Date of Award

5-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Chemical Engineering

Advisor

Goodwin, James G.

Committee Member

Bruce , David A.

Committee Member

Kitchens , Christopher L.

Committee Member

Hwu , Shiou-Jyh

Abstract

Biomass, such as animal fats and grease, is one of the better sources for transportation fuels, e.g. biodiesel. Use of such biomass in the biodiesel synthesis decreases the need for fossil energy, provides an outlet for utilizing the abundant resources effectively and economically, results in a cleaner fuel that is biodegradable, renewable, and non-toxic. Free fatty acid (FFA) esterification and triglyceride (TG) transesterification with low molecular weight alcohols are the central reactions for the biodiesel production. The focus of this research is to establish a better fundamental insight into heterogeneous catalysis for biodiesel forming reactions, in an attempt to design the catalyst systems more proficient and durable for applications concerning biodiesel synthesis.
Commercial processes normally involve low reaction temperatures (i.e. 60 °C) to maintain the alcohols in the liquid phase; however, the use of high reaction temperatures is required to improve the catalytic activity. Using gas-phase esterification of acetic acid with methanol (as a probe reaction) at the reaction temperatures exceeding the boiling point of water, the intrinsic activities of a variety of solid acid catalysts were investigated and compared. All catalysts (zeolite (Hβ), sulfated zirconia (SZ), tungstated zirconia (WZ), and Nafion/silica (SAC-13)) exhibited the similar capacity for effectively catalyze esterification. The nature of the active sites for solid acid catalysts (Brønsted vs. Lewis acid sites) was examined and discovered that Brønsted acid sites were found to be a key for the catalysis.
Knowledge of the reaction mechanism for heterogeneous esterification at high reaction temperatures was elucidated by using SAC-13 as a catalyst. The results suggested that the reaction proceeded via a single site mechanism and followed the same reaction pathway as homogeneous catalysis in which the adsorbed acetic acid appeared to react with the alcohol from the gas phase. As the reaction temperatures increased, a change in the reaction controlling step for esterification from surface reaction (at low temperature) to carboxylic acid adsorption (at high temperature) satisfactorily explained the experimental observations. As a result, the reaction orders for the alcohols were changed toward negative values, suggesting that use of a large excess of alcohol (as typically used at lower temperatures) could result in a significant lower reaction rate.
The negative impact of alcohols on the catalyst activity at high reaction temperatures was further remarked in a parallel study on the solid acid catalyzed liquid-phase transesterification of triglyceride. Operating under N2 atmosphere and temperature of 120 °C, a solid acid catalyst containing sulfur, i.e. sulfated zirconia, was deactivated by a permanent removal of its active sites in the presence of liquid alcohols. All of these results would lead to a better design for the reaction system dealing with the methanolysis of waste greases. Finally, the feasibility of a continuous multiphase reaction system was successfully demonstrated by using a simulative mixture of waste greases (lauric acid in tricaprylin). By conducting the reaction at high temperatures (>100 °C) and atmospheric pressure, a residue alcohol and byproduct water were continuously removed, resulting in the completion in esterification reaction and a better physical-chemical characteristics of ester products.

Share

COinS