Gas separation: apparatus – Apparatus for selective diffusion of gases – Plural separate barriers
Reexamination Certificate
2002-11-07
2004-03-23
Spitzer, Robert H. (Department: 1724)
Gas separation: apparatus
Apparatus for selective diffusion of gases
Plural separate barriers
C096S010000, C096S014000
Reexamination Certificate
active
06709494
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to semipermeable hollow fiber membrane devices that contain tubesheets with improved temperature capability. The physical characteristics of the uncured liquid potting compound have been tailored to facilitate the impregnation of the hollow fiber membrane bundle with said compound. Subsequent elevated temperature curing of this resinous material produces highly crosslinked castings with high glass transition temperatures.
BACKGROUND OF THE INVENTION
Processes that use membranes to separate mixtures of fluids including gases are accepted applications in many industries. Representative of these processes are microfiltration, ultrafiltration, reverse osmosis and gas separation. Membranes used to accomplish these separations have been fabricated in various geometries, such as flat sheet, spiral wound flat sheet, tubular and hollow fibers. The membrane geometry is usually dictated by the nature of the separation that is to be effected. When performing a separation on a viscous liquid mixture, for instance, it may be advantageous to use a membrane in a large diameter, tubular configuration in order to maintain fluid velocity and to minimize fouling of the membrane surface. Conversely, when separating fluids with low viscosities, such as gases, the use of membranes in a hollow fiber configuration is more appropriate.
The fine hollow fiber geometry for membrane fabrication is particularly advantageous because it can yield very high surface area-to-volume ratios. Much of this benefit is derived from the fact that the membrane support structure is integral to the hollow fiber; that is, in contrast to flat sheet membranes that are cast onto a nonwoven fabric, or to tubular membranes that are frequently cast onto a rigid porous backing tube. Thus a significant portion of the module volume of flat sheet, spiral wound, and tubular membranes is consumed by the membrane support structure. This volume is consequently unavailable for packing such modules with additional active membrane area.
Commercially large bundles of hollow fiber membranes are assembled into permeators or modules. The fibers in these modules are sometimes arranged in a parallel fashion, although it is often advantageous to wind the fibers around a core in order to impart structural integrity to the bundle. As part of the hollow fiber membrane module manufacturing process, at least one end of the fiber bundle is cast or potted in what is commonly referred to as a tubesheet. More commonly both ends of the bundle are so encapsulated. The tubesheet serves to hold the fibers in a fluid-tight relationship such that the feed fluid is isolated from the permeate fluid, thus allowing components to be separated by selective passage of one or more components through the membrane.
Tubesheets can be fabricated from any one of a number of liquid resinous materials that subsequently solidify, frequently through a chemical curing process. Among the preferred such resinous materials used as potting compounds are epoxy resins. Such resins are valued for the strength and durability of castings that can be produced from properly formulated mixtures that incorporate such resins. It is further recognized that there exists a large selection of commercially available raw materials that can be utilized to formulate these compounds. Thus tubesheet materials composed with epoxy resins can be tailored to meet the demands of various process applications.
The selection of a material for fabricating a membrane module tubesheet is dependent upon the properties of said material in both the cured, solid state and the uncured, liquid state. The properties of the material in each phase are important but for different reasons.
The properties of the cured resinous composition must meet the demands of the particular application of the membrane module. With respect to hollow fiber membrane modules used for gas separation, there are several properties of the tubesheet that are desirable. First, the cured resin must be of sufficient strength to withstand the pressure differential across the tubesheet during operation of the module. The feed pressure of the gas can be in excess of 80 atmospheres and consequently the pressure differential across the tubesheet will approach this value if the permeate pressure of the membrane approaches atmospheric pressure. In addition, the solidified resinous mixture comprising the tubesheet must be resistant to chemicals in the process fluid, including water vapor. Further, the tubesheet must be amenable to being cut or severed in a clean fashion such that the bores of the fibers can be opened to allow free passage of gas along the length of the hollow fibers. The cured resin must exhibit sufficient adhesion to the hollow fibers in order to maintain a fluid-tight relationship between the hollow fibers and the tubesheet, thus preventing unwanted species in the feed stream from mixing with the permeate. When a membrane module is to be operated at elevated temperature, it is essential that all structural components, including the tubesheet, be rated accordingly. Although cured epoxy resins are crosslinked materials, like all polymeric materials they are susceptible to creep when subjected to excessive pressure and temperatures. Heat and pressure induced deformation of a membrane module tubesheet can cause either gross mechanical failure of the tubesheet or a failure in the fluid-tight seal between the fibers and the tubesheet. Either condition can result in failure of the membrane module to operate as intended. The resistance of an epoxy resin to this type of failure is related to its glass transition temperature, T
g
. A cured resin is more susceptible to creep as it is subjected to temperatures approaching its T
g
. Therefore, it is desirable that the tubesheet material has T
g
well in excess of its intended operating temperature in order to provide an adequate margin of safety.
The properties of the uncured resinous material used to form the tubesheet must be given equal consideration, for the controlled and facile application of the liquid resinous compound into the hollow fiber membrane bundle is essential to the production of reliable commercial permeators. The liquid resin may be applied to the ends of the hollow fiber bundle by any suitable means. One method is directed by Fritzsche et al. in U.S. Pat. No. 4,323,454. The authors describe a process in which a hollow fiber bundle is placed in a mold while a liquid resinous composition of relatively low viscosity is poured into said mold. The liquid resinous material is then free to migrate through the interstices between the hollow fibers until the fibers are encapsulated. This method, and variations of it, is particularly amenable to large-scale production of commercial membrane modules. It is apparent, however, that if module potting is to be conducted by this method, the liquid resin properties must be carefully selected. Commercial membrane modules can comprise bundles of hollow fibers that range from 5 cm in diameter up to 15 cm in diameter, and frequently are as high as 30 cm in diameter. Those skilled in the art will recognize that to achieve complete penetration of a liquid resin throughout a 30 cm diameter bundle of hollow fibers is significantly more challenging that the encapsulation of a 5 cm diameter bundle.
Of paramount consideration, therefore, when selecting a liquid resinous compound for tubesheet formation, is the ability to control the manner in which it flows into the hollow fiber bundle such that all fibers are encapsulated. Two properties of liquid resins that define their ability to flow are viscosity and gel time. Viscosity is a measure of the liquid's thickness while gel time is an indicator of the time that a resinous material remains in a liquid state before it ceases to flow. It is desirable, then, that the liquid resinous compound has a viscosity that is low enough for a sufficient time before its gels such that all fibers in the hollow fiber bundle are adequately encapsulated. It is important to not
Giglia Salvatore
Macheras James Timothy
Praxair Technology Inc.
Rosenblum David M.
Spitzer Robert H.
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