Catalyst-Free Biodiesel Production Methods: A Comparative Technical and Environmental Evaluation

Oseweuba Okoro,John Birch,Zhifa Sun

doi:10.3390/su10010127

Abstract

In response to existing global focus on improved biodiesel production methods via highly efficient catalyst-free high temperature and high pressure technologies, this study considered the comparative study of catalyst-free technologies for biodiesel production as an important research area. In this study, therefore, catalyst-free integrated subcritical lipid hydrolysis and supercritical esterification and catalyst-free one step supercritical transesterification processes for biodiesel production have been evaluated via undertaking straight forward comparative energetic and environmental assessments. Energetic comparisons were undertaken after heat integration was performed since energy reduction has favourable effects on the environmental performance of chemical processes. The study confirmed that both processes are capable of producing biodiesel of high purity with catalyst-free integrated subcritical lipid hydrolysis and supercritical esterification characterised by a greater energy cost than catalyst-free one step supercritical transesterification processes for an equivalent biodiesel productivity potential. It was demonstrated that a one-step supercritical transesterification for biodiesel production presents an energetically more favourable catalyst-free biodiesel production pathway compared to the integrated subcritical lipid hydrolysis and supercritical esterification biodiesel production process. The one-step supercritical transesterification for biodiesel production was also shown to present an improved environmental performance compared to the integrated subcritical lipid hydrolysis and supercritical esterification biodiesel production process. This is because of the higher potential environment impact calculated for the integrated subcritical lipid hydrolysis and supercritical esterification compared to the potential environment impact calculated for the supercritical transesterification process, when all material and energy flows are considered. Finally the major contributors to the environmental outcomes of both processes were also clearly elucidated.

Highlights

In line with the global efforts to encourage a shift from fossil fuels to renewable and cleaner biofuels, many studies have been undertaken to develop sustainable liquid fuel alternatives
The pressurised mixed stream (1-P) is subsequently fed to the continuously stirred tank (CSTR) reactors (ST-1, ST-2, ST-3 and ST-4), arranged in series to improve the conversion of TAG to fatty acid methyl ester (FAME)
In the simulated direct supercritical transesterification (DST) process, the biodiesel (BIODIESL) stream is characterised by a FAME content (99 wt. %), that satisfies the European EN 14214 standard for the minimum required methyl ester content for biodiesel and is specified by a mass fraction of FAME of ≥96.5% per unit mass of the biodiesel product [49]

Summary

Introduction

In line with the global efforts to encourage a shift from fossil fuels to renewable and cleaner biofuels, many studies have been undertaken to develop sustainable liquid fuel alternatives These studies have resulted in the identification of biodiesel as a sustainable and environmentally benign fuel alternative that will aid in reducing peak global temperatures via the reduction in the volumes of greenhouse gas emissions such as CO2, NOx and SO2 [1]. This recognised importance of biodiesel as a renewable fuel has resulted in extensive work in the area of biodiesel research. Under such supercritical conditions the reacting alcohols usually exist in their supercritical state since the reacting conditions are at temperatures higher than the critical temperatures, 238.85 ◦C and 241 ◦C, of methanol and ethanol respectively and pressures higher than the critical pressures, 8.1 MPa and 6.41 MPa, of methanol and ethanol respectively [8,9]

Methods

Results

Conclusion