Process and apparatus for separating gases

Abstract

The invention relates to a process and apparatus for separation of gas mixtures with reduced maintenance costs. The process and the apparatus consist of a feed stream separation stage (1), and a retentate separation stage (2), of which both are membrane separation stages, wherein the first retentate stream (7) is heated to temperature higher than the temperature of the feed stream (5), before it is introduced to the retentate separation stage (2), and the total capacity of the membranes used in the retentate separation stage (2) is higher than the total capacity of the membranes used in the feed stream stage (1).

Claims

1. An apparatus, comprising: (i) a feed stream separation stage and a retentate separation stage which are membrane separation stages, wherein the feed stream separation stage is configured to separate a feed stream comprising at least two components into a first permeate stream and a first retentate stream, wherein the retentate separation stage is configured to separate the first retentate stream into a second permeate stream and a second retentate stream, and wherein the second permeate stream being combined with a raw gas stream to provide the feed stream; (ii) at least one device selected from the group consisting of a compressor arranged upstream of the feed stream separation stage, a vacuum device arranged in a permeate stream of the feed stream separation stage, and a vacuum device arranged in a retentate stream of the retentate separation stage; (iii) a heater arranged between the feed stream separation stage and the retentate separation stage; and (iv) optionally a purification treatment upstream of the feed stream separation stage; wherein the membranes of the retentate separation stage have a higher total capacity, measured for nitrogen Grade 4.8 under standard conditions, than the membranes of the feed stream separation stage, and wherein the retentate separation stage comprises a back pressure regulating valve in the second retentate stream, configured to adjust the retentate pressure to 1 to 100 bar.

2. The apparatus of claim 1, wherein a ratio of the total capacity of the membranes of the retentate separation stage to the total capacity of the membranes of the feed stream separation stage is in a range of from 1.05 to 10.

3. The apparatus of claim 1, wherein the membranes of the feed stream separation stage have a pure gas selectivity for carbon dioxide over methane of at least 30.

4. The apparatus of claim 1, wherein the material of a separation-active layer of the membranes is at least one selected from the group consisting of a polyamide, a polyetherimide, a polyaramide, a polybenzoxazole, a polybenzothiazole, a polybenzimidazole, a polysulfone, a cellulose acetate, a cellulose acetate derivative, a polyphenylene oxide, a polysiloxane, a polymer with intrinsic microporosity, a mixed matrix membrane, a facilitated transport membrane, a polyethylene oxide, a polypropylenexide, and a polyimide.

5. The apparatus of claim 4, wherein said material is a polyimide consisting of the monomer units A and B: ##STR00004## where x is from 0 to 0.5 and y is from 0.5 to 1, and R is the same or different radical selected from the group consisting of radicals L1, L2, L3 and L4: ##STR00005##

6. The apparatus of claim 1, wherein the compressor is present, and is a multistage compressor, and optionally the second permeate stream is introduced into the compressor between two compression stages.

7. The apparatus of claim 1, wherein the membrane separation stages consist of hollow fiber gas separation membrane modules.

8. A process for separating gases, carried out in an apparatus comprising: (i) a feed stream separation stage and a retentate separation stage which are membrane separation stages, the membranes of the retentate separation stage having a higher total capacity, measured for nitrogen Grade 4.8 under standard conditions, than the membranes of the feed stream separation stage; (ii) at least one device selected from the group consisting of a compressor arranged upstream of the feed stream separation stage, a vacuum device arranged in a permeate stream of the feed stream separation stage, and a vacuum device arranged in a retentate stream of the retentate separation stage; (iii) a heater arranged between the feed stream separation stage and the retentate separation stage; and (iv) optionally a purification treatment upstream of the feed stream separation stage; the process comprising: separating a feed stream comprising at least two components into a first permeate stream and a first retentate stream in the feed stream separation stage, separating the first retentate stream into a second permeate stream and a second retentate stream in the retentate separation stage; removing the first permeate stream as a first product, further processing the first permeate stream, or, if the second retentate stream is removed or further processed, discarding the first permeate stream; removing the second retentate stream as a second product, further processing the second retentate stream, or, if the first permeate stream is removed or further processed, discarding the second retentate stream; combining the second permeate stream with the feed stream; and heating the first retentate stream to a temperature higher than the temperature of the feed stream, before introducing the first retentate stream to the retentate separation stage.

9. The process of claim 8, wherein a ratio of the total capacity of the membranes of the retentate separation stage to the total capacity of the membranes of the feed stream separation stage is in a range of from 1.05 to 10.

10. The process of claim 8, wherein the membranes of the feed stream separation stage have a pure gas selectivity for carbon dioxide over methane of at least 30.

11. The process of claim 8, wherein the material of a separation-active layer of the membranes is at least one selected from the group consisting of a polyamide, a polyetherimide, a polyaramide, a polybenzoxazole, a polybenzothiazole, a polybenzimidazole, a polysulfone, a cellulose acetate, a cellulose acetate derivative, a polyphenylene oxide, a polysiloxane, a polymer with intrinsic microporosity, a mixed matrix membrane, a facilitated transport membrane, a polyethylene oxide, a polypropylenexide, and a polyimide.

12. The process of claim 11, wherein said material is a polyimide consisting of the monomer units A and B: ##STR00006## where x is from 0 to 0.5 and y is from 0.5 to 1, and R is the same or different radical selected from the group consisting of radicals L1, L2, L3 and L4: ##STR00007##

13. The process of claim 8, wherein the compressor is present, and is a multistage compressor, and optionally the second permeate stream is introduced into the compressor between two compression stages.

14. The process of claim 8, wherein the membrane separation stages consist of hollow fiber gas separation membrane modules.

15. The process of claim 8, further comprising adjusting the retentate pressure of the retentate separation stage with a back pressure regulating valve in the second retentate stream to 1 to 100 bar.

16. The process of claim 8, wherein the driving force used for the separation task is a partial pressure difference of the permeate gas between the retentate side and the permeate side in the respective membrane separation stages which is generated by a compressor in the feed stream, by a vacuum device in the second permeate stream, by a permeate-side flushing-gas stream, or a combination thereof.

17. The process of claim 8, wherein the first retentate stream is heated to a temperature 5 to 50° C. higher than the temperature of the feed stream before the first retentate stream is introduced to the retentate separation stage.

18. The process of claim 8, wherein the temperature of the feed stream is in the range of from 15 to 45° C. before the feed stream is introduced to the feed stream separation stage.

19. The process of claim 8, wherein the gas volume recycled with the second permeate stream totals less than 40% by volume of the raw gas stream.

20. The process of claim 8, wherein the raw gas stream is selected from the group consisting of a biogas, a natural gas, air, a gas mixture comprising carbon dioxide and methane, a gas mixture comprising hydrogen and methane, a gas mixture comprising carbon monoxide and methane, a gas mixture comprising helium and methane, a gas mixture comprising helium and nitrogen, a gas mixture comprising hydrogen and carbon monoxide, a gas mixture comprising a permanent gas having a boiling point of less than 110 K at 1 atm and a non permanent gas having a boiling point above or equal to 110 K at 1 atm, a gas mixture comprising carbon dioxide and a hydrocarbon, and a gas mixture comprising nitrogen and a hydrocarbon.

Description

FIGURE

(1) FIG. 1 shows an illustrative connection arrangement of membrane modules according to the present invention, wherein the numerals refer to the following items:

(2) 1: Feed stream separation stage 2: Retentate separation stage 4: One-stage or multistage compressor 5: Feed stream 6: First permeate stream 7: First retentate stream 8: Second retentate stream 9: Second permeate stream 12: Heater 17: Crude gas stream

(3) Measurement Methods:

(4) Selectivity of Membranes

(5) Gas permeabilities are reported in barriers (10.sup.−10 cm.sup.3 cm.sup.−2.Math.cm.Math.s.sup.−1.Math.cmHg.sup.−1). Permeances of hollow fiber membranes to gases are reported in GPU (Gas Permeation Unit, 10.sup.−8 cm.sup.3.Math.cm.sup.−2.Math.s.sup.−1cmHg.sup.−1).

(6) Flat Membranes

(7) For determination of the selectivity of flat membranes permeabilities to pure gases are measured by the pressure rise method. A flat sheet film between 10 and 70 μm in thickness has a pure gas applied to it from one side. On the other side, the permeate side, there is a vacuum (ca. 10.sup.−2 mbar) at the start of the test. Then, pressure rise on the permeate side over time is recorded. The polymer's permeability can be computed by the following formula:

(8) $P=\frac{V_{\mathrm{dead}}.Math.M{W}_{\mathrm{gas}}.Math.l}{\rho .Math.R.Math.T.Math.A.Math.\Delta p}.Math.\frac{\mathrm{dp}}{\mathrm{dt}}.Math.{10}^{10}$ P . . . Permeability in barrens (10.sup.−10 cm.sup.−3.Math.cm.sup.−2.Math.s.sup.−1.Math.cmHg.sup.−1) V.sub.dead . . . Volume of permeate side in cm.sup.−3 MW.sub.gas . . . Molar mass of gas in g.Math.mol.sup.−1 I . . . Thickness of film in cm p . . . Density of gas in g.Math.cm.sup.−3 R . . . Gas constant in cm.sup.3.Math.cmHg.Math.K.sup.−1mol.sup.−1 T . . . Temperature in kelvins (room temperature, ˜23° C.) A . . . Area of film in cm.sup.2 (˜12 cm.sup.2) Δp . . . Pressure difference between feed and permeate side in cmHg dp/dt. Pressure rise per time on permeate side in cmHg.Math.s.sup.−1

(9) The selectivity of the flat membrane according to the present invention for various pairs of gases is a pure-gas selectivity. It is calculated from the ratio of permeabilities of the pure gases as follows:

(10) $S=\frac{P_1}{P_2}$ S . . . pure gas selectivity P.sub.1 . . . permeability of gas 1 P.sub.2 . . . permeability of gas 2

(11) Hollow Fiber Membranes

(12) The permeance of hollow fibers is measured using a volume rise method. For this, the flux (at standard temperature and standard pressure) at the permeate site at constant pressure is measured.

(13) For hollow fibers it is necessary to measure the permeance P/l since the thickness of the separating layer is unknown. The permeance is computed by the following formula:

(14) $P/l=\frac{Q\left(\mathrm{STP}\right)}{R.Math.T.Math.A.Math.\Delta p}.Math.{10}^6$ P/l . . . permeance in GPU (gas permeation units. 10.sup.−6 cm.sup.3.Math.cm.sup.−2.Math.s.sup.−1.Math.cmHg.sup.−1) Q . . . gas flux of permeate side in cm.sup.3 (STP)/s R . . . gas constant in cm.sup.3.Math.cmHg.Math.K.sup.−1.Math.mol.sup.−1 T . . . temperature in kelvins (room temperature, ˜23° C.) A . . . membrane surface, i.e. external area as defined above, of hollow fiber in cm.sup.2 (between 60 and 80 cm.sup.2) Δp . . . pressure difference between feed and permeate side in cmHg

(15) The selectivity of the hollow fiber membrane according to the present invention for various pairs of gases is a pure-gas selectivity. It is calculated from the permeances of the pure gases as follows:

(16) $S=\frac{P_1}{P_2}$ S . . . pure gas selectivity P.sub.1 . . . permeance of gas 1 P.sub.2 . . . permeance of gas 2

(17) The examples which follow are intended to illustrate and describe the present invention in detail, but do not restrict it in any way.

(18) Membrane Capacity

(19) The calculation method as well as the reference gas and the standard conditions are described below.

(20) Hollow Fiber Membranes:

(21) The membrane capacity of the hollow fiber membrane (HFM) calculates as follows:
Capactity.sub.(HFM)=Permeance.sub.(HFM)*Surface.sub.(HFM)

(22) Wherein the permeance.sub.(HFM) is tested under the following standard conditions:

(23) TABLE-US-00001 Reference gas: Nitrogen, Grade 4.8 Temperature: Instead of room temperature as applied above for the permeance measurement to calculate the selectivities; for the capacity calculation, the permeance is determined at the average operating temperature of the membrane, which is usually the operating temperature of the separation stage Retentate pressure: 11 bara Permeate pressure: 1.1 bara
and the surface.sub.(HFM) correlates to the external membrane surface as defined in the definitions section in this disclosure.

(24) Flat Membranes:

(25) The membrane capacity of the flat membranes (FM) calculates as follows:
Capactity.sub.(FM)=Permeance.sub.(FM)*Surface.sub.(FM)
Wherein the permeability.sub.(FM) is tested under the following standard conditions:

(26) TABLE-US-00002 Reference gas: Nitrogen, Grade 4.8 Temperature: Instead of room temperature as applied above for the permeability measurement to calculate the selectivities; for the capacity calculation, the permeability is determined at the average operating temperature of the membrane, which is usually the operating temperature of the separation stage Retentate pressure: 11 bara Permeate pressure: 1.1 bara

(27) The permeance is then calculated by dividing the permeability through the thickness of the FM. The surface.sub.(FM) correlates to the external membrane surface as defined in the definitions section in this disclosure.

(28) The examples provided below are intended to illustrate the invention in more detail for a deeper understanding. They must not be construed in any way to limit the scope of the present invention.

EXAMPLES

(29) In all examples and comparative examples the membrane capacity relates to nitrogen (Grade 4.8) und were measured under the standard conditions described above.

(30) General Description of the Simulation:

(31) The simulation is based on the following assumptions: Steady state Ideal gas Ideal counter current in the membrane module No viscosity effects (no pressure loss on retentate or permeate flow) No sweep gas Constant temperature within each separation stage. Unless otherwise stated all separation stages in the examples are operated at identical temperatures.

(32) The simulation is done as follows:

(33) The following scheme of two flow channels separated by a membrane (double dotted line) was used to derive the equations needed for the simulation. NFi is a molar flow of component i at the high pressure side of the membrane. NPi is a molar flow of component i at the low pressure side of the membrane. Regarding to this scheme a feed entering the membrane is the sum of all molar component flows NFi (z=0) entering the membrane. Consequently, the retentate flow exiting the membrane is the sum of all molar component flows NFi (z=L). The permeate flow exiting the membrane is the sum of all molar component flows NPi (z=0). As there is no sweep flow entering the membrane on the opposite side of the permeate outlet these molar component flows NPi (z=L) are set to be zero.

(34) ##STR00003##

(35) The local molar flow of a component i through the membrane is its molar permeance Pi times the membrane area Udz times its local driving force, the local difference in partial pressure between feed and permeate side. pF and pP are the feed and the permeate pressure. The local molar concentration of component i on the feed or permeate side can be derived dividing the local molar feed or retentate flow of component i by the sum of all local molar component flows. From this the following set of equations can be derived.

(36) $\frac{d{\stackrel{.}{N}}_{\mathrm{Fi}}}{\mathrm{dz}}={\stackrel{.}{P}}_i.Math.U\left(\frac{{\stackrel{.}{N}}_{\mathrm{Fi}}}{\underset{j}{.Math.}{\stackrel{.}{N}}_{\mathrm{Fj}}}.Math.p_F-\frac{{\stackrel{.}{N}}_{\mathrm{Pi}}}{\underset{j}{.Math.}{\stackrel{.}{N}}_{\mathrm{Pj}}}.Math.p_P\right)$

(37) $\frac{d{\stackrel{.}{N}}_{\mathrm{Pi}}}{\mathrm{dz}}={\stackrel{.}{P}}_i.Math.U\left(\frac{{\stackrel{.}{N}}_{\mathrm{Fi}}}{\underset{i}{.Math.}{\stackrel{.}{N}}_{\mathrm{Fj}}}.Math.p_F-\frac{{\stackrel{.}{N}}_{\mathrm{Pi}}}{{.Math.}_j{\stackrel{.}{N}}_{\mathrm{Pj}}}.Math.p_P\right)$

(38) Including the boundary conditions described above the equations was solved in software Aspen Custom Modeler (ACM), however, other softwares like MATLAB, MathCad can also be used.

Example 1

(39) Separation of a Mixture of Methane and Carbon Dioxide with a Mixing Ratio of 60 to 40 with a Polyimide Membrane

(40) The FIG. 1 shown connection arrangement was used. Each stage consisted of a hollow fiber membrane module consisting of hollow polyimide fibers from Evonik Fibres GmbH (Sepuran® Green 4 inch, 1.2 meters length). The membranes used exhibited a pure gas selectivity for carbon dioxide over methane of 50.

(41) A 1000 m.sup.3/h biogas upgrading process was reached through simulation wherein the feed separation stage (1) consisted of 12 membrane modules, and the retentate separation stage (2) consisted of 24 membrane modules. All membrane modules have identical membrane area and identical permeance for N.sub.2 Grade 4.8 under standard conditions. Thus, the total capacity of the membranes of retentate separation stage (2) is twice as high as that of the feed stream separation stage (1).

(42) After steady state was reached, the feed stream (5) comprising the crude gas (17) and the second permeate stream (9) was compressed to 13 Bar and regulated to 25° C., and then was passed to the feed stream separation stage (1). The retentate stream (7) of the feed stream separation stage (1) was subsequently heated to 50° C. by a heater (12) and then passed to the retentate separation stage (2). A back pressure regulating valve (13) on the retentate side of the retentate separation stage (2) was set to 13 Bar and thus determined the driving force through the membrane of membrane separation stages (1) and (2).

(43) The permeate of the feed separation stage (1) had a content of 4.4% methane and 95.7% carbon dioxide. 401 m.sup.3/h of this mixture left the feed separation stage (1) as off-gas. The retentate of the retentate separation stage (2) had a content of 97.3% methane and 2.7% carbon dioxide. 598 m.sup.3/h of this mixture left the retentate separation stage (2) as product gas. The permeate of retentate separation stage (2) had a volume flow rate of 284 m.sup.3/h with a methane content of 30.9% and a carbon dioxide content of 69.1%, and was recycled via the second permeate stream (9) into the mixing chamber and compressed again by the compressor (4).

(44) The following Table 1 shows the process parameters and the result.

Comparative Example 1

(45) Example 1 was reproduced, except that the retentate stream (7) was not heated. The following Table 2 shows the process parameters and the result.

(46) Comparative Example 1 resulted in lower methane purity compared with Example 1, which shows a heating procedure between the feed stream separation stage (1) and the retentate separation stage (2) improves the methane purity of the retentate stream of the retentate separation stage (2).

Comparative Example 2

(47) Example 1 was reproduced, except that the feed stream (5) was heated to 50° C. instead of the retentate stream (7). The following Table 3 shows the process parameters and the result.

(48) Comparative Example 2 resulted in lower methane yield compared with Example 1, which shows a heating procedure between the feed stream separation stage (1) and the retentate separation stage (2) is better than a heating procedure up-stream the feed stream separation stage (1) in terms of the methane yield.

Comparative Example 3

(49) Example 1 was reproduced, except that the feed stream separation stage (1) and the retentate separation stage (2) had the same membrane capacity. The following Table 4 shows the process parameters and the result.

(50) Comparative Example 3 resulted in lower methane yield compared with Example 1, which shows a higher membrane capacity of the retentate separation stage (2) than the feed stream separation stage (1) resulted in a better methane yield. It further shows, that the purity of carbon dioxide in the first permeate stream (6) decreases. Thus, if it is desired to remove the first permeate stream (6) as the only or a second pure product, Example 1 with an increased higher membrane capacity in the retentate separation stage (2), due to the effects of the capacity on recycling stream (9), provides significantly better results.

Comparative Example 4

(51) Example 1 was reproduced, except that the retentate stream (7) of the feed stream separation stage (1) was not heated, and the membrane capacity of the retentate separation stage (2) was adjusted.

(52) A 1000 m.sup.3/h biogas upgrading process with methane purity and yield similar as Example 1 was reached through simulation until the retentate separation stage consisted of 34 membrane modules. The following Table 5 shows the process parameters and the result.

(53) Example 1 and Comparative Example 4 resulted in exactly the same methane purity and yield. However, Example 1 needs much less membrane modules (capacity) for the retentate separation stage (2).

Comparative Example 5

(54) A three stage process according to the FIG. 12 of WO2012/00727 was simulated for reference, with each of stages having the same membrane capacity. The membrane modules used in this comparative example was the same as Example 1.

(55) Comparative Example 5 differed from Example 1 in that:

(56) 1) the first retentate stream (7) was not heated;

(57) 2) the first permeate stream (6) was introduced to a third membrane stage;

(58) 3) the third retentate stream (10) and the second permeate stream (9) were recycled together into the mixing chamber and compressed again by the compressor (4);

(59) 4) the third permeate stream (11) was taken out as an off-gas.

(60) A 1000 m.sup.3/h biogas upgrading process with methane purity as Example 1 was reached through simulation until the each separation stage consisted of 23 membrane modules. The following Table 6 shows the process parameters and the result.

(61) Although Comparative Example 5 resulted in slightly higher methane yield compared to Example 1, the total number of the membrane modules (membrane capacity) is almost twice the total number of the membrane modules (membrane capacity) of Example 1, which shows Example 1 has a significant advantage in terms of the investment and maintenance cost.

Example 2

(62) Separation of a Mixture of Methane and Carbon Dioxide with a Mixing Ratio of 60 to 40 with a Polyimide Membrane

(63) The FIG. 1 shown connection arrangement was used. Each stage consisted of a hollow fiber membrane module consisting of hollow polyimide fibers from Evonik Fibres GmbH (Sepuran® Green 4 inch, 1.2 meters length). The membranes used exhibited a pure gas selectivity for carbon dioxide over methane of 40.

(64) A 1000 m.sup.3/h biogas upgrading process was reached through simulation wherein the feed separation stage consisted of 11 membrane modules, and the retentate separation stage consisted of 25 membrane modules. Alt membrane modules have identical membrane area and identical permeance for N.sub.2 Grade 4.8 under standard conditions. Thus, the total capacity of the membranes of retentate separation stage (2) is 2.27 times higher than that of the feed stream separation stage (1).

(65) After steady state was reached, the feed stream (5) comprising the crude gas (17) and the second permeate stream (9) was compressed to 13 Bar and regulated to 25° C. and then was passed to the feed stream separation stage (1). The retentate stream (7) of the feed stream separation stage (1) was subsequently heated to 50° C. by a heater (12) and then passed to the retentate separation stage (2). A back pressure regulating valve (13) on the retentate side of the retentate separation stage (2) was set to 13 Bar and thus determined the driving force through the membrane of membrane separation stages (1) and (2).

(66) The permeate of the feed separation stage (1) had a content of 4.8% methane and 95.2% carbon dioxide. 404 m.sup.3/h of this mixture left the feed separation stage (1) as off-gas. The retentate of the retentate separation stage (2) had a content of 97.4% methane and 2.6% carbon dioxide. 596 m.sup.3/h of this mixture left the retentate separation stage (2) as product gas. The permeate of retentate separation stage (2) had a volume flow rate of 359 m.sup.3/h with a methane content of 31.3% and a carbon dioxide content of 68.7%, and was recycled via the second permeate stream (9) into the mixing chamber and compressed again by the compressor (4).

(67) The following Table 7 shows the process parameters and the result.

Example 3

(68) Separation of a Mixture of Methane and Nitrogen with a Mixing Ratio of 80 to 20 with a Polyimide Membrane

(69) The simulation calculation of Example 1 was repeated for a natural gas of 80 mol-% methane and 20 mol-% nitrogen, membranes with a mixed gas selectivity for nitrogen over methane of 4 and a pure gas selectivity of 5.8, a feed separation stage (1) of 44 membrane modules, and a retentate separation stage (2) of 100 membrane modules.

(70) The following Table 8 shows the process parameters and the result.

Comparative Example 6

(71) Example 3 was repeated, except that the retentate stream (7) was not heated. Comparative example 6 corresponds to example 16 of U.S. Pat. No. 6,565,626.

(72) The following Table 9 shows the process parameters and the result.

(73) Comparative Example 6 resulted in lower methane purity compared with Example 3, which shows that heating the retentate of the feed stream separation stage (1) before feeding it to the retentate separation stage (2) improves the methane purity of the retentate stream of the retentate separation stage (2).

(74) TABLE-US-00003 TABLE 1 Example 1 Feed separation stage Retentate separation stage (12 modules) (24 modules) Mixing tank Permeate Retentate Location Biogas Circulation Mixing Feeding Retentate (Offgas) Feeding (Product) Permeate Pressure [Bar] 1 1 13 13 13 1 13 13 1 Flow [Nm.sup.3/h] 1,000 284 1,284 1,284 883 401 883 598 284 Temperature [° C.] 25 21 21 50 47 48 Composition [V/V] CH.sub.4 60.0% 53.6% 4.4% 97.3% 30.9% CO.sub.2 40.0% 46.4% 95.6% 2.7% 69.1% Circulation 28.4% Methane Yield 97.1%

(75) TABLE-US-00004 TABLE 2 Comparative Example 1 Feed separation stage Retentate separation stage (12 modules) (24 modules) Mixing tank Permeate Retentate Location Biogas Circulation Mixing Feeding Retentate (Offgas) Feeding (Product) Permeate Pressure [Barg] 1 1 13 13 13 1 13 13 1 Flow [Nm.sup.3/h] 1000 200 1200 1200 810 390 810 610 200 Temperature [° C.] 25 21 21 21 18 16 Composition [V/V] CH.sub.4 60.0% 53.6% 4.5% 95.4% 21.5% CO.sub.2 40.0% 46.4% 95.5% 4.6% 78.5% Circulation 20.0% Methane Yield 97.1%

(76) TABLE-US-00005 TABLE 3 Comparative Example 2 Feed separation stage Retentate separation stage (12 modules) (24 modules) Mixing tank Permeate Retentate Location Biogas Circulation Mixing Feeding Retentate (Offgas) Feeding (Product) Permeate Pressure [Barg] 1 1 13 13 13 1 13 13 1 Flow [Nm.sup.3/h] 1000 213 1213 1213 790 423 790 577 213 Temperature [° C.] 50 46 46 46 44 44 Composition [V/V] CH.sub.4 60.0% 57.4% 8.3% 97.9% 39.4% CO.sub.2 40.0% 43.6% 91.7% 2.1% 60.6% Circulation 21.3% Methane Yield 94.1%

(77) TABLE-US-00006 TABLE 4 Comparative Example 3 Feed separation stage Retentate separation stage (18 modules) (18 modules) Mixing tank Permeate Retentate Location Biogas Circulation Mixing Feeding Retentate (Offgas) Feeding (Product) Permeate Pressure [Barg] 1 1 13 13 13 1 13 13 1 Flow [Nm.sup.3/h] 1000 152 1152 1152 737 415 737 585 152 Temperature [° C.] 25 21 21 50 48 48 Composition [V/V] CH.sub.4 60.0% 58.2% 7.1% 97.5% 46.0% CO.sub.2 40.0% 41.8% 92.9% 2.5% 54.0% Circulation 15.2% Methane Yield 95.1%

(78) TABLE-US-00007 TABLE 5 Comparative Example 4 Feed separation stage Retentate separation stage (12 modules) (12 modules) Mixing tank Permeate Retentate Location Biogas Circulation Mixing Feeding Retentate (Offgas) Feeding (Product) Permeate Pressure [Barg] 1 1 13 13 13 1 13 13 1 Flow [Nm.sup.3/h] 1000 245 1245 1245 844 401 844 599 245 Temperature [° C.] 25 21 21 21 17 19 Composition [V/V] CH.sub.4 60.0% 53.1% 4.4% 97.3% 24.8% CO.sub.2 40.0% 46.9% 95.7% 2.7% 75.2% Circulation 24.5% Methane Yield 97.1%

(79) TABLE-US-00008 TABLE 6 Comparative Example 5 Retentate separation stage Permeate separation stage Feed separation stage (23 modules) (23 modules) Mixing tank (23 modules) Retentate Permeate Location Biogas Circulation Mixing Feeding Retentate Permeate Feeding (Product) Permeate Feeding Retentate (Offgas) Pressure [Barg] 1 1 16 16 16 3.6 16 16 1 3.6 3.6 1 Flow [Nm.sup.3/h] 1000 262 1262 1262 813 449 813 613 200 449 62 387 Temperature [° C.] 25 21 21 20 16 16 21 21 21 Composition [V/V] CH.sub.4 60.0% 97.1% 1.2% CO.sub.2 40.0% 2.9% 98.8% Circulation 26.2% Methane Yield 99.2%

(80) TABLE-US-00009 TABLE 7 Example 2 Feed separation stage Retentate separation stage (11 modules) (25 modules) Mixing tank Permeate Retentate Location Biogas Circulation Mixing Feeding Retentate (Offgas) Feeding (Product) Permeate Pressure [Barg] 1 1 13 13 13 1 13 13 1 Flow [Nm.sup.3/h] 1000 359 1359 1359 955 404 955 596 359 Temperature [° C.] 25 21 21 50 47 47 Composition [V/V] CH.sub.4 60.0% 52.4% 4.8% 97.4% 31.3% CO.sub.2 40.0% 47.6% 95.2% 2.6% 68.7% Circulation 35.9% Methane Yield 96.8%

(81) TABLE-US-00010 TABLE 8 Example 3 Feed separation stage Retentate separation stage Mixing tank (44 modules) (100 modules) Natural Permeate Retentate Location gas Circulation Mixing Feeding Retentate (Offgas) Feeding (Product) Permeate Pressure [bar] 1 1 55.2 55.2 55.2 13.8 55.2 55.2 13.8 Flow [Nm.sup.3/h] 1180 1676 2856 2856 2254 602 2254 578 1676 Temperature [° C.] 25 22 24 50 25 47 Composition [V/V] CH.sub.4 80.0% 79.4% 62.9% 98.0% 79.0% N.sub.2 20.0% 20.6% 37.1% 2.0% 21.0% Circulation 142% Methane Yield 60.0%

(82) TABLE-US-00011 TABLE 9 Comparative Example 6 Feed separation stage Retentate separation stage Mixing tank (44 modules) (100 modules) Natural Permeate Retentate Location gas Circulation Mixing Feeding Retentate (Offgas) Feeding (Product) Permeate Pressure [bar] 1 1 55.2 55.2 55.2 13.8 55.2 55.2 13.8 Flow [Nm.sup.3/h] 1180 861 2041 2041 1463 590 1463 590 861 Temperature [° C.] 25 21 24 21 3 19 Composition [V/V] CH.sub.4 80.0% 79.3% 63.9% 96.0% 78.3% N.sub.2 20.0% 20.7% 36.1% 4.0% 21.7% Circulation 73% Methane Yield 60.0%