Thesis for degree: Licentiate of Engineering

3.3.2 Interphase transport
The limitation of heat transport at the interphase transport level is normally less severe than
the interparticle transport level and the greater part of the resistance is often in the boundary
layer around the particle rather than within it [48]. This can be expected as the thermal
conductivity of the solid is often much larger than that for the gas. For a low Reynolds
number Re, Bizzi et al. [51] described the equation for the mass transfer coefficient k c as:
(3.40)
(3.41)
(3.42)
where ρ is the fluid density, v the fluid velocity, κ the permeability, μ the dynamic viscosity
and ψ the shape factor. For spherical particles it is assumed that the shape factor is ψ = 1 [51].
Then equation (3.41) reduces to:
(3.43)
The definition of the criterion for the concentration gradient across the gas boundary film
along the particle is [49]:
(3.44)
where r r is the reaction rate, r p the particle diameter, C the concentration, D eff the effective
diffusivity and n reaction order. The Damköhler number for interphase mass transport is
defined as [49]:
(3.45)
where, besides those mentioned in equation (3.44), k c is the mass transfer coefficient. The
Damköhler number for interphase heat transport is defined as [49]:
(3.46)
where ΔH is the enthalpy change of reaction, h heat transfer coefficient and T the bulk
temperature. Further, the dimensionless activation energy is defined as [49]:
(3.47)
where E a is the activation energy, R the gas constant and T the bulk temperature. The
dimensionless maximum temperature rise is used in the criterion for the concentration
gradient across the gas film and the criterion is defined as [49]:
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