Conversion of Waste Cooking Oil to Biodiesel via Enzymatic Hydrolysis Followed by Chemical Esterification

Abstract

Biodiesel (BD) is usually produced by alkali-catalyzed methanolysis of expensive edible oils. Although waste cooking oils (WCO) containing high free fatty acid (FFA) are inexpensive, they cannot be processed effectively by alkali catalysis because of the formation of soap, which reduces the BD yield and makes the downstream process complicated. A process consisting of enzymatic hydrolysis followed by chemical esterification was developed for the production of BD from WCO. The enzyme Candida rugosa lipase was used for the hydrolysis of WCO to fatty acid (FA). The complete conversion of WCO to FA was achieved after 10 h at a water to WCO ratio of 1:1 (v/v), 5 g of WCO, 5 mL of lipase−water solution (0.5 g/L), and temperature of 30 °C. After hydrolysis, FA was separated and converted to BD by chemical esterification using Amberlyst 15 (acidic styrene-divinylbenzene sulfonated ion-exchange resin) as a catalyst. The maximum BD yield of 99% was obtained after 2 h at a methanol to FA molar ratio of 4:1, 10 mL of FA−isooctane solution (0.35 M), 1 g of Amberlyst 15, and temperature of 60 °C. The activity of C. rugosa lipase slightly decreased with recycling, and FA yield after five cycles was 92%. Amberlyst 15 was repeatedly used for 100 cycles without loosing its activity. The developed two-step process has a potential to be used industrially as it can tolerate feedstocks containing a wide range of FFA and water.