Patent Application
20080029395
These and other objects and features of this invention will be better understood from the following detailed description taken in conjunction with the drawings, wherein:
The invention disclosed and claimed herein is a water purification system which is capable of purifying most water streams requiring purification including industrial wastewater, gas and oil field wastewater, and coal mine wastewater and is well-suited for desalination of salt water. The system incorporates electrochemical deionization, microfiltration, and carbon adsorption features to remove organic materials, inorganic materials, bacteria and solid particles. The system is compact, energy efficient, and low cost.
A water purification system in a single cell arrangement in accordance with one embodiment of this invention is shown in
The electrodes, which provide particle filtration, ionic species concentration and removal, and organic material and bacteria removal, are carbon-based porous structures. In accordance with the embodiments shown in the drawings, the electrodes are porous planar structures, i.e. plates, and the separator element is a perforated plate. However, any other configurations of electrodes and separator elements which provides the desired relationship between the electrodes and the separator element, such as tubular or rolled structures, may also be employed, and it is to be understood that such configurations are also considered to be within the scope of the invention claimed herein.
There are three basic requirements for an electrode for the water purification system of this invention—porosity, electrical conductivity, and mechanical strength. Accordingly, the electrodes are carbon-based porous structures comprising graphite for conductivity, at least one metal oxide, for increasing water adsorption by the electrode, and an ion-exchange, cross-linked, polarizable polymer for binding the components of the electrode together. In accordance with one embodiment of this invention, electrical conductivity of the electrode may be enhanced by the addition of carbon black.
Exfoliated graphite is the product of very rapid heating (or flash heating) of graphite intercalation compounds, such as graphite hydrogen sulfate, of relatively large particle diameter (flakes). The vaporizing intercalated substances force the graphite layers apart resulting in an accordion-like shape with an apparent volume typically hundreds of times that of the original graphite flakes. In accordance with one preferred embodiment of this invention, the graphite employed in the electrodes of this invention is exfoliated graphite. In accordance with one preferred embodiment of this invention, the exfoliated graphite is in the form of particles less than about 50μ in size.
In addition to graphite and at least one metal oxide, the electrodes of this invention comprise at least one ion-exchange component, which, in addition to providing ion-exchange, may also be used to bind the components of the electrodes into a cohesive structure. In accordance with one embodiment of this invention, the ion-exchange component is a cross-linked, polarizable polymer. Cross-linking of the polarizable polymer is required to avoid dissolution of the polymer in the wastewater being treated. Suitable ion-exchange, cross-linked, polarizable polymers may be selected from the group consisting of polyurethane, polyacrylic acid, sulfonated polystyrene, poly(vinyl alcohol) and combinations thereof. Suitable agents for cross-linking of the polarizable polymers may be selected from the group consisting of glyoxal, aldehydes, such as formaldehyde and glutaraldehyde, methylene amine, and combinations thereof.
The electrodes of this invention are necessarily hydrophilic and, as previously indicated, at least one metal oxide is employed in the electrode for the purpose of increasing water adsorption. Any metal oxide that is stable in water may be utilized. In accordance with one preferred embodiment, the at least one metal oxide is selected from the group consisting of TiO2, Al2O3 and mixtures thereof.
It will be appreciated that, depending upon the composition of the wastewater being treated, impurities such as oily tars and high organic species may collect on the electrode. Such impurities may be removed by periodic back-flashing of the electrode. To enhance this process, it is required that, in addition to hydrophilicity, the electrodes of this invention also possess hydrophobic properties. The balance between hydrophilicity and hydrophobicity of the electrode may be controlled, in accordance with one embodiment of this invention, by the appropriate selection of polarizable polymer and cross-linking agent. For example, poly(vinyl alcohol) (PVOH) has fewer —CH2 groups than poly(ethylene vinyl alcohol) (PEVOH). In PEVOH, the ethylene group provides hydrophobicity. Certain cross-linking agents, such as formaldehyde, have fewer —CH2 groups than glutaraldehyde and glyoxal. As the number of —CH2 groups increases, hydrophobicity increases and hydrophilicity decreases. It has been found that compositions with one (1) to five (5) —CH2 groups provide a desirable balance between hydrophilicity and hydrophobicity of the electrode. In accordance with one preferred embodiment of this invention, the electrode is provided with a hydrophobicity of up to about 50%.
In general, the electrodes of this invention may be produced by mixing metal oxide and carbon or graphite powders with a phenolic resin and a bubbler, such as ammonium bicarbonate, and molding the mixture at atmospheric pressure and elevated temperatures. The amount of ammonium bicarbonate or other bubbler employed depends on the desired porosity of the gas diffusion electrode. It has been found that a mixture comprising about 50-60 wt % graphite powders, 5-20 wt % carbon black, about 7 wt % phenolic resin and about 1-10 wt % ammonium bicarbonate molded at a temperature of about 200° C. produces a suitable electrode.
Exfoliated graphite was produced by mixing concentrated sulfuric acid and graphite powders. The mixture was heated in an oven at 600° C. The resulting expanded graphite includes C═O and C—OH bonds on the graphite particles, which crosslink with poly(ethylene vinyl alcohol) and glutaraldehyde. The resulting graphite powders are stable in the porous plate and can not wash out during the wastewater treatment process.
9 grams of exfoliated graphite powders were mixed with 10 grams of water and 10 grams of 10 wt % poly(vinyl alcohol), forming a first mixture. 10 grams of water were mixed with 2 grams of 40 wt % glutaraldehyde and 0.5 ml HCl (35 wt %), forming a second mixture. The two mixtures were mixed thoroughly and the resulting mixture was cast to produce a 1/16″ thick sheet which was then heat treated at 100° C. Water boiling from the plate generated bubbles, making the plate porous. Because glutaraldehyde binds with poly(vinyl alcohol) in an irreversible fashion, the resulting cross-linked polymer was entirely insoluble, even in hot water. Table 1 shows a comparison of surface resistance between the electrode produced in accordance with this example and other electrode materials.
One application for the water purification system of this invention is marine water desalination. Electrode voltage in the range of about 0 to 1.2 V is required for desalinating the water.
While in the foregoing detailed description this invention has been described in relation to certain preferred embodiments thereof, and many details have been set forth for purposes of illustration, it will be apparent to those skilled in the art that the invention is susceptible to additional embodiments and that certain of the details described herein can be varied considerably without departing from the basic principles of the invention.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11497092 | Aug 2006 | US |
| Child | 11515544 | US |
