A process for the distillation of a feed comprising a crude liquid methanol stream

Abstract

A process and system for the distillation of a feed comprising crude liquid methanol, the process comprising: (a) distilling the crude liquid methanol in a first distillation column 100 to form a firs

Description

This invention relates to a process for the distillation of crude liquid 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. Crude methanol is generally recovered by cooling the product gas stream to below the dew point and separating off the crude methanol 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 liquid methanol is typically purified by distillation. Distillation of crude methanol is normally performed in a distillation unit comprising a series of columns. 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 , which is produced in the final distillation step, is used to supply at least part of the heat for the pre-treatment topping step. Where electrolytic hydrogen, i.e. hydrogen produced by electrolysis, is used, together with carbon oxides, for the synthesis of methanol, it will be appreciated that the electrolysis, when powered by a renewable energy source, such as solar farms or wind farms, for example, will vary depending on the availability of the energy source which in turn results in varying amounts of electrolytic hydrogen being produced. “Electrolytic hydrogen” as referred to herein means hydrogen generated through the electrolysis of water or steam. The variance of the flowrate of electrolytic hydrogen will have a knock-on effect on the synthesis of methanol and the production rate of the methanol product gas mixture from which the crude liquid methanol stream is recovered. The variance will also impact on the amount of process heat available to heat feed streams, including heating streams within the distillation unit. This in turn impacts upon the distillation profile of a distillation column resulting in poorer separation of the components in the feed. Reference to “distillation profile” herein means the operating parameters of a distillation column that provide the desired separation of components. The applicants have realised that the distillation profile of a distillation column can be maintained, notwithstanding the fluctuating production rate of the crude liquid methanol stream during turndowns of the methanol synthesis unit, by providing a heat exchanger and an auxiliary heat exchanger, both coupled to a distillation column, wherein the auxiliary heat exchanger supplements the heat in the distillation column, the heat exchanger and the auxiliary heat exchanger being connected in series. Accordingly, the invention provides a process for the distillation of a feed comprising crude liquid methanol, the process comprising: (a) distilling the crude liquid methanol in a first distillation column to form a first distillation column overhead vapour and a first partially purified liquid methanol product; (b) recovering the first partially purified liquid methanol product from the first distillation column; (c) passing the first partially purified liquid methanol product through a first heat exchanger coupled to the first distillation column to form a first heat exchanger vapour which is returned to the first distillation column to supply heat thereto and a second partially purified liquid methanol product; (d) passing the second partially purified liquid methanol product from the first heat exchanger to a first auxiliary heat exchanger coupled to the first distillation column to form a third partially purified liquid methanol product and a second heat exchanger vapour, which is returned to the first distillation column to supplement the heat in the first distillation column; and (e) recovering the third partially purified liquid methanol stream. The first distillation column has a first column profile. The step of returning the second heat exchanger vapour to the first distillation column to supplement the heat in the first distillation column enables the first distillation column’s profile to be maintained despite variations in the feed flow or available heat recovery caused by fluctuations upstream. The third partially purified liquid methanol stream may be recovered from the first auxiliary heat exchanger, or it may be returned to the first distillation column, preferably at a position below the point of addition of the second heat exchange vapour, and recovered from the first distillation column, preferably at or near the bottom of the column. The feed of the crude liquid methanol is recovered from a methanol synthesis unit, up-stream of the first distillation column, which methanol synthesis unit may be fed with a feed synthesis stream comprising a combination of an electrolytic hydrogen containing stream produced by the electrolysis of water or a steam containing gas stream and a carbon oxide containing gas stream. The electrolysis unit may derive its energy from one or more sources of renewable energy. The crude liquid methanol recovered from the methanol production unit contains water, along with small amounts of higher alcohols and other impurities. Hence, distillation is required to produce a purified liquid methanol product. The crude liquid methanol is fed to the first distillation column, typically at a position in the upper half of the column, although it may be fed at a mid-point or lower half of the column. The first partially purified liquid methanol product is recovered from the first distillation column. The first partially purified liquid methanol product may be suitably recovered either from the bottom of the column or, preferably, from a position below the inlet for the crude liquid methanol feed via a collection device, which may be located under a lowermost packed or frayed section. The use of such a collection device allows recovery of the third partially purified liquid methanol product from the bottom of the first distillation column. In some arrangements, two distillation columns are used to produce the purified methanol product. The first distillation column and the second distillation column may be of any conventional design. The columns may be tray-columns or packed columns or a combination of both. The first and second distillation columns are preferably packed columns as this offers advantages in the operation of the process. The second distillation column may be operated to generate a fuel-grade or a higher purity AA grade methanol or their equivalents. In some arrangements, the first distillation column may be used to produce a stabilised or topped first partially purified liquid methanol stream by separating dissolved unreacted gases and light by-products from the crude liquid methanol in the first distillation column overhead vapour. The first distillation column overhead vapour may be passed through a condenser unit to cool it to below its dew point, to form a liquid which may be returned via a reflux drum to the first distillation column at or towards the top thereof. Heat is provided to the first distillation column by the first heat exchanger and the first auxiliary heat exchanger to produce the first partially purified methanol product, at or towards the bottom of the first distillation column. More particular, and in a continuous process, the first partially purified methanol product is passed through the first heat exchanger coupled to the first distillation column to form a first heat exchanger vapour which is returned to the first distillation column to supply heat thereto and a second partially purified liquid methanol product is produced. The first heat exchanger, coupled to the first distillation column, comprises a shell having a liquid inlet through which the first partially purified liquid methanol product enters. It also includes a source of heat and one or more heat transfer surfaces responsible for vaporising the first partially purified liquid methanol product to form a first heat exchanger vapour mixture comprising methanol and lighter volatile components. The heat exchanger further comprises a vapour outlet for returning the first heat exchanger vapour into the first distillation column to provide heat to the first distillation column to purify the crude methanol by separating the lighter volatile components in the first distillation column. Additionally, the heat exchanger includes a liquid outlet, for a second partially purified liquid methanol product. The first heat exchanger may include a weir that ensures the liquid level in the heat exchanger remains above the heating surfaces. The weir also enables separation of the incoming first partially purified liquid methanol product from the second partially purified liquid methanol product. However, a weir is not essential, and the first heat exchanger may instead comprise an instrumented level control system, an external piping lute or a side outlet, or other known arrangements, to achieve the same result. The source of heat for the first heat exchanger may suitably be a process gas. Alternatively, heating may be provided by steam, hot oil, or electric heating elements, but this is less preferred. In a preferred embodiment, the first heat exchanger is heated by a product gas mixture recovered from the methanol synthesis unit, more preferably a product gas stream recovered downstream of a feed gas interchanger in the methanol synthesis unit. The inlet temperature for the product gas mixture used to provide heat to the first heat exchanger coupled to the first distillation column is preferably in the range 100 to 140°C. The first heat exchanger is preferably a kettle reboiler. The first auxiliary heat exchanger may be used to provide supplemental heat to the first distillation column in cases where there is a reduction in flow of heat to the first heat exchanger and / or a reduction in the flow of the crude methanol feed to the first distillation column, for example during turn down in the methanol synthesis process. The first auxiliary heat exchanger ensures that sufficient vapour is generated in total to get the desired column profile or performance. The first auxiliary heat exchanger is in fluid flow communication with the first heat exchanger and comprises a shell having a liquid inlet through which the second partially purified liquid methanol product enters. The first auxiliary heat exchanger is therefore connected to the first heat exchanger in series. The first auxiliary heat exchanger also includes a source of heat and one or more heat transfer surfaces responsible for vaporising the second partially purified liquid methanol product to form a second auxiliary heat exchanger vapour comprising methanol and lighter volatile components. The first auxiliary heat exchanger further comprises a vapour outlet for returning the second auxiliary heat exchanger vapour into the first distillation column to provide supplemental heat to the first distillation column and to further purify the crude methanol by separating lighter volatile components in the first distillation column. Additionally, the first auxiliary heat exchanger preferably includes a weir that ensures the liquid level in the heat exchanger remains above the heating surfaces. The weir also enables separation of the incoming second partially purified liquid methanol product from the third partially purified liquid methanol product. However, a weir is not essential, and the first auxiliary heat exchanger may instead comprise an instrumented level control system or an external piping lute, or other known arrangements, to achieve the same result. The source of heatforthe first auxiliary heat exchanger may suitably be steam, hot oil, or electric heating elements. Process gas is not suitable for heating both the first heat exchanger and the first auxiliary heat exchanger. Where the first heat exchanger is heated with process gas, preferably the source of heat for the first auxiliary heat exchanger is hot oil , electrical heating or steam. Where the first heat exchanger is heated with hot oil, electrical heating or steam, preferably the source of heat for the first auxiliary heat exchanger is process gas. Both the first heat exchanger and first auxiliary heat exchanger comprise heat transfer surface(s) selected from the group consisting of tubes, coils or plate heat exchangers. In a preferred arrangement, the first auxiliary heat exchanger is a kettle reboiler. In some arrangements, the first heat exchanger and the first auxiliary heat exchanger each have a duty to supply heat to the first distillation column, wherein the duty on the first heat exchanger and first auxiliary heat exchanger fluctuates between 100% on the first heat exchanger and 0% on the first auxiliary heat exchanger, and between 0% on the first heat exchanger and 100% on the first auxiliary heat exchanger. In this latter arrangement the first distillation column may be operating in a hot standby mode. The duty on the first and first auxiliary heat exchangers may fluctuate because of a reduction in flow of heat to the first heat exchanger, for example by a reduction in the flow of process gas heating medium to the first heat exchanger, and / or a reduction in the flow of crude methanol feed to the first distillation column. Hence in some arrangements the duty may vary in response to a fluctuating flow of process gas heating medium to the first heat exchanger and / or a fluctuating flow of the third partially purified liquid methanol stream from the first distillation column as a result of the variation in the flow of the crude liquid methanol stream. A particular advantage of the present arrangement is that the series connection of liquid between the first heat exchanger and first auxiliary heat exchanger removes the need for the duty supplied by the first heat exchanger and first auxiliary heat exchanger to be precisely split. The total duty supplied to the first distillation column can be maintained at a required value by varying the heat input into the first auxiliary exchanger dependent on the heat supplied to the column by the first heat exchanger. This provides significantly improved operational flexibility for the process. The recovered third partially purified liquid methanol product may be passed to at least a second distillation column to recover a first purified methanol product, a fourth partially purified liquid methanol product and a second distillation column overhead vapour mixture. The second distillation column separates water and heavier compounds from methanol, and a first purified methanol product stream may be recovered from the column in the upper half of the column. The second distillation column overhead vapour mixture may be recovered from the top of the column and desirably used as a heat source for downstream distillation with formation of condensed overheads. The condensed overheads may be recycled to near the top of the second distillation column via a reflux drum. The fourth partially purified liquid methanol product recovered from the bottom of the second distillation column may be passed through a second heat exchanger, coupled to the second distillation column, to form a third heat exchanger vapour which is recycled to the second distillation column to supply heat thereto and a fifth partially purified liquid methanol product. In a preferred arrangement, the fifth partially purified liquid methanol product is passed to a second auxiliary heat exchanger, coupled to the second distillation column, to form a fourth heat exchanger vapour mixture, which is recycled to the second distillation column to supplement the heat in the second distillation column, and a sixth partially purified liquid methanol product. The second distillation column has a second column profile. The step of returning the fourth heat exchanger vapour to the second distillation column to supplement the heat in the second distillation column enables the second distillation column’s profile to be maintained through variations in the feed flow caused by fluctuations upstream. The sixth partially purified liquid methanol stream may be recovered from the second auxiliary heat exchanger, or it may be returned to the second column, preferably at a position below the point of addition of the fourth heat exchange vapour, and recovered from the second column, preferably at or near the bottom of the second column. In some arrangements, the second heat exchangerand the second auxiliary heat exchanger, coupled to the second distillation column, comprise the same components as the first heat exchanger and the first auxiliary heat exchanger, save that the second heat exchanger receives the fourth partially purified liquid methanol product through its liquid inlet and the second auxiliary heat exchanger receives the fifth partially purified liquid methanol product, from the second heat exchanger, through its liquid inlet. Process gas is not suitable for heating both the second heat exchanger and the second auxiliary heat exchanger. Where the second heat exchanger is heated with process gas, preferably the source of heat for the second auxiliary heat exchanger is hot oil, electrical heating or steam. Where the second heat exchanger is heated with hot oil, electrical heating or steam, preferably the source of heat for the second auxiliary heat exchanger is process gas. In some arrangements, the second heat exchanger and second auxiliary heat exchanger each have a duty to supply heat to the second distillation column wherein the duty of the second heat exchanger and second auxiliary heat exchanger fluctuates between 100% on the second heat exchanger and 0% on the second auxiliary heat exchanger, and between 0% on the second heat exchanger and 100% on the second auxiliary heat exchanger. In this latter arrangement the second distillation column may be operating in a hot standby mode. The duty on the second and second auxiliary heat exchangers may fluctuate because of a reduction in flow of heat to the second heat exchanger, for example by a reduction in the flow of process gas heating medium to the second heat exchanger, and / or a reduction in the flow of the third partially purified liquid methanol product passing to the second distillation column. The second heat exchanger and the second auxiliary heat exchanger are preferably kettle reboilers. The second heat exchanger and second auxiliary heat exchanger are in fluid flow communication and are therefore connected in series. The inlet temperature for the product gas mixture used to provide heat to the second heat exchanger coupled to the second distillation column is preferably in the range of 200 to 300 °C. The first heat exchanger and first auxiliary heat exchangers may be operated at an absolute pressure of about 1 to 2 bar and a temperature in the range of 85°C to 95°C and the second heat exchanger and second auxiliary heat exchanger may be operated at an absolute pressure of about 7 to 10 bar and a temperature in the range of 140°C to 160 °C. In some arrangements, the sixth partially purified liquid methanol product may be passed to a third distillation column. The third distillation column may be configured to produce a second purified methanol product stream as vapour overheads, a liquid water-rich stream from near the bottom of the column and a fusel oil stream, which may be recovered as a liquid or a vapour from an intermediate position on the column. In some arrangements the vapour overheads recovered from the second distillation column may be used to supply heat to the third distillation column. This may be achieved by passing the overheads vapour from the second distillation column to a heat exchanger, such as a kettle reboiler, through which the liquid off-take from the bottom of the column is passed, so as to generate a vapourised water-rich stream, which is returned to near the bottom of the third distillation column. The invention further provides a system for the process of the present invention, the system comprising: (a) a first distillation column to distil the crude liquid methanol to form a first distillation column overhead vapour and a first partially purified liquid methanol product; (b) a first heat exchanger coupled to the first distillation column through which a recovered first partially purified liquid methanol product is passed to form a first overhead heat exchanger vapour, which is returned to the first distillation column to supply heat thereto and a second partially purified liquid methanol product; and (c) a first auxiliary heat exchanger, coupled to the first distillation column through which the second partially purified liquid methanol product is passed to form a third partially purified liquid methanol product and a second auxiliary heat exchanger vapour which is returned to the first distillation column to supplement the heat in the first distillation column. The system may further comprise: (a) a second distillation column to distil the third partially purified methanol product to form a second distillation column overhead vapour, a first purified methanol product stream, and a fourth partially purified liquid methanol product; (b) a second heat exchanger, coupled to the second distillation column, through which a recovered fourth partially purified liquid methanol product is passed, to form a third heat exchanger vapour which is returned to the second distillation column, to supply heat thereto and a fifth partially purified liquid methanol product; and (c) a second auxiliary heat exchanger, coupled to the second distillation column, through which the fifth partially purified liquid methanol product is passed to form a sixth partially purified liquid methanol product, and a fourth heat exchanger vapour, which is returned to the second distillation column to supplement the heat in the second distillation column. The system may further comprise a third distillation column fed with the sixth partially purified methanol product from the second auxiliary heat exchangerand a third heat exchanger, coupled thereto, to supply heat into the third distillation column, the second overhead vapour mixture from the second distillation column being passed through the third heat exchanger as a source of heat, wherein the third distillation column is configured to produce a second purified methanol product stream as vapour overheads, a liquid water-rich stream from near the bottom of the column and a fusel oil stream, recovered as a liquid ora vapour from an intermediate position on the column. The system may further comprise a fourth distillation column coupled to the third distillation column and heated by process gas or steam that may be used to recover methanol from the fusel oil recovered from the third distillation column. The distillation columns used in the process may be any conventional design. They are normally elongate vessels arranged vertically with a top section and a bottom section. They may comprise packing and / or trays as described above. The inlets and outlets from the distillation columns may be arranged in a normal manner. For example, inlets for the liquid feeds to be distilled may be placed between the top and bottom sections. Off-takes or outlets at or near the top section may be used for vapour, and outlets at or near the bottom section used for liquids. So called side-draws may be taken from the columns for either vapour or liquid products. One or more collection devices may be placed at positions along the column to facilitate liquid or vapour recovery. The heat exchangers or reboilers take a liquid feed, vaporise it and return the vapour to the distillation column to drive the separation process. Various arrangements for the liquid recovery from the columns are described that are useful, but in the present process, a stream of liquid is fed from the first heat exchanger to the first auxiliary heat exchanger. It will be appreciated that the system and process may be further provided with a supply of crude liquid methanol from a crude methanol holding tank. 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 configuration of the distillation columns, heat exchangers and auxiliary heat exchangers in the process of the invention for the distillation of crude liquid methanol; and Figure 2 is a depiction of an alternative arrangement of the heat exchangers on the first and second columns depicted in Figure 1. It will be appreciated that the methanol synthesis process upstream for the distillation columns is not shown in Figure 1 nor is the further processing of the purified methanol product downstream from the distillation columns shown. The process for the synthesis 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, as is the further processing of the purified methanol product. It will also be understood by those skilled in the art that the Figures are 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 stream of crude liquid methanol, in line 1, is passed through a first distillation column 100. The first distillation column 100 is a packed column having a specific operating profile for the effective distillation of the crude liquid methanol which comprises methanol, water, higher alcohols and other impurities. Heat is supplied to the first distillation column 100 via heat exchangers, discussed below, causing the crude liquid methanol in the first distillation column 100 to evolve vapours that rise upwardly through the first distillation column 100 to form a first distillation column overhead vapour, which is recovered, in line 2, from the top of the first distillation column 100. The first distillation column overhead vapour includes water, methanol and light components, including methane, unreacted CO2 and H2 and potentially volatile by-products such as dimethyl ether. The first distillation column is therefore operated as a topping column. The recovered first distillation column overhead vapour in line 2 is further processed through a first distillation column primary condenser 102a and a second condenser 102b, so as to cool the first distillation column overhead vapour in line 2, below its dew point, to a liquid, recovered in lines 3a and 3b. The liquid is returned to the first distillation column 100 through a first distillation column reflux drum 104 and reflux pump 106. The first distillation column 100 is operated at a lower pressure than the crude methanol liquid stream in line 1. The bottoms product in the first distillation column 100 comprises a first partially purified liquid methanol product from the distillation. A stream, in line 4, of the first partially purified liquid methanol product is passed through the first heat exchanger 108, coupled to the first distillation column 100. The first heat exchanger 108 is a kettle reboiler. The first heat exchanger 108 is heated by means of a stream of product gas mixture recovered from an upstream methanol synthesis unit and fed via line 9. The heat of the product gas mixture is channelled through a heat transfer surface to heat the first partially purified methanol product to form a first heat exchanger overhead vapour which is returned at or near the bottom of the first distillation column 100 in line 5, to provide the heat required for the distillation of the crude liquid methanol in the first distillation column 100, and a second partially purified liquid methanol product which is recovered in line 6. The product gas mixture exiting the first heat exchanger 108 in line 10 is passed to a crude methanol condenser in the methanol synthesis unit (not shown). The second partially purified methanol in line 6 is passed to the first auxiliary heat exchanger 110 which is coupled to the first heat exchanger 108 in series. The first auxiliary heat exchanger 110 is heated by means of hot oil, in line 11, which exits the first auxiliary heat exchanger 110 through line 12. The heat from the hot oil is channelled through a heat transfer surface to heat the second partially purified liquid methanol product to form a second heat exchanger overhead vapour, which is returned to a point at or near the bottom of the first distillation column 100 via line 7, to provide any supplemental heat required for the first distillation column 100 to retain its distillation profile, and a third partially purified liquid methanol product which is recovered from the first auxiliary heat exchanger via line 8. The third partially purified liquid methanol product in line 8 is passed to a second distillation column 112, through pump 114. The second distillation column 112 is also a packed column. The third partially purified liquid methanol product is heated in the second distillation column 112 causing the third partially purified liquid methanol product to evolve vapours that rise upwardly through the second distillation column 112 to form a second distillation column overhead vapour which is recovered, in line 13, from at or near the top of the second distillation column 112, and a fourth partially purified liquid methanol product, recovered in line 16. The recovered distillation column overhead vapour is utilised to provide heat to a downstream heat exchanger 132 and then returned, in line 15, through a second distillation column reflux drum 116 and pump 118 to the second distillation column 112. A high-purity methanol product is recovered from near the top of the second distillation column via line 27. The fourth partially purified liquid methanol product recovered from the bottom of the second distillation column in line 16 is passed through a second heat exchanger 120, coupled to the second distillation column 112. The second heat exchanger 120 is a kettle reboiler. The second heat exchanger 120 is heated by a stream of product gas mixture recovered from an upstream methanol synthesis unit and fed via line 20. The heat of the product gas mixture is channelled through a heat transfer surface to heat the fourth partially purified liquid methanol product to form a third heat exchanger overhead vapour which is returned to a point at or near the bottom of the second distillation column 112, in line 17, to provide the heat required for the distillation of the third partially purified methanol liquid product in the second distillation column 112, and a fifth partially purified liquid methanol product which is recovered in line 18. The product gas mixture exits the second heat exchanger in stream 21 and is passed to a heat exchanger in the methanol synthesis unit (not shown). The fifth partially purified liquid methanol stream in line 18 is passed to a second auxiliary heat exchanger 122 which is coupled to the second heat exchanger 120 in series. The second auxiliary heat exchanger 122 is heated by means of hot oil which enters the second auxiliary heat exchanger through line 22 and exits through line 23. The heat from the hot oil is channelled through a heat transfer surface to heat the fifth partially purified liquid methanol product to form a fourth heat exchanger overhead vapour, which is returned to a point at or near the bottom of the second distillation column 112 through line 19, to provide any supplemental heat required for the second distillation column 112 to retain its distillation profile, and a sixth partially purified liquid methanol product is recovered in line 24. The sixth partially purified liquid methanol product in line 24 is passed through a third distillation column 124. The third distillation column 124 is a combination of a packed column and a plurality of distillation trays below the packed part of the column. The sixth partially purified liquid methanol product is heated in the third distillation column 124 to produce a third distillation column overhead vapour, recovered from the top of the third distillation column in line 26. The third distillation column overhead vapour is passed through a third distillation column condenser 126, to form a condensed purified methanol product which in turn is passed through a third distillation column reflux drum 128 and is recovered in line 28. The condensed purified methanol product is passed through a pump 130 and divided; a portion is returned to at or near the top of third distillation column 124 through line 29. The remaining portion of the condensed purified methanol product from line 28 is recovered from the pump 130 as a high-purity methanol product in line 30. The purified methanol stream 30 is combined with the purified methanol stream 27 to provide the purified methanol product from the process. Desirably the methanol streams 27 and 30 are cooled in heat exchangers 134 and 136 before they are combined. The bottoms product of the third distillation column 124 comprises water, and heavier hydrocarbons and is recovered in line 32. This stream is passed through the third distillation column heat exchanger 132 to produce an overhead vapour which is returned to the third distillation column 124 via line 34, and a liquid water stream which is recovered in line 36, cooled in heat exchanger 138 and sent for further processing. In one arrangement the water is purified and fed to an electrolysis unit to generate electrolytic hydrogen for use in the process and / or for power generation. A fusel oil fraction comprising higher alcohols and other minor by-products may be withdrawn from the side of the third distillation column 124 via line 38 and may be cooled in heat exchanger 140 or subjected to distillation in a further distillation column (not shown). In the embodiment of Figure 1, the first heat exchanger 108 and the second heat exchanger 120 are kettle reboilers depicted without a weir, whereas the first auxiliary heat exchanger 110 and the second auxiliary heat exchanger 122 are each depicted with a weir. In an alternative arrangement, the first heat exchanger 108 and second heat exchanger 120 each also include a weir to ensure that the liquid levels in the heat exchangers remains above the heating surfaces and to facilitate separation of the streams. In Figure 2 an alternative arrangement for the connection of the heat exchangers to the first and / or second distillation columns 100 and 112 in Figure 1 is depicted. The difference between the arrangement depicted in Figure 1 and that in Figure 2 is that instead of recovering the liquid partially purified methanol streams directly from the auxiliary heat exchangers 110 and 122, they are returned to the respective columns and new offtakes are provided from the bottom of the columns. Accordingly, in Figure 2 a feed stream 1 of crude methanol is passed to the first distillation column 100 where it is heated with separation of overheads that are recovered from the top of the column via line 2 and condensed (not shown). The first partially purified liquid methanol product is recovered from a collection device located underneath the packing in the first distillation column and passed via line 4 located near the bottom of the column to the first heat exchanger 108 to form the first heat exchanger vapour which is returned to the first distillation column 100 via line 5. The second partially purified liquid methanol product is fed from the first heat exchanger 108 via line 6 to the first auxiliary heat exchanger 110 coupled to the first distillation column to form a second heat exchanger vapour, which is returned to the first distillation column via line 7. The third partially purified liquid methanol product is recovered from the first auxiliary heat exchanger 110 via line 8 and is fed to near the bottom of the first distillation column. Accordingly, a new line 8a is provided from the bottom of the first distillation column for the recovery of the third partially purified liquid methanol product. This arrangement provides improved control of the hold-up time for the partially purified methanol stream in the first distillation column 100 sump and allows a smaller, simpler design for the auxiliary heat exchanger 110. As with Figure 1, whereas the first heat exchanger 108 is depicted without a weir, and the first auxiliary heat exchanger 110 is depicted with a weir, in an alternative arrangement, the first heat exchanger 108 also includes a weir to ensure that the liquid level in the heat exchangers remains above the heating surfaces and facilitate separation of the streams. 5 Whereas the arrangement of heat exchangers depicted in Figure 2 is provided for the first distillation column 100, it is also usefully applied to the second distillation column 112. The invention is further described by reference to the following example generated using conventional computer modelling software for the process depicted in Figure 1. The process streams and their compositions are set out in the following tables. 10 The heat to the first heat exchanger 108 was provided by 157400 kg / h of process gas in line 9 at a temperature of122°C and a pressure of 78.7 bar abs. The heat to the first auxiliary heat exchanger 110 was provided by hot oil. In this example, the flow in line 11 was initially at 0 kg / hr with all of the partially purified methanol in line 6 fed through the exchanger 110 to line 8 feeding the second distillation column 112. The heat to the second heat exchanger 120 was provided by 149500 kg / hr of 15 process gas at a temperature of 255°C and a pressure of 79.7 bar abs. The heat to the second auxiliary heat exchanger 122 was provided by 358400 kg / hr of hot oil at a temperature of 190°C and a pressure of 8.02 bar abs. Stream Number 1 2 4 5 6 8 Mass Flow kg / h 40320 19320 61440 21290 40150 40150 Temperature °C 47 75 86 87 87 87 Pressure bara 1.5 1.5 1.7 1.7 1.7 1.7 Molar Composition Unit Water mol% 51.35 0.33 42.79 23.75 51.52 51.52 Hydrogen mol% 0.02 0.05 Carbon dioxide mol% 0.04 0.12 Methanol mol% 48.56 99.20 57.20 76.23 48.47 48.47 Lights mol% 0.02 0.30 Heavies mol% 0.01 0.01 0.01 0.01 0.01 Stream Number 13 15 16 17 18 19 Mass Flow kg / h 50030 50030 73360 30610 42750 15090 Temperature °C 130 129 140 143 143 146 Pressure bara 8.35 8.10 8.60 8.60 8.60 8.60 Molar Composition Unit Water mol% 0.00 0.00 47.81 30.91 58.24 37.17 Hydrogen mol% Carbon dioxide mol% Methanol mol% 100.0 100.0 52.17 69.06 41.74 62.80 Lights mol% Heavies mol% 0.02 0.03 0.02 0.03 Stream Number 24 26 T1 29 30 36 38 Mass Flow kg / h 27650 46410 12500 33910 12500 14890 264 Temperature °C 146 71 130 67 67 113 88 Pressure bara 8.60 1.30 8.35 1.10 1.10 1.60 1.55 Molar Composition Unit Water mol% 67.89 0.00 0.00 0.00 0.00 100.0 63.28 Hydrogen mol% Carbon dioxide mol% Methanol mol% 32.10 100.0 100.0 100.0 100.0 0.00 35.05 Lights mol% Heavies mol% 0.02 1.67

Claims

1. A process for the distillation of a feed comprising crude liquid methanol, the process comprising: (a) distilling the crude liquid methanol in a first distillation column to form a first distillation column overhead vapour and a first partially purified liquid methanol product;(b) recovering the first partially purified liquid methanol product from the first distillation column;(c) passing the first partially purified liquid methanol product through a first heat exchanger coupled to the first distillation column to form a first heat exchanger vapour which is returned to the first distillation column to supply heat thereto and a second partially purified liquid methanol product;(d) passing the second partially purified liquid methanol product from the first heat exchanger to a first auxiliary heat exchanger coupled to the first distillation column to form a third partially purified liquid methanol product and a second heat exchanger vapour, which is returned to the first distillation column to supplement the heat in the first distillation column; and(e) recovering the third partially purified liquid methanol stream.

2. The process according to claim 1, wherein the feed of the crude liquid methanol stream is recovered from a methanol synthesis unit, up-stream ofthe first distillation column, which methanol synthesis unit is fed with a feed stream comprising electrolytic hydrogen produced by the electrolysis of water or steam and a carbon oxide containing gas stream.

3. The process according to claim 2, wherein the electrolysis of water or steam is performed using one or more sources of renewable energy.

4. The process according to any one of claims 1 to 3, wherein the third partially purified liquid methanol stream is recovered from the first auxiliary heat exchanger or returned to the first distillation column and recovered from the first distillation column.

5. The process according to any one of claims 1 to 4, wherein the first heat exchanger is process gas heated and wherein the first auxiliary heat exchanger is heated by a source of heat selected from the group consisting of hot oil, steam and direct electrical heating, or wherein the first heat exchanger is heated by a source of heat selected from the group consisting of hot oil, steam and direct electrical heating and wherein the first auxiliary heat exchanger is heated by process gas.

6. The process according to any one of claims 1 to 5, wherein the first heat exchanger and the first auxiliary heat exchanger each have a duty to supply heat to the first distillation column, wherein the duty on the first heat exchangerand first auxiliary heat exchanger fluctuates between 100% on the first heat exchanger and 0% on the first auxiliary heat exchanger, and between 0% on the first heat exchanger and 100% on the first auxiliary heat exchanger.

7. The process according to any one of claims 1 to 6, wherein the first heat exchanger and the firstauxiliary heat exchanger are each kettle boilers.

8. The process according to any one of claims 1 to 7, wherein the first heat exchanger and first auxiliary heat exchanger are operated at an absolute pressure of about 1 to 2 bar and at a temperature in the range of 85°C to 95 °C.

9. The process according to any one of claims 1 to 8, wherein the recovered third partially purified liquid methanol product is passed through a second distillation column to recover a first purified methanol product stream, a fourth partially purified liquid methanol product and a second overhead vapour mixture.

10. The process according to claim 9, wherein the fourth partially purified liquid methanol product is passed through a second heat exchanger coupled to the second distillation column to form a third vapour mixture which is returned to the second distillation column to supply heat thereto and a fifth partially purified liquid methanol product and wherein the fifth partially purified liquid methanol product is passed to a second auxiliary heat exchanger coupled to the second distillation column to form a fourth vapour mixture which is returned to the second distillation column to supplement the heat in the second distillation column and a sixth partially purified methanol product which is recovered.

11. The process according to claim 10, wherein the sixth partially purified liquid methanol stream is recovered from the second auxiliary heat exchanger or returned to the second distillation column and recovered from the second distillation column.

12. The process according to claim 10 or claim 11, wherein the second heat exchanger is process gas heated and wherein the second auxiliary heat exchanger is heated by a source of heat selected from the group consisting of hot oil, steam and direct electrical heating, or wherein the second heat exchanger is heated by a source of heat selected from the group consisting of hot oil, steam and direct electrical heating and wherein the second auxiliary heat exchanger is heated by process gas.

13. The process according to any one of claims 10 to 12, wherein the second heat exchanger and the second auxiliary heat exchanger are each kettle boilers.

14. The process according to any one of claims 10 to 13, wherein the second heat exchanger and the second auxiliary heat exchangereach have a duty to supply the second distillation column, wherein the duty of on the second heat exchanger and second auxiliary heat exchanger fluctuates between 100% on the second heat exchanger and 0% on the second auxiliary heat exchanger, and between 0% on the second heat exchanger and 100% on the second auxiliary heat exchanger.

15. The process according to any one of claims 10 to 14, wherein the second exchangerand second auxiliary heat exchanger are operated at an absolute pressure of about 7 to 10 bar and a temperature in the range of 140°C to 160 °C.

16. The process according to any one of claims 10 to 15, wherein the second overhead vapour mixture from the second distillation column is passed through a third heat exchanger, coupled to a third distillation column, so as to supply heat to the third distillation column for the distillation of the sixth partially purified methanol product wherein the third distillation column is configured to produce a second purified methanol product stream as vapour overheads, a liquid water-rich stream near the bottom of the column and a fusel oil stream, recovered as a liquid or a vapour from an intermediate position on the column.

17. The process according to claim 16, wherein a fusel oil produced in the third distillation column is recovered and optionally processed through a further distillation column.

18. The process according to claim 16 and claim 17, wherein the first distillation column, the second distillation column and the third distillation column are packed columns, tray columns, or a combination of packed and tray columns.

19. A system for the distillation of a feed comprising crude liquid methanol, the system comprising: (a) a first distillation column to distil the crude liquid methanol to form a first distillation column overhead vapour and a first partially purified liquid methanol product;(b) a first heat exchanger coupled to the first distillation column through which a recovered first partially purified liquid methanol product is passed to form a first heat exchanger vapour, which is returned to the first distillation column to supply heat thereto and a second partially purified liquid methanol product; and(c) a first auxiliary heat exchanger, coupled to the first distillation column through which the second partially purified liquid methanol product is passed to form a third partially purified liquid methanol product and a second heat exchanger vapour, which is returned to the first distillation column to supplement the heat in the first distillation column.

20. The system according to claim 19, further comprising:(a) a second distillation column to distil the third partially purified methanol product to form a second distillation column overhead vapour, a first purified methanol product stream, and a fourth partially purified liquid methanol product;(b) a second heat exchanger, coupled to the second distillation column, through which the fourth partially purified liquid methanol product is passed, to form a third heat exchanger vapour which is returned to the second distillation column, to supply heat thereto and a fifth partially purified liquid methanol product; and(c) a second auxiliary heat exchanger, coupled to the second distillation column, through which the fifth partially purified liquid methanol product is passed to form a sixth partially purified liquid methanol product, and a fourth heat exchanger vapour, which is returned to the second distillation column to supplement the heat in the second distillation column.

21. The system according to claim 20, further comprising a third distillation column fed with the sixth partially purified methanol product from the second auxiliary heat exchanger and a third heat exchanger, coupled thereto, to supply heat into the third distillation column, the second overhead vapour mixture from the second distillation column being passed through the third heat exchanger as a source of heat, wherein the third heat exchanger is configured to produce a second purified methanol product stream as vapour overheads, a liquid water-rich stream from near the bottom of the column and a fusel oil stream, recovered as a liquid or a vapour from an intermediate position on the column.

22. The system according to claim 21, further comprising a fourth distillation column coupled to the third distillation column and heated by process gas or steam and used to recover methanol from the fusel oil recovered from the third distillation column.A

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