Liquid purification or separation – Filter – Supported – shaped or superimposed formed mediums
Reexamination Certificate
2001-05-03
2004-02-10
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Filter
Supported, shaped or superimposed formed mediums
C210S500270, C210S500420, C264S041000, C096S004000, C427S244000
Reexamination Certificate
active
06688477
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable.
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT
Not Applicable.
FIELD OF THE INVENTION
The present invention relates to improved membranes for separation of gas or liquid or vapor mixtures. In particular, this invention relates to improved polymeric composite membranes and a process for preparation of the membranes.
BACKGROUND OF THE INVENTION
Gas or vapor or liquid separation via membranes is an established commercial technology with many applications and continues to find acceptance in new applications. Among the applications are: (a) separation of hydrogen from nitrogen, methane, or carbon monoxide in applications such as recovery of ammonia purge gas, oil refining, and synthesis gas manufacture; (b) separation of carbon dioxide and hydrogen sulfide from methane in the upgrading of natural gas; (c) separation of oxygen from nitrogen in the production of nitrogen enriched air or oxygen enriched air; (d) separation of water vapor from compressed air or natural gas to obtain a dry gas; (e) separation of volatile organic compounds (VOC) from air or nitrogen, (f) recovery of fuels from air or nitrogen in transloading operations; (g) recovery of fluorinated hydrocarbons from nitrogen in the manufacture of semiconductors; (h) pervaporative separation of water from liquid alcohol mixtures; and (i) pervaporative separation of trace organic compounds from aqueous streams. In each of these applications, membranes compete with other separation technologies, e.g., absorption in solvents, adsorption in molecular sieves or other adsorbents, distillation or refrigeration. The choice of separation technology employed depends upon a variety of factors, including capital cost of the separation equipment, energy cost per unit volume of gas produced, reliability, maintenance costs, ease and flexibility of operation, and size and weight of the separation equipment.
Useful membranes have a thin dense layer which provides the selectivity or separation characteristics and a porous substructure which provides mechanical support. Membranes used in gas or vapor or liquid separations may function based on one of three general transport mechanisms: (1) solution diffusion, (2) Knudsen diffusion, or (3) selective sieving by molecular size. Polymeric membranes used in commercial product offerings for gas or vapor or pervaporative liquid separation, function almost exclusively based on solution diffusion. Permeation via solution diffusion involves dissolution of a permeating species at one interface of the membrane, diffusion through the polymer membrane, and desorption at the opposite membrane interface. The driving force for permeation through the membrane is the partial pressure difference between the two sides. In pervaporative separation or pervaporation, a liquid mixture contacts one side of the membrane and the permeate is removed as a vapor from the other side.
The primary requirements of a commercial membrane are a high permeation coefficient (also referred to in membrane literature as “p/l” which is defined as the flux of the component per unit of partial pressure difference) for the faster permeating species, high selectivity (i.e., ratio of the permeation coefficient for the faster permeating species to the permeation coefficient for the slower permeating species), stability under the operating feed pressure and temperature, and tolerance to feed stream components and contaminants. Of these, the first two requirements appear to be diametrically opposed to each other because of the inverse relationship of the permeability of a polymer and the selectivity of that polymer for a given set of permeating species. For instance, usually, the more permeable the polymer, the lower its selectivity. This problem can be solved by applying a polymer of adequate selectivity as a coating or laminate to a support to prepare a thin composite membrane which simultaneously realizes a high permeation coefficient and a high selectivity.
Integrally skinned asymmetric membranes represent one class of commercial membranes with thin selective layers. The thin selective layer or skin and the substructure of an integrally skinned, asymmetric membrane are made of the same polymer in a single process. Their inherent limitation is that the permeation properties are derived from the support polymer, and there is a limited number of polymers from which integrally skinned, asymmetric membranes can be produced economically.
Composite membranes represent another class of thin polymer membranes. The selective layer and substructure of composite membranes are made of different polymers, usually in two or more separate process steps. Composite membranes are especially attractive when the selective layer polymer is expensive or lacks adequate mechanical properties to be a useful support. In such composite membranes, the porous support provides the mechanical strength while offering low resistance to transport of the gas or vapor. It is necessary that the surface pores of the porous support be sufficiently small that the thin selective layer bridging the mouths of the pores has adequate burst strength. Composite membranes overcome the inherent limitations of integrally skinned asymmetric membranes so that a wide range of materials can be used for the selective layer. Thus the properties of composite membranes can be tailored to particular applications.
Membranes provide an alternative to desiccant and refrigerant dehydrators used for compressed air drying and to glycol absorption or molecular sieve or deliquescent dehydrators used in natural gas drying. In order to be competitive with conventional technology, the membrane needs to possess a high permeation coefficient for water and high selectivity relative to the other components of the gas mixture. In addition, the membrane should be stable in contact with the feed components and contaminants under operating conditions.
Several methods for the preparation of membranes for dehydration have been described in the literature. One method involves coating a porous support with a solution of a polymer in a solvent mixture. The resulting separating layer may contain a single polymer or a blend of several materials, or multiple coatings with dissimilar polymers. Examples are U.S. Pat. Nos. 4,981,498 and 5,067,971 to Bikson et al. which describe a composite membrane for the dehydration of gases prepared by coating a porous support with a thin layer of a sulfonated polysulfone.
Interfacial polymerization has been used to form the selective layer directly on the porous support by reacting two immiscible reagents (e.g., polyamine in water with diacid chloride in an immiscible organic solvent) from which a cross-linked polymer film is formed on or in the support at the interface of the two reagents, as illustrated in U.S. Pat. No. 5,002,590 to Friesen et al.
As described above, many attempts have been made to provide composite membranes with both high permeation rates and high selectivity. The porous supports used in these membranes are desirably porous to provide low resistance to transport of gas or vapor species and still provide adequate mechanical support to a selective layer.
Permeation of species through a selective layer and the surface pores of the support has been described by a mathematical model (see Keller and Stein, J. Mathematical Biosciences, 1, 421-437, 1967). This type of model illustrates that not all the surface of the selective layer permits permeation; the fractional effective area for permeation increases as the surface porosity (void area fraction) of the support increases and the diameter of the surface pores decreases. Hence, high surface porosity values and small surface pore sizes are desirable in a porous support. In addition, smaller pore sizes provide better mechanical support to the thin selective layer thus preventing the rupture of the thin layer under operating conditions of pressure and temperature.
One of the problems in the preparation of composite membranes, especially when coating from
Air Products and Chemicals Inc.
Fortuna Ana
Rodgers Mark L.
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