(444d) Thermodynamic Modeling of the Nature of Speciation and Phase Behavior of Binary and Ternary Mixtures of Formaldehyde, Water and Methanol

Formaldehyde is an important industrial chemical with numerous applications. Due to its high reactivity, formaldehyde is usually produced, stored, sold and processed in the form of aqueous solutions, with methanol added for stability. In these solutions, formaldehyde reacts with the solvents to form a variety of reaction products, including oligomers. These chemical reactions can occur in the liquid and vapor phases and have a significant influence on the properties of formaldehyde-containing solutions. Of particular interest to industrial applications, is the prediction of the vapor-liquid equilibria (VLE) in formaldehyde-containing solutions, considering the chemical reactions that occur. In this work, the SAFT-g Mie group-contribution (GC) equation of state (EoS) [1-3], a predictive thermodynamic modelling technique, is used.

In the SAFT-g Mie framework, molecules are modelled as heteronuclear chains formed from fused spherical segments, which represent the distinct functional groups comprising the molecule. In this framework, it is assumed that the properties of a molecule or a mixture can be determined from the weighted contributions of the functional groups present in the system of interest, with the assumption that the parameters characterizing the functional groups are fully transferable across molecules.

In this work, the oligomerization reactions taking place in formaldehyde-containing aqueous and methanolic solutions are modeled implicitly using a physical approach which is possible within the SAFT-g Mie framework by adding association (reactive) sites to mediate the formation of the reaction products. Here, the reaction products are considered to be aggregates of the reactants. First, we use experimental data for the VLE in binary mixtures of formaldehyde-water and formaldehyde-methanol to obtain the optimal unlike interaction parameters between the corresponding SAFT-g Mie groups. Then, the newly developed parameters are used to predict the VLE of ternary formaldehyde-water-methanol mixtures, for a wide range of temperatures and pressures, with excellent agreement to experimental data. Finally, the distribution of reaction species (oligomers) in binary and ternary mixtures is predicted using the SAFT-g Mie EoS with remarkable agreement to experimental data.

[1] Papaioannou, V. et al. Chem. Phys. 140, 054107 (2014).

[2] Dufal, S. et al. J. Chem. Eng. Data 59, 3272–3288 (2014).

[3] Haslam, A. J. et al. J. Chem. Eng. Data 65, 5862–5890 (2020).