METHOD AND APPARATUS FOR PURIFYING GASEOUS PRODUCTS FROM A CO2 ELECTROLYSIS PROCESS

Abstract

A method for treating products from a CO.sub.2 electrolysis process where carbon dioxide (CO.sub.2) and water (H.sub.2O) are electrochemically reacted in a cathode chamber of an electrolysis cell. Gaseous cathode products are formed which comprise ethylene (C.sub.2H.sub.4), hydrogen (H.sub.2) and carbon monoxide (CO). The products are treated in a multi-stage separating process. First, the cathodic product gas stream is fed to a desublimation process, where CO.sub.2 and water (H.sub.2O) are frozen out of, and separated from, the product gas stream. Next, the product gas stream is compressed to a pressure, and then the compressed product gas stream is fed to a gas permeation process, wherein hydrogen (H.sub.2) is separated off by passing the hydrogen through a hydrogen-permeable membrane. Next, the retentate which remains and which comprises ethylene (C.sub.2H.sub.4) and carbon monoxide (CO) is subjected to distillative separation, such that ethylene (C.sub.2H.sub.4) and carbon monoxide (CO) are separated.

Claims

1. A method for processing products from a CO.sub.2 electrolysis, wherein carbon dioxide CO.sub.2 and water are electrochemically converted in a cathode space of an electrolysis cell, wherein gaseous cathode products which comprise at least ethylene (C.sub.2H.sub.4), hydrogen (H.sub.2) and carbon monoxide (CO) are formed, wherein the gaseous cathode products are processed in a multistage separation process, comprising: (T1) in a first step, delivering a cathodic product gas flow to desublimation so that CO.sub.2 and water are frozen out from the product gas flow and separated, (T2) in a second step, compressing the product gas flow purified with respect to CO.sub.2 to a pressure, (T3) in a third step, delivering the compressed product gas flow to gas permeation, hydrogen (H.sub.2) in the product gas flow being separated by passing the hydrogen (H.sub.2) through a hydrogen-permeable membrane, and (T4) in a fourth step, subjecting the retentate remaining in the product gas flow, containing ethylene (C.sub.2H.sub.4) and carbon monoxide (CO), to separation by distillation, so that ethylene (C.sub.2H.sub.4) and carbon monoxide (CO) are separated.

2. The method as claimed in claim 1, wherein the product gas flow is adjusted to a pressure of from 10 bar to 50 bar, in the third step (T3).

3. The method as claimed in claim 1, wherein compressed retentate from the membrane separation in a membrane unit is introduced for separation by distillation (T3) into a rectifying column, ethylene (C.sub.2H.sub.4) being obtained in the liquid phase and carbon monoxide (CO) being obtained in the gas phase.

4. The method as claimed in claim 1, wherein ethanol (C.sub.2H.sub.5OH) is formed as a cathode product, ethanol (C.sub.2H.sub.5OH) being condensed out and extracted in the liquid phase.

5. The method as claimed in claim 1, wherein formate and/or acetate in liquid or dissolved form as byproducts are furthermore formed as cathode products, these byproducts being separated.

6. An apparatus for processing a product gas flow from a CO.sub.2 electrolysis, containing at least ethylene (C.sub.2H.sub.4), hydrogen (H.sub.2) and carbon monoxide (CO) as gaseous cathode products, comprising: a desublimation unit for freezing CO.sub.2 and water out from the product gas flow, a compressor unit downstream of the desublimation unit, comprising a compressor, and a hydrogen-permeable membrane unit downstream of the compressor unit for separating hydrogen from the product gas flow.

7. The apparatus as claimed in claim 6, wherein the desublimation unit is configured redundantly, a first desublimation container and a second desublimation container connected in parallel therewith being provided.

8. The apparatus as claimed in claim 6, comprising: a cryodistillation unit downstream of the hydrogen-permeable membrane unit.

9. The apparatus as claimed in claim 6, wherein the compressor unit has a cooling apparatus downstream of the compressor, by means of which the compressed product gas flow be is cooled and heat of compression is dissipated.

10. The apparatus as claimed in claim 8, wherein the cryodistillation unit has a rectifying column, at the head of which a condenser that is designed to condense out ethylene (C.sub.2H.sub.4) is arranged.

11. The apparatus as claimed in claim 10, comprising: trays or packings inside the rectifying column, which cause intensive contact of ascending gas and downflowing liquid so that successive enrichment of ethylene (C.sub.2H.sub.4) in the liquid phase and corresponding enrichment of carbon monoxide (CO) in the gas phase is achieved.

12. The apparatus as claimed in claim 10, wherein the rectifying column has a column bottom which is designed in such a way that liquid ethylene (C.sub.2H.sub.4) with high purity is extracted at the column bottom.

13. The apparatus as claimed in claim 12, further comprising: a heating unit arranged at the column bottom, by means of which a part of the liquid flow arriving in the column bottom is evaporated so that a continuous counterflow of gas and liquid is achieved in the rectifying column.

14. The apparatus as claimed in claim 6, further comprising: a connection unit for receiving a product gas flow from a CO.sub.2 electrolysis.

15. An arrangement comprising: an apparatus as claimed in claim 14, and a CO.sub.2 electrolysis installation connected to the apparatus via the connection unit, so that a product gas flow from the electrolysis, containing gaseous cathode products, is delivered to the apparatus for purification.

16. The method as claimed in claim 2, wherein the product gas flow is adjusted to a pressure of from 10 bar to 45 bar, in the third step (T3).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] Exemplary embodiments of the invention will be explained in more detail below with the aid of a drawing, in which, schematically and in a highly simplified manner:

[0063] FIG. 1 shows an exemplary embodiment of an apparatus for processing the products from a CO.sub.2 electrolysis in various separation steps;

[0064] FIG. 2 shows an exemplary embodiment of an overall installation comprising a CO.sub.2 electrolysis installation and a processing apparatus for electrolysis products downstream on the product flow side.

DETAILED DESCRIPTION OF INVENTION

[0065] Identical reference signs have the same meaning in the figures.

[0066] FIG. 1 represents an exemplary embodiment of this separation step sequence according to the invention. It shows an apparatus for processing the products from a CO.sub.2 electrolysis in various separation steps. The separation, or purification, of the gaseous products on the cathode side of a CO.sub.2 electrolysis cell or electrolysis installation is shown in more detail in a schematic representation here. The input flow 11 into this separation sequence comprises gaseous cathode products and consists to a large extent of CO.sub.2 (about 80 m %). As secondary components, approximately 5 m % carbon monoxide CO and about 0.5-1 m % hydrogen H.sub.2 are present. As the final product, it contains ethylene C.sub.2H.sub.4 in a proportion of about 3-4 m %. The input flow 11 is saturated with water H.sub.2O at about 1 bar and 60 C., which corresponds to a content of about 10 m %.

[0067] In the method, it has been found particularly preferential to separate carbon dioxide CO.sub.2 and water H.sub.2O initially in a first separation step T1. In this desublimation step, selective separation is advantageously possible by cooling the gaseous input flow 11 to such an extent that water H.sub.2O and carbon dioxide CO.sub.2 freeze out. For this purpose, a process temperature below about 80 C. is provided in the separation step T1, this being possible even without energy- and cost-intensive compression. The freezing may in the simplest case for example take place in a cooled container in which dry ice and frozen water accumulate during the cooling.

[0068] The container or reactor of the desublimation unit 15 is preferably configured redundantly so that it is possible to switch over to a second container 15B when the holding capacity of a first container 15A is reached, which enables continuous operation. CO.sub.2 and water that have been frozen out can then be returned into the gas state by heating, and in this way a previously filled desublimation container 15A, 15B can be emptied again. Released CO.sub.2 and water H.sub.2O may be reused and are preferably recirculated back into the electrolysis process.

[0069] A further advantage of this procedure is that the product mass flow to be processed in this step is greatly reduced since carbon dioxide CO.sub.2 and water H.sub.2O together make up about 90 m % of the input flow 11 into the desublimation unit 15 here. This is important and advantageous insofar as the remaining reduced gaseous product mass flow is compressed to 10-50 bar in the following step T2 of the processing. For this purpose, a compressor unit 17 is provided, which has a compressor 19. Because of the reduced product mass flow, the following process step T2 can be carried out with much less plant-specific outlay and energy expenditure in respect of the compressor performance of the compressor 19.

[0070] The heating that results from the compression in the compressor 19 in separation step T2 is preferably compensated for by suitable cooling, so that the gaseous product mass flow is kept at a temperature of about 60 C. For this purpose, the compressor unit 17 has a cooling apparatus 25 which is connected downstream of the compressor 19 in the flow direction of the product mass flow.

[0071] Subsequent to the compression and cooling in the compressor unit 17, the next separation step T3 takes place in the form of gas permeation. This step purposely makes use of the fact that hydrogen is a very small molecule and therefore exhibits a comparatively high permeability by suitable use of gas permeation membranes. For this purpose, a hydrogen-permeable membrane unit 21 comprising a membrane is connected fluidically downstream of the compressor unit 17. Hydrogen H.sub.2 is therefore separated very efficiently from the product gas flow in the separation step T3, which is carried out in the gas permeation step T3 of the method. Hydrogen H.sub.2 that passes selectively through the hydrogen-permeable membrane is preferentially likewise recirculated into the upstream CO.sub.2 electrolysis process. As an alternative, particularly in the case of an undesired buildup, it is possible that this hydrogen gas flow or a part thereof is also discharged from the installation. Hydrogen is a gas which is inert for the electrochemical reaction per se, and actually promotes the diffusion of the production. After the buildup, it is preferentially separated and used separately. Since the membrane-based separation in step T3 does not take place fully selectively and the other volatile secondary components can also pass through the hydrogen-permeable membrane to a certain-albeit minor-extent, this material flow may be used as output so that secondary components do not become undesirably highly concentrated in the process.

[0072] The remaining retentate from the separation step T3 now contains predominantly carbon monoxide CO and ethylene C.sub.2H.sub.4, of which the difference in volatility is large enough to carry out separation by distillation in a step T4 by means of a cryodistillation unit 23 at a suitably low temperature. For this purpose, the already compressed retentate flow from the membrane separation is introduced into a rectifying column 27, which in a preferred embodiment of the method is adjusted and operated cryogenically, i.e. at about 50 bar and with about 100 C. at the column head, or is operated with about +6 C. in the column bottom.

[0073] A condenser 29 is fitted at the head of the rectifying column 27 and cools the ascending gas to such an extent that high-boiling ethylene C.sub.2H.sub.4 is condensed and recirculated into the rectifying column 27. The condenser 29 works only partially, and the resulting carbon monoxide gas flow in the condenser 29 is recirculated as gas into the electrolysis, or it is possible that here again a partial flow is delivered to thermal valorization. Carbon monoxide is advantageously reduced on the same catalyst likewise to form ethylene C.sub.2H.sub.4 or ethanol C.sub.2H.sub.5OH.

[0074] Inside the rectifying column 27, there are fixturesfor example so-called trays or packingswhich cause intensive contact of ascending gas and downflowing liquid according to a counterflow principle. This process leads to successive enrichment of ethylene C.sub.2H.sub.4 in the liquid phase and corresponding enrichment of carbon monoxide CO in the gas phase.

[0075] In this way, ethylene C.sub.2H.sub.4 can be obtained with high purity at the column bottom 31 of the rectifying column 27. In order to sustain the rectifying process T4 in the region below the gaseous feed entry as well, a part of the liquid flow arriving in the column bottom 31 may be evaporated with the aid of a heater unit 33 in order to ensure and continuously maintain a continuous counterflow of gas and liquid during operation.

[0076] As a further valuable product during the electrolysis of carbon dioxide CO.sub.2, ethanol C.sub.2H.sub.5OH is formed, which under the operating conditions of the electrolysis enters the multicomponent material flow after the gas separator 107 predominantly as a liquid. When connecting an electrolysis installation to the downstream apparatus for processing the product gas flow, it is possible and advantageous to introduce a partial flow from this material flow into a further distillation column 111. Such an interconnection is shown by way of example in FIG. 2. Here, ethanol C.sub.2H.sub.5OH may be obtained in an azeotropic composition as a head product in the distillation column 111. The bottom product is recirculated into the CO.sub.2 electrolysis.

[0077] A partial flow 13 may in this case be discharged in order to purposely remove further byproducts (for example formate and acetate), which occur in liquid or dissolved form, from the overall process.

[0078] FIG. 2 shows a schematic overview which is based on the method steps described above, so that a preferred integrated overall method is achieved, which comprises the CO.sub.2 electrolysis on the one hand and on the other hand the subsequent purification of the products in the substeps of desublimation T1, compression T2, membrane-based separation T3 and cryodistillation T4.

[0079] FIG. 2 in this case shows a schematic representation of the overall method in a flowchart of the process of an electrochemical production of ethylene C.sub.2H.sub.4 and ethanol C.sub.2H.sub.5OH from CO.sub.2 and water H.sub.2O and the separation of the cathode-side gaseous products according to the invention.

[0080] Here, as already described, in an electrolysis installation 100 or electrolysis cell, water H.sub.2O and carbon dioxide CO.sub.2 are initially converted electrochemically in the cathode space 101 essentially into ethylene, hydrogen, carbon monoxide, ethanol, formate and acetate. Oxygen is essentially formed in the anode space 103, and is separated from the anolyte with a sufficient purity with the aid of a gas-liquid separator 105.

[0081] The catholyte is fed with the reaction products likewise into a suitable gas-liquid separator 107, and the resulting gas and liquid flows are processed separately. This cathodic product gas flow 11 contains about 80 m % carbon dioxide CO.sub.2. As secondary components, approximately 5 m % carbon monoxide CO and about 0.5-1 m % hydrogen are present. As the final product for the processing and separation, it contains ethylene C.sub.2H.sub.4 in a proportion of about 3-4 m %. The cathodic product gas flow 11 is saturated with water at about 1 bar and 60 C., which corresponds to a content of about 10 m %.

[0082] This product gas flow 11 is also referred to as a feed flow, and is initially delivered into the desublimation unit 15 where the gas is cooled to at least 80 C., so that CO.sub.2 and water H.sub.2O are frozen out from the product gas flow 11. The desublimation is configured redundantly by means of two containers, a first container 15A and a second container 15B. It is therefore possible to switch over when the holding capacity of one of the containers 15A, 15B is reached, and continuous operation is therefore enabled. CO.sub.2 and water H.sub.2O that have been frozen out can then be returned into the gas state by heating, and the previously filled container 15A or 15B may thus be emptied. Released CO.sub.2 and water H.sub.2O may be reused and are recirculated into the electrolysis process of the electrolysis installation 100.

[0083] The product gas flow 11 dried and freed from CO.sub.2 in step T1 (cf. FIG. 1) is compressed in step T2 in the compressor 19 to 45 bar and regulated to 60 C., and then introduced into the membrane unit 21 in order to carry out a gas permeation, where hydrogen preferentially passes through the membrane and is thus separated selectively from the product gas flow 11. This hydrogen flow is preferentially likewise recirculated into the electrolysis installation 100, or in the event of an undesired high concentration, this hydrogen flow or a part thereof may also be discharged from the process. Since the membrane separation in the membrane unit 21 generally does not take place fully selectively and the other volatile secondary components 109 can therefore also pass through the membrane to a minor extent, this material flow 109 may be used as output so that secondary components do not build up in the process to an undesired extent.

[0084] The remaining product flow-predominantly comprising or consisting of carbon monoxide CO and ethylene C.sub.2H.sub.4is separated with the aid of a cryorectifying column 27 having a separating performance of about 10 theoretical plates in the rectifying part and about 20 theoretical plates in the stripping part. For sufficient separation, a reflux ratio of two is required. With a column pressure of 45 bar, condensation takes place at the column head with a temperature of 100 C. and the column bottom is regulated to +5.6 C. This results in ethylene at the column bottom 31 with a very high purity of 99.98 vol %.

[0085] The condenser 29 works only partially, and the resulting carbon monoxide flow in the condenser 29 is recirculated as gas into the electrolysis installation 100 or may also be delivered as a partial flow to another type of valorization, particularly in the case of an excessive concentration in the process.

[0086] The liquid catholyte flow which leaves the cathode-side gas separator 107 contains as a further valuable product about 10 m % ethanol C.sub.2H.sub.5OH. Besides further byproducts (acetate and formate <1%), the catholyte essentially contains 73 m % water, 11 m % potassium hydrogen carbonate and 7 m % potassium sulfate. This material flow is introduced into a further rectifying column 111, comprising a rectifying part with about 15 theoretical plates and a stripping part with about 30 theoretical plates, where an ethanol/water mixture having an azeotropic composition is obtained by distillation as the head product. This further rectifying column 111 is operated at atmospheric pressure with a reflux ratio of two. The condensation temperature is 78 C., and the reboiler is heated to 100 C. The catholyte freed from ethanol C.sub.2H.sub.5OH is obtained as the bottom product and recirculated into the electrolysis installation.

[0087] In order to remove further byproducts (for example formate and acetate), which occur in liquid or dissolved form, from the overall process, a partial flow 13 of the bottom product is discharged and delivered to further valorization.