(60b) Modeling Spills of Reactive Chemicals

When materials such as Oleums and Chlorsulfonic Acid are released into the environment they react with all available water to generate heat and form an acid mist. 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. 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. The algorithms do not always tolerate discontinuities, so smoothing techniques are used where needed. In the case of oleum, the thermodynamic behavior of the mixture is represented by the vapor-liquid equilibrium of the H2O-H2SO4-SO3-H2S2O7 system. The reaction for example between SO3 and H2O is represented as first order in the concentration of SO3 and H2O in the liquid phase. An arbitrarily large reaction rate constant is chosen, so that essentially all of the limiting reactant is consumed quickly. The mass transfer and heat transfer coefficients are computed assuming a flow over a flat plate.

For a release scenario the model computes the liquid and vapor phase compositions and conditions with time and the required vaporization rates for H2SO4 and SO3 resulting from for example a 65 wt% Oleum spill onto a dry or wet surface of a fixed area under specific meteorological conditions. The paper will present details regarding the modeling methodology and some preliminary validation using data from field spill studies.