Gas separation: processes – Selective diffusion of gases – Selective diffusion of gases through substantially solid...
Patent
1995-12-25
1997-12-30
Spitzer, Robert
Gas separation: processes
Selective diffusion of gases
Selective diffusion of gases through substantially solid...
95 51, 95 54, 96 13, 55524, 4274343, 4274395, 4283188, 4284735, B01D 5322, B01D 6912, B01D 7164
Patent
active
057025037
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
This invention relates to the field of composite gas separation membranes. More particularly, this invention relates to composite gas separation membranes comprising an asymmetric porous support and an ultrathin membrane forming polymer film.
BACKGROUND
Much technology has already been developed concerning composite gas separation membranes. Fundamentally, the purpose for a composite (or "multiple-layer") membrane structure is to allow the selection and combination of multiple materials which can each perform some of the necessary functions of the overall membrane better than any one of the materials could perform all of such functions. The process of selective passage of certain types of molecules in gaseous phase through a nonporous membrane material is a complex phenomenon occurring on a molecular level. Generally, the molecular selectivity is a combination of diffusion through the membrane material and gas solubility within the membrane material. Referring to the diffusion concept, the selective membrane material must have the very special performance property that certain types of molecules preferentially will pass through it, resulting in a concentration of such types of molecules on the permeate side of the membrane. Such selective membrane materials can be very expensive to develop and produce, and accordingly they command a high price. Further, since the gas molecules must physically pass through the selective membrane itself, overall membrane flux will be maximized when the selective membrane thickness is minimized. This is a crucial consideration in designing a gas separation membrane, because higher flux translates into higher productivity. Lower flux directly results in greater compression requirements to force the gas through the membrane, translating into increased operating costs. As a result of these considerations, the gas molecule selection function of a gas separation membrane is best performed by an ultrathin layer of a specially-selected, often expensive selective membrane material.
One class of such membrane forming polymers are those referred to as the "6FDA or 6FDA-type polyimides". These polymers can be formed by (A) the condensation of 5,5'-2,2,2-trifluoro-1-(trifluoromethyl)ethylidene-bis-1,3-isobenzo-furane dione (known as "6FDA") with an aromatic diamine such as 1,5-diaminonaphthalene or 1,3-diaminobenzene; and (B) dehydration to yield a 6FDA polyimide. The value and applicability of 6FDA polyimides as gas separation membranes is well known and documented, e.g., in the Hoehn et al U.S. Pat. Reissue No. 30,351 (based on U.S. Pat. No. 3,899,309), the Hayes U.S. Pat. No. 4,717,394, and the Ekiner et al U.S. Pat. No. 5,085,676. Other known membrane forming polymers include cellulose triacetate (CTA), polyethersulfone (PES), polytrimethylsilylpropyne (PTMSP), polyetherimide (PEI) and polypropylene oxide (PPO).
A process for laminating a polymer layer to a paper support is disclosed in the Lundstrom U.S. Pat. No. 3,767,737. In this process a polymer solution is spread out on the surface of a pool of water. When the solvent of the polymer solution evaporates the polymer solidifies and forms an ultrathin film on the water surface. This thin polymer film is then picked up by and laminated to a paper web.
Another important function of a gas separation membrane is to withstand the pressure drop across the membrane which is encountered in and necessary for its operation, and otherwise endure a reasonable lifetime as an integral material in the intended operating environment. This function is best performed by a structural support material which (1) can be prepared economically as a relatively thick layer which will provide adequate mechanical strength, and (2) is highly. permeable, so as not to markedly reduce the gas flux of the overall membrane.
It was an object of the invention to provide a composite membrane which would combine the features of (1) withstanding extensive pressure drops, (2) being highly permeable and (3) having an ultrathin membrane layer, wh
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McBride Thomas K.
Snyder Eugene I.
Spitzer Robert
UOP
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