Fig. 4.14 Vapor-liquid equilibrium in the system formaldehyde + water at 140°C. Left side: model with consistent description of chemical and phase equilibrium. Right side: inconsistent model
Experimental Studies of Phase Equilibria in Reacting Systems
For the design of RD processes, besides information on the reaction, information on phase equilibria is of prime importance, especially on vapor-liquid equilibria and in some cases also on liquid-liquid equilibria (see above). The systematic investigation of phase equilibria for the design of RD processes will generally involve also studies of reactive systems (see examples above). Studies of phase equilibria in reactive systems generally pose no problem if the reaction is either very fast or very slow as compared with the time constant of the phase equilibrium experiment (high or low Damkohler number Da). In the first case, the solution will always be in chemical equilibrium, in the second case, no reaction will take place. The definition of the time constant of the phase equilibrium experiment will depend on the type of apparatus used. If the RD process is catalyzed and the catalyst does not substantially influence the phase equilibrium, the phase equilibrium experiments can often be performed without catalyst and again no or only little conversion will take place.
Problems with phase equilibrium experiments in reactive systems arise mainly if the reaction time constant is of the same order magnitude as the time constants of phase equilibrium experiments (intermediate Damkohler number Da). For typical fluid phase equilibrium experiments, time constants are of the order of 10-1000 s, depending on the choice of the apparatus (and the definition of the time constant). These are however also typical time constants for many reactions, which are of interest for RD. It is therefore worthwhile to discuss measurements of reactive phase equilibria in more detail.
Measurements of reactive phase equilibria in the non-trivial case of intermediate Damkohler numbers Da can be classified into two groups: in the first, only the fully established equilibrium is investigated, whereas in the second the attempt is made to measure phase equilibrium without having reached chemical equilibrium. In experiments of the second type, reproducible results can only be expected, if their time constant is distinctly smaller than that of the chemical reaction.
Phase equilibrium experiments are usually classified into synthetic methods and analytical methods. In synthetic methods, the analysis of the coexisting phases is avoided by using information on the feed composition. The value of synthetic methods for measuring phase equilibria in reactive systems is obviously limited, even though they can in principle be applied to studies of fully equilibrated reacting mixtures, and are sometimes used together with extrapolation techniques to eliminate the influence of the reaction (see examples below).
Analytical methods for phase equilibrium measurements can be classified into methods that need sampling, such as gas chromatography or titration, and methods that do not need sampling, such as spectroscopic methods. Sampling is often a problem in chemically reactive systems. It may, for instance, shift the concentra-
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