Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
2000-03-24
2004-08-24
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S758000, C210S759000, C210S760000, C210S763000, C210S195100, C210S257100, C204S259000, C205S746000
Reexamination Certificate
active
06780328
ABSTRACT:
BACKGROUND OF THE INVENTION
Electrodialysis (ED), electrodeionization (EDI) and related methods and devices were initially developed during the 1950s, and have since that time been improved to the point that such systems are commonly employed to purify fluids for a variety of applications. In general, ED, EDI, and related methods and devices purify fluids through electric field-mediated transfer of ions through membranes from diluting Streams passing through “less concentrated”, ion depeleting compartments to concentrating or brine streams passing through “more concentrated”, ion concentrating compartments. Generally, anion transfer (i.e. cation rejecting) and cation transfer (i.e. anion rejecting) membranes are alternated in ED and EDI methods and devices, the membranes being placed between an anode (positive electrode) and a cathode (negative electrode) across which a DC electric field is applied transverse to the fluid flow directions. Anion transfer membranes allow passage only of low molecular weight negatively charged species (anions), and cation transfer membranes allow passage only of low molecular weight positively charged species (cations). Transfer of ions across membranes is mediated by the attraction of the anions to the positively charged anode and the cations to the negatively charged cathode. The combination of an anode, a cathode, and the alternating anion and cation transfer membranes there between is commonly referred to as an ED or EDI “stack”. Such stacks may also include cation or anion transfer membranes alternating with substantially non-ion-selective membranes, that is membranes which are not substantially selective for either anions or cations.
EDI differs from ED in that one or more EDI compartments formed by membranes include ion exchange media. The media, typically in the form of resin fibers, fabrics, beads or granules, is present in diluting compartments and sometimes also in concentrating compartments of an EDI device. An EDI compartment may contain either cation exchange resins, anion exchange resins, or a random or structured combination of cation exchange resins and anion exchange resins. The resins reside in the space between alternating anion and cation transfer membranes. In response to the transverse DC electric field, ions are transferred, for example, from diluting to concentrating compartments via the diluting compartment resins and adjacent membranes. The resins form a conductive bridge for movement of ions associated therewith to the ion exchange membranes and thus out of the diluting compartment. The resin facilitates mass transfer of ions by increasing the area available for mass transfer and by decreasing the distance in solution that the ions must travel in order to be removed from the diluting compartment, thus reducing the electrical resistance of the unit, especially in the diluting compartment. In EDI, as the product becomes more pure, the electric field splits water to hydrogen and hydroxide ions which continuously regenerate the membranes and the resins at least in part. The main advantages of EDI processes include continuous operation; stable product quality; the ability to produce high purity product without requiring periodic chemical regeneration; and reduced amounts of waste products.
One area in which EDT technology is gaining momentum is production of ultrapure makeup water for electric power plants. EDI was initially used in the electric power industry in 1991, and since that time more than 50 EDI devices have been installed in such plants. In these plants, EDI has partially or completely replaced the prior conventional (i.e. chemically regenerated) ion exchange resin beds, resulting in substantial operating cost savings. For example, ion exchange units are frequently used to purify blowdown (waste water) for recycling, requiring frequent regeneration, consuming large volumes of acid and caustic, and necessitating constant maintenance. Such exhaustion (degeneration) and regeneration can also result in variations in demineralization performance, thus affecting reliability of use. With the advent of EDI systems, deionization and regeneration are simultaneous and continuous, and problems associated with periodic regeneration are no longer present.
EDI is highly efficient in removing a substantial variety of ions from water. Strongly ionized substances such as sodium, calcium, magnesium, chloride, fluoride and sulfate are examples of ions which are routinely substantially completely removed from water using multi-step purification systems which include one or more EDI units. Weakly ionizable species such as CO
2
, silica, boric acid and ammonia may also be removed using EDI. Similarly, ethanolamine (ETA) and methoxypropylamine (MPA) are also readily removed by EDI. However, complete removal of non-ionized and non-ionizable organic substances such as ethanol and glyoxal, is not as easily accomplished.
U.S. Pat. No. 5,116,509 discloses use of an ultraviolet (UV) treatment step for deionized tap water prior to EDI treatment, but fails to teach or to suggest a system which processes such tap water water initially containing non-ionized or non-ionizable carbon compounds in addition to ionic or ionizable organic species. “Substantially complete removal” of total organic carbon (TOC) is purported to have been obtained using the system of U.S. Pat. No. 5,116,509. The patent teaches that following an initial deionization, organic species may be added to the deionized tap water and applied UV may break down the added organic species into smaller molecules, some of which are ionic and/or ionizable, allowing subsequent EDI to achieve the claimed TOC reduction. When some part of the added organic species is already in ionic and/or ionizable form, the claims for such a process may not be totally true, however. Although UV may convert non-ionizable organic species into ionic and/or ionizable organic species, UV may also convert ionic and/or ionizable organic species into non-ionizable organic species. If a significant amount of the organic species is in ionic and/or ionizable forms, application of UV may increase the concentration of non-ionizable organic species in the subsequent EDI feed, and may result in a lower organic carbon reduction than without UV. Further, the ionic or ionizable organic species may absorb UV further reducing UV available for converting non-ionizable organic species into ionic or ionizable organic species.
A need remains, therefore, for additional methods and devices capable of obtaining improved purity product from purification systems. In particular, additional methods and devices are needed which can remove non-ionized or non-ionizable carbon compounds in multi-step purification systems.
SUMMARY OF THE INVENTION
The invention is directed to methods and devices for removing substantially all inorganic and organic carbon compounds from water. In accordance with the present invention, one or more first deionization stages remove ionic and/or ionizable contaminants. The product stream of such first deionization stages is exposed to an organic carbon bond-breaking agent prior to becoming the feed stream of one or more second deionization stages. Such exposure causes non-ionized organic carbon compounds in the first deionization product stream to ionize or become ionizable, facilitating removal in such second deionization stages. Further, since ionic and/or ionizable contaminants are substantially removed in the first deionization stages, the amount of bond breaking agent required to convert the non-ionizable organic carbon compounds to ionic or ionizable organic carbon compounds is minimized.
In accordance with the invention, such first one or more deionization stages remove the ionic and/or ionizable inorganic and organic carbon species. The effluent from such first deionization stages is exposed to organic carbon bond-breaking agents, such as UV (preferably 184.9 nm wavelength or less) including catalyzed UV and/or other oxidizing agents (e.g., oxygen, ozone, singlet oxygen, hydrogen peroxide, hydroxide radi
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