Masashi Asaeda

Separation of inorganic/organic gas mixtures by porous silica membranes

Abstract The sol–gel procedures were applied to fabricate porous silica membranes coated on cylindrical porous α -alumina tubes by the hot coating methods. The pore diameters of the silica membranes were determined by the gas permeation methods to be less than 0.4 nm. The active layer thickness was about 1 μm, giving a large hydrogen permeance of around 3×10 −5 m 3 m −2 s −1 kPa −1 (1.3×10 −6 mol m −2 s −1 Pa −1 ) at 300°C. The permeance of H 2 and He were slightly dependent on temperature, while the observed CO 2 permeance showed a large temperature dependency. The largest permeance ratios observed are 150 for H 2 /CH 4 , 1100 for H 2 /C 2 H 6 , and 6300 for H 2 /C 3 H 8 at 300°C. As the CO 2 permeance increases at a lower temperature, CO 2 removal from organic gas mixtures can be preferably performed at a lower temperature near the room temperature. An example of observed CO 2 permeance at 35°C was approximately 2×10 −5 m 3 m −2 s −1 kPa −1 (0.9×10 −6 mol m −2 s −1 Pa −1 ). The permeance ratio of CO 2 /CH 4 was around 80–110 at 35–50°C and the ratio became smaller at higher temperature. The porous silica membranes fabricated in this work were quite stable when used in dry conditions. In humid conditions, however, the gas permeances of CO 2 and N 2 decreased drastically, giving a larger permeance ratio for H 2 /CO 2 .

Titania membranes for liquid phase separation: effect of surface charge on flux

Titania membranes were prepared by sol-gel processes. Molecular weight cut-offs (MWCO) were successfully controlled in the range from 500 to 1000 with pure water permeabilities of 0.6-1.5x10 -11 m 3 /m 2 .s.Pa. Nanofiltration experiments were carried out for various types of electrolytes (NaCl, Na 2 SO 4 , MgCl 2 , MgSO 4 ) using several membranes having different MWCOs (400, 500, >1000). Rejection showed the minimum values near the isoelectric point (IEP) which were determined by streaming potential measurements. Larger rejections were obtained for the case where electrolytes having divalent co-ions were nanofiltrated, while low rejection were observed for the case of electrolytes having divalent counterions. On the other hand, permeate volume fluxes were maximized near IEP and the fluxes were almost the same irrespective of types of electrolytes. However, permeate volume fluxes decreased at pH higher than IEP. The dependency was pronounced for the case of divalent counterions and smaller pore diameters, probably because of larger hydrodynamic resistance by ionically-adsorbed counter ions.

Characterization of sol–gel derived membranes and zeolite membranes by nanopermporometry

Silica-zirconia membranes having pore sizes in the range 0.5-2 nm, determined using water as a condensable gas (vapor) by nanopermporometry, were prepared by the sol-gel process, and used in gas permeation experiments. The permeability ratio of He/N 2 approached the Knudsen value (=2.6) for pore sizes larger than 2 nm. It then decreased with decreasing pore size, probably because of an enhanced contribution of surface diffusion in the small pore, and showed a minimum at an approximate pore size of 1 nm. It then increased to approximately 50 for a membrane having pore sizes of 0.3 nm. For the case of He/SF 6 , the curve appears to shift to a larger Kelvin diameter, probably because of the larger molecular size of SF 6 , as well as adsorption. The effects of non-condensable gases (He and N 2 ) were examined using silica-zirconia membranes of 2 and 0.8 nm in pore size. The pore size distribution (PSD) curves measured by He and N 2 were in good agreement with each other for membranes having a pore size as large as 2 nm. On the other hand, for the case of porous membranes having small pore sizes, PSD curves measured using He were shifted to a smaller pore size, compared with those measured by N 2 . This suggests the existence of micropores, which allowed the permeation of only He. Moreover, nanopermporometry was applied to MFI zeolite membranes to characterize selective (intracrystalline) and non-selective pores (intercrystalline) using hexane, and the data were in reasonable agreement with the observed separation performances.

CATALYTIC MEMBRANE REACTION FOR METHANE STEAM REFORMING USING POROUS SILICA MEMBRANES

Catalytic membranes, which have hydrogen permselectivity over other gaseous molecules and catalytic activity for methane steam reforming, were prepared by 2 different procedures and applied to methane steam reforming at 450–500°C. Type A catalytic membranes were manufactured by the preparation of a hydrogen separation layer from silica-zirconia colloidal sols, followed by the application of a nickel catalyst coating. Type B catalytic membranes were prepared via the impregnation of a nickel catalyst inside the α-alumina porous substrates, followed by the application of a coating on the hydrogen separation layer. Hydrogen permselectivity over nitrogen was degraded by coating the catalyst layer, as in the Type A membranes, and in addition, methane conversion decreased with time probably because of catalyst sintering or carbon deposition. Type B catalytic membranes showed a steady conversion for a longer period than did Type A, and the permeability ratio of hydrogen to nitrogen was approximately 200; therefore, Type B was found to be the effective route to preparing catalytic membranes. Methane steam reforming through the use of catalytic membranes revealed that methane conversion beyond the equilibrium conversion levels could be achieved either by sweeping the permeate stream or by pressurizing the feed stream at 6 bar and not using sweeping gas.

Nickel‐Doped Silica Membranes for Separation of Helium from Organic Gas Mixtures

Abstract: One of the problems in the use of inorganic silica membranes is their instability against water or water vapor, a problem that results from the dissolution and rearrangements of silica networks. In this work Ni(NO3)-6H2O was added to silica sol for fabrication of Ni-doped silica membranes by sol-gel techniques in order to prevent the densification of amorphous silica networks in a humid atmosphere at 50-3()0°C. A fresh Ni-doped silica membrane (Si/Ni = 2/1) fired at 500 C showed a large He permeance of about 2.6 × 10−5 [m3 (STP)/(m2 ·s · kPa)] with a selectivity of 600 (He/CH4) at 300’C. After the Ni-doped silica membrane was left in humid air (40°C, 60% RH) for 4 days, the He permeance decreased slightly (by 5%) with a larger selectivity of 800 (He/CH4) at 300’C. However, little change was observed in the activation energy of He permeation, suggesting that nickel oxides added to silica can preferably prevent the densification of silica networks through which only H2 and He can permeate. Humid He and CH4 showed smaller permeabilities, especially at temperatures below 150°C, than those of dry gases because of condensed and/or adsorbed H20 molecules in silica networks and on grain boundaries. Separation of He/CH4 mixtures with the fresh Ni-doped silica membrane (Si/Ni = 2/1) at 300’C gave relatively good results and coincided well with the predicted values with the ideal permeance ratio, 600.

Molecular dynamics studies on gas permeation properties through microporous silica membranes

Gas permeation mechanisms through a micropore of a vitreous silica (v-SiO 2 ) membrane were studied using a molecular dynamics (MD) simulation. Virtual v-SiO 2 membranes were prepared by the melt-quench methods using the modified Born-Mayer-Huggins pair potential and Stillinger-Waber three-body interactions. The particle-generating non-equilibrium MD technique was employed in order to simulate gas permeation phenomena, where permeating molecules were modeled as Lennard-Jones particles. This simulation method accommodates a change in the number of particles in a unit cell and, hence, an accurate simulation of the steady-state process of permeation can be achieved. The dependencies of permeance on temperature and pressure were discussed. For cylindrical pores of about 5 A in diameter, the calculated temperature dependencies of the permeance of He-like LJ (Lennard-Jones) particles were similar to those predicted by the normal Knudsen permeation mechanism, while, for CO 2 permeation, a temperature dependency larger than He and a significant deviation from the Knudsens could be observed. In the relatively high-temperature region (400-800 K), the simulated permeance of CO 2 was nearly independent of the upstream pressure, while at the temperature below 300 K, a pressure dependency of permeance was observed. Simulations of adsorption conducted on the same unit cell yielded a Henry-type isotherm at 400 K and a Langmuir-type isotherm at 260 K. These results indicate that gas-like permeation occurred in the higher-temperature region, where the permeation flux is proportional to the pressure drop across the pore. However, at lower temperatures, the transports of molecules as some type of adsorption phase might be dominant in such a small pore. A simple gas permeation model, considering the effect of the pore wall potential field and Langmuir type adsorption within a micropore explained those permeation properties of CO 2 well.

Molecular dynamics study of gas permeation through amorphous silica network and inter-particle pores on microporous silica membranes

A boundary driven non-equilibrium molecular dynamics simulation method was used to study gas permeation mechanisms through microporous amorphous silica membranes. Two types of silica membranes were prepared, one by random atom-removing and the other by regular pore-digging procedures. The former was a dense membrane that served as a model for network pores formed by silica polymers and the latter had a penetrating cylindrical pore which simulated an inter-particle pore. The permeances of He, H2 and Ne through network models with densities of 1.7 and 1.8 g cm−3 increased with decreasing temperature, while activated permeation was observed for the denser models. Deviations in the permeation properties from those predicted by the Knudsen model became greater with increasing membrane density as the result of molecular sieving effects. The permeance of H2 through a cylindrical pore 0.6 nm in diameter was greater than that for He at all temperatures examined as predicted by the Knudsen model, and the greater interaction of CO2 with the pore surface yielded a larger temperature-dependency curve for permeance, compared to He and H2. The simulated permeation properties of several gases were in agreement with experimental data on actual microporous silica membranes, indicating the qualitative validity of the microporous structure model composed of small openings in a silica network phase and larger inter-particle pores.

Effect of divalent cations on permeate volume flux through porous titania membranes

Abstract Titania (TiO2) membranes with molecular weight cut-offs controlled at 250, 400, and 2000 were successfully prepared by the sol-gel process. These porous membranes rejected electrolytes based on Donnan exclusion, and the permeate volume flux, based on the electroviscous effect, was pH-dependent, similar to polymeric porous membranes. Moreover, divalent counterions (Ca2+, Mg2+) which were strongly adsorbed by ion-exchange at alkali pH, drastically reduced volume flux, compared with monovalent cations (Na+, K+); this tendency was pronounced for porous membranes with smaller pore sizes. The evidence for hydrodynamic resistance by the adsorbed divalent cations was shown based on permeation and zeta potential measurements of mixtures.

Synthesis and Characterization of Microporous ZrO2 Membranes for Gas Permeation at 200°C

The development of gas separation membranes able to work at high temperatures require robust and thin ceramic layers. In this work, zirconia membranes have been prepared by the sol-gel method, following the colloidal sol route. The microporosity and crystallinity of the ZrO2 material was tested by N2 adsorption and XRD. The derived active zirconia layers were defect-free as seen by SEM. The optimum firing temperature range was set in the range 400–500°C. He, H2, CO2, N2 gas permeation was conducted at temperatures up to 200°C. High permeances were obtained and the microporosity of the zirconia layer was confirmed.

Condensable vapor permeation through miroporous silica membranes studied with molecular dynamics simulation

Abstract Molecular dynamics (MD) simulations of condensable vapor permeation through sub-nano scale pores were conducted for a virtual amorphous silica membrane, prepared by melt–quench procedures. The simulated permeance of C 2 H 6 ( T C =305 K)-like LJ particles through an 8 A in diameter pore showed a surface diffusion-like temperature dependency in the relatively high temperature region (400–800 K), while at around 300 K, the permeance decreased with decreasing temperature. That is, a maximum was observed in the temperature dependency curve for permeance. The critical temperature, T C of the permeating condensable vapor could be a contributing factor in the permeation properties through the micropore. The simulated permeance of C 2 H 6 at 260 K decreased with increasing mean pressure. At low pressure, where micropore filling would not be expected to occur, an almost gas like permeation was observed even at temperatures below the T C , while under micrpore filling conditions at a relatively high pressure, the permeance abruptly decreased. Adsorption simulations were also conducted on the same unit cell, and the mobility of the adsorbed molecules in the micropore filling phase were smaller than those in the lower density phase. Through this investigation of temperature and pressure dependency of permeance, it can be concluded that the development of the micropore filling phase led to a decrease in permeance, and transport as the condensed filling phase through the micropore was an activated process.