1. S,S&L Chapter 7 Terry A. Ring ChE
Reactor Design
S,S&L Chapter 7
Terry A. Ring
ChE

2. Reactor Types Ideal Real PFR CSTR
Unique design geometries and therefore RTD
Multiphase
Various regimes of momentum, mass and heat transfer

3. Reactor Cost Reactor is PRF CSTR Pressure vessel
Storage tank with mixer
Hydrostatic head gives the pressure to design for

4. Reactor Cost
PFR
Reactor Volume (various L and D) from reactor kinetics
hoop-stress formula for wall thickness:
t= vessel wall thickness, in.
P= design pressure difference between inside and outside of vessel, psig
R= inside radius of steel vessel, in.
S= maximum allowable stress for the steel.
E= joint efficiency (≈0.9)
tc=corrosion allowance = in.

5. Reactor Cost Pressure Vessel – Material of Construction gives ρmetal
Mass of vessel = ρmetal (VC+2VHead)
Vc = πDL
VHead – from tables that are based upon D
Cp= FMCv(W)

6. Reactors in Process Simulators
Stoichiometric Model
Specify reactant conversion and extents of reaction for one or more reactions
Two Models for multiple phases in chemical equilibrium
Kinetic model for a CSTR
Kinetic model for a PFR
Custom-made models (UDF)
Used in early stages of design

7. Kinetic Reactors - CSTR & PFR
Used to Size the Reactor
Used to determine the reactor dynamics
Reaction Kinetics

8. PFR – no backmixing Used to Size the Reactor Space Time = Vol./Q
Outlet Conversion is used for flow sheet mass and heat balances

9. CSTR – complete backmixing
Used to Size the Reactor
Outlet Conversion is used for flow sheet mass and heat balances

10. Review : Catalytic Reactors – Brief Introduction
Major Steps A B
7 . Diffusion of products
from pore mouth to
bulk
Bulk Fluid
CAb
External Diffusion
Rate = kC(CAb – CAS)
External Surface
of Catalyst Pellet
CAs
2. Defined by an Effectiveness Factor
6 . Diffusion of products
from interior to pore
mouth
Internal Surface
of Catalyst Pellet
3. Surface Adsorption
A + S <-> A.S
5. Surface Desorption
B. S <-> B + S
A  B
Catalyst
Surface
4. Surface Reaction

11. Catalytic Reactors Langmuir-Hinschelwood Mechanism (SR Limiting)
Various Mechanisms depending on rate limiting step
Surface Reaction Limiting
Surface Adsorption Limiting
Surface Desorption Limiting
Combinations
Langmuir-Hinschelwood Mechanism (SR Limiting)
H2 + C7H8 (T) CH4 + C6H6(B)

12. Catalytic Reactors – Implications on design
What effects do the particle diameter and the fluid velocity above the catalyst surface play?
What is the effect of particle diameter on pore diffusion ?
How the surface adsorption and surface desorption influence the rate law?
Whether the surface reaction occurs by a single-site/dual –site / reaction between adsorbed molecule and molecular gas?
How does the reaction heat generated get dissipated by reactor design?

13. Enzyme Catalysis Enzyme Kinetics S= substrate (reactant)
E= Enzyme (catalyst)

14. Problems Managing Heat effects Optimization
Make the most product from the least reactant

15. Optimization of Desired Product
Reaction Networks
Maximize yield,
moles of product formed per mole of reactant consumed
Maximize Selectivity
Number of moles of desired product formed per mole of undesirable product formed
Maximum Attainable Region – see discussion in Chap’t. 7.
Reactors (pfrs &cstrs in series) and bypass
Reactor sequences
Which come first

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

17. Temperature Effects
On Equilibrium
On Kinetics

18. Equilibrium Reactor- Temperature Effects
Single Equilibrium
aA +bB  rR + sS
ai activity of component I
Gas Phase, ai = φiyiP,
φi== fugacity coefficient of i
Liquid Phase, ai= γi xi exp[Vi (P-Pis) /RT]
γi = activity coefficient of i
Vi =Partial Molar Volume of i
Van’t Hoff eq.

19. Overview of CRE – Aspects related to Process Design
Le Chatelier’s Principle
Levenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd ed.

20. Unfavorable Equilibrium
Increasing Temperature Increases the Rate
Equilibrium Limits Conversion

21. Overview of CRE – Aspects related to Process Design
Levenspiel , O. (1999), “Chemical Reaction Engineering”, John Wiley and Sons , 3rd ed. 21

22. Feed Temperature, ΔHrxn
Adiabatic
Adiabatic
Cooling
Heat Balance over Reactor
Q = UA ΔTlm

23. Reactor with Heating or Cooling
Q = UA ΔT

24. Kinetic Reactors - CSTR & PFR – Temperature Effects
Used to Size the Reactor
Used to determine the reactor dynamics
Reaction Kinetics

25. PFR – no backmixing Used to Size the Reactor Space Time = Vol./Q
Outlet Conversion is used for flow sheet mass and heat balances

26. CSTR – complete backmixing
Used to Size the Reactor
Outlet Conversion is used for flow sheet mass and heat balances

27. Unfavorable Equilibrium
Increasing Temperature Increases the Rate
Equilibrium Limits Conversion

28. Various Reactors, Various Reactions

29. Reactor with Heating or Cooling
Q = UA ΔT

30. Temperature Profiles in a Reactor
Exothermic Reaction
Recycle

31. Best Temperature Path

32. Optimum Inlet Temperature Exothermic Rxn
CSTR
PFR

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

34. Inter-stage Cooler
Lowers Temp.
Exothermic Equilibria

35. Inter-stage Cold Feed Lowers Temp Lowers Conversion
Exothermic Equilibria

36. Optimization of Desired Product
Reaction Networks
Maximize yield,
moles of product formed per mole of reactant consumed
Maximize Selectivity
Number of moles of desired product formed per mole of undesirable product formed
Maximum Attainable Region – see discussion in Chap’t. 6.
Reactors and bypass
Reactor sequences

37. Reactor Design for Selective Product Distribution
S,S&L Chapt. 7

38. Overview Parallel Reactions Series Reactions Independent Reactions
A+BR (desired)
AS
Series Reactions
ABC(desired)D
Independent Reactions
AB (desired)
CD+E
Series Parallel Reactions
A+BC+D
A+CE(desired)
Mixing, Temperature and Pressure Effects

39. Examples Ethylene Oxide Synthesis CH2=CH2 + 3O22CO2 + 2H2O
CH2=CH2 + O2CH2-CH2(desired) O
parallel

40. Examples
Diethanolamine Synthesis
Series parallel

41. Examples Butadiene Synthesis, C4H6, from Ethanol
Series parallel , CH3CHO acetaldehyde

42. Rate Selectivity Parallel Reactions Rate Selectivity
A+BR (desired)
A+BS
Rate Selectivity
(αD- αU) >1 make CA as large as possible
(βD –βU)>1 make CB as large as possible
(kD/kU)= (koD/koU)exp[-(EA-D-EA-U)/(RT)]
EA-D > EA-U T
EA-D < EA-U T

43. Reactor Design to Maximize Desired Product for Parallel Rxns.

44. Maximize Desired Product
Series Reactions
AB(desired)CD
Plug Flow Reactor
Optimum Time in Reactor

45. Fractional Yield
(k2/k1)=f(T)

46. Real Reaction Systems More complicated than either
Series Reactions
Parallel Reactions
Effects of equilibrium must be considered
Confounding heat effects
All have Reactor Design Implications

47. Engineering Tricks Reactor types
Multiple Reactors
Mixtures of Reactors
Bypass
Recycle after Separation
Split Feed Points/ Multiple Feed Points
Diluents
Temperature Management with interstage Cooling/Heating

48. A few words about simulators
Aspen
Kinetics
Must put in with “Aspen Units”
Equilibrium constants
Must put in in the form
lnK=A+B/T+CT+DT2
ProMax
Reactor type and Kinetics must match!!
Kinetics
Selectable units
Equilibrium constants