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

Multiple-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 of different tube materials, various process parameters and sea water properties on tube wetting. The wetting behaviour of different tube materials such as aluminium brass, copper-nickel 90/10 and aluminium alloy AlMg2.5, which are commonly used in MED plants, was investigated in a horizontal tube falling film evaporator in pilot plant scale. In tests with stepwise increasing and decreasing wetting rate, pictures of the film flow over the horizontal tubes were taken and the percentages of the wet surface area were evaluated by means of an image processing software. The tube surfaces were characterized by measuring contact angles and determining surface free energies. Furthermore, the horizontal tube falling evaporator was used to study the effect of the wetting rate on composition and quantity of mixed salt scale. Experiments were performed with artificial sea water at wetting rates from 0.14 kg/(s m) down to 0.025 kg/(s m) for different tube materials such as aluminium brass, copper-nickel 90/10 and AlMg2.5. New insights into the wetting behaviour of different tube materials and the influence of the wetting rate on scale formation in the horizontal tube falling film evaporators are given. The effects of various process parameters such as wetting rate, temperature and salinity on the wettability of typical tube materials and the effect of the wetting rate on composition and quantity of mixed salt scale are presented and discussed.