Effect of Gas- and Liquid-Phase Axial Mixing on the Rate of Mass Transfer in a Pilot-Scale Distillation Column Packed with Mellapak 250Y

Abstract

Using the profile method, the vapor- and liquid-side volumetric mass-transfer coefficients (k V a, k L a) were evaluated by comparing the vapor and liquid concentration profiles with those calculated from a rate-based distillation model taking into account axial mixing in both phases. The method was applied for the atmospheric distillation of three different mixtures of primary alcohols: methanol-ethanol, ethanol-propanol, and methanol-propanol on structured packing Mellapak 250Y. The column was operated at total reflux; liquid and vapor samples were withdrawn at seven sampling points distributed regularly along the column. Bodenstein numbers Bo were determined by means of the cold air-water dynamic experiments performed under hydraulic conditions, by which the concentration profiles were measured in the distillation column. The mass-transfer coefficients were compared with those obtained in our previous work (Rejl et al. Ind. Eng. Chem.. Res, 2010, 49, 4383) using the plug flow model for their evaluation. The axial dispersion model reproduces the experimental profiles with a 2-fold decreased relative difference between the experimental and the calculated concentrations in comparison with the plug flow model. The individual transport coefficients are higher (by up to 2 times) and are less dependent on the phase flow rate in comparison with the plug flow model. The correspondence with the mass-transfer correlations designed for the packed columns (Rocha et al. Ind. Eng. Chem. Res. 1996, 35, 1660; Billet and Schultes TransIChemE 1999, 77,498; Olujic et al. Chem. Eng. Process. 1999, 38, 683) is not improved by using the dispersion model: the differences reach multiples of the measured values. The parameters k L a, k V a, Bo L , and Bo V were studied for their relative influences on the separation efficiency of the column. The relative contribution of k V a is predominant (60%), whereas the axial mixing of vapor Bo V has practically no effect. The contribution of k L a constitutes about 25%, and the one of the axial mixing of the liquid Bo L . increases from about 15 to 40% with a decrease in the liquid flow rate at the expense of the contribution of k V a.