(90b) Continuous Synthesis of Ethyl Esters from Free Fatty Acids over Metal Oxides in Sub/Supercritical Ethanol

Oct 29^th to Nov 3^rd, Minneapolis, MN

Continuous synthesis of ethyl esters from free fatty acids over metal oxides in sub/supercritical ethanol

Jiuxu Liu, Yue Nan and Lawrence L. Tavlarides

Department of Biomedical and Chemical Engineering, Syracuse University,

Email: lltavlar@syr.edu (Tavlarides); jliu23@syr.edu(Liu)

Abstract

In our previous study, we proposed a method for biodiesel synthesis by running the transesterification reactions under sub/supercritical conditions with trace amounts of homogeneous catalysts in order to achieve a high product yield and reduce the dosage of catalysts compared to the conventional technology. Another way to avoid using large amount of homogeneous catalysts (which bring burdens in product purification process) is switching to heterogeneous catalysts, acidic or basic. The common basic solid catalysts are hydrotalcites, dolomites, loaded and supported alkaline and alkaline metals and corresponding oxides, and mixed basic metal oxides. The basic solid catalysts reported in the literature face leaching problems. The active compounds can be detected in the product with high concentration, and the catalysts have to be regenerated after a few runs. The solid acid catalysts seem to be more promising compared to basic ones by considering the fact that they are insensitive to free fatty acid content, and esterification and transesterification occurs simultaneously. Commonly used acidic solids in biodiesel synthesis include acidic metal oxides, sulfonic ion-exchange resin, sulfated/ tungstated metal oxides, zeolites, heteropolyacids, amongst others. Similar with basic solids, the reported acidic catalysts deactivate along with reaction due to the leaching problem.

A literature review indicates that several compounds were found to be stable even at alcohol temperature higher than 180 ^oC including zirconia, titania, silica, and alumina. These simple metal oxides have been commonly used as catalyst supports due to their thermal and mechanical stability. However they did not show any catalytic ability under conventional conditions (T=70 ^oC, ambient pressure) due to their weak acid/base property.

In this study, we are investigating the possibility and mechanism of continuously synthesizing ethyl-ester biodiesel under sub and supercritical alcohol conditions from fatty acids, which largely exist in cheap feedstocks such as waste cooking oil, catalyzed by simple and stable metal oxides catalysts including ZrO₂, TiO₂ and Al₂O₃. The objectives include: a) kinetics of ethyl-ester biodiesel synthesis with oleic acid at SCTE conditions without the catalysts, b) effect of addition of the solid catalysts on reaction yields, and c) kinetics of the biodiesel synthesis over the selected metal oxides.

At this writing the first aim has been completed, which is the kinetic study of the non-catalytic esterification reaction of oleic acid with sub and supercritical ethanol. The reactions were conducted at temperatures from 175 to 325 ^oC, residence times from 1 to 8 min, and ethanol-to oleic acid molar ratio of 18:1. The pressure was kept as 200 bar. Under subcritical conditions, biodiesel yield slightly increased from 8% to 33% with temperature changing from 175 to 225 ^oC at 8 min residence time. Under supercritical conditions, a yield of 60% was reached at 275 ^oC and 8 min, and increased further to 80% at 325 ^oC and 8 min. A one-step reversible second-order reaction model was employed to best fit the data, and the activation energy for forward and reverse reaction step was calculated as 55 and 32 kJ/mol, respectively. A negative enthalpy change as -23 kJ/mol indicates that the reaction is exothermic. These kinetic parameters will be used when conducting the kinetic analysis of reaction with presence of the solid catalysts.

Experiments to complete the remaining objectives are in progress and will be reported.