Yoshihiro Kusuki

Gas Permeation Properties and Characterization of Asymmetric Carbon Membranes Prepared by Pyrolyzing Asymmetric Polyimide Hollow Fiber Membrane

Abstract Carbon membranes were continuously prepared by pyrolyzing an asymmetric polyimide hollow fiber membrane at temperatures ranging from 600 to 1000°C under nitrogen for 3.6 min. The asymmetric polyimide hollow fiber membrane was treated under atmospheric air at 400°C before pyrolysis. By scanning electron microscopy, the carbon membranes were observed to have an asymmetric structure of which a skin layer consisted of nodule aggregates. The asymmetric carbon membranes displayed high performance of gas permeability and selectivity. For example, the membranes prepared by pyrolyzing at over 700°C had permeation rates of hydrogen ranging from 10 −4 to 10 −3 cm 3 (STP)/(cm 2 s cmHg) and ratios of the permeation rate of hydrogen to that of methane ranging from 100 to 630 at a feed gas composition of 50% hydrogen in methane and at 80°C.

Relation of gas permeability with structure of aromatic polyimides II

Abstract We investigated relations between diffusivities on polyimides and physical parameters with mobilities of segments and examined viscoelasticity of polyimides on introducing the parameter as an index of mobilities of polymer chains. Each relaxation temperature of viscoelasticities on polyimide films having ordered regions was almost the same as that on the amorphous states. The CF 3 group as a substituent showed the similar effect to that of CH 3 group in elastic relaxation. The storage modulus was correlated well with the gas diffusivity. And in the case that relations between the diffusion coefficients and cohesive energy densities (CED) which were introduced as an index of cohesiveness of segments, the relation between CED and the gas diffusivities showed a good correlation. Mobilities of segments affected strongly the gas diffusivities even in the glassy polymers such as polyimides.

Vapor permeation and pervaporation separation of water-ethanol mixtures through polyimide membranes

Abstract Vapor permeation (VP) and pervaporation (PV) separation of water-ethanol mixtures were investigated for a series of polyimide membranes. Membrane performance for PV was described by both the permeability coefficient of water, P W , which was caluclated using the equilibrium partial pressure as the driving force, and the separation factor at the membrane separation step, α mem . For PV, the dependence of P W and α mem on feed composition (wt.% of ethanol, x E ) was rather small. For VP, with a decrease in x E , P W increased and α VP decreased. P W was larger for PV than for VP, while α mem was smaller for PV. The differences between PV and VP were larger for BPDA-DDBT and BPDA-ODA/DABA (4/6) polyimides having a higher total sorption in the liquid phase, but were small for Kapton-H and BPDA-ODA/DABA (8/2) polyimide having a lower sorption. In the case of the former polymers, the swelling effect was no large for PV that it spoiled membrane performance seriously, whereas it was not so large for VP.

Gas permeability and permselectivity in polyimides based on 3,3',4,4'-biphenyltetracarboxylic dianhydride

Abstract Permeation rates of hydrogen, carbon monoxide, carbon dioxide and methane were measured for amorphous films of a series of polyimides prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and each of the following diamines: 4,4′-oxydianiline (ODA), 4,4′-methylenedianiline (MDA), 4,4′-diaminodiphenyl sulfone (DDS), and dimethyl-3,7-diaminodibenzothiophene 5,5′-dioxide (DDBT). The permeability coefficients ( P ) at 10 atm and 50°C were in the order BPDA-DDBT > BPDA-DDS ≥ BPDA-MDA > BPDA-ODA. On going from BPDA-ODA to BPDA-DDBT, P increased by 6.0- and 9.4-fold for hydrogen and carbon dioxide, respectively. This was due to 1.4- and 3.2-fold increases in the diffusion coefficients accompanied by appreciable decreases in activation energies, and also to a 3.7-fold increase in the solubility coefficients. These results suggest that both high rigidity and bulkiness of a polymer backbone may prevent efficient chain packing and result in a more open structure for gas transport. BPDA-DDBT displayed high permselectivities for hydrogen/methane and carbon dioxide/methane in addition to high permeability coefficients, and has a high potential for use as a membrane material for gas separation.

Gas permeability and permselectivity in homo- and copolyimides from 3,3',4,4'-biphenyltetracarboxylic dianhydride and 3,3'- and 4,4'-diaminodiphenylsulfones.

The gas transport properties of homo- and copolyimides prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and 3,3′- and 4,4′-diaminodiphenylsulfones (m-DDS and p-DDS, respectively) are investigated. With increasing fraction of m-DDS, the density of the polyimide films increased by 1.1% and the glass transition temperature decreased by 70 K. With increasing fraction of m-DDS, permeability coefficients P of H2 and CO2 in the film decreased to 1/3 and 1/15, while P ratios of H2 to CO and CO2 to CH4 increased by 4.1 and 1.8 times, respectively, as a result of decrease in the diffusion coefficient and increase in diffusivity selectivity. These observations are interpreted in terms of changes in free volume fraction, distribution of size of free volume, and segmental mobility.

Sorption and transport of carbon dioxide in a polyimide from 3,3′,4,4′-biphenyltetracarboxylic dianhydride and dimethyl-3,7-diaminodibenzothiophene-5,5′-dioxide

Abstract Polyimide prepared from 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA) and dimethyl-3,7-diaminodibenzothiophene-5,5′-dioxide (DDBT) displayed CO2 plasticization effects on sorption and transport of CO2, in spite of its very rigid polymer backbone. Slow components of solubility and permeability coefficients (S and P) were observed which were attributed to the relaxation of the polymer chain caused by dissolved CO2 at a higher pressure. Increasing P with increasing feed pressure was represented by the extended dual-mode transport model taking into account the concentration dependence of the diffusion coefficient.

Permeation properties to hydrocarbons, perfluorocarbons and chlorofluorocarbons of cross-linked membranes of polymethacrylates with poly(ethylene oxide) and perfluorononyl moieties

Abstract Two kinds of cross-linked polymer membranes were prepared by photo polymerization. One of them (PEO membrane) is a co-polymer of methoxy-terminated poly(ethylene glycol) methacrylate (MEMA) and poly(ethylene glycol) dimethacrylate (EDMA) in feed ratio of 70/30 in wt%. The other (PF/PEO membrane) is a co-polymer of 1H,1H,9H-hexadecafluorononyl methacrylate and EDMA in feed ratio of 70/30 in wt%. The block lengths of poly(ethylene oxide) (PEO) are 9 and 14 for MEMA and EDMA, respectively. Permeation properties of inorganic gases, hydrocarbons, perfluorocarbons and chlorofluorocarbons (CFCs) were investigated for these membranes compared with a silicone rubber (SR) membrane. The PF/PEO membrane is inferior to the SR membrane as for CFCs/N 2 separation because the former has lower permeabilities. The PEO membrane has good performance for separation of hydrocarbons and CFCs from N 2 or perfluorocarbons.

Microporous carbon membranes

This article surveys the production and permeation of carbon membranes in the field of gas separation. Carbon membranes, which are prepared under optimized conditions, show appreciable selectivities for gas mixtures such as H2/N2, CO2/N2 and O2/N2. Separation between organic and inorganic gases, as well as alkane and alkene, is also possible using the carbon membranes. Permeances may be decreased by exposing the membranes to an oxidative or humidified atmosphere at temperatures below 100°C, but can be recovered by heat-treating in an inert atmosphere.