WO2025149735A1 - Process for synthesising methanol - Google Patents

Abstract

A process for the synthesis of methanol is described comprising the steps of: (i) forming a feed gas mixture comprising carbon dioxide and hydrogen; (ii) passing the feed gas mixture at an inlet temperature to a methanol synthesis unit comprising a methanol synthesis reactor containing a methanol synthesis catalyst to form a product gas mixture containing methanol, wherein the feed gas mixture is heated to the inlet temperature in heat exchange with the product gas mixture in a feed gas interchanger; (iii) cooling the product gas mixture and recovering a crude liquid methanol stream, and; (iv) distilling the crude liquid methanol stream in at least a first distillation column and a second distillation column in series to obtain a purified methanol product, wherein the first distillation column is coupled to a heat exchanger used to supply heat to the first distillation column that is heated by the product gas mixture downstream of the feed gas interchanger and the second distillation column is coupled to a heat exchanger used to supply heat to the second distillation column that is heated by steam or the product gas mixture upstream of the feed gas interchanger.

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 process is often operated in a loop: thus unreacted gas may be recycled to the synthesis reactor as part of the feed gas via a circulator. Fresh synthesis gas, termed make-up gas, is added to the recycled unreacted gas to form the feed gas stream. A purge stream is often taken from the circulating gas stream to avoid the build-up of inert gases in the loop.

The crude methanol is typically purified by distillation.

Distillation of crude methanol normally is performed in a series of column in which heat recovered from the overhead gaseous stream of one column is used in another column. For example US10039997 discloses a process comprising pre-treatment of the crude methanol in a topping stage for the separation of volatile components, at a defined topping pressure (p1); distillation of methanol with at least one final distillation step of methanol at a defined distillation pressure (p4), in which said distillation pressure (p4) is greater than the topping pressure (p1), and in which a gaseous stream of distilled methanol (440), which is produced in the final distillation step, is used to supply at least part of the heat for the pre-treatment topping step.

US10040738 discloses a process comprising at least three distillation stages operating in cascade at decreasing pressures, wherein a first stage operates at a maximum distillation pressure (p2), a second stage operates at a medium distillation pressure (p3), and a final distillation stage operates at a minimum distillation pressure (p4), wherein the first stage and the distillation stage each produce a respective gaseous stream of distilled methanol, and a respective solution containing methanol that is fed to the next distillation stage, and wherein at least one first gaseous stream of distilled methanol, produced in the first distillation stage, and a second gaseous stream of distilled methanol, produced in the second distillation stage, are used as heat sources to heat the second distillation stage and the final distillation stage, respectively.

EP3932897A1 discloses a process for distillation of methanol, comprising: (i) pre-treatment of a crude stream A of methanol in a stabilizing column V0 at pressure P0, for separation of volatile components, obtaining a stream of light gases L from the upper section of V0 and a liquid stream B0 comprising methanol from the lower section of V0; (ii) B0 is then directed towards concentration column V1 , at pressure P1 ; (iii) gaseous stream T1 recovered from the upper section of V1 is condensed in heat exchanger E2, supplying energy to concentration column V2; (iv) part of the condensed methanol obtained in step (iii) is sent to product C1 , and the remaining part of the condensed methanol is added to the upper section of V1 and used as reflux flow; (v) a liquid stream B1 comprising methanol is recovered from the lower section of V1 and passed on to V2, at pressure P2; (vi) gaseous stream T2 recovered from the upper section of V2 is split (S) in two separate streams, one stream being condensed in a heat exchanger E0, supplying energy to VO, and the other stream being condensed in a heat exchanger E3, supplying energy to concentration column V3; (vii) part of the condensed methanol obtained in step (vi) is sent to product, C2, and the remaining part of the condensed methanol is added to the upper section of V2 and used as reflux flow; (viii) a liquid stream B2 comprising methanol is recovered from V2 and passed on to V3, at pressure P3; (ix) gaseous stream T3, recovered from the upper section of V3, is condensed, part of the condensed methanol being sent to product, C3, and the remaining part being added to the upper section of V3 and used as reflux flow; (x) one or more side streams H comprising higher alcohols and other minor biproducts is withdrawn from V3 and a liquid stream B3, is drawn from V3; wherein: Columns V1 , V2 and V3 operate at decreasing pressures, such that P1 >P2>P3; P0> 0 barg and P1 > 0 barg; each column VO, V1 , V2 and V3 is correspondingly associated to a heat exchanger E0, E1 , E2 and E3, which is a reboiler for the same column; heat exchangers E0 and E3 are condensers for column V2; heat exchanger E2 is condenser for column V1 ; P1 <2 barg; and Heat exchanger E1 is supplied by an external energy source.

A four-column distillation arrangement is disclosed in Modern Production Technologies, by Max Appl, 1997 - CRU Publishing Ltd, pages 86-87 (ISBN1873387261), wherein a topping column is followed by two refining columns and recovery column that are heated by a combination of process gas and LP steam with the second refining column heated by the overheads from the first refining column.

The Applicants have realised that the efficiency of the process may be enhanced by using heat from the methanol product stream in specific arrangement of heat exchange stages before condensation of the crude methanol to drive the process.

Accordingly the invention provides a process for the synthesis of methanol comprising the steps of: (i) forming a feed gas mixture comprising carbon dioxide and hydrogen; (ii) passing the feed gas mixture at an inlet temperature to a methanol synthesis unit comprising a methanol synthesis reactor containing a methanol synthesis catalyst to form a product gas mixture containing methanol, wherein the feed gas mixture is heated to the inlet temperature in heat exchange with the product gas mixture in a feed gas interchanger; (iii) cooling the product gas mixture and recovering a crude liquid methanol stream, and; (iv) distilling the crude liquid methanol stream in at least a first distillation column and a second distillation column in series to obtain a purified methanol product, wherein the first distillation column is coupled to a heat exchanger used to supply heat to the first distillation column that is heated by the product gas mixture downstream of the feed gas interchanger and the second distillation column is coupled to a heat exchanger used to supply heat to the second distillation column that is heated by steam or the product gas mixture upstream of the feed gas interchanger.

The invention further provides a system for the synthesis of methanol comprising: (i) a syngas generation unit providing a feed gas mixture comprising carbon dioxide and hydrogen; (ii) a methanol synthesis unit configured to receive the feed gas mixture from the syngas generation unit, said methanol synthesis unit comprising a methanol synthesis reactor containing a methanol synthesis catalyst and a feed gas interchanger configured to heat the feed gas mixture with a product gas mixture recovered from the methanol synthesis unit to an inlet temperature for the methanol synthesis unit, (iii) cooling and gas-liquid separation equipment configured to receive the product gas mixture from the feed gas interchanger and recover a crude liquid methanol stream, and; (iv) a distillation unit comprising a first distillation column and a second distillation column in series configured to be fed with the crude methanol stream and produce a purified methanol product, wherein the first distillation column is coupled to a heat exchanger used to supply heat to the first distillation column that is heated by the product gas mixture downstream of the feed gas interchanger and the second distillation column is coupled to a heat exchanger used to supply heat to the second distillation column that is heated by steam or the product gas mixture upstream of the feed gas interchanger.

The process and system provide advantages in terms of increased thermal efficiency of the process by making better use of the process heat to drive the distillation by direct heat integration than the prior arrangements.

The feed gas mixture may be formed by combining a hydrogen gas stream and a carbon dioxide gas stream or may be formed by combining a make-up gas mixture comprising hydrogen and carbon dioxide, and optionally carbon monoxide, and a recycle stream of unreacted gas recovered from the methanol synthesis unit. The make-up gas may comprise, or may consist of, hydrogen and carbon dioxide. Carbon monoxide and inerts may be present in the make-up and/or feed gas mixture. The present invention is of particular effectiveness in utilising feed gases generated by combining electrolytic hydrogen with a carbon dioxide gas-containing stream. By the term “electrolytic hydrogen” we mean a hydrogen gas stream formed by the electrolysis of water/steam. The carbon dioxide gas stream may be a captured carbon dioxide gas stream recovered from a power plant that combusts carbon-containing fuels, such as coal or biomass, or may be a carbon dioxide stream separated from air, seawater or a gas produced by anaerobic digestion of biomass. Alternatively, the carbon dioxide stream may be a carbon oxides-containing synthesis gas mixture produced by the gasification of biomass or municipal solid waste (MSW). Alternatively, the carbon dioxide could be a relatively pure carbon dioxide stream, such as “recycled” or pipeline CO2, or CO2 extracted from an ammonia plant, or CO2 produced by a fermentation process, or from a geothermal source. Alternatively, off-gases from refineries or other chemical processes comprising principally hydrogen and carbon oxides may also be used. The feed gas mixture is used to synthesise methanol and so may be described in the following as “synthesis gas”.

The hydrogen and/or carbon dioxide streams may be subjected to one or more stages of purification to remove contaminants that might poison the methanol synthesis catalyst. For example, the hydrogen stream may be treated to remove oxygen, and/or the carbon dioxide stream may be treated to remove sulphur compounds. Known purification materials and methods may be applied. For example, oxygen may be removed by reacting it with hydrogen over an oxidation catalyst, such as a supported platinum catalyst. Sulphur compounds such as hydrogen sulphide may be adsorbed using zinc oxide adsorbent materials.

The hydrogen and carbon dioxide streams may be generated or provided at different pressures and compression of one or both streams may be performed using a single stage or multi-stage compressor. The make-up gas may be compressed to the operating pressure of the methanol synthesis unit, combined with the recycle gas and passed to the methanol synthesis unit. Preferably the compressors for the hydrogen, carbon dioxide stream, recycle gas and make-up gas are electrically driven.

The hydrogen and carbon oxide content of the gas mixture fed to the methanol synthesis unit should be adjusted so that the desired stoichiometry for the methanol synthesis reactions is achieved. There are two stoichiometric values that are commonly used. These are R and Z and may be determined from the molar concentrations of the components in the synthesis gas as follows;

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 .

The composition of feed gas mixture may be 15-45 mol% preferably 20-30 mol% carbon dioxide, 55-85 mol%, preferably 70-80 mol% hydrogen and the balance one or more inert gases.

The methanol synthesis unit may comprise one or more methanol synthesis reactors, each containing a bed of methanol synthesis catalyst. The methanol synthesis unit suitably comprises one or more methanol synthesis reactors, for example first, second and optionally third methanol synthesis reactors, each containing a bed of methanol synthesis catalyst, arranged in series and/or parallel that each produce product gas streams containing methanol. The methanol synthesis unit may therefore comprise one, two or more methanol synthesis reactors each containing a bed of methanol synthesis catalyst, and each fed with a feed gas comprising hydrogen and carbon dioxide, each producing a gas mixture containing methanol.

In some arrangements the methanol synthesis unit suitably comprises a single methanol synthesis reactor operated in a loop such that unreacted gas separated from the product mixture is recycled to the feed gas. In other arrangements, the methanol synthesis unit comprises a pair of methanol synthesis reactors operated in series and in loop such that unreacted gas separated from the product mixture from the second methanol synthesis reactor is fed to the feed gas for the first methanol synthesis reactor. If desired additional make up gas may also be fed to the second methanol synthesis reactor. Preferred arrangements of this type are disclosed in W02006/018610 A1 , W02017/121980 A1 and WO2017/121981 A1.

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

In 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 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. 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 (TCC), 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. In the present invention a methanol synthesis unit consisting of a single TCC or multiple parallel TCCs offers a particularly useful arrangement for the process as it does not require additional steam management and efficiently generates the process stream heat necessary for the downstream distillation duties.

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.

In a process comprising first and second methanol synthesis reactors, the first methanol synthesis reactor is preferably cooled by boiling water, such as in an axial-flow steam-raising converter or a radial-flow steam-raising converter, more preferably an axial-flow steam raising converter. The second methanol synthesis reactor may be a radial-flow steam-raising converter. Alternatively, the second methanol may be cooled by a synthesis gas comprising hydrogen and carbon dioxide. Accordingly, the second methanol synthesis reactor may be a cooled reactor selected from a tube cooled converter and a gas-cooled converter. A tube-cooled converter is preferred because of its simpler design. If a third methanol synthesis reactor is present, it is preferably cooled by boiling water. The third methanol synthesis reactor may then suitably be a steam-raising converter selected from an axial-flow steam-raising converter and a radial-flow steam-raising converter, most preferably an axial-flow steam raising converter. The first and second methanol synthesis reactors may be connected in series in which case the synthesis gas fed to the second methanol synthesis reactor comprises at least a portion of a methanol product gas stream recovered from the first methanol synthesis reactor. In such an arrangement, preferably the synthesis gas fed to the second methanol synthesis reactor comprises all of the methanol product gas stream recovered from the first methanol synthesis reactor.

The methanol synthesis catalysts in each of the methanol synthesis reactors may be the same or different. The methanol synthesis catalysts are preferably copper-containing methanol synthesis catalysts, which are commercially available. In particular, the methanol synthesis catalysts are one or more particulate copper/zinc oxide/alumina catalysts, which may comprise one or more promoters. Particularly suitable catalysts are Mg-promoted copper/zinc oxide/alumina catalysts as described in US4788175 and Si-promoted copper/zinc oxide/alumina catalysts as described in GB 2583185 B.

Methanol synthesis may be effected 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 feed gas at the reactor inlets maybe in the range 130°C to 250 °C. Heating of the feed gas is possible in some reactor configurations and the temperature of the feed gas at the catalyst bed inlets is preferably in the range 180-250°C. The temperature of the product gas mixture at the catalyst bed or reactor outlets is preferably in the range 220-280°C.

A product gas mixture containing methanol is recovered from at least one methanol synthesis reactor. Methanol is recovered from one or more of the product gas mixtures. This may be achieved by cooling one or more of the methanol product gas streams to below the dew point, condensing methanol, and separating a crude liquid methanol product from the unreacted gases.

In the process the product gas mixture is cooled in a feed gas interchanger. This is a gas-gas interchanger that uses the feed gas mixture to cool a methanol product gas stream. The feed gas interchanger therefore also pre-heats the feed gas mixture to the inlet temperature of the methanol synthesis unit. In the process, the feed gas interchanger provides a part of the cooling of the product gas. Additional cooling stages include the heat exchangers, e.g. gas-liquid interchangers, used to provide heat to the first and, optionally, the second distillation columns. Downstream of interchangers, additional cooling with water and/or air may be required to lower the product gas temperature to below the dew point at which the liquid crude methanol condenses. Where there is more than one methanol synthesis reactor and so multiple methanol product gas streams, these streams may be treated separately or may be combined before cooling and/or separating the crude liquid methanol. Separation of the crude liquid methanol product from one or more of the methanol product gas streams produces an unreacted gas mixture. A portion of the unreacted gas mixture is preferably returned as a recycle or loop gas stream to one or more of the methanol synthesis reactors in the methanol synthesis unit. Unreacted gas separated from a product gas mixture recovered from one methanol synthesis reactor may be returned to the same or a different methanol synthesis reactor. The unreacted gas mixture comprises hydrogen, carbon monoxide, and carbon dioxide and so may be used to generate additional methanol. The recycle gas stream may be recovered from at least one of one of the methanol product gas streams and recycled to at least one of the methanol synthesis reactors. If there is more than one recycle gas stream, these may be recycled separately to one or more of the methanol synthesis reactors or combined and fed to one or more of the methanol synthesis reactors.

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

The recycle ratios to form the feed gas mixtures to the one or more methanol synthesis reactors may be in the range 0.5:1 to 7:1 preferably 2:1 to 6:1 . By the term “recycle ratio”, we mean the molar flow ratio of the recycled unreacted gas stream to the make-up gas that form the gas mixtures fed to the one or more methanol synthesis reactors.

A portion of the unreacted gas mixture separated from the crude liquid methanol may be removed from the synthesis loop as the purge gas stream. The purge gas stream may be removed continuously or periodically to prevent the unwanted build-up of inert gases, such as nitrogen, argon and methane in the synthesis loop. The purge gas stream may be recovered from the separated unreacted gases before or after compression in the circulator.

The purge gas stream mixture may contain methanol and so, if desired, methanol may be recovered from the purge gas stream using a water wash, and the recovered methanol and water 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.

In the present invention, the process includes distilling the crude liquid methanol stream in a first distillation column and a second distillation column in series to obtain a purified methanol product. The first distillation column is coupled to a heat exchanger that is used to supply heat to the first distillation column and is heated by the product gas mixture downstream of the feed gas interchanger. This heating may be performed in a first distillation column reboiler. The second distillation column is coupled to a heat exchanger that is used to supply heat to the second distillation column and is heated by steam or the product gas mixture upstream of the feed gas interchanger. This heating may be performed in a second distillation column reboiler.

The heat exchanger coupled to the second distillation column maybe usefully heated with steam where one or more of the methanol synthesis reactors is a steam raising converter, such as an aSRC or rSRC. The steam may be medium-pressure or low-pressure steam. The steam may suitably be recovered from a steam drum coupled to one or more steam-raising converters, or less- preferably by a combustion-fired boiler. However, in a preferred arrangement, for example where the methanol synthesis reactor is a TCC or GCC, the heat exchanger coupled to the second distillation column is heated by the product gas mixture upstream of the feed gas interchanger. In this arrangement, using the product gas heated heat exchangers in the order claimed provides a more efficient recovery and use of the heat available for the distillation duties, and does not rely on steam generation, which can reduce the additional external heat input to produce refined methanol. For example, the inlet temperature range for the product gas to the shell side of the heat exchanger coupled to the second distillation column is preferably in the range 200 to 300 °C. The inlet temperature range for the product gas to the shell side of the feed gas interchanger is preferably in the range 150 to 250 °C. The inlet temperature range for the product gas to the shell side of the heat exchanger coupled to the first distillation column is preferably in the range 100 to 140°C. The cooled product stream from the heat exchanger coupled to the first distillation column may be fed to gas-liquid separation about 90 °C.

If desired, the crude liquid methanol stream may be fed to a low-pressure separation vessel or a stripping vessel upstream of the first distillation column. A low-pressure separation vessel may be provided between the gas-liquid separator used to recover the crude methanol and the first distillation column. This vessel, which is not heated, may be used to allow the flash separation dissolved gases from the crude methanol by reducing the pressure towards that of the first distillation column. The inclusion of the low-pressure separator may improve the efficiency of the first distillation column. Alternatively, a stripping unit fed with a hydrogen-containing gas may be provided between the gas-liquid separator used to recover the crude methanol and the first distillation column. The stripping unit provides recovery of dissolved carbon dioxide and improved process efficiency in recycling it to the process. A preferred stripping unit arrangement is described in WO2019/220073 A1.

Preferably the process may include one or more further distillation column downstream of the second distillation column that further refines the methanol product, preferably at least a third distillation column. Where a further distillation column is used, preferably it is coupled to a heat exchanger used to supply heat to the third distillation column that is heated by the overheads from the distillation column immediately upstream. Thus, where a third methanol distillation column is used, it is preferably coupled to a heat exchanger used to supply heat to the third distillation column that is heated by an overhead stream from the second distillation column. This heating may be performed in a third distillation column reboiler.

Conventional distillation column designs may be used. The columns may be tray-columns or packed columns. The first and second distillation columns are preferably packed columns as this offers advantages in the operation of the process.

In the process, the first distillation column may be used to produce a stabilised or topped methanol product by separating dissolved unreacted gases and light by-products from the crude liquid methanol. The stabilised or topped methanol product may be divided, a portion processed in the second distillation column.

The second distillation column may be used to separate methanol from the water by-product. The column maybe operated to generate a fuel-grade methanol or AA-grade high-purity methanol, however, to produce AA-grade methanol, the process preferably includes a third distillation column.

Where a third distillation column is provided, the process may comprise: (i) pre-treating the crude methanol in the first distillation column at a first pressure, P1 , to separate volatile components, obtaining a stream of light gases from an upper section of the first distillation column and a first liquid stream comprising methanol from a lower section of the first distillation column; (ii) passing the first liquid stream to the second distillation column operated at a second pressure, P2; (iii) recovering a gaseous stream from an upper section of the second distillation column, and condensing the gaseous stream in a heat exchanger supplying energy to the third distillation column; (iv) recovering a purified methanol product as a side draw of liquid from the second distillation column; and (v) adding condensed methanol obtained in step (iii) to the upper section of the second distillation column as a reflux flow.

The process may further comprise steps: (vi) recovering a second liquid stream comprising methanol from the lower section of the second distillation column and passing the second liquid stream to the third distillation column operated at a third pressure, P3; (vii) recovering a further gaseous stream from the upper section of the third distillation column and condensing the further gaseous stream, (viii) adding a part of the condensed stream obtained in step (vii) to the upper section of the third distillation column as a reflux flow and recovering a part of the condensed stream obtained in step (vii) as a further portion of purified methanol product; (ix) withdrawing one or more fusel oil side streams comprising higher alcohols and other minor biproducts from the side of the third distillation column and (x) recovering a third liquid stream consisting essentially of water from the base of the third distillation column.

The columns may operate at decreasing pressures, such that P2>P1>P3, with P2 being >6 bar abs and P3 being between 1 and 2 bar abs.

Fusel oil is a term used for one or more side streams comprising higher alcohols and other minor by-products formed in the methanol synthesis unit.

The first distillation column is coupled to a heat exchanger used to supply heat to the first distillation column that is heated by the product gas mixture downstream of the feed gas interchanger and the second distillation column is coupled to a heat exchanger used to supply heat to the second distillation column that is heated by steam or the product gas mixture upstream of the feed gas interchanger. In a preferred arrangement, for example where the product gas mixture is used to heat the first and second distillation columns, the first distillation column, the second distillation column, or the first and second distillation columns are also provided with a supplemental heater. This supplemental heater may be used to provide heat to the columns in cases where there is a reduction in flow of the product gas mixture through the heat exchangers, for example during turn down in the process due to variation in the flow of electrolytic hydrogen. The supplemental heater coupled to the first distillation column and/or the second distillation columns, which may be termed a supplemental distillation column reboiler, may be heated by steam, hot oil or may be electrically heated.

The stream of light gases recovered from the upper section of the first distillation column is preferably cooled to condense a methanol-containing stream, which is added to the upper section of the first distillation column as a reflux flow. The uncondensed light gases are separated from the condensed stream and may be used as a fuel gas. In particularly preferred arrangement, the cooling and separation are performed by passing the stream of light gases to a first heat exchanger cooled with water, dividing the resulting cooled stream into a first portion and a second portion, feeding the first portion to a reflux drum that returns the condensate to the first distillation column, passing the second portion to a second heat exchanger cooled with water to form a second cooled portion and feeding the second cooled portion to the reflux drum.

The fusel oil recovered from the third distillation column may if desired be subjected to a separate distillation in a further distillation column.

The purified methanol product recovered from the process 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 process according to one embodiment of the invention, and Figure 2 depicts a distillation arrangement for the process of Figure 1 .

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 stream 10 consisting essentially of hydrogen and carbon monoxide is combined with a recycle gas stream 12 to form a feed gas mixture which is fed via line 14 through a feed gas interchanger 16 where it is heated to an inlet temperature and fed via line 18 to the inlet of a tube-cooled converter 20 containing a bed of methanol synthesis catalyst 22. The feed gas is further heated as it passes through tubes disposed in the catalyst bed, thereby acting to cool the catalyst bed, and then passes through the catalyst bed 22, where the methanol synthesis reactions take place to form a product gas mixture. The product gas mixture is recovered from the tube-cooled converter via line 24 and passed to a first heat exchanger 26 fed with a methanol-containing liquid stream 28 recovered from a second distillation column 30 that acts to cool the product gas mixture. The partially cooled product gas mixture from the first heat exchanger 26 is fed via line 32 to the feed gas interchanger 16 where it is further cooled. From the feed gas interchanger 16, the further cooled product gas mixture is fed via line 34 to a second heat exchanger 36 fed with a methanolcontaining liquid stream 38 recovered from near the base of a first distillation column 40 that acts to cool the product gas mixture. The cooling in the first heat exchanger 26, the feed gas interchanger 16 and the second heat exchanger 36 acts to cool the product gas mixture to below the dew point such that crude methanol condenses. The cooled product mixture is fed from the second heat exchanger 36 via line 42 to a gas-liquid separator 44, where the liquid crude methanol product is separated from an unreacted gas. The unreacted gas is fed from the separator 44 via line 46 to a circulating compressor 48 that passes the unreacted gas as the recycle gas stream 12 to form the feed gas stream 14. A purge gas stream 50 is withdrawn from the unreacted gas stream 46 upstream of the circulating compressor 48 to prevent the unwanted build up of inerts in the process.

The liquid crude methanol stream is fed from the gas-liquid separator 44 via line 52 to the first distillation column 40. The first distillation column is operated at a lower pressure than the crude methanol stream such that dissolved gases in the methanol are evolved and recovered from the top of the column 40 via line 54. Heat to drive the separation is provided by a heated methanolcontaining liquid stream provided from the second heat exchanger 36 to the first distillation column 40 via line 56 at a position between the inlet for line 52 and the outlet for line 38. A degassed liquid crude methanol stream is recovered from the base of the first distillation column 40 and fed via line 58 to the second distillation column 30. The second distillation column 30 is operated at a pressure such that a gaseous purified methanol product is separated from the degassed liquid crude methanol. Heat to drive the separation is provided by a heated methanol-containing liquid stream provided from the first heat exchanger 26 to the second distillation column 30 via line 60 at a position between the inlet for line 58 and the outlet for line 28. A first portion of the gaseous methanol product is recovered from near the top of the second distillation column 30 via line 62, condensed in a third heat exchanger 64 and optionally one or more further heat exchangers cooled with water, to recover a liquid purified methanol product 66. A second portion of the gaseous methanol product is recovered from the top of the second distillation column 30 via line 68, condensed in a fourth heat exchanger 70 and recycled as a reflux stream 72 to the second distillation column 30. The fourth heat exchanger is cooled by a liquid stream fed via line 74.

A partially purified liquid crude methanol stream is recovered from the base of the second distillation column 30 and fed via line 76 to a third distillation column 78. The third distillation column 78 is operated at a pressure such that a gaseous purified methanol product is separated from the partially purified liquid crude methanol. Heat to drive the separation is provided by a heated methanol-containing liquid stream provided from the fourth heat exchanger 70 to the third column 78 via line 80 at a position between the inlet for line 76 and the outlet for line 74. A gaseous methanol product is recovered from the top of the third distillation column 78 via line 82, condensed in a fifth heat exchanger 84 and optionally one or more further heat exchangers cooled with water, to recover a further liquid purified methanol product 86.

A fusel oil fraction is withdrawn from the side of the third distillation column 78 via line 88 and may be subjected to further distillation if desired.

A liquid water stream is withdrawn from the bottom of the third distillation column 78 via line 90 and sent for further processing. In one arrangement the water is purified and fed to an electrolysis unit to generate hydrogen for use in the process and/or power generation.

In Figure 2, the distillation columns 40, 30, 78 are depicted as arranged in Figure 1 . The columns each contain one or more beds of inert distillation packing 100, 102, 104 to aid in the separation of components of the crude methanol 52. The third distillation column 78 further contains one or more distillation trays 106 beneath the lowest bed of distillation packing 104 to further enhance the separation of the by-product water 90 from the methanol product 82.

The light gas mixture recovered from the top of the first distillation column 40 via line 54 comprises methanol vapour and so to improve the efficiency of the process, a first water-cooled heat exchanger 108 is used to condense a methanol-containing stream 110, which is divided. A first portion 112 of the condensed stream is fed to a reflux drum 114 and then via line 116 to near the top of the column 40. A second portion of the condensed stream 118 is fed though a second water-cooled heat exchanger 120 and a further cooled portion fed to the reflux drum 114 via line 122. Light gases suitable for use as fuel are recovered downstream of the second water-cooled heat exchanger 120 via line 124.

The second distillation column 30 in addition to the process-gas heat exchanger 26 comprises a supplemental heater 130 that may be heated by steam, hot oil, or electrical power provided via line 132. Liquid is recovered from the near the bottom of the column 30 and fed to the supplemental heater 130 via line 134. A heated stream is returned to the column 30 above the off-take for line for line 134. The supplemental heater provides flexibility to the distillation process. For example, it may compensate for reduced heating from the process gas 28. Alternatively, the supplemental heater may increase the throughput of the second distillation column 30, thereby enabling purification of stored degassed methanol without harming the overall methanol production.

To improve control of the process, the purified methanol product 84 from the third distillation column 78 is fed to a reflux drum 140. A portion is returned to near the top of the column 78 via line 142. A further portion is fed from the drum 140 via line 86 to a methanol product heat exchanger, which may be water and/or air cooled, to provide a cooled purified methanol product, which is combined via line 146 with the cooled purified methanol product 66 recovered from the second distillation column 30.

The fusel oil stream 88 recovered from the side of the third distillation column 78 is cooled in a water-cooled heat exchanger 150 to form a cooled fusel oil stream, 152. The aqueous stream 90 recovered from the bottom of the third distillation column 78 is cooled on a water-cooled heat exchanger 154 to form a cooled aqueous stream 156.

Claims

Claims.

1 . A process for the synthesis of methanol comprising the steps of: (i) forming a feed gas mixture comprising carbon dioxide and hydrogen; (ii) passing the feed gas mixture at an inlet temperature to a methanol synthesis unit comprising a methanol synthesis reactor containing a methanol synthesis catalyst to form a product gas mixture containing methanol, wherein the feed gas mixture is heated to the inlet temperature in heat exchange with the product gas mixture in a feed gas interchanger; (iii) cooling the product gas mixture and recovering a crude liquid methanol stream, and; (iv) distilling the crude liquid methanol stream in at least a first distillation column and a second distillation column in series to obtain a purified methanol product, wherein the first distillation column is coupled to a heat exchanger used to supply heat to the first distillation column that is heated by the product gas mixture downstream of the feed gas interchanger and the second distillation column is coupled to a heat exchanger used to supply heat to the second distillation column that is heated by steam or the product gas mixture upstream of the feed gas interchanger.

2. A process according to claim 1 , wherein the feed gas mixture is generated by combining electrolytic hydrogen with a carbon dioxide gas stream.

3. A process according to claim 2, wherein the carbon dioxide gas stream is a carbon dioxide gas stream or a synthesis gas containing carbon oxides that has been generated by gasification of municipal solid waste or biomass.

4. A process according to any one of claims 1 to 3, wherein hydrogen and/or carbon dioxide streams are subjected to one or more stages of purification to remove contaminants that poison the methanol synthesis catalyst.

5. A process according to any one of claims 2 to 4, wherein the hydrogen and carbon dioxide streams are provided at different pressures and compression of one or both streams is performed using a single stage or multi-stage compressor.

6. A process according to any one of claims 1 to 5, wherein methanol synthesis unit comprises one, two or more methanol synthesis reactors each containing a bed of methanol synthesis catalyst, and each fed with a feed gas comprising hydrogen and carbon dioxide, each producing a product gas mixture containing methanol.

7. A process according to any one of claims 1 to 6, wherein the methanol synthesis reactor is an un-cooled adiabatic reactor, or a methanol synthesis reactor cooled by heat exchange with the feed gas, or a methanol synthesis reactor cooled by boiling water under pressure, preferably a methanol synthesis reactor cooled by heat exchange with the feed gas.

8. A process according to any one of claims 1 to 7, wherein the methanol synthesis unit comprises a single methanol synthesis reactor selected from a quench reactor, a tube- cooled converter or a gas-cooled converter.

9. A process according to any one of claims 1 to 8, wherein separation of the crude liquid methanol product from the methanol product gas streams produces an unreacted gas mixture and a portion of the unreacted gas mixture is returned as a recycle or loop gas stream to one or more of the methanol synthesis reactors in the methanol synthesis unit.

10. A process according to any one of claims 1 to 9, wherein the crude liquid methanol stream is fed to a low-pressure separation vessel or a stripping vessel upstream of the first distillation column.

11. A process according to any one of claims 1 to 10, wherein the first and second distillation columns are packed columns.

12. A process according to any one of claims 1 to 11 , wherein the first distillation column is used to produce a stabilised or topped methanol product by separating dissolved unreacted gases and light by-products from the crude liquid methanol.

13. A process according to claim 12, wherein the stabilised or topped methanol product is divided, a portion processed in the second distillation column and a portion sent for storage.

14. A process according to any one of claims 1 to 13, wherein a stream of light gases is recovered from the upper section of the first distillation column and cooled to condense a methanol-containing stream, which is added to the upper section of the first distillation column as a reflux flow, wherein the cooling is performed by passing the stream of light gases to a first heat exchanger cooled with water, dividing the resulting cooled stream into a first portion and a second portion, feeding the first portion to a reflux drum that returns the condensate to the first distillation column, passing the second portion to a second heat exchanger cooled with water to form a second cooled portion and feeding the second cooled portion to the reflux drum.

15. A process according to any one of claims 1 to 14, wherein the first distillation column and/or the second distillation column are also heated by a supplemental heater, preferably wherein the second distillation column is heated by a supplemental heater.

16. A process according to claim 15, wherein the supplemental heater is heated by steam, hot oil or is electrically heated.

17. A process according to any one of claims 1 to 16, wherein a liquid product recovered from the second distillation column is fed to a third distillation column to provide an additional purified methanol product stream.

18. A process according to claim 17, wherein the third distillation column is coupled to a heat exchanger used to provide heat to the third distillation column that is heated by an overhead stream from the second distillation column.

19. A process according to claim 17 or claim 18, wherein the third distillation column further produces a fusel oil stream and an aqueous stream.

20. A process according to any one of claims 17 to 19, wherein the process comprises: (i) pretreating the crude methanol in the first distillation column at a first pressure, P1 , to separate volatile components, obtaining a stream of light gases from an upper section of the first distillation column and a first liquid stream comprising methanol from a lower section of the first distillation column; (ii) passing the first liquid stream to the second distillation column operated at a second pressure, P2; (iii) recovering a gaseous stream from an upper section of the second distillation column, and condensing the gaseous stream in a heat exchanger supplying energy to the third distillation column; (iv) recovering a purified methanol product as a side draw of liquid from the second distillation column and (v) adding condensed methanol obtained in step (iii) to the upper section of the second distillation column as a reflux flow.

21 . A process according to claim 20 further comprising steps of: (vi) recovering a second liquid stream comprising methanol from the lower section of the second distillation column and passing the second liquid stream to the third distillation column operated at a third pressure, P3; (vii) recovering a further gaseous stream from the upper section of the third distillation column and condensing the further gaseous stream, (viii) adding a part of the condensed stream obtained in step (vii) to the upper section of the third distillation column as a reflux flow and recovering a part of the condensed stream obtained in step (vii) as a further portion of purified methanol product; (ix) withdrawing one or more fusel oil side streams comprising higher alcohols and other minor biproducts from the side of the third distillation column and (x) recovering a third liquid stream consisting essentially of water from the base of the third distillation column.

22. A process according to claim 21 , wherein P2>P1 >P3, with P2 being >6 bar abs and P3 being between 1 and 2 bar abs.

3. A system for the synthesis of methanol comprising: (i) a syngas generation unit providing a feed gas mixture comprising carbon dioxide and hydrogen; (ii) a methanol synthesis unit configured to receive the feed gas mixture from the syngas generation unit, said methanol synthesis unit comprising a methanol synthesis reactor containing a methanol synthesis catalyst and a feed gas interchanger configured to heat the feed gas mixture with a product gas mixture recovered from the methanol synthesis unit to an inlet temperature for the methanol synthesis unit, (iii) cooling and gas-liquid separation equipment configured to receive the product gas mixture from the feed gas interchanger and recover a crude liquid methanol stream, and; (iv) a distillation unit comprising a first distillation column and a second distillation column in series configured to be fed with the crude methanol stream and produce a purified methanol product, wherein the first distillation column is coupled to a heat exchanger used to supply heat to the first distillation column that is heated by the product gas mixture downstream of the feed gas interchanger and the second distillation column is coupled to a heat exchanger used to supply heat to the second distillation column that is heated by steam or the product gas mixture upstream of the feed gas interchanger.