Gas separation system with auto-refrigeration and selective recycle for carbon dioxide capture and compression

Abstract

A system and method for capturing and separating carbon dioxide from mixed gas streams. The gas stream is processed in a structure including a compression module comprising a plurality of compressors, intercoolers and inter-stage condensate separators. The flow path from the compression module includes a plurality of flow separators, gas stream splitters, heat exchangers and at least a first mixer and a first expander. The gas stream is sequentially compressed and cooled to form process condensate and separate it from the compressed gas stream. The gas stream is further dried and cooled to liquefy carbon dioxide and separate it from the non-condensable portion. Selective expansion of liquid carbon dioxide streams provides cooling for the system, and further energy efficiency is achieved by selective recycling of portions of gas streams, allowing for compact equipment and economical operation, while providing for high purity product streams of carbon dioxide.

Claims

1. A method for separating carbon dioxide from a mixed gas stream, the method comprising the steps of: (a) providing a processing structure including a compression module comprising at least a plurality of compressors, a plurality of intercoolers with inter-stage condensate separators, the processing structure further including a plurality of flow separators, at least one stream splitter, a plurality of heat exchangers, a plurality of mixers, at least a first expander, and a throttle valve; (b) delivering the mixed gas stream to the compression module; (c) (I) compressing and cooling the mixed gas stream through each of the plurality of compressors and intercoolers in sequence to form a compressed gas stream; (c) (II) generating output streams from each of the plurality of heat exchangers and the first expander, the output streams each including liquid carbon dioxide and a gaseous stream; (d) removing the liquid carbon dioxide from each of the said output streams in first, second, and third flow separators to generate from each selected flow separator a first, second, and third, respectively, liquid carbon dioxide stream and a first, second, and third, respectively, separated gaseous stream; (e) mixing in a first mixer of the plurality of mixers the first and the third liquid carbon dioxide streams generated in step (d) to form a first mixed liquid carbon dioxide stream and sending at least a first portion of the first mixed liquid carbon dioxide stream through to a second mixer to mix with the second liquid carbon dioxide stream to form a second mixed liquid carbon dioxide stream; (f) throttling at least a portion of the second mixed liquid carbon dioxide stream from step (e) in the throttle valve; (g) passing at least a portion of the throttled second mixed liquid carbon dioxide stream from step (f) through the plurality of heat exchangers to form a carbon dioxide product stream and removing the carbon dioxide product stream from the processing structure; (h) (I) splitting the first separated gaseous stream in a first one of the at least one stream splitter to form first and second portions of a split stream; (h) (II) using the second separated gaseous stream to cool the first portion of the split stream; (h) (III) using the third separated gaseous stream to cool the compressed gas stream to form a liquid carbon dioxide portion of the output stream of a first one of the plurality of heat exchangers; (h) (IV) removing the second separated gaseous stream from the processing structure; (i) recycling the third separated gaseous stream to at least one of the compressors selected from a second compressor and a compressor thereafter in the sequence in the compression module; and (j) expanding in the first expander a second portion of the split stream from the selected stream splitter in step (h) (I), to recover energy and to provide cooling to the processing structure.

2. The method according to claim 1, wherein: (c.1) the generating step of step (c) (II) comprises passing the compressed gas stream through a first one of the heat exchangers to form a first one of the output streams comprising a first two-phase flow; (c.2) the removing step of step (d) comprises separating the first two-phase flow into the first liquid carbon dioxide stream comprising at least 90% carbon dioxide and the first gaseous stream including residual carbon dioxide; (c.3) the splitting step of step (h) (I) comprises splitting the first separated gaseous stream wherein the first portion of the split stream is a first gas stream branch and the second portion of the split stream is a second gas stream branch; (c.4) the generating step of step (c) (II) further comprises passing the first gas stream branch through a second one of the heat exchangers to form a second one of the output streams comprising a second two-phase flow; (c.5) the removing step of step (d) further comprises separating the second two-phase flow into the second liquid carbon dioxide stream comprising at least 90% carbon dioxide and the second gaseous stream including impurities and residual carbon dioxide; (c.6) passing the second gaseous stream in sequence through the second heat exchanger and the first heat exchanger to form a fourth gas stream comprising impurities and residual carbon dioxide; (c.7) the generating step of step (c) (II) further comprises expanding the second gas stream branch to produce a third one of the output streams comprising a third two-phase flow; (c.8) the removing step of step (d) further comprises separating the third two-phase flow into the third liquid carbon dioxide stream comprising at least 90% carbon dioxide and the third gaseous stream including residual carbon dioxide; (c.9) passing the third gaseous stream through the first heat exchanger; (c.10) passing the third liquid carbon dioxide stream through a pressurizing means to raise the pressure of the third liquid carbon dioxide stream to match the pressure of the first liquid carbon dioxide stream, and thereafter performing the step of mixing the first liquid carbon dioxide stream with the third liquid carbon dioxide stream in the first mixer to form the first mixed liquid carbon dioxide stream; (c.11) passing the second liquid carbon dioxide stream through the second heat exchanger; (c.12) the throttling step of step (f) comprises passing the at least a portion of the second mixed liquid carbon dioxide stream through the throttle valve to form a first cold throttled stream including a gaseous portion and a liquid portion; and (c.13) the passing step of step (g) comprises passing at least a portion of the first cold throttled stream through the second heat exchanger and the first heat exchanger, in sequence, to form the carbon dioxide product stream comprising at least 90% carbon dioxide.

3. The method according to claim 2, further comprising the step of expanding the fourth gas stream in a vent stream expander to recover energy and to form a vent stream comprising impurities and residual carbon dioxide.

4. The method according to claim 3, further comprising splitting the vent stream into a first vent stream branch and a second vent stream branch.

5. The method according to claim 4, wherein, also after step (b) and before passing the compressed gas stream through one of the plurality of heat exchangers, passing the compressed gas stream through a first additional heat exchanger, and passing the first vent stream branch through the first additional heat exchanger.

6. The method according to claim 4, further including passing the second gas stream branch through a second additional heat exchanger and passing the second vent stream branch through the second additional heat exchanger.

7. The method according to claim 2, further comprising after step (c.13), the step of raising the pressure of the product stream to higher set pressure to form a pressurized product stream.

8. The method according to claim 2, wherein expanding the second portion of the split stream, which is the second gas stream branch of step (c.7), comprises expanding the second gas stream branch in the first expander to recover energy, and to produce the third two-phase flow.

9. The method according to claim 3, wherein expanding the second portion of the split stream, which is the second gas stream branch of step (c.7), comprises expanding the second gas stream branch in the first expander to recover energy, and to produce the third two-phase flow.

10. The method according to claim 2, further comprising removing oxygen from selected ones of each of the product or vent streams before removing the selected streams from the processing structure.

11. The method according to claim 3, further comprising removing oxygen from selected ones of each of the product or vent streams before removing the selected streams from the processing structure.

12. The method according to claim 1, further comprising pre-treating the mixed gas stream by removing at least one of water, particulate matter, mercury and other heavy metals, hydrogen chloride, hydrogen fluoride, nitrogen oxides, sulphur oxides and other sulphur derivatives from the mixed gas stream.

13. The method according to claim 2, further comprising, before passing the compressed gas stream through any of the plurality of heat exchangers to generate the output streams, drying the compressed gas stream to form a dried compressed gas stream having a dew point temperature at least one degree C. lower than a lowest operational temperature of each of the steps of the method.

14. The method according to claim 2, further including removing the fourth gas stream out of the processing structure.

15. The method according to claim 2, further including passing the second mixed liquid carbon dioxide stream through a second one of the at least one stream splitter to form the at least the portion of the second mixed liquid carbon dioxide stream and a splitted portion of the second mixed liquid carbon dioxide stream.

16. The method according to claim 15, further including recycling a first part of the splitted portion of the second mixed liquid carbon dioxide stream and a first part of the at least the portion of the second mixed liquid carbon dioxide stream by compressing and then mixing with the compressed gas stream, and removing a second part of the splitted portion of the second mixed liquid carbon dioxide stream and a second part of the at least the portion of the second mixed liquid carbon dioxide stream out of the processing structure.

17. The method according to claim 1, further including passing a second portion of the first mixed liquid carbon dioxide stream mixed in step (f) through at least one of the plurality of heat exchangers to form a second carbon dioxide product stream and removing the second carbon dioxide product stream from the processing structure.

18. A system for separating carbon dioxide from a mixed gas stream, the system comprising: a processing structure including a compression module comprising at least a plurality of compressors, a plurality of intercoolers with inter-stage condensate separators, the processing structure further including a plurality of flow separators, at least one stream splitter, a plurality of heat exchangers, a plurality of mixers, at least a first expander, and a throttle valve; the mixed gas stream being delivered to the compression module; the mixed gas stream being compressed and cooled through each of the plurality of compressors and intercoolers in sequence to form a compressed gas stream; output streams are generated from each of the plurality of heat exchangers and the first expander, the output streams each including liquid carbon dioxide and a gaseous stream; the liquid carbon dioxide is removed from each of the said output streams in first, second, and third flow separators to generate from each selected flow separator a first, second, and third, respectively, liquid carbon dioxide stream and a first, second, and third, respectively, separated gaseous stream; a first mixer of the plurality of mixers mixing the first and the third liquid carbon dioxide streams to form a first mixed liquid carbon dioxide stream; at least a first portion of the first mixed liquid carbon dioxide stream is sent through to a second mixer to mix with the second liquid carbon dioxide stream to form a second mixed liquid carbon dioxide stream; at least a portion of the second mixed liquid carbon dioxide stream is mixed in the throttle valve; at least a portion of the throttled second mixed liquid carbon dioxide stream is passed through the plurality of heat exchangers to form a carbon dioxide product stream and the carbon dioxide product stream is removed from the processing structure; the first separated gaseous stream is split in a first one of the at least one stream splitter to form first and second portions of a split stream; the second separated gaseous stream is used to cool the first portion of the split stream; the third separated gaseous stream is used to cool the compressed gas stream to form a liquid carbon dioxide portion of the output stream of a first one of the plurality of heat exchangers; the second separated gaseous stream is removed from the processing structure; the third separated gaseous stream is recycled to at least one of the compressors selected from a second compressor and a compressor thereafter in the sequence in the compression module; and a second portion of the split stream from the selected stream splitter is expanded to recover energy and to provide cooling to the processing structure.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described with reference to the drawings, in which

(2) FIG. 1 is a schematic representation of a first embodiment of the invention;

(3) FIG. 2 and is schematic representation of a second embodiment of the invention,

(4) while FIG. 3 showing further features of the embodiment of FIG. 2;

(5) FIG. 4 is a schematic representation of a second embodiment of the invention;

(6) FIGS. 5 and 6 are schematic representations showing further features of the embodiment of FIG. 4;

(7) FIGS. 7 to 10 are schematic representations showing further features of embodiments of the invention;

(8) FIG. 11 is a schematic representation of a start-up feature in a third embodiment of the invention;

(9) FIG. 12 is a schematic representation of a fourth embodiment of the invention;

(10) FIG. 13 is a schematic representation of a fifth embodiment of the invention; and

(11) FIG. 14 is a schematic representation showing further features of the first and second embodiments of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

(12) Referring to FIG. 1, a first embodiment of the method and system of the invention is shown schematically. As depicted in FIG. 1, the inlet carbon dioxide rich gas stream enters the process structure as inlet feed gas in path 1, and is compressed in compressor module CM1, and substantially dehydrated in dryer module D1. The compressor block must consist of a minimum of two compression stages, preferably three or four, with inter-stage coolers and condensate separators. Dryer module D1 can comprise any suitable dryer system, such as a molecular sieve, where the process gas will not be contaminated but only be dried, to achieve a water dew point temperature equal to or lower than the gas stream temperature in path 9. In the event that this dew point temperature is not practical for any reason, based on factors such as cost or equipment, the next plausible temperature to be used is the dew point temperature at path 21. After leaving the dryer the gas stream proceeds in path 3 to the heat exchanger E1, where it is cooled, leading to a two-phase flow in path 4, from heat exchanger E1 to separator S1, where the two phases are separated. The gas stream leaving separator S1 passes in path 5 to splitter SP1, where it is split into two branches as follows.

(13) The first branch flows in path 23 to and through heat exchanger E2, leaving in path 6 as a two phase flow, into separator S2. The gas stream leaving separator S2 consists of non-condensable gas phase impurities, such as argon, nitrogen, oxygen, and possibly NO.sub.x and SO.sub.x, and residual carbon dioxide, which have remained in the gaseous state. This stream flows in path 7 to and through heat exchanger E2, and from heat exchanger E2 in path 14 to heat exchanger E1 and flows therefrom in path 15 to be exhausted to atmosphere by any suitable means (not shown) through a vent with or without a silencer in path 15. The liquid stream leaving separator S2 flows in path 8 to and through heat exchanger E2 to mixer M2, where it is mixed with flow in path 18a, discussed below, and the mixed stream 9a flows in path 9 through throttle valve TV2, back through heat exchanger E2, and flows in path 10 to heat exchanger E1, and leaves the system as in path 11 as a product stream.

(14) The second branch from splitter SP1 flows in path 19 to and through expander module EM1, and as the gas passes through expander module EM1 it cools resulting in a two phase flow 19a to separator S3. The use of expander module EM1 allows for the production of necessary cooling for liquefaction in the system, and also for additional shaft output work, thereby enhancing the overall energy balance and improving the efficiency of the process. Once in separator S3, the gas and liquid phases are separated. The gas stream leaving separator S3 flows in path 21 through heat exchanger E1, leaving heat exchanger E1 in path 22, and is recycled back to an appropriate selected location in the compressor module CM1. This location must be subsequent to the inlet of path 1 into the compressor module CM1, i.e. the gas stream in path 22 must be returned to one of the intermediate compression stages within compressor module CM1.

(15) The liquid stream from separator S3 flows in path 20 to pump P1, where its pressure is raised to match that of the liquid stream leaving separator S1 in path 12. The liquid stream in path 12 and the liquid stream leaving pump P1 in path 20 are combined in mixer M1. The combined liquid streams flow from mixer M1 in path 18a to mixer M2, to be mixed with the flow in path 8 from separator S2 and heat exchanger E2, to flow in path 9a to enter the throttle valve TV2 as described above. This diversion which takes place in path 18a allows for the maximum cooling effect from the throttling process occurring in path 9.

(16) This configuration provides only one product stream, i.e. the flow in path 11, from the overall process, which allows for simplification of the carbon dioxide product piping, and of the multi-pass design for heat exchanger E1.

(17) FIGS. 2 and 3 are schematic representations of a high purity configuration respresenting a second embodiment of the method and system of the invention. In this configuration, an additional separator Sb is added to path 9 down stream of throttle valve TV2. The liquid stream from the separator Sb is a highly pure carbon dioxide product stream, which flows in paths 10 and 11, as in the configuration shown in FIGS. 2 and 3. However, the gaseous stream leaving separator Sb is recycled back in path 25 to the compressor module CM1.

(18) FIG. 3 is a further variant of the configuration of FIG. 2. In this configuration, the liquid stream from the separator Sb flows in paths 10 and 11 as in the configurations shown in FIG. 2. However, the gaseous stream from separator Sb flows in path 25 to an additional compressor, compressor module CMb, where it is further compressed. The compressed stream 25a leaving compressor module CMb flows to mixer M4, where it is combined with the outlet stream flowing in path 3 from dryer module D1, and the mixed flow passes from mixer M4 into heat exchanger E1. The configuration of FIG. 3 produces the same high purity of carbon dioxide in the product stream in path 11 as in the configuration of FIG. 2, but the configuration of FIG. 3 provides more flexibility to the overall operation of the high purity variant processes.

(19) FIG. 14 is yet another variant of FIG. 1. In this configuration, the gas stream 15 exiting heat exchanger E1 is expanded in a vent stream expander EM2 to recover energy and to form a vent stream 16 comprising impurities and residual carbon dioxide. Further, the vent stream 16 is split into two vent stream branches 16a and 16b; preferably, the vent stream branch 16a and the compressed gas stream 3 are each passed through an additional heat exchanger E1a, and the gas stream branch 19 and the vent stream branch 16b are each passed through another additional heat exchanger E2a.

(20) Referring now to FIGS. 4 to 6, a second embodiment of the the method and system of the invention is shown schematically, FIGS. 5 and 6 showing variants of the pathways depicted in FIG. 4.

(21) In the configuration shown in FIG. 4, the flow from mixer M1 in path 18a is diverted to mixer M2, as in the configuration shown in FIGS. 1 to 3. However, the combined stream leaving mixer M2 in path 8 flows to splitter SP2, where it is divided into two streams, flowing in paths 9 and 27 respectively. The stream in path 9 follows the same path as in the embodiment shown in FIGS. 1 to 3, to generate the carbon dioxide product stream in paths 10 and 11. However, the stream flowing from splitter SP2 in path 27 passes through throttle valve TVc, and provides cooling energy to heat exchanger E1, before exiting the process as a second carbon dioxide product stream in path 13.

(22) FIG. 5 is a schematic representation of a variant of the configuration of FIG. 4, in which two separators, Sc and Sd are added to the down stream flows in paths 9 and 27, respectively. The liquid stream from separator Sc flows in path 10 as a highly pure carbon dioxide product stream; similarly the liquid stream flowing from separator Sd in paths 28 and 13 is a highly pure carbon dioxide product streams. The gaseous streams (Stream 26 and Stream 29) from each of separators Sc and Sd in paths 26 and 29, respectively, are recycled back to the compressor module CM1, again at an intermediate stage, via mixer M5.

(23) FIG. 6 is a schematic representation of a variant of the configuration of FIG. 5, in which the gaseous streams in paths 26 and 29 from the separators Sc and Sd, respectively, are compressed by an additional compressor module CMc. The compressed stream from compressor module CMc flows in path 30 to be combined with the outlet stream flowing in path 3 from dryer module DM1 in mixer M6, before entering into heat exchanger E1. This option provides the same purity of carbon dioxide in the product streams in paths 11 and 13, but provides more flexibility to the overall operation of the high purity variant processes.

(24) For each of the configurations shown in FIGS. 4, 5 and 6, a slight variation can be achieved, in that the stream in path 8 can be diverted directly from mixer M2 to splitter SP2, instead of passing through heat exchanger E2. Other similar variations will be apparent to those with skill in the art.

(25) Various options can be provided to the configurations shown in each of FIGS. 1 to 6. These are illustrated by the schematic representations of FIGS. 7 to 10.

(26) Referring to FIG. 7, a second expander module EM2 is added to path 15 on the down stream side of heat exchanger E1, to harness more cooling energy and shaft output work, which reduces the overall energy demand and hence increases the overall efficiency of the process. In addition, the stream leaving the dryer module D1 in path 3 can be further cooled by the stream from second expander module EM2 in path 16, by the addition of heat exchanger E1a. The main advantageous features of this configuration option are:

(27) (1) Efficient use of the cooling energy available from the expanded stream in path 16, which ensures minimum losses in the available cooling energy from the stream in path 16;

(28) (2) The option of raising the temperature of the stream in path 16 to ambient condition before being sent to the atmosphere without using a heater; and

(29) (3) Significant reduction of the cooling load on heat exchanger E1 due to a lower inlet temperature from the stream in path 3 compared to a configuration without this option, which allows for the use of a smaller heat exchanger E1.

(30) Also shown in FIG. 7 is the addition of a second compressor module CM2 to the streams in paths 11 and 13 to increase the pressure of the carbon dioxide product streams in those paths to a level required for e.g. pipeline transportation. Further, considerable heating energy can also be harnessed from the inter-stage cooling of compressor module CM2 for use in another integrated energy conversion system with which the system of the present invention might be connected.

(31) Referring now to FIG. 8, as a further option, the stream leaving splitter SP1 in path 19 can be further cooled by the stream in path 16 by the addition of heat exchanger Eta. The main features of this option are:

(32) (1) Efficient use of the cooling energy available from the expanded stream in path 16, which ensures minimum losses in the available cooling energy from the stream in path 16; and

(33) (2) Ability to lower the temperature of the stream in path 19 rapidly during startup of the process, to facilitate quickly reaching steady state conditions after the startup.

(34) Referring now to FIG. 9, the options of FIG. 7 and FIG. 8 can be implemented simultaneously via a splitter SP2. In this configuration, splitter SP2 can be adjusted from 0 to 100% between the streams in paths 16a and 16b as desired. The main advantageous features of this option, in addition to those listed above in relation to FIGS. 7 and 8, include the important features

(35) (1) Significant reduction of the cooling load on heat exchanger E1 and eventually on heat exchanger E2 due to a low inlet temperature from the stream in path 3, which allows for the use of smaller heat exchangers than in the configurations without this option; and
(2) Efficient and near optimum temperature control over the whole process range during startup and steady state (normal operating) conditions.

(36) Referring now to FIG. 10, the expander module EM1, which receives the stream which flows in path 19 from splitter SP1, can be replaced by a Joule-Thompson Expansion Valve JT-1 and a chiller CH1. This option can be combined with the configurations of any of the options discussed above.

(37) For each embodiment of the system and method of the invention, additional features are provided for the startup of operation. A configuration exemplifying the start up procedure is shown schematically in FIG. 11. In this embodiment, the pressure of the stream in path 20 is increased by any suitable pressure boosting device, such as pump P1, to match the pressure of the stream in path 12, following which the streams from paths 12 and 20 can be combined in mixer M1. The combined stream is then diverted proportionally, and the proportion can be varied between 0 and 100% as desired, so as to flow from mixer M1 in the desired proportions in paths 18 and 18a. The stream in path 18a is directed to mixer M2, and thence to heat exchanger E2 throttle valve TV2, in path 9. The stream in path 18 flows from mixer M1 to throttle valve TV1 to heat exchanger E1 and leaves as product in path 13. This diversion and subsequent throttling provides maximum cooling to produce enough liquid carbon dioxide required for throttling and stabilizing the overall process. Also, this diversion of the portion of the combined streams in paths 12 and 20 which flows in path 18, balances the overall cooling load to the individual heat exchangers E1, E2.

(38) Referring now to FIG. 12, this is a schematic representation of a configuration of an embodiment of the invention in which the carbon dioxide purity in the product stream which flows in path 13 can be greater than 98% by volume. In this configuration, an additional separator Sa is added after the throttle valve TV1. Liquid from the separator Sa is taken out as a highly pure carbon dioxide product stream in paths 13 or 18, and the gaseous stream in path 24 is recycled back to the compressor module CM1 at an intermediate stage of that module.

(39) As further options, any of the additional optional features described above in relation to FIGS. 7 to 10 can be included in the configuration shown in FIG. 12.

(40) Referring now to FIG. 13, a further embodiment is shown, in which the exemplary startup operation of FIG. 11 is shown in relation to the embodiment of FIG. 6. In this embodiment, the combined stream in mixer M1 is diverted proportionally between the stream which flows in path 18a to mixer M2, as described in relation to FIG. 11, and a second stream, which flows from mixer M1 to throttle valve Tve, and thence to heat exchanger E1, to leave the system as a third product stream in path 11a. FIG. 13 illustrates pre-treating the gas stream in a Pretreater PT1 by removing at least one of water, particulate matter, mercury and other heavy metals, hydrogen chloride, hydrogen fluoride, nitrogen oxides, sulphur oxides and other sulphur derivatives from the gas stream. FIG. 13 also illustrates raising the pressure of the first product stream to a higher set pressure to form a first pressurized product stream, raising the pressure of the second product stream to a higher set pressure to form a second pressurized product stream, and raising the pressure of the third product stream to a higher set pressure to form a third pressurized product stream with the compressor module CM2 at 17.

(41) The embodiments described above thus provide for more efficient and cost-effective separation of carbon dioxide from carbon dioxide rich gas streams, by the use of the low-temperature gas separation processes of the invention, including the features described which provide for auto-refrigeration and gas recycling, by providing compression to the inlet gas streams in multiple stages with inter-stage cooling and condensate removal, while using the energy in the compressed gas to provide cooling to the incoming stream, and at the same time using an expansion stage before recycling a portion of the gas back to the compressor, at some intermediate stage within the multiple compression stages. The invention enables the reduction of the overall energy demand and temperature of the process without the use of external refrigeration means, in a simple and compact system, without the disadvantages of known processes and systems, using the novel arrangement of process flow pathways, described above in relation to the exemplary and non-restrictive embodiments, and more fully defined in the appended claims.