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 Columnwith Two Products

Feed (进料)

Bottoms(塔底产品)

Distillation Column(精馏塔系统)

Reboiler(再沸器)

Distillate(塔顶产品)

Reflux (回流液)

Reflux Pump(回流泵)

Condenser(冷凝器)

Accumulator(回流罐)

Steam(水蒸汽)

Vapors (上升蒸汽)

Coolant(冷却水)

StrippingSection

(提馏段)

(精馏段)Rectifying

Section

Vent(不凝气)

Bottom Pump(塔底泵)

Process Description,

Balance Conditions,

Control Objectives

& CVs Selection ?

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

F

B, xB

V

TR

TS

QH

P

QC

DG

D, xDL

LD

LB

MVs

Selection ?

Control Problems of Distillation Column

DistillationColumn

Distillate flow D

Disturbances(F, xF, TF)

Manipulated Variables Controlled Variables

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)

Column presure P

Accumulator level LD

Column level LB

Distillate product purity xD

Bottoms product purity xB

Pressure Control Problem

F

B, xB

V

TR

TS

QH

P

QC

DG

D, xDL

LD

LB

CV: P

MVs : QC, DG , V, L

DVs : F, xF

MV Selection & Control Schemes ?

Pressure Control Schemes

F

L

D

PC

F

L

D

PC

Scheme 1.1: MV is the unconden-sable vapors ( 不凝气 ) vent flow

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

MVs: L or D, QV or B

DVs : F, xF

CVs & MVs Pairing ?

F

BQV

FT11

FC11

LT21

DL

LT22

V LB

LD

Level Control Scheme 2.1

DF

BQV

L

FT11

FC11

FT12

FC12

LT21

LC21

LT22

LC22

FT13

FC13

Advantages & disadvantages ?

Level Control Scheme 2.2

DF

BQV

L

FT11

FC11

FT12

FC12

LT21

LC21

LT22

LC22

FT14

FC14

Advantages & disadvantages ?

Level Control Scheme 2.3

Advantages & disadvantages ?

F

BQV

FT11

FC11

FT12

FC12

LT21

LC21

DL

LT22

LC22

FT14

FC14

FT15

FC15

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

F, xF

V

B, xB

D, xD

VR

L

MDBDF

Overall material balance:

BDF BxDxFx

BD

FD

BD

BF

xx

xx

F

B

xx

xx

F

D

,

Balance for light component:

Material balance line:

BFD x

FD

x

FD

x

1

1111 Relation of D/F

to 1-xD, xB ?

Separation factor for a Binary Mixture

F, xF

V

B, xB

D, xD

VR

L

MD

)1(

)1(1

1 DB

BD

B

B

D

D

xx

xx

x

x

x

xS

Separation factor S ( 分离度 ) ：

Separation curve :

)1(1

Sx

Sxx

B

BD

)1(1

11

Sx

xx

B

BD

or

Relation of S to 1-xD, xB ?

Variables to Influence the Separation Factor

F, xF

V

B, xB

D, xD

VR

L

MD

relative volatility ( 相对挥发度 )

For variable reflux ratio L/D（回流比）：

nE

FxLD

S

11

n: number of stages

E: average stage efficiency

Influence of L/D, xF to S ?

Operating Points for a binary-component

column

Operating condition:

xF = 0.52,

n = 16, E = 0.816,

0.2

Analyze the influence of MVs !

Effects of V/F to D/F & L/D in Distillate & Bottom

F, xF

V

B, xB

D, xD

VR

L

MD

If the feed is in the form of liquid, and the material in the condenser keeps balance, then

DLVVR

(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

Column

D

L

B

V

xD / TR

LB

LD

WMVs CVs

Column

D

L

B

V LB

LD

WMVs CVs

xB / TS

(C)

(A) (B)

Column

D

L

B

V LB

LD

WMVs CVs

xB / TS

xD / TR

Distillate Purity Control Problem

CVs: LD, TR, LB

MVs: L, D, QV or B

DVs : F, xF

CVs & MVs Pairing ?

D

F

BQV

L

LT21

LT22

TT32 TR

LB

LD

Distillate ProductPurity Control Scheme 3.1A

D

F

BQV

L

FT11

FC11

FT12

FC12

LT21

LC21

LT22

LC22

FT13

FC13

TT32

TC32

TR

Advantages & disadvantages:(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 ProductPurity Control Scheme 3.1B

Advantages & disadvantages:(1) Positive influence of reflux smooth change ， (2) Easy to control LT22 if D<< L,

(3) Slow control effect of D to TR,

(4) Strong coupling between TC32 & LC22.

F

FC11

TT32

TC32

TR

DL

LT22

LC22

FT14

FC14

Distillate ProductPurity Control Scheme 3.1C

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.

F

TT32

TC32

TR

DL

LT22

LC22

FT14

FC14

FT15

FC15

Bottom Purity Control Problem

CVs: LD, TS, LB

MVs: L, D, QV or B

DVs : F, xF

CVs & MVs Pairing ?

TT31 TS

DF

BQV

L

LT21

LT22

LB

LD

Bottom ProductPurity Control Scheme 3.2A

D

F

BQV

L

FT11

FC11

LT21

LC21

LT22

LC22

FT13

FC13

TT31

TC31 TS

Advantages & disadvantages:(1) Fast control effect of QV to TS,

(2) Need enough reflux resulting in large energy consumption.

Bottom ProductPurity Control Scheme 3.2B

Advantages & disadvantages:(1) Easy to control LD if B<<V,

(2) Slow control effect of B to TS,

(3) Strong coupling between loops,

(4) Need enough reflux resulting in large energy consumption.

D

F

B

QV

L

FT11

FC11

LT21

LC21

LT22

LC22

FT13

FC13

TT31

TC31

TS

FT15

FC15

V

Bottom ProductPurity Control Scheme 3.2C

F

BQV

FT11

FC11

LT21

LC21

TT31

TC31 TS

DL

LT22

LC22

FT14

FC14

Advantages & disadvantages:(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 ProductPurity Control Scheme

3.2D

F

B

QV

FT11

FC11

LT21

LC21

TT31

TC31

TS

FT15

FC15

V

DL

LT22

LC22

FT14

FC14

Overall material balance can not be satisfied (Why ?).

Ex. light component in feed increases ……

Both Product Purity Control Problem

CVs: LD, LB , TR, TS

MVs: L, D, QV, B

DVs : F, xF

CVs & MVs Pairing ?

F

B

QV

LT21

TT31

TS

V

TT32 TR

DL

LT22

LB

LD

Controlled Process #1

Analyze the dynamic process to the change of u1, u2 as well as the steady-state gain matrix

F

QV

FT11

FC11

LT21

LC21

TT31

TS

L

LT22

LC22

TT32

TR

y2

y1

u1

u2

D, xD

B, xB

Controlled Process #2

Analyze the steady-state gain matrix to the change of u1, u2.

F

QV

FT11

FC11

LT21

LC21

TT32 TR

L

LT22

LC22

FT14

FC14

y1

TT31

TS

y2

u2

u1

D, xD

B, xB

Both ProductPurity Control Scheme 3.3A

Coupling Analysis between TC31 & TC32 ?

F

BQV

FT11

FC11

LT21

LC21

TT31

TC31 TS

DL

LT22

LC22

TT32

TC32

TR

Both ProductPurity Control Scheme 3.3B

Coupling Analysis for TC31、 TC32 & LC21 ?

F

FT11

FC11

B

QV

LT21

LC21

TT31

TC31

TS

FT15

FC15

V

DL

LT22

LC22

TT32

TC32

TR

Both ProductPurity Control Scheme 3.3C

Coupling Analysis for TC31、 TC32 & LC22 ?

F

BQV

FT11

FC11

LT21

LC21

TT31

TC31 TS

TT32

TC32

TR

DL

LT22

LC22

FT14

FC14

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

F

B

L

TT32

TC32

TR

×

FT11

D

QV

FT12

FC12

×

FT13

FC13KVF

How to reduce the influence of other main disturbances ?

Application of online analyzerin product purity control

D

F

BQV

L

FT11

FC11

FT12

FC12

FT13

FC13

TT32

TC32

TRAT52

AC52

How to introduce the purity information If the output of online analyzer is not continuous ?

Dynamic Simulation for a Column with Binary Mixture

Influence of Reflux Flow on System

0 50 100 150 2000.1

0.12

0.14

0.16

D,

kmol

/min

0 50 100 150 2000.95

1

1.05

LD,

kmol

0 50 100 150 2000.26

0.28

0.3

L, k

mol

/min

0 50 100 150 2000.92

0.93

xD

0 50 100 150 2000.16

0.18

0.2

0.22

B,

kmol

/min

0 50 100 150 200

1LB

, km

ol

0 50 100 150 200-1

0

1

2

V,

kmol

/min

min0 50 100 150 200

0

0.1

0.2

xB

min

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 SystemsOn-line analyzer control, multivariable predictive control, soft sensor/inferential measurement, etc.

Integrated Exercises See the attached DOC file