Cryogenic purification of biogas with pre-separation and external solidification of carbon dioxide

Abstract

A combined plant for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, including a mixing means, a compressor, a first exchanger, a distillation column, a second exchanger, a separating means, an expanding means, and a separator vessel. Wherein, the mixing means is configured such that the recycle gas is the overhead vapour stream, and the first exchanger and the expanding means are combined.

Claims

1. A combined facility for cryogenic separation and liquefaction of methane and carbon dioxide in a biogas stream, comprising: a mixing means for mixing a biogas stream with a recycle gas stream, thereby producing a mixed biogas stream, a compressor for compressing the mixed biogas stream to a pressure suitable for distillation, thereby producing a compressed stream, a first exchanger for cooling the compressed stream, thereby producing a cooled stream, a first separator vessel for receiving the cooled stream and for recovering an overhead vapour stream and a CO.sub.2-enriched liquid stream, a cooling means for cooling the overhead vapour stream, thereby producing a cooled overhead vapor stream, a cryogenic separation unit supplied with the cooled overhead vapour stream, comprising a distillation column, the distillation column comprising a top and a bottom, and producing a methane stream at the top of the distillation column and a CO.sub.2-enriched liquid stream at the bottom of the distillation column, a second exchanger for liquefying the methane stream produced at the top of the distillation column, thereby producing a liquefied methane stream, a separation means for separating the liquefied methane stream into two portions: a reflux portion and a product portion, an expanding means for expanding the CO-enriched liquid stream, thereby producing an expanded CO2-enriched liquid stream, and the first heat exchanger configured to heat the expanded CO2-enriched liquid stream for recovering cold from the expanded CO2-enriched liquid stream, thereby producing an expanded CO2 enriched stream a second separator vessel for receiving the expanded CO.sub.2-enriched stream from the expanding means and the first heat exchanger for producing an overhead vapour stream and liquid CO.sub.2 stream, wherein the mixing means is configured such that the recycle gas stream corresponds to the overhead vapour stream recovered at the outlet of the second separator vessel, and the cryogenic separation unit comprising: the distillation column comprising a cold section at the top of the distillation column and a hot section at the bottom of the distillation column, at least two containers external to the distillation column, for placing in contact a liquid from the cold section and a vapour rising from the hot section and for trapping all solid CO.sub.2, and a device for regenerating the at least two external containers, comprising a means for extracting a fluid from the distillation column which is configured to liquefy the solid CO.sub.2, a means for introducing the fluid into the at least two external containers in regeneration thereby bringing about the melting of the solid CO.sub.2 and a means for reintroducing the resulting liquid CO.sub.2-vapour stream into the distillation column.

2. The facility according to claim 1, wherein the cooling means is the first exchanger.

3. The facility according to claim 1, wherein the expanding means expands the CO.sub.2-enriched liquid stream from the first separator vessel.

4. The facility according to claim 1, further comprising, upstream of the mixing means, a compressing means for compressing the biogas stream to the pressure of the recycle gas stream.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) 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:

(2) FIG. 1 illustrates a refrigeration circuit in accordance with one embodiment of the present invention;

(3) FIG. 2 illustrates further details of FIG. 1, in accordance with one embodiment of the present invention; and

(4) FIG. 3 illustrates further details of FIG. 1, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

(5) 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.

(6) Next, it is mixed with a recycle stream R, the mixture is compressed in a compressor CO.sub.2, 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.

(7) Preferably, C01E and C02E are shell and tube exchangers (cooler of the compressors).

(8) The mixture of biogas-recycle stream R is sent to the exchanger E01. This exchanger E01 firstly makes it possible to cool the mixture in preparation for the separation in the separator vessel V05 to a temperature of between ?20? C. and ?35? C., preferably between ?25? C. and ?33? C. (the objective is to remain at least 2 degrees above the equilibrium temperature of solid formation). The separator vessel V05 receives the mixture from the exchanger E01, and an overhead vapour 8 and a CO.sub.2-enriched liquid 9 at the bottom of the vessel are recovered.

(9) The overhead vapour 8 from the separator vessel V05 is then cooled to a temperature of between ?45? C. and ?55? C., preferably between ?50? C. and ?55? C., in the exchanger E01 (the objective is still to remain at least 2 degrees above the equilibrium temperature of solid formation). The cooled overhead vapour 10 can then directly supply the distillation column K01 at an intermediate stage.

(10) A heat source is used at the bottom of the column (for example an electrical resistance heater, vapour or a portion of the hot biogas in indirect contact).

(11) The distillation column K01 comprises a cold section 2 at the top of the column and a hot section 3 at the bottom of the column and a means 4 for physically separating the cold section and the hot section.

(12) The external containers V04 A/B of the distillation column K01 operate in alternating sequences of filling and regeneration, with always at least one container being filled to allow continuous operation of the column, without said column being disturbed by the presence of solids. The formation of solids in the containers will be brought about by controlling the temperature. The CH.sub.4-rich cold liquid L coming from the top of the distillation column K01 will have a lower temperature than the equilibrium temperature of CO.sub.2 solidification, it will be brought into contact with the rising vapour V (mixture with similar concentrations of CH.sub.4 and CO.sub.2), the temperature of which is higher than the equilibrium temperature of CO.sub.2 solid formation (cf. FIG. 2). The contact of the two fluids will result in a liquid-vapour mixture whose concentrations and temperature are in the range for formation of solid CO.sub.2 F. The rate of saturation of the container with solids will depend on the process pressure and on the size of the containers.

(13) Melting (regeneration) of the solid CO.sub.2 trapped in the external containers will take place by direct contact with a stream of vapour extracted from a lower section of the column at a higher temperature. The resulting liquid-vapour mixture M will be reinjected into a lower stage of the column. And the overhead vapour T will be reinjected into the top of the column.

(14) At any time, there will be at least two regeneration containers in series (cf. FIG. 3). The first container R1 will be the one providing the main part of the cold, the second container R2 will ensure that the temperature and the liquid fraction of the mixture M to be reinjected in the column will remain constant in spite of the depletion of solid E in the first container, Once the first container has been completely regenerated, it can be put in service for the formation of solids. The second container will then act as a cold supply and the newly saturated container will be put into regeneration.

(15) The product at the top of the column is pure CH.sub.4 in the vapour state. The bottom product is a liquid rich in CO.sub.2, containing around 95%-98%.

(16) The methane at the top of the column is liquefied in the exchanger E02, against a fluid from a closed refrigeration circuit, A portion of the methane leaves the cycle as product and the other portion (reflux portion) is used as recycle for the column and reinjected into the top of the column.

(17) The CO.sub.2-enriched liquid recovered at the bottom of the column is mixed with the CO.sub.2-enriched liquid 9 from the separator vessel V05, then is expanded and heated in the exchanger E01 countercurrent to the biogas-recycle stream R mixture.

(18) The CO.sub.2-enriched stream from the exchanger E01 is sent to the separator vessel V01.

(19) The overhead vapour of the vessel V01 is reheated in the exchanger E01 and then mixed with the biogas. It corresponds to the stream previously named recycle stream R.

(20) The liquid from the bottom of the vessel V01 is the pure CO.sub.2. 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 CO.sub.2 could alternatively be reheated and vaporised in the exchanger E03 without passing through the exchanger E01.

(21) The exchanger E01 therefore uses, as sources of cold: the CO.sub.2-enriched liquid recovered at the bottom of the column and the CO.sub.2-enriched liquid from the separator vessel V05, the overhead vapour from the vessel V01 named recycle stream R at the outlet of the exchanger E01, and optionally the pure liquid CO.sub.2 recovered at the bottom of the vessel V01 in the case where the vaporisation thereof is desired.

(22) The process requires an input of refrigeration power in order to operate. This input of cold is represented in FIG. 1 by the refrigeration circuit: it is composed of: a compressor C03 with cooler C03E; an exchanger E03 which cools the compressed fluid using the recycled refrigerant fluid and the cold recovered from the separation cycle; a turbine ET01 and a JT valve PV05, for the expansion of the refrigerant fluid and production of cold; a separator vessel V02 separating the vapour and liquid phases of the refrigerant fluid; an exchanger E02 which uses the liquid phase of the refrigerant fluid to liquefy the biomethane at the top of the distillation column. The refrigerant fluid used in the scheme is CH.sub.4 but it can be replaced by other fluids such as N.sub.2, N.sub.2+H.sub.2, inter alia.

(23) 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: using a source of liquid nitrogen; by a Brayton cycle process.

(24) 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.