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

Abstract

AbstractScale 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. NOMENCLATURED=tube diameter (m)I=ionic strength (mol kg−1)L=tube length (m)mi=molality of ion i (mol kg−1)=mass flow rate (kg s−1)Ra=surface roughness (μm)S=salinity (g kg−1)t=temperature (°C)zi=charge number of ion i (—)Greek SymbolsΓ=wetting rate (kg s−1 m−1)τ=rest period (h)SubscriptsCO=condensationEV=evaporationo=outsideAdditional informationNotes on contributorsKerstin KrömerKerstin Krömer is currently working for a water supplier, Oldenburgisch-Ostfriesischer Wasserverband (OOWV) in Germany, in the area of system engineering. She is planning projects for the supply of drinking and process water. Prior to joining OOWV, she worked as a Ph.D. student on evaporation technology and seawater desalination in the Division of Technical Thermodynamics of the Faculty of Production Engineering at the University of Bremen, Germany. Her doctoral research has been focused on crystallization fouling in thermal seawater desalination. She studied production engineering at the University of Bremen, where she obtained her diploma degree in 2008.Stefan WillStefan Will is a full professor and head of the Institute of Engineering Thermodynamics at the University of Erlangen-Nuremberg, Germany. Before this he was a full professor at the University of Bremen, Germany, and also served as Dean of the Faculty of Production Engineering. After a diploma in physics he received a doctoral degree in chemical engineering at the University of Erlangen-Nuremberg in 1995. His research interests and activities cover a broad range in process and energy engineering including phase-change heat transfer, combustion, particle formation, and the development of optical techniques in these fields.Katell LoiselKatell Loisel holds the position of Global Development Head of Waste Water and Process Water of the Global Business Unit Water Solutions within BASF–The Chemical Company. She leads a broad scope of internal and external projects dedicated to water treatment technologies covering chemical products. Prior to BASF, she worked 11 years in the technical support group of CIBA within the Business Unit Water Treatment, where she took over different positions as Head of the European Technical Center and as New Business Development Manager. She holds a master's degree in chemistry and an engineer diploma in environmental chemical processes.Stephan NiedStephan Nied is currently Head of Technical Marketing within the Water Treatment Chemicals Platform at BASF SE in Ludwigshafen, Germany. Prior to joining the Water Treatment Chemicals Platform in 2012, he was Technical Manager of Water Treatment Chemicals in the business unit Care Chemicals & Formulators and member of the global team Water Treatment & Desalination at BASF SE. He is a state-certified environmental technician and joined BASF in 1983.Jürgen DeteringJürgen Detering studied chemistry at the University of Bochum and obtained his Ph.D. in organic chemistry from the University of Düsseldorf, Germany, in 1988. Shortly afterward, he joined the Polymer Laboratory of BASF SE, where he worked for 6 years in the field of water soluble functional polymers. He then worked for 2 years as a plant manager. Since 1997 he has been with the business unit Home Care & Formulation Technologies, being responsible for the development of detergent additives, dispersingagents, and polymeric antiscalants.Andreas KempterAndreas Kempter is a research scientist at BASF SE, Ludwigshafen, Germany. He received his Ph.D. in inorganic chemistry in 2007 from the Ruhr-University Bochum (RUB), Bochum, Germany. Prior to joining BASF SE, he worked as a postdoctoral research associate at the University of British Columbia in Vancouver, Canada, on nitrogen activation and at the Max-Planck Institute for Kohlenforschung in Mülheim/Ruhr, Germany, on copper catalysts for methanol synthesis. His current research interests are the optimization and development of novel polymeric antiscalants for water treatment applications.Heike GladeHeike Glade is a senior researcher in the Division of Technical Thermodynamics at the University of Bremen, Germany. She is head of the evaporation and desalination research group. Previously she was a junior professor for Heat Transfer and Distillation Techniques at the Martin-Luther-University of Halle-Wittenberg, Germany. She received her Ph.D. in production engineering from the University of Bremen in 2001. Her research interests include evaporation technology, development, analysis and optimization of desalination systems, heat and mass transfer, and crystallization fouling and its mitigation in heat exchangers. Her research work comprises modeling and simulation tasks as well as experimental studies in laboratory and pilot-plant scale.