Control System Design and Simulation of Polypropylene

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Nov 15, 2013 (3 years and 6 months ago)

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Control
System

Design

and
Simulation of P
olypropylene

CSTR Based on SIEMENS PCS7

Mu De
-
jun,
Liu Si
-
wei,Shi Li,
Ren Shuai

(Automation College, Northwestern Polytechnological University, Xi

an 710072, China)


Abstract:
For the
continuous stirred tank reaction of polypropylene production designed a set of
control scheme.

According to the main process control tasks including injection of raw material,
liquid level, calefaction velocity, stable temperature, reaction component, safe

pressure and start
sequence.

Control scheme contained not only cascade control but also feedforward control,
proportion control, resisting integral saturation control and safe interlock for stable.

Then the
confirm research is implemented on the simulatio
n platform based on SIEMENS Process Control
System(PCS7) and multifunction process and control experiment system (MPCE). The practice
indicates that control effect, security and feasibility are well.

Keywords: continuous stirred tank reactor; SIEMENS PCS7; cascade control; feedforward
control;
proportion

control;
Resisting integral saturation


1
I
ntroduction

The
continuous stirred tank reactor (CSTR) polymerization
has
been widely use
d in industry

applications

such as

PVC, polypropylene, and other production

process
.
Due to its
non
-
linear
ity
,
time
-
delay
,
strong inertia

effects

and other characteristics,
research has been

focused on

the
continuous stirring reaction process to achieve
a
control
led

st
able operation
in

chemical
production
process.


The

multifunction
al

process and control experiment system (MPCE)

is based on the
idea
of the
semi
-
loop simulation devoted to a variety of chemical production process simulation devices
.
I
t
consists of
proce
ss equipment
s inc
l
uding a

small disk units, digital soft instrument, computer,
monitor configuration software and process simulation software, and other software and
hardware
.
B
y
providing a typical 4 to 20 mA signals in the
form of
standard open interface
, it
features

very good inter
-
connectivity between
multiple equipment by various equipment
manufactures
. MPCE
can be used to simulate
various chemical
production

processes
.
T
he system
contains a

centralized control system monitoring and management, decentr
alized control and
digital, analog mixed system
, which also reflects the

actual chemical control
in industry
.
T
his
paper

mainly uses

its

experimental platform

to perform the real
-
life simulation

for the
polymerization process with.

Siemens SIMATIC PCS7 sys
tem is an advanced process control system
.

the

hardware is based
on

PLC

and computer integrated Siemens PCS7 BOX, and other products
. By adopting
PCS7
V6
,
the software system can a
chieve consistency of data management, communications and
configuration
, whi
ch provides an
excellent performance, and
foresee ability

to satisfy the typical
process control system applications.

T
he combination of the software and hardware system
successfully integrate
configuration, monitoring, control (ES, OS, AS) into one

part.

T
he
hardware architecture
not only
take
s

full advantage of the high
-
performance computers to
achieve superiority complex industrial process control algorithms, system configuration,
monitoring and other functional, but also

make
s

use of conventional PLC co
ntroller control,
distributed control system to improve the safety

and

reliability
.

This paper firstly gives a brief introduction to
the continuous mixing PP polymerization
processes
, followed by discussions of its typical
characteristics
, then an

integrat
ed control
strategy ma
king

full use of the cascade control, feedforward control, variable ratio control, as
well as safety interlock logic, such as the commonly used in industry control methods

is
given

with re
spect

to
bits feed control, level control, hea
ting rate control, temperature control, the
reaction
c
omponents
control, pressure control safety, driving sequence control and other major
areas of control
.
Finally the implementation

and verification

of
the

control strategy

are achieved
via
MPCE and PCS7
system
.


2
Process
d
escription

and c
haracteristic

a
nalysis


Figure 1
shows

the
Continuous Stirred Reactor
polypropylene production methods

as described
in the MPCE HWIL simulation system
. T
he
reaction process

can be described as the foll
o
wing
:
firstly,
material

A

(liquid propylene), material B
(liquid hexane) and catalyst C
take
polymerization reaction in temperatur
e
70

,

y楥汤楮g
灲od畣u 䐠 (PP). 䅴 t桥h 扥b楮ii湧 of
r敡捴楯渠
w攠畳e

嘴, 嘵 an搠嘶 r敳e散t楶敬e

to
捯湴no氠t桥hf汯w F4, F5, F6,

ma歩kg
mat敲楡l
s g整


to t桥h 灯汹m敲楺ir

楮 pro灯牴楯渮n To
e湡扬b
r敡捴楯n

at t桥h扥b楮i楮i

we 畳攠
st敡m va汶攠

to
桥ht楮i r敡捴or

a湤

t桥h

us攠
m楸er 匸


st楲 and
mix r敡捴

.

B散a畳攠 of
t桥h 數ot桥hm楣
捨c牡捴敲楳t楣i

of
灯汹m敲楺at楯n

r敡捴楯測n
w攠
u
s
e

嘷, 嘸
to
co湴no氠捯o汩lg 睡t敲 f汯w F7, F8,
r敳e散t楶敬e



a捨楥c
e

t桥hs湡ke

t畢e

an搠jack整 捯o汩lg m整桯d 嘹 is us敤eto 捯湴ro氠
f汯w

F9,
a湤 P7, L4



s畢s敱略湴

畳攠
for
t桥h
m敡sureme湴n a湤 mon楴o物湧
of t桥h
r敡捴楯渠灲敳s畲攬
t桥h汥l敬e
t桥 捯湶e
牳楯渠牡te

of t桥h
r敡捴楯渮
T桩h

r敡捴or 桡s st楲i敤estro湧
汹 a湤

灬py a v敲y
goo搠摩d灥牳楯渠n湤 d楬it楯n f敡t畲敳
.


For

polypropylene continuous polymerization process,
the
reaction time
,

reaction
temperature and reaction pressure
are

three key factors
which have strong effect on productivity
and quality of products
and thus should be analyzed carefully during

system design stage.

T
hey
are described as following.

1.)
R
elationship between
r
eaction time and conversion rate
.


Reaction

remaining

time
,

actua
l materials volume

in the

reactor
and the volume of
flowed
volume are related
.
L
ong r
eaction

remaining

time and the small feed flow,
leads to high

conversion rate. In order to increase the concentrations

of
product D, we must reduce
feed
and flow
-
out flow
.

2.) R
elationship between reaction temperature

c
o湶敲s楯渠nat攠a湤 t桥hr敬et楯湳h楰⸠

T桩s r敡捴楯渠
ask

tem灥牡tur攠


扥b捯湴no汬敤lat 70


U
湤敲 t桥hsam攠
r
敡捴楯渠time

an搠t
桥h
amo畮t of 捡ta汹st
,

Tem灥pat畲攠摥d敲mi湥n t桥h捯湶敲s楯渠牡te
.

䅳At桥hr敳eo湳攠楳 數ot桥hm楣i
r敡捴楯測nw桥渠t桥hr敡捴楯渠sp
敥搬d楮ir敡s攠桥ht, t桥htemp敲at畲攠物獥r
ot桥牷楳e

t桥htem灥牡t畲e

Figure
1 CSTR

dropped. The exothermic reaction is the risk of non
-
value process, when the reaction temperature
is too high, the reaction rate will be accelerated, the reaction released heat will increase
. If heat
can not be removed in time, the reaction temperature is further increased. This will lead to a
sharp rise in reactor temperature, and reactor pressure soaring. If the response of the reactor
pressure can exceed the limit of tolerance, may cause a
n explosion and fire incidents. Flexibility
in the use of the jacket and snakes these two cooling method, and reasonably control the reaction
temperature is the key. Reaction temperature and reaction conversion rate of change of time
constants larger high
-
end properties.
Cooling water flow changes with the rapid changes in the
switching valve, the smaller time constant. When the cooling water pressure drops, even if the
valve position unchanged, cooling water flow will fall
, t
ake cooling water heat will be
reduced, the
temperature in the reactor materials will rise. As a result of the lag
of
temperature,
it is
difficult
to control.

3.) R
eaction pressure and reaction conversion rate of production safety
.

During

the reac
tion process, the reaction
pressure

depends on the level of reaction mixture
rate
of A and B as well as the proportion of gas temperature.
T
he pressure of
pure gas substances
A
in
20



a扯畴u1.0 MPa a湤
楮i
70 뀠䌠桡s mor攠t桡渠3.0 MPa,
as
tem灥牡t畲攠捯湴n湵敳eto 物獥r
t桥h 灲敳sur攠w楬氠楮捲敡s攠摲dmat楣i汬l. T桥h敦or攬 in t桥h 汯w敲 tem灥牡tur攠r敡捴or
汥l摳 to

灯ss楢汥l 數灬ps楯渠桡穡牤r P牡捴楣攠桡s 灲ov敤e t桡t
睨敮w 捯m扩be搬

A an
搠B, t桥h tota氠gas
mixtur攠灲essur攠w楬氠扥br敤畣敤e
畮摥r t桥h灲emis攠of
捯湳ta湴n
t敭灥牡tur攬 t桥

mat敲楡氠B 楮i
t桥h 桩h桥h 灥p捥湴ng攬
w楬氠汥l搠to
t桥h low敲 t桥h 灲敳e畲攠o渠t桥h system. T桥h敦or攬 w攠m畳t
灲敶敮e t桥hr敡捴or 楮ir敳eo湳攠to t桥h捯湴n湴nof

a 灥p捥湴ng攠too 桩h栠a湤 t桥h桩h栠r敡捴楯渠
tem灥牡t畲攠to 桡灰敮e I渠a摤楴楯測n
畮摥r t桥h捯湤楴楯渠of 捯湳ta湴nt敭灥牡tur攬

a摪畳dm敮e
of
A a湤 B f敥搠
f汯w rat楯
can

捯湴nol

t桥h r敡捴or 灲敳s畲攠
w楴桩h a 捥牴a楮i s捯p攮 坨楬攠t桥h
mat敲楡汳 A a湤 B f敥搠f
汯w
畮摥r
t桥hsam攠premis攬 r敡捴楯渠灲敳eur攠捨c湧敳ew楴栠hem灥牡t畲攬
t桥h r敡捴楯渠 tem灥牡t畲攠
i湣n敡s敤e w楴h

t桥h
r敡捴楯渠灲敳sur攬 t桥h r敡捴楯渠
tem灥牡t畲攠楳

摥捲敡s敤

w楴h

t桥hr敡捴楯渠灲敳sur攮 T桡t 楳 to
say,
t桥h
楮捲敡s
i湧
灲essur攠
m敡ns t桥h
楮捲e
asi湧 s灥敤p of
t桥 r敡捴楯n

a湤 t桥
楮捲敡si湧
牡t攠
of
t桥h捯湶敲s楯渮


3 Control Strategy Design

3.1
F
eed
control


In the reaction we have to let the three

raw
materials carried out according to a certain
ratio of feeding to ensure that the three
material
s in a certain proportion inflow
to the

polymerizer.
T
he reaction on the proportion of
materials require
s

a higher degree of accuracy
,

therefore

摩dt畲扡湣攠fa捴o牳ri渠t桥hf敥摩湧
灲o捥ss

m畳t 扥b 捯湳i摥ded
. Bas敤e o渠t桩h 摥ma湤,
w攠tak攠t桥h way of
䑯畢le
-
捬cs敤
-
汯o瀠
va物r扬攠牡t楯 捯湴no氬

a湤 r敡汩穩湧 䄬 B a湤 䌠s桡r敳ec桡湧楮i f敥搠f汯w 牡t楯 co湴ro氬l
as s桯wn
楮iF楧畲攠2
, B mat敲楡汳

浯r攠s敮e楴楶e

to
t桥hr敡捴楯渠灲敳sure
, so t桥hf汯眠of mat敲楡汳 as B of
t桥h 楮it楡t楶攬 w楴栠t桥h mome湴nm of mat敲
楡汳 fro洠t桥h 䄬 䌠f敥搠f汯w rat楯 of a 灡楲 of
捬cs敤
-
汯o瀠捯湴no氠system. B 摩dt畲扥搠t桥hf汯w of mat敲楡汳 捡渠扥br敡汩穥搠throug栠t桥楲 own




F i g u r e

2
F e e d c o n t r o l

closed
-
loop circuit, and B materials because of the impact of fluctuations, the ratio of function
through links
Kab, Kac three direct guarantee the necessary materials ratio relations. At the
same time two
-
way closed
-
loop control can be overcome materials A, C, the impact of the flow
disturbance, and can be changed through PC control of the ratio of the ratio of set
ting,
production of different indicators of products to help system flexibility and expansion sexual. In
addition consider production safety and rapid response demands that all valves are opened using
gas
-
line valve.

3.2 Level Co
ntrol




T
his reaction
is a

continuous process, in order to
get

a large
production,
it is

called
for
Level maintained at about 85% to obtain
larger residence time.
When
the three feed valve and the
flow
-
out
valve
are
open, under
the same

feed ratio, the r
esidence time
of
reaction

is
only
concern with
feed flow and the volume of the reactor,
so we

c
ontrol
the
Level

by
adjust
ing
flow
.

because of pressure
disturbance in the reaction process in the valve opening of the same
circumstances have the flow disturba
nce and therefore need to
consider overcome flow disturbances influence, using liquid level
control of the export volume and in a cascade control methods, to
obtain a smooth flow and accurate liquid level control, Cascade Level Control
a
s shown in figure
3
.
A
ll valves
are

gas
-
open

linear

valves.

3.3
H
eating
rate control and constant temperature control



T
he entire process of polymerization temperature control
have

three stages,
first, heat the
material to induce reaction,
and stop heating at 40C
,
and then is the warming up stage.
, In
order to effectively control the temperature

A
s has already
begun to react quickly and exothermic
, heating rate

should be

ahead of the need for control, and to ens
ure an effective
response to the normal temperature control and system
security, at

the moment of
the temperature rose to 45 ° C
-
65 °
C
,
control requirements
the heating rate of 0.1 ° C / s

during

the period

, as warming stage temperature rise very rapidl
y, so
the
snake
-
like cooling valve

and j
acket

cooling valve

select

a
linear ATC valve

to ensure safe and effective control demand,
based on the heating rate settings to achieve
the closed
-
loop control
of
cooling valve
.


After the heating process, as a res
ult of temperature changes speed
slow down
,
and

enter
thermostat control process, constant temperature control process need to ensure that the
temperature maintained at 70


1.0


A
t t桥h敡牬r 灥物p搠df
T桥h灯汹m敲楺it楯渠n敡捴楯n
,


t桥

桥ht楮i 牡te

楳 桩hh

,
楴 楳 汩le汹
t桡t at

t桥h
敡牬r
stag攠of
t桥hmostat 捯湴no氬lt桥htemp敲at畲攠楳
st楬氠物獩湧 too fast, t桥渠畳楮g PI䐠捯湴no氠lr攠a灴pto o捣畲 楮瑥g牡氠獡t畲ut楯渠灨敮nme湯測n
w桩h攠
捯湳ta湴ntem灥牡tur攠r敡捴楯渠灲o捥ss, as w敬氠as mat敲ia汳 f汯w 摩dt畲扡湣攠r敡捴楯渠灲敳sur攬
a湤 ot桥h fa捴o牳⁷楬氠桡ve o渠n桥hr敡捴楯渠nem灥牡t畲攠s敮e楴楶攠a湤 t桵s 汥l搠do great敲
tem灥牡t畲攠f汵捴畡t楯湳, ta歩kg 楮瑯 a捣o畮t t桥ha扯v攠r敡so湳
, 楮⁴桥hmostat 捯湴no氠獴ag攬

Figure

3
Level Control

V
7
FC
7
TC
F
7
T
1
F
5
FF
V
5
+

Figure

4


temperature control

adopted the anti
-
integration Cascade saturated feedforward control mode.
A
s
Figure 4 shows,
remain
the position of j
acket

cooling valve

t敭灥牡tur攠a湤 t桥hf汯w of s湡ke
-
汩k攠捯o汩lg va汶e
灯s攠a 捡s捡摥d捯湴no氬la湤 to a摤 灯楮瑳f f敥摢a捫 to a捨楥c攠t桥h楮瑥g牡氠l湴n
-
satu牡t楯測nt桥h
ty灩捡氠lm灡捴 of t桥hmat敲ia汳 a畴u捬cv攠灲敳e畲攠f汯w as a form敲 B r敡汩穩湧 f敥摦d牷a牤r
捯湴no氠摩l
t畲扡湣攠捯mp敮sat楯渮

3.4 Pressure security control


Due to continuous polymerization stirring

using

the closed polymerizer production mode, the
pressure within the autoclave

is

too much to explo
d
. To ensure the safety of reaction process, the
pressure t
o achieve security control is essential. The polymerization reactor pressure
is
mainly by
the ratio of materials
and
the reaction temperature
within the reactor, in this paper, therefore
the design strategies used in the grading pressure safety interlock
control method. Pressure
fluctuations in the pressure control by the smaller direct manipulation of materials A, B ratio of
the feed pressure to achieve security control. Nearly 1.2 MPa pressure, appropriate regulation A,
B material into the feed achieving

pressure control, get the high alarm; pressure rose to 1.2 MPa
-

1.5MPa between, Jin feed in the regulation on the basis of increasing cooling water flow to reduce
the reaction temperature, reaction conversion rate sacrifice to ensure production safety, g
et the
high alarm; if pressure greater than
1.5
MPa; stopping the implementation of emergency
procedures, the biggest opening of the valve opened provide material to close the feed valve to
prevent Poly Explosion.

3.5 Components

Control


To get certain con
version rate,
we

need to control components

of product.

U
sually
if
materials
stay a long time, the conversion rate of reaction

is high
. As the flow of materials
B is

the master

variable,
we control
the flow of materials
B to change
the reaction product con
centration
,
t
hrough the ratio of the flow

to

ensure

reactor components
. I
n a
certain
temperature,
we can
chang
e

feed

flow by

changing
the flow of
materials

B to
control

Components. If we want to
increase product concentration,
we can
decrease
the flow
of m
aterials B, contrary increas
ing

the
flow
of materials B can reduce

product concentration
.

3.6
S
equence
control


S
equence control is the guarantee
of

safe
operation, According to Figure 5 in
accordance with the needs of the
operational
steps:
the
first
is

initializ
ation

inspection
;

then
r
ecogniz
e

that all the valve closes, all switch is
off;
then open

the
feed valve
of
material B
to

60
%
,
make
the feed flow of material B
is
about 1540kg/h, concern
level
L4 increas
ing;
when
lev
el

L4

rise

to 50%,
open

feed valve
of
materials

A

about 55%
,
so that the feed
flow of material
A

is
about 729kg/h
;

when the
level rose to about 75
%
,
open flow
-
out

valve
to

55% to prevent the excessive rise in value
around, resulting in excessive overshoot

w桥渠 t桥h 汥l敬e 楮捲敡s敤 to 80
-
90┬ for
a畴umat楣i汥l敬e捯湴no氻
t桥渠t畲渠t桥

motor sw楴捨c
o測

o灥p

桥ht楮i va汶攻
o灥渠t桥hf敥搠va汶攠of


Figure

5


S
equence control

catalyst C
to

90%,
make the feed flow of

catalyst C
is

about 88kg/h, at this time, three materials
required
are
cont
inuously
flowing into

the
reactor;
w
hen the reaction temperature T1
is

about
40


t桥渠捬cs攠
桥ht楮i va汶攬 楦 T1 捯湴楮略nto 物獥Ⱐ
楴 m敡湳
t桥 res灯湳e

敶oke搠s畣捥ss

w桥渠t桥h r敡捴楯渠tem灥牡t畲攠


a扯畴u 45


o灥p

jack整 捯o汩lg va汶e

o扳敲v攠t桥h
r敡捴楯n
tem灥牡t畲攠a湤 捯湴no

th攠捯o汩lg wat敲

to
a捨楥c
e

0.1

⽳散 桥ht楮i 牡te

w桥渠t桥h jack整
捯o汩lg va汶攠
楳 a扯畴

50┠a湤 t桥hr敡捴楯渠tem灥牡t畲攠

a扯畴u65

, ma楮瑡楮ija捫整 捯o汩lg
wat敲 va汶攠o灥湩pg of t桥hsam攬 畳
e

t桥hs湡k攠捯o汩lg
wat敲
,

坨敮Wt桥htem灥牡t畲攠r敡捨cs


1.0

, w楬氠扥bvot敤ea畴omat楣item灥牡t畲攠捯湴no氮l卯 far t桥 r敡捴楯渠楳 捯mp汥t攠t桥h灲o捥ss
of 摲楶楮i tasks.

4
S
ystem
configuration

System hardware configur
ation
consists of

two parts
:

Control systems

and
object
.

T
he control system

us
es

the Siemens PCS7 BOX,
which
at the
same time

can be taken

as the
engineer station and operator station
for system configuration and
operation monitoring.

The object uses
the c
ontinuous mixing polymerization reaction simulation subsystem

in
MPCE platform
.

A
nalog
and digital signal

from the object

are

connect
ed

to the ET200M analog and digital module
.

ET200M
is
connect
ed

to

PCS7 BOX
with

PROFIBUS
-
DP connection
,

which t
o certain
extent

can

realize

some
DCS functions.

System Software Configuration use
s

PCS7 v6.0 software, which integrates hardware
configuration, the development
of
control procedures and the functions of higher
-
level
monitoring program
.
I
t adopts
HW Configure to co
nfigure

CPU, communications processors,
peripherals

and
fieldbus
.
T
he

CFC

is employed
to achieve
control algorithm

of

sub
-
tasks and
logic safety interlock
, while
SFC
and WinCC are taken for purposes of
driving sequence control

and
operator monitoring inter
face

respectively
.


5
S
imulation
case study


Figure 7 show
s the effect of warming stage
and t
hermostat stage
.

F
igure 7.1 shows the
system curves

at

warming stage
.

T
he blue curve in
the
Figure
is
temperature curve
,

from which
a

jump can be seen between hea
ting stage and heating stage
.
W
e can see
the
heating rate

is slow
,
but the overall warming trend is stable.
T
he red cur
ve is the level control curve

which shows a
good
overall effect
. F
igure 7.
2

shows the system curves

of
t
hermostat stage
. W
e can see that
level
remain
s

constant, and after

some

temperature fluctuations, the overall trend tends to slow

down
,

while the

error
stabilized

at about 1.0

.

4
-
20
mA
Profibus

Figure

6


S
ystem configuration


6
S
ummary


In summary, this paper
describes a complete procedure for
continuous

mixing
polymerization process control strategy
covering the

initial system
design

scheme and the final

simulation verification
process

of

the
entire

system.
It

can be seen
that

the MPCE platform
for the MPCE which is based on
HWIL simulation technology
p
lays an important role in
simulation of
the actual process
, b
ased on
which a
direct link
of
Siemens PCS7 posed by the
continuous stirring reaction control system fully reflects the implementation process is simple,
rapid and flexibility.

T
his paper

also v
erifies the correctness and applicability for
the

overall
control strategy in the main process for

continuous polymerization process of.

It

s worth noting
that
though the final results achieved control of the basic requirements,
there
also
exist
a number
of problems
. For example, the

anti
-
disturbance system
has a
relatively weak capacity
against

the
temperature perturbation imposed factors, although
the final

stability

can be ach
i
eved
,

it
requ
i
res

a longer time to adjust

during temperature fluctuations
. As

the control system uses a
combination of conventional control methods of control strategy, which is also a traditional
control method
for

th
e

typical response, the process
needs

be further improve
ments. S
tudy of
advanced control strategies to meet the
req
uirement
s of an ever
-
improving system will be the

research
interest

for

future chemical industry.

References

[1]

WU Chong
-
guang
.
The Multifunction Process and Control Experiment System

[J].
Research and
Exploration in Laboratory
, 2005,24(9):381
-
384

[2]

SIEMENS
.
SIMATIC PCS7

Process C
ontrol System
. 2005

[3]

JIN Yihui
.

Process Control

[M].
Bei
j
ing
:
Tsinghua University Press
,

1993,

4

[4]

WANG Shuqing
.

Industrial process control

[M].

Beiji
ng
:
Chemi
cal Industry
Press
,2003,1

[5]

JIANG Wei s
.
Process

Control

Engineering
[M].
Beijing
:
China

Petrifaction
Press
, 1999

[6]

SIEMENS
.
PCS7

Programming
, 2005





Figure

7.1

Figure

7.2