Transport of Toluene in Microporous Vycor Class
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TRANSPORT OF TOLUENE IN MICROPOROUS VYCOR CLASS BEN ABELES, L. F. CHEN, J. W. JOHNSON AND J. M. DRAKE Exxon Research and Engineering Co., Route 22 East, Annandale,
NJ
08801
ABSTRACT A quasi-equilibrium model is developed for capillary condensation flow, in which surface diffusion is included. The model is in excellent agreement with the experimental data for the flow of toluene in untreated Vycor glass and in Vycor glass whose pores were derivatized with C1 8 H3 8 . The observed increase in viscosity with decreasing pore size is ascribed to a stagnant monolayer adsorbed to the pore walls. The surface diffusion coefficient at 22°C is 5 x 10-5 cm2 /sec for both the derivatized and untreated samples. INTRODUCTION Depending on the magnitude of the vapor and/or liquid pressures at the inlet and outlet sides of the porous materials, different flow regimes take place. When the inlet pressure p is smaller than the capillary evaporation pressure, Pe, so that there is no condensed liquid in the pores, transport is mainly due to surface diffusion of molecules adsorbed on the walls of the pores. Vapor phase transport is orders of magnitude smaller because of the relatively small number of molecules in the vapor phase compared to the adsorbed phase. When Pe < P < Po, where Po is the saturated vapor pressure, then the capillary pressures associated with the meniscus at the liquid-vapor interfaces must be taken into account. In pores of nanometer dimensions these pressures can reach magnitudes of the order of several hundred atmospheres and give rise to a correspondingly large flow of liquid. This flow regime is referred to as capillary condensation flow [1] (CCF). It plays an important role in processes occurring in nature and in technological applications such as flow of sap in trees, drying [2], dehumidification [3], separation [4], etc. We have studied transport of toluene through Vycor over the pressure range which encompasses all the different flow regimes. For the interesting intermediate regime, where both surface diffusion and CCF coexist, we have developed a quasi-equilibrium model of flow which is related in a simple manner to the critical evaporation pressure Pe. The model is in excellent agreement with the experimental data. EXPERIMENTAL Vycor used in this investigation was obtained from Corning glass and was cut and polished into discs 1.5 cm in diameter and 0.11 cm thick. Measurements were also made on Vycor discs in which the pores were derivatized with C1 8 H3 8 alkane molecules. The isotherms measured with toluene vapor at 23°C for the untreated Vycor is shown in Fig. la. We note that in the desorption branch of the isotherms the abrupt drop in the relative weight gain AW/W is due to a transition from pores filled with liquid toluene to toluene adsorbed on the pore walls. We refer to the corresponding transition pressure, indicated by the arrow in the figure, as the critical evaporation pressure Pe. The pressure Pe is determined through Kelvin equation by the neck radii i.e., radii of the smallest cross secti
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