19.5.3. Collision Diameter Data for Gas Species

The reaction rate involving a gas species and the particle cloud is proportional to the collision frequency between the gas molecule and the particle surface. When the size of gas molecule is comparable to the particle size, the collision diameter of the gas molecule should be included in the collision frequency calculation. Ansys Chemkin permits the collision diameter to be input as part of the reaction data via the DCOL keyword. Syntax and rules of the DCOL keyword are described below.

Figure 19.9: Syntax for DCOL keyword

<gas_reactants>+<surface_reactants>   jis<core_bulk>+<products>AisBisEis
             DCOL/diameter_in_cm/

Rules:

  1. Auxiliary keyword DCOL must be used in conjunction with a surface reaction.

  2. The surface reaction must be irreversible. If a reverse reaction is possible, the reverse reaction should be described by a separate irreversible reaction.

  3. The surface reaction must have only one gas species on the reactant side. The collision diameter given by the DCOL keyword corresponds to the collision diameter of this gas reactant.

  4. There must be at least one surface/bulk species appearing on the reactant side. This is to indicate the particle with which the gas molecules are colliding. The particle is identified by the core bulk species or unique species on its surface.

  5. The default value is 0 [cm], which assumes that the size of gas species is much smaller than the particle size.

  6. Like regular surface reactions, conservation of surface sites is desirable.

Units:

The reaction rate is given in [mole/cm2-sec] and the collision diameter is in [cm].

Example:

The surface reaction below describes the deposition of gas species C6H6 on a carbon particle whose core is represented by C(B):

Figure 19.10: Deposition of gas species C6H6 on a particle

C6H6 + 4(S) => 6C(B) + 4H(S) + H2        0.1  0.0  0.0
    DCOL/2.41E-8/
    FORD/(S)  2.0/
    STICK

The collision diameter for gas species C6H6 is 2.41E-8 [cm]. The parameters given in the reaction line are the sticking coefficient and the reaction rate is second-order with respect to (S) instead of the default fourth-order. Note that, when the sticking coefficient is used to specify the kinetic constant, the dependence is embedded in the rate formulation.