Studies of scale formation and optimization of antiscalant dosing in multi-effect thermal desalination units

Scaling of heat transfer surfaces is of great concern for the performance of the thermal desalination process working on seawater, whether it is for multistage flash or multi-effect evaporation. Lot of work has already been carried out for the development of antiscalants for inhibiting/minimizing the scaling. The normal concept of scale formation is due to inverse solubility of hardness salts in the seawater with temperature. Apart from temperature, reasons for scales differ by some factors in case of multi-effect desalination units than that of multi-flash units as experienced in operation of six multi-effect desalination units. As such for taking care of such additional scaling factors, dosing rates of the same antiscalant product is almost double or sometimes even more, in multi-effect evaporators than that in multistage flash units. The purpose of presentation of this paper is to identify these additional factors responsible for scale formation in multi-effect desalination units, other than temperature, with illustrations wherever possible and case study of optimization of dosing rate of antiscalant in 5 million gallon per day capacity multi-effect thermal desalination unit at one of the thermal desalination plants of Federal Electricity and Water, U.A.E. The paper is concluded with the general suggestions for minimizing scales in multi-effect thermal desalination units.