REGENERATION AND REUSE OF SPENT ACTIVATED CARBONS.

SOURCE:

Faculty: Engineering
Department: Chemical Engineering

CONTRIBUTORS:

Ejikeme, M. E.
Igbokwe, P. K.

ABSTRACT:

This research studied regeneration and reuse of spent activated carbons. The regeneration of spent carbons was done using chemical, microwave and conventional regeneration methods. The three different regeneration methods were compared based on the carbon loss (%), regeneration efficiency (%), and carbon yield (%). The activated carbons used were produced from hamburger seed shell (Mucuna slonei) and brewers’ spent grain via conventional and microwave heating techniques. Effect of microwave activation variables (power levels, irradiation time and KOH concentration) and conventional activation variables (time, temperature and KOH concentration) on the carbon yield and removal efficiency was studied. Microwave and conventional activation process were optimized with Central composite design (CCD). The Swiss blue dye loaded activated carbons were desorbed with 0.1M Hydrochloric acid, 0.1M Sodium hydroxide, chloroform, n-hexane and ethyl acetate. The effects of initial solid phase concentration, dosage, time and temperature on the desorption capacity were studied using chloroform. Desorption kinetic data were fitted to six models, while the isotherm data were fitted to eight model. Thermodynamics parameters, enthalpy (ΔH), entropy (ΔS), and Gibbs’ free energy (ΔG) were evaluated for the desorption processes. Effects of microwave regeneration conditions (power levels and radiation time), and conventional regeneration conditions (time and temperature) on the adsorption capacity of the regenerated activated carbons were studied with and without an oxidant (30%H2O2) for one cycle. Optimization of desorption process, microwave and conventional regeneration processes were done with RSM. Adsorption capacities and regeneration efficiencies of the regenerated activated carbons were studied for six cycles with an oxidant for each of the microwave, conventional, and chemical regeneration methods respectively. It was observed that the microwave activation technique had highest adsorptive removal efficiency with the highest carbon yield. The desorption kinetics data were found to fit pseudo second order model adequately. Freundlich model was found to fit the isotherm data properly. The negative values of Gibbs free energy and entropy confirmed the process as spontaneous and feasible. The positive value of enthalpy for the desorption process confirmed it as endothermic in nature. Quadratic models were each developed and validated for microwave and conventional activation, desorption, microwave regeneration and conventional regeneration processes. It was observed that chloroform had highest desorption efficiency and the presence of an oxidant enhanced the adsorptive capacities of the regenerated activated carbons. Regeneration efficiencies decreased with increase in the number of cycles for all the methods of regeneration. Comparative studies done using the three methods of regeneration after six cycles showed that microwave regeneration was the best with 98.71, 98.05, 97.94, 97.3 regeneration efficiencies (%), 0.76, 1.02, 1.98, 2.88 carbon loss (%), 88.23, 88.10, 87.62, 87.39 carbon yield (%), followed by conventional regeneration with 75.31, 74.62, 73.36, 70.18 regeneration efficiencies (%), 5.0, 7.7, 8.76, 11.08 carbon loss (%), 84.46, 82.17, 81.54, 80.01 carbon yield (%), and lastly was chemical regeneration with 45.08, 43.31, 42.0, 40.20 regeneration efficiencies (%), no carbon loss, 94.35, 95.87, 96.0, 96.51 carbon yield (%) for MPAHS, MPASG, CPASG and CPAHS respectively. It is concluded comparatively that microwave regeneration was the best method. It is recommended that microwave technique should be used in regeneration of spent carbons.