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Precipitation fouling of heat exchangers by magnesium (calcium) silicates Gavril, Marcela

Abstract

Fouling due to deposition of inverse solubility salts in cooling water systems is one of the major problems in chemical process industries. Deposition of calcium silicates as well as magnesium silicates can be a problem; however, literature information on both solubility data and fouling results for silicate containing cooling waters is sparse. In this work the effects of varying pH, concentrations of silicate and liquid velocity on the fouling process were investigated for both calcium and magnesium salts. Experiments were carried out with aqueous solutions containing either calcium or magnesium chloride and sodium silicate re-circulated at a bulk temperature of 25°C through a steam heated exchanger for periods up to 24 hours. The steam temperature was raised from about 106°C during fouling to maintain a constant heat flux. Magnesium (or calcium) was maintained in excess, with silicate concentrations in the range of 33 to 130 ppm as SiO32-. Over the range of conditions tested extent of fouling was low, and results were subject to considerable scatter. In the first set of experiments the velocity, heat flux, and concentration of reactants were kept relatively constant. The pH of the artificial cooling water was varied . For calcium solutions results after 6 hours of operation were scattered, with Rf values of 0.1 m²K/kW at pH 9 and 10.8 and much lower values at pH 11. For magnesium solutions the extent of fouling increased steadily with pH over the range 7-9, over 24 hour runs. Trends with pH are compared with results from previous literature. Experiments were carried out to study the effect of silicate concentration on the fouling process, both at constant magnesium concentration and at constant SiO3/Mg mol ratio. With velocity, pH and heat flux kept relatively constant, the silicate concentration was varied . The extent of fouling was found to increase with the silicate concentration over the range 33 to 130 ppm SiO3, irrespective of the magnesium concentration. The final set of experiments was carried out in order to determine the effect of velocity on the fouling rate. The rate of fouling appeared to go through a maximum with increasing velocity; however, additional experimentation is required to confirm this effect. Partial deposit analyses at pH from 8 to 9 indicated an SiO2/MgO mass ratio of 1.4±0.2, compared to 1.5 expected for pure MgSiO3. Deposits were a sticky yellow-brown solid, often with a mud-like consistency. Results are compared with literature guidelines for pH, magnesium and silicate concentrations to minimize fouling.

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