The Ansys Chemkin Multiphase Reactor model supports the use of one gas phase and multiple bulk phases. Here, a bulk phase is a phase defined in the context of the surface/liquid chemistry mechanism input. Composition of the individual phase, whether gas or bulk, is assumed to be homogeneous, and the phases are immiscible. A bulk phase can be liquid in the sense of the physical state of the phase (as gas, liquid, or solid). There are additional property and reaction type specifications available for such liquid-bulk phases. A liquid-bulk phase is treated as an ideal liquid mixture, that is, all the species in such a phase have unity activity coefficient and the mixture properties, including the mixture density (volume), are computed by the mole-fraction weighted average method.

Conservation of mass and energy are applied to individual phases in the multiphase reactor along with the conservation of species in each phase. In addition, global constraints on total reactor volume and reactor pressure are imposed. Note that all the phases in the reactor are assumed to have the same pressure.

Two canonical types of reactor models, closed and open, are available in the Ansys Chemkin Multiphase Reactor modeling framework. Both reactor models consider transient operation.

The closed reactor model doesn't allow any in-flow and out-flow whereas the open reactor model allows the flow of gas and bulk phases through the reactor. In essence, the gas phase and each bulk phase constitutes a batch reactor (for closed) and a perfectly stirred reactor (for open) in itself while interacting with other phases through the specified mass- and heat-transfer processes.

For the open reactor model, there could be multiple inlets (each carrying multiple phases) and one outlet. Gas and bulk phases can flow in and out of the reactor through these inlets and outlet.

The surface phases are bound to a bulk phase or to the reactor wall. Conceptually, as described in Surface Chemical Rate Expressions, a surface phase corresponds to an infinitesimal thin layer/membrane/surface sandwiched between the two adjoining phases. In this release of Chemkin, the surface area is user-specified and doesn't consider any volume-to-area dependence. For the Multiphase Open Reactor model, the surface areas that are bound to the bulk phases flow through the inlets and the outlet.