Process Integration Applied to the Design and Operation of Distillation Columns Hilde K. Engelien
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Transcript of Process Integration Applied to the Design and Operation of Distillation Columns Hilde K. Engelien
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Process Integration Applied to the Design and Operation of Distillation Columns
Hilde K. Engelien
22. March 2004
Department of Chemical Engineering, NTNU
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Introduction & Overview
• Process integration - definition• Motivation• Background
Overview of talk:• Introduction to multi-effect arrangements• Minimum vapour flowrate considerations
– Vmin as a target– Vmin-diagrams
• Multi-effect in practice– selecting controlled variables– industrial example
• Main contributions• Concluding remarks
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Introduction
• Process Integration - definition
“Systematic and general methods for design (and operation) of integrated process plants, focusing on efficient energy use and reduced environmental consequences”.
International Energy Agency (IEA), 1993
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Introduction
• Process Integration - definition• Motivation - energy savings, environment, innovation,...
• Distillation is a very common separation process:– performs about 95% of fluid separations in the chemical industries.
• Distillation is a very energy consuming process:– uses about 3% of the world total energy consumption.– accounts for around 25-40 % of energy usage in chemical and petroleum industry.
• Process integration – saves energy and reduces the environmental impact of a process
– reduce site utility costs (e.g. steam, cooling water)– may reduce capital costs
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Introduction
• Process Integration - definition• Motivation - energy savings, environment, innovation,…• Background to heat-integrated distillation columns
• Multi-effect prefractionator arrangements have high energy savings - is therefore an interesting arrangement to study.
• Operation of energy-integrated systems can be more difficult - want to operate so that the energy savings are achieved.
• Not many publications on the control of the integrated prefractionator/sidestream columns [Cheng & Luyben, 1985, Ding & Luyben, 1990, Bildea & Dimian, 1999, Emtir et al., 2003]
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Multi-Effect Distillation
= where pressure is used to adjust the temperature levels in two (or more) columns so that the condensing duty of one column can be used to provide heat in the reboiler of another column.
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A B C
A B
C
B C
Different Distillation Arrangements
Direct split (DS) Indirect split (IS)
A B C
A
BC
A BHP LP
forward-integration (F)
backward-integration (B)
LP HP
B
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Different Distillation Arrangements
Prefractionator columns
A B C
A
B
C
A B
B C
• 30 % less energy• prevents re-mixing effect of
middle component• Further energy savings can be
made with multi-effect integration.
Thermally coupled columns:• Single column shell (divided
wall column)• 30 % reduction in capital cost
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A B C
A
B
C
A B
B C
HP LP
Multi-Effect Prefractionator
Forward integrated prefractionator (PF)
Integrated reboiler/condenser
Heat input
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Multi-Effect Prefractionator
Backward integrated prefractionator (PB)
Integrated reboiler/condenser
A B C
A
B
C
A B
B C
HPLP
Heat input
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Energy ConsumptionPercentage Savings of Different Energy Integrated Arrangements
zF Columns = [4 2 1] = [5 4.5 1] = [5 1.5 1] = [2 1.5 1] = [10 5 1]
1/3
1/3
1/3
DS
IS
Petlyuk
DSF/DSB
ISF/ISB
PF/PB
-1.94
0.00
32.80
47.26
32.80
61.73
0.00
-0.51
7.59
7.59
8.21
37.44
-4.98
0.00
12.76
25.56
12.76
47.41
0.00
0.00
39.54
39.54
44.65
59.06
-0.28
0.00
32.71
32.71
34.03
50.05
0.10
0.80
0.10
DS
IS
Petlyuk
DSF/DSB
ISF/ISB
PF/PB
-0.09
0.00
32.99
37.69
32.99
65.55
0.00
-0.03
11.39
11.39
11.63
52.83
-0.23
0.00
12.43
16.20
12.43
54.54
0.00
0.00
47.37
47.34
49.02
71.71
-0.01
0.00
44.23
49.62
44.23
71.80
0.80
0.10
0.10
DS
IS
Petlyuk
DSF/DSB
ISF/ISB
PF/PB
0.00
-10.25
14.96
14.96
15.23
19.92
0.00
-2.14
1.73
1.73
1.77
10.78
0.00
-3.75
19.04
29.55
19.04
32.34
0.00
-17.43
12.45
12.45
12.86
19.68
0.00
-4.07
8.10
8.10
8.18
13.27
The integrated prefractionator arrangement is the best
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Minimum Vapour Flowrate, Vmin
• Vmin as a target - use to compare different designs– Minimum vapour flow at infinite number of stages– Can get within 10 % of Vmin target by using reasonable number of stages– Assumptions: ideal mixtures, constant relative volatility, constant molar
flows, sharp splits
• Can get within 10 % of Vmin-target using reasonable number of stages
E n e rg y
Num
ber o
f sta
ges
V m in
N m in
• Energy (V) vs. number of stages (N)– trade-off between number of stages and
energy– actual V approaches Vmin for N
approximately 2 x Nmin or larger, typically:
2Nmin + 20% Vmin3Nmin + 2 % Vmin4Nmin + 0.2 % Vmin
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Minimum Vapour Flowrate DiagramsA Visual Tool for Process Integration
= DC1/F
VT/F
PA/B
PB/C
PA/C
Vmin(C21)
Vmin(C22)
Vmin (PF/PB)
Vmin(C1)
A B C
A
B
C
A B
B C
C1
C21
C21
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Drawing the Vmin-diagram
Prefractionator column (C1):
Reference: Halvorsen (2003)
= DC1/F
VT/F
PA/B
PB/C
PA/C
zA zA + zB
Ref.: Halvorsen, Skogestad, 20003
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Extending the Vmin-diagram
Upper section main column (C21):
= DC1/F
VT/F
PA/B
PB/C
PA/C
PM3
PM1
PM4
PM2
Lower section main column (C22):
zA zA + zB
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Using the Vmin-diagramVapour Flowrate for Different Distillation Arrangements
• Vmin for different arrangements.
• Visualise how columns are (un)balanced.
• 5 cases identified - different operating options available.
= DC1/F
VT/F
PA/B
PB/C
PA/C
Vmin(C1)
Vmin (DSF/DSB)
Vmin (PF/PB)
Vmin (Petlyuk + ISF/ISB)
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Benefits of the Vmin-diagram
• Easy visualisation of minimum vapour flow.
• Different distillation arrangements are presented in same diagram.
• Tool for further design - balanced/unbalanced columns gives different design options.
• Starting point for further rigorous simulations - Vmin target, optimum recovery = (D/F)
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Control Problems with Heat-Integrated Distillation Columns
• Integrated columns have added complexity. • Integrated columns may be difficult to control as :
– dynamic upsets can propagate back & forth between columns.
– the system is non-linear, multivariable and interacting.
• Energy savings may not be achieved (or may be worse) if the columns are not operated correctly.
• The heat and mass integration of distillation columns causes additional control problems compared to single columns.
It is therefore essential to develop good control systems to ensure satisfactory operation
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Implementing Optimal Operation of Multi-effect Prefractionators
Objective: to implement a simple “optimal” control scheme for integrated distillation systems.
Want to find the controlled variables that will directly ensure optimal economic operation.
“Optimal” - means near-optimal operation. It is economically acceptable to be a certain distance from optimum (but not too far…).
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Objective : Selection of controlled variablesMethod: Self-optimizing control (Skogestad, 2000)
The method of self-optimizing control involves a search for the variables that, when kept constant, indirectly lead to near-optimal operation with acceptable loss.
Loss imposed by keeping constant setpoint for the controlled variable
d* Disturbance d
Cost J
C = constant2,s
C = constant1,s
Re-optimised J (d)opt
Loss
Steady State Optimisation
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The method of self-optimizing control consists of six steps:
1) Finding the DOF for optimisation.
2) Formulation of a of cost function, J, to be maximised for optimal operations &
constraints.
3) Identification of the most important disturbances.
4) Solving the nominal optimisation problem.
5) Identification of candidate controlled variables.
6) Evaluation of loss (at constant setpoints): L = J - Jopt
Ref.: Sigurd Skogestad, "Plantwide control: the search for the self- optimizing control structure”, Journal of Process Control, 10, 2000.
Steady State Optimisation
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DOF analysis for multi-effect columns : DOF = 11 - 4 = 7Objective function:
Operational constraints: – the LP column pressure must be 1 bar– the HP column pressure must be 15 bar– the purity of the products must be 99 mol%– there is a maximum area in the integrated reboiler/condenser– the duty of the HP condenser must equal the duty of the LP reboiler
(equality constraint)– non-negative flows
Process constraints - the mass, energy and component balances
J = pDD + pSS + pBB - pFF - pVV
Steady State Optimisation
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Results from optimisation:• Active constraints:
– Pressure in LP column– Product purity of sidestream– Product purity of bottom stream– Area in integrated exchanger
• Non-active constraints: – pressure in HP column– product purity in distillate
Implement active constraint control + control distillate composition
Steady State Optimisation
DOF Accounts:11 DOF total- 4 active constraints- 4 levels with no steady state effect- 1 fixed feedrate- 1 controlling distillate composition = 1 DOF left for self-optimising control
One DOF left for control - find a self-optimising control variable
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• Calculate loss L = (J - Jopt) for the selected disturbances (zF, F).• Identify the best variable(s) for control, where the loss is small.
Steady State Optimisation
Loss (%)
(feedrate disturbance)
Loss (%)
(composition disturbance)
Controlvariable, c
F + 20 % F - 20% zF,B + 0.1 zF,B - 0.1
PHP 2.50 0.95 21.94 71.70LT,HP/F 0.02 0.01 0.99 InfeasibleDHP/F 0.03 0.02 1.3 1.47xBD,HP 0.02 0.06 26.96 38.71QB,HP Infeasible 24.55 Infeasible 2.44DHP 8.40 19.14 1.33 1.41BHP 20.80 28.11 1.33 1.41QB,HP/F Infeasible Infeasible Infeasible InfeasibleBHP/F 0.03 0.02 1.30 1.47xBB,HP 0.02 0.14 40.92 InfeasibleLT,HP Infeasible 1.99 0.97 InfeasibleT4,HP 3.88 1.09 26.49 InfeasibleXAB,HP Infeasible Infeasible Infeasible Infeasible
Result: Control DHP/F
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Implementing Optimal Operation of Multi-Effect Prefractionators
F
D H PHP LPLC
LC
XC
PC
XCA
XC
LC
X LC
XC
L T,H P
B H PQ B ,H P
X A B ,H P s
(D /F ) s
Q C ,L P
D L P
S L P
B L P
X C
X B
X A
L T,L P
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Dynamic Simulations
0 10 20 30 40 500.98
0.985
0.99
0.995
1Product composition response in LP column
Time (h)
xAxBxC
0 10 20 30 40 500.98
0.985
0.99
0.995
1Product composition response in LP column
Con
cent
ratio
n
Time (h)
xAxBxC
5 % increase in feedrate F
0.5 increase in middle component feed (zF)
• System is controllable.
• System is sensitive to disturbances.
• The control of bottom composition (main column) is poor.
• Use of feed tank to reduce the feed disturbances (zF, F)
• Other control configurations possible.
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An Industrial Separation Example
3 cases for integration:– Column I and II– Column II and III– Column III and IV
Minimum vapour flowrates:– Case 3 has highest savings of
55 %
– PF/PB is the best
– ISF/ISB is 2. best
T = 9 2 C
Ro
E th a n eP ro p a n eB u tan e
E th a n e P ro p a n e
B u ta n e
I
N = 3 2 N = 4 8
I I
T = 2 CCo T = 3 4 CC
o
T = 9 9 C
Ro
P = 2 7 b a r P = 1 3 b a r g a s o l in e
T = 3 7 CCo
P = 4 b a r
N = 4 0
I II
T = 9 4 C
Ro
i- b u ta n e
n -b u ta n e
T = 3 8 CCo
P = 5 .7 b a r
N = 9 2
IV
T = 5 5 C
Ro
H y d ro- ca r b o nf ee d
Case 1 Case 2 Case 3
Vmin/F % Vmin/F % Vmin/F %
DS 4.23 0.0 1.63 0.0 3.38 -0.2
IS 4.33 -2.2 1.69 -3.76 3.38 0.0
DSF/DSB 3.48 17.7 1.17 28.1 2.49 26.4
ISF/ISB 3.43 19.0 1.16 28.5 2.42 28.4
Petlyuk 3.48 17.7 1.17 28.1 2.49 26.4
PF/PB 2.40 43.3 1.02 37.2 1.51 55.3
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Possible Integration for Case III
Indirect Split (IS) PF ISF (existing arrangement)
Energy savings from rigorous simulations:
i-b u ta n en -b u tan eP e n ta n e
P e n ta n e
n -b u ta n e
i-b u ta n e
i-b u ta n en -b u ta n eP e n tan e
i-b u ta n e
n -b u ta n eP e n ta n e
H P L P
T = 3 7 CCo T = 3 8 CC
o
N =4 0 N =9 2
P = 4 b a r
T = 9 4 C
Ro
T = 5 5 C
Ro
I II I IIIVIV
P = 5 .6 b a r
i-b u ta nen-b uta nep e nta n e
i-b uta ne
n-b uta nep e n ta n e
HP LP
ISF PFActual number of stages 42.7 % 28.6 %Infinite number of stages 43.7 % 56.9 %
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Lessons from the Industrial Example
• PF requires more stages to achieve potential energy savings.
• Revamp should therefore be accompanied by an increase in number of stages.
• If sufficient number of stages are allowed the rigorous simulations show that the PF arrangement has high energy savings (57 %).
• The challenge is to implement the arrangement and achieve the savings in practice !
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Main Contributions
• Comparison of multi-effect prefractionator with other multi-effect arrangements and non-integrated arrangements.
• Graphical visualisation of minimum energy for the multi-effect arrangements in a Vmin-diagram.
• Systematic method applied in the selection of controlled variables for the forward integrated prefractionator arrangement. Control variables are identified that will give low energy losses during operation.
• Analysis of the integrated prefractionator arrangement in an industrial setting .
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Concluding Remarks
• Focus of work is on the energy savings of multi-effect systems, especially the integrated prefractionator arrangement.
• Screening of multi-effect arrangements are based on minimum vapour flow at infinite number of stages (PF/PB can achieve up to 70 % savings).
• Minimum vapour flow (Vmin) is a good target, as by adding stages the actual value of vapour flow (V) is usually close to the minimum.
• The energy requirements for multi-effect arrangements are visualised in Vmin-diagrams.
• Selection of controlled variables using the systematic method of self-optimising control.
• Controlling the right variables can give low energy losses during operation.• Industrial case study - high energy savings if sufficient number of stages are
allowed.
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References
Bildea, C.S., Dimian, A.C., 'Interaction between design and control of a heat-integrated distillation system with prefractionator', Tans IChemE, 1999, Vol. 77, Part A, pp. 597-608
Cheng, H. C., Luyben, W., 'Heat-integrated distillation columns for ternary separations', Ind. Eng. Chem. Process Des. Dev., 1985, 24, 707-713
Ding, S.S., Luyben, W., ‘Control of a heat-integrated complex distillation configuration’, Ind. Eng. Chem. Res.¸1990, 29, 1240-1249
Emtir, M., Mizsey, P., Fonyó, Z., ' Economic and controllability investigation and comparison if energy integrated distillation schemes', Chem. Biochem. Eng. Q., 2003, 17(1), 31-42
Halvorsen, I.J, Skogestad, S., ‘Minimum energy consumption in multicomponent distillation. 1. Vmin diagram for a two product column’, Ind. Eng. Chem. Res., 2003, 42, 596-604
Hewitt, G., Quarini, J., Morell, M., ‘More efficient distillation’, The Chemical Engineer, 21 Oct. 1999
Skogestad, S., 2000, Plantwide control: the search for the self-optimizing control structure, J. Proc. Control, Vol.10, 487-507.
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Study Trip….
… sampling at the Glenfiddich Distillery, Scotland and Jameson Distillery, Ireland.
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Practical Considerations for the Multi-Effect Prefractionator
When considering a multi-effect distillation system for a practicalapplication it is important to look at:
• Operating costs (energy)• Capital costs• Total annual costs (operating + capital)• Control• Operability• Flexibility• Integration with overall process
Usually these factors are not independent and a trade-off must be made toachieve an “optimal” design.
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Possible Vmin DiagramsVmin-diagram Limiting
column sectionsPossible use ofexcess vapour
Case 1 (VC1 = VC22 > VC21)
VC22
VC21 Vmin
VC1
C 1
C 21
C 22
PF
a) Intermediate condenserbetween C22 and C21.
b) Overpurify C21 product.
c) Vapour sidestreamproduct.
d) Shorter column sectionC21.
PB
a) Overpurify C21 product.
b) Shorter column sectionC21.
Case 2 (VC1 = VC21 > VC22)
VC21
Vmin
VC22
VC1
C 1
C 21
C 22
PF
a) Overpurify C22 product.
b) Shorter column sectionC22.
PB
a) Intermediate reboilerbetween C22 and C21.
b) Overpurify C22 product.
c) Shorter column sectionC22.
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Case 3 (VC21 = VC22 > VC1)
VC22
VC21
Vmin
VC1
C 2 2
C 1
C 21
PF
Not common, unless αincreases with pressures.
PB
Overpurification in C1 ispossible, but not importantfor final products.
a) Use shorter column C1.
b) Intermediate condenserat top of C21 or C1.
Case 4 (VC21 = VC22 < VC1)
Vmin
VC21 VC22
VC1
C 22
C 1
C 21
PF
a) Intermediate condenserbetween C22 and C21, orbetween C1 and C22.
b) Overpurify productsfrom main column.
c) Vapour sidestreamproduct.
d) Shorter column sectionsC21 and C22.
PB
Not common, unless αincreases with pressures.
Case 5 (VC1 = VC21 = VC22)
VC21
VC22
Vmin
VC1
C 22
C 1
C 21
All column sections arebalanced.
No special measuresneeded.