Scale formation and mitigation of mixed salts in horizontal tube falling film evaporators for seawater desalination

CATEGORY: FOULING MITIGATION
Proceedings of International Conference on Heat Exchanger Fouling and Cleaning, June 9-14, 2013, Budapest, Hungary
Scale formation and mitigation of mixed salts in horizontal tube falling film evaporators for seawater desalination
H. Glade (1), K. Kromer (1), S. Will (2), K. Loisel (3), S. Nied (3), J. Detering (3), and A. Kempter (3)
heike.glade@uni-bremen.de
1 Technical Thermodynamics, University of Bremen, Bremen, Germany
2 Institute of Engineering Thermodynamics, Friedrich-Alexander-University Erlangen-Nuremberg, Erlangen, Germany
3 BASF SE, Ludwigshafen, Germany
Abstract
Scale formation on heat transfer surfaces is one of the most severe problems in the design and operation of multiple-effect distillers for seawater desalination.  Because of the complexity of crystallization fouling, research has mainly been restricted to single-salt precipitation, and not much attention has been given to the co-precipitation of calcium- and magnesium-containing salts from falling seawater films on horizontal tubes.
A horizontal tube falling film evaporator 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 CuNi 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 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 resulting in a locally enhanced OH^- concentration may promote a high degree of supersaturation of Mg(OH)₂ to drive its rapid precipitation on the tube surface. 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.
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