FR3067099A1 - GAS MIXTURE SEPARATION DEVICE - Google Patents

Abstract

The separating device (1) comprises a compression unit (6) and a unit (7) for separating the gaseous mixture (2, 8), intended to produce a permeate (3) and a retentate (4), and arranged in downstream of the compression unit (6). The compression unit (6) is shaped to perform a compression such that the temperature of the gas mixture remains substantially constant throughout the compression.

Description

The present invention relates to a device for separating a gas mixture and converting energy.

Cryogenic separation is typically preferred in the industry for producing large volumes of oxygen, nitrogen or the like, used in a variety of different energy conversion systems, for example for heating industrial furnaces or for power plants natural gas or coal electric.

However, cryogenic separation has very high operating energy costs, in particular involves a compression phase which is particularly costly in energy.

Already known in the prior art, in particular from US 2004/0016237, an integrated cryogenic air separation and energy conversion device, comprising an air separation unit capable of producing oxygen enriched air, and a compression unit to supply compressed air to the air separation unit. The device also includes a combustion unit supplied with oxygen-enriched air produced by the separation unit. The combustion unit is capable of delivering a drive gas to a turbine.

There is also known, in particular from US 6,382,958, a separation device with ceramic membrane, used in a heating process. In such a device, the air is heated to a very high temperature before separation.

US 8,752,391 also describes a separation device with a ceramic membrane operating at high temperature, in which the temperature of the gas mixture is raised to more than 540 ° C. before separation. This type of membrane is particularly expensive and is not suitable for use at low temperatures.

We also know, from WO 2008/013843, a device for implementing oxygen-enriched combustion, in which energy is produced by an independent Rankine cycle from heat recovery. However, this device does not make it possible to make a gas separation process more efficient.

Furthermore, according to US 7,637,109, the use of a refrigerant such as liquefied natural gas is known for cooling oxygen before compression. This cooling device is arranged upstream of a compression system, and therefore does not allow isothermal compression.

According to US 2012 0324944, a separation device is also known, operating at low pressures.

However, such devices are not entirely satisfactory.

In particular, known separation processes are usually costly in energy.

The object of the invention is in particular to provide an integrated device for separating a gas mixture and converting energy with better efficiency.

To this end, the subject of the invention is in particular a device for separating a gas mixture comprising:

- a compression unit;

- a unit for separating the gas mixture, intended to produce a permeate and a retentate, and arranged downstream of the compression unit;

characterized in that the compression unit is shaped to achieve compression such that the temperature of the gas mixture remains substantially constant during all of this compression.

The compression unit therefore achieves quasi-isothermal compression. Such quasi-isothermal compression requires less energy to bring the gas mixture to a predetermined pressure.

The invention is based on a gas separation process, using a thermodynamic cycle involving isothermal compression and planning to use the heat given off by combustion to heat one of the separated gases. An example of an application is oxy-combustion.

The device according to the invention may further comprise one or more of the following characteristics, taken in isolation or in any technically possible combination.

- During compression, the difference between the temperature of the gas mixture at all times and the initial temperature of the gas mixture is less than 50 ° C.

- The separation device comprises means for injecting a liquid into the compression unit, making it possible to add a liquid to the gas mixture in the compression unit to form a two-phase fluid, the liquid being capable of absorbing the heat from compression to keep the temperature substantially constant during this compression.

- The separation device comprises a heating unit, connected to the separation unit, and an expansion machine, arranged downstream of the heating unit.

The expansion machine is configured to achieve expansion such that the temperature of the gas mixture remains substantially constant throughout the expansion, in particular the difference between the temperature of the gas mixture at all times, and the temperature of the gas mixture prior to expansion, is less than 50 ° C.

- The expansion machine is mechanically connected to an electric generator.

- The heating unit includes a heat exchanger.

- The separation unit includes a membrane separation device.

- The gas mixture is formed of air, the permeate is air enriched in oxygen and the retentate is air depleted in oxygen.

- The gas mixture is formed by combustion gas, the permeate is air enriched in CO ₂ and the retentate is air depleted in CO ₂ .

The invention also relates to an industrial installation, characterized in that it comprises a separation device as defined above.

The installation according to the invention may further comprise one or more of the following characteristics, taken in isolation or in any technically possible combination.

- The industrial installation includes a combustion unit, connected to the separation unit so that the permeate or retentate feeds the combustion unit.

- The industrial installation includes an expansion machine arranged downstream of the combustion unit (10).

- The heating unit is connected to the combustion unit, so that the heating unit is supplied with an exhaust gas from combustion carried out in the combustion unit.

The invention also relates to a process for the separation of a gas mixture, characterized in that it is implemented by means of a separation device as defined above, and in that it comprises the following steps:

- A step of compressing the gas mixture, during which the temperature of the gas mixture remains substantially constant during compression, to obtain a compressed gas mixture;

- A step of at least partial separation of the compressed gas mixture to obtain a permeate and a retentate;

- a step of heating the permeate or the retentate;

- a step of relaxing the permeate or the heated retentate.

The invention will be better understood on reading the description which follows, given solely by way of example and made with reference to the appended figures, among which:

- Figure 1 is a schematic view of an industrial installation comprising a gas mixture separation device according to an exemplary embodiment of the invention; and

- Figure 2 is a diagram of a thermodynamic cycle representing the evolution of the gas mixture in the device of Figure 1.

It will be noted that in the present description, the terms “upstream” and “downstream” are defined relative to a direction of flow of a gas mixture in a circuit.

FIG. 1 shows an industrial installation 20.

For example, the industrial plant 20 forms a steelworks, a glassworks or a power plant.

The industrial installation 20 comprises a separation device 1 and a combustion unit 10.

The separation device 1 comprises a main circuit 5 comprising a compression unit 6 and a separation unit 7.

The compression unit 6 is supplied with a gas mixture 2 having a pressure PO and a temperature T0, and compresses this gas mixture 2 to obtain a compressed gas mixture 8 having a pressure P1 greater than PO and a temperature T1.

The pressure P1 is for example greater than 7 bar, preferably greater than 24 bar.

For example, the gas mixture 2 is formed of air. This air can optionally be dehydrated beforehand. As a variant, the gas mixture 2 is formed from combustion gases.

The compression unit 6 is capable of delivering almost isothermal compression. More particularly, the temperature of the gas mixture remains substantially constant throughout the compression.

Thus, by definition, the temperatures T0 and T1 are substantially equal.

Advantageously, the difference between the temperature TO of the gas mixture 2 and the temperature T1 of the compressed gas mixture 8 is less than 50 ° C.

Preferably, the difference between the temperature TO of the gas mixture 2 and the temperature T1 of the compressed gas mixture 8 is less than 15 ° C.

Optionally, the gas mixture 2 is previously cooled to room temperature before the compression unit.

To achieve such isothermal or quasi-isothermal compression, a liquid can be added to the gas mixture 2 in the compression unit 6 to form a two-phase fluid. Thus, the liquid, in direct contact with the gas, absorbs the heat from the compression of this gas, thereby obtaining an isothermal compression during which the temperature remains substantially constant throughout the compression. Said liquid may be capable of undergoing a phase change by evaporation during the absorption of the heat of compression. It should be noted that the temperature is considered to be substantially constant when the difference between this temperature and the initial temperature is in particular less than 50 ° C., preferably less than 15 ° C.

For example, said liquid used may be water or an oil.

Another example of a quasi-isothermal compression unit is described in the document "On The Strategies Towards Isothermal Gas Compression And Expansion", 2014, by Mahbod Heidari, Sylvain Lemofouet and Alfred Rufer.

Another example of a quasi-isothermal compression unit is described in US 4,027,993. This compression unit uses a mixture of gas and liquid, and has the advantage of keeping the gas temperature constant after compression.

US 4,446,698 or WO 2010/037980 describe isothermal compressors offering a larger heat transfer surface in a heat exchanger.

US 4,311,025, US 9,267,504 or US 7,401,475 describe devices into which liquid is injected during the compression and expansion phases to keep the system substantially isothermal.

The separation unit 7, arranged downstream of the compression unit 6, is intended to separate the compressed gas mixture 8 to obtain a permeate 3 and a retentate 4, having temperatures T1 a and T1 b respectively.

When the gas mixture 2, 8 is formed by combustion gas, the permeate 3 is air enriched in CO ₂ and the retentate 4 is air depleted in CO ₂ .

To achieve this separation, several alternative embodiments are envisaged, and if necessary combined, according to the present invention.

According to a first variant, the separation unit 7 comprises a membrane separation device 9 as visible in FIG. 1.

In a manner known per se, the membrane separation device 9 comprises a filter arranged transversely or radially to a flow of a compressed gas mixture. The filter then allows the selective passage of certain molecules and the stopping of others.

The membrane separation device 9 is able to operate at compressed gas mixture pressures 8 of between 7 bar and 80 bar.

Membrane separation processes usually have high efficiency as well as ease of design and maintenance.

According to a second variant, the separation unit 7 comprises an adsorption separation device.

In a manner known per se, the separation apparatus by adsorption, in particular by pressure inversion, removes a component from a compressed gas mixture by using its chemical affinity and its particular characteristics vis-à-vis an adsorbent exposed to a strictly controlled pressure oscillation.

According to a third variant, the separation unit 7 comprises a membrane separation apparatus and an adsorption separation apparatus in series between which a compressor is arranged. This variant makes it possible in particular to improve the efficiency of the separation.

The separation unit 7 does not exhibit significant dissipative phenomena, the temperature T1a of the permeate 3 and the temperature T1b of the retentate 4 are therefore substantially equal to the temperature T1 of the compressed gas mixture 8.

Conventionally, the permeate 3 or the retentate 4 feeds a combustion unit 10. In the embodiment described, the permeate 3 feeds the combustion unit 10.

Advantageously, the combustion carried out in the combustion unit 10 is part of the industrial process of the installation equipped with the device 1.

The separation device 1 comprises a secondary circuit 11 comprising a heating unit 12 and an expansion machine 13.

The heating unit 12 is connected to the separation unit 7, which supplies it with cold fluid 14. The cold fluid 14 is chosen from permeate 3 or retentate 4. More particularly, one from permeate 3 or the retentate 4 feeds the combustion unit 10, and the other feeds the heating unit 12. Thus, in the embodiment described, the retentate 4 forms the cold fluid 14.

The heating unit 12 is intended to increase the temperature of the cold fluid 14 to obtain a heated fluid 15 at a predetermined temperature T2.

Advantageously, the heating unit 12 includes a heat exchanger 16 intended to transfer thermal energy from a hot fluid 17 to the cold fluid 14 to obtain the heated fluid 15.

As can be seen in FIG. 1, the hot fluid 17 is formed by exhaust gases resulting from the combustion carried out in the combustion unit 10.

As shown in particular in FIG. 2, the heating unit 12 preferably maintains the pressure of the fluid constant.

The expansion machine 13 is intended to use the energy of the heated fluid 15 to produce mechanical energy.

The expansion machine 13 and the compression unit 6 are advantageously mounted on the same axis. Thus, the energy necessary for compression is supplied at least in part by the expansion machine 13. Preferably, the axis of the expansion machine 13 and the axis of the compression unit 6 are mechanically linked.

In the embodiment described, the expansion machine 13 is mechanically connected to an electrical energy generator 18, conventionally making it possible to convert mechanical energy from the expansion machine 13 into electrical energy.

The expansion machine 13 rejects a relaxed fluid 19.

The expansion machine 13 is preferably of the quasi-isothermal type, but the use of any other type of expansion machine, for example isentropic, is possible.

The expansion machine 13 can for example be an axial or radial turbine, a scroll or screw turbine or even a piston expansion machine,

The operation of the separation device 1 according to the invention will now be described with reference to FIG. 2.

FIG. 2 shows a diagram of a thermodynamic cycle of the operation of the separation device described above, according to an embodiment of the invention. The ordinate of this diagram shows the pressure P in the circuit and the abscissa shows the volume V in the circuit.

The state of the gas mixture circulating in the device can be represented by a polygonal curve. Each intermediate line on the curve, located between two points on this curve, represents an energy transformation in the separation device

1, that is to say that the different stages followed during the conversion of electrical energy are represented by the different points on the polygonal curve of the figure

2.

Thus, several parts of the polygonal curve can be described as follows.

A first part (shown in solid lines) between a point A and a point B of the diagram represents the almost isothermal compression of the gas mixture 2 to obtain the compressed gas mixture 8.

A second part between point B and point C represents the step of separation of the gas mixture. The gas mixture is separated into a retentate (shown in broken lines in Figure 2) and a permeate (shown in dotted lines in Figure 2). It should be noted that this separation step has little impact on the pressure of the retentate.

A third part between point C and point D represents the isobaric heating of the cold fluid 14 (retentate).

A fourth part between point D and point E represents the isothermal expansion of the heated fluid 15 (retentate).

In addition, a fifth part between point C and point F represents the evolution of the permeate following the separation.

Note that the invention is not limited to the embodiment described above, but could have various variants.

In particular, it is possible that the separation device 1, in particular the heating unit 16, is supplied with heat by any heat source.

Furthermore, the device 1 is able to operate with gas mixtures of various types.

It is clear that the invention has a number of advantages.

The separation device according to the invention makes it possible to reduce the energy losses of the industrial installation 20 while separating a gaseous mixture.

It should be noted that the equipment in thermal contact with the compressed gas mixture 8 is less stressed if the compressed gas mixture 8 has a relatively low temperature T1. The requirements imposed on this equipment are therefore simplified.

This is particularly advantageous for the membrane separation device 9 which usually does not operate at high temperatures.

In addition, by performing isothermal and non-adiabatic compression, energy expenditure is reduced.

Claims (15)

1. Device for separating (1) a gas mixture (2, 8) comprising: - a compression unit (6); - a unit (7) for separating the gas mixture (2, 8), intended to produce a permeate (3) and a retentate (4), and arranged downstream of the compression unit (6); characterized in that the compression unit (6) is shaped to achieve compression such that the temperature of the gas mixture remains substantially constant throughout this compression. 2. Separation device (1) according to claim 1, wherein, during compression, the difference between the temperature of the gas mixture at all times, and the initial temperature of the gas mixture, is less than 50 ° C. 3. Separation device (1) according to claim 1 or 2, comprising means for injecting a liquid into the compression unit (6), making it possible to add a liquid to the gas mixture in the compression unit to form a two-phase fluid, the liquid being able to absorb the heat from the compression to maintain the temperature substantially constant during this compression. 4. - Separation device (1) according to any one of claims 1 to 3, comprising a heating unit (12), connected to the separation unit (7), and an expansion machine (13), arranged downstream of the heating unit (12). 5. - separation device (1) according to claim 4, characterized in that the expansion machine (13) is configured to achieve an expansion such that the temperature of the gas mixture remains substantially constant during the entire expansion, in particular the difference between the temperature of the gas mixture at all times, and the temperature of the gas mixture prior to expansion, is less than 50 ° C. 6. -Separation device (1) according to claim 4 or 5, wherein the expansion machine (13) is mechanically connected to an electric generator (18). 7. - Separation device (1) according to any one of claims 4 to 6, wherein the heating unit (12) comprises a heat exchanger (16). 8. -Separation device (1) according to any one of the preceding claims, in which the separation unit (7) comprises a membrane separation device (9). 9. - separation device (1) according to any one of the preceding claims, in which the gas mixture (2, 8) is formed of air, the permeate (3) is air enriched in oxygen and the retentate (4) is oxygen-depleted air. 10. - Separation device (1) according to any one of claims 1 to 8, in which the gas mixture (2, 8) is formed by combustion gas, the permeate (3) is air enriched in CO ₂ and the retentate (4) is air depleted in CO ₂ . 11. - Industrial installation (20), characterized in that it comprises a separation device (1) according to any one of the preceding claims. 12. - Industrial installation (20) according to claim 11, comprising a combustion unit (10), connected to the separation unit (7) so that the permeate (3) or the retentate (4) feeds the unit combustion (10). 13. Industrial installation (20) according to claim 12, comprising an expansion machine arranged downstream of the combustion unit (10). 14. - Industrial installation (20) according to claim 12 or 13, wherein the heating unit (12) is connected to the combustion unit (10), so that the heating unit (12) is supplied by an exhaust gas from combustion carried out in the combustion unit (10). 15. - Process for the separation of a gas mixture (2, 8), characterized in that it is implemented by means of a separation device (1) according to any one of claims 1 to 10, and in that it comprises the following stages: - A step of compressing the gas mixture (2), during which the temperature of the gas mixture remains substantially constant during compression, to obtain a compressed gas mixture (8); - A step of at least partial separation of the compressed gas mixture (8) to obtain a permeate (3) and a retentate (4); - a step of heating the permeate (3) or the retentate (4); - A step of relaxing the permeate (3) or the heated retentate (4).