Liquid purification or separation – Filter – Supported – shaped or superimposed formed mediums
Reexamination Certificate
2001-12-07
2003-03-25
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Supported, shaped or superimposed formed mediums
C210S500380, C427S245000, C427S244000, C264S041000
Reexamination Certificate
active
06536605
ABSTRACT:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not applicable.
BACKGROUND OF THE INVENTION
This invention relates to permselective barriers in the form of thin membranes useful for the selective separation of mixtures of fluids, fluids and particulates, and solutions. More particularly, this invention relates to a method for fabrication of a permselective membrane suitable for reverse osmosis, nanofiltration, ultrafiltration and the like.
It is well known that dissolved substances, such as salts, minerals, and the like, can be separated from their solvents, such as water, by a technique known as reverse osmosis. For example, the mineral and salt content of seawater can be reduced substantially by utilizing reverse osmosis membranes to produce potable and/or commercially usable low salt water. Similarly, softened water for household or industrial use may be obtained from relatively hard water with high total dissolved solids content. Softened water is important for prolonging the life of various kinds of delicate machinery and for producing water which is usable in a variety of commercial and domestic applications. Perhaps the greatest impact of filtration technology to the increasingly industrialized world is the desalinization of brackish water or seawater to provide large quantities of relatively salt-free water for industrial, agricultural, or residential use. As the world continues to industrialize and population continues to increase at a rapid rate, increasing demands will be made on supplies of fresh water so that availability of efficient and effective mechanisms to convert brackish water and saltwater to productive uses will be increasingly important.
Of importance similar to reverse osmosis membranes are nanofiltration and ultrafiltration membranes, which are used to filter molecules and particulates from solutions and mixtures. For explanation purposes, the discussion below focuses upon reverse osmosis membranes, although one skilled in the art would realize that similar considerations apply to nanofiltration and ultrafiltration membranes. Reference herein to filtration membranes thus includes reverse osmosis, nanofiltration, and ultrafiltration membranes.
Reverse osmosis basically is a filtering out of dissolved ions or molecules by applying a hydraulic pressure to the water to be filtered to force it through a reverse osmosis membrane. Such membranes are selectively permeable for the water, but reject passage of unwanted constituents, typically salts and various dissolved minerals. Osmotic pressure, the tendency of solute components to spread evenly to both sides of a membrane, works against the reverse osmosis process. The more the feed water is concentrated with unwanted components, the greater is the osmotic pressure which must be overcome. Thus, to be practical, a reverse osmosis membrane must strongly reject passage of the unwanted components, commonly referred to in the art as having high rejection characteristics.
In osmosis, the application of pressure to a saline solution that causes movement of water through a solid or liquid barrier while preventing the phases from remixing rapidly requires a theoretical osmotic pressure of 115 psi to desalt a 1% NaCl solution at 25° C. Therefore, the more restrictive the solid or liquid barrier is to solution flow, the higher the pressure required to drive the desalting process. The term associated with restrictive flow is pressure drop. It is intuitive that the thicker the barrier layer existing between the salt and desalted solutions, the higher the pressure required to desalt the solution.
In many instances, reverse osmosis membranes currently are fabricated utilizing a porous substrate upon which is coated a monomer or polymer which is subsequently cross-linked, such as is disclosed in U.S. Pat. No. 4,277,344, issued to Cadotte, which is hereby incorporated by reference. The Cadotte patent reveals that good salt rejection and flow characteristics can be obtained with a reverse osmosis membrane made from cross-linked, interfacially polymerized aromatic polyamides. The membranes created with the Cadotte process represent an important advancement over prior art membranes due to significant improvements in the ion rejection characteristics, flow characteristics, and resistance to oxidative attack.
The process for making reverse osmosis membranes utilizing Cadotte technology is to coat a porous support layer with a polyamine component. The porous support layer with the polyamine coating is contacted with polyacyl halide, initiating an interfacial polymerization in situ on the support. The resulting product is dried to provide a composite membrane comprising the polyamide laminated to the porous support. The in situ cross-linking provides a mechanical adhesion of the resulting cross-linked reverse osmosis membrane to the support substrate.
Existing methods for performing a coating process like the Cadotte process utilize dip coating technology. In dip coating, the desired substrate is run on a continuous basis and is conveyed through a first liquid bath, coating both sides of the substrate with a first component, e.g., polyamine, and next is conveyed through a second liquid bath containing the second component, e.g., polyacyl halide, after which the membrane is conveyed through any desired rinsing processes, and then through a drying oven. The membrane layers thus formed are typically on the order of from about 0.5 microns to about 1.0 microns.
A primary drawback to the dip coating approach is that the results of the substrate coating process are dependent upon many hard to regulate factors, including the viscosity, cohesion, and adhesion properties of the coatings in the tanks, which properties vary with temperature, solution makeup, and other similar factors. For example, dip tanks depend upon gravity, whereby excess coating is allowed to run off from the substrate. Thus, due to the various factors affecting viscosity, adhesion, and cohesion of the coating, the thickness of the coating is difficult to control. The variations in thickness of the coating applied to the substrate can cause breaks or voids in the membrane coating, resulting in substantially reduced membrane effectiveness or failure of the product involved. It is common in the industry that up to twenty percent of the dip-coated substrate becomes scrap material due to such deficiencies in the resultant coated membrane products.
Further, a dip coating process may result in cross-contamination between the tanks as the substrate is passed from one tank to the next. A certain excess amount of the first constituent which does not sufficiently run off of the substrate is carried into the second tank, causing buildup of the constituents from the first dip tank in the second dip tank. This results in variations in the concentration and make-up of the constituents in the second tank, which variation progressively changes as the process continues, ultimately leading to variations in the effectiveness of the cross-linking occurring between the two coated layers, with corresponding variations in the final membrane. Often, these excess constituents remaining on the substrate must be extracted in subsequent baths of citric acid, bleach, and the like.
Further, the second dip tank is typically sized to be large enough so that an entire batch of membranes (e.g. an eight hour run of material) may be run before the constituent in the second tank becomes so contaminated that it must be discarded. As a result, large amounts of waste dip coating chemicals typically are created in the dip coating process.
In addition, the constituents tend to permeate the porous substrate during the dip coating process, creating several potential problems. First, the thickness of the filter layer of the filtration membrane inversely impacts the capacity of the membrane, whereby the thicker that the filter layer is, the lower is the membrane capacity. Because dip coating constituents tend to permeate the substrate, the resulting filter layer effectively extends into the substrate
Puglia John P.
Rice William C.
Fortuna Ana
Koch Membrane Systems, Inc.
Stinson Morrison & Hecker LLP
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