Reactive Vapor-Liquid Equilibria
Methods for measuring reacting vapor-liquid equilibria can roughly be classified into three groups: static or batch experiments, flow experiments and, as an intermediate between the first and the second type, recirculation experiments (Fig. 4.16).
For measuring vapor-liquid equilibria in reactive systems, analysis is needed in most cases. Most analytical techniques require sampling. This, however, often is non-trivial in batch cells. Special care has to be taken in sampling the gas phase, as this tends to alter the pressure. Batch cells are, therefore, especially attractive for measurements of reactive phase equilibria, when combined to feedback free analytical techniques such as spectroscopy. As time constants to establish equilibrium are rather high in batch cells, their application for studies of reacting systems is limited either to investigations of the fully established vapor-liquid and chemical equilibrium or to studies of slow reactions. Batch cells are often combined with sampling loops, which is a step in the direction of a recirculating still.
Batch cells have been used to study reactive phase equilibria, for instance by Patel and Young  and Arlt . These authors employ extrapolation techniques to gain information on the state, where no reaction has occurred. This allows them
Fig. 4.16 Schemes of different basic types of phase equilibrium experiments. (Single pass flow and recirculation techniques with evaporator in front of the phase separation/ equilibrium unit and condensation of the gas phase before sampling)
single pass flow r®-
recirculation a to use the information on the amounts of the different reactants charged into the batch cell and avoid problems with analysis. Interesting designs, in which batch cells have been coupled with spectroscopic analytics are described by Krissmann et al.  and Rogers et al. . Krissmann et al.  used UV spectroscopy for their studies of aqueous, sulfur dioxide containing systems. The liquid phase was analyzed in situ in the batch cell whereas for the gas phase a special optical cell was connected to the batch cell by a probe loop. Rogers et al.  studied aqueous amine solutions containing carbon dioxide using IR-spectroscopic analysis, employing probe loops both for the liquid and the gas phase. Roederer  describes a versatile cell with in situ IR-spectroscopic analysis of both phases, in which both vapor-liquid equilibria and reaction kinetics were studied.
The basic advantage of flow cells compared to batch cells is that sampling generally poses less a problem, especially for the gas phase. Furthermore, the time to establish phase equilibrium after evaporation in the phase separator-equilibrium unit (Fig. 4.16) can be kept low using suitable designs. For instance, in Cottrell-type units, such as the one described by Rafflenbeul and Hartmann  that time is of the order of 10-20 s. For reactions with time constants of the order of some minutes, flow techniques can therefore be used for measuring phase equilibrium without having reached chemical equilibrium. The disadvantage of the technique is the high amount of substance needed due to the single pass mode.
Single pass flow techniques have been used for studies of vapor-liquid equilibria of reacting systems by various authors. Maurer and coworkers describe a special type of thin film evaporator with a residence time of about 1-2 min, which they have extensively used for studies of vapor-liquid equilibria in formaldehyde containing systems in chemical equilibrium [20, 32]. In those studies, low evaporation ratios were used in order to only slightly shift the liquid-phase equilibrium upon evaporation. That idea has been extended by Hasse and Maurer  by using a gas saturation technique. More recently, a single pass Cottrell type flow cell was developed especially for phase equilibrium measurements in reacting systems by Reichl et al. . Residence times are about 30-90 s in that cell.
The problem of high amounts of substance needed in flow cells in a single pass mode can be circumvented using recirculation techniques. However, the recirculation can cause problems in studies of chemically reactive systems if an ongoing chemical reaction in the recirculating stream leads to shifting compositions of the evaporator feed, so that no steady state is reached.
Nevertheless, recirculation techniques have been employed for studies of reactive phase equilibria by several groups. In an early paper, Hirata and Komatsu  suggest an extrapolation technique, similar to the one discussed above, to eliminate problems with the ongoing chemical reaction. Applications of recirculating stills for measurements of phase equilibria in esterification systems are described, for instance, by Kang et al. , Lee and Kuo , Lee and Liang , and Lee and Lin . In these experiments enough time is given to reach a steady state. The question, how the comparatively quick evaporation with non-zero evaporation ratio affects the measurements, is, however, not discussed.
This brief survey shows that there are many options for measuring phase equilibria in reacting systems, which allow to carry out such studies for a wide range of systems and conditions. The main limitation for experimental investigations of reactive vapor-liquid equilibria is related to the velocity of the reaction itself: if phase equilibrium measurements of solutions are needed, which are not in chemical equilibrium, the reaction must be considerably slower than the characteristic time constant of the phase equilibrium experiment. Apparatus are available, where that time constant is distinctly below one minute. For systems with reactions too fast to be studied in such apparatuses, it should in many cases be possible to treat the reaction as an equilibrium reaction, so that the information on the phase equilibrium in mixtures, which are not chemically equilibrated is not needed.
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