Desalination with carbon aerogel electrodes. Revision 1

Abstract

An electrically-regenerated electrosorption process known as carbon aerogel CDI has been developed by Lawrence Livermore National Laboratory (LLNL) for continuously removing ionic impurities from aqueous streams. A salt solution flows in an unobstructed channel formed by numerous pairs of parallel carbon aerogel electrodes.Each electrode has a very high BET surface area (2.0-5 .4x1 OGft2 lb-i or 400-1100 m2 g-') and very low electrical resistivity (<40 mQ cm). BET surface areas of 1.3x107 ft2 lb-' (2600 m2 g-') have been achieved with thermal activation.After polarization, anions and cations are removed from the electrolyte by the imposed electric field and electrosorbed onto the carbon aerogel. The solution is thus separated into two streams, concentrate and purified water. Based upon thk analysis, it is concluded that carbon aerogel CDI may be an energy-efllcient alternative to electrodialysis and reverse osmosis for the desalination of brackish water (< 5000 ppm), provided that cell geometries and aerogel properties are c&efi.dly tailored for such applications. The intrinsic energy required by this process is approximately Q?72, where Q is the stored electrical charge and V is the voltage between the electrodes, plus losses due to parasitic electrochemical reactions, electrical resistance and pressure drop. The estimated requirement for desalination of a 2000 ppm feed is estimated to be -0.53-2.5 Wh gal-' (0.50-2.4 kJ L-'), depending upon voltage, flow rate, cell dimensions, carbon aerogel density, recovery ratio and other parameters. These estimates assume that 50-70'%0of the stored electrical energy is reclaimed during regeneration (electrical discharge). The possibility of such low power requirements for desalination of brackish water (BW), as well as the possibility of energy storage and recovery, may. make this process attractive for such applications. Though the intrinsic energy requirement for desalination of sea water (SW) are also relatively low, this application will be much more difficult. Additional work will be required to determine the suitability of carbon aerogel CDI for desalination of streams that contain more than 5000 ppm total dissolved solids (TDS). Applications at 2000 ppm will require the construction of electrochemical cells with extremely tight, demanding tolerances. At the present time, the process is best suited for streams with relatively dilute impurities, as recently demonstrated during a field test at LLNL Treatment Facility C.