Linda Benedict, Boldor, Dorin
LSU AgCenter researchers are investigating production of biodiesel, which has received worldwide attention as a renewable transportation fuel and blending agent. Biodiesel has potential to replace petroleum products, lower net carbon dioxide emissions, which contribute to global warming, and reduce emissions of particulate matter with carcinogenic compounds. Other advantages include portability, improved lubricity, higher flash point (the temperature at which it ignites) for increased safety during handling, the on-site manufacturing (especially for on-farm use), lower sulfur and aromatic content and higher biodegradability. One of the most attractive features of biodiesel is its domestic origin, which would help reduce U.S. dependency on imported petroleum.
Fundamentally, vegetable oils can be directly used in older (prior to the 1980s) or modified diesel engines, but their extended use would lead to poor engine performance and clogging, especially at low temperatures. Therefore, modifying the vegetable oil into biodiesel is essential for successful engine operation over the long term because this process reduces fuel viscosity and fuel-line clogging and improves the combustion process.
Transesterification (also called alcoholysis) is the chemical reaction of one unit of fat or oil (triglycerides) with three units of alcohol in the presence of a catalyst to form esters and glycerol. Because the reaction is reversible, extra alcohol is normally used to prevent reverse reactions. Among the suitable alcohols that can be used in the transesterification process, methanol and ethanol are used most frequently, especially methanol because of its low cost and its physical and chemical advantages. The catalyst can be a base, acid or enzyme, depending on oil quality, but most industrial processes use base catalysts – either sodium hydroxide or potassium hydroxide (lye) – because they act more rapidly with lower operational costs.
Base-catalyzed transesterification occurs at 140 degrees at atmospheric pressure, and it takes one hour using traditional heating sources. Recently, however, researchers discovered that microwaves used as a heat source, can decrease reaction time to less than two minutes, significantly reducing the costs of biodiesel production. This reaction can be performed continuously in a specially adapted microwave oven, with the possibility of controlling the reaction using a fiber optic temperature system.
The efficiency of microwave-assisted transesterification stems from the unique properties of the mixtures of vegetable oil, solvent and catalysts. In normal heating with an open flame or electric heater, energy is put into the system via conduction and convection through the different layers of liquids, and a mixer is needed to enhance the reaction rate through increased contact between fat and alcohol molecules. As such, the heating is relatively slow at the molecular level – each molecule has to get its thermal energy from the neighboring molecules and then patiently try to encounter its molecular counterpart in order to react.
In microwave irradiation, by comparison, rapid heating occurs because of direct energy absorption in the whole volume of the mixture. Microwave energy interacts with the material on a molecular level, generating inter-molecular friction and molecular mixing and agitation that increase contact between alcohol and oil molecules.
The reaction occurs at the same temperature, pressures and other operational parameters as in the normal transesterification, but special care and safety measures need to be observed when using the microwave process. In general, researchers strongly recommend against modifying home microwave ovens because microwaves can be extremely dangerous if they leak out of the system. In addition, home microwaves are not made to support the use of temperature measurement systems nor are they provided with a microwave power feedback control system. Therefore, the control of the reaction is extremely difficult, and the operation can become hazardous. The use of an industrial/commercial apparatus specially designed for this purpose is, therefore, mandatory.
Figure 1 presents a schematic version of the continuous system. In practice, a commercially available, fully instrumented, batch-type microwave system is modified to make it operate continuously. The setup includes a 2-quart cylindrical Teflon beaker acting as the reaction vessel in the center of the microwave application chamber and Teflon tubing serving as inflow and outflow conduits. Before the reaction, a catalyst (sodium hydroxide) is dissolved in alcohol, and the mixture is added to the oil (in a ratio of approximately 20 percent alcohol and 80 percent vegetable oil by weight) and mixed before being pumped into the microwave system.
The time in the chamber will depend on the flow rate and the volume of sample at any given time. The reaction temperature is monitored with a fiber optic probe and automatically controlled by the system. A magnetic stirrer maintains additional agitation in the chamber. The recommended time in the chamber is five minutes, even though one to two minutes are usually enough to complete the reaction. Glycerin separation and biodiesel washing, drying and solvent recovery follow the same recipe as in conventional biodiesel production process.
Quality analysis is performed similarly to other biodiesel production processes and includes determination of cloud point, flash point, viscosity at 104 degrees, acid number, oxidation stability index, free and total glycerin and free fatty acid composition. For example, LSU AgCenter researchers have determined that total and free glycerin and conversion rates of soybean and rice bran oil into biodiesel using this method meet the American Society for Testing and Materials (ASTM) requirements at reaction times as short as one minute and at temperatures as low as 122 degrees F. Cloud point, flash point, viscosity, acid number and oxidative stability index measured for the biodiesel made using the microwave-assisted technology met or exceeded the ASTM standard specifications.
Dorin Boldor, Assistant Professor, Department of Biological & Agricultural Engineering, LSU AgCenter, Baton Rouge, La.
(This article was published in the fall 2009 issue of Louisiana Agriculture.)