Method and apparatus for membrane separation of a mixture containing predominantly hydrogen and carbon dioxide

Abstract

Title: Membrane separation process for a mixture (1) containing mainly hydrogen and carbon dioxide. A membrane separation process for a mixture (1) containing mainly hydrogen and carbon dioxide comprises the following steps: Heating the mixture in a heat exchanger (E) to a first temperature; Permeating the heated mixture through a first membrane system (M1) to obtain a first residue (R1) depleted in hydrogen and carbon dioxide relative to the mixture; Permeating the first residue through a second membrane system (M2) to obtain a second residue (R2) depleted in hydrogen and carbon dioxide relative to the first residue; Expanding the second residue to a first pressure in a first turbine (T1) to obtain a second residue at a second pressure; Heating (E) at least a portion of the second residue at the second pressure to a second temperature.Expansion of at least part of the second residue at the second temperature in a second turbine (T2) from the second pressure to a third pressure lower than the second pressure. Figure from the abstract: FIG. 1,

Description

Title of the invention: Method and apparatus for membrane separation of a mixture containing mainly hydrogen and carbon dioxide

[0001] The present invention relates to a process for membrane separation of a mixture containing mainly hydrogen and carbon dioxide and in addition at least one other component chosen from carbon monoxide, methane and nitrogen.

[0002] A mixture containing predominantly hydrogen and carbon dioxide with a composition such that at least 50 mol% of the mixture is composed of hydrogen and carbon dioxide.

[0003] The at least one other component, which may be nitrogen, carbon monoxide or methane, constitutes at most 30 mol% of the mixture, preferably at most 20 mol% of the mixture.

[0004] The capture of carbon dioxide (CO2) by a distillation and / or partial condensation process fed by a waste gas from a hydrogen (H2) production unit comprising a hydrogen separation unit by pressure swing adsorption (in English "Pressure Swing Adsorption" or PSA) is known. The waste gas is depleted in hydrogen compared to the gas feeding this separation unit. The separation unit can be combined with a membrane separation of the carbon dioxide-depleted gas produced by the distillation and / or partial condensation process. This membrane separation aims to separate the CO2 and H2 from the rest of the gases in two stages.The permeate from a first membrane system is recycled to the PSA, while the permeate from a second membrane system, fed by the residue from the first system, is returned to the compression of the feed gas upstream of the separation by distillation and / or partial condensation to be compressed. The residue from the membrane systems is still under pressure.

[0005] According to the invention, the residual gas obtained under pressure is expanded in two turbines in series. The cold fluid at the outlet of the two turbines is sent to the cryogenic process to recover its cold. These two turbines in series make it possible to drive two boosters in series. These two boosters make it possible to lower the pressure of the permeate of the first membrane while being able to recycle it to the PSA which is at a higher pressure. The hot gas at the outlet of the second booster passes through the heat exchanger to exchange its heat.

[0006] The use of a turbine booster in the Cryogenic CO2 capture processes combined with membranes have already been described in WO2012 / 064938.

[0007] The invention makes it possible to maximize the energy extracted by turbines by making use of available heat, for example that produced by boosters. The process is thus more energy efficient because it uses heat, for example that produced by compression and typically dissipated in cooling water. The invention also makes it possible to obtain higher CO2 and hydrogen capture efficiencies because it makes it possible to reduce the pressure of the permeate of the first membrane. Finally, the invention makes it possible to control the risk of CO2 freezing at the outlet of the turbines. Description of the invention

[0008] According to an object of the invention, there is provided a process for membrane separation of a mixture containing mainly hydrogen and carbon dioxide as well as at least one more minor compound such as carbon monoxide, methane, nitrogen comprising the following steps: i. Heating the mixture in a heat exchanger to a first temperature ii. Permeation of the heated mixture in a first membrane system making it possible to obtain a first permeate loaded with hydrogen and carbon dioxide relative to the mixture and a first residue depleted in hydrogen and carbon dioxide relative to the mixture iii. Permeation of the first residue into a second membrane system to obtain a second permeate loaded with hydrogen and carbon dioxide relative to the first residue and a second residue depleted in hydrogen and carbon dioxide relative to the first residue iv. Expansion of the second residue to a first pressure in a first turbine in order to obtain a second residue at a second pressure characterized in that it comprises v. Heating at least a portion of the second residue at the second pressure to a second temperature, the same as or different from the first temperature and vi. Expanding at least a portion of the second residue at the second temperature in a second turbine from the second pressure to a third pressure lower than the second pressure.

[0009] According to other optional aspects: • the temperature of the second residue at the first pressure is regulated to maintain the temperature of the second residue at the second pressure above -80°C, preferably above -60°C or even above -55°C to avoid the risk of frost. • at least a portion of the second residue is heated according to step v) in the heat exchanger. • the first permeate is compressed in at least one booster and the first booster permeate cools in the heat exchanger. • the first permeate is compressed in two boosters in series, one booster coupled to the first turbine and the other booster coupled to the second turbine. • at least part of the second residue is heated according to step v) in a heat exchanger other than the heat exchanger where the mixture is heated. • at least part of the second residue is relaxed without having been reheated. • the outlet of the first turbine is at a temperature below 40°C, preferably below -20°C, preferably below -20°C, or even below -40°C. • -the mixture and at least part of the second residue are heated in the heat exchanger from the cold end thereof. • at least part of the second residue heats up to between 30 and 80°C. • the first pressure is between 45 and 60 bara and / or the second pressure is between 15 and 25 bara and / or the third pressure is between 2 and 4.5 bara. • the permeation of the heated mixture in a first membrane system making it possible to obtain a first permeate depleted in the other compound which is a smaller minority compared to the mixture and a first residue enriched in the other compound which is a smaller minority compared to the mixture • the permeation of the first residue in a first membrane system making it possible to obtain a second permeate depleted in the other compound which is a smaller minority compared to the first residue and a second residue enriched in the other compound which is a smaller minority compared to the first residue.

[0010] According to another object of the invention, there is provided a method for separating a feed gas containing predominantly hydrogen and carbon dioxide as well as at least one smaller compound such as carbon monoxide, methane, nitrogen in which the feed gas is separated by distillation and / or by partial condensation in an apparatus comprising a heat exchanger, at least one phase separator and / or at least one distillation column to form a gas mixture and a CO2-rich fluid, the gas mixture is separated by a membrane separation method as described above and the second permeate and / or the expanded residue in the second turbine is sent to the exchanger and / or to the at least one phase separator and / or to the at least one column of the apparatus.

[0011] According to another object of the invention, there is provided a method for separating a gas containing predominantly hydrogen and carbon dioxide as well as at least one more minor compound such as carbon monoxide, methane, nitrogen in which the gas is separated in a PSA to form a gas enriched in hydrogen and depleted in carbon dioxide and a feed gas enriched in carbon dioxide and depleted in hydrogen, the feed gas being separated by the method as described above.

[0012] According to another object of the invention, there is provided an apparatus for membrane separation of a mixture containing mainly hydrogen and carbon dioxide as well as at least one more minor compound such as carbon monoxide, methane, nitrogen comprising: a heat exchanger, means for sending a mixture to be heated in a heat exchanger up to a first temperature, a first membrane system, a second membrane system, a first turbine, means for sending the heated mixture to be separated by permeation in the first membrane system making it possible to obtain a first permeate loaded with hydrogen and carbon dioxide relative to the mixture and a first residue depleted in hydrogen and carbon dioxide relative to the mixture,means for sending the first residue to separate by permeation in the second membrane system making it possible to obtain a second permeate loaded with hydrogen and carbon dioxide relative to the first residue and a second residue depleted in hydrogen and carbon dioxide relative to the first residue, means for sending the second residue to expand to a first pressure in the first turbine in order to obtain a second residue at a second pressure characterized in that it comprises: , i. Means for heating at least a portion of the second residue at the second pressure to a second temperature, the same as or different from the first temperature and ii. a second turbine, means for sending at least a portion of the second residue at the second temperature to expand in the second turbine from the second pressure to a third pressure lower than the second pressure.

[0013] The invention consists of an embodiment of: • Reheat in a heat exchanger, for example one in which the gas to be separated in the first membrane system is reheated, the gas leaving the first turbine before carrying out the second expansion in the second turbine. The first pressure at the inlet of the first turbine is preferably between 45 bara and 60 bar while the second pressure at the discharge of this first turbine is between 15 bara and 25 bara depending on the type of machine. The gas temperature at the inlet of the first turbine is preferably between 50°C and 90°C. The gas leaving the first turbine is preferably at a temperature between -30°C and 40°C. A fraction of this gas (between 15% and 100% depending on the desired performance) is sent to the heat exchanger and / or to the partial condensation and / or distillation unit to be reheated. This fraction preferably passes through the entire exchanger to the hot end to preferably reach a temperature between 50°C and 100°C. This allows, after remixing, to obtain temperatures between 30°C and 100°C at the inlet of the second turbine. At the discharge of the second turbine, the third pressure is preferably between 2 bara and 4.5 bara.

[0014] This arrangement allows more energy to be recovered at the expansion level by having the hottest possible gas at the inlet of the second turbine. No external heat input is necessary because the system has excess heat. The heat is in this case produced by the boosters. • Control the gas temperature at the outlet of the second turbine using a bypass pipe. The objective is to maintain a sufficient margin relative to the CO2 condensation line and more preferably relative to the CO2 triple point temperature. A margin preferably greater than 1°C is maintained. To control the temperature at the outlet of the second expansion, the temperature at the inlet of this second turbine is regulated. This must be between 30°C and 80°C. To regulate this temperature, a portion of the gas at the outlet of the first turbine is heated through the heat exchanger as described previously. The proportions between hot gas heated to the hot end of the heat exchanger and cold gas taken directly from the outlet of the first turbine determine the temperature at the inlet of the second turbine.To have the most direct regulation possible, the regulation of the reheated fraction in the exchanger is done directly on the temperature at the outlet of the second turbine.

[0015] Two configurations are therefore possible: • Or we seek to maximize the energy extracted by the turbines without seeking to exploit the cold created by the turbines. In this case, the fraction sent through the heat exchanger is equal to 100% and the temperature of the gas at the inlet of the second turbine is the hottest temperature obtained by exchange through the heat exchanger, i.e. the temperature of the hot gas leaving the second booster less the approach in the exchanger. This temperature is preferably between 50°C and 100°C. • Or we seek to maximize the energy extracted by the turbines while wanting to use the cold at the outlet of the second turbine in the cryogenic part of the process. In this case, by using the cold bypass line and temperature control, we obtain by mixing a gas at the inlet of the second turbine at an intermediate temperature (preferably between 30°C and 80°C) which makes it possible to obtain a cold temperature at the outlet of the second turbine (preferably lower than -45°C but higher than -55°C to keep a sufficient margin compared to the triple point of CO2 and thus avoid CO2 freezing) allowing its use in the cryogenic part of the process. In fact, the temperature could be lower because the CO2 is diluted. But if we use this cold against pure CO2 in the cold part, there would be a risk of freezing it.

[0016] Alternatively, it is possible to reheat the residue going to the second turbine without going through the heat exchanger where the mixture to be separated is reheated. Alternatively, the residue can be heated first in the heat exchanger where the mixture to be separated is reheated and then in another exchanger. For this, a dedicated heat exchanger can be used using either heat from the SMR or water vapor. This solution makes it possible to reach even higher temperatures at the inlet of the second turbine. It is also possible, if the heat source allows it, to also reheat the residue at the inlet of the first turbine to further increase the amount of energy recovered during expansion.

[0017] On the other hand, the first pressure between the two turbines can be chosen in such a way that the temperature at the outlet of the first turbine is cryogenic, that is to say preferably less than -20°C and more preferably less than -40°C. This residue, expanded a first time, is therefore sent to the heat exchanger and / or to the partial condensation and / or distillation unit so that the frigories are recovered. The residue comes out at the hot end of the main exchange line at ambient temperature. Expansion in the second turbine can then be done in the various ways described previously. In order to obtain cryogenic temperatures at the outlet of the two turbines, it is also possible to cool the residue a little before entering the first turbine.

[0018] The invention will be described in more detail with reference to the figure where:

[0019] [Fig.l] represents a method according to a variant of the invention.

[0020] [Fig.l] shows a membrane separation process. The gas to be separated is produced in separating by distillation and / or partial condensation a feed gas containing mainly hydrogen and carbon dioxide as well as at least one smaller compound such as carbon monoxide, methane, nitrogen. This Feed gas is a waste gas from a PSA producing hydrogen from synthesis gas produced by a reformer, for example by steam reforming (SMR).

[0021] The gas mixture 1 may be a gas from a phase separator of a partial condensation process and / or a top gas from a distillation column, the phase separator and / or the column being part of an apparatus operating below 0°C to separate the feed gas.

[0022] The gas mixture 1 mainly contains hydrogen and carbon dioxide as well as at least one smaller compound such as carbon monoxide, methane, nitrogen. A mixture mainly contains hydrogen and carbon dioxide with a composition such that at least 50 mol% of the mixture is composed of hydrogen and carbon dioxide.

[0023] This mixture 1 is heated in a heat exchanger E, passing from the hot end to the cold end, and is separated in a first membrane system M1 to form a first permeate PI and a first residue RL. The first permeate PI is sent at least a part to cool in the heat exchanger E and is then compressed in two boosters in series C2, Cl and the first booster permeate is sent to cool in the heat exchanger E before being sent as feed gas to the PSA to recover the hydrogen it contains.

[0024] A part B3 of the first permeate PI can bypass the exchanger E and be sent directly from the membrane system M1 without having been cooled, the valve V3 being open.

[0025] Similarly, a part B2 of the first overpressured permeate can bypass the exchanger E and be sent directly to the PSA without having been cooled, the valve V2 being open.

[0026] In both cases, this makes it possible to regulate the temperature of the fluid 3 and by extension that of the permeate PI and the residue RL.

[0027] The first residue RI coming from the first membrane system M1 is separated in the second membrane system M2 forming a second permeate P2 and a second residue R2. The second permeate P2 cools in the heat exchanger E and is separated in a separation apparatus by partial condensation and / or distillation which produces the mixture 1. The second residue R2 leaves the second membrane system M2 at between 45 and 60 bara and between 50 and 90°C. Then it is expanded in a first turbine T1 to form a flow at between 15 and 25 bara. The temperature of the first expanded residue does not exceed 40°C, preferably below 0°C, or otherwise below -20°C, or even below -40°C. The second expanded residue is sent at least partly to heat up in the exchanger E by being sent to the cold end to exit at the hot end at a temperature between 50 and 100°C.Between 15 and 100% of the second residue expanded in the first temperature heats up in the exchanger. E. A part can bypass the exchanger E and join the rest of the second expanded residue via a bypass line B1 without having been heated, the valve V1 being open. In this way, it is possible to adjust the temperature of the gas sent to the second turbine T2, so that it varies between 30 and 100°C. The expanded gas in the turbine T2 is at between 2 and 4.5 bara and at a temperature preferably lower than -45°C but higher than -55°C and is sent to bring cold to the separation by partial condensation and / or distillation. This gas R2 expanded in the two turbines T1, T2 can be used to regenerate the dryers upstream of the cold separation.

[0028] The permeation of the heated mixture 1 in the first membrane system M1 makes it possible to obtain a first permeate PI depleted in the other compound which is more of a minority compared to the mixture and a first residue RI enriched in the other compound which is more of a minority compared to the mixture.

[0029] The permeation of the first residue RI in the second membrane system M2 makes it possible to obtain a second permeate P2 depleted in the other compound which is a smaller minority compared to the first residue and a second residue R2 enriched in the other compound which is a smaller minority compared to the first residue.

Claims

1. Claims Process for membrane separation of a mixture (1) containing mainly hydrogen and carbon dioxide as well as at least one smaller compound such as carbon monoxide, methane, nitrogen comprising the following steps: i. Heating the mixture in a heat exchanger (E) to a first temperature ii. Permeation of the heated mixture in a first membrane system (Ml) making it possible to obtain a first permeate (PI) loaded with hydrogen and carbon dioxide relative to the mixture and a first residue (RI) depleted in hydrogen and carbon dioxide relative to the mixture iii. Permeation of the first residue in a second membrane system (M2) making it possible to obtain a second permeate (P2) loaded with hydrogen and carbon dioxide relative to the first residue and a second residue (R2) depleted in hydrogen and carbon dioxide relative to the first residue iv. Expansion of the second residue to a first pressure in a first turbine (Tl) in order to obtain a second residue at a second pressure characterized in that it comprises v. Heating (E) at least a portion of the second residue at the second pressure to a second temperature, identical to or different from the first temperature and vi. Expansion of a portion of the second residue heated to the second temperature in a second turbine (T2) from the second pressure to a third pressure lower than the second pressure vii. the first permeate (PI) is compressed in two boosters in series (Cl, C2), one booster (Cl) coupled to the first turbine (Tl) and the other booster (C2) coupled to the second turbine (T2) and the first permeate boosted in the two boosters cools in the heat exchanger and viii. the temperature of the second residue (R2) at the first pressure is regulated to maintain the temperature of the second residue at the second pressure above -80°C, preferably above -60°C or even above -55°C to avoid the risk of freezing and at least a portion (Bl) of the second residue (R2) is relaxed without having been warmed.

2. Method according to claim 1 in which the at least part of the second residue (R2) is heated according to step v) in the heat exchanger.

3. Method according to one of the preceding claims in which at least a part of the second residue (R2) is heated according to step v) in a heat exchanger other than the heat exchanger (E) where the mixture is heated.

4. Method according to one of the preceding claims in which the outlet of the first turbine (T1) is at a temperature below 40°C, preferably below -20°C, preferably below -20°C, or even below -40°C.

5. Method according to one of the preceding claims in which the mixture (1) and at least part of the second residue are heated in the heat exchanger (E) from the cold end thereof.

6. A method according to any preceding claim wherein at least a portion of the second residue (R2) heats up to between 30 and 80°C.

7. Method according to one of the preceding claims in which the first pressure is between 45 and 60 bara and / or the second pressure is between 15 and 25 bara and / or the third pressure is between 2 and 4.5 bara.

8. A method for separating a feed gas containing predominantly hydrogen and carbon dioxide as well as at least one smaller compound such as carbon monoxide, methane, nitrogen, in which the feed gas is separated by distillation and / or by partial condensation in an apparatus comprising a heat exchanger, at least one phase separator and / or at least one distillation column to form a gas mixture and a CO2-rich fluid, the gas mixture is separated by a membrane separation method according to one of the preceding claims and the second permeate (P2) and / or the expanded residue in the second turbine (T2) is sent to the exchanger and / or to the at least one phase separator and / or to the at least one column of the apparatus.

9. Process for separating a gas containing mainly hydrogen and carbon dioxide as well as at least one smaller compound such as carbon monoxide, methane, nitrogen wherein the gas is separated in a PSA to form a hydrogen-enriched, carbon dioxide-depleted gas and a carbon dioxide-enriched, hydrogen-depleted feed gas, the feed gas being separated by the method of claim 8.

10. Apparatus for membrane separation of a mixture (1) containing mainly hydrogen and carbon dioxide as well as at least one more minor compound such as carbon monoxide, methane, nitrogen comprising: a heat exchanger (E), means for sending a mixture to be heated in the heat exchanger up to a first temperature, a first membrane system (Ml), a second membrane system (M2), a first turbine (Tl), means for sending the heated mixture to be separated by permeation in the first membrane system making it possible to obtain a first permeate (PI) loaded with hydrogen and carbon dioxide relative to the mixture and a first residue (RI) depleted in hydrogen and carbon dioxide relative to the mixture,means for sending the first residue to separate by permeation in the second membrane system making it possible to obtain a second permeate (P2) loaded with hydrogen and carbon dioxide relative to the first residue and a second residue (R2) depleted in hydrogen and carbon dioxide relative to the first residue, means for sending the second residue to expand to a first pressure in the first turbine in order to obtain a second residue at a second pressure characterized in that it comprises:, i. Means (E) for heating at least a portion of the second residue (R2) at the second pressure to a second temperature, identical to or different from the first temperature and ii. A second turbine (T2), means for sending at least a portion of the second residue (R2) heated to the second temperature to expand in the second turbine from the second pressure to a third pressure lower than the second pressure and means for regulating the temperature of the second residue (R2) at the first pressure to maintain the temperature of the second residue at the second pressure above -80°C, preferably above -60°C or even higher than -55°C to avoid the risk of freezing and means (T2) for expanding at least part (Bl) of the second residue (R2) expanded without having been reheated in the heat exchanger.

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