LAUSR

Abstract Spent caustic contains a great deal of toxic contaminants including sulfide and other complex organic compounds. Inefficient and poor management of treatment for spent caustic would impose sustainability challenges, deplete energy reserves and undermine water security. Suspension crystallization was utilized to remove sulfide and recover water from ethylene plant spent caustic for the first time in this study. The effects of variables, namely, coolant temperature, stirring rate, freezing time and ratio of ice phase on effective partition constant of Chemical Oxygen Demand (COD) and sulfide removal efficiency were thoroughly investigated. Response surface methodology (RSM) was applied to observe the interactive effects between variables and search for the optimum working conditions. The results show the best responses were 89.40% and 0.1817 for sulfide removal efficiency and effective partition constant while the ratio of ice phase could reach 75%, and the optimum working conditions were: coolant temperature at −9.95 °C, circumferential velocity of stirrer at 0.89 m/s and freezing time at 42.6 min. Based on these results, a conceptual hybrid process of suspension crystallization followed by a smaller incineration unit for treating spent caustic was proposed, of which energy consumption is only one fourth of that of direct incineration according to estimation.