Thermal performance of seawater desalination systems

This work describes a steady-state mathematical model developed to analyze both the multi-stage and multi-effect desalination systems. For MSF the model accounts for the geometry of the stages, the mechanism of heat transfer, and the variation of the physical properties of seawater with temperature and salinity. In addition, the model takes into consideration the role of fouling and its effect on the plant performance ratio. Relationships among the parameters controlling the product water cost to other operating and design parameters are presented. These parameters include plant performance ratio, specific flow rate of recirculating brine, top brine temperature, and specific heat transfer area. These relationships are used for studying a typical operating plant. The results obtained by the model are compared with data from the Sidi-Krir plant in west Alexandria, which contains 17 recovery and three rejection stages. There is good agreement between the calculated results and the plant data. The model results are also used to study the different operating conditions such as part load; for example, the effect of decreasing the recirculated flow rate and/or the top brine temperature on the unit performance ratio. In addition, the effect of decreasing the seawater temperature below the design value shows an increase in the unit production rate, while increasing the fouling factor reduces the plant production rate. For MED, the effect of the process variables on the performance of a "forward type" multi-effect boiling falling film is carried out. This includes the effect of number of effects and top brine temperature on performance ratio. In addition, the influence of these factors on the specific heat transfer area and the effect of seawater inlet temperature on the performance ratio are investigated. The effect of the top brine temperature and the temperature difference per effect on the performance ratio, the specific heat transfer area, and the cooling brine flow rate are also investigated. The study results indicate that the performance ratio is completely dependent on the number of effects and slightly dependent on the top brine temperature.