Novel methods for the removal of chlorine dioxide gas from aqueous solution and sodium chlorite production

Abstract

Chlorine dioxide (ClO2) is a key chemical synthesised on-site to combat bacterial contamination in water. Current technology uses two or more liquid solutions brought together in a reactor followed by a separate stripper unit, using large volumes of air to remove ClO2 as soon as it is formed. However, a business need required by Scotmas Ltd, identified a role for a novel reactor/stripper unit to open potential markets in the Middle East and Asia. The reactor had to be capable of producing 1,000 tpy of 31% w/w sodium chlorite solute, a precursor to the ClO2 production stage. A range of traditional mass transfer technologies were used to investigate the potential for developing a process to produce the required sodium chlorite solution. Novel technology based on a cyclonic gas/liquid contactor was also tried and a small test unit built. Initial work with the cyclonic contactor involved development of suitable test equipment, hydrostatic testing and then subsequent application of ClO2 to determine efficiencies. ClO2 production efficiency was found to be in the region of 80% on a molar basis from the sodium chlorate, hydrogen peroxide and sulphuric acid chemistry deployed. However, instability of the liquid reactant flow with the necessary high gas (air) rate to avoid decomposition of the ClO2 gas within the unit quickly rendered the unit obsolete. This led to a study of the potential pathways which cause rapid decomposition and therefore to avoid future issues in ClO2 production. As an alternative, a second approach was developed based on a novel combined ClO2 reactor/eductor system, integrated into a traditional stripper column. A detailed design based on Cornell’s Method was deployed and a fully integrated process design completed. Due to the nature of ClO2 and the chemical precursors, significant attention was paid to process and operational safety. Finally, the proposed efficiencies for the overall sodium chlorite production plant are detailed and examined in full, with respect to the ClO2 gas duty.