1. CH EN 5253 – Process Design II Effects of Impurities on Reactions and Reactor Design
February 11, 2019

2. Books
There is no chapter in the book on this subject

3. Impurity Effects Reactors Heat Exchange Separation Systems
Recycle Loops

4. Location of Separation Units

5. Impurities in Reactors
Point 1: Poisons for Catalysts
Kill Catalyst with time
Point 2: Impurities can cause side reactions altering
Reactor conversion
Generating additional undesirable products
Point 3: Impurities Impact Equilibrium Conversion
Point 4: Impurities Impact Reaction Rates
Lower concentrations
Point 5: Impurities have Reaction Heat Effects
Increase Cp

6. Point 1: Poisons for Catalysts
Kill Catalyst with time
Lead, Sulfur, Manganese in Gasoline kill Catalytic Converter
Platinum, Palladium, Rhodium
Note: Vehicles equipped with catalytic converters can run only on unleaded fuel

7. Review: Mechanism of Heterogeneous Catalysis

8. Catalytic Reactors Various Mechanisms depending on rate limiting step
Surface Reaction Limiting
Surface Adsorption Limiting
Surface Desorption Limiting
Combinations

9. Catalytic Reactors Toluene Hydrodealkylation (HDA)Process
H2 + C7Hi (T) CH4 + C6H6(B)
Catalyst: Chromium or molybdenum or platinum oxides
Langmuir-Hinshelwood Mechanism (Surface Reaction Limiting)
Impurities are adsorbed on catalytic sites
Decrease the Cv concentration of active sites

10. Point 2: Side reactions altering
Reactor conversion
Generating additional undesirable products
(undesired)
(undesired)

11. Point 3: Impurities Impact Equilibrium Conversion
Temperature Effects
Single Equilibrium
aA +bB  rR + sS
𝐾 𝑒𝑞 = 𝑎 𝑅 𝑟 𝑎 𝑆 𝑠 𝑎 𝐴 𝑎 𝑎 𝐵 𝑏 = exp −Δ 𝐺 𝑟𝑥𝑛 𝑜 𝑅𝑇 ,   𝑑 𝑙𝑛 𝐾 𝑒𝑞 𝑑𝑇 = Δ 𝐻 𝑟𝑥𝑛 𝑜 𝑅 𝑇 2
Van’t Hoff eq.

12. Unfavorable Equilibrium
Increasing Temperature Increases the Rate
Equilibrium Limits Conversion
Equilibrium line is repositioned and rate curves are repositioned due to impurities

13. Point 4: Impurities Impact Reaction Rates

14. Point 5: Reaction Heat Effects
Effect of Inert Addition Similar to Impurity Effects
Adiabatic
∆T=T2-T1
Q=external heat added
With increased inerts Cp rises, and these curves become more closely vertical.

15. Managing Heat Effects Reaction Run Away Reaction Dies
Exothermic
Reaction Dies
Endothermic
Preventing Explosions
Preventing Stalling

16. Impact on Reactor Design

17. PFR – no backmixing Used to Size the Reactor Space Time = Vol./Q
Outlet Conversion is used for flow sheet mass and heat balances
rK is smaller and V is larger due to impurities.

18. CSTR – complete backmixing
Used to Size the Reactor
Outlet Conversion is used for flow sheet mass and heat balances
rK is smaller and V is larger due to impurities.

19. Temperature Profiles in a Reactor
Exothermic Reaction
Impurities effect these curves and areas under these curves=size of reactor

20. Costs Higher capital cost for Higher operating cost for
Bigger reactors
Cooling and heating systems
Installation
Higher operating cost for
Utility – hot and cold water
Pumping
Separation

21. Consideration of Impurities in Catalytic Reactor Design
How the surface adsorption and surface desorption influence the rate law?
Whether the surface reaction occurs by a single-site or dual-site reaction between adsorbed molecule and molecular gas?
How does the reaction heat generated get dissipated by reactor design?