U.S. patent application number 14/064368 was filed with the patent office on 2014-02-20 for flash line for improving the devolatilization of polymer slurry produced in a polymerization reactor.
This patent application is currently assigned to Total Research & Technology Feluy. The applicant listed for this patent is Total Research & Technology Feluy. Invention is credited to Marc Moers, Lieven Van Looveren, Mathias Wijffels.
Application Number | 20140048152 14/064368 |
Document ID | / |
Family ID | 37866292 |
Filed Date | 2014-02-20 |
United States Patent
Application |
20140048152 |
Kind Code |
A1 |
Van Looveren; Lieven ; et
al. |
February 20, 2014 |
Flash Line for Improving the Devolatilization of Polymer Slurry
Produced in a Polymerization Reactor
Abstract
The present invention relates to a process for improving the
devolatilization of polymer slurry produced in an olefin
polymerization reactor. The process is characterized in that it
involves an adjustment of the temperature of the interior surface
of the flash line for periodically transferring polymer slurry from
the polymerization reactor to a downstream processing unit. In
particular, in accordance with the present process, the temperature
of the interior surface of the flash line is adjusted to a
temperature which is equal to or higher than the softening
temperature of the polymer passing through said flash line whereby
said temperature is higher in a first half of the length of the
flash line than in the remaining length of the flash line.
Inventors: |
Van Looveren; Lieven;
(Mielen-Boven-Aalst, BE) ; Moers; Marc;
(Antwerpen, BE) ; Wijffels; Mathias; (Antwerpen,
BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Total Research & Technology Feluy |
Seneffe |
|
BE |
|
|
Assignee: |
Total Research & Technology
Feluy
Seneffe
BE
|
Family ID: |
37866292 |
Appl. No.: |
14/064368 |
Filed: |
October 28, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
13693465 |
Dec 4, 2012 |
8618227 |
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14064368 |
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12447781 |
Sep 24, 2010 |
8372923 |
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PCT/EP2007/061547 |
Oct 26, 2007 |
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13693465 |
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Current U.S.
Class: |
137/340 |
Current CPC
Class: |
B01J 2219/00238
20130101; B01J 2219/00094 20130101; F16L 53/32 20180101; C08F 6/24
20130101; C08F 6/003 20130101; C08F 10/02 20130101; B01J 19/1837
20130101; B01J 2219/00006 20130101; C08F 6/003 20130101; Y10T
137/6579 20150401; C08F 6/24 20130101; C08L 23/04 20130101; C08F
110/02 20130101; C08F 10/02 20130101; B01J 2219/00204 20130101;
C08F 2/14 20130101; C08L 23/04 20130101; C08F 2/01 20130101; B01J
8/007 20130101 |
Class at
Publication: |
137/340 |
International
Class: |
F16L 53/00 20060101
F16L053/00 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 30, 2006 |
EP |
06123169.2 |
Claims
1-13. (canceled)
14. A flash line for transferring a polymer slurry from an upstream
polymerization reactor to a downstream flash tank comprising: a
pipe having an interior surface; and a control means adapted to
adjust a temperature of the interior surface of the pipe to heat a
first length of the pipe to a higher temperature than a remaining
length of the pipe, wherein the first length of the pipe is
upstream from the remaining length of the pipe.
15. The flash line of claim 14, wherein the control means
comprises: one or more jackets surrounding the pipe; and a heating
fluid in the one or more jackets.
16. The flash line of claim 15, wherein the control means further
comprises means for adjusting temperature, pressure and/or flow of
the heating fluid.
17. The flash line of claim 15, wherein the control means is
adapted to adjust a pressure of the heating fluid.
18. The flash line of claim 17, wherein the pressure of the heating
fluid is at most 2.5 barg.
19. The flash line of claim 17, wherein the pressure of the heating
fluid is higher in the first length of the pipe than in the
remaining length of the pipe.
20. The flash line of claim 19, wherein the pressure in the first
length of the pipe is at least 10% higher than the remaining length
of the pipe.
21. The flash line of claim 15, wherein the control means is
adapted to adjust a temperature of the heating fluid.
22. The flash line of claim 21, wherein the temperature of the
heating fluid is no more than 139.degree. C.
23. The flash line of claim 21, wherein the temperature of the
heating fluid is higher in the first length of the pipe than in the
remaining length of the pipe.
24. The flash line of claim 23, wherein the temperature of the
heating fluid in the first length of the pipe is at least 10%
higher than the remaining length of the pipe.
25. The flash line of claim 15, wherein a flow rate of the heating
fluid is higher in the first length of the pipe than in the
remaining length of the pipe.
26. The flash line of claim 26, wherein the flow rate of the
heating fluid in the first length of the pipe is at least 10%
higher than the remaining length of the pipe.
27. The flash line of claim 14, wherein the flash line comprises
different flash line zones, and wherein the temperature of the
interior surface of the flash line varies in the different flash
line zones.
28. The flash line of claim 27, wherein each different flash line
zone comprises a separate control means.
29. The flash line of claim 27, wherein the flash line comprises at
least three different flash line zones.
30. The flash line of claim 27, wherein the flash line comprises a
series of jackets, and wherein each jacket delimits one of the
different flash line zones.
31. The flash line of claim 14, wherein the control means is
adapted to adjust the temperature of the interior surface of the
pipe to a temperature that is equal to or higher than a softening
temperature of the polymer slurry passing through the pipe.
32. The flash line of claim 14, wherein the first length of the
pipe is at a temperature that is least 10% higher than the
remaining length of the pipe.
Description
TECHNICAL FIELD
[0001] The present invention relates to the field of olefin
polymerization. In particular, the present invention relates to a
process for improving the devolatilization of polymer slurry
produced in an olefin polymerization loop reactor.
BACKGROUND
[0002] Polyethylene (PE) is synthesized by polymerizing ethylene
(CH.sub.2.dbd.CH.sub.2) monomers. Because it is cheap, safe, stable
to most environments and easy to be processed polyethylene polymers
are useful in many applications. According to the properties
polyethylene can be classified into several types, such as but not
limited to LDPE (Low Density Polyethylene), LLDPE (Linear Low
Density Polyethylene), and HDPE (High Density Polyethylene). Each
type of polyethylene has different properties and
characteristics.
[0003] Ethylene polymerizations are frequently carried out in a
loop reactor using monomer, liquid diluent and catalyst, one or
more optional co-monomer(s), and hydrogen. The polymerization in a
loop reactor is usually performed under slurry conditions, with the
produced polymer usually in a form of solid particles which are
suspended in the diluent. The slurry in the reactor is circulated
continuously with a pump to maintain efficient suspension of the
polymer solid particles in the liquid diluent. Polymer slurry is
discharged from the loop reactor by means of settling legs, which
operate on a batch principle to recover the slurry. Settling in the
legs is used to increase the solids concentration of the slurry
finally recovered as product slurry. The product slurry is further
discharged through heated flash lines to a flash tank, where most
of the diluent and unreacted monomers are flashed off and recycled.
The polymer particles are dried, additives can be added and finally
the polymer may be extruded and pelletized.
[0004] In passing from the reactor to the flash tank the pressure
drops and the polymer slurry is partially devolatilized, i.e. the
volatile components thereof are removed. Insufficient
devolatilization of polymer slurry may ultimately result in polymer
end products that contain an undesired amount of diluent and/or
unreacted reactants, such as e.g. (co-)monomer(s). Devolatilization
is improved with higher temperature of the polymer slurry resulting
in lower content of volatiles in the polymer. However, high
temperatures in the flash lines induce fouling problems, i.e.
sticking of polymer product to the interior walls of the flash
lines, and degradation of the polymer particles in the flash lines.
On the other hand, in the flash lines where temperatures are too
low, devolatilization of the polymer slurry turns out to be
insufficient, resulting in polymer end products having
unsatisfactory properties and qualities.
[0005] U.S. Pat. No. 3,428,619 discloses a method for transferring
polymer slurry from a polymerization reactor to a flash tank via a
transfer line wherein the liquid diluent is substantially vaporized
and the polymer particles are dried during the transfer. The
transfer line comprises a plurality of externally heated zones, for
instance two or four zones, of varying size. The method comprises
controlling the flow rate of the materials flowing through the
different zones. The external heating of the zones is carried out
at a temperature sufficiently elevated to vaporize the liquid
present on the solid particles but which is below the softening
temperature of the polymer particles.
[0006] In view of the above, there remains a need in the art for
providing an improved process for the devolatilization of polymer
slurry issued from a polymerization reactor.
[0007] The present invention aims to provide an improved process
for the devolatilization of polymer slurry produced in a
polymerization reactor. More in particular, the present invention
aims to provide a process wherein devolatilization of polymer is
improved.
SUMMARY
[0008] In a first aspect, the present invention therefore relates
to a process for improving the devolatilization of polymer slurry
produced in a polymerization reactor. The process comprises the
steps of: [0009] introducing ethylene monomer, one or more optional
olefin co-monomer(s), and a diluent into a loop reactor, [0010]
feeding at least one polymerization catalyst into the reactor,
[0011] polymerizing said ethylene monomer and said optional
co-monomer(s) to produce a polymer slurry comprising essentially
liquid diluent and solid ethylene polymer particles, [0012]
periodically withdrawing said polymer slurry from said
polymerization reactor, and [0013] passing said withdrawn polymer
slurry through a heated flash line into a flash tank, said flash
line having an interior surface.
[0014] The process is in particular characterized in that the
temperature of the interior surface of the flash line is adjusted
to a temperature which is equal to or higher than the softening
temperature of the polymer passing through said flash line, whereby
said temperature is higher in a first half of the length of the
flash line than in the remaining length of the flash line.
[0015] The term "periodically" withdrawing or discharging polymer
as used herein refers to a process wherein there is a periodical
withdrawal or discharge of polymer slurry from the polymerization
reactor. In accordance with the present invention, the polymer
slurry is discharged through one or more settling legs, and
transferred through one or more heated flash lines into a flash
tank.
[0016] The softening temperature of the polymer passing through
said flash line is measured according to standard ISO 306:2004,
method A50. The melting point of the polymer passing through said
flash line is measured according to standard ISO 3146:2004, method
A50.
[0017] The present invention provides a proper design of the
temperature of flash lines in order to improve polymer slurry
devolatilization. In prior art systems, polymer slurry is heated in
flash lines to a temperature which is below its softening
temperature in order to avoid above-mentioned fouling and
degradation problems. In contrast, in accordance with the present
invention polymer slurry is heated in a flash line to a temperature
which is equal to or higher than its softening temperature without
inducing fouling or degradation problems. By adjusting the
temperature of the interior surface of flash lines improved
vaporization is obtained of unreacted reactants present in the
polymer slurry, such as diluent and/or (co)-monomer(s). The present
process thus provides improved separation of unreacted reactants
from the polymer particles.
[0018] Furthermore, in accordance with the present process a
mixture of gas and polymer solids is obtained that leaves the flash
lines to enter a flash tank. The polymer solids leave the flash
lines at a higher temperature. Such polymer solids are in
consequence easier to further devolatilize in downstream processes.
The need for expensive downstream flashing and drying operations of
the obtained polymer is therefore significantly reduced, since
substantial vaporization already occurs in the flash line zone and
also because of the higher temperature of the polymer particles
leaving the flash line.
[0019] In a second aspect, the invention relates to a flash line
for periodically transferring a polymer slurry from a
polymerization reactor to a flash tank comprising a pipe which is
adapted to receive said polymer slurry, and which is provided with
a jacket surrounding said pipe, said jacket being adapted for
receiving a heating fluid, whereby said flash line comprises
control means for adjusting the temperature of the interior surface
of the pipe to a temperature which is equal to or higher than the
softening temperature of the polymer passing through said pipe, and
which is higher in a first half of the length of the flash line
than in the remaining length of the flash line.
[0020] In a preferred embodiment the flash line according to the
invention is connected with one or more settling legs, and
preferably with at least two settling legs, for periodically
transferring polymer slurry from said settling legs to a flash
tank.
[0021] The present invention will now be disclosed in further
detail hereunder. The description is only given by way of example
and does not limit the invention. The reference numbers relate to
the hereto-annexed figure.
DESCRIPTION OF THE FIGURES
[0022] FIG. 1 is a schematic representation of a process and
apparatuses for the devolatilization of polymer slurry produced in
a polymerization reactor.
DETAILED DESCRIPTION OF THE INVENTION
[0023] The present invention is especially applicable to the
polymerization process of ethylene. Suitable "ethylene
polymerization" includes but is not limited to homo-polymerization
of ethylene or co-polymerization of ethylene and at least one
olefin co-monomer in presence of a polymerization catalyst.
Ethylene polymerization comprises feeding to a reactor the
reactants including the monomer ethylene, one or more optional
co-monomer(s), a diluent, a catalyst, optionally a co-catalyst, and
a terminating agent such as hydrogen.
[0024] Olefin co-monomers which are suitable for being used in
accordance with the present invention may comprise but are not
limited to aliphatic C.sub.3-C.sub.20 alpha-olefins. Examples of
suitable aliphatic C.sub.3-C.sub.20 alpha-olefins include
propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene,
1-decene, 1-dodecene, 1-tetradecene, 1-hexadecene, 1-octadecene and
1-eicosene. In a preferred embodiment of the present invention,
said co-monomer is 1-hexene. However, it should be clear from the
present invention that other co-monomers may as well be applied
according to the present invention.
[0025] Diluents which are suitable for being used in accordance
with the present invention may comprise but are not limited to
hydrocarbon diluents such as aliphatic, cycloaliphatic and aromatic
hydrocarbon solvents, or halogenated versions of such solvents. The
preferred solvents are C.sub.12 or lower, straight chain or
branched chain, saturated hydrocarbons, C.sub.5 to C.sub.9
saturated alicyclic or aromatic hydrocarbons or C.sub.2 to C.sub.6
halogenated hydrocarbons. Nonlimiting illustrative examples of
solvents are butane, isobutane, pentane, hexane, heptane,
cyclopentane, cyclohexane, cycloheptane, methyl cyclopentane,
methyl cyclohexane, isooctane, benzene, toluene, xylene,
chloroform, chlorobenzenes, tetrachloroethylene, dichloroethane and
trichloroethane. In a preferred embodiment of the present
invention, said diluent is isobutane. However, it should be clear
from the present invention that other diluents may as well be
applied according to the present invention.
[0026] As used herein, the term "polymerization slurry" or "polymer
slurry" or "slurry" means a substantially two-phase composition
including liquid and polymer solids. The solids include catalyst
and a polymerized olefin, such as polyethylene. The liquids may
comprise an inert diluent, such as isobutane, with dissolved
monomer such as ethylene, a molecular weight control agent such as
hydrogen, optional co-monomer(s), one or more antistatic agents,
antifouling agents, scavengers, or other process additives.
[0027] Suitable catalysts and co-catalysts for use in the
polymerization of ethylene are well known in the art. The
polymerization catalyst used in combination with the present
invention can for example be a metallocene-based catalyst, a
Ziegler-Natta catalyst or a chromium catalyst.
[0028] The present invention relates to a process for improving the
devolatilization of polymer slurry produced in a polymerization
loop reactor. The process involves an adjustment of the temperature
of the interior surface of the flash line. The term "flash line" as
used herein refers to an elongated jacketed pipe, the interior of
which is heated indirectly by running a heating medium through the
jacket. The polymer slurry is vaporized in the pipe utilizing the
heat supplied from condensing fluid in the jacket surrounding the
pipe. The fluid may be steam, which flows through the jacket in a
direction which is countercurrent to the flow direction of the
polymer slurry in the pipe. In a preferred embodiment, the flash
line comprises control means for adjusting the temperature of the
interior surface of the pipe to a temperature which is equal to or
higher than the softening temperature of the polymer passing
through said pipe.
[0029] The flash line may be divided in different flash line zones,
corresponding to different pipe parts, having a same or a different
diameter. Each of said flash line zones may comprise control means
for adjusting the temperature of the interior surface of the pipe
of said different flash line zones. In particular, each pipe part
may be provided with a jacket and heated using the heat supplied
from the fluid in the jacket. Preferably the flash line comprises
at least 3, and preferably at least 4 zones. In a preferred
embodiment the flash line comprises between 4 and 10 flash line
zones, and for instance 5, 6, 7, 8 or 9 zones.
[0030] The temperature of the interior surface of the flash line or
a zone thereof corresponds to the temperature of the interior
surface of the pipe of the flash line or a zone thereof. The
temperature of the interior surface of the flash line or a zone
thereof will be adjusted to a temperature which is equal to or
higher than the softening temperature of the polymer passing
through said flash line. The control means for adjusting the
temperature of the interior surface of the pipe of the flash line
(zones) comprise means for adjusting temperature, pressure and/or
flow of the heating fluid heating said flash line (zones).
[0031] While polymer slurry flows through the flash line, it will
be heated and vaporization occurs at least partially within the
flash line. In accordance with the invention, a large portion of
the liquid of the polymer slurry is vaporized by the time the
polymer material reaches the flash tank. The material flowing
through the flash line near the polymerization reaction will thus
mainly comprise liquid and polymer solids, while the material
flowing through the flash line near the flash tank will mainly
comprise vapour (gas) and polymer solids.
[0032] Surprisingly, despite the fact that the polymer slurry in
the flash line is heated to a temperature which is above its
softening point, the polymer does not accumulate and/or agglomerate
in the flash lines. No plugging problems of the flash lines occur.
A mixture of vapor and heated polymer solid particles arrives at
the flash tank, in which the vapors are separated and removed from
the heated dry polymer particles. The particles are collected for
further drying and processing as desired.
[0033] In one embodiment, the temperature of the interior surface
of the flash line may be the same in different zones of the flash
line.
[0034] In a preferred embodiment, the temperature of the interior
surface of the flash line varies in different zones thereof. For
instance, the temperature of the interior surface of the flash line
may be higher in zones located near the reactor outlet than in
zones located near the flash tank inlet. In a particularly
preferred embodiment, the invention relates to a process wherein
the temperature of the interior surface of the flash line is higher
in the first half, and preferably the first third of the length of
the flash line than in the remaining length of the flash line. With
the terms a or the "first third" or a or the "first half" of the
length of the flash line is meant that part of the flash line that
is closest to the polymerization reactor. Preferably, the
temperature of the interior surface of the flash line in the first
half, and preferably in the first third of the length of the flash
line is up to 70%, 60%, 50%, 40%, 35%, 30%, 25%, 20%, 15%, or 10%
higher than the temperature in the remaining length of the flash
line.
[0035] In a preferred embodiment, a process is provided according
to the invention, wherein the temperature of the interior surface
of the flash line is up to 10% higher, and for instance up to 10%,
15%, 20%, 25%, 30%, 35%, 40%, 50%, 60% or 70% in the first half,
and preferably the first third, of the length of the flash line
than in the remaining length of the flash line.
[0036] In a preferred embodiment, the process according to the
present invention, comprises periodically withdrawing (or
discharging) said polymer slurry from said polymerization reactor
by means of one or more settling legs, and passing said withdrawn
(or discharged) polymer slurry from said settling legs through one
or more heated flash lines into a flash tank. Preferably the
invention provides a process comprising periodically withdrawing
polymer slurry from a polymerization reactor by means of at least
two settling legs, and for instance by means of 2, 3, 4, 5, 6, 7,
8, 9, or 10 settling legs, and passing said withdrawn polymer
slurry from said settling legs through one or more heated flash
line, and for instance to through one single heated flash line,
into a flash tank.
[0037] The temperature of the interior surface of the flash line or
a zone thereof is adjusted by adjusting pressure, temperature,
and/or flow of the fluid heating said flash line or a zone
thereof.
[0038] In one embodiment, the process comprises adjusting the
temperature of the interior surface of the flash line by adjusting
the pressure of the liquid, preferably steam, heating said flash
line. Preferably, the steam pressure is at most 2.5 barg, and for
instance 0.5, 0.7, 0.9, 1.0, 1.1, 1.5, 2 or 2.5 barg. The term
"barg" as used herein stands for bar gauge and gives the difference
between atmospheric pressure and an internal pressure (bar=barg+1).
In one preferred embodiment, the present process comprises keeping
the pressure of the liquid, preferably steam, heating said flash
line at a maximum pressure of for instance 2.5 barg along the
length of the flash line. In another embodiment, however, the
present invention may also relate to a process wherein the pressure
of the fluid heating the flash line or a zone thereof is higher in
the first half, and preferably the first third of the length of the
flash line than in the remaining length of the flash line. For
instance, the pressure of the fluid heating the flash line in the
first half, and preferably in the first third of the length of the
flash line is up to 70%, 60%, 50%, 40%, 35%, 30%, 25%, 20%, 15%, or
10% higher than the pressure of the fluid heating the flash line in
the remaining length of the flash line.
[0039] In another embodiment, the process comprises adjusting the
temperature of the interior surface of the flash line by adjusting
the temperature of the liquid, preferably steam, heating said flash
line. In a preferred embodiment, the steam temperature is up to 95,
100, 105, 110, 115, 120, 125, 130, 135, or 139.degree. C. The
temperature of the fluid heating the flash line or a zone thereof
is higher in the first half, and preferably the first third of the
length of the flash line than in the remaining length of the flash
line. For instance, the temperature of the fluid heating the flash
line in the first half, and preferably in the first third of the
length of the flash line is up to 70%, 60%, 50%, 40%, 35%, 30%,
25%, 20%, 15%, or 10% higher than the temperature of the fluid
heating the flash line in the remaining length of the flash line.
In a preferred embodiment, the difference in temperature of the
liquid (steam) heating the flash line and the softening temperature
of the polymer transported in the flash line is equal to or more
than 10.degree. C., and for instance more than 11, 12, 13, 14 or
15.degree. C.
[0040] In yet another embodiment, the process comprises adjusting
the temperature of the interior surface of the flash line by
adjusting the flow of the liquid, preferably steam, heating said
flash line. In a preferred embodiment, the steam flow is higher in
the first half, and preferably the first third of the length of the
flash line than in the remaining length of the flash line. For
instance, the flow of the fluid heating the flash line in the first
half, and preferably in the first third of the length of the flash
line is up to 70%, 60%, 50%, 40%, 35%, 30%, 25%, 20%, 15%, or 10%
higher than the flow of the fluid heating the flash line in the
remaining length of the flash line.
[0041] In another preferred embodiment, the invention relates to a
process as defined herein, wherein the flash tank is operated at a
pressure of between 0.1 and 4 bar, and preferably at a pressure of
between 0.2 and 3 bar.
[0042] The present process may be applied in a double loop
polymerisation reactor consisting of two liquid full loop reactors.
The reactors are connected in series by one or more settling legs
of the first reactor connected for discharge of slurry from the
first reactor to said second reactor. Polymer slurry issued from
the second loop may be discharged through flash lines to a product
recovery zone, including a flash tank. The temperature of the
interior surface of the flash lines can be adjusted in accordance
with the invention to be at least the equal to or higher than the
softening temperature of the polymer particles in the line.
[0043] Referring now to FIG. 1 a polymerization loop reactor 1 is
represented consisting of a plurality of interconnected pipes. The
vertical sections of the pipe segments 2 are provided with jackets
3. Polymerization heat can be extracted by means of cooling water
circulating in these jackets of the reactor. Reactants are
introduced into the reactor 1 by feeding line 4. Catalyst,
optionally in conjunction with a co-catalyst or activation agent,
is injected in the reactor 1 by means of the conduct 5. Normally
the catalyst is introduced as a suspension in hydrocarbon diluent.
The polymerization slurry is directionally circulated throughout
the loop reactor 1 as illustrated by the arrows 6 by one or more
pumps, such as axial flow pump 7. The pump may be powered by an
electric motor 8. As used herein the term "pump" includes any
device from compressing driving, raising the pressure of a fluid,
by means for example of a piston or set of rotating impellers 9.
The illustrated reactor 1 is further provided with two settling
legs 10 connected to the pipes of the reactor 1. As the
polymerization progresses polymer slurry accumulates in these
settling legs 10. The settling legs 10 are provided with an
isolation valve 11. These valves 11 are open under normal
conditions and can be closed for example to isolate a settling leg
from operation. Further the settling legs are provided with product
take off or discharge valves 12. The discharge valves 12 may be any
type of valve, which can permit continuous or periodical discharge
of polymer slurry, when they are fully open. Polymer slurry settled
in the settling legs 10 is removed by means of flash lines 13 to a
product recovery zone, including a flash tank 14.
[0044] As shown on FIG. 1, two settling legs are provided for
allowing periodic discharge of polymer slurry from the
polymerization reactor and both settling legs are connected to a
single flash line 13. The withdrawn polymer slurry is passed from
said two settling legs through a single heated flash line into a
flash tank. It shall however be understood that more than two and
for instance 3, 4, 5, 6, 7, 8, 9, 10 settling legs can be provided,
that are all connected to one or more flash lines, and for example
to a single flash line.
[0045] Partial volatilization of unreacted reactants occurs in the
flash lines. Unreacted reactants are further separated from the
incoming polymer solids in the flash tank 14. Within the flash tank
14 unreacted reactants are removed as vapor via conduit 18. The gas
flowing through conduit 18 can be transferred to a recycle section
19, where the reactants in the gas are separated and if desired
re-fed to the polymerization reactor 1. Polyethylene powder is
removed from the tank 14 through conduit means 16 which conduct the
polyethylene powder to a purge column 17. Herein, the PE powder is
further treated to remove any remaining (co-)monomer(s) and
diluent. Treated PE powder is then removed through conduit means
20.
[0046] FIG. 1 represents a single flash line, although it shall be
clear that more than one flash line may be provided to connect the
reactor 1 with a flash tank 14. The flash line 13 is a conduit
which comprises a series of jackets 15, 115, 215. The jackets
delimit different flash line zones. In FIG. 1 three flash line
zones are represented. However, more than three flash line zones
may be provided on a flash line in accordance with the present
invention.
[0047] Advantageously, the present process and the present flash
line can be operated even when the flash tank is operated at a
pressure of between 0.1 and 4 bar, and preferably of between 0.2
and 3 bar, and for instance of 0.4 bar, although the operational
conditions of the flash lines are less flexible in view of such
relatively low pressure conditions. The present invention overcomes
this difficulty and advantageously provides a process which enables
to carefully adjust the operational conditions of the flash line(s)
and in particular the temperature of the interior surface of the
flash line(s).
[0048] For example, during transfer from reactor 1 to flask tank 14
the pressure drops from e.g. about 40 bar to e.g. about 400 mbar,
and polymer slurry is partially devolatilized in the flash line 13.
Further devolatilization is effected by adjusting the temperature
in the flash line 13. The temperature in the flash line is adjusted
by adjusting the temperature, pressure and/or flow, and preferably
by adjusting the pressure of a heated fluid, e.g. steam, flowing
through jackets 15, 115, 215. The steam flows in a direction
countercurrent to the direction the polymer slurry flow in the
flash line. The steam is introduced in the jackets 15, 115, 215 by
lines 21, 121, 221, respectively, and withdrawn from the jackets by
lines 22, 122, 222. The steam provides indirect heating of the
polymer slurry in flash line 13, such that the polymer slurry is
vaporized in the line 13. In accordance with the present invention,
the temperature, pressure and/or flow, of the steam is controlled
and adjusted such that the temperature of the interior surface 23
of the flash line 13 corresponds to or is higher than the softening
temperature of the polymer particles in the line 13. When the flash
line's interior surface is heated to such temperature, liquid
vaporization in addition to vaporization effected by pressure
reduction in line 13 is sufficient to volatilize a substantial part
of the liquid present in the line 13. Preferably, the conduit 13 is
of sufficient length and diameter to permit transporting polymer
slurry that expands due to pressure reductions and
vaporization.
EXAMPLES
[0049] The following example illustrates that degassing of a PE
polymer slurry produced in a polymerization loop reactor can be
improved by adjusting the temperature of the interior surface of
the flash line to a temperature which is equal to or higher than
the softening temperature of the polymer passing through said flash
line.
[0050] In this example, a first PE slurry was passed through a
flash line wherein temperature of the interior surface of the flash
line was not adjusted. A second PE slurry was passed through a
flash line wherein temperature of the interior surface of the flash
line was adjusted by adjusting pressure of the fluid heating said
flash line. In both cases, a mixture of gas and polymer solids was
obtained that left the flash lines to enter a flash tank. Results
of this comparative experiment are summarized in Table 1.
TABLE-US-00001 TABLE 1 first second PE slurry PE slurry isobutane
(kg/hour) 7195 5925 PE (kg/hour) 9800 8990 Temperature in
polymerization reactor (.degree. C.) 90 90 Pressure in
polymerization reactor (barg) 39 39 pressure in flash tank (barg)
0.41 0.40 action standard +1 bar(g) pressure profile flash
temperature (.degree. C.) 58 51
[0051] It can be derived from Table 1 that the polymer solids that
are derived from the second PE slurry leave the flash lines at a
higher temperature than the polymer solids derived from the first
PE slurry (Flash temperature of 58.degree. C. versus 51.degree.
C.--a gain of 7.degree. C.). As a consequence thereof, it is easier
and more efficient to further devolatilize polymer solids that are
derived from the second PE slurry in downstream processes.
* * * * *