# Process of Formaldehyde and Volatile Organic Compounds' Removal from Waste Gases
## 4 Upgrading the Waste Gas Purification Process
### Kinetics and Chemical Equilibrium in the Hydration of Formaldehyde  
Formaldehyde is highly reactive and, therefore, commonly handled in aqueous and/or methylene glycol solutions, where it forms different adducts with the solvents. Formaldehyde absorption by water is a mass-transfer process, complicated by a chemical reaction in the fluid phase [13]. In general, the basic resistance of mass transfer is represented by the Equation (8). \[\frac{1}{K}=\frac{1}{\beta_{g}}+\frac{m}{\beta_{l}} \tag{8}\]  
The basic design of such a process requires models of the phase equilibrium of aqueous and methylene glycol formaldehyde mixtures. Such models must consider not only differences in intermolecular forces but also a variety of chemical reactions [13]. Formaldehyde is unstable in its pure, gaseous state and is usually produced as an aqueous solution. In aqueous solutions, the most important chemical reaction product is methylene glycol (MG). In solution, formaldehyde is almost completely hydrated to methylene glycol. \[\mathrm{CH_{2}O}+\mathrm{H_{2}O}\leftrightarrow\mathrm{CH_{2}(OH)_{2}} \tag{9}\]  
Methylene glycol, depending on the strength of the solution, can be polymerized and forms a series of polyoxymethylene glycols [14]. \[\mathrm{CH_{2}(OH)_{2}}+\mathrm{HO(CH_{2}O)_{n-1}H}\leftrightarrow\mathrm{ HO(CH_{2}O)_{n}H}+\mathrm{H_{2}O} \tag{10}\]  
These chemical reactions have a significant effect on the properties of formaldehyde-containing solutions and should be taken into account in each thermodynamic model of these systems. The physicochemical model developed by [15] combines these chemical reactions with physical interactions and has proved to be very successful in describing the balance of volatile liquids and enthalpy in mixtures containing formaldehyde. This model is constantly updated by [16; 17; 18], and its application has been extended with new, reliable data. The equilibrium distribution of formaldehyde and its reaction products with water (W) significantly affects not only the equilibrium properties, but also the kinetics of the chemical reaction and transport properties. Equilibrium distribution data are available from spectroscopic studies, which show that in aqueous solutions, formaldehyde is present as methylene glycol and polyoxymethylene glycol,but rarely as free molecular formaldehyde [9]. Using a NMR (high resonance frequency nuclear magnetic resonance spectrometer), it is possible to separate signals from -CH\({}_{2}\)- groups in different polyoxymethylene glycols. Various models are proposed for describing the equilibrium of vapor-liquid in the formaldehyde water system. The model shown in [15] assumes that the vapors above the aqueous formaldehyde solution consist of three types: Water, formaldehyde monomers, and methylene glycol. These species are also present in the liquid phase. However, fluid with higher concentrations of formaldehyde also contains higher polyoxymethylene glycols. It is assumed that the vapor phase behaves as a mixture of ideal gases, but the chemical-reaction equilibrium for the formation of methylene glycol from formaldehyde and water is taken into account with the constant of the balance of chemical reactions [15]. The liquid phase is treated as a real, chemically reactive mixture of aqueous formaldehyde monomers, methylene glycols, and polyoxymethylene glycols at higher formaldehyde concentrations. The chemical potential of the components in the liquid phase is normalized according to [PERSON]'s law. The non-ideality of the liquid phase is taken into account, using the contribution method of the UNIFAC (**F**unctional-group **A**ctivity **C**oefficients) group [19]. Chemical reaction equations are taken into account with the true reaction equilibrium constants [14]. One of the most important features of this model is its capacity to be expanded to other chemically reactive components, such as methanol, but also to nonreactive, chemically inert components such as trioxane and methylal.