Mass transport model for the removal of para-chlorophenol by column adsorption using activated carbon

This project aims at purifying para-chlorophenol contaminated industrial effluents by column adsorption using Chemviron Filtrasorb 400 (F400) activated carbon. Two column adsorbers, namely, a constant diameter column (0° tapered) and a tapered column (5° tapered), have been operated under same conditions in the adsorption study. These two columns are operated with the same volumetric flowrate and activated carbon loading, but different values of initial concentration of para-chlorophenol. The para-chlorophenol concentration time curve (breakthrough curve) from each column run has been analyzed, and hence the characteristic of each column adsorber with different configuration has been evaluated. To predict the breakthrough curves for the para-chlorophenol/F400 carbon system, the Homogeneous Surface Diffusion Model (HSDM), which includes the effect of the external mass transfer and the effect of the intraparticle mass transfer by the homogenous surface diffusion, is used. The SIPS equilibrium isotherm model has been incorporated with this model to describe the adsorption equilibrium at the interface of adsorbent solid and solution. This HSDM applies the Crank-Nicholson implicit finite difference method which was proposed by Mathews and Weber[1] for solving diffusion equations. In literature, it has hitherto been applied to fixed-bed adsorption systems on activated carbon [2-5]. The HSDM is modified to describe the dynamic adsorption process in tapered fixed bed column adsorbers. Due to the unique configuration of the tapered columns, the interstitial velocity of adsorbate flow is changing along the column, and hence gives a variable external mass transfer coefficient (k_f), which has been used to represent the laminar boundary layer surrounding the F400 carbon particle at different bed heights. The surface diffusion coefficient (D_s) is assumed to be independent of the surface coverage of the F400 carbon solids in this project. A constant surface diffusion coefficient is used to represent the migration of para-chlorophenol molecules on the surface or pore walls of the F400 activated carbons. From the equilibrium time test, the para-chlorophenol/F400 carbon system has been shown to attain equilibrium after 20 days under constant agitation. The adsorption pilot plant column study, however, is a dynamic process, in which the F400 activated carbon did not possess a full adsorbing capacity in the duration of the column run. As a result, the bed utilization rate from the concentration time curve (breakthrough curve) is calculated to reflect the effective bed adsorption capacity towards para-chlorophenol in this dynamic process. With the introduction of this factor, the HSDM could provide a good fit to the experimental data. This new modified HSDM could be applied successfully to evaluate the dynamic adsorption behavior of para-ehlorophenol/ F400 activated carbons in two different kinds of columns - constant diameter columns and tapered columns with different inclinations.