In situ transesterification of rapeseed for production of biodiesel and secondary products

Abstract

In situ transesterification (IST) can potentially reduce the cost of biodiesel production by avoiding the oil extraction and refining stages of conventional transesterification, through the direct reaction of the oilseed with alcohol in the presence of a catalyst. However, a large excess of alcohol is currently required in IST to achieve comparable yields to conventional transesterification. Hence, in this study, methods for improving in situ transesterification of rapeseed for biodiesel production have been investigated. The focal point of this study is to reduce or utilize the excess alcohol. Pre-soaking seeds in methanol and reactive coupling were subsequently attempted. The respective rationales are to reduce the excess methanol requirement, and convert/use the excess methanol in a secondary process. Pre-soaking involved a chemical pre-treatment of the oilseed prior to the transesterification reaction. Pre-soaking was performed with methanol to oil molar ratio (MOMR) of 360:1 at 60°C using a catalyst (NaOH) concentration of 0.1M. A two-level factorial design was used to determine the optimum conditions for pre-soaking. It was found that a biodiesel yield of 85% was obtained for pre-soaking at 360:1 MOMR while the ‘un-soaked’ biodiesel yield was 75% at 475:1 MOMR. The higher biodiesel yield with 24% reduction in methanol requirement could potentially translate to energy savings in the downstream separation of biodiesel from excess methanol. Reactive coupling (transesterification + a glycerol polymerisation reaction) should increase the equilibrium conversion of biodiesel, whilst generating valuable secondary products. It was carried out in a pressure vessel at 10 bar, 140°C in an inert atmosphere. Polyglycerol was identified in the reaction mixture using FTIR and 1H-NMR. Using a MOMR of 375:1 with catalyst concentration (H2SO4) of 4.8 v/v% at 140°C, a biodiesel yield of 90% and polyglycerol (PG) yield of 10% were observed after 4 hours of reaction. Overall, the material balance indicated that at the end of the reaction, 19% of the unused methanol had been converted to dimethyl ether (DME). This would lead to energy savings in the separation of product. The Central Composite design of experiment for reactive coupling indicated that catalyst concentration was the most significant variable in biodiesel production, whilst molar ratio is significant for both polyglycerol and DME production. Moreover, the study demonstrates the practicality of FTIR online monitoring of IST, which could be valuable for on-line monitoring at industrial scales, where iii traditional off-line GC analysis is time-consuming and ineffective to correct immediate production problems. Furthermore, the online monitoring could be used for “fast IST” of rapeseed to biodiesel to detect the onset of saponification. This work has demonstrated co-production of valuable chemicals with biodiesel production via reactive coupling for the first time. This could be the initial step toward an integrated biodiesel-based bio-refinery.