TWO STEP AMINE ABSORPTION PROCESS FOR REMOVAL CO2/H2S FROM BIOGAS

Abstract

The present invention relates to a method for upgrading biogas, i.e. a method for removing carbon dioxide and/or hydrogen sulphide from biogas. Particularly the invention relates to a method for upgrading biogas by absorption in two absorbers, where the gas effluent of the first absorber is pressurized and fed to the absorber of the second absorption step and wherein the liquid effluents of the two absorbers are regenerated to form a regenerated absorption stream, which is then provided in two absorption streams which is fed to the absorber of the first and second absorption steps respectively. It also relates to a system for performing the method.

Claims

1-27. (canceled)

28. A method for upgrading a biogas stream, the biogas stream comprising methane, carbon dioxide, and optionally hydrogen sulphide, the method comprising the steps of: a. feeding the biogas stream and a first liquid absorption stream to an absorber of a first absorption step, b. absorbing carbon dioxide and hydrogen sulphide if present, from the biogas stream into the first liquid absorption stream, thereby obtaining a first gas effluent and a first liquid effluent, c. increasing the pressure of the first gas effluent, to obtain a pressurized biogas stream, d. feeding the pressurized biogas stream and a second liquid absorption stream to an absorber of the second absorption step of a second absorption step, e. absorbing carbon dioxide and hydrogen sulphide if present from the pressurized biogas stream into the second liquid absorption stream, thereby obtaining a second gas effluent and a second liquid effluent, f regenerating the first liquid effluent and the second liquid effluent in a regeneration system, thereby obtaining a regenerated absorption stream, g. providing the regenerated absorption stream in two streams to obtain the first liquid absorption stream and the second liquid absorption stream, and h. recovering or further processing the second gas effluent as an upgraded biogas stream.

29. The method according to claim 28, wherein at least the first absorption step and second absorption step are performed in an upgrade system, the first absorption step comprising the steps a. and b., and the second absorption comprising the steps c., d., and e., wherein the biogas stream is fed to the upgrade system, the regenerated absorption stream is fed to the upgrade system, the upgraded biogas is recovered from the upgrade system, and the first liquid effluent and second liquid effluent is fed from the upgrade system to the regeneration system.

30. The method according to claim 29, wherein one liquid absorption stream is provided to the upgrade system, the one stream being the regenerated absorption stream.

31. The method according to claim 28, wherein the upgrade system contains two absorption steps, the first and second absorption step.

32. The method according to claim 28, wherein either or both of the first and second absorption steps comprise two or more absorbers, the two or more absorbers of the first absorption step having substantially the same operating pressure and the two or more absorbers of the second absorption step having substantially the same operating pressure.

33. The method according to claim 28, further comprising a step of mixing the first and second liquid effluents.

34. The method according to claim 28, further comprising a step of feeding at least a part of the second liquid effluent to the absorber of the first absorption step, preferably at a lower section of the absorber.

35. The method according to claim 28, further comprising a step of feeding the first liquid effluent to the absorber of the second absorption step, preferably above a feeding point of the pressurized biogas stream, preferably at a midpoint of the second absorber.

36. The method according to claim 28, wherein carbon dioxide removed from the first liquid effluent and second liquid effluent is contained in an off-gas of the regeneration system.

37. The method according to claim 28, wherein step f. comprises heating the first liquid effluent and the second liquid effluent, to obtain the regenerated absorption stream.

38. The method according to claim 28, wherein the regeneration system comprises a stripper column with a reboiler.

39. The method according to claim 28, wherein the regeneration system comprises one single regeneration unit in which both the first and second liquid effluents are regenerated to provide the regenerated absorption stream, preferably wherein the regeneration unit is a stripper column with a reboiler.

40. The method according to claim 30, wherein the single regeneration unit provides the regenerated absorption stream as the only absorption liquid provided to the upgrade system.

41. The method according to claim 28, wherein the amount of carbon dioxide absorbed in step b. in to the first liquid absorption stream is greater than the amount of carbon dioxide absorbed in step e. into the second liquid absorption stream.

42. The method according to claim 28, wherein the flow rate of the first liquid absorption stream is greater than the flow rate of the second liquid absorption stream.

43. The method according to claim 42, wherein a total flow rate is the sum of the flow rates of the first and second liquid absorption streams, and the flow rate of the second liquid absorption stream is 1 to 30% the total flow rate, such as 1 to 25%, 1 to 20%, 2 to 15%, 3 to 15%, 3 to 12%.

44. The method according to claim 28, wherein a ratio of the flow rate of biogas stream and the flow rate of the first liquid absorption stream, abbreviated GL1, is less than a ratio of the flow rate of first gas effluent and the flow of rate of the second liquid absorption stream, abbreviated GL2.

45. The method according to claim 44, wherein GL1 is less than 20%, less than 10%, suitably 1 to 20%, 2 to 15%, 2 to 10% or 4 to 8%.

46. The method according to claim 44, wherein GL2 is greater than 20% or greater than 30%, suitably in the range of 20 to 150%, 25 to 50% or 25 to 35%.

47. The method according to claim 28, wherein the operating pressure of the second absorption step is 4 to 70 bara, 4 to 40 bara, 6 to 40 bara, 10 to 30 bara, or 15 to 25 bara.

48. The method according to claim 28, wherein the operating pressure of the first absorption step is 0.7 to 6 bara, 1 to 6 bara, 1 to 4 bara, 1 to 2 bara, or 1 to 1.5 bara.

49. The method according to claim 28, wherein the first and second liquid absorption streams are chemical absorption streams.

50. The method according to claim 49, wherein the chemical absorption streams comprise an amine, preferably selected from monoethanolamine, diethanolamine, diisopropanolamine, methyldiethanolamine, triethanolamine, and piperazine.

51. The method according to claim 28, wherein the first and second liquid absorption streams are physical absorption streams.

52. The method according to claim 28, wherein the upgraded biogas stream comprises less than 50 ppm carbon dioxide and less than 4 ppm hydrogen sulphide on dry basis.

53. The method according to claim 28, wherein the upgraded biogas stream is further processed into liquified biogas.

54. A biogas upgrade system for upgrading a biogas stream, the system being configured for performing the method according to claim 28.

Description

DETAILED DESCRIPTION

[0087] In the following, the invention is described with reference to the non-limiting examples and drawings, where

[0088] FIG. 1 shows a schematic diagram of biogas upgrade plant known in the art,

[0089] FIG. 2 shows a schematic diagram of an embodiment of the invention where the second liquid effluent is fed to the absorber of the first absorption step,

[0090] FIG. 3 shows a schematic diagram of an embodiment of the invention where the first and second liquid effluents are mixed prior to regeneration,

[0091] FIG. 4 shows a schematic diagram of an embodiment of the invention where the first liquid effluent is fed into the absorber of the second absorption step,

[0092] FIG. 5 shows a schematic diagram of an embodiment of the invention where a part of the second liquid effluent is fed to the absorber of the first absorption step, and

[0093] FIG. 6 shows a schematic illustration of the concept of the invention.

[0094] FIG. 1 shows a diagram of a process known in the art for upgrading biogas. Biogas comprising methane, carbon dioxide, and hydrogen sulphide, which is to be upgraded, is fed to the process as biogas stream 101. Biogas stream 101 is fed at the bottom of the absorber A1 where it is contacted counter-currently with a chemical absorption liquid which is fed at the top of the absorber A1 as the first liquid absorption stream 201. The gas phase exits the first absorber as first gas effluent 105 which has a reduced content of carbon dioxide and/or hydrogen sulphide compared to the biogas stream 101. The pressure of the first gas effluent 105 is increased in compressor C1 and recovered as the upgraded biogas stream which can be further processed or supplied to a gas grid. The pressurization is necessary for supplying the upgraded biogas to a gas grid or for further processing steps. The liquid phase exits the absorber A1 as the first liquid effluent 202, which is pumped to regeneration unit R1, a stripper with a reboiler. In the regeneration unit R1 the first liquid effluent 202 is heated by way of heater H1 (the reboiler) to release absorbed gas and regenerate the chemical absorption liquid, thereby providing the regenerated absorption stream 19. The regenerated absorption stream 19 is recycled into absorber A1 as the first liquid absorption stream 201. The gases which are desorbed or vaporized in the regeneration unit R1 exits as off-gas 113 which is processed in heat exchanger H5 and separator F1 to recycle condensed liquid 206 into the regeneration unit R1 and yielding the waste off-gas 114. Fresh chemical absorption liquid or make-up liquid, such as water, may be supplied to the first absorber as stream 115 to counter-act loss of liquid in the system and/or to absorb any absorption agent from the gas phase. Moreover, stripping gas (not shown) could be fed to the regeneration unit R1 to increase the mass transfer. In variations where the process is performed with a physical absorption liquid, such as water or methanol, a typical regeneration unit would comprise a stripper column to which a stripping gas is supplied to the system. As shown in FIG. 1 the process includes several heat exchangers H2, H3, H4, H5 for heating or cooling the streams, and, pumps P1, P2, P3 for circulating the streams. Heat exchanger H1 is provided to preheat first liquid effluent 202 with the excess heat of the regenerated absorption stream 19. The symbols used in the diagram are the typical representations used in the art and do not confer any information as to the nature of the equipment used beyond the type of unit operation.

[0095] FIG. 2 shows a diagram of an embodiment of the method according the invention. Unless otherwise stated the features and reference numbers are similar to those of FIG. 1. In the embodiment shown, the upgrade system (U) comprise one absorber in each of the first and second absorptions steps. The biogas stream 101 is fed to the bottom of the first absorber A1 while the first liquid absorption stream 201 is fed to the top. The resulting gas phase exits as the first gas effluent 105 which has a reduced content of carbon dioxide and/or hydrogen sulphide. The pressure of the first gas effluent 105 is increased in compressor C1, thereby obtaining the pressurized biogas stream 107 which is fed to the second absorber A2. The pressure of the pressurized biogas stream 107 is at least at the level of the operating pressure of the second absorber A2. The second liquid absorption stream 212 is fed at the top of the second absorber after having been pressurized by pump P4 to at least the operating pressure of the second absorber A2. In the second absorber A2, further carbon dioxide and/or hydrogen sulphide is absorbed from the gas phase, whereby the second gas effluent 108 and the second liquid effluent 214 are obtained. The first liquid effluent 202 and second liquid effluent 214 are regenerated in the regeneration system R thereby providing the regenerated absorption stream 19. The upgrade system U is indicated by dashed line and contains the absorber A1 of the first absorption step and the absorber of the second absorption step A2. The biogas stream 101 and regenerated absorption stream 19 are provided to the upgrade system U, while the upgraded biogas stream 108 and first liquid effluent 202 containing part of the second liquid effluent 214 are collected from the upgrade system U. The regeneration system R is indicated by a dashed line and contains a single regeneration unit R1 which regenerates the influent liquid by heating through heater H1 similar to the prior art process illustrated in FIG. 1. The condensed liquid stream 206 could in alternative configurations be added to stream 19 or fed to the top of one of the absorbers in place of or in addition to stream 115, in order to absorb any chemical absorption agent from the gas phase prior to the gas exiting the absorber. The regenerated absorption stream 19 is provided in two streams by splitting the regenerated absorption stream 19 to obtain the first liquid absorption stream 201 and second liquid absorption stream 212. As the first and second liquid absorption streams 201, 212 are provided from the regenerated absorption stream 19 they have the same composition. Prior to regeneration, the second liquid effluent 214 is mixed with the first liquid effluent 202 by feeding the second liquid effluent 214 to the first absorber A1, whereby the liquid from the second liquid effluent 214 is admixed with the first liquid effluent 202, both constituting the first liquid effluent 202, which is then pumped to the regeneration system R by pump P1. This recycling of the second liquid effluent 214 allows gas absorbed in the second liquid effluent 214 to be released by the reduced pressure in the first absorber A1, and subsequently to be absorbed in the first absorber. In this way, any methane absorbed in the absorber of the second absorption step A2 due to the increased pressure, is released into the gas phase of the first absorber A1 whereby the methane slip of the liquid effluents is reduced. In FIG. 2 a valve is provided for reducing the pressure of the second liquid effluent 214. The streams which are exchanged between the upgrade system U and regeneration system R are the first liquid effluent 202, containing the second liquid effluent 214, and the regenerated absorption stream 19. The regenerated absorption stream 19 is the only liquid absorption stream provided to the upgrade system U, the make-up stream 115 serving to counteract liquid loss in the process could also have been added to the regenerated absorption stream 19.

[0096] FIG. 3 shows a diagram of another embodiment of the method according the invention. Unless otherwise stated the features and reference numbers are similar to those of FIG. 2. In FIG. 3 the second liquid effluent 214 is mixed with the first liquid effluent 202 outside of the absorbers A1 and A2 and the resulting mixture is fed to the regeneration system R. In an alternative configuration the first liquid effluent 214 and second liquid effluent 202 are mixed in the regeneration unit R1 (not shown), and in yet another alternative, each liquid effluent is regenerated in separate regeneration units of the regeneration system, and the resulting liquid effluents are combined to obtain the regenerated absorption stream 19 (not shown)

[0097] FIG. 4 shows a diagram of another embodiment of the method according the invention. Unless otherwise stated the features and reference numbers are similar to those of FIG. 2. In FIG. 4 the first liquid effluent 202 and second liquid effluent 214 are mixed prior to regeneration by feeding the first liquid effluent 202 to the absorber of the second absorption step A2. A part of the biogas stream 101 is absorbed in the first absorber A1 thereby reducing the amount of gas to be compressed in compressor C1. The first liquid effluent 202 is pressurized by way of pump P1 and fed to the absorber of the second absorption step A2 at the midpoint of the absorber. By this configuration further gas is absorbed therein to increase the load of the liquid absorption stream before it is regenerated, thereby reducing the required circulation rate of liquid.

[0098] FIG. 5 shows a diagram of another embodiment of the method according the invention. Unless otherwise stated the features and reference numbers are similar to those of FIG. 2. In FIG. 5, a part of the second liquid effluent 214 is recycled into the first absorber. The recycled part is generated by flash separating the second liquid effluent 214 in separator F2 to provide a gas part 214a and a liquid part 214b thereof, allowing for the gas part 214a to be recycled into the first absorber A1 while mixing the liquid part 214b into the first liquid effluent 202. The separator F2 is not part of the regeneration system (R) as any carbon dioxide removed from the second liquid effluent 214 in separator F2 is not removed from the overall process but recycled into absorber A1. The off-gas 113 contains the carbon dioxide removed from the process.

[0099] FIG. 6 shows a schematic presentation of the steps of a method according to invention, thus illustrating the concept thereof. In step a. the biogas stream 101 and the first liquid absorption stream 201 are fed to the absorber A1 of the first absorption step. In step b. the gas and liquid phases in the first absorption step are brought into contact, whereby carbon dioxide and hydrogen sulphide are absorbed from the gas phase into the liquid phase. The liquid phase exits as the first liquid effluent 202 and the gas phase as the first gas effluent 105. The first gas effluent 105 is pressurized in step c. providing a pressurized biogas stream which in step d. is fed to the absorber A2 of the second absorption step along with the second liquid absorption stream 212. Carbon dioxide and hydrogen sulphide are absorbed from the gas phase into the liquid phase which in turn exit as the second liquid effluent 214 and second gas effluent 108 respectively. The second gas effluent 108 is recovered or further processed in step h. as the upgraded biogas. The first liquid effluent 202 and second liquid effluent 214 are regenerated in step f providing the regenerated absorption stream 19, which in step g. is provided in two streams as the first liquid absorption stream 201 and second liquid absorption stream 212. As regards step f it is to be understood that the conceptual illustration of FIG. 6 does not show how the first and second liquid effluents may be routed in a processing plant which employs the method of the invention.

[0100] The advantages of the invention will now be illustrated by way of the following non-limiting examples. The examples are process simulations performed in commercially available software such as CHEMCAD.

EXAMPLE I

Comparative

[0101] A biogas stream is upgraded according to a process as exemplified in FIG. 1. Stream properties and compositions as well as utility duties are as shown in Table I, where the reference numbers are those of FIG. 1. The absorption agent is monoethanolamine (MEA) and the absorber has a column height of 16 m. The absorber is in equilibrium, i.e. further column height would not improve the upgrade process. The upgraded biogas contains 95.11 mole % methane and 274.7 ppm carbon dioxide and 81.7 ppm hydrogen sulphide. On dry basis, this corresponds to 99.96 mole % methane, 288.7 ppm carbon dioxide and 85.8 ppm hydrogen sulphide.

TABLE-US-00001 TABLE I Lean Waste Upgraded Pressurized Name Biogas amine Off-gas Biogas Biogas Reference 101 201 114 105 107 Stream Properties Temperature 35 35 50 34.92 40 C. Pressure 1.1 4.5 1.8 1.1 11 bar(a) Mass flow 1261.25 19197.68 816.30 449.82 449.82 kg/h Stream Composition mole % Methane 56.37 0 0.04715 95.11 95.11 Carbon 37.58 3.404 90.83 0.02747 0.02747 dioxide Hydrogen 0.9394 0.01456 2.259 0.008166 0.008166 sulphide Water 5.117 85.68 6.861 4.850 4.850 MEA 10.90 0 0.000254 0.000254 Utility Duties kW Compressor C1 78.7 Heat Exchanger H1 750

EXAMPLE II

[0102] The feed biogas of Example I is in this example upgraded according to a process as exemplified in FIG. 2, i.e. according to an embodiment of the invention. Stream properties and compositions as well as utility duties are as shown in Table II, where the reference numbers are those of FIG. 2. The absorption agent is monoethanolamine (MEA) and the absorber of the first absorption step has a column height of 12 m and the absorber of the second absorption step has a height of 4 m. The upgraded biogas contains 99.47 mole % methane and 41.4 ppm carbon dioxide and 11.1 ppm hydrogen sulphide, which on dry basis is 99.99 mole %, 41.6 ppm and 11.2 ppm respectively.

[0103] The waste off-gas contains 394 ppm methane (methane slip).

[0104] The molar gas-liquid ratio of the absorber of the first absorption step, GL1, is about 0.06 (6%) and GL2 in the absorber of the second absorption step it is about 0.33 (33%).

[0105] Compared to example I, the present example has the same heating duty in heater H1 of 750 kW, practically the same compressor duty of about 79 kW, and the total absorber column height of the two examples are equivalent. As can be seen example II provides a purer upgraded biogas at practically the same utility duty and is thus more efficient. Further, the methane slip in the off-gas is also reduced compared to example I. The added pump duty associated with the second liquid absorption stream is insignificant due to the comparatively low utility demand of liquid pumps. The example is not necessarily optimized.

TABLE-US-00002 TABLE II 1.sup.st Lean First gas Pressurized 2.sup.nd Lean Upgraded Waste Name Biogas amine effluent biogas amine biogas Off-gas Reference 101 201 105 107 212 108 114 Stream Properties Temperature C. 35 35 34.97 40 35.27 35.6 50 Pressure bar(a) 1.1 4.5 1.1 11 22 11 1.8 Mass flow kg/h 1261.25 17210 450.6 450.6 2000 427.7 817.1 Stream Composition mole % Methane 56.37 0 95.05 95.05 0 99.47 0.03968 Carbon dioxide 37.58 3.672 0.3684 0.3684 3.672 0.004136 90.84 Hydrogen sulphide 0.9394 0.01660 0.01064 0.01064 0.01660 0.001115 2.262 Water 5.117 85.13 4.905 4.905 85.13 0.5275 6.862 MEA 11.17 0.000257 0.000257 11.18 0.000413 0 Utility Duties kW Compressor C1 78.8 Heater H1 750

EXAMPLE III

[0106] A biogas stream is upgraded according to a process as exemplified in FIG. 2 to achieve low emissions of carbon dioxide or hydrogen sulphide. Stream properties and compositions are as shown in Table II, where the reference numbers are those of FIG. 2. The absorption agent is methyldiethanolamine (MDEA). The upgraded biogas contains 98.68 mole % methane and 1.6 ppm carbon dioxide and 0 ppm hydrogen sulphide. On dry basis, this corresponds to 98.98 mole % methane, 1.6 ppm carbon dioxide and 0 ppm hydrogen sulphide.

[0107] The molar gas-liquid ratio of the first absorption step, GL1, is about 0.08 (8%) and in the second absorption step it, GL2, is about 1.35 (135%).

TABLE-US-00003 TABLE III 1.sup.st Lean First gas Pressurized 2.sup.nd Lean Upgraded Name Biogas amine effluent biogas amine biogas Off-gas Reference 101 201 105 107 212 108 113 Stream Properties Temperature C. 40 40 39.8 35 40.2 40.1 98.3 Pressure bar(a) 1.35 4.5 1.35 21.5 22 21.5 1.5 Mass flow kg/h 3242.5 41932.7 1131.7 1137.9 1500 1113.4 3017.0 Stream Composition mole % Methane 53.92 0 93.49 96.98 0 98.68 0.019 Carbon dioxide 38.35 0.046 0.13 0.13 0.046 0.00016 45.58 Hydrogen sulphide 0.020 0.000007 0.00005 0.00005 0.000007 0 0.00008 Oxygen 0.18 0 0.32 0.33 0 0.33 0.000048 Nitrogen 0.37 0 0.63 0.66 0 0.67 0.000033 Water 7.25 89.07 5.42 1.90 89.07 0.31 54.38 MDEA 0 10.89 0 0 10.89 0.000022 0.019

EXAMPLE IV

[0108] A biogas stream is upgraded according to a process as exemplified in FIG. 4, wherein the first liquid effluent 202 is pressurized to 8 bar(a) and fed to the middle of absorber A2. The second liquid effluent 214 is withdrawn at 8 bar(a) from the absorber of the second absorption step A2. The absorption agent in this example is commercially available MDEA based product sold under the name AdapT 201. Stream properties and compositions are as shown in Table IV, where the reference numbers are those of FIG. 4.

TABLE-US-00004 TABLE IV 1.sup.st Lean First gas Pressurized 2.sup.nd Lean Upgraded Name Biogas amine effluent biogas amine biogas Reference 101 201 105 107 212 108 Stream Properties Temperature C. 40 60 30 215.8 60.1 30 Pressure bar(a) 1.10 1.2 1.06 6.0 15.0 7.99 Mass flow kg/h 3122.5 26108.1 2432.6 2432.6 6228.4 1185.6 Stream Composition mole % Methane 57.4 0.00 66.5 66.5 0.00 95.7 Carbon dioxide 38.2 2.16 29.0 29.0 2.16 3.77 Hydrogen sulphide 0.596 0.0375 0.522 0.522 0.0375 0.052 Water 3.82 85.9 4.03 4.03 85.9 0.549 Adapt 201 11.894 0.00 0.00 11.894 0.00

EXAMPLE V

[0109] In this example the biogas of Example II is upgraded according to a process as exemplified in FIG. 3 at the conditions used in Example II. Stream properties and compositions as well as utility duties are as shown in Table V, where the reference numbers are those of FIG. 3. The absorption agent is monoethanolamine (MEA) and the absorber of the first absorption step has a column height of 12 m and the absorber of the second absorption step has a column height of 4 m. The upgraded biogas contains 99.47 mole % methane and 40.8 ppm carbon dioxide and 11.0 ppm hydrogen sulphide, which on dry basis is 99.99 mole %, 41.0 ppm and 11.1 ppm respectively. The composition of the upgraded biogas is thus substantially equal to that of Example II.

[0110] The methane slip in the waste off-gas which in this example is 1161 ppm. In example II, the methane slip was 394 ppm. The reduced methane slip in in Example II compared to this example is provided by recycling the second liquid effluent to the absorber of the first absorption step, and the methane not found in the waste-gas of Example II, is found in the upgraded biogas.

[0111] Hence, the configuration of FIG. 2 improves both methane production and reduces methane slip by recycling the second liquid effluent into the first absorption step. This is advantageous in two ways as it increases biomethane production and reduces methane slip, which is an emission subject to increasingly strict regulation and taxes.

[0112] The molar gas-liquid ratio of the first absorber is about 0.07 (7%) and in the second absorption step it is about 0.36 (36%).

[0113] The example above may be further optimized to increase the resulting effect shown here. This is within the skill of the skilled practitioner.

TABLE-US-00005 TABLE V 1.sup.st Lean First gas Pressurized 2.sup.nd Lean Upgraded Waste Name Biogas amine effluent biogas amine biogas Off-gas Reference 101 201 105 107 212 108 114 Stream Properties Temperature C. 35 35 48.70 40 35.27 35.6 50 Pressure bar(a) 1.1 4.5 1.1 11 22 11 1.8 Mass flow kg/h 1261.25 17134 495.5 495.5 2000 427.7 816.9 Stream Composition mole % Methane 56.37 0 88.61 88.61 0 99.47 0.1161 Carbon dioxide 37.58 3.667 0.6625 0.6625 3.667 0.004081 90.76 Hydrogen sulphide 0.9394 0.01652 0.7994 0.7994 0.01652 0.001101 2.267 Water 5.117 85.13 9.932 9.932 85.13 0.5277 6.862 MEA 11.19 0 0 11.19 0.000415 0 Utility Duties kW Compressor C1 Heater H1 750

EXAMPLE VI

[0114] In this example the biogas of Example II is upgraded according to a process as exemplified in FIG. 4 at the conditions used in Example II and V. Stream properties and compositions as well as utility duties are as shown in Table VI, where the reference numbers are those of FIG. 4. The absorption agent is monoethanolamine (MEA) and the absorber of the first absorption step has a column height of 12 m and the absorber of the second absorption step has a column height of 4 m.

[0115] The first liquid effluent is in this example fed to the absorber of the second absorption step. The second liquid effluent is cooled to 35 C. prior to being fed to the absorber of the second absorption step.

[0116] The upgraded biogas contains 99.46 mole % methane and 39.6 ppm carbon dioxide and 10.6 ppm hydrogen sulphide, which on dry basis is 99.99 mole %, ppm and 10.6 ppm respectively, which is equal to that achieved in Example II and Example V.

[0117] As can be seen the flow rate of liquid absorption agent, sum of stream 201 and 212, is 18171 kg/h, which is about 962 kg/h lower than in Example V and 1062 lower than in Example II, which is also reflected in the reboiler duty being 725 kW compared to 750 kW in Examples II and V.

[0118] Hence, this example demonstrates that a configuration as shown in FIG. 4 allows for a reduction amount of absorbing agent used.

[0119] The methane slip in this example is about 8790 ppm, which could be lowered by flashing the high-pressure second effluent and recycling the resulting gas phase into the first absorber, and regenerating the resulting liquid phase in the regeneration system.

[0120] The example above may be further optimized to increase the resulting effect shown here. This is within the skill of the skilled practitioner.

TABLE-US-00006 TABLE VI 1.sup.st Lean First gas Pressurized 2.sup.nd Lean Upgraded Waste Name Biogas amine effluent biogas amine biogas Off-gas Reference 101 201 105 107 212 108 114 Stream Properties Temperature C. 35 35 53.08 40 35.27 35.8 50 Pressure bar(a) 1.1 4.5 1.1 11 22 11 1.8 Mass flow kg/h 1261.25 16171 539.97 539.97 2000 425.1 816.6 Stream Composition mole % Methane 56.37 0 84.21 84.21 0 99.46 0.8790 Carbon dioxide 37.58 3.620 2.61 2.61 3.620 0.003962 90.01 Hydrogen sulphide 0.9394 0.01592 0.7759 0.7759 0.01592 0.001058 2.248 Water 5.117 85.20 12.40 12.40 85.20 0.5390 6.862 MEA 11.17 0 0 11.17 0.000416 0 Utility Duties kW Compressor C1 Heater H1 725

LIST OF REFERENCES

[0121] Reference Name

[0122] 101 Biogas stream

[0123] 105 First gas effluent

[0124] 107 Pressurized biogas stream

[0125] 108 Second gas effluent

[0126] 112 Dried biogas stream

[0127] 113 Off-gas

[0128] 114 Waste off-gas

[0129] 115 Make-up water or fresh absorption liquid

[0130] 201 First liquid absorption stream

[0131] 202 First liquid effluent

[0132] 206 Condensed liquid

[0133] 212 Second liquid absorption stream

[0134] 214 Second liquid effluent

[0135] 214a Gas part of second liquid effluent

[0136] 214b Liquid part of second liquid effluent

[0137] A1 Absorber of the first absorption step/First absorber

[0138] A2 Absorber of the second absorption step/Second absorber

[0139] C1 Compressor

[0140] H1 Heater (reboiler)

[0141] H2 Heat exchanger

[0142] H3 Heat exchanger

[0143] H4 Heat exchanger

[0144] H5 Heat exchanger

[0145] R Regeneration system

[0146] R1 Regeneration unit

[0147] P1 Pump

[0148] P2 Pump

[0149] P3 Pump

[0150] P4 Pump

[0151] U Upgrade system