Synthesis of Soybean Soapstock Methyl Ester through a Two-Step Transesterification Process

Soybean  soapstock (SS) as a lipid-rich by-product produced during vegetable oil refining process, is one of the most promising candidates for biodiesel production because of its easy collection, availability and low cost. In this work, the feasibility of biodiesel production from SS which contains high water and fatty contents was investigated in a two-stage process (esterification followed by transesterification). Sodium hydroxide (NaOH) catalyst, a methanol solvent and n-hexane co-solvent were used. Characterization using gas chromatography (GC) analysis and Fourier transform infrared (FTIR) spectroscopy were carried out to identify the constituent fatty acids and functional groups respectively in the oil and biodiesel produced. The GC results shows that the soybean  soapstock though having an unsaturated fatty acid with the highest percentage (linolenic with 32.261%), had a higher total percentage of saturated fatty acids when compared with the unsaturated fatty acids with myristic, stearic and palmitic acids being the more prominent saturated fatty acids with percentages of 15.696%, 15.542% and 14.898% respectively, while the biodiesel had a higher percentage of unsaturated fatty acids with oleic acid (26.045%) and linolenic acid (22.344%) having the highest percentages. Physico-chemical properties of the feedstock and biodiesel showed that the fuel properties of the biodiesel comparing favourably with standards. A biodiesel yield of 96.8wt% was obtained which thus confirms that soybean  soapstock was a viable feedstock for the production of biodiesel.