Mathematical modelling, design, fabrication and analysis of a detachable shell vertical tube evaporator for novel small scale multi-effect distillation system

Abstract

As the population continues to rise and water resources become scarce, desalination technologies are becoming increasingly essential. Large-scale desalination facilities have grown in popularity in recent years, but the rural population has recognised the necessity for small-scale decentralised desalination plants. Multi-effect desalination systems have a low specific heat consumption, making them ideal for decentralised operation. The task now is to determine system capacity and the best operational range for small-scale applications. The thermo-economic model for small-scale multi-effect desalination systems is developed in this study for various configurations. Best operational range for motive steam pressure and flow rate, along with feedwater mass flow rate is determined by optimising the small-scale plant for a variety of impacts. With small scale production, the emphasized objectives and constraints imposed over the input parameters are total distillate production and freshwater cost. According to the findings, the motive steam flow required for distillate production of 750 litres per day is estimated to be 25 to 35 kg/hr at a pressure range of 2 to 5 bar. This research presented a framework for determining the number of effects to be used based on the commercial aspects of overall production needs. The current study also addresses at the design and performance of a detachable shell vertical tube evaporator with variable feed water temperatures. Simultaneous evaporation and condensation in a vertical tube evaporator while developing a thin film on the exterior surface of the fluted tube results in a significant heat transfer coefficient. The system is quite easy to clean and maintain due to salt accumulation on the exterior surface. A removable exterior cover makes tube maintenance even simpler. The pilot plant was equipped with a baby boiler and a parabolic dish collector capable of producing 30 kg/hr steam. The evaporator used as a single effect with a condenser system, had a mean heat transfer coefficient of 9446 W/m2K at a gain output ratio of 1.80±02, with feed input temperature, motive steam pressure, and temperature in at 100°C, 1.30±028 bar, and 103±0.5°C, respectively. At a fresh water cost of 1.05 $/m3 , the pilot plant with a single effect generates up to 1282 litres per day. With a 4-effect system, a theoretical model for a multi effect system in parallel feed flow configuration estimates the lowest freshwater cost of 0.73 $/m3 and total output of 2668 litre/day Keywords: Multi effect distillation system, small scale, Process parameters, Optimization, Vertical tube evaporator.