Ethanol Conversion to Butadiene: A Thermodynamic Analysis

This work presents a review of the published studies approaching the thermodynamics of the ethanol conversion to 1,3-butadiene (BD), completed with our own calculation results. A spontaneity analysis, for the global reaction transforming ethanol into BD, has evidenced that a frequently used principle, considering negative variations of the standard Gibbs free energy, as a criterion for practically relevant equilibrium reactant conversions, appears as relatively conservative. The calculation of spontaneity domains, for the global reaction of BD synthesis from ethanol, showed that equilibrium ethanol conversions up to 80% can be obtained, even at positive values of standard Gibbs free energy variation. The characteristics of the ethanol conversion to BD, at chemical equilibrium, were calculated using reaction schemes describing the accepted process mechanisms and chemical species observed experimentally. The necessary properties of the chemical species were extracted mainly from the database of Aspen Plus simulator, whose computing facilities were also used for most of equilibrium calculations presented. The results confirm the previously published information, indicating that the process BD yield, at equilibrium, decreases to practically uninteresting values, when the calculation schemes include the secondary products observed experimentally. This proves that the practical catalytic processes of ethanol conversion to BD are kinetically controlled. An evaluation of the two process alternatives for producing butadiene, i.e., directly from ethanol and ethanol-acetaldehyde mixture, showed that the latter process is slightly favored thermodynamically, in comparison with the first (one-step) alternative.