Scale formation and control in thermal desalination systems

Water is the most important liquid in the world to maintain human, plant, and animal life. Moreover, high population growth, rapid urbanization, phenomenal industrial growth, and agricultural development make water one of the most precious resources in the world. Of the entire globe’s water, 94% is salt water present in the oceans and 6% is freshwater. Of the latter, about 27% is in glaciers and 72% is underground. While this water is important for transportation and fisheries, it is too salty to sustain human life or farming [1]. Besides salinity, other impurities in water come from many sources. It is important to understand the role of these impurities in desalination processes. Water may be purified by a number of desalination techniques in which the dissolved impurities are removed from water or pure water is removed from the impurities. The desalination process to be employed for given saline water depends on its impurities, level of salinity, colloidal matter, biological content, and its economics. Various desalination processes that are commonly in use are as follows: Thermal desalination-multi-stage-flash distillation, multi-effect distillation, solar evaporation or distillation, crystallization, or freeze distillation • Membrane-reverse osmosis, electrodialysis, membrane distillation • Ion exchange The economics of desalination process suggest that the more pure water that can be recovered from a stream, the higher the efficiency of the process. The recovery of pure water from saline water results in the increased concentration of brine, thus increasing the potential for fouling due to the precipitation of scale-forming salts/coagulation and the deposition of colloidal matter from brine. The fouling of heat exchangers and RO membranes is a complex phenomenon involving the deposition of several different types of foulants on the surfaces. The development of such deposits on heat exchanger surfaces in distillation plants leads to inferior thermal performance, decreased production, unscheduled shutdowns, poor product quality, and premature heat exchanger failure [2]. Figure 14.1 shows the fouling of a heat exchanger in a multistage flash desalination plant.