Plastic and nonmetallic article shaping or treating: processes – Direct application of electrical or wave energy to work – Using sonic – supersonic – or ultrasonic energy
Reexamination Certificate
1999-02-25
2001-08-07
Silbaugh, Jan H. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Direct application of electrical or wave energy to work
Using sonic, supersonic, or ultrasonic energy
C264S442000, C264S478000, C264S069000, C264S328100, C264S328200, C264S331110
Reexamination Certificate
active
06270714
ABSTRACT:
BACKGROUND OF THE INVENTION
Gas separation membranes are widely used today to separate gaseous mixtures in various industries including to recover hydrogen in the petrochemicals industry, generating nitrogen from air, and generating enriched oxygen from air to name just a few examples. Examples of the applications of such membranes can be found in a number of references (see for example,
Membrane Handbook
, Ho and Sirkar eds., Van Reinhold Nostrand, 1992). These membranes selectively permeate one gas more readily than another when subjected to a pressure difference between the high pressure and low pressure side of the membrane. Therefore it is critical to maintain a gas-tight seal between the high pressure and the low pressure side of the membranes, because any non-selective flow through leaks will destroy the separatory power of the membrane module. In the terminology of gas separation technology, such membrane modules are usually denoted as permeators.
A number of geometries have been used to pack a plurality of membranes into a pressure housing and maintain a gas-tight separation between the two sides of the membrane. The two most commonly used are spiral wound permeators and hollow fiber permeators. Because of the higher packing density afforded and the less difficult problem of pressure drops and fouling in gas as opposed to liquid separations, hollow fiber gas permeators tend to be the most widely used
The sealing of the hollow fiber permeators, so that the hollow fibers lumen is kept separate from the outer surface of the hollow fibers, is usually effected by forming a tube sheet around the end of a fiber bundle. By “tube sheet” is meant herein substantially disk-like structure, which comprises a plurality of open, hollow fibers and a solid substance completely filling and sealing the spaces between the outer surfaces of the fibers. The tube sheets are usually formed at or near one end or both ends of the fiber bundle and their thickness is a fraction of the length of the bundle. If the feed gas mixture is fed to the outside surface of the tube bundle, then the bundle can be bent over in two in a U shape and the both ends of the bundle can be potted in a single, common tube, the permeate being collected from the lumen. Alternatively, if the permeate is not collected from one end of the bundle, said end can be sealed with a resin. Further alternatively, both ends of the fiber bundle can be fixed in a tube sheet and the permeate can be removed from the fiber lumen at both ends of the module. If the feed gas mixture is fed to the fiber lumen, then a separate tube sheet must be formed around each end of the bundle. All of the above arrangements of hollow fibers in modules are well documented in the patent literature and well known to persons skilled in the art.
To form a tube sheet, some form of resinous or other plastic or glue-like material must be caused to flow between the individual fibers and fill all the interfiber interstices, so that no gaps are left between the fibers to allow a flow leak. A wide variety of thermoplastic and thermosetting materials are used for this purpose and reported in the patent literature. These include epoxy resins, polyurethane resins, silicone resis, liquid rubber, various other thermoplastic and thermosetting polymers.
These resins can be applied to the fiber bundle in several ways including by a:
a—Pouring the liquid resinous casting material into a mold in which the ends of the fiber bundle have been previously placed and then allowing the mixture to harden. This is often done under a centrifugal force field generated by sprig the fiber bundle on an axis perpendicular to the longitudinal axis running along the length of the bundle.
b—Injecting the liquid resinous casting material into a mold from the bottom out of which the ends of the fiber bundle extend. Often such a mold is formed by the pressure housing in which the hollow fiber bundle is retained (see e.g. Jap. 5,161,829). Similarly, the fiber ends are clogged with a quick-setting resin to prevent the casting resin from filling the fiber to the height of the tube sheet casting. After the tube sheet is formed, the fiber ends are cut open to unclog the pre-sealed fibers (see e.g. Jap.
5,161,829).
c—Slowly inserting the fiber bundle into a mold already containing the liquid resin.
In the terminology used in this patent application, casting refers to forming a tube sheet around a fiber bundle end within a mold, said mold being subsequently discarded. Potting will refer to forming a tube sheet around a fiber bundle end, in which the walls of the said mold, used to form the tube sheet, become a mechanical element in the pressure module assembly,
In casting or potting the tube sheet, the following mechanical/materials requirements must be met.
1. Ensuring adhesion between the liquid resinous casting material and the hollow fibers.
2. Ensuring complete penetration of the liquid casting material between the fibers of the fiber bundle, so that all the spaces are filed up and no gaps or channeling are caused.
3. Preventing wicking, which the climbing of liquid resin up individual fibers to a height much greater than the general level of the tube sheet. Such wicking leads to lack of control in determining the extent/dimension of the tube sheet, and can lead to formation of sharp stiff surfaces around the individual fibers against which they can break or crack.
4. Matching the linear coefficient of thermal expansion (LCTE) of the fibers, the tube sheet, and—if the tube sheet is formed to adhere and chemically seal against the pressure housing—the pressure housing. Otherwise operating at different temperatures than the setting temperature will generate differential expansions of the pot material and the fiber and/or housing, generating strains leading to cracks.
The first two requirements can be met by choosing a resin of low enough viscosity and properties which cause the resin to wet the fibers. However these are exactly the requirements that will exacerbate the problems of wicking.
Matching the LCTE can be done by filling the resins which have typical LCTE's of 80-250 ppm/° C. with a filler of powder with a lower LCTE, such as metal or metal aoides, or quartz or other ceramics, which have lower LCTE's. However as the percent of filler increases, the viscosity increases and penetration of the fiber bundle becomes more difficult.
One of the most practiced methods to solve these problems with polysulfone hollow fiber membranes is that of U.S. Pat. No. 4,323,454. In this patent an epoxy resin, filled e.g. with aluminum or silica powder, is formulated with a crosslinking system including two curing agents, a viscosity increasing agent and a solidifying agent. The viscosity increasing agent results in the resin viscosity increasing rapidly after injecting the potting resin into the bundle, which helps prevent wicking as the curing reaction continues and the temperature increases due to the cure exotherm. However, the solidification agent reacts much more slowly, so that the heat generated during the exotherm has time to be dissipated and there is a more uniform cure during the pot, leading to less strain. Wicking is usually less than 5 cm.
However, the system disclosed in. U.S. Pat. No. 4,323,454 is not effective for potting inorganic molecular sieve membranes. Such membranes, in the form of hollow fibers, have been developed and have combinations of high gas permeability and selectivities (e.g. J. D. Way and D. L. Roberts, Sep. Sci. and Technology, 27 (1992), pp. 29-41; Soffer et al, U.S. Pat. No. 4,685,940). These inorganic molecular sieve membranes have more extreme problems of potting because the LCTE of inorganic fibers is much lower than that of a polymer such as polysulfone (which is e.g. in the order of 50 ppm/C) and is no more than 10. The LCTE of glassy carbon hollow fiber membranes is about 5-6 ppm/C and that of silica hollow fiber membranes is even lower—<4 ppm/C.—while the LCTE of the tube sheet and of the pressure housing is higher, and this makes matching the LCTE
Azran Avraham
Dagan Gil
Carbon Membranes Ltd.
Hamilton Brook Smith & Reynolds P.C.
Poe Michael I.
Silbaugh Jan H.
LandOfFree
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