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 and a biofilm of microorganisms under anaerobic conditions wherein the pH of the aqueous solution is above 7.5 and wherein the aqueous solution comprises between 0.3 and 4 M sodium cations or between 0.3 and 4 M sodium and potassium cations and more than 20 mM phosphate ions.

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 the pH of the aqueous solution is above 7.5, wherein the aqueous solution comprises between 0.3 and 4 M sodium cations or between 0.3 and 4 M sodium and potassium cations and wherein the aqueous solution comprises more than 20 mM phosphate ions.

2. The process according to claim 1, wherein the aqueous solution comprises between 0.4 and 2 M sodium cations or between 0.4 and 2 M sodium and potassium cations.

3. The process according to claim 1, wherein the aqueous solution comprises between 0.5 and 1.5 M sodium cations or between 0.5 and 1.5 M sodium and potassium cations.

4. The process according to claim 1, wherein the carrier is comprised of activated carbon granules.

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

6. The process according to claim 5, 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.

7. 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.

8. The process according to claim 6, wherein the process is performed in more than one bioelectrochemical systems, each system comprising of the biocathode and an anode; 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.

9. The process according to claim 7, wherein the process is performed for between 0.03 and 12 hours when no power is supplied to the electron charged packed.

10. The process according to claim 7, wherein the power supply generating the potential is electricity generated by solar and/or wind.

11. The process according to claim 6, wherein the packed bed is charged by applying a current density to the cathode electrode of between 2 and 200 A/m.sup.2 or by applying a cathode potential to the current collector of the biocathode which is less negative than the hydrogen evolution potential.

12. The process according to claim 6, wherein the anode is a titanium mesh coated with iridium and or tantalum.

13. The process according to claim 6, wherein the power supply is generated by a chemical reaction at the anode.

14. 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.

15. 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 7.5 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.

16. The process according to claim 1, wherein the microorganisms are halophilic microorganisms.

Description

[0037] Preferably the gas comprising carbon dioxide is counter currently contacted with an aqueous solution having a pH of above 7.5 and comprising dissolved methane as obtained in the process according to this invention and wherein the gas strips the methane from the aqueous solution to obtain a gas comprising methane. In this way methane is effectively isolated from the aqueous reaction mixture while carbon dioxide is absorbed using the same unit operation.

[0038] FIG. 1 shows a possible process scheme for the process of this invention. A gas comprising carbon dioxide (1) is counter currently contacted in absorption column (3) with an aqueous solution (2) having a pH of above 7.5 and comprising dissolved methane as obtained in reactor (4). In column (3) the gas (1) strips the methane from the aqueous solution (2) to obtain a gas (5a) comprising methane. Part of the methane as formed in reactor (4) is separated as a gas (2d) in knockout vessel (2c) from aqueous reaction mixture (2a) and combined with the aforementioned gas (5a) to combined gas stream (5). A liquid aqueous solution (2e) is obtained in the knock-out vessel (2c). In this way methane is effectively isolated from the aqueous reaction mixture (2,2a) while carbon dioxide is absorbed in the same column (3). The methane rich gas is obtained as gas stream (5). In the obtained aqueous solution (6) comprising dissolved carbon a major part of the dissolved carbon dioxide is present as a bicarbonate ion and/or as a carbonate ion. This aqueous solution (6) is cooled in heat exchanger (7) and fed to an electron charged packed bed (8) comprising of a carrier and a biofilm of microorganisms under anaerobic conditions. In the electron charged packed bed (8) carbon dioxide as the bicarbonate ion and/or as the carbonate ion reacts to methane. It is believed that this is achieved without the formation of hydrogen as an intermediate reaction product. The electron charged packed bed (8) is part of a biocathode (8a) in a bioelectrochemical reactor (4) further comprising an anode (9) and an ion exchange membrane (10) to avoid oxygen as may be formed at the anode (9) to flow to the biocathode (8a).

[0039] A hydrogen rich stream (25), for example a rest stream of another process, may be supplied to biocathode (8a) of bioelectrochemical reactor (4). This hydrogen will be converted with carbon dioxide to methane in the bioelectrochemical reactor (4).

[0040] The aqueous reaction mixture (2,2a) obtained at the biocathode (8a) is fed to column (3) via a mixture vessel (13). To mixture vessel (13) make up water (14), make up caustic (15) and make up nutrients and vitamins (16) may be added. A catholyte bleed stream (17) discharges part of the catholyte from the process.

[0041] At the anode water is oxidised and the oxygen as formed is discharged via (18) to an anolyte buffer vessel (19). The anolyte compartment of the reactor (9) is fed with fresh anolyte via (20). In this vessel molecular oxygen is separated as (21). Make up water (22) and make up caustic (23) is added and an anolyte bleed stream (24) discharges part of the aqueous solution from the process.

[0042] Part of the anolyte (12) is fed to the mixture vessel (13) to become part of the catholyte and part of the catholyte (11) is fed to the anolyte buffer vessel (19) to become part of the anolyte. These streams (11,12) may be treated to lower the content of oxygen and methane as described above.

[0043] FIG. 2 shows a top view cross-sectional view of a possible bioelectrochemical reactor configuration (4). The bioelectrochemical reactor (4) is comprised of more numerous hexagonal shaped cells (30) functioning as a current collector. Each cell (30) comprises an electron charged packed bed of activated carbon granules (8) as part of a biocathode (8a), an anode (9) and an ion exchange membrane (10) to avoid oxygen as may be formed at the anode (9) to flow to the biocathode (8a) and methane as may formed at the biocathode (8a) to flow to the anode (9). The cells may advantageously be positioned next to each other as in a honeycomb as shown for two cells (30) in FIG. 2. The electron charged packed bed of activated carbon granules (8) contacts an inside current collector (32,33) made of stainless steel. Two adjoining cells (30) may share one inside current collector (32) as shown. When multiple cells are combined the current collector (32) suitably has a honeycomb structure where the cells of the honeycomb are the cells (30). The combined numerous hexagonal shaped cells (30) are further provided with an outer wall (31) as shown for the two combined cells (30). The outer wall (31) may optionally be spaced away from the honeycomb shaped current collector (32) and may have another shape.

[0044] FIG. 3 shows the cross-sectional view AA of a single hexagonal cell of FIG. 2. The anode (9) is shown in more detail as a metal plate submerged in the anolyte. Inside current collectors (32,33) are in contact with power source (34). Between inside current collector (33) and membrane (10) a membrane protector (35) made of a PTFE mesh is present. An inlet (36) for stream (6) of FIG. 1 is present at the lower end. The liquid flows via distribution plate (37) and liquid distribution material (38) to the packed bed (8). An outlet for steam (2a) of FIG. 1 is provided at the upper end of the cell (30). Further an inlet (40) for stream (20) and an outlet (41) for stream (18) of FIG. 1 is provided at the upper end of the cell (30).

[0045] The invention is illustrated by the following non-limiting examples. In these examples the energy efficiency of the process is shown. This energy efficiency is defined as follows. In general, the energy efficiency of an electron driven process as the process according to this invention is described as the external electrical energy that ends up in the aimed end-product methane. The energy efficiency is calculated as Equation 1.

[00001]$\begin{array}{cc}{}_{\mathrm{energy}}={}_{\mathrm{product}}{}_{\mathrm{voltage}}& \left(\mathrm{Eq}.l\right)\end{array}$

For the CH.sub.4 producing process of this invention, .sub.product is the current-to-methane efficiency. This is described as the efficiency of capturing electrons from the electric current in the form of CH.sub.4, which is calculated as shown in Equation 2.

[00002]$\begin{array}{cc}{}_{\mathrm{product}}=\frac{N_{CH4}.Math.8.Math.F}{{}_{t=0}^{t}I.Math.\mathrm{dt}}& \left(\mathrm{Eq}.2\right)\end{array}$

Where N.sub.CH4 is the amount of methane produced (in mole) during a certain amount of time (t); 8 is the amount of electrons required to produce 1 molecule of CH.sub.4; F is the Faraday constant (96485 C/mol e.sup.); I is the current (A).

[0046] The voltage efficiency (.sub.voltage) is described as the part of the energy input (i.e. the required cell voltage to run the system) which ends up in CH.sub.4, which is calculated as shown in Equation 3.

[00003]$\begin{array}{cc}{}_{\mathrm{voltage}}=\frac{-G_{\mathrm{CH}4}}{E_{\mathrm{cell}}.Math.8.Math.F}& \left(\mathrm{Eq}.3\right)\end{array}$

In this equation GC.sub.H4 is the change in Gibb's free energy of oxidation of CO.sub.2 to CH.sub.4 (89010.sup.3 J/mol CH.sub.4); Ecell is the applied cell voltage (V); 8 is the amount of electrons required to produce 1 molecule of CH.sub.4; F is the Faraday constant (96485 C/mol e.sup.).

Example 1

[0047] A biocathode was operated at halo alkaline medium, containing 0.6 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 BES setup is similar to the BES setup described in Liu, Dandan, Marta Roca-Puigros, Florian Geppert, Leire Caizn-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 cm2 cm1.5 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 cm2 cm). The catholyte and anolyte were recirculated over a catholyte and anolyte recirculation bottles. The total volume of anolyte and catholyte were 500 mL and 330 mL, respectively. 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.

[0048] The cathodic chamber was inoculated with 30 mL of anaerobic sludge from an upflow anaerobic sludge blanket (UASB) digestion in Eerbeek, The Netherlands. 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 (with a current density 4 A/m.sup.2). 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. After a start-up period (not shown), the following results were obtained.

[0049] In the first period (25 days), the catholyte contained trace amounts of dissolved phosphate, i.e. <0.1 mM. The coulombic efficiency in this period was between 35% and 60%. That means that the remaining electrons which were supplied to the biocathode ended up in biological growth and other end-products. It was found that acetate was one dominating by-products, which is typically formed by acetogenesis. No significant amount of H.sub.2 was detected. To suppress this process, at day 25 phosphate levels were increased to 5 mM in the catholyte. While an initial increase in coulombic efficiency could be noticed, following days did not show improvement in the coulombic efficiency. In contrast, more supplied electrons ended up in acetate. Hence, at day 50, the phosphate concentrations were increased to 50 mM. An immediate increase in CH.sub.4 formation was observed, bringing the coulombic efficiency towards >80%.

[0050] During the entire period of operation, the voltage efficiency did not change. This is because i) the applied potential at the biocathode always remained between 0.73 and 0.65 V, ii) the applied potential at the anode remained constant and iii) applied current density remained constant. Hence, a change in coulombic efficiency directly resulted in a change in energy efficiency. While in the period of day 0-50 the energy efficiency fluctuated between 20-50%, at higher phosphate levels the energy efficiency increased to 67%.

[0051] See also FIG. 4 where the current to methane efficiency is represented by the dotted line, the voltage efficiency by the solid line and the energy efficiency by the dashed line.

Example 2

[0052] A biocathode was operated at high saline medium, containing 0.6 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. Additionally, the phosphate concentration was 50 mM. Similar setup and process control was applied as in Example 1.

[0053] The methane-producing BES was galvanostatically controlled (fixed current) by a potentiostat (with a current density 2.5 A/m.sup.2). During a 70 days period of operation, energy efficiency was monitored over time. While maintaining the phosphate concentrations at 50 mM, also the energy efficiency over the entire period was between 55 to 67%, except for day 24 (49%). The latter might be explained as an outlier. Thus a stable process is illustrated as also shown in FIG. 5 where the current to methane efficiency is represented by the open circles, the voltage efficiency by the solid diamonds and the energy efficiency by the crosses and connecting line.