Reactor Design

1
Reactor Design

• Chapter 7
• Terry A. Ring
• ChE

2
Reactor Types

• Ideal
• PFR
• CSTR
• Real
• Unique design geometries and therefore RTD
• Multiphase
• Various regimes of momentum, mass and heat
  transfer

3
Reactor Cost

• Reactor is
• PRF
• Pressure vessel
• CSTR
• Storage tank with mixer and motor
• Pressure vessel
• 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.
• Pd 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)
• tccorrosion allowance 0.125 in.

5
Reactor Cost

• Pressure Vessel Material of Construction gives
  ?metal
• Mass of vessel ?metal (VC2VHead)
• Vc p((Dt)/2)2-(D/2)2L
• 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 (Requilib RGibbs)
• 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
Catalytic Reactors

• 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)

11
Enzyme Catalysis

• Enzyme Kinetics
• S substrate (reactant)
• E Enzyme (catalyst)

12
Problems

• Managing Heat effects
• Optimization
• Make the most product from the least reactant

13
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
  Chapt.7.
• Reactors (pfrs cstrs in series) and bypass
• Reactor sequences
• Which come first

14
Managing Heat Effects

• Reaction Run Away
• Exothermic
• Reaction Dies
• Endothermic
• Preventing Explosions
• Preventing Stalling

15
Temperature Effects

• On Equilibrium
• On Kinetics

16
Equilibrium Reactor-Temperature Effects

• Single Equilibrium
• aA bB ? rR sS
• ai activity of component I
• Gas Phase, ai fiyiP,
• fi fugacity coefficient of i
• Liquid Phase, ai ?i xi expVi (P-Pis) /RT
• ?i activity coefficient of i
• Vi Partial Molar Volume of i

Vant Hoff eq.
17
Unfavorable Equilibrium

• Increasing Temperature Increases the Rate
• Equilibrium Limits Conversion

18
Kinetic Reactors - CSTR PFR Temperature
Effects

• Used to Size the Reactor
• Used to determine the reactor dynamics
• Reaction Kinetics

19
PFR no backmixing

• Used to Size the Reactor
• Space Time Vol./Q
• Outlet Conversion is used for flow sheet mass and
  heat balances

20
CSTR complete backmixing

• Used to Size the Reactor
• Outlet Conversion is used for flow sheet mass and
  heat balances

21
Temperature Profiles in a Reactor
Exothermic Reaction
Recycle
Real Reactor
22
Feed Temperature, ?Hrxn
Adiabatic
Adiabatic
Cooling
Heat Balance over Reactor
Q UA ?Tlm
23
Reactor with Heating or Cooling
Q UA ?T
24
Strategies

• Optimum Inlet Temperature
• Best Temperature Path
• Combination of Two

25
Best Temperature Path
26
Managing Heat Effects

• Reaction Run Away
• Exothermic
• Reaction Dies
• Endothermic
• Preventing Explosions
• Preventing Stalling

27
Optimum Inlet TemperatureExothermic Rxn
CSTR
PFR
28
Various Reactors, Various Reactions
29
Managing Heat Effects

• Reaction Run Away
• Exothermic
• Reaction Dies
• Endothermic
• Preventing Explosions
• Preventing Stalling

30
Inerts Addition Effect
31
Inter-stage Cooler
Lowers Temp.
Exothermic Equilibria
32
Inter-stage Cold Feed
Lowers Temp Lowers Conversion
Exothermic Equilibria
33
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
  Chapt. 6.
• Reactors and bypass
• Reactor sequences

34
Reactor Design for Selective Product Distribution

• S,SL Chapt. 6

35
Overview

• Parallel Reactions
• AB?R (desired)
• A?S
• Series Reactions
• A?B?C(desired)?D
• Independent Reactions
• A?B (desired)
• C?DE
• Series Parallel Reactions
• AB?CD
• AC?E(desired)
• Mixing, Temperature and Pressure Effects

36
Examples

• Ethylene Oxide Synthesis
• CH2CH2 O2?2CO2 2H2O
• CH2CH2 O2?CH2-CH2(desired)

O
37
Examples

• Diethanolamine Synthesis

38
Examples

• Butadiene Synthesis, C4H6, from Ethanol

39
Rate Selectivity

• Parallel Reactions
• AB?R (desired)
• AB?S
• Rate Selectivity
• (aD- aU) gt1 make CA as large as possible
• (ßD ßU)gt1 make CB as large as possible
• (kD/kU) (koD/koU)exp-(EA-D-EA-U)/(RT)
• EA-D gt EA-U T?
• EA-D lt EA-U T?

40
Reactor Design to Maximize Desired Product
41
Maximize Desired Product

• Series Reactions
• A?B(desired)?C?D
• Plug Flow Reactor
• Optimum Time in Reactor

42
Fractional Yield
(k2/k1)f(T)
43
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

44
Engineering Tricks

• Reactor types
• Multiple Reactors
• Mixtures of Reactors
• Bypass
• Recycle after Separation
• Split Feed Points/ Multiple Feed Points
• Diluents
• Temperature Management