DEVELOPMENT OF A KINETIC-MODEL FOR THE ESTERIFICATION OF ACETIC-ACID WITH METHANOL IN THE PRESENCE OF A HOMOGENEOUS ACID CATALYST

The esterification kinetics of acetic acid with methanol in the presen ce of hydrogen iodide as a homogeneous acid catalyst was studied with isothermal batch experiments at 30-60 degrees C. The catalyst concentr ation was varied between 0.05 and 10.0 wt%. The experiments revealed t hat besides the main reaction, the esterification of acetic acid, a si de reaction appeared: the catalyst, hydrogen iodide, was esterified by methanol to methyl iodide. Plausible reaction mechanisms for methyl a cetate and methyl iodide formation were proposed. The rate-determining step in the acetic acid esterification was assumed to be the nucleoph ilic attack of methanol to the carbenium ion formed through proton don ation to acetic acid, whereas the rate-determining step in the hydroge n iodide esterification was presumed to be the substitution of the iod ide to protonated methanol. Hydrogen iodide and acetic acid act as pro ton donors; thus the protolysis equilibria of acetic acid and hydrogen iodide were included in the mechanism. Rate equations, concentration- based as well as activity-based with UNIFAC activity coefficient estim ations, were derived, and the kinetic and equilibrium parameters inclu ded in the rate equations were estimated from experimental data with r egression analysis. Simulation of the models with the estimated parame ters revealed that the rate equations predict correctly the experiment al trends in the acid catalyzed esterification. (C) 1997 Elsevier Scie nce Ltd.