HDA case study

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HDA case study

• S. Skogestad, May 2006

• Thanks to Antonio Araújo

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Process Description

• Benzene production from thermal-dealkalination of toluene (high-temperature, non-catalytic process).

• Main reaction:

• Side reaction

• 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.

CH3

+ H2 → + +CH4 Heat

H2+→2 ←

Toluene Benzene

Diphenyl

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Mixer FEHE Furnace PFR Quench

Separator

Compressor

Cooler

StabilizerBenzeneColumn

TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

Process Description

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Steady-state operational degrees of freedom

Process units DOF

External feed streams (feed rate) 2

Heat exchangers duties (including 1 furnace) 3

Splitters 2

Compressor duty 1

Adiabatic flash(*) 0

Gas phase reactor(*) 0

Distillation columns 6

Equality constraint

Quencher outlet temperature -1

Remaining degrees of freedom at steady state 13

(*) No adjustable valves (assumed fully open valve before flash)

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Steady-state operational degrees of freedom

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

1

2

7

4

3

56

8

1214

13 11 9

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Cost Function and Constraints

• The following profit is maximized (Douglas’s EP):

-J = pbenDben – ptolFtol – pgasFgas – pfuelQfuel – pcwQcw – ppowerWpower - psteamQsteam + Σ(pv,iFv,i)

• Constraints during operation:– Production rate: Dben ≥ 265 lbmol/h.– Hydrogen excess in reactor inlet: FHyd / (Fben + Ftol + Fdiph) ≥ 5.– Bound on toluene feed rate: Ftol ≤ 300 lbmol/h.– Reactor pressure: Preactor ≤ 500 psia.– Reactor outlet temperature: Treactor ≤ 1300 °F.– Quencher outlet temperature: Tquencher = 1150 °F.– Product purity: xDben ≥ 0.9997.– Separator inlet temperature: 95 °F ≤ Tflash ≤ 105 °F.– + Distillation constraints

• Manipulated variables are bounded.

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Disturbances

Disturbance Unit Nominal Lower Upper

Toluene feed flow rate lbmol/h 300 285 315

Gas feed composition mol% of H2 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

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2

2,5

3

3,5

4

4,5

5

5,5

6

6,5P

rofit

(M$/

year

)

Optimization

Benzene price

Disturbance

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• 14 steady-state degrees of freedom

• 10 active constraints:1. Pure toluene feed rate (UB)2. By-pass valve around FEHE (LB)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. Benzene mole fraction in benzene column bottom (UB) 8. Toluene mole fraction in benzene column distillate (UB)9. Toulene mole fraction in toluene column bottom (UB)10. Diphenyl mole fraction in toluene column distillate (UB)

• 1 equality constraint:11. Quencher outlet temperature

• 3 remaining unconstrained degrees of freedom.

Optimization

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Optimization – Active Constraints

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

8

2

1

4

3

5

6

7

10

9

11Equality

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Candidate Controlled Variables• Candidate controlled variables:

– Pressure differences;– Temperatures;– Compositions;– Heat duties;– Flow rates;– Combinations thereof.

• 138 candidate controlled variables might be selected.• 14 degrees of freedom.• Number of different sets of controlled variables:

• 10 active constraints + 1 equality constraint leaving 3 DOF:

• What should we do with the remaining 3 DOF?– Self-optimizing control!!!

18138 138!5.3 10

14 124!14!

127 127!333,375

3 124!3!

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Analysis of the linear model

• Select 3 of 127 candidate measurements.

• Scale variables properly and linearize.

• Find max σ(G3x3) by a branch-and-bound algorithm.

• Alternatively, find max σ(G3x3·Juu-1/2) by a branch-and-bound

algorithm.

• Calculate the loss by the exact local method for the most important disturbance: Namely, feed toluene rate.

“input scaling”

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Analysis of the linear model

σ(G3x3) = 0.02382, Loss = 8.6461 σ(G3x3·Juu-1/2) = 0.10257, Loss = 6.6803

Toluene column reflux flow rate Compressor powerLiquid (cooling) flow to quencher

Toluene mole fraction in stabilizer bottomCompressor powerFurnace heat duty


Diphenyl mole fraction in benzene column bottom Compressor powerLiquid (cooling) flow to quencher

Toluene mole fraction in separator liquid outletCompressor powerFurnace heat duty


Benzene mole fraction in benzene column bottomCompressor powerLiquid (cooling) flow to quencher

Diphenyl mole fraction in quencher outletCompressor powerFurnace heat duty

a. All measurements:σ(Gfull) = 0.0445; σ(Gfull·Juu

-1/2) = 0.1695

IIIIII

I

IIIII

II

IIII

II

III

III

III

III

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Self-optimizing variables

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

8

2

1

4

1

5

6

7

10

9

I

III

11

II

F

W

I

III

xtol

Q

L

Optimal set

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Analysis of the linear modelb. Distillation train excluded and separator pressure constant (controllability):σ(Gfull) = 0.0440; σ(Gfull·Juu

-1/2) = 0.1663


Toluene mole fraction in separator liquid outletCompressor powerSeparator pressure

Toluene mole fraction in separator liquid outletCompressor powerSeparator pressure


Benzene mole fraction in mixer outletCompressor powerSeparator pressure

Benzene mole fraction in mixer outletCompressor powerSeparator pressure


Quencher outlet diphenyl mole fractionCompressor powerSeparator pressure

Quencher outlet diphenyl mole fractionCompressor powerSeparator pressure

I

IIIII

I

IIIII

Note: The two methods give the same order in this case

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Alternative self-optimizing variables

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

8

2

1

4

1

5

6

7

10

9

11

P

W

I

xtol

II

III

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Conclusion steady-state analysis

• Many similar alternatives in terms of loss

• Need to consider dynamics (Input-output controllability analysis):– 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

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Bottom-up design of control system

• Start with stabilizing control– Levels– Pressure– Temperatures

• Normally start with fastest loops (often pressure) – but let is start with levels

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ol “Bottom-up”: Proposed Control Structure

Stabilizing Control: Control 7 liquid levels

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

LCLCLC

LCLCLC

LC

LV-configuration assumed for columns

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Avoiding “Drift” I – 4 Pressure loops

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

LCLCLC

LCLCLC

LC

PC PC PC

PC

Column pressures are given

Pressure with purge

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Avoiding “Drift” II – 4 Temperature loops

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

LCLCLC

LCLCLC

LC

TC

TCTC

TC

PC PC PC

PC

ps

Ts

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Now suggest pairings for supervisory control

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Control of 11 active constraints.

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

LCLCLC

LCLCLC

LC

TC

TCCC CC

CCCCCC

CC TC

CC

TC

FC

FC TC

PC

TC

3 DOF left

PC PC PC

SP

SPSP

SP

SP

SP methaneSPSP

SPSP SP

ps

Ts

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Control of 3 self-optimizing variables: Optimal set

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

LCLCLC

LCLCLC

LC

TC

TCCC CC

CCCCCC

CC TC

CC

TC

FC

FC TC

PC

TC

Supervisorycontrol problemseems difficult

PC PC PC

SP

SPSP

SP

SP

SP methaneSPSP

SPSP SP

ps

Ts

Ixtoluene

III

II

Q

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Control of 3 self-optimizing variables: Alternative set: SIMPLE

Mixer FEHE

Furnace

Reactor

Quencher

Separator

Compressor

Cooler


TolueneColumn

H2 + CH4

Toluene

Toluene Benzene CH4

Diphenyl

Purge (H2 + CH4)

LCLCLC

LCLCLC

LC

TC

TCCC CC

CCCCCC

CC TC

CC

TC

FC

FC TC

PC

TC

PC PC PC

SP

SPSP

SP

SP

SPSPSP

SPSP SP

ps

Ts

Ixtol

III

II

Could controlanother composition,e.g. quencher outlet diphenyl

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Conclusion HDA

• Follow systematic procedure

• May want to keep several candidate sets of “almost” self-optimizing variables

• Final evaluation: Non-linear steady-state simulations + Dynamic simulations using Aspen (ongoing!)

HDA case study

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