Method and apparatus for controlling polyolefine resin production plant

The names of a plurality of polyolefine resins are previously registered as index data in a name data base. An optimal operational pattern is stored in an operational pattern table for each combination of the name of a current resin under production and the name of a target resin to be produced. The optimal operation pattern is selected based on the name of the current resin under production and the name of the next target resin to be produced. Based on the selected optimal operational pattern, resin-type change control is executed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a control method for a production plant 
for producing plural types of polyolefine resins with a common system. 
More particularly, this invention relates to a control method and 
apparatus suitable for a production plant for polyolefine resins such as 
polyethylene and polypropylene. 
2. Description of the Related Art 
In a production plant for producing polyolefines such as polyethylene and 
polypropylene, a distributed control system (hereinafter called "DCS") is 
typically used as a process control system. There has been a proposal to 
accomplish high-precision process control by coupling a process computer 
to this DCS via the system bus of the DCS. 
There are multifarious needs for polyolefine resins like polypropylene. In 
the production process in a production plant for polyolefine resins such 
as polypropylene, it is common to frequently change the types of resins to 
be produced from one to another, e.g., from a high impact-strength resin 
having a high molecular weight to a high flow resin having a low molecular 
weight, or frequently change one polyolefine resin to a copolymer with 
another olefine. 
To meet such a demand, the present inventor proposed in Unexamined Japanese 
Patent Publication No. Sho 62-250010 a technique which detects a parameter 
concerning the physical property of polyethylene with a sensor or gas 
chromatography, performs operations on the detection value using a 
computer and controls the production of polyethylene having a 
predetermined physical property based on the operational results. It is 
however difficult to adapt this technique to the operational control 
needed at the time one of multifarious types of polyolefine resins is 
changed to another. 
Parameters involved in changing resins from one type to another in such a 
polyolefine resin production plant vary depending on the operational 
conditions. 
It is generally known to control the melt flow rate (hereinafter called 
"MFR") by the amount of hydrogen supply to a polymerization reactor. To 
change one type of polyolefine resin under production to another in a 
short period of time for the purpose of increasing the hydrogen 
concentration in the reactor to reduce the molecular weight, it is 
efficient to supply a large amount of hydrogen at a time. This approach 
however increases the activity of the polymerization catalyst in 
accordance with the increase in hydrogen concentration, causing abnormal 
polymerization (e.g., an increase in polymerization temperature or 
polymerization reaction failure) originating from the rapid increase in 
hydrogen concentration. This is dangerous as well as undesirable because 
of the amount of polyolefine resins which do not meet proper standards 
increases. 
Further, since the feed rate of hydrogen is closely associated with other 
parameters like the temperature of the reactor to be controlled, 
conventional control by the DCS alone is very likely to produce a lot of 
products which do not meet proper standards, i.e., defects, at the time of 
changing one type of resin to another. The suppression of the production 
of products which do not meet the standards therefore demands 
knowledge-based or experience-based control by skillful operators. 
In changing one type of resin to another in a polyolefine resin production 
plant, conventionally, skillful operators change a plurality of changeable 
elements based on their knowledge or experience. Those changeable elements 
are common elements in process data, which should be changed according to 
a variation in production conditions and which, when changed, would cause 
other process data to change. The common elements include the hydrogen 
concentration in a polymerization reactor and a heat transfer coefficient. 
If such common elements in process data are found and are used as control 
parameters to control the operation of changing the types of resins from 
one to another, it should be possible to suppress the occurrence of 
products which do not meet the standards and to change the resin type 
properly and effectively. 
SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a control 
method and apparatus for a polyolefine resin production plant, which, when 
needed, controls an operation of changing resins from one type to another 
in a polyolefine resin production plant using a proper process data 
element as a control parameter originating from a change in production 
conditions, thereby suppressing the occurrence of products which do not 
meet the standards and ensuring the proper and effective automation for 
changing the resin type. 
A control method for a polyolefine resin production plant according to the 
first aspect of this invention comprises: 
a name registering step of previously registering names of a plurality of 
polyolefine resins producible by a production system into a name data base 
as index data; 
a pattern storing step of previously storing an operational pattern for an 
optimal type changing operation as an operational pattern table into a 
pattern storage for each combination of a name of a current resin under 
production in a currently running production process by the production 
system and a name of a next target resin to be produced by the production 
system; 
a pattern selecting step of comparing the name of the current resin with 
the name of the next target resin to discriminate a combination thereof 
and selecting an operational pattern corresponding to a discrimination 
result from the operational pattern table; and 
a change control step of executing process control according to the 
operational pattern selected in the pattern selecting step to change a 
type of a resin to be produced by the production system. 
Each operation pattern to be stored in the operational pattern table may 
include a control target value for each resin type and information on a 
successive change in a feed rate of a raw material. 
The pattern selecting step may include a step of setting a start time for 
control by the selected operational pattern for a type changing operation 
to an optimal time for each operational pattern based on a given 
production end time for a type of a current resin. 
The production system may be used to produce multiple types of polyolefine 
resins different from one another in at least one of a molecular weight 
and a copolymer composition of polyolefine; and the pattern storing step 
may include a step of registering an optimal operational pattern given for 
each of a plurality of zones, obtained by previously dividing a hydrogen 
concentration in a polymerization reactor based on at least a relationship 
between a hydrogen concentration and activity of a polymerization 
catalyst, into the operational pattern table. 
In the previous case, the production system may be used to produce multiple 
types of polypropylene resins different from one another in at least one 
of a molecular weight and a copolymer composition of polypropylene. 
According to the second aspect of this invention, there is provided a 
control method for a polyolefine resin production plant for continuously 
changing a composition of a supplied element in a polymerization reactor 
for continuous polymerization to thereby produce different types of 
polymers, which method comprises: 
a change control step of changing a control parameter based on an 
operational pattern by a changing operation model for each of previously 
stored resin names at a time one type of resin is changed to another in 
the continuous polymerization; 
a monitor step of always detecting and monitoring a sub-parameter which 
changes in accordance with a change in the control parameter; and 
a fuzzy control step of performing fuzzy control of an operation based on 
the change control step in such a manner that when a predetermined period 
of time passes after a change in the control parameter, the control 
parameter and the sub-parameter approach target values corresponding to a 
target resin type to be changed. 
The fuzzy control step may include: 
an operation step of performing a fuzzy operation on a sub-parameter 
detected by the monitor step to obtain information on the control 
parameter; and 
a control step of changing the control parameter obtained in the operation 
step. 
The predetermined period of time may be a time from a point at which 
changing a resin type starts to a point at which the target values are set 
to values according to a target resin type to be changed after overaction 
of the control parameter. 
In the previous case, the method may further include a step of changing the 
control parameter to control only a pressure in the polymerization reactor 
as the sub-parameter during the predetermined period of time. 
The operational patterns may be obtained based on past production 
achievements. 
The polyolefines may be product selected from polypropylenes, polyethylenes 
and polyethylene waxes. 
The control parameter may be a feed rate of hydrogen. 
The control parameter may be a feed rate of a catalyst. 
The control parameter may be a feed rate of at least one of 
.alpha.-olefines having 2-8 carbon atoms. 
The sub-parameter may be one of a ratio of hydrogen to propylene and a 
ratio of hydrogen to ethylene. 
The sub-parameter may be at least one of a pressure and a temperature in 
the polymerization reactor. 
A control apparatus for a polyolefine resin production plant according to 
the third aspect of this invention comprises: 
a production system capable of producing plural types of polyolefine 
resins; 
a name data base section for previously registering names of a plurality of 
polyolefine resins producible by the production system as index data; 
a pattern storage for previously storing an operational pattern for an 
optimal type changing operation as an operational pattern table for each 
combination of a name of a current resin under production in a currently 
running production process by the production system and a name of a next 
target resin to be produced by the production system; 
a pattern selector for comparing the name of the current resin with the 
name of the next target resin to discriminate a combination thereof and 
selecting an operational pattern corresponding to a discrimination result 
from the operational pattern table; and 
a change controller for executing process control according to the 
operational pattern selected by the pattern selector to change a type of a 
resin to be produced by the production system. 
The pattern storage may store an operation pattern including a control 
target value for each resin type and information on a successive change in 
a feed rate of a raw material in the operational pattern table. 
The pattern selector may further include a function of setting a start time 
for control by the selected operational pattern for a type changing 
operation to an optimal time for each operational pattern based on a given 
production end time for a type of a current resin. 
The production system may have a polymerization reactor and may produce 
multiple types of polyolefine resins different from one another in at 
least one of a molecular weight and a copolymer composition of 
polyolefine; and the pattern storage may register an optimal operational 
pattern given for each of a plurality of zones, obtained by previously 
dividing a hydrogen concentration in the polymerization reactor based on 
at least a relationship between a hydrogen concentration and activity of a 
polymerization catalyst, into the operational pattern table. 
In the previous case, the production system may produce multiple types of 
polypropylene resins different from one another in at least one of a 
molecular weight and a copolymer composition of polypropylene. 
A control apparatus for a polyolefine resin production plant according to 
the fourth aspect of this invention comprises: 
a production system having a polymerization reactor, for continuously 
changing a composition of a supplied element in the polymerization reactor 
for continuous polymerization to thereby produce different types of 
polymers; 
a pattern storage for storing an operational pattern by a changing 
operation model for each resin name; 
a change controller for changing a control parameter based on the 
operational patterns stored in the pattern storage to thereby change one 
type of resin to another in the continuous polymerization; 
a monitor section for always detecting and monitoring a sub-parameter which 
changes in accordance with a change in the control parameter; and 
a fuzzy controller for performing fuzzy control of the change controller in 
such a manner that when a predetermined period of time passes after a 
change in the control parameter, the control parameter and the 
sub-parameter approach target values corresponding to a target resin type 
to be changed. 
The fuzzy controller may include: 
an operation section for performing a fuzzy operation on a sub-parameter 
detected by the monitor section to obtain information on the control 
parameter; and 
a controller of changing the control parameter obtained in the operation 
section. 
The fuzzy controller may set the predetermined period of time as a time 
from a point at which changing a resin type starts to a point at which the 
target values are set to values according to a target resin type to be 
changed after overaction of the control parameter. 
In the previous case, the fuzzy controller may change the control parameter 
to control only a pressure in the polymerization reactor as the 
sub-parameter during the predetermined period of time. 
As the operational patterns, the pattern storage may store operational 
patterns obtained based on past production achievements. 
The production system may produce one of polypropylenes, polyethylenes and 
polyethylene waxes as the polyolefines. 
The production system may use a feed rate of hydrogen as the control 
parameter. 
The production system may use a feed rate of a catalyst as the control 
parameter. 
The production system may use a feed rate of at least one of 
.alpha.-olefines having 2-8 carbon atoms as the control parameter. 
The production system may use one of a ratio of hydrogen to propylene and a 
ratio of hydrogen to ethylene as the sub-parameter. 
The production system may use at least on of a pressure and a temperature 
in the polymerization reactor as the sub-parameter. 
According to the control method and control apparatus for a polyolefine 
resin production plant which embody this invention, the names of a 
plurality of polyolefine resins producible by the production system are 
previously registered into the name data base as index data, an 
operational pattern for an optimal type changing operation is previously 
stored as an operational pattern table into the pattern storage for each 
combination of the name of a current resin under production in a currently 
running production process by the production system and the name of a next 
target resin to be produced by the production system, an operational 
pattern corresponding to the name of the current resin and the name of a 
given next target resin to be produced is selected from the operational 
pattern table, and process control according to the selected operational 
pattern is executed to change the type of a resin to be produced by the 
production system. This control system can suppress the occurrence of 
products which do not meet proper standards and can accomplish the proper 
and effective automation for changing one type of resin to another.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Control systems for a polyolefine resin production plant according to 
preferred embodiments of this invention will now be described with 
reference to the accompanying drawings. 
First Embodiment 
A control system for a polyolefine resin production plant according to the 
first embodiment of this invention will now be described with reference to 
FIG. 1. 
The control system in FIG. 1 comprises a bus 1, a process computer 2, an 
operator station 3, multifunction controllers 4, uninterrupted automatic 
controllers (UACs) 5, a wiring section 6, raw supply valves 7 and a 
magnetic disk apparatus 8. 
In FIG. 1, the process computer 2 and the operator station 3 are coupled to 
the bus 1. The process computer 2 and operator station 3 perform the 
general control of the system. 
Further coupled to the bus 1 is the multifunction controllers 4 which are 
controlled by the operator station 3. Control data for the multifunction 
controllers 4 are transferred to the respective uninterrupted automatic 
controllers 5 via the multifunction controllers 4, so that even when the 
multifunction controllers 4 are disabled, the uninterrupted automatic 
controllers 5 can perform desired processing. 
The raw supply valves 7 for supplying a raw material to the plant are 
coupled via the wiring section 6 to the associated multifunction 
controllers 4, which control the opening and closing of the associated raw 
supply valves 7. 
More specific functions of the control system which are realized by the 
above hardware structure is exemplarily illustrated in FIG. 2. 
A name data base 9 and an operational pattern table 10 shown in FIG. 2 are 
stored in storage areas set in the magnetic disk apparatus 8 as an 
external memory unit of the process computer 2 shown in FIG. 1. 
The names of about 100 types of resins, for example, are registered as 
indexes or index data in the name data base 9. The name data base 9 is a 
kind of an operation condition data base of each product type. For each 
type of resin, the specified values of plant operation process data such 
as the flow rate, temperature and pressure, and the target value for the 
product like MFR are registered in, for example, a sequential form in the 
name data base 9. 
Operational patterns each including the operation values of a raw-material 
supply pattern are registered in the operational pattern table 10 each in 
association with the combination of the target value for a resin type and 
the target value for the next target resin to be changed. Each operational 
pattern is equivalent to the combination pattern of a composition value 
and a raw-material supply pattern. 
The process computer 2 compares current operation process data or the 
operation process data of the type of the current resin with the specified 
value of the operation process data of the type of the next resin, reads 
the optimal operational pattern from the operational pattern table 10 
based on the combination of the compared operation process data, and 
controls the multifunction controllers 4 based on the operational pattern. 
When the end time for the current type of resin has been set previously or 
the like, it is desirable that the process computer 2 should select the 
optimal operational pattern from the operational pattern table 10 and 
should select the start time for the optimal operational pattern for each 
selected operational pattern based on the end time for the current type of 
resin. 
The raw-material supply pattern used in this system will now be explained 
with the pattern of the feed rate hydrogen taken as an example. 
FIG. 3 shows the relationship between the hydrogen concentration (%) in a 
polymerization reactor and the activity of a polymerization catalyst 
(relative value) or a change in the activity of a polymerization catalyst 
with respect to a change in the hydrogen concentration in the 
polymerization reactor. The graph shown in FIG. 3 is representative of 
solution polymerization, slurry polymerization, bulk polymerization and 
gas-phase polymerization. 
In FIG. 3, to reduce the molecular weight at the time of changing from one 
type of resin to another, for example, it should be efficient to feed the 
sufficient rate of hydrogen at a time until a polymer with a low molecular 
weight is produced. As is apparent from FIG. 3, however, as the hydrogen 
concentration increases, the activity of the polymerization catalyst 
increases and such a rapid rise in hydrogen concentration causes abnormal 
polymerization. It is therefore necessary to compare the hydrogen 
concentration at the operation value of the current type of resin with the 
target hydrogen concentration for the next type of resin to select the 
optimal supply pattern. 
According to this system, the hydrogen concentration in the reactor is 
previously divided into a plurality of zones based on the relationship 
between the hydrogen concentration and the activity of a polymerization 
catalyst, and the optimal operational pattern obtained for each segmented 
zone is registered in the operational pattern table 10. 
FIGS. 4A through 4D show the patterns of a change in the supply of a raw 
material or the patterns of a change in the feed rate of hydrogen for the 
individual segmented zones, which are to be registered in the operational 
pattern table 10. 
FIG. 4B shows the pattern which increases the feed rate of hydrogen in 
accordance with the condition for the next type of resin and is suitable 
in the case where the difference between the hydrogen concentration in the 
operation value for the current resin type and that in the target value 
for the next resin type is less than 0.5%. 
The pattern shown in FIG. 4C increases the feed rate of hydrogen in two 
steps and is suitable in the case where the hydrogen concentration in the 
operation value for the current resin type is less than 2% and the 
difference between the hydrogen concentration in the operation value for 
the current resin type and that in the target value for the next resin 
type is less than 0.5%. 
FIG. 4A shows the pattern which temporarily overfeeds the feed rate of 
hydrogen and is suitable in the case where the hydrogen concentration in 
the operation value for the current resin type lies off the range 
illustrated in TABLE 1. 
FIG. 4D shows the pattern for the case of reducing the feed rate of 
hydrogen. 
TABLE 1 shows what patterns should be selected based on the combination of 
the operation value for the current resin type and the target value for 
the next resin type, the rightmost column showing the feed rate of 
hydrogen per time. 
TABLE 1 
______________________________________ 
Target value 
Current 
for next 
operation Hydrogen 
type value Difference Pattern 
flow rate 
______________________________________ 
less than 2% 
less than 2% 
less than 5% 
FIG. 4B 
Quickly set it to 
next target value 
0.5% or above 
FIG. 4A 
10 Nm.sup.3 /Hr 
2% or above 
less than 2% FIG. 4A 
20 Nm.sup.3 /Hr 
less than 4% 
2% or above 
2% or above FIG. 4A 
20 Nm.sup.3 /Hr 
less than 4% 
less than 4% 
4% or above 
less than 2% FIG. 4C 
20 .fwdarw. 55 Nm.sup.3 /Hr 
4% or above 
2% or above FIG. 4A 
55 Nm.sup.3 /Hr 
less than 4% 
4% or above 
4% or above FIG. 4A 
55 Nm.sup.3 /Hr 
______________________________________ 
FIRST EXPERIMENTAL EXAMPLE 
The type changing operation for changing one type of resin to another in 
the polymerization reactor was executed in the case where the hydrogen 
concentration in the operation value for the current type of resin was 
less than 2% and the hydrogen concentration in the operation value for the 
next type of resin was 4% or above. 
The process computer 2 read the operation value for the current type of 
resin from the distributed multifunction controllers 4, read the name of 
the next type of resin to be changed from the name data base 9, and 
derived the difference between the target values of the hydrogen 
concentration for both resins. As a result, the pattern in FIG. 4C was 
selected based on the TABLE 1. 
FIG. 4C shows the two-step hydrogen supply pattern in which the feed rate 
of hydrogen is increased to 20 Nm.sup.3 /Hr and the feed rate of the 
catalyst is reduced at the first step, causing the polymerization amount 
in the polymerization reactor and thus reducing the catalyst 
concentration, and then the feed rate of hydrogen is further increased to 
55 Nm.sup.3 /Hr. When the hydrogen concentration reaches the target value, 
the feed rate of hydrogen is reduced to the specified value of 3.5 
Nm.sup.3 /Hr, after which the feed rate of hydrogen is adjusted to provide 
the target hydrogen concentration. 
It took 2 hours and 10 minutes for the physical property value of the 
produced polymer to meet the specified value under this control. 
First Comparative Example 
The same control as done in the first experimental example was executed 
with an operator manually operating only the distributed multifunction 
controllers. 
At the time of changing one type of resin to another, the operator 
simultaneously changed the set values of other multiple control loops and 
monitored those control loops, and performed the condition changing 
operation while discriminating the set values from the global point of 
view. Because the set values should be changed in full consideration of 
safety, the values were set to the minimum levels from the viewpoint of 
safety. 
In this case, to prevent abnormal polymerization, hydrogen was supplied for 
two hours at the rate of 20 Nm.sup.3 /Hr and the feed rate of hydrogen was 
then increased to 40 Nm.sup.3 /Hr while monitoring the rise in hydrogen 
concentration. After confirming that the hydrogen concentration rose to 
the specified level, the feed rate of hydrogen was dropped to 3.5 Nm.sup.3 
/Hr. 
It took 3 hours and 45 minutes under this control for the proper change of 
the type of a resin or for the physical property value of the product to 
meet the specified value. 
FIG. 5 shows changes in hydrogen concentration (%) of the first 
experimental example and the first comparative example. 
It is apparent from FIG. 5 that this invention can shorten the time for the 
physical property value of the next type of resin to reach the specified 
value. 
SECOND EXPERIMENTAL EXAMPLE 
A product with a lower molecular weight was changed to a product with a 
higher molecular weight in the same system structure as used for the first 
experimental example. 
To lower the hydrogen concentration, it is effective to temporarily stop 
supplying hydrogen and perform dehydrogenation. The "dehydrogenation" 
means to perform degassing from the polymerization reactor to discharge 
hydrogen together with the reactive monomer gas. This method can 
considerably shorten the time needed to reduce the hydrogen concentration. 
As this method drops the pressure in the reactor which may affect the 
reaction, however, a large amount of hydrogen should not be discharged. In 
this respect, this should be used as an auxiliary scheme. 
In this experimental example, the process computer 2 selected the pattern 
in FIG. 4D, temporarily stopped the supply of hydrogen and performed 
dehydrogenation, and set the feed rate of hydrogen back to the specified 
value of 4.5 Nm.sup.3 /Hr when the hydrogen concentration became the 
specified level. In this case, the changing of resins from one type to 
another took 3 hours. 
Second Comparative Example 
The operator manually performed the same control as done in the second 
experimental example. 
Because the operator should change the set values of other multiple control 
loops and monitor those control loops at the time of changing one type of 
resin to another in this case too, the operator performed only the 
operation to temporarily stop the supply of hydrogen first, and then 
returned the feed rate of hydrogen to the specified value of 4.5 Nm.sup.3 
/Hr. 
It took 5.5 hours to complete the proper change of the type of a resin in 
this case. 
FIG. 6 shows successive changes in hydrogen concentration (%) of the second 
experimental example and the second comparative example. It is apparent 
from FIG. 6 that this invention can shorten the time for the physical 
property value of the next type of resin to reach the specified value. 
The system shown in FIG. 1 may be realized by connecting a process computer 
to the bus of a conventional distributed control system so that this 
process computer accomplish the type change control. 
In executing the type change control, the operation process data of the 
current type of resin is compared with the specified value of the 
operation process data of the next type of resin and the difference 
between both values is obtained to select the proper operational pattern. 
Operational patterns to be registered in the operational pattern table are 
what is need for the operation process data of the plant, after changing, 
to reach the specified value of the operation process data of the next 
type of resin to be produced in order to ensure the continuous transition 
to the operation for the next type of resin without stopping the operation 
even after the completion of the operation for the current type of resin. 
They are considered the optimal operational patterns in that the quantity 
of products which do not meet the standards and are produced at proper 
time of changing the types of resins and in that the transition to the 
operation of producing the next type of resin is accomplished safely and 
efficiently. The operational patterns are obtained by dividing the 
conditions for changing operation process data into several types of 
patterns. 
As the target value of each type of resin and a successive change in the 
feed rate of each raw material are registered in the operational pattern 
table, it is possible to register the operational pattern which is based 
on the result of the analysis previously done through the manual operation 
for each type of resin. This can ensure the control that does not depend 
on the skill levels of the individual operators. 
Second Embodiment 
A control system for a polyolefine resin production plant according to the 
second embodiment of this invention will be described below. 
The second embodiment is adapted to a continuous polymerization system for 
polymers which automatically changes polymers from one type to another in 
the continuous polymerization. 
Polymers or high-molecular materials are produced by polymerization. 
Polymers can be produced by continuous polymerization in the 
polymerization reactor. This continuous polymerization sequentially 
produces plural types of polymers which have different molecular weights 
and different densities. 
The switching of compositions in the polymerization reactor should be 
executed promptly and stably in the continuous polymerization in order to 
reduce the number of transition products (the off-standards products which 
are produced transiently at the time of switching from one type of polymer 
to another) as much as possible when one polymer is changed to another. 
For example, the ratio of feed rates of supplied gases, such as the ratio 
of the feed rates of hydrogen, ethylene and propylene in changing the 
molecular weight or the ratio of the feed rates of propylene and ethylene 
in changing the molecular weight/density corresponds to a change in 
composition in the polymerization reactor and becomes an index for 
changing the polymer type or the transition. 
There is one conventional continuous polymerization technique which 
over-supplies a catalyst, hydrogen or propylene for a certain period of 
time in order to cause the ratio of the value measured by a gas 
chromatography (hereinafter called "gas chromatographic ratio") to 
approach the target value for the type of a transition product by 
adjusting the feed rate of the catalyst, the feed rate of hydrogen, the 
feed rate of propylene or the like with the feed rate of ethylene kept 
constant, and then returns that ratio to said target value. 
As already mentioned, conventionally, switching the polymer type is 
manually performed by an operator and thus greatly depends on the 
knowledge- and experience-based skill of that operator. Accordingly, the 
time for the transition varies depending on operators, thus making it 
difficult to stabilize the qualities of products around the transition for 
some operators. 
In this respect, the second embodiment of this invention provides a 
continuous polymerization system for polymers which is capable of 
automatically and quickly switches one type of polymer to another in the 
continuous polymerization while improving the quality of the products. 
That is, the second embodiment is a continuous polymerization system for 
polymers to which the control system of this invention for a polyolefine 
resin production plant is applied. FIG. 7 shows the hardware structure of 
the second embodiment. 
The continuous polymerization system for polymers sequentially produces 
different types of polymers through the continuous polymerization which is 
accomplished by continuously switching compositions in the polymerization 
reactor. 
The continuous polymerization system for polymers in FIG. 7 includes a 
model algorithm control section (hereinafter called "MAC section") 21, a 
distributed control system section (hereinafter called "DCS section") 23, 
and a polymerization reactor 28. 
The MAC section 21 sets a model pattern for the DCS section 23 and performs 
fuzzy control. Accordingly, the MAC section 21 manages model pattern 
parameters, fuzzy parameters and a name condition file and manages the 
fuzzy control. 
The DCS section 23 executes model-pattern based program control to perform 
the general control of the system and the sequence control. 
The MAC section 21 executes the model predictive control using a model 
pattern obtained by patterning the overaction part in a test sequence 
prepared by the DCS section 23. 
The "model pattern" is the operational pattern of the feed rates of 
hydrogen and propylene set on the basis of the past achievements for the 
individual polymer types. A scheme of patterning the transitional 
operations based on the experiences of skillful operators to construct 
models for the individual polymer types belongs to the model predictive 
control. For the optimization purpose, a control apparatus which employs 
the model predictive control should be used. 
The model predictive control includes MAC (Model Algorithmic Control) and 
DMC (Dynamic Matrix Control), for example. The typical method for the 
model predictive control uses the dynamic characteristic model of the 
process in the control algorithm to predict the future behaviors Y.sub.P 
(t), Y.sub.P (t+1), . . . of a control target from the previous 
operational data and to control it within a finite time, as illustrated in 
FIG. 8. 
With the use of the model pattern alone, the fuzzy control in the MAC 
section 21 cannot cope with the control of the physical property which 
changes from time to time. Therefore, the fuzzy control is used to control 
the physical property to the target value. In the fuzzy control, 
membership functions and fuzzy rules are set in association with the 
manual control operation so as to achieve a human-like control operation. 
The continuous polymerization system for polymers performs a fuzzy 
operation on the process feedback values and reflects the operation 
results on the process. More specifically, the continuous polymerization 
system for polymers executes sequence control which performs control based 
on model patterns attained from the past operational achievement data for 
the individual polymer types and indirect advanced control which performs 
a fuzzy operation at a higher level than the sequence control and sets the 
operation results in the DCS section 23 for advanced control. 
The DCS section 23 may be constructed by using a general-purpose computer 
system (e.g., "CENTUM" (product of YOKOGAWA ELECTRIC CO.) or "TDCS" 
(product of YAMATAKE HONEYWELL CO.)), which includes an operator station 
24, an operator console 25 and a field controller 26. 
The field controller 26 has a batch setter 55 to be discussed later. 
The MAC section 21 has a hard disk drive, a random access memory and an 
RS71 interface card, though not illustrated, and operates the system based 
on software. The software includes a line controller, BASIC interpreter, 
an interface card utility, an interactive graphics utility, a trend 
library, a data base utility, DOS (Disk Operating System), a conversion 
utility, a shell and a fuzzy processing program (such as "AdMAS" which is 
a packaged program). 
A catalyst, hydrogen, ethylene and propylene monomer or the like are 
properly supplied to the polymerization reactor 28 for polymerization. The 
field controller 26 receives data such as the feed rate of the catalyst, 
the feed rate of hydrogen, the feed rate of propylene and the like. The 
field controller 26 controls the pressure and the temperature in the 
polymerization reactor 28, and the gas chromatographic ratio. 
The polymerization reactor 28 performs continuous polymerization. The 
polymer may be polyolefine which is a high-molecule compound obtained by 
the polymerization of .alpha.-olefines each of which has a carbon-carbon 
double bond in the molecules. Polyolefine includes low-density 
polyethylene which is synthesized by radical polymerization. The 
polyolefine may also be polyethylene having an intermediate or high 
density or a polyolefine wax. 
The functional structure of the continuous polymerization system for 
polymers that can be achieved by software will now be described. 
FIG. 9 shows the functional structure of the MAC section 21, and FIG. 10 
shows the functional structure of the DCS section 23. 
The software of the continuous polymerization system for polymers includes 
the MAC function portion shown in FIG. 9 and the DCS sequence function 
portion also shown in FIG. 9. The MAC function portion includes a data 
base 29, a fuzzy control section 31, a trend monitor section 32, a data 
base input/output section 33, a data collecting section 36, a data setting 
section 37, a fuzzy processor 38, a simple simulator 41 a transition data 
collecting section 42, a transition setting section 43, a file selector 44 
and a data analysis/trend section 45. 
The fuzzy control section 31 manages the data input/output of the fuzzy 
processor 38 and name condition parameters, and performs fuzzy control 
like data processing and operation and pressure control of model patterns. 
In other words, the fuzzy control section 31 executes fuzzy control in 
such a way that sub-parameters like the gas chromatographic ratio, 
pressure and temperature, which change in accordance with the values of 
control parameters like the feed rate of hydrogen and the feed rate of a 
catalyst, are permitted to approach the target values associated with the 
next type of polymer to be changed, upon passage of a predetermined time 
after the values of the control parameters change. 
The trend monitor section 32 presents the trend display of the transition 
status in real time to display the contents and status of the fuzzy 
control. 
The data base input/output section 33 manages the input/output access to a 
data file 29A in the data base 29, such as the aforementioned model 
pattern parameters and name operation condition data, and inputs and 
changes those data. 
The simple simulator 41 has a process simulation function to store a simple 
model equation and a processing model equation of the process simulation, 
infer the physical property of a polymer and inspect the fuzzy output. 
The data collecting section 36 collects data from the DCS section 23 
through the communications software that is invoked on the line controller 
and stores the data in a common area. That is, the data collecting section 
36 serves as a detecting section to detect the values of the 
aforementioned sub-parameters. 
The fuzzy processor 38 incorporates a fuzzy operation section which is 
provided as a subprogram of the fuzzy processing program, and executes a 
fuzzy operation under the control of the fuzzy control section 31. That 
is, the values of the aforementioned control parameters are attained from 
the fuzzy operation on the values of the sub-parameters that are collected 
by the data collecting section 36. 
The data setting section 37 sets the model pattern data of the type of a 
resin and the values obtained through the fuzzy operation into the batch 
setter 55 of the field controller 26. 
The fuzzy processing program builds fuzzy rules, membership functions and 
the like. Each fuzzy rule has a condition (IF) part and a conclusion 
(THEN) part each consisting of a fuzzy set. 
The following are some examples of fuzzy rules. 
Rule 1 
IF x is A.sub.1 and y is B.sub.1 THEN z is C.sub.1 
Rule 2 
IF x is A.sub.2 and y is B.sub.2 THEN z is C.sub.2 
Both rules have two condition part variables "x is A" and "y is B" and one 
conclusion part variable "z is C." In the rules, A.sub.1, A.sub.2, 
B.sub.1, B.sub.2, C.sub.1 and C.sub.2 are fuzzy sets. 
The membership functions define the parts of a fuzzy rule for quantization. 
The fuzzy operation obtains the matchability ("goodness" of fit) of the 
condition part of each rule with given values, obtains the inference 
result of each rule based on the attained matchability, and then attains 
the final inference result from the inference result of each rule. 
The simple simulator 41 inputs data from the transition data file 29A to 
the fuzzy processor 38, provides the trend display of the output values on 
the trend monitor section 32 and adjusts the setting of the membership 
functions and fuzzy rules for the fuzzy control based on the displayed 
results. 
The transition data collecting section 42 stores, for example, 88 pieces of 
data from one hour before the beginning of the transition up to the end of 
the transition into the data file 29A in the data base 29. The transition 
setting section 43 sets the trend name and the time for starting the data 
collection. 
The file selector 44 selects the transition data file 29A to be displayed. 
The data analysis/trend section 45 presents the trend display of the 
transition data of the designated name in the data file 29A. 
Then, the DCS sequence section 50 has a system control sequence section 51, 
a parameter setting sequence section 52, a fuzzy control sequence section 
53, a model pattern sequence section 54 and the aforementioned batch 
setter 55. 
The fuzzy control sequence section 53 and model pattern sequence section 54 
control the polymerization reactor 28. 
The system control sequence section 51 controls the selection of the resin 
type, the selection of the sequence in use, the start and stop of the 
sequence, the selection of a model pattern parameter and the communication 
with the MAC section 21. The resin type is managed by an 8-bit code; a 
binary code is handled with a type number in a decimal notation. 
The parameter setting sequence section 52 communicates with the MAC section 
21 upon the initiation of the sequence after a resin type is selected, and 
sets model pattern parameters in the batch setter 55 of the DCS section 
23. 
The parameter setting sequence section 52 sets the same value as two model 
pattern parameters in the batch setter 55. This sequence section 52 
compares the two values, set in the batch setter 55, with each other and 
determines that the proper communication has been accomplished when those 
two values are the same, and sets this value in a program setter (not 
shown). When the two model pattern parameters in the batch setter 55 
differ from each other, the sequence section 52 sends a communication 
error message to the MAC section 21 and retries the communication. 
The fuzzy control sequence section 53 manages the fuzzy operation values 
which are to be set by the data setting section 37 of the MAC section 21 
and causes the batch setter 55 to write in a control tag. This sequence 
section 53 is enabled when a predetermined period of time passes after the 
setting of the model pattern parameters, and controls the communication 
between the MAC section 21 and the DCS section 23. 
The model pattern sequence section 54 is a patterning sequence which 
optimizes the patterning and constructs a model pattern for the resin 
types whose fuzzy parameters have not been built yet. This sequence 
section 54 changes the values of control parameters based on the model 
pattern for each type at the time of switching one type of polymer to 
another in the continuous polymerization. 
The control operation for the continuous polymerization in the continuous 
polymerization system for polymers will now be discussed. 
FIG. 11 is a flowchart illustrating the continuous polymerization control, 
and FIG. 12 illustrates the control characteristic of the polymerization 
transition. 
First, the MAC section 21 builds model patterns (step S00). In this case, 
the test pattern sequence performed patterning for 18 types, for example. 
Further, the patterning was performed with respect to 3 parameters: the 
feed rate of hydrogen, the feed rate of a catalyst and the feed rate of 
propylene. 
FIG. 13 exemplifies a model pattern for hydrogen. As shown in FIG. 13, the 
hydrogen pattern indicates the hydrogen supply of 30 Nm.sup.3 /Hr for 25 
minutes and the hydrogen supply of 120 Nm.sup.3 /Hr for 30 minutes at the 
time of overaction. 
With regard to the catalyst, the activity always varies, making it 
difficult to previously determine the feed rate. Therefore, the ratio of a 
change in activity is factorized based on the past achievements, and the 
feed rate is computed by the MAC section 21 spontaneously upon the 
activity at the transition time. The computed feed rate is set in the DCS 
section 23. 
Because the constructed model pattern has been optimized to a certain 
degree from past achievements, the polymerization transition time can be 
shortened. For instance, the time needed for the transition work 
(including feed-down and feed-up) is about 12 hours on average. The 
automation system for transition polymerization shortens this transition 
time to 10 hours, two hours shorter. 
The setting of the transition type, and the start and stop of the sequence 
are executed on the screen of the operator station 24 and a message is 
sent to the MAC section 21 from the DCS section 23 (step S01). 
The MAC section 21 receives the message from the DCS section 23 and sends 
an acknowledge message to the DCS section 23 (step S02). 
Next, the MAC section 21 determines if the message from the DCS section 23 
is correct (step S03). When the message from the DCS section 23 is 
correct, the data setting section 37 in the MAC section 21 sets model 
pattern parameters in the batch setter 55 in the DCS section 23 (step 
S04). In this example, two same model pattern parameters are set in the 
batch setter 55. 
Then, the DCS section 23 determines if the set model pattern parameters 
have been transferred by comparing a flag with the two same model pattern 
parameters (step S05). 
When the model pattern parameters have been transferred properly, it is 
determined if fuzzy setting of the type has been performed (step S06). 
When this fuzzy setting has been done, the system control sequence section 
51 selects and invokes the fuzzy control sequence section 53 (step S07). 
When the fuzzy control sequence section 53 is selected, the transition 
operation is initiated with the previously defined model pattern and is 
switched to fuzzy control after a predetermined period of time (step S08). 
More specifically, overaction is performed to change the feed condition to 
the condition for the next type of product in the initial state at the 
time of transition, and after the passage of the predetermined period of 
time, fuzzy control is performed to set the gas flow rate to the H/E 
target value shown in FIG. 12. 
The "predetermined period of time" is the time for setting the control 
target value to the value for the next type of product upon the execution 
of the overaction after the initiation of the transition, and is set for 
each type. 
In this case, the fuzzy control section 31 executes the gas chromatography 
control (H/E control) and pressure control based on the control algorithm 
during the fuzzy control period as shown in FIG. 12. The control algorithm 
can be changed for fine adjustment even during operation. The control 
cycle is separated into two parts for the gas chromatography control and 
pressure control, each of which can be set arbitrarily. For example, the 
chromatography control is carried out every 3 minutes while the pressure 
control is executed every minute. 
When the physical property reaches the target value and becomes stable, the 
transition is ended and the normal operation is initiated (step S09). The 
fuzzy control continues even in the normal operation to perform 
stabilization control of this operation. 
The fuzzy control sequence continues until the feed-up, the feed-down and 
the next transition. When the fuzzy control sequence is stopped 
arbitrarily based on the control table, control is switched to ordinary 
control by manual operation. 
When fuzzy setting has not bee made, the system control sequence section 51 
selects and invokes the model pattern sequence section 54 (step S10). 
This sequence section 54 starts the model pattern control shown in FIG. 12 
at the time of overaction (step S11). In the pattern compensation in this 
case, the pressure of n minutes later is inferred for the pressure 
control. 
Since overaction is taken in the predetermined period of time, the physical 
property is not controlled and only the pressure in the polymerization 
reactor is controlled. 
The data collecting section 36 of the high-rank MAC section 21 collects, 
for example, five pieces of pressure data each obtained every minute and 
outputs the ratio of change in the collected pressure data (step S12). 
Pressure change for one minute: 
dp1=P-P1, dp2=P1-P2, dp3=P2-P3, dp4=P3-P4, dp5=P5-P6 
Then, the data collecting section 36 infers the pressure one minute later 
from the current point, based on the data on the ratio of change (step 
S13). 
Inferred pressure after one minute: 
AVPA=SUM(dp1, dp2, dp3, dp4, dp5)/5+reactor pressure D201PS 
The data collecting section 36 determines if the inferred pressure value 
has reached the pressure point (upper limit/lower limit) previously 
determined by the type condition (step S14). 
At this point of time, the data collecting section 36 changes the amount of 
the catalyst to control the pressure (step S15). The amount of a single 
operation of the feed rate of the catalyst was the finite difference 
between the first point and the second point in the model pattern based on 
the past achievements. 
When the pressure reaches the point determined by the type condition, the 
MAC section 21 outputs an upper-limit pressure signal and a lower-limit 
pressure signal to the DCS section 23 to perform the sequence-based 
control (step S16). 
Since the gas chromatography control is not executed during this period, 
manual operation is necessary when the operation enters the stable state. 
The pattern sequence is used to optimize the model pattern, not for 
automation. 
The automation of the polymerization transition is carried out by the 
construction of the model patterns and the fuzzy control. The pattern of 
the test sequence which has constructed the model patterns under the 
model-pattern based program control can be automatically set by the MAC 
section 21 to be used for the optimization of the type-by-type patterns. 
It is therefore possible to develop a scheme of constructing the fuzzy 
parameters for the automatic switching of polymers as needed, even for 
other types of polymers than the typical polymers which are automatically 
changed from one to another. 
During the predetermined period of time, only the pressure in the 
polymerization reactor as a sub-parameter is controlled by changing the 
control parameters but the physical property is not controlled, so that 
the transition time can be shortened. 
The system may also use a feed rate of at least one of .alpha.-olefines 
having 2-8 carbon atoms, e.g., ethylene, propylene, butylene and the like, 
as the control parameter. 
It is apparent that, in this invention, a wide range of different working 
modes can be formed based on the invention without deviating from the 
spirit and scope of the invention.