Electrolysis: processes – compositions used therein – and methods – Electrolytic synthesis – Utilizing ac or specified wave form other than pure dc
Reexamination Certificate
1999-01-21
2002-02-12
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic synthesis
Utilizing ac or specified wave form other than pure dc
C205S743000, C210S748080
Reexamination Certificate
active
06346187
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is generally directed to the field of electrochemistry, and it relates to a new separation method and apparatus for removing ions, contaminants and impurities from water and other aqueous process streams. More specifically the invention relates to a new regeneration method for placing the removed ions back into solution.
2. Background Art
The separation of ions and impurities from electrolytes has heretofore been generally achieved using a variety of conventional processes including: ion exchange, reverse osmosis, electrodialysis, electrodeposition, or filtering. Several alternatives have been proposed that address the problems associated with the conventional separation processes. However, such alternatives have not been completely satisfactory for specific applications nor useful for all applications, and have not met with universal commercial success or complete acceptance.
The conventional ion exchange process has been used as a means for removing anions and cations, including heavy metals and radioisotopes, from process and waste water in various industries. This process generates large volumes of corrosive secondary wastes that must be treated for disposal through regeneration processes. Existing regeneration processes are typically carried out following the saturation of the columns by ions, by pumping regeneration solutions, such as concentrated acids, bases, or salt solutions through the columns. These routine maintenance measures produce significant secondary wastes, as well as periodic interruptions of the deionization process.
Secondary wastes resulting from the regeneration of the ion exchangers typically include used anion and cation exchange resins, as well as contaminated acids, bases and/or salt solutions. For example, H
2
SO
4
solutions have been used for the regeneration of cation columns in metal finishing and power industries; HNO
3
solutions have been used for the regeneration of cation columns used in processing nuclear materials; and NaCl solutions have been used in residential water softening processes.
Under an applied electrical field, ionic species in solution can be separated from the aqueous solution by a variety of means. The use of flow-through or flow-by electrochemical cells containing porous, high-surface-area, electrically conductive carbonaceous electrodes have been employed for separation of ionic species in solution. The electrodeposition of metals from aqueous solutions where electron transfer takes place between the carbon electrode and the ions in solution has been employed. Without apparent electron-transfer steps at potentials less than the reduction potentials of the soluble species, ionic species are thought to be separated from the solution by a simple electrostatic separation where they are held within the electrical double-layer formed at the solution-electrode interface, i.e., capacitive deionization. Such deionization of the solution by a capacitive process has been proposed in the early 1960's by Arnold and his colleagues [B. B. Arnold, G. W. Murphy, J. Phys. Chem. 65 1 (1961) 135-138.]. Their capacitive deionization (CDI) process was suggested for desalination of brackish water.
In one capacitive deionization method described in U.S. Pat. No. 5,425,858 issued to Farmer (herein referred as “Farmer”), a stream of electrolyte to be processed, which contains various anions and cations, electric dipoles, and/or suspended particles, is passed through a stack of electrochemical capacitive deionization cells, i.e., a capacitor. Each of these cells of the capacitor includes numerous electrodes having exceptionally high specific surface areas (for example, carbon aerogel having a surface area of 400-1000 m
2
/gm). By polarizing the cell, non-reductible and non-oxidizable ions are removed by electrodeposition. Electric dipoles also migrate to and are trapped at the electrodes. Small suspended particles are removed by electrophoresis. Therefore, the fluid stream leaving the cell is purified.
The Farmer method is an efficient deionization process since the pressure drop in the capacitive deionization cell is dictated by channel flow between parallel surfaces of monolithic, microporous solids (i.e., the electrodes) and across (rather than through) such surfaces. Hence, it is insignificant compared to that needed to force water through the permeable membrane required by the reverse osmosis process as well as capacitive deionization processes described by Andelman in, for example, U.S. Pat. Nos. 5,547,581, 5,415,768, 5,360,540, 5,200,068, and 5,192,432.
A feature of the Farmer separation system is that no expensive ion exchange membranes are required for the separation of the electrodes. All the anodes and cathodes of the electrode pairs define individual cells that are connected in series, each pair defining an open, unobstructed channel for fluid flow between the electrode pair. The system is modular and can be readily expanded to include several electrode pairs (i.e. cells or modules) thus forming a capacitor with a relatively large anode or cathode total surface area. Typically, the electrode pair modules are arranged so that fluid flow through the capacitor is in a serpentine pattern across, rather than through, a relatively large number of intermediate electrode pairs having no dimension open to the exterior of the cell(s) or capacitor (other than those electrode pairs at a single fluid input and single fluid output location). Ultimately, the Farmer system capacity can be increased to any desired level by expanding the capacitor to include a substantial number of electrode pairs. Although each electrode pair can define a single cell, in the Farmer system all of the closed series of cells formed by the intermediate electrode pairs act as a single cell or capacitor.
The Farmer system is not without its problems; underpotential deposition, electrodeposition, chemisorption, electrophoresis and other separation phenomena involving a charge transfer across the carbon surface and the liquid electrolyte can also occur. These types of processes affect the reversibility and robustness of the deionization-regeneration cycle (process). Under such conditions, the regeneration and/or rejuvenation of the saturated electrode and cell system would dictate the performance and overall effectiveness of the deionization process. Recent efforts have focused on the efficiency and effectiveness of the regeneration of multiple-cell stacks containing the above-described carbon aerogel compounds.
The electrosorption of simple ionic compounds such as NaCl, NH
4
Cl and NaNO
3
may be accomplished reversibly if cells can be regenerated or rejuvenated with deionized water over a long period of time and with voltage reversal as appropriate. However, several techniques for effective and optimal regeneration methods have been suggested. Even under a low or mild applied potential (i.e.,<1.2V in water-based streams) for removal of ions, certain charge transfer processes have been shown to occur which have given rise to deposited or strongly bound species. During regeneration, even with a reversed polarity, the applied voltage to capacitive systems may not be sufficient to conteract fractions of the ions that remain attracted strongly by other means such as underpotential deposition or chemisorption. Accordingly, a cycle efficiency (i.e., the ratio of the amount of salt recovered during regeneration over that determined during deionization expressed in terms of percentage) never reaches 100%, particularly over many successive cycles and especially when the total regeneration time and the total deionization time is the same. Certain features of such inherent inefficiency have been observed in results presented in previous investigations. Under special treatments, aged electrodes can, however, be rejuvenated to regain degraded capacity. Farmer et al. in the Journal of the Electrochemical Society, 143 pp. 159-169, 1996) has reported that electrodes that have been aged naturally
Lenz David J.
Tran Tri D.
Gorgos Kathryn
Nicolas Wesley A.
The Regents of the University of California
Thompson Alan H.
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