| Summary: | 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. === Thesis (M.Sc. (Microbiology))--North-West University, Potchefstroom Campus, 2008.
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