Cryogenic process for separating co 2 and h 2s

Abstract

The invention relates to a method for extracting, by freezing at atmospheric pressure, H2S and CO2 in dry gas mixtures containing variable concentrations of methane, hydrogen, nitrogen and CO, and for separating H2S and CO2 by controlling the thawing thermal flux, generating two successive sublimation pressures for the separate recovery, in the liquid phase, of H2S, then CO2.

Description

[0001] Cryogenic process for separating CO2 and H2S

[0002] Technical field and state of the art

[0003] The invention relates to the antisublimation and differential fusion of hydrogen sulfide and carbon dioxide.

[0004] In the remainder of this text, for simplification, the terms H2S and CO2 are used to refer to hydrogen sulfide and carbon dioxide. Antisublimation refers to the direct solidification of substances, here CO2 and H2S, at temperatures below their triple points. For example, see Pan et al. CO2 capture by antisublimation process and its technical economic analysis, doi.org / 10.1 002 / ghg. 1313.

[0005] The need to separate CO2 and H2S from other gases is encountered both in the purification of natural gas and in the purification of process gases, especially gases from biomass gasification. The extraction of H2S and CO2 is necessary for natural gas, in order to ensure high fuel power and to avoid acid corrosion in the presence of water.

[0006] For some processes, it is necessary to extract H2S without removing CO2, particularly in processes aimed at producing synthetic fuels from hydrogen, methane and CO2.

[0007] H2S contents in natural gas can vary from a few hundred ppm to several percent, and CO2 contents can vary from 0.5% to 2%.

[0008] For syngas, H2S concentrations can vary from a few hundred ppm to several percent, and CO2 concentrations vary from 20% to 35%.

[0009] Since 1950, H2S and CO2 have been extracted in both oil refineries and natural gas processing plants using amine-based solvents, such as MEA: monoethanolamine.

[0010] More recently, cryogenic devices for the removal of acid gases from a gaseous hydrocarbon stream using countercurrent separation devices have been proposed in WO 2012 / 01 5554 (Exxon Mobil, 2012). These devices, although interesting compared to the state of the art, are however energy-intensive, because they operate at pressures significantly higher than atmospheric pressure. WO 02060561 (Armines, 2002) presents the icing of CO2 at atmospheric pressure on flue gases without the presence of H2S and therefore without selective separation of these two substances.

[0011] Objects of the invention

[0012] The invention aims to overcome the drawbacks of the methods and devices known in the prior art.

[0013] According to a first aspect, there is proposed a method for the extraction by frosting at atmospheric pressure of H2S and CO2 in mixtures of dry gases to be treated, containing variable concentrations of methane CH4, hydrogen H2, nitrogen, carbon monoxide CO, H2S and CO2, to form a mixture of treated gases, the method ensuring the separation first of H2S then of CO2, the method comprising two successive defrosting steps, at two different sublimation pressures, namely a step of defrosting the H2S followed by a step of defrosting the CO2, for the recovery of H2S then of CO2 in separate liquid phases, the method comprising a step of injecting the mixture of dry gases to be treated into an enclosure containing an exchanger, the exchanger being connected to a heat transfer fluid circuit,the method comprising controlling the defrosting heat flow by adjusting the flow rate of the heat transfer fluid according to the pressure level measured in the enclosure during defrosting.,

[0014] Dry gas mixtures are gas mixtures with a water vapour content of less than 50 ppm (v).

[0015] For a liquid flow rate, heat flux means the mass flow rate of liquid multiplied by the temperature difference at the inlet and outlet of an exchanger.

[0016] Advantageously, the control of the defrosting heat flow of the H2S is carried out in such a way that the pressure in the enclosure is maintained between 80 kPa and 100 kPa during the liquefaction of the H2S.

[0017] Advantageously, the gaseous H2S is purged from the enclosure down to 1 ppm, after recovery in the enclosure of the liquid phase of H2S and before the CO2 defrosting step.

[0018] In some implementations, the final frosting temperature is advantageously adjusted between -140°C and -150°C, depending on the desired H2S content, respectively 560 ppm (v) and 100 ppm (v), in the treated gas mixture. In some implementations, the mixture of dry gases to be treated entering the exchanger is cooled to a temperature of the order of -150°C, a refrigerant circulating in the exchanger, the flow rate of the refrigerant being regulated so that the temperature of the refrigerant at the outlet of the exchanger is of the order of -90°C.

[0019] Advantageously, defrosting the enclosure comprises placing the enclosure under vacuum to a pressure preferably of the order of 10 Pa, followed by circulation of a heat transfer fluid in the exchanger.

[0020] Advantageously, the flow rate of the heat transfer fluid in the exchanger is controlled to maintain the pressure in the enclosure, preferably around 80 kPa, during defrosting of the H2S.

[0021] According to a second aspect, there is proposed a device for implementing the method as presented above, the device comprising at least two exchangers, one exchanger being in frosting mode while the other exchanger is in defrosting mode.

[0022] When cooling the gas mixture to atmospheric pressure, when the volume concentration of H2S in the gas mixture is greater than 2.34%, its liquefaction will occur by condensation: passage from the gas phase to the liquid phase and this up to the temperature of -85.5 °C which is that of its triple point.

[0023] For lower concentrations and at temperatures below its triple point, H2S will be frosted, more specifically antisublimated, passing directly from the gas phase to the solid phase.

[0024] In gas mixtures containing methane, hydrogen, nitrogen, CO, CO2 and H2S, and for volume concentrations of CO2 below 58% and below 2.34% for H2S, from -86°C, the two substances (H2S and CO2) will antisublimate together.

[0025] The following table shows the triple point temperatures and pressures of H2S and CO2, as well as the latent heats of fusion at the triple point.

[0026] These values ​​are taken from the N IST- National Institute of Standards and Technology database. According to one aspect of the invention, the large pressure difference of the triple points of H2S and CO2, (23.4 kPa for H2S and 520 kPa for CO2) is advantageously used to control the liquefaction of H2S, without liquefying CO2.

[0027] Liquefaction pressures are advantageously controlled by controlling the defrosting heat flow, for the efficient separation of the two substances (H2S and CO2) during defrosting sequences, at two successive sublimation pressures.

[0028] A process for extracting H2S and CO2 from gas mixtures with alternating icing and defrosting sequenced under sublimation pressures is proposed to separate these two molecules.

[0029] The gradual cooling at atmospheric pressure of gas mixtures containing methane, hydrogen, CO, nitrogen, or any of these gases with CO2 and H2S, is advantageously carried out down to cryogenic temperatures.

[0030] Once the temperature of the gas mixture to be treated reaches a value below -85.5°C, the H2S concentration is at most 2.34% and the H2S begins to antisublimate, i.e. to pass from the gas phase to the solid phase.

[0031] To reduce the H2S concentration, for example to 100 ppm(v), the antisublimation temperature will then advantageously be -150°C. From -85.5°C to -150°C, the CO2 will also be frosted in this temperature range.

[0032] A device for implementing the method advantageously comprises at least two exchangers, one exchanger being in frosting mode while the other exchanger is in defrosting mode. To ensure continuous cooling of the flow of the gas mixture, two exchangers or two series of exchangers advantageously operate in parallel, one in frosting mode and the other in defrosting mode.

[0033] In some implementations, the defrosting sequence is carried out by internal heating of the tubes of the exchanger(s), the CO2 and H2S sublimating.

[0034] Advantageously, when the partial pressure of H2S is greater than 23.4 kPa, the H2S begins its fusion at -85.5°C well before the fusion pressure of 520 kPa of CO2, the H2S is recovered in the liquid phase and the CO2 remains in the solid phase, then the CO2 is itself recovered in the liquid phase during the second sequence of defrosting at a pressure slightly greater than 520 kPa.

[0035] Advantageously, the control of the H2S fusion sequence is carried out by controlling the heating heat flow to stabilize the pressure in the closed enclosure where the exchanger is located at a pressure much lower than 520 kPa, typically between 50 kPa and 100 kPa.

[0036] Other objects and advantages of the invention will appear during the following description of embodiments, a description which will be carried out with reference to the appended figures in which:

[0037] - Figure 1 shows the sequences of a process for extracting H2S and CO2 separately;

[0038] - Figure 2 shows the two solid / vapor equilibrium curves of CO2 and H2S in a pressure diagram in Pascal and temperature in °C;

[0039] - Figure 3 shows a device for implementing a process for cooling a gas mixture, for icing CO2 and H2S and their sequential defrosting;

[0040] - Figure 4 shows the variation of the heat fluxes just necessary for the sequences of sublimation without fusion and fusion with quasi-zero sublimation.

[0041] Figure 1 shows the steps of a gas mixture treatment 1 1 , to extract H2S and CO2.

[0042] In a first step E 10, the gas mixture 1 1 is cooled, advantageously to -85°C.

[0043] For example, the volume compositions of this gas mixture 1 1 are as follows: CH454%; CO235%; H28.7%; H2S 2.3%.

[0044] During a second step E20, the temperature is lowered, advantageously to - 150°C, defining a new gas mixture 12.

[0045] In one implementation, for the gas mixture 1 2, the concentration of H2S is reduced to 100 ppm(v) and the concentration of CO2 is reduced to 75 ppm(v), the volume composition of the gas mixture 1 2 being CH486, 11%; CO20.018%; H213.87%; H2S: 0.001%, this mixture thus being purified of most of the H2S and CO2.

[0046] In a step E25, a first defrosting sequence is carried out. In one implementation, for this defrosting step E25, an enclosure containing an exchanger covered with the frosts 15 of H2S and CO2 is placed under vacuum.

[0047] The method described in Figure 1 includes a second defrosting step E30.

[0048] Advantageously, the second defrosting step E30 begins when the pressure is reduced in the enclosure to approximately 10 Pa and the frosted exchanger is heated, when the temperature has dropped from - 150°C to -85°C, the pressure in the enclosure being slightly greater than 76 kPa.

[0049] As shown in Figure 2, the sublimation pressure of H2S and CO2 increases when the temperature goes from -110°C to -86°C, and the difference between these two sublimation pressures increases.

[0050] Figure 4 shows that the energy required for this first phase, noted 1, is maximum; it is the sum of the sublimation energies of H2S and CO2 presented in the table below and serves as a reference, i.e. 100%.

[0051] The sublimation pressures of H2S and CO2 at -86°C are shown in the table below.

[0052] At -86°C, just before the fusion of H2S at -85.5°C, the partial pressure of CO2 is 53.28 kPa, the total pressure is therefore around 76 kPa, very far from the 520 kPa of the fusion pressure of CO2.

[0053] In step E32, the pressure is maintained between 76 kPa and 100 kPa. The solid H2S melts and produces the liquid phase 13 of H2S. The energy required is that of fusion of H2S (noted 2 in figure 4) which is simply 6% of the maximum energy.

[0054] The liquid phase of H2S is advantageously stored in a dedicated buffer volume, during a step E35.

[0055] Once all the H2S has passed into the liquid phase, the temperature rises quite quickly as does the pressure, during a step E40, by increasing the energy for the sublimation of CÛ2 alone, i.e. a little more than 45% of the maximum energy (noted 3 in figure 4).

[0056] In some implementations, the pressure in the enclosure will rise to above 520 kPa.

[0057] Advantageously, once this pressure is reached, a step E42 is initiated, corresponding to the fusion of CO2 at -56°C, the minimum energy supplied (noted 4 in figure 4) is approximately 16% of the maximum energy.

[0058] Liquid CO2 14 is advantageously stored in a dedicated buffer volume, during a step E45.

[0059] We now refer to Figure 3, which illustrates a device 1 for implementing a process for decarbonization and desulfurization of a gas mixture.

[0060] The arrows appearing in Figure 3 correspond to the direction of circulation of fluids in the conduits. The terms "inlet", "outlet", "upstream", "downstream" are used in the remainder of this description in reference to the direction of circulation of the fluids.

[0061] The device 1 comprises a pipe 1 10 for inlet of the gas mixture 1 1 to be decarbonized and desulfurized.

[0062] The piping 1 1 0 is connected to a supply branch 1 1 1 of an enclosure 71. A valve 1 13 is arranged on the branch 1 1 1 .

[0063] The piping 1 1 0 is connected to a branch 1 1 2 supplying an enclosure 72. A valve 1 14 is arranged on the branch 1 12.

[0064] The gas mixture 1 1 advantageously enters the device 1 at a temperature of -85°C, and a water content of 0.2 ppm (v), and arrives in one or other of the enclosures 71 or 72.

[0065] The enclosure 71 contains an exchanger 73, and the enclosure 71 is provided with a pressure gauge or sensor 79 and a sampling probe 81. Similarly, the enclosure 72 contains an exchanger 74, and is provided with a pressure gauge or sensor 80 and a sampling probe 82. The gas mixture leaves the enclosure 71 via a branch 121 on which a valve 123 is arranged.

[0066] The gas mixture leaves the enclosure 72 through a branch 122 on which a valve 124 is arranged.

[0067] The two branches 121, 122 are connected to a pipe 120 for the outlet of the decarbonized and desulfurized gas mixture 12.

[0068] The device 1 comprises a refrigerant fluid circuit 5. In the embodiment shown, this circuit comprises an inlet pipe 50 on which a variable speed pump 55 is placed.

[0069] The inlet pipe 50 is connected to the exchanger 73 of the enclosure

[0070] 71 by a branch on which is mounted a valve 58 and a temperature probe 772.

[0071] The inlet pipe 50 is connected to the exchanger 74 of the enclosure

[0072] 72 by a branch on which is mounted a valve 59 and a temperature probe 774.

[0073] At the outlet of the exchanger 73, the refrigerant fluid 5 is conducted into a pipe 51 by a branch on which a temperature probe 771 and a valve 56 are mounted.

[0074] At the outlet of the exchanger 74, the refrigerant fluid 5 is conducted into the pipe 51 by a branch on which a temperature probe 773 and a valve 57 are mounted.

[0075] The enclosure 71 is connected by a branch to a vacuum circuit 90, a valve 91 being mounted on the branch connecting to the vacuum circuit 90.

[0076] Likewise, the enclosure 72 is connected by a branch to the vacuum circuit 90, a valve 92 being mounted on the branch connecting to the vacuum circuit 90.

[0077] The device 1 comprises a heat transfer circuit, comprising an inlet pipe 60 and an outlet pipe 61.

[0078] In the embodiment shown, the inlet piping 60 for the heat transfer fluid 6 comprises a pump 65 and a flow meter 64.

[0079] The inlet pipe 60 of the heat transfer fluid 6 is connected to the exchanger 73 of the enclosure 71, by a branch on which a valve 66 is placed, this branch ending upstream of the valve 56, in the outlet branch of the refrigerant fluid 5.

[0080] Likewise, the pipe 60 of the heat transfer fluid 6 is connected to the exchanger 74 of the enclosure 72, by a branch on which a valve 67 is placed, this branch ending, upstream of the valve 57, in the outlet branch of the coolant fluid 5.

[0081] The outlet pipe 61 of the heat transfer fluid 6 is connected to the exchanger 73 of the enclosure 71, by a branch on which a valve 68 is placed, this branch ending, downstream of the valve 58, in the inlet branch of the heat transfer fluid 5. Similarly, the outlet pipe 61 of the heat transfer fluid 6 is connected to the exchanger 74 of the enclosure 72, by a branch on which a valve 69 is placed, this branch ending, downstream of the valve 59, in the inlet branch of the heat transfer fluid 5.

[0082] The device 1 comprises a buffer tank 131 for liquid H2S, the supply conduit 13 of this buffer tank 131 being provided with a temperature probe 133 and a valve 135.

[0083] The buffer tank 131 is connected to a purge circuit comprising a pump 132 and a valve 136.

[0084] The buffer tank 131 is provided with a pressure sensor 137.

[0085] The device 1 comprises a buffer tank 151 for liquid CO2, the supply conduit 15 of this buffer tank 151 being provided with a temperature probe 153 and a valve 155.

[0086] The buffer tank 151 is connected to a purge circuit comprising a pump 152 and a valve 156.

[0087] The buffer tank 1 51 is provided with a pressure sensor 1 57.

[0088] The enclosure 72 is connected to the supply circuit 13 of the buffer tank 131 and to the supply circuit 15 of the buffer tank 151 by a branch 31 on which a valve 34 is mounted.

[0089] Likewise, the enclosure 71 is connected to the supply circuit 13 of the buffer tank 131 and to the supply circuit 15 of the buffer tank 151 by a branch 32 on which a valve 35 is mounted.

[0090] The device 1 comprises a high-pressure CO2 supply circuit. This circuit comprises a pipe 95 connected via a valve 97 to the enclosure 71, and connected to the enclosure 72 via a valve 96.

[0091] The following description is carried out with, as an example, a gas mixture 1 1 of composition CH454%, CO2 35%, H28.7%, H2S 2.3%.

[0092] When the exchanger 73 of the enclosure 71 is in frosting mode, the valves 113 and 123 are open, the gas mixture 11 enters the enclosure 71 via the branch 111, and is advantageously cooled to -150°C, to reduce the H2S content to 100 ppm (v), the gas mixture 11 then being essentially decarbonized and desulfurized and becoming the gas mixture 12, leaving the enclosure 71 via the branch 121, which converges in the piping 120.

[0093] The volume composition of the gas mixture 12 is then CH486, 11%; CO2 0.018%; H213.87%; H2S: 0.001%. This composition is for example measured by a multi-gas analyzer 85, via a sampling probe 83.

[0094] In H2S and CO2 icing mode in enclosure 71, the variable speed pump 55 circulates the refrigerant fluid 5 via the piping 50.

[0095] The temperature of the refrigerant fluid 5 is advantageously -153°C, measured by the temperature probe 772 arranged at the inlet of the exchanger 73.

[0096] Valves 58 and 56 are open.

[0097] Valves 68, 66, 57 and 59 are closed.

[0098] The refrigerant fluid 5 leaves the enclosure 71 at a temperature advantageously of the order of -90°C, via the pipe 51, this temperature is measured by the temperature probe 771.

[0099] The refrigerant fluid 5 is cooled by a cryogenic system, not shown, and returns sucked by the pump 55 into the pipe 50.

[0100] The flow rate of refrigerant fluid 5, generated by the variable speed pump 55, is regulated by the temperature control 771, which must be less than -88°C.

[0101] H2S with a volume composition of 2.3% in the gas mixture 1 1 begins to freeze at -86°C and CO2 with a concentration of 35% begins to freeze at -90°C, so there is co-freezing of these two substances.

[0102] The other enclosure 72 is in defrost mode, and is isolated from the circulation of the gas mixture 11, the valves 114 and 124 of the branches 112 and 122 being closed.

[0103] Defrosting in enclosure 72 takes place as follows.

[0104] Valve 92 is open, valve 91 is closed and enclosure 72 is evacuated by vacuum circuit 90.

[0105] When the pressure in the enclosure 72, measured by the pressure gauge 80, reaches 10 Pa, the valve 92 closes.

[0106] Valves 67 and 69 of the heat transfer circuit 60 are open, and valves 66, 68, 57 and 59 are closed.

[0107] The heat transfer fluid 6 is circulated by the pump 65 and circulates towards the exchanger 74 in defrost mode.

[0108] The exchanger 74 is advantageously at an average temperature of - 120°C, and is heated by the heat transfer fluid 6, the temperature of which, measured by the temperature probe 773 at the inlet of the exchanger 74, is of the order of -50°C. The heat transfer fluid 6 cooled by defrosting recirculates via the outlet pipe 61, the coldness is recovered and the heat transfer fluid 6 returns to the temperature of -50°C, sucked in by the pump 65 from the inlet pipe 60.

[0109] The pressure in enclosure 72, measured by pressure gauge 80, gradually rises due to the sublimation of H2S and CO2.

[0110] When the temperature of the exchanger 74 reaches -85°C, the total pressure in the enclosure 72 is of the order of 76 kPa, composed of the partial pressures of H2S for 23.6 kPa and CO2 for 53.4 kPa, which is advantageously verified by the composition measurement by the gas analyzer 85, via the sampling probe 82.

[0111] The H2S then begins to liquefy, while the CO2 remains in the solid phase.

[0112] The pressure measured by the pressure gauge 80 is advantageously maintained around 80 kPa, and the flow rate of the pump 65 measured by the flow meter 64 is adjusted according to this pressure value.

[0113] The unit heat flow corresponding to the fusion of H2S, i.e. approximately 70 kJ / kg, is thus controlled for the fusion of H2S alone.

[0114] The temperature measured by probe 774 is almost constant, around -83°C.

[0115] When the mass of H2S is completely melted, the temperature measured by the probe 774 rises slightly by 1 to 2°C, the pressure in the enclosure 72 rises and the CO2 concentration rises, this concentration being advantageously measured by the gas analyzer 85 via the sampling probe 82, indicating that the energy supplied by the heat transfer fluid 6 is then used to sublimate the CO2.

[0116] Purging of liquid H2S can begin.

[0117] The buffer tank 131 is under partial vacuum, measured by the pressure gauge 137, the valves 34 and 135 are open, the valve 35 is closed as well as the valve 155.

[0118] Liquid H2S flows via pipes 31, 30 and 13 into buffer tank 131.

[0119] When liquid H2S flows, the temperature measured by probe 133 drops sharply to around -80°C.

[0120] When the flow stops, the temperature measured by probe 133 rises rapidly, valves 34 and 135 are then closed. Pipes 31, 30 and 13 are evacuated to approximately 10 Pa, to eliminate the residual gaseous H2S.

[0121] These pipes are flushed with CO2, via a pipe, not shown.

[0122] This rinsing of pipes 31, 30 and 15 is carried out to recover, during the second phase, the purest possible liquid CO2.

[0123] The liquid H2S stored in the buffer tank 131 is transferred to an external storage, the valve 136 is opened and the pump 132 is activated. The atmosphere of the enclosure 72, which is a mixture of CO2 and H2S, is purged by drawing a vacuum, by opening the valve 92.

[0124] Once the measurement of the composition by the analyzer 85, via the sampling probe 82, indicates an H2S concentration of less than 1 ppm, the valve 92 is closed and the pressure rises rapidly in the enclosure 72 by the sublimation of the CO2, due to the continuous heat supply from the heat transfer fluid 6.

[0125] Once the pressure reaches 520 kPa, the temperature of the exchanger 74 is at -56°C and the CO2 passes into the liquid phase, this liquefaction continues until the temperature probe 774 indicates a rise in temperature which goes from -56°C quickly to -45°C, thereby indicating that all the CO2 is melted.

[0126] A flow 10 of CO2 under pressure of 650 kPa is supplied via the pipe 95, the valve 97 is closed, the valve 96 is open, the pressure in the enclosure 72 is established at 650 kPa.

[0127] Once this pressure is reached, measured by the pressure sensor 80, the valve 34 is opened as well as the valve 155, the valve 35 is closed, the CO2 buffer tank 151 is at a pressure of 550 kPa, measured by the pressure sensor 157.

[0128] The liquid phase CO2 is therefore transferred by pressure difference via pipes 31, 30 and 15.

[0129] The temperature of the sensor 1 53 decreases sharply to -50°C when the liquid CO2 flows in the pipe 1 5.

[0130] When sensor 1 53 measures a temperature rise from -50°C to -40°C, this temperature rise indicates the end of the transfer into the liquid phase.

[0131] From there, valves 96, 34 and 155 are closed.

[0132] The CO2 contained in the buffer tank 151 is transferred, after opening the valve 156 by the pump 152, to a storage facility. The enclosure 72 is evacuated by opening the valve 92 of the vacuum pipe 90.

[0133] The enclosure 72 is ready to receive the gas mixture 1 1 , and the frosting / defrosting cycle is reversed.

[0134] The gas mixture 1 1 via the pipe 1 10 then the branch 1 12 enters the enclosure 72, because the valves 1 14, 1 24 are open and the valves 1 13 and 123 are closed.

[0135] The gas mixture 11 is decarbonized and desulfurized by the icing of CO2 and H2S down to -150°C on the exchanger 74, the gas mixture 11 changes composition as indicated above and becomes the gas mixture 12 whose composition is measured by the analyzer 85 via the sampling probe 84, exiting through the branch 122 and joining the piping 120.

[0136] The refrigerant enters the exchanger 74, the valves 59 and 57 are then opened and the valves 69, 67, 56 and 58 are closed.

[0137] The icing of H2S and CO2 on exchanger 74 takes place in the same way as presented above for exchanger 73.

[0138] The temperature of -90°C, measured by the temperature probe 773, is used to control the flow rate generated by the variable speed pump 55.

[0139] Defrosting in enclosure 71 takes place as follows.

[0140] Valves 113 and 123 of branches 111 and 121 are closed, valve 91 is open, valve 92 is closed and enclosure 71 is evacuated by vacuum circuit 90 and the pressure is measured by pressure sensor 79.

[0141] Valves 66 and 68 of the heat transfer circuit are open and valves 56, 58, 67 and 69 are closed.

[0142] The temperature measured by probe 772 is almost constant around -83°C.

[0143] The fusion of H2S takes place in the same way and the transfer of liquid H2S is made to the buffer tank 131 by opening the valves 35 and 135, the valve 34 being closed.

[0144] When the H2S flow stops, valves 35 and 135 are closed.

[0145] The defrosting of the CO2 on the exchanger 73 takes place in the same way as described above for the exchanger 74, the concentration measurement is carried out by the analyzer 85, via the sampling probe 81, to indicate the end of the H2S purge by drawing a vacuum from the enclosure 71. It is the temperature probe 772 which, by its rise, indicates the end of the defrosting of CO2 from the exchanger 73.

[0146] Liquid phase CO2 is transferred by pressure difference via pipes 32, 39, 15.

[0147] At the end of the CO2 transfer, valves 35 and 155 are closed.

[0148] The invention has many advantages.

[0149] It is not necessary to use a solvent, for example an amine-based solvent such as MEA, to extract CO2 or H2S from the gas mixture. Monoethanolamine (CAS 141-43-5) is harmful by inhalation or skin contact.

[0150] The cooling of the gas mixtures to be treated is carried out at atmospheric pressure, the process being less energy-intensive than the processes known from the prior art.

[0151] The choice of the antisublimation temperature allows a reduction of the H2S concentration of the treated gas, towards a target value, for example 100 ppm (v) with an antisublimation temperature of - 150°C.

[0152] The implementation of two exchangers operating in parallel, or two series of exchangers operating in parallel, one in frosting mode, the other in defrosting mode, allows continuous treatment of the gas mixtures to be treated.

Claims

Claims 1 . Method for extraction by atmospheric pressure frosting of H2S and CO2 in mixtures of dry gases (11) to be treated, containing variable concentrations of methane CH4, hydrogen H2, nitrogen, carbon monoxide CO, H2S, CO2, to form a mixture of treated gases (12), the method ensuring the separation first of H2S then of CO2, the method comprising two successive defrosting steps, at two different sublimation pressures, namely a step of defrosting of H2S followed by a step of defrosting of CO2, for the recovery of H2S then of CO2 in separate liquid phases, the method comprising a step of injecting the mixture of dry gases (11) to be treated into an enclosure (71, 72) containing an exchanger (73, 74), the exchanger (73, 74) being connected to a heat transfer fluid circuit (6),the method comprising controlling the defrosting heat flow by adjusting the flow rate of the heat transfer fluid (6) according to the pressure level measured in the enclosure (71, 72) during defrosting., 2. Method according to claim 1, characterized in that the control of the defrosting heat flow of the H2S is carried out in such a way that the pressure in the enclosure (71, 72) is maintained between 80 kPa and 100 kPa during the liquefaction of the H2S.

3. Method according to claim 1 or 2, characterized in that the gaseous H2S is purged from the enclosure (71, 72) down to 1 ppm, after the recovery in the enclosure (71, 72) of the liquid phase of H2S and before the CO2 defrosting step.

4. Method according to any one of claims 1 to 3, characterized in that the final frosting temperature is adjusted between - 140°C and - 150°C, depending on the desired H2S content, respectively 560 ppm (v) and 100 ppm (v), in the treated gas mixture (1 2).

5. Method according to any one of claims 1 to 4, characterized in that the mixture of dry gases (11) to be treated entering the exchanger (71, 72) is cooled to a temperature of the order of -150°C, a refrigerant fluid (5) circulating in the exchanger (73, 74), the flow rate of the refrigerant fluid (5) being regulated so that the temperature of the refrigerant fluid (5) at the outlet of the exchanger (73, 74) is of the order of -90°C.

6. Method according to any one of claims 1 to 5, characterized in that the defrosting of the enclosure (71, 72) comprises placing the enclosure (71, 72) under vacuum to a pressure preferably of the order of 10 Pa, followed by circulation of a heat transfer fluid (6) in the exchanger (73, 74).

7. Method according to claim 6, characterized in that the flow rate of the heat transfer fluid (6) in the exchanger (73, 74) is controlled to maintain the pressure in the enclosure (71, 72), preferably around 80 kPa, during defrosting of the H2S.

8. Device for implementing the method as presented in any one of the preceding claims, characterized in that it comprises at least two exchangers (71, 72), one exchanger being in frosting mode while the other exchanger is in defrosting mode.

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