HDA case study

1
HDA case study

• S. Skogestad, 10 Apr 2006
• Thanks to Antonio Araújo

2
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.

Benzene
Toluene
Diphenyl
3
Process Description
4
Steady-state operational degrees of freedom
14
5
Steady-state operational degrees of freedom
8
Purge (H2 CH4)
7
Compressor
1
Furnace
3
H2 CH4
Quencher
Toluene
Mixer
FEHE
Reactor
4
5
2
Cooler
6
13
11
9
Separator
Benzene
CH4
Toluene
Benzene Column
Toluene Column
Stabilizer
Diphenyl
12
14
10
6
Cost Function and Constraints

• The following profit is maximized (Douglass EP)
• -J pbenDben ptolFtol pgasFgas pfuelQfuel
  pcwQcw ppowerWpower - psteamQsteam
  S(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.

7
Disturbances
8
Optimization
Benzene price
Disturbance
9
Optimization

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

10
Optimization Active Constraints
Purge (H2 CH4)
Compressor
Equality
Furnace
11
2
H2 CH4
Quencher
Toluene
Mixer
FEHE
Reactor
3
1
Cooler
8
6
10
Separator
4
Benzene
CH4
Toluene
Benzene Column
Toluene Column
Stabilizer
Diphenyl
5
7
9
11
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!!!

12
Analysis of the linear model
a. All measurements (s(Gfull) 1.58)
I
II
III
II
III
II
III
13
Optimal self-optimizing variables
W
II
Purge (H2 CH4)
Compressor
xbenzene
I
Furnace
11
2
H2 CH4
Quencher
Toluene
Mixer
FEHE
Reactor
Flow
1
1
III
Cooler
8
6
10
Separator
4
Benzene
CH4
Toluene
Benzene Column
Toluene Column
Stabilizer
Diphenyl
9
5
7
14
Analysis of the linear model
b. Separator pressure constant (s(Gfull) 1.50)
I
II
III
15
Alternative self-optimizing variables
W
II
Purge (H2 CH4)
Compressor
xbenzene
I
Furnace
11
2
H2 CH4
Quencher
Toluene
Mixer
FEHE
Reactor
1
1
p
Cooler
III
8
6
10
Separator
4
Benzene
CH4
Toluene
Benzene Column
Toluene Column
Stabilizer
Diphenyl
9
5
7
16
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

17
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

18
Bottom-up Proposed Control StructureStabilizin
g Control Control 7 liquid levels
Purge (H2 CH4)
Compressor
Furnace
H2 CH4
Quencher
Toluene
Mixer
FEHE
Reactor
Cooler
LC
Separator
LC
LC
LC
Benzene
Toluene
CH4
Benzene Column
Toluene Column
Stabilizer
LC
LC
LC
Diphenyl
LV-configuration assumed for columns
19
Avoiding Drift I 4 Pressure loops
Pressure with purge
Purge (H2 CH4)
Compressor
Furnace
H2 CH4
Quencher
Toluene
Mixer
FEHE
Reactor
Cooler
PC
LC
Separator
PC
PC
PC
LC
LC
LC
Benzene
Toluene
CH4
Benzene Column
Toluene Column
Stabilizer
LC
LC
LC
Diphenyl
Column pressures are given
20
Avoiding Drift II 4 Temperature loops
Purge (H2 CH4)
Compressor
Furnace
H2 CH4
Quencher
Toluene
Ts
Mixer
FEHE
Reactor
TC
ps
Cooler
PC
LC
Separator
PC
PC
PC
LC
LC
LC
Benzene
Toluene
CH4
TC
TC
Benzene Column
Toluene Column
Stabilizer
TC
LC
LC
LC
Diphenyl
21
Now suggest pairings for supervisory control
22
Control of 11 active constraints.
Purge (H2 CH4)
Compressor
SP
CC
Furnace
SP
SP
FC
TC
H2 CH4
Quencher
Toluene
Ts
Mixer
FEHE
Reactor
TC
ps
FC
Cooler
PC
SP
LC
SP
TC
Separator
PC
PC
PC
LC
LC
LC
Benzene
Toluene
CH4
SP
SP
SP
TC
CC
CC
CC
TC
Benzene Column
Toluene Column
Stabilizer
SP
SP
SP
CC
TC
CC
CC
LC
LC
LC
3 DOF left
Diphenyl
23
Control of 3 self-optimizing variables Optimal
set
II
Purge (H2 CH4)
Compressor
I
xbenzene
SP
CC
Furnace
SP
SP
FC
TC
H2 CH4
Quencher
Toluene
Flow
Ts
Mixer
FEHE
Reactor
TC
III
ps
FC
Cooler
PC
SP
LC
SP
TC
Separator
PC
PC
PC
LC
LC
LC
Benzene
Toluene
CH4
SP
SP
SP
TC
CC
CC
CC
TC
Benzene Column
Toluene Column
Stabilizer
SP
SP
SP
CC
TC
CC
CC
Difficult supervisory control problem
LC
LC
LC
Diphenyl
24
Control of 3 self-optimizing variables
Near-Optimal set SIMPLE
II
Purge (H2 CH4)
Compressor
xbenzene
SP
I
CC
Furnace
SP
SP
FC
TC
H2 CH4
Quencher
Toluene
Ts
III
Mixer
FEHE
Reactor
TC
ps
FC
Cooler
PC
SP
LC
SP
TC
Separator
PC
PC
PC
LC
LC
LC
Benzene
Toluene
CH4
SP
SP
SP
TC
CC
CC
CC
TC
Benzene Column
Toluene Column
Stabilizer
SP
SP
SP
CC
TC
CC
CC
LC
LC
LC
Diphenyl
25
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