AU2003200254A1 - Method for operating a pressure swing adsorption process - Google Patents

Description

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION NAME OF APPLICANT(S): Linde Aktiengesellschaft ADDRESS FOR SERVICE: DAVIES COLLISON CAVE Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

INVENTION TITLE: Method for operating a pressure swing adsorption process The following statement is a full description of this invention, including the best method of performing it known to me/us:- -lA- Description The invention relates to a method for operating a pressure swing adsorption process, comprising at least two adsorbers which are connected in parallel, to which a gas mixture which is to be fractionated is fed via a charge gas line, from which a product stream is extracted via a product gas line and from which a residual gas stream is extracted via a residual gas line, preferably during a regeneration, purging and/or pressure-equalization step, the pressure swing adsorption process, in normal operation, being operated in accordance with a predetermined cycle schedule.

Pressure swing adsorption processes often represent only a preliminary process or subprocess within a larger overall process or an installation. In the context of methanol plants or steam reformers, the upstream adsorption process is used, for example, to generate a hydrogen product stream, the residual gas stream being fed, for example, to the underfiring of the steam reformer. Sudden failure of the residual gas stream extracted from the pressure swing adsorption process may in many types of installations lead to considerable problems which may even result in failure of the corresponding installation or the corresponding complex in the downstream consumers or installation parts to which the residual gas stream is fed.

Although there are generally mechanisms which enable the consumers or installation parts connected downstream of the pressure swing adsorption process to be "switched over to other gas sources", this switching needs to take place as smoothly as possible. Therefore, a sudden interruption in the residual gas stream needs 2 to be avoided.

In the event of a fault in the pressure swing adsorption process which means that the intended processing of the gas mixture to be fractionated, the intended generation of the product stream and/or the intended generation of the residual gas stream are no longer ensured for example as a result of the failure of one of the valves the feed of the gas stream to be fractionated i.e. of the charge gas stream and the discharging of the product gas stream are inevitably interrupted. Hitherto, in the event of faults of this nature in the pressure swing adsorption process, the valves of the residual gas line(s) have also been closed, so that it is neither possible for gas streams to be fed to the adsorbers nor for gas streams to be extracted therefrom.

It is an object of the present invention to provide a process of the generic type for operating a pressure swing adsorption process which avoids the abovementioned drawbacks.

To achieve this object, it is proposed that in the event of a fault in the pressure swing adsorption process which means that the intended processing of the gas mixture to be fractionated, the intended generation of the product stream and/or the intended generation of the residual gas stream are no longer ensured, the predetermined cycle schedule is reset in such a manner that a residual gas stream can be discharged continuously for a period of a few seconds to several minutes.

In this case, the composition of the residual gas stream which is discharged after the cycle schedule has been reset preferably substantially corresponds to that of the residual gas stream which is extracted during normal operation.

3 In this case, the residual gas stream can optionally be kept constant or reduced in a controlled manner with the aid of a downstream flow regulator over the course of the abovementioned period.

The method according to the invention makes it possible to ensure that a fault in the pressure swing adsorption process which means that the intended processing of the gas mixture to be fractionated, the intended generation of the product stream and/or the intended generation of the residual gas stream are no longer ensured, the discharge of the residual gas stream from the pressure swing adsorption process is not also interrupted.

Rather, the pressure swing adsorption process continues to be operated, by resetting the cycle schedule, in such a manner that a residual gas stream of substantially unchanged composition can continue to be extracted from the pressure swing adsorption process and fed to the downstream consumers or installation parts over a period of at least a few seconds to several minutes.

In principle, it is possible to further lengthen the time over which it is possible to discharge the residual gas stream in the event of a fault in the pressure swing adsorption process, but it is then no longer possible to ensure that the composition of the residual gas stream remains unchanged.

As a refinement of the method according to the invention, it is proposed for the change in the cycle schedule also to lead to a lengthening of the cycles.

This procedure leads to a control reduction in the quantity of residual gas stream, with the result that further lengthened discharging of the residual gas stream is made possible. However, a period of a few seconds to several minutes is generally sufficient to 4 enable downstream consumers to which the residual gas stream is supplied to be smoothly switched over to other sources.

If a fault occurs in the pressure swing adsorption process which means that the intended processing of the gas mixture to be fractionated, the intended generation of the product stream and/or the intended generation of the residual gas stream are no longer ensured, a certain quantity of gas is still stored in the adsorption vessels and the adsorption materials. This gas stored in the adsorption vessels and the adsorption materials is then used to form a residual gas stream over a period of a few seconds to several minutes. The extraction of the gas stored in the adsorption vessels and the adsorption materials does not damage the adsorption material(s) or lead to an out-of-spec product gas stream being delivered.

Even in the event of a fault in a residual gas valve of an adsorber, it is still possible to use the procedure according to the invention as a result of the new cycle schedule being modified in such a way that one or more other gas-containing adsorbers are used as residual gas suppliers. In this case, however, it is not always possible to ensure that the composition of the residual gas stream remains unchanged; in this respect, cf. also Example 2, Tables 3 and 4.

In the event of certain faults in the pressure swing adsorption process, it is possible to automatically switch over to an operating mode with a reduced number of adsorbers, for example from operating mode 6/1/3 to operating mode 5/1/2; changing the cycle schedule in this manner is also known as "switching back". In this context, the designations 6/1/3 and 5/1/2 stand for adsorption processes in which 6 or 5 adsorbers are arranged in parallel, in each case one adsorber is going through an adsorption cycle and 3 or 2 pressure- 5 equalization cycles are being performed between the adsorbers, respectively.

Depending on the nature of the previous fault, there are switching-back situations in which, although interruption-free, intended generation of the product stream is still ensured, a brief failure typically for 30 to 60 seconds of the residual gas stream cannot be prevented. If the continuity of the residual gas stream to be delivered has the highest priority, the procedure according to the invention can be used instead of switching over to the operating mode with a reduced number of adsorbers i.e. the switching back described above; cf. Example 3, Tables 5 and 6.

The process according to the invention will be explained in more detail below with reference to various pressure swing adsorption processes.

The abbreviations used in Tables 1 to 6 stand for the following process steps: A1/A2: Adsorption cycles El E3: Expansion cycles RO R3: Recompression cycles PPI: Provide purge gas PI: Purge D: Dump W: Waiting cycle Table 1 shows the cycle schedule of a 6/1/3 pressure swing adsorption process in normal operation, while Table 2 shows the altered cycle schedule of the pressure swing adsorption process after a fault (Example 1).

6 Table 1 Cycle 1 2 3 4 5 6 7 8 9 10 11 12 Cycle time T1 T2 TI T2 T1 T2 T1 T2 T1 T2 T1 T2 Ads. No.

Adsorber 1 El E2 E3 PP1 D P1 R3 R2 R1 RO Al A2 Adsorber 2 Al A2 El E2 E3 PP1 D P1 R3 R2 R1 RO Adsorber 3 R1 RO Al A2 El E2 E3 PP1 D P1 R3 R2 Adsorber 4 R3 R2 R1 RO Al A2 El E2 E3 PP1 D P1 Adsorber 5 D P1 R3 R2 R1 RO Al A2 El E2 E3 PP1 Adsorber 6 E3 PP1 D P1 R3 R2 R1 RO Al A2 El E2 Table 2 Cycle 1 2 3 4 5 6 7 8 9 10 11 12 Cycle time T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 Ads. No.

Adsorber 1 El E2 E3 PP1 D P1 R3 R2 R1 W W W Adsorber 2 W W El E2 E3 PP1 D P1 R3 R2 R1 W Adsorber 3 R1 W W W El E2 E3 PP1 D P1 R3 R2 Adsorber 4 R3 R2 R1 W W W El E2 E3 PP1I D P1 Adsorber 5 D P1 R3 R2 R1 W W W El E2 E3 PP1 Adsorber 6 E3 PP1 D P1 R3 R2 R1 W W W El E2 If a stop caused by a fault is triggered in the middle of a cycle, all the charge gas and product fittings and valves are closed immediately. The pressureequalization and purge sequences continue to operate unaffected. Then, the next cycle switches over to the altered cycle schedule corresponding to Table 2. If the stop takes place right at the end of the cycle, the next cycle likewise switches over to the altered cycle schedule shown in Table 2.

In the altered cycle schedule shown in Table 2, the RO, Al and A2 cycles become W cycles. In these "waiting cycles" all the valves of the corresponding adsorber i.e. including the residual gas valve(s) are closed.

7 In addition to the altered cycle schedule shown in Table 2, a large number of further cycle schedules are conceivable, making it possible to implement the method according to the invention.

Example 2: failure of the residual gas valve of an adsorber in a 4/1/1 process Table 3 shows a 4/1/1 pressure swing adsorption process in normal operation. If, for example, at the end of cycle 1 a fault in the residual gas valve of the adsorber 4 is detected, with the result that the residual gas valve cannot be opened, this has hitherto led to the pressure swing adsorption process being shutdown, since it is not usual to switch back to an operating mode with 3 adsorbers. According to the method according to the invention, the following cycle schedule can now be implemented.

Table 3 Cycle 1 2 3 4 5 6 7 8 Cycle time T1 T2 T1 T2 T1 T2 T1 T2 Ads. No.

Adsorber 1 El PP1 D P1 R1 RO Al A2 Adsorber 2 Al A2 El PP1 D P1 R1 RO Adsorber 3 R1 RO Al A2 El PP1 D P1 Adsorber 4 D P1 R1 RO Al A2 El PPl B T'able 4 Cycle 1 2 3 4 5 6 7 8 Cycle time T1 T2 T1 T2 T1 T2 T1 T2 Ads. No.

Adsorber 1 El DO D P1 R1 RO W W Adsorber 2 W W El PPI D P1 R1 W Adsorber 3 R1 W W W W PP1 D P1 Adsorber 4 W W R1 W W W El PP1 In this case, the sequence PP1 of the adsorber 1 is converted into a dump sequence DO, associated with a corresponding matching of the pressure stages of DO and D. The pressure swing adsorption installation can therefore still supply residual gas for at least a further 7 cycles while no further product is being delivered. In principle, of course, it is also possible for the adsorbers to be successively expanded into the residual gas system. However, in many cases this would lead to intolerable fluctuations in the composition of the residual gas delivered and to considerable pressure differences across the residual gas fittings.

Example 3: switching back with discontinuous delivery of residual gas If, for example, in the case of a 6/1/3 pressure swing adsorption process a fault in the adsorber 6 which requires the adsorber 6 to be shutdown immediately occurs towards the end of cycle 11 cf. Table 5 it is only possible to change over to cycle 10 of the 5/1/2 pressure swing adsorption process shown in Table 6, if the intended delivery of the product stream is to be maintained. If the process were switched to cycle 9, there would be a risk of the adsorber 1 being overloaded, which would lead to out-of-spec product being delivered. However, with the procedure described above it is necessary to accept a temporary interruption in the residual gas stream (adsorber 4: 9 D D) Table Cycle 1 2 3 4 5 6 7 8 9 10 11 12 Cycle time Tl T2 Tl T2 T1 T2 TI T2 T1 T2 Tl T2 Ads. No.

Adsorber 1 El E2 E3 PP1 D P1 R3 R2 R1 RO Al A2 Adsorber 2 Al A2 El E2 E3 PP1 D P1 R3 R2 R1 RO Adsorber 3 RI RO Al A2 El E2 E3 PP1 D P1 R3 R2 Adsorber 4 R3 R2 R1 RO Al A2 El E2 E3 PP1 D P1 Adsorber 5 D P1 R3 R2 Rl RO Al A2 El E2 E3 PP1 Adsorber 6 E3 PP1 D P1 R3 R2 R1 RO Al A2 El E2 Table 6 Cycle 1 2 3 4 5 6 7 8 9 Cycle time T1 T2 T1 T2 T1 T2 T1 T2 T1 T2 Tim 100 60 30 60 30 60 30 60 30 60 Ads. No.

Adsorber 1 El E2 PP1 D P1 R2 Rl RO Al A2 Adsorber 2 Al A2 El E2 PP1 D P1 R2 R1 RO Adsorber 3 RI RO Al A2 El E2 PPI D P1 R2 Adsorber 4 P1 R2 R1 RO Al A2 El E2 PP1 D Adsorber 5 PP1 D P1 R2 R1 RO Al A2 El E2 If interruption-free delivery of residual gas has the highest priority, however, it is possible to switch over to the cycle schedule according to the invention (cf. Table 2, cycle 12). In this case, adsorber 4 passes from the dump sequence D into the purge sequence P1 with continuous delivery of residual gas.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference to any prior art in this specification is not, and should not be taken as, an acknowledgement or any form of suggestion that that prior art forms part of the common general knowledge in Australia.

Claims (7)

1. Method for operating a pressure swing adsorption process, comprising at least two adsorbers which are connected in parallel, to which a gas mixture which is to be fractionated is fed via a charge gas line, from which a product stream is extracted via a product gas line and from which a residual gas stream is extracted via a residual gas line, preferably during a regeneration, purging and/or pressure-equalization step, the pressure swing adsorption process, in normal operation, being operated in accordance with a predetermined cycle schedule, characterized in that in the event of a fault in the pressure swing adsorption process which means that the intended processing of the gas mixture to be fractionated, the intended generation of the product stream and/or the intended generation of the residual gas stream are no longer ensured, the predetermined cycle schedule is reset in such a manner that a residual gas stream can be discharged continuously for a period of a few seconds to several minutes.

2. Method for operating a pressure swing adsorption process according to Claim 1, characterized in that the composition of the residual gas stream which is discharged after the cycle schedule has been reset substantially corresponds to that of the residual gas stream which is extracted during normal operation.

3. Method for operating a pressure swing adsorption process according to Claim 1 or 2, characterized in that the time for which it is possible to discharge the residual gas stream is at least 1 to 5 minutes.

4. Method for operating a pressure swing adsorption process according to one of the preceding Claims 1 to 3, characterized in that the residual gas stream is discharged while a constant quantitative flow of -12- residual gas is maintained.

Method for operating a pressure swing adsorption process according to one of the preceding Claims 1 to 4, characterized in that the change in the cycle schedule also leads to the cycles being lengthened. .V. -13-

6. A method for operating a pressure swing adsorption process, substantially as hereinbefore described with reference to the Examples.

7. The steps, features, compositions and compounds disclosed herein or referred to or indicated in the specification and/or claims of this application, individually or collectively, and any and all combinations of any two or more of said steps or features. DATED this TWENTY EIGHTH day of JANUARY 2003 Linde Aktiengesellschaft by DAVIES COLLISON CAVE Patent Attorneys for the applicant(s)