A Combined Reaction/Product Recovery Process for the Continuous Production of Biodiesel

Oak Ridge National Laboratory (ORNL) and Nu-Energie, LLC entered into a Cooperative Research And Development Agreement (CRADA) for the purpose of demonstrating and deploying a novel technology for the continuous synthesis and recovery of biodiesel from the transesterification of triglycerides. The focus of the work was the demonstration of a combination Couette reactor and centrifugal separator - an invention of ORNL researchers - that facilitates both product synthesis and recovery from reaction byproducts in the same apparatus. At present, transesterification of triglycerides to produce biodiesel is performed in batch-type reactors with an excess of a chemical catalyst, which is required to achieve high reactant conversions in reasonable reaction times (e.g., 1 hour). The need for long reactor residence times requires use of large reactors and ancillary equipment (e.g., feed and product tankage), and correspondingly large facilities, in order to obtain the economy of scale required to make the process economically viable. Hence, the goal of this CRADA was to demonstrate successful, extended operation of a laboratory-scale reactor/separator prototype to process typical industrial reactant materials, and to design, fabricate, and test a production-scale unit for deployment at the biodiesel production site. Because of its ease of operation, rapid attainment of steady state, high mass transfer and phase separation efficiencies, and compact size, a centrifugal contactor was chosen for intensification of the biodiesel production process. The unit was modified to increase the residence time from a few seconds to minutes*. For this application, liquid phases were introduced into the reactor as separate streams. One was composed of the methanol and base catalyst and the other was the soy oil used in the experiments. Following reaction in the mixing zone, the immiscible glycerine and methyl ester products were separated in the high speed rotor and collected from separate ports. Results from laboratory operations showed that the ASTM specification for bound acylglycerides was achieved only at extended reaction times ({approx}25 min) using a single-stage batch contact at elevated temperature and pressure. In the single-pass configuration, the time required gives no throughput advantage over the current batch reaction process. The limitation seems to be the presence of glycerine, which hinders complete conversion because of reversible reactions. Significant improvement in quality was indicated after a second and third passes, where product from the first stage was collected and separated from the glycerine, and further reacted with a minor addition of methanol. Chemical kinetics calculations suggest that five consecutive stages of 2 min residence time would produce better than ASTM specification fuel with no addition of methanol past the first stage. Additional stages may increase the capital investment, but the increase should be offset by reduced operating costs and a factor of 3 higher throughput. Biodiesel, a mixture of methyl esters, is made commercially from the transesterification of oil, often soy oil (see Reaction 1). The kinetics of the transesterification process is rapid; however, multiphase separations after the synthesis of the fuel can be problematic. Therefore, the process is typically run in batch mode. The biodiesel fuel and the glycerine product take several hours to separate. In addition, to push yields to completion, an excess of methoxide catalyst is typically used, which has to be removed from both the biodiesel and the glycerine phase after reaction. Washing steps are often employed to remove free fatty acids, which can lead to undesirable saponification. Standards for biodiesel purity are based either on the removal of contaminants before the oil feedstock is esterified or on the separation of unwanted by-products. Various methods have been examined to enhance either the pretreatment of biodiesel feedstocks or the posttreatment of reaction products, including the use of a cavitation reactor in the process intensification of the homogeneous acid catalysis of transesterification. Centrifugal mixing has been applied to biodiesel production, using the contactor as a low-throughput homogenizer, employing very low flow rates to increase residence times to tens of minutes. In this study, we have combined the reaction of oil and methoxide with the online separation of biodiesel and glycerine into one processing step, using a modified centrifugal contactor. Two distinct phases enter the reactor (reagents), and two distinct phases leave the reactor/separator (products), thus demonstrating the application of process intensification to high-throughput biofuel production. ORNL has been designing, fabricating, and operating centrifugal contactors for the selective extraction of actinide elements for over 25 years