A PROCESS TO TREAT A CARBON DIOXIDE COMPRISING GAS

Abstract

The invention is directed to a process to convert carbon dioxide to methane by contacting an aqueous solution comprising dissolved carbon dioxide with an electron charged packed bed comprising of a carrier, suitably activated carbon granules, and a biofilm of microorganisms under anaerobic conditions, wherein more than 90 mol % of the dissolved carbon dioxide in the aqueous solution is present as a bicarbonate ion and/or as a carbonate ion.

Claims

1. A process to convert carbon dioxide to methane by contacting an aqueous solution comprising dissolved carbon dioxide with an electron charged packed bed comprising of a carrier and a biofilm of microorganisms under anaerobic conditions, wherein more than 90 mol % of the dissolved carbon dioxide in the aqueous solution is present as a bicarbonate ion and/or as a carbonate ion.

2. The process according to claim 1, wherein the carrier is activated carbon granules or activated biochar granules.

3. The process according to claim 1, wherein no power is supplied to the electron charged packed bed.

4. The process according to claim 3, wherein the electron charged packed bed is part of a biocathode in a bioelectrochemical system further comprising an anode, an ion exchange membrane, and a cathode; wherein the packed bed is charged by applying a potential to the bioelectrochemical system resulting in a current between biocathode and anode for a certain time.

5. The process according to claim 4, wherein the aqueous solution as present at the anode is referred to as the anolyte and the aqueous solution as present at the cathode is referred to as the catholyte and wherein a recirculation is performed where part of the catholyte is fed to the anode to become part of the anolyte and part of the anolyte is fed to the cathode to become part of the catholyte.

6. The process according to claim 1, wherein the electron charged packed bed is part of a biocathode in a bioelectrochemical system further comprising an anode; and wherein at one moment in time the process is performed when the packed bed is charged by applying a potential to the bioelectrochemical system resulting in a current between biocathode and anode; and wherein at another moment in time the process is performed when no power is supplied to the electron charged packed bed.

7. The process according to claim 4, wherein the process is performed in more than one bioelectrochemical systems, each system comprising of the biocathode and an anode; and wherein in one or more bioelectrochemical systems the process is performed while no power is supplied to the electron charged packed bed of these one or more bioelectrochemical systems; and wherein power is supplied to the packed bed of one or more other bioelectrochemical system of the more than one bioelectrochemical systems such that these packed beds are charged with electrons while the process is not performed.

8. The process according to claim 6, wherein the process is performed for between 0.03 and 12 hours when no power is supplied to the electron charged packed.

9. The process according to claim 6, wherein the power supply is electricity generated by solar and/or wind.

10. The process according to claim 4, wherein the packed bed is charged by applying a cathode potential to the cathode electrode of between −0.50 and −0.60V vs. Ag/AgCl (3M KCl) or by applying a current density to the cathode electrode of between 5 and 200 A/m.sup.2.

11. The process according to claim 4, wherein the anode is a titanium mesh coated with iridium and/or tantalum.

12. The process according to claim 4, wherein the power supply is generated by a chemical reaction at the anode.

13. The process according to claim 1, wherein the packed bed is a packed bed of activated carbon granules having an activated surface area of between 500 and 1500 m.sup.2/g; and wherein the microorganisms are present as a biofilm on the surface of the activated surface area.

14. The process according to claim 1, wherein the pH of the aqueous solution is above 7.7.

15. The process according to claim 14, wherein the pH of the aqueous solution is above 8.

16. The process according to claim 14, wherein the aqueous solution comprises between 0.3 and 4 M sodium cations or sodium and potassium cations.

17. The process according to claim 16, wherein the aqueous solution comprises between 0.4 and 2 M, preferably between 0.5 and 1.5 M sodium cations or sodium and potassium cations.

18. The process according to claim 4, wherein the carrier and a biofilm of microorganisms is obtained in an activation step which activation step is performed at a pH greater than 8 and under anaerobic conditions and by supplying an amount of current at a cathode potential which is more positive than the theoretical hydrogen evolution potential at −0.61 V vs Ag/AgCl (3M KCl) at a pH of 7 to the packed bed comprising of carrier and biofilm of microorganisms; and wherein the microorganisms are a mixed culture microorganisms from a sludge of an anaerobic wastewater treatment plant.

19. The process according to claim 1, wherein the aqueous solution comprising dissolved carbon dioxide is obtained by contacting a gas comprising carbon dioxide with an aqueous solution having a pH of above 8 to obtain an aqueous solution wherein a major part of the dissolved carbon dioxide is present as a bicarbonate ion and/or as a carbonate ion.

20. The process according to claim 19, wherein the gas comprising carbon dioxide is counter currently contacted with the aqueous solution having a pH of above 8 and comprising dissolved methane as obtained a process comprising: contacting an aqueous solution comprising dissolved carbon dioxide with an electron charged packed bed comprising of a carrier and a biofilm of microorganisms under anaerobic conditions, wherein more than 90 mol % of the dissolved carbon dioxide in the aqueous solution is present as a bicarbonate ion and/or as a carbonate ion; and wherein the gas strips the methane from the aqueous solution to obtain a gas comprising methane.

21. A method to activate or reactivate a bioelectrochemical system comprising; an anode, and a biocathode comprising a packed bed comprising a carrier and a mixed culture of microorganisms from the sludge of an anaerobic wastewater treatment plant, by supplying an amount of current such that the cathode potential is more positive than the theoretical hydrogen evolution potential at −0.61 V vs Ag/AgCl (3M KCl) at a pH of 7 under anaerobic conditions and at a pH of greater than 8.

22. The method according to claim 21, wherein the carrier is activated carbon granules or activated biochar granules.

Description

EXAMPLE 1

[0039] A bioelectrochemical system (BES) was operated, for a 60-days long experiment. The BES setup is similar to the BES setup described in Liu, Dandan, Marta Roca-Puigros, Florian Geppert, Leire Caizán-Juanarena, Na Ayudthaya, P. Susakul, Cees Buisman, and Annemiek Ter Heijne. “Granular carbon-based electrodes as cathodes in methane-producing bioelectrochemical systems.” Frontiers in bioengineering and biotechnology 6 (2018): 78. The cathode electrode was 10.3 g of granular activated carbon, which was fully packed in the cathodic chamber. A plain graphite plate was used as a current collector. An anodic chamber and a cathodic chamber with a flow channel of 33 cm.sup.3 each (11 cm×2 cm×1.5 cm). The anodic chamber and cathodic chamber were separated by a cation exchange membrane with a projected surface area of 22 cm.sup.2 (11 cm×2 cm). The total volume of anolyte and catholyte were 500 mL and 330 mL, respectively. The catholyte circulation bottle was designed such that the H/D (height/diameter) ratio was increased to enable a better absorption of CO.sub.2. In order to remove O.sub.2 produced at the anode electrode, a high anolyte flow rate of 94 mL/min was used. Also, N.sub.2 was continuously bubbled at the rate of 80 mL/min in the anolyte recirculation bottle. The catholyte recirculation rate was 11 mL/min.

[0040] The cathodic chamber was inoculated with 30 mL of anaerobic sludge from an upflow anaerobic sludge blanket (UASB) digestion in Eerbeek. The volatile suspended solids of the inoculated anaerobic sludge was 30.6 g/L. The methane-producing BES was galvanostatically controlled (fixed current) by a potentiostat. In addition, cell voltage was manually monitored via a multimeter. Liquid samples for pH and conductivity measurements were taken twice per week for both anolyte and catholyte. The following results were obtained.

[0041] Initially, the catholyte consisted of a 50 mM phosphate buffer (1.36 g/L KH.sub.2PO.sub.4 and 5.67 g/L Na.sub.2HPO.sub.4) with 0.2 g/L NH.sub.4Cl, 1 mL/L Wolfe's vitamin solution and 1 mL/L Wolfe's modified mineral solution. The anolyte consisted only of the 50 mM phosphate buffer. Due to the use of the same phosphate buffer for both catholyte and anolyte, the initial pH and conductivity of catholyte and anolyte are the same (i.e. a pH of 6.7 and a conductivity of 7.68 mS/cm). After a start-up period (not shown), stable performance was obtained (day 0-day 30) when providing electrons at the biocathode with a current density of 5 A/m.sup.2. In this period, the obtained voltage efficiency was about 50% and the coulombic efficiency and a coulombic efficiency of 83-85% which leads to an energy efficiency of 40-42%.

[0042] After 30 days, the catholyte and anolyte were changed to a high saline medium, containing 1.0 M carbonate/bicarbonate buffer with a conductivity of around 40 mS/cm (Na:K of 4:1). The medium contained 0.2 g/L NH.sub.4Cl, 1 mL/L Wolfe's vitamin solution and 1 mL/L Wolfe's modified mineral solution. The resulting pH of the catholyte was 7.7-7.8. After the change of medium, voltage efficiency immediately increased to ˜83%. Coulombic efficiency initially dropped to 65%. This drop can be explained by a osmotic shock to the biocathode. However, after a few days of operation, the biofilm adapted and coulombic efficiency recovered to about 85%. As a result, due to the change in medium, the energy efficiency, η.sub.energy, of the methane-producing BES increased from 43% towards 65-70%. See also FIG. 2 where the Coulombic efficiency is represented by the circles, the voltage efficiency by the open triangles and the energy efficiency by the closed squares.