Patent Application
20240210106
The present invention relates to a process and to an apparatus for the separation of a flow rich in carbon dioxide by distillation in order to produce gaseous and/or liquid carbon dioxide.
The present invention may also relate to a process and to an apparatus for the separation of carbon dioxide by distillation, this distillation being carried out at a temperature of less than 0° C.
In a process for the liquefaction of feed CO2 rich in CO2 (>95 molar %) predominantly composed for the remainder 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 reduced load.
In particular, with the progressive development of capture units over the years to come, it may be advantageous to invest in a first liquefier in order to cover future demands: the liquefier operates at very low loads for the first few years, the time that the following emitters are ready to send their CO2 there.
In the CO2 liquefaction processes usually described, the cold process often consists of a distillation column acting as item of purification equipment. Most of the time, the column is used to separate the noncondensables at the top while the bottom of the column is at the specification of the CO2 produced.
The operability of the distillation columns is an important factor in the performance of the unit, particularly with regard to the column bottom specification. This is because the columns are composed of internal parts (packings, distributors, trays) which have a defined guaranteed operation in a restricted range. Too great a deviation with regard to the inlet conditions of the column can lead to a malfunction in the separation efficiency of the item of equipment.
The patent FR 2 100 737 provides a reflux at the distillation column top produced by a part of the subcooled inlet gas, the other part being directly injected into the column at an intermediate height. There thus exists a constraint when it is necessary to operate at a reduced load for the two sections of the column.
The present invention relates to a process for the liquefaction of feed CO2 rich in CO2 involving the CO2 itself in an open circuit or an external refrigeration cycle (ammonia or CO2, for example). The cold process, composed of a main exchanger and of a purification column, is capable of being operated in a wide range of capacities.
To deal with the problem related to operating at a reduced load noted supra, the addition of a pump to the liquefaction scheme is envisaged in order to increase the liquid load in the column by recycling the production liquid.
In addition to the problem of liquid load, the output of the unit being very sensitive to the temperature of the reflux, it is preferable to recycle a part of the bottom liquid of the column which has been cooled beforehand.
Moreover, in order to keep an unvarying profile of conditions throughout the column, it is necessary to recycle liquid not only at the reflux but also at the main feed, the aim of this being to avoid any problems at the level of the internal parts and of the sections of the column.
Certain embodiments of the invention thus make it possible to extend the application of the state of the art to the CO2 liquefaction scheme.
This is because it often happens that an operation having reduced load and of long duration is planned for CO2 liquefiers for the following reasons:
In all of the above cases, the CO2 liquefier must be able to operate at a reduced rate. In order to be able to operate the purification column under these conditions, a pump is added at the column bottom in order to recycle a part of the liquid to the feeding thereof and/or at the top of the column.
According to a subject matter of the invention, there is provided a process for the separation of a flow containing at least 95 mol % of carbon dioxide and also at least one impurity lighter than carbon dioxide by distillation, in which:
According to another subject matter of the invention, there is provided a process as described above having at least two operating modes, in which:
According to other optional subject matters:
In certain embodiments, the invention may include recycling a part of the bottom liquid at an intermediate level of the column and/or at the top of the column. It is preferable to recycle bottom liquid both at the reflux of the column but also, to feed the column, at an intermediate level. The amount is recycled so as to remain within the fixed operating limits of the referenced columns. Thus, there is no (or very minimal) impact on the performance of the item of equipment. The recycled flow will make it possible to have a reflux flow (4) and a main feed flow which will remain greater than 50%, preferentially greater than 70%, of the design throughputs of the column.
Moreover, the scheme of the CO2 liquefier, the reboiling (7) of the column of which is carried out by heating and vaporizing the bottom liquid and by injecting therein the gas formed, will be able to 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% of load (there is thus no hydraulic stress to reboil more).
This addition thus makes it possible to ensure separation conditions close to the dimensioning case (liquid and gaseous obstruction, liquid and vapor flow ratio, and the like).
The invention will be understood better from reading the following description and from studying the accompanying figures. These figures are given only by way of illustration and do not in any way limit the invention.
In normal operation, a liquid or gaseous mixture 0 containing at least 95 mol % of carbon dioxide and also at least one lighter impurity is separated by the process. If the flow rate of the stream 0 is greater than a threshold and if the carbon dioxide composition of the stream 8 is greater than a second threshold and if the temperature of the stream 12 becomes lower than a third threshold, the process is in a first normal operating mode. In this case, the flow of the stream 0 is cooled down to a first intermediate temperature between those of the cold end and of the hot end of a heat exchange means 10, in order to form a liquid stream 1 at a first temperature and at a first pressure. The stream 1 is divided into two in order to form two parts 2, 4. The part 2 is expanded in a valve V2 to the pressure of the column 20 and is introduced into the column at an intermediate level in order to be separated therein. The part 4 passes through the valve V1, cools down to the cold end of the exchange means 10 and is expanded in a valve V5 before being sent to the column top in order to form the reflux therein.
A liquid containing at least 99 mol % of carbon dioxide is withdrawn at the bottom of the column and the product 8 constitutes at least a part thereof. A stream 12 enriched in impurities exits at the top of the column and can be sent to the atmosphere, into a dedicated safety device or recovered. The column comprises means for detecting the purity of the bottom liquid of the column and means for measuring the flow of the stream 0 and the temperature of the gas 12.
In order to achieve the CO2 specification at the column bottom, a part 7 of the bottom liquid is heated up and vaporized in the exchange means 10 from an intermediate temperature to the hot end and is then slightly expanded by the valve V4 at the bottom of the column 20 in order to form therein the reboiling of the column. This is because the column 20 is slightly in excess pressure with respect to the exchanger 10, so that, despite the pressure drop in the exchanger 10, the flow 7 at the hot end is always at a higher pressure than the column 20.
If the flow of the stream 0 becomes lower than the first threshold and if the carbon dioxide composition of the stream 8 becomes lower than the second threshold and if the temperature of the stream 12 becomes greater than the third threshold, the process is modified in order to operate according to a second mode in which another fraction 3 of the bottom liquid is pressurized by a pump and sent to the top of the column 20 and/or to an intermediate level of the column 20 in order to be separated therein after expansion. In this example, the fraction 3 is divided into two after pumping in the pump P1, a part 6 being expanded in the valve V3 and mixed with the flow 2 in order to enter the column. The remainder 5 mixes with the second fraction 4 downstream of the valve V1 and is cooled in the exchange means 10 and sent to the reflux of the column 4.
To do this, the channel in the exchanger intended for the subcooling can be used. In the same way as for the reboiling, as this channel is sized for 100% of load, there will be no hydraulic stress.
Otherwise. the remainder 5 and the second fraction 4 can be subcooled independently of each other.
This subcooling thus makes it possible to obtain the lowest possible temperature for the reflux, making possible a yield which is always very high even during operations at low loads. This is all the more necessary if the reboiling has been increased.
To sum up, when the flow of the current 0 becomes lower than the first threshold and when the purity at 8 becomes lower than the second threshold and the temperature at 12 becomes greater than the third threshold, the pump P1 is started in order to recycle liquid at the column bottom to at least one of the inlets. The flow of the current 7 is increased in order to achieve the specification of the product at the outlet 8.
At the same time, in order to obtain a good yield and proper operation of the distillation column, the flows of the streams 5 and 6 are corrected depending on the measurement of the reboiling flow of the stream 7.
In the event of a change in the volume of carbon dioxide to be treated, if the amount to be treated is low to begin with, the pump is used to send the bottom liquid to the intermediate level and/or into the top of the column and, when the amount of carbon dioxide to be treated has increased sufficiently, the pump is no longer used.
The column 20 can operate at a pressure of greater than 7 bar or of greater than 10 bar.
[
[
In this instance, it is not necessary to remove the liquid to be separated from the exchange means 10 in order to separate it into parts 1, 4. The part 1 is expanded in the valve V2 to form the main feed 2 of the column 20.
[
The outlet of the pump P1 is connected to its inlet through a valve V8 in order to ensure a minimum suction flow, this valve V8 usually being closed. In the event of reduction in the flow 3, the valves V3 and V5 are first opened and subsequently the valve V8. This valve V8 can be present in all the schemes with the same operation in the event of reduction in flow 3.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.
“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.
All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.
| Number | Date | Country | Kind |
|---|---|---|---|
| FR2104519 | Apr 2021 | FR | national |
This application is a § 371 of International PCT Application PCT/EP2022/056139, filed Mar. 10, 2022, which claims the benefit of FR2104519, filed Apr. 29, 2021, both of which are herein incorporated by reference in their entireties.
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2022/056139 | 3/10/2022 | WO |
