HDA case study S. Skogestad, May 2006 Self- Self Thanks to - PowerPoint PPT Presentation

-Optimizing Control Optimizing Control HDA case study • S. Skogestad, May 2006 Self- Self • Thanks to Antonio Araújo 1

Process Description -Optimizing Control Optimizing Control • Benzene production from thermal-dealkalination of toluene (high- temperature, non-catalytic process). • Main reaction: CH 3 Self- Self + H 2 → + CH 4 + Heat Benzene Toluene • Side reaction + H 2 → 2 ← Diphenyl • Excess of hydrogen is needed to repress the side reaction and coke formation. • References for HDA process: • McKetta (1977) – first reference on the process; • Douglas (1988) – design of the process; • Wolff (1994) – discuss the operability of the process. • No references on the optimization of the process for control structure design purposes. 2

-Optimizing Control Optimizing Control Process Description Purge (H 2 + CH 4 ) Compressor Self- Self H 2 + CH 4 Toluene Quench Mixer FEHE Furnace PFR Separator Cooler CH 4 Toluene Benzene Toluene Stabilizer Benzene Column Column Diphenyl 3

-Optimizing Control Optimizing Control Steady-state operational degrees of freedom Process units DOF External feed streams (feed rate) 2 Heat exchangers duties (including 1 furnace) 3 Self- Self Splitters 2 Compressor duty 1 Adiabatic flash (*) 0 Gas phase reactor (*) 0 Distillation columns 6 14 Equality constraint Quencher outlet temperature -1 Remaining degrees of freedom at steady state 13 (*) No adjustable valves (assumed fully open valve before flash) 4

-Optimizing Control Optimizing Control Steady-state operational degrees of freedom 8 7 Purge ( H 2 + CH 4 ) Compressor 1 Furnace 3 Self- Self H 2 + CH 4 Quencher Toluene Mixer FEHE Reactor 4 5 2 6 Cooler 13 11 9 Separator Benzene CH 4 Toluene Toluene Benzene Stabilizer Column Column Diphenyl 14 12 10 5

-Optimizing Control Optimizing Control Cost Function and Constraints • The following profit is maximized (Douglas’s EP): -J = p ben D ben – p tol F tol – p gas F gas – p fuel Q fuel – p cw Q cw – p power W power - p steam Q steam + Σ (p v,i F v,i ) Self- Self • Constraints during operation: – Production rate: D ben ≥ 265 lbmol/h. – Hydrogen excess in reactor inlet: F Hyd / (F ben + F tol + F diph ) ≥ 5. – Bound on toluene feed rate: F tol ≤ 300 lbmol/h. – Reactor pressure: P reactor ≤ 500 psia. – Reactor outlet temperature: T reactor ≤ 1300 °F. – Quencher outlet temperature: T quencher = 1150 °F. – Product purity: x Dben ≥ 0.9997. – Separator inlet temperature: 95 °F ≤ T flash ≤ 105 °F. – + Distillation constraints • Manipulated variables are bounded. 6

-Optimizing Control Optimizing Control Disturbances Disturbance Unit Nominal Lower Upper Toluene feed flow rate lbmol/h 300 285 315 Self- Self Gas feed composition mol% of H 2 95 90 100 Benzene price $/lbmol 9.04 8.34 9.74 Energetic value of fuel to the furnace MBTU/lbmol 0.1247 0.12 0.13 7

Optimizing Control -Optimizing Control Optimization 6,5 6 5,5 Benzene price Profit (M$/year) 5 Self- Self 4,5 4 3,5 3 2,5 2 l a ) ) n ) ) ) ) r r ) ) r r r r e e r r i e e m e e e e w p w p w p w p o p o p p o p o o N u l u u l u ( l ( ( l ( ( ( ( e ( e e n e l n c l c t e t e o a o i a u i u r i r r i p r t p f t f i i d s d f s e f o e e o o e o n n e p e e p e e e f m f u m z u z e e l n o n a l o n a n e e v c e v c e B B u d u c d c l e l i o i e o t t e e e T e T g f f g s r s r e a e a n n G G E E 8 Disturbance

Optimization -Optimizing Control Optimizing Control • 14 steady-state degrees of freedom • 10 active constraints: 1. Pure toluene feed rate ( UB ) 2. By-pass valve around FEHE ( LB ) Self- Self 3. Reactor inlet hydrogen-aromatics ratio ( LB ) 4. Flash inlet temperature ( LB ) 5. Methane mole fraction in stabilizer bottom ( UB ) 6. Benzene mole fraction in stabilizer distillate ( UB ) 7. Toluene mole fraction in benzene column bottom ( LB ) 8. Benzene mole fraction in benzene column distillate ( LB ) 9. Diphenyl mole fraction in toluene column bottom ( LB ) ( LB ) 10.Toluene mole fraction in toluene column distillate • 1 equality constraint: 11. Quencher outlet temperature • 3 remaining unconstrained degrees of freedom. 9

-Optimizing Control Optimizing Control Optimization – Active Constraints Purge ( H 2 + CH 4 ) Compressor Equality Self- 11 2 Furnace Self H 2 + CH 4 Quencher Toluene Mixer FEHE Reactor 3 1 Cooler 8 6 10 4 Separator Benzene CH 4 Toluene Toluene Benzene Stabilizer Column Column Diphenyl 10 9 7 5

Optimizing Control -Optimizing Control Candidate Controlled Variables • Candidate controlled variables: – Pressure differences; – Temperatures; – Compositions; – Heat duties; Self- Self – Flow rates; – Combinations thereof. • 138 candidate controlled variables might be selected. • 14 degrees of freedom. • Number of different sets of controlled variables: 138 138! 18 5.3 10 14 124!14! • 10 active constraints + 1 equality constraint leaving 3 DOF: 127 127! 333,375 3 124!3! • What should we do with the remaining 3 DOF? 11 – Self-optimizing control!!!

Analysis of the linear model -Optimizing Control Optimizing Control a. All measurements ( σ (G full ) = 1.58): Branch-and-bound: σ (G 3x3 ) = 0.864 I Quencher outlet benzene mole fraction Self- Self II Compressor power III Liquid (cooling) flow to quencher Branch-and-bound: σ (G 3x3 ) = 0.853 Separator liquid outlet benzene mole fraction II Compressor power Liquid (cooling) flow to quencher III Branch-and-bound: σ (G 3x3 ) = 0.852 Benzene mole fraction in stabilizer bottom II Compressor power Liquid (cooling) flow to quencher III 12

Optimal self-optimizing variables -Optimizing Control Optimizing Control II W Purge ( H 2 + CH 4 ) Compressor x benzene I 11 2 Furnace Self- Self H 2 + CH 4 Quencher Toluene Mixer FEHE Reactor Flow 1 1 III Cooler 10 8 6 4 Separator Benzene CH 4 Toluene Toluene Benzene Stabilizer Column Column Diphenyl 13 9 7 5

Analysis of the linear model -Optimizing Control Optimizing Control b. Separator pressure constant ( σ (G full ) = 1.50): Branch-and-bound: σ (G 3x3 ) = 0.835 I Quencher outlet benzene mole Self- Self fraction II Compressor power Separator pressure III’ 14

Alternative self-optimizing variables -Optimizing Control Optimizing Control II W Purge ( H 2 + CH 4 ) Compressor x benzene I 11 2 Furnace Self- Self H 2 + CH 4 Quencher Toluene Mixer FEHE Reactor 1 1 p III’ Cooler 10 8 6 4 Separator Benzene CH 4 Toluene Toluene Benzene Stabilizer Column Column Diphenyl 15 9 7 5

-Optimizing Control Optimizing Control Conclusion steady-state analysis • Many similar alternatives in terms of loss • Need to consider dynamics (Input-output controllability analysis): Self- Self – RHP-zeros – RHP-poles – Input saturation – Easy of implementation (decentralized control of final 3x3 supervisory control problem)! • Now: Consider “bottom-up” design of control system 16

-Optimizing Control Optimizing Control Bottom-up design of control system • Start with stabilizing control Self- Self – Levels – Pressure – Temperatures • Normally start with fastest loops (often pressure) – but let is start with levels 17

“Bottom-up”: Proposed Control Structure -Optimizing Control Optimizing Control Stabilizing Control: Control 7 liquid levels Purge ( H 2 + CH 4 ) Compressor Self- Self Furnace H 2 + CH 4 Quencher Toluene Mixer FEHE Reactor Cooler LC Separator LC LC LC Benzene Toluene CH 4 Toluene Benzene Stabilizer Column Column LC LC LC 18 Diphenyl LV-configuration assumed for columns

-Optimizing Control Optimizing Control Avoiding “Drift” I – 4 Pressure loops Pressure with purge Purge ( H 2 + CH 4 ) Compressor Self- Self Furnace H 2 + CH 4 Quencher Toluene Mixer FEHE Reactor Cooler PC LC Separator PC PC PC LC LC LC Benzene Toluene CH 4 Toluene Benzene Stabilizer Column Column LC LC LC 19 Diphenyl Column pressures are given

-Optimizing Control Optimizing Control Avoiding “Drift” II – 4 Temperature loops Purge ( H 2 + CH 4 ) Compressor Self- Self Furnace H 2 + CH 4 Quencher Toluene T s Mixer FEHE Reactor TC p s Cooler PC LC Separator PC PC PC LC LC LC Benzene Toluene CH 4 TC TC Toluene Benzene Stabilizer Column Column TC LC LC LC 20 Diphenyl

-Optimizing Control Optimizing Control Now suggest pairings for supervisory control Self- Self 21

-Optimizing Control Optimizing Control Control of 11 active constraints. Purge ( H 2 + CH 4 ) Compressor Self- Self SP CC Furnace SP SP TC FC H 2 + CH 4 Quencher Toluene T s Mixer FEHE Reactor TC p s FC Cooler PC SP LC SP TC Separator PC PC PC LC LC LC Benzene Toluene CH 4 SP SP SP CC TC CC CC TC Toluene Benzene Stabilizer Column Column SP TC CC SP SP CC CC LC LC LC 3 DOF left 22 Diphenyl