Influence of current density and inlet gas flow in the treatment of gaseous streams polluted with benzene by electro-absorption

Abstract

This paper is focused on the evaluation of the electrochemically assisted absorption process (electro-absorption) as a treatment for volatile organic compounds contained in polluted air flows. Benzene degradation was used to test the technology, using a system consisting of a packed absorption column and a single compartment electrochemical cell equipped with boron-doped diamond (BDD) as anode and steel-steal as cathode. The influence of inlet gas flow rate and operation current intensity was evaluated by monitoring the concentration of benzene and intermediates both, in the liquid absorbent / electrolyte and in the outlet gaseous stream. Results show that steady-state values reached, during benzene absorption and electro-absorption, increase when the inlet gas flow rises from 1.5 to 6.0 L h−1 and that the bare absorption process retains benzene only up to this steady-state value of the liquid and then, it has no effect on the removal of benzene. Concentrations of benzene in the absorbent/electrolyte decreases with the increase in the current density during electrochemically assisted processes. As well, mineralization was found to be very important, as expected because of the very low concentrations of benzene into the liquid and the very high efficiencies of the electrolytic degradation with BDD electrodes, and benzoquinone and carboxylic acids were identified as the primary intermediates. These species were not detected in the outlet gas flow. The decrease in the concentration and mass flow of benzene in the outlet streams confirms that electro-absorption is a functional electrochemical application to remove benzene from gaseous streams. When current density rises (10, 50 and 100 mA cm−2), the degradation percentage increases. The values obtained were 43, 61 and 75% with 1,5 L h−1 of inlet gas flow and 10, 25 and 37 % with 6 L h−1. However, the degradation percentage relative to the energy consumption decreases when the current density rises. Results were, correspondingly, 24.7, 4.2 and 2.2 % kWh−1 with 1,5 L h−1 of inlet gas flow and 7.1, 2.1 and 1.4 % with 6 L h−1. This energy efficiency trend was explained because mass transport becomes the bottleneck in the oxidation of diluted organic solutions. These results contribute to expand the understanding of the mechanisms involved in the electro-absorption of benzene and recommend for further research in the topic, due to the relevance of the treatment results.