Method for separating a component from a gaseous mixture

Abstract

A method for separating a first component (2) from a gas mixture (1) comprising said first component (2) and a second component (3), the method comprising: a step of introducing the gas mixture (1) through a membrane separation system (4) having a permeate side (5) and a retentate side (6), the membrane separation system (4) comprising at least one membrane (8); a step of introducing a scavenging gas (7) comprising a third component into the permeate side (5) of the membrane separation system (4); a step of adjusting the pressure of the scavenging gas (7) on the permeate side such that the partial pressure of the third component in the scavenging gas (7) is substantially equal to the partial pressure of the first component (2) in the gas mixture (1) introduced into the membrane separation system (4);a recovery step of the first component (2) of the gas mixture (1) from the retentate side (6) of the membrane separation system (4) Abbreviated figure: Fig. 1;

Description

Title of the invention: Method for separating a component from a gaseous mixture

[0001] The invention relates to a method for separating molecules of interest by membrane permeation.

[0002] The invention relates more particularly to a method for separating a first component from a gaseous mixture comprising the first component and a second component.

[0003] Separating and recovering components present in low concentrations in sources can pose considerable technical and economic challenges. Not only does it require processing a large volume of sources to isolate a quantity of components of interest, but it also complicates achieving high purity and preventing losses of the component of interest.

[0004] By way of example, the argon present in certain gas sources, particularly fossil gases such as natural gas, has an isotopic composition distinct from that of argon present in ambient air. These specific isotopes, once isolated, can be of high value, making their recovery with minimal losses particularly desirable. However, their concentration in the sources is often low, on the order of trace amounts, which necessitates the processing of very large volumes of gas.

[0005] Existing separation technologies have limitations for this type of application. For example, adsorption systems are difficult to adapt for handling large flows where the component of interest is present in a small amount and must be recovered with high efficiency. If the component of interest is adsorbed, its subsequent desorption for recovery can be complex, often involving dilution by a carrier gas. Furthermore, finding a selective adsorbent for the component of interest is particularly difficult for rare gases.

[0006] Distillation systems, another separation method, are generally limited to mixtures in liquid-vapor equilibrium without solidification of a portion of the flow. They encounter difficulties when the gas mixtures contain components with very different liquefaction or solidification points. For example, the presence of a high concentration of carbon dioxide (CO2)—a relevant scenario if CO2 is the second component of the gas mixture, as envisaged in certain embodiments of the invention—can lead to its solidification and clogging of the equipment during the cooling required to separate more volatile gases such as argon.

[0007] Membrane systems operate on the basis of a driving force of the partial pressure difference of the components across a membrane. In conventional configurations, even if a membrane is designed to retain a component of interest on the retentate side, some loss of this component to the permeate side is often unavoidable. When the component of interest is already present in low concentration in the feed stream, its partial pressure is initially low, and any loss to the permeate, however minimal, can significantly reduce the recovery yield and the purity of the desired product in the retentate.

[0008] An object of the invention is to provide a solution for treating large gas flows, including those containing components of interest in low concentration, while minimizing, or even eliminating, the losses of these components of interest to the permeate side.

[0009] To this end, the method according to the invention, which otherwise conforms to the generic definition given in the preamble above, is essentially characterized in that the method comprises: - a step of introducing the gas mixture through a membrane separation system having a permeate side and a retentate side, the membrane separation system comprising at least one membrane; - a step of introducing a sweeping gas comprising a third component into the permeate side of the membrane separation system; - a step of adjusting the pressure of the scavenging gas on the permeate side such that the partial pressure of the third component in the scavenging gas is substantially equal to the partial pressure of the first component in the gas mixture introduced into the membrane separation system; - a step of recovering the first component of the gas mixture from the retentate side of the membrane separation system.

[0010] Furthermore, embodiments of the invention may include one or more of the following features: - said at least one membrane is more permeable to the second component than to the first component. - the gaseous mixture comprises at least 90 mol%, in particular at least 95 mol% of the second component. - the gas mixture comprises from 1 ppm to 10% of the first component. - the sweep gas is introduced in the counter-current of the gas mixture. - the first component and the third component are of the same chemical element. - the first component and the third component are different isotopes or isomers. - Isotopes or isomers are argon, oxygen, nitrogen, hydrocarbons or water. - the first component comprises argon isotopes from a fossil gas source and the third component comprises argon isotopes from the air. - the sweeping gas is ambient air. - the second component is carbon dioxide. - the process includes a step of measuring the partial pressure of the first component at the inlet of the membrane separation system.

[0011] The present invention proposes introducing a scavenging gas on the permeate side of a membrane separation system. This scavenging gas comprises a component of the same chemical nature as the target component that is to be recovered from a gas mixture by the membrane separation system. It is possible to adjust the pressure of the scavenging gas so that the partial pressure of the component on the permeate side is made substantially equal to the partial pressure of the component in the gas mixture on the retentate side. By thus eliminating the driving force related to the partial pressure difference for the component in the gas mixture to be isolated, its permeation through the membrane is largely suppressed or at least reduced to stochastic exchange effects, allowing its high-efficiency recovery on the retentate side.

[0012] The proposed solution has the advantage of being simple and relatively inexpensive to implement. It can eliminate a very large proportion of unwanted molecules without losing the target molecules, or at least minimize their loss. It is even possible to increase the type of molecule targeted in the stream to be treated, which can facilitate its treatment later in the process.

[0013] The invention will be described with reference to the figure.

[0014] Fig. 1 illustrates a method according to the invention in a schematic way.

[0015] Figure 2 shows an example of the parameters of a process according to the invention.

[0016] As illustrated in [Fig.1], the separation process according to the invention is implemented to separate a first component 2 from a gaseous mixture 1. The gaseous mixture 1 comprises the first component 2 and at least one second component 3.

[0017] A gaseous mixture 1 is introduced, for example via a feed line, into a membrane separation system 4.

[0018] The gas mixture 1 may comprise, for example, at least 90 mol%, in particular at least 95 mol% of the second component 3 and between 1 ppm and 10% of the first component 2 to be recovered.

[0019] In a particular embodiment, the first component 2 is argon, in particular argon isotopes from a fossil gas source, and the second component is carbon dioxide.

[0020] The membrane separation system 4 has a permeate side 5 and a retentate side 6, and comprises at least one membrane 8. Advantageously, the membrane may be more permeable to the second component 3 than to the first component 2.

[0021] The membrane separation system 4 is configured to provide, from the gas mixture 1, a permeate enriched in second component 3 on the permeate side 5 with respect to the gas mixture 1 and a retentate enriched in first component 2 and depleted in second component 3 on the retentate side 6 with respect to the gas mixture 1. The permeate 3 can be discharged from the membrane separation system 4 via a permeate outlet duct (not shown) and the retentate 2 can be recovered via a retentate recovery duct (not shown).

[0022] A sweep gas 7 is introduced, for example via a sweep gas supply conduit, into the permeate side 5 of the membrane separation system 4. The flow of sweep gas 7 on the permeate side 5 flows counter-currently to the flow of gas mixture 1. The sweep gas 7 comprises a third component.

[0023] According to a particular embodiment, the first component 2 and the third component are of the same chemical element. The third component and the first component may be different isotopes or isomers. These may be isotopes of argon, oxygen, nitrogen, hydrocarbons, or water.

[0024] The gas mixture 1 fed through the membrane separation system 4 can originate from a fossil gas source, for example, natural gas. For example, the gas mixture 1 can be natural gas comprising argon and carbon dioxide, and the sweep gas can be ambient air.

[0025] In a particular embodiment, the first component is argon, in particular argon isotopes from a fossil gas source, and the second component is carbon dioxide.

[0026] The first component 2 may be argon isotopes from a fossil gas source and the third component may be argon isotopes from air, the argon from air having a different argon isotopic composition than the argon from a fossil gas source.

[0027] The gas mixture 1 may comprise, for example, at least 80 mol%, in particular at least 95 mol% of the second component 3. The gas mixture 1 may comprise from 1 ppm to 10% of the first component 2 to be recovered.

[0028] The partial pressure of the third component in the sweep gas 7 and the partial pressure of the first component 2 are brought into equilibrium or brought close together to be substantially equal.

[0029] In particular, the pressure of the sweep gas 7 on the permeate side 5 is adjusted to ensure that the partial pressure of the third component in the sweep gas 7 is substantially equal to the partial pressure of the first component 2 in the gas mixture 1 introduced into the membrane separation system 4. This adjustment can be achieved, for example, by means of a control valve (not shown) on the sweep gas supply line or on the permeate outlet line, and / or a partial pressure sensor of the first component and / or the second component and / or the third component (not shown).

[0030] Adjusting the pressure of the scavenging gas 7 on the permeate side 5 may involve pumping, for example via a pumping system such as a vacuum pump. Such pumping makes it possible to lower the partial pressure of the third component, particularly when the partial pressure of the first component 2 in the feed gas mixture 1 is close to or below ambient pressure. This can also lower the partial pressure of the second component 3 on the permeate side, thereby promoting permeation.

[0031] Adjusting the partial pressure of the third component in the sweep gas 7 to be substantially equal to that of the first component 2 in the feed stream (gas mixture 1) reduces or eliminates the permeation of the first component 2 through the membrane(s) of the membrane separation system 4 and minimizes its loss. The first component 2 is thus recovered on the retentate side 6.

[0032] The method may include a step of measuring the partial pressure of the first component 2 at the inlet of the membrane separation system 4, for example using a partial pressure sensor located on the gas mixture supply line.

[0033] In a specific example, consider a feed stream (gaseous mixture to be treated) at 50 bar, mainly composed of carbon dioxide and containing 400 ppm of argon with an isotopic composition different from that of the ambient air, the latter being used as a scavenging gas. Given that the concentration of argon in the ambient air is 0.93%, the scavenging air pressure required to balance the partial pressures of argon on the retentate and permeate sides of the membrane separation system can be calculated as follows: p(Air) = 400 ppm x 50 bar / 0.93%, which gives approximately 2.15 bar.

[0034] By way of example, the evolution of the required air pressure as a function of the partial pressure of argon at the inlet of the membrane separation system is illustrated in [Fig.2],

[0035] The present invention makes it possible to minimize the partial pressure difference on the two sides of the membrane by scanning the permeate side of the system membrane separation. This results in a significant reduction in the loss of the first component in the permeate stream, thus allowing the recovery of almost 100% of this first component from the feed gas mixture 1, and this, on the retentate side 6.

[0036] In certain operating configurations, an increase in the total pressure (of the scavenging gas) – an increase in the total pressure on the permeate side – can reverse the permeation flow. This leads to a transfer of the third component from the permeate side 5 to the retentate side 6, which has the effect of enriching the flow of the initial gas mixture with this same third component from the scavenging gas.

[0037] When the first component 2 and the third component are of the same chemical nature, particularly distinct isotopes or isomers (for example, different isotopes of argon), this effect can lead to an enrichment of the retentate flow not only in the first component, the loss of which is then virtually eliminated, but also potentially in the third component from the sweep gas. This enrichment, combined with the resulting increase in the total retentate flow rate, can facilitate subsequent processing downstream of the membrane separation system.

Claims

Demands

1. A method for separating a first component (2) from a gas mixture (1) comprising said first component (2) and a second component (3), the method comprising: - a) a step of introducing the gas mixture (1) through a membrane separation system (4) having a permeate side (5) and a retentate side (6), the membrane separation system (4) comprising at least one membrane (8); - b) a step of introducing a scavenging gas (7) comprising a third component into the permeate side (5) of the membrane separation system (4); - c) a step of adjusting the pressure of the scavenging gas (7) on the permeate side such that the partial pressure of the third component in the scavenging gas (7) is substantially equal to the partial pressure of the first component (2) in the gas mixture (1) introduced into the membrane separation system (4);- d) a step of recovering the first component (2) of the gas mixture (1) from the retentate side (6) of the membrane separation system (4).;

2. A method according to the preceding claim, characterized in that said at least one membrane (8) is more permeable to the second component (3) than to the first component (2).

3. A process according to any one of the preceding claims, characterized in that the gas mixture (1) comprises at least 90 mol%, in particular at least 95 mol% of the second component (3), and from 1 ppm to 10% of the first component (2).

4. A method according to any one of the preceding claims, characterized in that the sweep gas (7) is introduced counter-currently to the gas mixture (1).

5. A method according to any one of the preceding claims, characterized in that the first component (2) and the third component are of the same chemical element, in particular different isotopes or isomers.

6.

7.

8.

9.

10. A process according to the preceding claim, characterized in that the isotopes or isomers are argon, oxygen, nitrogen, hydrocarbons, or water. A process according to any one of the preceding claims, characterized in that the first component (2) comprises argon isotopes from a fossil gas source and the third component comprises argon isotopes from air. Method according to the preceding claim, characterized in that the sweeping gas (7) is ambient air. A process according to any one of the preceding claims, characterized in that the second component (3) is carbon dioxide. A method according to any one of the preceding claims, comprising a step of measuring the partial pressure of the first component (2) at the inlet of the membrane separation system (4).