The use of stripped gas liquor (SGL) as process cooling water : analysis of the effects of operational conditions on microbial community dynamics, fouling, scaling and corrosion
Gubuza, Dumisani Mzwandile
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Sasol plants gasify more than 30 million tons per annum of coal via the Fischer-Tropsch process resulting in the production of synthesis gas and a condensate stream. 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. SGL has the potential to be used as process cooling water. Micro-organisms, scale deposit and corrosion are three major problems associated with the operation of cooling water systems. Parameters that have an influence on the fouling, scaling and corrosion of industrial cooling towers include cycles of concentration (COC), 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. Stripped gas liquor contains hydrocarbons which can interfere with PLFA analysis. There was, therefore, a need to optimise the selective extraction of hydrocarbons so as to eliminate their interference with PLFA analysis. The method used for the extraction of hydrocarbons before silicic acid fractionation succeeded in removing the hydrocarbons and their interference and was, therefore, determined to be appropriate method to use in subsequent analyses. Even though the two lab-scale cooling towers were of identical design their similarity or dissimilarity needed investigation. Physico-chemical analysis results confirmed that the two cooling towers were operated under identical operational conditions. The two cooling towers were found to be similar in terms of the fouling, scaling and corrosion rates as well as microbial community structure. The two cooling towers could be operated in parallel using different operational conditions with any differences in results being a reflection of the effects of operational conditions. The effects of pH, FV and COC on the rates of fouling, scaling and corrosion as well as microbial community dynamics were evaluated with the aid of a 2³ multi-factorial experimental design. Cycles of concentration was found to have the most profound effect on the fouling, scaling and corrosion rates followed by pH. Cycles of concentration and pH were found to have the most profound 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 SGE as feed water. These conditions were found to be 5 COC, a 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.