FR3153650A1 - Cryogenic process for separating CO2 and H2S - Google Patents

Abstract

Process for the extraction by atmospheric pressure frosting of H2S and CO2 in dry gas mixtures containing variable concentrations of methane, hydrogen, nitrogen, CO, and separation of H2S and CO2 by controlling the defrosting heat flow generating two successive sublimation pressures for the recovery in the separate liquid phase of H2S then CO2.

Description

Cryogenic process for separating CO2 and H2S

Technical field and state of the art

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

In the remainder of this text, for simplification, the terms _H2S and _CO2 are used to designate hydrogen sulphur and carbon dioxide.

Antisublimation refers to the direct solidification of substances, in this case _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/10.1002/ghg.1313.

The need to separate _CO2 , _H2S from other gases is encountered both in the purification of natural gas and in the purification of process gases, in particular 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.

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

_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%.

For synthesis gases, _H2S concentrations can vary from a few hundred ppm to several percent, and _CO2 concentrations vary from 20% to 35%.

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.

More recently, cryogenic devices for the removal of acid gases from a gaseous hydrocarbon stream using countercurrent separation devices have been proposed in WO 2012/015554 (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.

Document WO 02060561 (Armines, 2002) presents the icing of CO ₂ at atmospheric pressure on fumes without the presence of H ₂ S and therefore without selective separation of these two substances.

Objects of the invention

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

According to a first aspect, there is proposed a method for the extraction by frosting at atmospheric pressure of H ₂ S and CO ₂ in mixtures of dry gases to be treated, containing variable concentrations of methane CH ₄ , hydrogen H ₂ , nitrogen, carbon monoxide CO, H ₂ S and CO ₂ , to form a mixture of treated gases, the method ensuring the separation first of H ₂ S then of CO ₂ , the method comprising two successive defrosting steps, at two different sublimation pressures, namely a step of defrosting of H ₂ S followed by a step of defrosting of CO ₂ , for the recovery of H ₂ S then of CO ₂ 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 a control of the heat flow defrosting by adjusting the flow rate of the heat transfer fluid according to the pressure level measured in the enclosure being defrosted.

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

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.

Advantageously, the control of the defrosting heat flow of the H ₂ S 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 H ₂ S.

Advantageously, the gaseous H ₂ S is purged from the enclosure down to 1 ppm, after recovery in the enclosure of the liquid phase of H ₂ S and before the CO ₂ defrosting step.

In some implementations, the final frosting temperature is advantageously adjusted between -140°C and -150°C, depending on the desired H ₂ S content, respectively 560 ppm (v) and 100 ppm (v), in the treated gas mixture.

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

Advantageously, defrosting the enclosure includes 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.

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 H ₂ S.

According to a second aspect, a device is proposed 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.

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.

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.

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.

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

These values are taken from the NIST-National Institute of Standards and Technology database. Substance Triple point temperature (°C) Triple point pressure (kPa) Heat of fusion at the triple point (kJ/kg) Heat of sublimation (kJ/kg) H ₂ S -85.5 23,400 69.75 660 to -138°C _CO2 -56.6 520,000 200 573 at -78.8°C

According to one aspect of the invention, the large pressure difference of the triple points of H ₂ S and CO ₂ , (23.4 kPa for H ₂ S and 520 kPa for CO ₂ ) is advantageously used to control the liquefaction of H ₂ S, without liquefying the CO ₂ .

The liquefaction pressures are advantageously controlled by controlling the defrosting heat flow, for the efficient separation of the two substances (H ₂ S and CO ₂ ) during the defrosting sequences, at two successive sublimation pressures.

A process is proposed for the extraction of _H2S and _CO2 from gas mixtures with alternating icing and defrosting sequenced under sublimation pressures, to separate these two molecules.

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.

Once the temperature of the gas mixture to be treated reaches a value below -85.5°C, the concentration of H ₂ S is at most 2.34% and the H ₂ S begins to antisublimate, i.e. to pass from the gas phase to the solid phase.

To reduce the concentration of _H2S , 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.

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 gas mixture flow, two exchangers or two series of exchangers advantageously operate in parallel, one in frost mode and the other in defrost mode.

In some implementations, the defrost sequence is carried out by internal heating of tubes of the exchanger(s), the _CO2 and _H2S sublimating.

Advantageously, when the partial pressure of H ₂ S is greater than 23.4 kPa, the H ₂ S begins its fusion at -85.5°C well before the fusion pressure of 520 kPa of CO ₂ , the H ₂ S is recovered in the liquid phase and the CO ₂ remains in the solid phase, then the CO ₂ is itself recovered in the liquid phase during the second defrosting sequence at a pressure slightly greater than 520 kPa.

Advantageously, the control of the H ₂ S 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.

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:

FIG. 1 presents the sequences of a process for extracting _H2S and _CO2 separately;

FIG. 2 presents the two solid/vapor equilibrium curves of _CO2 and _H2S in a pressure diagram in Pascal and temperature in °C;

FIG. 3 presents a device for implementing a method of cooling a gas mixture, for the icing of _CO2 and _H2S and their sequential defrosting;

FIG. 4 presents the variation of the heat fluxes just necessary for the sequences of sublimation without fusion and fusion with quasi-zero sublimation.

There FIG. 1 shows the steps of a gas mixture treatment 11, to extract H ₂ S and CO ₂ .

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

For example, the volume compositions of this gas mixture 11 are as follows: CH ₄ 54%; CO ₂ 35%; H ₂ 8.7%; H ₂ S 2.3%.

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

In one implementation, for the gas mixture 12, the concentration of H ₂ S is reduced to 100 ppm(v) and the concentration of CO ₂ is reduced to 75 ppm(v), the volume composition of the gas mixture 12 being CH ₄ 86.11%; CO ₂ 0.018%; H ₂ 13.87%; H ₂ S: 0.001%, this mixture thus being purified of most of the H ₂ S and CO ₂ .

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 H ₂ S and CO ₂ is placed under vacuum.

The process described in FIG. 1 includes a second E30 defrosting step.

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.

As it appears 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.

There FIG. 4 shows that the energy required for this first phase noted 1, is maximum, it is the sum of the sublimation energies of H ₂ S and CO ₂ presented in the table below and serves as a reference, that is to say 100%.

The sublimation pressures of H₂S and CO₂at -86°C are shown in the table below. T (°C) P sublimation H ₂ S (kPa) P sublimation CO ₂ (kPa)
-86°C 22.3 53.28

At -86°C, just before the fusion of H ₂ S at -85.5°C, the partial pressure of CO ₂ is 53.28 kPa, the total pressure is therefore of the order of 76 kPa, very far from the 520 kPa of the fusion pressure of CO ₂ .

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

The liquid phase of H ₂ S is advantageously stored in a dedicated buffer volume, during a step E35.

Once all the _H2S has passed into the liquid phase, the temperature rises quite quickly as well as 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 the FIG. 4 ).

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

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 the FIG. 4 ) is about 16% of the maximum energy.

Liquid _CO2 14 is advantageously stored in a dedicated buffer volume, during a step E45.

We now refer to the FIG. 3 , which illustrates a device 1 for implementing a process for decarbonization and desulfurization of a gas mixture.

The arrows appearing in FIG. 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 fluids.

The device 1 comprises an inlet pipe 110 for the gas mixture 11 to be decarbonized and desulfurized.

The piping 110 is connected to a branch 111 supplying a chamber 71. A valve 113 is arranged on the branch 111.

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

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 arrives in one or other of the enclosures 71 or 72.

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.

Likewise, 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 through a branch 121 on which a valve 123 is arranged.

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

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

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.

The inlet pipe 50 is connected to the exchanger 73 of the enclosure 71 by a branch on which a valve 58 and a temperature probe 772 are mounted.

The inlet pipe 50 is connected to the exchanger 74 of the enclosure 72 by a branch on which a valve 59 and a temperature probe 774 are mounted.

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.

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.

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.

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.

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

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

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.

Likewise, 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 refrigerant fluid 5.

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.

Likewise, 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.

The device 1 comprises a buffer tank 131 for liquid H ₂ S, the supply conduit 13 of this buffer tank 131 being provided with a temperature probe 133 and a valve 135.

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

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

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.

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

The buffer tank 151 is provided with a pressure sensor 157.

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.

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.

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.

The following description is carried out with, as an example, a gas mixture 11 of composition CH ₄ 54%, CO ₂ 35%, H ₂ 8.7%, H ₂ S 2.3%.

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 H ₂ S 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.

The volume composition of the gas mixture 12 is then CH ₄ 86.11%; CO ₂ 0.018%; H ₂ 13.87%; H ₂ S: 0.001%.

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

In _H2S and _CO2 icing mode in enclosure 71, the variable speed pump 55 circulates the refrigerant fluid 5 via the piping 50.

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.

Valves 58 and 56 are open.

Valves 68, 66, 57 and 59 are closed.

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.

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

The flow 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.

_H2S with a volume composition of 2.3% in 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.

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.

Defrosting in enclosure 72 takes place as follows.

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

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

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

The heat transfer fluid 6 is circulated by the pump 65 and flows towards the exchanger 74 in defrost mode.

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 by the pump 65 from the inlet pipe 60.

The pressure in enclosure 72, measured by pressure gauge 80, rises gradually thanks to the sublimation of _H2S and _CO2 .

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 H ₂ S for 23.6 kPa and of CO ₂ for 53.4 kPa, which is advantageously verified by the composition measurement by the gas analyzer 85, via the sampling probe 82.

The _H2S then begins to liquefy, while the _CO2 remains in the solid phase.

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.

The unit heat flux corresponding to the fusion of _H2S , i.e. approximately 70 kJ/kg, is thus controlled for the fusion of _H2S alone.

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

When the mass of H ₂ S 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 CO ₂ 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 CO ₂ .

The purging of liquid _H2S can begin.

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.

Liquid _H2S flows via pipes 31, 30 and 13 into buffer tank 131.

When liquid _H2S flows, the temperature measured by probe 133 drops sharply to around -80°C.

When the flow stops, the temperature measured by probe 133 rises quickly, valves 34 and 135 are then closed.

Pipes 31, 30 and 13 are evacuated to approximately 10 Pa, to eliminate residual gaseous H ₂ S.

These pipes are rinsed with _CO2 , via a pipe, not shown.

This rinsing of pipes 31, 30 and 15 is carried out to recover, during the second phase, the purest possible liquid _CO2 .

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 enclosure 72, which is a mixture of _CO2 and _H2S , is purged by vacuum, by opening valve 92.

Once the measurement of the composition by the analyzer 85, via the sampling probe 82, indicates a concentration of H ₂ S less than 1 ppm, the valve 92 is closed and the pressure rises rapidly in the enclosure 72 by the sublimation of the CO ₂ , due to the continuous heat supply from the heat transfer fluid 6.

Once the pressure reaches 520 kPa, the temperature of the exchanger 74 is at -56°C and the CO ₂ 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 CO ₂ is melted.

A flow 10 of CO ₂ under pressure of 650 kPa is brought 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.

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 CO ₂ buffer tank 151 is at a pressure of 550 kPa, measured by the pressure sensor 157.

The liquid phase _CO2 is therefore transferred by pressure difference via pipes 31, 30 and 15.

The temperature of sensor 153 decreases sharply to -50°C when liquid _CO2 flows through pipe 15.

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

From there, valves 96, 34 and 155 are closed.

The _CO2 contained in the buffer tank 151 is transferred, after opening the valve 156 by the pump 152, to storage.

The enclosure 72 is evacuated by opening the valve 92 of the vacuum pipe 90.

Enclosure 72 is ready to receive gas mixture 11, and the frosting/defrosting cycle is reversed.

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

The gas mixture 11 is decarbonized and desulfurized by the icing of CO ₂ and H ₂ 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 the branch 122 and joining the piping 120.

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

The icing of _H2S and _CO2 on exchanger 74 takes place in the same way as presented above for exchanger 73.

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.

Defrosting in enclosure 71 takes place as follows.

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.

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

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

The fusion of H ₂ S takes place in the same way and the transfer of liquid H ₂ S is made to the buffer tank 131 by opening the valves 35 and 135, the valve 34 being closed.

When the _H2S flow dries up, valves 35 and 135 are closed.

The defrosting of the CO ₂ 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 H ₂ S 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 CO ₂ of the exchanger 73.

Liquid phase _CO2 is transferred by pressure difference via pipes 32,39,15.

At the end of the _CO2 transfer, valves 35 and 155 are closed.

The invention has many advantages.

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.

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.

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.

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 (8)

Atmospheric pressure frost extraction process for H₂S and CO₂in dry gas mixtures (11) to be treated, containing variable concentrations of methane CH₄, hydrogen H₂, nitrogen, carbon monoxide CO, H₂S, of CO₂, to form a treated gas mixture (12), the process ensuring the separation first of the H₂S then CO₂, the process comprising two successive defrosting steps, at two different sublimation pressures, namely a defrosting step of the H₂S followed by a CO defrosting step₂, for the recovery of H₂S then 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. Method according to claim 1, characterized in that the control of the defrosting heat flow of the H ₂ S 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 H ₂ S. Method according to claim 1 or 2, characterized in that the gaseous H ₂ S is purged from the enclosure (71, 72) down to 1 ppm, after the recovery in the enclosure (71, 72) of the liquid phase of H ₂ S and before the CO ₂ defrosting step. 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 H ₂ S content, respectively 560 ppm (v) and 100 ppm (v), in the treated gas mixture (12). 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. 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). 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 H ₂ S. 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.