Kew-Ho Lee

The transport of condensible vapors through a microporous vycor glass membrane

Abstract The flow of condensible vapors through microporous Vycor glass was investigated experimentally as well as theoretically. In porous materials, adsorbable gases frequently exhibit higher permeability than predicted from the flow of nonadsorbable gases. This enhanced flow has been attributed to the surface diffusion of adsorbed molecules along the surface of the porous media or to the viscous flow of capillary condensate at high relative pressures. In the present investigation, a new flow model of condensible vapors through microporous material was developed by considering the blocking effect of the adsorbed phase on the basis of a cylindrical capillary structure. Six different flow modes were considered depending on the pressure distribution and the film thickness of the adsorbed layer. Experimental measurements were also conducted on the transport of condensible vapors (Freon-113 and water) through microporous Vycor glass at steady state in the entire range of relative pressure. The maximum peak and scattering phenomena of permeabilities were observed. The estimated values of permeability from the developed model were compared with the experimental results. Also, it was attempted to explain the maximum peak and scattering phenomena of the experimentally observed permeabilities.

A NEW SILICON-BASED MATERIAL FORMED BY PYROLYSIS OF SILICON RUBBER AND ITS PROPERTIES AS A MEMBRANE

A new, highly porous silicon-based membrane was developed by pyrolyzing a silicon-rubber material (polydimethyl siloxane)in two steps. The first step was performed under an inert-gas environment below 800°C. The second step was performed in air below 950°C to oxidize and cross-link Si-O chains. The resulting silicon-based material was highly porous and had a fine pore structure (maximum porosity of 50%, BET surface area of I40m2/g) suitable for hot industrial gas separation even in a highly oxidizing environment. Gas permeability studies were performed at several different temperatures using a material derived by the pyrolysis of commercial silicon-rubber tubes. The results indicated that the flow through the membrane could be adequately explained by the Knudsen diffusion mechanism. The average permeabilities were 10 to 50 times those of porous Vycor glass.

Synthesis of Pd particle-deposited microporous silica membranes via a vacuum-impregnation method and their gas permeation behavior

Pd particle-deposited microporous silica membranes were synthesized to improve hydrogen permselectivity of the microporous silica membrane and to overcome high cost of palladium and crack formation through hydrogen embrittlement. Pd particles below 400 nm in diameter were readily deposited on the microporous silica membrane via a vacuum-impregnation method by using a Pd(C(3)H(5))(C(5)H(5)) precursor. After deposition of Pd particles on the microporous silica membrane, hydrogen permselectivity over nitrogen considerably increased from 11-28 to 30-115 in a permeation temperature range of 25-350 degrees C due to plugging membrane defects and hydrogen adsorption diffusion through the interface between the Pd and silica layer. The activation energy of the Pd-deposited silica membrane (6.32 kJ mol(-1)) was higher than that of the microporous silica membrane (4.22 kJ mol(-1)). In addition, the Pd-particle deposition led to an increase in the permselectivity of He and CO(2) with little chemical affinity for the Pd particles, which indicates that Pd-particle deposition gives the effect of plugging defects such as pinholes or cracks, which could be formed during the membrane preparation. Therefore it is demonstrated that Pd-particle deposition on the silica membrane is effective for induction of the hydrogen adsorption diffusion and plugging membrane defects.

Facile synthesis of mesoporous carbon and silica from a silica nanosphere–sucrose nanocomposite

We report a novel and simple synthetic method for preparing mesoporous carbon from silica nanosphere–sucrose (SN–S) nanocomposites. Sucrose was used as a carbon source, as well as a templating agent for the formation of the silica mesostructure, and a transparent colloidal silica sol, including 5 or 10 nm silica nanospheres, was employed as the silica source to easily control the pore size of the mesoporous carbon. The SN–S nanocomposite, with a wormhole-like mesostructure, was readily formed by the simple addition of sucrose into the colloidal silica sol. Using the SN–S nanocomposites, mesoporous carbon and silica were simultaneously synthesized. Mesoporous carbon could be obtained by the carbonization of the SN–S nanocomposite under vacuum and removal of the mesostructured silica. The pore size and pore wall thickness of the mesoporous carbon could be readily and independently controlled by the silica nanosphere size and sucrose concentration, respectively. Moreover, mesoporous silica could be also synthesized through calcination of the SN–S nanocomposite in air, and its pore size and pore wall thickness could be easily and independently controlled by the sucrose concentration and silica nanosphere size, respectively. In this article, we report that the sucrose plays a dual simultaneous role in the SN–S nanocomposite. The first role is as a non-surfactant template for the synthesis of wormhole-like mesoporous silica, and the second role is a carbon source for synthesis of mesoporous carbon.

Competitive adsorption-driven separation of water/methanol mixtures using hydrogen as a third competitor

In this study, we report competitive adsorption-driven separation of a water/methanol mixture in Pd-deposited silica membranes, which is induced by introducing hydrogen carrier gas as a third competitor. After replacing helium carrier gas by hydrogen carrier gas, water vapor permeance showed a slight decrease, whereas methanol vapor permeance significantly decreased. The water/methanol separation factor remarkably increased from 1.7-16.5 to 6.8-58.2 in the feed water content of 5.8-83.0 wt.%. From single vapor permeation tests in the presence of carrier gas (hydrogen or helium), it was confirmed that those permeation behavior was derived from stronger effect of the competitive adsorption between hydrogen and methanol vapor than that between hydrogen and water vapor. That is, hydrogen carrier gas dominantly inhibits adsorption of methanol vapor on the membrane surface, and the partial pressure of methanol on the membrane surface decreases, which leads to a decrease in methanol permeance with reduced driving force. In addition, temperature programmed desorption (TPD) results of water and methanol from Pd/silica particles also demonstrated that hydrogen carrier gas suppresses methanol adsorption on Pd/silica surface more dominantly than water adsorption.