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.
Multi-stage flash desalination: present and future outlook
Abstract Multi-stage flash desalination (MSF) is currently the workhorse of the desalination industry with a market share close to 60% of the total world production capacity. As the turning point of the new millennium nears, the process faces many challenges dictated by industrial demands and public needs. The conservative nature of the desalination owner, as well as the strategic characteristics of the product, makes the MSF process favored over other competitive thermal desalination methods. In addition, the process has several merits, which include a large production capacity, proven reliability and well-developed construction and operation experience. This study offers an overview of the present and future developments in the MSF process, which aims to reduce the production cost. Special attention is given to the process fundamentals, which are the key elements for any serious and physically sound development of the MSF process. Also, a summary of the novel (MSF-M) configuration is given, which has recently been proposed by the authors. The process is based on the modification of operational MSF plants as well as the concept of once-through MSF. The modification involves removal of the heat rejection section and the addition of a mixing tank for the feed stream and the unevaporated brine recycle. This eliminates the amount of energy rejected in the cooling seawater stream and reduces the amount of energy rejected in the brine blowdown stream. Analysis of the MSF-M process shows an increase in the thermal performance ratio by a factor of 2–3 over conventional MSF.
Solving equations simulating the steady-state behavior of the multi-stage flash desalination process
The paper presents an algorithm for solving the equations simulating the steady-state behavior of the multi-stage flash (MSF) desalination process. The model can be used either for designing new plants or anlysis and optimization of existing units. The set of equations relating the large number of design and operating variables for any stage (i-th stage) in MSF plants is broken down into three subsets. The first subset contains equations describing processes in the tubes of the preheater inside the flashing chamber. The second subset deals with the processes inside the flashing chamber. These two subsets of equations are subsequently solved iteratively involving only a single variable for each subset. These variables are: (1) saturation temperature Tvi of the flashed-off vapor and (2) temperature of the unevaporated brine flowing from the chamber. The third subset of equations considers the existing interactions between the first two subsets. This subset defines the mass of vapor formed by flashing Di in the i-th stage. All equations in the above-mentioned subsets are solved by a reliable and efficient one-dimensional fixed-point iteration. The main advantages of this method are less sensitivity to initial guesses, fewer iterations to obtain the required solution, and no need for calculating derivatives. The algorithm is implemented using the computer-aided design (CAD) interative package L-A-S (Linear Algebra and Systems). Detailed results are presented to show the dependence of the important factors controlling the fresh water cost, which are plant performance ratio, specific heat transfer area, specific brine flow rate, and specific cooling water flow rate, on the most significant two design variables, namely the total number of stages and the top brine temperature. The predicted data from the model are compared with published data of a typical MSF plant in operation at Kuwait. The agreement was found to be very good.