Measurements of transport rates of C4 hydrocarbons across a single-crystal silicalite membrane

Abstract

A ‘direct’ macroscopic experimental technique has been used to measure the rates of transport of C₄ hydrocarbons in a silicalite crystal. A silicalite membrane was fabricated by embedding a large (300 × 100 × 100 µm) silicalite crystal in an epoxy resin. The membrane was subjected to a known adsorbate pressure on the inflow side and the rates of transport of C₄ hydrocarbons across the membrane were determined by measuring the increase in pressure on the outflow side as a function of time. The data were analysed to determine the micropore diffusivities of the hydrocarbons in silicalite. This method is an attractive alternative to other macroscopic techniques such as chromatographic, gravimetric and zero-length chromatography which are commonly employed to determine the micropore diffusivities. It eliminates many of the extraneous mass-transfer resistances that may be present in these other methods. However, the membrane method also has several short-comings. As presently formulated, it can provide only an order of magnitude estimate of the micropore diffusivities and it can measure only integral diffusivity and not the differential diffusivity which is a more important fundamental quantity of interest. Experimental modifications and refinements of the mathematical model are presently being pursued to overcome these shortcomings.