WO2012114119A1

WO2012114119A1 - Process and apparatus for purification of carbon dioxide

Info

Publication number: WO2012114119A1
Application number: PCT/GB2012/050422
Authority: WO
Inventors: Caroline CORDEN; Timothy David EASTWOOD; Adrian Joseph Finn
Original assignee: Costain Oil, Gas & Process Limited
Priority date: 2011-02-25
Filing date: 2012-02-24
Publication date: 2012-08-30
Also published as: GB2489396B; GB2489396A; GB201103285D0

Abstract

A process for the low temperature purification of CO₂, wherein CO₂ is separated from a gaseous feed stream comprising at least 30 mol% CO₂ and at least one other gas having a lower boiling point than CO₂, the process comprising the steps of: (i) cooling and partially condensing the feed stream; (ii) passing the cooled and partially condensed feed stream from step (i) to a vapour-liquid separator to produce a vapour stream having reduced CO₂ content relative the feed stream and a liquid stream having increased CO₂ content relative to the feed stream; (iii) expanding and heating at least a portion of the liquid stream from step (ii); (iv) cooling and partially condensing at least a portion of the vapour stream from step (ii); (v) passing the cooled and partially condensed stream from step (iv) to a vapour-liquid separator to provide a vapour stream having reduced CO₂ content relative to the vapour stream from step (ii), and a liquid stream having increased CO₂content relative to the vapour stream from step (ii); (vi) expanding at least a portion of the liquid stream from step (v); and (vii) passing the expanded liquid stream from step (vi) to a vapour-liquid separator produce a vapour stream having reduced CO₂ content relative to the liquid stream from step (v), and a liquid stream having increased CO₂ content relative to the liquid stream from step (v); wherein cooling in step (i) is provided at least in part by heat exchange during heating of the liquid stream in step (iii).

Description

Process and Apparatus for Purification of Carbon Dioxide

This invention relates to processes and apparatus for the low temperature purification of carbon dioxide from a gaseous mixture containing carbon dioxide and one or more other gaseous contaminants.

The emission of carbon dioxide to the atmosphere from the combustion of fossil fuels is widely regarded as a significant contributor to global climate change. While a number of alternative "carbon-neutral" energy sources have been proposed, there are currently insufficient viable alternatives to the combustion of fossil fuels to meet global energy demands. There is therefore a need for technologies which are able to mitigate the environmental effects from the use of fossil fuels and from large-scale industrial processes such as steel and cement manufacture. "Carbon dioxide Capture and Storage" (CCS) is a technology which has been proposed to reduce carbon dioxide emissions from industrial plants, such as power stations, cement production plants and oil refineries. CCS involves the capture of carbon dioxide at source, transportation of the carbon dioxide to an injection site and sequestration of the carbon dioxide for long-term storage in suitable geological formations. In particular, captured carbon dioxide may be used in enhanced oil recovery techniques (EOR). One approach used in EOR involves the injection of gases into oil-bearing geological formations such that increased pressure of gas displaces oil deposits for recovery. Non- combustible gases are required for EOR purposes, since combustible gases (such as air) can cause the oil to ignite. Once the oil has been displaced from the reservoir, the carbon dioxide can be trapped in the depleted reservoir for long-term storage.

A conventional technique for carbon capture is "post-combustion capture". This involves the separation of carbon dioxide from flue gases prior to their emission to the atmosphere, and widely used techniques for post-combustion capture of carbon dioxide from power plants involve the use of amine scrubbers. Post-combustion capture technologies are an attractive solution in many cases since the necessary apparatus can readily be retrofitted at the effluent end of existing combustion apparatus. A potential disadvantage of conventional post-combustion capture processes, is that the concentration of carbon dioxide in the flue gas is relatively low (generally around 10 to 20% on a dry basis). Since extraction of CO₂ from streams containing high C0₂ content is easier than from those with lower C0₂ content, pre-combustion capture and oxy-fuel combustion processes have been proposed as alternatives to conventional post- combustion capture processes.

Pre-combustion capture involves the decarbonisation of carbon fuels with oxygen, or by steam reforming to form a mixture of hydrogen, carbon monoxide and water which is converted via a catalytic shift reaction to a mixture of carbon dioxide and hydrogen gas. Subsequent combustion of the separated hydrogen gas produces only water as a byproduct. However, there are a number of downsides to the use of pre-combustion capture particularly in terms of the relative immaturity of the technology and there is limited experience and know-how with large-scale hydrogen -fired gas turbines for power generation. Existing apparatus designed to combust fossil fuels will be essentially impossible to convert to the combustion of hydrogen.

Oxy-fuel combustion is a technique in which a fuel is burnt in the presence of a gas which is almost entirely composed of oxygen, usually 97% or more oxygen, instead of the air which is conventionally used as an oxidant. This technology is much more straightforward to retrofit into existing plants than the pre-combustion capture techniques described above but the very high combustion temperatures from using oxygen must be controlled by dilution of the gases in the combustion chamber for a conventional boiler to be used.

The gaseous effluent from oxy-fuel combustion is composed largely of carbon dioxide and water, with minor amounts of nitrogen, argon and oxygen, and combustion byproducts such as nitrogen oxides and sulphure oxides. Following the removal of water by inexpensive condensation and molecular sieve dehydration processes, a dry gas is obtained containing typically greater than 70% carbon dioxide (50% for retrofitted plants). In comparison, the concentration of carbon dioxide in flue gases from conventional combustion processes is around five times lower (10 to 15% on a dry basis). Despite the increased levels of carbon dioxide in oxy-fuel flue gas, it remains desirable to further increase the concentration of carbon dioxide prior to its sequestration. In particular, purer carbon dioxide is required to meet the specifications for EOR. Cryogenic processing is a robust and effective method for the bulk purification of carbon- dioxide containing gases. Due to the low relative volatility of carbon dioxide compared to the other gaseous components, cryogenic purification can be achieved by cooling, compressing and partially condensing gas streams to form two-phase vapour-liquid mixtures, followed by separation of the resulting carbon dioxide rich liquid phase. Cryogenic processing of carbon dioxide is an attractive technology for use in combination with CCS since it provides a high purity carbon dioxide product at elevated pressure which is thus integrated with the existing compression requirements for sequestration or EOR. There is therefore a need in the art for effective techniques which are able to process flue-gases, in particular oxy-fuel flue gases to provide a high-purity carbon dioxide product.

An example of a conventional cryogenic separation process is shown in Figure 1.

A combustion effluent gas (100) at essentially atmospheric pressure is passed to a multi- stage feed gas compression train (105). Each compression stage comprises a compressor (1 10), cooler (1 15) - typically air or water cooled, and a vapour liquid separator (120) to remove a condensed liquid (125), which comprises substantially water. The compressed feed (130) is passed to a pre-treatment unit (135), to remove the remaining water in the feed by passing the compressed feed over molecular sieves. If necessary, mercury may also be removed at this stage. The dry feed gas stream (140) containing carbon dioxide is routed to a high efficiency, multi-stream heat exchanger (200) where it is cooled and partially condensed.

The cooled, two phase stream (205) is passed to a vapour liquid separator (210) to give a C0₂ rich liquid stream (220) and a C0₂ lean vapour stream (215). The C0₂ rich liquid stream (220) is reduced in pressure across a valve (225) to give a low temperature, two phase stream (230). This stream is evaporated and reheated in the heat exchanger (200) to provide the refrigeration to cool the feed gas stream (140). The reheated stream (235) is passed to a multi-stage product compressor (300) where it is compressed and cooled in consecutive stages to provide a C0₂ product (310) meeting product pressure requirements.

The carbon dioxide lean gas (215), produced as the overhead vapour in the cold separator (210) is also reheated against feed gas. The reheated stream (400) is produced at essentially feed gas pressure and power can be recovered from this stream by heating in an exchanger (405) and passing the heated gas (410) to a turbo expander (415). A multi-stage expander arrangement may be used to obtain the desired high pressure C0₂ product - ca. 10,000 to 20,000 kPa absolute (as used herein the unit kPa refers to absolute pressure unless stated otherwise). The low pressure outlet gas (420) is subsequently vented to the atmosphere.

In this arrangement, the maximum purity of the carbon dioxide product is determined by the extent of condensation of the dry feed gas stream (140) in the multi-stream heat exchanger (200), and the carbon dioxide remaining in the vapour phase is an indicator of the loss of carbon dioxide in the off-gas stream and hence the maximum carbon dioxide recovery by the process. It will be appreciated that the partitioning of carbon dioxide is dependent on the temperature and pressure of the two phase stream (205). In addition, the equilibrium concentrations of carbon dioxide in the vapour and the liquid streams, and hence the maximum carbon dioxide recovery, are further limited by the freezing temperature of carbon dioxide. The minimum operating temperature is around -55 °C to avoid freezing of the carbon dioxide within the system.

The conventional process shown in Figure 1 therefore contains inherent limitations as to the purity of the C0₂ product obtained and the maximum C0₂ recovery. The use of lower separator pressure and/or higher operating temperatures provides a C0₂ product of increased purity, but at the expense of C0₂ recovery, since a greater proportion of C0₂ is lost to the overhead vapour stream (215). Furthermore, while the use of the expanded stream (230) to cool the feed gas stream (140) does provide some advantages in terms of energy efficiency, it is extremely difficult to match the expansion of the liquid stream (220) to the cooling requirements of the feed gas stream. Unnecessary expansion of the liquid stream (220) beyond the requirements to cool the feed gas is inefficient since it increases the energy required to recompress the heated stream (235) to provide a compressed product meeting the desired specifications for sequestration and EOR. Inadequate expansion of the liquid stream (220) means that the cooling requirements of the feed gas stream are not met and there is a corresponding reduction in C0₂ recovery through loss of C0₂ to the overhead vapour stream (215).

The present invention provides a C0₂ separation process in which enhanced separation techniques are used to provide a carbon dioxide product stream which is of increased purity relative to the known system described above, while the energy efficiency of the overall process is maintained through novel approaches to heat integration.

In a first aspect, the present invention provides a C0₂ separation process wherein C0₂ is separated from a gaseous feed stream comprising at least 30 mol% C0₂ and at least one other gas having a lower boiling point than C0₂, the process comprising the steps of:

(i) cooling and partially condensing the feed stream;

passing the cooled and partially condensed feed stream from step (i) to a vapour-liquid separator to produce a vapour stream having reduced C0₂ content relative the feed stream and a liquid stream having increased C0₂ content relative to the feed stream;

(iii) expanding and heating at least a portion of the liquid stream from step (ii); (iv) cooling and partially condensing at least a portion of the vapour stream from step (ii);

(v) passing the cooled and partially condensed stream from step (iv) to a vapour-liquid separator to provide a vapour stream having reduced C0₂ content relative to the vapour stream from step (ii), and a liquid stream having increased C0₂ content relative to the vapour stream from step (ii);

(vi) expanding at least a portion of the liquid stream from step (v); and (vii) passing the expanded liquid stream from step (vi) to a vapour-liquid separator produce a vapour stream having reduced C0₂ content relative to the liquid stream from step (v), and a liquid stream having increased C0₂ content relative to the liquid stream from step (v);

wherein cooling in step (i) is provided at least in part by heat exchange during heating of the expanded liquid stream in step (iii).

Expansion of at least a portion of the liquid stream from step (ii) in step (iii) provides a reduced pressure stream which is heated and evaporated in heat exchange contact with the gaseous feed stream so as to cool the gaseous feed stream. Further separation of the vapour stream from step (ii) in steps (v) and (vii) allows the initial separation in step (ii) to be carried out under conditions which provide a liquid stream of high purity to be obtained from step (ii). As noted above, under such conditions, a significant proportion of the CO₂ content of the feed stream is entrained in the vapour stream from step (ii). In the process of the invention, overall C0₂ recovery is maintained by further separation of the vapour stream from step (ii) in steps (v) and (vii). Furthermore, the use of two separation stages enables a liquid stream of high purity to be obtained from step (vii).

In this way, the present invention provides a clear advantage over the known process shown in Figure 1 , since in a single stage separation, manipulation of the separation conditions to maximise C0₂ recovery leads to a reduction in C0₂ purity. Similarly, manipulation of the separation conditions to maximise C0₂ purity leads to a reduction in C0₂ recovery. According to the present invention, it is possible to maximise both C0₂ recovery and C0₂ purity.

In more detail, it has been found that the process of the present invention may be used to obtain a C0₂ product stream having a purity of at least 94 mol%, more preferably at least 96 mol% and in many cases purity of 98 mol% and above, for example at least 99 mol% can be obtained.

In particular, the use of step (vii) enables further purification of the liquid stream from step (v). In this way, a liquid stream is obtained from step (vii) having a purity which is preferably at least 94 mol% or above, and in many cases 96 mol% or above, or even 98 mol% or above. Accordingly, the liquid stream from step (vii) may be combined with other high purity streams (e.g. the liquid stream from step (ii)) in downstream processing without detriment to the overall purity of the C0₂ product stream. In addition, due to recovery of the C0₂ content of the vapour stream from step (ii) in steps (iv) to (vii), it has been found that that the C0₂ recovery obtainable by the process of the invention is at least 94 mol% and in many cases the C0₂ recovery is 96 mol% or above, or even 98 mol% or above. It will of course be appreciated by the skilled person that the purity and recovery of the C0₂ product stream will depend to some extent on the composition of the gaseous feed stream. Nonetheless, in a like-for-like separation, the process of the present invention will provide an increase in both C0₂ recovery and C0₂ purity when compared with a like- for-like separation using the known process of Figure 1.

The gaseous feed stream is preferably supplied to step (i) of the process of the invention at a pressure in the range of from 1000 to 6000 kPa, more preferably 2000 to 4000 kPa, for example 3000 kPa. Generally, a C0₂ containing gas to be separated according to the invention will be supplied at atmospheric pressure and will be compressed to a pressure in the range of from 1000 to 6000 kPa to form the gaseous feed stream. For example, a multistage compression train may be used to form the gaseous feed stream. The temperature of the gaseous feed stream is preferably in the range of from 0 to 50 °C, for example 20 to 40 °C. In step (i), the gaseous feed stream is preferably cooled to a temperature in the range of from -15 to -40 °C, more preferably from - 20 to -35 °C, and most preferably -20 to -30 °C.

The expanded stream from step (iii) will generally have a pressure in the range of from 1000 to 3000 kPa, more preferably from 1000 to 2500 kPa, and most preferably from 1500 to 2500 kPa. Depending on the temperature and pressure of the liquid stream from step (ii), expansion in step (iii) may lead to cooling of the stream by the Joule Thomson effect. This can potentially lead to freezing of the expanded stream. In some embodiments it is therefore desirable to heat the liquid stream from step (ii) prior to expansion in step (iii), such that subsequent evaporation of the expanded stream provides effective cooling of the gaseous feed stream, whilst avoiding freezing of the expanded stream. For example, the at least one liquid stream from step (ii) may be heated to a temperature in the range of from -20 to -45 °C prior to expansion in step (iii). Heating of the first portion of the liquid stream from step (ii) prior to expansion is preferably by heat exchange during cooling of the gaseous feed stream in step (i).

The vapour stream from step (ii) is preferably cooled in step (iv) to a temperature in the range of from -35 to -55 °C, more preferably from -40 to -55 °C, still more preferably from -45 to -55 °C, and most preferably from -50 to -55 °C, for example -51 °C, -52 °C, -53 °C, or -54 °C.

In some embodiments, for example where there gaseous feed stream is supplied to step (i) of the process of the invention at a pressure in the range of from 1000 to 2500 kPa, step (iv) may further comprise the step of compressing the at least a portion of the vapour stream from step (ii).

Separation in step (v) is preferably conducted at a pressure in the range of from 2000 to 6000 kPa, more preferably 2500 to 4000 kPa, for example 3000 kPa. Since the liquid stream from step (v) is at a low temperature (e.g. from -50 to -55 °C) in preferred embodiments of the invention, it may be preferable to heat the at least a portion of the liquid stream from step (v) prior to expansion in step (vi). For example, the liquid stream from step (v) may be heated in heat exchange with the gaseous feed stream during cooling of the gaseous feed stream in step (i) and/or in heat exchange with the vapour stream from step (ii) during cooling of the vapour stream from step (ii) in step (iv). In preferred embodiments, the stream from step (v) may be heated to a temperature in the range of from -20 °C, preferably from -25 to -45 °C, prior to expansion in step (vii), for example from -30 to -40 °C. ln step (vi) the liquid stream from step (v) is preferably expanded to a pressure in the range of from 500 to 1500 kPa. Separation in step (vii) is preferably conducted at a temperature of from -30 to -55 °C, more preferably from -40 to -55 °C, still more preferably from -45 to -55 °C, and most preferably from -50 to -55 °C.

The vapour stream from step (vii) generally contains recoverable C0₂ content and is therefore preferably recycled to an earlier stage of the separation process. For example, at least a portion of the vapour stream from step (vii) may be recycled to the gaseous feed stream and/or at least a portion of the vapour stream from step (vii) may be recycled to the vapour stream from step (ii).

The liquid stream from step (vii) is at a low temperature (e.g. -40 to -55 °C) following expansion in step (vi) and may thus be reheated in heat exchange so as to contribute to the cooling duty in other parts of the process. In preferred embodiments, the liquid stream from step (vii) is heated by heat exchange during cooling of the gaseous feed stream in step (i) and/or by heat exchange during cooling of the at least a portion of the vapour stream from step (ii) in step (v). It will be appreciated that, according to preferred embodiments of the invention, there is a pressure differential between the expanded stream from step (iii) and the liquid stream from step (vii). More specifically, the expanded stream from step (iii) is preferably at a higher pressure than the liquid stream from step (vii). This provides the advantage that cooling of the gaseous feed stream in step (i) can be provided by both the expanded stream from step (iii) at higher pressure (e.g. in the range of from 1000 to 3000 kPa, as described above) and the liquid stream from step (vii) at lower pressure (e.g. in the range of from 500 to 1500 kPa, as described above). This enables the expansion in steps (iii) and (vi) to be closely matched to the cooling requirements in step (i) and, where appropriate, in step (v). In some cases it may be appropriate to expand the liquid stream from step (vii) prior to passing it in heat exchange with the gaseous feed stream in step (i) and/or the at least a portion of the vapour stream from step (ii) in step (v). In this way, the overall heat integration of the process is improved, and the energy requirements for downstream compression of the C0₂ product streams are minimised, thus improving the overall energy efficiency of the C0₂ recovery.

Thus, in preferred embodiments, the process of the invention further comprises the step of compressing at least a portion of the expanded heated stream from step (iii) and/or at least a portion of the liquid stream from step (vii) to obtain a compressed C0₂ product. The degree of compression is dependent on desired product specifications, but in preferred embodiments, the compressed C0₂ product will have a pressure in the range of from 8,000 to 20,000 kPa, preferably 10,000 to 20,000 kPa. This pressure range is preferred for all compressed C0₂ products referred to herein.

In a further embodiment, the expanded heated stream from step (iii), or a portion thereof, may be further purified. Thus, the process of the invention may further comprise the step of:

(viii) separating the expanded heated stream from step (iii) to produce a vapour stream having reduced C0₂ content relative to the liquid stream from step (ii), and a liquid stream having increased C0₂ content relative to the liquid stream from step (ii). In this way, the expansion and heating of the liquid stream from step (ii) in step (iii) may be exploited to obtain a two-phase stream which may be further separated to obtain a liquid C0₂ product stream of increased purity and a vapour phase of reduced purity.

In this way, a further increase in the purity of the C0₂ product is obtained when compared to a like-for-like separation using the known process of Figure 1.

Separation in step (viii) may be by way of a vapour-liquid separator (also known in the art as a flash drum or knock-out drum). Alternatively, and particularly where a higher level of purity of the C0₂ product is required, separation in step (viii) may be by way of a fractionation column. An advantage of using a fractionation column in step (viii) is that the fractionation column may be equipped with a reboil heat exchanger. Heat exchange between the gaseous feed stream and boiling liquids in the fractionation column may be used to further contribute to cooling of the gaseous feed stream in step (i), thus further improving the heat integration of the process of the invention.

Separation in step (viii) is generally carried out at an intermediate temperature, for example between -15 and -40 °C. The liquid and vapour streams obtained from step (iv) are thus obtained at a significantly lower temperature than the gaseous feed stream, and thus may also be used to further cool the gaseous feed stream in step (i) via heat exchange.

The liquid product stream obtained from step (viii) is preferably compressed to provide a compressed C0₂ product.

The vapour stream obtained from step (viii) may contain a recoverable quantity of C0₂. Thus, in some embodiments of the invention, at least a portion of the vapour stream from step (viii) is recycled to the gaseous feed stream. The vapour stream from step (v) is a waste material which may be vented to the atmosphere or passed to further processing to remove contaminants as appropriate. In a preferred embodiment, however, at least a portion of the vapour stream obtained from step (v) may be work-expanded, e.g. using a turbo-expander. Work-expansion of the vapour stream from step (v) may be used to generate power or to assist in boosting the pressure of the feed gas, e.g. by way of a turbo-expander having a compressor at the brake end. In addition, the cooling of the vapour stream due to work- expansion may be used to provide refrigeration to other parts of the process so as to improve the energy integration of the process.

In some embodiments, the at least a portion of the vapour stream from step (v) is heated prior to being passed to work-expansion. More preferably, the at least a portion of the vapour stream from step (v) is heated by heat exchange with the gaseous feed stream during cooling of the gaseous feed stream in step (i). In this way, there is provided a further contribution to the cooling of the gaseous feed stream, reducing the expansion requirement in step (iv) and thus reducing the energy required for compression of the C0₂ product stream(s).

Alternatively, or in addition, the at least a portion of the vapour stream from step (v) may be heated in heated exchange with the vapour stream from step (ii), during cooling of the vapour stream from step (ii) in step (iv). In some embodiments, it may be energy efficient to supplement one or more of the cooling steps described above with an external mechanical refrigeration cycle. In this way, the cooling duty borne by expanded streams as described above, and hence the power requirements for product gas compression, may be reduced. It will be appreciated that driving the compression stage of the external mechanical refrigeration cycle may be an additional application of the work-expansion of the at least a portion of the vapour stream from step (ii) and/or the at least a portion of the vapour stream from step (ix).

It will be appreciated that the process of the invention may involve compression of more than one stream to form a compressed C0₂ product. In a preferred embodiment of the invention, a multi-stage compression train may be used to compress multiple C0₂ containing streams. Streams having different pressures may be introduced into the compression train at a stage which corresponds to their pressure so as to provide a combined compressed C0₂ product stream. The gaseous feed stream preferably comprises at least 40 mol% C0₂, more preferably at least 50 mol% C0₂, still more preferably at least 60 mol% C0₂, and most preferably at least 70% by volume C0₂. In certain embodiments, the dry gaseous feed may comprise, for example at least 75 mol% C0₂, at least 80 mol% C0₂, at least 85 mol% C0₂, at least 90 mol% C0₂, or at least 95 mol% C0₂.

Preferably, the gaseous feed stream is substantially free of gases having a higher boiling point than C0₂. The content of such gases in the gaseous feed stream is preferably less th an 5 mol%, more preferably less than 2 mol%, still more preferably less than 1 mol%, and most preferably less than 0.5 mol%.

Still more preferably, the gaseous feed stream is substantially comprised of carbon dioxide and one or more of oxygen, nitrogen and argon. The content of gases other than carbon dioxide, oxygen, nitrogen and argon in the gaseous feed stream is preferably less than 5 mol%, more preferably less than 2 mol%, still more preferably less than 1 mol%, and most preferably less than 0.5 mol%. Where necessary, the gaseous feed stream is preferably treated to remove water prior to step (i), since water is likely to freeze under the operating conditions of the process of the invention, and therefore disrupt the operation of the processing apparatus. Preferably, the gaseous feed stream comprises less than 10 ppm by volume of water, more preferably less than 5 ppm by volume of water, still more preferably less than 2 ppm by volume of water, and most preferably less than 1 ppm by volume of water. Suitable approaches for the removal of water from a gas are well-known in the art, and include the use of a multistage compression train with vapour-liquid separators between compression stages to remove condensed water, and a subsequent dehydration process using a water absorber, such as molecular sieves.

In preferred embodiments, the gaseous feed stream comprises or consists of a dehydrated flue gas from a combustion process. In a particularly preferred embodiment, the gaseous feed stream comprises or consists of a dehydrated flue gas from an oxy-fuel combustion process.

The gaseous feed stream may contain other combustion effluent gases, such as oxides of sulfur and nitrogen. In some embodiments of the invention, these gases may be removed in an upstream processing step prior to step (i) of the process of the invention. However, in some cases it may be more efficient to remove these components from the compressed C0₂ product stream following the process of the invention. Accordingly, the process of the invention encompasses the use of a gaseous feed stream that comprises minor amounts of the oxides of sulfur and nitrogen, for example less than 2 wt% in total, more preferably less than 1 wt% in total. It will be appreciated that the process of the invention as described above may comprise a number of heat exchange steps. The configuration of the heat exchange steps is not particularly limited and may involve separate heat exchangers for each separate heat exchange step, or where appropriate, a number of different heat exchange steps may be combined within a single multistream heat exchanger.

In another aspect, the present invention provides a C0₂ separation apparatus for separating C0₂ from a gaseous feed stream comprising C0₂ and at least one other gas having a lower boiling point than C0₂, the apparatus comprising the following parts:

(i) means for cooling and partially condensing the gaseous feed stream;

(ii) a vapour-liquid separator adapted to separate the cooled and partially condensed stream from part (i) to provide a vapour stream having reduced C0₂ content relative the feed stream and a liquid stream having increased C0₂ content relative to the feed stream;

(iii) means for expanding and heating at least a portion of the liquid stream from part (ii);

(iv) means for cooling and partially condensing at least a portion of the vapour stream from part (ii);

(v) a vapour-liquid separator adapted to separate the cooled and partially condensed stream from part (iv) to provide a vapour stream having reduced C0₂ content relative to the vapour stream from part (ii), and a liquid stream having increased C0₂ content relative to the vapour stream from part (ii);

(vi) means for expanding the liquid stream from part (v); and

(vii) a vapour-liquid separator adapted to separate the expanded stream from part (vi) to produce a vapour stream having reduced C0₂ content relative to the liquid stream from part (v), and a liquid stream having increased C0₂ content relative to the liquid stream from part (v);

wherein the means for cooling in part (i) and the means for heating in part (iii) comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the liquid stream from part (ii). Preferably, the means for cooling and partially condensing the gaseous feed stream in part (i) further comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the liquid stream from part (v) to heat the liquid stream from part (v) prior to part (vi).

Alternatively, or in addition, the means for cooling the at least a portion of the vapour stream from part (ii) in part (iv) comprises one or more heat exchangers adapted to pass the at least a portion of the vapour stream from part (ii) in heat exchange contact with the liquid stream from part (v) to heat the liquid stream from part (v) prior to part (vi).

The apparatus may further comprise means for recycling at least a portion of the vapour stream from part (vii) to the gaseous feed stream and/or means for recycling at least a portion of the vapour stream from part (vii) to the vapour stream from part (ii). The apparatus may further comprise means for heating the liquid stream from part (vii). For example, the means for cooling and partially condensing the gaseous feed stream in part (i) may further comprise one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the liquid stream from part (vii) to heat the liquid stream from part (vii). Alternatively, or in addition, the means for cooling the at least a portion of the vapour stream from part (ii) in part (iv) may comprise one or more heat exchangers adapted to pass the at least a portion of the vapour stream from part (ii) in heat exchange contact with the liquid stream from part (vii) to heat the liquid stream from part (vii). In a further preferred embodiment, the apparatus of the invention may comprise :

(viii) means for separating the stream from part (iii) to produce a vapour stream having reduced C0₂ content relative to the liquid stream from part (ii), and a liquid stream having increased C0₂ content relative to the liquid stream from part (ii).

The means for separating the stream from part (iii) in part (viii) may comprise a vapour- liquid separator. Alternatively, the means for separating the stream from part (iii) in part (viii) may comprise a fractionation column. Where a fractionation column is used, it preferably comprises a reboil heat exchanger which is adapted to pass the gaseous feed stream in heat exchange contact with liquid in the fractionation column so as to cool the gaseous feed stream. A reboil heat exchanger may be an internal or external reboiler in accordance with the invention.

In some embodiments of the invention, the means for cooling the gaseous feed stream in part (i) may further comprise one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the liquid stream from part (viii) to heat the liquid stream from part (viii).

In some embodiments of the invention, the means for cooling gaseous feed stream in part (i) further comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the vapour stream from part (viii) to heat the vapour stream from part (viii).

The apparatus of the invention may further comprise means for recycling at least a portion of the vapour stream from part (viii) to the gaseous feed stream.

The apparatus of the invention may comprise means for heating and work-expanding at least a portion of the vapour stream from part (v). For example, the means for cooling the gaseous feed stream in part (i) may further comprise one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the at least a portion of the vapour stream from part (v) to heat the vapour stream from part (v). A turbo-expander is preferably used as the means for work-expanding.

Still further, the apparatus of the invention may comprise at least one compression system adapted to compress one or more of the liquid streams from parts (ii), (vii) and (viii) to provide a C0₂ product. For example, the compression system may comprise a multistage compression train.

In a further aspect, the present invention provides an oxy-fuel combustion apparatus having a flue gas outlet in flow communication with a C0₂ separation apparatus as defined above. The invention will now be described in greater detail with reference to preferred embodiments and with the aid of the accompanying figures, in which:

Figure 1 shows a conventional apparatus as described above for the purification of a carbon dioxide containing gaseous feed, such as a flue gas.

Figure 2 shows a process and apparatus in accordance with the present invention wherein the vapour stream from step (ii) is further purified in two vapour-liquid separator stages (605, 640). In this embodiment, the vapour stream from step (vii) (645) is recycled to the gaseous feed stream via an intermediate stage of the multistage compressor (105).

Figure 3 shows an alternative embodiment of the process and apparatus of the present invention wherein the vapour stream from step (vii) (645) is recycled to the vapour stream from step (ii) (215).

In the embodiment of the invention shown in Figure 2, a combustion effluent gas (100) at essentially atmospheric pressure is passed to a multi-stage feed gas compression train (105). Each compression stage comprises a compressor (1 10), cooler (1 15) - typically air or water cooled, and a vapour liquid separator (120) to remove a condensed liquid (125), which comprises substantially water.

The compressed feed (130) is passed to a pre-treatment unit (135), to remove the remaining water in the feed by passing the compressed feed over molecular sieves. If necessary, other contaminants such as mercury, sulfur oxides and nitrogen oxides may also be removed at this stage. The gaseous feed stream (140) containing carbon dioxide is routed to a high efficiency, multi-stream heat exchanger (200A) where it is cooled and partially condensed against returning product and off-gas streams. The cooled, two phase stream (205) is passed to a vapour liquid separator (210) to give a C0₂ rich liquid stream (220) and a C0₂ lean vapour stream (215). The C0₂ rich liquid stream (220) is expanded to an intermediate pressure across a valve (225) to give a low temperature, two phase stream (230), which is heated in the heat exchanger (200A) against the gaseous feed stream (140) and the resulting warmed stream (235) is passed to a multi-stage product compressor (300), where it is compressed to form a purified C0₂ product (310). The C0₂ lean vapour stream (215) is routed to a high efficiency, multi-stream heat exchanger (200B) where it is cooled and partially condensed against returning product and off-gas streams. The cooled, two phase stream (600) is passed to a vapour liquid separator (605) to give a C0₂ rich liquid stream (615) and a C0₂ lean vapour stream (610).

The C0₂ lean vapour stream (610) is warmed against the vapour stream (215) and the gaseous feed stream (140) in heat exchangers (200A and 200B). The reheated stream (400) is produced at essentially feed gas pressure, and power can be recovered from this stream by passing the stream to a heater (405) and passing the heated stream (410) to a turbo expander (415). A multi-stage expander arrangement may be used in some embodiments. The low pressure outlet gas (420) is subsequently vented to the atmosphere or is passed to further decontamination processes as required.

The C0₂ rich liquid stream (615) is heated to an intermediate temperature in heat exchanger (200B) against the vapour stream (215). The resulting stream (620) is expanded across an expansion valve (625) and the expanded stream (630) is passed to a further vapour-liquid separator (640) to give a C0₂ rich liquid stream (650) and a C0₂ lean vapour stream (645). The C0₂ lean vapour stream (645) is heated in heat exchangers (200A and 200B) against the vapour stream (215) and the gaseous feed stream (140). The resulting warmed stream (660) is returned to an intermediate stage of the multi-stage feed gas compression train (105). The C0₂ rich liquid stream (650) is heated in heat exchangers (200A and 200B) against the vapour stream (215) and the gaseous feed stream (140) and the resulting warmed stream (655) is passed to a lower pressure stage of the multi-stage product compressor (300), where it is compressed and recombined with the warmed stream (380) to form a purified C0₂ product (310).

In the embodiment of the invention shown in Figure 3, a compressed gaseous feed stream (140) containing carbon dioxide is routed to a high efficiency, multi-stream heat exchanger (200A) where it is cooled and partially condensed against returning product and off-gas streams.

The cooled, two phase stream (205) is passed to a vapour-liquid separator (210) to give a C0₂ rich liquid stream (220) and a C0₂ lean vapour stream (215). The C0₂ rich liquid stream (220) is expanded to an intermediate pressure across a valve (225) to give a low temperature, two phase stream (230), which is heated in the heat exchanger (200A) against the gaseous feed stream (140) to provide a warmed C0₂ rich stream (235) which is typically compressed in a subsequent stage, for example using a multi-stage product compressor (300) as described above.

The C0₂ lean vapour stream (215) is further cooled and partially condensed in heat exchanger (200B). The cooled, two phase stream (600) is passed to a compressor (670) and cooler (675), and is further cooled in the heat exchanger (200B). The cooled compressed stream (600) is passed to a vapour liquid separator (605) to give a C0₂ rich liquid stream (615) and a C0₂ lean vapour stream (610).

The C0₂ lean vapour stream (610) is warmed against the vapour stream (215) and the gaseous feed stream (140) in the heat exchangers (200A and 200B). The reheated stream (400) is produced at essentially feed gas pressure, and power can be recovered from this stream if required by passing the stream to a heater (405) and passing the heated stream (410) to a turbo-expander (415).

The C0₂ rich liquid stream (615) is heated to an intermediate temperature in heat exchanger (200B) against the compressed vapour stream (215). The resulting stream (620) is expanded across a valve (625), and the expanded stream (630) is passed to a further vapour-liquid separator (640) to give a C0₂ rich liquid stream (650) and a C0₂ lean vapour stream (645). The C0₂ lean vapour stream (645) is heated in the heat exchanger (200B) against the compressed vapour stream (215). The resulting warmed stream (665) is combined with the vapour stream (215) and passed to the compressor (670). The C0₂ rich liquid stream (650) is expanded across an expansion valve (680) and heated in the heat exchangers (200A and 200B) against the compressed vapour stream (215) and the gaseous feed stream (140) to produce a warmed C0₂ rich stream (655). This stream may subsequently be compressed as described above, so as to form a purified C0₂ product (310).

Examples

Comparative Example 1 Table 1 shows typical operating parameters for the conventional cryogenic separation process shown in Figure 1 when used to separate a gaseous mixture consisting of 83.5 mol% C0₂, 10 mol% N₂, 3.5 mol% Ar, and 3.0 mol% 0₂.

Table 1

Stream Number 215 400 410 420

Vapour (mole fraction) 1.0000 1.0000 1.0000 1.0000

Temperature (°C) -51.5 16.6 300.0 12.8

Pressure (kPa(a)) 3000 3000 3000 101

Mass Flow (kg/h) 68462 68462 68462 68462

Molar Flow:

C0₂ (kgmol/hr) 532 532 532 532

Nitrogen (kgmol/hr) 938 938 938 938

Argon (kgmol/hr) 265 265 265 265

Oxygen (kgmol/hr) 257 257 257 257

Total (kgmol/h) 1991 1991 1991 1991 Feed Compression Power 37.4 MW

Product Compression Power 17.4 MW

54.8 MW

Purity 94.9 mol% C0₂

Recovery of C0₂

Example 2

Table 2 shows typical operating parameters for the process of the invention using the apparatus shown in Figure 2 when used to separate a gaseous mixture consisting of 82.7 mol% C0₂, 3.8 mol% Ar, 10.3 mol% N₂, and 3.2 mol% 0₂, at a flow rate of 12,620 kg-mol-h^"1. It will be observed that, even with a feed stream comprising almost 6 mol% more material, the separation process provides a significant increase in the purity of the C0₂ product with lower overall energy consumption than in comparative Example 1. This advantage is obtained with only a minor reduction in overall C0₂ recovery.

Table 2

Stream Number 630 650 645 660 655 220 Vapour (mole 0.1513 0.0000 1.0000 1.0000 1.0000 0.0000 fraction)

Temperature (°C) -55.0 -55.0 -55.0 17.9 17.9 -20.0

Pressure (kPa(a)) 800 800 800 800 800 3000

Mass Flow (kg/h) 197988 170083 27905 27905 170083 244524

Molar Flow:

C0₂ (kgmol/hr) 4321 3845 476 476 3845 5457

Argon (kgmol/hr) 78 12 66 66 12 41

Nitrogen (kgmol/hr) 1 13 8 105 105 8 66

Oxygen (kgmol/hr) 48 5 43 43 5 26

Total (kgmol/h) 4560 3870 690 690 3870 5591

Feed Compression Power 38.4 MW

Product Compression Power 15.8 MW

54.2 MW

Purity 98.3 mol% C0₂

Recovery of C0₂ 93.4 %

Claims

A C0₂ separation process wherein C0₂ is separated from a gaseous feed stream comprising at least 30 mol% C0₂ and at least one other gas having a lower boiling point than C0₂, the process comprising the steps of:

(i) cooling and partially condensing the feed stream;

(ii) passing the cooled and partially condensed feed stream from step (i) to a vapour-liquid separator to produce a vapour stream having reduced C0₂ content relative the feed stream and a liquid stream having increased C0₂ content relative to the feed stream;

(iii) expanding and heating at least a portion of the liquid stream from step (ii);

(iv) cooling and partially condensing at least a portion of the vapour stream from step (ii);

(vi) expanding at least a portion of the liquid stream from step (v); and

(vii) passing the expanded liquid stream from step (vi) to a vapour-liquid separator produce a vapour stream having reduced C0₂ content relative to the liquid stream from step (v), and a liquid stream having increased C0₂ content relative to the liquid stream from step (v);

wherein cooling in step (i) is provided at least in part by heat exchange during heating of the liquid stream in step (iii).

A process according to Claim 1 , wherein the liquid stream from step (v) is heated prior to step (vi).

A process according to Claim 2, wherein the liquid stream from step (v) is heated by heat exchange during cooling of the gaseous feed stream in step (i).

4. A process according to Claim 2 or Claim 3, wherein the liquid stream from step (v) is heated by heat exchange during cooling of the at least a portion of the vapour stream from step (ii) in step (iv).

A process according to any one of the preceding claims, wherein at least a portion of the vapour stream from step (vii) is recycled to the gaseous feed stream.

A process according to any one of the preceding claims, wherein at least a portion of the vapour stream from step (vii) is recycled to the vapour stream from step (ii).

A process according to any one of the preceding claims, wherein the liquid stream from step (vii) is subsequently heated.

8. A process according to Claim 7, wherein the liquid stream from step (vii) is heated by heat exchange during cooling of the gaseous feed stream in step (i).

A process according to Claim 7 or Claim 8, wherein the liquid stream from step (vii) is heated by heat exchange during cooling of the at least a portion of the vapour stream from step (ii) in step (iv).

A process according to any one of the preceding claims wherein the liquid stream from step (vii) is subsequently compressed to provide a compressed C0₂ product.

A process according to any one of the preceding claims, wherein the heated stream from step (iii) is subsequently compressed to provide a compressed C0₂ product.

A process according to any one of Claims 1 to 10, further comprising the step of: (viii) separating the expanded heated stream from step (iii) to produce a vapour stream having reduced C0₂ content relative to the liquid stream from step (ii), and a liquid stream having increased C0₂ content relative to the liquid stream from step (ii).

13. A process according to Claim 12, wherein separation in step (viii) is by way of a vapour-liquid separator.

14. A process according to Claim 12, wherein separation in step (iv) is by way of a 5 fractionation column.

15. A process according to Claim 14, wherein further cooling of the gaseous feed stream in step (i) is provided by heat exchange with liquid in the fractionation column by way of a reboil heat exchanger.

10

16. A process according to any one of Claims 12 to 15, wherein the liquid stream from step (viii) is subsequently heated by heat exchange during cooling of the gaseous feed stream in step (i).

15 17. A process according to any one of Claims 12 to 16, wherein the vapour stream from step (viii) is subsequently heated by heat exchange during cooling of the gaseous feed stream in step (i).

18. A process according to any one of Claims 12 to 17, wherein the liquid stream from 20 step (viii) is subsequently compressed to form a compressed C0₂ product.

19. A process according to any one of Claims 12 to 18, wherein at least a portion of the vapour stream from step (viii) is recycled to the gaseous feed stream.

25 20. A process according to any one of the preceding claims, wherein at least a portion of the vapour stream from step (v) is heated and work-expanded.

21. A process according to Claim 20, wherein the at least a portion of the vapour stream from step (v) is heated by heat exchange during cooling of the gaseous

30 feed stream in step (i).

22. A process according to any one of the preceding claims, wherein the gaseous feed stream comprises at least 40 mol% C0₂.

A process according to any one of the preceding claims, wherein the gaseous feed stream comprises less than 10 ppm by volume of water.

A process according to any one of the preceding claims wherein the gaseous feed stream comprises a dehydrated portion of the flue gas from an oxy-fuel combustion process.

A C0₂ separation apparatus for separating C0₂ from a gaseous feed stream comprising C0₂ and at least one other gas having a lower boiling point than C0₂, the apparatus comprising the following parts:

(i) means for cooling and partially condensing the gaseous feed stream;

(vi) means for expanding the liquid stream from part (v); and

wherein the means for cooling in part (i) and the means for heating in part (iii) comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the liquid stream from part (ii).

26. An apparatus according to Claim 25, wherein the means for cooling and partially condensing the gaseous feed stream in part (i) further comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the liquid stream from part (v) to heat the liquid stream from part (v)

5 prior to part (vi).

27. An apparatus according to Claim 25 or Claim 26, wherein the means for cooling the at least a portion of the vapour stream from part (ii) in part (iv) comprises one or more heat exchangers adapted to pass the at least a portion of the vapour

10 stream from part (ii) in heat exchange contact with the liquid stream from part (v) to heat the liquid stream from part (v) prior to part (vi).

28. An apparatus according to any one of Claims 25 to 27, comprising means for recycling at least a portion of the vapour stream from part (vii) to the gaseous

15 feed stream.

29. An apparatus according to any one of Claims 25 to 28, comprising means for recycling at least a portion of the vapour stream from part (vii) to the vapour stream from part (ii).

20

30. An apparatus according to any one of Claims 25 to 29, comprising means for heating the liquid stream from part (vii).

31. An apparatus according to Claim 30, wherein the means for cooling and partially 25 condensing the gaseous feed stream in part (i) further comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the liquid stream from part (vii) to heat the liquid stream from part (vii).

30 32. An apparatus according to Claim 30 or Claim 31 , wherein the means for cooling the at least a portion of the vapour stream from part (ii) in part (iv) comprises one or more heat exchangers adapted to pass the at least a portion of the vapour stream from part (ii) in heat exchange contact with the liquid stream from part (vii) to heat the liquid stream from part (vii).

33. An apparatus according to any one of Claims 25 to 32, further comprising:

5 (viii) means for separating the stream from part (iii) to produce a vapour stream having reduced C0₂ content relative to the liquid stream from part (ii), and a liquid stream having increased C0₂ content relative to the liquid stream from part (ii).

10 34. An apparatus according to Claim 33, wherein the means for separating the stream from part (iii) in part (viii) comprises a vapour-liquid separator.

35. An apparatus according to Claim 33, wherein the means for separating the stream from part (iii) in part (viii) comprises a fractionation column.

15

36. An apparatus according to Claim 35, wherein the means for cooling and partially condensing the gaseous feed stream in part (i) further comprises a reboiler adapted to cool the gaseous feed stream by heat exchange with liquid in the fractionation column.

20

37. An apparatus according to any one of Claims 33 to 36, wherein the means for cooling the gaseous feed stream in part (i) further comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the liquid stream from part (viii) to heat the liquid stream from part (viii).

25

38. An apparatus according to any one of Claims 33 to 37, wherein the means for cooling gaseous feed stream in part (i) further comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the vapour stream from part (viii) to heat the vapour stream from part (viii).

30

39. An apparatus according to any one of Claims 33 to 38, comprising means for recycling at least a portion of the vapour stream from part (viii) to the gaseous feed stream. An apparatus according to any one of Claims 25 to 39, further comprising means for heating and work-expanding at least a portion of the vapour stream from part (ii) and/or at least a portion of the vapour stream from part (v).

An apparatus according to Claim 40, wherein the means for cooling in part (i) further comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the at least a portion of the vapour stream from part (ii) to heat the vapour stream from part (ii) and/or the at least a portion of the vapour stream from part (v) to heat the vapour stream from part (v).

An apparatus according to Claim 40 or Claim 41 , wherein the means for work- expanding the at least a portion of the vapour stream from part (ii) and/or the at least a portion of the vapour stream from part (v) comprises one or more turbo expanders.

An apparatus according to any one of Claims 25 to 42, comprising at least one compression system adapted to compress at least one of the liquid streams from parts (ii), (vii) and (viii) to provide a compressed C0₂ product.

20 44. Any oxy-fuel combustion apparatus having a flue gas outlet in flow communication with a C0₂ separation apparatus as defined in any one of Claims 25 to 43.