KINETICS OF ACTIVATED CARBON GASIFICATION AND REGENERATION

The kinetics of activated carbon oxidation and regeneration were studied using a thermal balance and a fluidized bed reactor. In the gasification experiments, the oxidation rate and the chemisorption rate were measured for several powdered activated carbons with surface areas of 500 to 2400 m('2)/g. Activation energies of adsorption and intrinsic adsorption rate constants were obtained from transient adsorption tests at 300 to 400 deg C. The data fitted a second-order Langmuir adsorption isotherm, suggesting that the oxygen molecules dissociated on the carbon surface. In the oxidation runs, the data indicated a clear transition from a high activation energy regime (at low to moderate temperature) to an adsorption control regime. The activation energy and oxidation rate at high temperature (above 628 deg C) correlated well with extrapolation of the adsorption data. The reactivity of the carbons could be correlated with the active surface area from the chemisorption data but not with the total surface area measured from low temperature adsorption of nitrogen.; In the regeneration experiments, the effects of substituted side groups on the pyrolysis and regeneration of eleven phenolic compounds were studied. It was found that the strongly adsorbed aromatic ring of a substituted phenol remained on the carbon surface while the side group and hydrogen atoms were lost in pyrolysis. The amount of residue formed was independent of the number and molecular weight of the side groups, and could be estimated by calculating an aromatic ratio. The residues from alkyl phenols were oxidized at rates up to 12 times that of a moderate surface area base carbon. The increase in rate was due to the formation of active sites. Halogens had no effects on the oxidation rate, but sodium catalysed both the reaction of residue and the base carbon. Oxygenated side groups could cause excessive loss of residue during pyrolysis. The adsorption capacity of spent carbons could be mostly recovered by pyrolysing the adsorbate. The residue could be removed by oxidation, but there was little change in adsorption capacity, and the efficiency of regeneration was less as the amount of carbon recovered decreased. The pyrolysed residue appeared to have a structure and adsorption capacity similar to that of base carbon. Due to pore mouth narrowing of pyrolysed residue, eventually oxidative regeneration was necessary to prevent pore blockage.