GB2630460A

GB2630460A - Process for synthesising methanol

Info

Publication number: GB2630460A
Application number: GB2406100.4A
Authority: GB
Inventors: John Cassidy Paul; David Yorath Neil; Chi Yiu Kar
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2023-05-26
Filing date: 2024-05-01
Publication date: 2024-11-27
Also published as: WO2024246479A1; GB202406100D0

Abstract

A process for synthesising methanol with low carbon dioxide emissions comprising: reforming a hydrocarbon feedstock, preferably natural gas, in a reforming section comprising a first autothermal reformer to form a synthesis gas containing hydrogen, carbon monoxide and carbon dioxide; converting the synthesis gas into a methanol product in a methanol loop (comprising one or more methanol synthesis reactors); and recovering a purge gas stream from the methanol loop; wherein at least some of the purge gas stream is used to generate a purge gas feedstock stream by treatment of the purge gas stream in a purge gas treatment unit by steps of reforming, water-gas shift and carbon dioxide removal to form a hydrogen stream, which is used as fuel for example to heat process streams within the reforming operations, methanol loop and/or purge gas treatment unit. The process does not comprise a steam methane reformer upstream of the first autothermal reformer and the removed carbon dioxide stream is recovered. The disclosure also relates to a method of retrofitting a methanol production unit by installing a purge gas treatment unit as described.

Description

Process for synthesising methanol

Technical Field

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.

Background to the invention

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 make-up 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.

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, VV02012/069821A1 describes a process in which a purge stream is removed from a synthesis loop, hydrogen is separated from the purge stream, and the remaining stream treated in steps of reforming, shift and hydrogen separation. The separated hydrogen is supplied to the synthesis loop.

Alternatively, instead of treating the tail gas (produced by separating hydrogen from the purge gas) to steps of reforming, shift and hydrogen separation, the tail gas may be used as a fuel that is 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 leads to CO2 emissions from the process.

Processes where this carbon is re-used in the process are known. For example, W02020/249924A1 discloses a methanol synthesis process utilising a reforming unit comprising a heat exchange reformer and an autothermal reformer in series for the synthesis gas generation, 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.

Alternatively, US2014/0058002A1 describes removing a purge stream from a methanol synthesis loop, separating hydrogen from the purge stream, passing the hydrogen-depleted purge stream to a reformer, carrying out shift, separating out the hydrogen from carbon dioxide and supplying the separated hydrogen to the synthesis loop.

Summary of the invention

The Applicant has found that, in order to reduce carbon emissions associated with methanol synthesis, at least a portion of the purge gas may be treated in a purge gas treatment unit to generate hydrogen which is then used as fuel e.g. to heat process streams and/or for steam raising and/or power.

Accordingly the invention provides a process for synthesising methanol comprising the steps of: (i) reforming a hydrocarbon feedstock in a reforming section comprising a first autothermal reformer to form a synthesis gas containing 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 reactors; and (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 feedstock stream which is treated in a purge gas treatment unit by subjecting the purge gas feedstock stream to: (iv) partial oxidation in a partial oxidation reactor to form a partially-oxidised purge gas or autothermal reforming in a second autothermal reformer to form a reformed purge gas, followed by: (v) one or more stages of water gas shift of the partially-oxidised or reformed purge gas in a water-gas shift unit to form a hydrogen-enriched gas, and (vi) a step of carbon dioxide removal from the hydrogen-enriched 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.

Steam methane reformers are known and generally comprise a radiant section containing a plurality of catalyst-containing reformer tubes through which a mixture of hydrocarbons and steam is passed. 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 that combust the fuel to generate heat for the endothermic steam reforming reactions.

Together, the partial oxidation reactor or second autothermal reformer, the water gas shift unit and the carbon dioxide removal unit may be described as a purge gas treatment unit. The use or installation of a purge gas treatment unit according to the present invention offer operators a means to significantly decarbonise the methanol process by replacing the fuel for fired equipment, which is typically a hydrocarbon stream such as natural gas, with hydrogen fuel generated by the purge gas treatment unit. For instance, the hydrogen fuel may be used in any fired heaters used in the methanol process (e.g. to preheat feeds or generate steam for the process).

This process may be established in a new methanol production unit, or an existing methanol production unit may be retrofitted.

Accordingly, the invention further provides a method for retrofitting a methanol production unit which initially comprises: 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 reactors, wherein the methanol loop is fed with a synthesis gas generated by the first autothermal reformer and generates a methanol product and a purge gas stream; wherein there is no steam methane reformer upstream of the first autothermal reformer; the method comprising installing a purge gas treatment unit which comprises: a partial oxidation reactor or a second autothermal reformer, arranged to receive the purge gas stream and produce a partially-oxidised purge gas or a reformed purge gas; a water-gas shift unit arranged to receive the partially-oxidised purge gas or the reformed purge gas and produce a hydrogen-enriched stream; a carbon dioxide removal unit arranged to receive the hydrogen-enriched stream and produce a hydrogen stream; the method further comprising adapting the methanol production unit to use at least part of the hydrogen stream as fuel.

Description of the Figures

Figure 1 is a flow sheet depicting a methanol production unit according to one embodiment of the invention comprising an autothermal reformer and a purge gas treatment unit, with hydrogen product supplied as fuel for any fired equipment.

Figure 2 is a flow sheet depicting one embodiment of a purge gas treatment unit suitable for use in the present invention.

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.

In Figure 1, a natural gas feed (101) is divided into a first portion (103) and a second portion (105). The first portion is pre-heated in a fired heater (106) which is fired by combusting at least a portion (104) of the hydrogen stream (157) generated by the purge gas treatment unit. The heated natural gas from the fired heater and an oxygen gas stream (107) from an air separation unit (not shown) are fed to the burner of a first autothermal reformer (109), 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. Steam is added upstream of the first autothermal reformer and/or upstream of the fired heater. The synthesis gas (111) is recovered from the first autothermal reformer at a temperature above 900°C and passed to a heat recovery unit (113), 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, condensate is recovered from cooled synthesis gas using one or more gas liquid separators (not shown) to generate a condensate stream (115), which is used as a source of steam used in the steam reforming stages of the process.

Separation of the condensate generates a make-up gas (117), which is recovered from the heat recovery unit, mixed with hydrogen generated by the purge-gas treatment unit (153) and the hydrogen separation unit (147), and the resulting hydrogen-enriched make-up gas compressed in syngas compressor (121). The compressed hydrogen-enriched make-up gas (123) is then combined with a recycle stream of unreacted gas (125) and the combined feed gas fed to a circulating loop compressor (127). The compressed feed gas is pre-heated in interchanger (129) and fed to a methanol synthesis unit (131) comprising one or more methanol synthesis reactors containing a methanol synthesis catalyst. The methanol synthesis unit 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.

The gaseous methanol product mixture (133) is recovered from the methanol synthesis unit, cooled in interchanger (129) and then in one or more further stages of cooling in heat exchangers (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) that separates a liquid crude methanol stream from the unreacted gas. Crude methanol (141) is recovered from the separator and sent for purification to provide a purified methanol product. The unreacted gas (143) is recovered from the separator. A purge gas stream (145) is taken and the remaining unreacted gas (125) is combined with the hydrogen-enriched make-up gas.

The purge gas stream is fed to a hydrogen separation unit (147) in which the purge gas stream 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 the make-up gas to form the hydrogen-enriched make-up gas.

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 feedstock stream (151), and fed to a purge gas treatment unit (153). In the purge gas treatment unit, further described by reference to Figure 2, the combined carbon-rich purge gas stream and natural gas mixture are heated and passed to a purge gas reforming unit in which it is subjected to autothermal reforming with an oxygen-containing gas (155), such as air, oxygen-enriched air or oxygen gas fed via line in a purge gas autothermal reformer. The reformed gas is subjected to 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 generates 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 generates a hydrogen stream (157), at least a portion of which (104) is used in the fired heater (106) as fuel. A further portion (185) is used for R ratio adjustment upstream of the methanol loop.

In Figure 2, one embodiment of a suitable purge gas treatment unit is depicted. The second portion (205) of the natural gas feed, optionally after a step of adiabatic pre-reforming, is combined with the carbon rich gas (249). Other carbon-containing gases, such as an off gas from a let-down vessel and/or distillation overheads may be compressed and combined with the feed gas. Steam (259) is optionally added and the resulting mixture, termed the purge gas feedstock stream, is heated in a fired heater (261). The fired heater is heated by combustion of a hydrogen stream (263) with air to produce a combustion flue gas (265).

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 oxygen-containing gas (255). The oxygen containing gas fed to the second autothermal reformer may be air and/or a portion of the oxygen containing gas recovered from the air separation unit used to provide the oxygen containing gas stream (255). The partially combusted gas is then adiabatically steam reformed in a bed of steam reforming catalyst disposed beneath the burner within the autothermal reformer. The autothermal reforming generates 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, the cooling reduces the temperature of the reformed purge gas to between 200 and 300°C and above the dew point, such that the cooled reformed gas may be fed directly, after optional steam addition, to a water-gas shift unit (273). Less preferably, the reformed purge gas may be cooled to below the dew point such that the steam condenses, and condensate recovered using one or more gas liquid separators and used as a source of steam for the process. Then, after heating and steam addition the de-watered reformed gas may be fed to the water gas shift unit.

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

The hydrogen-enriched reformed gas (275) recovered from the water gas shift unit is then fed to a heat recovery unit (277) that cools the hydrogen-enriched 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 (279), which is recovered for use in the process.

The resulting dewatered hydrogen-enriched gas (281) is fed from the heat recovery unit to a carbon dioxide removal unit (283) operating by means of an amine wash, which absorbs carbon dioxide from the dewatered hydrogen-enriched gas to produce a hydrogen stream. The hydrogen stream (257) is recovered from the carbon dioxide removal unit and divided between the portion (263) fed to the fired heater in the purge gas treatment unit and the portion (204) is used as fuel in one or more fired equipment, including the Fired Heater upstream of the first autothermal reformer. A further portion of the hydrogen stream (285) may also be used for R ratio adjustment upstream of the methanol loop.

Regeneration of the amine absorbent in the carbon dioxide removal unit generates a carbon dioxide stream (256), which may be compressed and sent for sequestration.

In a retrofit of an existing plant with hydrogen recovery, instead of feeding hydrocarbon-containing streams as fuel to one or more fired heaters, an installed purge gas treatment unit is used to generate a carbon dioxide stream which is recovered, and a hydrogen stream which is used as fuel in one or more fired heaters.

Detailed description of the invention

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

Feed to the first autothemmi reformer The hydrocarbon feed 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 gaseous mixture preferably comprises methane, associated gas or natural gas containing a substantial proportion, e.g. over 50% by volume methane. Natural gas is especially preferred. The hydrocarbon may be compressed to a pressure typically in the range 10-100 bar abs.

The hydrocarbon feedstock may be pre-treated, before or after compression, to remove impurities such as sulphur compounds, chloride contaminants and/or heavy metal contaminants. The hydrocarbon feedstock may also be subject to a step of pre-reforming carried out upstream of the first autothermal reformer.

First autothermal reformer The hydrocarbon feedstock is subjected to steam reforming in a reforming section comprising a first autothermal reformer. The term "first" is used to distinguish the first autothermal from the second autothermal reformer; the latter of which may be a component of the purge gas treatment unit as described below.

The hydrocarbon feedstock is typically mixed with steam upstream from the first autothermal reformer. Steam introduction may be effected for instance by direct injection of steam and/or by saturation of the hydrocarbon feedstock by contact of the latter with a stream of heated water in a saturator. The amount of steam introduced may be such as to give a steam to carbon ratio at the inlet to the first autothermal reformer of 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 moles of steam per gram atom of hydrocarbon carbon in the hydrocarbon feedstock. The amount of steam is preferably minimised as this leads to a lower cost, more efficient process.

The hydrocarbon feedstock, containing hydrocarbons and steam, is desirably pre-heated prior to reforming in the first autothermal reformer. If a fired heater is used, then it is preferably heated by combustion of a portion of the hydrogen stream produced in the purge gas treatment unit. Desirably, the mixed stream is heated to an inlet temperature in the range 450 to 700°C. Inlet temperatures in the range of 450 to 550°C are particularly suitable when there is no pre-reformer upstream of the first autothermal reformer, whereas higher inlet temperatures in the range of 550 to 700°C are particularly suitable when there is a pre-reformer upstream of the first autothermal reformer.

The hydrocarbon feedstock/steam gas mixture is then subjected to reforming in the first autothermal reformer. An autothermal reformer generally comprises a burner disposed near the top of the reformer, to which is fed the hydrocarbon feedstock 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 or secondary reforming, the heat for the endothermic steam reforming reactions is provided by combustion of hydrocarbon and hydrogen in the feed gas. The hydrocarbon feedstock 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. Whereas some steam may be added to the oxygen containing gas, preferably no steam is added so that the low overall steam ratio for the reforming process is achieved. 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.

The oxygen-containing gas used in the first autothermal reformer preferably comprises a95% vol. 02, which may be provided by an air separation unit (ASU) or from another oxygen source. An 02 concentration of a95% vol. is preferred instead of air, because if air is used then a greater proportion of the loop must be withdrawn as a purge to remove N2. Preferably the 02 content is a98% vol or a99% vol. The amount of oxygen-containing 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] / [CO] ratio to decrease and the proportion of carbon dioxide to decrease. 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.

In some embodiments the reforming section may comprise an adiabatic pre-reformer which is used in conjunction with the first autothermal reformer. In the pre-reforming stage, the hydrocarbon is subjected to a step of adiabatic steam reforming in which the hydrocarbon/steam mixture, is desirably heated to a temperature in the range 400-650°C, and then passed adiabatically through a bed of a suitable catalyst, usually a catalyst having a high nickel content, for example above 40% by weight. During such an adiabatic reforming step, any hydrocarbons higher than methane read with steam to give a mixture of methane, carbon oxides and hydrogen. The use of such an adiabatic reforming step, commonly termed pre-reforming, is desirable to ensure that the feed to the autothermal reformer contains no hydrocarbons higher than methane and also contains some hydrogen. This may be desirable in cases of low steam to carbon ratio mixtures in order to minimise the risk of soot formation in the autothermal reformer.

In some embodiments the reforming section may comprise a gas-heated reformer which is used in conjunction with the first autothermal reformer. The first autothermal reformer and gas-heated reformer may be arranged in series or in parallel. When arranged in series, the hydrocarbon feedstock stream is fed to catalyst filled tubes in the gas-heated reformer (tube-side) where steam reforming reactions take place to produce a partially reformed gas stream. The partially reformed gas stream is fed to the first autothermal reformer where further steam reforming reactions take place to produce a reformed gas stream. The reformed gas stream is fed to the shell of the gas-heated reformer to provide heat required for steam reforming taking place in the gas-heated reformer. When arranged in parallel, a first portion of the hydrocarbon feedstock stream is fed to catalyst filled tubes in the gas-heated reformer (tube-side) where steam reforming reactions take place to produce a first partially reformed gas stream. A second portion of the hydrocarbon feedstock stream is fed to the first autothermal reformer where steam reforming reactions take place to produce a second partially reformed gas stream. The first and second partially reformed gas streams are combined and used to provide heat required for steam reforming taking place in the gas-heated reformer. Alternatively, the first partially reformed gas stream may be combined with the second portion of the hydrocarbon feedstock fed to the first autothermal reformer.

The stream exiting the first autothermal reformer (and if present, the gas-heated reformer) is 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: 3 H2 + CO2 CH3OH + H2O 2 H2 + CO CH3OH 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]) / ([CO] + [CO2]). The ideal R-value for the methanol synthesis reaction is when R = 2. Typically, synthesis gas recovered from the first autothermal reformer has a R value of around 1.8. It is preferred that H2 is 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 to within the range 1.95 to 2.05. Preferred sources of hydrogen used for R value adjustment include a H2 rich stream from the hydrogen separation unit, a H2 stream from outside of the process (OSBL) or a portion of the hydrogen stream generated by the purge gas treatment unit. R value adjustment is preferably carried out after heat recovery, described below.

After leaving the first autothermal reformer (and if present, the gas-heated reformer) 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. Preferably, following such steam raising the reformed gas is cooled by heat exchange with one or more of the following streams; the hydrocarbon feedstock, water (including process condensate) used to generate steam, which may be used for heating or used in a pre-reforming stage, the mixture hydrocarbon and steam, a pre-reformed gas mixture, and in the distillation of crude methanol.

The cooling is preferably performed to lower the temperature of the synthesis gas from the first autothermal reformer to below the dew point such that steam present in the synthesis gas condenses.

The liquid process condensate may be separated from the synthesis gas, which may be termed makeup gas at this point, by conventional gas-liquid separation equipment.

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 loop.

The methanol loop (described later) produces an unreacted gas mixture, which is combined with the make-up gas to form a feed gas mixture. In addition, if a hydrogen-rich gas is recovered from the purge gas upstream on the purge gas treatment unit, this hydrogen-rich gas may also be combined with the make-up gas and the unreacted gas mixture to form the feed gas to the methanol loop. The R-value of the feed gas may be in the range 3 to 5 or higher. The addition of a hydrogen-rich gas that has been recovered from the loop purge gas means that the methanol synthesis reactors can be operated at their optimum R value of 3 to 5 with a make-up gas with an R value close to the stoichiometric optimum value of 2 that will maximise the methanol production.

Methanol loop Any methanol loop may be used to synthesise methanol in the process of the present invention. The methanol loop 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 loop 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.

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.

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 or loop 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.

The methanol synthesis reactor in the methanol loop 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.

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. If a third methanol synthesis reactor is present, it is preferably cooled by boiling water. The third methanol synthesis reactor may then suitably be a steam-raising converter selected from an axial-flow steam-raising converter and a radial-flow steam-raising converter, most preferably an axial-flow steam raising converter. 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 loops are described in US7790775A, W02017/121980A1 and W02017/121981A1.

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 US4788175A and SiO2-doped copper/zinc oxide/alumina catalysts as described in VV02020/212681A1.

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.

The portion of the unreacted gas mixture making up the recycle gas stream to the methanol loop 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 in the methanol loop.

The recycle ratios to form the feed gas mixtures to the one or more methanol synthesis reactors may be in the range 0.5:1 to 8:1, preferably 2:1 to 6:1, in the case of a single methanol synthesis reactor, or in the range of 0.5:1 to 5:1, preferably 1:1 to 3:1, in the case of a loop with two or more methanol synthesis reactors arranged in series. By the term "recycle ratio", we mean the molar flow ratio of the recycled unreacted gas stream to the make-up gas that form the gas mixtures fed to the one or more methanol synthesis reactors.

The crude liquid methanol recovered from the methanol loop contains water, along with small amounts of higher alcohols and other impurities. The crude methanol may first be fed to a flash column or letdown 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. Off-gases may be usefully recovered from the let-down vessel and at least the topping column in the distillation unit and fed to the purge gas treatment unit, to further reduce carbon dioxide emissions from the process. 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.

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.

Purge gas treatment unit 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. Purge gas streams in processes using steam reforming as a source of the make-up gas may be hydrogen rich. The purge gas stream preferably contains 50-90% by volume of hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane.

In the present invention, at least a portion of the purge gas stream is subjected to partial oxidation in a partial oxidation reactor to form a partially-oxidised 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 of the partially-oxidised or reformed purge gas 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 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.

The purge gas stream may contain methanol and so, if desired, upstream of the purge gas treatment 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.

At least a portion of the purge gas may be fed directly to the purge gas treatment unit, for example where the synthesis gas has an R-value of 1.95 or higher.

Alternatively, the purge gas stream may 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 feedstock stream. At least a portion of the hydrogen-rich gas stream may be added to the synthesis gas upstream of the methanol loop, e.g. to adjust the R value of the synthesis gas from the first autothermal reformer.

By "carbon-rich purge gas" we mean a gas stream that has a higher proportion of carbon-containing compounds (carbon monoxide, carbon dioxide and methane) than the parent purge gas. While individual components may have the same, or even lower, proportion than in the parent purge gas, the total of all carbon-containing compounds will be higher in the carbon-rich purge gas. Consequently, in such arrangements the portion of the purge gas fed to the purge gas treatment unit is the carbon-rich purge gas. Separating hydrogen from the purge gas upstream of the purge gas treatment unit also has the advantage that it reduces the loss of hydrogen from the process by combustion in the purge gas treatment unit. The separation of the hydrogen-rich and carbon-rich purge gas streams may be practiced using any known separation equipment, such as a hydrogen membrane separator or 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 parent purge gas stream may be recovered. Where 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 sent for partial oxidation or reforming in the purge gas treatment unit without further compression. This is highly desirable. Where a pressure swing absorption system is used to separate the hydrogen-rich gas, the carbon-rich purge gas may be at a lower pressure and so is less preferred. The hydrogen-rich gas stream recovered from the purge gas stream desirably comprises >95% by volume of H2. The separated hydrogen, in addition to being fed to the methanol loop may also be used upstream in hydrodesulphurisation of the hydrocarbon feedstock and/or used to strip dissolved gases from the crude methanol, and/or be used as fuel.

The feed to the purge gas treatment unit may be supplemented with a portion of the hydrocarbon feedstock and/or one or more hydrocarbon containing off-gases. This increases the flexibility of the purge gas treatment unit and is advantageous during start-up of the process. If desired, the hydrocarbon feedstock and/or one or more hydrocarbon containing off-gases may be treated in a pre-reformer upstream of the partial oxidation reactor or second autothermal reformer e.g. using an adiabatic pre-reformer as described above.

The feed to the purge gas treatment unit may, if desired, be supplemented with one or more other hydrocarbon containing off-gas stream(s).

Second autothermal reformer or partial oxidation reactor The purge gas treatment unit includes a partial oxidation reactor or a second autothermal reformer. Here, "second" is simply used to differentiate the autothermal reformer of the purge gas treatment unit from the first autothermal reformer described previously.

The scale of the second autothermal reformer in the purge gas reforming unit may be the same or different to that of the first autothermal reformer but the design, catalyst and operation are conveniently the same as described above. The portion of the purge gas fed to the second autothermal reformer may be compressed, if necessary, to a pressure in the range of 10-50 bar abs and heated, if necessary, to a temperature of 350-650°C prior to being fed to the second autothermal reformer. Heating of the portion of the purge gas may be performed by using a fired heater that is fired by a portion of the hydrogen stream produced by the purge gas treatment unit as fuel. The oxygen-containing gas fed to the partial oxidation reactor or second autothermal reformer in the purge gas reforming unit may be air, oxygen enriched air or oxygen gas, but is preferably air because the resulting hydrogen stream is used as fuel and therefore the presence of nitrogen may be tolerated. The use of air avoids the need to uprate or add a further air separation unit where an existing process is being retrofitted with a purge gas treatment unit. Steam may 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 comprise hydrogen, steam carbon monoxide, carbon dioxide and possibly small amounts of argon and nitrogen.

A partial oxidation reactor may alternatively be used to convert the portion of the purge gas into a partially oxidised purge gas by partial oxidation using a sub-stoichiometric amount of oxygen. Partial oxidation reactors, or POx reactors, are known and typically comprise a vessel to which the feed and an oxygen-containing gas are fed via a burner, analogous to that used in an autothermal reformer, disposed above a reaction chamber in which the partial combustion reactions take place. Unlike an autothermal reformer, a catalyst is not present in the vessel. The oxygen-containing gas may be the same or different from that used in the first autothermal reformer, although it is convenient to use air for the reasons set out above for the second autothermal reformer. The combustion temperature may be about 1300°C, or higher. Steam may be added to the feed and/or the oxygen lower the combustion temperature and reduce soot formation. The idealised formula for this reaction applied to methane in the feed is as follows: CH4 + 1/2 02 -> CO + 2 H2 However, yields are below stoichiometric because part of the feed is fully combusted, and the problem of soot formation requires generally increased amounts of steam or oxygen, and so the purge reforming unit preferably comprises an autothermal reformer rather than a partial oxidation reactor.

The exit temperature from the second autothermal reformer or partial oxidation reactor may be in the range 800-1300°C. It is desirable therefore to adjust the temperature of the partially-oxidised or reformed purge gas upstream of the water gas shift unit. This may conveniently be done by recovering heat in a heat recovery unit, including the generation of steam in one or more boilers, which steam may usefully be used in heating or in power generation using a steam turbine. In some embodiments, steam generated by the heat recovery from the purge gas reforming unit may be used to supplement the steam addition to the hydrocarbon feedstock and/or purge gas upstream of the respective reforming units. Extra steam generated in the purge gas treatment unit may be used to provide process heating or motive power for compressors or for generating electricity.

In some embodiments a gas-heated reformer may be used in conjunction with the partial oxidation reactor or second autothermal reformer. The partial oxidation reactor or second autothermal reformer and gas-heated reformer may be arranged in series or in parallel, analogously as described above in connection with the reforming section.

Water-gas shift unit The partially-oxidised or reformed purge 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 partially-oxidised or reformed purge gas, steam may be added upstream of the water gas shift unit, e.g. by direct addition.

The partially-oxidised or reformed purge gas may be passed through one or more beds of water-gas shift catalyst in one or more shift vessels to generate a hydrogen-enriched, or "shifted", gas. At the same time the water gas shift unit converts carbon monoxide in the partially-oxidised or reformed purge gas to carbon dioxide. The reaction may be depicted as follows; CO + H2O CO2 + H2 The one or more water-gas shift stages may include stages of high-temperature shift, medium-temperature shift, isothermal shift and low-temperature shift.

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.

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.

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 partially-oxidised or reformed purge gas may be subjected to a stage of isothermal water-gas shift in a shift vessel in which the catalyst, typically a copper/zinc oxide/alumina composition, 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. In some embodiments, steam generated by the isothermal shift stage may be used to supplement the steam addition to the hydrocarbon feedstock upstream of the first autothermal reformer and/or purge gas upstream of the purge gas treatment unit. This improves the efficiency of the process and enables the desired steam to carbon ratio to be achieved at low cost.

Following the one or more shift stages, the hydrogen-enriched reformed 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 partial oxidation or reforming, may be formed. Because the condensate may contain ammonia, methanol, hydrogen cyanide and 002, returning the condensate to form steam used in the partial oxidation or reforming stage offers a useful way of returning hydrogen and carbon to the process.

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.

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.

Carbon dioxide removal unit The role of the carbon dioxide removal unit is to separate the hydrogen-enriched gas stream into a hydrogen stream and a carbon dioxide-containing stream. Any suitable carbon dioxide removal unit suitable for this purpose may be used. For instance, 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.

Because the source of the hydrogen-enriched gas is a purge gas stream, inert substances such as nitrogen and argon may be present. An amine wash carbon dioxide removal unit conveniently leaves these inert gases within the hydrogen stream, so in this way they may be effectively removed from the process.

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 5 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.

Upon the separation of the carbon dioxide, the process provides a hydrogen stream. Where a pure oxygen-containing gas stream (i.e. ?95% vol, preferably z.98% 02 vol) is used in the partial oxidation reactor or second autothermal reformer in the purge gas reforming unit, 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. Where air is used in the partial oxidation reactor or autothermal reformer in the purge gas reforming unit, the hydrogen stream may comprise 50-60% vol hydrogen, with the balance comprising mostly nitrogen with minor amounts of one or more of methane, carbon monoxide, carbon dioxide and argon. The methane content of the hydrogen stream may be in the range 0.25-7.5% vol. The carbon monoxide content of the hydrogen stream may be in the range 0.5-7.5% vol. The carbon dioxide content of the hydrogen stream may be in the range 0.01-2.5% vol. When the purity of the hydrogen stream required for downstream purposes needs to be higher than produced by the carbon dioxide removal unit, the hydrogen stream may 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 may be used. For instance, 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, either to the hydrocarbon reforming unit or, preferably, the purge gas treatment unit.

Carbon dioxide recovered from the carbon dioxide removal unit may be optionally purified, compressed, and sent for storage or sequestration.

Uses of the hydrogen stream At least a portion of the hydrogen stream, either produced directly by the carbon dioxide separation unit or following further purification in the purification unit, is used as fuel. It is preferred that the hydrogen stream is combusted in one or more items of fired equipment. The hydrogen stream may be combusted as is' or may be blended with another fuel stream prior to combustion. If necessary, hydrogen fuel burners may need to be installed.

In a preferred embodiment at least a portion of the hydrogen stream is used to heat process streams and/or for steam raising.

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

In a preferred embodiment the hydrocarbon feedstock to the first autothermal reformer is heated using a fired heater that is fired by a portion of the hydrogen stream.

In a preferred embodiment one or more feeds to the purge gas treatment unit is heated using a fired heater that is fired by a portion of the hydrogen stream.

If desired, a portion of the crude hydrogen or a portion of the purified hydrogen may be recycled to the hydrocarbon feed for hydrodesulphurisation. If necessary, the portion may be compressed prior to being recycled.

The purge gas treatment unit may be fed by more than one methanol synthesis unit. The purge gas treatment unit may therefore be operatively connected to two or more methanol production units each having an autothermal reformer. Typically the purge gas streams from each methanol synthesis are combined to form the purge gas feedstock stream which is fed to the purge gas treatment unit.

Claims

Claims 1. A process for synthesising methanol comprising the steps of: (i) reforming a hydrocarbon feedstock in a reforming section comprising a first autothermal reformer to form a synthesis gas containing 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 reactors; and (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 feedstock stream which is treated in a purge gas treatment unit by subjecting the purge gas feedstock stream to: (iv) partial oxidation in a partial oxidation reactor to form a partially-oxidised purge gas or autothermal reforming in a second autothermal reformer to form a reformed purge gas, followed by: (v) one or more stages of water gas shift of the partially-oxidised or reformed purge gas in a water-gas shift unit to form a hydrogen-enriched gas, and (vi) a step of carbon dioxide removal from the hydrogen-enriched 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. A process according to claim 1, wherein the hydrocarbon feedstock is natural gas.
3. A process according to claim 1 or claim 2, wherein the purge gas stream is separated into a hydrogen-rich gas stream and a carbon-rich purge gas which is used to generate the purge gas feedstock stream.
4. A process according to claim 3, wherein the hydrogen-rich gas stream is added to the synthesis gas upstream of the methanol loop.
5. A process according to any of claims 1 to 4, wherein the methanol loop comprises one, two or more methanol synthesis reactors each containing a bed of methanol synthesis catalyst, wherein the methanol product is recovered from at least one methanol synthesis reactor.
6. A process according to any 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. A 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 reactor or second autothermal reformer.
8. A process according to any of claims 1 to 7, wherein the first autothermal reformer uses 95% vol. 02 as the oxygen-containing gas.
9. A process according to any of claims 1 to 8, wherein the partial oxidation reactor or second autothermal reformer in the purge gas reforming unit uses air as the oxygen-containing gas.
10. A process according to any of claims 1 to 9, wherein the hydrocarbon feedstock to the first autothermal reformer is heated using a fired heater that is fired by a portion of the hydrogen stream.
11. A process according to any of claims 1 to 10, wherein at least a portion of the hydrogen stream is used for steam raising.
12. A process according to any of claims 1 to 11, wherein a portion of the hydrogen stream is used to adjust the R value of the synthesis gas from the first autothermal reformer.
13. A process according to any of claims 1 to 12, wherein one or more feeds to the purge gas treatment unit is heated using a fired heater that is fired by a portion of the hydrogen stream.
14. A method for retrofitting a methanol production unit which initially comprises: 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 reactors, wherein the methanol loop is fed with a synthesis gas generated by the first autothermal reformer and generates a methanol product and a purge gas stream; wherein there is no steam methane reformer upstream of the first autothermal reformer; the method comprising installing a purge gas treatment unit which comprises: a partial oxidation reactor or a second autothermal reformer, arranged to receive the purge gas stream and produce a partially-oxidised purge gas or a reformed purge gas; a water-gas shift unit arranged to receive the partially-oxidised purge gas or the reformed purge gas and produce a hydrogen-enriched stream; a carbon dioxide removal unit arranged to receive the hydrogen-enriched stream and produce a hydrogen stream; the method further comprising adapting the methanol production unit to use at least part of the hydrogen stream as fuel.
15. A method according to claim 14, further comprising installation of hydrogen fuel burners.