EP3727649A1 - Cryogenic process for removing nitrogen from a discharge gas - Google Patents

Abstract

A process for producing biomethane (40) by purification of a biogas feed stream (1), comprising the following steps: Step a): introduction of the feed gas stream (1) into a pretreatment unit (5) in which said gas stream is partially separated from the CO₂ and the oxygen that it contains and is compressed at a pressure P1 greater than 50 bar abs; Step b): the gas stream to be treated (22) resulting from step a), depleted of CO₂, is subjected to cryogenic separation by introducing it into a distillation column (26) in order to separate the nitrogen from said gas stream to be treated; Step c): a stream (27) enriched with CH₄ resulting from the cryogenic separation is recovered by pumping the bottom product (37) of said column (26) at a pressure P2 greater than the critical pressure of said product, characterized in that said gas stream resulting from step a), depleted of CO₂ , used in step b) comprises between 0.3 mol% and 2 mol% of CO₂.

Description

 Process for producing a natural gas stream from a biogas stream

The invention relates to a process for producing bio methane by biogas purification, for example biogas from non-hazardous waste storage facilities (ISDND). It also relates to an installation for implementing the method.

More specifically, the present invention relates to a method of treatment by coupling a membrane permeation and a cryogenic distillation of a gaseous stream containing at least methane, carbon dioxide, air gases (nitrogen and oxygen) and pollutants (H ₂ S and volatile organic compounds (VOCs)). The objective is to produce a gaseous stream rich in methane whose methane content is in line with the needs of its use and to limit as much as possible the impact of CH ₄ discharges into the atmosphere (high greenhouse gas) ).

 The invention relates in particular to the purification of biogas from non-hazardous waste storage facilities, hereinafter ISDND (Non-Hazardous Waste Storage Facility), with the aim of producing biomethane in accordance with the injection into a natural gas system or in local use as a vehicle fuel.

 The anaerobic digestion of organic wastes in ISDNDs produces a significant amount of biogas throughout ISDND's lifetime and even several years after shutdown and closure of ISDND. By its main constituents - methane and carbon dioxide - biogas is a powerful greenhouse gas; At the same time, it constitutes a significant source of renewable energy in the context of the scarcity of fossil fuels.

 Biogas contains several polluting compounds and must be purified to allow commercial development. There are several processes for recovering and purifying biogas.

Biogas mainly contains methane (CH ₄ ) and carbon dioxide (CO ₂ ) in varying proportions depending on the method of production.

In the case of ISDND biogas, the gas also contains a proportion of air gases (nitrogen and oxygen) and, to a lesser extent, water, hydrogen sulphide, and volatile organic compounds (VOCs). According to the degraded organic matter, the techniques used and the particular conditions (climates, typologies ...) of each ISDND, the proportions of the biogas components differ. Nevertheless, on average, the biogas comprises, on dry gas, 30 to 60% of methane, 15 to 50% of CO2, 0 to 30% of nitrogen, 0 to 6% of oxygen, 0 to 1% of hosts and a few tens to thousands of milligrams per normal cubic meters of VOCs and a number of other trace impurities.

 Biogas is valued in different ways. It may, after partial treatment, be recovered near the production site to provide heat, electricity or both (cogeneration). The high content of carbon dioxide and nitrogen reduces its calorific value, increases the compression and transport costs and limits the economic interest of its valuation to this use of proximity.

 Further purification of the biogas allows its wider use. In particular, a thorough purification of the biogas makes it possible to obtain a biogas purified to the specifications of natural gas and which can be substituted for it. The biogas thus purified is called "biomethane". Biomethane thus completes the natural gas resources with a renewable part produced in the heart of the territories. It is usable for exactly the same uses as natural gas of fossil origin. It can feed a natural gas network, a filling station for vehicles.

 The modes of valorization of the biomethane are determined according to the local contexts: local energy needs, possibilities of valorization as biomethane fuel, existence close to networks of distribution or transport of natural gas in particular. Creating synergies between the different actors working on a territory (farmers, industrialists, public authorities), the production of biomethane helps the territories to acquire a greater energy autonomy.

 It should be noted that, depending on the country, environmental regulations often impose restrictions on releases to the atmosphere.

It is indeed necessary to put in place technologies to limit the impacts of greenhouse gases (CH ₄ ) and pollutants (H2S and VOCs) contained in biogas. It is therefore important to have a high CH ₄ yield (equal, in bulk, to the amount of CH ₄ recovered in relation to the amount of CH ₄ contained in the biogas) and to provide treatment systems for hhS and VOCs that avoid atmospheric emissions.

Moreover, an additional problem remains the presence of 0 ₂ , which, during the separation of the mixture, can generate an explosive atmosphere during the various stages of enrichment. This risk of creating an explosive mixture makes the landfill biogas particularly difficult to purify safely and economically.

The document US Pat. No. 8,221,524 B2 describes a process for enriching a gas with CH ₄ by up to 88% by different recycling steps. The process consists of compressing the gas stream and then passing it over an adsorbent to remove VOCs. The gas stream is then subjected to a membrane separation step and then to a pressure swing adsorption step (PSA). The adsorbent used in the PSA is of the CMS (carbon molecular sieve) type and makes it possible to eliminate the nitrogen and a small part of the oxygen.

 EP1979446 discloses a biogas purification process of removing hhS, compressing the gas, filtering it to remove particles. The gas is then subjected to a membrane separation step to remove CO2 and GO2, from drying by passing through a PSA then in different filters and finally again in a PSA to eliminate nitrogen. The gas is finally liquefied.

 US2004 / 0103782 discloses a biogas purification process of eliminating gas compression, filtering it to remove particles, subjecting it to a pressure swing adsorption (PSA) step to remove VOCs, and then membrane separation to remove most of the CO2 as well as a fraction of the oxygen.

 Document US Pat. No. 5,486,227 describes a process for purifying and liquefying a gaseous mixture consisting of subjecting the stream to a temperature-modulated adsorption (TSA) to eliminate hhS in particular and then to a pressure-swing adsorption (PSA) to eliminate CO2 in particular, then finally cryogenic separation to eliminate nitrogen and retain only methane.

US5964923 and US5669958 disclose a method of treating a gaseous effluent comprising dehydrating the gas, condensing it through an exchanger, subjecting the gas to membrane separation, and then cryogenic separation. US2010 / 077796 discloses a purification process of subjecting the gaseous stream to a membrane separation, treating the permeate in a distillation column, and then mixing the methane gas from the column, after vaporization, with the retentate obtained at room temperature. the outcome of membrane separation.

 Documents US3989478 and FR2917489 describe cryogenic systems for purifying a methane-rich stream. Both systems use an adsorption system to remove CO2 prior to the liquefaction stage.

 In US3989478, the regeneration of the adsorption systems is carried out through the nitrogen-rich distillate recovered at the top of the distillation column. In the document FR2917489, the regeneration of the adsorption system is carried out by liquid methane withdrawn at the bottom of the distillation column.

EP0772665 describes the use of a cryogenic distillation column for the separation of mine gas composed mainly of CFI ₄ , CO2 and nitrogen.

 None of the cited documents addresses the problem of providing safe biomethane from GO2, a methane concentration greater than 95%, a CO2 concentration of less than 2.5% and a methane yield greater than 85%.

One of the problems that the invention proposes to solve is that of providing a biogas purification process complying with the above constraints, that is to say a process that is safe, with optimal yield, producing a biomethane high quality substitutable for natural gas and which meets environmental standards including the destruction of polluting compounds such as VOCs and compounds with strong greenhouse effect such as CFI ₄ . The gas thus produced can be recovered in gaseous form either by injection into a gas network or for mobility applications.

 Furthermore, in the prior art, it is known to treat biogas in a gas purification unit that can use the following steps: a PSA (Pressure Swing Adsorption), an adsorbent screen (to remove VOCs) and a membrane floor.

The CO2 is mainly removed on the membrane step. This imperfect separation leaves in the purified gas a CO2 content frequently between 0.5 mol% and 1.5 mol%. It is possible to reduce the content of CO2 in the purified gas by sizing the unit of separation (involving consumption more important of the compressor). In all cases the CO2 content in the purified gas can never be much lower (same order of magnitude of concentration).

 This purified gas containing, among others, the remainder of CO2, methane, a little oxygen and nitrogen (between 1% and 20% mol) is then treated in a cryogenic unit.

 The temperatures reached in this unit are of the order of -100 ° C. or lower, which at low pressure (between Patm and about thirty bar) results in a solidification of the CO2 contained in the gas to be treated.

 A frequently used solution is to use a purification step based on adsorption technology (TSA, Temperature Swing Adsorption). This technology makes it possible to reach very low levels of CO2 (for example 50ppmv in the case of a liquefied natural gas). At these levels, the CO2 does not solidify at the temperatures considered even at low pressure because it is still soluble in methane. However, this purification unit is relatively expensive and requires the use of a so-called regeneration gas to be able to evacuate the stopped CO2. The gas frequently used is either the nitrogen that has been separated in the cryogenic stage or the methane product at the outlet of NRU. If nitrogen is used, it may be necessary to degrade the efficiency of the unit or add nitrogen to achieve the required flow rate. If production methane is used, CO2 concentration peaks associated with desorption may appear to make the gas out of specification.

 There is therefore a need to improve the processes as described above while reducing operating costs.

 The inventors of the present invention have then developed a solution to solve the problems raised above.

 The subject of the present invention is a method for producing biomethane by purifying a biogas feed stream, comprising the following steps:

Step a): introduction of the feed gas stream into a pre-treatment unit in which said gas stream is partially separated from the CO2 and oxygen it contains and is compressed at a pressure P1 greater than 25 bar absolute but preferably greater than 50 bar abs; Step b): subjecting the gaseous stream to be treated from step a) depleted in CO2 to a cryogenic separation by introducing it into a distillation column to separate the nitrogen from the gaseous stream to be treated,

Step c): a stream enriched in CH ₄ obtained from the cryogenic separation is recovered by pumping the bottom product from said column at a pressure P2 greater than 25 bar absolute and preferably greater than the critical pressure of said product, characterized in that said gaseous stream from step a) depleted of CO2 implemented in step b) comprises between 0.3 mol% and 2 mol% of CO2 _.

 The solution that is the object of the present invention is therefore not to further reduce the CO2 content at the outlet of the membrane step while ensuring a sufficient solubility of the CO2 in the gas to be treated (mainly methane) in order to avoid a crystallization and that at any point of the process.

 The TSA stage for slaughtering the majority of CO2 is therefore removed. The gas that supplies the cryogenic section therefore contains between 0.3 mol% and 2 mol% of CO2.

 According to other embodiments, the subject of the invention is also:

 - A method as defined above, characterized in that step a) further comprises a step of purifying the compressed gaseous gas stream at pressure P1.

 A process as defined above, characterized in that, in step a), the separation of CO2 and oxygen from the feed gas stream is carried out by a unit comprising at least two stages of separating membranes.

 A process as defined above, characterized in that the pressure P2 of step c) is greater than 40 bar abs.

 A process as defined above, characterized in that during step b), the gaseous stream depleted in CO2 from step a) undergoes expansion to a pressure P3 between 15 bar abs and 40 bar abs before entering said distillation column. Preferably, P3 is greater than 25 bar absolute.

 A process as defined above, characterized in that prior to expansion, the gaseous stream depleted of CO2 from step a) is at least partially condensed in a heat exchanger.

A process as defined above, characterized in that the gaseous stream depleted of CO2 from step a) is at least partially condensed in a heat exchanger countercurrent CH ₄ enriched stream from step c) and at least a portion of the nitrogen stream separated in step b).

 The subject of the invention is also:

 An installation for the production of bio methane by purification of biogas from non-hazardous waste storage facilities (ISDND) implementing the method as defined above.

 An installation as defined above, for the production of bio methane by purification of biogas from non-hazardous waste storage facilities (ISDND) according to the preceding claim, comprising successively:

 o a source of biogas;

 a pretreatment unit for removing all or part of the VOCs, the water, the sulfur compounds of the gas stream to be treated;

 o at least two stages of separating membranes capable of partially separating the CO2 and GO2 from said gas stream;

 a compressor capable of compressing said gaseous flow at a pressure of between 50 and 100 bar;

 o a heat exchanger able to cool the gaseous stream depleted in CO2, o a distillation column;

characterized in that it does not include TSA for removing CO2 at levels less than 0.3 mol%.

 The heat exchanger may be any heat exchanger, unit or other arrangement adapted to allow the passage of a number of flows, and thus allow a direct or indirect heat exchange between one or more lines of refrigerant, and a or multiple feed streams.

 The invention will be described in more detail with reference to the figure which illustrates a particular embodiment of a method according to the invention implemented by an installation as shown schematically in the figure.

 The reference refers to a liquid flow and the pipe that carries it, the pressures considered are absolute pressures and the percentages considered are molar percentages.

In the figure, the plant comprises a source of biogas to be treated (1), a pre-treatment unit (5) comprising a compression unit (2) and a unit for purifying CO2 and Ü2 (23), a VOC and water purification unit (3), a cryodistillation unit (4), and finally a methane gas recovery unit (6). All devices are interconnected by pipes.

 Upstream of the compression unit (2) is the CO2 purification unit (23) and any pre-treatment units.

The CO2 purification unit (23) combines, for example, two membrane separation stages. The membranes are chosen to allow the separation of at least 90% of the CO2 and about 50% of GO2. The retentate from the first separation is then directed to the second membrane separation. The permeate from the second membrane separation is recycled through a pipe connected to the main circuit upstream of the compressor. This step makes it possible to produce a gas (7) with less than 3% CO2 and with a CH ₄ yield greater than 90%. The temperature of this stream is typically ambient, if necessary air or water cooling steps can be incorporated.

 The compression unit (2) is for example in the form of a piston compressor.

 This compressor compresses the gas stream (7) at a pressure of, for example, between 50 and 80 bar. The outgoing flow is designated in the figure by the reference (8).

 The VOC and water purification unit (3) comprises two bottles (9, 10). They are loaded with adsorbents chosen specifically to allow the adsorption of water and VOCs, and their subsequent desorption during regeneration. The bottles work alternately in production mode and regeneration mode.

 In production mode, the bottles (9, 10) are supplied with gaseous flow at their lower part.

The pipe in which the gas flow (8) flows is split into two pipes (1 1, 12), each equipped with a valve (13, 14) and feeding the lower part respectively of the first bottle (9) and the second bottle (10). The valves (13, 14) will be alternately closed depending on the saturation level of the bottles. In practice, when the first bottle is saturated with water, the valve (13) is closed and the valve (14) is opened to begin charging the second bottle (10). From the upper part of each of the bottles opens a pipe respectively (15 and 16). Each of them splits into two pipes respectively (17, 18) and (19, 20). The purified flow of water and VOC from the first bottle flows through the pipe (18) while the purified flow of water and VOC from the second PSA flows through the pipe (20). The two pipes are joined to form a single pipe (21) supplying the cryogenic unit (4).

 In regeneration mode, the regenerative gas flows in the pipes (17, 19). It comes out at the bottom of the bottles.

 The cryodistillation unit (4) is fed by the pipe (21) in which circulates the gas stream (22) to be purified. It contains three elements respectively a heat exchanger (24), a reboiler (25), a distillation column (26).

 The exchanger (24) is preferably a brazed plate heat exchanger made of aluminum or stainless steel. It cools the gas stream (22) flowing in the pipe (21) by heat exchange with the flow of liquid methane (27) withdrawn from the distillation column (26). The gas stream (22) is cooled (28) to a temperature of about -100 ° C. The two-phase flow (28) resulting therefrom may alternatively ensure the reboiling of the bottom reboiler (25) of the column (26) and the heat generated (29) is transferred to the bottom of the column (26).

 The cooled fluid (28) is expanded by means of a valve (30) at a pressure for example between 20 bar absolute and 45 bar absolute bar absolute. The fluid then in the diphasic state or in the liquid state (31) is introduced into the column (26) at a stage E1 located in the upper part of said column (26) at a temperature, for example between -1 10 ° C and -100 ° C.

The liquid (31) then separates in the column (26) to form a gas (32) through the condenser (33). The cooling of the condenser (33) may, for example be provided by a refrigerating cycle using nitrogen and or methane. A portion (36) of the liquid (37) exiting the distillation column vessel (26) at a temperature between -120 ° C and -90 ° C is sent to the reboiler (25) where it partially vaporizes. . The formed gas (29) is returned to the column vessel (26). The other portion (38) of the remaining liquid (37) is pumped by means of a pump (39) to form the liquid methane stream (27) which vaporizes in the exchanger (24) to form a methane product pure gas (40). This pumping step is carried out at a high pressure, typically above 25 bar absolute, preferably above 50 bar absolute or the critical pressure of the fluid. This level of pressure makes it possible to avoid the accumulation of CO2 in the last drop of vaporization of the exchange line. The gas is very poor in heavy hydrocarbons, the dew point of the gas below the critical pressure is very low (typically below -90 ° C).

Claims

1. A process for producing biomethane (40) by purifying a biogas feed stream (1) comprising the steps of:

 Step a): introduction of the feed gas stream (1) into a pretreatment unit (5) in which said gas stream is partially separated from the CO2 and oxygen it contains and is compressed to a pressure P1 greater than 25 bar abs;

 Step b): the gaseous stream to be treated (22) from the CO2-depleted stage a) is subjected to a cryogenic separation by introducing it into a distillation column (26) in order to separate the nitrogen from the gaseous stream at treat

Step c): a stream (27) enriched in CH ₄ obtained from the cryogenic separation is recovered by pumping the bottom product (37) from said column (26) at a pressure P2 greater than 25 bar absolute and preferably greater than 25 bar. the critical pressure of said product,

characterized in that during step a), the separation of CO 2 and oxygen from the feed gas stream is carried out by a unit comprising at least two stages of separating membranes so that said gaseous stream resulting from the step a) depleted in CO2 implemented in step b) comprises between 0.3 mol% and 2 mol% of CO2 _.

2. Method according to the preceding claim, characterized in that step a) further comprises a water purification step of the gas stream (8) compressed at the pressure P1.

3. Method according to one of the preceding claims, characterized in that P1 is greater than 50 bar absolute.

4. Method according to one of the preceding claims, characterized in that the pressure P2 of step c) is greater than 40 bar abs.

5. Method according to one of the preceding claims, characterized in that during step b), the gas stream (22) depleted of CO2 from step a) is relieved (30) to a pressure P3 between 15 bar abs and 40 bar abs before entering said distillation column (26).

6. Method according to the preceding claim, characterized in that prior to the expansion (30), the gaseous stream (22) depleted of CO2 from step a) is at least partially condensed in a heat exchanger (24).

7. Method according to the preceding claim, characterized in that the gaseous stream (22) depleted of CO2 from step a) is at least partially condensed in a heat exchanger (24) against the current (27). enriched CH ₄ from step c) and at least a portion of the nitrogen stream separated in step b).

8. Installation for the production of bio methane by purification of biogas from non-hazardous waste storage facilities (ISDND) implementing the method according to one of the preceding claims, comprising successively:

 o a source of biogas (1);

 a pretreatment unit (3) for removing all or part of the VOCs, water, sulfur compounds from the gas stream to be treated;

 o at least two stages of separating membranes (23) able to partially separate the CO2 and GO2 from said gas stream;

 a compressor (2) capable of compressing said gas flow at a pressure of between 25 and 100 bar absolute;

 a heat exchanger (24) capable of cooling the depleted gaseous flow (22) to CO2;

 a distillation column (26);

characterized in that it does not include TSA for removing CO2 at levels less than 0.3 mol%.