Thermodynamic Study On Oilfield Wastewater Desalination System By Multi-Effect Evaporation

With the increasing of mining depth and exploration scale of Chinese oil fields, the oily wastewater generated in the process of oil extraction increased dramatically, how to desalinate oilfield wastewater depth to meet the boilers requirement has become one of the key issues to solve heavy oil production in the middle-late period of oilfield development.The multi-effect distillation (MED) technology to desalinate oilfield wastewater is presented for the high temperature, large amount, high ion concentration and complex composition of wastewater, and established MED model based on Aspen Plus, and then carried on the process calculation and optimization analysis, formed a complete computing package. The thermal performance of MED system with a capacity of100kg/h is experimentally studied, and the influence of feed flow rate, feed temperature and salt concentration on freshwater production, performance ratio and product water quality is obtained. The main research work and conclusions in this paper are as followed.(1) An experimental study is taken to research the main influence factors and rules of MED process, pure water, concentration of1.5%,2.5%, and3.5%NaCl solution was used as the experimental medium in the three-effect evaporation oilfield wastewater desalination system. The results shows that the feeding flux and temperature has significant effects on freshwater yield and performance ratio, which decrease with the feeding flux increasing and increased with the feeding temperature increasing, the trends consented with the theoretical results; the product water whose salinity is less than1000ppm meet the boiler feed water requirement(≤7000ppm), which demonstrated that the experimental device was capable of wastewater desalination and running well.(2) The process model of the MED system is developed by Aspen Plus. Based on the verified model, the influence of number of effects, heating steam temperature, concentration ratio, preheating pattern and feed water configuration on performance ratio and specific heat transfer area are investigated. This paper achieved the optimization design of oilfield wastewater MED system and offered a practical example which aimed at minimum unit cost of product water.(3) This paper made a comparative study of MED, MED-MVC (mechanical vapor compression) and MED-TVC (thermal vapor compression) system in order to improve the thermal and economic performance of MED system, results showed that the single-effect MVC system has the best thermal performance.(4) Influence of TVC ejecting location and operating steam pressure on entrainment ratio, the last effect steam condensate flow, performance ratio and specific heat transfer area are investigated for MED-MVC system. The research results indicated that increasing operating steam pressure can improve TVC performance, adjusting TVC ejecting location properly can improve the evaporation system economy greatly.