Dynamic Modeling and Control of Divided Wall Column

Authors:

László SzabóSándor NémethFerenc Szeifert

ABC

A

B

C

2010.02.18

8th Meeting of Young Chemical EngineersZagreb

University of Pannonia Department of Process Engineering

2

History of divided wall column

� Dividing-Wall Column (DWC) was introduced by Richard O.Wright in 1949 : Fractionation Apparatus

� However, lack of reliable design method and concerns about the operation and control of DWC have prevented the widespread application.

� People started to pay more attention to DWC after the Energy Crisis (1980).

� In 1985, BASF built the first commercial DWC.� More than 100 columns installed worldwide.

3

The number of the DWC

4

The divided wall column

Bottom stream

Boil up

Side stream

Reflux

Distillate stream

Feed

5

Concentration profile of the side product

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 5 10 15 20 25

Tray

Tol

uene

mas

sfra

c (m

/m)

Feed zoneProduct zone

6

Separation task

144,4

112,1

80,1

Boiling point(°C)

0,33o-Xylene

0,33Toluene

0,33Benzene

Feed concentration(kg/kg)

Feed90 kg/h

BenzenePurity: 0,99 kg/kg

Toluene

o-XylenePurity : 0,99 kg/kg

7

Flowsheeting implementation (AspenPlus)

Heat transferFeed

Benzene

Toluene

o-Xylene

8

Flowsheeting implementation(AspenPlus)

YesCondenser

NoReboiler

Feed

Benzene

Toluene

o-Xylene

Heat transfer

9

Flowsheeting implementation (AspenPlus)

NoCondenser

NoReboiler

Feed

Benzene

Toluene

o-Xylene

Heat transfer

10

Flowsheeting implementation (AspenPlus)

NoCondenser

YesReboiler

Feed

Benzene

Toluene

o-Xylene

Heat transfer

11

Inputs:

User interface of the simulation

12

Outputs:

User interface of the simulation

13

Parametric study

� Height of the wall� Vertical position of the wall � Splitting ratio� Comparing divided wall column to side

stream column

14

20

25

30

35

40

45

50

55

60

5 6 7 8 9 10 11 12 13

Height of the wall (tray)

Reb

oile

r dut

y (k

W)

Effect of the height of the wall

50-50%

15

20

25

30

35

40

45

50

55

60

-3 -2 -1 0 1 2 3

Relative position (midle of the column)

Reb

oile

r du

ty (k

W)

Effect of the vertical wall position

50-50%

16

20

25

30

35

40

45

50

55

60

65

70

0 20 40 60 80 100

Splitting ratio (%)

Reb

oile

r du

ty (

kW)

Effect of splitting ratio

17

Comparing DWC to side stream column

0

100

200

300

400

500

600

700

50 100 150 200 250 300

Feed rate (kg/h)

Reb

oile

r du

ty(k

W)

Divided wall column

Column with side product

18

Dynamics simulation (Aspen Dynamics)

� The reboiler, the condenser and the reflux drum were implemented

� The column was implemented by two blocks

� Pressure changers (pipe, valve, pump) were applied

19

Dynamics simulation (Aspen Dynamics)

20

Control structure of the DWC(PI controllers)

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��

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21

Load disturbance: feed rate

85

90

95

100

105

110

115

0 2 4 6 8 10 12 14Time (h)

Fee

d r

ate

(kg

/h)

22

Effect on temperature control of 22th stage

143

143.2

143.4

143.6

143.8

144

144.2

144.4

144.6

144.8

145

0 2 4 6 8 10 12 14

Time (h)

Tem

pera

ture

(°C

)

0

5

10

15

20

25

30

35

Reb

oile

r du

ty (

kW)

Process variable (°C)

Set point (°C)

Output (kW)

23

Effect on temperature control of 4thstage

88

88.05

88.1

88.15

88.2

88.25

0 2 4 6 8 10 12 14

Time (h)

Tem

pera

ture

(°C

)

0

5

10

15

20

25

30

Val

ve p

ositi

on (%

)

Process variable (°C)

Set point (°C)

Output (%)

24

Effect on mass rate of the productsMass rate of the products

25

27

29

31

33

35

37

39

41

0 2 4 6 8 10 12 14

Time (h)

Mas

sflo

w (k

g/h)

Head productSide productBottom product

25

Effect on massfraction of the products

0.91

0.92

0.93

0.94

0.95

0.96

0.97

0.98

0.99

1

0 2 4 6 8 10 12 14

Time (h)

Mas

sfra

c (k

g/kg

)

Head productSide productBottom product

26

Summary

� The DWC was implemented in dynamic and steady-state simulator

� Main parameters of the column were analysed: optimum wall position is determinated

� Comparing DWC to side stream column: significant energy saving

� Controllability of the divided wall column was analysed: feasible control structure

27

Thank you for your attention!

� László Szabó is grateful for the support of the PhD Fellowship of the MOL Plc

� The financial support from the TAMOP-4.2.2-08/1/2008-0018 (Livable environment and healthier people – Bioinnovation and Green Technology research at the University of Pannonia) project is gratefully acknowledged.