This invention relates to the separation of a relatively condensable gas from a mixture of gases. Aspects of the invention relate specifically, but not exclusively to the separation of carbon dioxide from a mixture of gases. Examples of the invention relate to the recovery of carbon dioxide and hydrogen in a concentrated form from a synthesis gas stream comprising hydrogen and carbon dioxide thereby generating an enriched carbon dioxide stream that may be used in a chemical process, or used for enhanced hydrocarbon recovery or may be stored, and an enriched hydrogen stream that may be used as fuel or as a hydrogen feed to a chemical process.

US 3,614,872 relates to an autorefrigeration process for separating a shifted synthesis gas feedstream into an enriched carbon dioxide product stream and an enriched hydrogen product stream comprising the steps of:

(1) cooling the shifted synthesis gas feedstream stepwise at superatmospheric pressure by noncontact counter flow heat exchange in a plurality of separate cooling zones, and where in each separate cooling zone one or two streams of coolant of different compositions which are produced subsequently in the process is passed in heat exchange relationship with one stream of synthesis gas feed thereby cooling the synthesis gas feedstream to a temperature below the dew point at the pressure of the synthesis gas feedstream, and where in at least one of the separate cooling zones, the synthesis gas flows in split streams, each split stream of which is cooled by separate product streams of different compositions out of heat exchange with each other, and separating in a gas-liquid separation zone a liquefied enriched carbon dioxide product stream and a gaseous enriched hydrogen product stream;

(2) withdrawing at least a portion of said liquefied enriched carbon dioxide product stream from the separation zone in (1), expanding at substantially the temperature at which it is removed from separation zone and passing said expanded portion through at least one cooling zone in (a) as one of said stream of coolant at reduced pressure relative to said separation zone, and removing the enriched carbon dioxide product stream departing form (1) is gaseous phase at a temperature higher than that in said separation zone;

(3) simultaneously withdrawing at least a portion of said gaseous enriched hydrogen product stream from the separation zone of (a) and passing said portion as said other stream of coolant through at least one cooling zone in (a) which is separate and distinct from any cooling zone cooled in (2) by said first stream of coolant; and

(4) withdrawing the gaseous enriched hydrogen product stream from (3) at a temperature higher than the temperature in said separation zone.

According to the description of US 3,614,872, the dried feed gas enters the separating portion of the system at substantially initial line pressure, that is a pressure of about 1400 psig (96.5 barg). It is also said that at start-up, back pressure valve 20 is closed and the enriched hydrogen product stream from the top of separator 19 is reduced in pressure from 1400 psig to about 140 psig (9.65 barg) by being passed across an expansion valve. By expansion across the valve, the temperature of this gaseous stream is dropped to -78 ⁰ F without solid formation. The cooled enriched hydrogen product gas is then used as internal refrigerant in cooler 13. It is also said that higher refrigeration efficiencies are possible if the compressed hydrogen enriched product gas instead of being expanded at constant enthalpy through a valve, is expanded at constant entropy; that is, the gas is made to operate an expansion engine or move the rotor of a turbo-electric generator. However, after start-up, it may no longer be necessary to supply refrigerant to cooler 13 at a temperature of -78 ⁰ F. Accordingly, the hydrogen enriched product gas may by-pass the expansion valve and is introduced into the cooler 13 at a temperature of about -55 to -65 ⁰ F. This scheme is said to avoid the large pressure drop previously experienced across the expansion valve.

As described in International Patent Application Number WO2010/012981, it has now been found that by pressurising a gas stream in a compression system and then passing the compressed gas feed stream through a heat exchanger system for example in heat exchange relationship with a plurality of internal refrigerant streams that are produced subsequently in the process, that the high pressure gas stream that exits the heat exchanger system may be cooled to a temperature in the range of -15 to -55°C. It has also been found that the cooled high pressure (HP) gas that exits the heat exchanger system may be separated in a gas-liquid separator vessel thereby forming a HP hydrogen rich vapour stream and a HP liquid carbon dioxide stream with the separation achieving capture of the CO ₂ from the synthesis gas feed stream. It has also been found that the hydrogen rich vapour stream may be reduced in pressure to any desired pressure by passing the HP hydrogen enriched vapour stream through a turboexpansion system that comprises a plurality of turboexpanders arranged in series. In particular, the hydrogen rich vapour stream may be obtained at the desired fuel gas feed pressure for a combustor of a gas turbine of a power plant (for example, at a pressure of 30 barg). It has also been found that the expanded H ₂ rich vapour streams that exit each turboexpander of the series may be used as internal refrigerant streams thereby providing a major portion of the refrigeration duty for the heat exchanger system. Also, expansion of the H ₂ rich vapour in the turboexpanders may be used to drive a rotor or shaft of the compressor(s) of the compressor system or to drive the rotor or shaft of a turbo-electric generator thereby achieving a net power consumption for the separation of the gas stream into a hydrogen rich vapour stream and liquid CO ₂ stream of less than 30 MW, preferably, less than 25 MW when processing 28,000 kmol/hour of syngas containing 56 mol% hydrogen and 43 mol% CO ₂ .

According to an aspect of the invention there is provided a method of separating carbon dioxide (CO ₂ ) and hydrogen (H ₂ ) from a gas stream into an H ₂ -HCh gas stream and a liquid CO ₂ stream comprising the steps of:

(i) cooling and optionally compressing the gas stream so as to obtain a high pressure multiphase stream comprising a gaseous phase and a liquid phase;

(ii) splitting the high pressure multiphase stream into a plurality of high pressure multiphase substreams;

(iii) further cooling each of the high pressure multiphase substreams;

(iv) recombining the plurality of high pressure multiphase substreams to form a low temperature, high pressure multiphase fluid stream; and

(v) separating the low temperature, high pressure multiphase fluid stream into a first H ₂ -rich gas stream and a first liquid CO ₂ stream.

An aspect of the invention provides a method of separating carbon dioxide (CO ₂ ) and hydrogen (H ₂ ) from a synthesis gas stream into an H ₂ -rich gas stream and a liquid CO ₂ stream comprising the steps of:

(i) if necessary, compressing the synthesis gas stream to a pressure in the range of 150 to 400 barg and removing at least part of the heat of compression therefrom to provide a high pressure vapour stream;

(ii) cooling the high pressure vapour stream so as to obtain a high pressure multiphase stream comprising a gaseous phase and a liquid phase;

(iii) splitting the high pressure multiphase stream into a plurality of high pressure multiphase substreams;

(iv) further cooling each of the high pressure multiphase substreams to a temperature offrom -5°C to -40°C;

(v) recombining the plurality of high pressure multiphase substreams to form a low temperature, high pressure multiphase fluid stream; and

(vi) separating the low temperature, high pressure multiphase fluid stream into a first H ₂ -rich gas stream and a first liquid CO ₂ stream.

Typically, the synthesis gas stream may comprise less than 5 mol%, preferably less than 3 mol% of impurities, i.e. species other than H ₂ and CO ₂ .

The synthesis gas stream contains carbon oxides and hydrogen in addition to other species, for example impurities. Preferably the carbon oxides comprises mainly carbon dioxide. For example the synthesis gas may comprise a shifted synthesis gas, for example a gas stream which has been subject to a shift reaction, for example a water gas shift reaction to convert at least part of a carbon monoxide content of the gas stream to carbon dioxide.

Preferably the synthesis gas stream may comprise from 40 to 60 mol% H ₂ and from 40 to 60 mol% CO ₂ . The proportions of H ₂ and CO ₂ within the synthesis gas stream may be approximately equal. Alternatively, the proportion of H ₂ may be higher or lower than that of CO ₂ . For instance, the synthesis gas stream may comprise between 40 and 44 mol% CO ₂ and between 54 and 60 mol% H ₂ .

Preferably, the gas stream may be delivered at an initial temperature of from 2O ⁰ C to 6O ⁰ C, e.g. approximately 40 ⁰ C. Typically, the gas stream is produced at a pressure of up to 120 bar, preferably, up to 95 bar e.g. between 40 and 60 bar.

In some examples, for example as described above, the pressure of the gas stream is at least 40 bar.

In particular in examples where a pressurised hydrogen enriched gas is advantageous, for example for use as a feed to a turbine, preferably the pressure of the gas stream in the process is at least 60 bar, for example at least 80 bar or more. The pressure of the stream fed to a separator vessel of the system may be for example 125 bar or less, for example 110 bar or less, 100 bar or less, or 90 bar. In some examples, there will be a compression step required to compress the syngas, to increase the pressure.

Thus the process may include the step of, prior to the separation, and preferably prior to the cooling, where the gas is compressed using a compression system such that the gas is increased in pressure to a pressure in the range of 60bar to less than 450 bar.

Accordingly, in some examples the feed pressure of the gas stream is 60 to 400 bar, for example 150 to 400 bar. However, gasifier technology is progressing rapidly and it may become feasible to produce gas at a pressure of at least 150 barg. Accordingly, where the gas is obtained at a high pressures, there will be no compression of the synthesis gas feed stream. In other arrangements, the separation might be carried out at relatively low pressures, and even where the gas is initially at a pressure of less than 150 barg, 120 barg, 90 barg or even less than 80 barg there might be no additional compression step carried out.

In examples of the invention, preferably the pressure of the multi-phase mixture fed to the separator is greater than 60 barg. The pressure may be greater than 75 barg, 80 barg or greater than 90 barg.

Preferably, the gas stream is compressed to a pressure of from 150 to 200 bar, e.g. from 160 to 190 bar.

Preferably, the compression of the gas stream is carried out using a multistage compression system, e.g. a compression system comprising a plurality of compressors arranged in series.

Preferably, the heat of compression may be removed, at least partially, by heat exchange with an external coolant such as air or water. For instance, a heat exchanger may be provided after each of the compressors within a multistage compression system.

Preferably, the high pressure vapour stream is cooled by being passed through a channel of a multichannel heat exchanger in heat exchange relationship with a cold CO ₂ product stream and with a cold H ₂ -rich product stream that are passed through further channels in the multichannel heat exchanger thereby forming the high pressure multiphase stream wherein the cold CO ₂ product stream and the cold H ₂ -rich product stream are produced subsequently in the process of the present invention.

Preferably the method includes removing at least a part of the liquid phase from the high pressure multiphase stream, prior to the splitting of the high pressure multiphase stream into a plurality of substreams. In some cases, substantially all of the liquid phase is removed from the high pressure multiphase stream, prior to the splitting of high pressure multiphase stream into a plurality of substreams. In such cases, it will be understood that a small amount of liquid phase may remain in the stream which is subsequently split.

The removed liquid phase, for example recovered liquid CO _2, may then be sequestrated or employed as an internal refrigerant stream in at least one multichannel heat exchanger.

At least one of the high pressure multiphase substreams may be cooled in a multichannel heat exchanger by heat exchange with at least one internal coolant stream.

The internal coolant stream may for example comprise a relatively cold stream generated elsewhere in the system. For example, the coolant stream may be a cold CO ₂ product stream or a cold H ₂ -rich product stream. The coolant may include a component which changes phase effecting cooling or additional cooling. For example, the internal coolant may include a CO ₂ refrigerant stream, for example a liquid CO ₂ stream, and evaporation of liquid CO ₂ of the stream effects significant additional cooling.

At least a part of a liquid CO ₂ stream may be at subcritical pressure and at least partly evaporates through a heat exchanger to provide cooling. The liquid CO ₂ stream may for example be derived from a CO ₂ product stream and/or liquid phase removed from the multiphase stream.

Preferably, the high pressure multiphase stream is split into two or more substreams.

One or more of the plurality of substreams may be cooled by being passed through a heat exchanger system by heat exchange with one or more internal refrigerant streams that are produced subsequently in the process of the present invention. For example, a substream or a pair of substreams may be passed through a heat exchanger system comprising one further multichannel heat exchanger or a pair of parallel multichannel heat exchangers (i.e. second and third multichannel heat exchangers) and cooled against one or more internal refrigerant streams. Preferably, the internal refrigerant streams may comprise low temperature, high pressure liquid CO ₂ streams and low temperature H ₂ -rich gas streams.

The substreams split from the multiphase stream may include an additional substream which is not subject to the further cooling of the substreams, wherein the additional substream is recombined with the plurality of cooled high pressure multiphase substreams. Preferably the amount of flow through one or more of the substream paths is controllable. A control system may be provided that allows for the flow to pass through only some of the paths at a particular time. In this way, the amount of cooling can be controlled and can be changed for different applications and/or during the operation of the system.

In one example of the invention, a first substream is passed through the second multichannel heat exchanger and a second substream is passed through the third

multichannel heat exchanger, wherein the, or each, internal refrigerant stream in the second multichannel heat exchanger is a liquid CO ₂ stream and the or each internal refrigerant stream in the third multichannel heat exchanger is an H ₂ -rich gas stream.

Thus the liquid CO ₂ may be used as an indirect refrigerant within the system, for example exchanging heat with other process streams of the system.

The system may be adapted such that the CO ₂ is at subcritical pressure and at least partly evaporates at, or upstream of, the heat exchanger. By evaporating the carbon dioxide, additional cooling can be provided within the system. For example, the liquid CO ₂ may be flashed, for example across a valve at, or upstream of, a heat exchanger. This feature of using the liquid CO ₂ stream as an internal coolant may be provided as a part of any of the examples described herein and may be provided in relation to any one of the aspects herein. At least a part, or all, of the liquid CO ₂ stream, may be used as an internal coolant.

Preferably, at least one of the plurality of substreams may be cooled using an external refrigeration system by heat exchange with an external refrigerant. The external refrigerant may comprise any suitable refrigerant. However, propane may be especially preferred, since it is able to cool the substream(s) to the desired temperature, for example from -5°C to -40 ⁰ C, and may be relatively inexpensive.

After cooling to the desired temperature of for example -5 to -40 ⁰ C, the plurality of substreams are recombined to generate the low temperature, high pressure multiphase stream.

The separation of the low temperature, high pressure multiphase stream may be carried out in a gas-liquid separator vessel, whereby the first Hj-rich gas stream may be withdrawn from at or near the top of the vessel and the first liquid CO ₂ stream may be withdrawn from at or near the bottom of the vessel. Preferably, the first liquid CO ₂ stream may comprise at least 95 mol%, more preferably at least 97 mol%, CO ₂ .

Preferably, the first H ₂ -rich gas stream may comprise at least 80 mol%, more preferably at least 84 mol%, H ₂ .

The pressure of the first liquid CO ₂ stream may be reduced to a desired export or pipeline delivery pressure. Typically, the desired export or pipeline delivery pressure may be between 100 and 170 bar, e.g. between 120 and 160 bar. The pressure of the first liquid CO2 stream may be reduced across a valve. Alternatively, the pressure of the first liquid CO ₂ stream may be reduced using a hydro turbine thereby recovering at least a portion of the energy of compression expended in step (i).

Typically, reducing the pressure of the liquid CO ₂ stream to the desired export or pipeline delivery pressure may result in the formation of a multiphase fluid stream comprising a liquid phase and a gaseous phase. Hence, the method may comprise the step of separating the multiphase fluid stream into a second H ₂ -HCh gas stream and a second liquid CO ₂ stream.

The second liquid CO ₂ stream may be employed as an internal refrigerant stream in at least one multichannel heat exchanger. In particular, the second liquid CO ₂ stream may be employed to cool the first substream in the second multichannel heat exchanger.

Having served as an internal refrigerant stream in the second multichannel heat exchanger, a cold CO ₂ product stream may be formed. This cold CO ₂ product stream may be used as a coolant for the high pressure vapour stream in the first multichannel heat exchanger. The CO ₂ product stream that exits the first multichannel heat exchanger may be a liquid or a supercritical fluid. For example, the CO ₂ product stream may be a supercritical fluid having a temperature of 31°C to 40 ⁰ C and a pressure of from 130 to 160 barg.

Preferably, the CO ₂ product stream may comprise at least 97 mol% CO ₂ .

The second H ₂ -rich gas stream may be further cooled, e.g. against one or more internal refrigerant streams in a fourth multichannel heat exchanger, to a temperature of from -41 °C to -6O ⁰ C, e.g. from -45°C to -55°C, thereby forming a multiphase fluid stream comprising a liquid phase and a gaseous phase.

The multiphase fluid stream may be separated, e.g. in a gas-liquid separator vessel, into a third H ₂ -rich gas stream and a third liquid CO ₂ stream. The third liquid CO ₂ stream may be used as internal refrigerant to cool the first substream in the second multichannel heat exchanger before being combined with the first liquid CO ₂ stream.

The third H ₂ -rich gas stream may be passed as internal refrigerant through at least one channel, preferably, sequentially through at least 2 channels, preferably 2 or 3 channels of the fourth multichannel heat exchanger in heat exchange relationship with the second H ₂ -rich gas stream. Preferably, after being passed through a channel of the multichannel heat exchanger, the third H ₂ -rich gas stream is subjected to isentropic expansion to a lower pressure thereby reducing its temperature before it is passed through a further channel of the fourth multichannel heat exchanger. This step of isentropic expansion of the third H ₂ - rich gas stream before passing the expanded stream through a channel of the fourth multichannel heat exchanger may be repeated one or more times. However, the third H ₂ - rich gas stream is preferably removed from the fourth multichannel at medium pressure, for example, a pressure of 170 to 175 bar.

Where reference is made herein to a channel of a heat exchanger, it will be understood that each "channel" may comprise a plurality of sub-channels by means of which the relevant stream passes through the heat exchanger. In some types of heat exchanger, each channel may be formed of hundreds or thousands of subchannels or paths through the heat exchanger. Such sub-channels or paths may in some cases be interleaved with the sub-channels or paths of a different channel carrying a different stream, or may be arranged separate to, but able to exchange heat with, other channels.

Preferably, the third H ₂ -rich gas stream that is removed from the fourth multichannel heat exchanger is combined with the second H ₂ -rich gas stream and the combined H ₂ -HCh gas stream may be passed as an internal refrigerant stream through at least one channel, preferably, sequentially through at least 2 channels, preferably, 2 or 3 channels of the third multichannel heat exchanger thereby cooling the second substream. Preferably, after being passed through a channel of the third multichannel heat exchanger, the combined H ₂ -rich gas stream is subjected to isentropic expansion to a lower pressure thereby reducing its temperature before it is passed through a further channel of the fourth multichannel heat exchanger. This step of isentropic expansion of the combined H ₂ -HcIi gas stream before passing the expanded stream through a channel of the fourth multichannel heat exchanger may be repeated one or more times. However, it is preferred that the combined H ₂ -rich gas stream that exits the fourth multichannel heat exchanger is at a pressure above the fuel gas feed pressure for the combustor of a gas turbine.

Isentropic expansion of the third H ₂ -rich vapour stream and of the combined H ₂ -rich vapour stream may be carried out in a turboexpander that generates motive power for driving a machine and/or driving an alternator of an electric generator. The machine that is driven by the turboexpander is preferably a compressor of the compression system and/or a pump (for example, for pumping liquid CO ₂ or supercritical CO ₂ ). Where the

turboexpander is used to drive an alternator of an electric generator, the electricity is preferably used to power one or more components of the system used for separating CO ₂ and H ₂ from the gas stream.

A cold H ₂ -rich gas stream is removed from the third multichannel heat exchanger and is passed through a channel of the first multichannel heat exchanger as described above. The H ₂ rich gas stream that exits the first multichannel heat exchanger may then be passed as fuel gas to the combustor of a gas turbine.

The CO ₂ product stream that exits the first multichannel heat exchanger may be exported from the process and sequestered and/or used in a chemical process. Where the CO ₂ product stream is sequestered, it is typically delivered to a pipeline that transfers the CO ₂ product stream as a liquid or supercritical fluid to a reception facility of an oil field where the CO ₂ product stream may be used as an injection fluid for an oil reservoir. If necessary, the CO ₂ product stream is pumped to above the pressure of the oil reservoir before being injected down an injection well and into the oil reservoir. The injected CO ₂ displaces the hydrocarbons contained in the reservoir rock towards a production well for enhanced recovery of hydrocarbons therefrom. If any carbon dioxide is produced from the production well together with the hydrocarbons, the carbon dioxide may be separated from the hydrocarbons for re-injection into the oil reservoir. It is also envisaged that the CO ₂ product stream may be sequestered by being injected into an aquifer or a depleted oil or gas reservoir for storage therein.

In some examples the gas mixture, for example a high pressure synthesis gas, which may have been partially or totally subjected to internal cooling may also be passed partially (with subsequent recombination of the feeds) or totally to a heat exchanger that employs an external refrigerant.

In an example, the gas mixture, for example synthesis gas, may be first cooled. The gas stream may then be separated into for example three streams. In an example, two streams may be subjected to internal refrigeration and another to external refrigeration. The cooled streams may then be recombined before further separation of for example carbon dioxide.

It will be understood that aspects of the invention relate broadly to the separation of carbon dioxide from a gas mixture including carbon dioxide. Indeed aspects of the invention can be applied to methods for separating a relatively incondensable gas fro ma mixture of gases. Thus a broad aspect of the invention provides a method of separating a relatively condensable gas from a gas mixture, the method comprising the steps of:

compressing and/or cooling the gas mixture to form a multi-phase mixture stream that includes a liquid comprising condensed condensable gas, splitting the stream after the compressing and/or cooling step to form a plurality of substreams; cooling at least one of the substreams and recombining a plurality of the substreams, and separating a liquid stream including condensed condensable gas from the recombined stream.

The method may further include the step of removing at least a part of the liquid from the multi-phase mixture prior to the splitting step.

A further aspect of the invention provides a method of separating CO ₂ from a gas mixture comprising CO ₂ , the method comprising the steps of:

(i) compressing and/or cooling the gas mixture to form a multi-phase mixture stream that includes liquid CO ₂

(ii) splitting the stream after the compressing and/or cooling step to form a plurality of substreams;

(iii) cooling at least one of the substreams;

(iv) recombining a plurality of substreams; and

(v) separating a first liquid CO ₂ stream from the recombined stream.

Thus this and other aspects of the invention may be applicable to applications where the feed gas is other than synthesis gas.

The recombined stream may be a multiphase stream on recombination and/or may be subject to further processing for example cooling and/or compression to form a multiphase stream prior to separation.

Preferably the method includes removing at least a part or substantially all of the liquid, for example liquid CO2, from the multi-phase mixture stream, prior to the splitting step. The pressure of the multi-phase mixture stream at the splitting step may be at least 60 bar, for example at least 80 bar or at least 120 bar.

At least one of the substreams is preferably cooled in a multichannel heat exchanger.

At least one of the substreams may be cooled by heat exchange with at least one internal coolant. The internal coolant may be selected from a cold CO ₂ product stream and a cold H ₂ -rich product stream.

At least a part of a liquid CO ₂ stream may at least partly evaporate through a heat exchanger to provide cooling.

At least one of the substreams may be cooled by heat exchange with an external refrigerant stream.

The substreams split from the multiphase stream may include an additional substream which is not subject to cooling, wherein the additional uncooled substream is recombined with at least one cooled substream.

The separation of the CO2 liquid from the recombined stream may leave a gas stream, wherein the gas stream is expanded in an expander to generate motive power.

The gas stream may be warmed against an internal stream for example a gas mixture stream and/or a substream.

The invention further provides apparatus for use in carrying out a method described herein.

Another aspect of the invention provides a system for separating CO ₂ and H ₂ from a gas stream into an H ₂ -HCh gas stream and a liquid CO ₂ stream comprising:

a compression system for compressing the gas stream to a pressure of from 60 to 400 barg, for example from 80 barg or 150 barg to 400 barg, and cooling means for removing at least part of the heat of compression, to provide a high pressure vapour stream;

a first multichannel heat exchanger for cooling the high pressure vapour stream against one or more internal refrigerant streams so as to obtain a high pressure multiphase stream comprising a gaseous phase and a liquid phase;

a flow splitting means for splitting the high pressure multiphase stream into a plurality of substreams;

a cooling means for reducing the temperature of each of the substreams in parallel to a temperature of from -5°C to -40 ⁰ C, wherein the cooling means comprises an external refrigeration circuit and a second multichannel heat exchanger; a flow recombination means for recombining the plurality of substreams to form a low temperature, high pressure multiphase fluid stream; and

a first separation means for separating the low temperature, high pressure multiphase fluid stream into a first H ₂ -rich gas stream and a first liquid CO ₂ stream.

Although the present invention has been described in relation to the separation of a hydrogen gas stream and a liquid carbon dioxide stream from a gas feed stream, it is also envisaged that a similar process may be used to separate a first hydrogen gas stream and a first liquid hydrogen sulfide stream from a feed stream comprising hydrogen and hydrogen sulfide (where carbon dioxide is either not a component of the feed stream or is present in the feed stream as an impurity). For example, where an acid gas removal plant (AGR) is used to separate hydrogen sulfide from a hydrogen rich gas stream, the resulting hydrogen enriched stream typically comprises in excess of 30% by volume, preferably, in excess of 40% by volume of hydrogen sulfide (the remainder comprising predominantly hydrogen). This stream could therefore be separated using the process of the present invention into a first H ₂ -rich gas stream and a first liquid H ₂ S stream.

Thus a broad aspect of the invention provides a method of separating a relatively condensable gas from a gas mixture additionally comprising relatively incondensable gas, the method comprising the steps of:

(i) compressing and cooling the gas mixture to form a multi-phase mixture stream that includes a liquid phase;

(ii) splitting the stream after the compressing and/or cooling step to form a plurality of substreams;

(iii) further cooling at least one of the substreams;

(iv) recombining a plurality of substreams; and

(v) separating a first liquid stream from the recombined stream.

The invention extends to methods and/or apparatus substantially as herein described optionally with reference to the accompanying drawings.

Any feature in one aspect of the invention may be applied to other aspects of the invention, in any appropriate combination. In particular, features of method aspects may be applied to apparatus aspects, and vice versa.

In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawing, in which: Figure 1 shows a detailed process flow diagram for a first embodiment of the process and CO ₂ condensation plant of the present invention; and

Referring to Figure 1, a dry synthesis gas feed gas stream 1 is fed at a pressure of 57 bar and a temperature of 4O ⁰ C to a first compressor 2 of a compression system. The synthesis gas feed stream 1 is fed at a rate of 28121 kmol/hour. The synthesis gas feed stream 1 contains approximately 56 mol% H ₂ , 43 mol% CO ₂ , 1 mol% CO and trace amounts of CH4, Ar and N ₂ . The feed stream may be free of hydrogen sulfide or more contain hydrogen sulfide as an impurity, in which case, the hydrogen sulfide will condense out of the synthesis gas feed stream together with the CO ₂ .

The compression system further comprises a second compressor 6, a third

compressor 10 and a fourth compressor 14, the four compressors 2, 6, 10, 14 being arranged in series.

A synthesis gas stream 3 exits the first compressor 2 at a pressure of 76 bar and a temperature of 68.3°C, the increase in temperature arising from heat of compression. In order to remove substantially all of the heat of compression, the stream 3 is then cooled against an external coolant in a first heat exchanger 4, while keeping the pressure drop across the heat exchanger 4 to a minimum. Accordingly, a gas stream 5 exits the first heat exchanger at a pressure of 75 bar and a temperature of 40 ⁰ C. Conveniently, the coolant may be air or water.

The cycle of compression followed by heat exchange with an external coolant to remove the heat of compression is repeated three times. Gas stream 5 is fed to the second compressor 6. A gas stream 7 exits the second compressor 6 and is fed to a second heat exchanger 8. A gas stream 9 exits the second heat exchanger 8 and is fed to the third compressor 10. A gas stream 11 exits the third compressor 10 and is fed to a third heat exchanger 12. A gas stream 13 exits the third heat exchanger 12 and is fed to the fourth compressor 14. A gas stream 15 exits the fourth compressor 14 at a pressure of 175 bar and a temperature of 69.5°C and is fed to a fourth heat exchanger 16.

A first high pressure gas stream 17 exits the fourth heat exchanger at a pressure of 174 bar and a temperature of 4O ⁰ C and is employed as a feed stream to a first multichannel heat exchanger 18.

The first multichannel heat exchanger 18 has three channels therethrough: a first channel; a second channel; and a third channel. The first high pressure gas stream 17 is fed to the first channel of the first multichannel heat exchanger 18. Internal refrigerant streams pass through the second and third channels of the multichannel heat exchanger 18 in counter-current direction to the fluid flowing through the first channel, in the form of a high pressure CO ₂ stream 67 and an H ₂ -rich fuel gas stream 48 respectively.

Thus, a first multiphase fluid stream 19, having a vapour fraction of 97.2 mol%, exits the first channel of the first multichannel heat exchanger 18 at a pressure of 173.5 bar and a temperature of 9.8°C.

The first multiphase fluid stream 19 is fed to a first manifold 20, where in this example it is split into three substreams: a first stream 21 comprises around 27 mol% of stream 19; a second substream 50 comprises around 31 mol% of stream 19; and a third stream 52 comprises around 42 mol% of stream 19. Although not shown here, a preferred example of the invention is where the first multiphase fluid stream 19 is split into only two substreams.

The first substream 21 is fed to a second multichannel heat exchanger 22.

The second multichannel heat exchanger 22 has three channels therethrough: a first channel; a second channel; and a third channel.

The first substream 21 is fed to the first channel of the second multichannel heat exchanger 22. Internal refrigerant streams pass through the second and third channels of the second multichannel heat exchanger 22 in counter-current direction to the fluid flowing through the first channel, the internal refrigerant streams comprising CO ₂ liquid streams.

Thus, a first cooled substream 23 exits the first channel of the second multichannel heat exchanger 22 at a temperature of around -27°C and is fed to a second manifold 24.

The second substream 50 is fed to a third multichannel heat exchanger 41.

The third multichannel heat exchanger 41 has four channels therethrough: a first channel; a second channel; a third channel; and a fourth channel.

The second substream 50 is fed to the first channel of the third multichannel heat exchanger 41. Internal refrigerant streams pass through the second, third and fourth channels of the third multichannel heat exchanger 41 in counter-current direction to the fluid flowing through the first channel, the internal refrigerant streams comprising H ₂ -rich gas streams.

Thus, a second cooled substream 51 exits the first channel of the third multichannel heat exchanger 41 at a temperature of around -27°C and is fed to the second manifold 24. The third substream 52 is cooled by a refrigeration system comprising two propane kettle heat exchangers 53, 55 arranged in series. Thus, the third substream 52 is passed to the first propane kettle 53 and is cooled by heat exchange with an external refrigerant, propane. A stream 54 that exits the first propane kettle 53 is then fed to the second propane kettle 55, where it is cooled by heat exchange with an external refrigerant, propane.

A third cooled substream 56 exits the second propane kettle 55 at a temperature of around -27°C and is fed to the second manifold 24.

The first, second and third cooled substreams 23, 51 , 56 recombine at the second manifold 24 to form a recombined low temperature multiphase stream 25. The recombined low temperature multiphase stream 25 comprises a liquid phase and a gaseous phase and has a vapour fraction of 64.6 mol%.

The recombined low temperature multiphase stream 25 is fed at a pressure of 172.5 bar and a temperature of -27°C to a first gas-liquid separator vessel 26. A first H ₂ -rich gas stream 27 is withdrawn from the top of the first gas-liquid separator vessel 26, while a first CO ₂ liquid stream 57 is withdrawn from the bottom of the first gas-liquid separator vessel 26.

The first CO ₂ liquid stream 57 comprises 97 mol% CO ₂ , 2.5 mol% H ₂ and trace amounts of CO, CH4, Ar and N ₂ . The first CO ₂ liquid stream 57 may be of sufficient purity for export purposes.

This first CO ₂ liquid stream 57 is combined at a third manifold 58 with a CO ₂ liquid stream 70 that exits the second channel of the second multichannel heat exchanger 22 at a temperature of -30 ⁰ C (discussed below), thereby forming a combined CO ₂ liquid stream 59.

The combined CO ₂ liquid stream 59 comprises 97.1 mol% CO ₂ , 2.3 mol% H ₂ and small amounts of CO, CH ₄ , Ar and N ₂ .

The pressure of the combined CO ₂ liquid stream 59 is reduced to pipeline delivery pressure of 150 bar by flashing it across a first valve 60. A combined multiphase CO ₂ stream 61 comprising a liquid phase and a gaseous phase and having a vapour fraction of 0.2 mol% is fed from the first valve 60 to a second gas-liquid separation vessel 62 at a pressure of 150 bar and a temperature of -27°C. A second H ₂ -HCh gas stream 63 is withdrawn from the top of the second gas-liquid separator vessel 62, while a second CO ₂ liquid stream 66 is withdrawn from the bottom of the second gas-liquid separator vessel 62.

The second H ₂ -rich gas stream 63 comprises 82.9 mol% H ₂ , 14.5 mol% CO ₂ , 1.9 mol% CO and small amounts of N ₂ , Ar and CH ₄ .

This second H ₂ -rich gas stream 63 is passed through a second valve 64 to reduce its pressure. Accordingly, an H ₂ -rich gas stream 65 is fed at a pressure of 74 bar and a temperature of -27 ⁰ C from the second valve 64 to a fourth manifold 39 where it combines with a similar pressure H ₂ -rich gas stream that exits the fourth multichannel heat exchanger 28 (see below).

The second CO ₂ liquid stream 66 comprises 97.3 mol% CO ₂ , 2.2 mol% H ₂ and trace amounts of CO, CH ₄ , Ar and N ₂ . The second CO ₂ liquid stream 66 may be of sufficient purity for export purposes.

The second CO ₂ liquid stream 66 has a molar flow of 11354 kmol/hour and is fed at a pressure of 150 bar and a temperature of -27°C to the third channel of the second multichannel heat exchanger 22, where it serves as an internal refrigerant stream.

A CO ₂ liquid stream 67 exits the third channel of the second multichannel heat exchanger 22 at a temperature of 5 ⁰ C and is then fed to the second channel of the first multichannel heat exchanger 18, where it serves to cool the high pressure gas stream 17.

A CO ₂ supercritical fluid stream 68 (which may be a supercritical, dense phase stream) having a molar flow of 11354 kmol/hour exits the second channel of the first multichannel heat exchanger 18 at a temperature of 27°C and a pressure of 149 bar. The CO ₂ supercritical fluid stream 68 may subsequently be delivered, e.g. by pipeline, to another location, e.g. for storage, sequestration or use in enhanced oil recovery.

The first H ₂ -rich gas stream that is removed from the top of the first separator vessel

27 comprises 84.7 mol% H ₂ , 13.0 mol% CO ₂ , 4.9 mol% N ₂ , 1.7 mol% CO, 1.1 mol% Ar and a trace amount of CH ₄ .

The first H ₂ -rich gas stream 27 has a molar flow of 18172 kmol/hour and is fed to the fourth multichannel heat exchanger 28.

The fourth multichannel heat exchanger 28 has four channels therethrough: a first channel; a second channel; a third channel; and a fourth channel.

The first H ₂ -rich gas stream 27 is fed to the first channel of the fourth multichannel heat exchanger 28. Internal refrigerant streams pass through the second, third and fourth channels of the fourth multichannel heat exchanger 28 in counter-current direction to the fluid flowing through the first channel, the internal refrigerant streams comprising low temperature H ₂ -rich gas streams.

A low temperature H ₂ -rich multiphase fluid stream 29 comprising a gaseous phase and a liquid phase and having a vapour fraction of 92.2 mol% exits the first channel of the fourth multichannel heat exchanger 28 at a temperature of around -49 ⁰ C. The low temperature H ₂ -HcIi multiphase fluid stream 29 is then fed to a third gas-liquid separator vessel 30. A third H ₂ -rich gas stream 31 is withdrawn from the top of the third gas-liquid separator vessel 30, while a third CO ₂ liquid stream 69 is withdrawn from the bottom of the third gas-liquid separator vessel 30.

The third H ₂ -rich gas stream 31 comprises 91.8 mol% H ₂ , 5.7 mol% CO ₂ and small amounts OfN ₂ , CO, Ar and CH ₄ .

The third CO ₂ liquid stream 69 comprises 98.1 mol% CO ₂ , 1.4 mol% H ₂ and trace amounts of CO, CH ₄ , Ar and N ₂ . The third CO ₂ liquid stream 69 may be of sufficient purity for export purposes.

The third CO ₂ liquid stream 69 has a molar flow of 1427 kmo I/hour and is fed at a pressure of 172 bar and a temperature of -49°C to the second channel of the second multichannel heat exchanger 22, where it serves as an internal refrigerant stream for the first substream 21. Stream 70 that exits the second channel of the multichannel heat exchanger is passed to manifold 58 where it is combined with the first liquid CO2 stream 57 thereby forming combined liquid CO2 stream 59 (see above).

The third H ₂ -rich gas stream 31 that is withdrawn from the third gas-liquid separator vessel 30 is passed to the second channel of the fourth multichannel heat exchanger 28, where it serves as an internal refrigerant stream.

An H ₂ -rich gas stream 32 then exits the second channel of the fourth multichannel heat exchanger 28 at a pressure of 171.5 bar and a temperature of -32°C and is fed to a first turboexpander 33, in which its pressure is reduced, the work of the expanding gas being used to drive a compressor of the compression system, a pump or an alternator of an electric generator.

An H ₂ -rich gas stream 34 exits the first turboexpander 33 at a pressure of 112 bar and a temperature of -54 ⁰ C and is fed to the third channel of the fourth multichannel heat exchanger 28, where it serves as an internal refrigerant stream.

An H2-rich gas stream 35 then exits the third channel of the fourth multichannel heat exchanger 28 at a temperature of -32°C and is fed to a second turboexpander 36, in which its pressure is reduced, the work of the expanding gas being used to drive a compressor of the compression system, a pump or an alternator of an electric generator.

An H ₂ -HCh gas stream 37 exits the second turboexpander 36 at a pressure of 75 bar and a temperature of -53°C and is fed to the fourth channel of the fourth multichannel heat exchanger 28, where it serves as an internal refrigerant stream.

An H ₂ -rich gas stream 38 then exits the fourth channel of the fourth multichannel heat exchanger 28 at a pressure of 74 bar and a temperature of -30 ⁰ C and is fed to the fourth manifold 39, where it combines with the second tb-rich gas stream 65 to form a combined H ₂ -HCh gas stream 40, which comprises 91.8 mol% H ₂ , 5.7 mol% CO ₂ , and small amounts of CO, N ₂ , Ar and CH ₄ .

The combined H ₂ -rich gas stream 40 is fed to the second channel of the third heat exchanger 41 , where it serves as an internal refrigerant stream for the second substream 50.

An H ₂ -rich gas stream 42 exits the second channel of the third heat exchanger 41 at a temperature of -4 ⁰ C and is fed to a third turboexpander 43, in which its pressure is reduced, the work of the expanding gas being used to drive a compressor of the compression system, a pump or an alternator of an electric generator.

An H ₂ -rich gas stream 44 exits the third turboexpander 43 at a pressure of 48 bar and a temperature of -30 ⁰ C and is fed to the third channel of the third multichannel heat exchanger 41, where it serves as an internal refrigerant stream.

An H ₂ -rich gas stream 45 exits the third channel of the third heat exchanger 41 at a temperature of -4 ⁰ C and is fed to a fourth turboexpander 46, in which its pressure is reduced, the work of the expanding gas being used to drive a compressor of the

compression system, a pump or an alternator of an electric generator.

An H ₂ -rich gas stream 47 exits the fourth turboexpander 46 at a pressure of 31 bar and a temperature of -29°C and is fed to the fourth channel of the third multichannel heat exchanger 41, where it serves as an internal refrigerant stream.

An H ₂ -rich gas stream 48 exits the fourth channel of the third multichannel heat exchanger 41 at a temperature of 3.5 ⁰ C and is fed to the third channel of the first multichannel heat exchanger 18, where it serves as a coolant for the high pressure stream 17.

A final H ₂ -rich gas stream 49 exits the fourth channel of the first multichannel heat exchanger 18 at a temperature of pressure of 30 bar and a temperature of 27°C. The final Hrrich gas stream 49 is suitable for use as an H ₂ -HCh fuel gas.

The refrigeration system that cools the third multiphase substream uses propane as a refrigerant. The system comprises a series of three compressors consisting of a low pressure compressor 78, a medium pressure compressor 79 and a high pressure compressor 80. The high pressure compressor 80 is in fluid communication with a series of two air cooled desuperheaters 71 , 72 for removing a portion of the heat of compression. A liquid propane stream exits the second desuperheater 72 at a pressure of 13.9 bar and a temperature of 4O ⁰ C.

The liquid propane stream is passed through a first valve 73, thereby reducing the pressure of the stream to 7.1 bar and the temperature to 13.7°C and producing a multiphase stream comprising a gaseous phase and a liquid phase having a vapour fraction of 21.5 mol%.

The multiphase stream is fed to a first gas-liquid separator 74 of the refrigeration system. A gas stream is withdrawn from the top of the separator 74 and is then routed via a second valve 81 to a point in the circuit where it is reunited with the fluid flowing between the medium pressure compressor 79 and the high pressure compressor 80.

A liquid stream is withdrawn from the bottom of the separator 74 and is then fed through a third valve 75, thereby reducing the pressure of the stream to 3.4 bar and the temperature to -10.1 °C and producing a multiphase stream comprising a liquid phase and a gaseous phase and having a vapour fraction of 15.9 mol%. This multiphase stream is then fed to the first propane refrigeration kettle 53 to serve as a refrigerant stream. A multiphase propane stream having a vapour fraction of 60.1 mol% exits the first propane refrigeration kettle 53 and is fed to a second gas-liquid separator 76 of the refrigeration circuit.

A gas stream is withdrawn from the top of the second separator 76 and is then routed via a fourth valve 82 to a point in the circuit where it is reunited with the fluid flowing between the low pressure compressor 78 and the medium pressure compressor 79.

A liquid stream is withdrawn from the bottom of the second separator 76 and is then fed through a fifth valve 77, thereby reducing the pressure of the stream to 1.7 bar and the temperature to -30 ⁰ C and producing a multiphase stream comprising a liquid phase and a gaseous phase and having a vapour fraction of 1 1.6 mol%. This multiphase stream is then fed to the second propane refrigeration kettle 55 to serve as a refrigerant stream. A gaseous propane stream exits the first propane refrigeration kettle 53 and is fed to a second gas-liquid separator 76 and is fed to the low pressure compressor 78.

The low pressure compressor 78 boosts the pressure of the gaseous propane stream from 1.7 bar to 3.3 bar. The medium pressure compressor 79 boosts the pressure of the gaseous propane stream from 3.3 bar to 7.0 bar. The high pressure compressor 80 boosts the pressure of the gaseous propane stream from 7.0 bar to 14.7 bar.

The process described above and shown in Figure 1 recovers 98.4% of the H ₂ from the initial syngas stream 1 in the H ₂ rich fuel gas stream 49. It also provides a carbon capture level of 89.7%, 92.0% if only CO ₂ is considered.

It will be understood that the present invention has been described above purely by way of example, and modification of detail can be made within the scope of the invention.

Each feature disclosed in the description, and (where appropriate) the claims and drawings may be provided independently or in any appropriate combination.