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PULSED CAPACITIVE DEIONIZATION

IP.com Disclosure Number: IPCOM000208871D
Publication Date: 2011-Jul-20
Document File: 7 page(s) / 120K

Publishing Venue

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Abstract

Embodiments of the present invention provide a capacitive deionization water purification system and membrane capacitive deionization water purification system with a pulsed, rather than continuous, electric field applied across the electrodes. Embodiments of the present invention eliminate boundary layer polarization effects at the surface of the electrodes or ion exchange membranes, thereby increasing the efficiency and decreasing the cost of operating such systems.

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PULSED CAPACITIVE DEIONIZATION

BACKGROUND

Embodiments of the present invention relate generally to water purification and more particularly to capacitive deionization water purification systems with a pulsed, rather than continuous, electric field applied across the electrodes.

Water is one of the most abundant natural resources on earth yet, paradoxically, one of the scarcest natural resources. While fully seventy-five percent of the surface of the planet is water, actually very little of it is available for use by man without further treatment since most of it is saline or brackish. Furthermore, the distribution of water, especially potable water, on the planet is such that many inhabited areas do not have an abundance of potable water. Thus, there is a need for purifying saline or brackish water to obtain potable water in many parts of the world.

The principles of water purification through capacitive deionization (CDI) and membrane capacitive deionization (MCDI) have been known for many years.  CDI works to desalinate saline or brackish water by applying an electric field between two porous electrodes such that the anions in solution, such as chloride, nitrate, carbonate, and sulfide ions, flow towards and are adsorbed into the anode while the cations, such as calcium, magnesium, and sodium, flow towards and are adsorbed into the cathode.  This allows the electrodes to adsorb the majority of the total dissolved solids and produce a stream of treated water.  After the ion adsorption capacity of the electrodes has been reached, the applied voltage is reduced to zero or reversed and a concentrate stream is released for subsequent disposal.  MCDI is a modification of CDI that comprises an anion-exchange membrane inserted in front of the anode and a cation-exchange membrane inserted in front of the cathode.  Cation exchange membranes are relatively permeable to cations with a low molecular weight and relatively impermeable to anions while anion exchange membranes are relatively permeable to anions with a low molecular weight and relatively impermeable to cations. The ion-exchange membrane eliminates the co-ion expulsion effect, where at high surface charge densities co-ions are pushed away from the electrodes, which negatively influences the salt adsorption rate and ion removal capacity in CDI.

Although the primary advantage of CDI and MCDI are their low operating cost, which is about one third of the cost of reverse osmosis, boundary layer polarization effects may provide an upper limit to the rate at which ions may be adsorbed by the electrodes.  Boundary layer polarization is characterized by a local change of concentration caused by the depletion of salt from the solution in the immediate vicinity of the electrodes or ion exchange membranes that results from an ion-transport phenomena and causes  an undesirably high ohmic resistance. This typically occurs when the ion concentration in the solution close to the electrode or membrane...