A simple non-catalytic supercritical biodiesel process with adsorption-based refining

DEBORA L. MANUALE; VANINA A. MAZZIERI; CARLOS R. VERA; JUAN C. YORI

ENERGY & FUELS (PRINT)

Año: 2009

0887-0624

Different feedstocks of varying acidity ranks and water contents were subjected to a series of discontinuous steps that simulated a biodiesel production process.  The three steps comprised:  (i) the non-catalytic transesterification with supercritical methanol at 280 ºC; (ii) the stripping of the unreacted methanol, water and volatile products; (iii) the adsorption of the impurities with silica.  Refined soy oil, chicken oil and waste cooking oil were subjected to the same simple procedure.  The process produced biodiesel according to the EN 14212 standard. Biodiesel production by the reaction of oils in supercritical methanol at 280 ºC and methanol-to-oil molar ratios of 15 and 20 produced amounts of glycerol as small as 0.02%.  This simplified the subsequent refining of the biodiesel and is considered and advantage over the classic alkali-catalyzed process (that produces 10% of glycerol by-product) because washing steps can be spared.  Methanolysis and thermal degradation transformed glycerol into water and organic products of smaller molecular size. The contents of methyl esters, water and free fatty acids showed a volcano pattern when plotted as a function of the reaction time.  This was attributed to the initial reaction of water and triglycerides to form free fatty acids that increased the acidity of the product mixture.  At longer reaction times these acids would be esterified into methyl esters.  The elimination of glycerol made a final step of refining by silica adsorption amenable.  After methanol and water stripping, silica refining enabled the elimination of trace free fatty acids and glycerides from biodiesel and enabled the compliance of the EN 14214 limits for acid, methyl ester and glycerides content. The attractive feature of this process is its simplicity, its aplicability to virtually any feedstock and its relatively good yield of methyl esters.  A greater yield of 94-96% can also be obtained at the expense of an additional unit for the selective separation of unreacted glycerides and acids and their recycle to the reactor feed.