Modeling of Reactive Distillation
Modeling of Reactive Distillation
John Schell
Dr. R. Bruce Eldridge
Dr. Thomas F. Edgar
OutlineOutline
• Overview of Reactive Distillation
• Project Overview– Tower Design
– Steady-State Models
– Dynamic Models and Control
• Individual Work– Column Design and
Operation
– Validation of Models
– Preliminary Dynamics and Control Studies
• Future Work
Reactive DistillationReactive Distillation
• Homogeneous or Heterogeneous/ Catalytic Distillation
• First Patents in 1920s• Applied in 1980s to
Methyl Acetate• Common applications:
– Ethylene Glycol– MTBE, TAME, TAA
Favorable ApplicationsWesterterp (1992)
Favorable ApplicationsWesterterp (1992)
• Match between reaction and distillation temperatures
• Difference in relative volatility between product and one reactant
• Fast reaction not requiring a large amount of catalyst
• Others: liquid phase reaction, azeotrope considerations,exothermic reactions
Subawalla Approach (Dissertation)Subawalla Approach (Dissertation)
1. Decide on a Pre-reactor- Rate of reaction
- >1/2 of initial reaction rate at 80% of equilibrium conversion
2. Pressure
3. Location of Zone
4. Estimate Catalyst- Isothermal Plug-flow reactor
with ideal separators
5. Design Tower- Size reaction zone
• Catalyst requirements• Column diameter
- Determine reactant feed ratio
- Feed location- Reflux ratio
• High reflux rate - 2-3 times non-rxtive column
- Diameter• Through-put• Catalyst density
Project Overview
• Design and Construct TAME Column
• Validate Steady State Models
• Develop Dynamic Models
• Test Control Algorithms
TAME ChemistryTAME Chemistry
• Exothermic• Equilibrium Limited
– 45-62% at 50-80 C
• Azeotropes• Catalyst: Amberlyst-15
• Methanol can inhibit rates.
• Rihko and Krause (1995)
MeOHSa MeOH Sa
TAMES a KB1
KB2
MeOHSa 2M1B
TAMES a KB3
KB4
MeOHSa 2M2B
TAMES a TAME Sa
2M2BKB5
KB62M1B
Sa is a vacant adsorption site.
Pilot Plant (SRP)Pilot Plant (SRP)
• 0.152-meter diameter column
• Finite reflux
• 7 meters of packing in 3 sections
• Fisher DeltaV Control
• Koch’s Katamax packing
Makeup MeOH
C5 from Cat
Cracker Pre-Reactor
ReactiveDistillation
Column
Mixing Tank
Back - CrackingReactor
Recycle
TAME
Unreacted C5, MeOH
3.7 atm
SRP Pilot PlantSRP Pilot Plant
•Koch – Spool section, Katamax, Catalyst
•SRP - $145K
Steady-State MultiplicitySteady-State Multiplicity
• Bravo et al. (1993)– Observed multiple steady-states in TAME CD
• Hauan et al. (1997)– dynamic simulation provided evidence in MTBE
system
• Nijuis et al. (1993)– found multiplicity in MTBE system
• Jacobs and Krishna (1993)– found multiplicity in MTBE system
Steady-State Distillation ModelsSteady-State Distillation Models
Trayed Tower:
Equilibrium Model
Rate Model
Packed Tower:
Continuous Model
ii
jijijjij
jijjij
Kxy
RyVxL
yVxL
,,,
1,11,1
Li
Vi NN
kkLi
Lii RAANLx
z
TAME Reaction RatesTAME Reaction Rates
Comparison of Reaction Rates
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Stage (Condenser=1)
Rea
ctio
n R
ates
(lb
mol/h
r)
RADFRAC
RateFRAC
TAME Concentration ProfileTAME Concentration Profile
Comparison of TAME Profiles
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Stage (Condenser=1)
Mole
Fra
ctio
n
RADFRAC
RateFRAC
Effective Reaction RateEffective Reaction Rate
• Traditionally simulations use intrinsic reaction rate.
• Effective rate is a function of intrinsic rate and diffusion limitations. Molefraction
Eff
ecti
ve R
ate
Control for TAME TowerControl for TAME Tower
• Fisher DeltaV– Visual Basic
– Matlab, Visual Studio
• State Estimation– Temperature Profiles
– Online Analyzers
• Control Algorithms– PID
– Linear MPC
– Non-Linear MPC
Individual Work
• Design and Construct RD Column for Novel System
• Steady State Model Validation
• Dynamic Models and Control Study
Novel System
• Kinetic Reaction– Not Equilibrium limited
– Equilibrium Isomers
• Exothermic
• Kinetics from CSTR Experiments
• Feed is dominated by inerts
• Replace hazardous heterogeneous catalyst
A + B C1
C1 C3C2Isomer Distribution for Reactive Systems
0
5
10
15
20
25
30
35
40
45
50
1 2 3 4 5
Isomer
Mo
le %
Plug-flow Reactor
CD Column
Novel System DataNovel System DataStandard Conditions at 50 psig Over 26 Experiments
OverheadVaporTemp
DA-220-1 DA-220-2 DA-220-3 DA-220-4 TI-215 DA-210-1 DA-210-2 DA-210-3 DA-210-4 ReboilerTemp
Te
mp
era
ture
(C
)
0
5
10
15
20
25
High
Low
Average
Standard Deviation
Reactive Zone
Novel System DataNovel System DataProfiles for 35 psig at Standard Conditions
OverheadVapor Temp
DA-220-1 DA-220-2 DA-220-3 DA-220-4 TI-215 DA-210-1 DA-210-2 DA-210-3 DA-210-4 ReboilerTemp
Tem
per
atu
re (
C)
0
5
10
15
20
25
Hi
Lo
Average
Stnd Dev
Reactive Zone
Simulation Validation - 50 psigSimulation Validation - 50 psigColumn Data and Simulation for Standard Flows at 50 psig
0 5 10 15 20 25
Tem
per
atu
re (
C)
Simulation Validation – 35 psi
Simulation and Data for Standard Flows at 35 psig
0 5 10 15 20 25
Tem
per
atu
re (
C)
Effect of PressureEffect of Varying Pressure
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Tem
per
atu
re (
C)
25 psig
35 psig
50 psig
75 psig
Effect of Varying Feed RateEffect of Varying Reactant Feed Rates
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Tem
per
atu
re (
C)
25 g/min A and 10 g/min B
75 g/min A and 10 g/min B
100 g/min A and 10 g/min B
150 g/min A and 20 g/min B
Dynamic Modeling and Control Study
• Aspen Custom Modeler/ Aspen Dynamics– Validate Steady State
Solution
– Validate Dynamic Studies
• Develop Control Algorithms– PID
– Linear MPC
– NLMPC
Aspen Custom ModelerAspen Custom Modeler• Formerly Speed-Up
and DynaPlus• Equation Solver• Aspen Properties Plus• Tear Variables
automatically selected• Solves Steady-State
and Dynamic• Dynamic Events and
Task Automation
1 2 3 4 5 6 7 8 9 10
1 X X
2 X X
3 X X T T
4 X X T T
5 X X T T
6 X X T T
7 T T T T T T
8 T T T T T T
9 X
10 X
Equations vs. Variables
Validation of Dynamic SimulatorValidation of Dynamic SimulatorComparison of ACM and Aspen Plus Radfrac Results
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
Tem
per
atu
re (
C)
ACM w/Tear
Aspen Plus
Feed Disturbance With Manual ControlFeed Disturbance With Manual Control
Stream Results
Time Hours0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3
Pre
ssur
e N
/m2
Tem
pera
ture
K
Mol
ar F
low
rat
e km
ol/s
3500
0036
0000
520
540
560
2e-5
2.5e
-53e
-53.
5e-5
C - Production
Time Hours0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3
B-F
eed
Rat
e
B-P
rodu
ctC
1.5e
-52e
-52.
5e-5
3e-5
-0.0
50
0.05
0.1
0.15
0.2
Control of Reactive DistillationControl of Reactive Distillation
• Configurations– DB
– LV
– BV, LB…
• Goals– Conversion
– Product Purity
F
R
D
B
VL
Duty
Control of Reactive DistillationControl of Reactive Distillation
• Bartlett and Wahnschafft (1997)– Simple Feed-Forward/
Feed-Back PI Scheme
• Sneesby et al. (1999)– Two point control with
linear conversion estimator
• Kumar and Daoutidis (1999)– Showed linear
controllers unstable for ethylene glycol systems
– Demonstrated possible Nonlinear MPC scheme
Dependency of Conversion on Reboiler Duty and Reflux RatioDependency of Conversion on Reboiler Duty and Reflux Ratio
Conversion vs Reboiler DutyConversion vs Reboiler Duty
Conversion of Olefin for Molar Reflux Ratio of 1.9
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Reboiler Duty (MMkcal/hr)
Convers
ion
Single Tray Conversion Estimation
Dependency of Conversion on Temperature
0
50
100
150
200
250
300
350
400
0.00
0.10
0.20
0.30
0.40
0.50
0.60
0.70
0.80
0.90
1.00
Conversion
Tem
per
ature
(C
)
T8
T6
Single Tray Conversion Estimation
Single Tray Purity Estimation
Purity of Alkylate
230
235
240
245
250
255
260
265
0.00
000E
+00
5.00
000E
-08
1.00
000E
-07
1.50
000E
-07
2.00
000E
-07
2.50
000E
-07
3.00
000E
-07
3.50
000E
-07
4.00
000E
-07
4.50
000E
-07
5.00
000E
-07
Benzene Concentration
Tem
per
ature
(C
) T6
T7
T8
Single Tray Purity Estimation
Feed Disturbance With Manual ControlFeed Disturbance With Manual Control
Stream Results
Time Hours0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3
Pre
ssur
e N
/m2
Tem
pera
ture
K
Mol
ar F
low
rat
e km
ol/s
3500
0036
0000
520
540
560
2e-5
2.5e
-53e
-53.
5e-5
C - Production
Time Hours0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3
B-F
eed
Rat
e
B-P
rodu
ctC
1.5e
-52e
-52.
5e-5
3e-5
-0.0
50
0.05
0.1
0.15
0.2
Feed Disturbance with Simple PID Control
Feed Disturbance with Simple PID Control
S trea m R esu lts
T im e H o u rs
0 0 .2 5 0 .5 0 .7 5 1 1 .2 5 1 .5 1 .7 5 2 2 .2 5 2 .5 2 .7 5 3
Pressu
re N/m
2
Tem
pera
ture K
Mo
lar F
low
rate k
mo
l/s
37
00
00
38
00
00
52
05
40
56
05
80
1.5
e-52
e-52
.5e-5
3e-5
3.5
e-5
C-Production
Time Hours0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3
B-F
eed
Rat
e
B-P
rodu
ctC
1.5e
-52e
-52.
5e-5
3e-5
-0.0
50
0.05
0.1
0.15
Conclusion and Future WorkConclusion and Future Work
• TAME Tower– Collect Data– Validate Models– Developing Advanced
Models– Improvements
• New chemical system• Adjust for better dynamic
studies
• Novel System– Validate Dynamic Models– Develop Control
Algorithms
Comparison of Reaction Rates
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0
0.01
0.02
0.03
0.04
0.05
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Stage (Condenser=1)
Rea
ctio
n R
ates
(lb
mol/h
r)
RADFRAC
RateFRAC
C-Production
Time Hours0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9 3
B-F
eed
Rat
e
B-P
rodu
ctC
1.5e
-52e
-52.
5e-5
3e-5
-0.0
50
0.05
0.1
0.15