GB2073863A

GB2073863A - Gas recovery method and apparatus

Info

Publication number: GB2073863A
Application number: GB8107007A
Authority: GB
Original assignee: Cryoplants Ltd
Current assignee: Cryoplants Ltd
Priority date: 1980-03-05
Filing date: 1981-03-05
Publication date: 1981-10-21
Also published as: GB2073863B

Abstract

A mixture of nitrogen and hydrogen is recovered from feed gas mixture comprising carbon monoxide, nitrogen and hydrogen. The feed gas mixture is cooled, washed with liquid nitrogen in a column 2 to remove carbon monoxide from the gas. The resulting liquid has hydrogen stripped therefrom in column 4 and is then passed into a third gas-liquid contact column 6 in which nitrogen is separated from carbon monoxide. The nitrogen is liquefied and part of it is used as the source of liquid nitrogen in the column 2. Another part is added to the gas mixture leaving the column 2 so as to adjust its composition to that required for ammonia synthesis. The hydrogen stripped from the liquid in the column 4 may be compressed and returned to the column 2 or may be purified in a further column (not shown in Fig. 1). <IMAGE>

Description

SPECIFICATION Gas recovery method and apparatus This invention relates to a method of and apparatus for recovering gas. In particular, it relates to a method of and apparatus for recovering hydrogen from a feed gas mixture comprising carbon monoxide, hydrogen and nitrogen. The hydrogen is recovered with nitrogen in a mixture that is suitable for use in the synthesis of ammonia.

Ammonia is synthesised commercially from hydrogen and nitrogen. One way of producing the necessary hydrogen is by the partial oxidation of substances containing carbon and hydrogen. A cryogenic air separation unit is conventionally used to provide the necessary oxygen. The unit may also provide a nitrogen stream for the synthesis of ammonia. The partial oxidation yields a gas mixture comprising hydrogen and carbon monoxide. Hydrogen is sepaated from the carbon monoxide and mixed with the nitrogen stream to form an ammonia synthesis gas.

In order to obviate the need to use a cryogenic air separation unit and hence the need to handle substantially pure oxygen, it has been proposed to use air in the aforesaid process of partial oxidation. Thus, the resulting gas mixture contains not only hydrogen and carbon monoxide but also nitrogen. Such a gas mixture may be produced at elevated pressure (e.g. at least 50 bar).

Conventional processes for separating hydrogen and carbon monoxide are unsuitable for recovering hydrogen (mixed with nitrogen) from a gas stream comprising hydrogen, nitrogen and carbon monoxide.

It is an aim of the present invention to provide a method of and apparatus for recovering substantially all the hydrogen from a gas mixture comprising carbon monoxide, hydrogen and nitrogen, the hydrogen being recovered with nitrogen in a mixture that is suitable for use in the synthesis of ammonia.

According to the present invention there is provided a method of recovering hydrogen from a feed gas mixture comprising carbon monoxide, hydrogen, and nitrogen, the hydrogen being recovered with nitrogen in a mixture that is suitable for use in the synthesis of ammonia, which method comprises: (a) introducing the feed gas mixture at a temperature below ambient into a first gas-liquid contact region at elevated pressure in which the feed gas mixture is washed with liquid nitrogen to produce a purified gas mixture comprising a major portion of the hydrogen in the feed gas mixture and a liquid comprising nitrogen and carbon monoxide with hydrogen dissolved in it; (b) stripping hydrogen from the said liquid in a second gas-liquid contact region to leave a residual liquid substantially free of hydrogen, the pressure in the second region being lower than in the first region;; (c) purifying the stripped hydrogen, or compressing the stripped hydrogen, cooling it and returning it to the first region; (d) passing the residual liquid from the second region into a third gas-liquid contact region in which nitrogen substantially free of carbon monoxide is separated by rectification from the residual liquid, and (e) liquefying the nitrogen and returning at least a portion of the soformed liquid nitrogen to the first region.

The invention also provides apparatus for performing the above method comprising: (a) means for reducing the temperature of the feed gas mixture to below ambient; (b) a first gas-liquid contact column having an inlet for the feed gas mixture; an inlet for liquid nitrogen; an outlet for the purified gas mixture, and an outlet for the residual liquid; c) a second gas-liquid contact column having an inlet for the residual liquid from the first column, an outlet for the hydrogen and an outlet for the residual liquid; (d) means for purifying the hydrogen or a compressor for compressing the hydrogen, means for cooling the compressed hydrogen and means for returning it to the first column; (e) a third gas-liquid contact column having an inlet for the liquid from the second column, and an outlet for the nitrogen separated from the liquid in use of the apparatus;; (f) means for liquefying the said nitrogen; and (g) means for passing the so-formed liquid nitrogen into the first column.

The step of stripping hydrogen from the residual liquid in the second gas-liquid contact region or column makes it possible to reduce the energy consumption from what it would otherwise be were the residual liquid be passed directly to a column in which carbon monoxide was removed therefrom.

If, for example, the feed gas mixture formed by the partial oxidation with air of a substance containing carbon and hydrogen it will typically be at a pressure of 50 to 75 bar and have a mole ratio of hydrogen to nitrogen of less than 3:1. It will also typically contain from 0.5 to 3.5% by volume'of carbon monoxide. In addition, it will typically contain impurities such as argon, methane, carbon dioxide and water vapour. Any carbon dioxide and water vapour present may be removed from the gas mixture by conventional means upstream of the first gas-liquid contact region or column. Substantially all the argon and methane will generally be separated from the hydrogen in the second region or column, leaving that region or column as part of the residual liquid.Owing to the relatively high pressure of the feed gas mixture substantially all the refrigeration required for the operation of the method according to the invention may, on occ.sions, be generated without recourse to an outside source of refrigeration. If necessary, extra refrigeration may be generated by expansion in a turbine of a high pressure gas stream obtained directly or indirectly from the feed gas mixture. Alternatively or in addition, use may be made of externally generated refrigeration, for example, a conventional refrigeration cycle employing say Freon (Registered Trade Mark) as the refrigerant. In addition, owing to the relatively high proportion of nitrogen in the feed gas mixture, all the liquid nitrogen required for the first gas-liquid contact region or column may be obtained from the feed gas mixture and not from an external source.

The method and apparatus according to the invention will now be described by way of example with reference to the accompanying drawings, of which: Figure 1 is a schematic flow sheet illustrating a plant having three gas-liquid contact regions or columns on which the method according to the invention can be performed; Figure 2 is a schematic flow sheet illustrating a plant having three gas-liquid contact regions or coiumns on which the method according to the invention can be performed; Figure 3 is a flow sheet illustrating one plant for performing the method according to the present invention; and Figure 4 is a flow sheet illustrating an alternative plant for performing the method according to the invention.

In Figs. 1 and 2 the main heat exchangers for the plant are not shown. This is so as to simplify the flow sheets. Passage of the stream through a heat exchanger is indicated in the drawings by a symbol of a circle containing a substantially sinusoidal line.

The plant shown in Fig. 1 has three gas-liquid contact columsn 2, 4 and 6. In the column 2 the feed gas mixture comprising nitrogen, hydrogen and carbon monoxide is washed with liquid nitrogen. This produces a product gas containing nitrogen and hydrogen substantially free of carbon monoxide and a liquid comprising carbon monoxide and nitrogen with hydrogen dissolved in it. Hydrogen is stripped from this liquid in the column 4. In the column 6, a nitrogen stream free of carbon monoxide is separated as a gaseous fraction from the liquid. The resulting nitrogen is then liquefied and at least a portion of the so-formed liquid is returned to the first column 2.

The feed gas mixture is typically produced at a pressure of at least 50 bar (typically in the range of 50 to 75 bar) and at a temperature slightly above ambient (e.g. 40"C). Any carbon dioxide and water vapour present may be removed from the feed gas mixture by conventional means. The feed gas mixture is cooled to below ambient temperature by heat exchange. It is then introduced into the column 2, typically just above the sump of the column. The column 2 may, for example, be of the sieve tray kind. Typically, it may have in the order of 20 to 40 theoretical trays. The feed gas mixture ascends column 2 and comes into contact with liquid nitrogen descending the column from tray to tray. Liquid nitrogen is introduced into the column 2 near its top through an inlet 10.Contact between the feed gas mixture and the liquid nitrogen condenses carbon monoxide and any impurity such as methane in the feed gas mixture.

Although hydrogen does not condense at the temperature of liquid nitrogen, it is soluble therein and accordingly some of the hydrogen in the feed gas mixture is dissolved in the liquid as it descends from tray to tray. Accordingly, a liquid comprising liquid nitrogen and liquid carbon monoxide with dissolved hydrogen is collected at the bottom of the column 2, while at the top of the column a gas mixture comprising hydrogen is obtained. This gas mixture is substantially free of any argon and/or methane in the feed gas mixture as well as being free of carbon monoxide. The gas mixture is passed out of the column 2 through an outlet 1 2. The residual liquid is passed out of the column 2 through an outlet 14. Most of the hydrogen is removed from the feed gas mixture in the column 2.

The operating pressure of the column 2 is typically substantially the same as the pressure of the incoming feed gas mixture.

The residual liquid passed out of the column 2 through the outlet 14 is typically cooled by heat exchange and it is then introduced into the column 4 through an inlet 1 6 near the top of the column 4. The column 4 typically contains from 10 to 20 theoretical trays. Sieve trays may, for example, be employed. The operating pressure in the column 4 is below the critical pressure of the liquid entering the column but preferably is close to that critical pressure. Typically, the operating pressure of the column 4 is in the order of 30 bar.

The liquid entering the column 4 descends passing from tray to tray. As shown in Fig. 1, the liquid at the bottom of the column 4 may be used to cool a gas stream. For example, the feed gas mixture may be passed through the heat exchange coil 1 8 in the column 4 before being introduced into the column 2. Hydrogen is stripped from the liquid and when the liquid collects at the bottom of the column 4, it contains substantially no hydrogen.

Hydrogen stripped from the liquid in the column 4 leaves the column 4 through an outlet 20.

It is then compressed by a compressor 22, cooled by heat exchange and passed into the column 2 through an inlet 24 which is at a level just above the liquid in the bottom of the column 2.

The compressor 22 may receive hydrogen for compression at a temperature below 0 C, or alternatively, the hydrogen leaving the column 4 through the outlet 20 may be warmed by heat exchange upstream of the compressor 22.

The liquid collecting at the bottom of the column 4 leaves column 4 through an outlet 26 and is introduced into the column 6 through an inlet 28 situated about halfway up the column 6.

The column 6 typically contains from 80 to 90 theoretical trays. The trays may, for example, be sieve trays. The column 6 is fitted with a condenser 30 and reboiler 32. The condenser 30 provides reflux for rectification of the fluid entering the column 28. Since nitrogen is more volatile than carbon monoxide a vapour relatively rich in nitrogen is formed. The vapour ascends the column 6 and some of it is condensed by condenser 30 so that a liquid condensate rich in nitrogen is formed. This condensate provides reflux for the column and descends the column tray by tray and becomes progressively leaner in nitrogen as it contacts ascending vapour. By the time the liquid reaches the bottom of the column 6 it contains all the carbon monoxide in the incoming liquid.If the incoming liquid contains argon and/or methane substantially all the argon and/or methane are separated from the nitrogen and are collected in the liquid at the bottom of the column 6. The reboiler 32 effects vaporisation of the liquid collecting in the bottom of the column 6 and thus causes a vapour stream too ascend the column.

Substantially pure nitrogen leaves the column 6 through an outlet 34 and is liquefied by heat exchange in a liquefier 36. The resulting liquid nitrogen is pumped by a pump 38 through the inlet 10 of the first column and thus provides the liquid nitrogen for the first column 2. A portion of the liquid nitrogen is, as necessary, combined with the purified hydrogen/nitrogen gas mixture leaving the column through the outlet 1 2 so as to form a gas mixture of hydrogen and nitrogen containing 3 moles of hydrogen for each mole of nitrogen.

The liquid in the bottom of the column 6 leaves the column 6 through an outlet 48 and is passed through an expansion valve 42. The resulting fluid is then passed through the heat exchanger 36 countercurrently to the nitrogen stream so as to liquefy the latter. It is then warmed to ambient temperature (or just above ambient) typically by heat exchange with the incoming feed gas mixture.

The refrigeration necessary for the condenser 30 and the heat necessary for the reboiler 32 are provided by a working fluid in the following cycle of operation. After passing through the reboiler 32 the working fluid is pumped by means of a pump 44 through an expansion valve 46 and the resulting cold fluid is passed through the condenser 30 in which it is warmed by condensing nitrogen. The working fluid is then compressed in a compressor 48, cooled by heat exchange and returned to the reboiler 32. If desired, the working fluid may be warmed by heat exchange downstream of the condenser 30 and upstream of the compressor 48 so as to avoid the expedient of having to compress cold gas.

The amount of power consumed by the compressor 48 depends on the operating pressure of the column 6. The lower the operating pressure of column 6, the less power that needs to be consumed by the compressor 48. Typically, however, it is desirable to obtain a pressurised waste gas stream for the liquid at the bottom of the column 6. Preferably, the operating pressure in the column 6 is less than 1 5 bar. Thus, for example, the operating pressure of the column 6 may be chosen to be 10 bars at which pressure adequate separation of the nitrogen is possible.

The working fluid that is passed through the condenser 30 and the reboiler 32 is typically nitrogen.

In Fig. 2 are shown a number of improvements that can be made to the plant shown in Fig.

1. Like parts in Figs. 1 and 2 are identified by the same reference numerals in these figures.

If it is desired to reduce the carbon monoxide concentration of the feed gas mixture from, say, 2 to 32% by volume under 1% by volume, the feed gas mixture may be passed into an auxiliary gas-liquid contact column 50. Typically there are about 3 to 6 theoretical trays in this column which typically operates at substantially the pressure of the feed gas mixture. The trays may, for example, be sieve trays. The column is provided with reflux by passing through a condenser 58 in the column a portion of the liquid which has been passed through the expansion valve 42. A large proportion of the carbon monoxide in the feed gas mixture condenses acts as reflux, and is collected at the bottom of the column 50.This liquid is then united with that leaving the column 2 through the outlet 1 4. The gas collecting at the top of the column 50 passes into the column 2 through the inlet 8. By so reducing the concentration of carbon monoxide in the gas mixture entering the column 2 through the inlet 8 it is possible to reduce the overall power consumption of the plant. This is because less liquid nitrogen will be required for the column 2 and, in consequence, less power needs to be consumed by the compressor 48.

The coolant from the condenser is vented from the process typically after heat exchange with the feed gas mixture. Under some operating conditions, the plant shown in Fig. 1 may be selfrefrigerating. In other instances, however, depending on the composition of the feed gas mixture and the operating pressure chosen for the column 6, it is necessary to generate extra refrigeration. This can be done in one of two ways. First, a portion of the cooled feed gas mixture may be passed through an expansion turbine 52, and the resulting cold gas, after heat exchange, is introduced into the hydrogen stripper.Alternatively, a portion of the working fluid being recirculated to and from the reboiler 32 may be taken from immediately upstream of the reboiler 32, expanded in the expansion turbine 54 and the resulting cold gas, after heat exchange, combined with the working fluid at a region downstream of the condenser 30 but upstream of the compressor 48. In some instances it may be desirable to sub-cool the working fluid downstream of the pump 44 but upstream of the expansion valve 46 in a heat exchanger 56. This may be done by heat exchange with the fluid passing from the expansion valve 42 to the heat exchanger 36.

Referring now to Fig. 3 of the accompanying drawings, a feed gas mixture comprising hydrogen, nitrogen and carbon monoxide is introduced into the plant through a conduit A. The feed gas mixture is typically derived from a partial oxidation unit in which air is used as the oxidising gas. Carbon dioxide and water vapour are typically removed from the feed gas mixture upstream of the conduit A. The feed gas mixture is passed into a heat exchanger 102. As the feed gas mixture passes through the heat exchanger 102 so it is cooled to below 0 C. A side stream is taken from the feed gas mixture passing through the heat exchanger 102 and is passed through a heat exchange coil 104 in which it is cooled further before being reunited with.

the feed gas mixture at a region in the heat exchanger 102 downstream of the location from which the feed gas stream is taken. The feed gas mixture after leaving the heat exchanger 102 is passed through another heat exchanger 106 in which it is further cooled and reduced in temperature. The feed gas mixture is then introduced into a first gas-liquid contact column 108 through an inlet 11 0. In the column 108, the gas mixture is contacted or washed with liquid nitrogen passed into the column 108 through an inlet 11 2 situated near the top of the column.

The contact or washing with liquid nitrogen condenses carbon monoxide from the incoming gas mixture and results in the production of a gas mixture of nitrogen and hydrogen substantially free of carbon monoxide. Such a gas mixture leaves the column 108 through an outlet 114 at its top and then passes through the heat exchangers 106 and 102 countercurrently to the feed gas mixture, thereby providing refrigeration for the heat exchangers 106 and 1 02. The gas mixture then passes into the conduit B from which it is taken for the synthesis of ammonia.

The residual liquid collecting at the bottom of the column 108 passes through an outlet 11 6 at the bottom of the column and is passed through an expansion valve 11 8 into the inlet 1 22 to a second gas-liquid contact column 1 20 in which the heat exchange coil 104 is located.

Hydrogen is evolved or stripped from the liquid flowing through the column 1 20 and this hydrogen leaves the column through an outlet 1 26 at the top thereof. The hydrogen will contain some nitrogen and carbon monoxide mixed with it. The hydrogen passes through the heat exchange 102 providing cooling therefor and is compressed in a compressor 1 26 before being united with the uncooled feed gas mixture.

The liquid collecting at the bottom of the column 1 20 provides cooling for the feed gas mixture passing through the heat exchange coil 104. Liquid is passed out of the column 1 20 through an outlet 1 30 at the bottom thereof. The liquid is passed through an expansion valve 132, and then through an inlet 1 36 of a third gas-liquid contact column 1 34 which is fitted with a condenser 1 38 and a reboiler 140.

The fluid entering the column 1 34 is substantially free of hydrogen and thus consists essentially of nitrogen and carbon monoxide. In operation, the vapour at the top of the column 1 34 is condensed by condenser 1 38. The thus formed liquid acts as reflux and flows along the gas-liquid contact surfaces in the column, gradually descending the column, and comes into contact with vapour ascending the column, such vapour being formed in part by the reboiler 140 causing the liquid collecting at the bottom of the column to boil. As the liquid descends the column it becomes gradually richer in carbon monoxide, whereas the vapour reaching the top of the column 1 34 is substantially free of carbon monoxide. This vapour is thus essentially pure nitrogen. It passes out of the column 1 38 through an outlet 148 and is liquefied by being cooled in a heat exchanger 142. The resulting liquid nitrogen is then passed through a subcooler 144. A portion of the so-formed liquid nitrogen is taken by pump 146 and some of this portion is distributed to the inlet 11 2 of the column 108 thereby fulfilling the liquid nitrogen requirements of the column 108 while the rest of the portion is united with the gas mixture leaving the column 108 at a region intermediate the outlet 114 and the heat exchanger 1 06.

By this means the ratio of hydrogen to nitrogen in the gas mixture leaving the column 108 can be adjusted to a value suitable for the synthesis of ammonia.

That portion of the liquid nitrogen not taken by the pump 146 is passed into storage through a conduit D.

The liquid collecting at the bottom of the column 1 34 consists essentially of nitrogen and carbon monoxide. The waste gas stream is typically formed from this liquid and used for the purposes of recovering power. Thus, liquid is discharged from the column 1 34 through an outlet 1 50 at its bottom. The liquid is then divided into major and minor streams. The minor stream is passed through an expansion valve 1 52 and then used to provide cooling for the heat exchangers 144 and 142 being passed therethrough countercurrently to the nitrogen stream from the outlet 148 of the column 1 34. The resulting mixture of carbon monoxide and nitrogen vapour leaves the heat exchanger 142 and then passes through the heat exchanger 102 countercurrently to the feed gas mixture.After leaving the heat exchanger 102 the mixture of nitrogen and carbon monoxide is vented from the process. The major stream is passed through an expansion valve 1 54 and then through the heat exchanger 142 countercurrently to the nitrogen stream from the outlet 148 of the column 134, thereby providing cooling for the heat exchanger 142. After leaving the heat exchanger 1 42 the stream from the outlet 1 50 of the column 1 34 is passed through the heat exchanger 102 countercurrently to the feed gas mixture 102, thereby providing cooling for the heat exchanger 102. The gas after leaving the heat exchanger 102 is pssed to the conduit C for power recovery and venting from the process.

The cooling for the condenser 1 38 and the heating for the reboiler 140 is provided by a working fluid, preferably nitrogen, which undergoes the following cycle of operation starting immediately downstream of the condenser 1 38. Nitrogen gas leaves the condenser 1 38 and passes through the heat exchanger 102, providing cooling therefor, countercurrently to the feed gas mixture. After leaving the heat exchanger 102 the working fluid is compressed by a compressor 1 56 and a so-formed compressed gas is returned through the heat exchanger 102, being cooled and reduced in temperature as it passes therethrough.A side stream is taken from the compressed nitrogen as it is flowing through the heat exchanger and passed through an expansion turbine 1 58. This cools the nitrogen and the resulting cold nitrogen is united with a part of the working fluid cycle intermediate the condenser 1 38 and the heat exchanger 102 so as to enable further cooling to be provided to the heat exchanger 102. The compressed nitrogen, after being cooled in the heat exchanger 102, is passed through the reboiler 140 in which it provides heat necessary to boil the liquid at the bottom of the column 1 34 and is itself liquefied.The resulting liquid nitrogen is pumped by pump 1 60 through an expansion valve 1 62 and the resulting cold nitrogen is passed through the condenser 1 39 thus completing the cycle of operation.

If desired, rather than combining the expanded nitrogen with the working fluid between the condenser 1 38 and the heat exchanger 102, it could be expanded in the turbine 1 58 to a lower pressure than the working fluid, warmed to ambient temperature by being passed through the heat exchanger 102 countercurrently to the feed gas mixture, compressed in an additional compressor (not shown) to a pressure equal to that of the gas immediately upstream of the compressor 156, and then united with such gas. Another alternative is to employ the expansion turbine 1 58 and an additional compressor (not shown) in a separate refrigeration cycle.

In the following table, information is given about the composition pressure and flow rate of the streams A, B, C and D shown in Fig. 3. Typically, the column 1 08 operates at a pressure of 73.5 bar, the column 120 at a pressure of 30 bar and the column 134 at a pressure of 12 bar. STREAM DESCRIPTION FLOW PRESSURE COMPOSITION (mole fraction) (moles per unit time) (bars) H2 N2 CO Ar CH4 A Feed gas mixture 100 75 .465 .503 .004 .006 .022 B Ammonia synthesis gas 62 73 .75 .25 < 10ppm negligible C Waste gas 35.5 9 0 .915 .011 .016 .058 2.4 1 0 .915 .011 .016 .058 D Liquid nitrogen to storage 0.1 12 0 1 < 10ppm negligible Referring now to Fig. 4 of the accompanying drawings, a feed gas mixture comprising hydrogen, nitrogen and carbon monoxide is introduced into the plant through a conduit A.The feed gas mixture is typically derived from a partial oxidation unit in which air is used as the oxidising gas. Carbon dioxide and water vapour are typically removed from the feed gas mixture upstream of the conduit A. The feed gas mixture is passed into a heat exchanger 202. As the feed gas mixture passes through the heat exchanger 202 so it is cooled to below O"C. A side stream is taken from the feed gas mixture passing through the heat exchanger 202 and is passed through a heat exchange coil 204 in which it is cooled further before being reunited with the feed gas mixture at a region in the heat exchanger 202 downstream of the location from which the feed gas stream is taken. The feed gas mixture after leaving the heat exchanger 202 is passed through another heat exchanger 206 in which it is further cooled and reduced in temperature.The feed gas mixture is then introduced into a first gas-liquid contact column 208 through an inlet 210. In the column 208, the gas mixture is contacted or washed with liquid nitrogen passed into the column 208 through an inlet 21 2 situated near the top of the column.

The contact or washing with liquid nitrogen condenses carbon monoxide from the incoming gas mixture and results in the production of a gas mixture of nitrogen and hydrogen substantially free of carbon monoxide. Such a gas mixture leaves the column 208 through an outlet 214 at its top and then passes through the heat exchangers 206 and 202 countercurrently to the feed gas mixture, thereby providing refrigeration for the heat exchangers 206 and 202.

The gas mixture then passes into the conduit B from which it is taken for the synthesis of ammonia.

The residual liquid collecting at the bottom of the column 108 passes through an outlet 21 6 at the bottom of the column and is passed through an expansion valve 218 into the inlet 222 to a second gas-liquid contact column 1 20 in which the heat exchanger coil 204 is located.

Hydrogen is evolved or stripped from the liquid flowing through the column 220 and this hydrogen leaves the column through an outlet 226 at the top thereof. The hydrogen will contain some nitrogen and carbon monoxide mixed with it. This impure hydrogen then passes into a gas-liquid contact column 201 through an inlet 203. The impure hydrogen passes upwardly through the column 201 and comes into contact with the liquid nitrogen passing through the column 201, having been introduced through an inlet 205. The impurities in the hydrogen enter the liquid phase.Substantially pure hydrogen leaves the top of the column 201 through the outlet 207, passes through the heat exchanger 202 countercurrently to the feed gas mixture, is compressed in a compressor 209 to the pressure of the synthesis gas leaving the column 208 through the outlet 214, and is mixed with such synthesis gas downstream of the heat exchanger 202. Impure liquid nitrogen collected at the bottom of the column 201 is passed out of an outler 211 into the column 220 through an inlet 213.

The liquid collecting at the bottom of the column 220 provides cooling for the feed gas mixture passing through the heat exchange coil 104. Liquid is passed out of the column 220 through an outlet 230 at the bottom thereof. The liquid is passed through an expansion valve 232, and then through an inlet 236 of a gas-liquid contact colun 234 which is fitted with a condenser 238 and a reboiler 240.

The fluid entering the column 234 is substantially free of hydrogen and thus consists essentially of nitrogen and carbon monoxide. In operation, the vapour at the top of the column 234 is condensed by condenser 238. The thus formed liquid acts as reflux and flows along the gas-liquid contact surfaces in the column, gradually descending the column, and comes into contact with vapour ascending the column, such vapour being formed in part by the reboiler 240 causing the liquid collecting at the bottom of the column to boil. As the liquid descends the column it becomes gradually richer in carbon monoxide, whereas the vapour reaching the top of the column 234 is substantially free of carbon monoxide. This vapour is thus essentially pure nitrogen. It passes out of the column 238 through an outlet 248 and is liquefied by being cooled in a heat exchanger 242.The resulting liquid nitrogen is then passed through a subcooler 244. A portion of the so-formed liquid nitrogen is taken by pump 246 and some of this portion is distributed to the inlet 21 2 of the column 208 thereby fulfilling the liquid nitrogen requirements of the column 208 while another part of the portion is passed to the inlet 205 of the column 201 thereby fulfilling the liquid nitrogen requirements of the column 201, and the rest of the portion is united with the gas mixture leaving the column 208 at a region intermediate the outlet 21 4 and the heat exchanger 206. By this means the ratio of hydrogen to nitrogen in the gas mixture leaving the column 108 can be adjusted to a value suitable for the synthesis of ammonia.

That portion of the liquid nitrogen not taken by the pump 246 is passed into storage through a conduit D.

The liquid collecting at the bottom of the column 234 consists essentially of nitrogen and carbon monoxide. The waste gas stream is typically formed from this liquid and used for the purposes of recovering power. Thus, liquid is discharged from the column 234 through an outlet 250 at its bottom. The liquid is then divided into major and minor streams. The minor stream is passed through an expansion valve 252 and then used to provided cooling for the heat exchangers 244 and 242 being passed therethrough countercurrently to the nitrogen stream from the outlet 248 of the column 234. The resulting mixture of carbon mnoxide and nitrogen vapour leaves the heat exchanger 242 and then passes through the heat exchanger 202 countercurrently to the feed gas mixture. After leaving the heat exchanger 202 the mixture of nitrogen and carbon monoxide is vented from the process.The major stream is passed through an expansion valve 254 and then through the heat exchanger 242 countercurrently to the nitrogen stream from the outlet 248 of the column 234, thereby providing cooling for the heat exchanger 242. After leaving the heat exchanger 242 the stream from the outlet 250 of the column 234 is passed through the heat exchanger 202 countercurrently to the feed gas mixture 102, thereby providing cooling for the heat exchanger 202. The gas after leaving the heat exchanger 202 is passed to the conduit C for power recovery and venting from the process.

The cooling for the condenser 238 and the heating for the reboiler 240 is provided by a working fluid, preferably nitrogen, which undergoes the following cycle of operation starting immediately downstream of the condenser 238. Nitrogen gas leaves the condenser 1 38 and passes through the heat exchanger 202, providing cooling therefor, countercurrently to the feed gas mixture. After leaving the heat exchanger 202 the working fluid is compressed by a compressor 256 and a so-formed compressed gas is returned through the heat exchanger 202, being cooled and reduced in temperature as it passes therethrough. A side stream is taken from the compressed nitrogen as it is flowing through the heat exchanger and passed through an expansion turbine 258. This cools the nitrogen and the resulting cold nitrogen is united with a part of the working fluid cycle intermediate the condenser 238 and the heat exchanger 202 so as to enable further cooling to be provided to the heat exchanger 102. The compressed nitrogen, after being cooled in the heat exchanger 202, is passed through the reboiler 240 in which it provides heat necessary to boil the liquid at the bottom of the column 234 and is itself liquefied. The resulting liquid nitrogen is pumped by pump 260 through an expansion valve 262 and the resulting cold nitrogen is passed through the condenser 238 thus completing the cycle of operations.

Claims

1. A method of recovering hydrogen from a feed gas mixture comprising carbon monoxide, hydrogen and nitrogen, the hydrogen being recovered with nitrogen in a mixture that is suitable for use in the synthesis of ammonia, which method comprises: (a) introducing the feed gas mixture at a temperature below ambient into a first gas-liquid contact region at elevated pressure in which the feed gas mixture is washed with liquid nitrogen to produce a purified gas mixture comprising a major portion of the hydrogen in the feed gas mixture and a liquid comprising nitrogen and carbon monoxide with hydrogen dissolved in it; (b) stripping hydrogen from the said liquid in a second gas-liquid contact region to leave a residual liquid substantially free of hydrogen, the pressure in the second region being lower than in the first region;; (c) purifying the stripped hydrogen, or compressing the stripped hydrogen, cooling it and returning it to the first region; (d) passing the residual liquid from the second region into a third gas-liquid contact region in which nitrogen substantially free of carbon monoxide is separated by rectification from the residual liquid, and (e) liquefying the nitrogen and returning at least a portion of the so-formed liquid nitrogen at the first region.

2. A method as claimed in claim 1, in which the pressure in the first region is at least 50 bars.

3. A method as claimed in claim 1 or claim 2, in which the pressure in the second region is about 30 bar (but below the critical pressure of the liquid leaving the column.)

4. A method as claimed in any one of the preceding claims, in which the pressure in the third region is less than 1 5 bar.

5. A method as claimed in any one of the preceding claims, in which all the liquid nitrogen passed into the first region to wash the feed gas mixture is obtained from the nitrogen produced in the third region.

6. A method as claimed in any one of the preceding claims, in which the hydrogen stripped from the residual liquid in the second region is united with the feed gas mixture after compression and cooling.

7. A method as claimed in any one of the preceding claims, in which liquid collected at the bottom of the third gas-liquid contact region is expanded by passage through an expansion valve and the resulting fluid is heat exchanged with a stream of gaseous nitrogen taken from the third region so as to liquefy the nitrogen.

8. A method as claimed in claim 7, in which at least some of the liquid nitrogen is pumped to the first region.

9. A method as claimed in any one of the preceding claims, in which the concentration of the carbon monoxide in the feed gas mixture is reduced by rectification upstream of the first gas-liquid contact region, the resulting liquid being united with residual liquid taken from the first region.

10. A method as claimed in claim 9, in which cooling for the rectification of the feed gas mixture is provided by a stream of coolant formed by expanding liquid collected at the bottom of the third region through an expansion valve.

11. A method as claimed in any one of the preceding claims, in which additional refrigeration is generated by expanding a part of the incoming feed gas mixture in a turbine.

1 2. A method as claimed in any one of the preceding claims in which the third region is in a rectification column fitted with a reboiler and a condenser.

1 3. A method as claimed in claim 12, in which a working fluid undergoes the following cycle of operations to provide cooling for the condenser and heating for the reboiler: (a) compression (b) passage through the boiler (c) expansion through an expansion valve,and (d) passage through the condenser.

14. A method as claimed in claim 13, in which the working fluid is sub-cooled downstream of the reboiler and upstream of the expansion valve by heat exchange with the stream of coolant formed by taking liquid from the bottom of the third region and expanding it through a valve.

1 5. A method as claimed in claim 1 3 or claim 14, in which a portion of the working fluid is taken from a location downstream of a compressor employed to compress the working fluid and upstream of the reboiler, is expanded in a turbine to provide a refrigerated stream, and after refrigeration has been extracted from the stream is returned to the inlet of the compressor.

1 6. A method as claimed in any one of the preceding claims, in which the feed gas mixture includes argon and/or methane, and the stripped hydrogen is obtained substantially free of argon and/or methane, substantially all the argon and/or methane being separated from the feed gas mixture as part of the liquid collected in the third region after separation of nitrogen from the liquid entering the third region.

1 7. A method as claimed in claim 1, in which the stripped hydrogen is purified in a further gas-liquid contact column in which it is contacted with liquid nitrogen.

1 8. A method of recovering hydrogen from a feed gas mixture comprising hydrogen, nitrogen and carbon monoxide substantially as herein described with reference to Fig. 1, 2, 3 or 4 of the accompanying drawings.

19. Apparatus for performing the method claimed in claim 1, comprising: (a) means for reducing the temperature of the feed gas mixture to below ambient; (b) a first gas-liquid contact column having an inlet for the feed gas mixture; and inlet for liquid nitrogen; and inlet for the purified gas mixture, and an outlet for the residual liquid; (c) a second gas-liquid contact column having an inlet for the residual liquid from the first column; an outlet for the hydrogen and an outlet for the residual liquid; (d) (i) means for purifying the said hydrogen, or (ii) a compressor for compressing the hydrogen, means for cooling the compressed hydrogen and means for returning it to the first column; (e) a third gas-liquid contact column having an inlet for the liquid from the second column, and an outlet for the nitrogen separated from the liquid in use of the apparatus; (f) means for liquefying the said nitrogen; (g) means for passing the so-formed liquid nitrogen into the first column.

20. Apparatus for recovering hydrogen from a feed gas mixture comprising carbon monoxide, hydrogen, and nitrogen, substantially as herein described with reference to Fig. 1, Fig. 2, Fig. 3 or Fig. 4 of the accompanying drawings.