(536f) Thermodynamic Modeling of Ion Exchange Resin Catalyzed Liquid Phase Esterification

The simultaneous chemical and phase equilibrium as well as the reaction kinetics of ion exchange resin catalyzed esterification of acetic acid with ethanol were investigated. The modeling approaches applied in the literature to the reaction kinetics of this type fall into three categories: 1) pseudo-homogeneous approach, 2) surface adsorption models (Eley?Rideal, Langmuir?Hinshelwood?Hougen?Watson, etc.), and 3) heterogeneous two-phase models where the ion exchange resin is seen as a polymer solution surrounded by an external fluid phase. In this work, the last-mentioned approach was adopted, and thermodynamic models derived for polymer solutions and gels were used to calculate the activity coefficients of the chemical species. The results obtained were interpreted by considering the swelling behavior of the resins (elastic properties) and the distribution of the species between the phases (selective sorption).

Esterification of acetic acid with ethanol was studied in batch and fixed-bed reactors using three sulfonated PS?DVB ion-exchange resins of different cross-link densities (5.5 wt-% DVB, 7.0 wt-%, and 20 wt-%) and porosities (microporous, mesoporous, and macroporous). The reactants to dry catalyst mass ratio and the reaction temperature were varied. Prior to the reaction kinetics experiments, the phase equilibrium behavior of the non-reactive binaries with the polymer, as well as of the reactive five-component system, were investigated with independent experiments. The extent of swelling of the resins as a function of liquid phase composition was also recorded.

The simultaneous chemical and phase equilibrium behavior of the systems were correlated with a modified Flory?Huggins model coupled with an appropriate expression for the swelling pressure in the resin phase. The reaction rate equation was written in terms of resin phase concentrations and activity coefficients. The effect of resin swelling on the H+ ion concentration was explicitly included in the rate equation.

As to the equilibrium state of the system in a batch reactor, the selective sorption effect (i.e., lower concentration of water than of ethyl acetate in the liquid phase) was found to decrease with increasing cross-link density of the resin. The thermodynamic model was able to reproduce this phenomenon. Further, the model predicted that the fractional extent of reaction in a batch reactor increases with the amount of resin catalyst in the system.

In the kinetic experiments it was found that the overall reaction rate per unit mass of resin catalyst decreases with increasing cross-link density, which is in accordance with the previous works. However, it was shown that this does not originate from decreasing accessibility of the sulfonic acid groups, which is the classical explanation, but is linked to the phase equilibrium behavior of the reactive system containing an elastic polymeric component.