Pressure swing adsorption process

0001The invention relates to a pressure swing adsorption process for cleaning or decomposing a gas stream using six cyclically operated adsorbers, in which the gas stream is passed through an adsorber during an adsorption phase at the highest process pressure and cleaned or disassembled gas is withdrawn from the outlet end of the adsorber, after the adsorption phase the adsorber is subjected to a multi-stage direct current relaxation and the resulting expansion gases are used partly to build up the pressure of other, previously purged adsorbers and partly to purge another adsorber at the lowest process pressure, which is followed by a countercurrent expansion to the lowest process pressure and a purging with DC expansion gas from another adsorber, followed by a multi-stage pressing on the adsorption pressure with DC expansion gas and purified or decomposed product gas.

0002A method of this type is already known from DE-OS 28 51 847. The process diagram shown there in FIG. 5 relates to a system with six adsorbers, the adsorption phases of three adsorbers overlapping in time. The direct current relaxation takes place in two stages, whereby in a first phase there is pressure equalization with another adsorber which is going through a pressure build-up phase, while in a second phase purge gas is produced for another adsorber which is then being purged. After the subsequent countercurrent expansion and purging, a two-stage pressure build-up takes place, first with expansion gas from an adsorber then in a DC expansion phase and finally with product gas.

0003Furthermore, from DE-OS 26 24 346 a pressure swing adsorption process with nine adsorbers is known, in which it is also essential that at least three adsorbers are operated simultaneously in adsorption. In this method, four direct current relaxation stages are provided for each adsorber within a cycle, three of which are carried out in pressure equalization with adsorbers to be pressed on. Finally, there are a number of pressure swing adsorption processes which are operated using two to a maximum of five adsorbers and which have in common that only one adsorber goes through an adsorption phase at a time. This is necessary in such processes, since otherwise the remaining elementary process steps could not be carried out. As an example of such processes, reference is made to US Pat. Nos. 3,430,418 and 3,564,816.

0004From the known prior art there is the teaching that if more than five adsorbers are used, several adsorbers are operated simultaneously in one adsorption phase. This is expressly stated in DE-PS 30 06 836. The parallel adsorption in several adsorbers was considered to be particularly desirable with regard to the constant amount of both the product gas and the residual gas flow, that is to say the countercurrent expansion gas and the purge gas loaded with desorbed components.

0005The invention was based on the object of further developing a method of the type mentioned at the outset such that not only does a constant and uniform amount of gas be obtained, but that, in addition, particularly energetically favorable operation is possible.

0006This object is achieved in that there is only one adsorber in each adsorption phase and that four or five DC relaxation phases are provided, of which one purge gas supplies another adsorber and the rest are carried out in pressure equalization with other adsorbers to be pressed on.

0007In deliberate departure from the usual procedure, the possibility according to the invention of operating several adsorbers simultaneously in one adsorption phase is not used and only one adsorber is operated in one adsorption phase, which enables the number of pressure equalization stages compared to the known six adsorber method to increase two or even three levels. The advantage that can be achieved in this way can be seen in particular in the fact that the product gas, ie the non-adsorbed gas, can be obtained in higher yield, since the loss of the product component in the residual gas is reduced as the number of pressure equalization stages increases. In addition, the pressure potential of the expansion gases is used to a greater extent by increasing the number of pressure compensation stages.

0008Surprisingly, it has been shown that when operating a six-adsorber system with only one adsorber in adsorption there are no difficulties with regard to the amount of product gas and residual gas, but that the constancy of these streams can be ensured by relatively simple control measures. In order to remove a constant amount of product even if the last phase of repressurizing an adsorber with product gas is shorter than the adsorption phase, i.e. if product gas only has to be branched off during this part of the adsorption phase, one can either provide a buffer container for the product gas or , which is usually cheaper because without additional investment is possible Conduct part of the product gas together with direct current expansion gas from another adsorber during a previous pressure build-up stage into another adsorber which is then in a pressure build-up phase by pressure equalization. The best way to do this is to use an adsorber that is going through the last press-on phase due to pressure equalization, since the additionally introduced product gas then experiences the least pressure loss and, moreover, the introduction of product gas via the outlet end of the adsorber to be pressed on ensures that the adsorber outlet end of adsorbable is kept completely clean Components supported, because such components which may still be present in the adsorber to be pressed on are then pushed back in a particularly pronounced manner to the inlet end of this adsorber. With such a procedure, it can also be advantageous to make the last pressure build-up phase under pressure equalization relatively short and the pressure build-up phase subsequently to be carried out relatively long, since on the one hand only a small partial flow of the product is diverted for this purpose when pressure is applied relatively slowly with product gas must and on the other hand also during the previous one, only a correspondingly small partial flow of the product flow has to be branched off by pressure equalization, which in turn is associated with only a small loss of pressure potential.

0009To set a constant residual gas flow when using the invention, a preliminary container in the residual gas line or a simple control can be provided. A possibility of regulation is given, for example, by the fact that the residual gas drawn off during a countercurrent expansion phase is set to a constant current by a control valve provided in the residual gas line, and that the residual gas generated during a flushing phase is regulated by regulating the purge gas to be purged to the adsorber to be purged via a in the purge gas line arranged control valve is set to such a value, that a constant gas flow over time emerges from the adsorber to be purged.

0010In a first embodiment of the invention, four DC relaxation phases are provided. In view of a high product yield and good use of the pressure potential, it is advisable to carry out the first three DC relaxation phases in pressure equalization with other adsorbers to be pressed on and to obtain the DC relaxation gas required for purging another adsorber from the fourth DC relaxation phase.

0011In the second embodiment of the invention with five DC relaxation phases, on the other hand, it has proven to be advantageous to use the expansion gas from the first three and the fifth DC relaxation phases to build up the pressure of other adsorbers, while purging with the fourth DC relaxation gas. The gas obtained in the fifth DC relaxation phase is preferably used for the first pressure build-up phase of an adsorber which has been purged immediately before.

0012The method according to the invention can be used in a large number of gas separation processes, for example for air separation, the extraction of noble gases, the purification of natural gas and in particular for the purification of synthesis gases in order to obtain a hydrogen stream. The adsorption can in each case be carried out by choosing any adsorbent suitable for the special separation process, for example activated carbon, silica gel, aluminum gel or molecular sieves. The gas flows to be processed in the method according to the invention can be in quantitative terms within a wide range. The method according to the invention is suitable, for example, for the production of pure hydrogen, in particular for gas flows between approximately 2000 and 50,000 Nm³/ h, whereby the upper limit is due to the flow if the usual size is adhered to, while if the lower limit is undershot it should be checked whether simpler processes with fewer adsorbers and lower product yield are more economical overall. For technical reasons, however, the lower limit can be reduced much further, whereby the evaluation of the product gas also plays a role. For example, when cleaning helium, it makes sense to use the process with gas quantities of around 50 Nm or more³/ h.

0013Further details of the invention are explained below with reference to the exemplary embodiments shown schematically in the figures.

0014Show it:<ul id="ul0001" list-style="none"><li>FIG. 1 shows an installation for carrying out the invention,</li><li>Figures two timing schemes for the implementation of the</li><li>2nd and 3 method in a plant according to FIG. 1.</li></ul>

0015In the system shown in Figure 1, the six adsorbers are identified by the numbers 1 to 6. Valves 11 and 16 are assigned to adsorber 1 on the raw gas inlet side and valves 12, 13, 14 and 15 on the product gas outlet side. Correspondingly, 2 to 6 valves 21-26 to 61-66 are assigned to the adsorbers. The system contains a raw gas supply line 70, which can be connected to the adsorbers 1 to 6 via the valves 11 to 61, and a product gas line 71, which can be connected to the outlet ends of the adsorbers 1 to 6 via the valves 12 to 62. In addition, a residual gas line 72 is provided which can be connected via the valves 16 to 66 to the inlet ends of the adsorbers 1 to 6, and finally lines 73, 74 and 75 are provided which are connected via the valves 13 to 63, 14 to 64 and 15 to 65 can be connected to the outlet ends of the adsorbers 1 to 6. Lines 73 and 74 are pressure equalization lines, while 75 is a purge gas line. If the process is carried out with four pressure equalization stages, an additional pressure equalization is carried out via line 75. The line 73 is connected to the product gas line 71 via valve 76.

0016The processes taking place in succession in the individual adsorbers during a complete switching cycle will be explained with reference to the adsorber 1, it being initially assumed that three pressure compensation stages are used in accordance with the cycle diagram shown in FIG. The details ADS, E1 to E5, S, B0 to B3 added in brackets refer to FIG. 2.

0017The raw gas to be broken down or cleaned passes through line 70 and the open valve 11 into the adsorber 1. The more easily adsorbable components are held in the adsorber 1, while non-adsorbed components emerge and are released into the product gas line 71 via the open valve 12 . The adsorption phase (ADS) is continued until a desired loading state of the adsorber 1 is reached, after which the valves 11 and 12 are closed and the valves 21 and 22 are opened so that the adsorption is continued in the adsorber 2 with continuous product delivery via line 71. In the adsorber 1, the pressure is now reduced to a first intermediate pressure (E1). This is done by pressure equalization with the adsorber 3 via line 73 and the open valves 13 and 33. The adsorber 3 runs through its third pressure equalization press-on phase B1 during this phase. After the pressure has been equalized, valve 13 is closed and valves 14 and 44 are opened, so that the adsorber 1 releases further direct current expansion gas into the adsorber 4, which is currently going through its second pressure build-up phase (B2), via line 74. After this pressure equalization has ended, valve 44 is closed and valve 54 is opened, so that further direct current expansion gas is drawn off from adsorber 1 via line 74 and is now introduced into adsorber 5. The adsorber 5 passes through its first pressure build-up stage B3. After this third pressure equalization has ended, valve 14 is closed and a last direct current expansion gas is discharged into line 75 via valve 15, which is now open. This gas is passed as purge gas through the open valve 65 through the adsorber 6 and, after loading with desorbed components, is passed through the valve 66 into the residual gas line 72. After the rinsing phase has ended, valve 15 is closed and adsorber 1 is expanded to the lowest process pressure by opening valve 16 in counterflow to the direction of adsorption (E5). Then the purge (S) of the adsorber 1 takes place with expansion gas from the adsorber 2, which is currently in its fourth DC relaxation phase (E4), for which purpose the valves 15 and 25 are opened. The purging gas loaded with desorbed components is released into the residual gas line 72 via the opened valve 16. After flushing, the adsorber 1 must be pressed back onto the adsorption pressure. This is done first by a first pressure build-up (B3) in pressure equalization with the adsorber 3 via the line 74, for which purpose the valves 14 and 34 are opened when the valve 16 is now closed. After this first pressure equalization, the adsorber 1 is exposed to a second pressure equalization via line 74 with the valve 14 still open and the valve 34 closed, but the valve 44 now open, this time with the adsorber 4, which is currently undergoing its second direct current relaxation phase (E2). After completion of this second press-on phase (B2), valve 14 is closed and a third pressure equalization with the adsorber 5 via line 73 and the opened valves 13 and 53 is initiated. During this press-on phase (B1) of the adsorber 1, the adsorber 5 runs through its first DC relaxation phase (E1). Finally, after the valve 53 has been closed, the adsorber 1 is brought back to adsorption pressure with product gas from line 71, which is led via the valves 76 and 13 to the outlet end of the adsorber 1, after which the cycle can begin again.

0018Each adsorber is operated in an adsorption phase (ADS) for 1/6 of the cycle time. While the adsorber 1 is going through its adsorption phase, the adsorber 2 is first in the pressure build-up phase B1 and then in the pressure build-up phase B0, the adsorber 3 first in the pressure build-up phase B3 and then in the pressure build-up phase B2, the adsorber 4 first in the countercurrent relaxation phase E5 and then in the rinsing phase S, the adsorber 5 in the DC relaxation phase E3 and then in the DC relaxation phase E4 and finally the adsorber 6 first in the DC relaxation phase E1 and then in the DC relaxation phase E2.

0019So that each adsorber delivers a constant amount of product via line 71 for the duration of its entire adsorption phase, product gas is not only passed through the open valve 76 into the respective adsorber to be pressed open during phase B0, but it also becomes so during the press-on phase B1 which takes place in pressure compensation Valve 76 opened. In this case, a quantity of product gas is also branched off via valve 76 to build up pressure by means of a conventional control measure, not shown in the figure, in such a way that the product gas stream drawn off via line 71 remains constant.

0020The timing diagram shown in FIG. 3 contains four pressure compensation stages and five DC relaxation phases. The choice of four pressure compensation stages is particularly useful when there is a high pressure ratio between the adsorption pressure and the residual gas pressure, for example a pressure ratio of 15 or more. The procedure is largely the same as that of the timing diagram shown in FIG. 2, so that instead of a detailed explanation, only the differences are to be shown. In contrast to the clock scheme described above, the fourth DC relaxation phase (E4) is divided into two individual steps E41 and E42. The corresponding adsorber is purged (S) only during the relaxation phase E41, while the adsorber which has been purged immediately before is subjected to a first pressure equalization (B4) during the relaxation phase E42. Based on the adsorber 1, the system shown in Figure 1, this means that during the DC relaxation phase E41 expansion gas is passed through the open valve 15 and line 75 into the adsorber 6, for which the valves 65 and 66 are open so that the residual gas over Line 72 can be withdrawn. After the expansion phase E41 has ended, the expansion gas in phase E42 continues to be conducted via line 75 and the opened valves 15 and 65 into the adsorber 6, but valve 66 is now closed.

0021The duration of a complete cycle can vary in both cycle schemes in the ranges customary for pressure swing adsorption processes; typical cycle times range from a few minutes to about 30 minutes, for example 24 minutes.

0022In the following, the advantages of the method according to the invention are compared with a method with seven adsorbers, two of which are simultaneously in adsorption and in which three pressure equalizations are provided, as well as with a method with five adsorbers, with only one adsorber simultaneously in adsorption is operated and as it is known from US-PS 3,564,816.

0023In all three cases it was assumed that hydrogen was obtained from a steam reformer gas containing 75% by volume, hydrogen, 5% by volume carbon monoxide, 5% by volume methane and 15% by volume carbon dioxide. The desired amount of hydrogen product is 10,000 Nm in all cases³/ h with a hydrogen purity of 99.999 vol%. The raw gas pressure is 20 bar, the residual gas pressure is 1.3 bar and the raw gas temperature is 303 K.

0024With the method according to the invention with three pressure equalizations, as shown in FIG. 2, a raw gas quantity of 15 504 Nm is achieved³/ h required, which corresponds to a hydrogen yield of 86%. A comparably high hydrogen yield could previously only be achieved using a process with at least seven adsorbers, which requires a significant additional outlay on containers and valves. The process with five adsorbers, on the other hand, only achieves a hydrogen yield of 81%. The investment costs for the pressure swing adsorption system are lower in this case, but the savings are more than compensated for by the higher investment costs for the gas generator, which has to be about 6% greater to achieve the same amount of hydrogen product than in the process according to the invention, and by the higher operating costs.

0025The main comparative data between the three methods are given in the table below.<tables id="tabl0001" num="0001"><img file="EP0146124A2_D0001.tif" /></tables>