The chemistry and engineering behind the biodiesel process
By Christina Borgese and Marc Privitera | September 08, 2011
Biodiesel reaction and separation methods range from the time-honored large batch tanks with long residence time reactions and water washes for product separation to intensification, enzymatic, and supercritical reactions coupled with distillation and mechanical separation methods. Selecting the best reaction and separation method for the process depends upon the feedstock characterization, a process we covered in the August issue of Biodiesel Magazine. This installment highlights the specific drivers behind selecting the best reaction and separation techniques while considering process throughput, overall conversion efficiency and plant economics.
The biodiesel reactions have three elements for driving the conversion of the feedstock to the finished product: mixing, molar ratio, and residence time. In simple terms, you have to present the reagent molecules with the opportunity, enough energy and enough time to react. It’s kind of like a seventh-grade school dance.
Mixing is the first operation to consider. Mixing drives the reagent interface surface area. The interface surface area is increased by decreasing through shear the dispersed phase liquid droplet size to the smallest size possible. In the usual batch reactor, the mixing is motivated by an agitator. Many high-shear flow strategies have been successfully employed to intensify phase interaction. The number of molecules motivated to react is driven by the surface area of the two immiscible phases.