Excess Free Energy of the Reactive Systems Ethanol + Acetic Acid, I-Butanol + Acetic Acid and 3-Methyl-1-Butanol + Acetic Acid At 323 K

Excess Free Energy of the Reactive Systems ethanol + acetic acid, i-butanol + acetic acid and 3-methyl-1-butanol + acetic acid at 323 K.

Jairo Duran, Yohana Páez, Alejandro López, Alvaro Orjuela

Grupo de Procesos Químicos y Bioquímicos. Department of Chemical and Environmental Engineering. Universidad Nacional de Colombia. Bogotá. Colombia

Reliable thermodynamic data is important for evaluation and design of chemical processes especially in systems where biobased chemicals are involved. For example phase equilibrium predictions obtained from generalized methods (e.g. UNIFAC) tend to fail with polar or branched structures common in biobased molecules. Among the variety of biobased chemicals, esters play an important role in the current chemical industry; therefore, experimental evaluation on the thermodynamics of such materials is required .  Esterification systems have been studied widely and in most cases, thermodynamic information has been evaluated more than once. However, reactive systems, i.e., alcohol + carboxylic acid show problems related with reaction products (ester and water) which can change phase equilibrium behavior of the mixture. Several authors have pointed the importance of carrying out phase equilibrium measurements under mild conditions where reaction extent is negligible.  

In this work, isothermal VLE of the binary systems containing ethanol, i-butanol and 3-methyl-1-butanol with acetic acid have been measured at 323 K. A hermetic and jacketed-glass cell (100 mL) equipped with a magnetic stirrer was used in experiments. Degasified mixture of acid + alcohol was charged into the cell continuously stirring at 500 rpm and the temperature inside the cell was maintained at 323 K ± 0.1 K by using heating fluid circulation from an isothermal bath. Using a turbo pump (Pfeiffer Vacuum GmbH, Germany), pressure was reduced in the chamber below 0.05 kPa ± 0.03 kPa. Equilibrium pressure in the cell was measured with an absolute capacitance manometer (Baratron® model 625 D, MKS Instruments, US) with an accuracy of ± 0.03 kPa. Procedure was repeated more than once in each point in order to ensure a correct pressure reading. Samples were analyzed by gas chromatography. Acetate fractional yield was not greater than 0.7% (mole basis).

Activity coefficients for the liquid phase and compositions of the vapor phase were calculated taking into account the dimerization of acetic acid in the vapor phase. Additionally, Gibbs – Duhem equation was used to evaluate excess free energy in the systems. All systems showed negative deviations from the ideality. The Herington test of thermodynamic consistency was performed to evaluate data reliability. The effect of excess volume with total pressure was neglected in thermodynamic consistency calculations. Three data sets were consistent using the criteria of Herington and Kojima at al. Equilibrium data were correlated with three parameter NRTL model. Maximum deviation of 0.8 kPa was found. Negative values of G^E showed the strong interactions in liquid phase due the formation of hydrogen bonds. This effect is stronger in ethanol + acetic acid than 3-methyl-1-butanol + acetic acid system.      

Keywords: Esterification, vapour – liquid equilibrium, excess free energy, thermodynamic consistency.