ADSORPTION RATE OF METHANE AND CARBON-DIOXIDE ON ACTIVATED CARBON BY THE SEMIBATCH CONSTANT MOLAR FLOW-RATE METHOD

The adsorption of methane and carbon dioxide on activated carbon pelle ts have been studied by a semi-batch constant molar flow rate method, which was first proposed by Do (1995). In this method, a very low and constant flow of adsorbate was introduced into a pre-evacuated adsorpt ion cell. The pressure of the cell was monitored as a function of time and then analysed to extract dynamic parameters such as D-app (the ap parent diffusivity if the diffusion process is the transport mechanism ) and k(d) (rate constant for desorption if the Langmuir kinetics cont rols the uptake). For the range of pressure of 0 to 5 Torr and the ran ge of molar flow rate used of 2 x 10(-9) to 12 x 10(-9) gmol/s, the eq uilibrium isotherms for both methane and carbon dioxide are linear and no heat effect was observed, supporting the isothermal analysis of li near systems. The analysis of the experimental data of different parti cle size suggests that the diffusion process, rather than the Langmuir kinetics, is the controlling mass transfer mechanism, and the dual di ffusion (pore and surface) mechanism adequately explains the adsorptio n rate of both methane and carbon dioxide. The surface diffusivities a t 293, 303, and 323 K for methane are 1.3 x 10(-4), 1.5 x 10(-4) and 2 x 10-4 cm(2)/s, respectively, and those for carbon dioxide are 1.9 x 10(-5), 2.3 x 10(-5) and 3.2 x 10(-5) cm(-2)/s, respectively. The acti vation energies for surface diffusion are found to be half of the heat of adsorption at zero loading. The proposed technique has been proven to be a reasonably quick and reliable method in determining diffusivi ties of gases in porous materials such as activated carbon. (C) 1998 E lsevier Science Ltd. All rights reserved.