(255e) Solid Acid Catalyzed Esterification of Free Fatty Acids in Oil Using Co2 Enhanced Media

Stagg-Williams, S. M. - Presenter, University of Kansas
Zhao, L. - Presenter, The University of Kansas

Technologies of biodiesel production via transesterification of waste oils are attracting much attention because of its low cost compared to natural oil. However these high free fatty acid (FFA) containing feeds present significant process problems because the free fatty acids are saponified by the homogeneous alkali catalysts used in triglycerides transesterification reaction in biodiesel production. Therefore, these free fatty acids need to be converted into their corresponding esters before transesterification. Esterification reactions are conventionally carried out homogeneously in batch processes using Bronsted acids such as sulfuric acid or Lewis acids like Sn-octoate as catalysts. But the applications of these homogeneous acids cause some problems such as corrosion, loss of catalyst, difficult separation, and environment problems. Using of the homogeneous acid catalyst adds neutralization and separation steps. It is well known that the use of heterogeneous catalysts can give a wide array of benefits, for example, facile separation, avoidance of corrosion, and easy recovery of catalysts. These advantages will significantly simplify the process. However, conventional solid acids have been found to exhibit low activity, ascribed to the inaccessibility of the acid sites due to the poor miscibility of the reactants. Our work focused on using solid acids combined with media (CO2-based and hexane) to increase the miscibility of the methanol/fatty acid/oil system. The initial catalysts studied were commercially available solid acids Nafion (SAC-13, Engelhard) and Amberlyst-15 (Rohm and Haas) for benchmarking purposes. The media is being explored as a means of increasing the accessibility of the active sites, simplifying product separation, reducing the operating temperatures and pressures, and eliminating liquid acids and bases. The expansion studies are performed in a Jurgeson® view cell. High free fatty acid content oils (10 ? 20 wt%) are simulated using palmitic acid mixed with soybean oil. The reactions are studied in a view cell/batch reactor using CO2 expanded methanol/hexane as the solvent. Activity studies will also be performed without catalyst in CO2-epxanded methanol and supercritical hexane to study the reactivity of the uncatalyzed media. Reaction products are analyzed using HPLC with data provided in the literature used as a benchmark. At room temperature, dense CO2 resulted in a V/Vo ratio of 10 for methanol at 6 MPa. However, the CO2 and soybean oil formed two immiscible phases at room temperatures with little expansion (V/V0 near 1.2) being observed at 50°C. The effects of temperature and feed ratio on the expansion have been studied for a methanol and soybean oil mixture. A methanol/soybean oil molar ratio of 6:1 resulted in a V/V0 near 2.0 at 5.5 MPa. When the molar ratio increased to 9:1 the V/V0 increased to 3.8. It appeared as though an emulsion was formed in the presence of the CO2, enhancing the mass transfer area for the esterification reaction. Increasing the temperature had a negative effect on the degree of expansion with the V/V0 for a 6:1 molar ratio of methanol/soybean oil system at 65°C, 50°C, and 26°C being 2.0, 1.55, and 1.29, respectively. However, the temperature did not appear to affect the formation of the emulsion. Expansion studies performed with 5wt% palmitic acid in the system are very similar to the methanol/soybean oil studies. Esterification of palmitic acid with methanol using Amberlyst-15 has been performed and 45.3% conversion of the palmitic acid was achieved after 3 hours at 85°C with 85wt% presence of soybean oil. This result is consistent with what the literature reported. Small increases in the conversion of both palmitic acid and the soybean oil were observed in the presence of CO2. While only small increases in conversion were observed with the resins, larger increases in conversion are expected when solid acids that traditionally exhibit higher diffusion limitations due to pore size restrictions are employed, such as zeolites. These solid acids as well as new solid acids are currently being explored.