Maximizing the Yield of Saturated Hydrocarbons from Hydroprocessing of Used Cooking Oil by Control of Oligomer Formation

The feedstock for the process of hydroprocessed esters and fatty acids (HEFA) is being shifted to renewable waste and residues, e.g., used cooking oil (UCO) containing (un)saturated triacylglycerides (TAG), polar substances including diacylglycerides (DAG), monoacylglycerides (MAG) and free fatty acids (FFA), and dimeric/oligomeric products. However, the yield of saturated hydrocarbons during hydroprocessing of refined UCO (r-UCO) can be lowered by the formation of oligomers via double bonds present in unsaturated TAG and via those produced by dehydration of hydroxy groups of MAG and DAG and β-elimination of (un)saturated TAG. Herein we attempted firstly to remove polar substance from the r-UCO through liquid–liquid extraction and secondly to pre-hydrogenate unsaturated fatty acid chains before the main hydroprocessing reaction. The first attempt was effective for reduced oligomer formation in batch-mode experiments but not in continuous-flow ones. The other attempt was to prove very successful in both modes of experiments regardless of the presence of polar substances in r-UCO; thus, continuous-flow hydroprocessing of r-UCO could be sustained for 100 h without oligomer formation. Consequently, coupling pre-hydrogenation with main hydroprocessing is very powerful in maximizing the yield of hydrocarbons from the conversion of r-UCO because removing polar substances is meaningful but not economic.