POLYMER SEPARATION MEMBRANE FOR PURIFYING METHANE

Abstract

The use of polymer separation membranes to selectively separate CO.sub.2 and H.sub.2 from CH.sub.4 in a membrane separation step for purifying methane contained in an optionally pre-dried product gas mixture of a methanation method which contains CH.sub.4, H.sub.2 and CO.sub.2 is described. a) The separation is carried out at an operation temperature T.sub.B between 20 C. and 100 C.; and b) the polymer membranes b1) are able to simultaneously separate CO.sub.2 and H.sub.2 from CH.sub.4, b2) have a higher selectivity for the separation of CO.sub.2 than of H.sub.2 from CH.sub.4, i.e., a ratio 1/2<1, and b3) have a glass transition temperature T.sub.g that is lower than the operation temperature T.sub.B.

Claims

1. A polymer separation membrane for selectively separating CO.sub.2 and H.sub.2 from CH.sub.4 in a membrane separation step for purifying methane contained in an optionally pre-dried product gas mixture of a methanation method which comprises CH.sub.4, H.sub.2 and CO.sub.2, wherein a) the separation is carried out at an operation temperature T.sub.B between 20 C. and 100 C.; and b) the polymer membrane b1) is able to simultaneously separate CO.sub.2 and H.sub.2 from CH.sub.4, b2) has a higher selectivity for the separation of CO.sub.2 than of H.sub.2 from CH.sub.4, i.e., a ratio 1/2<1, and b3) has a glass transition temperature T.sub.g that is lower than the operation temperature T.sub.B.

2. A method for producing methane, comprising the following steps: a methanation step in which, by reducing CO.sub.2 with H.sub.2, a product gas is formed that comprises H.sub.2O, H.sub.2 and CO.sub.2 in addition to CH.sub.4; optionally a drying step in which H.sub.2O is removed from the product gas; and a membrane separation step for purifying the methane, wherein the gas mixture obtained by drying and containing CH.sub.4, H.sub.2 and CO.sub.2 is subjected to separation using separation membranes being able to selectively separate CO.sub.2 and H.sub.2 from CH.sub.4; wherein a) the separation in the membrane separation step is conducted at an operation temperature T.sub.B between 20 C. and 100 C.; and b) polymer membranes are used that b1) are able to simultaneously separate CO.sub.2 and H.sub.2 from CH.sub.4, b2) have a higher selectivity for the separation of CO.sub.2 than of H.sub.2 from CH.sub.4, i.e., a ratio 1/2<1, and b3) have a glass transition temperature T.sub.g that is lower than the operation temperature T.sub.B.

3. The method according to claim 2, wherein in the membrane separation step, the content of CO.sub.2 in the purified methane is lowered to below 2 vol %, below 1 vol % or below 0.5 vol %; and/or the content of H.sub.2 in the purified methane is lowered to below 10 vol %, below 8 vol %, below 4 vol %, or below 2 vol %.

4. The method according to claim 2, wherein the separation membranes used are those made of polyethers, poly(urethane-urea) elastomers, polyethers, polysiloxanes, and thermoplastic polyether-blockpolyamide (PEBA) copolymers.

5. The method according to claim 3, wherein the separation membranes used are PEBA copolymer membranes.

6. The method according to claim 2, wherein the separation is conducted at an operation temperature T.sub.B between 0 C. and 60 C.

7. The method according to claim 6, wherein the separation is conducted at an operation temperature T.sub.B between 5 C. and 30 C.

8. The method according to claim 7, wherein the separation is conducted at an operation temperature T.sub.B between 10 C. and 25 C.

9. The method according to claim 2, wherein the separation membranes are able to, in addition to CO.sub.2 and H.sub.2, simultaneously also separate residual amounts of H.sub.2O from CH.sub.4.

10. The polymer separation membrane according to claim 1, wherein in the membrane separation step, the content of CO.sub.2 in the purified methane is lowered to below 2 vol %, below 1 vol % or below 0.5 vol %; and/or the content of H.sub.2 in the purified methane is lowered to below 10 vol %, below 8 vol %, below 4 vol %, or below 2 vol %.

11. The polymer separation membrane according to claim 1, wherein the separation membrane comprises a polyether, a poly(urethane-urea) elastomer, a polyether, a polysiloxane, and/or a thermoplastic polyether-block-polyamide (PEBA) copolymer.

12. The polymer separation membrane according to claim 10, wherein the separation membrane comprises a PEBA copolymer membrane.

13. The polymer separation membrane according to claim 1, wherein the separation is conducted at an operation temperature T.sub.B between 0 C. and 60 C.

14. The polymer separation membrane according to claim 13, wherein the separation is conducted at an operation temperature T.sub.B between 5 C. and 30 C.

15. The polymer separation membrane according to claim 14, wherein the separation is conducted at an operation temperature T.sub.B between 10 C. and 25 C.

16. The polymer separation membrane according to claim 1, wherein the separation membrane is able to, in addition to CO.sub.2 and H.sub.2, simultaneously also separate residual amounts of H.sub.2O from CH.sub.4.

Description

SHORT DESCRIPTION OF THE DRAWINGS

[0045] In the following, the present invention will be described in more detail by means of nonlimiting examples and referring to a single drawing,

[0046] FIG. 1, schematically showing the procedure of a method or plant, respectively, for producing methane by methanation according to the state of the art using the inventive membranes during the membrane separation step.

EXAMPLES

[0047] As mentioned above, the method and corresponding plant schematically shown in FIG. 1 correspond to a relatively simple embodiment according to the state of the art. Here, the actual methanation reaction through hydrogenation of carbon dioxidepreferably originating from ambient airis conducted in reactor 01 according to the reaction equation

4H.sub.2+CO.sub.2.fwdarw.CH.sub.4+2H.sub.2O

resulting in a product gas mixture 101 rich in water and methane (and, as mentioned at the beginning, optionally further hydrocarbons, which will, however, not be discussed in further detail).

[0048] At position 02, before gas separation, this mixture is subjected to a pretreatment step normally comprising (pre-)drying as well as an optional temperature adjustment and/or removal of particles and other components (e.g., from the environmental air) potentially detrimental to the membranes such as ammonia or higher hydrocarbons, as well as the application of pressure required for membrane separation to the gas flow. The pre-treated product gas 102 passes through a control valve 11 into the gas membrane separator 03, which comprises at least one membrane separation step using polymer membranes to be used according to the invention and separates the gas mixture into at least one high-pressure retentate flow 107 and at least one low-pressure permeate flow 103.

[0049] Due to the higher selectivity of the membranes for the gas components CO.sub.2 and H.sub.2 compared to CH.sub.4, CO.sub.2 and H.sub.2 are simultaneously enriched in the permeate flow 103 and depleted in the retentate flow 107 according to the present invention.

[0050] According to the state of the art, separator 02 uses membranes having the highest possible selectivities for H.sub.2 and CO.sub.2 compared to CH.sub.4, i.e., membranes having the highest possible values for 1 and 2, in order to separate the largest possible amount of these two gases from the product gas flow in each separation step. These are all polymer membranes, in particular polyimide membranes, in their glassy states below their glass transition temperatures and they all show a higher selectivity for the separation of H.sub.2 than of CO.sub.2 from CH.sub.4, i.e., a ratio 1/2>1. However, this is particularly disadvantageous in view of feeding the purified methane into a natural gas grid because a larger number of membrane separation steps or cycles or larger membrane surfaces are required in order to lower the CO.sub.2 content of the methane to the admissible limit value. At the same time, according to the state of the art, the H.sub.2 concentration is decreased to values that are far below the admissible limit values, which unnecessarily increases the recyclate volume flow 103 requiring larger amounts of energy for recompression by a compressor 05.

[0051] For this reason, the present invention uses polymer membranes showing higher selectivities for the separation of CO.sub.2 than of H.sub.2 from CH.sub.4, i.e., a ratio 1/2<1, because the limit values for the CO.sub.2 concentration are, as mentioned at the beginning, often only half of those for H.sub.2. This considerably reduces the number of required membrane separation steps before feeding into the gas grid.

[0052] In preferred embodiments, the separator according to the invention still comprises a plurality of membrane separation stages of the polymer membranes to be used according to the invention so that in the retentate flow 107, i.e., in the purified methane, [0053] the content of CO.sub.2 is decreased below 2 vol %, more preferably below 1 vol %, most preferably below 0.5 vol %; and/or [0054] the content of H.sub.2 is decreased below 10 vol %, below 8 vol %, below 4 vol %, or below 2 vol %, particularly preferably below 10 vol % or below 8 vol %;

[0055] in particular both, because in this way the purified methane has a sufficiently low concentration of CO.sub.2 and H.sub.2 in the retentate 107 in order toafter the concentration is measured using an analyzer 13be able to be fed into a natural gas grid shown as bold line 21.

[0056] Subsequently, in a compressor 105, the pressure desired for methanation is applied to permeate 103 which is refed into the reactor 01 as compressed recyclate 105.

[0057] Due to the lower limit value of CO.sub.2, gas analyzer 13 is preferably mainly a CO.sub.2 analyzer. Based on the concentration measurement values from analyzer 13, the control valve 11, the control valve 12, the compressor 05, and the gas pretreatment 02 can be controlled, if required, to adjust the temperature and/or the pressure. In this way, the ratio of the volume flows of retentate 107 and permeate 103 can also be adjusted.

[0058] As mentioned above, the pretreatment step at position 02 may also comprise temperature adjustment in order to adjust the inventive operation temperature T.sub.B between 20 C. and 100 C. or to set an operation temperature preferred according to the invention between 0 C. and 60 C., more preferably between 5 C. and 30 C., most preferably between 10 C. and 25 C., limits included, if required. This guarantees that the operation temperature T.sub.B is higher than the glass transition temperature T.sub.g of the polymer membrane to be used according to the invention when a particular type of membrane is to be used.

[0059] Here, the respective selection of the polymer membranes mainly depends on their selectivity ratio 1/2 and the composition of the product gas mixture produced in the respective reactor 01, i.e., on the concentration of CO.sub.2 and H.sub.2 therein. For example, when excessive hydrogen is used for a catalytic methanation and the H.sub.2 concentration in the product gas flow 101 is (considerably) higher than that of CO.sub.2, the polymer membranes used in separator 03 are, for obtaining suitable H.sub.2 concentrations in the retentate 107, preferably those having a selectivity ratio 1/2 less far below or even just below 1, i.e., which are able to separate CO.sub.2 and H.sub.2 almost equally well from CH.sub.4. In this way, when a H.sub.2 concentration in retentate 107 of, for example, below 4 vol %, which is admissible for feeding into a natural gas grid according to OEVGW guideline G31, is obtained, very probably the CO.sub.2 concentration also lies below the admissible 2 vol %. However, in other cases, for example when excessive CO.sub.2 is available for methanation, e.g., when obtaining CO.sub.2 from environmental air, the invention preferably uses membranes having the smallest possible selectivity ratio 1/2 in order to separate considerably more CO.sub.2 than H.sub.2 from the product gas flow in every separation step.

Example 1, Comparative Example 1

[0060] For a theoretical calculation of the energy consumption of a continuous operation of a plant constructed as shown in FIG. 1, it was assumed that a methanation method was conducted through hydrogenation of CO.sub.2 according to the equation

4H.sub.2+CO.sub.2.fwdarw.CH.sub.4+2H.sub.2O

in reactor 01, followed by 100% drying of the product gas 101 in dryer 02 and subsequent purification of the product gas 103 in separator 03 by separating CO.sub.2 and H.sub.2 from CH.sub.4 by means of a respective polyimide membrane commonly used therefor in its glassy state and a polymer membrane according to the invention in its rubbery state, both at ambient temperature. In addition, it was assumed that a pressure of 60 bar is maintained in reactor 01 in order to shift the chemical equilibrium towards the product side, that drying is ideally conducted without pressure loss, and that permeate 103 enriched in CO.sub.2 and H.sub.2 is continuously recycled from separator 03 to reactor 01 after having been brought back to the reaction pressure of 60 bar in compressor 05. For the membranes, the following selectivities 1 and 2 were assumed for the H.sub.2/CH.sub.4 (1) and CO.sub.2/CH.sub.4 (2) separations.

Comparative Example 1

[0061] Polyimide membrane (state of the art): 1=70 2=30 1/2=2.33

Example 1

[0062] Polyether-block-polyamide (PEBA) membrane: 1=2 2=20 1/2=0.10

[0063] These lie within the common selectivities and selectivity ratios for the respective membrane types, as will be shown by the examples and comparative examples below.

[0064] Finally, a maximum admissible CO.sub.2 content in retentate 107 of only 0.5 vol % was assumed, which is well below the limit value according to the OEVGW guideline G31, however, is taken with regard to reductions of this limit value planned for the future, as mentioned above, in order to be allowed to keep feeding the purified methane into the natural gas grid after such a reduction. At the same time, however, the limit value for the H.sub.2 content assumed is above this guideline because it is planned to increase it to up to 10 vol %.

[0065] Here, the difference in energy consumption for operation of the method is essentially based on the compression power of compressor 05, which has to compress different permeate volume flows depending on the gas separation membranes used in the separator. The higher the pressure in the reactor, the higher are the compression efforts saved by the present invention.

[0066] The values calculated based on the above assumptions are shown in Table 1 overleaf.

TABLE-US-00001 TABLE 1 Comparative Description Unit Example 1 Example 1 Membrane selectivity 1, H.sub.2/CH.sub.4 70 2 Membrane selectivity 2, CO.sub.2/CH.sub.4 30 20 CO.sub.2 content in methanation product gas [vol %] 2.0 2.0 H.sub.2 content in methanation product gas [vol %] 8.0 8.0 CH.sub.4 content in methanation product gas [vol %] 90.0 90.0 Methanation product gas overpressure [bar] 60.0 60.0 Methanation product gas volume flow rate [Sm.sup.3/h] 6000.0 6000.0 CO.sub.2 content in permeate [vol %] 9.6 13.2 H.sub.2 content in permeate [vol %] 44.8 13.3 CH.sub.4 content in permeate [vol %] 45.6 73.5 Permeate overpressure [bar] 2.0 2.0 Permeate volume flow rate [Sm.sup.3/h] 993 385 CO.sub.2 content in retentate before feeding into grid [vol %] 0.5 0.5 H.sub.2 content in retentate [vol %] 0.7 7.3 Retentate volume flow rate [Sm.sup.3/h] 5007 5615 Required compressor power [kW] 378 265 Improvement of energy efficiency in gas treatment by [%] 30%

[0067] Since the PEBA membrane is only able to separate H.sub.2 and CO.sub.2 less selectively from CH.sub.4 and thus has considerably lower absolute values for 1 and 2 (1=2, 2=20) compared to the polyimide membrane (1=70, 2=30), the permeate contains larger amounts of CH.sub.4 (73.5 vol % compared to 45.6 vol %). This is also the main reason why such membranes have so far not been used for the inventive purpose according to the state of the art.

[0068] However, the inventive gas membrane separation results in a permeate volume flow of only 385 Sm.sup.3/h compared to 993 Sm.sup.3/h according to the state of the art, which is why 30% less compressor power is required in order to repressurize the permeate with a pressure of 60 bar. For even higher pressures, energy savings would be correspondingly higher.

Examples 2 to 7, Comparative Examples 2 to 4

[0069] Table 2 overleaf shows several membrane types together with their respective selectivities 1 and 2 and selectivity ratios 1/2, namely polymer membranes known according to the state of the art to be used for gas membrane separation of a methanation product gas in their glassy state below their glass transition temperatures T.sub.g as Comparative Examples 2 to 4 (C2 to C4) as well as polymer membranes to be used according to the invention in their rubbery state above their glass transition temperatures having inverted selectivity ratios as Examples 2 to 7 of the invention (E2 to E7).

[0070] Here, the values for 1 and 2 were either taken from relevant literature or determined by the inventor in own experiments. Here, pure gas permeation experiments with the respective gas, i.e. CH.sub.4, CO.sub.2 or H.sub.2, were conducted at room temperature with different feed gas pressures, the linear proportionality factor was calculated from the measurement results as the quotient of the arithmetic mean of the measured flow rates at different pressures and the respective pressure (m.sup.2/bar), and the quotient of the proportionality factors for H.sub.2 and CH.sub.4 was taken as 1 and that of the factors for CO.sub.2 and CH.sub.4 was taken as 2 for the respective membrane.

TABLE-US-00002 TABLE 2 Temperature Example Membrane material [ C.] 1 2 1/2 Source C2 Polyimide BPDA - arom. diamine 40 130 40 3.25 Tanihara et al. .sup.c C3 Polyimide BPDA - arom. diamine 25 190 70 2.71 Experiment C4 Polyimide 6FDA-DBBT 35 80 45 1.777 Yang et al. .sup.d E2 Terathane 2900 (PolyTHF) .sup.a 35 1.5 7 0.21 Li et al. .sup.e E3 Polydimethylsiloxane (PDMS) 23 1.5 4 0.375 Experiment E4 Pebax MH 1657 .sup.b 30 2 16 0.125 Car et al. .sup.f E5 Pebax MH 1657 .sup.b 10 2.5 26 0.096 Car et al. .sup.f E6 Pebax MV 1074 .sup.b 27 2 16 0.125 Car et al. .sup.f E7 PVC/Pebax MH 1657 20 2.5 35 0.07 Ahmadpour et al. .sup.g .sup.a Commercially available membrane made of poly(tetramethyleneglycol) ether (polytetrahydrofuran, PolyTHF) .sup.b Commercially available membranes made of polyether-block-polyamide copolymers (PEBA) .sup.c Tanihara et al., J. Membr. Sci. 160, 179-186 (1999). .sup.d Yang et al., Polymer 42, 2021-2029 (2001). .sup.e Li et al., J. Membr. Sci. 369, 49-58 (2011). .sup.f Car et al., J. Membr. Sci. 307, 88-95 (2008). .sup.g Ahmadpour et al., J. Nat. Gas Sci. Eng. 21, 518-523 (2014).

[0071] The results from Table 2 show that the selectivity ratios 1/2 of the inventive polymer membranes arecontrary to the membranes according to the state of the art in their glassy statesnot only below 1 but are typically also an order of magnitude below those of commonly used membranes.

[0072] In addition, a comparison of Examples 4 and 5 shows that the selectivity for H.sub.2 and CO.sub.2 with regard to CH.sub.4, i.e., 1 and 2, for membranes used according to the present invention in their rubbery states increase with decreasing temperatures, with 2 increasing more than 1, so that the selectivity ratio 1/2 decreases further when lowering the operation temperature. Consequently, according to the present invention, a targeted increase of the temperature during gas separation will be unnecessary in most cases.

Examples 8 and 9, Comparative Examples 5 to 7

[0073] A calculation of further examples of the present invention and of comparative examples was based on the operation of a plant analogous to Example 1 and Comparative Example 1, using the selectivities of the commercially available membranes of Comparative Examples 2 to 4 and Examples 5 and 6 listed in Table 2 above.

[0074] The results are shown Table 3 overleaf.

TABLE-US-00003 TABLE 3 Description Unit Comp. 5 Comp. 6 Comp. 7 Ex. 8 Ex. 9 Membrane selectivity 1, H.sub.2/CH.sub.4 130 190 80 2.5 2 Membrane selectivity 2, CO.sub.2/CH.sub.4 40 70 45 26 16 CO.sub.2 content in methanation product gas [vol %] 2.0 2.0 2.0 2.0 2.0 H.sub.2 content in methanation product gas [vol %] 8.0 8.0 8.0 8.0 8.0 CH.sub.4 content in methanation product gas [vol %] 90.0 90.0 90.0 90.0 90.0 Methanation product gas overpressure [bar] 60.0 60.0 60.0 60.0 60.0 Methanation product gas volume flow rate [Sm.sup.3/h] 6000.0 6000.0 6000.0 6000.0 6000.0 CO.sub.2 content in permeate [vol %] 10.0 11.6 10.9 13.3 10.3 H.sub.2 content in permeate [vol %] 48.9 55.3 49.7 14.2 12.0 CH.sub.4 content in permeate [vol %] 41.1 33.1 39.4 72.5 77.7 Permeate overpressure [bar] 2.0 2.0 2.0 2.0 2.0 Permeate volume flow rate [Sm.sup.3/h] 946 812 864 702 915 CO.sub.2 content in retentate before feeding [vol %] 0.5 0.5 0.5 0.5 0.5 into grid H.sub.2 content in retentate [vol %] 0.34 0.6 1.0 7.2 7.3 Retentate volume flow rate [Sm.sup.3/h] 5054 5188 5136 5298 5085 Required compressor power [kW] 360 309 329 267 348

[0075] The values for the required compressor power of compressor 05 show that the membrane used according to the present invention in Example 8, whichlike the one in Example 1had a selectivity ratio 1/2 of approximately 1:10, again yielded better results than all commercially available membranes having inverted selectivity ratios regularly used for product gas purification according to the state of the art.

[0076] The required compressor power calculated for inventive Example 9 is just above the average of the three comparative examples, however, for identical CO.sub.2 contents, the two inventive examples are able to achieve an H.sub.2 content in the purified methane that is even up to approximately 20 times higher than according to the state of the art, after that of Example 1 was already 10 times higher than that of Comparative Example 1.

[0077] In addition, the very high selectivity ratio 1/2 of approximately 2.7 in Comparative Example 5 was based on laboratory measurement values of the inventor (see Table 2, Comparative Example 3, Experiment), which will certainly not be achievable in practice during operation of a gas purification plant, which is why also in this case significantly larger amounts of permeate would have to be recycled and recompressed, which would further increase the required compressor power. Thus, for Comparative Example 6 a realistically required compressor power would lie between those of Comparative Examples 5 and 7and thus in the range of Example 9.

[0078] Examples 10 to 17, Comparative Examples 8 to 15 In the calculation examples overleaf, pair comparisons were made between the membrane of Example 8 according to the invention and the prior art membrane of Comparative Example 7 by varying various process parameters, again assuming a maximum CO.sub.2 content of 0.5 vol % and a maximum H.sub.2 content of 10 vol % in the purified methane.

TABLE-US-00004 TABLE 4 Membrane Methanation product gas Permeate Retentate Compressor selectivity CO.sub.2 H.sub.2 CO.sub.2 H.sub.2 CO.sub.2 H.sub.2 req. H.sub.2/CH.sub.4 CO.sub.2/CH.sub.4 Vol. flow Pressure content content Pressure content content content content power Saved Example (1) (2) [Sm.sup.3/h] [bar] [vol %] [vol %] [bar] [vol %] [vol %] [vol %] [vol %] [kW] energy B10 2.5 26 6000.0 60.0 3.0 12.0 0.5 16.5 19.9 0.23 10.0 535 11% V8 80 45 13.5 59.9 0.5 0.6 604 B11 2.5 26 6000.0 30.0 3.0 12.0 0.5 15.1 19.7 0.3 10.0 419 12% V9 80 45 12.6 55.3 0.5 0.7 475 B12 2.5 26 6000.0 30.0 3.0 12.0 2.0 13.1 19.4 0.48 10.0 317 16% V10 80 45 11.0 47.5 0.5 0.9 379 B13 2.5 26 6000.0 30.0 3.0 12.0 5.0 9.4 18.4 0.5 9.5 283 10% V11 80 45 8.6 36.2 0.47 1.0 313 B14 2.5 26 6000.0 30.0 4.0 12.0 5.0 11.8 17.8 0.5 9.4 312 7% V12 80 45 10.9 34.3 0.5 0.7 336 B15 2.5 26 6000.0 30.0 2.0 10.0 2.0 10.0 17.1 0.5 8.7 251 15% V13 80 45 8.5 47.4 0.5 1.4 295 B16 2.5 26 6000.0 30.0 3.0 10.0 2.0 13.3 16.4 0.5 8.5 310 12% V14 80 45 11.7 42.3 0.5 0.8 353 B17 2.5 26 6000.0 30.0 3.0 10.0 2.0 15.6 16.4 1.0 9.0 218 17% V15 80 45 13.0 49.2 1.0 2.2 265

[0079] It can be seen that the same membrane, when used according to the invention in Example 8, provides compressor power savings of 11.5% compared to the common membrane from Comparative Example 7, effects energy savings between 7% and 17% when varying various other process parameters in Examples 10 to 17, and at the same time results in an increase of the H.sub.2 content in the purified methane to at least 8.5 vol %, which is especially desirable in the future.

Example 18, Comparative Example 16

[0080] Finally, the process parameters selected for the comparison of membranes in Example 12 and Comparative Example 10 were used again in order to compare the same membrane (see also Comparative Examples 4 and 7) having a selectivity ratio 1/2 of 80/45=1.8 as well as the one of Comparative Examples 3 and 6 having a selectivity ratio 1/1 of 190/70=2.7 to the membrane of Example 7.

[0081] The latter is, according to Ahmadpour et al. (see above), a PVD/PEBA composite membrane and has a selectivity ratio 1/2 of 2.5/35=0.07 and thus the lowest ratio found in the literature for separations of H.sub.2 or CO.sub.2, respectively, from CH.sub.4.

[0082] In addition, no fixed upper limits for the H.sub.2 content in the purified methane were preset in these comparisons.

[0083] The results are summarized in Table 5 overleaf.

TABLE-US-00005 TABLE 5 Membrane Methanation product gas Permeate Retentate Compressor selectivity CO.sub.2 H.sub.2 CO.sub.2 H.sub.2 CO.sub.2 H.sub.2 req. H.sub.2/CH.sub.4 CO.sub.2/CH.sub.4 Vol. flow Pressure content content Pressure content content content content power Saved Example (1) (2) [Sm.sup.3/h] [bar] [vol %] [vol %] [bar] [vol %] [vol %] [vol %] [vol %] [kW] energy B18 2.5 35 6000.0 30.0 3.0 12.0 2.0 14.8 19.2 0.5 10.5 277 27% V10 80 45 11.0 47.5 0.5 0.9 379 B18 2.5 35 6000.0 30.0 3.0 12.0 2.0 14.8 19.2 0.5 10.5 277 23% V16 190 70 11.5 51.1 0.5 0.5 360

[0084] It is obvious that energy savings due to the reduced required compressor power were much higher in this case than in Table 4 above in case of the inventive use of the membrane having a selectivity ratio 1/2 of 2.5/26=0.1, namely another 50% higher than before.

[0085] This entailed an H.sub.2 content in the purified methane of 10.5 vol %, however, it is obvious that the results would not have been any worse if it had been limited to 10.0 vol %.

[0086] For a person ordinarily skilled in the art it follows that with the development of polymer membranes, such as elastomer membranes, with even lower selectivity ratios 1/2, the present invention will most likely allow even higher energy efficiency when purifying the product gases of methanations.

[0087] In any case, the inventor is at the moment conducting further research and experiments to determine other gas separation membranes suitable according to the present invention in analogy to the ones described above.

[0088] The present invention thus provides a new method for producing methane by methanation and subsequent purification via gas membrane separation, which method is not only, but mainly extremely advantageous compared to the method of the state of the art when very low limit values for the concentration of CO.sub.2 in the purified methane have to be complied with.