formed. One or more further stages of cooling are desirable. The cooling may be performed in heat exchange in one or more stages using demineralised water, air, or a combination of these. In a preferred embodiment, cooling is performed in heat exchange with one or more liquids used in the CO2 separation unit. One, two or three stages of condensate separation may be performed. Any condensate not used to generate steam may be sent to water treatment as effluent.

[0054] Typically, the hydrogen-enriched gas stream contains 10 to 30% vol of carbon dioxide (on a dry basis). In the present invention, preferably after separation of the condensed water, carbon dioxide is separated from the hydrogen-enriched gas stream in a carbon dioxide removal unit.

[0055] The carbon dioxide removal unit may operate by means of adsorption of carbon dioxide into a solid adsorbent, such as a molecular sieve, in a pressure swing absorption (PSA) unit, separation of a hydrogen-rich gas using a hydrogen -permeable membrane, or alternatively by absorption into a liquid in a physical wash system or a reactive wash system. Solid adsorbent and membrane systems may be used where the amount of purge gas and / or the purity of the hydrogen stream are not high. However, for improved carbon dioxide removal, a reactive wash system, especially an amine wash system, is preferred. The carbon dioxide may therefore be separated by an acid gas recovery (AGR) process. In the AGR process, a de-watered hydrogen-enriched reformed gas stream (i.e. a de-watered shifted gas) is contacted with a stream of a suitable absorbent liquid, such as an amine, for example monoethanolamine, diethanolamine, methyl diethanolamine and diglycolamine, particularly methyl diethanolamine (MDEA) solution so that the carbon dioxide is absorbed by the liquid to give a laden absorbent liquid and a gas stream having a decreased content of carbon dioxide. The laden absorbent liquid is then regenerated by heating, to desorb the carbon dioxide and to give a regenerated absorbent liquid, which is then recycled to the carbon dioxide absorption stage. The heating may suitably be provided by steam, hot condensate or another suitable heating medium generated by the process. Alternatively, chilled methanol or a glycol may be used to capture the carbon dioxide in a similar manner as the amine. If the carbon dioxide separation step is operated using a liquid washing step as a single pressure process, i.e. essentially the same pressure is employed in the absorption and regeneration steps, only a little recompression of the recycled carbon dioxide will be required. Carbon dioxide removal units of the types described above are commercially available.

[0056] The recovered carbon dioxide is relatively pure and so may be compressed and used for the manufacture of chemicals, purified for use in the food industry, or sent to storage or sequestration or used in enhanced oil recovery (EOR) processes. In cases where the CO2 is to be compressed for storage, transportation, use in EOR processes or conversion to other chemical products, the CO2 may be first dried to prevent liquid water present in trace amounts, from condensing. For example, the CO2 may be dried to a dew point < 10°C by passing it through a bed of a suitable desiccant, such as a zeolite, or contacting it with a glycol in a glycol drying unit.

[0057] Upon the separation of the carbon dioxide, the process provides a hydrogen product gas stream. Where a pure oxygen-containing gas stream (i.e. >95% vol, preferably >98% O2 vol) is used in the autothermal reformer, the hydrogen stream may comprise 75-99% vol hydrogen, preferably 90-99% vol hydrogen, with the balance comprising one or more of methane, carbon monoxide, carbon dioxide and inert gases.

[0058] When the purity of the hydrogen product stream required for downstream purposes needs to be higher than produced by the carbon dioxide removal unit, the hydrogen gas stream may be passed to a purification unit to provide a purified hydrogen product stream. The purification unit may comprise 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 units comprise regenerable porous adsorbent materials that selectively trap gases other than hydrogen and thereby purify it. The purification unit produces a pure hydrogen stream preferably with a purity greater than 99.5% vol, more preferably greater than 99.9% vol. Such systems are commercially available. The purification unit also produces an off gas. The off gas contains carbon compounds and so is preferably not used as a fuel, but rather is fed back into the process as a feed to the hydrocarbon reforming unit.

[0059] Carbon dioxide recovered from the carbon dioxide removal unit may be optionally purified, compressed, and sent for storage or sequestration. Alternatively, at least a portion of the carbon dioxide may be sent to other processes that utilise carbon dioxide as a feed. If desired, depending on the stoichiometry of the feed to the methanol synthesis unit, it may be possible, especially if there is an external source of hydrogen, such as hydrogen from an electrolysis unit, to feed a portion of the recovered carbon dioxide to the methanol synthesis unit to generate additional methanol.

[0060] If desired, a portion of the crude hydrogen or a portion of the purified hydrogen may be compressed, if necessary, and recycled to the hydrocarbon feed for hydrodesulphurisation. The fired heater is fired using at least a portion of the hydrogen product produced by the hydrogen production unit. This offers a potential reduction in CO2 emissions from an existing process using purge gas and natural gas mixtures as fuel of at least 90% and potentially 95%, or higher. The replacement of the conventional carbon-containing fuel gas may require adjustment of one or more of the burners in the fired heater, or replacement of one or more of the burners. Therefore, where an existing fired heater is retrofitted with a hydrogen production unit as described above, the retrofitting method may include installation of new H2 fuel burners in the fired heater.

[0061] The invention will now be further illustrated by reference to the Figures in which;

[0062] Figure 1 is a flow sheet depicting a methanol synthesis unit according to one embodiment of the invention comprising a fired heater and an autothermal reformer passing synthesis gas portions to a methanol synthesis unit and a hydrogen production unit, with hydrogen product supplied as fuel for the fired heater.

[0063] It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as feedstock drums, pumps, vacuum pumps, compressors, gas recycling compressors, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks and the like may be required in a commercial plant. Provision of such ancillary equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

[0064] In Figure 1 , natural gas supplied by line 10 at a pressure in the range of 15 to 50 bar abs is combined with steam (not shown) at a steam to carbon ratio in the range of 0.5:1 to 4:1 and an off gas recycle stream 12 and the resulting feed gas mixture fed via line 14 to a hydrocarbon reforming unit comprising a fired heater 16, where a hydrogen fuel gas stream fed by line 18 is combusted to heat the mixture to a temperature in the range of 300°C to 750°C,. A flue gas 20 is recovered from the fired heater 16. The heated feed gas mixture is passed from the heater 16 via line 22 to an autothermal reformer 24. In the autothermal reformer 24, the heated feed gas mixture 22 and an oxygen gas stream provided by line 26 from an air separation unit (not shown) are fed to a burner, where partial combustion takes place generating a hot gas mixture that is adiabatically reformed in a bed of steam reforming catalyst disposed below the burner, to generate a synthesis gas mixture comprising hydrogen, carbon monoxide, carbon dioxide and steam.

[0065] The synthesis gas is recovered from the autothermal reformer 24 at a temperature above 900°C via line 28. The synthesis gas is divided into a first portion 30 and a second portion 32. The first portion 30 comprises 20 to 90% of the synthesis gas 28 on a dry gas basis. The first portion is passed to a heat recovery unit 34, where the synthesis gas is cooled in two or more stages by heat exchange in an interchanger with a process stream, and / or using water and air as coolant to cool the synthesis gas to below the dew point such that the steam condenses. In the heat recovery unit 34, condensate is recovered from cooled synthesis gas using one or more gas liquid separators (not shown) to generate a condensate stream 36, which is used to saturate the natural gas feed with steam or as a source of steam used in the process.

[0066] Separation of the condensate generates a de-watered synthesis gas or make-up gas, which is recovered from the heat recovery unit 34 via line 38, mixed with a hydrogen-enriched gas stream provided by line 40, and the resulting hydrogen-enriched make-up gas compressed in syngas compressor 42. The compressed hydrogen-enriched make-up gas 44 is then combined with a recycle stream of unreacted gas provided by line 46 and the combined feed gas fed to a circulating loop compressor 48. The compressed feed gas 50 is pre-heated in interchanger 52 and fed to a methanol synthesis unit 54 comprising one or more methanol synthesis reactors containing a methanol synthesis catalyst. The methanol synthesis unit 54 may comprise one, two or more methanol synthesis reactors, which may be cooled or uncooled, and connected in parallel or series. Methanol synthesis reactions take place over the methanol synthesis catalyst to convert hydrogen, carbon monoxide and carbon dioxide to a gaseous methanol product mixture comprising methanol and steam.

[0067] The gaseous methanol product mixture is recovered from the methanol synthesis unit 54 via line 56, cooled in interchanger 52 and then in one or more further stages of cooling in heat exchangers 58 to below the dew point at which the methanol and steam condense. The cooled mixture is then fed via line 60 to a gas-liquid separator 62 that separates a liquid crude methanol stream from the unreacted gas. Crude methanol is recovered from the separator 62 via line 64 and sent for purification to provide a purified methanol product. The unreacted gas is recovered from the separator 62 via line 66. A purge gas stream is taken from line 66 via line 68 and the remaining unreacted gas is mixed with the hydrogen-enriched make-up gas via line 46.

[0068] The purge gas stream 68 may be divided. In one arrangement all of the purge gas is fed via line 68 to a hydrogen recovery unit 70 in which the purge gas stream is separated into a hydrogen-rich stream and a carbon-rich off gas stream by passing the purge gas stream through a suitable membrane. The hydrogen-rich gas stream is recovered from the separation unit 70 via line 40 and mixed with the make-up gas in line 38 to form the hydrogen-enriched make-up gas. The carbon-rich off gas stream is recovered from the hydrogen recovery unit 70 via line 72 and recycled to the reformer feed via off-gas recycle line 12. Optionally, at least a portion of the purge gas stream 68 may by-pass the hydrogen recovery unit 70 via line 74 (shown as a dotted line) and be recycled to the off-gas recycle stream 12. The second portion of synthesis gas recovered from the autothermal reformer via line 32 is fed to a hydrogen production unit comprising a water gas shift unit 76 and a carbon dioxide removal unit 80. The second portion is desirably cooled in a heat recovery unit (not shown) upstream of the water gas shift unit 76 to reduce the temperature of the second portion to the inlet temperature for the water-gas shift unit. In one arrangement, the cooling reduces the temperature to between 200 and 300°C and above the dew point, such that the cooled gas may be fed directly, after optional steam addition (not shown), to the water-gas shift unit 76. The water gas shift unit 76, desirably comprises an isothermal shift vessel containing an isothermal shift catalyst in which the synthesis gas becomes enriched in hydrogen by the water-gas shift reaction to form a hydrogen-enriched shifted gas stream.

[0069] The hydrogen-enriched shifted gas recovered from the water gas shift unit 76 is then fed to a heat recovery unit (not shown) that cools the gas to below the dew point such that remaining steam condenses. The heat recovery unit comprises one of more gas liquid separators that separate the condensate, which is recovered for use in the process. The resulting dewatered hydrogen-enriched gas is fed via line 78 to a carbon dioxide removal unit 80 operating by means of an amine wash, which absorbs carbon dioxide from the dewatered hydrogen-enriched gas to produce a hydrogen product stream. The hydrogen product stream is recovered from the carbon dioxide removal unit 80 and fed via line 18 to the fired heater 16. If desired, a portion of the hydrogen product may be taken from line 18 via line 82 (shown as a dotted line) and used to supplement the recovered hydrogen stream 40 added to the make-up gas 38 fed to the methanol synthesis reactor 54. Regeneration of the amine absorbent in the carbon dioxide removal unit 80 generates a carbon dioxide stream, which is recovered from the unit 80 via line 84. The recovered carbon dioxide may be compressed and sent for sequestration.

[0070] In a retrofit of an existing plant with hydrogen recovery, instead of feeding the carbon rich off gas to the fired heater as a fuel, a hydrogen production unit is installed and fed with the second synthesis gas portion to generate a carbon dioxide stream which is recovered, and a hydrogen stream which is used as fuel in the fired heaters.

[0071] The invention is further illustrated by reference to the calculated mass balance prepared using standard process modelling software for the process depicted in Figure 1 where 20% of the synthesis gas from the autothermal reformer is passed to the hydrogen production unit.

[0072] The combination of features provides an efficient low CO2 emissions methanol process.

Claims

1. A method for synthesizing methanol, comprising the steps of (i) reforming a hydrocarbon-containing feed gas in a hydrocarbon reforming unit comprising a sequentially fired heater and an autothermal reforming unit to form a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide; (ii) separating the synthesis gas into a first portion and a second portion; (iii) converting the first portion of the synthesis gas into a methanol product in a methanol synthesis unit comprising one or more methanol synthesis reactors;(iv) extracting a purge gas stream from a methanol synthesis unit and (v) separating hydrogen from the purge gas stream in a hydrogen recovery unit to form a separated hydrogen stream and an off-gas stream, and combining at least a portion of the separated hydrogen stream with a first portion of synthesis gas supplied to the methanol synthesis unit, wherein a second portion of the synthesis gas is passed to a hydrogen production unit wherein it is subjected to one or more water gas shift stages in a water gas shift unit to form a hydrogen-rich gas, wherein the hydrogen-rich gas is subjected to a carbon dioxide removal step in a carbon dioxide removal unit to form a hydrogen product stream and a carbon dioxide stream, wherein the carbon dioxide stream is extracted and at least a portion of the hydrogen product stream is supplied to a fired heater as fuel.

2. The method according to claim 1, wherein the feed gas comprises natural gas or pre-converted gas containing methane.

3. The method according to claim 1 or 2, wherein a portion of the purge gas stream is combined with the feed gas.

4. The method according to any one of claims 1 to 3, wherein a portion of the exhaust gas stream is combined with the feed gas.

5. The method according to any one of claims 1 to 4, wherein oxygen-containing gas containing ≥95 vol.% O2 is fed to the autothermal reforming unit.

6. The method according to any one of claims 1 to 5, wherein the first portion comprises from 20 to 90 vol.% of the synthesis gas extracted from the autothermal reforming unit (calculated as dry gas).

7. The method according to any one of claims 1 to 6, wherein the first portion of the synthesis gas is cooled in one or more stages to a temperature below the dew point, and the condensate is separated to obtain dehydrated synthesis gas, which is fed to a methanol synthesis unit.

8. The method according to any one of claims 1 to 7, wherein the methanol synthesis unit comprises one, two or more methanol synthesis reactors, each of which comprises a bed of methanol synthesis catalyst, and wherein the methanol product is recovered from at least one methanol synthesis reactor.

9. The method according to claim 8, wherein the mixture of unreacted gas separated from the methanol product extracted from one methanol synthesis reactor is returned to the same or another methanol synthesis reactor.

10. The method according to any one of claims 1 to 9, wherein the methanol product is fed to a flash column or pressure relief vessel where dissolved gases are separated and recovered from the crude liquid methanol stream, and wherein the crude liquid methanol stream is subjected to one or more purification stages in a methanol purification unit comprising one, two or more distillation columns to obtain a purified methanol product.

11. The method of claim 10, wherein the dissolved gases released from the flash column or pressure relief vessel and one or more purification stages are recycled back to the hydrocarbon reforming unit, the methanol synthesis unit, or directly to the hydrogen production unit.

12. The method according to any one of claims 1 to 11, wherein the hydrogen recovery unit comprises a membrane separation unit, a pressure swing absorption unit or a cryogenic separation unit.

13. The method according to any one of claims 1 to 12, wherein one or more water gas reforming stages comprise an adiabatic high-temperature reformer, an adiabatic medium-temperature reformer, or an isothermal reformer cooled by boiling water under pressure.

14. The method according to any one of claims 1 to 13, wherein the carbon dioxide removal unit operates by adsorbing carbon dioxide into a solid adsorbent, separating hydrogen-rich gas using a hydrogen-permeable membrane, or by absorption into a liquid in a physical washing system or a reactive washing system, preferably by absorption using an amine wash.

15. The method according to any one of claims 1 to 14, wherein a portion of the hydrogen product stream is combined with a first portion of the synthesis gas fed to the methanol synthesis unit.

16. The method according to any one of claims 1 to 15, wherein at least part of the carbon dioxide extracted from the carbon dioxide removal unit, optionally with additional hydrogen from an external source, is fed to the first portion of the synthesis gas passed to the methanol synthesis unit.

17. A method for upgrading a methanol production process, comprising a hydrocarbon reforming unit comprising a series-connected flame heater and an autothermal reforming unit, and a methanol synthesis unit comprising one or more methanol synthesis reactors, wherein synthesis gas from the hydrocarbon reforming unit is fed to the methanol synthesis unit and a methanol product and a purge gas stream are generated; wherein said method comprises the steps of (i) installing a separating means downstream of the autothermal reforming unit and upstream of the methanol synthesis unit for forming a first portion of the synthesis gas and a second portion of the synthesis gas, and also means for passing the first portion of the synthesis gas to the methanol synthesis unit;(ii) installing a hydrogen production unit and means for passing a second portion of the synthesis gas to a hydrogen production unit, wherein said hydrogen production unit comprises a water gas shift unit, a carbon dioxide removal unit, and means for feeding a hydrogen product stream recovered from the carbon dioxide removal unit to a fired heater as fuel; (iii) installing a hydrogen recovery unit for separating hydrogen from the purge gas stream to form a separated hydrogen stream and an off-gas stream, and means for combining at least a portion of the separated hydrogen stream with the first portion of the synthesis gas fed to a methanol synthesis unit.

18. The method according to claim 17, in which the hydrogen recovery unit is arranged to separate the hydrogen-enriched gas from the purge gas stream and means configured to feed at least a portion of the hydrogen-enriched gas into a first portion of the synthesis gas fed to the methanol synthesis unit.

19. The method according to claim 17 or 18, further comprising installing H2 fuel burners in the flame heater.

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