Hydrotreated Vegetable Oils

Hydroprocessing is an alternative process to transesterification of oils and fats into biodiesel. The products are referred to as Hydrotreated Vegetable Oils (HVO) or synonymously as Hydroprocessed Esters and Fatty Acids (HEFA). They are considered to be among the advanced fuels because their NOx emissions during combustion are lower, they have better storage stability and better cold flow properties (EAFO 2020). Since 2011 HEFA is also approved for commercial use as aviation fuel (biokerosene). A big variety of oil and fat feedstocks can be used for HVO/HEFA processing. Also waste materials and low quality residues are suitable. The Finnish company Neste Oil owns currently the largest production capacities for HEFA worldwide.

In the first step of the process, contaminants like solids and water are removed. Then esters and double bonds of triglycerides in oils and fats have to be saturated. Hydrogen is used for this purpose, which is why this process is called hydrogenation. This process takes place in special fixed bed reactors with precious metal catalysts (e.g. cobalt or nickel-molybdenum) and requires temperatures from 280 to 340°C and pressures between 50 and 100 bar. As a result, vegetable oil is transformed into fully saturated n-alkanes.

The n-alkanes are then further processed in a following process step, called isomerization and cracking. During this step oxygen and double bonds are removed and branched molecules are created. Catalysts break n-alkanes into shorter molecules, open bonds are saturated with hydrogen and partially recombined to a complex branched molecule. Branched alkanes that neither contain oxygen nor double bonds have a lower freezing point which is for example important for aviation fuels.

Cracking and isomeration takes place at temperatures between 280 und 400°C and pressures between 30 and 100 bar. Higher temperatures and low pressures favour cracking, since isomeration takes place at lower temperatures and high pressures. The process is followed by distillation and rectification (see chapter 2.1) to seperate the products. Hydrogen is removed and diesel, naphta or kerosene are provided.