Plantwide Control for Economically Optimal Operation of Chemical

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

Outline q Ch. 2 Introduction q Ch. 4 Economically optimal operation of CO 2 capturing process; selection of controlled variables q Ch. 5 Economically optimal operation of CO 2 capturing process; design control layers q Ch. 6 Modeling and optimization of natural gas to liquids (GTL) process q Ch. 7 Self-optimizing method for selection of controlled variables for GTL process q Ch. 8 Conclusions and future works M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 2

Outline q Ch. 2 Introduction q Ch. 4 Economically optimal operation of CO 2 capturing process; selection of controlled variables q Ch. 5 Economically optimal operation of CO 2 capturing process; design control layers q Ch. 6 Modeling and optimization of natural gas to liquids (GTL) process q Ch. 7 Self-optimizing method for selection of controlled variables for GTL process q Ch. 8 Conclusions and future works M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 3

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 CV 1 s (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 (CV 2 s; local CVs)? – Pairing of inputs and outputs • Step 6: Supervisory control (MPC layer) • Step 7: Real-time optimization M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 4

Optimal Operation Mode I: maximize efficiency Mode II: maximize throughput 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 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 5

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 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 6

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 S 1 For scalar case, which usually happens in many cases, the maximum expected loss is: Maximum gain rule is useful for prescreening the sets of best controlled variables M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 7

Exact local method for selection the best CVs F is optimal sensitivity of the measurements with respect to disturbances; M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 8

Applications of plantwide procedure to two important processes 1. Post-combustion CO 2 capturing processes (Chapters 3, 4 and 5) 2. Converting of natural gas to liquid hydrocarbons (Chapters 6 and 7) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 9

Importance of optimal operation for CO 2 capturing process Dependency of equivalent energy in CO 2 capture plant verses recycle amine flowrate An amine absorption/stripping CO 2 capturing process* *Figure from: Toshiba (2008). Toshiba to Build Pilot Plant to Test CO 2 Capture Technology. http: //www. japanfs. org/en/pages/028843. html. M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 10

Gas commercialization options and situation of GTL processes A simple flowsheet of a GTL process M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 11

Outline q Ch. 2 Introduction q Ch. 4 Economically optimal operation of CO 2 capturing process; selection of controlled variables q Ch. 5 Economically optimal operation of CO 2 capturing process; design control layers q Ch. 6 Modeling and optimization of natural gas to liquids (GTL) process q Ch. 7 Self-optimizing method for selection of controlled variables for GTL process q Ch. 8 Conclusions and future works M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 12

Economically optimal operation of CO 2 capturing Step 1. Objective function: min. (energy cost + cost of released CO 2 to the air) Step 2. 10 steady-state degrees of freedom Step 3. 3 main disturbances Step 4. Optimization 4 equality constraints and 2 inequality 2 unconstrained degrees of freedom; 10 -4 -4=2 Steps 5&6. Exact Local method: The candidate CV set that imposes the minimum worst case loss to the objective function M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 13

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 - CO 2 recovery in the absorber and CO 2 content at the bottom of the stripper - Recycle amine flowrate and reboiler duty Applying a bidirectional branch and bound algorithm for finding the best CVs The best self-optimizing CV set in region I: CO 2 recovery (95. 26%) and temperature of tray no. 16 in the stripper These CVs are not necessarily the best when new constraints meet M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 14

Optimal operational regions as function of feedrate Region I. Nominal feedrate Region II. Feedrate >+20%: Max. Heat constraint Region III. Feedrate >+51%: Min. CO 2 recovery constraint M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 15

Proposed control structure with given flue gas flowrate (region I) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 16

Region II: in presence of large flowrates of flue gas (+30%) Flowrateof flue gas (kmol/hr) Pumps duty (k. W) Optimal nominal point 219. 3 +5% feedrate Self-optimizing CVs in region I Cooler Duty (k. W) Reboiler duty (k. W) Objective function (USD/ton) CO 2 recovery % Temperature of tray no. 16 °C 3. 85 95. 26 106. 9 321. 90 1161 2. 49 230. 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 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 17

Proposed control structure with given flue gas flowrate (region I) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 18

Proposed control structure with given flue gas flowrate (region II) Reboiler duty at the maximum M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 19

Region III: reaching the minimum allowable CO 2 recovery Flowrate of flue gas (kmol/hr) Pumps Duty (k. W) CO 2 recovery % Self-optimizing CV in region II Cooler Duty (k. W) Reboiler Duty (k. W) Objective function (USD/ton) Temperature of 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 CO 2 recovery 335. 1 4. 54 80 109 284. 6 1393 3. 39 A controller needed to set the flue gas flowrate M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 20

Outline q Ch. 2 Introduction q Ch. 4 Economically optimal operation of CO 2 capturing process; selection of controlled variables q Ch. 5 Economically optimal operation of CO 2 capturing process; design control layers q Ch. 6 Modeling and optimization of natural gas to liquids (GTL) process q Ch. 7 Self-optimizing method for selection of controlled variables for GTL process q Ch. 8 Conclusions and future works M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 21

Design of the control layers Regulatory layer: Control of secondary (stabilizing) CVs (CV 2 s), 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 (CV 1 s), MPC • CO 2 recovery in the absorber, • Temperature at tray 16 in the stripper, • Condenser temperature. M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 22

RGA analysis for selection of pairings 1. Dynamic RGA Recycle amine CO 2 recovery Reboiler duty Temp. no. 16 in the stripper 2. Steady-State RGA M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 23

”Break through” of CO 2 at the top of the absorber (Uni. Sim simulation) tray 15 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 24

Proposed control structure with given flue gas flowrate, Alternative 1 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 25

Proposed control structure with given flue gas flowrate, Alternative 2 (reverse pairing) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 26

Proposed control structure in region II, Alternative 3 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 27

Modified alternative 2 Modified Alternative 2: Alternative 4 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 28

Control of self-optimizing CVs using a multivariable controller M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 29

Performance of the proposed control structure, Alternative 1 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 30

Performance of the proposed control structure, Alternative 3 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 31

Performance of the proposed control structure, Alternative 4 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 32

Performance of the proposed control structure, MPC M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 33

Comparison of different alternatives • 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 Alternative 4 is recommended for implementation in practice M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 34

Outline q Ch. 2 Introduction q Ch. 4 Economically optimal operation of CO 2 capturing process; selection of controlled variables q Ch. 5 Economically optimal operation of CO 2 capturing process; design control layers q Ch. 6 Modeling and optimization of natural gas to liquids (GTL) process q Ch. 7 Self-optimizing method for selection of controlled variables for GTL process q Ch. 8 Conclusions and future works M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 35

A simple flowsheet of GTL process CO+H 2+CH 4 CO 2 CH 4 CO+H 2 (CH 2)n M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 36

Pre-reformer reactions Converting higher hydrocarbons than methane, For Methanation Shift Reaction Auto-thermal reformer (ATR) reactions Oxidation of methane: Steam reforming of methane: Shift Reaction: Fischer-Tropsch (FT) reactions M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 37

Fischer-Tropsch (FT) reactor Simulation of a slurry bubble column reactor (SBCR) Reactions: Kinetics (the model developed by Iglesia et al): FT products distribution (ASF model): 41 reactions: 21 reactions for Cn. H 2 n+2 and 20 reactions for Cn. H 2 n FT products: C 1, C 2, C 3 -C 4 (LPG), C 5 -C 11 (Naphtha, Gasoline), C 12 -C 20 (Diesel), C 21+ (wax) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 38

Detailed flowsheet of GTL process (Uni. Sim) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 39

Different methods for calculation of α 1) α 1 Real roots α as a function of the selectivity (1. 2 ≤ H 2/CO ≤ 2. 15) 2) α 2 3) Constant α = 0. 9 (α 3) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 40

FT reactor performance (single pass) at H 2/CO=2 feed for α 1, α 2 and α 3 parameter α 1 α 2 α 3 CO conversion, % 83. 56 86. 52 86. 97 H 2 conversion, % 90. 98 91. 82 91. 97 CH 4 formation (kg/kgcat. hr) 0. 0106 0. 011 Other hydrocarbons formation (kg/kgcat. hr) 0. 1877 0. 1924 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 41

FT Products distribution when α 1 is used M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 42

FT Products distribution when α 2 is used M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 43

FT Products distribution when α 3 is used M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 44

Dependency of different α calculation methods vs. feed H 2/CO M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 45

Optimization formulation Objective function Variable income (P): sales revenue – variable costs Steady-state degrees of freedom 1. H 2 O/C (fresh + recycled hydrocarbons to pre-reformer) 2. O 2/C (hydrocarbons into ATR) 3. Fired heater duty 4. CO 2 recovery percentage 5. Purge ratio 6. Recycle ratio to FT reactor Operational constraints 1. 2. 3. 4. Molar ratio H 2 O/C ≥ 0. 3 ATR exit temperature ≤ 1030ºC, active at the max. Inlet temperature to ATR ≤ 675ºC, active at the max. The purge ratio is optimally around 2%, it is bounded at a higher value, active at the min. M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 46

Optimality of objective function (α 2 model) with respect to decision variables and active constraints, wax price= 0. 63 USD/kg M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 47

Outline q Ch. 2 Introduction q Ch. 4 Economically optimal operation of CO 2 capturing process; selection of controlled variables q Ch. 5 Economically optimal operation of CO 2 capturing process; design control layers q Ch. 6 Modeling and optimization of natural gas to liquids (GTL) process q Ch. 7 Self-optimizing method for selection of controlled variables for GTL process q Ch. 8 Conclusions and future works M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 48

Optimal Operation of GTL process - Mode I: Natural gas is given - Mode II: Natural gas is also a degree of freedom (maximum throughput) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 49

Process flowsheet of GTL process with data for optimal nominal point (mode I) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 50

Mode I: Natural gas flowrate is given Step 1: Define the objective function and constraints Variables income Inequality Constraints Three inequality constraints + capacity constraints on the variable units; fired heater (duty +40% compared to nominal), CO 2 recovery unit (+20% feedrate), oxygen plant (+20% oxygen flowrate). Equality Constraints (Specs: 9) Step 2: Identify degrees of freedom (DOFs) for optimization 15 degrees of freedom M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 51

Step 3: Identification of important disturbances Natural gas flowrate, Natural gas composition, Natural gas price, FT reactions kinetic parameter, Change in active constraints value. M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 52

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: H 2 O/C, CO 2 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 Step 6. Selection of CVs Exact local method for selection of the best CVs M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 53

Individual measurements (mode I) worst-case loss for the best 5 individual measurement sets no. Sets Loss (USD/hr) 1 y 3: CO 2 recovery y 9: CO mole fraction in fresh syngas y 12: CO mole fraction in tail gas 1393 2 y 3: CO 2 recovery y 2: H 2 O/C y 6: H 2/CO in tail gas 1457 3 y 3: CO 2 recovery y 2: H 2 O/C y 5: H 2/CO in fresh syngas 1698 4 y 3: CO 2 recovery y 6: H 2/CO in tail gas y 5: H 2/CO in fresh syngas 2594 5 y 10: CH 4 mole fraction in fresh syngas y 6: H 2/CO in tail gas y 5: H 2/CO in fresh syngas 2643 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 54

Control structure for mode I of operation with proposed CVs and possible pairings with MVs (red lines are by-pass streams) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 55

Mode II: Natural gas feedrate is also a degree of freedom Point A: oxygen flowrate saturates 1 extra DOF, 1 new active constraint M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 56

Optimal values in: nominal point, saturation of oxygen flowrate and maximum throughput Recycle ratio to FT Purge of tail gas H 2/CO fresh H 2/CO into FT H 2 O/C O 2/C CO 2 recovery opt. nominal 0. 6010 0. 523 75. 73% 73. 79% 3% 2. 1 max. oxygen 0. 5357 0. 516 76. 80% 90% 3% max. through put 0. 4084 0. 504 76. 04% 97. 13% 3% CO conversion % H 2 conversion % Carbon efficiency Objective function (USD/hr) per pass overall 2. 03 85. 74 95. 50 89. 93 96. 92 0. 87 74. 59% 49293 2. 092 1. 91 67. 08 94. 14 74. 705 95. 88 0. 86 74. 30% 59246 2. 095 1. 80 51. 25 94. 79 60. 69 96. 39 0. 87 74. 31% 59634 FT reactor volume is the bottleneck M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 57

Individual measurements (mode II) worst-case loss for the best 5 individual measurement sets no. Loss (USD/hr) Sets 1 y 3: CO 2 recovery y 2: H 2 O/C y 7: H 2/CO into FT reactor 3022 2 y 3: CO 2 recovery y 2: H 2 O/C y 6: H 2/CO in tail gas 3316 3 y 3: CO 2 recovery y 2: H 2 O/C y 5: H 2/CO in fresh syngas 3495 4 y 3: CO 2 recovery y 2: H 2 O/C y 17: tail gas flowrate to syngas unit 4179 5 y 3: CO 2 recovery y 9: CO mole fraction in fresh syngas y 15: CO mole fraction into FT reactor 4419 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 58

Control structure for mode II of operation with proposed CVs and possible pairings with MVs (red lines are by-pass streams) M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 59

Concluding remarks of self-optimizing application for GTL process • 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: CO 2 recovery H 2/CO in fresh syngas H 2 O/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 M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 60

Outline q Ch. 2 Introduction q Ch. 4 Economically optimal operation of CO 2 capturing process; selection of controlled variables q Ch. 5 Economically optimal operation of CO 2 capturing process; design control layers q Ch. 6 Modeling and optimization of natural gas to liquids (GTL) process q Ch. 7 Self-optimizing method for selection of controlled variables for GTL process q Ch. 8 Conclusions and future works M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 61

Conclusions and future works Systematic plantwide procedure of Skogestad was applied for a post-combustion CO 2 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 Uni. Sim/Hysys linked with MATLAB showed to be a very good tool for optimal operation studies M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 62

Conclusions and future works Implementation of the final control structure for CO 2 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! M. Panahi ’Plantwide Control for Economically Optimal Operation of Chemical Plants’ 63