Miniplant experiments showed precipitation of the components cinnamic
acid, aldehyde, and alcohol from the liquid phase, because their
concentration exceeded the saturation concentration. Especially the low
water saturation concentration of cinnamic acid led to precipitation. In
order to be able to simulate substance precipitation and dissolution in
the aqueous phase and thus achieve a higher model depth, a storage term\(\dot{n}\)precipitation,i is introduced for any given
component i. In every time step of the dynamic simulation, the
concentration of each component in the aqueous phase is compared to the
saturation concentration of that same component. If the concentration
exceeds the saturation concentration, the overlap is stored in the
storage term, from which it can dissolute again at lower concentrations.
The mass balance of each component in the aqueous phase is therefore
described through differential equations in dependency on the reactions
of the respective enzymes, the feed streams, mass transfer to the
organic phase, and the precipitated amount in each time step. Equation
(6) shows the generalized form of the mass balance for any given
component i in the volume specific form. Equations (7) and (8) show the
modified forms of the mass balance for the cofactors NADH and
NAD+ respectively, considering the auto-oxidation of
NADH instead of the non-existing precipitation of the cofactors.