(762a) The Rate-Limiting Step in Liquid-Liquid Transesterification Reactions for Biodiesel Production

Transesterification reactions between a nonpolar triglyceride-rich oil phase and a polar alcohol phase have received much interest over the past decade due to the growing interest in biodiesel production. A quantitative analysis of the liquid-liquid phase behavior of these systems has been carried out which has facilitated the thorough investigation of the rate-limiting step in these reactions. In this study, the transesterification reaction was carried out in a unique experimental apparatus consisting of a thin graduated cylinder. Under these conditions, the two liquid phases exist as distinct liquid layers with the alcohol phase resting on top of the triglyceride-rich oil phase. The mixing that was selected was unique in that each phase was mixed independently so that the composition of each phase could be assumed homogeneous and yet remains as a distinct liquid layer. This experimental setup allowed each phase to be sampled independently as well as kept the interfacial area between the two phases constant; it is simply the cross sectional area of the cylinder. By first measuring the partition coefficient of the homogeneous potassium-based catalyst, it was determined that the alcohol phase acts as the primary reaction volume. Once the primary reaction volume was established, the rate-limiting step was investigated. The transesterification reaction rate is defined as being under mass transfer control when the rate of triglyceride mass transfer into the alcohol phase is less than the reaction rate of triglycerides to form biodiesel methyl esters. To experimentally compare the rates of mass transfer and reaction, the primary reaction volume, i.e. the alcohol phase, was sampled and its contents were analyzed using a HPLC. Mass transfer control is physically signified by the absence of triglycerides in the alcohol phase. When this is observed it is because triglycerides react to form methyl esters as soon as they enter the alcohol phase. On the other hand, the transesterification reaction rate was defined as being under reaction control when the rate of triglyceride mass transfer into the alcohol phase is greater than the reaction rate to form methyl esters. When this occurs, triglycerides will accumulate in the reaction volume and can be measured by the HPLC. The two variables with the greatest affect on the rate-limiting step are the catalyst concentration and the agitation speed; the effects of both of these variables were therefore investigated.