(305e) Production of Biodiesel From Alternative Feedstocks – a Bubble Reactor for Converting Oils Containing Free-Fatty-Acids to Biodiesel

Cairncross, R. A., Drexel University
Melick, C., Drexel University
Mohammed, M., Drexel University

Biodiesel is a diesel substitute that is typically produced by transesterification of refined vegetable oils (soybean oil in USA) with methanol; this chemistry is simple and effective with base catalysts and refined oils. However, crude vegetable oils and many alternative oil feedstocks contain significant (> 1%) concentrations of free-fatty-acids (FFA); for example waste fryer oil typically is 10-15% FFA and trap grease is >85% FFA. Free-fatty-acids cause low conversion and difficult separation with the traditional chemistry because they form soaps with base catalysts. There are several alternative techniques for producing biodiesel from high-FFA oils including two-step reactions, high pressure reactions, and supercritical reactions. In this research a two-phase bubble column reactor is used to esterify FFA at moderate temperatures (100-150°C) and atmospheric pressure. Methanol added to the reactor boils at these temperatures and produces bubbles that rise through the reactor. Despite having lower methanol concentrations in the reacting liquid phase, acceptable reaction rates and high conversions are achieved with this configuration. Our research has also shown that bubble reactors are effective at overcoming equilibrium limitations of the esterification reaction. Because the bubbles remove by-product water, the reactor is much more robust for feedstocks containing water, and conversions approaching 100% are achieved with up to 10% water in methanol. Similar results have been obtained for reactions with ethanol/water mixtures showing that it is not necessary to break the ethanol-water azeotrope when purifying ethanol for this reactor. A model of the reactor has been developed that accurately predicts trends of the experimental data and can be used to design improved reactors. Feasibility studies of full-scale production of biodiesel from trap-grease have been performed using the bubble reactor and two other reaction schemes ? supercritical reactors and medium pressure CSTR's in series. The bubble reactor process has the highest return on investment and lowest greenhouse gas emissions.