Supercritical method converts low-value feedstocks

By Jerry W. Kram | January 17, 2008
More heat and pressure may be the solution to converting recalcitrant feedstocks into biodiesel. Researchers at the University of Arkansas successfully used supercritical methanol to convert feedstocks with high levels of free fatty acids (FFAs) into biodiesel. At high temperatures and pressures, many substances become supercritical fluids that exhibit properties of both a liquid and a gas. Supercritical fluids are used in many organic chemistry reactions as both solvents and reactants.

R. E. "Buddy" Babcock, professor of chemical engineering, and graduate student Brent Schulte heated methanol until it became a supercritical fluid and reacted it with low-grade chicken fat and tall oil, a byproduct of the pulp and paper industry. The methanol reacted with the fats to create biodiesel in a one-step process that didn't require any catalyst. Babcock and Schulte found that FFAs were preferentially converted into biodiesel in the reaction. In traditional biodiesel production, FFAs form soaps in the presence of caustic catalysts. The soaps have to be removed from the biodiesel in a washing step.

The researchers found the process to be highly efficient. With chicken fat, the biodiesel yield was more than 94 percent, and with tall oil, the yields were in excess of 89 percent. The reaction proceeded most efficiently at a temperature of 325 degrees Celsius (617 degrees Fahrenheit).

Babcock said the process would be ideal for converting feedstocks such as feed-grade chicken fat, which can have as much as 6 percent FFAs. Results using tall oil were mixed. Crude tall oil contains rosins and resin acids that don't combine with methanol to form biodiesel under the conditions that the researchers tested. Tall oil fatty acids (TOFA), which can be extracted from tall oil, worked well in the experiments. However, TOFA doesn't have much of a price advantage over other biodiesel feedstocks, according to Babcock. He said future research will look at ways to make the process work better with crude tall oil.
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