DEVELOPMENT OF REACTION KINETIC MODELS FOR VAPOR-PHASE HYDROGENOLYSIS OF GLYCEROL TO 1,2-PROPANEDIOL

Abstract

Catalytic hydrogenolysis of the excess amount of glycerol obtained as a byproduct from the biodiesel industry has attracted the attention of researchers since last two decades. A good amount of information is available on liquid phase hydrogenolysis however, liquid phase hydrogenolysis presents few difficulties such as high operating pressure and separation of catalyst from the reaction system. Due to this nowadays, researchers are moving towards more economic and process friendly vapor phase hydrogenolysis. In this study, kinetics of vapor phase hydrogenolysis of glycerol to 1,2-propanediol over 35 wt % Cu-Zn (7:3)/MgO catalyst is studied using various models such as Langmuir-Hinshelwood-Houson-Watson (LHHW) non-dissociative model, LHHW dissociative model and Eley-Rideal model. The kinetic experiments were performed at atmospheric pressure, in the temperature range of 190-240 oC and contact time (W/FGo) of 203-609 kgcat h-1. The products of the reactor were analyzed using an offline gas chromatography. Acetol, 1,2-propanediol were the major products identified while 1-propanol, 2-propanol, methanol, ethanol, ethylene glycol and glycerol were the other products comprising of less than 5%. The kinetic models were developed considering a new reaction scheme, i.e. glycerol gets first dehydrated to form acetol which is subsequently hydrogenated to form 1,2-propanediol. The reaction mechanism was developed considering adsorption, surface reaction and desorption steps. The external mass transfer resistance was neglected based on Mears’ criterion of external diffusion [APPENDIX II]. The intra-particle mass transfer resistance was neglected considering the Weisz-Prater criterion for internal diffusion [APPENDIX I]. The surface reaction was assumed to be rate-deterring step and irreversible in nature. A mathematical model equation was formulated based on mole in terms of partial pressure of individual species i.e., glycerol, acetol and 1,2-propanediol (neglecting the less significant compounds) which resulted in a system of ordinary differential equation. The system of ODE was solved numerically by employing inbuilt ODE solver ode23s for stiff system. The solution was integrated with the Genetic algorithm optimization technique to minimize the fitness function. Fitness function was basically a sum of squares between simulated and experimental values of partial pressure of individual species. ii| P a g e The two model namely LHHW non-dissociative type model and Eley-Rideal model successfully correlated the experimental data with simulated data. For LHHW non-dissociative model, activation energy estimated as 63.35 ± 3.29 kJ kmol-1 for formation of acetol from glycerol. Whereas the activation energy for formation of 1,2-Propanediol from acetol was calculated as 109.23 ± 3.77 kJ kmol-1. For Eley-Rideal model, the activation barrier for dehydration and hydrogenation reaction was estimated to be 82.62 ± 8.30 kJ kmol-1 and 146.18 ± 3.94 kJ kmol-1 respectively.