Modelling of gas–liquid and gas–liquid-solid reactors for water treatment chemicals – From ferrous sulfate to ferric sulfate

Ferric sulfate is an efficient coagulant in water treatment. Ferric sulfate is produced via oxidation of ferrous sulfate. The reaction proceeds spontaneously in the absence of an added catalyst, but the rate can be enhanced by solid catalysts, such as active carbon and metal-doped active carbon. The reaction environment is a complex gas–liquid (GL) or a gas–liquid-solid (GLS) system, with a strong interaction of gas solubility, interfacial mass transfer and kinetic effects. Because of the large volumes in the water treatment, selection of continuous reactor technology is an evident option. The aim of this work was to perform model simulations for continuous stirred tank reactors and tubular reactors by using a multiscale approach, from the kinetics of catalytic surface reactions to transport phenomena and flow pattern. The kinetic equations for non-catalytic and catalytic reactions were extracted from previous studies, as well as the Henry's constant for oxygen solubility. Mass balance equations permitted to obtain the reactor models which were solved numerically. The results showed that the non-catalytic reaction is non-negligible but not sufficient for effective oxidation. Oxygen can be introduced in excess to compensate for the oxygen remaining in gas phase during the process. The tubular reactor concept and the series of two continuous stirred tank reactors (both catalytic or one non-catalytic and one catalytic) showed very satisfactory results, enabling a high conversion of ferrous sulfate to ferric sulfate.