The kinetics of adsorption of carbon tetrachloride and chloroform from air mixtures by activated carbon

Abstract

The kinetics of gas adsorption were studied in an air flow apparatus using CCl 4 and CHCl 3 as the adsorbate vapors and activated carbon as the adsorbent. Five fractions of uniformly activated carbon granules, in size ranges of 0.130 to 0.036 cm diam, were packed in glass columns to various bed depths and weights, and subjected at several temperatures to a 0.1 relative pressure of the adsorbate vapor at various fixed flow velocities. On plotting the ratio of exit to inlet vapor concentrations against time, sigmoid shaped curves were obtained at all flow rates, carbon weights, and granule diameters. Vapor breakthrough of the bed was taken as the time when 1% of the inlet concentration appeared in the exit flow stream, ( C x C 0) = 0.01 . The Wheeler adsorption equation was used to analyze the experimental data for a fixed temperature at this breakthrough time ( t b ). The adsorption rate constant at this breakthrough time was first order with respect to gas molecules and essentially independent of carbon granule diameters. The values were 4000 min −1 and 7255 min −1 at 25 °C for CCl 4 and CHCl 3, respectively. The sigmoid curve for the complete breakthrough of CCl 4 was found to contain three adsorption rate constants. The pseudo first-order constant was operative when the concentration of active sites was much larger than the concentration of gas molecules ( 0 < C x C 0 < 0.04 ); a second-order rate constant was operative when active sites and concentrations of gas molecules were both affecting the rate ( 0.04 < C x C 0 < 0.65 ); and a pseudo first-order rate constant with respect to active sites when the concentration of gas molecules was much greater than active sites ( 0.65 < C x C 0 < 0.95 ). Arguments are advanced to explain the variation of the rate constants between large and small carbon granules in terms of a combined mechanism which involves internal diffusion and surface adsorption.