Estudo termodinâmico, cinético e otimização da produção de etileno a partir de etanol em alumina e óxido misto de cério-zircônio

Abstract

The ethylene production through catalytic dehydration of ethanol is a process that became stagnated over the last decades due to viability and low costs of natural gas and naphtha. Nowadays ethylene production is usually done by cracking of hydrocarbons. However, the manufacture of ethylene through ethanol is getting more attention mainly because of environmental reasons. Thus, this work had as objectives to study the kinetics of ethylene production from ethanol using Al2O3 and CeZrO2 as catalysts to optimize reaction conditions and to analyze thermodynamically this system using the minimization of the Gibbs free energy using experimental values at standard state and using methods based in quantum chemistry. A design of experiment methodology was used to obtain kinetic data varying temperature, ethanol concentration in feed and residence time and the data was fitted to obtain response surfaces. The maximization of ethanol conversion and molar fraction of ethylene was done using non-linear optimization method generalized reduced gradient (GRG). Kinetic models proposed in this work were Power- Law type and others simple models based on Langmuir-Hinshelwood kinetic. Quantum computational calculations were performed using software Gaussian03 applying B3LYP/6-31G+ and G2 methods, while the others calculations used the free software Scilab. The CeZrO2 catalyst was more selective for ethylene production than Al2O3, besides an expressive hydrogen formation. This could bring economic advantages, since this catalyst leads to products with high added value (C2H4 and H2), while alumina forms ethylene and a large quantity of water. Results of thermodynamic equilibrium state indicated that ethylene production through ethanol is highly favorable. The quantum method ab initio pos Hartree-Fock (G2) was more efficient that hybrid functional B3LYP in predicting chemical equilibrium in systems analyzed in this work, when compared with results obtained through experimental data on standard state. Effect of water addition in feed was studied and from a thermodynamical point of view, it may be said that the addition of small quantities of water does not affect expressively the equilibrium. This result can contribute to process economy as a whole, because an important part of costs in obtain ethylene through ethanol route is related to the need of using ethanol with high purity level.