Performance optimization of a multi stage flash desalination unit with thermal vapor compression using genetic algorithm

Abstract

In the present study, a multi stage flash desalination system with brine recirculation and thermal vapor compression (MSF-BR-TVC) is analyzed. From four different configurations of MSF-BR-TVC systems, the one with two ejectors the receives secondary vapor from two successive middle stages of the heat recovery section is studied in more detail due to its higher performance ratio which is 9.90. In this configuration, the compression ratio for the first and second ejectors are 4.55 and 4.06, respectively. Also, the mixing number for these two ejectors are 2.62 and 2.33. Then, an optimization process based on genetic algorithm is carried out to evaluate the effect of most important parameters on the performance ratio. It is observed that optimum performance ratio of 12.80 is achieved for the heat steam, last stage and top brine temperatures of 90.18°C, 39.75°C and 89.97°C, respectively. Although such performance ratio is the highest value in the range of decision variables, it leads to a large, uneconomical specific heat transfer area. To resolve this issue, two-objective optimization is performed using NSGA II method. The two objectives are selected as performance ratio and specific heat transfer area with the latter considered as cost function due to its direct connection with cost. Two-objective optimization using NSGA II leads to a Pareto front curve. Selection of each point on this curve depends on the designer’s decision, namely priority of the performance ratio or the specific heat transfer area. Moreover, the effect of the number of total stages on the optimum performance ratio for the selected MSF-BR-TVC is investigated. Results show that increasing the total number of stages from 20 to 30 increases the optimum performance ratio from 9.97 to 15.27.