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SUPERCRITICAL DRYING PROCESS MODELING AND EQUIPMENT DESIGN
Abstract
Aerogels production consists of two main stages: preparation of the gel filled with solvent (alcohols, acetone or hexane) by polymerization or sol-gel process and its supercritical drying. The most energy consuming and complicated from a technical point of view is the drying step [1]. Supercritical drying allows keeping the internal structure by preventing collapse of pores. Supercritical conditions require the use of expensive highpressure apparatus and the process rate is limited by diffusion within the pores of the gel. To reduce drying time and increase the efficiency of the process theoretical studies and simulations using high-precision methods are required. The supercritical drying process can be divided into several stages: loading and sealing of the apparatus, pressurization, solvent displacement of the free volume of the apparatus and diffusional exchange of the solvent within a porous media (gel). During pressurization two-component heterogeneous system appears in apparatus. Its physical and chemical properties, composition are strongly dependent on the initial apparatus loading, process pressure and temperature. A theoretical study of phase equilibria in heterogeneous system "solvent - carbon dioxide" was conducted. Various alcohols (methanol, ethanol, isopropanol), acetone and hexane was used as a solvent. The results of this study allowed to choose the most appropriate solvent and the effective parameters of the supercritical drying process depending on the type of dried samples. The obtained data were used to calculate the equations of mathematical model of supercritical drying [2] in apparatus of different geometry and different loading. The theoretical dependence between the process time and the apparatus loading at different temperature and pressure was shown. After analysis of the results the effectiveness criterion was proposed. Criterion can be used to reduce the economic costs of aerogels production.
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