Abstract
Scaling is a major problem in cooling water circuits as it reduces water flow and therefore affects the efficiency of the circuit. The natural organic matter has been suggested in earlier studies to limit the formation of calcium carbonate scaling by complexing the calcium ion. It was therefore the aim of this study to characterize the natural organic matter in the cooling water circuits at Lethabo power station (Vereeniging) and to investigate its potential to complex with calcium.
The cooling water and raw water samples were comprehensively analyzed for major metal ions, anions and dissolved organic carbon using AAS, ICP-OES, IC and TOC analyzer and the results entered into MINTEQA2 speciation program to determine the precipitation potential of aragonite and calcite in the water samples. The natural organic matter from the cooling water and raw water were isolated initially through the cross-flow ultrafiltration using a polysulfone membrane with a molecular weight cut-off of 45 kDa. The collected isolates were characterized by ultraviolet-visible spectrophotometer, Fourier transform-infrared spectroscopy and (carbon, hydrogen and nitrogen) elemental analysis. The natural organic matter was also fractionated on ultrafiltration stirred cells using membranes with molecular weight cut-off of 1 kDa, 10 kDa and 100 kDa and then characterized using high performance size-exclusion chromatography. The isolated fractions were also titrated with sodium hydroxide and with calcium chloride to determine the amounts of carboxylic and phenolic groups available for complexation and the extent of the complexation of the fractions with calcium respectively.
The speciation results from MINTEQA2 indicated that the raw water had a potential to be corrosive and the cooling water had a potential to scale. Higher concentrations of the natural organic matter were detected in the permeate from the cross-flow ultrafiltration process which suggested that it had predominantly a low molecular weight fraction.
This low molecular weight fraction was further confirmed by the results from the high performance size-exclusion chromatography analysis of the fractions obtained from the fractionation process using ultrafiltration stirred cells. The specific ultraviolet absorbance and ratios of 465 to 656 nm absorbances results indicated that the organic compounds were mostly aliphatic in character. Titration of the concentrated organic isolates with sodium hydroxide solution revealed that the fractions had a high content of titratable acidic groups and titrating with calcium chloride showed that there is considerable complexation with calcium to affect calcium carbonate precipitation.
Dr. M. Shumane