Liquid purification or separation – Filter – Material
Reexamination Certificate
2001-07-19
2004-09-21
Fortuna, Ana M. (Department: 1723)
Liquid purification or separation
Filter
Material
C210S500270, C210S500420, C264S041000, C264S185000, C264S263000, C264S561000
Reexamination Certificate
active
06793820
ABSTRACT:
BACKGROUND ART
Hollow fiber membranes have gained acceptance for use in treating many aqueous streams. In some cases, use of hollow fiber membranes is essential for the supply of clean drinking water and for treatment of wastewater. Hollow fiber membranes can also be used to de-water sludges and other streams containing suspended solids. Key to the successful use of such membranes for these purposes is that the membrane be hydrophilic, allowing the membrane to “wet” when in contact with the stream to be treated. For this to occur, the membrane should advantageously be made from a hydrophilic polymer.
One such polymer that has proved suitable for making hydrophilic hollow-fiber membranes is an ethylene-vinyl alcohol (EVAL) copolymer. Such a copolymer is known to be useful in blood dialysis and, because of its hydrophilicity and excellent rejection of high molecular weight substances such as proteins, has many other uses in medical and laboratory applications. Typically, EVAL hollow fiber membranes are cast by forcing a solution of EVAL copolymer through an orifice along with a lumen-forming solution and into a coagulation bath to form membranes having different morphologies and pore structures, depending upon the composition of the casting dope and the process conditions. See, for example. U.S. Pat. Nos. 4,134.837, 4,269,713, 4,317,729 4,362,677, 4,385,094, and Japanese Published Application No. 57-18924. Although a variety of these patents report the use of either a low molecular weight pore-former or a high molecular weight pore-former, there is no recognition of the value of a mixture of both low and high molecular weight pore-formers, and the EVAL membranes prepared according to the processes reported still suffer from a relatively low water flux and limited structural integrity and lifetime when used in applications requiring higher fluid pressures.
DISCLOSURE OF INVENTION
According to the present invention there is provided a process for the fabrication of a strong, durable microporous hydrophilic hollow fiber membrane having high water flux. The process comprises casting the membrane by conventional spinneret technology from a casting dope comprising an EVAL copolymer having a particular composition, followed by a series of post-casting steps.
The casting dope comprises EVAL copolymer in a solvent; a small amount of water; and two pore-formers, one low molecular weight and one high molecular weight. The lumen-forming fluid and the coagulation bath are of conventional composition. After precipitation or coagulation, the hollow fiber membranes are preferably stretched, soaked in hot water, and crosslinked.
BEST MODE FOR CARRYING OUT THE INVENTION
An ideal microporous hydrophilic hollow fiber membrane has three essential characteristics. First, the fiber should have a high water flux. Generally, water fluxes greater than about 2 m
3
/m
2
·d·0.1 MPa at 25° C. will lead to commercially practical processes. Second, the fiber should have a high wet tensile strength. This will ensure that the fiber has a long lifetime when operating under high pressure differentials, or when the fiber is under stress during operation. Generally, the wet tensile strength of the fiber should be on the order of at least about 180 g/fil. Third, the fiber should have a high wet elongation at break so as to ensure long fiber lifetimes and durability under operating conditions. Generally, the wet elongation at break should be greater than about 40%.
A microporous hydrophilic hollow fiber membrane with such characteristics will be useful for a wide range of applications, including water purification, wastewater treatment and dewatering sludges. The present invention describes a process for the fabrication of such a membrane.
The first step in preparing a microporous hydrophilic hollow fiber membrane according to the present invention is to prepare a spinning solution, comprising a mixture of an EVAL copolymer, a low molecular weight pore-former, a high molecular weight pore-former, water, and a solvent.
Although virtually any EVAL copolymer may be used in the present invention, copolymers with an ethylene content (relative to vinyl alcohol content) of 27 mol % to 48 mol % are especially suitable.
In making a high-performance membrane, the concentration of EVAL copolymer in the spinning solution should be greater than about 25 wt % based upon the total weight of the spinning solution. If the concentration of copolymer is less than this, the strength of the resulting fiber is too low. Conversely, if the concentration of copolymer is too high, the water flux through the fiber is too low. It has been found that the concentration of EVAL copolymer should be kept in the range of 25 to 40 wt % to obtain practical water fluxes.
The spinning solution preferably contains at least two pore-formers: one with a low molecular weight and one with a high molecular weight. The term “low molecular weight” means ≦1000 Daltons; and “high molecular weight” means ≧1000 Daltons. It has been found that this combination of pore-formers results in a structure suitable for a high-performance membrane. If only a low molecular weight pore-former is used, it has been found that the wall of the resulting fiber contains large voids. These voids reduce the strength of the fiber and are likely to result in defects or damage. In addition, use of only a low molecular weight pore-former leads to an outside surface with little or no porosity, which leads to low water fluxes. Conversely, if only a high molecular weight pore-former is used, it has been found that both the wall and the outside surface of the resulting fiber has low porosity, also leading to low water fluxes. Preferably, the weight ratio of the low molecular weight pore-former to the high molecular weight pore-former should be greater than about 0.3 but less than about 3. The spinning solution preferably contains the low molecular weight pore-former and the high molecular weight pore-former in an amount of 5 to 15 wt %, respectively, based on the total weight of the spinning solution.
Virtually any low molecular weight pore-former may be used, provided that the compound is not a solvent for the EVAL copolymer and provided it is miscible with the other components of the spinning solution and with the quench baths. Exemplary classes of suitable low molecular weight pore-formers include alcohols, ketones, amines, and esters. It has been found that the most effective low molecular weight pore-formers are mono- and polyhydric alcohols, such as n-propanol, isopropanol (IPA), n-butanol, ethylene glycol (EG), and glycerol.
The high molecular weight pore-former preferably is soluble in the solvent used to form the spinning solution and miscible in the spinning solution, resulting in solutions that are clear as opposed to cloudy. Exemplary suitable high molecular weight pore-formers include polyols such as polyethylene glycol (PEG), polypropylene glycol (PPG), and polyvinyl alcohol (PVA), polyvinyl pyrrolidone (PVP), and polyethylene oxide (PEO).
The spinning solution preferably also contains a small amount of water, in the order of 0.05 to 1 wt % based on the total weight of the spinning solution. The majority of this water is preferably introduced to the spinning solution by reason of the addition of the spinning solution's other components since such other components are very hydrophilic and tend to have non-zero concentrations of absorbed water. Since the concentration of water in these other components will depend on the methods used to dry them prior to formulating the spinning solution, it is desirable to add a small amount of water to maintain a total water concentration of between about 0.05 and 1 wt %.
Suitable spinning solution solvents include dimethyl sulfoxide (DMSO), dimethylacetamide (DMAC), dimethylformamide (DMF), and N-methyl pyrrolidone (NMP).
To form the solution, all components should first be thoroughly dried. Then, the components are mixed at elevated temperature, generally 80° C. to 100° C., for a suitable length of time, say, 16 to 48 hours. The resu
Friesen Dwayne T.
Lyon David K.
McCray Scott B.
Sakashita Daichi
Sidwell Delores R.
Ebara Corporation
Fortuna Ana M.
Wenderoth Lind & Ponack LLP
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