Cryogenic process for separating CO2 and H2S

Abstract

Process for extracting H2S and CO2 from dry gas mixtures containing varying concentrations of methane, hydrogen, nitrogen, CO, by atmospheric pressure freezing, and for separating H2S and CO2 by controlling the defrosting heat flow, generating two successive sublimation pressures for the separate liquid-phase recovery of H2S and then CO2.

Description

Title of the invention: Cryogenic process for separating CO2 and H2S

[0001] Technical field and prior art

[0002] The invention relates to the antisublimation and differential fusion of hydrogen sulfide and carbon dioxide.

[0003] In the remainder of this text, for the sake of simplicity, the terms H2S and CO2 are used to refer to hydrogen sulfide and carbon dioxide.

[0004] Antisublimation here refers to the direct solidification of substances, in this case CO2 and H2S, at temperatures below their triple points. See, for example, the document Pan et al., CO2 capture by antisublimation process and its technical economic analysis, doi.org / 10.1002 / ghg.1313.

[0005] The need to separate CO2 and H2S from other gases arises both in the purification of natural gas and in that of process gases, particularly gases from biomass gasification. The extraction of H2S and CO2 is necessary for natural gas to ensure high combustion efficiency and to prevent 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] The H2S content of natural gas can vary from a few hundred ppm up to several percent, and the CO2 content can vary from 0.5% to 2%.

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

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

[0010] More recently, cryogenic devices for removing acid gases from a gaseous hydrocarbon stream using countercurrent separation devices were proposed in document WO 2012 / 015554 (Exxon Mobil, 2012). These devices, while interesting compared to the state of the art, are nevertheless energy-intensive, as they operate at pressures significantly higher than atmospheric pressure.

[0011] Document WO 02060561 (Armines, 2002) presents the freezing of CO2 at atmospheric pressure on fumes without the presence of H2S and therefore without selective separation of these two substances.

[0012] Objects of the invention

[0013] The invention aims to overcome the drawbacks of methods and devices known in the prior art.

[0014] According to a first aspect, a process for extracting H2S and CO2 from dry gas mixtures to be treated by atmospheric pressure freezing is proposed, containing varying concentrations of methane CH4, hydrogen H2, nitrogen, carbon monoxide CO, H2S and CO2, to form a treated gas mixture, the process ensuring the separation first of the H2S and then of the CO2, the process comprising two successive defrosting steps, at two different sublimation pressures, namely a defrosting step of the H2S followed by a defrosting step of the CO2, for the recovery of the H2S and then the CO2 in separate liquid phases, the process comprising a step of injecting the dry gas mixture to be treated into a chamber containing a heat exchanger, the heat exchanger being connected to a heat transfer fluid circuit,The process includes 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.

[0015] By dry gas mixture, we mean here mixtures of gases whose water vapor content is less than 50 ppm (v).

[0016] For a liquid flow rate, heat flux is defined as the mass flow rate of liquid multiplied by the temperature difference at the inlet and outlet of an exchanger.

[0017] Advantageously, the control of the H2S defrosting heat flow 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 H2S.

[0018] Advantageously, 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.

[0019] In certain implementations, the final freezing 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.

[0020] In certain implementations, the dry gas mixture to be treated entering the exchanger is cooled to a temperature of around -150°C, a heat transfer fluid circulating in the exchanger, the flow rate of the heat transfer fluid being regulated so that the temperature of the heat transfer fluid at the outlet of the exchanger is around -90°C.

[0021] Advantageously, the defrosting of the enclosure includes a vacuuming of the enclosure up to a pressure preferably on the order of 10 Pa, followed by a circulation of a heat transfer fluid in the exchanger.

[0022] 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 the defrosting of the H2S.

[0023] According to a second aspect, a device is proposed for implementing the process as presented above, the device comprising at least two exchangers, one exchanger being in frosting mode while the other exchanger is in defrosting mode.

[0024] During the cooling of the gas mixture at atmospheric pressure, when the volume concentration of H2S in the gas mixture is greater than 2.34%, its liquefaction will occur by condensation: passing 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.

[0025] For concentrations below and at temperatures below that of its triple point, H2S will be frosted, more particularly antisublimed, passing directly from the gas phase to the solid phase.

[0026] In gas mixtures containing methane, hydrogen, nitrogen, CO, CO2 and H2S, and for volume concentrations of CO2 less than 58% and less than 2.34% for H2S, from -86°C, the two substances (H2S and CO2) will antisublimate together.

[0027] The following table presents the temperatures and pressures of the triple points of H2S and CO2 as well as the latent heats of fusion at the triple point.

[0028] These values ​​are extracted from the NIST- National Institute of Standards and Technology database. Substance Triple point temperature (°C) Triple point pressure (kPa) Heat of fusion at triple point (kJ / kg) Heat of sublimation (kJ / kg) h2s -85.5 23,400 69.75 660 at -138°C co2 -56.6 520,000 200 573 at -78.8°C

[0029] 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.

[0030] 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.

[0031] A process for extracting H2S and CO2 from gas mixtures is proposed with alternating freezing and defrosting sequenced in sublimation pressures, to separate these two molecules.

[0032] 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.

[0033] Once the temperature of the gas mixture to be treated reaches a value below -85.5°C, the concentration of H2S is at most 2.34% and H2S begins to antisublimate, that is to say to pass from the gas phase to the solid phase.

[0034] To reduce the concentration of H2S, for example to 100 ppm(v), the antisublimation temperature will advantageously be -150°C. From -85.5°C to -150°C, CO2 will also freeze within this temperature range.

[0035] A device for implementing the process advantageously comprises at least two exchangers, one exchanger being in frosting mode while the other exchanger is in defrosting mode.

[0036] To ensure continuous cooling of the gas mixture flow, two exchangers or two series of exchangers advantageously operate in parallel, one in freezing mode and the other in defrosting mode.

[0037] In some implementations, the defrosting sequence is carried out by internal heating of tubes of the exchanger(s), with CO2 and H2S sublimating.

[0038] Advantageously, when the partial pressure of H2S is greater than 23.4 kPa, H2S begins its melting at -85.5°C well before the melting pressure of 520 kPa of CO2, H2S is recovered in liquid phase and CO2 remains in solid phase, then CO2 is itself recovered in liquid phase during the second defrosting sequence at a pressure slightly greater than 520 kPa.

[0039] Advantageously, the control of the H2S melting sequence is achieved 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.

[0040] Other objects and advantages of the invention will become apparent during the following description of embodiments, which will be made with reference to the accompanying figures in which:

[0041] [Fig-1] presents the sequences of a process for extracting H2S and CO2 separately;

[0042] [Fig.2] shows the two solid / vapor equilibrium curves of CO2 and H2S in a pressure diagram in Pascals and temperature in °C;

[0043] [Fig.3] presents a device for implementing a process for cooling a mixture of gases, for the freezing of CO2 and H2S and their sequential defrosting;

[0044] [Fig.4] shows the variation of the heat fluxes just required for the sequences of sublimation without fusion and of fusion with almost no sublimation.

[0045] Fig. 1 shows the steps of a gas mixture treatment 11, to extract H2S and CO2.

[0046] In a first step E10, the gas mixture 11 is cooled, advantageously down to -85°C.

[0047] By way of example, the volume compositions of this gas mixture 11 are as follows: CH4 54%; CO2 35%; H2 8.7%; H2S 2.3%.

[0048] During a second step E20, the temperature is lowered, advantageously down to -150°C, defining a new gas mixture 12.

[0049] In one embodiment, for the gas mixture 12, the concentration of H2S is reduced to 100 ppm(v) and the concentration of CO2 is reduced to 75 ppm(v), the volumetric composition of the gas mixture 12 being CH4 86.11%; CO2 0.018%; H2 13.87%; H2S: 0.001%, this mixture thus being purified of the essential part of the H2S and CO2.

[0050] In a step E25, a first defrosting sequence is carried out.

[0051] In one embodiment, for this defrosting step E25, an enclosure containing a heat exchanger covered with frosts containing 15% H2S and CO2 is placed under vacuum.

[0052] The process described in [Fig.1] includes a second defrosting step E30.

[0053] Advantageously, the second defrosting stage E30 begins when the pressure in the enclosure is reduced to about 10 Pa and the frosted exchanger is heated, when the temperature has gone from -150°C to -85°C, the pressure in the enclosure being slightly above 76 kPa.

[0054] As shown in [Fig.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.

[0055] Fig. 4 shows that the energy required for this first phase, noted as 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%.

[0056] The sublimation pressures of H2S and CO2 at -86°C are shown in the table below. T (°C) P sublimation H2S (kPa) P sublimation CO2 (kPa) -86°C 22.3 53.28

[0057] At -86°C, just before the melting 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 melting pressure of CO2.

[0058] During 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 H2S fusion (denoted 2 in [Fig. 4]), which is simply 6% of the maximum energy.

[0059] The liquid phase of H2S is advantageously stored in a dedicated buffer volume, during a step E35.

[0060] Once all of the H2S has passed into the liquid phase, the temperature rises quite rapidly, as does the pressure, during a step E40, by increasing the energy for the sublimation of CO2 alone, i.e. a little more than 45% of the maximum energy (noted 3 on [Fig.4]).

[0061] In certain implementations, the pressure in the enclosure will rise to above 520 kPa.

[0062] Advantageously, once this pressure is reached, a step E42 is initiated, corresponding to the melting of CO2 at -56°C, the minimum energy supplied (noted 4 in [Fig.4]) is about 16% of the maximum energy.

[0063] Liquid CO2 14 is advantageously stored in a dedicated buffer volume, during a step E45.

[0064] We now refer to [Fig.3], which illustrates a device 1 for implementing a process for decarbonizing and desulfurizing a gas mixture.

[0065] The arrows appearing in [Fig.3] correspond to the direction of fluid flow in the conduits. The terms "inlet", "outlet", "upstream", "downstream" are used in the remainder of this description with reference to the direction of fluid flow.

[0066] The device 1 includes an inlet pipe 110 for the gas mixture 11 to be decarbonized and desulfurized.

[0067] The piping 110 is connected to a supply branch 111 of an enclosure 71. A valve 113 is arranged on the branch 111.

[0068] The piping 110 is connected to a supply branch 112 of an enclosure 72. A valve 114 is arranged on the branch 112.

[0069] The gas mixture 11 advantageously enters the device 1 at a temperature of -85°C, and a water content of 0.2 ppm (v), and reaches one or the other of the enclosures 71 or 72.

[0070] The enclosure 71 contains an exchanger 73, and the enclosure 71 is provided with a pressure gauge or pressure sensor 79 and a sampling probe 81.

[0071] Similarly, the enclosure 72 contains an exchanger 74, and is equipped with a pressure gauge or pressure sensor 80 and a sampling probe 82.

[0072] The gas mixture exits the enclosure 71 through a branch 121 on which a valve 123 is disposed.

[0073] The gas mixture exits the enclosure 72 through a branch 122 on which a valve 124 is disposed.

[0074] The two branches 121, 122 are connected to an outlet pipe 120 of the decarbonized and desulfurized gas mixture 12.

[0075] Device 1 includes a heat transfer fluid circuit 5.

[0076] In the embodiment shown, this circuit includes an inlet pipe 50 on which a variable speed pump 55 is placed.

[0077] The inlet piping 50 is connected to the heat exchanger 73 of the enclosure 71 by a branch on which is mounted a valve 58 and a temperature probe 772.

[0078] The inlet piping 50 is connected to the heat exchanger 74 of the enclosure 72 by a branch on which is mounted a valve 59 and a temperature probe 774.

[0079] At the outlet of the heat exchanger 73, the heat transfer fluid 5 is conveyed into a pipe 51 by a branch on which a temperature probe 771 and a valve 56 are mounted.

[0080] At the outlet of the heat exchanger 74, the heat transfer fluid 5 is conveyed into the piping 51 by a branch on which are mounted a temperature probe 773 and a valve 57.

[0081] 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.

[0082] Similarly, 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.

[0083] Device 1 includes a heat transfer circuit, comprising an inlet pipe 60 and an outlet pipe 61.

[0084] In the embodiment shown, the inlet piping 60 for the heat transfer fluid 6 includes a pump 65 and a flow meter 64.

[0085] The inlet pipe 60 of the heat transfer fluid 6 is connected to the heat 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 heat transfer fluid 5.

[0086] Similarly, the piping 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 heat transfer fluid 5.

[0087] The outlet pipe 61 of the heat transfer fluid 6 is connected to the heat 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.

[0088] Similarly, the outlet pipe 61 of the heat transfer fluid 6 is connected to the heat exchanger 74 of the enclosure 72, by a branch on which a valve 69 is placed, this branch ending, downstream of valve 59, in the inlet branch of the heat transfer fluid 5.

[0089] The device 1 includes 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.

[0090] The buffer tank 131 is connected to a purge circuit comprising a pump 132 and a valve 136.

[0091] The buffer tank 131 is equipped with a pressure sensor 137.

[0092] The device 1 includes 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.

[0093] The buffer tank 151 is connected to a purge circuit comprising a pump 152 and a valve 156.

[0094] The buffer tank 151 is equipped with a pressure sensor 157.

[0095] 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.

[0096] Similarly, 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.

[0097] Device 1 includes a high-pressure CO2 supply circuit. This circuit includes piping 95 connected via a valve 97 to enclosure 71, and connected to enclosure 72 via a valve 96.

[0098] The following description is made with, as an example, a gas mixture 11 of composition CH4 54%, CO2 35%, H2 8.7%, H2S 2.3%.

[0099] When the heat exchanger 73 of the enclosure 71 is in frost mode, the valves 113 and 123 are open, the gas mixture 11 enters the enclosure 71 through branch 111, and is advantageously cooled down 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, exiting the enclosure 71 through branch 121, which converges in the piping 120.

[0100] The volume composition of the gas mixture 12 is then CH4 86.11%; CO2 0.018%; H213.87%; H2S: 0.001%.

[0101] This composition is for example measured by a multi-gas analyzer 85, via a sampling probe 83.

[0102] In H2S and CO2 freezing mode in enclosure 71, the variable speed pump 55 circulates the heat transfer fluid 5 via the piping 50.

[0103] The temperature of the heat transfer fluid 5 is advantageously -153°C, measured by the temperature probe 772 located at the inlet of the heat exchanger 73.

[0104] Valves 58 and 56 are open.

[0105] Valves 68, 66, 57 and 59 are closed.

[0106] The heat transfer fluid 5 exits the enclosure 71 at a temperature advantageously of the order of -90°C, through the piping 51, this temperature is measured by the temperature probe 771.

[0107] The heat transfer fluid 5 is cooled by a cryogenic system, not shown, and returns drawn by the pump 55 into the piping 50.

[0108] The flow rate of the heat transfer fluid 5, generated by the variable speed pump 55, is regulated by the temperature control 771, which must be less than -88°C.

[0109] H2S with a volume composition of 2.3% in the gas mixture 11 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.

[0110] 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 branches 112 and 122 being closed.

[0111] The defrosting in enclosure 72 takes place as follows.

[0112] Valve 92 is open, valve 91 is closed and enclosure 72 is evacuated by vacuum circuit 90.

[0113] When the pressure in the enclosure 72, measured by the manometer 80, reaches 10 Pa, the valve 92 closes.

[0114] Valves 67 and 69 of the heat transfer circuit 60 are open, and valves 66, 68, 57 and 59 are closed.

[0115] The heat transfer fluid 6 is put into circulation by the pump 65 and circulates towards the exchanger 74 in defrost mode.

[0116] 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 around -50°C.

[0117] 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, drawn by the pump 65 from the inlet pipe 60.

[0118] The pressure in the enclosure 72, measured by the manometer 80, gradually increases due to the sublimation of H2S and CO2.

[0119] When the temperature of the exchanger 74 reaches -85°C, the total pressure in the enclosure 72 is on the order of 76 kPa, composed of the partial pressures of H2S for 23.6 kPa and of CO2 for 53.4 kPa, which is advantageously verified by the composition measurement by the gas analyzer 85, via the sampling probe 82.

[0120] H2S then begins to liquefy, while CO2 remains in solid phase.

[0121] 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.

[0122] The unit heat flux corresponding to the melting of H2S, i.e. approximately 70 kJ / kg, is thus controlled for the melting of H2S only.

[0123] The temperature measured by probe 774 is almost constant, around -83°C.

[0124] 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 concentration in CO2 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.

[0125] The purging of the liquid H2S can begin.

[0126] The buffer tank 131 is under partial vacuum, measured by the manometer 137, valves 34 and 135 are open, valve 35 is closed as well as valve 155.

[0127] Liquid H2S flows via pipes 31, 30 and 13 into buffer tank 131.

[0128] When the liquid H2S flows, the temperature measured by probe 133 drops sharply to around -80°C.

[0129] When the flow dries up, the temperature measured by probe 133 rises rapidly, valves 34 and 135 are then closed.

[0130] Pipes 31, 30 and 13 are evacuated to about 10 Pa, to remove residual gaseous H2S.

[0131] These pipes are flushed with CO2, via a pipe, not shown.

[0132] This rinsing of the pipes 31, 30 and 15 is carried out to recover, during the second phase, the purest possible liquid CO2.

[0133] 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.

[0134] The atmosphere of enclosure 72, which is a mixture of CO2 and H2S, is purged by vacuuming, by opening valve 92.

[0135] 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 CO2, due to the continuous heat input from the heat transfer fluid 6.

[0136] Once the pressure reaches 520 kPa, the temperature of the exchanger 74 is at -56°C and the CO2 passes into liquid phase, this liquefaction continues until the temperature probe 774 indicates a rise in temperature which goes from -56°C rapidly to -45°C, thereby indicating that all the CO2 has melted.

[0137] A flow rate 10 of CO2 under pressure of 650 kPa is brought via piping 95, valve 97 is closed, valve 96 is open, the pressure in enclosure 72 is established at 650 kPa.

[0138] 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.

[0139] CO2 in liquid phase is therefore transferred by pressure difference via pipes 31, 30 and 15.

[0140] The temperature of the sensor 153 drops sharply to -50°C when liquid CO2 circulates in the pipe 15.

[0141] When sensor 153 measures a temperature rise from -50°C to -40°C, this temperature rise indicates the end of the transfer in the liquid phase.

[0142] From there, valves 96, 34 and 155 are closed.

[0143] The CO2 contained in the buffer tank 151 is transferred, after opening of the valve 156 by the pump 152, to a storage.

[0144] The enclosure 72 is evacuated by opening the valve 92 of the vacuum piping 90.

[0145] The enclosure 72 is ready to receive the gas mixture 11, and the freezing / defrosting cycle reverses.

[0146] The gas mixture 11 via piping 110 and then branch 112 enters enclosure 72, because valves 114, 124 are open and valves 113 and 123 are closed.

[0147] The gas mixture 11 is decarbonized and desulfurized by the freezing of CO2 and H2 S 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 branch 122 and joining the piping 120.

[0148] The heat transfer fluid enters the heat exchanger 74, valves 59 and 57 are then open and valves 69, 67, 56 and 58 are closed.

[0149] The icing of H2S and CO2 on the exchanger 74 takes place in the same way as shown above for the exchanger 73.

[0150] The temperature of -90°C, measured by the temperature probe 773, is used to control the flow generated by the variable speed pump 55.

[0151] The defrosting in enclosure 71 takes place as follows.

[0152] 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.

[0153] Valves 66 and 68 of the heat transfer circuit are open and valves 56, 58, 67 and 69 are closed.

[0154] The temperature measured by probe 772 is almost constant around -83°C.

[0155] The melting of H2S takes place in the same way and the transfer of liquid H2S occurs to buffer tank 131 by opening valves 35 and 135, valve 34 being closed.

[0156] When the H2S flow dries up, valves 35 and 135 are closed.

[0157] The CO2 defrosting on the heat exchanger 73 takes place in the same way as described above for exchanger 74, the concentration measurement is carried out by analyzer 85, via sampling probe 81, to indicate the end of the H2S purging by vacuuming enclosure 71.

[0158] It is the temperature probe 772 which, by its rise, indicates the end of the CO2 defrosting of the exchanger 73.

[0159] CO2 in liquid phase is transferred by pressure difference via pipes 32, 39, 15.

[0160] At the end of the CO2 transfer, valves 35 and 155 are closed.

[0161] The invention has many advantages.

[0162] 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.

[0163] 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 in the prior art.

[0164] 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.

[0165] The implementation of two exchangers operating in parallel, or of two series of exchangers operating in parallel, one in freezing mode, the other in defrosting mode, allows continuous treatment of the gas mixtures to be treated.

Claims

Demands

1. A process for extracting H2S and CO2 from dry gas mixtures (11) to be treated by atmospheric pressure freezing, containing varying concentrations of methane CH4, hydrogen H2, nitrogen, carbon monoxide CO, H2S, and CO2, to form a treated gas mixture (12), the process ensuring the separation of H2S first and then CO2, the process comprising two successive freezing steps at two different sublimation pressures, namely an H2S freezing step followed by a CO2 freezing step, for the recovery of H2S and then CO2 in separate liquid phases, the process comprising a step of injecting the dry gas mixture (11) to be treated into a vessel (71, 72) containing a heat exchanger (73, 74), the heat exchanger (73, 74) being connected to a fluid circuit heat transfer fluid (6),the process 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. A method according to claim 1, characterized in that the control of the H2S defrosting heat flow 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 H2S.

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

4. A method according to any one of claims 1 to 3, characterized in that the final freezing 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 (12).

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

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

7. A 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 the defrosting of the H2S.

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