(279a) Experimental Investigation to Characterize the Influence of Single Components On a Complex Multicomponent Liquid-Liquid-Equilibrium

The most commonly used separation process
in chemical industry is distillation, followed by liquid-liquid-extraction. In
general, the separation process can be modeled using the equilibrium method.
While vapor-liquid-equilibria (VLE) are the basis for distillation processes,
extractive separations can be described using liquid-liquid-equilibria (LLE).
The NRTL-equations are a common model used to account for the real behavior of
the respective liquid phases by activity coefficients. Experimentally
determined binary interaction parameters are commonly used to describe the
behavior of multicomponent systems. Furthermore, predictive methods, like
UNIFAC, often estimate the VLE behavior of simple mixtures with a satisfactory
accuracy. The complexity increases with two liquid phases in equilibrium (VLLE
or LLE). Due to strong interactions in the liquid phases, the thermodynamic
modeling of such industrial relevant systems is very complex. The exact
description of these systems especially when polar molecules or electrolytes
are included is an enormous challenge. A
theoretical or predictive behavior of such complex LLE systems often does not
reflect the reality since there is a huge number
of influencing parameters. In order to reduce the experimental effort, a
systematic approach, based on a minimal necessary number of fundamental
experiments is needed to describe such complex mixtures.

This systematic approach is required
to understand the influence of every component on the thermodynamic
equilibrium. The characterization of a multicomponent liquid-liquid-mixture can
be realized in four steps that are schematically shown in Figure 1. In the first
step the components which are forming two liquid phases have to be
identified. Their binary or ternary mixing behavior has to be investigated to a
sufficient extent. The second step includes the investigation of the
influence of single additives on the thermodynamic equilibrium. Therefore, the
respective components are added separately in different quantities to a binary
or ternary model mixture in order to characterize their influence on the
equilibrium by use of the partition coefficient. In the third step of
the routine, the additive interactions are investigated. All relevant
components are combined into a multicomponent reference system, with which the
combined effect can be compared to the influence of the single components. In
the fourth step, the key components that show the largest impact on the
equilibrium have to be identified evaluating the results of steps one to three.
The resulting reduced mixture of key components is less complex but shows
equivalent characteristics compared to the real process solution including even
unknown components.

Figure 1:Approach for
the identification of key compounds

This practical approach is illustrated
in an example where the optimization of a complex industrial process is
subsequently accomplished. A liquid-liquid extraction process is the key
operation of the entire process. The investigated mixture consists of water,
different alkyl-pyridines, an ammonium salt, other organic nitrogen compounds
and unknown components, where water has to be separated from the pyridines. In
the first step, water and two alkyl-pyridines have been identified as
main components. The isothermal LLE of the three main components is presented
in Figure 2. Pyridine A and
water form two liquid phases. The closed miscibility gap has its critical point
near the water/pyridine B binary system. In the second step, components
were added to model solution. The thermodynamic behavior shows a strong
dependence on the species of the additives and the added amount. A correlation
between the polarity of the additives and the impact on the equilibrium can be
seen. Interactions between additives in a multicomponent model mixture and the
process solution will be investigated in step three. Step four
ends with the definition of the resulting key components. The thorough
application of the systematic approach will help to optimize the extractive
separation unit incorporated in this complex industrial process and to reduce
the complexity of the downstream processing.

Figure 2: Ternary LLE
of Pyridine A, Pyridine B and water, experimental results, T= 60°C