Modeling of
Reactive Distillation
John Schell
Dr. R. Bruce Eldridge
Dr. Thomas F. Edgar
Outline
• 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 Distillation
• Homogeneous or
Heterogeneous/
Catalytic Distillation
• First Patents in 1920s
• Applied in 1980s to
Methyl Acetate
• Common applications:
– Ethylene Glycol
– MTBE, TAME, TAA
Favorable Applications
Westerterp (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)
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 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
KB 2
TAMESa  MeOHS a  2M1B
KB1
KB 4
TAMESa  MeOHS a  2M2B
KB3
TAMESa  TAME  Sa
KB6
2M2B  2M1B
KB5
Sa is a vacant adsorption site.
Pilot Plant (SRP)
• 0.152-meter diameter
column
• Finite reflux
• 7 meters of packing in 3
sections
• Fisher DeltaV Control
• Koch’s Katamax packing
Unreacted C5,
MeOH
Reactive
Distillation
Column
Recycle
Back - Cracking
Reactor
3.7
atm
C5 from Cat
Cracker
Mixing
Tank
Pre-Reactor
Makeup MeOH
TAME
SRP Pilot Plant
•Koch – Spool
section, Katamax,
Catalyst
•SRP - $145K
Steady-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 Models
Trayed Tower:
Packed Tower:
Equilibrium
Model
Continuous
Model
L j 1 xi , j 1  V j 1 yi , j 1 
L j xi , j  V j yi , j  Ri , j
yi  Kxi
Rate Model
N iV  N iL

xi L   ANiL  i AL Rk
z
k
TAME Reaction Rates
Comparison of Reaction Rates
0.05
RADFRAC
RateFRAC
0.03
0.02
0.01
-0.01
Stage (Condenser=1)
-0.02
-0.03
-0.04
-0.05
-0.06
15
14
13
12
11
10
9
8
7
6
5
4
3
2
0
1
Reaction Rates (lbmol/hr)
0.04
TAME Concentration Profile
Comparison of TAME Profiles
0.90
0.80
0.60
0.50
RADFRAC
RateFRAC
0.40
0.30
0.20
0.10
Stage (Condenser=1)
15
14
13
12
11
10
9
8
7
6
5
4
3
2
0.00
1
Mole Fraction
0.70
• Traditionally
simulations use
intrinsic reaction rate.
• Effective rate is a
function of intrinsic
rate and diffusion
limitations.
Effective Rate
Effective Reaction Rate
Molefraction
Control 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
– Not Equilibrium limited
– Equilibrium Isomers
• Exothermic
• Kinetics from CSTR
Experiments
• Feed is dominated by
inerts
• Replace hazardous
heterogeneous catalyst
C1
C1
C3
C2
Isomer Distribution for Reactive Systems
50
45
40
Plug-flow Reactor
CD Column
35
Mole %
• Kinetic Reaction
A + B
30
25
20
15
10
5
0
1
2
3
Isomer
4
5
Novel System Data
Standard Conditions at 50 psig Over 26 Experiments
25
High
Low
Average
Standard Deviation
Temperature (C)
20
15
10
Reactive Zone
5
0
Overhead DA-220-1 DA-220-2 DA-220-3 DA-220-4
Vapor
Temp
TI-215
DA-210-1 DA-210-2 DA-210-3 DA-210-4
Reboiler
Temp
Novel System Data
Profiles for 35 psig at Standard Conditions
25
Hi
Lo
Average
Stnd Dev
Temperature (C)
20
15
10
Reactive Zone
5
0
Overhead DA-220-1
Vapor Temp
DA-220-2
DA-220-3
DA-220-4
TI-215
DA-210-1
DA-210-2
DA-210-3
DA-210-4
Reboiler
Temp
Simulation Validation - 50 psig
Temperature (C)
Column Data and Simulation for Standard Flows at 50 psig
0
5
10
15
20
25
Simulation Validation – 35 psi
Temperature (C)
Simulation and Data for Standard Flows at 35 psig
0
5
10
15
20
25
Effect of Pressure
Effect of Varying Pressure
Temperature (C)
25 psig
35 psig
50 psig
75 psig
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
Effect of Varying Feed Rate
Effect of Varying Reactant Feed Rates
Temperature (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
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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 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
Equations vs. Variables
1
2
3
4
1
X
X
2
X
X
3
X
4
X
5
6
7
8
X
T
T
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
10
9
10
X
X
Validation of Dynamic Simulator
Comparison of ACM and Aspen Plus Radfrac Results
ACM w/Tear
Temperature (C)
Aspen Plus
1
2
3
4
5
6
7
8
9
10 11 12 13 14 15 16 17 18 19 20 21 22 23
1
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Time Hours
2
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
Stream Results
Pressure N/m2
350000
360000
Temperature K
540
560
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
520
0
C - Production
Molar Flow rate kmol/s
2e-5 2.5e-5 3e-5 3.5e-5
3e-5
B-Feed Rate
2e-5
2.5e-5
1.5e-5
0.2
0
-0.05
C
B-Product
0.05 0.1
0.15
Feed Disturbance With Manual Control
0
0.25
0.5
0.75
1
1.25
1.5
1.75
Time Hours
2
2.25
2.5
2.75
3
3
Control of Reactive Distillation
• Configurations
– DB
– LV
– BV, LB…
• Goals
– Conversion
– Product Purity
D
R
F
L
V
Duty
B
Control 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 Ratio
Conversion vs Reboiler Duty
Single Tray Conversion
Estimation
Dependency of Conversion on Temperature
400
350
T8
T6
250
200
150
100
50
Conversion
1.00
0.90
0.80
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0
0.00
Temperature (C)
300
230
Benzene Concentration
5.00000E-07
4.50000E-07
4.00000E-07
250
3.50000E-07
3.00000E-07
2.50000E-07
2.00000E-07
1.50000E-07
1.00000E-07
5.00000E-08
0.00000E+00
Temperature (C)
Single Tray Purity Estimation
Purity of Alkylate
265
260
255
T6
T7
T8
245
240
235
1
1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9
Time Hours
2
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
Stream Results
Pressure N/m2
350000
360000
Temperature K
540
560
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
520
0
C - Production
Molar Flow rate kmol/s
2e-5 2.5e-5 3e-5 3.5e-5
3e-5
B-Feed Rate
2e-5
2.5e-5
1.5e-5
0.2
0
-0.05
C
B-Product
0.05 0.1
0.15
Feed Disturbance With Manual Control
0
0.25
0.5
0.75
1
1.25
1.5
1.75
Time Hours
2
2.25
2.5
2.75
3
3
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
Time Hours
2
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
Temperature K
540
560
Pressure N/m2
370000
380000
580
Stream Results
520
Molar Flow rate kmol/s
2e-5
2.5e-5
3e-5
3.5e-5
3e-5
B-Feed Rate
2e-5
2.5e-5
1.5e-5
0
C-Production
1.5e-5
0.15
0
-0.05
C
B-Product
0.05
0.1
Feed Disturbance with Simple PID
Control
0
0.25
0.5
0.75
1
1.25
1.5
1.75
Time Hours
2
2.25
2.5
2.75
3
3
Conclusion and Future Work
• TAME Tower
0.05
0.04
RADFRAC
RateFRAC
0.03
0.02
0.01
15
14
13
12
11
9
10
8
7
6
5
4
3
2
0
1
Reaction Rates (lbmol/hr)
– Collect Data
– Validate Models
– Developing Advanced
Models
– Improvements
Comparison of Reaction Rates
-0.01
Stage (Condenser=1)
-0.02
-0.03
-0.04
-0.05
-0.06
3e-5
C-Production
B-Feed Rate
1.5e-5
2e-5
2.5e-5
0
-0.05
– Validate Dynamic Models
– Develop Control
Algorithms
C
B-Product
0.05
0.1
• Novel System
0.15
• New chemical system
• Adjust for better dynamic
studies
0
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
Time Hours
2
2.1 2.2 2.3 2.4 2.5 2.6 2.7 2.8 2.9
3