• Energy Research

Abstract The catalyst-free two-step process has been developed for biodiesel production using low-grade feedstocks. The first step consists of triglycerides hydrolysis under subcritical water conditions to generate and increase free fatty acid (FFA) content for ethyl ester production. In its subcritical state, water can be used as both a solvent and a reactant for the hydrolysis of triglycerides. The hydrolyzed product mixture is separated by decantation into the oil phase of FFA (upper layer) and a water phase that contains glycerol (lower layer). In the second step, the hydrolyzed products of free fatty acids were successfully esterified to their ethyl ester in supercritical ethanol conditions without any catalyst. Under the sub- and supercritical conditions of water and ethanol, the hydrolysis and the esterification reactions proceed quickly, with a conversion of greater than 98 % after 10−20 min. This two-step process for biodiesel production offers several advantages, such as milder reaction conditions and pollution reduction due to the use of water instead of organic solvents. Also, the glycerol is removed after the hydrolysis reaction so that the backward reaction between the glycerol and the ethyl ester disappears, and lead to the biodiesel yield and quality improvement. The aim of this study is making a comparison between our previous one-step process and the two-step reaction process to find the best pathway for designing and building an integrated reactor. Indeed, the two-step process is more applicable for low-grade feedstocks with a high amount of FFA and water.

Abstract The continuous biodiesel production process under sub- and supercritical conditions using a trace amount of potassium hydroxide (KOH) as a catalyst has been studied. CO2 was added as a co-solvent to reduce the reaction time and increase biodiesel yield. The proposed procedure enables simultaneous transesterification and esterification of triglycerides and free fatty acid (FFA), respectively. The shorter reaction time and milder reaction conditions may reduce energy consumption due to the simplification of the separation and purification steps. The process variables, including reaction temperature, ethanol to oil molar ratio, catalyst amount, and process pressure, were systematically optimized. The highest biodiesel yield (98.12%) was obtained after a 25-min reaction time using only 0.11% wt. of KOH and a 20:1 ethanol to oil ratio. The process optimum temperature and pressure were 240 °C and 120 bar, respectively. The proposed kinetic model suggested a first-order reaction with an activation energy of 15.7 kJ·mol−1 and a reaction rate constant of 0.0398/min−1. The thermodynamic parameters such as Gibbs free energy, enthalpy, and entropy were calculated as 144.82 kJ·mol−1, 11.4 kJ·mol−1, −0.26 kJ·mol−1 and at 240 °C, respectively.