Economic and Environmental Analyses of Biodiesel Production Processes From Unused Low-grade Oil

Two two-step transesterification processes are presented for biodiesel (BD) production from 300 t/d unused low-grade oil (LGO) with 24.5 wt% of free fatty acid (FFA). Acid-catalyzed (case 1) and enzymatic (case 2) esterifications were used for FFA reduction. The FFA in LGO was converted into fatty acid methyl esters (FAME) by H₂SO₄-catalyzed esterification (case 1) or transformed into sodium salts (soap) via a neutralization reaction with NaOH (case 2). In case 2, FFA was separated from soap and transformed into monoesters via enzymatic esterification. The two de-acidification processes decreased the FFA content of LGO to 0.5 wt%, enabling the production of 294 t-BD/d through subsequent alkali-catalyzed transesterification. Case 2, using an enzyme, was proposed to reduce the concentration of H₂SO₄, resulting in less corrosion to downstream equipment. The total production cost of case 2 ($62 million/y) was 32% higher than that of case 1 ($47 million/y) because of the greater consumption of CH₃OH, H₂SO₄, NaOH, and enzyme during FFA reduction. The total capital investment for case 2 ($41 million) exceeded that of case 1 ($31 million). Consequently, the minimum fuel selling price of case 2 (0.58 $/kg-BD) is higher than that of case 1 (0.42 $/kg-BD). The net CO₂ emissions reduction of the produced BD is 2.47 kg-CO₂/kg-BD for case 1 and 2.34 kg-CO₂/kg-BD for case 2. Given the variability in the acidity and composition of the feedstocks, future studies should include comparative economic and environmental analyses of various raw materials.