Sorptive separation of ethanol-water mixtures with a bi-dispersed hydrophobic molecular sieve, silicalite: determination of the controlling mass transfer mechanism

Sorption of ethanol and water in bi-dispersed pellets formed by compaction of a hydrophobic crystalline molecular sieve, silicalite, is examined. The difficulties inherent in the measurement and interpretation of mixed liquid adsorption equilibria are discussed and the individual adsorption isotherms for {ethanol-water}/silicalite crystals presented. The adsorbed ethanol exhibits a density similar but smaller than that of liquid ethanol. The adsorbed water, however, is as much as four times lighter than liquid water. Diffusion of ethanol in the silicalite micropores is reasonably well described with a constant diffusivity of 5 × 10−14m 2s−1. The intracrystalline diffusivity of water is higher than that of ethanol at low loading but is drastically reduced as saturation is approached. A sensitivity analysis based on a detailed theoretical model yields the characteristic time constants of 3.3 s for external boundary layer mass transfer, 83.9 s for diffusion through the liquid filled macropores of the pellets and 0.03 s for diffusion in the micropores of the embedded crystals. The intraparticle transport is therefore firmly controlled by the macropore diffusion and can be described in terms of a pore diffusion model with the embedded crystals assumed in point wise local equilibrium with the macro pore fluid.