Stripped gas liquor (SGL) as process cooling water: analysis of the effects of operational conditions on microbial community dynamics, fouling, scaling and corrosion

Abstract

More than 30 million tons per annum of coal are gasified at the various Sasol plants in South Africa via the Fischer-Tropsch process. This process results in the production of synthesis gas and a condensate stream (van Dyk et al., 2006; van Nierop et al., 2000). After gravimetric separation of the tar products, the solvent extraction of phenolics and the steam stripping of ammonia from the condensate stream, a complex effluent referred to as stripped gas liquor (SGL) is generated. Due to the current emphasis on the reduction of raw water intake and zero effluent discharge, this SGL has the potential to be used as process cooling water (van Dyk et al., 2006). The objective of this study was to determine the suitability of using SGL as cooling water and to identify optimal operational conditions. Microorganisms (fouling), scale deposition and corrosion are three major problems associated with the operation of industrial cooling water systems. Parameters that have an influence on the fouling, scaling and corrosion of industrial cooling towers include cycles of concentration (COC), linear flow velocity (FV) and pH. The aim of this study was to evaluate the effects of COC, pH and FV on the rates of fouling, scaling and corrosion as well as on the microbial community dynamics in cooling towers using SGL as process cooling water. Effects of pH, FV and COC on the rates of fouling, scaling and corrosion as well as microbial community dynamics determined using PLFA and DGGE analyses were evaluated with the aid of a 23 multi-factorial experimental design. Results obtained indicated that the cycles of concentration had the most profound effect on the fouling, scaling and corrosion rates followed by pH. It was demonstrated that cycles of concentration and pH had the greatest effect on microbial community dynamics. The use of the 23 multi-factorial experimental design also allowed the determination of optimal conditions for the operation of the cooling towers using SGL as feed water. These conditions were 5 COC, pH of 7.5 and a FV of 0.6 m/s. In conclusion, the use of a 23 factorial experimental design, physico-chemical, PLFA and DGGE analyses were instrumental in the successful determination of the effects of COC, pH and FV on the rates of fouling, scaling and corrosion as well as on the microbial community dynamics in the cooling towers using SGL as process cooling water