U.S. patent application number 13/869272 was filed with the patent office on 2013-09-05 for dynamic pressure control in double loop reactor.
This patent application is currently assigned to TOTAL RESEARCH & TECHNOLOGY FELUY. The applicant listed for this patent is Alain Brusselle, Daan Dewachter, Louis Fouarge. Invention is credited to Alain Brusselle, Daan Dewachter, Louis Fouarge.
Application Number | 20130231446 13/869272 |
Document ID | / |
Family ID | 36579357 |
Filed Date | 2013-09-05 |
United States Patent
Application |
20130231446 |
Kind Code |
A1 |
Dewachter; Daan ; et
al. |
September 5, 2013 |
Dynamic Pressure Control in Double Loop Reactor
Abstract
The present invention discloses a slurry loop reactor comprising
at least two loop reactors connected in series and wherein the line
connecting the two loops is subject to a dynamic pressure
difference.
Inventors: |
Dewachter; Daan; (Mechelen,
BE) ; Fouarge; Louis; (Dilbeek, BE) ;
Brusselle; Alain; (Wilrijk, BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Dewachter; Daan
Fouarge; Louis
Brusselle; Alain |
Mechelen
Dilbeek
Wilrijk |
|
BE
BE
BE |
|
|
Assignee: |
TOTAL RESEARCH & TECHNOLOGY
FELUY
SENEFFE (FELUY)
BE
|
Family ID: |
36579357 |
Appl. No.: |
13/869272 |
Filed: |
April 24, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12159302 |
Jan 21, 2011 |
8455595 |
|
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PCT/EP2006/012534 |
Dec 27, 2006 |
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13869272 |
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Current U.S.
Class: |
526/64 |
Current CPC
Class: |
B01J 2219/00033
20130101; C08F 10/00 20130101; B01J 19/1837 20130101; C08F 110/02
20130101; B01J 2219/0004 20130101; B01J 8/007 20130101; B01J
19/1881 20130101; C08F 10/00 20130101; C08F 110/06 20130101; C08F
2/001 20130101; B01J 2219/00166 20130101; C08F 2/01 20130101; B01J
2219/00094 20130101 |
Class at
Publication: |
526/64 |
International
Class: |
C08F 2/00 20060101
C08F002/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 30, 2005 |
EP |
05028726.7 |
Claims
1-7. (canceled)
8. A method for controlling pressure in a double loop reactor
comprising: operating a first loop reactor of the double loop
reactor at a first initial pressure; operating a second loop
reactor of the double loop reactor at a second initial pressure,
wherein the first loop reactor and the second loop reactor are
connected in series via a line adapted for transferring polymer
from the first loop reactor to the second loop reactor, and wherein
the line is subject to a dynamic pressure difference; linking a set
point pressure value of the second loop reactor to a process
pressure value of the first loop reactor; and discharging polymer
from the second loop reactor each time a pressure in the second
loop reactor equals the set point pressure value, wherein the set
point pressure value is equal to the process pressure value minus
the dynamic pressure difference.
9. The method of claim 8, wherein when polymer is discharged from
the first loop reactor the process pressure value of the first loop
reactor drops below the first initial pressure, and wherein
pressure in the second loop reactor directly increases while
maintaining the dynamic pressure difference.
10. The method of claim 9, wherein the polymer is discharged from
the first loop reactor and the second loop reactor by dumping legs
of the first loop reactor and the second loop reactor.
11. The method of claim 8, wherein the process pressure value of
the first loop reactor increases after the polymer is discharged
from the second loop reactor.
12. The method of claim 8, wherein the set point pressure value is
between 0 and 5 bars.
13. The method of claim 8, wherein the set point pressure value is
between 0.5 and 5 bars.
14. The method of claim 8, wherein pressure in the second loop
reactor is controlled in real time by pressure variations in the
first loop reactor.
15. The method of claim 14, wherein pressure in the second loop
reactor is controlled in cascade by pressure in the first loop
reactor through a differential pressure measurement.
16. The method of claim 8, wherein a predetermined pressure
difference is maintained between the first loop reactor and the
second loop reactor.
17. The method of claim 16, wherein the predetermined pressure
difference is not more than 5 bars.
18. The method of claim 16, wherein the predetermined pressure
difference is from 0.5 bars to 2 bars.
19. The method of claim 16, wherein the predetermined pressure
difference is from 1.5 bars to 2 bars.
20. The method of claim 8, further comprising setting a minimum
time between discharges of polymer from the second loop
reactor.
21. The method of claim 8, further comprising: introducing an
olefin monomer into the first loop reactor; contacting the olefin
monomer with a first catalyst system within the first loop reactor
to form a first polyolefin; withdrawing the first polyolefin from
the first loop reactor; transferring the first polyolefin from the
first loop reactor to the second loop reactor via the line;
contacting the first polyolefin with a second catalyst system
within the second loop reactor to form a second polyolefin; and
withdrawing the second polyolefin from the second loop reactor.
22. The method of claim 21, wherein the second polyolefin is a
bimodal polymer, and wherein the first catalyst system and the
second catalyst system are both metallocene catalyst systems.
23. The method of claim 8, further comprising collecting growing
polymer exiting the first loop reactor by continuous discharge or
settling legs.
24. The method of claim 8, wherein the first loop reactor and the
second loop reactor are linked by a by-pass line and an alternate
route, and wherein the by-pass line collects growing polymer
exiting the first loop reactor at exit points and sends the growing
polymer to an entry point in the second loop reactor.
25. The method of claim 24, wherein a velocity in the by-pass line
is larger than 3 m/s.
26. The method of claim 8, wherein the first loop reactor operates
in batch operation.
27. The method of claim 26, wherein the second loop reactor
operates in continuous operation.
Description
[0001] This invention is related to the field of olefin
polymerisation in double loop reactors.
[0002] High density polyethylene (HDPE) was first produced by
addition polymerisation carried out in a liquid that was a solvent
for the resulting polymer. That method was rapidly replaced by
polymerisation under slurry conditions according to Ziegler or
Phillips. More specifically slurry polymerisation was carried out
continuously in a pipe loop reactor. A polymerisation effluent is
formed which is a slurry of particulate polymer solids suspended in
a liquid medium, ordinarily the reaction diluent and unreacted
monomer (see for Example U.S. Pat. No. 2,285,721). It is desirable
to separate the polymer and the liquid medium comprising an inert
diluent and unreacted monomers without exposing the liquid medium
to contamination so that said liquid medium can be recycled to the
polymerisation zone with minimal or no purification. As described
in U.S. Pat. No. 3,152,872, a slurry of polymer and the liquid
medium is collected in one or more settling legs of the slurry loop
reactor from which the slurry is periodically discharged to a flash
chamber thus operating in a batch-wise manner. The mixture is
flashed in order to remove the liquid medium from the polymer. It
is afterwards necessary to recompress the vaporised polymerisation
diluent to condense it to a liquid form prior to recycling it as
liquid diluent to the polymerisation zone after purification if
necessary.
[0003] Settling legs are typically required to improve the polymer
concentration in the slurry extracted from the reactor; they
present however several problems as they impose a batch technique
onto a continuous process.
[0004] EP-A-0,891,990 and U.S. Pat. No. 6,204,344 disclose two
methods for decreasing the discontinuous behaviour of the reactor
and thereby for increasing the solids concentration. One method
consists in replacing the discontinuous operation of the settling
legs by a continuous retrieval of enriched slurry. Another method
consists in using a more aggressive circulation pump.
[0005] More recently, EP-A-1410843 has disclosed a slurry loop
reactor comprising on one of the loops a by-pass line connecting
two points of the same loop by an alternate route having a
different transit time than that of the main route for improving
the homogeneity of the circulating slurry.
[0006] The double loop systems are quite desirable as they offer
the possibility to prepare highly tailored polyolefins by providing
different polymerising conditions in each reactor. It is however
often difficult to find suitable space to build these double loop
reactors as in the current configuration they need to be close to
one another in order to insure adequate transfer of growing polymer
from one loop to the other. The average velocity of the material
circulating in the transfer line is of less than 1 m/s: these lines
must therefore be very short in order to avoid sedimentation and
clogging due to the polymerisation of residual monomers. There is
thus a need to provide means either to connect two existing
reactors that may be distant from one another or to build two new
reactors that do not need to be close to one another if available
space so requires.
[0007] It is an aim of the present invention to provide control
means for connecting two or more loop reactors.
[0008] It is also an aim of the present invention to decrease the
residence time of the material in the line connecting the
reactors.
[0009] It is yet another aim of the present invention to improve
the homogeneity of the flow in the loop reactors.
[0010] It is a further aim of the present invention to increase the
concentration of olefin in the first reactor.
[0011] It is yet another aim of the present invention to increase
the solids content.
[0012] Accordingly, the present invention discloses a slurry loop
reactor comprising at least two loop reactors connected in series
and wherein the line connecting the two loops is subject to a
dynamic pressure difference.
[0013] It is difficult to maintain a constant pressure difference
between the two loop reactors, since the control applies to a
mixture of batch discharge and continuous operations. The present
invention thus discloses a system wherein the pressure in the
second reactor is controlled in real time by the pressure
variations in the first reactor in order to maintain a
predetermined pressure difference.
[0014] The present invention provides a method for the slurry
polymerisation of olefins that comprises the steps of: [0015]
providing at least two loop reactors connected in series; [0016]
providing a line connecting two loop reactors wherein the line
connecting two loops is subject to a dynamic pressure difference;
characterised in that the pressure in the second reactor is
controlled in real time by the pressure variations in the first
reactor in order to maintain a predetermined pressure
difference.
[0017] Typical pressure differences are of at most 5 bars,
preferably from 0.5 to 2 bars and more preferably from 1.5 to 2
bars. It must be noted that at the end of each dump the pressure
difference between the two loops may be greater than or equal to
the differential set-point value.
[0018] In a first embodiment according to the present invention,
the two loop reactors are linked by a conventional line connecting
the settling legs of the first reactor to the second reactor.
[0019] In another, preferred, embodiment according to the present
invention, the two loop reactors are linked a by-pass line (11), as
represented in FIG. 1, for connecting two points of the same loop
reactor (12) and (13) by an alternate route having a different
transit time than that of the main route, said by-pass line (11)
also collecting the growing polymer exiting the first loop reactor
(1) at exit points (14) and sending said growing polymer to an
entry point (13) in the second reactor (2).
LIST OF FIGURES
[0020] FIG. 1 represents a double loop reactor configuration
wherein the two reactors are connected by a by-pass line.
[0021] FIG. 2 represents typical pressure profiles, expressed in
bars, in the first and second reactors as a function of time
expressed in h:min:s.
[0022] FIG. 3 represents a double loop configuration including the
low pressure readings indicated as LPn and the system of valves
that can be activated in order to control operations in the
reactor.
[0023] In all embodiments, pressure is typically controlled by
interaction between a set-point value and dumping of the legs. Each
time the set-point value is reached one leg is dumped and
consequently, pressure drops to a value that is lower than the
set-point value: this is essential to maintain control of the
pressure. If the pressure drop is not sufficient, there exists a
scenario for recovering control. This type of control is necessary
for linking leg dumping that is a batch-wise process, to
polymerisation in a loop reactor that is a continuous process.
[0024] In prior art, the conventional way of operating the double
loop reactor was to work with a static set-point value and with a
static differential pressure.
[0025] The present invention links the set-point value of the
second reactor directly to the process value of the first reactor.
It uses a dynamic control system that is able to link the
batch-wise dumping process in both reactors to the continuous
polymerisation process.
[0026] This invention thus allows maintaining the desired
differential pressure at all times.
[0027] As a consequence, the dumping dynamic of the second reactor
needs to be more constrained than that of the first reactor in
order to cope with all the upsets of both the first and second
reactors. Typical pressure profiles in the first and second
reactors are represented in FIG. 2.
[0028] When one leg of the first reactor dumps, the pressure drops
in the first reactor and the pressure in the second reactor
directly increases while maintaining the differential pressure.
When the second reactor reaches the set-point value, that is the
actual process value of the first reactor, thus now at a lower
pressure than the initial pressure, minus the differential
pressure, a leg of the second reactor is fired.
[0029] The cycle then re-starts and the pressure in the first
reactor increases again.
[0030] The growing polymer exiting the first reactor can be
collected either by continuous discharge or by settling legs
technology. Preferably, settling legs are used.
[0031] Throughout the present description the loops forming the
slurry loop reactor are connected in series and each loop can be
folded.
[0032] Optionally, the lines may be jacketed.
[0033] When a by-pass line is used, the velocity of the material
circulating in the line connecting the loop reactors, must be
sufficient to avoid sedimentation and possibly clogging: it must be
of at least 3 m/s.
[0034] The present invention may be used with all types of catalyst
systems. It can be used for the homo- or co-polymerisation of
olefins, preferably of ethylene and propylene. It has proven
particularly useful for preparing bimodal polymers with metallocene
catalyst systems
EXAMPLE
Differential Pressure Control.
[0035] In normal operations, the pressure in second reactor A was
controlled in cascade by the pressure of first reactor B through a
differential pressure measurement. This control had a fixed,
manually adjustable set-point value that could be varied between 0
and 5 bars.
[0036] It was also possible to switch the cascade control on/off
manually and start control separately and independently from the
bimodal regulation package, in order to allow start-up.
[0037] The general set-up is represented in FIG. 3.
[0038] In drift cases, several possibilities were considered and
studied.
1. The Pressure in Second Reactor A was too High.
[0039] Setting the "Minimum Waiting Time Between Dumps" at 1 second
should prevent the increase of pressure in the second reactor. If
however said pressure had increased and if the additional criteria
described hereafter was not fulfilled then the reactors were
killed. These additional criteria were related to the differential
pressure. Typically, the differential pressure (DP) set-point value
was adjusted between 0.5 and 5 bars. If during operation of the
reactors, the differential pressure dropped below half the
set-point value for 30 consecutive seconds while the differential
pressure control was activated, then both reactors were killed.
This was handled by the first differential pressure interlock in
the cascade.
2. The Pressure in Second Reactor A was too Low.
[0040] In this drift case no action was required on differential
pressure interlock: it was covered by the low pressure indicator
LP6 represented on FIG. 3. If LP6 was lower than 35 barg, then
valves 12, 13, 14 and 15 were automatically closed and if it
dropped below 30 barg, then valves 10 and 11 were automatically
locked.
3. The Pressure in First Reactor B was too Low.
[0041] This situation occurred for example when a product take-off
(PTO) valve remained blocked in open position. The pressure in
first reactor B dropped and so did the pressure in second reactor
A. It occurred that the pressure difference remained too high to
activate the first differential pressure interlock. The Borsig
valve above the blocked PTO was closed if the blockage resulted
from a feedback error and if simultaneously, the pressure in second
reactor A was smaller than 37 barg for more than 5 s. This was
handled by differential pressure interlocks 2, 3 and 4 in the
cascade.
[0042] In another example according to the present invention, low
pressure in first reactor B resulted from low temperature in that
reactor. Large amount of hydrogen in the reactor slowed down the
reaction thereby reducing the temperature. If the differential
pressure control was unable to compensate for such pressure drop in
first reactor B, the reactor was killed.
4. The Pressure in First Reactor B was too High.
[0043] This situation was not critical and it was not necessary to
implement any specific action.
* * * * *