Document Type: Review Paper
Department of Chemical and Biological Engineering, Iowa State University, Ames, IA 50011, USA.
Department of Chemical and Biomolecular Engineering, Ohio University, Athens, OH 45701, USA.
Serious environmental concerns regarding the use of fossil-based fuels have raised awareness regarding the necessity of alternative clean fuels and energy carriers. Biodiesel is considered a clean, biodegradable, and non-toxic diesel substitute produced via the transesterification of triglycerides with an alcohol in the presence of a proper catalyst. After initial separation of the by-product (glycerol), the crude biodiesel needs to be purified to meet the standard specifications prior to marketing. The presence of impurities in the biodiesel not only significantly affects its engine performance but also complicates its handling and storage. Therefore, biodiesel purification is an essential step prior to marketing. Biodiesel purification methods can be classified based on the nature of the process into equilibrium-based, affinity-based, membrane-based, reaction-based, and solid-liquid separation processes. The main adverse properties of biodiesel – namely moisture absorption, corrosiveness, and high viscosity – primarily arise from the presence of oxygen. To address these issues, several upgrading techniques have been proposed, among which catalytic (hydro)deoxygenation using conventional hydrotreating catalysts, supported metallic materials, and most recently transition metals in various forms appear promising. Nevertheless, catalyst deactivation (via coking) and/or inadequacy of product yields necessitate further research. This paper provides a comprehensive overview on the techniques and methods used for biodiesel purification and upgrading.
- Various biodiesel purification methods, i.e., equilibrium-based, affinity-based, and reaction-based separation techniques along with membrane technology and solid-liquid separation processes were reviewed.
- Deoxygenating process via hydrodeoxygenation and decarboxylation/decarbonylation pathways is a common way to upgrade bio-based oils to produce biorenewable diesel with excellent fuel properties.
- Catalysts and operating conditions, i.e., pressure, temperature, and contact time, are the most important variables in biodiesel upgrading discussed herein.