Method and apparatus for separating a mixture containing co2, at least one component lighter than co2 and mercury

Abstract

In a method for separating a mixture containing CO2, at least one component lighter than CO2 and mercury, the mixture (11) is cooled in a heat exchanger (43) and is partially condensed forming a first two-phase fluid, the first two-phase fluid exits the heat exchanger at a first temperature between -25°C and -38°C and is separated in a first phase separator (12) forming a gas (14) that is enriched in the at least one lighter component relative to the mixture and a liquid bath that is enriched in CO2 and in mercury relative to the mixture, a first liquid (16) is drawn off by means of a pipe that is in communication with the upper portion of the liquid bath and a flow of a second liquid (18) corresponding to less than 1% of the liquid generated by the partial condensation of the mixture is drawn off at most once per year.

Description

[0001] Method and apparatus for separating a mixture containing CO2, at least one component lighter than CO2, and mercury

[0002] The present invention relates to a method and apparatus for separating a mixture containing CO2, at least one component lighter than CO2 and mercury and optionally at least one NOx.

[0003] State of the art:

[0004] CO2 separation units using partial condensation and / or distillation, installed on flue gases from an industrial source (cement plant, boiler, for example), allow for the removal of some of the mercury contained in the flue gases. Some of the elemental mercury is eliminated with the dust during the filtration stage, while almost 100% of the mercury in oxidized form is eliminated with the condensate generated during the compression stage. This condensate is then sent to a liquid effluent treatment plant.

[0005] The reader can refer to the associated papers and presentations: “Measuring Trace Elements during the December 2012 Callide Oxyfuel Trial” by Nelson et al 3rd Oxyfuel Combustion Conference, Ponferrada, 2013 or “The Air Products- Vattenfall Oxyfuel CO2 Compression and Purification Pilot Plant at Schwarze Pumpe” for more information about the mercury balance.

[0006] The residual elemental mercury that is not removed by the filtration or compression stage can cause damage to downstream processes operating at low temperatures, particularly aluminum equipment. In this case, the remaining mercury must be removed.

[0007] The conventional solution involves using an adsorbent charge that stores the mercury throughout the unit's lifetime. In WO 2014009675 A2, a solution allows for the cessation of mercury in the cryogenic section. In this case, the mercury is stored in phase separators and must be purged periodically. A cold mixture of mercury and impure CO2 is then treated due to the high toxicity of mercury. The purges can be stored in pressurized cylinders and then sent to a company specializing in the treatment of toxic waste streams. This solution presents implementation problems (particularly cylinder filling) and costs, as the external company's services can be expensive. In some CO2 capture processes at temperatures below 0°C, the incoming gas is compressed, dried, cooled, and separated by a single partial condensation step.In the case of gas containing NOx, gas scrubbing is carried out upstream of partial condensation as illustrated in WO13 / 135993 and WO15 / 148927.

[0008] Invention:

[0009] The present invention makes it possible to avoid sending the purges from the cryogenic section for external treatment. These purges are preferably recycled to the unit generating a gas from which the gas to be separated by the process of the invention is derived. Due to the high temperatures (up to 1500°C in the case of a cement plant) and the presence of oxygen, elemental mercury will be oxidized. The mercury oxide will preferably be returned to the process for separating a mixture containing CO2 according to the invention from the flue gases and removed during compression and condensate generation.

[0010] According to one aspect of the invention, a process is provided for separating a mixture containing CO2, at least one component lighter than CO2, and mercury. The mixture is cooled in a heat exchanger and partially condensed, forming a first two-phase fluid. This first two-phase fluid exits the heat exchanger at a first temperature between -25°C and -38°C and is separated in a first phase separator, forming a gas enriched in the at least one lighter component relative to the mixture, and a liquid bath enriched in CO2 and mercury relative to the mixture. A first liquid is drawn off by means of a pipe opening into the upper part of the liquid bath and optionally sent to a distillation separation apparatus in a stripping column to separate, forming a liquid enriched in CO2 in the column tank. A flow rate of a second liquid corresponding to less than 1% is drawn off.or even less than 0.5%, of the liquid generated by the partial condensation of the mixture at most once a year, the second liquid being richer in mercury than the first liquid and is sent upstream of the heat exchanger.

[0011] Depending on other optional characteristics:

[0012] • The flow of the second liquid is drawn at most once every two years.

[0013] • The gas from the first phase separator is cooled in the heat exchanger forming a second two-phase fluid. The second two-phase fluid exits the heat exchanger at a temperature lower than the first temperature and is separated in a second phase separator, forming a gas enriched in at least one lighter component compared to the second two-phase fluid and a liquid enriched in CO2 compared to the second two-phase fluid, which is eventually sent to the stripping column to separate.

[0014] • the second liquid flow is drawn from the first phase separator at a pressure between 15 and 45 abs.

[0015] • The flow of the first liquid from the first phase separator is drawn off more often than once a year

[0016] • The flow rate of the second liquid is the only liquid drawn from the tank of the first phase separator

[0017] • The flow rate of the second liquid constitutes a purge fluid

[0018] • the second liquid is sent to a boiler or cement plant from which comes the mixture containing CO2, at least one component lighter than CO2 and mercury.

[0019] According to another aspect of the invention, a combustion process is provided in which a fuel is burned in the presence of oxygen in a combustion unit generating heat and fumes, the fumes being treated to constitute the mixture separated by a process as described above and the flow of the second liquid is sent to the combustion unit to participate in the combustion at most once a year.

[0020] According to another aspect of the invention, a process for producing a synthesis gas is provided in which a synthesis gas containing carbon dioxide and hydrogen is produced by a reformer, a combustion unit generates heat for the reformer and fumes, the combustion unit operating according to the process as described above.

[0021] Optionally, the flow of the second liquid is mixed with air or oxygen sent to the combustion unit and / or flue gases recycled to the combustion unit. Optionally, the flow of the second liquid is sent to the combustion unit without having been mixed with any other gas.

[0022] The mercury present in the second liquid can be transformed into mercury oxide in the combustion unit. The humid flue gases are treated by compression and / or scrubbing with water or an alkaline liquid, and the mercury oxide is removed in a liquid produced by compression and / or scrubbing. According to another aspect of the invention, an apparatus for separating a mixture containing CO2, at least one component lighter than CO2, and mercury is provided, comprising a heat exchanger, means for directing the mixture to cool in the heat exchanger, forming a first two-phase fluid, means for extracting the first two-phase fluid from the heat exchanger at a first temperature between -35°C and -37°C, which is an intermediate temperature for the heat exchanger, and a first phase separator designed to contain a liquid bath.a first pipe opening at a first level in an upper part of the liquid bath of the first phase separator to draw off a first liquid, a second pipe opening at a level lower than the first level, for example in a tank, of the first phase separator to draw off a second liquid enriched with mercury, the second pipe being connected upstream of the heat exchanger to send the second liquid there, possibly a stripping column, means for drawing off the first liquid by means of the first pipe and possibly for sending the first liquid to the stripping column forming a liquid enriched with CO2 in the column tank.

[0023] According to another object of the invention, a combustion apparatus is provided comprising a combustion unit in which a fuel is burned in the presence of oxygen generating heat and fumes, treatment means for treating the fumes to constitute the mixture to be separated connected to an apparatus of one of claims 10 or 11.

[0024] According to another object of the invention, a synthesis gas production apparatus is provided comprising a reformer in which a synthesis gas containing carbon dioxide and hydrogen is produced and a combustion apparatus according to claim 12.

[0025] Description of the invention:

[0026] Since the purging of the cryogenic part is available under pressure (approximately between 15 and 45 bar), no compression element is necessary as the boiler or cement plant is operated at pressures close to atmospheric pressure.

[0027] Since the purge is cold (approximately -25°C), a compatible material should be chosen for the piping. If the temperature is too low, stainless steel is the preferred choice. The piping can potentially be flexible due to its small diameter (related to the low flow rate). Insulation of the line will not be necessary since maintaining the cold temperature is not required, as the fluid will be heated in the boiler or cement plant.

[0028] Thus, the phase separator purge will be directly connected to a valve which will be connected to the boiler or cement plant. According to the invention, it is recommended to wait for mercury to accumulate in a phase separator and then purge more thoroughly but temporarily.

[0029] If the separator is continuously purged, there is a risk of CO2 being drawn in. Therefore, this temporary purging is preferred, which could be triggered by an operator by manually opening the valve.

[0030] Regarding injection into the boiler or the cement plant itself, the purge can be mixed at different points:

[0031] • With recycled fumes (primary or secondary) where applicable

[0032] • With veins of air or oxygen (in the case of oxycombustion) injected for combustion

[0033] • With combustible gas such as natural gas, where applicable

[0034] • Directly into the combustion chamber.

[0035] In the case of oxycombustion, the first option is preferred for its simplicity of execution.

[0036] Apart from oxycombustion, injection into the air stream was preferred.

[0037] The invention will be described in more detail with reference to the figures: [FIG. 1] shows a method according to the invention.

[0038] [FIG. 2] shows a part of [FIG. 1] in a more detailed modified manner.

[0039] Figure 1 shows a process according to the invention, for removing mercury from a gas 11 containing carbon dioxide, mercury, and a component lighter than carbon dioxide such as oxygen or carbon monoxide. The compressed mixture 11 is cooled in a heat exchanger 43 to partially condense it, forming a first two-phase fluid. This first two-phase fluid exits the heat exchanger 43 at a temperature between -35°C and -37°C, i.e., an intermediate temperature of the exchanger, and is separated in a first phase separator 12, operating at a temperature close to, but warmer than, the triple point of mercury. The gas 14 from the first phase separator 12 is cooled in the heat exchanger 43 forming a second two-phase fluid, the second two-phase fluid exits the heat exchanger at a temperature below -38.8°C, between -45°C and -54°C for example.The second two-phase fluid 14 is separated in a second phase separator 13, forming a gas 15 enriched in at least the lighter component compared to the second two-phase fluid and a liquid bath enriched in CO2 and mercury compared to the second two-phase fluid.

[0040] The temperature in separator 13, if present, may be below the triple point of mercury, since the mercury has been removed in separator 12.

[0041] The gas 15 from the phase separator 13 is heated in the first exchanger 43. A liquid 20 is taken out of the phase separator 13 which is mixed with a first liquid 16 from the separator 12 to form a liquid 23.

[0042] The presence of the separator 13 is not essential and the column 25 can be supplied solely by the liquid 16, 19.

[0043] In normal operation, a first liquid 16 is continuously drawn off via a pipe opening into the upper part of the liquid bath. The liquid 23, formed by mixing liquids 16 and 20, is then sent to a distillation separation apparatus in a stripping column 25, forming a CO2-enriched liquid 29 in the column tank. A valve 21 reduces the pressure of the liquid 23 upstream of the column 25. A gas enriched in oxygen and / or carbon monoxide and depleted in CO2 exits at the top of the column 25, and a liquid 29 enriched in CO2 and depleted in oxygen and / or carbon monoxide exits the column tank 25 to form a product.

[0044] In addition to liquid 16, a flow of a second liquid 18 is withdrawn at a pressure between 15 and 45 bar abs, corresponding to less than 1% of the liquid generated by the partial condensation of gas 14 from the first phase separator, the withdrawal taking place at most once a year, by opening valve 22, the second liquid 18 being richer in mercury than the first liquid 16 and possibly richer in CO2 than the first liquid 16 since the mercury accumulates at the bottom of the tank, no liquid being withdrawn from the separator tank 12 other than liquid 18.

[0045] Preferably, the flow of the second liquid 18 is drawn off at most once every two years.

[0046] Gas 1 can consist of fumes. For example, in a synthesis gas production process in which a synthesis gas containing carbon dioxide and hydrogen is produced by a reformer fed, for example, by natural gas containing mercury, a combustion unit generates heat for the reformer and fumes.

[0047] The fumes are treated to form gas 1 and then separated by a process. The flow rate of the second liquid 18 can be drawn off and sent to the combustion unit to participate in combustion at most once a year.

[0048] For example, in a combustion process in which a fuel is burned in the presence of oxygen in a combustion unit generating heat and fumes, the fumes being treated to constitute mixture 1 separated by and the flow of the second liquid 18 is sent to the combustion unit to participate in combustion at most once a year.

[0049] Alternatively, gas 11 could originate from a cement plant where natural gas and / or oils are burned. This cement plant therefore includes a combustion unit.

[0050] The flow of the second liquid can be mixed with oxygen or air sent to the combustion unit and / or flue gases recycled to the combustion unit.

[0051] Otherwise, the flow of the second liquid can be sent to the combustion unit without having been mixed with another gas.

[0052] In the combustion unit, the mercury present in the second liquid 18 is transformed into mercury oxide by the unit's high temperatures. The humid flue gases are treated by compression and / or scrubbing with water or an alkaline liquid before forming the mixture to be treated by the process shown in Figure 1. The mercury oxide is removed in a liquid called condensate, produced by compression and / or scrubbing, and sent to a liquid effluent treatment plant.

[0053] Figure 2 shows the phase separator 12 of Figure 1. The separator 12 comprises a two-phase fluid inlet line 11, a gas outlet 14, and two liquid outlets 16 and 18 opening at different heights below the liquid level in the separator. The liquid line 16 opens into an upper part of the liquid zone and includes a pressure-reducing valve 19 and an outlet to the column 25. The liquid line 18 opens at a lower level in the separator than the liquid line 16, preferably into the separator tank, the line being opened or closed by a valve 22. The presence of the column 25 and the second phase separator is not essential. For example, the liquid 16 and 19 can be the only process product if the column 25 and the separator 13 are absent.

[0054] Otherwise, liquid 23 may be the only product of the process if column 25 is absent.

Claims

Claims 1. A process for separating a mixture containing CO2, at least one component lighter than CO2, and mercury, in which the mixture (11) is cooled in a heat exchanger (43) and partially condensed, forming a first two-phase fluid. The first two-phase fluid exits the heat exchanger at a first temperature between -25°C and -38°C and is separated in a first phase separator (12), forming a gas (14) enriched in the at least one lighter component relative to the mixture, and a liquid bath enriched in CO2 and mercury relative to the mixture. A first liquid (16) is withdrawn by means of a pipe opening into the upper part of the liquid bath and optionally sent to a distillation separation apparatus in a stripping column (25) to separate, forming a liquid enriched in CO2 in the column tank. A flow rate of a second liquid (18) corresponding to less than 1%, or even less than 0.5%,liquid generated by the partial condensation of the mixture is collected at most once a year; this second liquid is richer in mercury than the first liquid and is sent upstream of the heat exchanger.

2. A method according to claim 1 in which the flow of the second liquid (18) is withdrawn at most once every two years.

3. A method according to claim 1 or 2 wherein the gas (14) from the first phase separator (12) being cooled in the heat exchanger (43) forming a second two-phase fluid, the second two-phase fluid exits the heat exchanger at a temperature lower than the first temperature and is separated in a second phase separator (13), forming a gas (15) enriched in the at least lighter component compared to the second two-phase fluid and a liquid (20) enriched in CO2 compared to the second two-phase fluid which is optionally sent to the stripping column (25) for separation.

4. A method according to any one of the preceding claims wherein in the flow of a second liquid (18) is drawn from the first phase separator (12) at a pressure between 15 and 45 abs.

5. A method according to any one of the preceding claims wherein the second liquid (18) is sent to a boiler or cement plant from which comes the mixture containing CO2, at least one component lighter than CO2 and mercury.

6. Combustion process in which a fuel is burned in the presence of oxygen in a combustion unit generating heat and fumes, the fumes are treated to constitute the mixture to be separated by a process according to any one of claims 1 to 5 and the flow of the second liquid (18) is sent to the combustion unit to participate in the combustion at most once a year.

7. A process for producing a synthesis gas in which a synthesis gas containing carbon dioxide and hydrogen is produced by a reformer, a combustion unit generates heat for the reformer and fumes, the fumes are treated, the combustion unit and the fume treatment operating according to the process of claim 6.

8. Method according to claim 5 wherein the flow of the second liquid (18) is mixed with air or oxygen sent to the combustion unit and / or fumes recycled to the combustion unit.

9. Method according to claim 6 wherein the flow of the second liquid (18) is sent to the combustion unit, without having been mixed with another gas.

10. A method according to claim 6 wherein the mercury present in the second liquid (18) is transformed into mercury oxide in the combustion unit, the fumes which are wet are treated by compression and / or washing with water or an alkaline liquid and the mercury oxide is removed in a liquid produced by compression and / or washing.

11. Apparatus for separating a mixture containing CO2, at least one component lighter than CO2, and mercury, comprising a heat exchanger (43), means for sending the mixture (11) to cool in the heat exchanger, forming a first two-phase fluid, means for extracting the first two-phase fluid from the heat exchanger at a first temperature between -35°C and -37°C, which is an intermediate temperature of the heat exchanger, a first phase separator (12) designed to contain a liquid bath, a first conduit opening at a first level in an upper part of the liquid bath of the first phase separator for drawing off a first liquid (16), a second conduit opening at a level lower than the first level, for example in a tank, of the first phase separator to draw off a second liquid (18) enriched in mercury, the second pipe being connected upstream of the heat exchanger to send the second liquid there and means for drawing off the first liquid by means of the first pipe.

12. Apparatus according to claim 11 comprising a dewatering column (25), the means for drawing off the first liquid by means of the first conduit being connected to send the first liquid to the dewatering column forming a liquid enriched in CO2 in the tank of the dewatering column.

13. Combustion apparatus comprising a combustion unit in which a fuel is burned in the presence of oxygen generating heat and fumes, treatment means for treating the fumes to constitute the mixture to be separated connected to an apparatus of one of claims 11 or 12.

14. Apparatus for the production of a synthesis gas comprising a reformer in which a synthesis gas containing carbon dioxide and hydrogen is produced and a combustion apparatus according to claim 13.

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