Scale Formation of Mixed Salts in Multiple-Effect Distillers

Abstract

In multiple-effect distillers (MEDs) with horizontal tube falling film evaporators, difficult-to-control scale is formed on the outside of the tubes, causing considerable capital, operating, and maintenance costs. Because seawater is a multicomponent electrolyte solution, scale formation is primarily caused by coprecipitation of inorganic salts, such as calcium carbonate, magnesium hydroxide, and calcium sulphate, which are insoluble at elevated temperatures. Therefore, solubility data and rate constants obtained under single-salt precipitation are not applicable. The presence of another precipitating salt in the solution affects the precipitation thermodynamics and kinetics, as well as scale structure and strength. Because the scaling process is complex, research primarily has been restricted to single-salt precipitation. A horizontal tube falling film evaporator in pilot-plant scale was used to study scale formation under conditions approximating industrial MEDs at an evaporation temperature of 75 °C, exceeding the top brine temperatures currently used in MEDs. To investigate the interactive effect of mixed salts, experiments were conducted with artificial seawater and model-solutions based on artificial seawater with varying calcium or magnesium concentrations. Scale on the outside of horizontal tubes was analyzed with scanning electron microscopy, energy-dispersive X-ray spectroscopy, wide-angle X-ray diffraction, and atomic absorption spectroscopy to obtain structural, chemical, and quantitative information.