Methanol distillation

Abstract

A process for the distillation of crude methanol comprising: (i) passing crude methanol stream 10 to a distillation unit comprising distillation column 12 from which separate water stream 30 and fusel

Description

This invention relates to an optimisation of a process for the distillation of a stream of crude methanol. More particularly it relates to the optimisation of the recovery of fusel oil, generated as a by-product from the synthesis of methanol, from a distillation unit, wherein the crude methanol stream fed to the distillation has a high-water content and to which, optionally, an alkali is added. Methanol is conventionally produced from synthesis gas which comprises hydrogen, carbon monoxide and carbon dioxide as the main components. These gases are reacted in the presence of a suitable catalyst at suitable temperatures and pressures to produce methanol. Water is formed as a by-product of the reaction. The crude liquid methanol product is then usually subjected to distillation to separate the methanol from the water and other by-products. These by-products include “fusel oil”, which is also known as fusel alcohol. Fusel oil comprises a mixture of alcohols. These alcohols have a higher boiling point than methanol. The water content of the crude liquid methanol depends on the composition of the synthesis gas feed to an upstream methanol synthesis unit, specifically to the relative CO / CO2 content. Simplified reaction stoichiometry for methanol synthesis demonstrates the production of water where there is CO2 in the synthesis gas feed: CO + 2 H2 CH3OH CO2 + 3 H2 CH3OH + H2O Usually, processes for the production of methanol use a synthesis gas typically derived from reforming natural gas, or from coal gasification. These all have a relatively high CO content and therefore the production of water as a by-product is tolerable. For conventional methanol production processes, the feed of crude liquid methanol to the distillation unit typically contains between about 25 mol% to 35 mol% of water in natural gas-based plants and about 7 mol% to about 15 mol% of water in coal-based plants. The desire towards producing greener products i.e. products with a low carbon intensity, has led to the development of methanol plants utilising hydrogen and CO2-rich feeds. It will be appreciated therefore that by using CO2-rich feed the crude liquid methanol that is produced in an upstream synthesis unit will have a greater amount of water than conventional plants where the methanol synthesis feed gas comprised larger quantities of carbon monoxide. For typical methanol production plants, a distillation unit may comprise one or more columns and desirably comprises three or more columns to allow effective and efficient separation of water and fusel oil from the desired methanol product. In these arrangements the distillation column from which the fusel oil is recovered usually includes a side draw taken near to the bottom of the distillation column. The fusel oil will normally be contaminated with water and other contaminants. A disadvantage of having a feed of liquid methanol with a high-water content is that more water will be purged together with the fusel oil. This is undesirable as it reduces the lower heating value (LHV) of the fusel oil making it less attractive as a fuel, and increases the energy requirements if the fusel oil is to be further processed. Furthermore, we have found that when there is more water in the feed, methanol losses in the fusel oil and heat input to produce the desired methanol purity are both increased. Both are undesirable. It is also known, in conventional distillation processes of liquid methanol, to add an alkali, such as sodium hydroxide (NaOH), to the feed. The alkali, being substantially non-volatile, is retained in the liquid phase. A disadvantage in using an alkali is that the fusel oil can be contaminated with the alkali, which makes it difficult to process further and / or dispose of. WO2010 / 026412 A1 discloses a process for distillation, in a distillation column, of a crude stream of methanol produced from synthesis gas, said crude stream comprising fusel oil and alkali wherein said fusel oil is removed from the distillation column as a vapour side draw from a point in the distillation column below the feed and wherein said vapour side draw is substantially free of alkali. It is therefore an object of the invention to provide an optimised process, at least minimising the above-mentioned disadvantages, for the distillation of a feed stream of liquid methanol, wherein the feed stream has a high water content and comprises fusel oil, and, optionally, an alkali. It is a further object of the invention to provide an optimised process for the distillation of a feed stream of liquid methanol wherein a recovered fusel oil has a low water content and is substantially free of alkali. Accordingly, the invention provides a process for the distillation of a crude methanol stream comprising (i) passing the crude methanol stream to a distillation unit comprising a distillation column from which separate water and fusel oil streams are recovered, (ii) distilling the crude methanol stream in the distillation unit and (iii) recovering a purified methanol product stream, a water stream and a fusel oil stream from the distillation unit, wherein the distillation column from which the separate water and fusel oil streams are recovered is fed with a water-rich stream comprising at least 55 mole% water via a feed inlet and the fusel oil stream is recovered from a fusel oil outlet positioned above the feed inlet. The water rich stream comprises or consists of methanol, water and fusel oil. In a preferred embodiment, the water-rich stream may comprise at least 60 mol% water, more preferably at least 64 mol% The water content in the water-rich stream may be up to 75 mol% or 85 mol%. Typically, the amount of water in the crude methanol fed to the distillation unit will be greater than, or equal to, 40 mol% and potentially as high as 55 mol% where additional water is added from upstream synthesis gas washing stages. Where the crude methanol comprises 55 mol% water, it is the water-rich stream, and the distillation unit may comprise a single column from which separate methanol, water and fusel oil streams are all recovered. Where the crude methanol stream comprises below 55 mol% water, preferably where the crude methanol comprises about 40 to 54 mol% water, the distillation unit may comprise two or more distillation columns connected in series. Accordingly, the distillation unit preferably comprises a first distillation column in which at least dissolved gases present in the crude methanol are separated from the liquid crude methanol, and one or more further distillation columns operated in series that are used to separate the components from the resulting liquid stream. At least one of the further distillation columns in the distillation unit is fed with the water-rich stream, from which at least methanol, water and fusel oil are recovered as separate streams. The further distillation column fed with the water-rich stream may be the final distillation column in a series of 2, 3, 4 or more distillation columns connected in series in the distillation unit. Conventional distillation column designs may be used. The columns may be tray-columns or packed columns ora combination thereof. The first distillation column may be used to produce a stabilised or topped methanol product by separating dissolved unreacted gases and light by-products from the crude liquid methanol. The stabilised or topped methanol product may be processed in a second distillation column. The second distillation column may be used to separate methanol from the water by-product. The second distillation column may be operated to generate a fuel-grade methanol orAA-grade high-purity methanol, however, to produce AA-grade methanol, the process preferably includes a third distillation column. The stream of crude methanol is produced in a methanol synthesis unit upstream from the distillation unit. In the synthesis unit, hydrogen is reacted with a CO2-containing gas over a suitable methanol synthesis catalyst in one or more reactors operated in series and / or parallel. The hydrogen may be sourced from the electrolysis of water or steam using renewable power, or from other sources, and the CO2-containing feed may be a pure CO2 gas stream recovered from a CO2 source, or a CO2-rich synthesis gas produced by gasification of a biogenic carbon source, such as biomass or municipal solid waste (MSW). In a preferred arrangement, the crude methanol fed to the distillation unit is produced from a methanol synthesis unit fed with hydrogen and CO2. For pure hydrogen and CO2 feeds to the methanol synthesis unit, without addition of wash water, the amount of water in the crude methanol will be about 50 mol%. The fusel oil may comprise one or more of ethanol, 1-propanol, 2-propanol and various isomers of butanol. The fusel oil may be drawn as a liquid or vapour, preferably as a liquid, from the distillation column. The fusel oil may comprise about 5 mol% to 40 mol% water and may be subject to distillation in a separate distillation column located downstream of the distillation unit. According to one aspect of the invention, the water-rich stream may further comprise alkali. The alkali may be sodium hydroxide (NaOH). The alkali may be combined with the crude methanol prior to it being fed into the distillation unit or it may be brought into contact with the crude methanol or water-rich stream in a distillation column, or both. The fusel oil that is drawn from the final distillation column is substantially free of alkali where there is a draw of fusel oil above the feed. The invention may now be further described by reference to the following drawings in which; Figure 1 is a depiction of one embodiment of the invention, and Figure 2 is a depiction of a further embodiment of the invention. Figure 1 represents the final column in the distillation unit which is fed with a crude methanol stream comprising at least 55 mol% water. The crude methanol feed has been treated in at least one upstream distillation column to remove dissolved gases and volatile components. Optionally it may have been treated in a further one or more distillation columns to remove part of the contained methanol as a purified product stream. In this final column a purified methanol product stream is withdrawn overhead, a purified water stream is withdrawn as the bottom product and a fusel oil stream is withdrawn as a side draw. The side draw is withdrawn from the distillation column as a liquid or vapour stream from above the feed. Thus, in Figure 1, a feed stream of liquid crude methanol comprising at least 55 mol% water is fed via line 10 to the side of a distillation column 12 comprising a plurality of trays 14. The feed inlet in the side of the column 12 is located below the mid-point of the column. Heat is supplied to the column 12 by means of a reboiler 16, heated via line 18 with either steam, product gas recovered from the methanol synthesis unit, or a vapour stream recovered from an upstream distillation column in the distillation unit. The heat provided to the column causes the boiling and separation of the components in the crude methanol as it passes though the trays 14. A methanol vapour stream is recovered from the top of the column 12 via line 20, condensed in water-cooled condenser 22 and fed to a reflux drum 24. From the reflux drum, the liquid methanol is compressed in a pump (not shown), divided, and part returned to the column via line 26. The part that is not returned to the column is drawn off as a pure product methanol stream via line 28. A liquid water stream is recovered from the bottom of the column 12 via line 30. A liquid or vapour fusel oil stream is recovered from the side of the column via line 32. The fusel oil outlet in the side of the column 12 is positioned above the inlet for the feed 10 and preferably at or more preferably below the mid-point of the column. Figure 2 shows a three-column distillation unit. A first crude methanol stream comprising about 50 mol% water from an upstream methanol synthesis unit is fed via line 40 to a first multi-tray distillation column 42 which removes dissolved gases and volatile components as an overhead stream via line 44. The light gas mixture recovered from the top of the first distillation column 42 via line 44 comprises methanol vapour and so to improve the efficiency of the process, a first heat exchanger 46 which may be water and / or air cooled, is used to partially condense a methanol-containing stream 48, which is divided. A first (liquid) portion of the condensed stream is fed to a reflux drum 50 and then via line 52 to near the top of the column 42. A second (vapour) portion of the condensed stream 48 is fed though a second water-cooled heat exchanger 54, and a further condensed liquid portion fed to the reflux drum 50 via line 56. Light gases suitable for use as fuel are recovered downstream of the second water-cooled heat exchanger 54 via line 58. A reboiler 60 provides heat to the first distillation column 42. Heat for the first distillation column may be provided via line 62 by an external heating medium such as steam, or by heat recovered from elsewhere in the process. A second crude methanol stream is removed as the bottom product from column 42 and fed via line 64 to a second multi-tray distillation column 66. A purified methanol stream is withdrawn from at or near the top of the second distillation column 66 via line 68. A third crude methanol stream with an elevated water content of at least 55 mol%, e.g. at least 60 mol% or at least 64 mol%, is withdrawn from the bottom of the second distillation column 66 and fed via line 70 to a third multi-tray distillation column 72. The second distillation column 66 preferably is operated at a pressure that enables useful heat integration with the third distillation column 72. A pair of reboilers 74 and 76 are used to provide heat to the second distillation column. Heat for the reboilers may be provided by an external heating medium such as steam, or by heat recovered from elsewhere in the process, or by a combination of external heating and recovered heat, via lines 78 and 80. The second distillation column 66 operates at a higher pressure than the third distillation column 72, which allows an overhead vapour stream recovered from the top of the second distillation column via line 82 to be condensed in a reboiler 84 coupled to the third distillation column to provide heat to the third distillation column 72. The condensed overhead vapour stream is returned from the reboiler 84 to near the top of the second distillation column 66 via line 86. Purified methanol is withdrawn from at or near the top of the third distillation column via line 88, a purified water stream is withdrawn from the bottom via line 90 and a fusel oil stream is withdrawn as a liquid or vapour side draw via line 92 from above the feed 70. If desired, the fusel oil stream may be subjected to further distillation, to recover any remaining methanol. The methanol product recovered from near the top of the second distillation column 66 via line 68 is cooled in a third heat exchanger 94 which may be water and / or air cooled, and optionally one or more further heat exchangers cooled with water, to recover a liquid purified methanol product 96. The gaseous methanol product recovered from the top of the third distillation column 72 via line 88, is condensed in a fifth heat exchanger 98 which may be water and / or air cooled and optionally one or more further heat exchangers cooled with water, to recover a further liquid purified methanol product 100. To improve control of the process, the purified methanol product 100 is fed to a reflux drum 102. A portion is returned to near the top of the third distillation column 72 via line 104. A further portion is fed from the drum 102 via line 106 to a methanol product heat exchanger 108, which may be water and / or air cooled, to provide a cooled purified methanol product, which is combined with the cooled purified methanol product 96 recovered from the second distillation column. The liquid water stream withdrawn from the bottom of the third distillation column 72 via line 90 may be cooled in exchanger 110 and sent via line 112 for further processing. In one arrangement, the water is purified and fed to an electrolysis unit to generate hydrogen for use in the methanol synthesis unit and / or power generation. The fusel oil stream 92 recovered from the side of the third distillation column 72 is cooled in a further heat exchanger 114 which may be water and / or air cooled to form a cooled fusel oil stream, 116. The Invention is further illustrated by reference to the following example where a computer model of the distillation system was used to compare the separation and heat requirements of the arrangement of Figure 2, configured to produce ‘AA grade purified methanol, of which a key specification is an ethanol content of less than 10 ppm by weight. Example 1 The first crude methanol stream fed to the distillation unit comprises 50 mol% water, along with methanol, fusel oil, volatile components and dissolved gases. Before entering the first distillation column the first crude methanol stream is dosed with sodium hydroxide solution. Dissolved gases and volatile components are removed as the overhead product in the first distillation column, leaving a second crude methanol stream comprising methanol, water and fusel oil. Half of the methanol contained in the second crude methanol stream is removed as a purified product from near the top of the second distillation column. The third crude methanol stream fed to the third or final distillation column in the unit thus comprises 67 mol% water, methanol and fusel oil. For a particular heating duty supplied to the reboiler of the second distillation column, when the third distillation column is configured with a liquid fusel oil draw below the feed, the fusel oil draw recovered from the column contains about 71 mol% water and some sodium hydroxide. In comparison, according to the present invention, if the fusel oil is withdrawn as a liquid side draw from above the feed, less heat is required to achieve the ethanol specification in the methanol product, or the ethanol content of the purified methanol product from the third distillation column is reduced by about 25% with no change to the heating duty supplied to the reboiler of the second distillation column. In this case the fusel oil draw contains about 32 mol% water and is also substantially free of sodium hydroxide. In both cases the methanol content of the fusel oil draw is the 5 same. Example 2 The column arrangement is the same as Example 1, however in this case the reboil duty to the second distillation column is varied to achieve an ethanol content of 5 ppm by weight in the combined purified 10 methanol product from the second and third columns. When, according to the invention, the fusel oil is withdrawn as a liquid from above the feed, the reboil duty is reduced by about 2% versus when the fusel oil is withdrawn as a liquid from below the feed. The same benefits as Example 1 are also realised when the fusel oil is withdrawn from above the feed in terms of water and sodium hydroxide content of the fusel oil. 15

Claims

1. A process for the distillation of a crude methanol stream comprising (i) passing the crude methanol stream to a distillation unit comprising a distillation column from which separate water and fusel oil streams are recovered, (ii) distilling the crude methanol stream in the distillation unit and (iii) recovering a purified methanol product stream, a water stream and a fusel oil stream from the distillation unit, wherein the distillation column from which the separate water and fusel oil streams are recovered is fed with a water-rich stream comprising at least 55 mole% water via a feed inlet and the fusel oil stream is recovered from a fusel oil outlet positioned above the feed inlet2. The process of claim 1, wherein the water-rich stream has a water content of at least about 60 mol%, preferably at least about 64 mol%.

3. The process of claim 1 or claim 2, wherein the water-rich stream contains an alkali.

4. The process of claim 3, wherein the alkali is combined with crude methanol prior to it being fed into the distillation unit or is brought into contact with the water-rich stream in the distillation column, or both.

5. The process of any one of claims 1 to 4, wherein the distillation unit comprises two, three or four distillation columns and the distillation column from which the separate water and fusel oil streams are recovered is the second, third or fourth distillation column in the distillation unit, preferably the third and / or final distillation column in the distillation unit.

6. The process of any of claims 1 to 5, wherein the distillation unit is fed with a crude methanol stream from a methanol synthesis unit, which methanol synthesis unit is fed with a CO2-containing gas stream and hydrogen.

7. The process of claim 6, wherein the CO2-containing gas stream is a pure CO2 gas stream or a CO2-rich gas.

8. The process of claim 6 or claim 7, wherein the hydrogen is electrolytic hydrogen produced by the electrolysis of water or steam, or hydrogen produced from other sources.

9. The process of any one of claims 1 to 8, wherein the fusel oil is drawn as a liquid or vapour, preferably as a liquid, from the distillation column.

10. The process of any one of claims 1 to 9, wherein the fusel oil that is drawn from the distillation column is drawn as a liquid and comprises about 5 mol% to 40 mol% water.

11. The process of any one of claims 1 to 10, wherein the fusel oil that is drawn from the distillation column is subject to distillation in a downstream distillation column located downstream of the distillation unit.A

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