Chemistry: electrical and wave energy – Processes and products – Electrophoresis or electro-osmosis processes and electrolyte...
Reexamination Certificate
2001-06-22
2002-04-02
Phasge, Arun S. (Department: 1741)
Chemistry: electrical and wave energy
Processes and products
Electrophoresis or electro-osmosis processes and electrolyte...
C204S533000, C204S536000
Reexamination Certificate
active
06365023
ABSTRACT:
FIELD
This invention relates in general to an electrodeionization (EDI) process wherein liquid to be purified is passed through an ion depletion compartment containing anion and/or cation resin beads under the influence of a polar field to effect ion transfer from the liquid in the ion depletion compartment to a liquid in an ion concentration compartment.
BACKGROUND
The purification of a liquid by reducing the concentration of the ions or molecules in the liquid has been an area of substantial technological interest. Many techniques have been used to purify and isolate liquids or to obtain concentrated pools of the specific ions or molecules from a liquid mixture. Known processes for purifying liquids include distillation, electrodialysis, reverse osmosis, liquid chromatography, membrane filtration and ion exchange. Another method is electrodeionization.
An early apparatus and method for treating liquids by electrodeionization was disclosed in U.S. Pat. Nos. 2,689,826 and 2,815,320. U.S. Pat. No. 2,689,826, issued to P. Kollsman on Sep. 21, 1954, describes an apparatus and process for the removal of ions within a liquid mixture in a depletion chamber through a series of anionic and cationic diaphragms into a second volume of liquid in a concentration chamber under the influence of an electrical potential which causes the pre-selected ions to travel in a predetermined direction. The volume of the liquid being treated is depleted of ions while the volume of the second liquid becomes enriched with the transferred ions and carries them in concentrated form. U.S. Pat. No. 2,815,320, issued to P. Kollsman on Dec. 3, 1957, describes the use of microporous beads formed of ion exchange resins as a filler material positioned between the anionic or cationic diaphragms. This ionic exchange resin acts as a path for ion transfer and also serves as an increased, conductivity bridge between the membranes for the movement of ions.
The term “electrodeionization” refers to the process wherein an ion exchange material is positioned between anionic and cationic diaphragms. The term “electrodialysis” refers to such a process which does not utilize ion exchange resins between the anionic and cationic diaphragms. Illustrative of other prior art attempts to use a combination of electrodialysis and ion exchange materials or resins to purify saline from brackish water are described in U.S. Pat. Nos. 2,794,770; 2,796,395; 2,947,688; 3,384,568; 2,923,674; 3,014,855; and 4,165,273. Attempts to improve electrodeionization apparatus are shown in U.S. Pat. Nos. 3,149,061; 3,291,713; 3,515,664; 3,562,139; 3,993,517; and 4,284,492.
A commercially successful electrodeionization apparatus and process is described in U.S. Pat. No. 4,632,745, issued to A. Giuffrida et al. on Dec. 30, 1986. The apparatus utilizes ion depletion compartments containing an ion exchange solid composition and a concentration compartment which is free of an ion exchange solid material. The electrodeionization apparatus includes two terminal electrode chambers containing an anode and a cathode respectively which are utilized to pass the direct current transversely through the body of the apparatus containing a plurality of ion depletion compartments and ion concentrations compartments. In operation, the dissolved ion salts of the liquid are transferred through the appropriate membranes from the ion depletion compartments to the ion concentration compartments. The ions collected in the ion concentration compartments are removed through discharge outlets and then directed to waste.
In present electrodeionization processes, feed water is initially pretreated in a reverse osmosis step to reduce the ionic load and colloidal contaminants therein, prior to being directed towards electrodeionization. This practice extends the useful life of the resin beads used in electrodeionization. However, even when using a reverse osmosis pretreating step, the presence of certain carbonic species (including dissolved CO
2
, H
2
CO
3
, HCO
3
−
and CO
3
−2
) in the feed water causes problems in the overall process. Generally, ionized carbonic species such as HCO
3
−
and CO
3
−2
are retained by the reverse osmosis (RO) membrane. However un-ionized species such as CO
2
and H
2
CO
3
readily pass through the RO membrane. In electrodeionization, carbonate producing species such as CO
2
and H
2
CO
3
can cause so-called “scaling” in the ion concentration compartments due to precipitation of calcium ion and magnesium ion at the anionic membrane, particularly at neutral to high pH conditions. Scaling can result in a substantial reduction of the useful life of the electrodeionization apparatus.
Thus, two common problems encountered in the practice of EDI are (a) inadequate ionic removal which can lead to poor water quality and (b) scaling, which when unattended, can quickly lead to premature failure of an EDI module.
A number of factors can lead to poor water quality. However, for a well designed and constructed EDI module, insufficient electrical current is the most common source of poor water quality. This occurs because a certain minimum current is required to remove the ionic contaminants. Furthermore, the higher the ionic content of the feed water, the higher the current required to effectively remove contaminants and produce good water quality. Good water quality is defined herein by the resistivity of the water which is typically desired to be no lower than approximately 3 mega-ohm-cm, more preferably above 5 mega-ohm-cm, and most preferably greater than 10 mega-ohm-cm.
A number of factors can lead to scaling, for example, the presence of hard ions such as Ca
−2
or Mg
−2
in high concentrations in the water feeding the EDI module. Some manufacturers of EDI modules specify that Ca
−2
levels be maintained below 0.5 ppm to prevent scaling. Scaling typically occurs in the boundary layers adjacent to the cathode and to the anionic membrane on the side facing the waste compartments due to the high pH conditions typical of these regions. In the cathode, electrochemical reactions typically produce hydroxide reactions (OH
−
); in the waste side of the anion membrane high hydroxide ion concentration occurs as the result of their transport through the membrane. It is believed that the pH at the boundary layer increases with current. Therefore, the current needs to be maintained at a sufficiently low level to prevent or, at least ameliorate, the incidence of scaling.
In view of the above, it is necessary to maintain the current passing through the EDI module within an acceptable range. If the current is too low, poor water quality is obtained. If the current is too high, the incidence of scaling increases.
Presently, EDI modules typically operate using a constant voltage power supply. Unfortunately, it has been observed that the electrical impedance of EDI modules increases with the age of the module. Although the cause of the impedance increase is not known, it is thought to be due to absorption of contaminants into the ion exchange media which, in general tends to increase the specific impedance of ion exchange resins. This impedance increase means that as the EDI module ages, the current passing through the module decreases when powered with a constant voltage power supply. Thus, over time a low enough current may be reached as to result in inadequate water quality. Likewise, a new module having a low impedance and run at constant voltage can produce a very high current thereby increasing the incidence of scaling. Therefore, the aging of the EDI module results in a large variation in current during its lifetime; variations that can produce electrical currents outside a desired operating range.
In addition to reduced longevity, it is also well-known that the impedance of an EDI module increases with decreasing temperature. Thus, during warm summer months, EDI modules may produce very good water quality, while, during the cold winter months when the tap water temperature may be as low was 4 degrees centigrade, the mod
De Los Reyes Gastón
Denoncourt Linda M.
Garcia Bienvenido
Cook Paul J.
Hubbard John Dana
Millipore Corporation
Phasge Arun S,.
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