Control of Distillation Column
（精馏塔控制）
Lei Xie
Institute of Industrial Control,
Zhejiang University, Hangzhou, P. R. China
2012/05/23
Contents





Distillation Principle & Control Problems
Column Pressure Control
Material Balance (物料平衡) Control
Product Purity Control
(1) Distillate Purity Control
(2) Bottom Purity Control
(3) Both Distillate & Bottom Purity Control
Other Control Schemes
A Typical Distillation Column
with Two Products
Coolant
(冷却水)
Condenser
(冷凝器)
Distillation Column
(精馏塔系统)
(精馏段)
Rectifying
Section
Reflux (回流液)
Accumulator
(回流罐)
Feed (进料)
Stripping
Section
(提馏段)
Vent
(不凝气)
Process
Description,
Distillate
(塔顶产品)
Balance
Conditions,
Reflux Pump
(回流泵)
Vapors (上升蒸汽)
& CVs
Selection ?
Steam
(水蒸汽)
Reboiler
(再沸器)
Bottom Pump
(塔底泵)
Control
Objectives
Bottoms
(塔底产品)
Control Objectives





Guarantee operation safety
Decrease disturbances
Control purity and composition of
products
Increase the product yield
Reduce the operational energy
Selection of CVs & MVs
QC
DG
P
LD
MVs
TR
F
L
D, xD
TS
V
QH
LB
B, xB
Selection ?
Control Problems
of Distillation Column
Disturbances
(F, xF, TF)
Manipulated Variables
Controlled Variables
Distillate flow D
Distillate product purity xD
Reflux flow L
Bottoms flow B
Steam flow QV
Coolant flow QC
Top uncondensable
vapor vent flow DV
(塔顶不凝气流量)
Accumulator level LD
Distillation
Column
Bottoms product purity xB
Column level LB
Column presure P
Pressure Control Problem
QC
CV: P
DG
P
LD
TR
F
MVs : QC, DG , V, L
DVs : F, xF
D, xD
L
TS
V
QH
MV Selection &
Control Schemes ?
LB
B, xB
Pressure Control Schemes
PC
PC
L
L
F
D
Scheme 1.1: MV is the uncondensable vapors (不凝气) vent flow
F
D
Scheme 1.2: MV is the coolant
flow to control column pressure
Application case based on effectiveness
Level Control Problem
(or Material Balance Control)
CVs: LD, LB
LD
MVs: L or D, QV or B
LT
22
DVs : F, xF
FC
11
FT
11
L
D
F
V
LB
CVs & MVs Pairing ?
LT
21
QV
B
Level Control Scheme 2.1
FC
13
FC
11
LC
22
LT
22
FT
13
FT
11
F
L
D
FC
12
FT
12
LT
21
Advantages &
disadvantages ?
LC
21
QV
B
Level Control Scheme 2.2
LC
22
FC
11
FC
14
LT
22
FT
14
FT
11
F
L
D
FC
12
FT
12
LT
21
LC
21
QV
B
Advantages &
disadvantages ?
Level Control Scheme 2.3
LC
22
LT
22
FC
15
FC
11
FT
15
FT
11
L
F
LT
21
FT
14
D
FC
12
FT
12
FC
14
Advantages &
disadvantages ?
LC
21
QV
B
Measurement of Product Purity

Direct measurement approaches
(1) On-line composition/quality analyzers (在线分析仪),
(2) soft sensors / inferential measurement (软测量仪).

Indirect measurement approaches
(1) Sensitive tray temperature (灵敏板温度) or bubble point
of the liquid, if the column pressure is constant;
(2) Pressure compensated temperature;
(3) Difference between tray temperature and top/bottom
temperature.
Material Balance (物料平衡)
for a Column with Binary Mixture
VR
MD
F, xF
L
D, xD
V
Overall material balance:
F  DB
Balance for light component:
FxF  DxD  Bx B
D xF  xB

,
F xD  xB
B xD  xF

F xD  xB
B, xB
Material balance line:

  1

1



1  xD  1 
xF 
 1 xB
 D
 D

F   F 

Relation of D/F
to 1-xD, xB ?
Separation factor
for a Binary Mixture
Separation factor S (分离度)：
VR
MD
F, xF
S
xD
1  xB xD (1  xB )


1  xD
xB
xB (1  xD )
L
D, xD
V
Separation curve :
xD 
B, xB
or
SxB
1  xB ( S  1)
1  xB
1  xD 
1  xB ( S  1)
Relation of S to 1-xD, xB ?
Variables to Influence the
Separation Factor
For variable reflux ratio L/D
（回流比）：
VR
MD
F, xF






S 
 1  D  1 
L
xF 

L
D, xD
V
B, xB
relative volatility
(相对挥发度)
Influence of L/D, xF to S ?
nE
E: average
stage efficiency
n: number of stages
Operating Points
for a binary-component column
Operating
condition:
xF = 0.52,
n = 16, E = 0.816,
  2.0
Analyze the
influence of
MVs !
Effects of V/F to D/F & L/D
in Distillate & Bottom
VR
MD
F, xF
L
D, xD
V
B, xB
If the feed is in the form of
liquid, and the material in the
condenser keeps balance, then
VR  V  L  D
(1) If L/F is constant, V/F↑→D/F↑&
D/L↑;
(2) If D/F is constant, V/F↑→ D/L↓;
(3) If V/F is constant, D/F↑→ L/F↓→
L/D ↓.
Distillation Product Purity
Control Problems
MVs
CVs
W
D
xD / TR
L
LD
Column
B
(A)
L
Column
B
W
D
L
CVs
xD / TR
Column
LD
xB / TS
LB
V
MVs
CVs
D
LB
V
W
MVs
LD
B
xB / TS
V
LB
(C)
(B)
Distillate Purity Control Problem
CVs: LD, TR, LB
MVs: L, D, QV or B
LD
TT
32
LT
22
TR
L
DVs : F, xF
D
F
LB
CVs & MVs Pairing ?
LT
21
QV
B
Distillate Product
Purity Control Scheme 3.1A
TC
32
TT
32
TR
FC
11
LC
22
LT
22
FC
13
FT
13
FT
11
L
D
F
Advantages & disadvantages:
FC
12
FT
12
LT
21
LC
21
QV
B
(1) Fast & strong control effect of L to TR,
(2) Negative influence of reflux (回流量)
fast change to column operation,
(3) Difficult to control LT22 if D<< L.
Distillate Product
Purity Control Scheme 3.1B
TC
32
TT
32
LC
22
TR
FC
14
FT
14
FC
11
L
F
LT
22
D
Advantages & disadvantages:
(1)
(2)
(3)
(4)
Positive influence of reflux smooth change，
Easy to control LT22 if D<< L,
Slow control effect of D to TR,
Strong coupling between TC32 & LC22.
Distillate Product
Purity Control Scheme 3.1C
TC
32
LC
22
TT
32
LT
22
TR
FC
15
FC
14
FT
15
L
FT
14
D
F
Advantages & disadvantages:
(1) Week coupling between TC32 & LC22, and smooth reflux change,
(2) Easy to control LT22 for any D/L,
(3) Fast control effect of D to TR.
Bottom Purity Control Problem
CVs: LD, TS, LB
LD
MVs: L, D, QV or B
LT
22
DVs : F, xF
F
L
TT
31
D
TS
LB
CVs & MVs Pairing ?
LT
21
QV
B
Bottom Product
Purity Control Scheme 3.2A
FC
11
LC
22
LT
22
FC
13
FT
13
FT
11
L
D
F
TC
31
TT
31
Advantages & disadvantages:
TS
LT
21
(1) Fast control effect of QV to TS,
(2) Need enough reflux resulting in
large energy consumption.
LC
21
QV
B
Bottom Product
Purity Control Scheme 3.2B
FC
11
LC
22
LT
22
FC
13
FT
13
FT
11
L
D
F
TS
TT
31
Advantages & disadvantages:
TC
31
V
FC
15
LT
21
FT
15
QV
B
LC
21
(1)
(2)
(3)
(4)
Easy to control LD if B<<V,
Slow control effect of B to TS,
Strong coupling between loops,
Need enough reflux resulting in
large energy consumption.
Bottom Product
Purity Control Scheme 3.2C
LC
22
LT
22
FC
14
FT
14
FC
11
FT
11
L
D
Advantages & disadvantages:
F
TC
31
TT
31
TS
LT
21
LC
21
QV
B
(1) Fast control effect of QV to TS,
(2) Automatic balance between L & V,
to reduce energy consumption,
(3) Strong coupling between control
loops TC31 & LC22.
Bottom Product
Purity Control Scheme 3.2D
LC
22
LT
22
FC
14
FT
14
FC
11
FT
11
L
D
F
TS
TT
31
Overall material balance can
not be satisfied (Why ?).
TC
31
V
FC
15
LT
21
FT
15
QV
LC
21
B
Ex. light component in feed
increases ……
Both Product Purity
Control Problem
CVs: LD, LB , TR, TS
LD
TT
32
MVs: L, D, QV, B
LT
22
TR
DVs : F, xF
L
D
F
TS
TT
31
CVs & MVs Pairing ?
V
LT
21
QV
LB
B
Controlled Process #1
y1
TT
32
LT
22
u1
TR
LC
22
FC
11
FT
11
F
u2
D, xD
L
TS
LT
21
TT
31
y2
LC
21
QV
B, xB
Analyze the dynamic
process to the change of
u1, u2 as well as the
steady-state gain matrix
Controlled Process #2
u1
TT
32
y1
LC
22
TR
LT
22
FC
14
FT
14
FC
11
FT
11
L
F
u2
TS
LT
21
D, xD
TT
31
y2
LC
21
QV
B, xB
Analyze the steadystate gain matrix to
the change of u1, u2.
Both Product
Purity Control Scheme 3.3A
TT
32
TC
32
LC
22
LT
22
TR
FC
11
FT
11
L
D
F
TC
31
TT
31
Coupling Analysis
between TC31 & TC32 ?
TS
LT
21
LC
21
QV
B
Both Product
Purity Control Scheme 3.3B
TT
32
TC
32
LC
22
LT
22
TR
FC
11
FT
11
L
D
F
TS
TT
31
TC
31
V
FC
15
LT
21
FT
15
QV
B
LC
21
Coupling Analysis for
TC31、TC32 & LC21 ?
Both Product
Purity Control Scheme 3.3C
TC
32
TT
32
LC
22
TR
LT
22
FC
14
FT
14
FC
11
FT
11
L
D
F
TC
31
TT
31
Coupling Analysis for
TC31、TC32 & LC22 ?
TS
LT
21
LC
21
QV
B
Scheme Discussion
to Control Both Product Purity




Not easy to control both distillate & bottom product purity
simultaneously, but sometime necessary to save energy or other
requirements;
Only one of product flow can be used to control product purity,
the other product purity has to be controlled by V or L;
Possible to exist strong coupling among loops, especially
between purity control loops;
To reduce the coupling, special control strategies can be used
such as decoupling, predictive control, and other APC
(Advanced Process Control) algorithms
Other Control Systems for
distillation column




Feed Enthalpy Control (进料热焓控制)
to reduce the effect of feed enthalpy change on product
composition control.
Feed Flow Feedforward Control (进料前馈控制)
to reduce the effect of feed flow change on product
purity control.
Minimize Energy Consumed in Column Operation
Analyzer Based Control for Product Composition
Key technology: On-line composition analyzer
Feed Flow-rate
Feed-forward Control
TC
32
TT
32
L
TR
FT
13
×
FT
11
×
D
FC
13
KVF
F
How to reduce the
influence of other
main disturbances ?
FC
12
FT
12
QV
B
Application of online analyzer
in product purity control
TC
32
TT
32
TR
AC
52
FC
13
FC
11
FT
13
AT
52
FT
11
L
F
How to introduce the purity
information If the output of online
analyzer is not continuous ?
FC
12
FT
12
QV
D
B
Dynamic Simulation for a Column
with Binary Mixture
Influence of Reflux Flow on System
1.05
LD, kmol
D, kmol/min
0.16
0.14
0.12
0.1
0
50
100
150
1
0.95
200
0
50
100
150
200
0
50
100
150
200
0
50
100
150
200
0
50
100
min
150
200
xD
L, kmol/min
0.3
0.28
0.26
0
50
100
150
0.93
0.92
200
LB, kmol
0.2
0.18
0.16
0
50
100
150
1
200
2
0.2
1
xB
V, kmol/min
B, kmol/min
0.22
0.1
0
-1
0
50
100
min
150
200
0
Other Simulation Study
1. Level Control Scheme: D  LD, B  LB
(1.1) Influence of V, L on CVs & D, B;
(1.2) Distillate Purity Control: L  xD, V const.;
(1.3) Bottom Purity Control: V  xB, L const.;
(1.4) Both Product Control: L  xD, V  xB.
2. Level Control Scheme: L  LD, B  LB
3. Level Control Scheme: L+D LD, B  LB
Summary


Column Control Objectives
Regular Control Schemes
Column pressure control, feed enthalpy control,
product purity control including four detailed
objectives , etc.

Application of APC Systems
On-line analyzer control, multivariable
predictive control, soft sensor/inferential
measurement, etc.
Integrated Exercises

See the attached DOC file