The invention relates to apparatus and methods for carrying out electrodeionization to purify water.
Electrodeionization is a process for removing ions from liquids by sorption of these ions into a solid material capable of exchanging these ions for either hydrogen ions (for cations) or hydroxide ions (for anions) and simultaneous or later removal of the sorbed ions into adjacent compartments by the application of an electric field. (See Glueckauf, E., "Electro-Deionization Through a Packed Bed", December 1959, pp. 646-651, British Chemical Engineering for a background discussion.) The hydrogen and hydroxide ions needed to drive the ion exchange process are created by splitting of water molecules at the interface of anion and cation exchanging solids which contact each other in the orientation that depletes the contact zone of ions, when in the presence of an electric field. This orientation requires that the anion exchanging material face the anode and the cation exchanging material face the cathode. The created hydroxide ions enter the anion exchanging material, and the created hydrogen ions enter the cation exchanging material.
The electrodeionization process is commonly carried out in an apparatus consisting of alternating diluting compartments and concentrating compartments separated by anion permeable and cation permeable membranes. The diluting compartments are filled with porous ion exchanging solid materials through which the water to be deionized flows. The ion exchanging materials are commonly mixtures of cation exchanging resins and anion exchanging resins (e.g., U.S. Pat. No. 4,632,745), but alternating layers of these resins have also been described (e.g., U.S. Pat. Nos. 5,858,191 and 5,308,467). Ion exchanging materials consisting of woven and non-woven fibers have also been described. E.g., U.S. Pat. No. 5,308,467 describes a fabric in which bundles of cation-exchange fibers and are woven alternately with bundles of anion-exchange fibers, and U.S. Pat. No. 5,512,173 describes a cloth containing cation exchange fibers, anion exchange fibers and ionically inactive fibers. The compartments adjoining the diluting compartment into which the ions are moved by the applied electric field, called concentrating compartments, may be filled with ion exchanging materials or with inert liquid permeable materials. An assembly of one or more pairs of diluting and concentrating compartments, referred to as a "cell pair", is bounded on either side by an anode and a cathode which apply an electric field perpendicular to the general direction of liquid flow.
The diluting compartments are each bounded on the anode side by an anion permeable membrane and on the cathode side by a cation permeable membrane. The adjacent concentrating compartments are each correspondingly bounded by a cation permeable membrane on the anode side and an anion permeable membrane on the cathode side. The applied electric field causes anions to move from the diluting compartment across the anion permeable membrane into the concentrating compartment nearer the anode and cations to move from the diluting compartment across the cation permeable membrane into the concentrating compartment nearer the cathode. The anions and cations become trapped in the concentrating compartments because the movement of anions toward the anode is blocked by a cation permeable membrane, and the movement of cations toward the cathode is blocked by an anion permeable membrane. A flow of water is set up to remove the ions from the concentrating compartments. The net result of the process is the removal of ions from the water stream flowing through the diluting compartments and their concentration in the water flowing through the concentrating compartments.
The removal of the ions from the diluting compartment is a multi-step process involving diffusive steps as well as electrically driven steps. First, it is clear that the movement of ions directly from the diluting solution across the bounding membranes, under the influence of the applied electric field, contributes insignificantly to the overall removal of these ions, because the concentration of ions in the diluting solution is typically 1,000 to 100,000 times smaller than the concentration of ions in the solid ion exchanging materials. While the mobility of ions in the solid material may be on the order of 20 times smaller than their mobility in the solution, the electric field acting on the ions in the two phases is the same, so the product of mobility times concentration times electric field strength, which determines the rate of ion removal, is 50 to 5,000 times as large in the solid ion exchanging material.
Glueckauf showed that the mechanism of ion removal from the diluting compartment solution includes two steps. The first step is the diffusion of cations to the cation exchanging solids and the diffusion of anions to the anion exchanging solids. The second step is electrical conduction within the solids phases to the bounding membranes of the diluting compartment. Because the concentration of ions in ion exchanging solids is so high, the process that controls the overall removal of ions is their rate of diffusion from the solution to the surface of the ion exchanging solids. This diffusion rate is a function of three factors; the diffusion rate is proportional to surface area between the ion exchanging solids and the flowing solution, inversely proportional to the thickness of the liquid layer through which the ions must diffuse, and proportional to the difference in concentration of the ions in the bulk of the diluting solution and their concentration next to the ion exchanging solid. In order to achieve high rates of ion removal, the product of the above three factors should thus be as high as possible. The ratio of the surface area to the diffusion distance is inversely proportional to the characteristic dimension of the ion exchanging solid material; the characteristic dimension is particle radius for ion exchange resins and is fiber diameter for ion exchange fibers. In designing electrodeionization apparatus, this characteristic dimension can be made as small as possible, commensurate with avoidance of excessive pressure drops or plugging by particles in the water to be treated. Particle diameters on the order of 500 to 600 micrometers are typical, and fiber diameters can be on the order of several tens of microns.
As noted above, the third factor controlling the rate of ion removal is the difference in concentration of the ion being removed between the bulk of the solution and its concentration in the liquid adjacent to the surface of the ion exchanging solid where it is being exchanged for either a hydrogen or a hydroxide ion. The concentration of the ion in question at the surface of the ion exchanging solid is in equilibrium with the concentration of that ion in the solid. For cations, the equilibrium concentration is approximately equal to the ratio of the cation concentration to the hydrogen ion concentration in the cation exchanging solid times the concentration of the cation in solution. For anions, the equilibrium concentration is approximately equal to the ratio of the anion concentration to the hydroxide ion concentration in the anion exchanging solid times the concentration of the anion in solution. In order for this equilibrium concentration to be low, and the rate controlling concentration difference to be large, the cation exchanging solid should be predominantly in the hydrogen form, and the anion exchanging solid should be predominantly in the hydroxide form. In fact, if the two solids are completely in the ionic form rather than in the hydrogen or hydroxide form, there is no concentration difference, and ions will not be removed by this diffusive mechanism.
In order for the ion exchanging solids to be predominantly in the hydrogen and hydroxide forms, the so-called "regenerated forms," the rate of hydrogen ion and hydroxide ion creation (water splitting) must be both high and spatially uniform. A high average rate of water splitting can be achieved by applying a high voltage drop across the diluting compartment. With equinormal mixtures of ion exchange particles, voltages of between 1 and 5 volts are adequate for the purpose. The achievement of a uniform distribution of water splitting is a more difficult problem and much effort has gone into designing structures that achieve this (e.g., U.S. Pat. Nos. 5,858,191, 5,868,915 and 5,308,467). The random nature of mixtures of cation and anion exchanging particles tends to cause some portion of the particles to be regenerated to a needlessly high degree and others inadequately regenerated. Water flowing through the regions of inadequately regenerated material will be inadequately purified. The essence of the difficulty that existing approaches have had in dealing with the problem is that the number of contacts between cation exchanging and anion exchanging material where water splitting can take place is limited by the relatively large characteristic dimension of the ion exchanging material. This results in regions of inadequately regenerated resin between the water splitting sites.