PROCESS FOR TREATING A CARBON DIOXIDE-RICH GAS CONTAINING WATER

Abstract

In a process for treating a carbon dioxide-rich gas (1) containing water, the treatment by compression and/or washing and/or drying of the gas produces acidified water (W1, W2, W3, W4, W7) which is sent to a cooling circuit (W8, W10).

Claims

1. A process for treating a carbon dioxide-rich gas containing water and at least one other acid gas, comprising: a) compressing the gas in at least one compression stage and cooling the compressed gas and separating at least one condensate from the cooled gas after at least one compression stage, the condensate consisting of water in which the CO.sub.2 and/or the at least one other acid gas is dissolved, forming acidified water having a pH of at most 6.5, and/or b) purifying the carbon dioxide-rich gas, upstream of the compression of step a), in a unit for washing with water so as to produce a purified gas and water containing impurities, and/or c) drying the gas compressed in step a), in a drying unit and recovering the water contained in the compressed gas, forming recovered water, wherein: i) at least a portion of the acidified water of step a) and/or ii) at least a portion of the water containing impurities of step b) and/or iii) at least a portion of the recovered water of step c) is/are mixed with water circulating in a cooling circuit thereby forming a mixture with a desired degree of acidity.

2. The process as claimed in claim 1, wherein the mixture serves to cool an indirect-contact heat exchanger.

3. The process as claimed in claim 2, wherein the heat exchanger is of aluminum brazed plate-fin type or corrugated plate type or shell-and-tube type.

4. The process as claimed in claim 2, wherein the heat exchanger serves to cool carbon dioxide-rich gas compressed in step c), downstream of at least one compression stage, and/or to be purified in step c).

5. The process as claimed in claim 2, wherein the dryer operates by water adsorption and is periodically regenerated by a dry regeneration gas which leaves the adsorption unit having recovered therein water accumulated in the drying unit.

6. The process as claimed in claim 2, wherein the heat exchanger serves to cool the regeneration gas having recovered the water in the drying unit, the water thus condensed constituting the recovered water of step c).

7. The process as claimed in claim 1, wherein the at least one other gas is chosen from the group: SO.sub.2, SO.sub.3, H.sub.2S, HCl, NO, N.sub.2O, NO.sub.2, N.sub.2O.sub.4, HNO.sub.3, HNO.sub.2, H.sub.2CO.sub.3.

8. The process as claimed in claim 1, wherein an acid flow originating from storage is added to the water of the cooling circuit.

9. The process as claimed in claim 1, wherein the flow of water from the flow or flows i) and/or ii) and/or iii) is adjusted so as to obtain a desired pH in the cooling circuit.

10. The process as claimed in claim 8, wherein the acid flow originating from storage and also at least one of the flows i) and/or ii) and/or iii) is adjusted so as to obtain a desired pH in the cooling circuit.

11. The process as claimed in claim 1, wherein the carbon dioxide-rich gas is purified upstream of the compression in the unit for washing with water so as to produce a purified gas and water containing impurities; and the temperature and/or flow rate of washing water sent to the cooling tower is/are adjusted in order to vary the flow rate and/or the acidity of the water containing impurities sent to the cooling circuit.

12. The process as claimed in claim 1, wherein the cooling circuit serves to cool a gas compressed in a compression stage, which may be that of step a) or a stage for compression of a gas produced by treating a gas treated in at least one of steps a) and/or b) and/or c).

13. A process for treating a carbon dioxide-rich gas containing water and at least one other acid gas; comprising: a) at least one compression stage, for compressing the gas, cooling means for cooling the compressed gas and at least partially condensing the water that it contains, and means for separating at least one condensate of the cooled gas, the separating means being placed after at least one compression stage, the condensate consisting of water in which the CO.sub.2 and/or the at least one other acid gas is dissolved, forming acidified water having a pH of at most 6.5, and/or b) a unit for washing with water so as to produce a purified gas, and water containing impurities, and/or c) a drying unit and also means for recovering the water removed by the drying, which comprises means for mixing: i) at least a portion of the acidified water of option a) and/or ii) at least a portion of the water containing impurities of option b) and/or iii) at least a portion of the recovered water of option c) with water circulating in a cooling circuit so as to form a mixture with a desired degree of acidity.

Description

BRIEF DESCRIPTION OF THE FIGURE

[0077] For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:

[0078] FIG. 1 illustrates a process according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0079] In FIG. 1, the carbon dioxide-rich gas can originate from any source capable of producing such a gas. Here, two combustion gas flows FG1 and FG2 are mixed so as to form the carbon dioxide-rich gas 1 containing for example at least 10 mol % of CO.sub.2, at least 40 mol % of CO.sub.2, at least 80 mol % of CO.sub.2 or at least 90 mol % of CO.sub.2. This gas is sent to a tower for washing with water Q supplied at the top by pressurized water, W12, capable of absorbing at least one acid gas present in the carbon dioxide-rich gas, for example SO.sub.2, SO.sub.3, NO.sub.2, NO, N.sub.2O, HNO.sub.3, HNO.sub.2, CO.sub.2, etc. The washing tower Q can also operate at atmospheric pressure or sub-atmospheric pressure. The flow rate of water W12 sent to the top of the tower can be varied in order to adjust the flow rate and/or the acidity of liquid formed in the tank of the tower Q.

[0080] The water in the tank of the tower Q is acidified and can be pumped by a pump P1. A portion W5 is optionally mixed with the water W12 and the remainder W7 can be sent according to the invention to the cooling circuit in order to vary the acidity level thereof (not shown in the FIGURE).

[0081] The carbon dioxide-rich gas 3 purified in the tower Q is, in this example, compressed in a compressor C1, then divided into two and cooled in two heat exchangers E1, E2 by indirect heat exchange with different nitrogen flows W13 or with another refrigerant, such as water. Next, the gas flows are combined and cooled in the heat exchanger E3 in order to condense a portion of the water contained in the gas. This water will contain carbonic acid in a dissolved form and/or sulfuric acid and/or sulfurous acid and/or nitric acid, and/or nitrous acid. It is separated from the gas in a separator S1 or by another equivalent means, the condensate W1 of which is drawn off into the separator tank S1. The gas is again compressed, in a compressor C2, and cooled in a heat exchanger E4 and the water formed is separated in a phase separator S2 or another equivalent means, the condensate W2 of which is drawn off into a tank. The gas from the separator S2 is again compressed, in a compressor C3, and cooled in a heat exchanger E5 and the water formed is separated in a phase separator S3, the condensate W3 of which is drawn off into a tank. The gas from the separator S3 or another equivalent means is compressed by a compressor C4, is cooled by a heat exchanger E6 and then in an exchanger E7 in order to further condense water. This water in the gas 5 is removed in the separator S4 or another equivalent means in order to form a gas 7 and condensed water W4.

[0082] The gas 7 is then dried in a dryer D which may be an adsorption unit. The dried gas 9 may then be used as dry product or be purified by another means such as another adsorption unit and/or a membrane and/or a unit for separation at a temperature below 0° C., for example by partial condensation and/or distillation.

[0083] If the dryer D operates by adsorption, it will be necessary to regenerate it by sending thereto a dry regeneration gas. This gas R leaves the dryer loaded with water and can be mixed with the flow 5, in this case between the exchangers E6, E7, the mixing point depending on the amount of water contained and on the temperature of the gas R. It goes without saying that the composition of the gas R must be compatible with the definitive use of the gas 9. It may for example be rich in carbon dioxide.

[0084] It will be understood that the dryer does not necessarily operate via adsorption. The gas 7 may simply be cooled and/or pressurized in order to condense the water that it contains, this acidified water being recovered as flow R for supplying the cooling cycle.

[0085] The water flows W1, W2, W3, W4 are mixed so as to form a flow of water containing at least one dissolved acid gas. The mixture formed may nevertheless only contain at least two of the flows mixed together. Alternatively, just one flow W1, W2, W3, W4 can be recovered. At least one portion of the recovered water (mixed or not mixed) is sent to the water of a refrigeration cycle.

[0086] In the case of the example, the water is sent as flow W6 to the heat exchanger E7, where the gas 5 cools downstream of the exchanger E6, in this case mixed with the regeneration gas R.

[0087] Thus, the water recovered in S4 comes partially from the regeneration gas R.

[0088] Here, the water W6 is not cooled upstream of the exchanger E7. Nevertheless, it is possible to cool it, for instance in the cooling tower T.

[0089] The possibility of varying a flow sent to join the water circuit and/or mixing at least two flows makes it possible to compensate for acidity levels that are too high or too low.

[0090] For example, if one of the flows W1 to W4 or W7 has an exceptionally high acidity level, it can nevertheless serve to supply the cooling cycle, by reducing its flow rate and/or by mixing it with a less acid flow.

[0091] A nitrogen circuit W serves to cool the exchanger E1 and the exchanger E2, optionally after expansion in a turbine. The nitrogen expanded in the turbine supplies a tower T by the bottom. While rising up the tower, it cools the water W10 sent to the top of the tower, producing a reheated nitrogen flow at the top of the tower, which is discharged into the atmosphere, and cooled water in a tank which supplies the cooling circuit. A nitrogen flow W11 can be sent into the atmosphere. For example, if the nitrogen W arrives too cold or if there is no need to cool very much the flow of water W10 depending on how much heat is introduced by the heat exchanger E7 into the cooling circuit, not all the nitrogen is sent to the tower T, but the portion W11 can be sent directly into the atmosphere. This constitutes a way to adjust the temperature of the water W8 originating from the tower T.

[0092] It will be understood that the precise type of nitrogen circuit is not important and, for this reason, it is not described in detail.

[0093] It should be noted that the cooling water circuit is optionally supplied with an acid flow W9, which may be sulfuric acid and/or may have a pH of less than 7.

[0094] This sending of acid can be eliminated or reduced since the water circuit is acidified by water originating from at least one of three sources, namely; [0095] the washing tower Q, when present, the condensate W7 of which can be mixed with the water circuit; [0096] the regeneration gas R, when present, the water of which can optionally be separated by mixing it with the gas to be dried; [0097] at least one of the compression stages C1, C2, C3, C4 after cooling and partial condensation.

[0098] It should be noted that the compressors C1, C2, C3, C4 can each consist of at least one stage of a compressor.

[0099] The tower Q can be supplied with water originating from at least one of the compressors C1 to C4 so as to form the flow W12. This flow W12 may otherwise be completely independent.

[0100] It is possible to adjust the flow rate and/or the acidity of the flow W12 by varying the flow rate and/or the acidity of the flow W12. The flow rate can be varied by means of a valve on the flow W12. Alternatively, at least one of the flows W1, W2, W3 can be adjusted by means of a valve Since the flows W1, W2, W3 may have different acidity levels, this may be sufficient to vary the acidity of the purge of water removed in the column tank.

[0101] The water having served to cool the heat exchanger E7 can be sent to a washing tower T at the top thereof, so as to cool a nitrogen flow W15.

[0102] The acid W9, if present, is preferably mixed with the liquid from the tank of the tower T and then the water formed is mixed with the flow W6.

[0103] The water in the gas R is astutely recovered by mixing the gas R with the gas compressed between the exchangers E6 and E7, so that the exchanger E7 cools the gas R and the separator S4 recovers the water originating from the compressor and from the regeneration gas R.

[0104] The cooling cycle supplied with the water from at least one of the sources a), b), c) may be that of the compressor of the source a) or any other compressor, for example a compressor of product from a separation device supplied with the carbon dioxide-rich gas.

[0105] In the example of the FIGURE, water is taken from the washing tower, the washed gas compressor and the dryer of the gas originating from the compressor in order to supply the cooling circuit.

[0106] Nevertheless, it will be understood that the washing tower, the compressor and the dryer are not necessarily all present.

[0107] For example, the process may be solely a washing process (and the device a washing tower with associated means) producing acidified water for acidifying water from a cooling circuit. In this case, the compressor and/or the dryer is not present. The washing tower can therefore produce a gas to be treated by compression and/or drying.

[0108] Likewise, the process may be solely a drying process (and the device a dryer with associated means) producing acidified water for acidifying water from a cooling circuit. In this case, the compressor and/or the washing tower is not present, Therefore, the dryer can dry a gas originating from the compression of the gas to be dried and/or from the washing in a washing tower, so as to produce the gas to be treated by drying.

[0109] The cooling circuit may cool gas from a compression stage, which may be the gas that has been purified in the washing tower and/or gas to be dried in the dryer, Otherwise, it may cool a cooling circuit independent of steps a), b) and c).

[0110] It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.