1 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ Plantwide...
-
Upload
chrystal-parsons -
Category
Documents
-
view
217 -
download
0
Transcript of 1 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ Plantwide...
1M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Plantwide Control for Economically Optimal Operation of Chemical Plants
- Applications to GTL plants and CO2 capturing processes
Mehdi PanahiPhD defense presentation
December 1st, 2011Trondheim
2M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Outline
Ch.2 Introduction
Ch.4 Economically optimal operation of CO2 capturing process; selection of controlled variables
Ch.5 Economically optimal operation of CO2 capturing process; design control layers
Ch.6 Modeling and optimization of natural gas to liquids (GTL) process
Ch.7 Self-optimizing method for selection of controlled variables for GTL process
Ch.8 Conclusions and future works
3M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Outline
Ch.2 Introduction
Ch.4 Economically optimal operation of CO2 capturing process; selection of controlled variables
Ch.5 Economically optimal operation of CO2 capturing process; design control layers
Ch.6 Modeling and optimization of natural gas to liquids (GTL) process
Ch.7 Self-optimizing method for selection of controlled variables for GTL process
Ch.8 Conclusions and future works
4M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Skogestad plantwide control procedure
I Top Down • Step 1: Identify degrees of freedom (MVs)• Step 2: Define operational objectives (optimal operation)
– Cost function J (to be minimized)– Operational constraints
• Step 3: Select primary controlled variables CV1s (Self-optimizing) • Step 4: Where set the production rate? (Inventory control)
II Bottom Up
• Step 5: Regulatory / stabilizing control (PID layer)– What more to control (CV2s; local CVs)?– Pairing of inputs and outputs
• Step 6: Supervisory control (MPC layer)• Step 7: Real-time optimization
5M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Mode I: maximize efficiency
Mode II: maximize throughput
Optimal Operation
Self-optimizing control is when we can achieve acceptable loss with constant setpoint values for the controlled variables without the need to reoptimize the plant when disturbances occur
6M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Selection of CVs: Self-optimizing control procedure
Step 3-1: Define an objective function and constraints
Step 3-2: Degrees of freedom (DOFs)
Step 3-3: Disturbances
Step 3-4: Optimization (nominally and with disturbances)
Step 3-5: Identification of controlled variables (CVs) for
unconstrained DOFs
Step 3-6: Evaluation of loss
7M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Maximum gain rule for selection the best CVs
Let G denote the steady-state gain matrix from inputs u (unconstrained degrees of freedom) to outputs z (candidate controlled variables). Scale the outputs using S1
1i
1S =diag
span(z )
i i i,opt i,opt id,e d e
span(z )= max z -z = max e (d)+ max e
max 2 -1/21 uu
1 1L =
2 σ (S GJ )
For scalar case, which usually happens in many cases, the maximum expected loss is:
uumax 2
1
J 1L =
2 S G
Maximum gain rule is useful for prescreening the sets of best controlled variables
8M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Exact local method for selection the best CVs
21max. Loss= σ(M)
2-11/2 y
uu nM=J G (FW W )d
y -1 yuu ud dF=G J J -G
opt.ΔyF=
ΔdF is optimal sensitivity of the measurements with respect to disturbances;
9M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Applications of plantwide procedure to two important processes
1. Post-combustion CO2 capturing processes (Chapters 3, 4 and 5)
2. Converting of natural gas to liquid hydrocarbons (Chapters 6 and 7)
10M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
An amine absorption/stripping CO2 capturing process*
Dependency of equivalent energy in CO2 capture plant verses recycle amine flowrate
*Figure from: Toshiba (2008). Toshiba to Build Pilot Plant to Test CO2 Capture Technology. http://www.japanfs.org/en/pages/028843.html.
Importance of optimal operation for CO2 capturing process
11M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Gas commercialization options and situation of GTL processes
A simple flowsheet of a GTL process
12M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Outline
Ch.2 Introduction
Ch.4 Economically optimal operation of CO2 capturing process; selection of controlled variables
Ch.5 Economically optimal operation of CO2 capturing process; design control layers
Ch.6 Modeling and optimization of natural gas to liquids (GTL) process
Ch.7 Self-optimizing method for selection of controlled variables for GTL process
Ch.8 Conclusions and future works
13M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Economically optimal operation of CO2 capturing
Absorber
Stripper
Pump 1 Reboiler
V-6
Rich/Lean Exchanger
Surge TankPump 2
V-8Condenser
Cooler
V-5
V-2
V-4
V-9
V-3
Steam
Cooling Water in
Amine Makeup
Water Make up
Flue Gas from Power
Plant
V-7
To Stack
CO2
Condensate
Cooling Water out
Cooling Water out
Cooling Water in
n=1n=1
n=15
n=20
V-1
V-10
Step 1. Objective function:
min. (energy cost + cost of released CO2 to the air)
Steps 5&6. Exact Local method: The candidate CV set that imposes the minimum worst case loss to the objective function
Step 2. 10 steady-state degrees of freedom
Step 3. 3 main disturbances
Step 4. Optimization4 equality constraints and 2 inequality
2 unconstrained degrees of freedom;10-4-4=2
14M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Exact local method for selection of the best CVs
39 candidate CVs- 15 possible tray temperature in the absorber- 20 possible tray temperature in the stripper- CO2 recovery in the absorber and CO2 content at the bottom of the stripper- Recycle amine flowrate and reboiler duty
The best self-optimizing CV set in region I: CO2 recovery (95.26%) and temperature of tray no. 16 in the stripper
These CVs are not necessarily the best when new constraints meet
Applying a bidirectional branch and bound algorithm for finding the best CVs
15M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Optimal operational regions as function of feedrate
Region I. Nominal feedrate
Region II. Feedrate >+20%: Max. Heat constraint
Region III. Feedrate >+51%: Min. CO2 recovery constraint
16M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Proposed control structure with given flue gas flowrate (region I)
17M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Region II: in presence of large flowrates of flue gas (+30%)
Flowrateof flue gas (kmol/hr)
Pumpsduty
(kW)
Self-optimizing CVs in region I CoolerDuty(kW)
Reboilerduty(kW)
Objectivefunction
(USD/ton)CO2 recovery
%
Temperatureof
tray no. 16°C
Optimal nominal point
219.3 3.85 95.26 106.9 321.90 1161 2.49
+5% feedrate 230.3 4.24 95.26 106.9 347.3 1222 2.49
+10% feedrate 241.2 4.22 95.26 106.9 371.0 1279 2.49
+15% feedrate 252.2 4.64 95.26 106.9 473.3 1339 2.49
+19.38% feedrate,reboiler duty
saturates
261.8 4.56(+18.44%)
95.26 106.9 419.4 (+30.29%)
1393(+20%)
2.50
+30% feedrate (reoptimized)
285.1 4.61 91.60 103.3 359.3 1393 2.65
Saturation of reboiler duty (new operations region, region II); one unconstrained degree of freedom left
Maximum gain rule for finding the best CV: 37 candidates
Temp. of tray no. 13 in the stripper: the largest scaled gain
18M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Proposed control structure with given flue gas flowrate (region I)
19M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Proposed control structure with given flue gas flowrate (region II)
Reboiler duty at the maximum
20M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Region III: reaching the minimum allowable CO2 recovery
Flowrateof
flue gas (kmol/hr)
Pumps Duty(kW)
CO2
recovery %
Self-optimizingCV
in region II
Cooler Duty(kW)
Reboiler Duty(kW)
Objective function
(USD/ton)
Temperatureof tray 13
°C
Optimal nominal case in +30% feedrate
285.1 4.61 91.60 109 359.3 1393 2.65
+40% feedrate 307.02 4.58 86.46 109 315.5 1393 2.97
+50% feedrate 328.95 4.55 81.31 109 290.3 1393 3.31
+52.78% feedrate,reach to minimum
CO2 recovery
335.1 4.54 80 109 284.6 1393 3.39
A controller needed to set the flue gas flowrate
21M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Outline
Ch.2 Introduction
Ch.4 Economically optimal operation of CO2 capturing process; selection of controlled variables
Ch.5 Economically optimal operation of CO2 capturing process; design control layers
Ch.6 Modeling and optimization of natural gas to liquids (GTL) process
Ch.7 Self-optimizing method for selection of controlled variables for GTL process
Ch.8 Conclusions and future works
22M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Design of the control layers
Regulatory layer: Control of secondary (stabilizing) CVs (CV2s), PID loops • Absorber bottom level,
• Stripper (distillation column) temperature,
• Stripper bottom level,
• Stripper top level,
• Stripper pressure,
• Recycle surge tank: inventories of water and amine,
• Absorber liquid feed temperature.
Supervisory (economic) control layer: Control of the primary (economic) CVs (CV1s), MPC
• CO2 recovery in the absorber,• Temperature at tray 16 in the stripper,• Condenser temperature.
23M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
RGA analysis for selection of pairings
2 2
dyn. -2s
2 2
6.85s+1.74 -0.76s 0.038
2400s +107s+119.7s +11.4s+1G (s)=(-9.51s-1.02)e 0.45s+0.0754
218s +17.3s+1 205s +18.8s+1
dyn.RGA (0)=0.77 0.23
0.23 0.77
10-3
10-2
10-1
100
101
102
0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Frequency [rad/min]
||RG
A -
I|| su
m
Diagonal pairing alt.1
Off-diagonal pairing alt.2Recycle amine
Reboiler
dutyCO2 recovery
Temp. no.16 in the stripper
2SS
0.5232 1.48G 10
8.47 5.17
SSRGA =0.27 1.27
1.27 0.27
1. Dynamic RGA
2. Steady-State RGA
24M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
”Break through” of CO2 at the top of the absorber (UniSim simulation)
Liquid mole fraction of CO2 in trays of the Absorber
0,015
0,02
0,025
0,03
0,035
0,04
0,045
0,05
0,055
0 50 100 150 200 250 300 350 400 450
Time (min)
mo
le f
rac
tio
n
tray 15
tray 14
tray 13
tray 12
tray 11
tray 10
tray 9
tray 8
tray 7
tray 6
tray 5
tray 4
tray 3
tray 2
tray 1
tray 1
tray 15
25M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Proposed control structure with given flue gas flowrate, Alternative 1
26M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Proposed control structure with given flue gas flowrate,Alternative 2 (reverse pairing)
27M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Proposed control structure in region II,Alternative 3
28M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Modified alternative 2 Modified Alternative 2: Alternative 4
29M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Control of self-optimizing CVs using a multivariable controller
30M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Performance of the proposed control structure, Alternative 1
31M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Performance of the proposed control structure, Alternative 3
32M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Performance of the proposed control structure, Alternative 4
33M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Performance of the proposed control structure, MPC
34M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
• Alternative 1 is optimal in region I, but fails in region II
• Alternative 2 handles regions I (optimal) and II (close to optimal), but more interactions in region I compare to Alternative 1. No need for switching
• Alternative 3 is optimal in region II. Need for switching
• Alternative 4 is modified Alternative 2 ,results in less interactions. No need for switching
• MPC, similar performance to Alternatives 2 & 4
Comparison of different alternatives
Alternative 4 is recommended for implementation in practice
35M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Outline
Ch.2 Introduction
Ch.4 Economically optimal operation of CO2 capturing process; selection of controlled variables
Ch.5 Economically optimal operation of CO2 capturing process; design control layers
Ch.6 Modeling and optimization of natural gas to liquids (GTL) process
Ch.7 Self-optimizing method for selection of controlled variables for GTL process
Ch.8 Conclusions and future works
36M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
A simple flowsheet of GTL process
CH4 CO+H2(CH2)n
CO+H2+CH4
CO2
(CH2)n
37M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Auto-thermal reformer (ATR) reactions
2n
Oxidation of methane:
Steam reforming of methane:
Shift Reaction:
4 2 2
32
2CH O CO H O
4 2 23CH H O CO H
2 2 2CO H O CO H
( )2n m 2 2
mC H +nH O n H +nCO
2 4 2CO+3H CH +H O
2 2 2CO+H O CO +H
Pre-reformer reactionsConverting higher hydrocarbons
than methane, For
Methanation
Shift Reaction
Fischer-Tropsch (FT) reactions
2 2 22 (- -)nnCO nH CH nH O
38M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Fischer-Tropsch (FT) reactor
2 2 22 (- -)nnCO nH CH nH O
Simulation of a slurry bubble column reactor (SBCR)
Reactions:
Kinetics (the model developed by Iglesia et al):
2 4
4
8 0.05
5
1.08 10( )g-atom surface metal. s1 3.3 10
H CO CH
CHCO
P P molr
P
2
8 0.6 0.65
5
1.96 10( )g-atom surface metal. s1 3.3 10
H CO COCO
CO
P P molr
P
FT products distribution (ASF model): 2 1(1 ) nnw n
41 reactions: 21 reactions for CnH2n+2 and 20 reactions for CnH2n
FT products: C1, C2, C3-C4 (LPG), C5-C11 (Naphtha, Gasoline), C12-C20 (Diesel), C21+ (wax)
39M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Detailed flowsheet of GTL process (UniSim)
Heater
Natural Gas8195 kmol/hr3000 kPa, 40 °CCH4:0.955C2H6:0.030C3H8:0.005n-C4H10:0.004N2:0.006
Pre-Reformer
Sep
ara
tor
CO
2
Rem
ov
al
Steam
Pure Oxygen(from ASU)5236 kmol/hr
Water8589 kmol/hr
Fired Heater
Autothermal Reformer
(ATR)
FT Reactor
3-Phase Separator
Compressor
MP Saturated Steam
48887 kmol/hr252 °C, 4000 kPa
MP Steam Light Ends
Purge to fired heater (as fuel)
392 kmol/hr
Water38°C
4743 koml/hr
Tail Gas12673 kmol/hrCH4: 0.3587CO: 0.1454H2: 0.2142N2: 0.1173CO2: 0.0838H2O: 0.0024Other: 0.0782
MP Steam
Syngas35100 kmol/hr
Vapor
Liquid
CO2
936 kmol/hr
To Upgrading Unit721 kmol/hr, 30°C(141.4 m3/hr)vol. mole fractionLPG: 0.0468
Naphtha/Gasoline: 0.3839
Diesel: 0.3216
Wax: 0.2242
Other: 0.0235
To fired heater (not shown) to produce superheat steam
ASU Turbines
Superheated Steam
Extra Steam
27020 kmol/hr
Water
Natural Gas (fuel)
2937 kmol/hr3000 kPa
200 °C
675 °C
5204 kmol/hr455 °C, 3000 kPa
1030 °C
16663 kmol/hr400 °C, 4000 kPa 10 kPa
210°C2000 kPa
210°C
Water9736 kmol/hr
40M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
4 2
0.4
10.6
12 25
2
22 2 19 2
20000.55
1 1 1 1 1 116 [ 2 ( ) ... 25 ( )]
16 1 30 1 28 1 352 1 350
(1 ) 2000(1 ) 1 1 1 1 1 1
16 [ 2(2 (1 ) )( ) ... 25(1 2 3 ...20 )(1 ) ( )]16 1 30 1 28 1 352 1 350
CH H
CO CO
r P wwr P w w
0.02 0.03 0.04 0.05 0.06 0.070.7
0.8
0.9
1
1.1
1.2
1.3
1.4
1.5
1.6
Left hand side value in 6.12
Val
ue
of
real
ro
ots
Real roots α as a function of the selectivity (1.2 ≤ H2/CO ≤ 2.15)
1) α1
2) α2
3) Constant α = 0.9 (α3)
2
(0.2332 0.633)[1 0.0039( 533)]CO
CO H
yT
y y
Different methods for calculation of α
41M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
FT reactor performance (single pass) at H2/CO=2 feed for
α1, α2 and α3
parameter α1 α2 α3
CO conversion, % 83.56 86.52 86.97
H2 conversion, % 90.98 91.82 91.97
CH4 formation (kg/kgcat.hr) 0.0106 0.011 0.011
Other hydrocarbons formation (kg/kgcat.hr)
0.1877 0.1924 0.1924
42M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
FT Products distribution when α1 is used
43M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
FT Products distribution when α2 is used
44M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
FT Products distribution when α3 is used
45M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Dependency of different α calculation methods vs. feed H2/CO
46M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Optimization formulation
Objective function
Variable income (P): sales revenue – variable costs
Steady-state degrees of freedom1. H2O/C (fresh + recycled hydrocarbons to pre-reformer)
2. O2/C (hydrocarbons into ATR)
3. Fired heater duty
4. CO2 recovery percentage
5. Purge ratio
6. Recycle ratio to FT reactor
Operational constraints1. Molar ratio H2O/C ≥ 0.3
2. ATR exit temperature ≤ 1030ºC, active at the max.
3. Inlet temperature to ATR ≤ 675ºC, active at the max.
4. The purge ratio is optimally around 2%, it is bounded at a higher value, active at the min.
47M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Optimality of objective function (α2 model) with respect to decision variables and active constraints, wax price= 0.63 USD/kg
48M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Outline
Ch.2 Introduction
Ch.4 Economically optimal operation of CO2 capturing process; selection of controlled variables
Ch.5 Economically optimal operation of CO2 capturing process; design control layers
Ch.6 Modeling and optimization of natural gas to liquids (GTL) process
Ch.7 Self-optimizing method for selection of controlled variables for GTL process
Ch.8 Conclusions and future works
49M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Optimal Operation of GTL process
- Mode I: Natural gas is given
- Mode II: Natural gas is also a degree of freedom (maximum throughput)
50M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Process flowsheet of GTL process with data for optimal nominal point (mode I)
Heater
Natural Gas8195 kmol/hrCH4:0.955C2H6:0.030C3H8:0.005n-C4H10:0.004N2:0.006
Pre-Reformer
Se
pa
rato
r
CO
2
Re
mo
va
l
Steam
Oxygen(from ASU)5190 kmol/hr
Water8560 kmol/hr
Fired Heater
Autothermal Reformer
(ATR)
FT Reactor
3-Phase Separator
Compressor I0.47 MW
HP Saturatied Steam
Light Ends
Purge to fired heater (as fuel)325 kmol/hr
Water906 kmol/hr
Tail Gas10846 kmol/hr
MP Steam
Syngas34627 kmol/hr
Vapor210°C
2500 kPa
Liquid
CO2906 kmol/hr
Liquid fuels to upgrading unit: 751 kmol/hr (144 m3/hr)vol. mole fractions:LPG: 0.0525Naphtha/Gasoline: 0.3759Diesel: 0.3156 Wax: 0.2257
To fired heater (not shown) to produce superheat steam
ASU TurbinesSuperheated Steam
Extra Steam
Water
Natural Gas (fuel)
455°C
4896 kmol/hr 455°C, 3000 kPa
200°C
675°C
1030°C
Compressor II0.55 MW
V-1∆P=f(Q)
Recycle Tail gas to FT7762 kmol/hr
2700 kPa
2500kPa3000kPa
210°C2700 kPa3000 kPa
Steam Drum
Water
51M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Mode I: Natural gas flowrate is given
Step 1: Define the objective function and constraintsVariables income
Inequality ConstraintsThree inequality constraints + capacity constraints on the variable units; fired heater
(duty +40% compared to nominal), CO2 recovery unit (+20% feedrate), oxygen plant (+20% oxygen flowrate).
Equality Constraints (Specs:9)
Step 2: Identify degrees of freedom (DOFs) for optimization15 degrees of freedom
52M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Step 3: Identification of important disturbances
Natural gas flowrate,
Natural gas composition,
Natural gas price,
FT reactions kinetic parameter,
Change in active constraints value.
53M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Step 4: Optimization
MIXED method: combines the advantage of global optimization of BOX and efficiency of SQP method
15 degrees of freedom, 9 equality constraints and 3 active constraints:
unconstrained degrees of freedom: 15 – 9 – 3 = 3, which may be viewed as:
H2O/C, CO2 recovery, tail gas recycle ratio to FT reactor
Step 5. Identification of candidate controlled variables
18 candidate measurements including the three unconstrained degrees of freedom 18 18!
8163 3!15!
Step 6. Selection of CVs
Exact local method for selection of the best CVs
54M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
no. Sets Loss (USD/hr)
1 y3:CO2 recoveryy9: CO mole
fractionin fresh syngas
y12: CO mole
fractionin tail gas
1393
2 y3:CO2 recovery y2: H2O/Cy6: H2/CO
in tail gas1457
3 y3:CO2 recovery y2: H2O/Cy5: H2/CO
in fresh syngas1698
4 y3:CO2 recoveryy6: H2/CO
in tail gas
y5: H2/CO
in fresh syngas2594
5y10:CH4 mole
fractionin fresh syngas
y6: H2/CO
in tail gas
y5: H2/CO
in fresh syngas2643
Individual measurements (mode I)
worst-case loss for the best 5 individual measurement sets
55M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Control structure for mode I of operation with proposed CVs and possible pairings with MVs (red lines are by-pass streams)
Heater
Natural Gas
Pre-Reformer
Se
par
ato
r
CO
2 R
emo
va
l
Steam
Oxygen(from ASU)
Water
Fired Heater
Autothermal Reformer
(ATR)
FT Reactor
3-Phase Separator
Compressor I
HP Saturated Steam
Light Ends
Purge to fired heater (as fuel)
Water
Tail Gas
MP Steam
Syngas
Vapor
Liquid
CO2
Liquid fuels to upgrading unit
To fired heater (not shown) to produce superheat steam
ASU Turbines
Superheated Steam
Extra Steam
Water
Natural Gas (fuel)
SP=455°C
SP=200°C
SP=675°C SP=1030°C
Compressor II
V-1∆P=f(Q)
Recycle Tail gas to FT
Steam Drum
Water
TC
TC
TC
TC
TC
TC
PC
TC
PC
PC
SP=30 bar
SP=38°C
SP=210°C
SP=12.5bar
SP=30°C
SP=27bar
CCSP (H2O/C)
CC
Self-optimizing CV1 SP (CO%)=25.67
SP=455°CTC
CC
Self-optimizing CV2SP (CO2 recovery%)=
75.73
RCCC
Self-optimizing CV3 SP (CO%)=12.96
RC
SP=3% purge
SP (splitter ratio)
56M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Mode II: Natural gas feedrate is also a degree of freedom
Point A: oxygen flowrate saturates
1 extra DOF, 1 new active constraint
57M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Optimal values in: nominal point, saturation of oxygen flowrate and maximum throughput
H2O/C O2/CCO2
recovery
Recycle ratio to FT
Purge of
tail gas
H2/CO
fresh
H2/CO
into FT
CO conversion % H2 conversion %
Carbonefficiency
Objective function(USD/hr)
per pass
overallper pass
overall
opt. nominal
0.6010 0.523 75.73% 73.79% 3% 2.1 2.03 85.74 95.50 89.93 96.92 0.87 74.59% 49293
max. oxygen
0.5357 0.516 76.80% 90% 3% 2.092 1.91 67.08 94.14 74.705 95.88 0.86 74.30% 59246
max. through
put0.4084 0.504 76.04% 97.13% 3% 2.095 1.80 51.25 94.79 60.69 96.39 0.87 74.31% 59634
FT reactor volume is the bottleneck
58M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
no. SetsLoss
(USD/hr)
1y3:CO2
recoveryy2: H2O/C
y7: H2/CO
into FT reactor3022
2y3:CO2
recoveryy2: H2O/C
y6: H2/CO
in tail gas3316
3y3:CO2
recoveryy2: H2O/C
y5: H2/CO
in fresh syngas3495
4y3:CO2
recoveryy2: H2O/C
y17: tail gas
flowrate to syngas unit
4179
5y3:CO2
recovery
y9: CO mole
fractionin fresh syngas
y15: CO mole
fractioninto FT reactor
4419
Individual measurements (mode II)
worst-case loss for the best 5 individual measurement sets
59M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Control structure for mode II of operation with proposed CVs and possible pairings with MVs (red lines are by-pass streams)
Heater
Natural Gas
Pre-Reformer
Se
par
ato
r
CO
2 R
em
ov
al
Steam
Oxygen(from ASU) flowrate is at max.
Water
Fired Heater
Autothermal Reformer
(ATR)
FT Reactor
3-Phase Separator
Compressor I
HP Saturated Steam
Light Ends
Purge to fired heater (as fuel)
Water
Tail Gas
MP Steam
Syngas
Vapor
Liquid
CO2
Liquid fuels to upgrading unit
To fired heater (not shown) to produce superheat steam
ASU Turbines
Superheated Steam
Extra Steam
Water
Natural Gas (fuel)
SP=455°C
SP=200°C
SP=675°C SP=1030°C
Compressor II
V-1∆P=f(Q)
Recycle Tail gas to FT
Steam Drum
Water
TC
TC
TC
TC
TC
TC
PC
TC
PC
PCSP=30 bar
SP=38°C
SP=210°C
SP=12.5bar
SP=30°C
SP=27bar
CCSP (H2O/C=0.4084)
Self-optimizing CV1
SP=455°CTC
CC
Self-optimizing CV2SP (CO2 recovery%)=
76.04
RC
CC
Self-optimizing CV3SP (H2/CO)=1.8
RC
SP=3% purge
SP (splitter ratio)
60M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
• Self-optimizing method was applied for selection of the CVs for GTL
• There are 3 unconstrained DOFs in both modes of operation
• One common set in the list of the best individual measurements in two modes:
CO2 recovery
H2/CO in fresh syngas
H2O/C setpoint reduces from 0.6 to o.4
• Operation in Snowballing region should be avoided
• Saturation point of oxygen plant capacity is recommended for operation in practice
Concluding remarks of self-optimizing application for GTL process
61M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Outline
Ch.2 Introduction
Ch.4 Economically optimal operation of CO2 capturing process; selection of controlled variables
Ch.5 Economically optimal operation of CO2 capturing process; design control layers
Ch.6 Modeling and optimization of natural gas to liquids (GTL) process
Ch.7 Self-optimizing method for selection of controlled variables for GTL process
Ch.8 Conclusions and future works
62M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Conclusions and future works
Systematic plantwide procedure of Skogestad was applied for a post-combustion CO2 capturing process; a simple control configuration was achieved, which works close to optimum in the entire throughput range without the need for switching the control loops or re-optimization of the process
A GTL process model suitable for optimal operation studies was modeled and optimized. This model describes properly dependencies of important parameters in this process
Self-optimizing method was applied to select the right measurements for the GTL process in two modes of operation
UniSim/Hysys linked with MATLAB showed to be a very good tool for optimal operation studies
63M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’
Conclusions and future works
Implementation of the final control structure for CO2 capture plant is recommended for implementation in practice
Dynamic simulation of the GTL process should be done to validate the proposed control structures
The application of plantwide control procedure is strongly recommended for other newer energy-intensive processes
Developing a systematic method for arriving at a simple/single control structure, which works close to optimum in all operational regions can be a good topic for future work
Thank you for your attention!