Carbon molecular sieve membranes separation factor with

Carbon molecular sieve membranes suitable for gas separation have been prepared by pyrolyzing thermosetting polymers. CMSMs with pore diameter 3-5 A have ideal separation factors, ranging from 4 to more than 170 for various gas pairs [2]. The permeation characteristics of the molecular sieve carbon membrane can be varied by changing the high temperature treatment parameters [8]. 

It has been observed that membranes of carbon molecular sieves can exceed the upper bound of conventional polymeric membranes. The carbon molecular sieve membranes are produced by carbonization of aromatic polymers (e.g., polyimides), yielding pore dimensions in the range of O2 and N2 molecular dimensions. Polyimide/poly(vinyl pyrrolidone) blends subjected to carbonization conditions also yielded carbon molecular sieve membranes that exceeded the upper bound limit for conventional polymeric membranes [195, 196]. Specific values were O2 permeability of 560-810 barrers with a O2/N2 separation factors of 10-7 well above the upper bound. 

The aromatic polyimides, cited earlier, separate carbon dioxide over methane with a-valves of 150-160. Aromatic polyoxadiazoles have a-valves of 100-2 00.157 The permeability of one aromatic polyimide was improved by two to four orders of magnitude by carbonizing it on porous alumina,158 the final a-valve being 100. In this case, the final actual membrane was probably a porous carbon molecular sieve. Facilitated transport has also been used to increase the separation factor. A porous poly(vinylidene difluoride) membrane with ethanolamine or diethanolamine in the pores gave a separation factor of carbon dioxide over methane of 2000.159 Such a method is less energy-intensive than when an amine is used in the usual solvent method. 

Ethylene has been separated from ethane by a silver nitrate solution passing countercurrent in a hollow fiber poly-sulfone.165 This separation has also been performed with the silver nitrate solution between two sheets of a polysilox-ane.166 A hydrated silver ion-exchanged Nafion film separated 1,5-hexadiene from 1-hexene with separation factors of 50-80.167 Polyethylene, graft-polymerized with acrylic acid, then converted to its silver salt, favored isobutylene over isobutane by a factor of 10. Olefins, such as ethylene, can be separated from paraffins by electroinduced facilitated transport using a Nafion membrane containing copper ions and platinum.168 A carbon molecular sieve made by pyrolysis of a polyimide, followed by enlargement of the pores with water at 400 C selected propylene over propane with an a-valve greater than 100 at 35°C.169... 

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