Ken-ichi Okamoto

Tubular-type pervaporation module with zeolite NaA membrane☆

Zeolite NaA membranes were prepared on the surfaces of porous tubular supports composed of mullite, α-alumina and/or cristobalite using the hydrothermal synthesis. The zeolite NaA crystals and the surface morphologies of the membranes were characterized by X-ray diffraction and SEM. EPMA analysis revealed that the zeolite composite membranes consisted of three layers — the zeolite layer, the intermediate layer and the mullite support. According to pervaporation measurements using H2OEtOH mixture, the permselectivity increased in the alumina content of the substrate and reached a constant at >70 wt% Al2O3. Zeolite NaA membrane showed very high water-selective permeation through all the ethanol concentrations. The tubular-type module with ca. 0.5 m2 membrane area was composed of 17 pieces of zeolite membrane using 65 wt% alumina content substrate, which has high pervaporation performance and can be purchased at a low price. The efficiency of the module on permeation rate increased in proportion to the feed flow rate and exhibited 100% at Reynolds number >80. Pervaporation flux and separation factor of the module for 5 wt% water/95 wt% ethanol mixture at 95°C was ca. 2.35 kg m−2 h−1 and was greater than 5 000, respectively.

Permeability and permselectivity of gases in fluorinated and non-fluorinated polyimides

Abstract Solubility and permeability coefficients for gases in a series of fluorinated and non-fluorinated polyimides were measured to investigate the effects of chemical structure on permeability and permselectivity. Polyimides with higher concentration of fluorine had lower chain packing density and higher permeability. Polyimides with C(CF3)2 linkages had higher permeability with relatively higher permselectivity. 2,2-Bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride/2,2-bis(4-aminophenyl)hexafluoropropane (6FDA-BAHF) polyimide with two C(CF3)2 linkages in the polymer repeat unit displayed excellent performance for CO 2 CH 4 and O 2 N 2 separations; its permeabilities for O2 and CO2 were 3.7 and 3.1 times, respectively, larger than those of 6FDA-pp′ODA (4,4′-oxydianiline), a conventional 6FDA-based polyimide, while the permselectivities for O 2 N 2 and CO 2 CH 4 were 13% and 22%, respectively, smaller. On the other hand, polyimides having two C(CF3)2 linkages together with two ether ones in the repeat unit had lower permeability with a similar level of permselectivity as compared with 6FDA-BAHF polyimide. For H 2 CH 4 and H 2 CO separations, polyimides prepared from 6FDA and meta-oriented non-fluorinated diamines were preferable. These separation properties were discussed in connection with packing density and local segmental mobility.

Permeation and separation properties of polyimide membranes to olefins and paraffins

Pure and mixed gas permeation experiments for olefins and paraffins of C2 and C3 were carried out for several polyimide and other polymer membranes at pressures up to 8 atm and temperatures from 308 to 423 K. The olefins were more permeable to the corresponding paraffins due to their preferential diffusion based on the difference in their molecular size. Polyimide prepared from 4,4′-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) and 2,4,6-trimethyl-1,3-phenylenediamine (TrMPD) displayed relatively high performance: permeability coefficient to propylene, PC3H6 = 20–40 Barrer (1 Barrer = 1 × 10−10 cm3(STP)/(cm s cmHg)) and ideal separation factor (permeability ratio of pure propylene and propane). αid(C3H6/C3H8) = 11 at 323 K and 2 atm. Polyimide from 6FDA and dimethyl-3,7-diaminodiphenylthiophene-5,5-dioxide (DDBT) displayed low permeability and high permselectivity: PC3H6 = 0.8 Barrer and αid(C3H6/C3H8) = 27 at 323 K and 2 atm. Their performance was much better than that of other polymers such as poly(2,6-dimethyl-1,4-phenyleneoxide). For mixed gas permeation, the separation factor was lower by about 40% than the αid due to the increase in PC3H8 caused by coexisting propylene.

Synthesis and properties of highly sulfonated proton conducting polyimides from bis(3-sulfopropoxy)benzidine diamines

Two novel sulfonated diamine isomers, 2,2′-bis(3-sulfopropoxy)benzidine (2,2′-BSPB) and 3,3′-bis(3-sulfopropoxy)benzidine (3,3′-BSPB), were successfully synthesized and the highly sulfonated polyimides (SPIs) with sulfonic acid groups in the side chains were prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and BSPB monomers. Transmission electron microscopy (TEM) analysis revealed that these side-chain-type SPI membranes have a microphase-separated structure composed of hydrophilic side chain domains and hydrophobic polyimide main chain domains. They showed high proton conductivities similar to or higher than those of Nafion 117 in the high relative humidity range (>70% RH). The proton conducting behavior was analyzed by percolation theory. Despite their high ion exchange capacity (IEC = 2.89 meq. g−1) and high water uptakes, they displayed much better water stability than common sulfonated polyimides with the sulfonic acid groups directly bonded to the polymer backbone. This is probably due to the microphase-separated structure of the membrane and the strong basicity of BSPB diamine moieties resulting from the electron donating effect of the propoxy groups. The sulfopropoxy groups were stable against aging in acidic aqueous solution at 373 K. Both the polyimide membranes showed good mechanical strength under high moisture conditions (about 6 GPa at 90% RH) and displayed anisotropic membrane swelling.

Application of zeolite membranes to esterification reactions

Pervaporation-aided esterification of acetic acid with ethanol was investigated at 343 K using zeolite T membranes. Almost complete conversion was reached within 8 h when initial molar ratios of alcohol to acetic acid were 1.5 and 2. The reaction time courses were well described by a simple model based on the assumptions that the reaction obeyed second-order kinetics and the permeation flux of each component was proportional to its concentration. The influence of operating parameters on variation in conversion with reaction time was investigated by means of the simulation using the model.

Preparation and gas separation performance of zeolite T membrane

Zeolite T membranes were prepared by hydrothermal synthesis on porous mullite tubes seeded with zeolite T crystals, using milk-like aluminosilicate gel with a molar composition of SiO2∶Al2O3∶Na2O∶K2O∶H2O = 1∶0.05∶0.26∶0.09∶14. A zeolite T crystal layer of about 20 µm in thickness was formed on the outer surface of the support after the synthesis at 373 K for 30 h. Single-gas and mixed-gas permeation experiments through zeolite T membranes were carried out by a vacuum method at 303∼473 K using He, H2, CO2, O2, N2, CH4, C2H6 and C3H8 single-component gases and CO2/N2, CO2/CH4 and other CO2/hydrocarbon mixtures, respectively. In single-gas permeation experiments, with increasing kinetic diameter from 0.33 nm for CO2 to 0.43 nm for C3H8, the gas permeance decreased by four orders in magnitude, indicating a kind of molecular sieving behavior for the zeolite T membranes prepared in this study, that is, they had little defects and their permeation behavior was controlled by zeolitic pores of erionite. Permeance of CO2 was much higher than those of N2 and CH4 and the ideal selectivities for CO2/N2 and CO2/CH4 were 31 and 266 at 343 K, respectively. In mixed-gas permeation experiments, zeolite T membranes showed the high selectivities for CO2/N2 and CO2/CH4 pairs of 107 and 400, respectively, at 308 K. The selectivity α decreased with an increase in temperature, but was still in a high level of 20 and 52 for CO2/N2 and CO2/CH4, respectively, even at 473 K. This is due to the synergetic effects of competitive adsorption of CO2 and molecular sieving of zeolitic pores. Because of the increasing effect of single file diffusion, the selectivities for CO2/C2H6 (α = 61) and CO2/C3H8 (α = 17) were rather low.

Rain type classification algorithm for TRMM precipitation radar

A TRMM precipitation radar (PR) standard algorithm for classifying precipitation types is designated as the algorithm 2A-23. This algorithm classifies precipitation type into three categories: stratiform, convective, and other. In the case of convective precipitation, a further examination is made to determine whether it is warm rain or not. The algorithm 2A-23 also detects bright band and determines the height of a bright band when it is detected.

Effect of photocrosslinking on permeability and permselectivity of gases through benzophenone-containing polyimide

Permeability of H2, CO2, O2, CO, N2 and CH4 in polyimides prepared by polycondensation of equimolar amount of 3,3′,4,4′-benzophenone tetracarboxylic dianhydride (BTDA) or BTDA/2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (1:1 or 1:3 molar ratio) with 2,4,6-trimethyl-1,3-phenylene-diamine was measured at 35°C to investigate the effect of photocrosslinking on permeability and permselectivity. The permeability coefficients decreased with increasing UV-irradiation time as a result of crosslinking. The decreases in the permeability coefficients were attributed to a change in diffusivity, while solubility was not greatly affected. Permselectivity of photocrosslinked BTDA-containing polyimides is controlled by the diffusivity selectivity and is higher than that of the polyimides with similar permeability. The largest change in selectivity was obtained for the H2/CH4 gas pair which has a large difference in the molecular sizes of the constituents. The fact that the selectivity for the H2/CH4 separation increased by a factor of 50 after 30 min UV-irradiation with a decrease in H2 permeability by a factor of 5 is especially interesting from a membrane separation standpoint.

Application of zeolite T membrane to vapor-permeation-aided esterification of lactic acid with ethanol

Zeolite T membranes were applied to vapor-permeation-aided esterification of lactic acid with ethanol. The hybrid process provided almost complete conversion within a short reaction time by removing water from the reaction mixture. Zeolite T membrane worked steadily for a long time. The reaction time-courses were described by a model based on the assumptions that the esterification obeyed second-order kinetics and the permeation flux of each component was proportional to its concentration in the reaction mixture. The final reaction liquid mixtures consisted mostly of ethyl lactate and ethanol with little ester of polylactic acids, although concentrated lactic acid solution was used as a source.

The pervaporation properties of sulfonyl-containing polyimide membranes to aromatic/aliphatic hydrocarbon mixtures

Abstract Membranes of polyimide from 3,3′,4,4′-diphenylsulfone tetracarboxylic dianhydride (DSDA) and dimethyl-3,7-diaminobenzothiophene-5,5′-dioxide (DDBT), in which each monomer has a sulfonyl group, were investigated for pervaporation (PV) separation of aromatic/aliphatic binary and multi-component systems. The sorption amounts of benzene (Bz) and toluene (Tol) in the membrane were 11 and 15 g 100 g dry polymer, and those of aliphatics were much lower. The diffusion coefficient of the pure liquid was in the order of n-hexane (n-Hx>n-octane (n-Ot)> Bz> Tol ⪢ cyclohexane (CHx)>iso-octane (i-Ot). The membranes were preferentially permeable to aromatics over aliphatics. The dependence of separation factor α and total specific permeation flux Q on the feed composition was more significant for the Bz/CHx and Tol/i-Ot systems than for the Bz/n-Hx and Tol/n-Ot systems.The dependence of α and Ql on temperature was investigated. The membrane displayed good durability and higher PV performance as compared with other common polyimides, for example, α = 32, Ql = 0.93 kg μm/m2 h for Bz/CHx and α = 113 and Ql = 2.8 kg μm/m2 h for Tol/i-OtxBz or xTol of 60 wt% and 351 K.