Aspen for Process Creation
Chapter 4&5
Terry A. Ring
Onion Model of Process Design
Process Synthesis-1
• Reaction 3
– Direct Chlorination
– Pyrolysis
Reactor Types
• Ideal
– PFR
– CSTR
• Real
– Unique design geometries and therefore RTD
– Multiphase
– Various regimes of momentum, mass and
heat transfer
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 Used in early stages of design
• Kinetic model for a PFR
• Custom-made models (UDF)
Stoichiometric Reactor
C

j 1
i, j
Aj  0, i  1,2,...,R
• C chemical Species
• υi,j stoichiometric coefficient
for ith species in jth reaction
• Aj chemical formula of jth
species
• R chemical reactions
Stoichiometric Reactor Example
CO  2 H 2  CH 3OH
CO  H 2  H 2O  C ( s)
  2 H2 
  1 CO 
v1, j  

1 CH 3OH 


 1 H 2 



1
CO

v2 , j  
 1 H 2O 


 1 C (s) 
• Reactions
– 1 Methane Synthesis
– 2 Coking
• Conversion, Xk, of key
component, k
– Xk=(nk-in – nk-out)/ nk-in
n j-out
 j 
 n j-in - n k -in X k  ,
 k 
j  1,2,...,C
• Extent of Reaction
– ξi= (ni,j-in – ni,j-out)/ νi,j
R
n j-out  n j-in   i ij ,
i 1
j  1,2,...,C
Reactions with low
conversions?
• Due to slow kinetics
• Due to non-favorable Equilibrium
– Solution
• Set up reactor
• Followed by Separator
• Recycle reactant to extinction
Equilibrium Reactor-1
• Single Equilibrium
• aA +bB  rR + sS
o

a a
 Grxn 
K eq 
 exp
,
a a
 RT 
r s
R S
a a
A B
– ai activity of component I
Van’t Hoff eq.
o
d
ln
K

H rxn
eq 

 
2
 dT  RT
• 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
Equilibrium Reactor-2
• Total Gibbs Free Energy is minimized at T&P
C
GTot   ni Gi
i 1
– Specify components that are entering system and T&P
of System
– Specify possible reaction products
• Note, no reactions are defined!
• Gives outlet composition at equilibrium
Equilibrium Reactors
• To determine the potential products given some
reactants
– Use Gibbs Reactor
• To determine equilibrium for a given reaction(s)
– Use Equilibrium Reactor
• To determine what operating conditions for a
given reactor
– Use Equilibrium Reactor
• Vary T and P to get highest conversion
• To determine heat of reaction
– Use Stoichiometric Reactor
• Conversion at 100%
Examples
• Which reaction takes place?
– C + ½ O2  CO
– C + O2  CO2
• What temperature (or pressure) is required
for a reaction?
– Run equilibrium reactor at different T and P.
Separations
• What conditions are to be used for a flash
after reaction?
• Run flash with different T and P.