GB2606855A - Process for synthesising methanol - Google Patents

Abstract

Process for synthesising methanol comprising the steps of (i) passing a feed gas comprising a make-up gas having a stoichiometry value, R, less than 2.0 to a methanol synthesis unit comprising a first MeOH synthesis reactor and a second methanol synthesis reactor connected in series, wherein the second methanol synthesis reactor is operated in a loop, and (ii) recovering a purge gas stream from the loop and a crude methanol product from the methanol synthesis unit, wherein a hydrogen rich gas is recovered from the purge gas stream and combined with the make-up gas fed gas to the methanol synthesis unit. Preferably, the make-up gas has a stoichiometry of 1.70 to 1.94 prior to addition of the H2 rich gas. The hydrogen-rich gas may be used to strip the crude methanol product before being combined with the make up gas. The make-up gas may be generated by a reforming unit comprising an adiabatic pre-reformer and an autothermal reformer connected in series.

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 (Al) and W02017121981 (Al) 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 Applicant has discovered that recovering a hydrogen-rich gas from a purge gas stream taken from the loop around the second methanol synthesis reactor and adding it to the gas fed to the first methanol synthesis reactor provides an improved process.

Accordingly the invention provides a process is for synthesising methanol comprising the steps of (i) passing a feed gas comprising a make-up gas having a stoichiometry value, R, less than 2.0 to a methanol synthesis unit comprising a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein the second methanol synthesis reactor is operated in a loop, and (ii) recovering a purge gas stream from the loop and a crude methanol product from the methanol synthesis unit, wherein a hydrogen rich gas is recovered from the purge gas stream and combined with the make-up gas fed gas to the methanol synthesis unit.

The make-up gas for the process typically comprises hydrogen, carbon monoxide, and/or carbon dioxide.

There are two stoichiometric values that are commonly used to describe the proportions of the reactants fed to a methanol synthesis reactor. These are R and Z and may be determined from the molar concentrations of the components in the synthesis gas as follows.

R = ([H2] -[CO2]) / ([CO] + [CO2]) Z = [H4 / (2[CO] + 3[CO2]) The ideal stoichiometric mixture arises when there is enough hydrogen to convert all of the carbon oxides into methanol. This is when R = 2 and Z = 1. However different synthesis gas generation techniques produce different synthesis gases having different proportions of the reactants. The stoichiometry value R of the make-up gas is less than 2.0 i.e. is sub-stoichiometric, and preferably is in the range 1.70 to 1.94. The R value of the feed gas after addition of the hydrogen-rich gas, excluding the any recycle gas stream, is preferably in the optimal range for methanol synthesis of from 1.95 to 2.05.

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. Accordingly, the process may include a reforming unit comprising an autothermal reformer (ATR) as the sole reformer, to which a hydrocarbon feedstock, optionally mixed with steam, may be fed. However, in a preferred arrangement, the reforming unit comprises an adiabatic pre-reformer and autothermal reformer connected in series. A mixture of the hydrocarbon feedstock and steam are fed to the pre-reformer to convert C2+ hydrocarbons to methane and form a pre-reformed gas mixture containing hydrogen, steam, carbon monoxide, carbon dioxide and methane, which is fed to the autothermal reformer. The addition of a pre-reformer upstream of the autothermal reformer allows a greater amount of heat to be put into the process upstream of the ATR and allows a better make up gas stoichiometry for methanol synthesis to be achieved at the ATR exit than use of an ATR alone.

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%. In a preferred embodiment a portion of the hydrogen-rich stream is mixed with the hydrocarbon feed stream. The hydrogen stream may be combined with the hydrocarbon upstream and/or downstream of any hydrodesulphurisation stage.

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, i.e. 0.3 to 3 moles of steam per gram atom of hydrocarbon carbon in the hydrocarbon feedstock. It is preferred that the steam to carbon ratio is 51.5:1, more preferably in the range 0.3 to 0.9:1. 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, which preferably includes a portion of a carbon-rich gas obtained after recovery of the hydrogen-rich gas.

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 reformer will generally comprise a burner disposed near the top of the 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 therefore 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. 02, 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 [Hz] / [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 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, the mixture hydrocarbon and steam, the pre-reformed gas mixture, and 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. A hydrogen-rich gas recovered from the purge gas is added to the make-up gas. The hydrogen-rich gas may be added to the make-up gas before or after compression in the synthesis gas compressor.

The feed gas to the methanol synthesis unit, prior to water or steam addition, may consist of the make-up gas and the hydrogen-rich gas, or where the first reactor in the methanol synthesis unit is operated in a loop, the feed gas to the methanol synthesis unit may consist of the make-up gas, the hydrogen rich 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, second and optionally third or further methanol synthesis reactors, each containing a bed of methanol synthesis catalyst, arranged in series that each produce product gas streams containing methanol. The methanol synthesis unit may therefore comprise 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 crude methanol product stream is recovered from one or more of the product gas mixtures. This may be achieved by cooling the one or more product gas mixtures to below the dew point, condensing a crude methanol product, and separating the crude liquid methanol product from the unreacted gases. Separation of the crude liquid methanol product from one or more of the methanol product gas streams produces one or more unreacted gas mixtures.

The methanol synthesis unit includes a second methanol synthesis reactor operated in a loop. The first methanol synthesis reactor is also preferably operated in a loop. Accordingly, a portion of an unreacted gas mixture recovered from the second methanol synthesis reactor and optionally the first methanol synthesis reactor 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. A recycle gas stream is recovered from at least the methanol product gas stream recovered from the second methanol synthesis reactor and recycled to at least one of the methanol synthesis reactors. If there are multiple recycle gas streams, 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 unit comprises a first methanol synthesis reactor and a second methanol synthesis reactor connected in series. In such an arrangement, gas fed to the second methanol synthesis reactor may comprise at least a portion of the unreacted gases from the methanol product gas stream recovered from the first methanol synthesis reactor. Whereas the gas fed to the second methanol synthesis reactor may consist of all of the unreacted gases from methanol product gas stream from the first methanol synthesis reactor and a recycle gas stream, if desired a portion of the unreacted gas stream from the first methanol synthesis reactor not fed to the second methanol synthesis reactor, may be recycled to the feed to the first methanol synthesis reactor. Suitable methanol loops are described in US7790775, W02017/121980 and W02017/121981.

For example, the methanol synthesis unit may comprise a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein a portion of an unreacted gas stream recovered from the first methanol synthesis reactor is recycled to the first methanol synthesis reactor and a portion of an unreacted gas stream recovered from the second methanol synthesis reactor is recycled to the second methanol synthesis reactor.

Alternatively, the methanol synthesis unit may comprise a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein a portion of an unreacted gas stream recovered from the second methanol synthesis reactor is recycled to the first methanol synthesis reactor.

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 bot 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 steam-raising 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 Onwards 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 US 5827901.

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 U5441 1877.

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 stream is preferably recovered from each product gas mixture before it is further used. This may be achieved by cooling the one or more product gas mixtures to below the dew point, condensing a crude methanol product, and separating the 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. Using a gas-gas interchanger usefully allows improved control of steam generation in steam-raising converters. Where there are two or more methanol synthesis reactors, the product gas mixtures may be separately cooled or combined and cooled together to produce the crude methanol stream. The cooling of the product gas mixture is preferably performed to a temperature in the range 40 to 50°C.

Where the liquid crude methanol recovered from the one or more product gas mixtures 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.

The carbon dioxide-depleted crude methanol may then be treated conventionally by distillation to produce a purified methanol product.

The portion of the unreacted gas mixture making up the gas stream recycled to the loop is compressed by one or more compressors or circulators. The compression may take place before the stream is divided, e.g. to provide a purge gas stream, or after it is divided, or after combination of the recycle gas stream with the feed gas. The recycle ratio to form the gas mixture fed to the first methanol synthesis reactor may be in the range 0.1:1 to 1:1. Whereas the recycle ratio to form the gas mixture fed to the second synthesis reactor may be 1.1:1 to 6:1, it is preferably in the range 1.5:1 to 6:1, more preferably 2:1 to 6:1. By the term "recycle ratio", we mean the molar flow ratio of the recycled loop gas to the make-up gas that form the gas mixtures fed to the first and second methanol synthesis reactors. Accordingly, the recycle ratio for the first synthesis gas arises from the proportion of the loop recycle gas combined with a make-up gas, and the recycle ratio for the second synthesis gas arises from the proportion of the loop gas combined with the unreacted gas mixture, both expressed relative to the make-up gas.

A portion of the unreacted gas mixture separated from the loop around the second methanol synthesis reactor is removed from the loop as the purge gas stream, which is used to prevent the build-up of unwanted inerts in the process. The purge gas stream may be removed continuously or periodically. The purge gas stream may be recovered from the separated unreacted gas before or after compression in the circulator. A purge gas recovered downstream of compressor can give more driving force and aid membrane separation.

If desired, a portion of the unreacted gas mixture fed to the second methanol synthesis reactor may be fed to the first methanol synthesis reactor. Thus, in one arrangement, a compressed gas mixture comprising unreacted gas recovered from the first methanol synthesis reactor and unreacted gas recovered from the second methanol synthesis reactor is fed to the first methanol synthesis reactor, preferably prior to a heating step.

In the process, at least a portion of the purge gas stream is separated into a hydrogen-rich gas stream, which is recycled to the make-up gas for the feed to the first methanol synthesis reactor. This will result in a carbon-rich off gas stream. By "carbon-rich off gas stream" we mean a gas stream that has a higher proportion of carbon containing compounds (carbon monoxide, carbon dioxide and methane) than the purge gas. While individual components may have the same, or even lower, proportion than in the purge gas, the total of all carbon-containing components will be in a higher proportion in the carbon-rich gas compared to the purge gas. Preferably all of the purge gas stream is subjected to a separation step. 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 purge gas stream may be recovered.

It will be understood that by adding the hydrogen-rich gas stream to the make-up gas, that the stoichiometry value R of the feed gas will be increased. If desired additional hydrogen from an external source may also be added.

The carbon rich off-gas, which contains inerts, is desirably sent as fuel, e.g. to a fired heater, such as a fired heater used to preheat and/or reheat the feeds in the reforming unit. Alternatively, the carbon rich off gas may be exported from the process for use as a fuel.

The hydrogen-rich gas recovered from the purge gas stream desirably comprises >80% by volume of H2. The separated hydrogen, in addition to being recycled to the methanol loop may also be used upstream in hydrodesulphurisation of the hydrocarbon feedstock and/or exported from the process for other use. The crude methanol contains dissolved carbon dioxide. In a preferred arrangement, the hydrogen-rich gas is used to strip the crude liquid methanol product before it is fed to the makeup gas. The hydrogen-rich gas thereby recovers and recirculates carbon dioxide that might otherwise be lost form the process. Preferably the carbon-dioxide-enriched Hydrogen-rich gas stream is desirably recycled to the suction or first stage of the make-up gas compressor because the pressures will match.

The purge gas stream mixture may contain methanol and so, if desired, upstream of the separation of the hydrogen-rich gas and the carbon-rich gas, methanol may be recovered from the purge gas stream using a water wash. Preferably at least a portion of the resulting wash water containing methanol is added to the feed gas fed to methanol synthesis unit. Any purge gas wash water containing methanol not added to the feed gas to the methanol synthesis unit may be sent for purification with the crude methanol.

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 may first be fed to a flash vessel or column where dissolved gases are released and separated from the crude liquid methanol stream. The crude liquid methanol may also be subjected to one or more purification stages including one or more, preferably two or three, stages of distillation in a methanol purification unit comprising one, two or more distillation columns. The de-gassing stage and distillation stages may be heated using heat recovered from the process, for example in the cooling of a product gas stream, a reformed gas stream 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 will be further described by reference to the figures in which: Figure 1 depicts a further process according to one embodiment of the invention, and Figure 2 depicts a further process according to another embodiment of the invention.

Figures 1 and 2 include a reforming unit comprising a pre-reformer and an autothermal reformer. In Figure 1, the methanol synthesis unit comprises two stages connected in series, with the first stage operated on a once-through basis and the second stage operated in a loop. In Figure 2, the methanol synthesis unit comprises two methanol stages connected in series, with the both stages operated in loops.

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 mixture of natural gas and steam supplied by line 10 is fed to a fired heater 12 where it is heated. The heated gas mixture is fed from the fired heater 12 by line 14 to a pre-reformer 16 containing a fixed bed of particulate steam reforming catalyst. The heated gas mixture is reformed adiabatically over the catalyst thereby converting higher hydrocarbons present in the natural gas to methane. The pre-reformed gas mixture is fed from the pre-reformer 16 by line 18 to the fired heater 12 where it is heated to the autothermal reformer inlet temperature. The re-heated pre-reformed gas mixture is fed from the fired heater 12 via line 20 to an autothermal reformer 22 fed with an oxygen stream 24. In the autothermal reformer, the pre-reformed gas mixture is partially combusted with the oxygen in a burner mounted near the top and the resulting hot, partially-combusted gas brought to equilibrium through a bed of steam reforming catalyst disposed beneath the burner. The resulting autothermally reformed synthesis gas stream is fed from the autothermal reformer 22 via line 26 to a heat recovery unit 28 comprising one or more heat exchangers, where it is cooled to below the dew point to condense steam. Process condensate 30 is removed from the cooled gas mixture using gas-liquid separation equipment in the heat recovery unit to produce a make-up gas. A portion of the condensate 30 may be used to generate the steam used to prepare the feed gas 14 provided to the pre-reformer 16. The make-up gas is recovered from the heat recovery unit 28 via line 32, combined with a hydrogen-rich stream provided via line 34, and the resulting mixture fed via line 36 to a syngas compressor 38 where it is compressed. A compressed gas mixture is recovered from the syngas compressor 38 via line 40 and fed to a gas-gas interchanger 42, where it is heated to the methanol converter inlet temperature and then fed via line 44 to the inlet of a methanol synthesis reactor 46. Whereas a single converter is depicted, it will be understood that this flowsheet may operate with two or more converters operated in parallel. The methanol synthesis reactor 46 is an axial-flow steam-raising converter comprising methanol synthesis catalyst-filled tubes 48 cooled by boiling water under pressure 50. Methanol synthesis reactions take place in the reactor 46 to generate a product gas mixture comprising methanol, unreacted hydrogen, and carbon dioxide. The product gas mixture is recovered from the reactor 46 via line 52, cooled in the interchanger 42 and fed via line 54 to one or more further heat exchangers 56 where it is cooled to below the dew point to condense a liquid crude methanol. The cooled mixture is passed from the one or more heat exchangers 56 via line 58 to a gas liquid separator 60, where the unreacted gases are separated from the liquid crude methanol, which is recovered from the separator 60 via line 62. The crude methanol is sent to a distillation and purification unit (not shown) where it is degassed and distilled in two or three stages of distillation to produce a purified methanol product. A first unreacted gas mixture 64 recovered from the gas-liquid separator 60 is combined with a second unreacted gas mixture from line 66 and the combined feed gas passed via line 68 to circulating compressor 70. The compressed feed gas is fed from the compressor 70 via line 72 to a second gas-gas interchanger 74 where it is heated and then fed via line 76 to the inlet of a radial-flow steam-raising converter 78 containing an annular bed of 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, or a tube-cooled converter. A second product gas mixture is recovered from the reactor 78 via line 84 and cooled in the interchanger 74 and one or more further heat exchangers 86 to below the dew point. The cooled mixture is then passed from the heat exchangers 86 via a line 87 to a second gas-liquid separator 88, from which a second stream of liquid crude methanol is recovered via line 90. The first and second crude liquid methanol streams may, if desired, be combined before being sent for purification to produce a purified methanol product as described above. An unreacted gas mixture 92 is recovered from the second gas-liquid separator 88 and divided into the second unreacted gas stream 66 and a purge stream 94. The purge 94 is fed to a purge gas washing unit 96, which is fed with a water stream 98 and which generates a purge gas wash stream 100 containing a small amount of methanol. The purge gas wash stream may be sent for recovery of the methanol with the crude methanol streams 62 and 90. A washed purge gas is recovered from the purge 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 purge gas and supplied via line 34 to the make-up gas stream in line 32. A carbon rich off gas is recovered from the hydrogen recovery unit 104 via line 106 and sent as fuel to be combusted in the fired heater 12.

If desired, a portion of compressed gas mixture 72 may be fed to the feed gas 40 prior to heating in interchanger 42 (not shown).

Figure 2, the reforming unit and methanol synthesis unit are the same as depicted in Figure 1, except that the purge gas mixture 94 taken from the unreacted gas mixture 92 is divided. A first portion is passed to the purge gas washing unit 96 and hydrogen separation unit 104 for generation of the hydrogen-rich stream 34. A second portion bypasses these and is recycled to the compressed feed gas in line 40 for the axial-flow steam-raising converter 46. In this arrangement, the second unreacted gas stream 66 is not combined with the unreacted gas stream in line 64 but is instead fed directly to the circulating compressor 70.

Thus, in Figure 1 just the hydrogen-rich gas 34 is used to achieve the desired stoichiometry and in Figures 2 the bypass line 110 means that both untreated purge gas and the hydrogen-rich gas stream are used to achieve the desired stoichiometry.

Although not shown, the hydrogen-rich gas may be compressed and used to strip the crude liquid methanol of gasses and the resulting enriched stripping gas then combined with the make-up gas stream in lines 40 or 44 fed to the reactor 46.

Each fiowsheet offers a benefit compared to W02017121980 (Al) and W02017121981 (Al) The arrangement of Figure 1 recycles a hydrogen rich stream 34 recovered from the loop purge 94 rather than recycling part of the loop gas from separator 88. This is convenient when a relatively small quantity of recovered hydrogen is required to achieve the required stoichiometry and avoid high reaction temperatures in converter 46. The arrangement of Figure 2 recycles both a hydrogen rich stream 34 and some of the loop gas 110. This is convenient when a relatively small quantity of recovered hydrogen is required to achieve the required stoichiometry but additional diluent gas 110 is needed to avoid high reaction temperatures in converter 46.

Claims (12)

1. Claims.1 A process for synthesising methanol comprising the steps of (i) passing a feed gas comprising a make-up gas having a stoichiometry value, R, less than 2.0 to a methanol synthesis unit comprising a first methanol synthesis reactor and a second methanol synthesis reactor connected in series, wherein the second methanol synthesis reactor is operated in a loop, and (ii) recovering a purge gas stream from the loop and a crude methanol product from the methanol synthesis unit, wherein a hydrogen rich gas is recovered from the purge gas stream and combined with the make-up gas fed gas to the methanol synthesis unit.
2. 2. A process according to claim 1, wherein the make-up gas is 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. 3. A process according to claim 1 or claim 2, wherein the first methanol synthesis reactor and the second methanol synthesis reactor are operated in a loop.
4. 4. A process according to any one of claims 1 to 3, wherein the make-up gas, before hydrogen-rich gas is added has a stoichiometry value R in the range of 1.70 to 1.94 and the feed gas has a stoichiometry value R which is higher than that of the make-up gas
5. 5. A process according to any one of claims 1 to 4, wherein an unreacted gas mixture separated from a product gas mixture recovered from one methanol synthesis reactor is returned to the same or a different methanol synthesis reactor.
6. 6. A process according to any one of claims 1 to 5, wherein a portion of an unreacted gas stream recovered from the first methanol synthesis reactor is recycled to the first methanol synthesis reactor and a portion of an unreacted gas stream recovered from the second methanol synthesis reactor is recycled to the second methanol synthesis reactor.
7. 7. A process according to any one of claims 1 to 5, wherein a portion of an unreacted gas stream recovered from the second methanol synthesis reactor is recycled to the first methanol synthesis reactor.
8. 8. A process according to any one of claims 1 to 5, wherein a compressed gas mixture comprising unreacted gas recovered from the first methanol synthesis reactor and unreacted gas recovered from the second methanol synthesis reactor, is fed to the first methanol synthesis reactor, preferably prior to a heating step.
9. 9. A process according to any one of claims 1 to 8, 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.
10. 10. A process according to any one of claims 1 to 9, wherein a carbon-rich off gas obtained by separation of the hydrogen-rich gas from the purge gas is used as a fuel in a fired heater to heat one or more feed streams to the reforming unit, or is exported to a separate process.
11. 11. A process according to any one of claims 1 to 10, 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.
12. 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.