EXERGY AND THERMO-ECONOMIC ANALYSIS OF MODIFIED MSF DESALINATION PLANTS CONFIGURATIONS
This paper presents the exergy and thermo-economic analysis of a Modified MSF thermal Desalination Plant Configurations. The modified configuration is based on the filed patent of Fath, [1] for MSF with inter-stage brine re-heater(s). The brine reheaters, placed at different inter-stages, are introduced to increase the unit flashing range. The Visual Desalination and Simulation (VDS) package, developed by authors [2], is used as a tool of analysis. The results are compared with the basic 5000 m3/day MSF configuration of Eoun Mousa (Egypt), [3]. An MSF process flow-sheet is developed of basic MSF desalination plant with interstage re-heater(s). The results showed that the use of reheating in the MSF desalination configuration reduces the unit water cost by 5.8 % and increases the distillate product by 3.7 % than that of the conventional MSF desalination plant. Using two re-heaters in the MSF flow-sheet cause a reduction in the unit cost of 6 % and increase in the distillate product of 5.2 % at the expense, of the performance ratio. Using re-heater in by pass of cooling stream (MSF_RH_CS) configuration shows 12.5 % increase in the distillate water product over that of the conventional MSF however, the performance ratio decreased. As a result, the water cost becomes higher than the conventional MSF by 3 %. The use of the re-heater in the MSF_M flow-sheet reduces the unit water cost by 9 % than that of the conventional MSF. The distillate product of MSF_M_RH is 4 % higher than that of the conventional MSF. Also the salt concentration is lowered by 20 % for the same brine recirculation.
Energy consumption and costs with a multi-stage flashing (MSF) desalting system
Energy consumption and costs are among the main parameters affecting the choice of desalting systems and the final unit cost of the desalted water. In addition to the performance of the plant, the cost of the consumed energy depends on the quality of energy, i.e., either mechanical or thermal and, if thermal, at what temperature. The cost also depends on the way this energy is applied (e.g., thermal energy can be directly applied from a boiler or from cogeneration steam turbines or from heat recovery steam generators of gas turbines). The use of the concept of availability (exergy) to bring different kinds of energy to a common basis of comparison is presented here. The way energy applied was considered in comparing the energy consumption of different desalting systems, namely, multi-stage flash (MSF), conventional multi-effect boiling (MEB), thermovapor compression (TVC), mechanical vapor compression (MVC), and reverse osmosis (RO) desalting systems. A rational basis for costing thermal and electrical energy was used to compare the energy costs of different types of desalting systems.
Analysis of mechanical vapour compression desalination process
Characteristics of the single-effect mechanical vapour-compression (MVC) are analysed as a function of the system design and operating parameters. The analysis focuses on prediction of the specific power consumption for the vapour compressor and the specific heat transfer areas for the evaporator/condenser unit and the plate-type feed preheaters. The mathematical model includes material and energy balance equations as well as a set of correlations for evaluation of the thermophysical properties and the heat transfer coefficients. The correlations are dependent on the transport properties of the flowing streams, i.e. temperature, concentration, and velocity. In addition, the model considers the effects of the boiling point elevation and the pressure losses in the demister. Results show decrease in the specific power consumption and the specific heat transfer area of the evaporator/condenser at higher top brine temperatures. On the other hand, the specific power consumption decreases at low-temperature differences between the boiling brine and steam condensate, while the specific heat transfer area increases. Model predictions are found consistent with the available industrial data. Finally, comparison of the performance of the MVC system and the single-effect thermal vapour compression (TVC) is made as a function of design and operating parameters. Although, the results show comparable performance, choice between the two systems should take into consideration other operational parameters. Copyright © 1999 John Wiley & Sons, Ltd.