Master'sOpen AccessEN
Silika alkojelin süperkritik karbon dioksit ile kurutulmasının matematiksel modellenmesi
Supercritical extraction is an important and the most time consuming step in aerogel production. An understanding of the drying process is crucial to provide information about scale-up and optimization of the process. As a result, drying times of the wet gels can be shortened and thus, more economically feasible aerogel production can be achieved. In this study, mathematical models for supercritical drying have been developed to predict drying times of alcogels. The models based on Fick's Law with a concentration-dependent diffusion coefficient were used to simulate the supercritical drying of the wet gels using a parallel pore model in which all pores of various dimensions are positioned in parallel. The model equations were developed for two types of supercritical drying: batch supercritical drying and continuous supercritical drying. In the first part, extraction of solvent fully occupying the pores of spherical, cylindrical and rectangular alcogels into supercritical CO2 (sc-CO2) in the batch system was studied. The model equations were one-dimensional and unsteady state. In the second part, model equations for supercritical extraction of the cylindrical gel, with length much longer than its thickness (1.19 cm in diameterx15.92cm in length), in continuous sc-CO2 flow were developed. Two dimensional and unsteady state model equation governing the mass transfer inside of the pores of the wet gel was developed. Additionally, a model equation for variation of ethanol concentration in the bulk flow as a function of time and position was also derived. The coupled partial differential equations that govern the drying of the wet gel in continuous supercritical drying were solved using numerical techniques. In this study, supercritical extraction of the gel (1.19 cm in diameter x15.92cm in length) was also performed. Mass of ethanol collected during the supercritical extraction of the wet gel prepared in the laboratory was compared to model results. It was found that the present model proposed for the continuous supercritical dying of the wet gels with sc-CO2 for replacement of ethanol from the porous structure of the gel fits sufficiently well with the experimental data. Furthermore, the mass transfer coefficient regressed from experimental data was compared with the correlations proposed in the literature for supercritical extraction for the operation conditions (1.5 L sc-CO2/min volumetric flow rate and 313.15 K and 100 bar) at which supercritical extraction was executed. The predicted and calculated values were found to be close each other. The governing model equations for supercritical drying with carbon dioxide were solved using the finite difference method and MATLAB program. The effect of operation variables, including temperature, mass transfer coefficient, gel porosity and tortuosity, as well as thickness of the gels on drying time in the batch system were investigated using simulations. It was found that mass transfer coefficient and gel dimensions are the most predominant parameters which affect the extraction time. Time for the diffusion of sc-CO2 notably decreased with increasing mass transfer coefficient and increased with increasing thickness.
