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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 (U c ) and fouling factors (FF) are stage dependent and conversely the operating overall heat transfer coefficient (U D ) 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.

Salt carry-over due to excessive foaming in a multistage flash (MSF) distiller and detrimental foaming in the deaerator section have been reported in the Saline Water Conversion Corporation (SWCC) of Saudi Arabia and other desalination plants. The application of a suitable antifoaming agent has prevented distillate contamination and improved release of dissolved oxygen in the deaerator section, thus improving the operating efficiency of MSF plants. The developer of a polyoxypropylene glycol-based antifoaming agent collaborated with SWCC in carrying out trial runs to show that their product was as effective and economical as the one currently being used (a polyglycol blend in a hydrocarbon solvent) in SWCC plants. Trial tests were carried out at laboratory, pilot and commercial plant levels. Miscibility of this agent in water was determined initially in the laboratory, followed by field trial runs in an MSF pilot plant and the commercial plants of Al-Jubail I and II. Foam control, non-interference with antiscalant performance, distillate purity and stability at a low dose rate over a wide range of temperatures were some of the important criteria evaluated during testing of this antifoaming agent. During MSF plant runs, distillate conductivity and dissolved oxygen level in the feed were regularly monitored and found within the acceptable limits. Compatibility of this agent with antiscalant was also assessed. Heat transfer and plant performance data were found satisfactory confirming that this agent did not exhibit any adverse effect on the antiscalant used in Al-Jubail plants. Results of laboratory and field trial runs of this antifoaming agent are discussed.

The Saline Water Conversion Corporation (SWCC) has been actively involved in the investigation of scale formation in order to improve the performance of its distillers. A number of optimization tests have been carried out resulting in successful operation of MSF distillers at low antiscalant dose rates. The additives which were tested include polymaleic, polyphosphonate and polycarboxylates. The present study reports the results of the tests which have been carried out to evaluate the effectiveness of polyphosphonate in controlling scale formation on the heat transfer surfaces at a top brine temperature (TBT) of 90.6°C and a dose rate of 0.8 ppm, which is considered low compared to previous practices. These tests were firstly performed on a 20 t/d MSF pilot plant. The test results at the pilot plant showed stable overall heat transfer coefficients and fouling factors through out the test period, thus, indicated insignificant scaling of the heat transfer surfaces. Based on satisfactory results at the pilot plant, the test was performed in Al-Jubail Phase II at a TBT of 90.6°C and dose rate of 0.8 ppm for a total test duration of 93 days. The overall heat transfer coefficient of the brine heater was almost constant at a value of 2700 W/m 2 K, thus consistently greater than the design value (2258 W/m 2 K). The unit performance ratio and distillate production rate were also fairly close to the design value. Chemical analysis results showed that product water conductivity and loss of total alkalinity of brine blowdown were maintained within acceptable limits. The brine heater and flash chambers were inspected at the end of the test. The brine heater tubes were found clean and also no sign of fresh scaling was noted on the demister pads, which confirmed the capability of polyphosphonate in controlling alkaline scale formation at TBT of 90.6°C and a dose rate of as low as 0.8 ppm.

The Saline Water Conversion Corporation (SWCC) of Saudi Arabia is currently producing around 16% of the total worldwide capacity of desalted water [1]. The majority of SWCC desalination plants employs the multistage flash (MSF) distillation process which produce 94% of SWCC's total desalinated water. SWCC various MSF distillers are characterized by a wide range of operating and design conditions. The capacity of operating distillers ranges between 2.5 to 7.5 migd. Some large distillers of rated capacity up to 10 migd are under construction in Shoibah and Yanbu. The number of stages of these distillers varies between 16 and 34, while the operating top brine temperature varies between 90°C and 115°C. Design and operating parameters of various SWCC MSF distillers have been collected and effectively utilized to simulate and analyze the thermal performance of these distillers. The thermal performance of each distiller is quantitatively assessed using a computer program which is based on the first and the second law of thermodynamics. A comparative study on energy and exergy analyses is conducted for different MSF plants. This paper also presents a comprehensive micro-thermal analysis to identify the potential for improving plant efficiency. The exergy losses due to irreversibility in different subsystems of each MSF distiller are evaluated. Exergy destruction in the brine heater, heat recovery and heat rejection stages and friction in liquid paths are determined. The impact of different design and operating parameters such as top brine temperature and number of stages on the thermal performance of the MSF system is evaluated.