CATALYSIS AND CATALYTIC REACTION MECHANISM

PART II

BY PUAN AZDUWIN KHASRI6 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): catalystactivity

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.

The sintering decay constant, kd, follows the Arrhenius equation

Decay law

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

Design Equation

Solution

Wrdt

dXN AAO '

AA Ctakr )(''

tkta

d1

1)(

Reaction rate law

Decay Law

Stoichiometry

)1()1( XV

NXCC AO

AOA

Combining

)1)((' XtakV

W

dt

dX

tk

dt

V

Wk

X

dX

d

1

'

)1(

tk

dt

V

Wk

X

dX

d1

'

)1(

Integrate

)1ln('

1

1ln tk

Vk

Wk

X dd

Solving

136.0

)21801ln(180200

4520

1

1ln 1

1

1

X

hhhkg

hkg

X

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

Temperature of the catalytic reactor should be increased with

time in order for the reaction rate to remain constant.

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.

MOVING BED REACTOR

The design equationfor 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