Prediction of Liquid-Liquid Equilibria in Oligomer-Containing Biotechnological Systems Using Associating Lattice Cluster Theory (ALCT)

In bioprocess engineering often oligomer-containing complex mixtures occur, which show liquid-liquid demixing and are hard to describe with standard thermodynamic models. The situation is complicated further by the fact that many of these systems are only poorly characterized and/or contain components which are not available as pure substances. Standard process engineering workflows fail in these cases. Recent advances in molecular thermodynamics enable tackling these issues. In the present study, we combine lattice cluster theory^1–3 for modeling the molecular architecture with Wertheim theory for modeling H-bonding yielding the Associating Lattice Cluster Theory (ALCT).

As a test system for the application of ALCT, we choose mixtures of oleic acid (OA), formic acid (FA), and formoxystearic acid (FSA). That system is of interest for the production of FSA-based biopolymers from the renewable resource OA. FSA can be produced from OA and FA in the presence of strong acids. It is, however, presently not available commercially as a pure substance. OA + FA show a miscibility gap, but it is unclear if this is also the case for FA + FSA. Both OA and FSA are oligomeric substances, i.e. their molar mass is relatively high in comparison with FA, but end group effects are still important. Thermodynamic modeling of the system FA + OA + FSA is therefore extremely challenging, but a prerequisite for studies of the production of biopolymers using the route over OA and FA. ALCT lends itself to tackle the problem as it is well suited for predictions of properties of mixtures containing oligomers of different architecture.

Liquid-liquid equilibria (LLE) in the system FA + OA + FSA were studied both experimentally and by modeling and simulation with ALCT. In the experiments, the main challenge is the analysis, for which quantitative NMR spectroscopy was used. The ALCT model was parameterizes using experimental data on LLE and excess enthalpies for the system FA + OA and available data on related systems. The description of FSA is predictive and based only on information on its molecular structure. The predictions of ALCT are compared to experimental data on the LLE in the system FA + OA + FSA and show very good agreement. This demonstrates the applicability of ALCT for modeling and predicting the phase behavior of oligomer-containing biotechnological systems.

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