Valorization of palm oil soap stock waste toward for biodiesel production: Process optimization under zero waste concept

Highlights: • Palm oil soap stock was firstly used as low cost material for biodiesel production. • One simple step of enzymatic biodiesel production from soap stock was developed. • The desirable biodiesel yield (91.50%) was achieved under optimal condition. • The biocatalyst presented high performance and reusability up to 16 cycles. • The predicted physio-chemical properties of biodiesel met the European standard. Palm oil soap stock (POSS) was recycled from bio-waste to bio-fuel. Firstly, POSS was converted through acidification to be acidified POSS (APOSS) coupled with solvent extraction. APOSS with a high content of free fatty acids (FFAs) (90.94%) was obtained. Subsequently, the FFAs in APOSS were converted to biodiesel via esterification, catalyzed by immobilized alginate-polyvinyl alcohol (ALG-PVA) lipase. The key factors affecting the reaction, such as the temperature (30–50 °C), methanol to FFA molar ratio (2:1 to 4:1), and agitation rate (150–250 rpm) were optimized by statistical response surface methodology (RSM). The highest biodiesel yield was achieved with a reaction temperature of 40 °C, methanol/FFA molar ratio of 3:1, and 200 rpm agitation rate, as the optimal conditions. Furthermore, the biocatalyst can be reused up to 16 cycles. Interestingly, the alternative biodiesel production process reported herein promotes zero waste from the palm oil refinery process.