Assessment and Modeling of a Sequential Process for Water Treatment-Adsorption and Batch CWAO Regeneration of Activated Carbon

A new sequential process, AD-OX, for (post)treatment of water polluted by poorly biodegradable organic compounds has been investigated. It is based on hybridizing classical adsorption on a fixed bed of activated carbon (AC) followed by batch wet catalytic oxidation at higher temperature and pressure on the same bed of AC, which is then regenerated in situ. The basic idea is to take advantage of both operations: 1-efficient water purification at room temperature by adsorption, 2-effective concentrated pollutant degradation by batch air oxidation achieving simultaneously AC regeneration. A small fixed bed reactor, 10 mm diameter, 0.18 m long, filled with granular activated carbon and equipped with convenient 3-way valves, may achieve successively the two main steps. Several AD-OX cycles have been performed with a phenol solution to quantify the regeneration of the activated carbon adsorption capacity. Results show that activated carbon is quickly damaged during the first cycles, due to oxidative coupling, but then a quasi-steady state is obtained proving that significant oxidative regeneration has been achieved. A dynamic model of the adsorption step has been first developed, including intraparticle diffusion, liquid solid external mass transfer, and axial dispersion of the liquid phase. It has been applied to simulate the performance of the regenerated activated carbon. Using oxidation kinetics over this aged carbon and its adsorption isotherm, separately determined in an autoclave at reaction temperature, the oxidation step after several cycles has been simulated including the heating period, where desorption and oxidation simultaneously occur. The proposed model conveniently predicts the complex phenol concentration-time profile and gives insight to the hydrodynamic behavior of the recycle reactor and the role of mass transfer resistances.