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Design and Control of Acetaldehyde Production Process
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Trends in Chemical Engineering
Volume 1, Issue 1
www.stmjournals.com
Design and Control of Acetaldehyde Production Process
K. Nagamalleswara Rao1, M. Venkata Ratnam1, P. Rajendra Prasad2, V. Sujatha2
1
Department of Chemical Engineering, Bapatla Engineering College (Autonomous),
Andhra Pradesh, India
2
Department of Chemical Engineering, AU College of Engineering, Andhra University, Vizag, India
Abstract
This paper discusses the steady state and dynamic simulations of acetaldehyde
production process by using Aspen Plus and Aspen Dynamics. The model developed in
the present study is the modification of the process developed by Eastman Chemical
Company of U.S. Patent 6,121,498. The proposed design consists of a FEHE (feed
effluent heat exchanger), reactor, absorption column, compressors and two distillation
columns. Acetic acid reacts with hydrogen and gives acetaldehyde as main product and
ethanol, ethyl acetate, acetone, and water as byproducts. The paper is divided into two
stages. In the first stage, the entire process plant is developed for steady-state simulations
and in the second stage, dynamic simulations are performed by providing decentralized
controllers for the entire plant. Tray temperature control loop and composition control
loop are designed to maintain final product purity. Results of dynamic simulation are
analyzed for different process conditions. The proposed plant with control structure
performs very well in rejecting various disturbances while maintaining the acetaldehyde
product purity at 95.17 mol%, with a settling time of less than 9 h.
Keywords: Apen Plus,
U.S. Patent 6,121,498
Aspen
Dynamics,
temperature
control
loop,
*Author for Correspondence E-mail: kanidarapunag2001@gmail.com
INTRODUCTION
Many efforts are being made to expand the
manufacture of chemical products in such a
way as to minimize pollution. The conceptual
design of such production plants involves
interplay between design and operational
requirements, which attempts to balance noncommensurable objectives such as economics,
safety health, and environmental impact
[1–20].
Dynamic simulation and modeling have
become mandatory parameters for process and
control
engineers;
therefore,
dynamic
simulation software is being used constantly
for the assessment of control and optimization
of technologies. Aspen Dynamics and Aspen
Custom
Modeler
enable
commercial
simulation software users to perform quick and
reliable dynamic model bifurcation analysis,
without resorting to non-chemical engineering
specialized software [4].
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
Plant-wide control system design is useful for
economical operation over a wide range
(design throughput to maximum throughput)
encompassing multiple active constraint
regions [16].
Acetaldehyde is a versatile chemical
intermediate. It is commercially produced via
the Wacker process, the partial oxidation of
ethylene. The process is corrosive, requiring
expensive materials of construction. Eastman
Chemical Company has developed a selective
palladium catalyst that gives acetaldehyde
with selectivity of up to 86% at 46%
conversion. Byproducts formed include
ethanol, acetone and ethyl acetate, all of which
can be sold after purification [21].
This paper focuses on the development of a
decentralized plant-wide control structure for
the
US
patented
process
(patent
US006121498A) with modifications in the
process. A custom flow sheet is developed and
plant-wide control structure is developed for
Page 1
Acetaldehyde Production Process
process controllability analysis performed
using Aspen Dynamics.
Rao et al.
distillation column (C2) in pure form. The
Aspen NRTL physical property model is used
in all units of the process.
PROCESS STUDIED
The process flow diagram is shown in Figure
1. The process contains mixer, feed effluent
heat exchanger (FEHE or HX1), absorption
column, heat exchangers, compressors and
distillation columns.
Mixing and Reactor Preheating
Acetic acid, hydrogen and ethanol at 298 K,
9.8 atm pressure are mixed and preheated to
reactor temperature 335 K in HX1. Heat duty
of the HX1 is 47444.2 cal/s.
The fresh feed contains 100 kmol/h of
hydrogen, 100 kmol/h of acetic acid,
50 kmol/h of ethanol. The absorber is operated
at low pressure (1.41 atm), which results in a
loss of acetic acid of quantity 133 kmol/h. The
hydrogen is vent off in the distillation column
(C1). Acetaldehyde is obtained from second
Reactor (R1)
A vapor stream is fed into the reactor. The
reactor heat duty is 700341 cal/s and reactor
exit temperature is 673.15 K. Since the
reaction is exothermic, the lower the
temperature the higher the chemical
equilibrium constant.
Fig. 1: Process Flow Sheet for Production of Acetaldehyde.
Absorber
The column in the process has 10 stages and
feed is fed on stage 5. The operating pressure
is 1.41 atm. Diameter of the column is 2 m.
Distillation Column (C1)
The column has 10 stages and feed stage is
stage 5, which is the optimum feed stage to
minimize reboiler heat input. The operating
pressure is 1.97 atm, which gives a reflux
drum temperature of 278.15 K. The reflux
ratio is low (RR = 2), energy consumption
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
3.96 MW, and the column diameter is 2 m.
Reboiler temperature is 485 K and heat input
is 2.82 MW.
Distillation Column (C2)
Distillate from column C1 is rich of
acetaldehyde product, so it is separated from
the unwanted products in column C2. The
number of trays in the column is 10.
Condenser is total condenser; reboiler is kettletype, reflux ratio 2. Feed stream enters above
stage 5 of the column. Heat duty of the
Page 2
Trends in Chemical Engineering
Volume 1, Issue 1
condenser is 3.2 MW and temperature is
102.18 K. Reboiler temperature is 418.7 K
heat duty is 2.96 MW.
Figures 2–6 give T-xy diagrams for several
binary pairs. They are water/ethanol,
water/ethyl acetate, ethanol/ethyl acetate,
acetic acid/ethyl acetate, and acetic
acid/ethanol. Three azeotropic compositions
are found from the ternary plots. The
azeotropes are formed between water/ethanol,
water/ethyl acetate, and ethanol/ethyl acetate.
From the plots, it is evident that separation of
ethyl acetate and acetic acid is easy due to
difference in boiling points. The normal
boiling points of water 100.02 °C, acetic acid
118 °C, ethanol 78.31 °C, ethyl acetate
77.2 °C. Figures 7 and 8 show the ternary
diagrams for ethyl acetate/water/acetic acid
and ethyl acetate/water/ethanol, respectively,
at 1 bar. Azeotrope is formed for ethyl acetate
and water system at 71.39 °C for ethyl acetate
and ethanol system at 71.78 °C and for ethanol
and water system at 70.33 °C.
REACTION KINETICS
The production of acetaldehyde involves the
following main and side reactions.
CH3COOH + H2 → CH3CHO + H2O (main)
CH3COOH + 2H2 → C2H5OH + H2O
CH3COOH + C2H5OH → C2H5COOCH3
2CH3COOH + 4H2 → CH3COCH3 + CH4 + 3H2O
(1)
(2)
(3)
(4)
Reaction (1) is the main reaction and the
selectivity of the acetaldehyde is up to 86% at
46% conversion. Byproducts formed include
ethanol, acetone and ethyl acetate.
PHASE EQUILIBRIUM
Temperature, C
NRTL physical properties are used in Aspen
simulations. There are three basic separations,
namely,
acetic
acid/acetaldehyde,
acetaldehyde/ethanol and acetaldehyde/water.
120
115
110
105
100
95
90
85
80
75
70
65
60
55
50
45
40
35
30
25
20
0.00
T-xy diagram for ACETICAC/ACEHYD
T-x 1.0133 bar
T-y 1.0133 bar
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
Liquid/vapor mole fraction, ACETICAC
Fig. 2: T-xy Plot for Acetic acid/Acetaldehyde.
T-xy diagram for ETHANOL/ACEHYD
80
75
T-x 1.0133 bar
T-y 1.0133 bar
70
65
Temperature, C
60
55
50
45
40
35
30
25
20
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0.45
0.50
0.55
0.60
0.65
0.70
0.75
0.80
0.85
0.90
0.95
1.00
Liquid/vapor mole fraction, ETHANOL
Fig. 3: T-xy Plot for Ethanol/Acetaldehyde.
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
Page 3
Acetaldehyde Production Process
Rao et al.
T-xy diagram for ETHANOL/EACETATE
78.5
78.0
77.5
T-x 1.0133 bar
T-y 1.0133 bar
77.0
Temperature, C
76.5
76.0
75.5
75.0
74.5
74.0
73.5
73.0
72.5
72.0
71.5
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
Liquid/vapor mole fraction, ETHANOL
Fig. 4: T-xy Plot for Ethanol/Ethyl Acetate.
T-xy diagram for WATER/ETHANOL
102
100
T-x 1.0133 bar
98
T-y 1.0133 bar
96
Temperature, C
94
92
90
88
86
84
82
80
78
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
Liquid/vapor mole fraction, WATER
Fig. 5: T-xy Plot for Water/Ethanol.
120
115
T-xy diagram for ACETACID/ETHANOL
T-x 1.0133 bar
T-y 1.0133 bar
Temperature, C
110
105
100
95
90
85
80
75
0.00 0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00
Liquid/vapor mole fraction, ACETACID
Fig. 6: T-xy Plot for Acetic Acid/Ethanol.
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
Page 4
Trends in Chemical Engineering
Volume 1, Issue 1
Fig. 7: Ternary Map for Ethanol/Water/Ethyl
Acetate.
Fig. 9: Residue Curve Mapping for Acetic
Acid/Hydrogen/Acetaldehyde.
Fig. 10: Residue Curve Mapping for
Water/Ethanol/Ethyl Acetate.
Fig. 8: Ternary Map for Hydrogen/Acetic
Acid/Acetaldehyde.
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
Figures 9 and 10 show the residue curves; they
lead from the minimum-boiling azeotrope to
the highest-boiling component.
Page 5
Acetaldehyde Production Process
Rao et al.
PROCESS DYNAMICS AND
CONTROL
Tyreus-Luyben (TL) and Ziegler-Nichols
tuning to find controller settings.
This section deals with a systematic approach
to design a control structure for the
acetaldehyde production process. The design
steps followed are: (1) determination of
temperature control tray, (2) placing the
decentralized PI controllers, (3) application of
relay feedback to find ultimate gain (Ku) and
ultimate period (Pu), and (4) application of
Control Structure
Figure 11 shows the plant-wide control
structure developed for the acetatldehyde
plant. Before performing dynamic simulations
on the developed control structure,
conventional PID controllers are arranged in
all loops.
Fig. 11: Plant-Wide Control Structure for Acetaldehyde Production Process.
Controller Design
Once the decentralized control structure was
designed,
dynamic
simulations
were
performed
using
Aspen
Dynamics.
Proportional-integral controllers were used for
flow, pressure, composition, temperature, and
level controls. Relay feedback tests were
performed on temperature loops to find the
ultimate gain (Ku) and ultimate period (Pu).
Initial controller parameters were calculated
according to Kc (Ku/3) and tI (2Pu). In order
to obtain an acceptable damping, further
detuning from the initial settings was done.
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
Pressure Controller
Pressure controller is arranged for distillation
columns and condensers and the tuning
constants are with a gain of 20 and integral
time of 12 min. Controller output is the heat
removal rate in the condenser to do this task
the controller action assigned as reverse.
Level Controllers
There are a total of five level controllers
arranged for the absorber and for the two
distillation columns. All level loops are
proportional with KC = 10 and integral time of
60 min. For column C1 at base level, one level
controller is arranged and its action is direct.
Page 6
Trends in Chemical Engineering
Volume 1, Issue 1
Its steady state value for the base level is
0.625 m. Gain is 10 and integral time 60 min.
The second level controller is arranged for the
reflux drum. PV signal comes from the level
on stage 1. OP goes to valve V15. For this
controller, a direct-acting proportional-only
controller is specified. For this, set point is
0.625 m and bias 50%, gain 10, and integral
time 60 min. Similarly, for second column C2
and for absorption column-level controllers,
pressure controllers are arranged.
Selecting Temperature/Composition Control
Tray Location
Figure 12 shows a large change in the
temperature profile in the lower part of the
column. Stage 3 is selected for temperature
control in column C1. Figure 13 shows the
column composition profile. Figures 14 and 15
show the vapor-to-liquid flow rate and relative
volatility profiles, respectively.
Block C1: Temperature Profile
220
200
180
Te mperature C
160
Temperature C
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
Stage
Fig. 12: Temperature Profile for Distillation Column C1.
Block C1: Liquid Composition Profiles
1.00
0.90
0.80
ACETICAC
H2
X (mole frac)
0.70
ACEHYD
WATER
0.60
EACETATE
0.50
ACETONE
0.40
ETHANOL
METHANE
0.30
0.20
0.10
0.00
1
2
3
4
5
6
7
8
9
10
Stage
Fig. 13: Composition Profile for Column 1.
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
Page 7
Acetaldehyde Production Process
Rao et al.
Block C1: Molar Flow Ratios
3.50
3.25
Vapor to liquid flow ratio
3.00
2.75
Flow Ratio (Mole)
2.50
2.25
2.00
1.75
1.50
1.25
1.00
0.75
0.50
0.25
0.00
1
2
3
4
5
6
7
8
9
10
9
10
Stage
Fig. 14: Vapor-to-Liquid Flow Rate in C 1.
Block C1: Relative Volatility
55
RelVol-ACETICAC
50
ACETICAC
45
ACEHYD
40
EACETATE
WATER
35
30
25
20
15
10
5
0
1
2
3
4
5
6
7
8
Stage
Fig. 15: Relative Volatility Profile in C1.
Tray Temperature Control
A temperature controller is arranged to hold
the temperature on stage 3 by adjusting the
reboiler heat input. PID controller is selected
for its built-in relay feedback test capability.
The PV is selected is the temperature on stage
3. The OP is selected to be the reboiler heat
input. Values are initialized. The normal
controller output is 1.96 MMkcal/h. The
controller action is set at reverse because for
the increase of tray temperature reboiler heat
input decreases. Then closed loop ATV test is
performed. The default value of the relay
output amplitude is 5%, which is usually good.
Relay feedback test is performed.
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
Composition Control
Controllers are arranged to the hydrogen and
acetic acid feed streams and the controller
action is specified as “Reverse.” The PV is the
mole fraction of hydrogen in the feed. In the
next step, initialization and dynamic runs are
performed for convergence of steady state
conditions
Performance Evaluation
For disturbances, the performance is evaluated
from transient response plots. From the closed
loop ATV tests for temperature controller
arranged for column C1, results are reported
as: Ultimate gain 69, ultimate period 1.2 min.
Tyreus-Luyben parameters are gain 21.5
Integral time 2.64 min.
Page 8
Trends in Chemical Engineering
Volume 1, Issue 1
300.0
25. 0
S TRE AMS (" S 24" ).F kmol/hr
15. 0
20. 0
10. 0
Response of column C1 for top and bottom
flow rates is shown in Figure 16.
S TRE AMS (" S 26" ).F kmol/hr
50. 0 100.0 150.0 200.0 250.0
100.0
S TRE AMS (" S 28" ).F kmol/hr
25. 0
50. 0
75. 0
0.0
Ziegler-Nichols setting test results are gain
31.3, integral time 1 min. The values show
good controlling action of the controllers.
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Tim e Hours
2.0
S et P oint ba r
1.0
1.5
Controller Output kmol/hr
24 5.524 24 5.526 24 5.528 24 5.53
P roce ss V ariable bar
0.79 999 7
0.80 000 2
0.80 000 7
Fig. 16: Response Plot for Tops and Bottoms for C1.
0.0
1.0
2.0
3.0 4.0
Time Hours
5.0
6.0
Controller Output kg/hr
10000.015000.020000.0
Process Variable m
Set Point m
0.6 0.65 0.7 0.75 0.8 0.85
Fig. 17: Pressure Controller Response Plot Absorber.
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
Time Hours
Fig. 18: Level Controller Response Plot for C1.
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
Page 9
Rao et al.
S TREAMS ("S 26").F kmol/hr
150.0 200.0 250.0 300.0
S TREAMS ("S 24").F kmol/hr
10.0
15.0
20.0
25.0
Acetaldehyde Production Process
0.0
5.0
10 .0
Time Hours
15 .0
Fig. 19: Tops and Bottoms Response Plot for 20% Increase in Hydrogen Flow Rate.
Figures 17 and 18 show the response of
pressure and level controllers of the absorber.
Figure 19 shows the tops and bottoms of
column C1 response corresponding to the
+ 20% disturbance in hydrogen feed rate.
Process reached the steady state at 8.54 h.
maintained despite the large disturbances
present in the process and the plant settling
time is identified as of less than 9 h. The
developed control structure parameters are:
ultimate gain 69 and ultimate period 1.2 min
which is the desired result for a stable
operation of a plant.
CONCLUSIONS
This paper presents a realistic steady-state
model and a plant-wide control structure for
production of acetaldehyde. No other design
and control study of this system has been
reported in open literature. Three distillation
regions are identified from the residue curves;
they are between water, ethanol, and ethyl
acetate. Due to the presence of azeotropic
regions, two distillation columns are used to
separate the products acetaldehyde, ethanol,
ethyl acetate, water, acetone, hydrogen, and
methane. The design and operating variables
identified in this process are temperature of
absorber, distillation columns, and the total
number of stages in distillation columns.
Acetic acid feed flow rate and the hydrogen
feed flow rate are controlled using flow
controllers. The overall control strategy of this
system is provided with decentralized
controllers. The developed temperature-control
loop structure for distillation column (C1)
adjusts the reboiler heat input to maintain the
temperature of the trays in the column. The
recommended control strategy uses the
reboiler duty to control the third stage
temperature of the distillation column (C1).
This simple control strategy does not need any
online composition measurements. The
acetaldehyde product purity is 95.17 mol%.
The purity of the acetaldehyde product is
TCE (2014) 1-11© STM Journals 2014. All Rights Reserved
The byproducts, namely, acetic acid, ethanol,
ethyl acetate, and acetone can be collected at
the outlet streams of the distillation columns
and can be stored. The above results show that
the developed plant-wide control structure is
stable for large disturbances and the designed
plant is environmentally friendly and safe to
operate.
ACKNOWLEDGMENT
The authors are grateful to Bapatla
Educational Society, Bapatla, Guntur District,
Andhra Pradesh, India.
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