Removal of binary and ternary synergistic mixtures of ozone and VOCs by activated carbon filter: Mathematical modelling

Evaluating the activated carbon filter's performance in removing gaseous pollutants is essential to determining the filter maintenance schedule for indoor environments. This paper describes the development of a mathematical model to predict an activated carbon filter's performance for removing mixtures of ozone and volatile organic compounds (toluene and limonene) at their low-level concentrations in typical indoor environments. Mass transfer equations were employed to estimate the filter's dynamic behaviour by including the reaction rates of ozone with carbon surfaces and the reactive volatile organic compound, as well as the adsorption isotherm for a single volatile organic compound or binary mixtures of volatile organic compounds. The reaction rate constant for the ozone-limonene reaction was calculated by fitting the model to the experiment results of their binary mixture at 9 ppm of limonene and 0.1 ppm of ozone. Additionally, the ozone-exposed filters at 0.1 ppm were used to perform adsorption tests to determine the parameters of the Dubinin-Radushkevich isotherm using experimental results at 9, 30, 50, 70, and 90 ppm of volatile organic compounds. The Dubinin-Radushkevich isotherm was extended using the volume exclusion theory to consider the effect of by-products on the removal modelling of the binary mixture of ozone and limonene and the ternary mixture of ozone, limonene, and toluene. The model was able to show the reduced accessibility of the surface of activated carbon for reaction with ozone because of the adsorption of volatile organic compounds and generated by-products (keto-limonene and carvone). The initial influence is of minor magnitude; however, as the surface load intensifies, its significance can progressively amplify over time. Pre-exposing the filter to ozone prior to conducting adsorption tests was able to capture the average change in the adsorption properties of the filter. These considerations in the modelling result in a remarkable predictive capability in determining the breakthrough behaviour of the filter in removing the binary or ternary mixture of ozone and volatile organic compounds at various concentrations. The insignificant effect of the homogeneous reaction between ozone and limonene for the investigated experimental setup was confirmed by comparing the predictions made by the proposed model with those made by another model that considers the homogeneous reaction. Finally, the sensitivity analysis revealed that the model is more sensitive to adsorption isotherm parameters compared to the reaction kinetic parameter between ozone and limonene.