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View Online One of the most attractive uses for CO2 extracted from the ocean is the synthesis of a renewable, carbon-neutral liquid fuel via reaction of the extracted CO2 with H2.16,17,18 If the entire process (CO2 extraction, H2 generation, and fuel synthesis) is powered using a carbon-neutral source such as wind, solar, or nuclear, then no net CO2 is emitted into the atmosphere during the combustion of the synthesized fuel. While cost estimates suggest that fuel produced in this way could be cost-competitive in the commercial market in the near future,19 this process also represents a very attractive possibility for on-site, on-demand fuel synthesis at sea for military applications,19,20 and for remote communities or research stations.21 We note that CO2 extracted from seawater can also be used as an enriched CO2 source in the synthesis of renewable, carbon-neutral algal biofuels.22 It is important to note that for both the non-biological16,18 and bio- logical22 routes to liquid fuel synthesis, CO2 separation from the atmosphere (in this case indirectly through the extraction of CO2 from seawater) allows a truly carbon-neutral fuel, in contrast to CO2 separated from flue-gas, which could produce a fuel with reduced, but not zero, net CO2 emissions. Previous investigations have demonstrated the extraction of CO2 from the DIC in seawater,23,24,25 but challenges remain, including slow extraction rates for membrane-based extraction without acidification,23 poor selectivity of CO2-containing ions for ion-exchange approaches without acidification,24 and the need for water with low conductivity and hardness (such as deionized (DI) water) for ion-exchange approaches with acidification.25 In this article, we describe a novel approach to these chal- lenges – efficient CO2 extraction from seawater using BPMED.26 Seawater is pumped through a BPMED system, resulting in two output streams: acidified seawater and basified seawater. In the acidified stream, the HCO3 and CO32 ions in the input seawater area converted into dissolved CO2, which is subsequently vacuum stripped, producing a stream of pure CO2 gas. The CO2-depleted acidified solution can then be combined with the basified solution, creating a neutral-pH solution that can be returned to the ocean. Experimental methodology Setup and equipment Fig. 1 shows a schematic and labelled photograph of the exper- imental setup. A BPMED unit consisting of a nine-cell membrane stack that converts input seawater into two separate output streams: acidified seawater and basified seawater. CO2 gas is vacuum stripped from the acid solution using commercial membrane contactors (two Liqui-Cel! X50 fibre type 2.5 8 membrane contactors from Membrana-Charlotte), and then the acid and base output solutions are recombined into a neutral-pH waste solution. The input seawater flow is divided so that some flows through the acid compartment of the BPMED unit and the remainder flows through the base compartment, while electrode solution is pumped through both the anode and cathode compartments. Any gases present in the anode and cathode output solutions are separately vented before recombining the solutions in the electrode solution tank. Variable pumps (IDNM 3534 motor and VS1MX Microdrive, Baldor Electric Company) and valves are used to control the flow rate and pressure of seawater and electrode solution. Balancing the pressure among all the compartments in the BPMED unit is crucial to avoiding mechanical damage to the BPMED membrane stack and the unwanted mixing of different solution streams within the membrane stack. We measure and automatically record with control software the following experimental parameters every 5 s: current, voltage, solution flow rates, solution pH values, pressure in each compartment, and the CO2 flow rate. A programmable DC power supply (XHR40-25, Xantrex Technology, Inc.) is used to supply constant current. The sensors include (all from Omega Engineering, Inc.): FP5603 for measuring liquid solution flow rates, FMA1620A for CO2 gas flow rate, pressure sensor PX 309, and pH sensor PHE-4581. Fig. 2 shows the custom-designed BPMED unit in more detail. The unit consists of nine cells in series, with each cell consisting of: a basified solution compartment, a bipolar membrane (BPM, Neosepta BP-1E, Ameridia Corp.) an acidified solution compartment, and an anion exchange membrane (AEM, Neo- septa ACS, Ameridia Corp.). At each end of the membrane stack, a cation exchange membrane (CEM, Neosepta CMX-S, Ameridia Corp.) is used to separate the membrane stack from electrode compartment. Broadly speaking, under an applied voltage, water dissociation inside the BPM and the ion-selective membranes comprising a BPM will result in the transport of H+ ions from one side of a BPM, and OH ions from the opposite side. AEMs/CEMs, as their names suggest, only allow the transport of negatively/positively charged ions through the Fig. 1 (a) Schematic of the experimental setup. BPMED 1⁄4 bipolar membrane electrodialysis. (b) Photo of experimental setup. Energy Environ. Sci. This journal is a The Royal Society of Chemistry 2012 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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