A process for stripping methanol

Abstract

A process and system for stripping liquid crude methanol containing methanol and dissolved gases comprising hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane. Th

Description

This invention relates to a process for the stripping of methanol. In particular, the invention relates to a process for stripping methanol in a vessel having a stripping section and a wash section. The invention further relates to an apparatus for the stripping of methanol. BACKGROUND 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 recovered by cooling the product gas stream to below the dew point and separating off the product as a liquid. The crude methanol generally contains dissolved gases, including carbon dioxide, that are separated as part of the purification. 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 gasses in the loop. It is known, in a methanol synthesis process, to pass a stream of liquid crude methanol through a stripping vessel before passing the stream of liquid crude methanol into one or more distillation columns. The purpose of doing so is to strip out dissolved gases from the liquid crude methanol, the dissolved gases comprising hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane. WO2013 / 144041 A1 discloses a continuous process for the preparation of methanol by hydrogenation of carbon dioxide in the presence of a solid catalyst comprising the steps of (i) feeding a feed comprising carbon dioxide; and at least part of a first recycle gas stream comprising carbon dioxide and hydrogen to a reactor, to obtain a gaseous feed with a hydrogen:carbon dioxide molar ratio of between 2-18:1; (ii) contacting said gaseous feed with a catalyst at a temperature of between 200 and 300°C and a pressure of between 40 and 200 bar, thereby forming an outlet stream comprising methanol, water, carbon monoxide, carbon dioxide, and hydrogen; (iii) cooling the outlet stream; (iv) subjecting said outlet stream to a separation step, while optionally at least part of a second recycle gas stream is added to said outlet stream comprising methanol prior to and / or during said separation step, in which separation step methanol and water are separated from non-condensable components, thereby forming a methanol-comprising product stream and a first recycle gas stream; (v) stripping the methanolcomprising product stream using a hydrogen stream, thereby forming a purified methanol product stream and a second recycle gas stream; and (vi)feeding at least part of the first recycle gas stream to step (i) and at least part of the second recycle gas stream to steps (i) and / or (iv). WO2019 / 220073 A1 discloses a process for synthesising methanol comprising the steps of (i) passing a feed gas comprising a make-up gas containing hydrogen and carbon dioxide to a methanol synthesis loop, (ii) recovering a product gas mixture containing methanol from the methanol synthesis loop, (iii) cooling the product gas mixture to below the dew point to condense crude methanol, (iv) separating the crude methanol from an unreacted gas mixture, (v) passing a portion of the unreacted gas mixture to the methanol synthesis loop and (vi) recovering a portion of the unreacted gas mixture as a purge gas stream, characterised by contacting the crude methanol and a portion of the purge gas in a stripping unit to strip dissolved gases from the crude methanol thereby forming a stripped crude methanol and an enriched gas mixture, and feeding at least a portion of the enriched gas mixture to the methanol synthesis loop. The Applicants have realised that the enriched gas mixture formed in the stripping unit contains residual methanol. This residual methanol is fed back into the methanol synthesis loop and hence is fed to the one or more methanol synthesis reactors in the loop. Given that the methanol production reaction is an equilibrium reaction, adding methanol to the methanol synthesis reactor results in inefficient methanol production. Furthermore, residual methanol may also be lost byway of condensates as it is returned to the methanol loop. CA685045 A discloses a process for purifying methanol containing higher acetylenes and aromatic compounds. OBJECT OF THE INVENTION The object of the invention is to provide a process for stripping crude methanol in a vessel having a stripping section and a wash section in order to minimise the loss of crude methanol product in an enriched gas stream formed in a stripping unit and to at least partially overcome the above disadvantages. The object of the invention is further to provide an apparatus for the stripping of a crude methanol product wherein the loss of crude methanol product in an enriched gas stream formed in a stripping unit is minimised. SUMMARY OF THE INVENTION According to the present invention there is provided a process for stripping liquid crude methanol, the process comprising the steps of: (a) passing a feed stream of liquid crude methanol into a stripping section of a vessel, the liquid crude methanol comprising methanol and dissolved gases comprising hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane; (b) passing a stream of stripping gas into the stripping section of the vessel, the stripping gas comprising hydrogen; (c) contacting the liquid crude methanol and the stripping gas in the stripping section of the vessel to form a liquid stripped crude methanol product and an enriched gas mixture comprising hydrogen, the dissolved gasses and methanol vapour; (d) passing the enriched gas mixture through a water wash section, fed with wash water, which water wash section is in fluid communication with the stripping section of the vessel, such that at least a portion of the methanol vapour in the enriched gas mixture is washed out of the enriched gas mixture to form a methanol depleted gas mixture and a methanol-enriched wash water, which is passed to the stripping section of the vessel. In an embodiment of the invention, the wash section is configured to operate between a liquid crude methanol inlet in the vessel and a methanol depleted gas mixture outlet in the vessel. In a preferred form of the invention, the wash section may form part of the vessel. More preferably, the wash section is integrated into the vessel. By “integrated”, it is meant herein that the wash section is included in a housing for the vessel, the housing including the stripping section as well, both the wash section and stripping section being in fluid communication. In one embodiment of the invention, the stripping gas comprises or consists of electrolytic hydrogen generated by electrolysis of water or aqueous solution. This electrolytic hydrogen may usefully be a portion of the hydrogen used to synthesis the liquid crude methanol, the stripping gas may comprise a fresh hydrogen stream. The electrolysis of the water or an aqueous solution may be powered by one or more renewable energy resources In another embodiment of the invention, the stripping gas comprises a portion of purge gas recovered from one or more methanol synthesis units. A stream of the purge gas may be removed continuously or periodically to prevent the unwanted build-up of inert gases, such as nitrogen, argon and methane in a methanol synthesis loop. Purge gas streams, especially in processes using steam reforming as a source of make-up gas, used in the production of methanol, are hydrogen rich and contain either or both of carbon dioxide and carbon monoxide. The purge gas stream preferably contains 50-90% by volume of hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane. Accordingly, a portion of the purge gas stream may be used to strip or remove dissolved gases from the liquid crude methanol. The dissolved gases are suitably removed in one or more vessels, each vessel having a stripping section to which a liquid crude methanol stream and a stripping gas is fed. The liquid crude methanol and a portion of the purge gas may be continuously fed to the vessel and contacted therein in a counter-current manner to remove the dissolved gases from the liquid crude methanol. Alternatively, the liquid crude methanol may be sparged with the portion of the purge gas in the vessel. Reference to “a portion of the purge gas” herein includes a portion of the purge gas itself or a gas obtained from the purge gas. Thus, in one arrangement, a portion of the purge gas stream itself comprising hydrogen and carbon oxides is used to strip the dissolved gases from the crude methanol. The portion used may be 10-90% by volume of the purge gas stream. The remaining portion is removed to reduce the build-up of inert gases in the loop. Whereas the portion of the purge gas stream may be fed directly to the stripping unit, it may be desirable to increase or decrease the hydrogen content of gas fed to the vessel. Thus, in another arrangement, at least a portion of the purge gas stream is separated into a hydrogen-rich gas stream and a hydrogen-depleted gas stream and at least a portion of the hydrogen-rich gas stream is fed to the vessel. In this arrangement, preferably all of the purge gas is subjected to the separation step. Some or all of the hydrogen-rich gas stream may be fed to the vessel. The separation of the hydrogen-rich and hydrogen-depleted gas streams may be practiced using known separation equipment such as hydrogen membrane separator or a pressure swing adsorption unit, or a cold box separation system. Using these techniques over 50% of the hydrogen present in the purge gas may be recovered. In a preferred arrangement, a hydrogen-rich gas stream is recovered from the purge gas and the hydrogen-rich gas stream is then used to strip the dissolved gases from the liquid crude methanol. Using the hydrogen-rich gas instead of the purge gas as the stripping gas allows re-use of the separated carbon oxides and methane in the hydrogen-depleted gas stream in the make-up gas generation, and minimises the inert gases returned to the loop and so passed forward to the purification stage. The hydrogen-rich gas recovered from the purge gas stream desirably comprises >95% by volume of H2. The portion of the purge gas or hydrogen-rich gas stream may instead be fed to the methanol synthesis unit to generate additional methanol. In an embodiment of the invention, the source of water fed to the wash section of the vessel is fresh water. In another embodiment of the invention the water may be recycled water. The source of water may be a combination of both fresh water and recycled water. The source of water may usefully be a purge gas water stream recovered from a purge gas washing column used to treat purge gas from a methanol synthesis loop upstream of the stripping vessel. The wash water may also be recovered from a methanol distillation unit downstream of the vessel. These sources of water are advantageous in the present invention because they contain methanol and dissolved gases. The wash section may be provided such that, in use, it is located above the stripping section and the vessel configured such that the stripping gas is added to near the bottom of the stripping section, rises through the liquid crude methanol in the stripping section to form the enriched gas containing methanol vapour, which passes upwards to the wash section where it is washed by supplying wash water to the top of the wash section. A washed enriched gas is recovered from the top on the wash section. Wash water containing methanol is passed under gravity downwards from the wash section to the stripping section where it is combined with the liquid crude methanol. A combined stripped methanol and wash water containing methanol may then be recovered from the bottom of the stripping section. In one embodiment of the invention, the source of water is supplied into the vessel, between a liquid crude methanol inlet and a methanol depleted gas mixture outlet of the vessel. Wash water may also be added to the vessel at a point opposite to the liquid crude methanol inlet in the vessel. In a further embodiment of the invention at least a portion of the washed enriched gas mixture is returned to a synthesis or make-up gas compressor. The recovered synthesis gas is recovered and fed into the methanol synthesis loop from which the feed stream of liquid crude methanol is recovered. According to another aspect of the invention there is provided a system for stripping liquid crude methanol, the system comprising a vessel having a: (a) a stripping section having an inlet for a feed stream of liquid crude methanol and an inlet for a stripping gas, wherein the liquid crude methanol comprises methanol and dissolved gases comprising hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane, and the stripping gas comprises hydrogen; the stripping section being configured to allow the stripping gas to contact the liquid crude methanol to form an enriched gas mixture containing methanol vapour and a stripped liquid crude methanol; and (b) a water wash section, in fluid communication with the stripping section, having an inlet for the enriched gas mixture recovered from the stripping section and an inlet for wash water, wherein the water wash section is configured to contact the wash water with the enriched gas mixture such that the methanol vapour in the enriched gas mixture is washed out of the enriched gas mixture to form a methanol depleted gas mixture, which is recovered from the wash section, and a methanol-enriched wash water stream that is combined with the liquid crude methanol in the stripping section. The vessel preferably comprises a housing which houses the stripping section and the wash section such that the wash section is integrated in the vessel. The wash section is provided in the vessel between a liquid crude methanol inlet in the vessel and an enriched gas mixture outlet in the vessel. The vessel preferably comprises trays or packing materials or a combination of both. One or more of the trays or packing materials may be used in the stripping section and wash water section of the vessel. The vessel may also include a device to prevent or reduce liquid carryover from the vessel, such as a demister pad, which may be located between the washing section and the outlet for the washed enriched hydrogen gas stream. The vessel preferably further has a sump zone, located in the stripping section between the inlet for the stripping gas and the outlet for the stripped liquid methanol. This sump zone may collect the stripped liquid methanol after it has passed through the stripping section. In normal operation the vessel may be oriented with an off-take zone for the washed enriched hydrogen located above the washing section in turn located above the stripping section in turn above the sump zone. The washing section and stripping section preferably comprise a packing material. Wash water may be fed to the top of the packing in the washing zone where it passes down under gravity over the packing counter-current to the enriched hydrogen containing methanol coming up from the stripping section below. The washing water flowing downwards becomes enriched in methanol and is combined with the liquid crude methanol between the washing section and the stripping section. The combined methanol-water mixture then passes downwards over the packing in the stripping section under gravity where it contacts the hydrogen stripping gas as it passes upwards from near the bottom of the packing in the stripping section. The stripped liquid crude methanol and water mixture may then be recovered from the bottom of the sump zone. In the invention, the methanol depleted gas mixture is desirably passed to a methanol synthesis unit. The methanol synthesis unit may be any methanol synthesis unit but is preferably a methanol synthesis unit operating in a synthesis loop and fed by a synthesis gas compressor. The methanol depleted gas mixture is preferably fed to the inlet of the synthesis gas compressor. In this way the hydrogen and caron dioxide are useful returned to the methanol synthesis. DETAILED DESCRIPTION OF THE INVENTION The invention is further illustrated by reference to the accompanying Figures, in which: Figure 1 is a depiction of a flowsheet of one embodiment of the process of the invention for stripping liquid crude methanol, and Figure 2 is a depiction of an integrated stripping and washing vessel suitable for the process. FIGURES Figure 1: is a depiction of a flowsheet of one embodiment of the invention. It will be appreciated that the methanol synthesis process upstream of the vessel for the stripping of a feed stream of liquid crude methanol is not limiting. The downstream purification process, which would include one or more distillation columns is also not limiting. The process forthe synthesis and distillation of methanol, and the necessary equipment and feeds therefor are known to the person skilled in the art in accordance with conventional chemical engineering practice. It will also be understood by those skilled in the art that Figure 1 hereto is diagrammatic and that further items of equipment such as feedstock drums, pumps, vacuum pumps, compressors, gas recycling compressors, pressure sensors, pressure relief valves, control valves, 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. Referring to Figure 1, a feed stream of liquid crude methanol 10 is fed into a vessel 100 for purposes of stripping the liquid crude methanol into a liquid stripped crude methanol product and a washed enriched gas mixture comprising hydrogen. The feed stream of liquid crude methanol in line 10 is recovered from a methanol synthesis unit, generally designated by the number 110. The methanol synthesis unit 110 comprises a methanol synthesis reactor operated at elevated temperature and pressure in a loop with downstream cooling and recovery of the liquid crude methanol and recirculation of unreacted gases to the methanol synthesis rector. A stream of synthesis-gas is fed into the methanol synthesis unit 110 from a synthesis gas compressor 120. The synthesis gas compressor is in turn fed with a feed stream of carbon dioxide in line 2, a portion of a stream of hydrogen in line 4 and a stream of recovered hydrogen in line 6.. The hydrogen stream 4 is preferably electrolytic hydrogen. The recovered hydrogen stream 6 is obtained from the methanol synthesis unit 110 by separation of hydrogen from a methanol loop purge gas. A portion of the stream of hydrogen in line 4, in line 8, is fed to the vessel 100 where the hydrogen contacts the liquid crude methanol from line 10 in a stripping section 112 of the vessel 100. The hydrogen from line 8 is used as the stripping gas for the liquid crude methanol. The stripping gas is fed into the vessel 100 at a point between the liquid crude methanol 10 inlet of the vessel 100 and a stripped liquid crude methanol product 12 outlet of the vessel 100. The hydrogen from line 8 is therefore fed into the stripping section 112 of the vessel 100. The stripping gas contacts the liquid crude methanol from line 10 in the stripping section 112 of the vessel 100 to form a stripped liquid crude methanol product recovered in line 12 and an enriched gas mixture comprising hydrogen, the dissolved gasses and methanol vapour. The enriched gas mixture rises through the stripping section 112 of the vessel 100 to the wash section 114 in the vessel 100. The stripping section 112 and wash section 114 of the vessel 100 are in fluid flow communication, each section being integrated within the shell of the vessel 100. A feed stream of wash water in line 14 is introduced into the vessel 100 at a point between the inlet for the liquid crude methanol and an outlet for the methanol depleted gas mixture, which gas mixture is recovered from the vessel in line 16. The recovered methanol depleted gas mixture in line 16 is returned to a feed stream in line 18 into the synthesis-gas compressor 120 which is subsequently passed into the methanol synthesis unit 110. In the embodiment as depicted in Figure 1, a purge gas washing liquid, in line 20, is fed into the vessel opposite to the liquid crude methanol inlet of the vessel 110 so that it comes into contact with the liquid crude methanol from line 10. An additional stream of water is added to the vessel 100 in line 20. The stream of water in line 20 is purge gas wash water recovered from a separate purge gas washing column (not shown) used to treat offgas (not shown) from the methanol synthesis unit 110. In one embodiment the hydrogen feed in line 8 is provided by an electrolysis unit (not shown) that generates hydrogen by the electrolysis of water. In another embodiment, the hydrogen feed in line 8 is provided by recovering a hydrogen-rich gas from the purge gas from the methanol synthesis unit 110 in a hydrogen recovery unit, which separates hydrogen from the purge gas using a membrane. Figure 2 depicts an integrated washing and stripping vessel comprising a single shell containing the washing section 114 and stripping section 112, each containing a bed of a suitable packing material. Liquid crude methanol 10 is fed to an inlet located between the washing section 114 and stripping section 112. Wash water in line 14 is fed to an inlet near the top of the packing in the wash section 114. Additional purge gas wash water is optionally fed via line 20 to an inlet between the wash section 114 and stripping section 112. Stripping gas comprising hydrogen is fed via line 8 to an inlet near the bottom of the packing in the stripping section 112. Stripped liquid crude methanol is recovered via line 12 from an outlet at the bottom of the vessel. Washed enriched gas is recovered via line 16 from an outlet located at the top of the vessel. A demister pad 200 is located between the inlet for the wash water 14 and the outlet for the washed enriched gas 16. 5 A sump zone 210 is located between the inlet for the stripping gas fed via line 8 and the outlet for the stripped liquid crude methanol recovered via line 12.

Claims

1. A process for stripping liquid crude methanol, the process comprising the steps of:(a) passing a feed stream of liquid crude methanol into a stripping section of a vessel, the liquid crude methanol comprising methanol and dissolved gases comprising hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane;(b) passing a stream of stripping gas into the stripping section of the vessel, the stripping gas comprising hydrogen;(c) contacting the liquid crude methanol and the stripping gas in the stripping section of the vessel to form a liquid stripped crude methanol product and an enriched gas mixture comprising hydrogen, the dissolved gasses and methanol vapour;(d) passing the enriched gas mixture through a water wash section, fed with wash water, which water wash section is in fluid communication with the stripping section of the vessel, such that at least a portion of the methanol vapour in the enriched gas mixture is washed out of the enriched gas mixture to form a methanol depleted gas mixture and a methanol-enriched wash water, which is passed to the stripping section of the vessel.

2. The process of claim 1, wherein an additional source of water is supplied into the vessel, the additional source of water being sourced from a purge gas water stream recovered from a purge gas washing column used to treat purge gas recovered from a methanol synthesis loop upstream of the vessel.

3. The process according to claim 1 or claim 2, wherein the wash water is recovered from a methanol distillation unit downstream of the vessel.

4. The process according to anyone of claims 1 to 3, wherein a portion of the hydrogen in the stripping gas is electrolytic hydrogen recovered from an electrolysis unit powered by renewable energy.

5. The process according to anyone of claims 1 to 4, wherein the stripping gas comprises >95% by volume of hydrogen.

6. The process according to anyone of claims 1 to 5, wherein the stripping gas comprises a portion of a purge gas recovered from a methanol synthesis loop upstream of the vessel.

7. The process according to claim 6, wherein the portion of the purge gas used in the stripping gas contains 50-90% by volume hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane.

8. The process according to anyone of claims 1 to 7, wherein the vessel comprises trays or packing materials or a combination of both.

9. The process according to anyone of claims 1 to 8, wherein the liquid crude methanol and the stripping gas are contacted in the vessel in a counter current.

10. The process according to anyone of claims 1 to 9, wherein the liquid crude methanol is sparged with the stripping gas in the vessel.

11. The process according to anyone of claims 1 to 10, wherein the stripped liquid crude methanol product is subjected to one or more distillation steps, downstream of the vessel, to produce a purified methanol product.

12. The process of any one of claims 1 to 11, wherein the wash section and stripping section of the vessel are integrated within the vessel.

13. A system for stripping liquid crude methanol, the system comprising a vessel having:(a) a stripping section having an inlet for a feed stream of liquid crude methanol and an inlet for a stripping gas, wherein the liquid crude methanol comprises methanol and dissolved gases comprising hydrogen and one or more of carbon monoxide, carbon dioxide, nitrogen, argon and methane, and the stripping gas comprises hydrogen; configured to allow the stripping gas to contact the liquid crude methanol to form an enriched gas mixture containing methanol vapour and a stripped liquid crude methanol and(b) a water wash section in fluid communication with the stripping section, the water wash section having an inlet for an enriched gas mixture recovered from the stripping section and an inlet for wash water, wherein the water wash section is configured to contact the wash water with the enriched gas mixture such that the methanol vapour in the enriched gas mixture is washed out of the enriched gas mixture to form a methanol depleted gas mixture, which is recovered from the wash section, and a methanol-enriched wash water stream that is combined with the liquid crude methanol in the stripping section.

14. The system according to claim 13, wherein the vessel comprises a housing and wherein the wash section and stripping section are integrated in the vessel.

15. The system according to claim 13 or claim 14, wherein the stripping section and the water wash section each comprise a packing.

16. The system according to any of claims 13 to 15, wherein the vessel comprises a device to minimise the liquid carry over located between the inlet for the water wash and an outlet for the methanol depleted gas mixture.

17. The system according to any of claims 13 to 16, wherein the stripping section comprises a sump zone between the inlet for the stripping gas and an outlet for the stripped liquid crude methanol.A

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