WO2015105438A1

WO2015105438A1 - Apparatus comprising a membrane unit and a water scrubber unit for removing carbon dioxide from a gas

Info

Publication number: WO2015105438A1
Application number: PCT/SE2014/000149
Authority: WO
Inventors: Tobias Persson; Gunnar BENJAMINSSON; Johan BENJAMINSSON
Original assignee: Gasefuels Ab
Priority date: 2014-01-13
Filing date: 2014-12-16
Publication date: 2015-07-16
Also published as: SE538348C2; SE1400012A1; EP3094398A4; EP3094398A1

Abstract

Apparatus, where a water scrubber unit (20), mainly consisting of an absorption column (21), flash column (22) and desorption column (23) is integrated with a membrane unit (10), for purification/separation/enrichment of a gas or biogas containing significant amount of methane and carbon dioxide, wherein the carbon dioxide concentration of the gas is to be reduced, where an incoming gas stream (1) is merged with flash gas stream (24) from flash column (22) and pressurized in compressor (30) and then led into a membrane unit (10) for separation of carbon dioxide in a membrane (11), if necessary separation of water or heating in separation unit for water (12), separation of hydrogen sulfide in the separation unit for hydrogen sulfide (13) and separation of particles and other contaminants in the separation unit for particles and other contaminants (14), whereby a residue gas stream (3) departs and the purified gas stream (4) is led to water scrubber unit (20), wherein the energy for cooling and pumping of the recirculating water flow through the columns (21, 22 and 23) drops, alternatively, that the capacity for the production of upgraded biogas (5) increases.

Description

APPARATUS COMPRISING A MEMBRANE UNIT AND A WATER SCRUBBER

UNIT FOR REMOVING CARBON DIOXIDE FROM A GAS

Field of the Invention

The invention relates to a process for upgrading of biogas and other methane containing gas mixtures to a methane-rich gas. Upgrading processes are executed in different types of upgrading plants in order to remove carbon dioxide, hydrogen sulfide, water and particles from biogas and other methane containing gas mixtures. A gas with an enriched content of methane remains after upgrading of biogas or other methane containing gas mixtures.The methane-rich gas may be used as a vehicle fuel or for injection into a natural gas grid. Common upgrading technologies used today are water scrubbing technology, PSA, amine scrubbing and membrane technology.

Object of the Invention

The object of the invention is an upgrading process with water scrubber technology that is integrated with a process that mainly removes carbon dioxide and, if necessary, also hydrogen sulfide, particles and water. In the integrated process, membrane technology is used to separate the bulk of the carbon dioxide and in some applications also hydrogen sulfide and water.

The production capacity of upgraded gas from an upgrading plant using the water scrubber technology may increase with the invention. The invention also significantly decreases the specific energy consumption per produced unit volume upgraded gas. It is also possible to obtain a carbon dioxide-rich gas from the membranes.

Another objective of the invention is to enable a lower pressure in the flash column without decreased capacity of the water scrubber. A lower flash pressure reduces the methane loss and the invention allows the methane loss to be reduced without a significant increase of the specific energy consumption per cubic meter upgraded gas. The invention may thus be an alternative to other treatment methods for methane in off-gas streams in countries where the methane loss requirement is that the loss has to be lower than around 0.5 vol%. One example of a treatment method of off-gas streams that the invention may replace is RTO.

Background to the Invention

The purpose of biogas upgrading is to obtain a methane-rich product gas by separating mainly carbon dioxide, water and hydrogen sulfide from the biogas. Biogas is today mainly upgraded by PSA, water scrubber, amine scrubber and membrane tehnology. The marker share of the technologies are PSA (23%), water scrubber (40%), amine scrubber (22%) and membrane technology (8%). Other upgrading technologies are genosorb scrubber and cryogenic technology (Biogas upgrading - technology overview, comparison and perspectives for the future, Bauer et. al, DOI: 10.1002/bbb.l423; Biofuels, Bioprod. Bioref. 7:499-511 (2013)).

PSA is an upgrading technology where biogas is compressed and purified from hydrogen sulfide and water before carbon dioxide is removed in columns containing a material that, under pressure, adsorbs carbon dioxide but not methane. Methane can therefore pass through the columns while the carbon dioxide is trapped. The adsorption material is then regenerated by depressurization wherein the carbon dioxide is discharged from the column.

Biogas is usually compressed to 6-10 bar with the water scrubber technology and led to an absorption column in which carbon dioxide and hydrogen sulfide is dissolved in water while methane can be extracted from the top of the absorption column and then dried. The water is regenerated by a first pressure reduction in a flash column from which a part of carbon dioxide and methane dissolved in the gas goes back to the incoming biogas flow. Then the water is led to a desorption column where mainly carbon dioxide and hydrogen sulfide is discharged from the water with air. When upgrading with an amine scrubber carbon dioxide is separated from biogas by a chemical reaction between carbon dioxide and for example activated amines, so-called aMDEA, in an absorption column. Methane does not react with the amine and can be led out from the top of the absorption column. The amine is then regenerated by heating the amine in a desorption column.

A membrane is a filter that can separate various gas molecules because the molecules have different diffusion velocity and solubility in the membrane. Membranes for biogas upgrading often have high permeability of carbon dioxide, water and hydrogen sulfide but not for methane. An upgrading plant with membrane technology usually first purifies the gas from water and contaminants such as hydrogen sulfide before the gas is compressed, typically 5 - 20 bar, and then led into membrane modules.

The membrane technology represents an increasing proportion of upgrading plants (Biogas upgrading - technology overview, comparison and perspectives for the future, Bauer et al., DOI: 10.1002 / bbb.1423; Biofuels, Bioprod. Bioref. (2013)). The membrane technology is under development and the patents US2012111052A1, JP2005023211 (A) and US20130098242A1 describe gas separation processes by membrane technology. Lie et al also have the patent US20110072965A1 which describes how a particular type of membrane can be produced that is suitable for separating gases.

Today, there are examples of how upgrading techniques can be combined. Baker (Membrane Technology and Applications, ISBN 0-470-85445-6) describes how membrane technology and amine scrubber can be combined where carbon dioxide is pre-separated with a membrane module before the last carbon dioxide is separated by an amine scrubber. One advantage is that the amine scrubber can be built at lower cost, while the disadvantage is that the complexity of the plant increases. According to Bhide (Hybrid processes for the removal of acid gases from natural gas, Journal of MEMBRANE SCIENCE 1997), however, there is a higher cost of combining membrane technology with an amine scrubber in comparison with purification of gas with different membrane combinations. The patent US005407466A relates to an invention of how two different types of membranes can be combined and integrated with an amine scrubber or other technology with physical absorption in a genosorb process and thereby obtain an improved process for gas treatment.

Membranes can also be combined with the PSA technology. The patent ES2411332 (T3) describes how landfill gas can be upgraded by combining membrane technology and PSA. Carbon dioxide is first separated by membranes and then nitrogen is separated with PSA technology. Another example is patent US002332424A relating to an invention of how membranes can be integrated with a PSA in order to increase the methane yield and purity of the gases.

The report "Biogas to biomethane technology review" (Technische Universitat Wien, IEE / 10/130, May 2012) describes that membrane technology has lower capital cost than water scrubber technology and also that the total upgrading cost is usually lower for membrane technology than the water scrubber technology. It also shows that the electricity consumption is 0.46 kWhel/m³ biomethane for water scrubber technology and 0.25 to 0.43 kWhel/m³ biomethane for membrane technology.

Advantages of the Invention

Starting with a water scrubber plant, the invention shows how membrane technology can be integrated in a way so that methane losses are reduced, the capacity increases and the energy consumption is reduced. Inventions in this field have instead focused on integrating membrane technology with other upgrading plants in order to complement the membrane properties. The patent US002332424A is an invention of how membranes can be integrated with a PSA to increase the methane yield and purity of the gases. The patent US005407466A is an invention of how two different types of membranes can be combined with an amine scrubber or another technique of physical absorption in a genosorb process in order to obtain an improved process for gas treatment. However, this patent does not integrate off-gas streams from the amine scrubber or genosorb scrubber with membrane technology in a way so that the process methane losses and energy use decreases and capacity increases.

By separating carbon dioxide from the input biogas flow and/or from the flash gas using membrane technology, the specific energy consumption of the water scrubber per unit volume of upgraded gas may decrease.

The production capacity of upgraded gas of a water scrubber plant increases with the invention since its capacity primarily depends on the total incoming flow of gas to the absorption column. An integrated process that raises the methane content of the biogas before the gas is upgraded in the water scrubber, or reduces the internal circulation of gas, thus leads to a higher capacity of the water scrubber. In cases where the compressor of the water scrubber limits the capacity of the plant, it can be supplemented with additional compressor capacity.

The invention allows the pressure in the flash column to be decreased without significant increased energy consumption or decreased capacity of the water scrubber. This creates the possibility of reaching lower methane losses than previously applicable. Methane losses from a water scrubber with the integrated invention may be less than 0.5 vol%. In markets where a methane loss of 0.5 vol% is sufficient, the invention provides a saving to investing in treatment facilities for residual gas such as RTO. A reduced methane loss also gives a higher production of upgraded gas and thus higher revenues.

The process design of the water scrubber does not allow, unlike the amine scrubber and PSA, a separate gas stream of high carbon dioxide content to be obtained. With the invention, a separate carbon dioxide-rich gas stream may be obtained which for example can be used for catalytic or biological methanation.

Membrane technology can be integrated with existing water scrubbers and thereby increase the production capacity of upgraded gas of the water scrubber. Advantages of the membrane technology with relatively low investment cost and specific energy consumption can then be applied to a portion of the gas flow, while the total capacity will be higher and a significant part of the equipment at the existing plant can be used. The investment to complement an existing water scrubber plant with membrane technology and related equipment, as well as additional compressor capacity in case that the existing compressor capacity cannot handle an increasing gas flow, is expected to be lower than investing in a brand new upgrading plant to increase the biogas upgrading capacity. The invention also provides the advantage that a purchaser of a water scrubber plant has a good alternative to increase the capacity in the future.

The invention can also be used in the construction of a new upgrading plant. The integration between membrane technology and a water scrubber plant enables lower methane losses, higher capacity, lower energy consumption and utilization of carbon dioxide in comparison with a conventional water scrubber plant.

Brief Description of the Drawings

Examples of embodiment of the invention will be described with reference to the accompanying drawings. Figure 1 schematically shows a water scrubber that is integrated with a process that mainly separates carbon dioxide, hydrogen sulfide, particles and water, where membrane technology is used to separate the bulk of the carbon dioxide. Gas streams coming to the membrane unit can be biogas and flash gas from the flash column of the water scrubber. Figure 2 shows an application where the invention is used for separation of gases such as carbon dioxide from the flash gas without prior pressure increase of the flash gas. Figure 3 shows an application where a water scrubber is integrated with processes which separate gases such as carbon dioxide from the flash gas, from the incoming gas stream to the plant and from the outgoing gas stream of the process separating gases such as carbon dioxide from the flash gas.

Detailed Description of the Invention

Figure 1 schematically shows a plant for biogas upgrading where a water scrubber unit 20, mainly consisting of absorption column 21, flash column 22 and desorption column 23, is integrated with the membrane unit 10 for the separation of gases such as carbon dioxide with membrane module 11 and, when necessary, separation unit for water 12, separation unit for hydrogen sulfide 13 and separation unit for particles and other contaminants 14. An alternative configuration for separating water in the separation unit 12 is instead to heat the gas with a heat exchanger.

Gas stream 1 in Figure 1 is merged with gas stream 24 and is led into a compressor 30 to raise the pressure of the gas stream. Gas stream 2 has after compressor 30 a pressure of 2-18 bar, essentially 4-16 bar, preferably 8-12 bar and contains mainly methane, carbon dioxide, water, hydrogen sulfide and particles. The carbon dioxide concentration in the incoming gas stream 1 is typically between 20-60 vol%, essentially 30-50 vol%, preferably 40-50 vol%. The methane content of the gas stream 1 is typically between 40-80 vol%, essentially 50-70 vol%, preferably 50-60 vol%.

For the process configuration of Figure 1, gas stream 2 is led into the membrane unit 10 consisting of the separation unit for water 12, separation unit for hydrogen sulfide 13, separation unit for particles and other contaminants 14 and membrane module for separation of carbon dioxide 11. The order of the steps incorporated in the membrane unit 10 may vary. The units 12, 13 and 14 are established and used to the extent that the membrane configuration and the composition of the biogas requires. The units 12, 13 and 14 are placed after the compressor, but one or more of them can also be placed before the compressor.

After passing the different steps of purification/separation in membrane unit 10 in Figure 1, the gas goes to the membrane 11 for separation of gases such as carbon dioxide, hydrogen sulfide and water. The membranes may be of the type polymeric hollow fiber membranes, carbon membranes, or other types of membranes and different types of membranes may be combined with each other as well as only one type is used. The membranes may be arranged in series or parallel or a combination of both.

From membrane 11 in Figure 1, a residue gas stream 3 which mainly contains carbon dioxide and small amounts of methane leaves. The residue gas stream may be burned in a boiler, be destroyed by thermal oxidation or otherwise handled.

According to the process configuration of Figure 1, gas stream 4 leaves the membrane 11 to the water scrubbing unit 20 with a higher methane concentration compared to the gas stream 2 that enters the membrane unit 10. The higher methane concentration in gas stream 4 lead to an increase in capacity of the absorber column 21 since less carbon dioxide requires to be separated in the absorption column 21, compared to if the methane concentration was the same in gas stream 4 as gas stream 2. As it is the total incoming flow of gas to the absorption column that affect the capacity of the absorption column 21, more upgraded gas is produced since the gas stream 4 has a higher methane content than the biogas stream 1 . Also the energy consumption in the water scrubber unit 20 can be reduced due to less need for cooling, and due to the fact that the water amount required for separating the carbon dioxide in the gas stream 4 is less than the amount of water needed to separate the carbon dioxide in gas stream 2.

Under typical conditions with 50 vol% carbon dioxide in gas stream 1 the capacity for the production of upgraded methane-rich gas in the absorption column 21 ca be increased by 40-70% and energy consumption for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 be reduced by 10-20% with a process configuration as in Figure 1. Under typical conditions with 40 vol% carbon dioxide in gas stream 1 the capacity for the production of upgraded methane-rich gas in the absorption column 21 ca be increased by 30-40% and energy consumption for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 be reduced by 5-15% with a process configuration as in Figure 1.

Under typical conditions with 35 vol% carbon dioxide in gas stream 1 the capacity for the production of upgraded methane-rich gas in the absorption column 21 can be increased by 20-30% and energy consumption for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 be reduced by 5-10% with a process configuration as in Figure 1.

In the process configuration of Figure 1 the gas stream from the flash column 22 is returned to gas stream 1 according to gas stream 24. The invention allows that the pressure in flash column 22 can be lowered without that the capacity for production of upgraded biogas 5 of the absorption column 21 is reduced or that energy use for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 increases significantly. This makes it possible to get down to total methane losses from the upgrade installation of below 0.5 vol%, which can provide increased revenues for gas sales and reduced costs for disposal of residue stream 6.

From the absorption column 21 in Figure 1 a gas stream 5 leaves which has a methane content of >95 vol%. From the desorption column 23 a residue stream 6 leaves containing mainly carbon dioxide and air.

Figure 2 schematically shows a plant for biogas upgrading where a water scrubber installation 20, mainly consisting of an absorption column 21, flash column 22 and desorption column 23 is integrated with the membrane unit 10 for the separation of gases such as carbon dioxide with membrane module 11 and, when necessary, separation unit for water 12, separation unit for hydrogen sulfide 13 and separation unit for particles and other contaminants 14. An alternative configuration for separating water in separation unit 12 is instead to heat the gas with a heat exchanger. The order of the steps incorporated in membrane unit 10 may vary. The units 12, 13 and 14 are established and used to the extent that the membrane configuration and composition of biogas requires.

In Figure 2 gas streams 1 and 7 are merged and led into a compressor 30 to raise the pressure. Gas stream 2 has after compressor 30 a pressure of 2-18 bar, essentially 4-16 bar, preferably 8-12 bar and contains methane, carbon dioxide, water, hydrogen sulfide and particles. The carbon dioxide concentration in the incoming gas stream 1 is typically between 20-60 vol%, essentially 30-50 vol%, preferably 40-50 vol%. The methane content of gas stream 1 is typically between 40- 80 vol%, essentially 50-70 vol%, preferably 50-60 vol%.

For the process configuration according to Figure 2 gas stream 2 is led into the water scrubber unit 20. The water scrubber unit is integrated with membrane unit 10 via gas stream 24" which is led from the flash column 22 to the membrane unit 10. From the membrane module 11 a gas stream with high methane content is obtained which either is merged with gas stream 1 before the compressor according to the gas stream 7 or merged with the outgoing gas stream 5 from the absorption column 21 as gas stream 8. The membranes may be of the type polymeric hollow fiber membranes, carbon membranes, or other types of membranes and different types of membranes may be combined with each other as well as only one type is used. The membranes may be arranged in series or parallel or a combination of both. From membrane 11 residue stream 3 which mainly contains carbon dioxide and methane leaves. The residual gas stream can be burned in a boiler, be destroyed by thermal oxidation or otherwise handled.

In a conventional water scrubber set up the flash gas stream is reversed directly to the suction side of the compressor. The invention with the process configuration according to Figure 2 makes gas stream 2 to have lower carbon dioxide content in comparison to a water scrubber set up where the invention is not used. The high methane content in the gas stream 2 leads to an increase of the production capacity of upgraded biogas 5 in the water scrubber unit 20 since less carbon dioxide needs to be separated in the absorption column 21, compared to if the flash gas would have been returned directly to the suction side of the compressor. Also the energy consumption in the water scrubber unit 20 can be reduced due to less need for cooling, and due to the fact that the water amount required for separating the carbon dioxide in the gas stream 2 is less than the amount of water needed to separate the carbon dioxide in the flash gas if it would be reversed to the suction side of the compressor.

Under typical conditions with 80-90 vol% carbon dioxide in the flash gas the capacity of the water scrubber can be increased by about 10-20% and the energy consumption reduced by 10-20% with the process configuration of Figure 2.

In the process configuration as shown in Figure 2 the gas stream from the flash column 22 is led to the membrane unit 10 according to flash gas stream 24". The invention allows that the pressure in the flash column 22 can be lowered without that the capacity of absorption column 21 is reduced or that energy use for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 increases significantly. This creates the opportunity to get down to a total methane loss from the biogas upgrading plant described below 0.5 vol%, which can provide increased revenues for gas sales and reduced costs for disposal of residue stream 6.

Figure 3 schematically shows a plant for biogas upgrading where a water scrubber unit 20 is integrated with the membrane units 10a and 10b. Water scrubber unit 20 consists mainly of an absorption column 21, flash column 22 and desorption column 23. Membrane unit 10a consists of membrane module 11a for the separation of gases such as carbon dioxide and, when necessary, separation unit for water 12a, separation unit for hydrogen sulfide 13a and separation unit for particles and other contaminants 14a. An alternative configuration for separating water in the separation unit 12a is instead to heat the gas with a heat exchanger. Membrane unit 10b consists of membrane module lib for the separation of gases such as carbon dioxide and, when necessary, separation unit for water 12b, separation unit for hydrogen sulfide 13b and separation unit for particles and other contaminants 14b. An alternative configuration for separating water in the separation unit 12b is instead to heat the gas with a heat exchanger.

In Figure 3 gas stream 1 is merged with the gas stream 7 and is led into a compressor 30 to raise the pressure. Gas stream 2 has before compressor 30 a pressure of 2-18 bar, essentially 4-16 bar, preferably 8-12 bar and contains methane, carbon dioxide, water, hydrogen sulfide and particles. The carbon dioxide concentration in gas stream 1 is typically between 20-60 vol%, essentially 30- 50 vol%, preferably 40-50 vol%. The methane content in gas stream 1 is between 40-80 vol%, essentially 50-70 vol%, preferably 50-60 vol%.

For the process configuration of Figure 3 gas stream 2 is led into the membrane unit 10a consisting of the separation unit for water 12a, the separation unit for hydrogen sulfide 13a, separation unit for particles and other contaminants 14a and membranes for separating gases such as carbon dioxide 11a. The order of the steps incorporated in the membrane unit 10a may vary. The units 12a, 13a and 14a are established and used to the extent that the membrane configuration and composition of the biogas requires. The units 12a, 13a and 14a are in Figure 3 placed after the compressor 30, but one or more of them can also be placed before the compressor 30.

After the different steps of purification/separation in the membrane unit 10a in Figure 3, a gas stream goes into the membrane 11a for the separation of gases such as carbon dioxide, hydrogen sulfide and water. The membranes may be of the type polymeric hollow fiber membranes, carbon membranes, or other types of membranes and different types of membranes may be combined with each other as well as only one type is used. The membranes may be arranged in series or parallel or a combination of both. From membrane 11a in Figure 3, a residue stream 3a which mainly contains carbon dioxide and minor amounts of methane leaves. The residual gas stream can be burned in a boiler, be destroyed by thermal oxidation or otherwise handled.

For the process configuration of Figure 3 gas stream 4 is led into the water scrubber unit 20. The water scrubber unit is integrated with membrane unit 10b via gas stream 24" which is led from the flash column 22 to the membrane unit 10b. From the membrane module lib a gas stream with high methane content is obtained that either is merged with gas stream 1 on the suction side of the compressor according to gas stream 7 or merged with the outgoing gas stream 5 from the absorber column 21 as gas stream 8. The membranes may be of the type polymeric hollow fiber membranes, carbon membranes, or other types of membranes and different types of membranes may be combined with each other as well as only one type is used. The membranes may be arranged in series or parallel or a combination of both. From membrane lib in Figure 3, a residue stream 3b which mainly contains carbon dioxide and minor amounts of methane leaves. The residual gas stream can be burned in a boiler, be destroyed by thermal oxidation or otherwise handled.

The invention with the process configuration according to Figure 3 makes the gas stream 4 to have lower carbon dioxide content in comparison to a water scrubber set up where the membrane unit 10a and 10b are not used. The high methane content in the gas stream 4 leads to an increase of the production capacity of upgraded biogas in the water scrubber unit 20 since less carbon dioxide needs to be separated in the absorption column 21. Since it is the total incoming flow of gas to the absorption column that affects the capacity of the water scrubber 20, more upgraded gas is produced. Also energy consumption can be reduced as a result of reduced need for cooling in the water scrubber unit 20, and due to that the water required to separate the carbon dioxide in the gas stream 4 is less than the amount of water that would be needed if membrane unit 10a and 10b had not been integrated with the water scrubber plant.

Under typical conditions with 50 vol% carbon dioxide in gas stream 1 the capacity for the production of upgraded methane-rich gas in the absorption column 21 can be increased by 40-80% and the energy consumption for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 be reduced by 10-30% with a process configuration as in Figure 3.

Under typical conditions with 40 vol% carbon dioxide in gas stream 1 the capacity for the production of upgraded methane-rich gas in the absorption column 21 can be increased by 30-50% and the energy consumption for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 be reduced by 5-25% with a process configuration as in Figure 3.

Under typical conditions with 35 vol% carbon dioxide in gas stream 1 the capacity for the production of upgraded methane-rich gas in the absorption column 21 can be increased by 20-40% and the energy consumption for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 be reduced by 5-15% with a process configuration as in Figure 3.

In the process configuration as shown in Figure 3 the gas stream from the flash column 22 is led to the membrane unit 10b according to flash gas stream 24". The invention allows that the pressure in the flash column 22 can be lowered without that the capacity of absorption column 21 is reduced or that energy use for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 increases significantly. This creates the opportunity to get down to a total methane loss from the biogas upgrading plant described below 0.5 vol%, which can provide increased revenues for gas sales and reduced costs for disposal of residue stream 6.

From the absorption column 21 in Figure 3 gas stream 5 which has a methane content of 95 vol% leaves. From the desorption column 23 residue stream 6 containing mainly carbon dioxide and air leaves.

Claims

1. An apparatus, where a water scrubber unit 20, mainly consisting of an absorption column 21, flash column 22 and desorption column 23 is integrated with a membrane unit 10, for purification/separation/enrichment of a gas or biogas containing significant amount of methane and carbon dioxide, wherein the carbon dioxide concentration of the gas is to be reduced, which is characterized by that incoming gas stream 1 is merged with flash gas stream 24 from flash column 22 and pressurized in compressor 30 and then led into a membrane unit 10 for separation of carbon dioxide in a membrane 11, if necessary separation of water or heating in separation unit for water 12, separation of hydrogen sulfide in the separation unit for hydrogen sulfide 13 and separation of particles and other contaminants in the separation unit for particles and other contaminants 14, whereby a residue gas stream 3 departs and the purified gas stream 4 is led to water scrubber unit

20, wherein the energy for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 drops, alternatively, that the capacity for the production of upgraded biogas 5 increases.

2. An apparatus according to the claims in 1, which is characterized by that the pressure in the flash column 22 can be lowered without significant reduction of the production capacity of upgraded biogas 5 in the absorption column 21 or that the energy use for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 significantly increases, whereby the methane losses decrease.

3. An apparatus according to the claims in 1 or 2, which is characterized by that the

apparatus is supplemented by additional compressor capacity.

4. An apparatus, where a water scrubber unit 20, mainly consisting of an absorption column

21, flash column 22 and desorption column 23 is integrated with a membrane unit 10, for purification/separation/enrichment of a gas or biogas containing significant amount of methane and carbon dioxide, wherein the carbon dioxide concentration of the gas is to be reduced, which is characterized by that flash gas stream 24" from the flash column 22 in water scrubber unit 20 is led to a membrane unit 10 for separation of carbon dioxide in a membrane 11, if necessary separation of water or heating in separation unit for water 12, separation of hydrogen sulfide in the separation unit for hydrogen sulfide 13 and separation of particles and other contaminants in the separation unit for particles and other contaminants 14, whereby a residue gas stream 3 departs and the methane-rich gas stream 7 is led to the suction side of the compressor, whereby the energy use for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 decreases, alternatively, that the production capacity of upgraded gas 5 increases.

5. An apparatus according to the claims in 4, which is characterized by that the pressure in the flash column 22 can be lowered without significant reduction of the production capacity of upgraded biogas 5 in the absorption column 21 or that the energy use for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 significantly increases, whereby the methane losses decrease.

6. An apparatus according to the claims in 4 or 5, which is characterized by that the plant is supplemented by additional compressor capacity.

7. An apparatus, where a water scrubber unit 20, mainly consisting of an absorption column 21, flash column 22 and desorption column 23 is integrated with a membrane unit 10, for purification/separation/enrichment of a gas or biogas containing significant amount of methane and carbon dioxide, wherein the carbon dioxide concentration of the gas is to be reduced, which is characterized by that flash gas stream 24" from the flash column 22 in water scrubber unit 20 is led to a membrane unit 10b for separation of carbon dioxide in a membrane lib, if necessary separation of water or heating in separation unit for water 12b, separation of hydrogen sulfide in the separation unit for hydrogen sulfide 13b and separation of particles and other contaminants in the separation unit for particles and other contaminants 14b, whereby a residue gas stream 3b departs and the methane-rich gas stream 7 is merged with gas stream 1 and pressurized in compressor 30 and then led into a membrane unit 10a for separation of carbon dioxide in a membrane 11a, if necessary separation of water or heating in separation unit for water 12a, separation of hydrogen sulfide in the separation unit for hydrogen sulfide 13a and separation of particles and other contaminants in the separation unit for particles and other contaminants 14a, whereby a residue gas stream 3a departs and the purified gas stream 4 is led to water scrubber unit 20, wherein the energy for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 drops, alternatively, that the capacity for the production of upgraded biogas 5 increases.

8. An apparatus according to the claims in 7, which is characterized by that the pressure in the flash column 22 can be lowered without significant reduction of the production capacity of upgraded biogas 5 in the absorption column 21 or that the energy use for cooling and pumping of the recirculating water flow through the columns 21, 22 and 23 significantly increases, whereby the methane losses decrease.

9. An apparatus according to the claims in 7 or 8, which is characterized by that the plant is supplemented by additional compressor capacity.