Optimization of soapstock splitting process to reduce the concentration of impurities in wastewater

Soapstock is a by-product of crude vegetable oil refining which contains soaps, neutral oil and different impurities removed from oil during the process. This substance is predominantly treated with mineral acid (splitting) to recover fatty acids from soaps. High treatment costs make it advisable to reduce the pollutants of this wastewater “at source”. The Response Surface Methodology was employed to optimize the splitting of the rapeseed soapstock. The experiments were conducted in an industrial plant operating on a continuous basis. The study factors included: phosphorus content in oil (X₁), flow rate of NaOH to the saponification section (X₂), saponification temperature (X₃), soapstock splitting temperature (X₄) and pH (X₅). The concentration of organic phosphorus compounds (Porg), COD and the acid number (AN) of post-refinery fatty acids were measured during the experiments. All the study factors substantially affect the AN, whereas the pH value during soapstock splitting has no material effect on Porg concentration, and soapstock splitting temperature has no material effect on COD. Phosphorus content in oil has a very large effect on all the study values, and when was reduced, the wastewater concentrations of Porg and COD dropped considerably while the AN of post-refinery fatty acids increased. For average value of X₁ = 185 mg/kg, the optimal conditions were as follows: X₂ = 3.08 kg/h, X₃ = 137 °C, X₄ = 91.5 °C and X₅ = 3.50. It is possible under such conditions to produce post-refinery fatty acids with an of 171 mg KOH/g and wastewater with Porg concentration 425 mg/dm³ and COD 7958 mg/dm³. The Response Surface Methodology is highly suitable for mathematical modelling of the rapeseed soapstock splitting process under industrial conditions.