Biodiesel Production From Cottonseed Soapstock Acid Oil via Two-Step Acid Esterification and Alkaline Transesterification

Soapstock, the main by-product of alkali refining of cottonseed oil, is a low-cost lipid feedstock whose high free fatty acid (FFA) content prevents its direct use in conventional alkali-catalysed biodiesel processes. In this study, acid oil recovered from cottonseed soapstock by sulphuric acid acidulation (acid value 118.6 mg KOH/g, FFA ≈ 59.7 %) was converted into fatty acid methyl esters (FAME) by a two-step process. In the first step, FFAs were esterified with methanol over H₂SO₄; the effects of methanol and catalyst loadings were examined at 65 °C for 120 min. A methanol loading of 40 wt% and 2 wt% H₂SO₄ reduced the acid value to 1.7 mg KOH/g (98.6 % FFA conversion). In the second step, the pretreated oil was transesterified with methanol (6:1 molar ratio) using KOH at 60 °C for 60 min, and the crude esters were purified by vacuum distillation to remove gossypol-derived pigments and unsaponifiable matter. The overall yield of distilled FAME was 76.8 % relative to acid oil. The product had an ester content of 97.6 %, kinematic viscosity of 4.31 mm²/s and acid value of 0.38 mg KOH/g, meeting the main EN 14214 and ASTM D6751 limits, while its oxidation stability (4.9 h) requires antioxidant addition. The results show that cottonseed soapstock, abundant in Uzbekistan, can serve as a viable feedstock for biodiesel production.