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Abstract—In this study, the dynamic responses of three C5
fraction separation processes: conventional, simplified, and
intensified processes with control systems are investigated. The
simplified process provides higher concentration of DCPD in the
product and can be stable in the dynamic state. While an
intensified process can substantially reduce energy consumption,
it is necessary to fix the liquid split ratio in the divided wall
column that separate IP from extractive solvent NMP and
DCPD to maintain the steady state. Otherwise loss of NMP to
DPCD will upset the process.
I. INTRODUCTION
The separated wall distillation column, referred to as the dividing-wall column (DWC), was first established by Wright and Elizabeth in 1949 [2]. It is a heat-integrated distillation column. The DWC is a promising energy saving alternative for separating multi-component mixtures, as compared to traditional distillation columns. However, the DWC might lead to weak stabilizations and increase control difficulties owing to its complexity and two more degrees of freedom. Owing to difficulties in control of thermally coupled distillation and divided wall distillation, there has been extensive research on control strategies for these processes [3–8]. However, there is no comprehensive control solution for the divided wall distillation system. In this research, the control strategies used in conventional and intensified processes are established. Additionally, we compare and apply different control strategies for fixing the split ratio in the staggered stage in the DWC to maintain control stability in the intensified process. In these cases, the differences in the input rates and concentrations can affect the split ratio in the DWC, which might cause a slight difference in the temperature and composition distribution in the DWC. It is observed from the comparison of the control of conventional and intensified processes that maintaining the split ratio at a constant value can maintain the controllability of the entire process.
II. OVERVIEW OF PROCESSES
The following three kinds of processes are investigated in
this study: (1) the conventional process, which is commonly
Haiso-Ching Hsu is with ChangChun Plastics. Co. Ltd., Kaohsiung,
Taiwan (email: [email protected]).
San-Jang Wang is with the Center for Energy and Environmental
Research, National Tsing Hua University, Hsinchu, 30013, Taiwan
(corresponding author; phone: 886-3-5715131×33624; fax: 886-3-5715408;
e-mail: [email protected]).
John Di-Yi Ou is with the Department of Chemical Engineering, National
Tsing Hua University, Hsinchu, 30013, Taiwan (e-mail:
David Shan-Hill Wong is with the Department of Chemical Engineering,
National Tsing Hua University, Hsinchu, 30013, Taiwan (corresponding
author; phone: 886-3-5715131×33641; fax: 886-3-5715408; e-mail:
used in industry, has the advantage of trouble-free operation
and mature in manufacturing; (2) the simplified process,
which has been investigated in literature, can obtain higher
concentration of the primary product, i.e., dicyclopentadiene
(DCPD). However, it has rarely been used in actual
production; (3) the intensified process, which employs the
technique of the DWC and external heat integration [1]. This
process is predictably hard to control in a dynamic system.
The processes and control strategies are presented in the
subsequent paragraphs.
A. Conventional Process
This process is designed using the control strategy shown in Fig. 1. This process has been referred to from Tain et al.’s [9] and Sun et al.’s [10] processes, and is already being used in industry. Therefore, it can be considered as a standard condition.
PC1
1
30
60
LC1
LC2TC
34
IPC105
Feed
FC
PC1
1
37
77
LC1
LC2TC1
15
48
TC2
L1
PC1
1
29
56
LC1
LC2TC1
4538
TC2
L1
C101
C102
R1-1
R1-2
R1-3
R1-4
R1-5
H1
H2
R1-6
M1
C103
C104
DMF recycle
DMF
make-up
H1
1
108
8182
1
3
13
RecycleFC
LC4
FC
FC
X
LC1
LC2
LC3
TC1
TC2
55
PC1
PC2
TC3
TC4
5
41
3
L2
PC1
1
10
22
LC1
LC2
TC1
19
12
TC2
C106
M2
R2
PD+CP
C107
50
1
90
35
PC
LC2
3TC1
TC2
78
PC1
1
15
LC1
LC2
TC1
10
3
TC2
L1
D1
C108
8
Figure 1. Control diagram of conventional process.
B. Simplified Process
In Hsu’s study [1], the simplified process has been proven effective for obtaining higher concentration of DCPD. This process is shown in Fig. 2. Developing a control system for this process can make it suitable for commercialization.
PC5
1
30
60
LC51
LC52TC4255
IP
Feed
FC1
PC1
1
37
77
LC11
LC12TC1154
L1
PC2
1
30
56
LC21
LC22
TC212817
TC22
C101
C103
C104
DMF recycle
DMF
make-up
H1
1
108
6165 1
20
LC42
RC1
X
LC31
LC32
TC31
TC32
68
PC3
PC4
TC42
4
30
3
L2
M2
R2PD+CP
C107
5
1
40
35
PC7
RC2
TC72
36
X
DCPD
LC71
LC72
M1
C105LC41
C102TC12
18
TC42
3
Figure 2. Control diagram of simplified process.
Plant-wide Design and Control of C5 Separation Processes
Haiso-Ching Hsu, San-Jang Wang, John D. Ou, and David S. H. Wong
6th International Symposium onAdvanced Control of Industrial Processes (AdCONIP)May 28-31, 2017. Taipei, Taiwan
978-1-5090-4396-5/17/$31.00 ©2017 IEEE 354
C. Intensified Process
Using the technologies of external heat integration and internal heat integration in the DWC, the intensified process can be applied in made suitable for application in industry.
DMF
make-upDMF recycle
M1
M2
C104&105
L1
IP
PD+CP
L3
DCPD
C107'
Reactor
L2
C101&102
C103'H1
Feed
20
6473
1
1
38
34 5433
7653
4
1
4039
H2
FC
PC1 PC2
1
69
70
9798
127
LC11
LC21
LC22
TC11
20
103
TC1275
TC13
LC31
LC32
TC3129
42 TC32
PC3
TC4150
LC42
LC51
LC52TC43
72
PC4
PC5
LC71
LC73
RC2
TC72
X
RC1
X
35
S_C103input
Figure 3. Control diagram of intensified process.
Improvement in any of the abovementioned processes can help in improving the separation of high value-added products and development of strategies of C5 separation.
III. CONTROL STRATEGY
The control strategies of the conventional, simplified, and
intensified processes are established using singular value
decomposition (SVD) analysis. The following control
strategies are used for these processes: (1) The flow rate of
fresh feed is controlled; (2) the reflux drum levels at the top of
the column are controlled by manipulating distillate flow rates;
(3) the sump levels at the bottom of the column are controlled
by manipulating bottom flow rates; (4) column pressure is
controlled using condenser duty; (5) temperature control is
used to maintain product purity, where controlled stage
temperature is selected through steady-state analysis (SVD
and relative gain array); (6) the sump levels at the left-side
bottom of C104&105 in Fig. 3 are controlled by manipulating
the dimethylformamide (DMF) make-up flow rate; (7) the
flow rate of the DMF recycle stream is in proportion to that of
the S_C103 stream [11].
The primary control objective is to maintain product
purities under feed flow and feed composition disturbances.
The disturbances used in this study include ±5% and ±10%
changes in feed flow rate and cyclopentadiene (CPD),
isoprene (IP), and pentadiene (PD) feed compositions.
However, the dynamic control in C105 in the conventional
and simplified processes and C104&105 in the intensified
process is unstable under ±10% changes in feed flow rate and
concentrations. In the DWC of C104&105, disturbances in the
input make the liquid split ratio close to zero or infinity,
resulting in failure of control valves. To maintain the
operability and controllability of the process, C105 and
C104&105 should be modified to maintain the reflux ratio on
the top using one point proportional-integral-derivative
control in C105 and the right section of C104&105, as shown
in Figs. 1, 2, and 3.
However, there are a few instabilities and not controlled
situations resulting from 10% change in input flow rate in the
intensified process. The temperature differences on the
left-hand side cannot be maintained using typical control
strategies, causing a slight difference in concentration, as
shown in Figs. 4 and 5. This difference affects the distribution
of the DMF in C104&105. The right section of C104&105 has
a higher concentration of extractant, because of which
separation cannot be operated properly owing to leakage of
the extractant.
Figure 4. Temperature of C104&105 stages, shown in Fig. 3,
under steady state and 23 h after input disturbance.
Figure 5. Concentration of extractant in C104&105 stages,
shown in Fig. 3, under steady state and 23 h after input
disturbance.
Owing to this control failure, the two extra degrees of
freedom of the DWC can be used to modify the process; the
strategy of fixing the liquid split ratio and feed-in stage (stage
33) in C104&105 is used in the intensified process.
For the conventional and simplified processes, the control
strategies have been well developed in previous works, thus,
the results are stable in the columns. The following section
shows the comparison of the conventional, simplified, and
DWC processes with and without fixing the liquid split ratio.
IV. SIMULATION RESULTS
For comparison of the results of the processes, primary
product recoveries and concentrations are presented as figures
355
to demonstrate the stabilities of the processes. If the system is
maintained in a stable state, the outputs of the products should
lie in a small range.
A. Comparison of the Conventional and Simplified
Processes
The comparisons of the concentrations and recoveries of
DCPD, IP, and PD plus cyclopentene (CP), for the
conventional and simplified processes are shown in Figs. 6 to
17. It can be concluded from the figures that both processes
can be maintained in a stable state. Additionally, the results
show that the simplified process provides a considerably
higher concentration of DCPD and maintains stability.
Figure 6. Concentration of DCPD in conventional process.
Figure 7. Concentration of DCPD in simplified process.
Figure 8. Recovery of DCPD in conventional process.
Figure 9. Recovery of DCPD in simplified process.
Figure 10. Concentration of IP in conventional process.
Figure 11. Concentration of IP in simplified process.
Figure 12. Recovery of IP in conventional process.
356
Figure 13. Recovery of IP in simplified process.
Figure 14. Concentration of PD+CP in conventional process.
Figure 15. Concentration of PD+CP in simplified process.
Figure 16. Recovery of PD+CP in conventional process.
Figure 17. Recovery of PD+CP in simplified process.
B. Comparison of the DWC process with and without Fixing
Liquid Split Ratio
The control strategy fails after slight difference occurs in
the column temperature of C104&105 resulting from a higher
range of disturbances in the input stream in the DWC process,
without fixing the liquid split ratio in C104&105, which is
shown in Figs. 4 and 5. This problem is due to leakage of
extractant from the right side of the column. The leaked
extractant acts as a thinner in the reactor and decreases
reaction rates, and pollutes the product of stream L3, as shown
in Fig. 18. In addition, this leakage causes failure of the
C104&105 system. This failure can be prevented by fixing the
liquid split ratio in C104&105. Fig. 19 shows the results of the
leakage after fixing the split ratio.
Figure 18. Leakage of extractant in DWC process without
fixing the liquid split ratio.
Figure 19. Leakage of extractant in DWC process after fixing
the liquid split ratio.
357
Consequently, the leakage can be recovered after a period.
Moreover, the entire process can be feasible and maintained in
a range. The comparisons of the results obtained with and
without fixing the liquid split ratio are shown in Figs. 20 to 31.
The results show that without fixing the liquid split ratio, the
entire system stops working under 10% change in input or
lower input of IP. This is owing to slight changes in a few
stages of the column.
Figure 20. Concentration of DCPD in DWC process without
fixing the liquid split ratio.
Figure 21. Concentration of DCPD in DWC process after
fixing the liquid split ratio.
Figure 22. Recovery of DCPD in DWC process without fixing
the liquid split ratio.
Figure 23. Recovery of DCPD in DWC process after fixing
the liquid split ratio.
Figure 24. Concentration of IP in DWC process without fixing
the liquid split ratio.
Figure 25. Concentration of IP in DWC process after fixing
the liquid split ratio.
Figure 26. Recovery of IP in DWC process without fixing the
liquid split ratio.
358
Figure 27. Recovery of IP in DWC process after fixing the
liquid split ratio.
Figure 28. Concentration of PD+CP in DWC process without
fixing the liquid split ratio.
Figure 29. Concentration of PD+CP in DWC process after
fixing the liquid split ratio.
Figure 30. Recovery of PD+CP in DWC process without
fixing the liquid split ratio.
Figure 31. Recovery of PD+CP in DWC process after fixing
the liquid split ratio.
V. CONCLUSION
Three processes can be used for C5 separation. The
comparison of the conventional and simplified processes
shows that the simplified process provides higher
concentration of DCPD in the product and can be stable in the
dynamic state. The comparison of the two processes with the
DWC process shows that liquid split ratio should be fixed in
the DWC process to achieve improved control of column
temperature and concentration. Thus, the DWC process can
be feasible and applicable in industry.
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