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Fixed Bed Reactor. Quak Foo Lee Chemical and Biological Engineering The University of British Columbia. C A,out. Regeneration. 1. 2. C A,in. Fixed Bed Reactor. Solids take part in reaction  unsteady state or semi-batch mode Over some time, solids either replaced or regenerated.

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fixed bed reactor

Fixed Bed Reactor

Quak Foo Lee

Chemical and Biological Engineering

The University of British Columbia

fixed bed reactor2

CA,out

Regeneration

1

2

CA,in

Fixed Bed Reactor
  • Solids take part in reaction  unsteady state or semi-batch mode
  • Over some time, solids either replaced or regenerated

Breakthrough

curve

CA,out/CA,in

t

isothermal reaction plug flow reactor
Isothermal Reaction:Plug Flow Reactor
  • Plug flow of fluid – no radial gradients, and no axial dispersion
  • Constant density with position
  • Superficial velocity remains constant
plug flow model
Plug Flow Model

z + dz CA,f + dCA,f

z CA,f

U0 (m/s) superficial velocity

mass balance
Mass Balance

Input – Output – Reaction = Accumulation

Divide by ∂z and take the limits as ∂z  0

ε is void fraction in bed

slide6

Void fraction

For first order reaction, fluid only:

For steady state:

Volume of reactor

Therefore,

conversion as a function of height
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

balance on solid
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

solve these equations
Solve These Equations

= 0 (In quasi steady state, we ignore the

accumulation of A in gas)

Substitute rAv

non isothermal packed bed reactor
Non-Isothermal Packed Bed Reactor
  • For mass continuity  did balance on fluid and on solid
  • For energy balance, we do balance on each phase
non isothermal packed bed reactor12
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)
modeling
Modeling

q =0

Tf + dTf

z + dz

Tf

z

Tf,0

U0

governing equations
Governing Equations

Solve all these equations together.

  • Fluid
  • Solid

Quasi Steady State

for heat transfer
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.