Patent Application
20220397339
This application claims the benefit of priority under 35 U.S.C. § 119 (a) and (b) to French Patent Application No. 2106085, filed Jun. 9, 2021, the entire contents of which are incorporated herein by reference.
The present invention relates to a plant and a process for producing liquid methane and liquid carbon dioxide from a biogas flow.
Biogas is the gas produced during the degradation of organic matter in the absence of oxygen (anaerobic fermentation), also known as methanization. This may be natural degradation—it is thus observed in marshland or in household waste landfills—but the production of biogas may also result from the methanization of waste in a dedicated reactor referred to as a methanizer or digester.
By virtue of its main constituents—methane and carbon dioxide—biogas is a powerful greenhouse gas; at the same time, it also constitutes a source of renewable energy which is appreciable in the context of the increasing scarcity of fossil fuels.
Biogas predominantly contains methane (CH4) and carbon dioxide (CO2), in proportions which can vary according to the way in which it is obtained, but also contains, in smaller proportions, water, nitrogen, hydrogen sulfide, oxygen, and also other organic compounds, in the form of traces.
Depending on the organic matter which has undergone decomposition, and on the techniques used, the proportions of the components differ; on average, however, biogas comprises, on a dry gas basis, from 30% to 75% of methane, from 15% to 60% of CO2, from 0% to 15% of nitrogen, from 0% to 5% of oxygen and trace compounds.
After a step of pretreating these contaminants, the biogas can be used as is in order to supply a boiler or a cogeneration unit, or else purified in order to obtain a gas which meets the specifications for injection into the natural gas network (e.g.: 3% CO2 max).
In numerous regions of Europe and throughout the world, the natural gas network is not always accessible close to the areas of production of fermentable waste. Furthermore, while there is no need for heat on the biogas production site, depending on the purchase price of electricity, the cogeneration does not always have a sufficient output to render profitable the major investment in a digestion unit. It is then advantageous in these two cases to transport the biogas to a distribution or consumption point. The liquefaction of biogas after purification would make it possible to transport biomethane at a lower cost. According to the regulations in certain geographic zones, it is forbidden to release CH4 into the environment; this adds an additional constraint and restricts the choice of biogas separation processes to highly effective methods.
Today, biogas purification processes are mainly based on absorption, permeation or adsorption techniques. These systems then require the addition of a supplementary module in order to obtain biomethane in the liquid form. Moreover, in the majority of cases, the content of CO2 in the biogas at the end of this purification step is still too high to supply such liquefaction systems.
A system of cryotrapping based on the principles of reversible exchangers has been proposed. The system is based on the solidification of the CO2 present in the biogas on a cold surface (trapping), followed by a step of sublimation or liquefaction of the CO2 using a hot source. For a continuous production of biomethane, is then necessary to work with several exchangers in parallel. Their solution makes it possible to separate and liquefy the methane and the CO2 into separate steps, but it is not possible to recover the cold used in the solidification of the CO2.
Starting from there, one problem that arises is that of providing a method of separating and liquefying methane and CO2 from biogas with a minimum loss of methane and using a minimum number of operations.
A solution of the present invention is a combined plant for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas flow, comprising:
Depending on the case, the plant according to the invention can have one or more of the features below:
The present invention also relates to a combined process of cryogenic separation and liquefaction of methane and carbon dioxide within a biogas flow, using the plant as defined previously, and comprising:
Note that the withdrawal of vapour V then the reinjection of a two-phase flow D as described above makes it possible to reduce the reflux at the top of the column which is at a much lower temperature level and is supplied directly by an external source of cold.
Depending on the case, the process according to the invention may have one or more of the features below:
The process according to the invention makes it possible to separate and liquefy the products of the biogas in a single combined distillation/liquefaction operation. The operating conditions of the products at the inlet and outlet of the column and in the recycle section have been calculated to prevent the formation of solid CO2.
The thermal integration between the flows of the separation section and those of the refrigeration cycle enable the recovery of the cold used in the liquefaction of the CO2 and in the recycling of the liquid methane. It is possible to completely or partly recover the energy used in the liquefaction of the CO2 if this CO2 is not desired as a product or when it can be used in the gaseous state.
For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawings, in which like elements are given the same or analogous reference numbers and wherein:
The pretreated biogas 1 (pretreated by drying, desulfurization) is introduced into the process at atmospheric pressure and temperature, it is compressed a first time in a compressor C01, to the pressure of the recycle circuit (around 8 bar). After compression, it is cooled in C01E to ambient temperature with CW(=Cooling Water) or air.
Next, it is mixed with a recycle flow R, the mixture is compressed in a compressor CO2, to the pressure of the distillation column (around 15 bar) or more depending on the requirements of the downstream exchanger E01 and it is cooled to ambient temperature in C02E, with CW or air.
Preferably, C01E and C02E are shell and tube exchangers (coolers of the compressors)
The mixture of biogas—recycle flow R is sent to the exchanger E01. The main objective of this exchanger is to cool the mixture in preparation for the distillation. The mixture can then be expanded or supplied directly to the column where it will be used as reboiler.
If there is no heat source at the bottom of the column, it is necessary to inject the mixture into the bottom to ensure the circulation of vapour from the bottom. If there is a heat source in the bottom of the column (reboiler), the mixture is introduced higher up in the column.
The distillation column K01 separates the methane from the carbon dioxide. The feed for the column is the biogas+recycle flow R mixture. This feed acts as main reboiler; an additional source of heat may also be used (for example an electrical resistance heater, vapour or a portion of the hot biogas in indirect contact). The product at the top of the column is pure CH4 in the vapour state. The bottom product is a liquid rich in CO2, containing around 95%-98%.
Vapour V is withdrawn from an intermediate stage of the distillation column K01. This vapour V is sent into the exchanger E01 and partially condensed so as to produce a two-phase flow D. The two-phase flow D is reinjected into the column at the stage corresponding to the equilibrium temperature.
The methane at the top of the column is liquefied in the exchanger E02, against a fluid from a closed refrigeration circuit. A portion 2 of the methane leaves the cycle as product and the other portion 3 (reflux portion) is used as recycle for the column and reinjected at the top of the column.
The CO2-enriched liquid recovered at the bottom of the column is expanded and heated in the exchanger E01 countercurrent to the biogas—recycle flow R mixture.
The CO2-enriched flow from the exchanger E01 is sent to the separator vessel V01.
The overhead vapour of the vessel V01 is reheated in the exchanger E01 and then mixed with the biogas. It corresponds to the flow previously named “recycle flow R”.
The liquid from the bottom of the vessel V01 is the pure CO2 4. This can, depending on the requirements, leave the process as product or be reheated in the exchanger E01 and in another exchanger E03 of the refrigeration circuit in order to be completely vaporized before leaving the cycle. Note that the pure CO2 could alternatively be reheated and vaporised in the exchanger E03 without passing through the exchanger E01.
The exchanger E01 therefore uses, as sources of cold: the CO2-enriched liquid recovered at the bottom of the column, the overhead vapour from the vessel V01 named “recycle flow R” at the outlet of the exchanger E01, and optionally the liquid pure CO2 recovered at the bottom of the vessel V01 in the case where the vaporisation thereof is desired.
The process requires an input of refrigeration power in order to operate. This input of cold is represented in
This refrigeration cycle can be replaced by other sources of cold (depending on the amount of liquid biomethane to be produced). By way of example but not exclusively:
It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2106085 | Jun 2021 | FR | national |
