Method and apparatus for separating a gaseous mixture containing carbon dioxide

Abstract

Title: Process and apparatus for separating a gaseous mixture containing carbon dioxide. Process for separating a first gaseous mixture (05) containing carbon dioxide and at least one impurity lighter than carbon dioxide, the first gaseous mixture originating from a storage (70) of liquid carbon dioxide, wherein the first gaseous mixture is sent to the column vessel if its CO2 content is above a CO2 purity threshold and is sent to a level at least two theoretical trays above the column vessel if its CO2 content is below the CO2 purity threshold. Single figure

Description

Title of the invention: Method and apparatus for separating a gas mixture containing carbon dioxide

[0001] The invention relates to a method and apparatus for separating a gas mixture containing carbon dioxide and at least one impurity lighter than carbon dioxide by phase separation and / or by distillation. According to such a method, a gas mixture containing carbon dioxide and at least one light impurity (02, H2, CO etc.), which is for example a boil-off gas from a liquid carbon dioxide storage facility, can be separated forming a liquid depleted in light impurities which can be returned to the storage facility in liquid form or reused elsewhere in the process. This makes it possible to produce purified carbon dioxide from the boil-off gas from the storage facilities in liquid form, the liquid being returned to at least one fixed storage facility or one which can be transported through loading systems.

[0002] Boil-off gases (BOGs) are generally generated on the final equipment of a CO2 separation plant: • In at least one fixed CO2 storage and / or • In the CO2 loading systems and / or • During CO2 loading operations into transportable storage (e.g. on ships / tankers / trains) or into a pipeline.

[0003] These CO2-rich gases, generated by several phenomena, are CO2 losses if they are not recycled or liquefied and returned because they are generally sent to a vent.

[0004] With the increase in demand for CO2 capture and separation units and the increase in the capacity of these units, it becomes necessary to treat the BOG (boil-off gas) generated.

[0005] KR20130047951 describes a method in which the evaporated gas from a CO2 storage liquid is liquefied by heat exchange with LNG and returned to storage.

[0006] KR101242949 describes a process in which evaporated gas from a CO2 storage facility is reheated, compressed and expanded to partially liquefy it, with the produced liquid being returned to the storage facility.

[0007] In a small-scale CO2 separation plant, the evaporated gases generated in the storage and loading systems are currently: • Used for the regeneration of the adsorbent which is used to dry the gas to be liquefied and then released into the atmosphere or • Released directly into the atmosphere.

[0008] The method according to the invention consists in recycling the evaporated gas, for example from storage and loading systems, to a unit for separating a CO2 mixture by distillation and / or partial condensation at a suitable location where the conditions (pressure / temperature) are favorable. In addition, recycling the BOG to the separation unit increases the demand for coolant. The invention describes the best location for recycling it in order to optimize the increase in fluid circulating on the liquefier.

[0009] According to an object of the invention, it is planned to return the evaporated gases to a phase separator and / or a distillation column located upstream or downstream of the storage or in another CO2 separation process.

[0010] A distillation column contains means for promoting the exchange of heat and matter, such as trays, structured or bulk packings.

[0011] A phase separator does not contain means to promote the exchange of heat and matter, essentially performing a separation equivalent to a single theoretical plate.

[0012] Refrigeration for separation may come from a refrigerant cycle containing CO2 in an open or closed cycle. It is also possible to have a cascade scheme containing or other type of compound such as NH3 or any other refrigerant allowing the liquefaction of CO2.

[0013] According to an object of the invention, there is provided a method for separating a first gas mixture containing carbon dioxide and at least one impurity lighter than carbon dioxide, the first gas mixture originating from a liquid carbon dioxide storage or being produced during the filling of a liquid carbon dioxide storage in which the first gas mixture as well as a second gas mixture containing carbon dioxide and at least one impurity lighter than carbon dioxide is separated in a distillation column of a separation system comprising a heat exchanger and the at least one distillation column to form a liquid flow purer in carbon dioxide than the second gas mixture and a gas less pure in carbon dioxide than the second gas mixture, the second gas mixture cools in the heat exchanger upstream of the separation,the liquid flow is withdrawn from a distillation column of the separation system, at least a first part of the liquid flow is optionally pressurized and sent to at least one liquid carbon dioxide storage or even to the liquid carbon dioxide storage from which the first gas mixture comes, characterized in that the first gas mixture is sent to the bottom of the column if its content of at least one light impurity is lower than a purity threshold for this at least one impurity and / or if its CO2 content is higher than a CO2 purity threshold and is sent to a level at least two theoretical plates, above the column tank if its content of at least one light impurity is greater than the purity threshold for this at least one impurity and / or if its CO2 content is less than the CO2 purity threshold.

[0014] According to other optional features: • the first gas mixture is sent to the column without having been heated or cooled in the heat exchanger. • the first gas mixture comes from storage operating at a pressure equal to or greater than the pressure of the distillation column. • the first gas mixture comes from storage operating at a pressure higher than the pressure of the distillation column and is expanded upstream of the column. • the second gas mixture is sent to the column at a level at least two theoretical plates above the column tank. • the first gas mixture is mixed with the second gas mixture upstream of the column and the mixed flow is sent to the level at least two theoretical plates above the column tank if the content of the first gas mixture in the at least one light impurity is greater than a purity threshold and / or if the flow rate of the first gas mixture is less than a flow rate threshold. • a reboiling flow is sent to the column tank. • the first gas mixture is mixed with the reboil flow and the mixed reboil flow is sent to the column bottom if its content of at least one light impurity is less than a purity threshold for this at least one impurity and / or if its CO2 content is greater than a CO2 purity threshold. • the reboiling flow rate is reduced, possibly to zero, if the first gas mixture is sent to the column bottom. • the first gas mixture comes from a source other than storage supplied by liquid from the column tank. • the first gaseous mixture containing carbon dioxide and at least one impurity is at a temperature below -45°C in a pipe bringing it from the storage to the at least one column. • the first gas mixture is sent to the atmosphere if its CO2 content is less than a value below the CO2 purity threshold and / or if its content of at least one impurity is greater than a value above the purity threshold of the at least one impurity. • the storage from which the gas comes can operate at a pressure of at least 10 bars

[0015] the column operates at between 10 and 20 bara.

[0016] According to another object of the invention, there is provided an apparatus for separating a first gas mixture containing carbon dioxide and at least one impurity lighter than carbon dioxide, comprising means for supplying the first gas mixture from a liquid carbon dioxide storage or a filling line of a liquid carbon dioxide storage, a separation system comprising a heat exchanger, at least one distillation column, and optionally at least one phase separator, means for sending the first gas mixture to separate in the separation system, means for sending a second gas mixture containing carbon dioxide and at least one impurity lighter than carbon dioxide to cool, or even condense, in the heat exchanger,means for causing the second mixture cooled in the heat exchanger to separate in the at least one phase separator and / or the at least one distillation column to form a liquid flow purer in carbon dioxide than the second gaseous mixture and a gas less pure in carbon dioxide than the second gaseous mixture, means for withdrawing the liquid flow from a phase separator or a distillation column of the separation system, possibly a pump for pressurizing at least a first part of the liquid flow, means for sending the at least a first part of the liquid flow to a liquid carbon dioxide storage or even to the liquid carbon dioxide storage from which the first gaseous mixture comes, characterized in that it comprises an analyzer for analyzing the content of CO2 and / or of the at least one impurity,in that the means for bringing the first gas mixture from a liquid carbon dioxide storage or a filling line of a liquid carbon dioxide storage are connected to the tank of the distillation column to introduce the first gas mixture therein to reboil the column as well as to a level of the distillation column at a first level at least two theoretical plates above the tank to introduce the first gas mixture therein to separate in the column and in that it comprises means for opening the means for bringing the first mixture to the tank or to the first level depending on the content or contents detected by the analyzer.

[0017] The apparatus may comprise means for sending the first gas mixture to the atmosphere and means for enabling the sending of the first mixture to the atmosphere as a function of the CO2 content and / or the at least one impurity in the first mixture.

[0018] The invention will be described in more detail, with reference to the figure where:

[0019] [Fig-1] represents a method according to the invention.

[0020] [Fig. 1] shows a method adapted for example as a function of the pressure of a liquid CO2 storage, the pressure and / or the purity of the BOG of the storage and the quantity of BOG to be recycled.

[0021] In [Fig.l], the apparatus for separating the first gas mixture 05 comprising CO2 and at least one impurity lighter than CO2 such as hydrogen, helium, carbon monoxide, nitrogen, argon or oxygen is constituted by a heat exchanger 20 and a distillation column 40.

[0022] The source 70 of the gas mixture 05 can be: • at least one fixed storage of liquid CO2 in which an evaporation gas O5 is generated by the heat inputs and / or • at least one liquid CO2 loading system connected to a storage producing an 05 evaporation gas and / or • at least one transportable storage (e.g. on ships / tankers / trains), at least part of the 05 gas mixture being generated during the liquid CO2 loading operations.

[0023] The gas source 70 can be supplied exclusively with liquid CO2 by the apparatus of [Fig.l], exclusively by at least one other apparatus for producing liquid CO2 or by both.

[0024] Here the column 40 is heated in the bottom by a flow of reboiling gas 07, which is cooled in the heat exchanger 20. Alternatively, this CO2-rich gas 07 can be formed by vaporizing a portion of the bottom liquid of the column 40 in the exchanger 20 and returning it to the column. Other sources of the reboiling gas are possible. The reboiling gas arrives directly in the bottom of the column and is itself separated by distillation.

[0025] The first gas mixture 05 contains at least 90 mol% CO2, preferably at least 95 mol% CO2, or even at least 97 mol% CO2 or at least 99% CO2.

[0026] A second mixture 11 contains at least 20 mol% CO2, preferably at least 40 mol% CO2, or even at least 60 mol% CO2, at least 80 mol% CO2, or at least 90% CO2.

[0027] The first mixture 05 is at least as pure in CO2 as the second mixture 11 but may have a variable purity, for example due to stratification in the storage 50, gas leaks for example air.

[0028] The gas 05 is at a temperature of at most -45°C and is preferably neither heated nor cooled in the heat exchanger 20 in [Fig.l].

[0029] The first gas mixture 05 is sent to the bottom of the distillation column 40 to be separated, forming a gas enriched in at least one light impurity and depleted in CO2 at the top of the column and a liquid enriched in CO2 and depleted in light impurity at the bottom of the column.

[0030] Liquid formed in the bottom of column 40 is withdrawn from the column and divided into two parts. This liquid is purer in CO2 than the second fluid 07 and possibly purer in CO2 than the first fluid 05. On the other hand, it may be less pure in CO2 than the first fluid 05. At least a first part 33 is pressurized by a pump P and is sent directly to a storage 70, which may be the gas source 05 or another storage, or treated to form a liquid to be sent to the gas source 70 or to another storage.

[0031] Optionally, a second portion 03 of the tank liquid is expanded to partially vaporize it, forming a two-phase flow. According to one variant, the two-phase flow is reheated in the heat exchanger to form the gas 01. According to another variant, the two-phase flow is sent to a tank 50. The gas and the liquid formed in the tank are sent to the heat exchanger (in order to provide the necessary refrigeration energy) where the liquid vaporizes to form a gas, this gas and the gas from the separator 50 heated in the heat exchanger being mixed to form a gas 01. The gas 01, in both cases, is compressed in a compressor 10 to form a compressed gas which is then cooled in the heat exchanger 20, then expanded in a valve to be sent as liquid 2 to an intermediate level of the column 40.

[0032] A gas 11 containing carbon dioxide and at least one impurity lighter than carbon dioxide may be sent to the inlet of the compressor or to an intermediate level of the compressor to be compressed. This gas may be the second mixture to be separated.

[0033] According to [Fig.l], the gas 05 is at a pressure higher than that of the column 40 and may need to be expanded. For example, the storage from which the gas 05 comes may operate at a pressure of at least 10 bar while the column operates at between 10 and 20 bara. Preferably, the storage operates at a pressure equal to or higher than that of the column. In the case where the quantity of BOG 05 is sufficiently low so as not to significantly affect the refrigerant charge required in the unit, at least a portion of the gas 05 is returned as flow 08 to the compressed 02 stream in the compressor 10 and / or as flow 06 to the reboiler gas flow 04. A regulation of respective flow rates 06, 07 sent to the middle of the column with flow 02 or the bottom of the column with flow 04 is ensured by valves 60 and 80, depending on the purity of the BOG 05 to be treated.An analyzer A detects the CO2 content and / or the content of at least one impurity (02, N2, CO) of the flow 05. The first gas mixture is sent to the bottom of the column if its content of at least one light impurity detected by the analyzer A is lower than a purity threshold for this at least one impurity and / or if its CO2 content detected by the analyzer A is higher than a CO2 purity threshold. The flow 05 is sent to a level at least two theoretical plates above the bottom of the column if its content of at least one light impurity detected by the analyzer A is higher than the purity threshold for this at least one impurity. and / or if its CO2 content is lower than the CO2 purity threshold detected by analyzer A.

[0034] The thresholds can be chosen according to the purity of the flow 11; for example, if the flow 05 contains more CO2 and / or less of the light impurity than the flow 11, it will be sent to the tank and if the flow 05 contains at most as much CO2 and / or at least as much of the light impurity as the flow 11, it will be sent to a higher level of the column than the tank.

[0035] If the flow 05 has a content incompatible with distillation in the column, the first gas mixture is sent to the atmosphere. Thus, if its CO2 content detected by the analyzer A is less than a value lower than the CO2 purity threshold and / or if its content in the at least one impurity detected by the analyzer A is greater than a value higher than the purity threshold of the at least one impurity, no part of the flow 05 will be sent to the column. A control system regulated by the analyzer A determines whether the flow 05 is sent to the column or not. If it must be sent to the column, the analyzer A determines to which level of the column it will be sent.

[0036] In this description, only two points of introduction of the flow rate 05 are possible but it is obviously possible to design a more complicated process with at least three possible injection points for the flow rate 05, depending on the purity.

[0037] This arrangement may also be suitable for the case where the light impurity content of the first mixture 05 is higher than in [Fig.l]. The elements common to [Fig.l] are not described in detail.

[0038] In certain cases, the flow rate 08 can make it possible to reduce or even eliminate the reboiling flow rate.

[0039] Insulation is provided for the gas line 05 so that the gas does not heat up between the source 70 and the separation apparatus, since the source 70 operates at a subambient temperature.

Claims

Claims

1. Method for separating a first gas mixture (05) containing carbon dioxide and at least one impurity lighter than carbon dioxide, the first gas mixture originating from a storage (50) of liquid carbon dioxide or being produced during the filling of a storage of liquid carbon dioxide in which the first gas mixture as well as a second gas mixture (11) containing carbon dioxide and at least one impurity lighter than carbon dioxide is separated in a distillation column (40) of a separation system comprising a heat exchanger (20) and the at least one distillation column (40) to form a liquid flow purer in carbon dioxide than the second gas mixture and a gas (45) less pure in carbon dioxide than the second gas mixture, the second gas mixture cools in the heat exchanger upstream of the separation,the liquid flow is withdrawn from a distillation column of the separation system, at least a first part (33) of the liquid flow is optionally pressurized and sent to at least one storage (50) of liquid carbon dioxide or even to the storage of liquid carbon dioxide from which the first gas mixture comes, characterized in that the first gas mixture is sent to the bottom of the column if its content in the at least one light impurity is lower than a purity threshold for this at least one impurity and / or if its CO2 content is higher than a CO2 purity threshold and is sent to a level at least two theoretical plates above the bottom of the column if its content in the at least one light impurity is higher than the purity threshold for this at least one impurity and / or if its CO2 content is lower than the CO2 purity threshold.,

2. A method according to claim 1 wherein the first gas mixture (05) is sent to the column (40) without having been heated or cooled in the heat exchanger (20).

3. A method according to claim 1 or 2 wherein the first gas mixture is obtained from a storage (50) operating at a pressure equal to or greater than the pressure of the distillation column (40).

4. Method according to claim 3 in which the first gas mixture (05) comes from a storage (50) operating at a pressure higher than the pressure of the distillation column (40) and is expanded upstream of the column.

5. A method according to claim 1 to 4 wherein the second gas mixture (11) is sent to the column (40) at a level at least two theoretical plates above the column vessel.

6. Method according to claim 5 in which the first gas mixture (05) is mixed with the second gas mixture (11) upstream of the column (40) and the mixed flow is sent to the level at least two theoretical plates above the column tank if the content of the first gas mixture in P or at least one light impurity is greater than the purity threshold and / or if the CO2 content of the first mixture is less than the CO2 purity threshold.

7. Method according to one of claims 1 to 6 in which a reboiling flow (07) is sent to the bottom of the column (40).

8. Method according to claim 7 in which the first gas mixture (05) is mixed with the reboil flow (07) and the mixed reboil flow is sent to the bottom of the column (40) if its content of the at least one light impurity is lower than the purity threshold for this at least one impurity and / or if its CO2 content is higher than the CO2 purity threshold.

9. Method according to one of the preceding claims in which the reboiling flow rate (07) is reduced, possibly to zero, if the first gas mixture (05) is sent to the bottom of the column.

10. Method according to one of the preceding claims in which the first gaseous mixture (05) comes from a source other than a storage (50) supplied with liquid from the bottom of the column (40).

11. Method according to one of the preceding claims in which the first gaseous mixture (05) containing carbon dioxide and at least one impurity is at a temperature below -45°C in a pipe bringing it from the storage (70) to the at least one column (40).

12. Method according to one of the preceding claims in which the first gas mixture (05) is sent to the atmosphere if its CO2 content is less than a value lower than the CO2 purity threshold and / or if its content of the at least one impurity is greater than a value higher than the purity threshold of the at least one impurity.

13. Apparatus for separating a first gas mixture containing carbon dioxide and at least one impurity lighter than carbon dioxide, comprising means for supplying the first gas mixture (05) from a storage (50) of liquid carbon dioxide or a filling line for a liquid carbon dioxide storage, a separation system (40, 40', 50) comprising a heat exchanger (20), at least one distillation column (40), and possibly at least one phase separator (40', 50), means for sending the first gas mixture to separate in the separation system, means for sending a second gas mixture (07, 11,) containing carbon dioxide and at least one impurity lighter than carbon dioxide to cool, or even condense, in the heat exchanger, means for causing the second mixture cooled in the heat exchanger to separate in the at least one phase separator and / or the at least one distillation column to form a liquid flow purer in carbon dioxide than the second gas mixture and a gas less pure in carbon dioxide than the second gas mixture,means for withdrawing the liquid flow from a phase separator or from a distillation column of the separation system, possibly a pump (P) for pressurizing at least a first part (33) of the liquid flow, means for sending the at least a first part of the liquid flow to a storage (70) of liquid carbon dioxide or even to the storage of liquid carbon dioxide from which the first gaseous mixture comes, characterized in that it comprises an analyzer (A) for analyzing the content of CO2 and / or of the at least one impurity,in that the means for bringing the first gas mixture (05) from a storage (70) of liquid carbon dioxide or a filling pipe of a storage of liquid carbon dioxide are connected to the tank of the distillation column to introduce the first gas mixture therein to reboil the column as well as to a level of the distillation column at a first level at least two theoretical plates above the tank to introduce the first gas mixture therein to separate in the column and in that it comprises means for opening the means for bringing the first mixture towards the tank or towards the first level depending on the content or contents detected by the analyzer.,

14. Apparatus according to claim 13 comprising means for sending the first gas mixture to the atmosphere and means for enabling the sending of the first mixture to the atmosphere as a function of the CO2 content and / or the at least one impurity in the first mixture (05).