FR3114516A1 - Biogas treatment process – associated facility - Google Patents

Abstract

The present invention relates to a method for treating biogas according to which said biogas is passed in the gaseous state through a first membrane stage comprising at least a first separating membrane, the permeability to methane of said first separating membrane being greater than its permeability carbon dioxide, the temperature and pressure conditions of said biogas allowing the passage of methane downstream of said membrane. Characteristically, according to the invention, the gaseous mixture enriched in carbon dioxide which accumulates upstream of said membrane is recovered, said gaseous mixture enriched in carbon dioxide is distilled under temperature and pressure conditions making it possible to obtaining liquid carbon dioxide and recovering the liquid carbon dioxide. The present invention relates to an associated installation. FIG. 1

Description

Biogas treatment process – associated facility

Technical field of the invention

The present invention relates to a method for treating biogas originating, for example, from an anaerobic digester and an installation allowing the implementation of this method.

State of the art

Document GB 2 247 518 describes a method for starting a cold distillation column of a mixture of gases containing methane and acid gases including carbon dioxide.

The publication entitled “processing of carbon dixide rich gas” by A. J. Finn et al and published in the journal Engineering tomorrow … today following the conference of September 14, 2014 reviews the processes for the separation of natural gases rich in carbon dioxide. This publication indicates that cryogenic fractionation makes it possible to produce pure carbon dioxide in the form of a pressurized liquid in an economically profitable way.

Document US 2013/0298598 A1 describes a method of condensing a gas rich in carbon dioxide using a second fluid which may be ammonia. According to this method, the second fluid is in liquid/vapor equilibrium, heat is then exchanged between the liquid phase of the second fluid and the gas rich in carbon dioxide so as to vaporize the second fluid and to mix it with the gaseous fraction outside the phase separator.

Document WO2019/238488 A1 describes a method for separating gaseous carbon dioxide or a mixture of water vapor and carbon dioxide from a gaseous mixture chosen from air and biogas. This process uses an adsorbent material and recovers the latent heat from the separated carbon dioxide.

Document FR 17 62860 A1 describes a fractional distillation process which makes it possible to supply methane without risk linked to oxygen at a methane concentration greater than 95%, a carbon dioxide concentration less than 2.5% and a methane yield greater than 85%. This document indicates that the carbon dioxide is first separated from the biogas by passage over one or more membranes. The gas then contains between 0.5% mol and 1.5% mol of carbon dioxide. Without oversizing the compressor that allows the mixture to pass through the membrane, the carbon dioxide content cannot be reduced below the values indicated. The gas obtained after passing over the membranes is then cooled so as to solidify the carbon dioxide it contains. When the carbon dioxide content is very low, for example 50ppmv, the carbon dioxide does not solidify and remains soluble in methane. An adsorbent material is then used to extract the remaining carbon dioxide. The carbon dioxide is then desorbed using a stream of nitrogen or the methane produced. In the case of biogas, the latter contains oxygen which is only partially separated on the membranes and whose concentration increases or remains constant. Oxygen tends to accumulate in an area of the distillation column creating risks of explosion. The aforementioned document proposes to replace the step of adsorption of carbon dioxide by the introduction of the gas obtained after passing through the membrane at the level of plate n-4, plate n being the last plate of the column, when the oxygen concentration of the mixture is greater than 0.1% mol.

US 4,681,612 B describes a process for treating biogas generated by solid waste landfills. In this process, a cryogenic separation is carried out in order to separate the carbon dioxide and the impurities from the methane. The methane recovered at the top of the column in the gaseous state is then passed over membranes to extract the residual carbon dioxide.

Document US 4,936,887 B describes a process similar to the preceding document but applied to natural gas which contains methane, ethane and other higher molecular weight hydrocarbons.

Document US 2010/0077796 A1 describes a hybrid process combining membranes and cryogenic distillation applied to a gas mixture containing mainly methane and nitrogen as a minority species.

Document WO 2008/092603 A1 describes a process for treating biogas in which the biogas is scrubbed using a solution containing an amine to remove carbon dioxide and sulfur compounds contained in the biogas. The resulting methane-enriched mixture is then liquefied and separated from the remaining carbon dioxide, oxygen and/or nitrogen.

Document US 4,639,257 B describes two methods for recovering carbon dioxide from either an effluent rich in carbon dioxide or an effluent poor in carbon dioxide. An effluent low in carbon dioxide contains at most the concentration of carbon dioxide at equilibrium at the freezing temperature of the effluent, which depends on its composition, in particular. In the illustrative example of this document, the effluent contains 12% by mole of carbon dioxide. The carbon dioxide-rich off-gas is the light fraction of natural gas; it may contain at least 40% by volume of carbon dioxide. The illustrative example given is a gas mixture containing mainly carbon dioxide, methane and ethane. According to this process, the gaseous mixture to be treated and previously purified of any constituent likely to solidify during the process is compressed; the gas is then cold distilled so as to collect the carbon dioxide in the liquid state at the bottom of the distillation column. The vapor at the head of the column is heated in order to be compressed and passed through a selective membrane which makes it possible to recover the carbon dioxide contained in the vapor. This carbon dioxide recovered from the gas phase is then reinjected into the gas mixture to be distilled.

Technical problem to solve

An object of the present invention is to provide a biogas treatment process which allows the recovery of the methane it contains and avoids the release of the carbon dioxide it contains into the environment.

Another object of the present invention is to provide a biogas treatment process which allows energy savings in terms of pressure, temperature and heat exchange.

Brief description of the invention

The present invention relates to a process for treating biogas according to which said biogas is passed in the gaseous state through a first membrane stage comprising at least a first separating membrane, the permeability to methane of said first separating membrane being greater than its permeability carbon dioxide, the temperature and pressure conditions of said biogas allowing the passage of methane downstream of said membrane. According to the invention, typically, the gaseous mixture enriched in carbon dioxide which accumulates upstream of said membrane is recovered, said gaseous mixture enriched in carbon dioxide is distilled under temperature and pressure conditions making it possible to obtaining liquid carbon dioxide and recovering the liquid carbon dioxide. The carbon dioxide is then advantageously stored or injected into a gas network.

The distillation of the mixture enriched in carbon dioxide makes it possible to liquefy the carbon dioxide and to evacuate the methane which it contains in the form of gas. The methane thus separated is thus also recovered either for direct use, for example, in a boiler or a turbine, or by mixing with the gaseous mixture enriched in methane which is recovered downstream of the membrane, or by reinjection into the digester which produces biogas.

The distillation is carried out under conditions which do not generate the formation of solid carbon dioxide.

The membrane or membranes of the first membrane stage and of the second stage when there is a second stage and the batteries of separating membranes are advantageously permeable to oxygen. Their permeability to oxygen is such that the gas mixture enriched in carbon dioxide contains less than 0.1% by mole of oxygen. The biogas is advantageously conditioned before passing over the membranes. In particular, volatile organic compounds, hydrogen sulphide and water contained in the biogas at the digester outlet are removed.

It is obvious that in the process of the invention, the mixture enriched in methane which is collected downstream of the first membrane can be upgraded by injection into a gas network or storage as in any known biogas treatment process which has intended to separate the methane from the other constituents of the biogas. Use within a private network or as fuel is also possible.

Distillation is carried out on a liquid gas mixture containing mainly liquid carbon dioxide in which bubbles of methane and possibly hydrogen sulphide are present. Distillation allows the separation of carbon dioxide and its recovery as a liquid.

Advantageously, the liquid/gas mixture to be distilled does not contain hydrogen sulphide.

Advantageously, the liquid/gas mixture to be distilled contains at least 80%, in particular at least 85% and more particularly at least 90% by mole of carbon dioxide, the remainder being methane.

According to a particular embodiment, the liquid/gas mixture to be distilled contains at least 97% in moles of carbon dioxide and more particularly at least 99%, or even 99.5% in moles of carbon dioxide.

According to a particular mode of implementation, the mixture enriched in carbon dioxide which accumulates upstream of said first membrane is gaseous and contains at least 80%, in particular at least 85% and more particularly at least 90% by mole of carbon dioxide, in particular at least 92% by mole of carbon dioxide and more particularly at least 94% by mole of carbon dioxide and more particularly 94.54% of carbon dioxide. This mixture also contains methane. It may contain traces of water which will be separated before distillation.

According to one mode of implementation, which can be combined with the other modes of implementation, the gaseous mixture enriched in methane which is formed by passing through said first separating membrane contains water, and for example at least 94% by mole methane, nitrogen, oxygen (from 0% or 0.5% to 3% by moles, 3% inclusive) and hydrogen sulphide.

According to a particular mode of implementation that can be combined with each of the other aforementioned modes of implementation -in particular those with reference to the composition of the mixture to be distilled- during the distillation, said mixture of liquid carbon dioxide to be distilled has a temperature equal or greater than -5°C and in particular of the order of -1.01°C.

According to a particular embodiment, the liquid carbon dioxide/methane gas mixture to be distilled contains at least 80%, in particular at least 85% and more particularly at least 90% by mole of carbon dioxide, the remainder being methane and/or said liquid carbon dioxide/methane gas mixture to be distilled is at a temperature equal to or greater than -5° C. and in particular of the order of -1.01° C. during the distillation.

According to a particular mode of implementation of the distillation, which can be combined with each of the modes of implementation of the process according to the invention, the distillation is implemented at a negative temperature but higher than -5° C. and in particular order of -2°C or -1°C or -1.01°C or at a positive temperature equal to or greater than 0.5°C, in particular equal to 1°C. The pressure in the column is adjusted to always obtain a liquid/gas balance, the liquid preferably being boiling and therefore subcooled. These temperatures are easy to obtain and require little energy to maintain them. The material used does not have to have specific mechanical characteristics; it is therefore standard and therefore inexpensive. The pressure in the column can be equal to or greater than 16 bars, in particular equal to or greater than 20 bars, more particularly equal to or greater than 30 bars and less than 40 bars and more particularly less than 35 bars for the aforementioned temperature values.

When the temperature in the distillation column is positive, the formation of ice flakes due to the presence of a few residual ppm of water in the mixture to be distilled is avoided. These flakes turn out to be very abrasive and quickly deteriorate the installation, especially the valves.

Advantageously, in the distillation column, the carbon dioxide is subcooled, that is to say it is at a temperature below the vapor/liquid equilibrium temperature at the pressure prevailing in the distillation column. We thus have a liquid carbon dioxide in constant boiling. This sub-cooling is advantageously equal to or greater than 5°C and equal to or less than 15°C and in particular equal to 10°C.

Advantageously, before the distillation, the gaseous mixture enriched in carbon dioxide is compressed; the latter remains gaseous or in liquid/gas equilibrium.

Advantageously, before the distillation step and after the compression step if the latter is implemented, a Joule-Thomson expansion of the gaseous mixture enriched in carbon dioxide is carried out so as to liquefy it. A liquid/gas mixture is obtained, the carbon dioxide being liquid and containing bubbles of methane. Joule-Thomson expansion is implemented in such a way that no solid carbon dioxide is formed. Advantageously, said gas mixture enriched in carbon dioxide is cooled down to room temperature or a temperature below room temperature before carrying out said Joule-Thomson expansion. By cooling the gaseous mixture enriched in carbon dioxide, it facilitates its liquefaction during the Joule-Thomson expansion. Expansion leads to liquefaction. It is thus also possible to adapt the compression of the aforementioned gaseous mixture as precisely as possible according to the cooling needs of the installation.

Advantageously, before distillation, the gaseous mixture is compressed, then it is cooled to room temperature or to a temperature below room temperature and then it is expanded in a Joule-Thomson expansion valve so as to liquefy it. The compression rate of the gas mixture is adjustable according to the desired cold production.

Advantageously, said gaseous mixture enriched in carbon dioxide is cooled down to room temperature or a temperature below room temperature before carrying out the Joule-Thomson expansion.

Advantageously, during Joule-Thomson expansion, the gaseous mixture is subcooled. The subcooling can be, for example, equal to or greater than 1°C and equal to or less than 5°C and in particular equal to 3°C for a flow rate of 250 Nm ³ /h (the normal temperature of for the Nm ³ being 0°C)

In all cases, according to the process of the invention, the gaseous mixture enriched in methane which forms downstream of said first membrane is recovered and, depending on the composition of said gaseous mixture enriched in methane, either it is recycled for the to process again, either it is injected into a distribution network or into a storage tank.

According to a particular mode of implementation, the gaseous mixture enriched in methane obtained is passed downstream of said first separating membrane through a second membrane stage which comprises at least one second separating membrane whose permeability to methane is greater than the permeability to carbon dioxide, the temperature and pressure conditions of said gaseous mixture enriched in methane allowing the passage of methane downstream of said second membrane. A methane of improved purity is thus obtained. The carbon dioxide retained upstream of the second membrane is mixed with the biogas, preferably with the purified biogas at the outlet of the purification unit described below.

Advantageously, the liquid carbon dioxide formed during the distillation is used to produce cold and in particular to cool said biogas upstream of said first membrane stage and/or of said second membrane stage when the process comprises passage over a second membrane stage. It is the merit of the inventors to have found that it was possible to use part of the liquid carbon dioxide present in the boiler of the distillation column to cool the biogas upstream of the first membrane and/or the second membrane possibly, in several places of the installation without using a specifically dedicated cold production.

Each membrane stage can comprise several membranes connected in parallel which make it possible to divide the flow and to treat simultaneously less significant flows.

Advantageously, during distillation, the liquid carbon dioxide is separated by gravity from the gaseous methane it contains. This separation is implemented by introducing the liquid carbon dioxide, containing the methane bubbles, at the top of the column. The liquid flows by gravity and allows the separation of the gaseous methane which is recovered at the top of the column. Liquid carbon dioxide is collected at the bottom of the column.

The present invention also relates to an installation allowing the implementation of the method according to the invention. This installation comprises a source of biogas, this source possibly being a digester, in particular anaerobic, capable of producing biogas, a methane separation unit connected to the output of said digester which comprises at least a first battery of separating membranes which comprises at least a first separating membrane whose permeability to methane is greater than the permeability to carbon dioxide. Characteristically, according to the invention, it comprises a distillation column arranged so as to allow the distillation of the gaseous mixture enriched in carbon dioxide which accumulates upstream of said first membrane.

According to a particular embodiment, it comprises a heat exchanger, advantageously an air cooler mounted upstream of the boiler of said distillation column and/or a valve capable of carrying out a Joule-Thomson expansion, said valve is mounted between said distillation column distill and said boiler, downstream of the boiler on the pipe which connects said heat exchanger to said column when said installation comprises such an exchanger. The air cooler makes it easy to cool the gaseous fluid to ambient temperature without major energy expenditure. Similarly, the valve that allows the Joule-Thomson expansion does not require any energy consumption.

Advantageously, the installation comprises a heat exchanger making it possible to cool the biogas upstream of said first battery of separating membranes with the liquid carbon dioxide present at the bottom of the distillation column. The heat exchanger is, for example, of the thermosiphon type and cools the heat transfer fluid circulating in the installation's cooling system. This heat exchanger is connected to the boiler of the distillation column.

The distillation column is preferably a packed column, in particular a Koch type ring column. A packed column facilitates gas/liquid separation.

Advantageously, the installation of the invention further comprises a conditioning unit connected at the outlet of the digester and upstream of said first battery of separating membranes and which comprises at least one column capable of adsorbing/absorbing hydrogen sulphide and the volatile organic compounds contained in the biogas leaving the digester (purification unit), a dehydration unit and an oil separator.

Furthermore, the installation of the invention can be easily obtained from an existing biogas treatment installation without major modifications to the latter. The distillation column of the installation of the invention can be quickly and inexpensively integrated into an existing biogas treatment installation which makes it possible to extract methane from the latter.

Definitions

The term “biogas” defines, within the meaning of the present invention, the gaseous mixture resulting from a digester in which organic waste decomposes in the absence of air (anaerobic digestion). Raw biogas, within the meaning of the invention, mainly contains methane and carbon dioxide. It also contains water vapor, hydrogen sulphide, nitrogen, oxygen and potentially pollutants such as siloxanes, organochlorine compounds, mercaptans, nitrogen compounds including ammonia, other volatile organic compounds and metals. The digestion reaction being anaerobic, the raw biogas contains very little oxygen and nitrogen. Within the meaning of the invention, the raw biogas is free of hydrocarbons other than methane; in particular, it is free of other light hydrocarbons; most notably, it is free of ethane and propane.

The biogas within the meaning of the invention contains, for example, at least 50% by mole of methane, in particular at least 60% by mole of methane, at least 30% by mole of carbon dioxide. Oxygen represents, for example, less than 2 mol%. Nitrogen can represent from 0 to 6% in moles to the detriment of methane. Water represents, for example, less than 6% by mole and hydrogen sulphide represents, for example, less than 0.0048% by mole.

The term "unit" defines any device or combination of devices suitable for a given purpose. Thus a unit for purifying raw biogas is a unit capable of obtaining from raw biogas biogas as previously defined.

tricks

The present invention, its characteristics and the various advantages it provides will appear better on reading the following description of two particular embodiments of the invention, presented by way of illustrative and non-limiting examples and which refers to the attached drawings on which:

FIG. 1 represents a first embodiment of the installation of the invention, which only comprises a single stage of separating membranes;

FIG. 2 represents a modeling diagram of a second embodiment of the installation of the invention which comprises two stages mounted in series of separating membranes.

EXAMPLE

With reference to FIG. 1, the embodiment of the installation of the invention represented comprises a digester 1. The digester 1 is the seat of an anaerobic digestion reaction which produces biogas. Biogas mainly contains methane and carbon dioxide. It also contains water vapor, hydrogen sulphide, nitrogen, oxygen and potentially pollutants such as siloxanes, organochlorine compounds, mercaptans, nitrogen compounds including ammonia, other volatile organic compounds and metals. Since the digestion reaction is anaerobic, biogas contains very little oxygen and nitrogen.

The digester 1 is connected to a compressor C1 by means of a pipe 11. The output of the compressor C1 is connected to two columns 2A and 2B, connected in series. Columns 2A and 2B are filled with a packing capable of fixing hydrogen sulphide and other volatile organic compounds potentially contained in very small quantities in the biogas. The outlet of columns 2A and 2B is connected to a particle filter 2C which makes it possible to retain the particles coming from column 2A and 2B. Columns 2A, 2B and filter 2C form the biogas purification unit. The output of the particulate filter 2C is connected to a heat exchanger EC1 which cools the gas leaving the filter 2C. The output of the heat exchanger EC1 is connected to a compressor C2. Compressor C2 is connected to an EC2 heat exchanger which recovers the heat generated at compressor C2. The output of the compressor C2 is connected to a heat exchanger EC31 capable of cooling the gaseous flow which passes through it at the output of the compressor C2. Line 22 exiting heat exchanger EC31 is connected to a dehydration loop by means of a thermostatic valve V3. The valve V3 is connected to the dehydration loop, which comprises a pipe 221. The pipe 221 crosses the exchanger EC31 so as to heat the gaseous flow by recovering the heat captured during the cooling of the gaseous flow at the outlet of the compressor C2. The output of the EC31 exchanger is connected to an EC32 exchanger which cools the gas flow. The outlet of the EC32 exchanger is connected to a condensate collector 37 which allows all condensed liquids, including water, to be evacuated. The output of the condensate collector 37 is connected to the thermostatic valve V3. Downstream of the bypass to valve V3, line 22 opens into an oil separator SH which removes all oil droplets from the compressors and possibly present in the gas flow. The output of the SH oil separator is connected to two M1 membranes connected in parallel.

With reference to FIG. 1, the biogas conditioning unit comprises the purification unit formed by the compressor C1, by the columns 2A, 2B and the filter 2C and the assembly formed by the compressor C2, the dehydration loop and the separator of oil.

With reference to FIG. 1, the membranes M1 are capable of separating methane from carbon dioxide; they are also permeable to oxygen which passes through them at the same time as methane. The gas mixture enriched in carbon dioxide therefore contains less than 0.1% by mole of oxygen. The term “part upstream of the M1 membranes” designates the part located upstream of the M1 membranes and which therefore fills with a gas depleted in methane but enriched in carbon dioxide since carbon dioxide passes through the membrane less than methane. The downstream part of the membranes corresponds to the outlet after passing through the membrane(s). The output of the M1 membranes is connected to a pipe 3 equipped with an Ana1 analyzer. In the event of non-compliance, the flow contained in pipe 3 can be returned to the digester 1. Otherwise, pipe 31 allows the recovery/use of the methane produced. The upstream part of the membranes M1 is connected via a single pipe 51 to a mixer 53. The outlet of the mixer 53 is connected to a compressor C3. An EC3 heat exchanger combined with the C3 compressor makes it possible to recover the heat produced during the compression of the gas. The output of the compressor C3 is connected to an air cooler 6 which allows the gas in the pipe to be cooled down to ambient temperature. The outlet of air cooler 6 is connected to a 2D column containing zeolites, which allows the gas to be dried. The outlet of the column 2D is connected to a heat exchanger EC4 which is the boiler of the distillation column 7, by means of a pipe 71. The pipe 71 passes through the heat exchanger EC4 and opens into the distillation column 7 at the zone containing the vapor phase. Line 71 is equipped with an expansion valve V4 which enables Joule-Thomson expansion before entering the distillation column. The pressure in the distillation column is such that there is subcooling of the liquid carbon dioxide. The bottom of the distillation column 7 makes it possible to recover a liquid. It is connected via a line 75 to a liquid reservoir RL. An Ana2 analyzer is mounted on line 75 upstream of the RL tank. The bottom of column 7 is also connected to a recirculation pipe 77 on which a heat exchanger EC5 is mounted. The recirculation line 77 opens into the distillation column 7 after passing through the exchanger EC5, in the zone occupied by the gas phase. The area of the distillation column 7 which contains the gas phase is connected to a starting tank RD containing carbon dioxide. Pipe 75 is connected upstream of the analyzer Ana2 to a restart pipe 76 which is connected to the mixer 53 and which is equipped with a valve V1 upstream of the mixer 53. A valve V2 is mounted on the pipe 75 upstream from the bypass to the pipe 76. A valve V5 is mounted upstream of the mixer 53 on a pipe which connects the pipe 75, downstream of the valve V2 (the downstream being in the direction of the liquid reservoir RL) to the mixer 53 .

Heat exchanger EC5 is thermally connected to heat exchanger EC1 and heat exchanger EC32. The cold generated in the EC5 exchanger is thus used to cool the biogas at the outlet of the 2C filter and in the dehydration loop.

The cold storage loop operates at -1°C (temperature of carbon dioxide in the boiler of the distillation column /5°C (maximum refrigeration temperature of the gas mixture upstream of the M1 membranes). current, the biogas enters EC1 at a pressure of between 50 and 300 mbar. The temperature at the inlet of the exchanger EC1 is between 50 and 15°C. The temperature of the biogas at the outlet of the exchanger EC1 is 5° C which allows the condensation of most of the water contained in the gaseous mixture.At the level of the exchanger EC32, the gas pressure is between 7 and 13 bars. Its temperature is between 50°C and 20°C at the inlet of the EC32 exchanger and from -1°C to 5°C (limits included) at the outlet of the EC32 exchanger.

Any excess cold that may be available can be used elsewhere in the installation.

An example of implementation of the method of the invention will now be described with reference to FIG. 1. The biogas leaving digester 1 is slightly compressed by compressor C1; it is at a temperature of around 35°C. It contains, for example, 60% methane, 30.8% carbon dioxide. The biogas purified in columns 2A, 2B and having passed through filter 2C is then cooled by exchanger EC1 so as to condense the water it may contain, then compressed by compressor C2. The heat from compressor C2 is recovered in exchanger EC2. The purified and compressed biogas flow is first cooled in the EC31 exchanger using the heat of the incoming flow. It then passes through the EC32 exchanger where it is cooled again. All the condensates are recovered at the level of the condensate collector 37. The dried gas then passes through the three-way valve V3 to be directed in whole or in part to the exchanger EC31. After joining the lines, it enters the SH oil separator. Oil extraction is important so as not to damage the M1 membranes. The biogas stream is then split into two and each sub-stream passes through a separating membrane M1. Methane passes through the M1 membranes while carbon dioxide is retained upstream of the M1 membranes. The methane therefore passes through line 3 and is analyzed by the Ana1 analyzer. If the composition of the methane complies with the standards for use, it is injected into a network for direct use. If the composition of the methane does not comply with the standards, it is reinjected into the digester 1 through line 4. The gaseous mixture enriched in carbon dioxide which accumulates upstream of the membranes M1 is introduced into the mixer 53. It then passes through compressor C3. The heat generated by the compression is evacuated at the level of the exchanger EC3. At the outlet of compressor C3, the gaseous mixture enriched in carbon dioxide passes through an air cooler 6, passes into column 2D in order to be dried and purified a second time. It then passes through the heat exchanger EC4 which is in fact the boiler of the distillation column 7. It is cooled at the level of the heat exchanger EC4 and at the level of the valve V4 which performs a Joule-Thomson expansion . The mixture rich in carbon dioxide and poor in methane liquefies at valve 104 (Joule-Thomson expansion). However, some gas bubbles remain in the liquid. This mixture is introduced into the distillation column 7 at the portion of this column containing the gas phase (at the top of the column). The liquid and the gas are then separated. The liquid carbon dioxide falls to the bottom of the column while the gaseous methane escapes at the top of the column and is reintroduced into methanizer 1.

The distillation column 7 is a packed column of the ring type, in particular Koch rings. The liquid phase condenses on the packing and drops to the bottom of the column. In the example shown, the temperature at the bottom of the column is maintained at a constant value plus or minus one degree by the heat exchanger EC4 and by a temperature regulation system, not shown. In the distillation column 7, the liquid carbon dioxide is still boiling; there is therefore a liquid/gas phase equilibrium which corresponds to the boiling point of carbon dioxide at the pressure prevailing in the distillation column 7. The distillation conditions will be more fully described with reference to FIG. 2 and the tables below.

Still with reference to FIG. 1, The liquid carbon dioxide is drawn off at the bottom of the column through pipe 75. It can be stored in the RL tank, be released into the atmosphere or be used in another installation.

The start-up of the installation takes place thanks to the reservoir RD which contains gaseous carbon dioxide and which is connected to the head of the column 7. The pipe 75 which is connected to the reservoir RL of liquid carbon dioxide makes it possible to fill the bottom of the column and to create with the flow of gaseous carbon dioxide coming from the tank RD the desired balance in the distillation column 7. A gaseous nitrogen tank can also be connected to the top of the distillation column 7 to perform a purge. The Ana2 analyzer makes it possible to detect the composition of the liquid carbon dioxide drawn off and, depending on its composition, to redirect it to the atmosphere, the RL tank or another installation. The valve V5 makes it possible to introduce gaseous carbon dioxide into the mixer 53 so as to start the installation.

With reference to FIG. 1, the cold generated in exchanger EC5 is used to cool the biogas to be treated in exchangers EC1 and EC32.

It is also noted that the installation of the invention produces heat at the level of the exchangers EC2 and EC3. This heat can be used by the customer for digester 1, for example.

The second embodiment is the same as the first shown in FIG. 1 except that it includes a second battery of methane-permeable membranes. These membranes are mounted in parallel to form a block of membranes. This second block is mounted in series at the outlet of the first stage of membranes M1. These second membranes M2 make it possible to improve the purity of the methane obtained. The gas depleted in methane and enriched in carbon dioxide which is formed upstream of the second membranes M2 is reinjected into the compressor C1 at the level of a mixer 12.

Fig. 2 represents a diagram used for modeling the method of the invention. This diagram does not represent the actual installation itself but the latter in the form of modelable unit operations. Thus, the distillation column 7 of FIG. 1 which is also present in the second embodiment is modeled in the form of a column 7 and a liquid/gas phase separator 109. The boiler of the distillation column (referenced EC4 in FIG. 1) corresponds to the boiler 109 and to the heat exchanger 151. A mixer 12 is added to allow the mixing of the purified biogas coming from the digester (not represented in FIG. 2) with the mixture enriched in carbon dioxide produced upstream of the membranes M2. This mixer 12 is connected to the compressor C1. Compressor C1 is modeled by compressors 31 and 13 in FIG. 2. Exchanger EC2 corresponds to exchanger 16 in Fig. 2. Exchanger 152 combined with exchanger 9 in FIG. 2 model the EC31 exchanger. Exchanger 16 corresponds to exchanger EC32 in Fig. 1. Condensate collector 37 is modeled by condenser 17. Mixer 53 is not shown in Fig. 1. The M1 and M2 membranes are grouped into two separate blocks. An analyzer 105 is mounted on the pipe at the outlet of the second membrane stage M2. Compressor C3 is modeled by compressor 87. Exchanger 98 models air cooler 6 in FIG. 1. Valve V4 is modeled by valve 104.

The method implemented in this second embodiment is identical to the first except that the methane from the membranes M1 is purified a second time through the membranes M2. The methane leaving the M2 membranes therefore contains less carbon dioxide than the methane coming from the M1 membranes.

On the diagram of Fig. 2 stages are modeled from the compression in compressor C1.

The following tables show temperature, pressure and composition values at various points in the installation, referenced in Fig. 2. These points are referenced by numbers or more explicit terms indicated in rectangles located on the pipe or indicating the point of measurement on the pipe by means of an arrow. The biogas which enters the mixer 12 of FIG. 2 corresponds to the biogas treated by the columns 2A, 2B and by the filter 2C of FIG. 1. In table 1 below, the term “raw biogas” refers to the biogas leaving digester 1.

setting Raw_biogas ret_stage1 output_CO2 output_CH4 unit pressure 0.002 9.48 5.48 8.38 barg temperature 36.85 17.48 12.24 16.86 °C Mole fraction [water] 0.053 0.0014 0 0.002 Mole fraction [carbon dioxide] 0.308 0.076 0.945 0.0005 mole fraction [methane] 0.606 0.888 0.05 0.947 mole fraction [hydrogen sulphide] 4.82e-05 5.173e-05 0 7.76e-05 mole fraction [oxygen] 0.014 0.015 0 0.023 mole fraction [nitrogen] 0.016 0.017 0 0.026 flow 303.3 209.1 166.6 125.2 kg/h Molar mass 0.025 0.018 0.042 0.016 kg/mol density 39.51 446.17 285.14 398.45 mol/m³ Internal energy -2167.05 -2828.85 -3027.65 -2820.70 J/mol enthalpy 432.14 -455.78 -708.03 -443.79 J/mol entropy 9.33 -17.06 -15.86 -17.77 J / mol K Heat capacity Cp 36.42 36.61 37.98 36.20 J / mol K Thermal conductivity 0.029 0.032 0.017 0.033 W/mK

Setting Perm_stage3 Biogas_sec In_sep unit pressure 4.38 -0.009 10.58 barg temperature 14.31 3.99 18.03 °C molar fraction [Water] 0 0.006 0.001 mole fraction [Carbon dioxide] 0.229 0.324 0.302 molar fraction [Methane] 0.770 0.636 0.672 mole fraction [Hydrogen sulphide] 0 5.06e-05 3.83e-05 molar fraction [Oxygen] 0 0.015 0.011 molar fraction [Nitrogen] 0 0.017 0.013 Debit 83.9 292.9 375.8 kg/h Molar mass 0.022 0.025 0.024 kg/mol Vapor phase composition molar fraction [Water] 0 0.006 0.001 mole fraction [Carbon dioxide] 0.229 0.324 0.302 molar fraction [Methane] 0.770 0.636 0.672 mole fraction [Hydrogen sulphide] 0 5.06e-05 3.83e-05 molar fraction [Oxygen] 0 0.015 0.011 molar fraction [Nitrogen] 0 0.017 0.013 density 229.69 43.74 497.01 mol/m³ Internal energy -2859.50 -3069.77 -2864.21 J/mol enthalpy -485.23 -747.99 -505.19 J/mol entropy -10.98 4.29 -15.52 J / mol K Heat capacity Cp 36.39 35.36 37.35 J / mol K Thermal conductivity 0.029 0.025 0.028 W/mK

Tables 3 and 4 below show the same values for other points in the installation. Thus, upstream of compressor C3,

Setting 78 62 61 107 unite Work W Low temperature °C High temperature °C Pressure 5.4 45.4 45.4 45.45 barg temperature 12.2 19.8 202.9 -9.6 °C molar fraction [Water] 0 0 0 0 mole fraction [Carbon dioxide] 0.9455 0.945 0.945 0.945 molar fraction [Methane] 0.054 0.054 0.054 0.054 Debit 166.6 166.6 166.6 166.6 kg/h Molar mass 0.042 0.042 0.042 0.042 kg/mol Vapor phase composition molar fraction [Water] 0 0 0 N / A mole fraction [Carbon dioxide] 0.945 0.945 0.945 N / A molar fraction [Methane] 0.054 0.054 0.054 N / A Liquid phase composition molar fraction [Water] N / A N / A N / A 0 mole fraction [Carbon dioxide] N / A N / A N / A 0.945 molar fraction [Methane] N / A N / A N / A 0.054 Energy -3027.6 -4322.8 2620.9 J/mol enthalpy -708.0 -2600.1 6454.8 J/mol Gibbs energy 0.024 0.053 0.080 kWh/kg entropy -15.86 -36.65 -12.43 J / mol K Heat capacity Cp 37.98 64.78 47.44 J / mol K thermal conductivity 0.017 0.022 0.034 W/mK

setting 88 101 87 unit work 2908.6836 W pressure 30.48 32.68 barg temperature -15.61 -40 °C molar fraction [Water] 0 0 mole fraction [Carbon dioxide] 0.945 0.360 mole fraction [methane] 0.0545 0.639 mole fraction [Hydrogen sulphide] 0 0 molar fraction [Oxygen] 0 0 molar fraction [Nitrogen] 0 0 flow 166.69 8.66 kg/h Molar mass 0.042 0.026 kg/mol Vapor phase composition molar fraction [Water] 0 0 mole fraction [Carbon dioxide] 0.757 0.360 mole fraction [methane] 0.242 0.639 Liquid phase composition mole fraction [Carbon dioxide] 0.959 0.871 mole fraction [methane] 0.040 0.128 enthalpy -3080.63 -3694.44 J/mol entropy -33.65 -36.52 J / mol K Heat capacity Cp 52.48 49.60 J / mol K

Table 5 below groups together the values of the parameters of the conditions for the distillation of the gaseous mixture in the distillation column 7,

Steam Input2 Entrance 2 entry1 Entry1 High Down plateau 8 8 14 14 1 20 Pressure (bar) 31.49 33.83 31.49 33.89 33.70 33.95 Vapor fraction (-) 1.00 1.00 2.38E-08 0.00 1.00 0.00 Temperature (C) -15.61 -12.55 -15.61 -15.52 -40.00 -1.01 Total molar flux (mol/s) 0.075 0.075 1.01z 1.014 0.092 0.997 Total mass flux (kg/s) 0.002 0.002 0.043 0.043 0.002 0.043 Molar flux (mol/s) CO2 0.057 0.057 0.973 0.973 0.033 0.997 CH4 0.018 0.018 0.041 0.041 0.058 5.9E-04 molar fractions(-) CO2 0.757 0.757 0.95 0.959 0.36 0.99 CH4 0.242 0.242 0.040 0.040 0.63 5.96E-04 mass flows (kg/s) CO2 0.002 0.002 0.042 0.042 0.001 0.043 CH4 2.92E-04 2.92E-04 6.61E-04 6.61E-04 9.45E-04 9.54E-06 Density (kg/ ^m3 ) 864,160 863,765 837,439

The term "inlet" designates the flow of liquid or vapor arriving in the distillation column 7 after the valve V4 (referenced 104 in the diagram of FIG. 2).

In view of the aforementioned Table 5, it can be seen that the temperature in the boiler of the distillation column is -1.01° C. for a pressure of 33.95 bars. After valve V4 (Joule-Thomson expansion), the pressure is 33.83 bars for steam and 33.89 bars for liquid. The mixture no longer contains water but only carbon dioxide and methane. In the boiler, the carbon dioxide is liquid; the boiler contains more than 99% by mole of carbon dioxide. The steam leaving the distillation column is at -40° C. for a pressure of 33.7 bars. It contains methane and possibly some hydrogen sulphide.

Claims (10)

Process for the treatment of biogas according to which said biogas is passed in the gaseous state through a first membrane stage comprising at least a first separating membrane, the permeability to methane of the said first separating membrane being greater than its permeability to carbon dioxide, the temperature and pressure conditions of said biogas allowing the passage of methane downstream of said membrane, characterized in that the gaseous mixture enriched in carbon dioxide which accumulates upstream of said membrane is recovered, in that the said gaseous mixture enriched in carbon dioxide is distilled under temperature and pressure conditions making it possible to obtain liquid carbon dioxide and in that the liquid carbon dioxide is recovered. Process according to Claim 1, characterized in that the liquid carbon dioxide/methane gas mixture to be distilled contains at least 80%, in particular at least 85% and more particularly at least 90% by mole of carbon dioxide, the remainder being methane and/or in that said liquid carbon dioxide/methane gas mixture to be distilled is at a temperature equal to or greater than -5° C. and in particular of the order of -1.01° C. during the distillation. Process according to Claim 1 or 2, characterized in that, before the distillation stage, a Joule-Thomson expansion is carried out on the gaseous mixture enriched in carbon dioxide so as to liquefy it. Process according to Claim 3, characterized in that the said gaseous mixture enriched in carbon dioxide is cooled down to ambient temperature or a temperature below ambient temperature before carrying out the said Joule-Thomson expansion. Process according to any one of the preceding claims, characterized in that the gaseous mixture enriched in methane obtained is passed downstream of the said first separating membrane through a second membrane stage which comprises at least one second separating membrane whose permeability to methane is greater than the permeability to carbon dioxide, the temperature and pressure conditions of said gaseous mixture enriched in methane allowing the passage of methane downstream of said second membrane. Process according to any one of the preceding claims, characterized in that the liquid carbon dioxide formed during the distillation is used to produce cold and in particular to cool the said biogas upstream of the said first membrane stage and/or of the said second membrane stage when claim 6 is taken in combination with claim 5. Process according to any one of the preceding claims, characterized in that during the distillation, the liquid carbon dioxide is separated by gravity from the gaseous methane which it contains. Installation allowing the implementation of the method according to any one of the preceding claims, comprising a source of biogas, in particular a digester capable of producing biogas, a methane separation unit connected to said digester, which comprises at least a first battery separating membranes, which comprises at least a first separating membrane whose permeability to methane is greater than the permeability to carbon dioxide, characterized in that it comprises a distillation column arranged so as to allow the distillation of the enriched gaseous mixture in carbon dioxide which accumulates upstream of said first membrane. Installation according to claim 8, characterized in that it comprises a heat exchanger, mounted upstream of the boiler of the said distillation column and/or a valve capable of carrying out a Joule-Thomson expansion, the said valve being mounted between the said distillation column and the said boiler and downstream of said boiler on the pipe which connects said heat exchanger to said distillation column when said installation comprises such a heat exchanger. Installation according to any one of Claims 8 to 10, characterized in that it comprises a heat exchanger making it possible to cool the said biogas upstream of the said first battery of separating membranes with the liquid carbon dioxide present in the bottom of the column. to distill.