Method for purifying liquefied carbon dioxide from collected liquefied gas

Abstract

The invention relates to a method for purifying liquefied carbon dioxide from a liquefied gas comprising more than 70% by volume of carbon dioxide, the method comprising the following consecutive steps: a) a step of collecting the liquefied gas from one or more satellite production sites; b) a step of centrally purifying the collected gas.

Description

[0001] Process for purifying liquefied carbon dioxide from collected liquefied gas

[0002] technical field

[0003] The present invention relates to a process for purifying carbon dioxide in liquid form comprising more than 70% by volume of carbon dioxide from liquefied gas collected at satellite sites and purified at a centralized site.

[0004] Technical background

[0005] Currently, there is a search for processes to valorize waste, particularly waste from biomass. To this end, methanization processes are being developed.

[0006] Anaerobic digestion is a process of decomposing organic matter. It involves the fermentation of livestock effluents or by-products from the agri-food industry. The principle is as follows: the organic effluents are stored in a sealed tank called a "digester," where they are subjected to the action of microorganisms (bacteria) in the absence of oxygen (anaerobic fermentation) for a specific period (generally around 60 days). This process generates biogas, which contains, among other things, methane (CH4, in proportions of 50% to 70%), carbon dioxide (CO2), and an organic residue (called "digestate," used as fertilizer). The biogas can be converted into electricity or into gas and fuel for vehicles.

[0007] Anaerobic digestion represents a substantial economic interest; the biogas produced can replace natural gas in all its current uses: heat production, electricity production and fuel for vehicles, and the digestate is fully utilized, notably in the form of fertilizer whose composition is much more complete than that of a chemical fertilizer.

[0008] Anaerobic digestion also offers environmental benefits, as biogas is a renewable energy source. While its production and use do generate polluting emissions into the atmosphere, such as carbon dioxide, these emissions remain lower than those from fossil fuels. During biogas purification, a gas composed primarily of carbon dioxide, known as "lean gas," is produced. The present invention relates to the valorization of this "lean gas," rich in carbon dioxide, from these digesters. However, the carbon dioxide generated during this process is only of interest if it is purified to a very high purity level while recovering the residual methane remaining in the lean gas. This would allow for the recovery of all the methane and carbon dioxide produced by anaerobic digestion.

[0009] It is also possible to utilize carbon dioxide-rich gases produced by cement plants, or by steam reforming or oxy-combustion processes. Indeed, the process according to the invention is not limited to carbon dioxide-rich gases from methanizers but applies to carbon dioxide-rich gases in general.

[0010] In general, the process according to the invention aims to valorize carbon dioxide-rich gases, which are reaction by-products and are generally sent to the atmosphere.

[0011] Therefore, it is necessary to develop a carbon dioxide purification process that aims for a high level of purity, and whose process is not too energy-intensive.

[0012] Technical problem to solve

[0013] There is therefore a real need for an efficient and energy-saving carbon dioxide purification process that yields very high-purity carbon dioxide. In other words, the process according to the invention aims to valorize a by-product of methanation, namely a "lean gas," by isolating and purifying the carbon dioxide to obtain very high-purity carbon dioxide. The process according to the invention aims to treat a carbon dioxide-rich gas in liquid form and purify it to achieve food-grade quality, in particular by complying with EIGA Doc 126-11 (Appendix 1, page 6). The carbon dioxide produced can be used, for example, in the production of sparkling water.

[0014] The process according to the invention also aims to obtain liquid carbon dioxide of pharmaceutical grade, in compliance with the European Pharmacopoeia and GMP. It can, for example, be used for cryogenics.

[0015] In the agricultural field, it can also be used to enrich the atmosphere of agricultural greenhouses with carbon dioxide by increasing the carbon dioxide level from 350 ppm to 1200 ppm.

[0016] This process has the advantage of being energy-efficient, leading to good yields and providing very high purity carbon dioxide.

[0017] This process can also have the advantage of recycling all or part of other components of the gas, such as methane, oxygen or nitrogen.

[0018] Furthermore, according to one embodiment, the liquefied gas can originate from one or more collection sites. It is then possible to recover the liquefied gas from each site and process it at a single, central site using the purification process according to the invention. The centralized site is thus shared. This results in savings in energy, infrastructure, and cost. Indeed, it is no longer necessary to install a purification plant for each collection site. This process therefore has the advantage of being able to recover gas from very small-scale collection sites, whose production volumes do not justify the installation of a purification plant.

[0019] Brief description of the invention

[0020] Thus, the invention relates to a process for purifying liquefied carbon dioxide from a liquefied gas comprising more than 70% by volume of carbon dioxide, comprising the following successive steps: a) a step of collecting the liquefied gas from one or more satellite production sites, b) a step of centralized purification of the collected gas.

[0021] Other advantageous features of the process according to the invention are detailed below. The gas from the satellite production sites can undergo pretreatment comprising the following successive steps:

[0022] 1) a step involving the supply of raw gas containing more than 70% carbon dioxide,

[0023] 2) a possible gas compression stage, the gas pressure at the outlet of this stage being between 15 and 20 bar,

[0024] 3) a possible step of drying the liquefied gas and then,

[0025] 4) a liquefaction stage, the liquefied gas exiting this stage being at a temperature between -50 and -25°C and a pressure between 15 bars and 20 bars.

[0026] The process may comprise the following successive steps: b1) a step of vaporizing the liquefied gas using a vapor condenser, the temperature of the gas exiting this step being between -10°C and 10°C and the pressure of the gas exiting this step being between 15 and 25 bar; then b2) a step of compressing the gas leading to an increase in the pressure of said gas from step b1) from 3 to 5 bar, the gas exiting this step being at a pressure between 20 and 25 bar; then b3) at least one step of filtering the gas; then b4) a step of auto-liquefaction of the gas in the vapor condenser of step b1), the temperature of the gas exiting this step b4) being between -15°C and -30°C and the pressure of the gas exiting this step b4) being between 20 and 25 bar; then b5) a step of expanding the gas liquefied via a pressure-reducing valve, the pressure of the liquefied gas at the outlet of this valve being between 15 and 20 bar,then b6) a step of distilling the fluid so as to separate the pure liquefied carbon dioxide from the other components of the gas, b7) a step of recovering the carbon dioxide in liquid form from step b6).

[0027] The process may include the following successive steps: b8) a liquefaction step of the gas recovered at the top of the distillation column in step b6), the gas exiting this step being at a temperature between -55 and -30°C and at a pressure between 15 and 20 bars, then b9) a separation step of the liquid phase from the gaseous phase of the fluid from the liquefaction step b8), then b10) a recycling step of the liquid phase from the separation step b9) to the distillation step b6).

[0028] The process involves the following successive steps:

[0029] - a heating step of the gaseous phase from step b9),

[0030] - a membrane filtration step of the heated gas phase,

[0031] - a step of recycling the gas from the previous filtration step, before step b3) of compression.

[0032] The process may include, after step 4), a step 5) of storing the liquefied gas to be purified.

[0033] Detailed description

[0034] Other features, aspects, objects and advantages of the present invention will become even clearer upon reading the description that follows.

[0035] It is specified that the expressions "from ... to ..." and "between ... and ...." used in this description should be understood as including each of the mentioned limits.

[0036] Unless otherwise stated, all temperatures given below are in degrees Celsius and all pressures are in bar and are absolute pressures. The notation "bar" in this disclosure is therefore equivalent to the notation "bar a" or "bara" (denoting absolute pressure).

[0037] The raw material for the process is a liquefied gas comprising more than 70% by volume of carbon dioxide relative to the total volume of the liquefied gas, preferably more than 80% by volume.

[0038] The process according to the invention can also be implemented on pure liquefied carbon dioxide gas. The aim of the process is then to increase the purity level of this gas. As indicated above, the initial gas can come from various sources. For example, it can come from a cement plant, a methanizer, steam reforming processes, or oxy-combustion processes.

[0039] Its composition is therefore diverse and depends on the process from which it originates. The other components of the liquefied gas can thus be methane, oxygen, nitrogen, hydrogen, carbon monoxide, hydrogen sulfide, water vapor and volatile organic compounds referred to below as VOCs.

[0040] The process according to the invention comprises the following steps: a) a step of collecting the liquefied gas containing carbon dioxide from one or more so-called "satellite" production sites. b) a step of centralized purification of the collected gas.

[0041] The collection stage allows the liquefied gas to be recovered for purification and transported to a central site, where the purification of the collected gas takes place.

[0042] The term "satellite sites" refers to sites where the initial gas is produced. A satellite site can be a biogas plant, a biogas treatment plant, a steam reforming plant, an oxy-combustion plant, or a unit for recovering gases produced by a cement plant and their mixtures. Satellite sites may have identical or different production volumes, and with identical or different gas purities.

[0043] Satellite sites enable the production of raw gas. This raw gas is a mixture of gases consisting primarily of carbon dioxide. The composition of this gas mixture can vary from one satellite site to another. Even within the same satellite site, the composition of the gas mixture can vary. The raw gas produced at a satellite site is in gaseous form and contains more than 70% carbon dioxide by volume.

[0044] The term "central site" refers to the site where the liquefied gas purification process is carried out, preferably the process described below. Preferably, the gas produced by the satellite sites undergoes pretreatment before being transported to the central site for further purification. This is because the produced gas must arrive at the central site in liquefied form.

[0045] This preprocessing may include the following successive steps:

[0046] Step 1) Supply of raw gas

[0047] As mentioned above, raw gas can originate from one or more methanation units, one or more biogas purification units, one or more steam reforming units, one or more oxy-combustion units, or one or more units for recovering gases produced by a cement plant and their mixtures. Raw gas contains more than 70% carbon dioxide by volume relative to the total gas volume.

[0048] Step 2) Possible compression of the raw gas

[0049] The raw gas can undergo a compression step.

[0050] At the compressor inlet, the gas pressure can be between 1.05 and 1.40 bar. At the compressor outlet, the gas pressure is between 15 and 20 bar.

[0051] Increasing the gas pressure reduces the power required to liquefy carbon dioxide and improves the efficiency of the process.

[0052] During this compression stage, the gas must be compressed to at least a pressure greater than the triple point pressure of carbon dioxide, i.e. 5.18 bars, in order to avoid icing.

[0053] Preferably, compression is carried out in at least two stages using a multi-stage compressor. At each stage, at the compressor outlet, at least two heat exchangers connected in series are used to recover the energy generated by gas compression at different temperature levels. Furthermore, cooling the gas protects the compressor from potential overheating. Preferably, an oil-free, dry compressor is used. This prevents contamination of the gas with oil residues.

[0054] Preferably, the process uses a two-stage compressor. At the outlet of the first compressor, the gas temperature is between 110 and 210 °C, preferably between 110 and 190 °C. At the outlet of the second compressor, the gas temperature is between 150 and 220 °C. At least two heat exchangers connected in series reduce this temperature range to between 25 and 50 °C, preferably to ambient temperature.

[0055] At the compressor outlet, the gas is preferably between 15 and 20 bar.

[0056] The gas then passes through a buffer tank, which stabilizes the gas pressure within the process. In other words, the buffer tank dampens fluctuations in gas flow, which depend on the gas production process. This tank is positioned downstream of the compressor and upstream of the dryer so that the pressure at the dryer's inlet remains constant.

[0057] Step 3) Possible drying of the gas

[0058] The gas can then be dried to eliminate any remaining traces of water and achieve a dew point temperature at the outlet of the drying device of between -65 and -45 °C at 20 bars or a water content of less than 20 ppm.

[0059] Preferably, a zeolite that selectively adsorbs water is used. The apparatus used to perform this drying step can be equipped with two columns: one adsorbing traces of moisture from the gas and the other desorbing the water, i.e., regenerating the column. Continuous, cyclic operation of the two columns is preferred.

[0060] This drying step is essential if a food grade of the final liquefied carbon dioxide is targeted.

[0061] Step 4) Gas liquefaction

[0062] The gas, possibly compressed and dried, then undergoes a liquefaction stage. The gas passes through a liquefier, which allows the carbon dioxide to change state from gas to liquid. The liquefier is a heat exchanger that uses a refrigerant. At the liquefier's outlet, a two-phase fluid containing liquefied carbon dioxide is obtained. It is at a temperature between -50 and -25°C and a pressure between 15 and 20 bar.

[0063] Step 5) Storage of the liquefied gas to be purified

[0064] The liquefied gas can be directed to a storage tank. Preferably, the stored gas has a temperature between -50 and -25 °C and a pressure between 15 and 20 bar.

[0065] The liquefied gas can then be transported to the central site where it will be purified.

[0066] This implementation of the process, which combines satellite sites and a central site, offers the advantage of pooling equipment where possible, thereby reducing the costs associated with the production and purification of liquefied carbon dioxide. This can be achieved by collecting raw gas containing carbon dioxide from multiple production sites. The raw gas can be liquefied at the satellite sites. The resulting gas can then be transported to a central site for storage. The central site is equipped with the necessary purification equipment. Thus, a single distillation column can be used to purify liquefied carbon dioxide from numerous independent production sites.

[0067] Preferably, step b) of centralized purification of the collected gas is carried out according to the following steps. The process comprises at least the following 7 consecutive steps: steps bl) to b7).

[0068] At the process inlet, the temperature of the liquefied gas is preferably between -40°C and -15°C, and preferably between -20°C and -15°C. The temperature of the liquefied gas depends on its chemical composition, which directly influences its liquefaction temperature.

[0069] At the process inlet, the pressure of the liquefied gas is preferably between 15 bar and 25 bar. Generally, the initial liquefied gas is transported in tanks at a pressure of 16 to 17 bar.

[0070] Step bl) Vaporization of the liquefied gas

[0071] The liquefied gas is conveyed to a vapor-condenser. A pump can be used to regulate the flow rate at the inlet of the device.

[0072] Vaporization is carried out via a vapor-condenser, which transforms a liquid into a gas in step 1b) of the process according to the invention and a gas into a liquid in step 1b4) of the process according to the invention. Thus, the energy produced for the liquid / gas phase change is used for the gas / liquid phase change.

[0073] The vapor condenser is a heat exchanger with two inlets and two outlets, one for each fluid. The liquefied gas, containing more than 70% carbon dioxide by volume relative to the total volume of the liquefied gas, enters through the first inlet of the vapor condenser. The vapor condenser then vaporizes the liquefied gas.

[0074] The vaporized gas exiting the vaporizer has a temperature between -10°C and 10°C and a pressure between 15 and 25 bars.

[0075] Step b2) Gas compression

[0076] The gas from the vapor condenser is then compressed using a compressor. The purpose of this step is to create a pressure differential when the gas re-enters the vapor condenser in step b4), this time to be liquefied. The pressure differential between the two inlets of the vapor condenser allows for a greater temperature difference between the inlet fluids and the saturation temperature of carbon dioxide. This pressure differential allows the use of a single fluid to perform both phase changes within the vapor condenser. Increasing the gas pressure reduces the power required to liquefy the carbon dioxide and improves the energy efficiency of the process. This compression step thus eliminates the need for a thermodynamic cycle to reliquefy the gas. The gas compression step results in an increase in the initial gas pressure of 3 to 5 bar.

[0077] At the compressor outlet, the gas pressure is between 20 and 25 bar and the gas temperature generally corresponds to the ambient temperature + 5 °C.

[0078] Step b3) Filtration

[0079] The process includes at least one filtration step to remove impurities present in the initial gas and increase the purity level of the carbon dioxide.

[0080] The process according to the invention may include different types of filtration, each filtration step allowing the removal of a specific type of impurity.

[0081] Filtration can be activated carbon filtration, particle filtration, adsorption filtration, or bacterial filtration.

[0082] The process according to the invention can comprise a single filtration step up to twenty steps. It is possible to use a single device comprising several layers of filter materials, or several devices each comprising a single type of filter material, or several devices each comprising several types of filter materials.

[0083] The number of filtration stages and the type of filtration required depend on the quality of the initial gas, in other words, its initial carbon dioxide content, i.e., its purity level, and also its origin. These successive filtrations aim to remove fine particles and dust, bacteria, volatile sulfur compounds, volatile hydrocarbon compounds, water molecules, NVORs (Non-Volatile Organic Residues), such as traces of grease, sulfur and sulfur derivatives, oil, and other organic compounds. Preferably, the process according to the invention comprises three filtration stages: b31) at least one or more filtration stages on an activated carbon filter, then b32) one or more filtration stages on a particulate filter, b33) one or more filtration stages on an adsorption filter.

[0084] Activated carbon filtration aims to remove volatile organic compounds (VOCs) from the gas. Particulate filtration aims to remove solid particles and dust from the gas. When used after activated carbon filtration, this filtration stage removes any remaining activated carbon residue from the previous filtration process, whether in powder or dust form. A molecular sieve can be used, for example, to filter the particulate material.

[0085] Preferably, the process comprises between one and five particle filtration stages, and more particularly between two and three particle filtration stages.

[0086] Step b4) Self-liquefaction

[0087] The filtered gas then undergoes an auto-liquefaction step. The gas enters the vapor-condenser of step bl) through the second inlet, preferably at a pressure between 20 and 25 bar and at a temperature between 0 and 40 °C.

[0088] For the purposes of this invention, self-liquefaction means liquefaction independent of any external energy input. In this particular case, as indicated above, the energy generated by the condensation in step bl) is used to carry out this self-liquefaction.

[0089] The pressure difference between the carbon dioxide in liquid form at the first inlet of the vaporizer and the carbon dioxide in gaseous form at the second inlet of the vaporizer allows both the vaporization of the liquid at the first inlet of the vaporizer, which is relatively cold (Temperature below -20 °C) and the liquefaction of the gas at the second inlet of the vaporizer liquefier, which is relatively hot (temperature close to 20°C, without additional external energy input).

[0090] The liquefied gas at the second outlet of the vapor-condenser has a temperature between -15°C and -30°C and a pressure between 20 and 25 bars.

[0091] Step b5) Expansion of the liquefied gas

[0092] Upon exiting the vapor-condenser, the liquefied gas undergoes pressure reduction via a pressure-reducing valve. This step lowers the gas pressure after the self-liquefaction stage. The gas pressure is thus adjusted to the pressure required for the subsequent distillation stage. Preferably, the pressure after this pressure reduction stage is between 15 and 20 bar.

[0093] The liquefied gas exiting the expansion valve preferably has a temperature between -45 °C and -20 °C and a pressure between 15 and 20 bar.

[0094] Step b6) Distillation

[0095] The liquefied gas from the previous step undergoes distillation to isolate the carbon dioxide from other potentially present gases and impurities.

[0096] Thus, liquefied carbon dioxide is collected at the bottom of the column, while gaseous impurities are collected at the top. The temperature within the column is preferably between -45 °C and -18 °C, at a pressure generally between 15 and 20 bar.

[0097] Step b7) Recovery of liquefied carbon dioxide

[0098] The liquefied and purified carbon dioxide is collected. It can be sent to a storage tank or used directly for a subsequent application. The liquefied carbon dioxide is then at a temperature between -20 and -30°C and a pressure between 15 and 20 bar. The purified carbon dioxide, collected at the bottom of the column, can be analyzed. Depending on the purity of the liquid obtained, the circuit may include a bypass valve.

[0099] If the purity of the recovered liquid is insufficient, the liquid can be returned to the circuit to undergo the purification process according to the invention again. In one embodiment, the liquid can be sent back to the initial storage tank.

[0100] If the purity of the recovered liquid is satisfactory, the liquid can be sent directly to a storage tank or used directly for a later application.

[0101] Step b8) Possible liquefaction of the gas

[0102] The gas recovered at the top of the distillation column in step b6) is a gas with a low carbon dioxide content, that is, a content necessarily lower than that of the gas entering the process according to the invention. This gas may optionally contain oxygen, nitrogen, methane, hydrogen, carbon monoxide, or other impurities depending on the origin of the initial gas.

[0103] This gas can undergo a liquefaction step to be cooled to a temperature between -55°C and -35°C and a pressure between 15 and 20 bar. A liquefier using glycol water as a coolant can be used.

[0104] This step liquefies the carbon dioxide, which is not yet liquefied and isolated at this stage of the process. At the end of this step, a two-phase fluid is obtained.

[0105] Step b9) Phase separation

[0106] The two-phase fluid from the previous step b8) is brought to a separator, which allows the liquid phase to be separated from the gaseous phase.

[0107] Step bl0) Possible recycling of the liquid phase to distillation The liquid phase from the previous step b9), containing the liquefied carbon dioxide, can be recycled to step b6) of distillation.

[0108] Step k) Possible recycling of the gaseous phase

[0109] The gaseous phase from the separation step b9) can be heated using a heat exchanger.

[0110] The heated gas can be filtered through a membrane to separate a gas rich in carbon dioxide from a gas poor in carbon dioxide.

[0111] Carbon dioxide-rich pargas is defined as a gas composed of more than 50% carbon dioxide by volume relative to the total volume of the gas.

[0112] A gas low in carbon dioxide is defined as a gas composed of less than 50% carbon dioxide by volume relative to the total volume.

[0113] The carbon dioxide-rich gas can be recycled and returned to the circuit, before step b3) of gas compression.

[0114] Low-carbon gas can be recycled, for example, back to a unit producing the original gas, or used on-site. If it is high-methane gas, it can, for example, be injected into the city gas network. If the original gas comes from a methanization unit, it can be recycled back to the methanization unit or to the wastewater treatment unit. In the case of gas from an oxy-combustion process, if the gas is predominantly oxygen, it can be recycled back to the combustion unit. In the case of gas from steam reforming, if the gas is predominantly hydrogen, it can be recycled back to the steam reforming unit.

[0115] Therefore, depending on the gas being treated, it is possible to recycle, and thus recover, the gases recovered following the liquefaction of carbon dioxide.

[0116] The following examples illustrate the present invention, but are in no way exhaustive.

[0117] Examples 1. Purity of the liquefied carbon dioxide produced

[0118] A gas from a biogas purification unit produced by a methanizer of the following composition is treated by the process according to the invention:

[0119] Table 1

[0120] At the end of the process, the gas has the following composition:

[0121] Table 2

[0122] The carbon dioxide produced complies with the European Pharmacopoeia, standard EN936 / EIGA / ISBT, and Regulation (EC) No 231 / 2012. 2. Energy consumption of the process

[0123] The process according to the invention consumes from 0.17kW / h to 0.27kW / h per kg of liquefied CO2 produced, depending on the composition of the gas treated.

[0124] The energy impact of the purification process according to the invention is reduced by the use of the vapor liquefier. This device allows the energy required to carry out the phase changes of the gas to be purified to be conserved and reused.

[0125] When liquefied gas is recovered from satellite sites and processed at the centralized site, pooling resources for gas purification reduces the energy cost of this process.

Claims

DEMANDS 1. Process for purifying liquefied carbon dioxide from a liquefied gas comprising more than 70% by volume of carbon dioxide comprising the following successive steps: a) a step of collecting the liquefied gas from one or more satellite production sites, b) a step of centralized purification of the collected gas.

2. A process according to claim 1, characterized in that the gas from the satellite production sites undergoes pretreatment comprising the following successive steps: 1) a step involving the supply of raw gas containing more than 70% carbon dioxide, 2) a possible gas compression stage, the gas pressure at the outlet of this stage being between 15 and 20 bar, 3) a possible step of drying the liquefied gas and then, 4) a liquefaction stage, the liquefied gas exiting this stage being at a temperature between -50 and -25°C and a pressure between 15 bars and 20 bars.

3. A process according to claim 1 or 2, characterized in that step b) comprises the following successive steps: b1) a step of vaporizing the liquefied gas using a vapor condenser, the temperature of the gas at the outlet of this step being between -10 °C and 10 °C and the pressure of the gas at the outlet of this step being between 15 and 25 bar, then b2) a step of compressing the gas leading to an increase in the pressure of said gas from step b1) from 3 to 5 bar, at the outlet of this step, the gas is at a pressure between 20 and 25 bar, then b3) at least one gas filtration step, then b4) a gas auto-liquefaction step in the vapor-condenser of step bl), the temperature of the gas at the outlet of this step b4) being between -15°C and -30°C and the pressure of the gas at the outlet of this step b4) being between 20 and 25 bar, then b5) a step of de-expansion of the liquefied gas via a de-expansion valve, the pressure of the liquefied gas at the outlet of this valve being between 15 and 20 bar, then b6) a step of distillation of the fluid so as to separate the pure liquefied carbon dioxide from the other components of the gas, b7) a step of recovery of the carbon dioxide in liquid form from step b6).

4. The process according to claim 3, characterized in that it comprises the following successive steps: b8) a liquefaction step of the gas recovered at the top of the distillation column in step b6), the gas exiting this step being at a temperature between -55 and -30°C and at a pressure between 15 and 20 bars, then b9) a separation step of the liquid phase from the gaseous phase of the fluid from the liquefaction step b8), then b10) a recycling step of the liquid phase from the separation step b9) to the distillation step b6).

5. A method according to any one of the preceding claims, characterized in that it comprises the following successive steps: - a heating step of the gaseous phase from step b9), - a membrane filtration step of the heated gas phase, - a step of recycling the gas from the previous filtration step, before step b3) of compression.

6. A process according to claim 2, characterized in that it comprises, after step 4), a step 5) of storing the liquefied gas to be purified.