(202b) Phase Equilibria of a Reactive Distillation Process for Isobutyl Acetate Production

Environmental concerns on the use of petrochemical solvents have boost the production and use of greener substitutes. Among these, biobased esters of short and medium chain alcohols stand out as direct or functional substitutes for a large variety of petrochemical solvents for different applications. However, the main challenge for a most biobased esters are the higher production costs compared with the traditional fossil chemicals. To overcome this drawback, there is need to developed more efficient and integrated processes for synthesis and separation.

In the last few years, reactive distillation has been applied to the synthesis of a large variety of bioderived esters. The synergic effect of combining reaction and separation helps overcoming chemical equilibrium limitations, improving process productivity and reducing energy consumption. Further improvement can be achieved by developing even highly integrated processing schemes such as the dividing wall reactive distillation, reactive HIDIC columns, reactive centrifuge distillation, etc. In any case, the conceptual development of such processes requires a reliable representation of the phase equilibria in the multicomponent system.

Amongst the large variety of biobased esters of industrial interest, isobutyl acetate has found a variety of applications in high value-added niche markets such as in fragrance, flavors and cosmetics enamels. Currently, isobutyl acetate is produced by Fischer esterification of isobutanol with acetic acid under batch operation. The process is limited by the chemical equilibrium, so continuous water removal during reaction is required. However, there are several azeotropes between all the components even between alcohol and acetic acid, so excess alcohol or a make-up alcohol stream are required to achieve high conversions. Thus, the implementation of a reactive distillation process seems suitable to overcome major limitations during isobutyl acetate synthesis. While the idea has been mentioned in the past, a suitable feasibility study has yet to be done.

Modeling of reactive distillation operations require a reliable thermodynamic model of the reactive mixture. In particular, the isobutyl acetate system is characterized for being highly non-ideal; the system exhibits different azeotropes (binary, ternary, and possibly quaternary), immiscibility of reactants, and dimerization in the vapor phase. Also, incomplete phase equilibria data are available in the literature, so process modeling can be affected by the unknown behavior of the mixtures. Then, the aim of this work was to do a complete review of available phase equilibria data of the mixtures involved in the synthesis of isobutyl acetate. Taking into account the missing data, LLE measurements were done for mixtures of acetic acid - isobutanol - water at 10, 20 and 30 °C. Then, by using results from the whole set of experiments, together with reported VLE and LLE data of binary and ternary mixture, a complete set of binary interaction parameters for an activity-based model were regressed. The model agrees reasonably well with fitted data, and I was used to create a residue curve map for the conceptual design of an isobutyl acetate reactive distillation process.