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 CO ₂ from streams containing high C0 ₂ content is easier than from those with lower C0 ₂ 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 sulphur 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 C0 ₂ rich liquid stream (220) and a C0 ₂ lean vapour stream (215). The C0 ₂ 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 C0 ₂ 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 C0 ₂ 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 C0 ₂ product obtained and the maximum C0 ₂ recovery. The use of lower separator pressure and/or higher operating temperatures provides a C0 ₂ product of increased purity, but at the expense of C0 ₂ recovery, since a greater proportion of C0 ₂ 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 C0 ₂ recovery through loss of C0 ₂ to the overhead vapour stream (215).

The present invention provides a novel process and apparatus for the purification of a gaseous mixture comprising carbon dioxide and other gases which aims to address one or more of the difficulties encountered with prior art C0 ₂ separation techniques. In particular, the present invention provides a C0 ₂ separation process comprising novel heat integration techniques to improve the efficiency of the process and to minimise the power requirements for downstream compression. In preferred embodiments of the invention, 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 C0 ₂ separation process wherein C0 ₂ is separated from a gaseous feed stream comprising at least 30 mol% C0 ₂ and at least one other gas having a lower boiling point than C0 ₂ , 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 C0 ₂ content relative the feed stream and a liquid stream having increased C0 ₂ content relative to the feed stream;

(iii) dividing the liquid stream from step (ii) into at least two liquid streams; and (iv) expanding and heating at least one of the at least two liquid streams from step (iii); wherein cooling in step (i) is provided at least in part by heat exchange during heating of the liquid stream in step (iv).

Expansion of the at least one liquid stream in step (iv) provides a reduced pressure stream which is heated and evaporated in heat exchange with the gaseous feed stream. By expanding only a portion of the liquid stream from step (ii), in step (iv), the present invention provides the advantage that the cooling provided by expansion of the at least one liquid stream in step (iv) can be closely matched with the cooling requirements in step (i). In this way, unnecessary expansion of at least one other liquid stream obtained from step (iii) is avoided, thus surprisingly reducing overall power requirements for compression during subsequent downstream processing. This advantage is obtained without any detriment to product purity and recovery rates, which are equivalent to those obtained in the conventional process described above. More specifically, it has been found that the process of the present invention can be used to obtain a C0 ₂ product stream with at least 90 mol% recovery of the C0 ₂ from the gaseous feed stream. Preferably, the C0 ₂ recovery is at least 92 mol%, more preferably at least 94 mol% and most preferably at least 96 mol%. The purity of the C0 ₂ recovered according to the process of the invention is generally at least 90 mol%, and in preferred embodiments is at least 92 mol%, more preferably at least 94 mol% and most preferably at least 96 mol%.

It will of course be appreciated by the skilled person that the purity and recovery of the C0 ₂ 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 C0 ₂ recovery and purity which is equivalent to that obtained by the known process of Figure 1 , but with a reduction in overall energy consumption. Preferably, the proportion of the liquid stream from step (ii) that is expanded and heated in step (iv) is 45 wt% or less, more preferably 40 wt% or less, still more preferably 35 wt% or less, still more preferably 30 wt% or less, still more preferably 25 wt% or less, and most more preferably 20 wt% or less. Preferably, the proportion of the liquid stream from step (ii) that is expanded and heated in step (iv) is 5 wt% or greater, and more preferably 10 wt% or greater. Depending on the temperature and pressure of the liquid stream from step (ii), expansion in step (iv) 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 at least one liquid stream from step (ii) prior to expansion in step (iv), such that 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 (preferably from -25 °C) prior to expansion in step (iv). 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).

As noted above, at least one further liquid stream is obtained from step (iii). The at least one further liquid stream may be recovered from the process as a C0 ₂ product stream at substantially the same pressure as the gaseous feed stream. Alternatively, or in addition the at least one further liquid stream may be subjected to further processing to increase its purity and/or to contribute to the cooling of the gaseous feed stream.

Thus, in a preferred embodiment, the process of the invention further comprises the step of:

(v) expanding at least one more of the at least two streams from step (iii).

The expanded stream from step (v) is preferably passed in heat exchange contact with the gaseous feed stream so as to contribute further to cooling of the gaseous feed stream in step (i). In this embodiment, there are at least two streams which contribute to the cooling of the gaseous feed stream in step (i), namely the at least one expanded stream from step (iv) and the at least one expanded stream from step (v). The degree of expansion and the flow rate of each of the at least two streams may be controlled so as to closely match the cooling requirements in step (i). As above, this advantage is obtained without any detriment to product purity. Where both the at least one expanded stream from step (iv) and the at least one expanded stream from step (v) contribute to cooling in step (i), it is desirable that the expanded stream from step (v) be at a higher pressure than the expanded stream from step (iv).

The at least one expanded stream from step (v) that is passed in heat exchange contact with the gaseous feed stream preferably comprises a larger proportion of the liquid stream from step (ii) than the at least one expanded stream from step (iv). Preferably the proportion of the liquid stream from step (ii) that is expanded in step (v) and passed in heat exchange contact with the gaseous feed stream is at least 40 wt%, more preferably at least 50 wt%, still more preferably at least 60 wt% and most preferably at least 70 wt%.

Cooling in step (i) is thus preferably provided in by at least one low pressure (preferably 300 to 1200 kPa) expanded stream comprising from step (iv) a minor proportion of the liquid stream from step (ii), and by at least one intermediate pressure (preferably 1000 to 3000 kPa) expanded stream from step (v) comprising a major proportion of the liquid stream from step (ii). As suggested above, by controlling the contribution of the at least one low pressure expanded stream and the at least one intermediate pressure expanded stream to the cooling requirements in step (i), the distribution of heat in the process is highly efficient, thus minimising the power required for subsequent compression of the C0 ₂ product.

In some preferred embodiments, the expanded heated stream from step (iv), or a portion thereof, may subsequently be compressed in order to provide a compressed C0 ₂ product. The degree of compression is dependent on desired product specifications, but in preferred embodiments, the compressed C0 ₂ product will have a pressure in the range of from 8,000 to 20,000 kPa preferably 10,000 to 20,000 kPa. [as used herein the unit kPa refers to absolute pressure unless stated otherwise]. This pressure range is preferred for all compressed C0 ₂ products referred to herein.

In other preferred embodiments, the expanded heated stream from step (iv), or a portion thereof, may subsequently be recycled to the gaseous feed stream. This option may be preferred when the purity of the expanded heated stream from step (iv) is inadequate for desired product specifications. In such cases, the at least one other stream obtained from step (iii) is preferably subjected to further purification as described in more detail below.

The at least one expanded stream from step (v), or a portion thereof, following heating in heat exchange with the gaseous feed stream, may also subsequently be compressed to form a C0 ₂ product stream. In a further embodiment, the at least one expanded stream from step (v), or a portion thereof, may be further purified - either following heat exchange contact of the at least one expanded stream from step (v) with the gaseous feed stream, or without a heat exchange step. Thus, the process of the invention may further comprise the step of:

(vi) separating the at least one expanded stream from step (v) to produce a vapour stream having reduced C0 ₂ content relative to the at least one stream from step (v), and a liquid stream having increased C0 ₂ content relative to the at least one heated stream from step (v).

The process of the invention may further comprise the step of:

(vii) separating the at least one expanded stream from step (iv) to produce a vapour stream having reduced C0 ₂ content relative to the at least one stream from step (iv), and a liquid stream having increased C0 ₂ content relative to the at least one heated stream from step (iv). In this way, the expansion of the at least one stream in step (iv) and/or step (v) can be exploited to obtain a two-phase stream which may be separated to obtain a liquid C0 ₂ product stream of increased purity and a vapour phase of reduced purity. Of course, the feasibility of step (vii) will depend on the temperature and pressure of the expanded stream from step (iv). Where the expanded stream from step (iv) is at low pressure (e.g. 300 to 1200 kPa) and/or high temperature (e.g. -10 °C or greater) following heat exchange with the gaseous feed stream then the expanded stream from step (iv) will generally be routed to compression or recycled to the gaseous feed stream as discussed above. By the use of process steps (vi) and/or (vii) it has been found that the purity of the recovered CO ₂ product stream may be increased to at least 94 mol%, and more preferably at least 96 mol%. In many cases purity of 98 mol% and above, for example 99 mol% and above can be obtained. This increase in purity is obtained with negligible reduction in overall C0 ₂ recovery, which remains substantially as described above. Of course, it will be appreciated that the actual purity of the CO ₂ product stream will depend to some extent on the composition of the gaseous feed stream. Nonetheless, by the use of process steps (vi) and/or (vii), the process of the present invention provides a CO ₂ stream of increased purity when compared with a like-for-like separation using the process shown in Figure 1.

In this way, the present invention provides a further advantage over the known process shown in Figure 1 , since in a single stage separation, manipulation of the separation conditions to maximise CO ₂ recovery leads to a reduction in CO ₂ purity. Similarly, manipulation of the separation conditions to maximise CO ₂ purity leads to a reduction in CO ₂ recovery. According to the present invention, it is possible to maximise both CO ₂ recovery and CO ₂ purity. Furthermore, the use of steps (vi) and/or (vii) does not increase the overall power consumption of the process.

Separation in either of steps (vi) and (vii), where used, 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 is required of the CO ₂ product, separation in either of steps (vi) and (vii) may be by way of a fractionation column.

In a preferred embodiment, where a fractionation column is used in step (vi) to obtain a CO ₂ product stream of high purity, the at least one expanded heated stream from step (iv), or a portion thereof, is preferably recycled to the gaseous feed stream, or is compressed to form a separate CO ₂ product stream of lower purity.

An advantage of using a fractionation column in step (vi) or step (vii) 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 either of steps (vi) and (vii) is generally carried out at an intermediate temperature, for example between -15 and -40 °C. The liquid and/or vapour streams obtained from either of steps (vi) and (vii) may also therefore be used to further cool the gaseous feed stream in step (i) via heat exchange.

As above, the liquid product streams obtained from step (vi) and/or step (vii) are preferably compressed to provide a compressed C0 ₂ product.

The vapour stream obtained from step (vi) and/or step (vii) may contain a recoverable quantity of C0 ₂ . Thus, in some embodiments of the invention, at least a portion of the vapour stream from step (vi) or step (vii) is recycled to the gaseous feed stream.

The vapour stream from step (ii) is a waste stream 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 (ii) may be work-expanded, e.g. using a turbo-expander. In some embodiments, the at least a portion of the vapour stream from step (ii) is heated prior to being passed to work-expansion. More preferably, the at least a portion of the vapour stream from step (ii) 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 C0 ₂ product stream(s).

Work-expansion of the vapour stream from step (ii) 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 other embodiments of the invention, the vapour stream from step (ii) may be passed to further processing so as to recover the residual C0 ₂ content of the vapour stream. Thus, the process of the invention may further comprise the steps of:

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

(ix) passing the cooled and partially condensed stream from step (viii) to a vapour-liquid separator to provide a vapour stream having reduced C0 ₂ content relative to the vapour stream from step (ii), and a liquid stream having increased C0 ₂ content relative to the vapour stream from step (ii).

By the use of process steps (viii) and (ix) to recover residual C0 ₂ from the vapour stream from step (ii), it has been found that that the C0 ₂ recovery obtainable by the process of the invention may be increased to at least 94 mol% and in many cases the C0 ₂ recovery is 96 mol% or above, or even 98 mol% or above. In this way, the separation in step (ii) may be operated under conditions in which an increased proportion of the C0 ₂ content from the gaseous feed stream is obtained in the vapour stream from step (ii), such that the liquid stream from step (ii) has increased purity. The reduction in C0 ₂ recovery which would result using the known process shown in Figure 1 is avoided in the present invention by the use of steps (viii) and (ix).

It will be appreciated that the actual C0 ₂ recovery will depend to some extent on the composition of the gaseous feed stream. Nonetheless, by the use of process steps (viii) and (ix), the process of the present invention provides increased C0 ₂ recovery when compared with a like-for-like separation using the process shown in Figure 1.

In accordance with this aspect of the invention, at least a portion of the vapour stream obtained from step (ix) may be work-expanded, e.g. using a turbo-expander. As described above, work-expansion 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. ln some embodiments, the at least a portion of the vapour stream from step (ix) is heated prior to being passed to work-expansion. More preferably, the at least a portion of the vapour stream from step (ix) 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, further reducing the expansion requirement in step (iv) and thus further reducing the energy required for downstream compression of the C0 ₂ product stream(s).

Alternatively, or in addition, the at least a portion of the vapour stream from step (ix) is heated by heat exchange during cooling of the vapour stream from step (ii) in step (viii).

At least a portion of the liquid stream obtained from step (ix) may be recovered from the process as a C0 ₂ product stream at substantially the same pressure as the gaseous feed stream. Alternatively, or in addition, at least a portion of the liquid stream obtained from step (ix) may be subjected to further processing to increase its purity and/or to contribute to the cooling of the gaseous feed stream.

In preferred embodiments, the process of the invention further comprises the step of:

(x) expanding the liquid stream from step (ix).

Since the liquid stream from step (ix) is generally at low temperature (e.g. from -50 to -55 °C) it may be preferable in some embodiments to heat the stream from step (ix) prior to expansion in step (x). For example, the liquid stream from step (ix) may be heated in heat exchange with the gaseous feed stream and/or the vapour stream from step (ii). In preferred embodiments, the stream from step (ix) may be heated to a temperature in the range of from -25 to -45 °C prior to expansion in step (x), for example from -30 to -40 °C.

The expanded stream from step (x), or a portion thereof, may be passed in heat exchange contact with the gaseous feed stream so as to contribute further to cooling of the gaseous feed stream in step (i). Alternatively, or in addition, the expanded stream from step (x), or a portion thereof, may be passed in heat exchange contact with the vapour stream from step (ii) so as to contribute to the cooling of the vapour stream from step (ii) in step (viii). The at least one expanded stream from step (x), or a portion thereof, following heating in heat exchange with the gaseous feed stream and/or the vapour stream from step (ii), may subsequently be compressed to form a C0 ₂ product stream. Alternatively, where a 5 high purity C0 ₂ product is desired, the expanded stream from step (x), or a portion thereof, may be recycled to the gaseous feed stream.

In a further embodiment, the expanded stream from step (x), or a portion thereof, may be further purified - either following heat exchange contact of the expanded stream from 10 step (x) with the gaseous feed stream and/or the vapour stream from step (ii), or without a heat exchange step. Thus, the process of the invention may further comprise the step of:

(xi) passing the expanded stream from step (x) to a vapour-liquid separator produce a vapour stream having reduced C0 ₂ content relative to the liquid 15 stream from step (ix), and a liquid stream having increased C0 ₂ content relative to the liquid stream from step (ix).

The use of step (xi) enables further purification of the liquid stream from step (ix). In this way, a liquid stream is obtained from step (xi) having a purity which is preferably at least 20 94 mol% or above, and in many cases 96 mol% or above, or even 98 mol% or above. In this way, the liquid stream may be combined with other purified streams (e.g. from steps (vi) and/or (vii)) in downstream processing without detriment to the overall purity of the C0 ₂ product stream.

25 As noted above, the actual purity of the C0 ₂ product stream will depend to some extent on the composition of the gaseous feed stream. Nonetheless, by the use of process step (xi), the process of the present invention enables an increase in both C0 ₂ recovery and purity when compared with a like-for-like separation using the process shown in Figure 1.

30

The vapour stream from step (xi) may contain recoverable C0 ₂ 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 (xi) may be recycled to the gaseous feed stream and/or at least a portion of the vapour stream from step (xi) may be recycled to the vapour stream from step (ii).

The liquid stream from step (xi) is at a low temperature (e.g. -40 to -55 °C) following expansion in step (x) 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 (xi) 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 (viii).

Preferably, the liquid stream from step (xi), or a portion thereof, is compressed to provide a compressed C0 ₂ product.

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 C0 ₂ product. In a preferred embodiment of the invention, a multi-stage compression train may be used to compress multiple C0 ₂ 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 C0 ₂ product stream. The gaseous feed stream preferably comprises at least 40 mol% C0 ₂ , more preferably at least 50 mol% C0 ₂ , still more preferably at least 60 mol% C0 ₂ , and most preferably at least 70% by volume C0 ₂ . In certain embodiments, the dry gaseous feed may comprise, for example at least 75 mol% C0 ₂ , at least 80 mol% C0 ₂ , at least 85 mol% C0 ₂ , at least 90 mol% C0 ₂ , or at least 95 mol% C0 ₂ .

Preferably, the gaseous feed stream is substantially free of gases having a higher boiling point than C0 ₂ . The content of such gases 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%.

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 C0 ₂ 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. 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 C0 ₂ 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.

The temperature to which the gaseous feed stream is cooled in step (i) depends on the other process steps that are included. Where the vapour stream from step (ii) is not passed to a further separation stage (i.e. it is simply removed from the process as waste and/or passed to a work-expansion step to recover power), the gaseous feed stream may be supplied to step (ii) at a low temperature, for example 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, for example -51 °C, -52 °C, -53 °C, or -54 °C It will be appreciated that carbon dioxide freezes and -56.6 °C and thus -55 °C is an effective lower limit for the operating temperature in the process of the invention.

Where the vapour stream from step (ii) is passed to further separation in steps (vii) and (viii), the gaseous feed stream may be supplied to step (ii) at an intermediate temperature, for example in the range of from -15 to -40 °C, more preferably from - 20 to -35 °C. The vapour stream from step (ii) is then subsequently cooled to low temperature in step (viii), for example 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.

The at least one expanded stream from step (iv) will generally have a pressure in the range of from 300 to 1200 kPa, more preferably 500 to 1000 kPa, and most preferably 600 to 800 kPa.

The at least one expanded stream from step (v) 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.

The separation in step (vi) and/or step (vii) is preferably carried out at an intermediate temperature, for example in the range of from -15 to -40 °C, more preferably from - 20 to -35 °C.

Separation in step (ix) 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 pressure in step (ix) is as described above for the gaseous feed stream.

Separation in step (xi) 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 pressure is preferably from 500 to 1500 kPa. 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. ln another aspect, the present invention provides a C0 ₂ separation apparatus for separating C0 ₂ from a gaseous feed stream comprising C0 ₂ and at least one other gas having a lower boiling point than C0 ₂ , 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 C0 ₂ content relative the feed stream and a liquid stream having increased C0 ₂ content relative to the feed stream; and

(iii) means for dividing the liquid stream from part (ii) into at least two streams; (iv) means for expanding and heating at least one of the at least two streams from part (iii);

wherein the means for cooling in part (i) and the means for heating in part (iv) comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the at least one stream of part (iv).

In some embodiments the apparatus of the invention may further comprise means for recycling the expanded heated stream from part (iv) to the gaseous feed stream.

In a preferred embodiment, the apparatus further comprises:

(v) means for expanding at least one more of the at least two streams from part (iii).

In accordance with this embodiment, the apparatus of the invention preferably comprises one or more heat exchangers adapted to pass the gaseous feed stream in heat exchange contact with the at least one expanded stream from part (v).

The apparatus may further comprise:

(vi) means for separating the at least one stream from part (v) to produce a vapour stream having reduced C0 ₂ content relative to the at least one stream from part (v) and a liquid stream having increased C0 ₂ content relative to the at least one stream from part (v). The apparatus may further comprise:

(vii) means for separating the at least one stream from part (iv) to produce a vapour stream having reduced C0 ₂ content relative to the at least one stream from part (iv) and a liquid stream having increased C0 ₂ content relative to the at least one stream from part (iv).

The means for separating the at least one stream from part (v) in part (vi) and/or the means for separating the at least one stream from part (iv) in part (vii) may comprise a vapour-liquid separator. Alternatively, the means for separating the at least one stream from part (v) in part (vi) and/or the means for separating the at least one stream from part (iv) in part (vii) 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 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 (vi) and/or the liquid stream from part (vii).

In some embodiments of the invention, 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 vapour stream from part (vi) and/or the vapour stream from part (vii).

The apparatus of the invention may comprise means for recycling at least a portion of the vapour stream from part (vi) and/or part (vii) to the gaseous feed stream. In a preferred embodiment, the apparatus further comprises:

(viii) means for cooling and partially condensing at least a portion of the vapour stream from part (ii); and (ix) a vapour-liquid separator adapted to separate the cooled and partially condensed stream from part (viii) to provide a vapour stream having reduced C0 ₂ content relative to the vapour stream from part (ii) and a liquid stream having increased C0 ₂ content relative to the vapour stream from part (ii).

The apparatus may further comprise

(x) means for expanding the liquid stream from part (ix). 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 expanded stream from part (x) to heat the expanded stream from part (x). Alternatively, or in addition, the means for cooling the at least a portion of the vapour stream from part (ii) in part (viii) 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 expanded stream from part (x) to heat the expanded stream from part (x). In a further preferred embodiment, the apparatus according to the invention may further comprise:

(xi) a vapour-liquid separator adapted to separate the expanded stream from part (x) to produce a vapour stream having reduced C0 ₂ content relative to the liquid stream from part (ix), and a liquid stream having increased C0 ₂ content relative to the liquid stream from part (ix).

The apparatus may further comprise means for recycling at least a portion of the vapour stream from part (xi) to the gaseous feed stream and/or means for recycling at least a portion of the vapour stream from part (xi) to the vapour stream from part (ii).

The apparatus may further comprise means for heating the liquid stream from part (xi). 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 liquid stream from part (xi) to heat the liquid stream from part (xi). Alternatively, or in addition, the means for cooling and partially condensing the at least a portion of the vapour stream from part (ii) in part (viii) may comprise one or more heat exchangers adapted to pass the vapour stream from part (ii) in heat exchange contact with the liquid stream from part (xi) to heat the liquid stream from part (xi).

The apparatus of the invention may comprise 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 (ix). 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 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 (ix) to heat the vapour stream from part (ix). A turbo-expander is preferably used as 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 (ix).

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 (iv), (v), (vi), (vii) (ix) and (xi) to provide a compressed C0 ₂ 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 C0 ₂ 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 liquid stream from step (ii) is divided into two separate streams.

Figure 3 shows a process and apparatus as shown in Figure 2, wherein one of the liquid streams from step (iii) is passed to downstream separation according to step (vii).

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 high efficiency, multi-stream heat exchangers (200A and 200B) where it is cooled and partially condensed. The cooled, two phase stream (205) is passed to a vapour liquid separator (210) to give a C0 ₂ rich liquid stream (220) and a C0 ₂ lean vapour stream (215). The C0 ₂ rich liquid stream (220) is divided into two streams (240 and 245). The first stream (240) is expanded to an intermediate pressure across a valve (250) to give a low temperature, two phase stream (255). This stream is evaporated and reheated in the heat exchangers (200A and 200B) thereby providing a minor part of the refrigeration duty required to cool the feed gas stream (140). A warmed intermediate pressure C0 ₂ product stream (260) is then passed to an intermediate pressure stage of multi-stage product compressor (300), where it is compressed and cooled in consecutive stages to provide a C0 ₂ product (310) meeting product pressure requirements.

The second stream (245) is warmed in heat exchanger (200B) and the warmed stream 265 is expanded to low pressure across a valve (270) to give a low temperature, two phase stream (275). This stream is evaporated and reheated in the heat exchangers (200A and 200B) thereby providing a major part of the refrigeration duty required to cool the feed gas stream (140). A warmed low pressure C0 ₂ product stream (280) is then passed to a low pressure stage of multi-stage product compressor (300), where it is compressed and cooled in consecutive stages to provide a C0 ₂ product (310) meeting product pressure requirements.

The C0 ₂ lean gas (215), produced as the overhead vapour in the cold separator (210) is also reheated against the gaseous feed stream. The reheated stream (400) is produced at essentially feed gas pressure and power can be recovered from this stream by heating the stream in exchanger (405), and passing the heated gaseous stream (410) to a turbo expander (415). A multi-stage expander arrangement may be used. The low pressure outlet gas (420) is subsequently vented to the atmosphere.

The embodiment of the invention shown in Figure 3, corresponds to that of Figure 2, except that the low temperature, two phase stream (255) at intermediate pressure is warmed in heat exchanger (200B) against the gaseous feed stream and the resulting stream (285) is then passed to a further vapour-liquid separator (500) to provide a C0 ₂ rich liquid stream (510) and a C0 ₂ lean vapour stream (505). The C0 ₂ rich liquid stream (510) is further warmed in heat exchanger (200A) against the gaseous feed stream and the resulting stream (515) is then passed to an intermediate pressure stage of multistage product compressor (300), where it is compressed and cooled in consecutive stages to provide a C0 ₂ product (310) meeting product pressure requirements.

The C0 ₂ lean vapour stream (505) is also reheated in heat exchanger (200A) against the gaseous feed stream and is the reheated stream 520 is recycled to an intermediate stage of the multistage compression train (105).

Figure 3 also shows an alternative embodiment (see dashed lines) in which the low pressure stream (280A) is instead recycled to the multistage compression train (105). 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% C0 ₂ , 10 mol% N ₂ , 3.5 mol% Ar, and 3.0 mol% 0 ₂ .

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:

C0 ₂ (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% C0 ₂

Recovery of C0 ₂

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 83.5 mol% C0 ₂ , 3.5 mol% Ar, 10 mol% N ₂ , and 3.0 mol% 0 ₂ , at a flow rate of 1 1 ,930 kg-mol-h ^"1 . It will be observed that the process of the invention is capable of separating this mixture to obtain a compressed C0 ₂ product with the same purity and recovery as in comparative Example 1 , but wherein the energy requirement for product gas compression are reduced from 17.4 MW to 15.1 MW (i.e. a 13.2% reduction in product gas compression energy requirements). Table 2

Stream Number 140 205 245 255 260 265

Vapour (mole 1.0000 0.1669 0.0000 0.0276 1.0000 0.0270 fraction)

Temperature (°C) 30.0 -51.5 -51.5 -52.8 20.6 -32.9

Pressure (kPa(a)) 3000 3000 3000 2100 2100 3000

Mass Flow (kg/h) 499960 499960 75681 355817 355817 75681

Molar Flow:

C0 ₂ (kgmol/hr) 9961 9961 1654 7776 7776 1654

Argon (kgmol/hr) 418 418 27 126 126 27

Nitrogen (kgmol/hr) 1 193 1 193 45 21 1 21 1 45

Oxygen (kgmol/hr) 358 358 18 84 84 18

Total (kgmol/h) 1 1930 1 1930 1743 8196 8196 1743 Stream Number 275 280 310 215 420

Vapour (mole 0.1826 1.0000 0.0000 1.0000 1.0000

fraction)

Temperature (°C) -55.0 20.6 30.0 -51.5 12.8

Pressure (kPa(a)) 740 740 15000 3000 101

Mass Flow (kg/h) 75681 75681 431498 68462 68462

Molar Flow:

C0 ₂ (kgmol/hr) 1654 1654 9429 532 532

Argon (kgmol/hr) 27 27 153 265 265

Nitrogen (kgmol/hr) 45 45 255 938 938

Oxygen (kgmol/hr) 18 18 101 257 257

Total (kgmol/h) 1743 1743 9939 1991 1991

Feed Compression Power

Product Compression Power

Purity 94.9 mol% CO ₂

Recovery of CO ₂ 94.7 %

Example 3

Table 3 shows typical operating parameters for the process of the invention using the apparatus shown in Figure 3 when used to separate a gaseous mixture consisting of 83.0 mol% CO ₂ , 4.0 mol% Ar, 10 mol% N ₂ , and 3.0 mol% O ₂ , at a flow rate of 14,328 kg-mol-h ^"1 . It will be observed that the process of the invention is capable of separating this mixture to obtain a compressed CO ₂ product significantly increased purity (98.5 mol%) relative to comparative Example 1 , but still with reduced energy requirements for product gas compression (15.8 MW, i.e. a 9.2% reduction relative to comparative Example 1 ). This large increase in purity and reduction in power requirements is obtained with only a small reduction in overall CO ₂ recovery. Table 3

Stream Number 310 215 420

Vapour (mole

fraction) 0.0000 1.0000 1.0000

Temperature (°C) 30.0 -51.5 12.1

Pressure (kPa(a)) 15000 3000 101

Mass Flow (kg/h) 413908 86055 86055

Molar Flow:

CO ₂ (kgmol/hr) 9297 665 665

Argon (kgmol/hr) 54 363 363

Nitrogen (kgmol/hr) 60 1 133 1 133

Oxygen (kgmol/hr) 29 329 329

Total (kgmol/h) 9440 2491 2491 Feed Compression Power 38.7 MW

Product Compression Power 15.8 MW

54.5 MW

Purity 98.5 mol% C0 ₂

Recovery of C0 ₂ 93.3 %