reactive water ethanol
Fig. 4.10 RD lines (full lines) and isotherms (dashed lines) in the system ethanol + acetic acid + ethyl acetate + water at 1 bar. The reactive azeotrope and the temperature minimum do not coincide as a result of thermodynamic inconsistency
188.8.131.52 Formaldehyde + Water + Methanol: Intrinsically Reactive Complex Mixture
Formaldehyde is one of the most important CI building blocks in the chemical industry and is the basis for a large variety of products. It is usually handled in aqueous solutions, which generally also contain methanol. In those solutions, formaldehyde (CH20) and water react to methylene glycol (HOCH2OH) and a series of oligomer poly(oxymethylene) glycols (H0[CH20]„H, n > 1). Similar reactions occur in methanolic formaldehyde solutions, yielding hemiformal (H3C0CH20H) and poly (oxymethylene) hemiformals (H3C0[CH20]nH, n > 1).
Analysis of formaldehyde solutions by conventional methods such as titration only gives the overall formaldehyde concentration that results from the sum of the free formaldehyde (usually a very small amount) and that bound in its various reaction products. The true concentrations in the solution can only be measured with NMR-spectroscopy.
It should be kept in mind that while formaldehyde + water + methanol mixtures are ternary from an overall standpoint, they are polynary mixtures with more than 20 components in appreciable concentrations if the true species are taken into account.
MG methylene glycol MGn poly(oxymethylene) glycol
HFn poly(oxymethylene) hemiformal
Fig. 4.11 Scheme of a physicochemical vapor-liquid equilibrium model of the system formaldehyde + water + methanol
A predictive thermodynamic model for thermodynamic properties of formaldehyde solutions, has been developed by Maurer and coworkers [20-23]. It explicitly takes into account the chemical reactions in these complex systems. Fig. 4.11 shows a scheme of the vapor-liquid equilibrium model. The methylene glycol and hemiformal formation take place both in the gas and the liquid phase. The oligomers are present only in the liquid phase and do not have to be considered in the gas phase due to their low vapor-pressures. The gas phase is a mixture of ideal gases, non-idealities in the liquid phase are taken into account by the UNIFAC group contribution method. Some of the UNIFAC parameters are adjusted to experimental vapor-liquid equilibrium data of systems containing formaldehyde. The chemical equilibrium model is based on NMR spectroscopic data of the liquid phase reactions and gas density measurements . The results obtained with that model are usually given in overall concentrations to allow direct comparison with results from standard analysis. The agreement with experimental data on vapor-liquid equilibria is excellent in binary, ternary, and multicomponent formaldehyde containing mixtures over a wide range of compositions and temperatures [22, 23].
Fig. 4.12 gives an example of results of the application of that model. RD lines are shown for the system formaldehyde + water + methanol at 1 bar in overall concentrations. The distillation boundary between the low-boiling azeotrope in the formaldehyde + water system and methanol, the global low boiler, can clearly be seen.
One advantage of such physicochemical models is that they can easily be extended to include effects of reaction kinetics. This is shown in Fig. 4.13 where results from a case study on reactions kinetic effects on separations of mixtures of formaldehyde + water + methanol are shown. Whereas the equilibrium model
4 Thermodynamics of Reactive Separations | 85 formaldehyde azeotrope
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