Optimum Design And Safety Analysis Of Ethylene Recovery Process For Refrigeration System

Safety is very important in chemical industry. Realizing the production safety and economic benefits are the goal of enterprise. Owing to the minus pressure operation, air (mainly O2 and N2) would penetrate into the refrigeration system. The mixture of O2 and ethylene is easy to form explosive mixture. In order to ensure production safety, a certain amount of non-condensable gas would be exhausted. Ethylene in the exhausted gas is wasted. In the driving process of ethylene compression refrigeration system, we should use high purity ethylene to replace of nitrogen. At present, factory emits the replaced ethylene gas directly to the flare without recycling, and this also cause waste of ethylene. It will improve the economic benefits of enterprises to recycle ethylene in a reasonable manner.Compression refrigeration system which under minus pressure operation emite a total of 420 tons ethylene annually in normal condition and driving condition. Existing technology use condensation method and membrane method recycling emitted ethylene of normal condition. Due to the serious entrainment phenomenon of gas-liquid separator, lead to incomplete gas/liquid separation and exhausted gas with liquid. When reach to critical liquefaction temperature, O2 will be absorbed by liquefied ethylene, can’t be discharged and then accumulate in the system. There is hazardous of oxygen content exceeding bid. By reducing the amount of liquid of tail gas and changing the turning point of condensation, the condensation temperature operation elasticity can be improved. The membrane permeate side with minus pressure can increase the probability of seeping into the air, also lead to O2 accumulation in the system. By using the current limit orifice plate to produce pressure drop and increase the membrane permeability side pressure to plus pressure, can avoid air infiltration effectively.In view of the oxygen accumulated safety problems of the existing recycling process, proposed the method of using the precision filter to reduce exhaust gas liquid volume and setting membrane permeate side to plus pressure directly. And designed the improved schemes to solve the problem of safety. UniSim Design software was utilized to optimize the operating parameters and examined the influence of condensing temperature on oxygen content in the system and the economic efficiency. The simulation result showed that the condensation temperature ranges of improved schemes could increase from -44～-46℃ to-50～-55℃. The improved schemes improved condensation operating elasticity, reduced oxygen content in the system, improved the security and could keep higher economic benefits. Using membrane recycling method, the oxygen content in system could reduce to 150 ppm and economic efficiency was as high as 3.75 million yuan·a-1.In driving condition, the tail gas discharge intermittently, instantaneous emissions is big (8500 Nm3·h-1), ethylene content change in big range (C2H4 wt%:0～100%) and exhaust gas is with pressure. In view of these characters, we proposed the turbine expansion refrigeration recycling scheme. UniSim Design software was utilized to simulate the operating parameters. The results showed that economic benefit could obtain 145000 yuan·a-1 when the condensation temperature is-120～-130℃. Design of cold box heat exchange area is 450 m2 and turbine expander output power is 90 kw.