Pre-treatment of used cooking oils for the production of green chemicals: A review

Abstract

Exploitation of waste lipids in the production of higher value-added and green chemicals will become a common practice in the future to enhance the sustainability of the oleochemical industry. However, waste lipids are highly heterogeneous and contain a large variety of impurities that can affect potential valorization routes. Thus, there is need for carry out refining processes to obtain suitable oleochemical feedstocks. This work reviews the available and potential technologies for the pre-treatment, purification and refining of waste lipids, emphasizing the transformation of used cooking oils (UCOs) into oleochemical raw materials. Initially, a description of the world generation and supply of UCOs is presented, and some potential valorization alternatives are reported. Then, the chemical nature and composition of typical UCOs is considered, and the main physicochemical properties are recognized in order to identify suitable separation processes for impurities removal. In particular, acidity and moisture reduction are priorities considering that they affect further thermal, chemical and biochemical transformations. Also, toxic compounds need to be reduced to enable the use as fermentation substrates, or as raw material for value-added application. Nitrogen-, phosphorus-, and sulfur-containing compounds as well as polar compounds must to be removed to enhance sensorial properties, and to avoid negative effects during subsequent transformations. Among the most popular up-grading methods, esterification, adsorption, solvent extraction, and distillation have been adopted industrially. Also, some intensification alternatives have been effectively implemented in UCO refining (e.g. reactive separations). • Used cooking oils (UCOs) pre-treatment processes are presented. • Typical chemical composition and impurities are described in depth. • Potential effects in further valorization processes are explained in detail. • Comprehensive review of processes for removal of impurities of different nature. • Current process intensifications alternatives for UCOs pre-treatment are reported.