The multiple effect evaporation system is formed a sequence of single effect evaporators, where the vapor formed in one effect is used in the next effect. The vapor reuse in the multiple effect system allows reduction of the brine and the temperature to low values and prevent rejection of large amount of energy to the surrounding, which was the main drawback of the single effect system. In addition to the desalination industry, the main bulk of the multiple effect evaporation processes is found in the food, pulp and paper, petroleum, and petrochemical industries. As discussed in chapter 1 the origins of the multiple effect evaporation dates back to the 19*^ century with the growth of the sugar industry, where it was necessary to devise an efficient evaporation process to produce good quality sugar crystal at low prices.
Although, the first desalination plants were of the evaporation type their use was not expanded to full industrial scale because of limited design and operating experience. Such systems were plagued with excessive fouling, scaling, and corrosion. However, accumulated experiences during the 2^^ half of the past century in thermal desalination processes, headed by the MSF process, have resulted in rapid progress and development of efficient and inexpensive chemical treatment for reduction and prevention of fouling, scaling, and corrosion. Such advances made it possible to maintain plant factors as high as 90% and to keep plants on-line for more than 2 years of operation. As a result, recent research, development, pilot plant operation, and field results show superior performance and the many attractive features of the multiple effect evaporation in comparison with the predominant MSF process.
The multiple effect evaporation process can be configured in forward, backward, or parallel feed. Fig. 1. The three configurations differ in the flow directions of the heating-steam and the evaporating brine. Selection among the three configurations relies on variation in the salt solubility as a function of the top brine temperature and the maximum brine concentration. At higher temperatures or higher brine concentrations, scale formation takes place inside and outside the tube surfaces. This results in the following: