This invention relates to a methanol synthesis process and in particular to a method for revamping a methanol synthesis process operating in a synthesis loop.

Methanol synthesis is generally performed by passing a synthesis gas comprising hydrogen, carbon oxides and any inert gases 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 methanol synthesis reactor, to form a product gas stream containing methanol. A crude methanol product is generally recovered by cooling the product gas stream to below the dew point of the methanol and separating off the product as a liquid from a downstream catch-pot. The process is often operated in a loop: thus unreacted gas recovered from the catch-pot is often recycled to the synthesis reactor via a circulator. Fresh synthesis gas, termed make-up gas, is added to the recycled unreacted gas to form the synthesis gas. A purge stream is often taken from the circulating gas stream to avoid the build-up of inert gasses.

Methanol may be synthesised from the purge gas. Thus, US5424335 discloses a process in which the purge gas from a methanol synthesis loop is subjected to a further step of methanol synthesis, preferably while in indirect heat exchange with the purge gas undergoing heating to the synthesis inlet temperature so that the reacted purge gas, containing synthesized methanol, leaves the synthesis catalyst at a temperature below the maximum temperature achieved by the reacting purge gas during passage over the catalyst. The synthesis loop can be operated less efficiently, e.g. at a lower pressure or with added carbon dioxide, than is conventional since the additional methanol produced from the purge gas compensates for the loss of loop efficiency. The addition of the purge gas synthesis stage enables existing plants to be uprated by lowering the loop pressure, thus enabling a greater throughput through the synthesis gas compressor to be achieved.

US6258860 discloses a process for the production of methanol from synthesis gas derived from a carbonaceous feedstock which comprises the following steps: (1) part of the unreacted gas stream from a first methanol synthesis zone is recycled to the first methanol zone; (2) another part of the unreacted gas stream from the first methanol synthesis zone is supplied to a second methanol synthesis zone; (3) part of the unreacted gas stream from the second methanol synthesis zone is recycled to the second methanol synthesis zone; (4) hydrogen is recovered from another part of the unreacted gas from the second methanol synthesis zone to give a hydrogen enriched gas stream and a hydrogen depleted gas stream; and (5) recycling the hydrogen depleted gas stream to the second methanol synthesis zone. There is a desire to improve the flexibility of such methanol processes, in particular in revamping methanol processes where the existing loop is constrained rather than to operate the loop in a less efficient manner. Accordingly, the invention provides a method for revamping a methanol synthesis process operating in a synthesis loop, comprising the steps of (i) installing a methanol synthesis reactor containing a methanol synthesis catalyst outside of the synthesis loop, (ii) recovering a purge gas stream from the synthesis loop, (iii) passing the at least a portion of the purge gas stream through the installed methanol synthesis reactor to form a product gas stream containing methanol, and (iv) recovering methanol from the product gas stream to form a methanol- depleted gas mixture, wherein a hydrogen stream is recovered from the methanol-depleted gas mixture and fed, along with a carbon dioxide stream, to the synthesis loop.

By the term "revamping" we include adapting an existing methanol plant and process to improve one or more of the process efficiency, the methanol synthesis catalyst lifetime and methanol production. Passing the portion of the synthesis loop purge gas through the installed methanol synthesis reactor can increase methanol production significantly.

The installed methanol synthesis reactor may be an un-cooled adiabatic reactor. 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 reactor may also be a quench reactor, or a reactor selected from a tube-cooled converter or a gas- cooled converter, wherein the catalyst bed is cooled in heat exchange with the synthesis gas. Alternatively, the reactor may be cooled by boiling water under pressure, such as an axial flow steam-raising converter, or a radial flow steam-raising converter. In each case, the reactors contain fixed beds of methanol synthesis catalyst through which the synthesis gas is passed.

In a preferred arrangement, the installed methanol synthesis reactor is a gas-cooled converter (GCC) or a tube-cooled converter (TCC), preferably a tube-cooled converter. In a TCC, the catalyst bed is cooled by feed synthesis gas passing through open ended tubes disposed within the bed that discharge the heated gas to the catalyst. As an alternative to a TCC, a GCC may be used to cool the catalyst bed by passing the synthesis gas though tubes in a heat exchanger-type arrangement. A GCC is described for example in the aforesaid US 5827901 . The use of a TCC is preferred over the GCC in that it is simpler and cheaper to fabricate due to the use of open topped tubes and the elimination of the upper header and all of the differential expansion problems that the gas cooled converter raises. A TCC therefore has the advantage of low equipment cost and lower outlet temperature, which favours the synthesis reaction equilibrium.

In an alternative preferred arrangement, the installed methanol synthesis reactor is an axial- flow steam-raising converter (aSRC). In such reactors, the synthesis gas typically passes axially through vertical, catalyst-containing tubes that are cooled in heat exchange with boiling water under pressure. 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 WO2012146904 (A1). Steam raising converters in which the catalyst is present in tubes cooled by boiling water under pressure offer a useful means to remove heat from the catalyst.

Both TCC and aSRC reactors provide a similar proportion of uprate to an existing process, so the decision of which type to use may be based upon whether additional steam is beneficial to the plant or not. The aSRC is easier to start-up and easier to control when considering the various operating cases such as beginning of life (BOL), end of life (EOL), loss of C02 and partial load. The TCC will also handle all of these cases, but desirably includes a start-up heater which may need to be used under low load operation.

If the steam generated from the aSRC can be used beneficially by integration into the existing plant system, then this may be the preferred option. If the recovered heat has little or no value, the TCC may be the better option as the heat will simply be rejected to air or cooling water.

The methanol synthesis catalyst in the installed methanol synthesis reactor is preferably a copper-containing methanol synthesis catalyst, in particular a particulate copper/zinc oxide/alumina catalyst. Particularly suitable catalysts are Mg-doped copper/zinc oxide/alumina catalysts as described in US4788175. Methanol synthesis in the installed methanol synthesis reactor operating outside of the synthesis loop may be effected at pressures in the range 10 to 120 bar abs, and temperatures in the range 130°C to 350°C. The pressure of the synthesis gas at the reactor inlet is preferably 50-100 bar abs, more preferably 70-90 bar abs. The temperature of the synthesis gas at the synthesis reactor inlet is preferably such that the temperature inlet the bed of methanol synthesis catalyst is 200-250°C and at the outlet preferably 230-285°C.

Methanol is synthesised from the portion of the purge gas in the installed methanol synthesis reactor thereby forming product gas stream containing methanol. Methanol is recovered from the product gas stream to form a methanol-depleted gas mixture. In the present invention, hydrogen is recovered from the methanol-depleted gas mixture to generate a hydrogen stream and a hydrogen-depleted gas stream.

The installed methanol synthesis reactor may be operated on a once-though basis in which case the product gas stream is cooled to below the dew point to condense methanol therefrom, a liquid methanol product stream is recovered using a gas-liquid separator, the resulting methanol-depleted gas stream, treated to recover hydrogen therefrom and the remaining hydrogen-depleted gas stream used as fuel. Alternatively, the installed methanol synthesis reactor may be operated in a loop, outside the main synthesis loop, in which case the product gas stream from the installed methanol synthesis reactor is cooled to below the dew point to condense methanol, a liquid methanol product stream is recovered using a gas-liquid separator, the methanol-depleted gas recovered from the separator is divided into a recycle stream that is compressed and returned to the installed methanol synthesis reactor and a purge stream, which is treated to recover hydrogen therefrom. The recovered hydrogen stream and a carbon dioxide stream are fed to the main synthesis loop. However, a once-through scheme is preferred for the installed methanol synthesis reactor because it is simpler to install due to the smaller number of equipment items and the limited number of tie-ins required. One of the key benefits of the once-through scheme versus a loop arrangement is that an additional circulator is not required. In addition, the installed methanol synthesis reactor will be smaller and therefore cheaper than a reactor used in a loop arrangement.

In the present invention, a hydrogen stream is recovered from the methanol-depleted gas mixture and fed, along with a carbon dioxide stream, to the synthesis loop. This offers improved process efficiency compared with US6258860 in which streams are recycled to the purge methanol synthesis reactor.

Adding carbon dioxide to the synthesis loop will allow an increase in methanol production, but the percentage conversion of carbon oxides to methanol falls as more and more carbon dioxide is added, limiting the increase in methanol production and wasting an ever-increasing fraction of the valuable carbon oxides that are lost in the purge gas.

In the synthesis loop, particularly for reactor designs that remove the heat of reaction in such a way that the reaction trajectory is close to the maximum rate line, the Applicant has found that the methanol production of the synthesis loop can be increased by adding hydrogen to increase the stoichiometric ratio, R (where R = ([H2]-[C02])/([CO]+[C0 ₂ ]). Although the percentage conversion of carbon oxides to methanol is still lower than for the original synthesis loop, it is higher than for the synthesis loop with only carbon dioxide added. If sufficient hydrogen is available, the economic maximum carbon dioxide that can be added to the synthesis loop is more than double the economic maximum carbon dioxide that can be added without additional hydrogen.

In the present invention, a synthesis loop has been provided with both additional carbon dioxide and additional hydrogen that will produce a purge gas with more reactants, both carbon oxides and hydrogen, than the synthesis loop without additional carbon dioxide and additional hydrogen. This purge gas can be economically converted to make additional methanol by installing a new methanol synthesis reactor fed by the said purge gas. Hydrogen recovered from the methanol-depleted gas mixture is sufficient that the overall conversion of carbon oxides for the synthesis loop and the installed methanol reactor is greater than the conversion of the original synthesis loop without additional carbon dioxide and additional hydrogen. The hydrogen may be recovered from the methanol depleted gas mixture by any means suitable, such as by using a Pressure-Swing Adsorption (PSA) unit, a membrane unit, a liquefaction unit, or a combination of two or more of these. However, in a preferred arrangement, the hydrogen is recovered by PSA. PSA units suitable for recovering hydrogen from gas streams are available commercially. The hydrogen stream is preferably >90% vol H2, more preferably >95% vol H2.

In a particularly preferred arrangement, the recovered hydrogen stream is recycled to the existing synthesis gas loop compression suction and the tails are sent to the fuel system. This allows a low-pressure type PSA unit to be used, which reduces cost and increases reliability significantly.

The carbon dioxide stream may be any carbon dioxide stream, for example a carbon dioxide stream recovered from one or more C02-containing process gases. The CO2 recovery may be by a wet CO2 recovery method using liquid chemical or physical absorbents, or dry CO2 recovery using a membrane separation unit or a PSA unit. The CO2 may, for example, be suitably recovered from a flue gas, such as a flue gas from a fired-heater and or a fired primary steam reformer. The carbon dioxide stream may alternatively be recovered from downstream processing of off-gases recovered from the methanol recovery process. The carbon dioxide stream is preferably >70% vol CO2, more preferably >90% vol CO2, most preferably >95% vol

The carbon dioxide stream may be injected to the existing syngas loop compressor. Injecting the hydrogen and carbon dioxide streams into the existing synthesis loop may require installing a new compressor for the synthesis loop. The method may be applied to an existing methanol process, in which case the method may form part of a re-vamp of the methanol process, for example to improve methanol recovery. The method may also form part of a new methanol plant and process.

Accordingly, the invention further provides a process a process for synthesising methanol comprising the steps of: (i) passing a feed gas mixture comprising a make-up gas and at least a portion of a recycle gas stream to one or more methanol synthesis reactors containing a methanol synthesis catalyst operating in a synthesis loop and recovering a first product gas stream containing methanol from said one or more reactors, (ii) cooling the first product gas stream using one or more heat exchangers and recovering methanol from the first product gas stream thereby forming a first methanol-depleted gas mixture, (iii) dividing the first methanol- depleted gas mixture into a purge gas stream and a loop gas stream, (iv) combining the loop gas stream with the make-up gas to form the feed gas mixture for the synthesis loop, (v) passing at least a portion of the purge gas stream to a methanol synthesis reactor containing a methanol synthesis catalyst installed outside of the synthesis loop to form a second product gas stream containing methanol, and (vi) recovering methanol from the second product gas stream to form a second methanol-depleted gas mixture, wherein a hydrogen stream is recovered from the second methanol-depleted gas mixture and fed, along with a carbon dioxide stream, to the synthesis loop.

In the above process, passing the portion of the purge gas stream through the methanol synthesis reactor containing a methanol synthesis catalyst installed outside of the loop generates a second product gas stream, which is "the product gas stream" of the revamp method. The methanol containing product gas mixture is cooled to below the dew point to recover methanol therefrom. The resulting second methanol-depleted gas mixture contains hydrogen and is used as a source of hydrogen for synthesising methanol in the one or more methanol synthesis reactors in the synthesis loop. The carbon dioxide stream is used to augment the feed gas to the one or more methanol synthesis reactors in the synthesis loop.

The make-up gas is a synthesis gas. Synthesis gases typically comprises hydrogen and carbon dioxide. Carbon monoxide may also be present. The make-up gas is not a gas stream recovered from a synthesis loop. Rather, the make-up gas may be generated by the steam reforming of methane, natural gas or naphtha using established steam reforming processes or partial oxidation processes, or by a combination of reforming processes, such as pre-reforming and/or or steam reforming and/or autothermal reforming. Alternatively, the make-up gas may be generated by the gasification of a carbonaceous feedstock such as coal or biomass. For the production of methanol, the desired stoichiometry ratio of the make-up gas, R, is preferably in the range 1 .9 to 3.0. Where hydrogen is recovered from the methanol depleted gas mixture, the optimum addition of carbon dioxide can be calculated from the stoichiometric ratio, R.

The mixture of the make-up synthesis gas plus the carbon dioxide stream desirably has a value of R between 1 .95 and 2.00 for the optimum carbon dioxide flow. Where carbon dioxide is recovered from the crude methanol, for example by using a scrubber, then the R value will be close to 2.00. If carbon dioxide is not recovered from the crude methanol, then losses due to the solubility of carbon dioxide in the crude methanol mean that the R value will be closer to 1 .95. After the hydrogen stream has been added to the make-up synthesis gas and carbon dioxide mixture, the resultant feed gas mixture to the methanol synthesis reactor or reactors in the synthesis loop preferably has an R value in the range 2.2 to 2.4. For reactor designs that remove the heat of reaction in such a way that the reaction trajectory is close to the maximum rate line, it has been found that an R value of about 2.3 is optimum. The exact R value may be more than 2.3 if the hydrogen recovery is high enough, or may be less than 2.3 if the hydrogen recovery is lower. If the R value is less than the optimum then the percentage conversion of carbon oxides to methanol in the synthesis loop will be less, requiring the installed methanol reactor to be made larger. If the R value is more than the optimum value, then the percentage conversion of carbon oxides to methanol in the synthesis loop will be slightly higher, allowing the installed methanol reactor to be made smaller.

Detailed calculations have been done for a synthesis loop using radial-flow steam raising converter (rSRC), which is a design that removes the heat of reaction in such a way that the reaction trajectory is close to the maximum rate line. For the rSRC, the optimum R value was calculated to be 2.304 for the mixture of make-up synthesis gas, hydrogen and carbon dioxide fed to the synthesis loop. For other converter designs the optimum R value may be slightly different, but will still be in the range of 2.2 to 2.4 for the optimum overall conversion of carbon oxides to methanol.

The make-up gas, hydrogen and carbon dioxide feed gas mixture fed to the synthesis loop may be compressed using conventional compression equipment, combined with the recycle gas and passed to the one or more methanol synthesis reactors containing a methanol synthesis catalyst. Methanol is synthesised in the one or more reactors. The reactions may be depicted as follows;

CO2 + 3H ₂ ≠ CH3OH + H2O In the process of the invention, at least part of the first methanol-depleted gas mixture is used as a recycle gas fed to the one or more methanol synthesis reactors in the loop. Where there are two or more methanol synthesis reactors, the recycle gas may be fed to one or more of them. The first methanol-depleted gas is preferably compressed in a compressor to the loop pressure. The compressor may be the existing circulator or a new larger compressor.

The one or more methanol synthesis reactors in the synthesis loop may be an un-cooled adiabatic reactor. Alternatively, one or more cooled reactors 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 reactor may be a quench reactor, or a reactor selected from a tube-cooled converter or a gas-cooled converter, wherein the catalyst bed is cooled in heat exchange with the synthesis gas. Alternatively, the reactor may be cooled by boiling water under pressure, such as an axial flow steam-raising converter, or a radial flow steam- raising converter. In each case, the reactors contain fixed beds of methanol synthesis catalyst through which the synthesis gas is passed. One or more methanol synthesis reactors may be used in the synthesis loop of the process.

Thus, the synthesis loop may be operated with a single cooled methanol synthesis reactor such as a tube-cooled converter or a gas-cooled converter. Where two or more reactors are used, they may be the same or different. Thus, in one arrangement the synthesis loop is arranged with a first water-cooled reactor such as an axial flow steam-raising converter or a radial flow steam-raising converter, followed by a gas-cooled or tube-cooled converter, or vice versa. Alternatively, the synthesis loop may be operated with an axial flow steam raising converter followed by a radial flow steam raising converter, or vice-versa.

The methanol synthesis catalyst used in the one or more reactors in the synthesis loop again is preferably a copper-containing methanol synthesis catalyst, in particular the methanol synthesis catalyst in the one or more reactors in the synthesis loop is a particulate copper/zinc oxide/alumina catalyst. Particularly suitable catalysts are Mg-doped copper/zinc oxide/alumina catalysts as described in the aforesaid US4788175. The same or different catalysts may be used in different methanol synthesis reactors in the process to enhance the methanol synthesis under different operating conditions and feed gas compositions over the lifetime of the catalysts.

Methanol synthesis in the one or more methanol synthesis reactors in the synthesis loop may be effected at pressures in the range 10 to 120 bar abs, and temperatures in the range 130°C to 350°C. The pressure of the synthesis gas at the reactor inlet is preferably 50-100 bar abs, more preferably 70-90 bar abs. The temperature of the synthesis gas at the synthesis reactor inlet is such that the temperature inlet the bed of methanol synthesis catalyst is preferably 200- 250°C and at the outlet preferably 230-285°C.

The first and second product gas streams comprise unreacted hydrogen and carbon dioxide, along with methanol vapour. The second product gas stream typically will contain higher amounts of nitrogen and methane than the first product gas stream. The first and second product gas streams are preferably separately cooled in one or more heat exchangers before passing the cooled product gas mixtures containing methanol to gas-liquid separators. The one or more heat exchangers may be water-cooled heat exchangers but preferably include a gas-gas-interchanger in which the product gas mixture containing methanol is cooled in heat exchange with the gas mixture fed to the methanol synthesis reactor. Such a gas-gas interchanger may be used alone or in combination with one or more downstream water-cooled heat exchangers.

The purge gas stream is recovered from the first methanol-depleted gas mixture to prevent the build-up of inert gases such as nitrogen and methane in the synthesis loop. The purge stream is desirably recovered from the first methanol-depleted gas mixture upstream of the circulator before it is recycled to the one or more methanol synthesis reactors.

In the process, a portion of the first methanol-depleted gas mixture is passed to said one or more methanol synthesis reactors in the synthesis loop as the recycle gas stream. Thus, the process is operated in a loop with unreacted recycle gas depleted in methanol being mixed with fresh synthesis gas, termed make-up gas, the hydrogen stream and the carbon dioxide stream and the resulting mixture fed to the one or more methanol synthesis reactors. The recycle ratio of methanol-depleted recycle gas to make-up gas may be in the range 0.01 :1 to 25:1 . By the term "recycle ratio", we mean the molar flow ratio of the recycled gas to the make-up gas that forms the synthesis gas mixture fed to the one or more reactors.

The second methanol depleted gas mixture recovered after separation of methanol from the second product gas stream, is treated as described above to recover hydrogen. The recovered hydrogen stream is fed with the carbon dioxide stream to the synthesis loop. The resulting hydrogen-depleted gas remaining after hydrogen recovery may be used as a fuel gas, e.g. in upstream steam generation or in a fired primary reformer.

The methanol product recovered from the first and second product gas streams contains water and often small amounts of other alcohols and so may be termed "crude methanol". The crude methanol streams recovered from the first and second product gas streams may be combined or treated separately. Preferably they are combined to save on equipment costs. The crude methanol may be further processed, for example by one or more, preferably two or three, stages of distillation to produce a purified methanol product. Alternatively, the crude methanol may be recovered and stored.

The methanol product, with or without purification, may be subjected to further processing, for example to produce dimethyl ether or formaldehyde, or may be stored for use in electrical power generation, for example using a direct methanol fuel cell to generate electrical power. 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 in which an installed methanol synthesis reactor operates on a once-through basis, and

Figure 2 depicts a process according to another embodiment of the invention in which an installed methanol synthesis reactor operates in a loop.

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 synthesis gas stream 10 comprising hydrogen, carbon dioxide and carbon monoxide is combined with a carbon dioxide stream 12 and the resulting mixture fed to the first stage of a compressor 14, where it is compressed and combined with a hydrogen stream 16 fed to the second stage of the compressor. A compressed feed gas mixture comprising the make-up gas, carbon dioxide stream and hydrogen stream is fed from the compressor by line 18 to a methanol synthesis unit 20. The methanol synthesis unit 20 may be a pre-existing methanol synthesis unit. The methanol synthesis unit 20 comprises one or more methanol synthesis reactors containing a methanol synthesis catalyst (not shown). The one or more methanol synthesis reactors in the methanol synthesis unit 20 operate in a loop. Thus, the compressed feed gas mixture 18 is fed to the one or more methanol synthesis reactors within the unit where methanol is synthesised. A first product gas stream is recovered from the one or more methanol synthesis reactors and cooled to below the dew point in one or more stages of heat exchange (not shown). A crude methanol product stream 22 is separated from a first methanol-depleted gas mixture. The first methanol-depleted gas mixture is divided. A portion is recycled to the feed gas mixture for the one or more methanol synthesis reactors in the unit 20 (not shown). The remaining portion is recovered as a purge gas stream 24. The purge gas stream 24 is heated in gas-gas interchanger 26 and the heated gas fed to the inlet of an installed methanol synthesis reactor 28, operating outside the synthesis loop of unit 20. In this arrangement, the installed reactor is a tube-cooled converter. Thus, the heated gas is passed upwards through a plurality of tubes 30 disposed within a fixed bed of catalyst 32, where it is further heated. The heated gas discharges into a space above the catalyst bed 32 and then passes downwards through the bed where methanol synthesis takes place. A second product gas stream 34 is recovered from the tube-cooled converter 28 and passed through the gas-gas interchanger 26 where it is cooled in exchange with the purge gas stream 24. The partially-cooled product gas is further cooled in one or more heat exchangers 36 where it is cooled to below the dew point to condense methanol. The cooled product stream is fed from the one or more heat exchangers 36 via line 38 to a gas-liquid separator 40. A crude methanol product stream 42 is recovered from the separator and combined with the crude methanol product stream 22 recovered from the methanol synthesis unit 20. The combined crude methanol streams may be processed, e.g. by one or more stages of distillation (not shown) to produce a purified methanol product. A second methanol-depleted gas mixture 46 is recovered from the gas-liquid separator 40 and fed to a hydrogen recovery unit 48. In this arrangement, the hydrogen recovery unit comprises a pressure-swing adsorption unit. A hydrogen stream is recovered from hydrogen recovery unit 48 and passed via line 16 to the compressor 14. A hydrogen-depleted gas mixture 50, rich in nitrogen and methane, is removed from the hydrogen recovery unit 48.

The process of Figure 2 is the same as that in Figure 1 except that it additionally comprises a recycle off-take line 60 that takes a portion of the second methanol-depleted gas steam 46 recovered from the separator 40, passes it through a compressor circulator 62 and feeds the compressed gas via a line 64 to be combined with the purge gas stream 24 fed to the installed methanol synthesis reactor 28. The remaining portion of the second methanol-depleted gas mixture 46 is fed via line 66 to the hydrogen recovery unit 48 and is the source of the hydrogen stream 16 fed to the methanol synthesis unit 20.