Multi objective optimization of MSF and MSF-TVC desalination systems with using the surplus low-pressure steam (an energy, exergy and economic analysis)

Abstract

Water is consumed as one of the main fluids in the natural gas refinery which becomes wastewater after undergoing different process. Therefore, it is not possible to reuse or dispose the wastewater to the environment due to contaminant issues. At the same time, a huge amount of heat is wasted in surplus low pressure (LP) steam flow, which is used in natural gas refining process. In present study, it is attempted to model Multi Stage Flash (MSF) and multi stage flash coupled with thermal vapor compressor (MSF-TVC) systems for desalination of existing refinery wastewater utilizing different heat losses occur in the refinery process. In order to optimize the MSF and MSF-TVC systems, effective parameters in the field of energy, exergy and economic including gain output ratio (GOR), exergy efficiency and payback ratio (PBR) are used simultaneously. For this purpose, the Pareto solution and TOPSIS methods are used as the multi objective optimization. As a result, the optimum values of the first stage temperature (maximum system temperature) and number of desalination stages are obtained 130 °C and 32 stages for MSF and 115 °C and 25 stages for MSF-TVC systems. Finally, it is found that the amount of 46,335 ton/year CO2 emission reduction can be achieved by using surplus LP steam heat potential in refinery wastewater desalination.