Optimal conditions determination for hydrodeoxygenation of free fatty acids to obtain green diesel

Abstract

In this work, the production of green diesel involving the hydrodeoxygenation reaction of vegetable oil, such as oleic acid, producing CO or CO2 as byproducts, was studied. This reaction takes place under hydrogen saturation to produce stearic acid; subsequently, heptadecane is produced by the decarbonylation and decarboxylation pathways, while octadecane is obtained by the subsequent hydrodeoxygenation of octadecanal. In the formation of octadecane, water is obtained as a byproduct, while the production of heptadecane leads to CO and CO2 as byproducts of the decarbonylation and decarboxylation reactions. When considering the products and reaction routes in the hydrotreatment of triglycerides and free acids, a mathematical model was found to be reliable in determining the minimization of the Gibbs free energy of reaction over a range of reaction temperatures and pressures. This model followed the methods of Marrero‐Gani, Joback‐Reid, and Chen‐Dinivahi‐Jeng for the estimation of thermodynamic properties of the pure components. The fugacity was calculated by considering the partial fugacity coefficients, and the Peng‐Robinson equation of state and the van der Waals mixing rule were used. The mole fractions at equilibrium were estimated by least squares analysis, using MATLAB, in order to account for non‐idealities in reaction thermodynamic properties and to find the equilibrium composition that minimized the total Gibbs free energy. Likewise, the reaction coordinates of the equilibrium reaction were estimated together with the known values of the composition in order to know the effect of pressure and temperature on the aforementioned reactions.