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Fig. 2 (a) Picture and (b) schematic of the CO2-from-seawater BPMED unit. ES 1⁄4 electrode solution, SW 1⁄4 seawater, CEM 1⁄4 cation exchange membrane, AEM 1⁄4 anion exchange membrane, BPM 1⁄4 bipolar membrane. In panel (a), the opposite side of the unit that is not visible contains the cathode (), Electrode solution in and out for the cathode, Seawater out (acid), and Seawater out (base). membrane.27 The properties of these membranes, such as elec- trical resistance, burst strength, and thickness are provided by the manufacturer;28 the Neosepta ACS and CMX-S are mono- valent-anion and monovalent-cation permselective membranes, respectively. The electrodes are titanium with an iridium-ruthe- nium based coating (custom electrodes, De Nora Tech Inc.). The solution compartments between adjacent membranes are filled with 762 mm thick polyethylene mesh spacers, and these compartments are sealed against leaks using axial pressure and 794 mm thick EPDM rubber gaskets. Each membrane has an active area of 180 cm2. The next section describes in detail how this system extracts CO2 from seawater. Procedure The experiments described below are all continuous, ‘‘flow- through’’ experiments (in contrast to batch mode) in which each volume element of seawater passes through the system exactly once. For each experiment, constant flow rates are chosen for the acid, base, and electrode solutions. Depending on these flow rates and the Faradaic efficiency of the BPMED process, a constant current value is chosen that will result in the desired pH for the output acid solution. Acid and base flow rates of 3.1 lpm, 3.6 lpm, 4.1 lpm, and 6 lpm and acid output pH values in the range 3–6 were investigated in the experiments described below. A membrane stack consisting of nine cells (a shown in Fig. 2b) was used for all experiments described below, except for the 6 lpm seawater experiment, for which an eight-cell membrane stack was used. The input seawater solutions are prepared by adding 35.95g of Instant Ocean! sea salt per litre of DI water. The electrode tanks are filled with a 0.1M H2SO4/0.25M Na2SO4 solution (solutes were ACS reagent grade, purchased from Sigma-Aldrich.). For some experiments (labelled ‘‘RO’’ in Fig. 3, a solution with twice the ionic concentration of seawater 71.9 g of Instant Ocean! sea salt per litre of DI water – was used in order to mimic a typical reverse osmosis (RO) brine solution). Once the seawater and electrode solutions are prepared and loaded into the input solution tank and electrode solutions tanks, respectively, the pumps and flow-control are adjusted to achieve the desired solution flow rates and pressures. The seawater and electrode solutions are started flowing together to prevent pres- sure drops between different compartments. The degassing vacuum pump is turned on, with its output initially venting to atmosphere. After the initial turn-on, the output of the degassing vacuum pump is directed through the CO2 flow meter in order to measure the rate of CO2 extraction. The rate measured by the CO2 flow meter before current is applied to the BPMED unit is recorded to determine the flow-meter offset, and this value is subtracted from the recorded flow rate during operation to calculate the net CO2 extraction. The power supply is then connected to the BPMED-unit electrodes and the software is used to set a constant-current value, at which point the pH of the acid solution begins to drop and the pH of the base solution begins to rise. As the acid solution pH drops, the rate of CO2 gas extraction increases until the solution pH values and the CO2 extraction rate reach steady-state values. The experiment is run and data is recorded until we observe a constant acid pH reading for 15 s (3 data points, with each data recorded every 5 s). These last three data points, for which the value of the acid output pH is constant, are averaged to obtain the measured value shown in Fig. 3. After each experiment, the acid and base compartments are rinsed with seawater to remove any residual acidic or basic solutions from the BPMED membrane stack that might influence the data for the next experiments. The value of constant current for a given experiment is chosen to achieve a desired end-pH value for the output acid solution, given the other experimental parameters such as solution flow rate and Faradaic efficiency. For each of the solution flow rates studied in our experiments, several constant-current experiments were performed, with the current chosen to achieve output acid solution pH values in the range of 3–6. When a voltage is applied across the BPMED stack, and the current density exceeds the limiting current density for the BPM (typically < 10 mA cm2), water dissociation is induced in the BPM, producing H+ and OH ions which move in the applied field (H+ toward the negatively-charged cathode, and OH toward the positively-charged anode). A theoretical minimum View Online This journal is a The Royal Society of Chemistry 2012 Energy Environ. Sci. 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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