Scale Formation and Mitigation of Mixed Salts in Horizontal Tube Falling Film Evaporators for Seawater Desalination

Abstract

Scale formation on heat transfer surfaces is one of the most severe problems in multiple-effect distillers for seawater desalination. A horizontal tube falling film evaporator in pilot-plant scale was used to study crystallization fouling under conditions close to those prevailing in industrial multiple-effect distillers. Experiments were performed with artificial seawater and model solutions based on artificial seawater under various process conditions. In experiments with artificial seawater, the surface of copper–nickel 90/10 tubes was covered with a two-layer scale comprising a thin, flaky magnesium-rich and calcium-free base layer underneath a thick layer of calcium carbonate crystals in the form of aragonite. Analyses indicated Mg(OH)2 (brucite) and iowaite in the thin base layer. The magnesium-rich scale layer was formed even at a low evaporation temperature of 50°C, which promotes the assumption of locally high pH values at the metal-solution interface. A shift of pH to high values in the thin seawater film due to CO2 release and, additionally, cathodic reactions may promote a high degree of supersaturation of Mg(OH)2. Once the tube surface is completely covered with the thin Mg-rich scale layer, it seems that the growth of the Mg-rich layer ceases and aragonite crystals start to precipitate. A decrease in the Mg2+ ion concentration in the solution results in an increase in the mass of calcium carbonate as aragonite in the scale layer. Results suggest that Mg2+ ions retard the calcium carbonate crystallization.