1. Fixed Bed Reactor Quak Foo Lee Chemical and Biological Engineering
The University of British Columbia

2. Fixed Bed Reactor
Solids take part in reaction  unsteady state or semi-batch mode
Over some time, solids either replaced or regenerated 1 2
CA,in
CA,out
Regeneration
Breakthrough
curve
CA,out/CA,in t

3. Isothermal Reaction: Plug Flow Reactor
Plug flow of fluid – no radial gradients, and no axial dispersion
Constant density with position
Superficial velocity remains constant

4. Plug Flow Model z + dz CA,f + dCA,f z CA,f
U0 (m/s) superficial velocity

5. Input – Output – Reaction = Accumulation
Mass Balance
Input – Output – Reaction = Accumulation
Divide by ∂z and take the limits as ∂z  0
ε is void fraction in bed

6. For first order reaction, fluid only:
Void fraction
For first order reaction, fluid only:
For steady state:
Volume of reactor
Therefore,

7. Conversion as a function of Height
Integrating with CA,f = CA,f,in at z = 0
Note 1: Same equation as for catalytic reactor with 1st order reaction
Note 2: Can be used in pseudo-homogeneous reaction

8. Balance on Solid aA (fluid) + S (solid)  Products
Input – Output – Reaction = Accumulation
Over increment of dz: input = 0, output =0
Volume fraction of solid =
m3 of solid m3 of reactor volume
mol
m3 of solid · s

9. Balance on Solid

10. Solve These Equations = 0 (In quasi steady state, we ignore the
accumulation of A in gas)
Substitute rAv

11. Non-Isothermal Packed Bed Reactor
For mass continuity  did balance on fluid and on solid
For energy balance, we do balance on each phase

12. Non-Isothermal Packed Bed Reactor
Assumptions:
Adiabatic reaction – no heat lost through shell to surroundings (no radial temperature gradients) q = 0
Biλ is small – uniform T within particle (an exothermic reaction Tp > Tg)
Plug flow of gas and use Tref =0 for enthalpy calculations
Assume an average density can be used (ρg = constant)

13. Modeling
q =0
Tf + dTf
z + dz Tf z
Tf,0 U0

14. Governing Equations Fluid Solid Solve all these equations together.
Quasi Steady State

15. For Heat Transfer Z=0 all t Tf = Tf,0; Ts = Ts,0
T = 0 all z Ts = Ts,0; Tf = Tf,0
Generally, require a numerical solution for the solution of the set of equations.