METHOD FOR SEPARING ALL OR PART OF THE COMPOUNDS OF A BIOGAS IN LIQUID OR TWO-PHASIC STATE
A two-stage cryogenic distillation process with a C3 to C7 hydrocarbon liquefaction agent effectively separates methane and carbon dioxide from biogas, addressing purity and recovery challenges in existing methods, achieving over 99.9% purity.
Patent Information
- Authority / Receiving Office
- FR · FR
- Patent Type
- Patents
- Current Assignee / Owner
- SUBLIME ENERGIE
- Filing Date
- 2021-10-01
- Publication Date
- 2026-06-26
AI Technical Summary
Existing biogas purification methods, such as membrane separation, PSA, and cryogenic distillation, fail to achieve high purity levels of methane and carbon dioxide recovery, especially when biogas contains hydrocarbons like butane, leading to membrane deterioration, high heat consumption, and solid carbon dioxide particle formation.
A two-stage cryogenic distillation process using a liquefaction agent from the C3 to C7 hydrocarbon family is injected into a first distillation column to separate methane, followed by a second distillation to recover carbon dioxide, preventing dry ice formation and ensuring high purity recovery.
The process achieves high purity separation of methane and carbon dioxide, exceeding 99.9%, while avoiding membrane degradation and solid carbon dioxide issues, facilitating efficient biogas compound recovery.
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Abstract
Description
Title of the invention: METHOD FOR SEPARING ALL OR PART OF THE COMPOUNDS OF A BIOGAS IN LIQUID OR TWO-PHASIC STATE TECHNICAL FIELD OF THE INVENTION
[0001] The present invention relates to the field of biogas valorization, and relates more particularly to a process for separating all or part of the compounds of a biogas in liquid or two-phase liquid / vapor state, the biogas containing methane, carbon dioxide and possibly hydrocarbons of the C3 to C7 family allowing its liquefaction.
[0002] The process according to the invention is intended in particular, but not exclusively, for the purification of biogas in liquid or two-phase liquid / vapor state for the purpose of recovering all or part of its compounds, and in particular methane for use as fuel for vehicles or for injection into a natural gas network as well as carbon dioxide. STATE OF THE ART
[0003] Among the known biogas purification methods, the conventionally used methods for separating methane from carbon dioxide are membrane separation, PSA (Pressure Swing Adsorption) and cryogenic distillation.
[0004] Membrane separation is based on the use of membranes made of materials that allow carbon dioxide to pass through while retaining methane. PSA is based on the adsorption of carbon dioxide into the solid components of the high-pressure adsorption column and desorption at low pressure. Cryogenic distillation, on the other hand, relies on successive temperature reductions until the production of liquid carbon dioxide and liquefied biomethane, or on the desublimation of carbon dioxide, which produces liquid carbon dioxide and gaseous biomethane.
[0005] These conventionally used methods, however, have drawbacks for separating biogas in liquid or two-phase states. Either they do not allow for satisfactory purity levels of the recovered methane, or they do not allow for the recovery of carbon dioxide, or they prove unsuitable for processing liquefied biogas or biogas containing hydrocarbons, particularly butane, used to liquefy the biogas. Thus, membrane separation is not the preferred technique when the biogas contains hydrocarbons, and especially butane, as the latter rapidly deteriorates the membranes. PSA does not This method, however, does not allow for achieving the desired purity levels. Furthermore, it requires a significant amount of heat, which contradicts the use of cold previously applied to liquefy the biogas. Cryogenic distillation, on the other hand, has the disadvantage of producing solid carbon dioxide particles (dry ice crystals) that can cause blockages in heat exchangers and the packing in the column head.
[0006] The invention aims to remedy these problems by proposing a separation process allowing the separation of the compounds of a biogas in liquid or two-phase liquid / vapor state in order to recover methane at a high purity level and to recover carbon dioxide while avoiding the problems of clogging and formation of dry ice. SUBJECT OF THE INVENTION
[0007] To this end, and according to a first aspect, the invention proposes a process for separating all or part of the compounds of a biogas in liquid or two-phase liquid / vapor state, containing methane, carbon dioxide and optionally a hydrocarbon or a mixture of hydrocarbons from the C3 to C7 family, the process being remarkable in that a first separation to separate the methane from the other compounds is carried out by cryogenic distillation in a first distillation column having a column head heated to the condensation temperature of methane at a given pressure, the first separation being carried out by injecting into the first distillation column: - as the primary feedstock, biogas is liquefied at an equilibrium temperature, resulting in a two-phase mixture that ensures the separation of the different compounds. - in secondary feed, a liquid liquefaction agent composed of one or a mixture of hydrocarbon(s) from the C3 to C7 family, the liquefaction agent being injected at the top of the first distillation column, above the biogas inlet level, at a temperature Tl causing the desublimation of carbon dioxide and in a quantity proportional to the flow rate of carbon dioxide vapor rising at the top of the column.
[0008] The injection of a liquefaction agent at the top of the column above the biogas inlet thus makes it possible to dissolve the dry ice crystals which form at the top of the distillation column used to separate methane from other compounds by ensuring a satisfactory separation of gaseous methane from the liquid mixture composed of carbon dioxide and the liquefaction agent.
[0009] Advantageously, the liquefaction agent is injected at the methane reflux point.
[0010] Advantageously, the liquefaction agent is injected into the first dis- column tillation at a temperature Tl of the order of -100°C.
[0011] Advantageously, the liquefaction agent is subjected to two-stage cooling under pressure before being injected into the head of the first distillation column to reach temperature Tl.
[0012] Advantageously, the head of the first distillation column is cooled in whole or in part by the liquefaction agent.
[0013] Advantageously, the process includes a step of recovering methane in gaseous state after the first separation.
[0014] According to an advantageous embodiment, a second separation is carried out on the components of the liquid mixture recovered after the first separation, consisting of carbon dioxide, the liquefaction agent, and any hydrocarbons present. This second separation is performed by cryogenic distillation in a second distillation column after the liquid mixture has been decompressed to reach an equilibrium temperature and pressure that allows the separation of carbon dioxide as condensate at the top of the column and the liquefaction agent, along with any hydrocarbons present as residues, in liquid form at the bottom of the column.
[0015] Advantageously, the second separation is carried out by cooling the head of the second column to the condensation temperature of carbon dioxide, which is between -50° Celsius and -60° Celsius, depending on the pressure reached after the expansion.
[0016] Advantageously, the separation process includes a step of recovering carbon dioxide in gaseous form (condensate) at the end of the second separation.
[0017] Advantageously, the process includes a step of recovering the liquefaction agent, along with any hydrocarbons, after the second separation for its rerouting in whole or in part to the first distillation column.
[0018] Advantageously, the liquefaction agent is a linear or non-linear hydrocarbon of the alkene type from the C3 to C7 family or a mixture of hydrocarbons from the C3 to C7 family.
[0019] Advantageously, the liquefaction agent is a hydrocarbon or a mixture of hydrocarbons identical or having physico-chemical properties equivalent to the possible hydrocarbon or mixture of hydrocarbons present in the biogas.
[0020] The invention also relates to an installation enabling the separation of all or part of a biogas in liquid or two-phase liquid / vapor state, containing methane, carbon dioxide and optionally a hydrocarbon or a mixture of hydrocarbons, implementing the separation process as described above.
[0021] The advantage of the separation process according to the invention and the associated installation is that it allows for high purity levels of the compounds separated from the biogas, on the order of 99.9% for each of the compounds (methane and carbon dioxide) and of the The same order applies to the liquefaction agent recovered at the outlet of the second separation.
[0022] The advantage of the separation process according to the invention and the installation, in the case of non-liquid biogas or biogas in a two-phase state, is to facilitate the anti-sublimation of carbon dioxide. BRIEF DESCRIPTION OF THE FIGURES
[0023] Other features and advantages of the invention will become apparent from the detailed description of the invention which follows with reference to the accompanying figures, in which:
[0024] [Fig.1] Fig.1 represents a schematic view of an example of the implementation of an installation for the separation of all or part of the compounds of a biogas in liquid state or in a two-phase liquid / vapor state implementing a separation process according to the invention;
[0025] [Fig.2] Fig.2 represents the main steps of a process for separating any or part of the compounds of a biogas in liquid state or in a two-phase liquid / vapor state according to the invention. DETAILED DESCRIPTION OF THE INVENTION
[0026] In relation to figures 1 and 2, a process for separating the compounds of a biogas in the liquid state or in a two-phase liquid / vapor state and the installation provided for this purpose are described.
[0027] In the example described, the biogas to be treated contains methane, carbon dioxide, and a hydrocarbon or mixture of hydrocarbons from the C3 to C7 family. The hydrocarbon or mixture of hydrocarbons was introduced to ensure the prior liquefaction of all or part of the biogas. The liquefied biogas will also be referred to as a "ternary mixture" in this application, being composed mainly of methane, carbon dioxide, and a hydrocarbon. In the example described below, it is considered to be biogas liquefied or rendered mainly liquid by n-butane (C4). Since the invention is not limited to this hydrocarbon, the following description remains applicable to biogas liquefied or rendered mainly liquid by a hydrocarbon or mixture of hydrocarbons from the C3 to C7 family other than C4.
[0028] The separation of the biogas compounds is based on a double distillation. The installation 1, illustrated in [Fig. 1], thus comprises a first distillation column 10 for purifying the biogas and a second distillation column 20 for separating the carbon dioxide from the other compounds.
[0029] The first distillation column 10 comprises theoretical trays defining the stages of the column, a packing to promote bubbling, and an exchange surface suitable for the rising vapors through descending liquids, a condenser 11 fluidly connected to the first distillation column 10, at the top of the latter, as well as heating means, of the boiler type, provided at the foot of the first column.
[0030] The first distillation column 10 further comprises a dual feed: a first feed, designated the main feed 12, for the injection of biogas and a second feed, designated the secondary feed 13, for the injection of a liquefaction agent in liquid form. The latter will be described in detail later. The main feed 12 separates the upper part of the column (column head) from the lower part of the column. The secondary feed 13 is arranged to allow the injection of the liquefaction agent at the top of the column, advantageously at the theoretical first stage of said column, but preferably at the reflux 17 of the condenser.
[0031] The second distillation column 20 comprises an arrangement similar to that of the first column except that it has only one feed to inject the residues from the separation operation into the first distillation column 10 and conveyed via a circuit 14 having as its inlet a reservoir 16 in which the residues have been stored after recovery from the bottom of the first distillation column 10 and as its outlet the second distillation column 20. It thus comprises trays defining the theoretical stages of the column, a packing to promote the proper bubbling of rising vapors through descending liquids, a condenser fluidly connected to the second distillation column 20, at the top of the latter, as well as heating means, of the type of boiler, provided at the bottom of the first distillation column 10.
[0032] The installation 1 further comprises a rerouting circuit 15 of the liquefaction agent supplemented with the hydrocarbon(s) initially present in the biogas to the first distillation column 10, the rerouting circuit 15 comprising as an entry point the bottom of the second distillation column 20 and as an exit point, the secondary feed 13 of the first column.
[0033] According to the separation process ([Fig.2]), the biogas undergoes a first separation (step 100) to separate the methane from the other compounds (carbon dioxide and hydrocarbons). This first separation is carried out by cryogenic distillation in the first distillation column 10.
[0034] More particularly, the first separation is carried out by injecting the liquefied biogas at an equilibrium temperature into the first distillation column 10, as the main feed, to obtain a two-phase mixture allowing the separation of the different compounds (step 103).
[0035] Prior to its introduction into the first distillation column 10, the The ternary mixture is heated to a temperature between -60°C and -50°C, at a pressure of approximately 21 bar. The heated mixture is then introduced into the first distillation column 10 to separate the methane from the carbon dioxide and hydrocarbons. Methane, being the lightest compound compared to the hydrocarbon and carbon dioxide mixture, rises as a gas to the top of the column, while the carbon dioxide and hydrocarbons descend as liquids to the bottom. The methane forms the condensate at the top of the first column, while the hydrocarbon and carbon dioxide mixture forms the residue at the bottom.
[0036] To prevent carbon dioxide from freezing, a liquefaction agent is injected via the secondary feed 13 at a temperature T1, causing the carbon dioxide to desublimate, in a quantity proportional to the upward flow rate of carbon dioxide vapor at the top of the column (step 102). Advantageously, the injected liquefaction agent is composed of a hydrocarbon or a mixture of hydrocarbon(s) from the C3 to C7 family in liquid form. Preferably, it is a linear or nonlinear hydrocarbon (alkene type) or a mixture of hydrocarbons from the C3 to C7 family. In the example described, the liquefaction agent chosen is identical to the hydrocarbon present in the biogas to be purified and which had been used to liquefy the biogas, namely n-butane (C4). In this case, the temperature Tl at which the liquefaction agent is injected will be around -100°C.
[0037] Advantageously, the liquefaction agent is injected at the same level as the reflux of the condenser.
[0038] Advantageously, the liquefaction agent is subjected to pressure cooling before being injected into the head of the first distillation column 10 to reach temperature Tl (step 50). In the example described, it is thus brought from ambient temperature to -100°C by passing through two cooling stages via heat exchangers, a first stage at -56°C followed by a second stage at -100°C.
[0039] For the purpose of carrying out the first separation, the column head is pre-cooled to the condensation temperature of methane (step 101). In the example described, the column head is advantageously cooled to -106 °C at a pressure of approximately 20 bar. Advantageously, the head of the first distillation column 10 is also cooled by the liquefaction agent (this cooling is represented in [Fig. 2] by the double arrow between steps 101 and 102). The latter, pre-cooled (step 50), is injected into the column at a temperature of -100 °C with a flow rate sufficient to generate a ternary mixture with the methane, carbon dioxide, and butane initially present.
[0040] Once the column has reached equilibrium, the depotting of the ternary mixture begins with the aid of a pump. The mixture is then heated to -56°C. Once separated from the rest of the compounds, the methane is recovered (step 104) while the residue composed of carbon dioxide and the liquefaction agent plus the hydrocarbon initially present is decompressed after passing through the boiler and continues the second phase of distillation in the second distillation column.
[0041] In the example described, the carbon dioxide is separated. To do this, the residue composed of carbon dioxide and the hydrocarbon mixture, in this case a butane mixture (a mixture composed of the hydrocarbon present in the biogas and the liquefaction agent injected at the top of the column 10) is subjected to a second cryogenic distillation in the second distillation column 20 (step 200).
[0042] Similarly, the top of the second column must be pre-cooled before starting the distillation (step 201). In the example described, it is cooled to the condensation temperature of carbon dioxide, which is between -50° Celsius and -60° Celsius, depending on the pressure reached after expansion (expansion to 5.5 bar). Once the second column has reached its equilibrium point, the binary mixture is introduced into the second distillation column (step 203) after first being subjected to expansion (to 5.5 bar) (step 202) to reach an equilibrium temperature and pressure that allows the separation of carbon dioxide as condensate at the top of the column and the liquefaction agent, along with any hydrocarbon residues, as liquid at the bottom of the column.When the binary mixture is injected into the second distillation column 20, the carbon dioxide undergoes vaporization and rises to the top of the second column in a gaseous state while the hydrocarbon mixture descends in a liquid state to the bottom of the second distillation column 20.
[0043] Once separated from the rest of the compounds, the carbon dioxide is recovered (step 204) while the hydrocarbon mixture is recovered at the bottom of the distillation column to possibly be routed in whole or in part to the first distillation column 10 or a buffer storage tank to possibly be reused to liquefy a biogas (step 205).
[0044] In the embodiment described above, all the compounds constituting the biogas are separated. However, it may be provided that only a portion of the compounds are separated in order to recover only a portion of the biogas, in this case the methane.
[0045] Similarly, in the embodiment described above, the liquefied biogas subjected to the separation process according to the invention is a ternary mixture composed mainly of methane, carbon dioxide and a hydrocarbon of the family C3 to C7. It can be envisaged, however, that the biogas, composed mainly of methane and carbon dioxide, will be suitable for the separation process and installation used to purify such biogas without departing from the scope of the invention. Similarly, the biogas fed into the column may be a mixture in a liquid state or in a two-phase liquid / vapor state.
[0046] The invention is described above by way of example. It is understood that a person skilled in the art is able to carry out different embodiments of the invention without departing from the scope of the invention.
Claims
1.
2.
3. Demands A process for separating all or part of the compounds of a biogas in liquid or two-phase liquid / vapor state, containing methane, carbon dioxide and optionally a hydrocarbon or a mixture of hydrocarbons from the C3 to C7 family, characterized in that a first separation to separate the methane from the other compounds is carried out by cryogenic distillation in a first distillation column (10) having a column head brought to the condensation temperature of methane at a given pressure, the first separation being carried out by injecting into the first distillation column (10): - as the primary feedstock, biogas is liquefied at an equilibrium temperature, resulting in a two-phase mixture that ensures the separation of the different compounds. - in secondary feed, a liquid liquefaction agent composed of a mixture of hydrocarbon(s) from the C3 to C7 family, the liquefaction agent being injected at the top of the first distillation column (10), above the biogas inlet level, characterized in that the liquefaction agent is injected into the first distillation column at a temperature Tl causing the desublimation of carbon dioxide and in a quantity proportional to the flow rate of carbon dioxide vapor rising at the top of the column and in that a second separation of the compounds of the liquid mixture recovered at the end of the first separation and composed of carbon dioxide, the liquefaction agent and any hydrocarbons is carried out by cryogenic distillation in a second distillation column (20) after expansion of the liquid mixture to reach an equilibrium temperature and pressure allowing the separation of carbon dioxide as condensate at the top of the column and the liquefaction agent with any hydrocarbons added as residues in liquid form at the bottom of the column. A separation process according to claim 1, characterized in that the liquefaction agent is injected at the methane reflux point. A separation process according to claim 1 or claim 2, characterized in that the liquefaction agent is injected into the first distillation column (10) at a temperature Tl of the order of -100°C.
4. Separation process according to any one of the preceding claims, characterized in that the liquefaction agent is subjected to two-stage cooling under pressure before being injected into the head of the first distillation column (10) to reach temperature Tl.
5. Separation process according to any one of the preceding claims, characterized in that the head of the first distillation column (10) is cooled in whole or in part by the liquefaction agent.
6. Separation process according to any one of the preceding claims, characterized in that it comprises a step of recovering methane in the gaseous state after the first separation.
7. Separation method according to any one of the preceding claims, characterized in that the second separation is carried out by cooling the head of the second distillation column (20) to the condensation temperature of carbon dioxide between -50 ° Celsius and -60 ° Celsius depending on the pressure reached after expansion.
8. Separation process according to any one of the preceding claims, characterized in that it comprises a step of recovering carbon dioxide in the gaseous state at the end of the second separation.
9. Separation process according to any one of the preceding claims, characterized in that it comprises a step of recovering the liquefaction agent with added hydrocarbons at the end of the second separation for its rerouting in whole or in part to the first distillation column (10).
10. Separation process according to any one of the preceding claims, characterized in that the liquefaction agent is a linear or non-linear hydrocarbon of the alkene type of the C3 to C7 family or a mixture of hydrocarbons of the C3 to C7 family.
11. Separation process according to any one of the preceding claims, characterized in that the liquefaction agent is a hydrocarbon or a mixture of hydrocarbons identical or having physico-chemical properties equivalent to the possible hydrocarbon or mixture of hydrocarbons present in the biogas.
12. Installation (1) enabling the separation of all or part of a biogas in liquid or two-phase liquid / vapor state, containing methane, carbon dioxide and optionally a hydrocarbon or a mixture of hydrocarbons, implementing the separation process according to any one of the preceding claims, said installation comprising a first distillation column (10) for separating methane from the other compounds by cryogenic distillation, said first distillation column (10) comprising: - a main feed for injecting liquefied biogas at an equilibrium temperature, allowing for the creation of a two-phase mixture that ensures the separation of the different compounds and - a secondary feed for injecting a liquid liquefaction agent composed of a mixture of hydrocarbon(s) from the C3 to C7 family, the secondary feed being arranged to inject the liquefaction agent at the top of the first distillation column (10), above the biogas inlet level and a second distillation column (20) for the separation of the compounds of the liquid mixture recovered after the first separation and composed of carbon dioxide, the liquefaction agent and any hydrocarbons.