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L -29 Diffusion Effects in catalyst:
effectiveness factor
Prof. K.K.Pant
Department of Chemical EngineeringIIT Delhi.
[email protected]
mailto:[email protected]:[email protected]
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The total rate of consumption Of A inside a pellet
Evaluated at the surface conditions
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Internal Effectiveness Factor
As s
Actual overall rate of reaction =
Rate of reaction that would result if entire
interior surface were exposed to the externalpellet surface
conditions C ,T
' "
A A A
' "
As As As
-r -r -r = = =
-r -r -r
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For STRONG PORE DIFFUSION
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Internal Effectiveness Factor
1 121
3
= coth -1
Internal effectivenessFactor, is: ranged 01
for a first-orderreaction in a spherical
catalyst pellet
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Calculation of Catalytic Effectiveness FactorCatalytic
Effectiveness Factor:
where
- Thiele Modulus
1storder reaction rate:
Spherical Pellet
Cylindrical Pellet
Slab Pellet
)313(1
Coth
DekSaR p/3
DekSaR
p/2
DekSaL p/
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How can the rate of reaction be increased?
Decrease the radius of the pellet.
increase the temperature.
Increase the concentration
Increase the internal surface area
For large value of Thiele modulus, the effectiveness factor (for
n>1
efffectiveness factor decrease with increase concentration at
the
external surface of the pellet
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For nthorder reaction
= ( 2/n+1 )
1/2
3/= R( k CASn-1/ De)
1/2
OR
2= R2kSap CASn-1/De
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Uses the measured values of the rate of reaction to
determine if Internal diffusion controls the rate.
Weisz-Prater Parameter CWP2= 3( Coth -1)
2 = (observed rate/rate cal. at CAS) x (rate
calculated at CAS) / diffusion Rate)
= (-rA(obs)/ -rAs
2= -rASSapR2/De CAs = -rAS pR
2/De CAs
WeiszPrater Criterion for internal diffusion
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CWP= (-rA(obs)/ -rAs )(-rASpR2/De CAS)
CWP = (-rA(obs) (pR2/De CAS)
These are measured or known terms.
if CWP > 1, Internal diffusion limits the rate.
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Falsified Kinetics
Measurement of the apparentreaction orderand activation energy
results primarily wheninternal diffusion limitations are
present.
This becomes serious if the catalyst pelletshape and size
between lab (apparent) andreal reactor (true) regime were Too
different.
Smaller catalyst pelletreduces the diffusion
limitationhigher activation energy moretemperature sensitive
RUNAWAY REACTION CONDITIONS!!!!
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Falsified Kinetics
With the same rate of production, reaction order
and activation energy to be measured
