WO2023281238A1

WO2023281238A1 - Process for synthesising methanol

Info

Publication number: WO2023281238A1
Application number: PCT/GB2022/051507
Authority: WO
Inventors: Simon Robert Early
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2021-07-09
Filing date: 2022-06-15
Publication date: 2023-01-12
Also published as: GB202109905D0; GB202208786D0

Abstract

A process is described for synthesising methanol comprising the steps of (a) passing a feed gas comprising a make-up gas to a methanol synthesis unit comprising a first methanol synthesis reactor and a second methanol synthesis reactor, (b) recovering a first product gas mixture from the first methanol synthesis reactor and a second product gas mixture from the second methanol synthesis reactor, and (c) cooling the first and second product gas mixtures to condense methanol therefrom, thereby forming a first condensate mixture and a second condensate mixture, characterised in that said first and second condensate mixtures are fed to a common gas-liquid separator having an internal partition forming first zone into which the first condensate mixture is fed via first inlet, and a second zone into which the second condensate mixture is fed via a second inlet, wherein the common gas- liquid separator further comprises a liquid off-take zone in fluid communication with the first and second zones from which liquid crude methanol is recovered via a crude methanol outlet, and a combined gas offtake zone in fluid communication with the first and second zones, wherein (i) at least a portion of a first unreacted gas is recovered from the combined gas off-take zone via a first gas outlet, compressed, heated and fed to the second methanol synthesis reactor, and (ii) a second unreacted gas is recovered from the second zone via a second gas outlet located between the second inlet and the first gas outlet, and recycled to the feed gas to first methanol synthesis reactor.

Description

Process for synthesising methanol

This invention relates to a process for synthesising methanol.

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 often taken from the circulating gas stream to avoid the build-up of inert gasses in the loop.

Large-scale plants generally employ two or more stages of methanol synthesis.

W02017121980 (A1) and WO2017121981 (A1) disclose processes for making methanol using cooled first and second methanol synthesis reactors in series, with the second methanol synthesis reactor operated in a loop and wherein the first synthesis reactor has a higher heat transfer per cubic metre of catalyst than the second synthesis reactor. The product gas streams from each methanol synthesis reactor are cooled to condense methanol and the condensate mixtures fed to distinct gas liquid separators, from which are recovered liquid crude methanol streams and unreacted gases, which have different compositions, for further use in the processes. Beneficially a purge gas is recovered from the unreacted gas recovered from the second methanol synthesis reactor as this minimises the loss of hydrogen from the process.

The Applicant has found that a common gas-liquid separator with unreacted gases recovered from different zones separated by a partition simplifies the process without sacrificing the benefits arising from the different compositions of the unreacted gases.

Accordingly the invention provides a process for synthesising methanol comprising the steps of (a) passing a feed gas comprising a make-up gas to a methanol synthesis unit comprising a first methanol synthesis reactor and a second methanol synthesis reactor, (b) recovering a first product gas mixture from the first methanol synthesis reactor and a second product gas mixture from the second methanol synthesis reactor, and (c) cooling the first and second product gas mixtures to condense methanol therefrom, thereby forming a first condensate mixture and a second condensate mixture, characterised in that said first and second condensate mixtures are fed to a common gas-liquid separator having an internal partition forming first zone into which the first condensate mixture is fed via first inlet, and a second zone into which the second condensate mixture is fed via a second inlet, wherein the common gas-liquid separator further comprises a liquid off-take zone in fluid communication with the first and second zones from which liquid crude methanol is recovered via a crude methanol outlet, and a combined gas offtake zone in fluid communication with the first and second zones, wherein (i) at least a portion of a first unreacted gas is recovered from the combined gas off-take zone via a first gas outlet, compressed, heated and fed to the second methanol synthesis reactor, and (ii) a second unreacted gas is recovered from the second zone via a second gas outlet located between the second inlet and the first gas outlet, and recycled to feed gas to the first methanol synthesis reactor.

The make-up gas for the process typically comprises hydrogen and carbon dioxide and may contain carbon monoxide. The make-up gas may be generated by combining a hydrogen stream with a carbon dioxide stream, preferably at a molar ratio of about 3:1 , or the make-up gas may be derived from hydrocarbon sources or carbonaceous sources using steam reforming, partial oxidation and gasification steps. Hence, the make-up gas may be generated by processes including the steam reforming of methane, natural gas, or naphtha using established steam reforming processes. Alternatively, the make-up gas may be generated by gasification of coal, biomass or municipal waste. However, the present invention is of particular effectiveness in utilising reactive synthesis gases generated by processes including a step of partial oxidation of a hydrocarbon, biomass or carbonaceous feedstock. By “reactive synthesis gases” we mean a synthesis gas comprising hydrogen, carbon monoxide and carbon dioxide in which the ratio (by volume) of carbon monoxide to carbon dioxide is typically >2:1 , preferably >5:1. Such processes include processes in which a hydrocarbon feedstock is subjected to autothermal reforming in an autothermal reformer, autothermal reforming of a pre-reformed hydrocarbon feedstock and so-called combined reforming in which a first portion of a hydrocarbon feedstock is subjected to steam reforming and a second portion is subjected to autothermal reforming. In one suitable arrangement the reforming is performed using a gas-heated reformer (GHR) or heat-exchange reformer (HER) in series with an autothermal reformer, where the autothermally-reformed gas recovered from the autothermal reformer is used to heat the GHR or HER.

The hydrocarbon feedstock may be any gaseous or low boiling hydrocarbon-containing feedstock such as natural gas, associated gas, LPG, petroleum distillate or naphtha. It is preferably methane, associated gas or natural gas containing a substantial proportion, e.g. over 85% v/v methane. Natural gas is an especially preferred feedstock. The feedstock may be available at a suitable pressure or may be compressed to a suitable pressure, typically in the range 10-100 bar abs.

If the hydrocarbon feedstock contains sulphur compounds, before or after compression, the feedstock may be 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 and/or reduce the risk of soot formation in the reforming process, hydrogen may be added to the hydrocarbon feedstock. 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%.

Where a pre-reformer or steam reformer is used, the hydrocarbon feedstock is mixed with steam: this steam introduction may be effected 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. One or more saturators may be used. If desired, a portion of the hydrocarbon feedstock may bypass the steam addition, e.g. the saturator. The amount of steam introduced may be such as to give a steam ratio of 0.3 to 3:1 , i.e. 0.3 to 3 moles of steam per gram atom of hydrocarbon carbon in the hydrocarbon feedstock. The hydrocarbon/steam mixture may then be pre-heated prior to reforming in the pre-reformer. This may be achieved by using a fired heater. The fired heater may be heated by combustion of a portion of the hydrocarbon feedstock, typically with waste fuel gases separated from downstream processing. The resultant hydrocarbon feedstock/steam mixture may then be subjected to reforming in the reforming unit to generate a synthesis gas mixture, which is used to prepare the make-up gas.

In a preferred arrangement, the reforming unit is operated in two stages in series, which may be termed pre-reforming and autothermal reforming. In the first stage, the hydrocarbon is subjected to a step of adiabatic steam reforming. In such a process, the hydrocarbon/steam mixture, is desirably heated to a temperature in the range 300-650°C, and then passed adiabatically through a bed of a suitable steam reforming 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 react 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 a significant amount of hydrogen. This may be desirable in cases of low steam to carbon ratio feeds in order to minimise the risk of soot formation in the autothermal reformer. The pre-reformed gas, which comprises methane, hydrogen, steam, and carbon oxides, is then fed, optionally after addition of steam and/or a hydrogen-containing stream, to an autothermal reformer in which it is subjected to autothermal reforming. If desired the temperature and/or pressure of the pre-reformed gas may be adjusted before feeding it to the autothermal reformer. The steam reforming reactions are endothermic and therefore, especially where natural gas is used as the hydrocarbon feedstock, it may be desirable to re-heat the pre-reformed gas mixture to the autothermal reformer inlet temperature. If the pre-reformed gas is heated, this may conveniently also be performed in the fired heater used to pre-heat the feed to the pre-reformer.

The autothermal reformerwill generally comprise a burner disposed near the top ofthe reformer, to which is fed the hydrocarbon feedstock or pre-reformed gas 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 a portion of the hydrocarbon and any hydrogen present in the feed gas. The hydrocarbon feedstock or pre-reformed gas is typically fed to the top of the 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, magnesium aluminate, alpha-alumina, titanium dioxide, zirconium dioxide and mixtures thereof. In a preferred embodiment, the autothermal reforming catalyst comprises a layer of a higher activity supported Rh catalyst such as Rh on alpha-alumina or Rh on stabilised zirconia over a conventional Ni on alumina catalyst to reduce catalyst support volatilisation.

The oxygen-containing gas fed to the autothermal reformer is preferably >95% vol. O2, which may be provided by an air separation unit (ASU) or from another oxygen source.

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. The amount of oxygen-containing gas added is preferably such that 50 to 70 moles of oxygen are added per 100 gram atoms of carbon contained in the feed to pre-reforming and autothermal reforming stages. Preferably the amount of oxygen added is such that the autothermally reformed gas leaves the autothermal reforming catalyst at a temperature in the range 750-1100°C. For a given feedstock/steam mixture, amount and composition of the oxygen-containing gas and reforming pressure, this temperature largely determines the composition of the autothermally-reformed gas.

The reformed gas recovered from the reforming unit is a crude synthesis gas comprising hydrogen, carbon monoxide, carbon dioxide, methane, and steam. The amount of methane is influenced by the reformer exit temperature. High exit temperatures lower the methane content of the reformed gas. The synthesis gas produced by the autothermal reformer may contain 2.5 to 7% by volume of carbon dioxide on a wet basis, preferably 3 to 5% by volume of carbon dioxide on a wet basis. On a dry-gas basis, i.e. without the steam, the carbon dioxide content may be in the range 2.8 to 13% by volume, preferably 4 to 8% by volume.

After leaving the reforming unit, the synthesis gas is then 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 the reforming stage, for preparing the mixture hydrocarbon and steam, the pre-reformed gas mixture, or in the distillation of crude methanol. For safety reasons the reformed gas is preferably not used to heat the oxygen-containing gas fed to the autothermal reformer.

The cooling is performed to lower the temperature of the synthesis gas to below the dew point such that steam condenses. The liquid process condensate may be separated from the synthesis gas, which may be termed make-up gas at this point, by conventional gas-liquid separation equipment.

If desired, a portion of the make-up gas may be exported to external processes.

The make-up gas may be compressed in a synthesis gas compressor to the desired loop pressure before feeding the make-up gas to the methanol synthesis unit.

The feed gas to the methanol synthesis unit may consist of the make-up gas or the make-up gas may further comprise a portion of a gas rich in hydrogen, or a hydrogen-rich gas recovered from an unreacted gas stream. Using a hydrogen-rich gas recovered from the unreacted gas can improve the feed gas stoichiometry to maximise methanol productivity in the methanol synthesis unit. Where the first reactor in the methanol synthesis unit is operated in a loop, the feed gas to the methanol synthesis unit may comprise the make-up gas and a recycled gas stream comprising unreacted gases recovered from the first methanol synthesis reactor and/or a subsequent methanol synthesis reactor in the methanol synthesis unit.

The methanol synthesis unit comprises first and second methanol synthesis reactors, each containing a bed of methanol synthesis catalyst, arranged in series that each produce product gas streams containing methanol.

The first methanol synthesis reactor may be an un-cooled adiabatic reactor. Alternatively, the first methanol synthesis reactor may be cooled by heat exchange with a synthesis gas, such as in a quench reactor, a tube-cooled converter or a gas-cooled converter. Preferably, the first methanol synthesis reactor is cooled by boiling water under pressure, such as in an axial-flow steam-raising converter or a radial-flow steam-raising converter. Most preferably, the first methanol synthesis reactor is an axial-flow steam-raising converter as this has been found to be the most effective arrangement for recovering heat. The second methanol synthesis reactor may be the same or different. Thus, in one arrangement both the first and second methanol synthesis reactors are axial- flow steam-raising converters. Preferably, the first synthesis reactor has a higher heat transfer per cubic metre of catalyst than the second synthesis reactor. Accordingly, preferred arrangements comprise a combination of an axial-flow steam raising converter followed by a tube-cooled or gas- cooled converter, or a combination of an axial-flow steam-raising converter, followed by radial-flow steam raising converter. Such arrangements are particularly useful in the present invention due to the characteristics and performance of these reactors with different feed gas mixtures. 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 steamraising converter and a radial-flow steam-raising converter, most preferably an axial-flow steam raising converter.

In an adiabatic reactor, the synthesis gas may pass axially, radially, or axially and radially through a fixed bed of particulate methanol synthesis catalyst. The exothermic methanol synthesis reactions occur resulting in an increase in the temperature of the reacting gases. The inlet temperature to the bed therefore is desirably cooler than in cooled reactor systems to avoid over-heating of the catalyst which can be detrimental to selectivity and catalyst life. Alternatively, a cooled reactor may be used in which heat exchange with a coolant within the reactor may be used to minimise or control the temperature. A number of cooled reactor types exist that may be used. In one configuration, a fixed bed of particulate catalyst is cooled by tubes or plates through which a coolant heat exchange medium passes. In another configuration, the catalyst is disposed in tubes around which the coolant heat exchange medium passes. The methanol synthesis reactors may be cooled by the feed gas or by boiling water, typically under pressure. For example, the methanol synthesis reactor may be an axial-flow steam-raising converter, a radial-flow steam raising converter, a gas-cooled converter, or a tube cooled converter.

In an axial-flow, steam-raising converter (aSRC), the synthesis gas typically passes axially through vertical, catalyst-containing tubes that are cooled in heat exchange with boiling water under pressure flowing outside the tubes. The catalyst may be provided in pelleted form directly in the tubes or may be provided in one or more cylindrical containers that direct the flow of synthesis gas both radially and axially to enhance heat transfer. Such contained catalysts and their use in methanol synthesis are described in US8785506. Steam raising converters in which the catalyst is present in tubes cooled by boiling water under pressure offer a particularly useful means to remove heat from the catalyst.

In a radial-flow steam raising converter (rSRC) the synthesis gas typically passes radially (inwards or outwards) through a bed of particulate catalyst which is cooled by a plurality of tubes or plates through boiling water under pressure is fed as coolant. Such reactors are known and are described for example in US4321234. They offer a lower pressure drop than an aSRC but have a more complicated internal construction. In a tube-cooled converter, the catalyst bed is cooled by synthesis gas passing through tubes disposed within the bed that are open-ended and discharge the heated gas to the space above the catalyst within the reactor shell. The heated gas may then pass directly through the bed of catalyst without leaving the converter. TCC’s can provide sufficient cooling area for a range of synthesis gas compositions and may be used under a wide range of conditions. As an alternative to a TCC, a gas- cooled converter (GCC) may be used to cool the catalyst bed by passing the synthesis gas though tubes or plates in a heat exchanger-type arrangement. In this case the heated synthesis gas is withdrawn from the converter before being returned back to the catalyst bed. An example of a GCC is described in US5827901 .

Alternatively, the methanol synthesis reactor may be a quench reactor in which one or more fixed beds of particulate methanol synthesis catalyst are cooled by a synthesis gas mixture injected into the reactor within or between the beds. Such reactors are described, for example, in US4411877.

The methanol synthesis catalysts are suitably copper-containing methanol synthesis catalysts, which are commercially available. In particular, suitable methanol synthesis catalysts are particulate copper/zinc oxide/alumina catalysts, which may comprise one or more promoters. The methanol synthesis catalysts in each of the methanol synthesis reactors may be the same or different. The copper oxide content of the catalyst (expressed as CuO) may be in the range of 60 to 70% by weight. The weight ratio of Cu:Zn (expressed as CuO:ZnO) is preferably in the range of 2:1 to 3.5:1 , especially 2.5:1 to 2.75:1 for methanol synthesis catalysts. In the methanol synthesis catalysts, the catalyst preferably contains 20-30% by weight zinc oxide. The catalyst typically contains alumina, which may be in an amount in the range 5 to 20% by weight.

Methanol synthesis may be performed in the 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 reactor inlets is preferably 50-100 bar abs, more preferably 70-90 bar abs. The temperature of the synthesis gas at the reactor inlets is preferably in the range 200-250°C and at the outlets preferably in the range 230-280°C.

Each methanol synthesis reactor produces a product gas mixture. In the current process, a liquid methanol-containing condensate stream from each methanol synthesis reactor is fed to the common gas-liquid separator. This may be achieved by cooling the product gas mixtures to below the dew point. 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. Using a gas-gas interchanger usefully allows improved control of steam generation in steam-raising converters. In the process, the product gas mixtures are separately cooled. The cooling of the product gas mixtures is preferably performed to a temperature in the range 40 to 50°C. The process comprises recovering a first product gas mixture from the first methanol synthesis reactor and a second product gas mixture from the second methanol synthesis reactor and cooling the first and second product gas mixtures to condense methanol therefrom, thereby forming first and second condensate mixtures.

In the process, the first and second condensate mixtures are fed to a common gas-liquid separator having an internal partition forming first zone into which the first condensate mixture is fed via first inlet, and a second zone into which the second condensate mixture is fed via a second inlet, wherein the common gas-liquid separator further comprises a liquid off-take zone in fluid communication with the first and second zones from which liquid crude methanol is recovered via a crude methanol outlet, and a combined gas offtake zone in fluid communication with the first and second zones, wherein (i) at least a portion of a first unreacted gas is recovered from the combined gas off-take zone via a first gas outlet, compressed, heated and fed to the second methanol synthesis reactor, and (ii) a second unreacted gas is recovered from the second zone via a second gas outlet located between the second inlet and the first gas outlet, and recycled to the first methanol synthesis reactor.

In one arrangement, all of the first unreacted gas, minus any purge stream, may be fed to the second methanol synthesis reactor. In another arrangement, a portion of the first unreacted gas may be returned to the first methanol synthesis reactor. It this arrangement it may be more desirable to recover the purge from the second unreacted gas.

The common gas-liquid separator suitably is in the form of an elongate shell, for example a cylindrical pressure vessel, having a first end and a second end, which may be domed, with the first and second inlets located at opposite sides of the shell. Where the shell is cylindrical, the first and second inlets may be diametrically opposed. The crude methanol outlet may be located at one end and the first gas outlet at the opposite end. The second gas outlet is located between the second inlet and the first gas outlet. The gas liquid separator comprises an internal partition forming the first and second zones. The partition therefore divides the first zone from the second zone within the gas liquid separator such that the first zone is fed with the first condensate mixture and the second zone with the second condensate mixture. The partition is suitably an impermeable plate extending across the inside of the separator dividing the internal volume into the first and second zones. The partition may be equidistant from the first and second inlets so, for example where the shell is cylindrical, the partition may extend across the diameter of the shell, although the partition may be located nearer or further from the first and second inlets if desired. The liquid off-take zone and the combined gas off-take zone are in fluid communication with the first and second zones to permit recovery of crude liquid methanol and the first unreacted gas from the gas-liquid separator. Accordingly in some embodiments, the partition has gaps or orifices near the first and second ends of the shell to allow the liquid condensates from the first and second zones to combine, and the unreacted gas from the first and second zones to combine. Accordingly, the liquid offtake zone contains the liquid crude methanol from both first and second zones, and the combined gas off-take zone contains unreacted gas from both first and second zones. The amounts of unreacted gases from the first and second zones in the combined gas off-take zone will in part be dictated by their pressure and volumetric flow rate. The gas-liquid separator further comprises a second gas outlet to recover unreacted gas from the second zone. The second zone is fed with the condensate mixture from the second methanol synthesis reactor in the methanol synthesis unit. This mixture will generally contain a higher hydrogen concentration than the unreacted gas in the first condensate mixture recovered from the first methanol synthesis reactor. Accordingly, rather than send a portion of the combined unreacted gases recovered from the first gas outlet to the first methanol synthesis reactor the Applicant has found it advantageous to use the second unreacted gas as the recycle gas to the first methanol synthesis reactor because this improves the stoichiometry of the feed gas to the process and enhances the overall methanol productivity.

The common gas-liquid separator may be sized according to the scale of the process using normal engineering practices and fabricated using conventional materials suitable for gas-liquid separators used in methanol plants.

A purge may be taken from the first unreacted gas prior to or after its compression, or from the second unreacted gas mixture if desired, to prevent the build-up of inert gases such as methane, argon or nitrogen, in the process.

In one arrangement, the Applicant has found it desirable to recover a hydrogen-rich gas from the second unreacted gas and feed the hydrogen-rich gas to the first methanol synthesis reactor. Thus, in some embodiments, the process includes passing the second unreacted gas through a hydrogen recovery unit to generate a hydrogen-rich gas and passing the hydrogen-rich gas to the first methanol synthesis reactor. In this way, the more-hydrogen-rich unreacted gas from the second methanol synthesis reactor becomes the feed for the hydrogen recovery step, thereby reducing waste from the process. The recovery of a hydrogen-rich gas from the unreacted gas creates hydrogen-depleted gas, which may be termed a carbon-rich gas stream. The separation of the hydrogen-rich and carbon-rich gas streams may be practiced using known separation equipment such as 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 second unreacted gas stream may be recovered. The carbon-rich gas stream, which will generally also comprise inert gases, may suitably be used as the purge gas from the process. The purge may be combusted as fuel, e.g. in a fired heater used to preheat and/or reheat the feeds in the reforming unit or generate steam. Alternatively, the carbon rich off gas may be exported from the process for use as a fuel. The hydrogen-rich gas recovered from the second unreacted gas stream desirably comprises >80% by volume of hh. The separated hydrogen, in addition to being recycled to the methanol synthesis unit may also be used upstream in hydrodesulphurisation of the hydrocarbon feedstock and/or exported from the process for other use. The crude methanol stream recovered from the gas-liquid separator contains dissolved carbon dioxide. In a preferred arrangement, at least a portion of the hydrogen-rich gas is used to strip the crude liquid methanol product before the hydrogen-rich gas is fed to the first methanol synthesis unit. The hydrogen-rich gas used in this way thereby recovers and recirculates carbon dioxide that might otherwise be lost from the process. The resulting carbon- dioxide-enriched hydrogen-rich stripping gas stream may conveniently be recycled to the suction or first stage of the make-up gas compressor because the pressures will match.

If desired, a portion of the second unreacted gas may bypass the hydrogen recovery unit and be combined with the make-up gas fed to the first methanol synthesis reactor. This provides for additional control of the stoichiometry of the feed gas to the first methanol synthesis reactor, which can improve productivity.

The Applicant has found it desirable to wash residual methanol present in the second unreacted gas recovered from the second zone of the gas-liquid separator to maximise its recovery from the process. Accordingly, in some embodiments a washing stage, such as a water washing stage, is applied to the second unreacted gas in a wash vessel, and the washed gas is then recycled to the first methanol synthesis reactor. In some embodiments, the washing stage may be performed upstream of the hydrogen recovery unit such that the washed unreacted gas is provided to the hydrogen recovery unit. The washings from the washing stage may conveniently be fed to a methanol purification unit.

The first unreacted gas recovered from the combined gas offtake zone and comprising the unreacted gases from the first and second zones is fed to the second methanol synthesis reactor. In this way the first and second reactors are connected in series. The first unreacted gas is desirably compressed in one or more stages and heated prior to its introduction into the second methanol synthesis reactor.

Whereas the gas fed to the second methanol synthesis reactor may consist of all of the first unreacted gas, if desired a portion of the first unreacted gas may be recycled to the feed to the first methanol synthesis reactor.

A liquid crude methanol stream is recovered from the liquid off-take zone of the common gas-liquid separator via a crude methanol outlet. The liquid crude methanol may be further processed to produce a purified methanol product. The crude methanol stream recovered from the methanol production unit contains water, along with small amounts of higher alcohols and other impurities. The crude methanol, whether subjected to the above hydrogen stripping stage or not, may first be fed to a flash vessel or column where dissolved gases are released and separated from the crude liquid methanol stream. Where the liquid crude methanol contains dissolved carbon dioxide it may be treated by first reducing its pressure/and or increasing its temperature and separating vapourised carbon dioxide using a flash-gas vessel. This carbon dioxide may be recycled to the process but is preferably exported from the process either as a raw material for a separate chemical synthesis process or for sequestration and carbon capture. The crude liquid methanol may also be subjected to one or more purification stages including one or more, preferably two orthree, 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, a reformed gas stream, steam generated in the methanol synthesis unit, or other sources. Typically, 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.

The invention is further described by reference to the figures in which:

Figure 1 depicts a process according to one embodiment of the invention,

Figure 2 depicts a further process according to another embodiment of the invention, and Figure 3 depicts an embodiment of a common gas-liquid separator used in Figures 1 and 2.

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 make-up gas comprising hydrogen, carbon dioxide and optionally carbon monoxide is provided via line 10, combined with an unreacted gas stream provided via line 12, and the resulting mixture fed via line 14 to a syngas compressor 16 where it is compressed. A compressed feed gas mixture is recovered from the syngas compressor 16 via line 18 and fed to a gas-gas interchanger 20, where it is heated to the methanol reactor inlet temperature and then fed via line 22 to the inlet of a first methanol synthesis reactor 24. Whereas a single reactor is depicted, it will be understood that this flowsheet may operate with two or more reactors operated in parallel. The first methanol synthesis reactor 24 is an axial-flow steam-raising converter comprising methanol synthesis catalyst-filled tubes 26 cooled by boiling water under pressure 28. Methanol synthesis reactions take place in the reactor 24 to generate a product gas mixture comprising methanol, unreacted hydrogen, and carbon dioxide. The product gas mixture is recovered from the reactor 24 via line 30, cooled in the interchanger 20 and fed via line 32 to one or more further heat exchangers 34 where it is cooled to below the dew point to condense a liquid crude methanol. The resulting first condensate mixture is passed from the one or more heat exchangers 34 via line 36 to a common gas liquid separator 38.

Turning to Figure 3, the common gas-liquid separator 38 comprises an elongate cylindrical shell, oriented vertically having a first domed upper end 40 and a second domed lower end 42. The first upper end 40 has first gas outlet 44 for recovering a first unreacted gas from a combined gas offtake zone 46 within the upper end 40 of the separator. The second lower end 42 comprises a crude methanol outlet 48 for recovering liquid crude methanol from a liquid off-take zone 50 within the lower end 42 of the separator. The separator contains an internal partition 52 extending across the diameter of the shell and aligned vertically, which divides the interior of the gas-liquid separator into a first zone 54 into which the first condensate mixture is fed via a first inlet 56 connected to line 36, and a second zone 58 into which a second condensate mixture is fed via a second inlet 60 connected to a second mixed condensate line 62. The first and second inlets 56, 60 are positioned approximately equidistant from the first and second ends 40, 42, and are diametrically opposed on the shell. The partition 52 extends from a position in the combined gas off-take zone 46 near the first end 40 to a position in the liquid off-take zone 50 near the second end 42. The partition therefore extends to below the normal liquid surface level in use, as shown by the shaded area. Gaps at the top and bottom of the partition 52 ensure that the combined gas off-take zone 46 and the liquid offtake zone 50 are in fluid communication with the first and second zones 54, 58. In consequence, the liquid crude methanol withdrawn from via outlet 48 is a mixture of condensed methanol from the first and second condensate mixture streams 36, 62, and the first unreacted gas withdrawn via the first gas outlet 44 is a mixture of unreacted gases present in the first and second condensate mixture streams 36, 62. Within the first and second zones 54, 58, between the first and second inlets 56, 60 and the combined gas off-take zone 46, first and second demister or vane packs 64, 65 are present to aid liquid droplet separation from the condensate mixtures 36, 62. The gas-liquid separator 38 further comprises a second gas outlet 66 on the shell in fluid communication with the second zone 58. The second outlet 66 is located between the second inlet 60 and the first outlet 44 and conveniently between the second demister or vane pack 65 and the first outlet 44. A second unreacted gas mixture is recovered from the gas-liquid separator from outlet 66 via line 90.

By connecting the first and second zones, the gap between the shell and partition 52 near the first end 40 allows surplus unreacted gas from stream 62 not removed in the second unreacted gas 90 to exit via line 68. Similarly, if the desired second unreacted gas stream 90 is greater than the gas from stream 62, the gap allows some of the unreacted gas that entered via line 36 to exit via line 90. Most commonly and preferably, the second unreacted gas stream flowrate in line 90 will be less than the gas in stream 62. The gap near the lower second end of separator 42 is below the liquid level. This allows a common liquid outlet with a single level control to adjust the flow of crude methanol 88.

Returning to Figure 1 , a first unreacted gas mixture is recovered from the gas-liquid separator 38 via the first gas outlet and fed via line 68 to a circulating compressor 70 where it is compressed to the desired pressure and thence via line 72 to a gas-gas interchanger 74 where it is heated to the desired temperature. The heated unreacted gas is then passed from the interchanger 74 via line 76 to a second methanol synthesis reactor in the form of a radial-flow steam-raising converter 78 containing an annular bed of methanol synthesis catalyst 80 cooled by tubes or plates containing boiling water under pressure 82. Whereas a radial flow steam-raising converter is depicted, the process may equally be performed using an axial-flow steam-raising converter, a gas-cooled converter, ora tube-cooled converter. Methanol is synthesised in the catalyst bed. A second product gas mixture is recovered from the converter 78 via line 84, cooled in interchanger 74 and one or more further heat exchangers 86 to condense a further amount of crude liquid methanol. The resulting second cooled condensate mixture is passed from the heat exchanger 86 via line 62 to the second inlet of the common gas-liquid separator 38.

The second unreacted gas mixture is recovered from the gas-liquid separator from outlet 66 via line 90 and circulated, optionally after further treatment, as line 12 to the feed to the first methanol synthesis reactor 24. By taking the unreacted gas 90 from the second zone, the hydrogen content is higher, and the carbon oxide content is lower than in stream 68. This provides for a higher loop efficiency (measured by conversion of carbon oxides to methanol) than prior art processes where purge gas in is taken from the equivalent of stream 68, which will have a lower hydrogen content and higher carbon oxide content than the claimed process.

Optionally, as shown by the dashed line, a portion of the unreacted gas 90 may be compressed by a compressor (not shown) and fed via line 94 to the compressed feed gas in line 18 upstream of the gas-gas interchanger 20. In some embodiments, all of the unreacted gas may be compressed and passed via line 94 to the feed gas for the first methanol synthesis reactor 24. The compressor may be a single stage compressor.

Liquid crude methanol is recovered from the crude methanol outlet of the gas-liquid separator 38 via line 88 and sent for purification in one or more stages of distillation (not shown) to produce a purified methanol product.

A purge stream 92 may be taken from the first unreacted gas mixture in line 68 as depicted, or line 72, for processes where the make-up synthesis gas is sub-stoichiometric (i.e. with R<2). Where the make-up synthesis gas is hydrogen-rich it may be more convenient to take the purge from the second unreacted gas stream in line 90 or even the compressed feed gas mixture in line 18.

In Figure 2, the methanol synthesis reactor arrangement and common gas-liquid separator are the same as depicted in Figure 1. However, in the arrangement shown in Figure 2, the second unreacted gas stream in line 90 is fed to a gas washing unit 96, which is fed with a water stream 98, which generates a wash stream 100 containing a small amount of methanol. The wash stream may be sent for recovery of the methanol with the crude methanol stream 88. A washed unreacted gas is recovered from the gas washing unit 96 and fed via line 102 to a membrane hydrogen recovery unit 104, where a hydrogen-rich stream is separated from the washed gas and supplied via line 12 to the make-up gas stream in line 10. A carbon rich offgas is recovered from the hydrogen recovery unit 104 via line 106 and used as fuel. This line may also be used to purge inerts from the loop.

Optionally, as shown by the dashed line, a bypass 110 around the hydrogen recovery unit can be included. Thus, in some embodiments, a bypass line 110 may convey a portion of the unreacted gas stream 90 to the hydrogen-rich gas steam in line 12. Again, in some embodiments, the by-pass stream may be compressed and fed downstream of the compressor 16 to the compressed gas in line 18 or to the heated compressed gas in line 22. The inclusion of the bypass line 110 means that both unreacted gas and the hydrogen-rich gas stream may be used to achieve the desired stoichiometry.

Although not shown, in some embodiments, the hydrogen-rich gas may be compressed and used to strip the crude liquid methanol recovered in line 88 of gasses and the resulting enriched stripping gas then combined with the make-up gas stream in lines 18 or 22 fed to the reactor 24.

Whereas in Figures 1 and 2, all of the first unreacted gas mixture 72, minus any purge stream 68, is fed to the second methanol synthesis reactor 78, if desired a portion of the first unreacted gas mixture may be returned to the first methanol synthesis reactor 24, for example via a conduit between line 72 and line 18 or line 22 (not shown).

Claims

Claims.

1. A process for synthesising methanol comprising the steps of (a) passing a feed gas comprising a make-up gas to a methanol synthesis unit comprising a first methanol synthesis reactor and a second methanol synthesis reactor, (b) recovering a first product gas mixture from the first methanol synthesis reactor and a second product gas mixture from the second methanol synthesis reactor, and (c) cooling the first and second product gas mixtures to condense methanol therefrom, thereby forming a first condensate mixture and a second condensate mixture, characterised in that said first and second condensate mixtures are fed to a common gas-liquid separator having an internal partition forming first zone into which the first condensate mixture is fed via first inlet, and a second zone into which the second condensate mixture is fed via a second inlet, wherein the common gas-liquid separator further comprises a liquid off-take zone in fluid communication with the first and second zones from which liquid crude methanol is recovered via a crude methanol outlet, and a combined gas offtake zone in fluid communication with the first and second zones, wherein (i) at least a portion of a first unreacted gas is recovered from the combined gas off-take zone via a first gas outlet, compressed, heated and fed to the second methanol synthesis reactor, and (ii) a second unreacted gas is recovered from the second zone via a second gas outlet located between the second inlet and the first gas outlet, and recycled to the feed gas to first methanol synthesis reactor.

2. A process according to claim 1 , wherein the feed gas comprises a make-up gas generated by a reforming unit comprising an adiabatic pre-reformer and autothermal reformer connected in series or a gas-heated reformer and autothermal reformer connected in series.

3. A process according to claim 1 or claim 2, wherein the make-up gas, before unreacted gas is added has a stoichiometry value R in the range of 1.70 to 1 .95 and the feed gas has a stoichiometry value R which is higher than that of the make-up gas

4. A process according to any one of claims 1 to 3, wherein the first methanol synthesis reactor is an axial-flow steam-raising converter and the second methanol synthesis reactor is an axial- flow steam-raising converter, a radial-flow steam-raising converter, a gas-cooled converter or a tube-cooled converter.

5. A process according to any one of claims 1 to 4, wherein all of the first unreacted gas, minus any purge stream, is fed to the second methanol synthesis reactor.

6. A process according to any one of claims 1 to 4, wherein a portion of the first unreacted gas is returned to the first methanol synthesis reactor.

7. A process according to any one of claims 1 to 6, wherein a washing stage is applied to the second unreacted gas in a wash vessel to form a washed gas, and the washed gas is recycled to the first methanol synthesis reactor.

8. A process according to any one of claims 1 to 7, wherein the second unreacted gas is passed through a hydrogen recovery unit to generate a hydrogen-rich gas and the hydrogen-rich gas is passed to the first methanol synthesis reactor.

9. A process according to claim 8, wherein a carbon-rich off gas obtained by separation of the hydrogen-rich gas from the second unreacted gas is used as a fuel in a fired heater to heat one or more feed streams, or is exported to a separate process.

10. A process according to claim 8 or clam 9, wherein the hydrogen-rich gas is used to strip the crude liquid methanol product before the hydrogen-rich gas is combined with the make-up gas and passed to the first methanol synthesis reactor.

11. A process according to any one of claims 8 to 10, wherein a portion of the second unreacted gas bypasses the hydrogen recovery unit and is combined with the make-up gas passed to the first methanol synthesis reactor.

12. A process according to any one of claims 1 to 11 , wherein the crude methanol is subjected to one or more steps of distillation to produce a purified methanol product.