CATALYSIS AND CATALYTIC
REACTION MECHANISM
PART II
BY PUAN AZDUWIN KHASRI
6 DISEMBER 2012
DESIGN EQUATION
• Catalyst weight W substitute V (reactor
volume)
• Batch : NAO dX/dt = -r'A W
• Packed-bed (tubular) :FAO dX/dW = -r'A
• CSTR : W = FAOX / -r'A (less frequently used)
CATALYST DEACTIVATION
• Catalytic deactivation adds another level of
complexity to sorting out the reaction rate
law parameters and pathways.
a(t): catalyst
activity
Catalyst Decay
TYPES OF CATALYST DECAY
Sintering/
Aging
Fouling/
Coking
Poisoning
SINTERING/AGING
• The loss of catalytic activity due to a loss of
active surface area resulting from the prolonged
exposure to high gas-phase temperatures.
Decay law
The sintering decay
constant, kd, follows the
Arrhenius equation
Example 1:Calculating conversion with
catalyst decay in batch reactor
• Consider A B
• Reaction information,
– Isothermal, liquid phase
– batch reactor,
– catalyst due to decaying of aging
– W=20 kg, k’=45 h-1, kd=180 h-1,V occupies 200 kg
feed.
– Find the X at t=2 hours
Solution
Design Equation
N AO
dX
 r ' A W
dt
Reaction rate law
 r ' A  k ' a(t )CA
Decay Law
1
a (t ) 
1  kd t
Stoichiometry
N AO
C A  C AO (1  X ) 
(1  X )
V
Combining
dX W
 k ' a (t )(1  X )
dt V
dX
Wk ' dt

(1  X ) V 1  k d t
Integrate
dX
Wk '
dt
 (1  X )  V  1  kd t
Solving
1
Wk '
ln

ln( 1  k d t )
1  X Vkd
1
20kg45h 1
1
ln

ln(
1

180
h
2h)
1
1  X 200kg180h
X  0.136
DECAY RATE LAW
FOULING/COKING
• It results from a carbonaceous (coke) material
being deposited on the surface of a catalyst.
The amount of coke on the surface
after a time t
Activity for deactivation by coking;
POISONING
• Deactivation by this mechanism occurs when
the poisoning molecules become irreversibly
chemisorbed to active sites thereby reducing
the number of sites available for the main
reaction.
REACTOR TYPE VS SPEED OF CATALYST
DECAY
TEMPERATURE TIME TRAJECTORIES
To maintain a constant conversion
with a decaying catalyst in a
packed or fluidized bed is to
increase the reaction rate by
steadily increasing the feed
temperature to the reactor.
Temperature of the catalytic
reactor should be increased with
time in order for the reaction
rate to remain constant.
MOVING BED REACTOR
The design equation
for moving-bed
reactor
Straight-Through Transport Reactors
(STTR)
The catalyst deactivates very
rapidly.
Catalyst Decay in Reactors
• EXERCISE:
– Example 10-6
– Example 10-7
– Example 10-8
THE END