1. CATALYSIS AND CATALYTIC REACTION MECHANISM PART II BY PUAN AZDUWIN KHASRI 6 DISEMBER 2012

2. DESIGN EQUATION Catalyst weight W substitute V (reactor volume) Batch : N AO dX/dt = -r' A W Packed-bed (tubular) :F AO dX/dW = -r' A CSTR : W = F AO X / -r' A (less frequently used)

3. 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

4. TYPES OF CATALYST DECAY Sintering/ Aging Fouling/ Coking Poisoning

5. 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, k d, follows the Arrhenius equation Decay law

6. 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, k d =180 h -1,V occupies 200 kg feed. – Find the X at t=2 hours

7. Design Equation Solution Reaction rate law Decay Law Stoichiometry

8. Combining Integrate Solving

9. DECAY RATE LAW

10. 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;

11. 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.

12. REACTOR TYPE VS SPEED OF CATALYST DECAY

13. 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.

14. MOVING BED REACTOR The design equation for moving-bed reactor

15. Straight-Through Transport Reactors (STTR) The catalyst deactivates very rapidly.

16. Catalyst Decay in Reactors EXERCISE: – Example 10-6 – Example 10-7 – Example 10-8

17. THE END