Development of an environmentally benign and optimised biodiesel production process

Abstract

The challenges of reducing the world’s excessive dependence on fossil fuels and atmospheric accumulation of greenhouse gases have led to the development of alternative sustainable biodiesel. Recently, non-catalytic biodiesel production using supercritical technology has received a significant interest due to its numerous advantages including short reaction time, high yield of biodiesel, elimination of catalyst preparation and separation costs and its applicability for various feedstock. This study has introduced an in-depth assessment for the valorisation of both low and high acid values waste cooking oils (WCO) into biodiesel using supercritical methanolysis. The effects of different process variables have been investigated including methanol to oil (M:O) molar ratio, temperature, pressure and time. Both transesterification and esterification reactions have been extensively studied. Different responses have been investigated for this study including overall biodiesel yield, glycerol yield and FFAs conversion. Response surface methodology (RSM) via Box-Behnken Design (BBD) and Central Composite Design (CCD) has been used to investigate the effect of the process variables and their interactions on the reaction responses. In addition, overall reaction kinetics for both transesterification and esterification reactions have been studied where both have been reported as pseudo-first order reactions. Thermodynamics of the reaction has been analysed to report the thermodynamic data of the reaction including Arrhenius constant and activation energy. The kinetic studies have resulted in 50.5 kJ/mol for transesterification reaction and 34.5 kJ/mol for esterification reaction. Numerical and graphical optimisation have been employed to minimise the process conditions and to maximise the production of biodiesel where the optimal conditions of the low acidity WCO have been developed at M:O molar ratio of 37:1, reaction temperature of 253.5 oC, reaction pressure of 198.5 bar in 14.8 min reaction time for 91% biodiesel yield. However, for high acidity WCO the optimal conditions have been developed at M:O molar ratio of 25:1, reaction temperature of 265 oC and reaction pressure of 110 bar in 20 min for 98% biodiesel yield. Further, this work has developed a heat exchanger network (HEN) that has achieved the optimal process energy requirements based on Pinch method.