Effect of gas strirring on external mass transfer, intraparticle diffusion and energy consumption during adsorption

Abstract

Gas stirring was used to study the adsorption of basic dyes onto montmorillonite clay. This technique had been demonstrated previously as being more efficient than mechanical stirring. The experimental set-up employed was a fixed-bed column connected to an air-supply system with provision for controlling different airflow rates at different temperatures. The dyestuff used was Basic Red (a commercial dye). The rate of dye removal was found to increase with increasing gas flow and increasing temperature. Dye removals of 86% to 55% could be achieved for contact times of 35 min to 50 min using dye concentrations between 200 mg/l and 500 mg/l at a gas flow rate of 100 cm3/s. The calculated Langmuir constants demonstrated that clay is a favourable adsorbent for the basic dye. Compared to powdered activated carbon (PAC , the adsorption capacity of the clay was 41.3% while the relative cost of basic dye removal was 0.024-times that of PAC. The external mass-transfer coefficient (Ks) and the intraparticle diffusion rate parameter (Kp) were calculated and evaluated as functions of gas stirring and temperature. It was found that an increase in the rate of adsorption effected by gas stirring was double that obtained by increasing the temperature due to the high turbulence induced by axial and radial flow created by gas stirring. The activation energy (E) of 5.95 kcal/mol demonstrated that adsorption of the basic dye onto clay was diffusion-controlled. The energy consumption (W) for removing a known percentage of the dye was evaluated and found to correlate with the equation W = 8.56Q1.02X8.63 for 50 < Q < 100 cm3/s, where Q is the gas flow rate and X is the percentage dye removal.