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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• Kinetic model for a PFR• Custom-made models (UDF)
Used in early stages of design
Stoichiometric Reactor
• C chemical Species• υi,j stoichiometric coefficient
for ith species in jth reaction• Aj chemical formula of jth
species• R chemical reactions
C
jjji RiA
1, ,...,2,1,0
Stoichiometric Reactor Example
• Reactions– 1 Methane Synthesis– 2 Coking
• Conversion, Xk, of key component, k– Xk=(nk-in – nk-out)/ nk-in
• Extent of Reaction– ξi= (ni,j-in – ni,j-out)/ νi,j
)(1
1
11
1
12
)(
2
2
2
,2
3
2
,1
22
32
sC
OH
COH
v
OHCH
COH
v
sCOHHCO
OHCHHCO
j
j
Cjij ,...,2,1, n nR
1iiin-jout-j
CjXk
jk ,...,2,1,n- n n in-kin-jout-j
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
– 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
2
ln,exp
RT
H
dT
Kd
RT
G
aa
aaK
orxneq
orxn
aB
aA
sS
rR
eq
Van’t Hoff eq.
Equilibrium Reactor-2
• Total Gibbs Free Energy is minimized at T&P
– 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
i
C
iiTot GnG
1
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.