A Different Path to Biodiesel

By Jerry W. Kram
In 2005, a graduate student of R.E. "Buddy" Babcock, professor of chemical engineering at the University of Arkansas, looked at the possibility of making biodiesel from low-cost, low-quality feedstocks. That research showed that high levels of free fatty acids (FFAs) were one of the primary components that made low-quality fats unsuitable for biodiesel production by the traditional transesterification reaction.

Another grad student, Brent Schulte, picked up the research and found that Japanese researchers had discovered a different way of making biodiesel using supercritical methanol. Above a certain temperature and pressure, many substances pass what is called the critical point where they can exist either as a liquid or a gas. Above this point, they become supercritical fluids that have properties of both a gas and a liquid. In particular, a supercritical fluid can dissolve another substance like a liquid and penetrate a solid like a gas. "Basically the methanol molecules are just in better contact [with the fatty acids] because they are in a single phase," Babcock explains. "It is a homogeneous-rather than a heterogeneous-reaction, so there is no need for a catalyst. If you go into a single phase, it is just easier for the reactants to convert into biodiesel."

Fats aren't very soluble in methanol, which is one of the reasons that caustic catalysts like sodium hydroxide or potassium hydroxide are needed to speed up the transesterification reaction. One of the advantages of supercritical fluids is that carefully making small adjustments to the temperature and pressure conditions of the reactor has large effects on the properties of the supercritical fluid. Babcock and Schulte found that of the conditions they tested, a pressure of 1,650 pounds per square inch and a temperature of 325 degrees
Celsius (617 degrees Fahrenheit) produced a reaction that transformed the fats into biodiesel quickly and completely.

Another reason supercritical methanol may be a good choice for future biodiesel production is that it does a good job converting FFAs to biodiesel. In fact, Babcock says, the reaction works even better with FFAs than with triglycerides. "This method prefers the free fatty acids, so that's not a problem," he says. In one set of experiments, Schulte reacted tall oil fatty acids (TOFA) with supercritical methanol. Tall oil is a byproduct of the pulp and paper industry that can be distilled to make TOFA, a mixture of nearly pure FFAs.

Compared with the low-quality chicken fat, which had an FFA level of up to 12 percent, the TOFA needed just half as much methanol by weight to achieve similar yields of biodiesel.

The downside to using TOFA is that it isn't much cheaper than soybean oil at this time. "Right now, a gallon of TOFA is about the same price as a gallon of diesel," Babcock says. "However, it is price-competitive with some of the other feedstocks. If we get enough of these feedstocks, maybe it will give some relief to the price pressure on the industry."

Crude tall oil is cheaper. However, crude tall oil contains 30 percent to 45 percent resin acids, which don't react with methanol in this method and can't be made into acceptable biodiesel. However, Babcock says this method shows a lot of promise for economically converting many low-quality feedstocks, including brown and trap grease, into biodiesel. Another possible avenue for future research will be looking for ways to use crude tall oil for biodiesel production.