Experimental and Theoretical Analysis of Phase Equilibria in a Two-phase System Used for Biocatalytic Esterifications

The partitioning behavior of the reactants 1-butanol, propionic acid and butyl propionate in an aqueous-organic two-phase system consisting of alginate beads suspended in hexane was investigated. Partitioning experiments with a single reactant showed that, even in the dilute region, the equilibrium concentrations of 1-butanol and propionic acid cannot be described by constant partition coefficients as is normally done in the field of biocatalysis. Besides the aqueous alginate beads, two other aqueous phases with different compositions (solutions with and without electrolytes) were also used for partitioning experiments. The equilibrium concentrations of the reactants obtained from the systems with the three different aqueous phases (water, water plus electrolytes, alginate beads) demonstrated that the partitioning behavior of the reactants is scarcely influenced by the presence of the electrolytes or by the alginate matrix, at least up to reactant concentrations of 80 mmol/l in the organic phase. The comparison of the experimental equilibrium concentrations with predicted values obtained from simulations with the modified UNIFAC (Dortmund) model showed a generally good agreement. However, in the dilute region, differences of up to 100% occurred between experimental and predicted values. Thus, for the later detailed mathematical modeling of processes occurring inside the alginate beads (such as mass transfer and enzymatic reaction), the modified UNIFAC (Dortmund) model is not adequate. Therefore, empirical correlations were derived for the mathematical description of the reactants' partitioning behavior. Experiments, conducted with two reactants simultaneously present in the two-phase system, showed that at reactant concentrations in the organic phase higher than 10 mmol/l the partitioning behavior of the investigated reactants is influenced by the presence of the second component. Thus, in systems with multiple reactants the derived correlations are strictly only valid up to this concentration.

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