Device and method for separating gases

Abstract

A device for separating gases comprises the following components: a source for the gases and flow adjustment means; a membrane unit for the production of a permeate gas and a retentate gas, one of which is the product gas; purity determining means for the product gas; a first control unit for the device; a retentate control system and a product gas pressure measurement, whereby the source has a second control unit for the flow adjustment means as a function of a target value of the gases and the first control unit is connected to the second control unit and to the retentate control system, whereby the first control unit can determine the target value and can control the retentate control system.

Claims

1. A device for separating a mixture of gases into a product gas with certain purity requirements and a residual gas, comprising: a source for supplying the mixture under pressure and adjustment means to adapt the flow to be supplied by the source; a membrane unit connected to the source that can separate the mixture into a permeate gas and a retentate gas, one of which is the product gas; means for determining the purity of the product gas; and a first control unit for controlling the device, wherein: the source is equipped with a second control unit that can control the adjustment means as a function of a target value of the mixture of the gases at a point between the source and the membrane unit; the device is equipped with a retentate control system and a measuring instrument for the pressure of the product gas; the first control unit has a control connection to the second control unit and to the retentate control system whereby the first control unit comprises an algorithm that can determine the target value on the basis of the pressure and purity of the product gas and can control the retentate control system, wherein the product gas is the retentate gas; and wherein the device is provided with a permeate control system to which the first control unit has a control connection.

2. The device according to claim 1, wherein the source is a compressor unit that comprises adjustment means, with an outlet that is connected to the membrane unit and whereby the target value is a target value at the outlet.

3. The device according to claim 1, wherein the first control unit can determine and pass on the target value to the second control unit the basis of the pressure of the product gas, and can control the retentate control system on the basis of the purity of the product gas.

4. The device according to claim 1, wherein the target value is a target pressure.

5. The device according to claim 1, wherein the target value is a target flow rate.

6. The device according to claim 2, wherein the device is equipped with an inlet gas control system between the compressor unit and the membrane unit, and in that the first control unit has a control connection to the inlet gas control system.

7. The device according to claim 1, wherein the device is a device for separating air.

8. The device according to claim 1, wherein the product gas is nitrogen.

9. The device according to claim 1, wherein the purity requirements constitute both a minimum concentration and a maximum concentration, whereby this minimum and maximum concentration can relate to the main component of the product gas or to a certain impurity or a group of impurities in the product gas.

10. The device according to claim 1, wherein the means for determining the purity of the product gas comprise a measuring instrument for the purity of the product gas.

11. The device according to claim 10, wherein the device is provided with a buffer for product gas, and in that the measuring instrument for the purity is positioned between the buffer and the membrane unit.

12. The device according to claim 1, wherein the retentate control system is a retentate control valve.

13. A device for separating a mixture of gases into a product gas with certain purity requirements and a residual gas, the device comprising: a source for supplying the mixture under pressure and adjustment means to adapt the flow to be supplied by the source; a membrane unit connected to the source that can separate the mixture into a permeate gas and a retentate gas, one of which is the product gas; means for determining the purity of the product gas; and a first control unit for controlling the device, wherein: the source is equipped with a second control unit that can control the adjustment means as a function of a target value of the mixture of the gases at a point between the source and the membrane unit; the device is equipped with a retentate control system and a measuring instrument for the pressure of the product gas; the first control unit has a control connection to the second control unit and to the retentate control system whereby the first control unit comprises an algorithm that can determine the target value on the basis of the pressure and purity of the product gas and can control the retentate control system; wherein the source is a compressor unit that comprises adjustment means, with an outlet that is connected to the membrane unit and whereby the target value is a target value at the outlet; wherein the device is equipped with an inlet gas control system between the compressor unit and the membrane unit, and in that the first control unit has a control connection to the inlet gas control system; and wherein the inlet gas control system is an inlet gas control valve.

14. The device according to claim 1, wherein the permeate control system is a permeate control valve.

15. A method for separating a mixture of gases into a product gas with certain purity requirements and a residual gas, wherein: the pressure of the mixture is increased; the mixture at an increased pressure is driven through a membrane unit and is separated into a permeate gas and retentate gas, one of which is the product gas; the pressure of the product gas is measured; the pressure and purity of the product gas are controlled against reference values by a first control unit that adjusts a flow of the retentate gas and by imposing a target value for the pressurised mixture before it flows into the membrane unit at a second control unit that controls adjustment means to obtain the target value; the purity of the product gas is determined; the first control unit controls a retentate control system, the first control unit having an first control connection to the second control unit and a second control connection to the retentate control system, wherein the product gas is the retentate gas, and wherein, if the purity of the product gas is too high while the product gas pressure cannot be reduced, this purity is controlled by adjusting the permeate pressure.

16. The method according to claim 15, wherein the pressure of the mixture in a compressor unit equipped with adjustment means is increased.

17. The method according to claim 15, wherein the pressure of the product gas is controlled towards a reference value by imposing the target value for the pressurised gas before it flows into the membrane unit at a second control unit and the purity of the product gas is controlled by adjusting the retentate flow rate.

18. The method according to claim 15, wherein the target value is a target pressure.

19. A method for separating a mixture of gases into a product gas with certain purity requirements and a residual gas, wherein: the pressure of the mixture is increased; the mixture at an increased pressure is driven through a membrane unit and is separated into a permeate gas and retentate gas, one of which is the product gas; the pressure of the product gas is measured; the pressure and purity of the product gas are controlled against reference values by a first control unit that adjusts a flow of the retentate gas and by imposing a target value for the pressurised mixture before it flows into the membrane unit at a second control unit that controls adjustment means to obtain the target value; the purity of the product gas is determined; the first control unit controls a retentate control system, the first control unit having an first control connection to the second control unit and a second control connection to the retentate control system, wherein the pressure of the mixture in a compressor unit equipped with adjustment means is increased, and wherein if the compressor unit operates at its minimum capacity, the pressure of the mixture that is brought to the membrane unit is made lower than the pressure at the outlet of the compressor unit by expanding this mixture.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) With the intention of better showing the characteristics of the invention, a device according to the state of the art and two variants of devices according to the invention are described hereinafter, with their accompanying methods, by way of an example, without any limiting nature, with reference to the accompanying drawings, wherein:

(2) FIG. 1 schematically shows a known device; and

(3) FIGS. 2 and 3 schematically show two variants of the device according to the invention.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS

(4) The device 1 shown in FIG. 1 is a device for producing nitrogen from air, whereby the nitrogen has a certain minimum purity.

(5) This device 1 comprises a compressor unit 2 with a compressor 3, whose outlet 4 is connected via a gas treatment unit 5 to a membrane unit 6 that is provided with a membrane 7 that has greater permeability for oxygen than for nitrogen. The membrane 7 separates the retentate side 8 from the permeate side 9 of the membrane unit 6.

(6) The compressor unit 2 can also comprise a gas dryer, not shown.

(7) The retentate side 8 is connected to a minimum pressure valve 10 and a three-way valve 11. Between the minimum pressure valve 10 and the membrane unit 6 is an oxygen sensor 12.

(8) The oxygen sensor 12 is connected in a signal-transferring manner to a control unit 13 that is provided with an algorithm to adjust the capacity at which the compressor operates by means of a slide valve at the inlet, depending on the oxygen concentration measured by the oxygen sensor 12.

(9) The control unit 13 is also connected controllingly to the three-way valve 11.

(10) The operation of this device 1 to produce nitrogen from air is as follows.

(11) The compressor 3 draws in air from the outside, compresses it and drives it at an increased pressure to the gas treatment unit 5 where impurities are removed and the air is heated.

(12) Then the compressed air flows to the membrane unit 6 where it is separated by means of the membrane 7 into a permeate that consists of a nitrogen/oxygen mixture with an increased oxygen content with respect to air, and a retentate that is nitrogen with a low oxygen content.

(13) The permeate is residual gas and can be vented to the outside air.

(14) The retentate, which is the product gas, is then supplied via the minimum pressure valve 10 and the three-way valve 11 to an installation, that does not belong to the device, that needs this product, for example a nitrogen network 14.

(15) The pressure in the membrane unit 6 is kept at a desired level by the minimum pressure valve 10, which is set to a certain pressure.

(16) The oxygen sensor 12 continually measures the oxygen content of the retentate and passes on this measured value to the control unit 13, which uses this value to control the compressor 3.

(17) If the oxygen content is higher than desired, the position of the slide valve is adjusted to this end so that the compressor operates at a higher capacity. If the oxygen content is less than desired, the opposite happens.

(18) If the measured oxygen content exceeds certain limits, the three-way valve 11 is controlled by the control unit 13, with the result that, to prevent the supply of a product gas that is outside the required specifications, the three-way valve 11 goes to such a position that the product gas is vented off.

(19) Such situations primarily occur when starting up/switching off the device, or when the flow demanded by the nitrogen network 14 is greater than the design limit of the device 1.

(20) The device 15 according to the invention shown in FIG. 2 has a number of differences to the device 1 shown in FIG. 1, which are set out hereinafter.

(21) A first difference is that there is no minimum pressure valve 10.

(22) A second difference is that the compressor unit 2 is provided with a controller 16 that can control a large number, in this case but not necessarily two, of compressors 3A and 3B, as a function of an imposed pressure at the outlet 4, whereby the control unit 13 is connected to the controller 16, instead of directly to an adjustment means in order to adjust the capacity of a compressor, such as a slide valve or a variable speed motor.

(23) In order to be able to do this there must of course be a pressure sensor in the compressor unit, which is not shown.

(24) A third difference is that between the compressor unit 2 and the gas treatment unit 5 there is an inlet gas control valve 17, to which the control unit 13 has a control connection.

(25) A fourth difference is that at the outlet of the membrane unit 6 on the permeate side 9 there is a permeate control valve 18 to which the control unit 13 has a control connection.

(26) A fifth difference is that at the outlet of the membrane unit 6 on the retentate side 8, there is a retentate control valve 19 between the membrane unit 6 and the oxygen sensor 12, to which the control unit 13 has a control connection.

(27) A sixth difference is that between the three-way valve 11 and the retentate control valve 19 there is a pressure sensor 20, that is connected in a signal transferring manner to the control unit 13.

(28) A seventh difference is that there is a connection point 21 between the compressor unit 2 and the inlet gas valve 17, to which another installation 22 that requires compressed air can be connected.

(29) The operation of the device 15 according to the invention, in this case described for the production of nitrogen from air, is as follows.

(30) The compressors 3A and 3B draw in air from the outside and compress it to a certain target pressure. This target pressure is a pressure known at the controller 16, which is imposed by the control unit, whereby the controller 16 controls the compressors 3A, 3B such that this target pressure is reached, thereby taking account of the optimisation of operational aspects of the compressors 3A, 3B, such as safety, lifetime, energy consumption, pressure control range and maintenance periods.

(31) The controller 16 hereby also takes account of the minimum pressure requirement of any other installation 22 connected to the connection point 21.

(32) The air pressurised to the target pressure is guided via the inlet gas control valve 17 to the gas treatment unit 5, where impurities are removed and the gas mixture is heated.

(33) Then the air flows to the membrane unit 6, where it is separated by means of the membrane 7 into a permeate that consists of a nitrogen/oxygen mixture with an increased oxygen content with respect to air, and a retentate that is nitrogen with a low oxygen content.

(34) The permeate is removed by the permeate control valve 18.

(35) The retentate, which is the product gas, is then supplied via the retentate control valve 19 and the three-way valve 11 to an installation, that does not belong to the device, that needs this product, for example a nitrogen network 14.

(36) The device 15 is controlled by the control unit 13. This controls the product gas pressure and the product purity, that are both imposed by the installation that needs the product gas, for example a nitrogen network 14.

(37) The control of the product gas pressure operates such that the product gas pressure, which is measured by the pressure sensor 20, is controlled at a certain reference value because the control unit adjusts the target pressure at the outlet 4 of the compressor unit 2 that the control unit 13 passes on to the controller 16.

(38) If this target pressure lies within the region in which the compressors 3A, 3B can operate within the pre-imposed conditions regarding their operational aspects, and within any limits imposed by another connected installation 22, the controller 16 controls the compressors 3A, 3B such that they operate optimally. If applicable this can also mean that only one of the two compressors 3A, 3B is operating, and the other is stopped.

(39) If the target pressure imposed by the control unit 13 is less than the pressure at which the compressor unit 2 must operate, the target pressure will be the lowest acceptable pressure and the pressure supplied to the rest of the device 15 is reduced by the inlet gas valve 17, so that a low operating pressure of the membrane unit 6 is nonetheless obtained.

(40) The inlet gas control valve 17 is optional, whereby without this inlet gas control valve 17 the product gas pressure and the product gas purity can also be guaranteed by the device 15, but with a smaller control range of the product gas flow.

(41) The product purity is controlled because the retentate control valve 19 is controlled as a function of the oxygen concentration measured by the oxygen sensor 12.

(42) If this oxygen concentration is too high and the purity of the product gas is thus too low, the retentate control valve 19 will be closed, such that the pressure in the membrane unit 6 increases and a greater fraction of permeate is obtained and thus the purity of the product gas is increased.

(43) For the rest, the product gas pressure will fall somewhat, which is compensated by increasing the target pressure of the compressor unit 2.

(44) If the purity of the product gas is too high, the control will be reversed.

(45) During this operation the permeate control valve 18 is fully open.

(46) If however the pressure in the membrane unit 6 is low, i.e. equal or practically equal to the desired product gas pressure, the retentate control valve 19 is fully open. This occurs for example if the device 15 has to operate at a low flow rate and/or a low purity.

(47) In such a situation the product gas purity cannot be reduced further if it is too high, by further opening the retentate control valve 19.

(48) In this case the permeate control valve 18 can be used to increase the pressure on the permeate side 9 and thereby produce a less pure product gas.

(49) The control described above can also be implemented differently. It is also possible that the product gas pressure is controlled at a set point by adjusting the position of the retentate control valve 19, and the purity of the product gas is controlled to a set point by adjusting the target pressure of the compressor unit 2.

(50) A combined control in which both the target pressure of the compressor unit 2 and the position of the retentate control valve 19 are adjusted on the basis of the measured product gas purity and product gas pressure is also possible.

(51) Although shown in the device 15 of FIG. 2, a permeate control valve 18 is optional, whereby the device 15 can still operate well in the vast majority of circumstances, but only lacks the possibility to limit the purity of the product gas when operating below its design capacity.

(52) The device 23 according to the invention shown in FIG. 3 is a similar device to device 15 of FIG. 2, with the difference that this device is adapted to a situation in which the permeate, thus not the retentate, is the desired product gas.

(53) The actual differences to the device 15 of FIG. 2 are:

(54) The oxygen sensor 12, pressure sensor 20, and three-way valve 11 are on the permeate side 9 instead of on the retentate side 8, and there is no permeate control valve 18.

(55) The operation of the device 23 is largely similar to device 15, except that due to the absence of the permeate control valve 18 the control relating to it can no longer take place, and that the response of the retentate control valve 19 to an anomalous purity value is the other way around, which means that as the retentate control valve 19 opens further, the purity of the product gas will increase, and vice versa.

(56) For both the device 15 and the device 23, just as for the traditional device 1, if the measured purity exceeds certain limits, the three-way valve 11 is controlled by the control unit 13, with the result that to prevent the supply of a product gas that is outside the required specifications, the three-way valve 11 is put to such a position that the product gas is vented out.

(57) To adjust the flow resistance that the inlet gas, the permeate and the retentate experience, valves are used in the above example.

(58) Alternative systems for this are also possible, such as a combination of parallel open/closed valves, each with a different flow resistance in an open state, and this combination can form a specific flow resistance by selectively opening and closing a number of valves.

(59) The present invention is by no means limited to the embodiments described as an example and shown in the drawings, but a device and method according to the invention, can be realised in all kinds of variants, without departing from the scope of the invention.