This invention relates to a method for removing methanol from a gas mixture recovered from a methanol synthesis reactor.

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 gas-liquid separator. The process is often operated in a loop: thus unreacted gas recovered from the gas-liquid separator may be 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.

It has been proposed to install a chiller after the crude methanol product cooler, or additional cooling water exchangers, to reduce the temperature inlet the gas-liquid separator thereby reducing the amount of methanol left in the unreacted gas stream recycled to the synthesis reactor. This has two effects; firstly, less methanol is lost in the purge gas thereby increasing production, and furthermore, the methanol entering the methanol synthesis reactor is reduced, which will allow for an improved conversion. While effective in improving the efficiency of the methanol production, it may not be possible, especially in existing methanol plants, to install such additional cooling apparatus.

US6258860 discloses recovering methanol from waste gas and liquid streams, for example by scrubbing flash gases or synthesis loop purge gases with water, however there is no disclosure of recycling scrubbed gases to the main synthesis loop.

US4219492 discloses a process for manufacturing methanol, wherein the pressure in the step of preparation of the synthesis gas is selected equal to the pressure in the step of preparation of methanol from the synthesis gas. In one arrangement, methanol is scrubbed from the loop gas with water and returning the scrubbed gas to the loop.

US3950369 discloses scrubbing methanol from a gas using a catchpot scrubber. In one arrangement, a cooled product gas/liquid mixture is fed with scrubbing liquid and the scrubbed gas, minus a purge stream, recycled to the loop. We have found a method that overcomes problems with the prior art methods.

Accordingly, the invention provides a method for revamping a methanol synthesis process comprising a methanol synthesis reactor fed with a make-up gas and a recycle gas and operating in a synthesis loop that produces a gas mixture containing methanol, one or more heat exchangers that cool the gas mixture containing methanol to form a product mixture and a gas-liquid separator that separates a crude methanol product from a methanol-depleted gas mixture, said method comprising (i) installing gas washing apparatus within or downstream of the gas-liquid separator, (ii) passing a gas mixture containing methanol selected from the product gas mixture or the methanol-depleted gas mixture to the gas washing apparatus, (iii) contacting the gas mixture containing methanol with a water wash liquid in a counter-current manner, (iv) recovering a water-methanol mixture and a washed gas mixture from the gas washing apparatus, and (v) recycling at least a portion of the washed gas mixture to the methanol synthesis reactor, wherein the water-methanol mixture and the crude methanol stream are not combined and are processed separately.

By the term, "processed separately" we mean that the streams are subjected to a further processing step, such as a purification step, to which they are fed separately. By not combining the streams, the water-methanol mixture may be integrated into the process separately from the crude methanol product, for example the streams may be fed separately to a distillation unit to improve the efficiency of separation of purified methanol.

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.

The invention includes a number of possible arrangements of the gas washing apparatus. In these arrangements, the gas washing apparatus is installed either within or downstream of the gas-liquid separator, which is the existing gas-liquid separator.

The gas-liquid separator is typically an elongate cylindrical vessel with an upper end having a methanol-depleted gas outlet and a lower end having a crude methanol liquid outlet. The product gas mixture is usually fed to the separator at a point between the methanol depleted gas outlet and crude liquid methanol outlets, typically at or below a mid-point along its length. The separator may also include a so-called vane pack, which comprises a mesh or other device that entrains or captures liquid droplets. Such vane packs are installed between the product gas inlet and the methanol-depleted gas outlet. In one arrangement, the method comprises (i) installing gas washing apparatus within the gas- liquid separator, (ii) passing the product gas mixture to the gas washing apparatus, and (iii) contacting the product gas mixture containing methanol with a water wash liquid in a counter- current manner. The gas washing apparatus installed in this arrangement may comprise a plurality of wash trays or a fixed bed of packing, such as random packing or structured packing, and a wash-water inlet connected to a suitable water supply. The gas washing apparatus is installed between the product gas inlet and the methanol-depleted gas outlet. In this arrangement, the resulting methanol-depleted gas is the washed gas and the methanol- depleted gas outlet is therefore the washed gas outlet. The wash water inlet is installed at the top of the gas washing apparatus so that the water passes vertically downwards under gravity over the trays or packing and the product gas passes upwards from the product gas inlet through the trays or packing to the methanol-depleted gas outlet. In one arrangement, the gas washing apparatus may replace an existing vane pack. In another arrangement, the gas washing apparatus may be installed below the vane pack. In another arrangement, the gas washing apparatus may be installed above the vane pack. Each arrangement has different advantages. For example, replacing the vane pack with trays or packing will still provide the separation of the liquid from the gas and any carryover of liquid will contain mostly water, with very little methanol. Installing the gas washing apparatus below the vane pack means that the carryover of liquid from the gas leaving the top of the wash section will mostly be water, which is much easier to separate than droplets of crude methanol liquid. Installing the gas washing apparatus above the vane pack may be a more convenient retrofit in some plants where it is easier to extend the length of existing separator vessel.

In these arrangements, the existing product gas inlet and methanol-depleted gas outlet in the may be used. The existing crude methanol outlet may also be used. However, in these arrangements, the gas washing apparatus further comprises a water-methanol outlet installed at or adjacent the bottom of the wash section, i.e. between the trays or packing and the product gas inlet. This additional liquid stream outlet allows recovery of the water-methanol mixture separately from the crude methanol. The water-methanol mixture may then be integrated into the process differently from the crude methanol product, for example the streams may be fed separately to a distillation unit to improve the efficiency of separation of purified methanol. A separate water-methanol recovery is particularly advantageous for processes that produce crude methanol with a low water content, for example where the methanol is synthesised from a synthesis gas obtained by coal gasification. In this case, it is desirable not to mix the streams and thereby increase the water content of the crude methanol. A separately collected water- methanol mixture may be distilled separately to obtain a low-water crude methanol product that may be blended with the crude methanol recovered from the crude methanol outlet and then the blend distilled as normal. In another arrangement, the method comprises (i) installing gas washing apparatus downstream of the gas-liquid separator, (ii) passing the methanol-depleted gas mixture to the gas washing apparatus, and (iii) contacting the methanol-depleted gas with a water wash liquid in a counter-current manner. In this case, the existing gas-liquid separator is used as normal to produce a crude methanol stream and a methanol-depleted gas stream and it is the methanol- depleted gas stream recovered from the gas-liquid separator that is subjected to gas washing. In this arrangement, the gas washing apparatus comprises an additional gas-washing vessel, which may be a cylindrical vessel like the gas-liquid separator described above, containing a plurality of wash trays or a fixed bed of packing, a methanol-depleted gas inlet, a washed gas outlet, a wash-water inlet connected to a suitable water supply and a water-methanol mixture outlet. The methanol-depleted gas inlet is suitably provided between the water-methanol outlet and the trays or packing. The wash water inlet is installed at or near the top of the gas washing vessel so that the water passes vertically downwards under gravity over the trays or packing and the methanol-depleted gas passes upwards from the methanol-depleted gas inlet through the trays or packing to the washed gas outlet.

The water-methanol mixture recovered from the gas washing vessel is not combined with the crude methanol recovered from the existing gas-liquid separator. For the reasons set out above, it is advantageous not to combine the water-methanol mixture and crude methanol streams but rather to process them separately.

The product gas mixture and methanol-depleted gas mixture each comprises methanol vapour, steam, hydrogen, carbon dioxide and/or carbon monoxide. The methanol vapour content in the product gas mixture may be in the range 5-15% by volume. The methanol vapour content in the methanol-depleted gas mixture may be in the range 0.01 -5% by volume. The temperature of the gas mixture containing methanol inlet the gas washing apparatus may be in the range 10 to 150 °C, preferably 20 to 80 °C. The pressure of the gas mixture containing methanol may be in the range 1 to 120 bar absolute (bar abs). The temperature of the gas mixture containing methanol inlet the gas washing apparatus is preferably at or below the dew point.

The gas mixture containing methanol is passed upwards through the gas washing apparatus, where it is contacted with a water wash liquid in a counter-current manner. The gas washing apparatus may contain a plurality of wash trays, for example 2-50 wash trays. The wash trays may be in the form of perforate plates or trays or other designs that permit counter current flow of the gas mixture and the wash water though apparatus. Additionally, or alternatively, the apparatus may contain a fixed bed of a packing, i.e. a porous bed of inert material, e.g. ceramic or metal shaped units, through which the gas and liquid may flow in a counter-current manner. The wash water becomes enriched in methanol as it contacts the gas mixture and produces the washed gas mixture. The wash water enriched in methanol, or methanol-enriched wash water, is the water-methanol mixture. The wash water may be any suitable liquid water stream, including water recovered from downstream processes, such as a crude methanol distillation process. Preferably the water is essentially free of methanol and soluble salts. The wash water may be fed to the gas washing apparatus at a temperature in the range 1 -60 °C, preferably 5-50 °C. Using colder water is preferred but not essential because the water will be cooled by partial evaporation into the gas stream. The flowrate of the water wash stream is preferably minimised to reduce the burden on any downstream methanol purification apparatus. The water flowrate may usefully be in the range 0.001 to 1 .0 kmol/kmol of methanol-containing gas mixture. Below this range the recovery of methanol is too low and above this range the methanol content of the bottoms water is too dilute. The Applicant has found that relatively low amounts of water can be very effective in recovering methanol. Preferably, the water flowrate is in the range the range 0.01 to 0.1 kmol/kmol of methanol-containing gas mixture. The suitability of such low water wash flowrates is surprising and greatly enhances the utility of the process for recovering methanol.

The dissolution and condensation of methanol from the gas mixture into the wash water gives rise to the evolution of heat, and this would be expected to lower the effectiveness of water- washing methods such that they may not be as efficient as merely chilling the gas mixture as previously proposed. The Applicant has found surprisingly that the temperature rise associated with the water wash does not impact significantly on the efficiency of the methanol removal and that the present method offers significant advantages in both the methanol recovery and the efficiency of methanol processes in which it is used. The Applicants have realised that contrary to previous methods, the present method may be operated effectively where the temperature of the washed gas mixture at the outlet of the gas washing apparatus is greater than the temperature of the methanol-containing gas mixture at the inlet. 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. Where the methanol synthesis process is operated in a loop, i.e. where a portion of the methanol-depleted gas mixture is recycled to an upstream methanol synthesis reactor, the method also offers the ability to improve conversion of synthesis gas to methanol, thereby increasing production efficiency. Accordingly, the invention further provides 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, recovering a gas mixture containing methanol from said one or more reactors, cooling the gas-mixture containing methanol in one or more heat exchangers to form a product gas mixture, and passing the product gas mixture to a gas-liquid separator that separates a crude methanol product from a methanol-depleted gas mixture, wherein the process comprises (ii) passing a gas mixture containing methanol selected from the product gas mixture or the methanol-depleted gas mixture to a gas washing apparatus, (iii) contacting the gas mixture containing methanol with a water wash liquid in a counter-current manner, (iv) recovering a water-methanol mixture and a washed gas mixture from the gas washing apparatus, and (v) recycling at least a portion of the washed gas mixture to the methanol synthesis reactor as the recycle gas, wherein the water-methanol mixture and the crude methanol stream are not combined and are processed separately.

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 in the present invention 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, which refers to the ratio of molar concentrations of the gas components,

R = ([H2]-[C02])/([CO]+[C0 ₂ ]), is preferably in the range 1 .9 to 3.0. In contrast, purge gas streams typically have a R value > 5 and contain much higher amounts of inert gases such as nitrogen and methane.

The make-up gas is compressed using conventional compression equipment, combined with the recycle gas and passed to 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 methanol process, at least part of the washed gas mixture is used as a recycle gas fed to the one or more methanol synthesis reactors. Where there are two or more methanol synthesis reactors, the recycle gas may be fed to one or more of them. The washed gas mixture is preferably compressed in a compressor to the loop pressure. The compressor may be the existing circulator.

The one or more methanol synthesis reactors 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 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, which may be the same or different, may be used in the process. Thus the process may be operated with a single cooled methanol synthesis reactor such as a tube-cooled converter or a gas-cooled converter, or may be operated with two different reactors, such as a tube-cooled converter or gas cooled converter followed by a steam-raising converter such as an axial flow steam-raising converter or a radial flow steam- raising converter, or vice-versa. Alternatively, the process may be operated with an axial flow steam raising converter followed by a radial flow steam raising converter or vice-versa.

The methanol synthesis catalysts are preferably copper-containing methanol synthesis catalysts, in particular the methanol synthesis catalyst in the synthesis reactor is a particulate copper/zinc oxide/alumina catalyst. Particularly suitable catalysts are Mg-doped copper/zinc oxide/alumina catalysts as described in US4788175.

Methanol synthesis may be effected in the reactor 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 200-250°C and at the outlet preferably 230-280°C. The gas mixture recovered from the one or more methanol synthesis reactors comprising unreacted hydrogen and carbon dioxide, along with methanol vapour and steam is preferably cooled in one or more heat exchangers before passing the cooled product gas mixture containing methanol to the gas washing apparatus. The one or more heat exchangers may be water-cooled heat exchangers or, preferably, may include a gas-gas-interchanger in which the product gas mixture containing methanol is cooled in heat exchange with the feed gas mixture fed to the one or more methanol synthesis reactors. Such a gas-gas interchanger may be used alone or in combination with one or more downstream water-cooled heat exchangers. The cooling of the product gas mixture containing methanol is desirable to improve methanol recovery. The temperature of the methanol-containing gas mixture at the inlet of the gas washing apparatus is preferably in the range 10 to 150 °C, preferably 20 to 80 °C. The pressure is preferably in the range 10 to 120 bar absolute (bar abs).

The gas washing apparatus used in the process may be as described above for the revamp method.

A purge gas stream is desirably recovered from the washed gas mixture to prevent the build-up of inert gases such as nitrogen and methane. The purge stream is desirably recovered from the washed gas mixture upstream of the circulator to prevent the build-up of inert gases such as nitrogen and methane in the loop before it is recycled to the one or more methanol synthesis reactors. The purge gas typically comprises hydrogen and carbon oxides and may be used for hydrogen recovery, for example by pressure-swing absorption or by using suitable membranes, or may be subjected to one or more further processing stages including autothermal reforming, water-gas shift and methanol synthesis.

In the process, a portion of the washed gas mixture is passed to said one or more methanol synthesis reactors 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 makeup gas, 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 crude methanol stream and the water-methanol mixture are further processed, for example by one or more, preferably two or three, stages of distillation to produce a purified methanol product. The water recovered from the crude methanol and/or water-methanol mixture in downstream processing may usefully be used as at least part of the wash water stream. The flow-rate of the water-wash liquid is desirably controlled within the capacity of the distillation apparatus so that the quality of the product methanol may be maintained. 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 figure in which;

Figure 1 depicts a process according to one embodiment of the present invention; and

Figure 2 depicts a gas liquid separator adapted in accordance with the invention with gas- washing apparatus installed within.

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 , make-up gas stream fed by line 10 is combined with a recycle gas stream in line 12 and the resulting synthesis gas fed via line 14 to a gas-gas heat exchanger 16 where it is heated in exchange with a product gas stream 26. The temperature of the heated synthesis gas 18 is adjusted if necessary by means of an optional heat exchanger 20 and the synthesis gas then fed to a plurality of tubes 22 disposed within a tube-cooled converter 24 containing a bed of a particulate copper-based methanol synthesis catalyst 26. The synthesis gas is heated as it passes through the tubes 22 and is discharged into a space above the catalyst bed 26. The heated synthesis gas then passes through the bed 26 where methanol synthesis takes place to form a gas mixture comprising unreacted synthesis gas, steam and methanol vapour. The gas mixture is fed from the reactor 24 via line 28 to the gas-gas heat exchanger 16 where it is cooled in heat exchange with the synthesis gas 14. The cooled product gas 30 is fed to one or more optional further heat exchangers 32 where it is cooled to below the dew point. The cooled product gas stream containing unreacted gases and liquid methanol is fed to a gas liquid separator 34. A liquid crude methanol stream 36 is recovered from the bottom of the separator 34. A methanol- depleted gas stream is recovered from the top of the separator and fed via line 38 to a cylindrical wash vessel 40. The longitudinal axis of the cylinder is aligned vertically. The wash vessel 40 contains a plurality of horizontal perforate trays 42. The line 38 is connected to an inlet below the trays 42. A liquid water wash stream 44 is provided to an inlet near above the trays 42. The liquid water wash and methanol-depleted gas are passed in a counter-current manner through the wash vessel. The gas mixture becomes depleted in methanol as it passes upwards through the wash vessel and the water becomes enriched in methanol. A water-methanol mixture 46 is recovered from an outlet at the bottom end of the wash vessel. A washed gas mixture 48 is recovered from an outlet at the top end of the wash vessel. A purge stream 50 is taken from the washed gas mixture. The remaining washed gas mixture is passed to a circulator 52 that raises the pressure of the washed gas to the loop pressure. The pressurised washed gas mixture is then fed as the recycle gas stream 12 to the make-up gas 10.

In Figure 2, a cooled product gas 100 stream containing unreacted gases and liquid methanol is fed to an existing gas liquid separator 134. A liquid crude methanol stream 136 is recovered from the bottom of the separator 134. The gas-liquid separator contains a plurality of horizontal perforate trays 142 in a wash section installed above the inlet for the cooled product gas 100. A liquid water wash stream 144 is provided to an inlet above the trays 142. The liquid water wash and methanol-depleted gas are passed in a counter-current manner through the wash section. The gas mixture becomes depleted in methanol as it passes upwards through the wash section and the water becomes enriched in methanol. A water-methanol outlet is installed at or adjacent the bottom of the wash section, i.e. between the trays 142 and the cooled product gas inlet. A washed gas mixture 148 is recovered from an outlet at the top end of the gas liquid separator. The wash section may replace an existing vane pack in the gas-liquid separator, or it may be disposed below an existing vane pack or above an existing vane pack (not shown).

The Invention is further illustrated by reference to the following Example.

Example 1

A methanol process operating at 80 bara exit the converter was modelled to show the effect of the water-wash applied to a wash vessel containing perforate trays in comparison to a chiller installed upstream of a conventional gas-liquid separator.

Gas-liquid separator Mol% methanol exit

inlet temperature gas-liquid separator

45 °C 0.57

40 °C 0.46

35 °C 0.37

30 °C 0.29

25 °C 0.23

20 °C 0.18

15 °C 0.14

10 °C 0.1 1

5°C 0.08 To reduce the methanol content to 0.08 mol%, the chiller would need to cool down the gas- liquid separator inlet to 5 °C. Using the wash water system, water at 50 °C needs only a modest flowrate of 0.25 kg water per kmol of gas (0.014 kmol/kmol) to reduce the methanol content of a gas stream at 45 °C from 0.57% to 0.08%. There is an associated increase in the temperature of the washed gas stream from 45 °C to 49 °C, but the change in crude methanol stream temperature is less than 0.5 °C.