Process for separating a carbon dioxide-rich stream by distillation to produce liquid carbon dioxide

Description

[0001] The present invention relates to a method for separating a carbon dioxide-rich stream by distillation to produce gaseous and / or liquid carbon dioxide.

[0002] More particularly, the present invention relates to a process for separating carbon dioxide by distillation, this distillation being carried out at a temperature below 0°C.

[0003] In a CO2 liquefaction process with a CO2-rich feed (>95% mol) composed mainly of impurities (O2, N2, CO, H2 for example), the CO2 liquefier is designed to be operated at 100% of its capacity but is also suitable for producing liquid CO2 at a very low load.

[0004] The present invention relates to a process for liquefying CO 2 from a CO 2 -rich feedstock using the CO 2 itself in an open circuit or an external refrigeration cycle (ammonia or CO 2 for example). The cold process, consisting of a main exchanger and a purification column, is capable of being operated in a wide range of capacities.

[0005] In particular, with the progressive development of capture units over the coming years, it may be interesting to invest in a first liquefier covering future demands: the liquefier operates at very low loads in the first years, until the following emitters are ready to send their CO2 there.

[0006] In the CO2 liquefaction processes usually described, the cold process is often composed of a distillation column acting as purification equipment. In most cases, it is used to separate the incondensables at the top while the bottom of the column is at the specification of the CO2 produced.

[0007] The operability of distillation columns is an important factor in the performance of the unit, particularly with regard to the column bottom specification. Indeed, the columns are composed of internals (packings, distributors, trays) which have a guaranteed operation defined within a restricted range. Too great a deviation in the column inlet conditions can lead to a malfunction in the separation efficiency of the equipment.

[0008] Document EP 4 285061 A1 proposes a reflux at the top of the distillation column carried out by a part of the subcooled inlet gas. The other part is directly injected into the column at an intermediate height. There is therefore a constraint when it is necessary to operate at a reduced load for both sections of the column.

[0009] To address this issue, adding a pump to the liquefaction flowsheet is being considered to increase the liquid load in the column by recycling production liquid.

[0010] In addition to the liquid load problem, the unit's efficiency being very sensitive to the reflux temperature, it is necessary to recycle part of the column bottom liquid having been previously cooled.

[0011] Furthermore, in order to maintain a constant condition profile throughout the column, it is necessary to recycle liquid not only to the reflux but also to the main feed, in order to avoid any problems with the internals and sections of the column.

[0012] The invention therefore makes it possible to extend the application of the state of the art to the CO2 liquefaction scheme.

[0013] Indeed, it often happens that a reduced load and long duration operation is planned for CO2 liquefiers for the following reasons: Long-term maintenance. Phasing of unit construction in the case of a liquefier collecting CO2 emissions from multiple sources. Shutting down one or more sources for various reasons. Limiting product exports, etc.

[0014] In all of the above cases, the CO2 liquefier must be able to operate at reduced speed. In order to be able to operate the purification column under these conditions, a pump is added to the column tank in order to recycle part of the liquid to its feed and / or to the top of the column.

[0015] According to an object of the invention, there is provided a method for separating a flow containing at least 95 mol% of carbon dioxide as well as at least one impurity lighter than carbon dioxide by distillation in which: i. The flow is cooled to a first temperature intermediate between the cold end and the hot end of a heat exchange medium to form a liquid flow at a first temperature and a first pressure and is divided into at least two to form a first fraction and a second fraction. ii. The first fraction is expanded to the pressure of a distillation column called the second pressure, lower than the first pressure, and is sent to an intermediate level of the distillation column. iii. The second fraction is cooled in the heat exchange medium to the cold end thereof, expanded to the pressure of the distillation column, and sent to a level of the distillation column above the arrival point of the first fraction. iv. A liquid flow containing at least 99 mol% carbon dioxide is withdrawn from the bottom of the column. v.A fraction of the liquid flow is liquid product and another fraction of the liquid flow is pressurized in a pump and sent to the top of the column and / or to an intermediate level of the column.

[0016] According to another object of the invention, there is provided a method as described above having at least two modes of operation in which: a. In a first mode of operation, the flow containing at least 95 mol% of carbon dioxide has a flow rate above a first threshold, the carbon dioxide composition of the liquid at the bottom of the column is above a second threshold and the temperature of the column head gas is below a third threshold and no part of the liquid flow is sent to the column after pumping and preferably a part of the liquid withdrawn from the bottom of the column constitutes the product of the process and b. In a second mode of operation, the flow containing at least 95 mol% of carbon dioxide has a flow rate below the first threshold, the carbon dioxide composition of the liquid at the bottom of the column is below the second threshold and the temperature of the column head gas is above the third threshold and the fraction of the liquid flow is pressurized in the pump and sent to the top of the column and / or to the intermediate level of the column.

[0017] According to other optional objects: at least a portion of the other fraction of the liquid flow is cooled in the heat exchange means after being pressurized in the pump and is then sent to the top of the column and / or to the intermediate level of the column. a portion of the liquid pressurized in the pump mixes with the second fraction and the flow formed is cooled in the heat exchange means. the flow that cools in the exchange means is a liquid flow and the other fraction of the liquid is pressurized and then sent to the hot end to cool. the flow that cools in the exchange means returns to the hot end of the exchange means in gaseous form. the other fraction of the liquid flow is not cooled in the heat exchange means after being pressurized in a pump and is then sent to the top of the column and / or to an intermediate level of the column.a fraction of the liquid flow is not pressurized by the pump and is heated in the heat exchange means from an intermediate temperature thereof and is then sent to the column for separation. the fraction of the liquid flow which is not pressurized by the pump vaporizes in the heat exchange means and is sent to the bottom of the column in gaseous form. the fraction of the liquid flow forming the product is pressurized in the same pump as the other fraction of the liquid flow. the inlet of the pump is connected to the outlet of the pump by a bypass line, this line being able to be opened by a valve and in which in the event of a reduction in the flow rate of liquid to be pumped in the pump, the flow rate of pumped liquid sent to the column is first increased before opening the valve of the bypass line.

[0018] The invention mainly consists of recycling a portion of the tank liquid at an intermediate level of the column and / or at the top of the column. It is better to recycle tank liquid both at the reflux of the column but also to feed the column at an intermediate level. The quantity is recycled in such a way as to remain within the fixed operating limits of the referenced columns. Thus there is no impact on the performance of the equipment. The recycled flow rate will allow to have a reflux flow rate (4) and a main feed flow rate which will remain higher than 50%, preferably higher than 70% of the design flow rates of the column.

[0019] On the other hand, the diagram of the CO 2 liquefier whose reboiling (7) of the column is carried out by heating and vaporizing the tank liquid and injecting the gas formed into it can be increased in the same proportions mentioned above in order to guarantee good separation. This is due to the fact that the main exchanger (10) is sized for 100% load (therefore there is no hydraulic constraint to reboil more).

[0020] This addition thus makes it possible to ensure separation conditions close to the dimensioning case (liquid and gaseous congestion, liquid and vapor flow rate ratio, etc.).

[0021] The invention will be described in more detail with reference to the figures: [ Fig.1 ] represents a method according to the invention. [ Fig.2 ] represents a method according to the invention. [ Fig.3 ] represents a method according to the invention. [ Fig.4 ] represents a method according to the invention.

[0022] [ Fig.1 ] represents a method according to the invention.

[0023] In normal operation, a liquid or gaseous mixture 0 containing at least 95 mol% carbon dioxide and at least one lighter impurity is separated by the process. If the flow rate of stream 0 is greater than a threshold, and if the carbon dioxide composition in stream 8 is greater than a second threshold and if the temperature of stream 12 becomes lower than a third threshold, the process is in a first normal operating mode. In this case, the flow rate of stream 0 is cooled to a first intermediate temperature between those of the cold end and the hot end of a heat exchange means 10 to form a liquid stream 1 at a first temperature and a first pressure. Stream 1 is divided into two to form two parts 2, 4. Part 2 is expanded in a valve V2 to the pressure of column 20 and is introduced into the column at an intermediate level to separate therein.Part 4 passes into valve V1, cools to the cold end of the exchange medium 10 and is expanded in a valve V5 before being sent to the column head to form the reflux.

[0024] A liquid containing at least 99 mol% carbon dioxide is withdrawn from the bottom of the column and the product 8 constitutes at least a part of it. A stream 12 enriched with impurities leaves the top of the column and can be sent to the atmosphere, in a dedicated safety device or recovered. The column comprises means for detecting the purity of the column bottom liquid and means for measuring the flow rate of stream 0 and the temperature of gas 12.

[0025] In order to achieve the CO2 specification in the column bottom, a portion 7 of the bottom liquid heats up and vaporizes in the exchange means 10 from an intermediate temperature to the hot end, then is slightly expanded by the valve V4 in the bottom of the column 20 to form the reboiling of the column. Indeed, the column 20 is slightly overpressured compared to the exchanger 10, so that despite the pressure drop in the exchanger 10, the flow rate 7 at the hot end is always at a higher pressure than the column 20.

[0026] If the flow rate of stream 0 becomes lower than the first threshold and if the carbon dioxide composition in stream 8 becomes lower than the second threshold and if the temperature of stream 12 becomes higher than the third threshold, the method is modified to operate according to a second mode in which another fraction 3 of the tank liquid is pressurized by a pump and sent to the top of column 20 and / or to an intermediate level of column 20 to separate there after expansion. In this example, fraction 3 is divided in two after pumping in pump P1, a part 6 being expanded in valve V3 and mixed with flow 2 to enter the column. The remainder 5 mixes with the second fraction 4 downstream of valve V1 and is cooled in the exchange means 10 and sent to the reflux of column 4.

[0027] To do this, the pass in the exchanger intended for subcooling can be used. In the same way as for reboiling, this pass being sized for 100% load, there will be no hydraulic constraint.

[0028] Alternatively, the remainder 5 and the second fraction 4 can be subcooled independently of each other.

[0029] This subcooling thus makes it possible to obtain the lowest possible temperature for reflux, allowing for consistently high efficiency even during low load operations. This is all the more necessary if reboiling has been increased.

[0030] In summary, when the flow rate of stream 0 becomes lower than the first threshold and the purity at 8 becomes lower than the second threshold and the temperature at 12 becomes higher than the third threshold, pump P1 starts up to recycle liquid at the bottom of the column to at least one of the inlets. The flow rate of stream 7 is increased to reach the product specification at outlet 8.

[0031] At the same time, in order to obtain good efficiency and good operation of the distillation column, the flow rates of streams 5 and 6 are corrected according to the reboiling flow rate measurement in stream 7.

[0032] In the event of a change in the volume of carbon dioxide to be treated, if the quantity to be treated is low at first, the pump is used to send the tank liquid to the intermediate level and / or to the top of the column and when the quantity of carbon dioxide to be treated has increased sufficiently, the pump is no longer used.

[0033] Column 20 can operate at a pressure greater than 7 bars or greater than 10 bars.

[0034] [ Fig.2 ] shows a variant of the [ Fig.1 ] where column 20 operates at low pressure (for example >7 bara but <10 bara). Here the liquid pumped in pump P1 is cold enough so that liquid 5 can be reinjected directly downstream of reflux expansion 4 in valve V5 with minimal or no impact on the process efficiency. Liquid 5 is expanded in a valve V6 without having been cooled in the exchange means 10 and without having been mixed with liquid 4 upstream of valve V5. This makes it possible to reduce the discharge pressure of pump P1 and therefore to optimize the scheme.

[0035] [ Fig.3 ] shows a variant of the [ Fig.1 ] in the case where the CO 2 0 is in the liquid state at the inlet of the main exchanger 10 and / or when there is no intermediate expansion via the valve V1 to form the reflux of the column; in this case, the tank liquid 3 can be returned from the pump P1 to the inlet of the main exchanger 10 to mix with the stream 0. This aims to minimize the pipe interfaces as well as to optimize the overall energy of the process at reduced operation. Indeed, this induces returning a cold liquid pure in CO 2 to the inlet of the exchanger 10.

[0036] Here it is not necessary to remove the liquid to be separated from the exchange means 10 to separate it into parts 1, 4. Part 1 is expanded in the valve V2 to form the main feed 2 of the column 20.

[0037] [ Fig.4 ] shows a variant of the [ Fig.1] in the case where a P1 pump intended for the export of production 8 from the column is used to carry out this recycling in conjunction with the export of production 8. This thus makes it possible to limit investments. A V7 valve can nevertheless be added in order to bring the pressure of product 8 back to that at which it must be stored.

[0038] The outlet of pump P1 is connected to its inlet through a valve to ensure a minimum suction flow, this valve being usually closed. In case of flow reduction 3, valves V3, V5 are first opened and then the valve. This valve can be present in all diagrams with the same operation in case of flow reduction 3.

Claims

1. Process for the separation of a flow (0) containing at least 95 mol% of carbon dioxide and also at least one impurity lighter than carbon dioxide by distillation, in which: i. the flow (1) is cooled down to a first temperature intermediate between those of the cold end and of the hot end of a heat exchange means (10), in order to form a liquid flow at a first temperature and at a first pressure, and it is divided into at least two in order to form a first fraction (2) and a second fraction (4), ii. the first fraction is expanded to the pressure of a distillation column (20), referred to as second pressure, which is lower than the first pressure, and it is sent to an intermediate level of the distillation column, iii. the second fraction is cooled in the heat exchange means down to the cold end of the latter, it is expanded to the pressure of the distillation column and it is sent to a level of the distillation column above the point of arrival of the first fraction, iv. a liquid flow containing at least 99 mol% of carbon dioxide is withdrawn at the bottom of the column, v. one fraction (8) of the liquid flow is a liquid product and another fraction (3) of the liquid flow is pressurized in a pump (P1) and sent into the top of the column and / or to an intermediate level of the column.

2. Process according to Claim 1, having at least two operating modes in which: a. in a first operating mode, the flow (0) containing at least 95 mol% of carbon dioxide has a flow above a first threshold, the carbon dioxide composition of the liquid at the column bottom is greater than a second threshold and the temperature of the column top gas (12) is below a third threshold and no part of the liquid flow is sent to the column after pumping and preferably a part of the liquid withdrawn at the bottom of the column constitutes the product of the process and b. in a second operating mode, the flow containing at least 95 mol% of carbon dioxide has a flow below the first threshold, the carbon dioxide composition of the liquid at the column bottom is lower than the second threshold and the temperature of the column top gas is above the third threshold and the fraction of the liquid flow is pressurized in the pump (P1) and sent into the top of the column and / or to the intermediate level of the column.

3. Process according to either of the preceding claims, in which at least a portion (5) of the other fraction of the liquid flow is cooled in the heat exchange means (10) after having been pressurized in the pump (P1) and subsequently is sent into the top of the column and / or to the intermediate level of the column.

4. Process according to Claim 3, in which a part of the liquid pressurized in the pump (P1) mixes with the second fraction and the flow formed is cooled in the heat exchange means (10).

5. Process according to Claim 3 or 4, in which the flow (0) which cools in the exchange means (10) is a liquid flow and the other fraction (3) of the liquid is pressurized and then sent to the hot end in order to cool.

6. Process according to one of Claims 1 to 4, in which the flow (0) which cools in the exchange means (10) goes back in at the hot end of the exchange means in gaseous form.

7. Process according to either of Claims 1 and 2, in which the other fraction (6) of the liquid flow is not cooled in the heat exchange means (10) after having been pressurized in a pump (P1) and subsequently is sent into the top of the column and / or to an intermediate level of the column.

8. Process according to one of the preceding claims, in which a fraction (7) of the liquid flow is not pressurized by the pump (P1) and is reheated in the heat exchange means (10) from an intermediate temperature of the latter and is then sent to the column (20) in order to be separated therein.

9. Process according to one of the preceding claims, in which the fraction (8) of the liquid flow forming the product is pressurized in the same pump (P1) as the other fraction (3) of the liquid flow.

10. Process according to one of the preceding claims, in which the inlet of the pump (P1) is connected to the outlet of the pump by a bypass pipe, it being possible for this pipe to be opened by a valve (V8), and in which, in the event of a reduction in the flow of liquid (3) to be pumped in the pump, the flow of pumped liquid sent to the column is first increased before opening the valve of the bypass pipe.