Question 10.46: Derive the partial differential equation for unsteady-state ...
Derive the partial differential equation for unsteady-state unidirectional diffusion accompanied by an nth-order chemical reaction (rate constant k):
\frac{∂C_{A}}{∂t} =D\frac{∂^{2}C_{A}}{∂y^{2}} -kC_{A}^{n}
where C_{A} is the molar concentration of reactant at position y at time t. Explain why, when applying the equation to reaction in a porous catalyst particle, it is necessary to replace the molecular diffusivity D by an effective diffusivity D_{e}. Solve the above equation for a first-order reaction under steady-state conditions, and obtain an expression for the mass transfer rate per unit area at the surface of a catalyst particle which is in the form of a thin platelet of thickness 2L.
Explain what is meant by the effectiveness factor ; for a catalyst particle and show that it is equal to (1/\phi) tanh \phi for the platelet referred to previously where \phi is the Thiele modulus L\sqrt{(k/D_{e})} .
For the case where there is a mass transfer resistance in the fluid external to the particle (mass transfer coefficient h_{D}), express the mass transfer rate in terms of the bulk concentration C_{Ao} rather than the concentration C_{AS} at the surface of the particle. For a bed of catalyst particles in the form of flat platelets it is found that the mass transfer rate is increased by a factor of 1.2 if the velocity of the external fluid is doubled.
The mass transfer coefficient h_{D} is proportional to the velocity raised to the power of 0.6. What is the value of h_{D} at the original velocity?
k = 1.6 × 10^{-3} s^{-1} , D_{e} = 10^{-8} m²/s
catalyst pellet thickness (2L) = 10 mm.
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