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.
Modelling, simulation, optimization and control of multistage flashing (MSF) desalination plants Part I: Modelling and simulation
Abstract Multistage flashing (MSF) processes can be represented by both steady-state and dynamic models. The former is a useful tool for the design, understanding and optimization of existing as well as new plants. Dynamic models are required for solving problems in the transient phase. This, in turn, includes problems such as control strategies, stability assessment, process interactions, trouble shooting, start-up, load changes and shutdown scheduling. This part of the paper offers an assessment of modelling and simulation studies for the MSF plants. Normally an MSF process is a nonlinear recycle process with a closed loop information flow. Solution methods recognising these features must be used. One is the simultaneous approach (equation oriented) and the other is stage by stage (sequential) approach. Advantages and disadvantages of both the solution procedures are discussed, in view of minimizing the computational effort. In general, computer based process simulation can be done either by using a specific (specially developed) program or by applying a general purpose simulation package. For several reasons, a specific program is preferred for the MSF process simulation. An available general simulator, however, can be used to get initial simulation results. In this paper, use of both types of programs is evaluated for the steady-state and dynamic simulations of the MSF plants.