Supercritical Methanol for Biodiesel Production

University of Arkansas researchers find that using supercritical methanol in the biodiesel production process could alleviate some of the challenges to the cost-competitive production of the fuel.
By Jessica Ebert | March 17, 2008
Defined as a substance that takes up space and has mass, matter in its simplest form consists of particles that combine to form all the elements regarded as the building blocks of the physical world; things such as carbon, nitrogen, oxygen and hydrogen. As most people with a basic science background know, matter comes in three main phases: solids, liquids or gases. Several factors determine the phase in which a molecule exists, including temperature and pressure. Changing these factors, in particular, changing the temperature shifts a substance from one phase to another. Heat a solid ice cube in a cooking pot, for example, and it quickly melts forming a liquid. Boil that liquid further and it eventually evaporates forming a gaseous water vapor.

When pressure is factored in, however, a new phase of matter can be reached. By increasing both the temperature and the pressure, a critical point is obtained at which gases and liquids are indistinguishable fluids. Matter that exists in this new phase is called a supercritical fluid (SCF). These fluids exhibit properties of both a liquid and a gas: they can penetrate solids like a gas and dissolve materials into their components like a liquid. In addition, small manipulations of temperature and pressure result in changes in the density of the SCF. Taken together, these "tunable" fluid properties make SCFs suitable solvents for a range of chemical and biological applications. Supercritical carbon dioxide, for instance, is an established industrial solvent for dissolving caffeine in the decaffeination of coffee and tea.

Now, researchers at the University of Arkansas report that using supercritical methanol in the biodiesel production process shows promise for the economic conversion of low-quality feedstocks like chicken fat and tall oil fatty acids (TOFA). "Bascially, methanol at supercritical conditions becomes an excellent solvent and dissolves the feedstock so that the molecules of the reactants are in close proximity of each other and therefore react readily without a distinct catalyst," explains Robert Babcock, a chemical engineer at the University of Arkansas. Although Japanese scientists first reported the use of supercritical methanol for the conversion of rapeseed-oil-to-biodiesel in 2001, this new work likely represents the first published data making use of supercritical methanol for the conversion of chicken fat or TOFA to biodiesel, Babcock says. Tall oil is a byproduct of pulp and paper milling that can be distilled to a mixture of nearly pure free fatty acids (FFAs). Feedstocks with a high FFA content are considered low quality.

Figure 1

Phase diagram of methanol showing the point where it becomes a supercritical fluid

The U of A study represents Babcock's graduate student, Brent Schulte's master's thesis. Recognizing that the price per gallon of biodiesel is dominated by feedstock costs and that conventional, base-catalyzed transesterification doesn't work well on the high FFA content typical of cheaper, lower-quality feedstocks, Schulte-with Babcock's guidance-decided to focus his research on alternative production processes. "I spent quite a bit of time investigating other potential methods of biodiesel production, including chemical catalysis, enzymatic catalysis and supercritical methanol treatment," Schulte explains. "Each has a set of advantages and disadvantages. Bottom line, supercritical methanol treatment is a very robust process." However, "the tradeoff is the higher temperature and pressure requirements," Babcock says. "From a safety standpoint, this process is not something people can be doing in their garages."

For each experimental trial, the reaction time and pressure were held constant at 20 minutes and about 1,650 pounds-per-square-inch respectively. On the other hand, a range of reaction temperatures and ratios of methanol to feedstock were tested because, according to Schulte, these two parameters have been identified as the most important with regard to the cost of large-scale production of biodiesel using a supercritical methanol treatment. Temperatures varied from 275 to 325 degrees Celsius (527 to 617 degrees Fahrenheit) while the ratio of methanol to chicken fat or TOFA ranged from 10:1 to 40:1.

The chicken fat was acquired from Tyson Foods Inc. in Scranton, Ark. It was a lower-cost chicken fat that contained about 12 percent FFA. The TOFA came from Georgia-Pacific's pulp and paper mill in Crossett, Ark. After liquefying the feedstock, weighing it and pouring it into a two-liter, stainless steel reactor, water was added in part to take up volume to help achieve the desired pressure, Babcock explains. In addition, there is some evidence that the presence of water in a limited amount improves the yield of the reaction, he says. Methanol was the last reactant to be added before the reactor was placed in a heater assembly and the temperature increased. After letting the reaction run for 20 minutes, the reactor was cooled and the contents were removed for analysis.

Comparison of various biodiesel/diesel substitute production methods


The reactor soup consisted of methyl esters, glycerol, water and any methanol and feedstock that didn't react. The density differences between these various layers of products allowed for the easy separation of the biodiesel from the other components. Overall, the U of A researchers found that these low-quality, high-FFA feedstocks can be converted to biodiesel in a single step with yields exceeding 89 percent for chicken fat and 94 percent for TOFA.

Interestingly, compared with chicken fat, less methanol was required for the TOFA conversion, which greatly decreased the cost. The team also successfully tested the conversion of a mixture of chicken fat and TOFA. In this case, however, a two-step process worked the best; the first being a hydrolysis step that liberated any FFAs from the triglycerides while the second was a supercritical methanol step for the production of biodiesel.

Finally, Babcock and Schulte tested cold flow properties of the resulting biodiesel and found that the viscosity of the fuel exceeded ASTM D6751 specifications for biodiesel suggesting that the widespread, commercial use of biodiesel made from a supercritical methanol treatment would require blending with a biodiesel fuel made from a feedstock like soybean oil. "The most significant finding of this work is that a process exists that can convert not only lower cost feedstocks like chicken fat and tall oil fatty acids into biodiesel, but that same process can convert these very different feedstocks at the same time," Schulte says. "Hopefully, my research helps those in the biodiesel industry think even more 'outside-the-box.' There are other materials we can utilize for biodiesel production and I've verified some."

Although Schulte was awarded his master's of science in chemical engineering and now works as a field completions engineer with Southwestern Energy Co. in Conway, Ark., Babcock plans to continue the work. "Our work adds to the mounting data bank on the biodiesel reaction," Babcock says. "It is not the economic breakthrough that everyone is looking for but hopefully it may lead to such a breakthrough." To that end, Babcock's team hopes to obtain additional funding to study crude tall oil and algae oil as a feedstock. The tall oil used in this initial research was refined crude tall oil, which Babcock explains is an economic constraint of the supercritical methanol process. "The refined tall oil has a value of almost as much as biodiesel," he says. "If we could develop the process to the point of using crude tall oil as a feedstock then the economics of the process improve drastically." In addition, Babcock wants to conduct comparative economic studies of the various processes being employed to make biodiesel.

Jessica Ebert is a Biodiesel Magazine staff writer. Reach her at or (701) 738-4962.
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