Andreas Kempter

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

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.

New insights into silica scaling on RO-membranes

Abstract The polymerization of silica in hard water was investigated by using different supersaturation levels of silica at pH 7 and 8. The results indicate a kinetically stable range up to ∼400 ppm [SiO2] under the chosen conditions at pH 7, whereas no such range is present at pH 8. Additionally, polyvinylpyrrolidone and polyethers were tested for their silica inhibition capabilities. It was found that the performance is depending on molecular weight as well as dosage of the polymers, respectively. From these results, a mechanism for inhibition at pH 7 and 8 was suggested.

Wetting behaviour of different tube materials and its influence on scale formation in multiple-effect distillers

AbstractMultiple-effect distillation (MED) plants with horizontal tube falling film evaporators for sea water desalination exhibit relatively high heat transfer coefficients achieved by film flow under clean surface conditions. However, they are susceptible to heat transfer deterioration and scale formation accompanied by film breakdown. Thus, maintaining all heated tubes in a fully wetted state is one of the key issues to be considered in designing falling film evaporators. Surface tension, hydrophilicity, hydrophobicity and wettability of materials play an important role in scale formation. The wetting rate, that is the brine mass flow rate on one tube per unit tube length, is one of the most important parameters in the design and operation of MED plants. It has an impact on heat transfer, tube bundle design, scale formation, the capital and operational costs of the plant and its operational flexibility. Thus, it is very important to acquire a better understanding of the wetting behaviour and the impact...

Phosphate removal from waste water by a seeding approach using novel seeding material

AbstractA seeding concept was successfully applied for water treatment application, in particular to phosphate removal from wastewater. Here, several different seeding materials based on hydroxylapatite and fluoroapatite were synthesized and the P-removal capabilities were tested in a test water system. Besides high chemical stability, the fluoroapatite seeding material successfully and repeatedly removes >95% of phosphorous from the test water. A small pilot test was carried out, showing that also in continuous operation the seeds have high removal capacity of >90%.