1 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ Plantwide Control for Economically Optimal Operation of Chemical Plants - Applications to GTL plants and CO 2 capturing processes Mehdi Panahi PhD defense presentation December 1 st , 2011 Trondheim
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1 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ Plantwide Control for Economically Optimal Operation of Chemical.
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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
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%)
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’
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