Control of gas composition of a gas separation system having membranes

Abstract

The present invention relates to a method of controlling a gas separation plant, to a plant thus controlled and also to its use for separation of gas mixtures, especially in the processing of biogas or natural gas, or syngas.

Claims

1. An apparatus for separating gases, comprising as membrane separation stages at least a feed stream separation stage, a retentate separation stage and a permeate separation stage and also at least one compressor arranged on the feed side of said feed stream separation stage and/or at least one vacuum pump arranged downstream of said feed stream separation stage, wherein said feed stream separation stage separates a feed stream, consisting of two or more components, into a first permeate stream and a first retentate stream, said retentate separation stage divides said first retentate stream into a second permeate stream and a second retentate stream, wherein the second permeate stream is supplied to a permeate control means located downstream of said retentate separation stage and wherein said second permeate stream is supplied to said feed stream downstream of said permeate control means, and wherein said second retentate stream is removed as product or further processed, said permeate separation stage divides said first permeate stream into a third retentate stream and a third permeate stream, wherein said third retentate stream is supplied to a retentate control means located downstream of said permeate separation stage and wherein said third retentate stream is supplied to said feed stream downstream of said retentate control means, and wherein said third permeate stream is removed as product or further processed or discarded, said permeate control means can raise or lower the permeate pressure of said retentate separation stage and is controlled on the basis of measured values from one or more measuring means in said first retentate stream and/or one or more measuring means in said second retentate stream, and/or said retentate control means can raise or lower the retentate pressure of said permeate separation stage and is controlled on the basis of measured values from one or more measuring means in said first permeate stream and/or one or more measuring means in said third permeate stream.

2. The apparatus according to claim 1, wherein said first permeate stream is not subjected to recompression, and/or gas separation membrane modules having a mixed gas selectivity CO.sub.2/CH.sub.4 of not less than 30 are used at least in said feed stream separation stage, and/or at least one of said membrane separation stages comprises more than one gas separation membrane module interconnected in parallel and/or series, and/or the gas separation membrane module(s) consist(s) of hollow fibre membranes and/or flat membranes, and/or said apparatus is configured such that the gas volume recycled in said second permeate stream and in said third retentate stream amounts in total to less than 60% by volume of the volume of a raw gas stream, and/or said apparatus is configured such that a concentration of at least one permeate gas of said feed stream separation stage, after returning said second permeate stream and said third retentate stream, is raised in said feed stream by not less than 2% as compared with the concentration in a raw gas stream.

3. The apparatus according to claim 2, wherein gas separation membrane modules having a mixed gas selectivity CO.sub.2/CH.sub.4 of not less than 30 are used in all three membrane separation stages.

4. The apparatus according to claim 2, wherein said apparatus is configured such that a concentration of at least one permeate gas of said feed stream separation stage, after returning said second permeate stream and said third retentate stream, is raised in said feed stream by 3 to 40% compared with the concentration in said raw gas stream.

5. The apparatus according to claim 1, wherein said second permeate stream and said third retentate stream are led to the suction side of said compressor, and/or in that the compressor is a multi-stage compressor and/or that the compressor is a multi-stage compressor and said second permeate stream and/or said third retentate stream is/are introduced into said compressor between two compression stages, and/or in that said compressor is arranged in said apparatus such that it generates a pressure gradient in said feed stream separation stage.

6. The apparatus according to claim 1, wherein said apparatus comprises a controller means which adapts a rotary speed of said compressor to changes in said second permeate stream and/or said third retentate stream and/or said raw gas stream, and/or in that said apparatus is configured such that the supplied amount of raw gas is regulated to adjust to changes in the amount of recycled gas from said second permeate stream and/or said third retentate stream.

7. The apparatus according to claim 1, wherein flow meters are used as measuring means in said first retentate stream and/or in said first permeate stream or in that an online or offline measuring means is used in said second retentate stream and/or in said third permeate stream to determine the composition of the particular gas mixture.

8. The apparatus according to claim 1, wherein the membranes in the membrane separation stages comprise a separation-active layer of amorphous or partly crystalline materials chosen from polyimides, polyamides, polysulphones, cellulose acetates and derivatives, polyphenylene oxides, polysiloxanes, polymers having intrinsic microporosity, mixed matrix membranes, facilitated transport membranes, polyethylene oxides, polypropylene oxides and mixtures thereof.

9. The apparatus according to claim 8, wherein the material used for the separation-active layer of the membranes is a polyimide of units of the general formulae A and B: ##STR00002## wherein x is in the range of from 0 to 0.5 and y is in the range of from 0.5 to 1, and wherein R.sup.1 and R.sup.2 are each independently chosen from one or more of L1, L2, L3 and L4 . ##STR00003##

10. The apparatus according to claim 1, wherein not less than 95% of the feed stream separation stage retentate component led into said apparatus is removed via said second retentate stream, and/or in that not more than 5% of feed stream separation stage retentate component led into said apparatus is removed via said third permeate stream.

11. A method of controlling a gas separation plant, wherein a gas mixture comprising a more readily permeating component A and a less readily permeating component B is separated in an apparatus according to claim 1, said method comprising i. selecting a setpoint range for a concentration of said component B in the second retentate stream, or for a parameter correlating with said concentration of component B in the second retentate stream, lowering the pressure of the second permeate stream by the permeate control means until said concentration or parameter correlating with concentration of component B in the second permeate stream is within the setpoint range for component B in the second permeate stream, if the concentration or parameter correlating with concentration for a component B is below the setpoint range in the second retentate stream, and raising the pressure of the second permeate stream by the permeate control means until said concentration or parameter correlating with concentration of component B in the second permeate stream is within the setpoint range for component B in the second permeate stream, if the concentration or parameter correlating with concentration for a component B is above the setpoint range in the second retentate stream; and/or ii. selecting a setpoint range for a concentration of said component B in the third permeate stream, or for a parameter correlating with said concentration of a component B in the third permeate stream, raising the pressure of the third retentate stream by the retentate control means until said concentration or parameter correlating with concentration of component B in the third retentate stream is within the setpoint range for component B in the third retentate stream, if the concentration or parameter correlating with concentration for a component B is below the setpoint range in the third permeate stream, and lowering the pressure of the third retentate stream by the retentate control means until said concentration or parameter correlating with concentration of component B in the third retentate stream is within the setpoint range for component B in the third retentate stream, if the concentration or parameter correlating with concentration for a component B is above the setpoint range in the third permeate stream.

12. The method according to claim 11, wherein the concentration of component B in the second permeate stream and/or the third retentate stream is determined online and/or offline.

13. A method of controlling a gas separation plant, wherein an apparatus according to claim 1 is used, said method comprising i. selecting a setpoint range for a parameter of the second retentate stream correlated by a calibration curve with a volume flow of the first retentate stream, lowering pressure of said second permeate stream by the permeate control means until said parameter of the second retentate stream is within the setpoint range when the volume flow of first retentate stream increases, and raising pressure of said second permeate stream by the permeate control means until said parameter of the second retentate stream is within the setpoint range when the volume flow of said first retentate stream decreases; and/or ii. selecting a setpoint range for a parameter of the third permeate stream correlated by a calibration curve with a volume flow of the first permeate stream, raising pressure of said third retentate stream by said retentate control means until said parameter of the third permeate stream is within the setpoint range when the volume flow of said first permeate stream increases, and lowering a pressure of said third retentate stream by said retentate control means until said parameter of the third permeate stream is within the setpoint range when the volume flow of said first permeate stream increases.

14. The method according to claim 13, wherein a calibration curve containing a correlation between a volume flow rate and pressure of a gas stream is used as a control curve to maintain a concentration of a component in a different gas stream.

15. The method according to claim 11, wherein a pressure drop across said feed stream separation stage is set at from 1 to 30 bar, and/or a pressure drop across said feed stream separation stage and said retentate separation stage is set at from 1 to 100 bar.

16. The method according to claim 15, wherein the pressure drop across said feed stream separation stage is set at from 3 to 10 bar, and/or the pressure drop across said feed stream separation stage and said retentate separation stage is set at from 10 to 70 bar.

17. The method according to claim 11, wherein a driving force used for gas separation is a partial pressure difference between a retentate side and a permeate side of at least one of the membrane separation stages, wherein said partial pressure difference is generated by said at least one compressor, which is arranged on the feed side of said feed stream separation stage, and optionally by said at least one vacuum pump in said second and/or third permeate stream and/or by a permeate-side purge gas stream, and/or in that a pressure of the permeate of said feed stream separation stage is in an equal or elevated state relative to an ambient pressure, so there is still a partial pressure difference between a retentate and a permeate of said permeate separation stage and hence there is a driving force in the event that said permeate of said permeate separation stage is at ambient pressure or negative pressure is applied.

18. The method according to claim 11, wherein a controller means adapts a rotary speed of said compressor to changes in the second permeate stream and/or said third retentate stream and/or a raw gas stream, and/or changing amounts of recycled gas from said second permeate stream and/or said third retentate stream are equalized, by a regulation of the supplied amount of raw gas, preferably via a raw gas control means, preferably without changing the rotary speed of said compressor or a performance of the gas separation plant is raised or lowered by changing a volume throughput of said compressor, wherein a resultant change in a volume flow of said first retentate stream is counteracted by selecting a setpoint range for a parameter of the second retentate stream correlated by a calibration curve with a volume flow of the first retentate stream, lowering pressure of said second permeate stream by the permeate control means until said parameter of the second retentate stream is within the setpoint range when the volume flow of first retentate stream increases, and raising pressure of said second permeate stream by the permeate control means until said parameter of the second retentate stream is within the setpoint range when the volume flow of said first retentate stream decreases and/or a resultant change in a volume flow of said first permeate stream is counteracted by selecting a setpoint range for a parameter of the third permeate stream correlated by a calibration curve with a volume flow of the first permeate stream, raising pressure of said third retentate stream by said retentate control means until said parameter of the third permeate stream is within the setpoint range when the volume flow of said first permeate stream increases, and lowering a pressure of said third retentate stream by said retentate control means until said parameter of the third permeate stream is within the setpoint range when the volume flow of said first permeate stream increases.

19. The method according to claim 11, wherein said method is practised in the context of operating a biogas plant, wherein the rotary speed of the compressor and hence the volume throughput of said compressor is controlled according to said biogas plant fill level and/or via fermenter pressure or intermediate store fill level, in order that the fill level in the fermenter and/or intermediate store may be changed or kept constant, or the gas mixture used is chosen from mixtures of predominantly but not exclusively carbon dioxide and methane, predominantly but not exclusively hydrogen and methane, predominantly but not exclusively carbon monoxide and hydrogen, raw biogas, and raw natural gas.

20. A biogas plant comprising an apparatus according to claim 1.

Description

EXAMPLE 1

(1) The purpose of this test was to find a calibration line with which product gas quality in retentate stream (8) and off-gas quality in permeate stream (11) can be maintained in the event of a change in the feed flow in feed stream (5), or the compressor speed, by changing the permeate pressure of retentate separation stage (2) and by changing the retentate pressure of permeate separation stage (3), respectively.

(2) To this end, compressor performance in a running 3-stage interconnected arrangement as per the general experimental set-up was raised in stages. The pressures of the permeate of retentate separation stage (2) and of the retentate of permeate separation stage (3) were then changed in an attempt to maintain the off-gas concentration (11) and the product gas concentration (8) within a narrow range. As the compressor performance increases from initially 60% to finally 75%, the feed volume flow (5) increases from 3.83 m.sup.3/h to 5.23 m.sup.3/h, i.e. by 36%. Within this interval, the permeate pressure of retentate separation stage (2) decreases from 951 mbara to 241 mbara and the retentate pressure of permeate separation stage (3) increases from 3.6 bara to 4.43 bara. At all compressor performances, product gas concentration (8) fluctuates between 95.23 and 95.75% methane and the off-gas concentration of methane between 0.5 and 0.62%. Both the values have been regulated within a narrow range, subject to experimental error. Detailed data regarding this test are presented below in Table 1:

(3) TABLE-US-00001 TABLE 1 Per- Reten- Reten- Per- Reten- Per- Per- Per- Calcu- Com- Feed meate tate tate meate tate meate meate meate lated pressor stream stream stream stream stream stream stream stream stream retentate perfor- flow (9a) (10a) (8) (11) (8) (9a) (6) (11) stream mance (5) pressure pressure c(CH.sub.4) c(CH4) flow flow flow flow (7) flow [%] [m.sup.3/h] [mbara] [bara] [%] [%] [m.sup.3/h] [m.sup.3/h] [m.sup.3/h] [m.sup.3/h] [m.sup.3/h] 60 3.83 951 3.6 95.75 0.5 1.665 0.622 1.641 1.28 2.287 62.5 4.1 760 3.8 95.68 0.62 1.807 0.756 1.669 1.372 2.563 65 4.3 660 3.9 95.54 0.58 1.907 0.838 1.715 1.427 2.745 67.5 4.53 560 4.03 95.23 0.62 2 0.94 1.76 1.5 2.94 70 4.77 460 4.16 95.52 0.55 2.086 1.044 1.828 1.57 3.13 72.5 5.01 320 4.29 95.43 0.55 2.175 1.16 1.894 1.646 3.335 75 5.23 241 4.43 95.34 0.62 2.267 1.28 1.925 1.697 3.547

(4) In addition, volume flow was measured for the second retentate stream (8), the first permeate stream (6), the third permeate stream (11) (=off-gas) and the second permeate stream (9a). The volume flows of the first retentate stream (7) can be determined from the sum total of the volume flow values of the second retentate stream (8) and of the second permeate stream (9a).

(5) The permeate pressure of retentate separation stage (2) can then be plotted against the volume flow of the first retentate stream (7) to determine a calibration curve for maintaining product gas concentration when the feed rate of retentate separation stage (2) changes, for example as a result of a change in the compressor speed or as a result of a change in the composition of the raw gas (see FIG. 4).

(6) FIG. 4 shows that a linear regression with good correlation is obtained. This relationship can then be used in a control system for the plant of the present invention. This control system uses a flow value determined for the first retentate stream (7) by means of a volume flow meter (20a) by calculating the permeate pressure as per the straight-line equation in FIG. 4 to determine the permeate pressure required in retentate separation stage (2) to maintain the product gas concentration. This pressure is then appropriately set using a control means (18) in the second permeate stream.

(7) The retentate pressure of permeate separation stage (3) can then be plotted against the volume flow of the first permeate stream (6) to analogously determine a calibration curve for maintaining off-gas concentration in permeate stream (11) when the feed rate of permeate separation stage (3), i.e. the first permeate stream (6), changes, for example as a result of a change in the compressor speed or as a result of a change in the composition of the raw gas (see FIG. 5).

(8) A linear regression with good correlation is also obtained in FIG. 5. This relationship can then be used similarly to the procedure described above for retentate separation stage (2) in a control system for the plant of the present invention. First the flow value of the first permeate stream (6) is determined by measurement with a volume flow meter (21a) and used in the straight-line equation in FIG. 5 to determine the retentate pressure needed in permeate separation stage (3)—and set using the control means (19) in the third retentate stream (10)—to maintain the off-gas concentration in permeate stream (11).

EXAMPLE 2

(9) The issue to be examined is whether, by changing the retentate pressure of permeate separation stage (3), using the control means (19) in the third retentate stream (10), it is possible to achieve a change in the methane concentration in the off-gas from the plant (third permeate stream (11)) and to obtain a calibration curve. In the event that a measurement of the off-gas concentration shows that a change has occurred, it would then be possible to use this calibrated relationship to adjust the methane content of the off-gas.

(10) To this end, while keeping the compressor speed constant, the retentate pressure of permeate separation stage (3) was changed with a control means (19) in the third retentate stream (10) and the resulting change in the methane concentration of the third permeate stream (11) (off-gas) was measured. The volume flows of the plant were also recorded. The values are shown in Table 2.

(11) TABLE-US-00002 TABLE 2 Com- pres- Reten- Per- Per- Reten- Per- Per- Per- Calcu- sor Feed tate meate meate tate meate meate meate lated per- stream stream stream stream stream stream stream stream retentate form- (5) (10a) (9a) (11) (8) (9a) (6) (11) stream Double ance flow pressure pressure c(CH4) flow flow flow flow (7) flow com- [%] [m.sup.3/h] (bara) (mbara) [%] [m.sup.3/h] [m.sup.3/h] [m.sup.3/h] [m.sup.3/h] [m.sup.3/h] pression 60 3.3 3.5 950 0.99 1.494 0.47 1.37 1.166 1.964 24.1% 60 3.3 3.4 950 0.94 1.482 0.456 1.399 1.147 1.938 25.5% 60 3.3 3.3 950 0.88 1.462 0.44 1.434 1.122 1.902 27.7% 60 3.3 3.2 950 0.82 1.44 0.427 1.464 1.09 1.867 30.4% 60 3.3 3.1 950 0.76 1.406 0.409 1.509 1.062 1.815 33.7% 60 3.3 3 950 0.69 1.375 0.394 1.555 1.027 1.769 37.4% 60 3.3 2.9 950 0.63 1.347 0.38 1.596 0.986 1.727 41.4% 60 3.3 2.8 950 0.56 1.283 0.36 1.663 0.955 1.643 47.5% 60 3.3 2.7 950 0.5 1.247 0.345 1.713 0.911 1.592 52.9% 60 3.3 2.6 950 0.44 1.177 0.33 1.789 0.868 1.507 61.4%

(12) As Table 2 shows, the methane concentration in off-gas stream (11) increases as a result of increasing the retentate pressure in permeate separation stage (3). This is shown in graph form in FIG. 6. The regression is linear with very good correlation. This curve can be used as a calibration curve for control purposes. By inserting the desired methane concentration in the equation of FIG. 6 the corresponding retentate pressure required can be determined.

(13) As a point of interest the fast rising double compression rate for a decreasing retentate pressure of permeate separation stage (3) and hence a decreasing methane concentration in the off-gas is depicted in graph form in FIG. 7.

EXAMPLE 3

(14) A change in the permeate pressure of retentate separation stage (2) with a control means (18) in the second permeate stream (9a) can be used to achieve a change in the methane concentration in the product gas of the plant (=second retentate stream (8)). In the event that a measurement of the product gas concentration reveals that a change has occurred, this calibrated relationship can be used to adjust the methane content of the product gas.

(15) To this end, the permeate pressure of retentate separation stage (2) was changed while keeping compressor speed constant, and the resultant change in the methane concentration in the product gas was measured. The values are shown Table 3.

(16) TABLE-US-00003 TABLE 3 Stage 2 c(CH.sub.4) in retentate stream permeate (8) pressure [bara] [%] 1.005 96.44 0.95 96.77 0.9 97.03 0.85 97.23 0.8 97.48 0.75 97.68 0.7 97.93 0.65 98.14 0.6 98.34 0.55 98.55 0.5 98.80 0.445 99.07 0.4 99.28 0.35 99.47 0.3 99.59 0.284 99.66

(17) As is apparent, the methane concentration of product gas (8) increases on reducing the permeate pressure in retentate separation stage (2). This is shown in graph form in FIG. 8. The regression is linear with very good correlation. This curve can be used as a calibration curve for control purposes. By inserting the desired methane concentration in the equation of diagram 5, the corresponding permeate pressure required can be determined.

DESCRIPTION OF FIGURES

(18) FIG. 1: Exemplary inventive interconnected arrangement comprising measuring means (20a) and (20b), (21a) and (21b), (22) and (23) and also the control means (18) and (19). The control means in raw gas stream (17) and the controller and data-processing means are not shown. However, their arrangement and use is clearly apparent from the overall context of the description. FIG. 1 shows an inventive arrangement with the recycling of streams (9b) and (10b) onto the suction side of the compressor. Alternative arrangements explained in the above description, for example the recycling of one or more of streams (9b) or (10b) into an elevated compression stage of compressor (4) or without measuring means (22) and (23) or with only some of measuring means (20a) and (20b) and/or (21a) and (21b), are easily derived by a person skilled in the art as a modification of FIG. 1 and therefore are not shown separately. FIG. 1 is merely provided by way of elucidation of the present invention and not in any way as limitation of its scope of protection.

(19) FIG. 2: The permeate pressure needed in retentate separation stage (2) to achieve a retentate quality of 98.3% of component B in the second retentate stream (8) and of 0.7% of component B in the third permeate stream (11) is shown as a function of the volume flow of the first retentate stream (7). The area ratio chosen for the membranes in the membrane separation stages was as follows: stage 1:stage 2:stage 3=2:2:3. Three curves are depicted for different concentrations of component B (CH.sub.4 in this case) of 45, 55 and 65% in raw gas stream (17).

(20) FIG. 3: The retentate pressure needed in permeate separation stage (3) to achieve a retentate quality of 98.3% of component B in the second retentate stream (8) and of 0.7% of component B in the third permeate stream (11) is shown as a function of the volume flow of the first permeate stream (6). The area ratio chosen for the membranes in the membrane separation stages was as follows: stage 1:stage 2:stage 3=2:2:3. Three curves, which merge into each other, are depicted for different concentrations of component B (CH.sub.4 in this case)—45%, 55% and 65%—in raw gas stream (17).

(21) FIG. 4: Dependence of permeate pressure of retentate separation stage (2) on feed gas rate of retentate separation stage (2) to maintain product gas quality

(22) FIG. 5: Dependence of retentate pressure of permeate separation stage (3) on feed gas rate of permeate separation stage (3) to maintain off-gas quality

(23) FIG. 6: Dependence of methane concentration in off-gas (11) on retentate pressure of permeate separation stage (3)

(24) FIG. 7: Dependence of recycling rate on methane content in permeate (11) of permeate separation stage (3)

(25) FIG. 8: Dependence of methane concentration in product gas (8) on permeate pressure of retentate separation stage (2)

LIST OF REFERENCE SIGNS

(26) 1: feed stream separation stage 2: retentate separation stage 3: permeate separation stage 4: single-stage or multi-stage compressor 5: feed stream 6: first permeate stream 7: first retentate stream 8: second retentate stream 9: second permeate stream consisting of sub-streams 9a, between control means 18 and retentate separation stage 2, and 9b downstream of control means 18 10: third retentate stream consisting of sub-streams 10a, between control means 19 and permeate separation stage 3, and 10b downstream of control means 19 11: third permeate stream 12: optional pressure-reducing valve in first retentate stream 7 (not shown in the Figures) 13: optional pressure-reducing valve in second retentate stream 8 (not shown in the Figures) 14: optional pressure-reducing valve in third retentate stream 10 (not shown in the Figures) 15: vacuum pump (not shown in the Figures) 16: mixing chamber (not shown in the Figures) 17: raw gas stream 18: permeate control means in 2.sup.nd permeate stream (also simply referred to as control means 18 in the description) 19: retentate control means in 3.sup.rd retentate stream (also simply referred to as control means 19 in the description) 20a: 1.sup.st retentate measuring means for analysis of 1.sup.st retentate stream (also simply referred to as measuring means 20a in the description) 20b: 2.sup.nd retentate measuring means for analysis of 2.sup.nd retentate stream (also simply referred to as measuring means 20b in the description) 21a: 1.sup.st permeate measuring means for analysis of 1.sup.st permeate stream (also simply referred to as measuring means 21a in the description) 21b: 2.sup.nd permeate measuring means for analysis of 3.sup.rd permeate stream (also simply referred to as measuring means 21b in the description) 22: 3.sup.rd permeate measuring means for analysis of 2.sup.nd permeate stream (also simply referred to as measuring means 22 in the description) 23: 3.sup.rd retentate measuring means for analysis of 3.sup.rd retentate stream (also simply referred to as measuring means 23 in the description) 24: controller means of the compressor (not shown in the Figures) 25: raw gas control means to control the raw gas stream (17) (not shown in the Figures)