An experimental-based energy integrated process for biodiesel production from waste cooking oil using supercritical methanol

Abstract

Biodiesel has been recognised as one of the effective, green, renewable and sustainable fuels. It is derived from renewable living resources either animal fats or vegetable oils. Biodiesel production in the absence of catalyst using supercritical methanol has recently been receiving significant attention. Non-catalytic transesterification reaction eliminates the difficulties of catalyst preparation and separation processes. Although it has shown high conversion for the reactants with relatively short reaction time in comparison with the conventional catalytic transesterification processes, it has some disadvantages including higher reaction temperature and pressure, large excess of methanol to oil (M:O) molar ratio and higher energy consumption. In an attempt to mitigate these problems, an experimental study followed by process design/integration for biodiesel production from waste cooking oil (WCO) has been performed. A low-quality WCO collected from local restaurants has been selected as a feedstock for the reaction. The experimental phase of the transesterification reaction together with an optimisation procedure resulted in the optimised conditions of M:O molar ratio of 37:1, reaction temperature of 253.5 oC, reaction pressure of 198.5 bar in 14.8 min. The maximum yield was 91%. In addition, kinetics of the reaction has been studied concluding an irreversible pseudo first order reaction with a reaction rate constant of 0.0006 s-1. Moreover, thermodynamics of the reaction has been studied at a temperature range of 240 - 270 oC with resulting frequency factor and activation energy of 4.05 s-1 and 50.5 kJ/mol. A kinetic reactor has been simulated using the experimentally determined kinetic and thermodynamic data. The enthalpy content of the reactor product stream has been used to separate most of the unreacted methanol in an adiabatic flash drum. Finally, a scheme has been developed for an energy integrated process in order to maximise the heat recovery. Energy savings resulted from the developed heat exchanger network (HEN) have been concluded as 32.2 % and 23.8 % for both heating and cooling energies respectively, in comparison with an existing process energy requirements in the literature. The amount of heat exchanged for each unit has been determined in addition to composition, temperature and pressure of the streams. Vacuum distillation column has been designed to separate the unreacted triglycerides from biodiesel in order to fulfil the quality restrictions of the final biodiesel product.