Electrolysis: processes – compositions used therein – and methods – Electrolytic material treatment
Reexamination Certificate
1999-04-12
2001-10-30
Gorgos, Kathryn (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic material treatment
C204S267000, C204S269000, C204S278500, C204S551000, C204S554000, C205S688000
Reexamination Certificate
active
06309532
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an electrochemical separation method and apparatus for removing ions, contaminants and impurities from water, fluids, and other aqueous process streams, and for placing the removed ions back into solution during regeneration.
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, and filtering. Other methods have been proposed and address the problems associated with the conventional separation processes. However, these proposed methods have not been completely satisfactory and have not met with universal commercial success or complete acceptance. One such proposed ion separation method is a process for desalting water based on periodic sorption and desorption of ions on the extensive surface of porous carbon electrodes.
The conventional ion exchange 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 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.
In some instances, the secondary radioactive wastes are extremely hazardous and can cause serious environmental concerns. For instance, during plutonium processing, resins and solutions of HNO
3
become contaminated with PuO
2
++
and other radioisotopes. Given the high and increasing cost of disposal of secondary wastes in mined geological repositories, there is tremendous and still unfulfilled need for reducing, and in certain applications, eliminating the volume of secondary wastes.
Another example is the use of the ion exchange process for industrial purposes, such as in the electroplating and metal finishing industries. A major dilemma currently facing the industry relates to the difficulties, cost considerations and the environmental consequences for disposing of the contaminated rinse solution resulting from the electroplating process. A typical treatment method for the contaminated rinse water is the ion exchange process.
Other exemplary processes which further illustrate the problems associated with ion exchange include residential water softening and the treatment of boiler water for nuclear and fossil-fueled power plants. Such water softeners result in a relatively highly concentrated solution of sodium chloride in the drinking water produced by the system. Therefore, additional desalination devices, such as reverse osmosis filters are needed to remove the excess sodium chloride introduced during regeneration.
Therefore, there is still a significant and growing need for a new method and apparatus for deionization and subsequent regeneration, which significantly reduce, if not entirely eliminate secondary wastes in certain applications. The new method and apparatus should enable the separation of any inorganic or organic ion or dipole from any ionically conducting solvent, which could be water, an organic solvent, or an inorganic solvent. For example, it should be possible to use such a process to purify organic solvents, sudh as propylene carbonate, for use in lithium batteries and other energy storage devices. Furthermore, it should be possible to use such a process to remove organic ions, such as formate or acetate from aqueous streams.
The new method and apparatus should further be adaptable for use in various applications, including without limitation, treatment of boiler water in nuclear and fossil power plants, production of high-purity water for semiconductor processing, removal of toxic and hazardous ions from water for agricultural irrigation, and desalination of sea water.
In the conventional reverse osmosis systems, water is forced through a membrane, which acts as a filter for separating the ions and impurities from electrolytes. Reverse osmosis systems require significant energy to move the water through the membrane. The flux of water through the membrane results in a considerable pressure drop across the membrane. This pressure drop is responsible for most of the energy consumption by the process. The membrane will also degrade with time, requiring the system to be shut down for costly and troublesome maintenance.
Therefore, there is a need for a new method and apparatus for deionization and ion regeneration, which substitute for the reverse osmosis systems, which do not result in a considerable pressure drop, which do not require significant energy expenditure, or interruption of service for replacing the membrane(s).
U.S. Pat. No. 3,883,412 to Jensen describes a method for desalinating water. Another ion separation method relating to a process for desalting water based on periodic sorption and desorption of ions on the extensive surface of porous carbon electrodes is described in the Office of Saline Water Research and Development Progress Report No. 516, March 1970, U.S. Department of the Interior PB 200 056, entitled “The Electrosorb Process for Desalting Water”, by Allan M. Johnson et al., (“Department of the Interior Report”) and further in an article entitled “Desalting by Means of Porous Carbon Electrodes” by J. Newman et al., in J. Electrochem. Soc.: Electrochemical Technology, March 1971, Pages 510-517, (“Newman Article”). A comparable process is also described in NTIS research and development progress report No. OSW-PR-188, by Danny D. Caudle et al., “Electrochemical Demineralization of Water with Carbon Electrodes”, May, 1966.
The Department of the Interior Report and the Newman Article review the results of an investigation of electrosorption phenomena for desalting with activated carbon electrodes, and discuss the theory of potential modulated ion sorption in terms of a capacitance model. This model desalination system
10
, illustrated in
FIG. 1
, includes a stack of alternating anodes and cathodes which are further shown in
FIG. 2
, and which are formed from beds of carbon powder or particles in contact with electrically conducting screens (or sieves). Each cell
12
includes a plurality of anode screens
14
interleaved with a plurality of cathode screens
16
, such that each anode screen
14
is separated from the adjacent cathode screen
16
by first and second beds
18
,
20
, respectively, of pretreated carbon powder. These two carbon powder beds
18
and
20
are separated by a separator
21
, and form the anode and cathode of the cell
12
. In operation raw water is flown along the axial direction of the cells
12
, perpendicularly to the surface of the electrode screens
14
,
16
, to be separated by the system
10
into waste
23
and product
25
.
However, this model system
10
suffers from several disadvantages, including:
1. The carbon powder beds
18
and
20
are used as electrodes and are not “immobilized”.
2. Raw water must flow axially through these electrode screens
14
and
16
, beds of carbon powder
18
and
20
, and separators
21
, which cause significant pressure drop and large energy consumption.
3. The carbon bed electrodes
18
and
20
are quite thick, and a large potential drop is developed across them, which translates into lower removal efficiency and higher energy consumption during operation.
4. Even though the carbon particles “touch”, i.e., adjacent particles are in contact with each other, they are not intimately and entirely electrically connected. Therefore, a substantial electrical resistance is developed, and significantly contributes to the process inefficiency. Energy is wasted and the electrode surface area is not utilize
Farmer Joseph C.
Murguia Laura
Tran Tri D.
Feely Michael J
Gorgos Kathryn
Regents of the University of California
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