Modeling Reactive Mixtures: Case Study for Spills of Formaldehyde Solutions

When a liquid mixture such as an aqueous formaldehyde solution is released into the environment, the vaporization depends strongly on chemical reactions that regulate the evolution of formaldehyde vapors. Most of the consequence modeling methods and models currently used in the industry usually ignore the reaction phenomena. This paper describes a dynamic model developed using the Aspen® Custom Modeling (ACM) tool kit. In the case of formaldehyde solutions, the thermodynamics and reaction kinetics of a mixture of HCHO-H₂O-CH₃OH are computed from models available in the open literature. The liquid-phase, reversible, chemical reactions between water and formaldehyde, as well as methanol and formaldehyde, form oligomers, most of which are non-volatile. The mass transfer and heat transfer coefficients between the surface of the spill and the atmosphere are computed by established correlations for flow over a semi-infinite flat plate.

The DuPont-ACM model employs various methods for solving simultaneous systems of nonlinear algebraic and ordinary differential equations. This makes it possible, for example, to solve coupled differential mass and energy balances which are implicit in temperature and composition. Also, even though temperature and composition are not state variables, they can be given initial conditions.

For a release scenario, the model computes the time-varying liquid and vapor phase compositions and conditions. It also computes the required vaporization rates for HCHO and CH₃OH resulting from, for example, a 30 wt% formaldehyde solution spill onto a dry surface of a fixed area under specific meteorological conditions. The paper will present details regarding the formaldehyde spill modeling methodology, with examples.