CHEMICAL ENGINEERING TRANSACTIONS

VOL. 81, 2020

A publication of

The Italian Association
of Chemical Engineering
Online at www.cetjournal.it

Guest Editors: Petar S. Varbanov, Qiuwang Wang, Min Zeng, Panos Seferlis, Ting Ma, Jiří J. Klemeš

ISBN 978-88-95608-79-2; ISSN 2283-9216

Pressure Swing Thermal-Dividing Wall Integrated Light

Hydrocarbon Distillation Design and its Control

Chenyang Fan, Zhe Cui, Haoran Zhang, Wende Tian*

College of Chemical Engineering, Qingdao University of Science & Technology, Qingdao 266042, P.R. China

tianwd@qust.edu.cn

Light hydrocarbon separation has received much attention due to its high energy consumption. In this paper, a

novel five-column pressure swing thermal-dividing wall (PSTDW) integrated light hydrocarbon separation

process is designed to replace the conventional six-column scheme and its control structure is proposed to

effectively intensify the integrated distillation process. First, dividing wall column (DWC) as an energy-saving

technology is introduced in the distillation process, which can save energy by 13.49 MW. Second, the heat

duty between ethylene column and propylene column in the process is matched by means of thermal

integration to further improve thermal integration efficiency, with energy saved by 33.7 %. Finally, one control

strategy of the PSTDW process is designed to guarantee the product purity qualification. Its dynamic

simulation results show that the control structure can effectively make the process back to a steady state in

the face of disturbance.

1. Introduction

The efficient separation of light hydrocarbon has greatly promoted energy saving and sustainable production.

Light hydrocarbon mixture, which typically separated by distillation process are major chemical raw material

for the petrochemicals production (Pivovarova, 2019). Its main products, ethylene, and propylene, are the

most indispensable chemicals used to produce polymers, plastics, membranes, rubbers, and chemical

intermediates in many industries. Their separations are difficult because of low relative volatility and high

energy consumption. For example, Methanol to Olefins (MTO) distillation as an important part of MTO process

to obtain pure ethylene and propylene through separation, consumes two-thirds of the energy of MTO process.

A new energy-saving process for separating light hydrocarbon mixture is thus the key to improve the market

competitiveness.

As a representative of energy-saving equipment, the Dividing Wall Column (DWC) has the advantages of low

energy consumption and capital cost (Feng et al., 2018). Dividing wall column can achieve multi-component

separation through a single column. It not only reduces equipment investment but also improves thermal

efficiency. Compared with a conventional distillation column, DWC can achieve 10 − 60 % energy savings and

10 − 50 % capital cost savings (Li et al., 2019).

In addition, thermal integration has been widely used in energy conservation as a kind of process integration

(Patrascu et al., 2018). For example, pressure-swing heat integration can achieve energy integration by

adjusting the operating pressure of column and changing the temperature of condenser and reboiler (Wang et

al., 2018). Lü et al. (2018) proposed a pressure-swing distillation to separate a minimum-boiling azeotropic

system of ethyl acetate and n-hexane, which can reduce the energy cost and Total Annual Cost (TAC) by

62.61 % and 49.26 % respectively compared with continuous homogenous azeotropic distillation process.

The dynamic controls for distillation processes have been widely studied in recent years, but there is relatively

little research for the control study of the light hydrocarbon distillation with high purify requirement. Wang et al

(2018) proposed the optimal design and control of the Kaibel and multi-side stream column for separating five-

component hydrocarbon mixtures, and the control structure of KDWC can handle feed disturbance more

effectively with the purity was closer to the design value. Design and control of DWC for the separation of

DOI: 10.3303/CET2081050

Paper Received: 24/03/2020; Revised: 12/05/2020; Accepted: 13/05/2020
Please cite this article as: Fan C., Cui Z., Zhang H., Tian W., 2020, Pressure Swing Thermal-Dividing Wall Integrated Light Hydrocarbon
Distillation Design and its Control, Chemical Engineering Transactions, 81, 295-300 DOI:10.3303/CET2081050

295

hydrocarbon mixtures was studied by Kim (2016), in which the controllability of DWC was improved by utilising

the side-rectifier.

Ma et al. (2017) optimised front-end depropanisation process through thermal integration between columns,

which can reduce the energy consumption by 38.8 %. However, the hydrocarbon separation process still has

great energy-saving potential. In this work, a novel integrated light hydrocarbon distillation process is

proposed to replace the conventional distillation scheme, which can greatly reduce energy consumption and

meet high purify requirement. To improve the stability of the new process, a control structure of the high

integration process is developed, which can effectively handle feed disturbance, and the purity is closer to the

design value.

2. Pressure swing thermal-dividing wall (PSTDW) integrated distillation process

2.1 The conventional six-column separation process

The conventional distillation flowsheet is shown in Figure 1. The hydrocarbon mixture is fed into depropaniser

via drying and pressurising operation. The bottom product of depropaniser enters into the debutaniser to

separate C4 components from C5 and other heavy components, and the top product is compressed into

demethaniser to remove methane and other light gases. The bottom product of demethaniser is introduced

into deethaniser to separate C2 and C3. The mixture of ethane and ethylene obtained from the top of

deethaniser enters into ethylene distillation column to realise ethylene refining, and the bottom stream of

deethaniser is introduced into propylene distillation column to obtain pure propylene.

In the distillation process, the propane obtained from propylene distillation column is used as absorbent in the

demethaniser to make the operating temperature not too low and avoids cryogenic process. Owing to the

quite close boiling points of propylene and propane, a large number of trays and extensive energy are

required to separate them. Hence, column with heat pump as an energy-saving technology is used in this

process. The top stream of the column is taken as heat source for the column reboiler. Composition of the

hydrocarbon mixture is listed in Table 1. Product purity specifications of the important components are::

ethylene and propylene more than 99.95 % and 99.6 %. The content of C4 components in C4 stream exceeds

95 %, and the content of C5 components in C5+ stream is more than 95 %.

Depropanizer DebutanizerDemethanizer Deethanizer Propylene columnEthylene column

Cooler

Pump

Cooler

Heater

Cooler

PumpFeed

Ethylene
C4

C5+

PropyleneC O , H 2 ,

CH4

Cooler
Pump

-11.90 MW

262.30 K

235.79 K

-21.58 MW

247.72 K

-4.79 MW

322.5 K

-16.89 MW

247.73 K

2.0 MPa

321.25 K

180

10.55 MW

348.60 K

2.28 MW

326.92 K
17.60 MW

327.94 K

-4.62 MW

369.50 K

16.53 MW

327.94 K

Figure1: Process flow diagram of the pre-depropanation separation process

Table1: Composition of hydrocarbon mixture

Component Mass fraction %

H2 0.18

CO 0.19

N2 0.19

CH4 1.76

C2H4 41.27

C2H6 0.79

C3H6 40.32

C3H8 2.81

C4 10.45

C5+ 2.04

2.2 The novel Multi-olefin distillation process based on dividing wall column (MODP-DWC)

The distillation column sequence is designed based on relative volatility of the components. The phase

equilibrium constant and relative volatility of the light hydrocarbon mixture is listed in Table 2. It can be seen

that the relative volatility between C4H10-01 and C5H10-01 is the highest, so C4H10-01 and C5H10-01 should be

296

separated first. However, the contents of C4 and C5 components are the lowest. It is unreasonable to

separate the C4 and C5 components firstly. The separation of CH4 and C2H4, C2H6 and C3H6, C3H8 should be

carried out first.

Table2: The phase equilibrium constant and relative volatility of the components

Component i CH4 C2H4 C2H6 C3H6 C3H8 C4H8-01 C4H10--01 C5H10-01 C5H12-01

Ki 5.16 3.22 2.95 1.12 0.96 0.44 0.39 0.15 0.16

αij

1.60

1.09

2.64

1.16

2.18

1.13

2.68

0.93

Process flow diagram based on MODP-DWC system is shown in Figure 2. The process involves five columns,

namely, dividing wall column, ethylene distillation column, depropaniser, propylene distillation column, and

debutaniser. Light hydrocarbon mixture is fed into dividing wall column (DWC) to separate C1, C2 and C3

components. Lighter components (CO, H2, CH4) are left from the top of the dividing wall column, while C2

components are extracted from the side of DWC and enter ethylene distillation column to obtain pure

ethylene. The bottom products of DWC containing C3, C4, and C5 components are introduced into

depropaniser for separating C3 components from C4 and C5 components, and the mixture of propylene and

propane obtained from the top of depropaniser are fed into propylene distillation column for propylene

purification. The bottom products of the depropaniser are introduced into debutaniser to separate C4 and C5

components.

The mass fraction and feed flow rate of major streams are summarised in Table 3. It can be seen that the

PSTDW process can realise light hydrocarbon product separation while saving heat public utility and cool

public utility by 8.73 MW and 4.76 MW.

Dividing

wall column

Ethylene

column
Depropanizer Debutanizer

Propylene

column

C3+ C4, C5+

C3Ethylene C4

C5+

Propylene

Figure 2: Process flow diagram based on dividing wall column

Table 3: The mass fraction and feed flow rate for the system

Stream C1 Ethylene Propylene C4 C5+

Flowrate/ kg/h 2,447.37 39,318.00 37,157.29 10,051.05 2,055.54

wt/ % H2 7.21 0 0 0 0

CO 7.61 8.76e-12 0 0 0

N2 7.61 7.93e-13 0 0 0

CH4 69.68 0.05 0 0 0

C2H4 7.89 99.95 2.92×10-7 0 0

C2H6 2.35×10-4 2.61×10-4 6.39×10-6 0 0

C3H6 1.38×10-9 2.10×10-15 99.99 6.28×10-3 4.39×10-13

C3H8 5.75×10-12 9.18×10-19 5.16×10-3 3.80×10-4 2.71×10-12

C4H10-01 7.29×10-19 3.67×10-30 7.59×10-32 4.02 2.72

C4H8--01 2.30×10-18 3.47×10-30 5.75×10-36 95.98 48.64

C5H10-01 5.23×10-29 1.19×10-46 2.22×10-80 2.95×10-4 48.64

C5H12-01 6.20×10-31 1.09×10-49 4.71×10-88 2.44×10-5 0.01

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2.3 Feasibility analysis of thermal integration in MODP-DWC system

There are five columns in MODP-DWC system. The feasibility of thermal integration for the five columns is

shown by T-H diagram in Figure 3. The x-coordinate represents the heat load, the y-coordinate represents the

temperature, and the energy input and output of five columns are represented by quadrilateral. The ethylene

distillation column and propylene distillation column consume a large amount of heat which can only be

provided by public utility. Thus, the two distillation columns cause a lot of energy consumption. The overhead

stream temperature of the propylene distillation column is 278 K, 2 K higher than the bottom temperature of

ethylene distillation column (276 K). In addition, the heat load of ethylene distillation column is close to the

cold load of propylene distillation column. The heat integration between the two columns could be realised by

adjusting their operating pressure.

0 10000 20000 30000 40000 50000

240

250

260

270

280

290

300

T
e

m
p

e
ra

tu
re

/
K

Heat Duty /KW

Ethylene column

Propylene column

Figure 3: T-H diagram of distillation system

The relation between ethylene mass fraction and operating pressure of the ethylene column are shown in

Figure 4(a). The mass fraction of ethylene is decreased from 0.999502 to 0.99949 when the operating

pressure of the ethylene column is increased from 1.4 to 2.6 MPa. When the pressure approaches to 2.05

MPa, the mass fraction of ethylene reduces to 0.9995. Thus, the upper pressure limit of ethylene column is set

at 2.05 MPa to meet the quality of ethylene.

The relation between temperature and operating pressure of the ethylene column are shown in Figure 4(b).

The operating pressure of ethylene column is in a range of 1.5-2.05 MPa, and the pressure of the propylene

column is in a range of 0.5-1.1 MPa. In pressurised heat integration process, the operating pressure of the

ethylene distillation column is 2.05 MPa and the bottom temperature of the ethylene distillation column is 282

K. In order to heat the reboiler by using the top stream of propylene distillation column, the top stream

temperature of the propylene distillation column should be set at least 292 K, 10 K higher than that of the

reboiler temperature of ethylene column with a corresponding pressure of 1.02 MPa. In depressurised heat

integration process, the minimum operating pressure of ethylene distillation column is 1.5 MPa and its

corresponding reboiler temperature is 268 K. The condenser temperature of the propylene distillation column

should be higher than 278 K to keep 10 K difference with the reboiler of ethylene distillation column. There are

a lot of combinations of heat integration in this pressure range discussed above. Although the depressurised

heat integration process has advantages in equipment investment and operating cost, it has higher

requirements for refrigerant of ethylene distillation column condenser. Consequently, in pressurised heat

integration process, the pressure of ethylene column is increased to 2.01 MPa, and the matching scheme

realises heat integration between propylene column and ethylene column.

1.4 1.6 1.8 2.0 2.2 2.4 2.6

0.9994899

0.9994952

0.9995005

P
u

ri
ty

o
f

e
th

y
le

n
e

k
g

/k
g

Pressure/ MPa

1.4 1.6 1.8 2.0 2.2 2.4 2.6
265

270

275

280

285

290

295

Pressure of propylene column/ MPa

T
e

m
p

e
ra

tu
re

/
K

Pressure of ethylene column/ MPa

1.0 0.9 0.8 0.7 0.6 0.5

10 K

(a) (b)

Figure 4: (a) The relation between ethylene mass fraction and operating pressure of the ethylene column; (b)

The relation between temperature and operating pressure of the ethylene column

298

2.4 The PSTDW integrated distillation process

In the MODP-DWC system, when the pressure of ethylene distillation column is increased to 2.01 MPa, the

corresponding heat load of ethylene column reboiler is 38.88 MW. When the propylene distillation column is

increased to 1.02 MPa, the corresponding heat load of propylene column condenser is reduced to 43.39 MW,

and the remaining heat of propylene column condenser is removed by cold public utility. Process flow diagram

of the MODP-DWC heat integration process is shown in Figure 5. Compared to the integrated conventional

distillation process, the PSTDW process can save 73.47 MW energy.

Dividing wall

column

Ethylene column Depropanizer DebutanizerPropylene column

-11.41 MW

164.52 K
-40.67 MW

240.43 K
-8.63 MW

280.82 K

-46.34 MW

278.34 K

-3.13 MW

319.87 K

40.60 MW

273.87 K

11.11 MW

329.41 K

7.13 MW

334.89 K

122

46.28 MW

286.54 K

3.07 MW

366.51 K

Figure 5: The PSTDW distillation system

3. Plant wide control for the PSTDW distillation system

The control structure is shown in Figure 6. According to the basic principles of the control design methodology,

some conventional controllers, such as flow controller (FC), level controller (LC), temperature controller (TC),

and pressure controller (PC), should be arranged first to establish the control structure. The controllers added

in the control scheme are summarised as follows:

(1) The flow controller is employed to adjust the feed flow rate (reverse acting);

(2) The operating pressure of column is controlled by condenser heat removal (reverse acting);

(3) The flow rates of distillate and bottom product are used as manipulated variables to control the levels of

column sump and reflux drum, respectively (direct acting);

(4) The return flow rate of column is proportional to the feed flow rate;

(5) The temperature of sensible stage is adjusted by manipulating condenser or reboiler heat duty (reverse

acting).

For flow controller, the default adjustment constant gain is 0.5, the integration time is 0.3 min. For temperature

controller, the tuning parameters are Kc = 2 and Ti = 10 min (Luyben, 2013). The default gain and integration

time of other controllers are given similarly.

Feed

Ethylene

Propylene

Pump

Dividing

wall column

Ethylene

column

Depropanizer

Debutanizer

ΔT

QR/F

ΔT TC

ΔT

ΔT TC

PC
LC

TCΔT

Figure 6: Control scheme of PSTDW distillation system with top temperature controller

After the dynamic running 3 h, the ±10 % feed flow rate disturbances are added to investigate the stability of

the control structure. Figure 7 shows the response curves for the control scheme. It can be seen that the

desired product purities at the new steady state can be achieved after 8 h. At the new steady state, the

purities of ethylene and propylene are 99.92 % and 99.99 % corresponding to the 10 % flow rate increase in

feed, whereas they are 99.98 % and 99.99 % corresponding to the 10 % flow rate decrease in fresh feed. And

the purities of the C4 and C5 are quickly stabilised above 95 %. These results demonstrate that the proposed

control structure can achieve the controllable operating of the PSTDW distillation system.

299

0 5 10 15 20 25 30
0.9990

0.9992

0.9994

0.9996

0.9998

1.0000(a)

P
u
ri
ty

o
f
e
th

y
le

n
e
/
k
g
/k

Time/ h

+10% feed rate

- 10% feed rate

0 5 10 15 20 25 30
0.99990

0.99992

0.99994

0.99996

0.99998

1.00000
(b)

+10% feed rate

- 10% feed rate

P
u
ri
ty

o
f
p
ro

p
y
le

n
e
/
k
g
/k

Time/ h

0 5 10 15 20 25 30
0.99996

0.99997

0.99998

0.99999

1.00000

+10% feed rate

P
u
ri
ty

o
f
C

4
/
k
g
/k

Time/ h

(c)

- 10% feed rate

0 5 10 15 20 25 30
0.95

0.96

0.97

0.98

0.99

1.00(d)

+10% feed rate

- 10% feed rate

P
u
ri
ty

o
f
C

5
/
k
g
/k

g
Time/ h

Figure 7: Dynamic responses of the control scheme. (a) Purity of ethylene; (b) Purity of propylene; (c) Purity of

C4; (d) Purity of C5

4. Conclusions

In this work, a novel pressure swing thermal-dividing wall integrated distillation process is proposed to replace

conventional six-column scheme for separating light hydrocarbon mixture. Dividing wall column is introduced

to improve thermodynamic efficiency. In order to further save energy consumption of this system, the

overhead steam of the propylene distillation column is used to heat the reboiler of the ethylene distillation

column, which reduces energy consumption by 33.7 %. Finally, one control strategy of the PSTDW distillation

system is established, which can resist ±10 % fresh feed flow rate disturbances effectively. The economy of

the PSTDW process will be taken into consideration in the near future.

Acknowledgements

The authors gratefully acknowledge financial support provided by National Natural Science Foundation of

China (Grant number: 21576143).

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