Treatment of Spent Caustic Effluent of Oil Refinery with Catalytic Oxidation and Optimization of Relevant Parameters using Response Surface Methodology

Caustic soda is a substance used for sweetening hydrocarbon products in oil refinery, gas and petrochemical units. The caustic soda-generated salts are increasingly formed during the process of removing impurities like mercaptans, sulfides and nitrates, which finally leave process system through spent caustic discharge. Spent caustic discharge into environment is prohibited due to the presence of compounds like organic and toxic substances, sulfide salts, mercaptans as well as high chemical oxygen demand (COD), emphasizing the need to treat such effluents. The present study aimed to apply catalytic wet air oxidation method for treatment of spent caustic effluent from Bandar Abbas oil refinery and to optimize related parameters. Experimental processes were performed in a 500-ml batch reactor. Catalyst concentration, residence time and stoichiometric air flow parameters were optimized by Box-Behnken design. Based on the results, optimal conditions for this process were the catalyst concentration of 123 ppm, the residence time of 3 h and the stoichiometric air flow of 4.1 l/h, with a maximum reduction of spent caustic COD of 42.2%. The results of optimization experiments were analyzed by Design Expert 11 software, the results of which documented that the three mentioned parameters had the greatest effect on reducing spent caustic COD. Prolonging residence time had no significant effect on COD removal. Polynomial equation based on the three mentioned variables was presented to predict the spent caustic COD changes. In this treatment method, in addition to a significant reduction in COD, the available toxic substances also reached less than 1 ppm.