Abstract
A process and apparatus for providing a carbon monoxide rich gas product. Carbon dioxide gas is partially converted in an electrolyser to obtain carbon monoxide gas and oxygen gas. A gas mixture containing at least carbon dioxide, carbon monoxide and hydrogen is withdrawn from the electrolyser. At least a part of the gas mixture is introduced into a separation system to provide a first gas or gas mixture enriched in carbon dioxide and a second gas or gas mixture enriched in carbon monoxide. The first gas or gas mixture or a part thereof is reintroduced into the electrolyser which is operated at a conversion rate for carbon dioxide 20% to 40% below a maximum conversion rate for the electrolyser.
Claims
1. A process for providing a carbon monoxide rich gas product, wherein carbon dioxide gas is partially converted in an electrolyser by electrolytic reduction to carbon monoxide gas and oxygen gas and wherein a gas mixture containing at least carbon dioxide gas, carbon monoxide gas and hydrogen gas is withdrawn from the electrolyser, characterised in that at least a part of the gas mixture withdrawn from the electrolyser is introduced into a separation system providing a first gas or gas mixture enriched in carbon dioxide when compared to the gas mixture withdrawn from the electrolyser and a second gas or gas mixture enriched in carbon monoxide when compared to the gas mixture withdrawn from the electrolyser, in that the first gas or gas mixture or a part thereof is reintroduced into the electrolyser, and in that the electrolyser is operated at a conversion rate for carbon dioxide which is 20% to 40% below a maximum conversion rate the electrolyser is practically able to achieve.
2. A process according to claim 1, wherein the electrolyser is operated at a electrolysing pressure level of at least 3 bar.
3. A process according to claim 2, wherein the gas mixture withdrawn from the electrolyser or the part thereof is submitted to the separation system at a separation pressure level of at least 3 bar.
4. A process according to claim 3, wherein the electrolysing pressure level is differing by no more than 1 bar from the separation pressure level.
5. A process according to claim 1, wherein the electrolyser is operated at an electrolysing pressure level of 0.5 to 1.5 bar.
6. A process according to claim 5, wherein the gas mixture withdrawn from the electrolyser or the part thereof submitted to the separation system is compressed in a gas compressor from the electrolysing pressure level to the separating pressure level.
7. A process according to claim 1, wherein exactly one separation step is performed in the separation system, and wherein the second gas or gas mixture is withdrawn from the exactly one separation step.
8. A process according to claim 1, wherein more than one separation step is performed, wherein at least a portion of the gas mixture withdrawn from the electrolyser is submitted to one of the more than one separation steps, and wherein the second gas or gas mixture is withdrawn from another one of the more than one separation steps.
9. A process according to claim 1, wherein the carbon dioxide gas is provided to the electrolyser at a temperature level from 35 to 100 C.
10. A process according to claim 1, wherein the carbon dioxide gas is provided to the electrolyser with a purity of at least 99.9 mol % on a dry basis.
11. A process according to claim 2, wherein the carbon dioxide gas is provided to the electrolyser at the electrolysing pressure level.
12. An apparatus for providing a carbon monoxide rich gas product, including an electrolyser adapted to partially convert carbon dioxide gas by electrolytic reduction to carbon monoxide gas and oxygen gas and means to withdraw a gas mixture containing carbon dioxide gas, carbon monoxide gas and hydrogen gas from the electrolyser, characterised in that the apparatus comprises a separation system and means adapted to submit at least a part of the gas mixture withdrawn from the electrolyser to the separation system, the separation system being adapted to provide a first gas or gas mixture enriched in carbon dioxide when compared to the gas mixture withdrawn from the electrolyser and a second gas or gas mixture enriched in carbon monoxide when compared to the gas mixture withdrawn from the electrolyser, in that means are provided to reintroduce the first gas or gas mixture or a part thereof into the electrolyser, and in that means are provided that are adapted to operate the electrolyser at a conversion rate for carbon dioxide which is 20% to 40% below a maximum conversion rate the electrolyser is practically able to achieve.
Description
SHORT DESCRIPTION OF THE FIGURES
[0026] FIG. 1A illustrates a process according to a preferred embodiment of the present invention.
[0027] FIG. 1B illustrates an alternative to the process illustrated in FIG. 1A according to a further preferred embodiment of the present invention.
[0028] FIG. 2A illustrates a process according to a further preferred embodiment of the present invention.
[0029] FIG. 2B illustrates an alternative to the process illustrated in FIG. 2A according to a further preferred embodiment of the present invention.
[0030] FIG. 3A illustrates a process according to a further preferred embodiment of the present invention.
[0031] FIG. 3B illustrates an alternative to the process illustrated in FIG. 3A according to a further preferred embodiment of the present invention.
[0032] In the figures, like and/or functionally corresponding elements are identified with identical reference numerals. For reasons of conciseness, repeated explanations of such elements are omitted.
DETAILED DESCRIPTION OF THE FIGURES
[0033] In FIG. 1A, a process according to a preferred embodiment of the present invention is schematically illustrated and designated 100.
[0034] According to the process 100 as shown in FIG. 1A, a carbon dioxide rich gas stream A, preferably containing at least 99.9 mol % carbon dioxide on a dry basis, is fed together with an optional recycle stream B described below at a pressure level of at least 5 bar and at a temperature level of 35 to 100 C. into a carbon dioxide electrolyser 10, wherein, under application of electricity C illustrated as a dashed arrow, carbon dioxide is electrochemically reduced forming carbon monoxide and oxygen. The electrolyser 10 is preferably embodied as a stack of electrolyser cells and operates at the pressure level (also referred to as an electrolysing pressure level herein) at which the carbon dioxide rich gas stream A is provided, i.e. at least 5 bar. Oxygen gas leaves an anode of the electrolyser 10 as an oxygen rich gas stream D. A carbon monoxide enriched gas stream E still containing residual hydrogen from the carbon dioxide rich gas stream A and carbon dioxide not converted in the electrolyser 10 is supplied to a separation system 20. In the process 100 as shown in FIG. 1A, the separation system 20 also operates at a pressure level (also referred to as a separation pressure level herein) of at least 5 bar. From the separation system 20, a carbon monoxide rich gas stream F (previously referred to as a second gas or gas mixture) and a further gas stream G mostly containing the separated carbon dioxide and hydrogen (previously referred to as a first gas or gas mixture) are withdrawn. The further gas stream G is in part be recycled in form of the recycle stream B to the electrolyser 10 in order to increase the carbon dioxide conversion. As mentioned, the electrolyser 10 is particularly operated at a conversion rate below a practical maximum.
[0035] As, according to the process 100 as shown in FIG. 1A, the carbon monoxide rich gas stream F still contains noticeable amounts of hydrogen, this process is preferred if a corresponding content in the carbon monoxide product corresponding to the carbon monoxide rich gas stream F is desired or can be tolerated.
[0036] In FIG. 1B, an alternative to the process illustrated in FIG. 1A according to a preferred embodiment of the present invention is schematically illustrated and designated 200.
[0037] In contrast to process 100 shown in FIG. 1A, in the process 200 as shown in FIG. 1B the carbon dioxide rich gas stream A is provided at a pressure level of only about I bar. The temperature level and the carbon dioxide content may be comparable to that explained for the process 100 as shown in FIG. 1A. The electrolyser 10 correspondingly operates at a lower electrolysing pressure level which corresponds to the lower pressure level of the carbon dioxide rich gas stream A, i.e. at about 1 bar. The carbon monoxide enriched gas stream E is compressed in a gas compressor 30 before being provided to the separation system 20, which operates at the same separation pressure level at as the separation system 20 according to the process 100 as shown in FIG. 1A, i.e. at a pressure level of at least 5 bar. The carbon monoxide enriched gas stream E leaves the gas compressor 30 at this pressure level, i.e. at least 5 bar.
[0038] Like the process 100 as shown in FIG. 1A, the carbon monoxide rich gas stream F of the process 200 according to FIG. 1B still contains noticeable amounts of hydrogen. The process 200 is therefore also preferred if a corresponding content in the carbon monoxide product is desired or can be tolerated. Furthermore, the process 200 according to FIG. 1B is preferably used if the electrolysis cannot be viably achieved at the electrolysing pressure used according to process 100.
[0039] In FIG. 2A, a one-stage process according to a further preferred embodiment of the present invention is schematically illustrated and designated 300.
[0040] According to the process 300 as shown in FIG. 2A, the carbon dioxide rich gas stream A is provided under the conditions used in the process 100 as shown in FIG. 1A, i.e. at a pressure level of at least 5 bar, a temperature level of 35 to 100 C. and with a carbon dioxide content of preferably at least 99.99 mol % carbon dioxide on a dry basis. The electrolyser 10 used according to the process 300 shown in FIG. 2A operates at the electrolysing pressure level which corresponds to the pressure level at which the carbon dioxide rich gas stream A is provided, i.e. at a pressure level of at least 5 bar. In contrast to the process 100 as shown in FIG. 1, however a two-stage separation system with steps or units 21 and 22 is performed. Both steps or units 21 and 22 operate at a separation pressure level which corresponds to the electrolysing pressure level at which the electrolyser 10 operates and at which the carbon dioxide rich gas stream A is provided, i.e. at a pressure level of least 5 bar. The process 300 according to FIG. 2A essentially differs from the process 100 according to FIG. 1A in that the carbon monoxide rich gas stream F is not directly provided as a product but is further purified in the (second) step or unit 22. This carbon monoxide rich gas stream F which, as mentioned, still contains noticeable amounts of hydrogen, is separated in the second step or unit 22 into a hydrogen product gas withdrawn from the second step or unit 22 as a product stream I and into a further purified carbon monoxide product gas withdrawn from the second step or unit 22 as a product stream K.
[0041] As according to the process 300 as shown in FIG. 2A, a further step or unit 22 is used, the carbon monoxide product gas withdrawn as the product stream K is further depleted in hydrogen as compared to the carbon monoxide rich gas stream F. Process 300 as shown in FIG. 2A is thus preferred if higher purity levels of the carbon monoxide product are desired or necessary.
[0042] In FIG. 2B, an alternative to the process illustrated in FIG. 2A according to a further preferred embodiment of the present invention is schematically illustrated and designated 400.
[0043] In contrast to process 300 as shown in FIG. 2A, in the process 400 as shown in FIG. 2B the carbon dioxide rich gas stream A is provided at a pressure level of only about 1 bar. Therefore, the process 400 as shown in FIG. 2B partially resembles the process 200 as shown in FIG. 1B. The temperature level and the carbon dioxide content of the carbon dioxide rich gas stream A may be comparable to that used in the processes 100, 200 and 300 as discussed before. The electrolyser 10 correspondingly operates at the lower electrolysing pressure level of the carbon dioxide rich gas stream A, i.e. at about 1 bar. The carbon monoxide enriched gas stream E is compressed in a gas compressor 30 before being provided to the (first) step or unit 21 which operates at the same separation pressure level as the separation system 20 according to the processes 100, 200 and the steps or units 21 and 21 of the process 300 as discussed before, i.e. at a separation pressure level of at least 5 bar. The carbon monoxide enriched gas stream E leaves the gas compressor 30 at this pressure level, i.e. at least 5 bar. For further explanations, reference is made to the explanations to FIG. 2A.
[0044] As according to the process 400 as shown in FIG. 2B, like in the process 300 as shown in FIG. 2A, a further step or unit 22 is provided, the carbon monoxide product gas withdrawn from the activated carbon PSA as the gas stream K is further depleted in hydrogen as compared to the carbon monoxide rich gas stream F. The process 400 as shown in FIG. 2B is thus preferred if higher purity levels of the product streams are desired or necessary. Furthermore, in the process 400 as shown in FIG. 2B, the electrolyser 10 operates at a lower electrolysing pressure level than the electrolyser 10 according to the process 300 as shown in FIG. 2A. Therefore, process 400 according to FIG. 2B is desirable if the electrolysis cannot be viably achieved at the pressure level used according to process 300.
[0045] In FIG. 3A, a membrane-based gas purification process according to a further preferred embodiment of the present invention is schematically illustrated and designated 500.
[0046] The pressure level and further specifications of the carbon dioxide rich gas stream. A correspond to that of the carbon dioxide rich gas stream A used in the processes 100 and 300 as shown in FIGS. 1A and 2A. These conditions are not repeated for the sake of conciseness. Also the electrolysing pressure level of the carbon dioxide electrolyser 10 in the process 500 according FIG. 3A resembles that of the carbon dioxide electrolysers 10 in the processes 100 and 300 as shown in FIGS. 1A and 2A. In contrast to the processes 100, 200, 300 and 400 as shown in FIGS. 1A, 1B, 2A and 2B, however, a different separation system 40 is used. Depending on the operating performance of the separation system 40, the specifications of the streams F, G and H may resemble that of the corresponding streams F, G and H of the processes 100, 200, 300 and 400 as discussed before or may differ.
[0047] In FIG. 3B, an alternative to the process illustrated in FIG. 3A according to a further preferred embodiment of the present invention is schematically illustrated and designated 600.
[0048] The essential difference between the process 500 illustrated in FIG. 3A and the process 600 illustrated in FIG. 3B is that the carbon dioxide rich gas stream A is provided at a pressure level which is comparable to that used in the processes 1B and 2B, i.e. at about 1 bar. The electrolysing pressure level at which the electrolyser 10 is operated, is also about 1 bar. As the different separation system 40, however, operates at a separating pressure level comparable to that used in the process 500 according FIG. 3A, like in the processes 200 and 400 illustrated in FIGS. 1B and 2B a gas compressor 30 is used.
[0049] It is understood that the separation system used may incorporate a pressurizing device such as a blower or a compressor to overcome the pressure drops through the system and return the CO2 rich stream for recycle to the electrolyzer.
