Process for cooling a mixture of CO2 and water
By employing chilled water from a water-nitrogen column to optimize CO2 capture processes, the method addresses inefficiencies in high-temperature regions, reducing energy consumption and investment costs through improved liquefaction and vaporization control.
Patent Information
- Application Number
- FR2025002848
- Authority / Receiving Office
- FR · FR
- Patent Type
- Utility models
- Current Assignee / Owner
- Filing Date
- 2025-03-20
- Publication Date
- 2025-09-26
- Estimated Expiration
- 2035-03-20
AI Technical Summary
Existing CO2 capture processes face challenges in regions with high cooling water temperatures, leading to inefficient condensation and densification, high energy consumption, and oversized machinery due to high vaporization rates, especially in industrial sites lacking ample cooling water supplies.
Utilizing chilled water from a water-nitrogen column to reduce CO2 cycle temperatures, optimizing the cooling loop by integrating chilled water exchangers to lower the discharge pressure and minimize vaporization, thereby reducing energy consumption and investment costs.
Significantly reduces energy consumption and investment costs by lowering cycle temperatures, enhancing liquefaction efficiency, and minimizing vaporization, while maintaining dryer performance with minimal additional absorbent needs.
Smart Images
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Abstract
Description
Title of the invention: Method for cooling a mixture of CO2 and water
[0001] The present invention relates to a method for cooling a mixture of CO2 and water, for use in a method for separating a mixture of CO2 and water. Methods for capturing and liquefying CO2 from low-content flue gases by cryogenic means require a cold supply, which can be achieved through the use of a CO2 cycle or another cycle. This cycle, which can be closed or semi-open, comprises the following steps: compression, cooling / condensation, expansion and vaporization. The order of these steps can be modified according to the diagram.
[0002] This invention provides new ways to carry out the cooling / condensation or densification step of this CO2 stream by using the excess cold available from an air / nitrogen tower to produce chilled water for cooling wet flue gases before a drying unit. The idea is to find an optimal compromise between the size of the dryers and the electrical consumption of the cycle, optimizing the total capture cost (TCO) for liquid CO2 production schemes. In particular, in some regions, climatic conditions impose very high cooling water temperatures in summer.
[0003] For the production of liquid CO2, a pure CO2 cycle is often used to provide the cold necessary for condensation. This cycle includes a compression step followed by cooling in an exchanger with cooling water, a step which allows the fluid to be condensed. The fluid is then expanded to a pressure close to the suction of the last compressor stage in order to liquefy the CO2. This cycle can also contain a brazed aluminum plate and fin type exchanger before expansion in order to allow the fluid to reach a density very close to that of liquid CO2.
[0004] The use of chilled water (5-10°C) to cool a gas stream is known and very widespread, particularly in CO2 capture processes. In processes for separating fumes containing CO2, a chilled water loop is very often used to cool the previously compressed fumes just before entering the dryer. This operation makes it possible to condense a large part of the water remaining in the fumes and thus to reduce the quantity of adsorbent required in the dryers.
[0005] This chilled water is generated by a cold supply coming from a flow rich in Nitrogen (for example: high pressure product from a pressure swing adsorption unit or PSA, previously expanded in turbines or the nitrogen co-product of a Cryogenic distillation air separation unit. This heat exchange occurs in a dedicated column by direct contact between water and the N2-rich gas. The direct contact water vaporization allows for a fairly low approach at the bottom of the column resulting in water temperatures lower than the gas temperature.
[0006] The generated flow rate of chilled water depends strongly on the flow rate of the PSA product and thus on the CO2 concentration upstream of the PSA.
[0007] An example of such a method is found in FR2884307A.
[0008] On an industrial site located in the middle of the land, it is not uncommon for large quantities of water to be used as cooling water to be unavailable. Indeed, cement, lime or steel plants rarely have utilities available in large quantities as on large industrial sites such as refineries. Thus, when choosing the type of cooling loop for the process, the choice is often made for a closed water loop. Such a cycle cools the process and returns hot to air-water exchangers.
[0009] Unfortunately in many countries, a similar cooling loop design can generate cooling water at temperatures above 35°C during the summer period. Such a high temperature has direct consequences on the condensation and densification stage of the fluid passing through the cycle. Considering an approach of 5°C, it is common to obtain temperatures above 40°C at the outlet of the first heat exchanger to the cooling water.
[0010] The pressure required to obtain sufficient density at these temperatures is often very high and it happens that in certain extreme cases a pressure higher than 110 bara is not sufficient. On the other hand, expanding a fluid at very high pressure cooled only to 40°C or more to pressures of the order of 45 bara results in a very high proportion of vaporization of the latter. This compression at very high pressure combined with a large volume of vaporized molecules to be recycled leads to over-sizing the machine but also induces very high energy consumption.
[0011] The use of a portion of the chilled water from the column makes it possible to significantly reduce the temperature of the fluid in this cycle to temperatures of the order of 20°C if the outlet of the last exchanger is 40°C. This makes it possible to significantly reduce the discharge pressure of the cycle compressor to obtain a density necessary for liquefaction. Similarly, a lower temperature will also have the consequence of drastically reducing the proportion of liquid vaporizing at the time of expansion. Finally, such a scheme makes it possible to directly liquefy the cycle fluid over a very large period of the year (in European climates) and to produce no vaporization during expansion. No molecules will therefore be recycled to compressor inducing a strong gain in energy and investment on the latest compression wheels
[0012] The counterpart of the use of chilled water at the cycle level is the reduction of water allocated to the cooling of the fumes entering the dryers. However, this aspect has a relatively low impact on the design of the dryers for several reasons. Firstly, the chilled water cycle is often underutilized and it is thus possible to allocate part of the water to the CO2 cycle without changing the inlet temperature of the dryers. This aspect is all the more important around average climatic conditions where part of the cold gas rich in N2 is not sent into the water-nitrogen tower in order not to produce water that is too cold and potentially freeze.
[0013] On the other hand, an increase of a few degrees in the inlet temperature of the dryers induces only a small amount of additional water to the dryers and therefore a low need for additional absorbent. Such an increase in the price of the dryers would be largely offset by the reduction in the price of the cycle compressor and lower energy consumption. The TSA regeneration cycle can also be adapted between the summer and nominal cases in order to compensate for faster saturation of the adsorbent.
[0014] Chilled water is produced in the water-nitrogen column by direct contact with the PSA product. The latter, after being expanded in a series of turbines, enters the column at a temperature between 2 and 5°C.
[0015] The chilled water produced in this column will mainly be used in an exchanger upstream of the dryers. This will allow the fumes to be cooled and more of the water present in the latter to be condensed in order to optimize the adsorbent volume of the dryers.
[0016] The other part of the refrigerated water produced is used in a heat exchanger at the CO2 cycle level. The latter is located after the cycle compressor and after a first CO2 cooling exchanger downstream of the compressor. This first exchanger can be either a cooling water exchanger or a Gas - Air exchanger which allows for a better approach.
[0017] The chilled water exchanger will reduce the temperature of the CO2 to around 20-25°C just before it is expanded and liquefied. The liquid produced is then sent to the main exchanger to be sub-cooled while the part vaporized in the expansion is recycled to the cycle compressor. For example, for a 2500 t / d separation process, a reduction in the cycle temperature from 38°C to 21°C allows a reduction in the cycle compressor discharge pressure of 10%, 30% less flow to be recycled to the compressor and therefore a gain on the specific energy of 1.3%. The investment in the dryers will be increased by 24% but the impact on the TCO will be low thanks to to the improvements mentioned above. For this example, we can expect a TCO gain of 0.7% thanks to this invention.
[0018] According to an object of the invention, there is provided a method for cooling a mixture of CO2, nitrogen and water in which the mixture of CO2, nitrogen and water is compressed in a compressor then cooled to condense part of the water it contains and then dried in dryers, the dried mixture is a. Cooled and separated by partial condensation and distillation or b. Separated by adsorption forming a carbon dioxide-enriched gas and a nitrogen-enriched gas, the carbon dioxide-enriched gas being cooled and separated by partial condensation and distillation
[0019] forming a fluid rich in CO2 and in which a refrigerated water loop serves i. cooling the dried gas mixture from step a) or the carbon dioxide-enriched gas from step b) by means of a cooling cycle and ii. to cool a compressed cycle gas in a cycle compressor of a closed cooling cycle.
[0020] According to other characteristics of the invention: • the cooling cycle is a CO2, ammonia or propane cycle • the chilled water loop cools, or even condenses the CO2 from the cycle to the outlet of a cycle compressor just before an expansion stage • the loop includes a chilled water exchanger on the cycle is a plate exchanger to cool and condense the CO2. • the loop includes cooling water and chilled water heat exchangers consisting of a single double-tube exchanger • the loop includes a chilled water exchanger which is not necessarily following an exchanger with cooling water or a gas-air exchanger • all the water generated by a water-nitrogen column is used to cool and condense the CO2 at the cycle level. • the nitrogen-enriched gas is expanded in a turbine and sent to a water-nitrogen column to cool water sent to the column to produce chilled water.
[0021] The invention will be described in more detail with reference to the figures where:
[0022] [Fig-1] represents a method according to the invention
[0023] [Fig.2] represents a method according to the invention
[0024] [Fig. 1] represents a method according to the invention in which a gas containing CO2, nitrogen and water 1 is compressed in a compressor then cooled in a cooler RI against water CW and then in a cooler R2 forming a gas 5. Cooling in RI and R2 causes condensation of water in the gas 1, 2 forming water flows W, Wl. The dried gas 5 is dried in dryers, cooled in a main exchanger and separated by partial condensation and / or distillation forming a CO2-rich liquid. A portion 23 of the CO2-rich gas from a cycle compressor is cooled in coolers R3 against water CW and R4 against chilled water. The cooled gas is expanded in a valve V forming a two-phase mixture which is separated in a phase separator forming a liquid 27 and a gas 25. The liquid 27 is sent to the main exchanger and the gas 25 to the cycle compressor. A refrigeration cycle is used to cool the coolers R2, R4. The cooled water in a tower T is pumped by a pump PI and a part 15 goes to the cooler R2 and a part 17 to the cooler R4, the part 15 having a larger flow rate than the part 17. The two heated parts are mixed to form the liquid 19 which is returned to the top of the tower T.The tower is cooled by a gas 11 which leaves at the top of tower T as gas 21.
[0025] In some variants, the gas 5 is separated by pressure swing adsorption forming a CO2-enriched gas and a nitrogen-enriched gas. It is the CO2-enriched gas that is compressed, cooled and separated by partial condensation or distillation to form the CO2-rich liquid.
[0026] The nitrogen-enriched gas is expanded in a turbine and constitutes gas 11.
[0027] The R4 cooler can also be used to cool / condense a gas other than CO2, for example ammonia or propane.
[0028] [Fig.2] illustrates a variant of the previous figure in which the cooler R3 is replaced by an AC air cooler.
Claims
[Claim 1] Claims A method of cooling a mixture of CO2, nitrogen and water in which the mixture of CO2, nitrogen and water (1) is compressed in a compressor and then cooled to condense part of the water (W, Wl) it contains and then dried in dryers, the dried mixture is a. Cooled and separated by partial condensation and distillation or b. Separated by adsorption forming a carbon dioxide-enriched gas and a nitrogen-enriched gas, the carbon dioxide-enriched gas being cooled and separated by partial condensation and distillation forming a CO2-rich fluid and in which a chilled water loop (13, 15, 17, 19) serves c. cooling (R2) the dried gas mixture (3) from step a) or the carbon dioxide-enriched gas from step b) by means of a cooling cycle and d. to cool (R4) compressed cycle CO2 (23) in a cycle compressor of a closed cooling cycle.