Multi-stage absorption of rendering plant odours using sodium hypochlorite and other reagents

Abstract

Conditions for using sodium hypochlorite solution as the main component of a multi-stage absorption system for the treatment of malodorous process emissions were studied, together with the additional reagents needed for effective odour control. In laboratory experiments, mixtures containing vpm levels in air of trimethylamine, hydrogen sulphide, methyl mercaptan, dimethyl disulphide, n-butanal and sometimes ammonia were passed through three bubble-plate columns containing some of the following: water, dilute sulphuric acid, sodium hypochlorite solutions (varying in pH and available chlorine content) sodium hydroxide and sodium hydrogen sulphite. Odour removal was monitored by Chromatographic and sensory methods. Conclusions from laboratory experiments were supported by field-tests at four rendering plants in the U.K., treating both ventilation and process gases. Alkaline hypochlorite with considerable excess available chlorine removes many sulphur-compounds and aldehydes but effective odour control requires an acid pre-wash to prevent the generation of odorous chlorinated compounds from ammonia and amines. Acidic hypochlorite solution followed by sodium hydrogen sulphite (to remove aldehyde) and sodium hydroxide was a most effective combination in both laboratory and field tests. Odour generated in chlorination reactions involving acidic hypochlorite solution was analysed by GC-MS and GC-MPD-odour-port and the odour key compounds identified.