(576a) A New Solubility Model to Capture the Intrinsic Kinetics of Biodiesel Formation From Palm Oil

A new solubility model, which takes into account the limited solubility between the reactants during the early stage of the reaction and the immiscibility of the reaction by-product, namely glycerol, has been developed to describe the kinetics of the base-catalysed transesterification of palm oil. Associated phase equilibria have been determined and tie lines have been constructed in order to identify the reaction path within the multiphase system. Under conditions of kinetic control (high mass transfer rates), the multiphase system is considered to be limited by interphase solubility, which in turn is modified by the reaction product (methyl esters). It was found that the solubility model constructed offers a good representation of the kinetics of the reaction (Figure 1). A comparison between the solubility model and a conventional pseudo-homogeneous reaction model reveals that for the reaction conditions studied in this work, i.e. 6:1 methanol to oil molar ratio and temperature of 30oC-60oC, due to a very short period of the multiphase stage in relation to that of the homogeneous stage, the latter model also provides a good description of the kinetics of the reaction. The use of a solubility model is deemed more crucial for the transesterification of triglycerides that have extremely limited solubility with methanol or for the reaction carried out at higher methanol to oil molar ratio or lower temperature, in which case the multiphase stage of the reaction would be longer and the assumption of a homogeneous reaction for the whole course of the reaction may no longer hold. In order to be able to collect a sufficient amount of early kinetic data (for data fitting and model comparison), a novel continuous flow reactor (mini-CSTR/PFR combination for segregated flow) had to be developed to determine the intrinsic kinetics of the fast base-catalysed transesterification reaction of palm oil for this purpose. Compared with a conventional batch apparatus, the mini-CSTR/PFR is more effective as it allows the collection of ultra low residence time kinetic data. The small dimensions of the reactor and the use of a high shear rotor-stator homogeniser present the advantage of substantially reduced mass transfer influence, which allows the reactor to operate in a kinetic (solubility)-controlled regime. Currently, the lowest possible conversion obtained from the reactor is approximately 15%. The design of scaled versions of this mini-CSTR/PFR might be useful for the development of industrial processes for a continuous production of biodiesel via base-catalysed reaction since very high conversions can be achieved within extremely short residence times employing reaction conditions normally used in the commercial production of biodiesel. For instance, a conversion of 86% can be obtained within 1 minute at the reaction temperature of 60oC, a methanol to oil molar ratio of 6:1, and a catalyst concentration of 1 wt%. Figure 1: Fit of solubility model to experimental results to extract intrinsic kinetic data.