Transcript L4-Reactor Heat Effects.ppt

Reactor Heat Effects
S,S&L Chapter 7
T&S Chapter 15
Problems
• Managing Heat effects
• Optimization
– Make the most product from the least reactant
Managing Heat Effects
• Reaction Run Away
– Exothermic
• Reaction Dies
– Endothermic
• Preventing Explosions
• Preventing Stalling
Equilibrium ReactorTemperature Effects
• Single Equilibrium
• aA +bB  rR + sS
o

a a
 Grxn 
K eq 
 exp 
,
a a
 RT 
r s
R S
a a
A B
Van’t Hoff eq.
o
 d ln K eq  H rxn

 
2
 dT  RT
– 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
Kinetic Reactors - CSTR & PFR –
Temperature Effects
• Used to Size the Reactor
• Used to determine the reactor dynamics
• Reaction Kinetics
C
dC j
 rj  
 k (T ) Cii
dt
i 1
  EA 
k (T )  ko exp 
 RT 
Unfavorable Equilibrium
• Increasing Temperature Increases the Rate
• Equilibrium Limits Conversion
PFR – no backmixing
• Used to Size the Reactor
Xk
dX
V  Fko 
 rk
0
• Space Time = Vol./Q
• Outlet Conversion is used for flow sheet
mass and heat balances
CSTR – complete backmixing
• Used to Size the Reactor
Fko X k
V
 rk
• Outlet Conversion is used for flow sheet
mass and heat balances
Temperature Profiles in a Reactor
Exothermic Reaction
Reactor with Heating or Cooling
Q = UA ΔT
Best Temperature Path
Optimum Inlet Temperature
Exothermic Rxn
Various Reactors, Various
Reactions
Managing Heat Effects
• Reaction Run Away
– Exothermic
• Reaction Dies
– Endothermic
• Preventing Explosions
• Preventing Stalling
Reactor with Heating or Cooling
Q = UA ΔT
Inerts Addition Effect
Managing Heat Effects
• Reaction Run Away
– Exothermic
• Reaction Dies
– Endothermic
• Preventing Explosions
• Preventing Stalling
Inter-stage Cooler
Lowers Temp.
Exothermic Equilibria
Inter-stage Cold Feed
Lowers Temp
Lowers Conversion
Exothermic Equilibria
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
SS&L.
• Reactors and bypass
• Reactor sequences
Reactor Problem on Design I Final
Exam
Feed Temperature, ΔHrxn
Adiabatic
Cooling
Heat Balance over Reactor
Q = UA ΔTlm
Adiabatic