Simulation of multistage flash desalination process

Abstract The majority of large-scale desalination plants in the Arab Gulf area employ the multistage flash (MSF) processes. MSF processes are energy intensive and it is, therefore, essential to search for design and operating conditions which lead to reduction of energy dissipation and consequently lower water production cost. This paper reports a simulation study which was carried out to examine to what extent the thermal irreversibility of an MSF process is influenced by variation of the most important design and operating parameters. The impact of variation of top brine temperature, number of stages and terminal temperature approach on the distiller performance ratio and irreversibility were explored and presented graphically in the thermal grids. The simulation study revealed that within the selected range of number of stages (16–40) and top brine temperature (90–120°C), the MSF distiller irreversibility is highly influenced by the number of stages and to a lesser extent by the variation of the top brine temperature. The simulated results are compared with one year operating data obtained from an MSF distiller. The distiller is a cross-tube configuration, which consists of 22 stages and operating at a top brine temperature ranging between 90 to 98°C. Using a steady state simulation program, an envelop of possible operating conditions is constructed for the distiller to interrelate performance ratio, production, recycle flow and top brine temperature. Stage-wise simulation of individual heat transfer coefficients and fouling factors showed that both clean overall heat transfer coefficients (Uc) and fouling factors (FF) are stage dependent and conversely the operating overall heat transfer coefficient (UD) is to a great extent less dependent. The dependence of the distiller irreversibility on the process conditions is reported. The distiller energy losses varied between 52 and 62 kJ/kg of distillate. Process details, which are responsible for distiller irreversibility are pinpointed and opportunities for better utilization of available energy are discussed.