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a much higher normalized current density to achieve a given pH because the concentration is twice that of the seawater solution. Fig. 3b shows the CO2 extraction efficiency as a function of the pH of the acid solution. We define this efficiency as the measured rate of CO2 extraction divided by the rate at which DICx flows through the acid compartment of the BPMED unit (see ESI,† Section S3). We performed separate tests of the membrane contactors in order to optimize the membrane contactor efficiency (measured rate of CO2(g) extracted divided by rate at which CO2(aq) is flowed through the contactor via the input solution) as a function of the solution flow rate (see ESI,† Section S1). Our measure- ments indicated that the membrane contactors (Liqui-Cel! X50 fibre type 2.5 8 membrane contactors from Membrana-Char- lotte) extracted dissolved CO2 gas from solution most efficiently for flow rates in the range 3.75 lpm–5 lpm, with relatively decreased efficiency at lower (2.5 lpm and 3 lpm) and higher (6.25 lpm) flow rates. For the optimum flow-rate range of 3.75 lpm–5 lpm, we found that one, two, and three passes through a membrane contactor gave an extraction efficiency of about 33%, 60%, and 75%, respectively. Based on these results, and the need to minimize pressure imbalances in the system, we chose to use two membrane contactors connected in series for the prototype used for the experiments described in this paper (see Fig. 1). Fig. 3c shows the rate of CO2 extraction (in slpm) as a function of the pH of the acid solution. This rate depends on the DICx in the input solution, the flow rate of the input solution, the pH of the acid solution (which determines the fraction of HCO3 and CO3(2) in the input solution that is converted into CO2(aq), and depends on the applied current density and the Faradaic effi- ciency of the electrodialysis process), and the efficiency with which the membrane contactors extract CO2(aq) from solution. From Fig. 3c we see that for a given input solution at a fixed acid pH value (and therefore a fixed concentration of CO2(aq)), the rate of CO2 extraction increases with increasing flow rate due to the increasing rate of CO2(aq) flowing through the membrane contactors. It is important to note, however, that the CO2 extraction rate shown in Fig. 3c doesn’t simply scale linearly with solution flow rate for a fixed solution and pH value due to the dependence of membrane contactor extraction efficiency on flow rate (see ESI,† Section S1). We also note that the CO2 extraction rate for our ‘‘RO’’ solution flowed at 3.6 lpm is approximately equal to that of the seawater solution flowed at 6 lpm because the DICx of the RO brine solution is twice that of the seawater solution. Finally, Fig. 3d shows the energy of CO2 extraction in kJ mol1(CO2). The energy plotted in Fig. 3d is calculated by multiplying the current times the voltage to yield the applied power, dividing the result by the measured rate of CO2 extraction shown in Fig. 3c, and converting the units to kJ mol1(CO2). As such, this energy represents the electrochemical energy that must be applied to the BPMED unit to acidify the incoming solution to the desired pH, and does not include the energy required to pump the seawater solution through the system or the energy needed to vacuum strip CO2 from the acidified seawater. Addi- tionally, the energy shown in Fig. 3d was calculated using the total voltage measured over the entire nine-cell BPMED membrane stack, including the contribution from the end Energy Environ. Sci. electrodes. Previous measurements in our lab have shown that approximately 30% of the total voltage for a seven-cell BPMED unit is due to the electrodes.3 Since the fractional contribution of the electrode voltage to the total voltage scales as the inverse of the number of cells, then for a commercial-scale unit with over 100 cells, the contribution of the electrode voltage to the total voltage becomes negligible, and so we would expect a commer- cial-scale system to consume approximately 23% less electro- chemical energy (30% 7/9) than the data shown in Fig. 3d. The minimum energy shown in Fig. 3d (see ESI,† Table S1 for a table of numerical values) is 242 kJ mol1(CO2) for seawater at a flow rate of 3.1 lpm acidified to pH 5 with 59% CO2 extraction effi- ciency. A higher CO2 extraction efficiency of 68% was observed for seawater at 6 lpm acidified to a pH of 3.7, with a small increase in the energy to 285 kJ mol1(CO2). Conclusions We have described the design, construction, and characterization of a novel prototype for the efficient extraction of dissolved inorganic carbon from seawater as CO2 gas using BPMED. We characterized the performance of the prototype and demon- strated the ability to extract 59% of the total dissolved inorganic carbon from seawater as CO2 gas with an electrochemical energy consumption of 242 kJ mol1(CO2). To put this number into context, simplistically considering the reaction of three moles of H2 and one mole of CO2 to form one mole of methanol (CH3OH, LHV 1⁄4 644 kJ mol1)31 with an overall energy storage efficiency of 20%, the CO2 extraction energy of 242 kJ mol1(CO2) repre- sents only 7.5% of the required energy input. This technology has the potential for significant impact in a wide range of fields, including CO2 mitigation and carbon- neutral liquid fuels. Importantly, BPMED is a technology that is already deployed at commercial scale for many diverse applica- tions, and the performance of a commercial-scale CO2-from- seawater system is expected to be even better than the results demonstrated here due to various favourable effects with the scale-up of BPMED systems, such as the reduced fractional contribution of electrode voltage to the total voltage. Acknowledgements We thank F. Torres, D. Bar, and M. Steinberg for helpful discussions, and three anonymous reviewers for useful sugges- tions. This work was supported by DARPA contract HR0011- 10-C-0147. The views, opinions, and/or findings contained in this article are those of the authors and should not be interpreted as representing the official views or policies, either expressed or implied, of the Defense Advanced Research Projects Agency or the Department of Defense. Approved for Public Release, Distribution Unlimited. References 1 M. M. Halmann and M. Steinberg, Greenhouse Gas Carbon Dioxide Mitigation: Science and Technology, Lewis Publishers, New York, 1999. 2 N. MacDowell, et al., Energy Environ. Sci., 2010, 3, 1645–1669. 3 M. D. Eisaman, L. Alvarado, D. Larner, P. Wang, B. Garg and K. A. Littau, Energy Environ. Sci., 2011, 4, 1319–1328. This journal is a The Royal Society of Chemistry 2012 View Online Downloaded by University of Oxford on 26 March 2012 Published on 06 February 2012 on http://pubs.rsc.org | doi:10.1039/C2EE03393CPDF Image | Co2 Extration from Seawater
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