Looking at biodiesel production in new ways

By Staff | September 10, 2012

Jon Van Gerpen, a professor at the University of Idaho, wants people to think more fundamentally about biodiesel production. “There are a lot of things we don’t know about the biodiesel process,” he said at the Collective Biofuels Conference. “It’s deceptively simple.” He raised questions that perhaps should have been asked 20 years ago during biodiesel’s infancy, including, “What affects the reaction, and why?” and “Can we guide the reaction in some way?” Methanol’s solubility in oil is limited, so excess methanol is used to facilitate reaction. There’s only 1 percent oil in excess methanol droplets. “The reaction only occurs on the surface of those droplets,” he explained. “The diffusion process moves the oil to reaction zone, and then moves the methyl esters away.” Transesterification is conventionally referred to as a mass transfer limited reaction, but it could be chemical reaction limited. “We usually don’t know which,” he said. “There’s a balance between the relative significance of diffusion and reaction.” Van Gerpen is developing equations to address this. “Diffusion we can do,” he said, “but the piece we’re missing is we don’t know what the chemical reaction rate is in the droplets.” Some calculations have been done by industry but only for specific reactors and parameters. He cited David Boocock’s (Toronto) pioneering work in cosolvent use for biodiesel, used by Biox today. A cosolvent changes a two-phase reaction into one. With a cosolvent there is no longer a mass transfer limitation so, as Van Gerpen said, “The measured rates should be the true chemical reaction rate.” He said modeling can tell us where monos and di’s reside in the droplets. Also, it can help understand how methanol droplets get smaller as the reaction proceeds, and how glycerin behaves in methyl esters. “Can we optimize the timing of agitation and settling for the fastest overall process?” he asked. Ultimately, he said, “This knowledge has to help design a better process, but I’m not sure how just yet.” An important conclusion is that the process should no longer be called mass transfer limited.

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