Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2000-05-17
2002-08-27
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S490000, C210S500420, C210S500270, C096S004000, C095S045000
Reexamination Certificate
active
06440309
ABSTRACT:
BACKGROUND OF THE INVENTION
It is suggested in the art that separation of liquid mixtures is feasible by the process of pervaporation. Pervaporation is a separation process whereby a liquid mixture is separated based on the preferred solubility and diffusivity of the components of the mixture through an active polymer membrane phase. On the permeate side of the membrane the partial vapor pressure of the species to be removed is maintained at a level much lower than its value under ambient conditions by maintaining vacuum or using a sweep gas (for example, air or nitrogen). The permeating species undergo a phase change (from liquid to vapor) as they diffuse through the membrane and the permeating species are condensed in a cold trap. The process of pervaporation can be used effectively to separate mixtures of close-boiling point components and it is suitable for the removal of volatile compounds from aqueous as well as organic liquid mixtures. Application of pervaporation technology include dehydration of alcohol-water streams, removal of organic pollutants from dilute aqueous wastes, and separation of close boiling point mixtures and isotropic mixtures.
To date, most pervaporation studies have involved pure or asymmetric composite polymer membranes. Well-documented drawbacks of polymeric pervaporation membranes include lack of physical stability and chemical vulnerability to various industrial solvents. Conversely, ceramic membranes are known to have excellent structural integrity and high chemical and thermal resistance (Charpin, et al., 1991). The benefits of ceramic membranes are largely offset by poor selectivity and a limited selection of pore sizes. Such shortcomings are overcome in my invention by surface modification of the ceramic substrate with grafted polymer chains, forming an asymmetric covalently-bonded polymer surface layer. This thin polymer layer provides the desired chemical selectivity and allows controlled reduction of pore size near the membrane surface. The resulting grafted asymmetric ceramic-supported polymer (CSP) membrane possess polymer layer stability far superior to membranes which are simply coated or pore-filled with a polymer phase (Sakohara, et al., 1990).
Polymer modification methods of membranes include the filling of porous polymeric membranes with a gel, made consisting of cross-linked polyvinyl alcohol, for use in fractional liquid extraction [Hongbing, D. and E. L. Cussler, Fractional Extraction with Hollow Fibers with Hydrogel-Filled Walls,
AICHE J.,
37, N6 (1991) 855-862.] The membrane proposed by Hongbing and Cussler is different from the present invention because it deals with a polymeric membrane and the modifying polymeric gel is not chemically bonded to surface of the membrane.
Modification of a ceramic membrane by solution coating of preformed polymer was reported by Rezac and Koros [Rezac, M. E. and W. J. Koros, Preparation of Polymer-Ceramic Composite Membranes with Thin Defect-Free Separating Layers,
J. Appl. Polymer Sci.,
46 (1992) 1927-1938.]. Polymer-ceramic composite membranes were prepared by solution deposition of a thin selective polymer layer onto a microporous ceramic support. This membrane was formed by physical adsorption of pre-formed polymer onto the surface of the membrane. Thus, the polymeric phase is not a stable phase, thus polymer attrition will occur upon exposure to a good solvent. The present invention, in contrast, consists of a ceramic membrane support with polymeric chains chemically attached to the ceramic support.
Another example of a polymer-coated membrane was reported by Song and Hong [Song, K. M. and W. H. Hong, Dehydration of Ethanol and Isopropanol Using Tubular Type Cellulose Acetate Membrane with Ceramic Support in Pervaporation Process,
J. Membrane Sci.,
123 (1997) 27-33.] for the dehydration of isopropanol. A tubular-type cellulose-acetate membrane was formed on either the inner or outer surfaces of a porous ceramic support by the dip-coating and a rotation-drying technique. The Song and Hong membrane consists of a ceramic support physically-coated with a dense polymeric phase. Such a membrane will be unstable when exposed to high concentrations of the permeating species or good solvent for the polymer. In contrast, the present invention consists of a polymeric phase which is covalently bonded to the ceramic surface.
Another example of a polymer-coated membrane was reported by Song and Hong [Song, K. M. and W. H. Hong, Dehydration of Ethanol and Isopropanol Using Tubular Type Cellulose Acetate Membrane with Ceramic Support in Pervaporation Process,
J. Membrane Sci.,
123 (1997) 27-33.] for the dehydration of isopropanol. A tubular-type cellulose-acetate membrane was formed on either the inner or outer surfaces of a porous ceramic support by the dip-coating and a rotation-drying technique. The Song and Hong membrane consists of a ceramic support physically-coated with a dense polymeric phase. Such a membrane will be unstable when exposed to high concentrations of the permeating species or good solvent for the polymer. In contrast, the present invention consists of a polymeric phase which is covalently bonded to the ceramic surface.
A polymer-coated ceramic membrane for pervaporation was also proposed by Zhu et al. [Zhu, Y. R. G. Minet and T. T. Tsotsis, A Continuous Pervaporation Membrane Reactor for the Study of Esterification Reactions Using A Composite Polymeric/Ceramic Membrane,
Chem. Eng. Sci.,
51, No. 17, (1996) 4103-41]. The membrane was prepared by dip-coating a &ggr;-Alumina ceramic support tube with polyetherimide solution. The membrane is intended for in situ water removal from reaction systems. In this membrane the polymeric phase is physically coated onto the ceramic support. As a result, such a membrane has a short life-time due to polymer attrition when subjected to good solvent conditions. Such a membrane cannot be effectively used for organic-organic separations. In a later study Zhu and Chen [Zhu, Y. and H. Chen, Pervaporation Separation and Pervaporation-Esterification Coupling Using Crosslinked PVA Composite Catalytic Membranes on Porous Ceramic Plate,
J. Membrane Sci.,
138 (1998) 123-134.] reported on a pervaporation membrane prepared by crosslinking a PVA dense active layer was coated on the porous ceramic plate. Then, Zr(SO
4
)
2
.4H
2
O, an inorganic solid acid which was used as the esterification catalyst in the experiment, was immobilized on the dense active layer. This membrane belongs to the class of coated polymeric membranes with cross-linking to improve membrane stability. However, the polymeric phase is not chemically attached to the porous ceramic support. In contrast, in the present invention the polymeric phase is chemically attached to the support surface.
Another example of a polymer-ceramic composite pervaporation membrane was proposed by Goldman et al. [Goldman, M., D. Fraenkel and G. Levin, A Zeolite/Polymer Membrane for Separation of Ethanol-Water Azeotrope,
J. Appl. Polymer Sci.,
37 (1989) 1791-1800.] who added a fine zeolite to a solution of dissolved poly(vinyl chloride), with the final membrane formed by subsequent casting of the mixture followed by a drying step. The membrane was used for ethanol/water azeotropic pervaporation. The above membrane differs from the present invention in that it deals with a powdered ceramic dispersed in a polymeric phase. In contrast, the present invention is a ceramic membrane with a polymeric phase of single chains chemically bonded to the ceramic membrane surface.
A ceramic membrane modified by pore-filling with acrylamide-based polymer was reported by Sakohara et al. [Sakohara, S.: F. Muramoto; T. Sakata and M. Asaeda, Separation of Acetone/Water Mixture by Thin Acrylamide Gel Membrane Prepared in Pores of Thin Ceramic Membrane,
J. Chemical Engineering of Japan,
23, N1 (1990) 40-45.] A thin polyacrylamide gel membrane was formed within the pores of a thin porous ceramic membrane of silica-alumina by copolymerizing acryl
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