U.S. patent application number 10/849393 was filed with the patent office on 2004-09-30 for method and apparatus for high solids slurry polymerization.
Invention is credited to Cymbaluk, Teddy H., Franklin, Robert K. III, Hensley, Harvey D., Hottovy, John D., Perez, Ethelwoldo P., Przelomski, David J..
Application Number | 20040192860 10/849393 |
Document ID | / |
Family ID | 25401189 |
Filed Date | 2004-09-30 |
United States Patent
Application |
20040192860 |
Kind Code |
A1 |
Hottovy, John D. ; et
al. |
September 30, 2004 |
Method and apparatus for high solids slurry polymerization
Abstract
An olefin polymerization process wherein monomer, diluent and
catalyst are circulated in a continuous loop reactor and product
slurry is recovered by means of a continuous product take off. The
continuous product allows operating the reaction at significantly
higher solids content in the circulating slurry. In a preferred
embodiment, the slurry is heated in a flash line heater and passed
to a high pressure flash where a majority of the diluent is
separated and thereafter condensed by simple heat exchange, without
compression, and thereafter recycled. Also an olefin polymerization
process operating at higher reactor solids by virtue of more
aggressive circulation.
Inventors: |
Hottovy, John D.;
(Bartlesville, OK) ; Hensley, Harvey D.;
(Bartlesville, OK) ; Przelomski, David J.;
(Houston, TX) ; Cymbaluk, Teddy H.; (Pasadena,
TX) ; Franklin, Robert K. III; (Houston, TX) ;
Perez, Ethelwoldo P.; (Sugar Land, TX) |
Correspondence
Address: |
Michael G. Fletcher
FLETCHER YODER
P.O. Box 692289
Houston
TX
77269-2289
US
|
Family ID: |
25401189 |
Appl. No.: |
10/849393 |
Filed: |
May 19, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10849393 |
May 19, 2004 |
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10176289 |
Jun 20, 2002 |
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10176289 |
Jun 20, 2002 |
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09586370 |
Jun 2, 2000 |
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09586370 |
Jun 2, 2000 |
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08893200 |
Jul 15, 1997 |
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6239235 |
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Current U.S.
Class: |
526/64 ;
526/72 |
Current CPC
Class: |
C08F 210/16 20130101;
C08F 10/02 20130101; B01J 4/008 20130101; B01J 19/1837 20130101;
C08F 6/003 20130101; B01J 8/007 20130101; C08F 10/00 20130101; B01J
2219/00006 20130101; B01J 8/0055 20130101; C08F 10/00 20130101;
C08F 2/14 20130101; C08F 210/16 20130101; C08F 210/14 20130101;
C08F 10/02 20130101; C08F 2/14 20130101 |
Class at
Publication: |
526/064 ;
526/072 |
International
Class: |
C08G 085/00; C08F
002/00 |
Claims
What is claimed is:
1. A method of continuously obtaining polymer product from an
olefin polymerization reactor comprising an endless loop of pipe,
the method comprising: circulating within the loop a slurry of
polymer particles and liquids while maintaining the reactor at a
first pressure above 400 psia; continuously conveying an amount of
the polymer particles from the reactor first through a discharge
means located below a horizontal midline of a cross-section of the
reactor pipe and then through a transfer line; and receiving the
polymer particles at the inlet of a non-cyclonic primary flash
vessel having vertical sidewalls and a conical bottom and
maintained at a second pressure less than 25 psia, whereupon the
particles settle to the bottom of the flash vessel.
2. The method of claim 1 in which the reactor is a horizontal loop
reactor.
3. The method of claim 2 in which the discharge means is located
upstream of a reactor-circulating pump in the reactor.
4. The method of claim 1 in which the first pressure is above 600
psia.
5. The method of claim 2 in which the first pressure is from 635 to
675 psia and the liquids include isobutane and ethylene.
6. The method of claim 1 including controlling the flow through the
discharge means in response to the pressure of the reactor, and
adding fresh olefin feedstock to the reactor at a constant
rate.
7. The method of claim 6 in which the reactor is a horizontal loop
reactor.
8. The method of claim 6 in which the reactor is a vertical loop
reactor.
9. The method of claim 1 in which the particles enter the
non-cyclonic primary flash vessel at a tangent to the vertical
sidewall in the upper half of the vessel.
10. The method of claim 1 further comprising removing a portion of
the polymer particles via an outlet at the bottom of the flash
vessel while retaining an amount of particles sufficient to
maintain a dynamic seal between the inlet and the outlet of the
vessel.
11. The method of claim 1 in which the reactor is a vertical loop
reactor.
12. The method of claim 1 1 in which the particles enter the
non-cyclonic primary flash vessel at a tangent to the vertical
sidewall in the upper half of the vessel.
13. The method of claim 11 further comprising removing a portion of
the polymer particles via an outlet at the bottom of the flash
vessel while retaining an amount of particles sufficient to
maintain a dynamic seal between the inlet and the outlet of the
vessel.
14. The method of claim 13 in which polymer particles are removed
while maintaining a level which fills the conical bottom of the
vessel.
15. The method of claim 11 including the additional step, before
the polymer particles enter the primary flash vessel, of receiving
the polymer particles at the inlet of an intermediate non-cyclonic
flash vessel with an upper section having vertical sidewalls and
the inlet being tangential to the sidewall, a conical bottom with
an outlet therein, and operated at a third pressure intermediate
between the first and second pressures.
16. The method of claim 15 in which the first pressure is above 600
psia and the third pressure in the intermediate flash vessel is
above 180 psia, and the second pressure in the primary flash vessel
is below 25 psia.
17. A method of continuously obtaining polymer product from an
olefin polymerization reactor comprising an endless loop of pipe,
the method comprising: circulating within the loop a slurry of
polymer particles and liquids while maintaining the reactor at a
first pressure above 400 psia; continuously conveying an amount of
the polymer particles from the reactor first through a discharge
means located below a horizontal midline of a cross-section of the
reactor pipe and then through a transfer line; receiving the
polymer particles at the inlet of a non-cyclonic primary flash
vessel having vertical sidewalls and a conical bottom and
maintained at a second pressure less than 25 psia, whereupon the
particles settle to the bottom of the flash vessel; and including
the additional step, before the polymer particles enter the primary
flash vessel, of receiving the polymer particles at the inlet of an
intermediate non-cyclonic flash vessel with an upper section having
vertical sidewalls and the inlet being tangential to the sidewall,
a conical bottom with an outlet therein, and operated at a third
pressure intermediate between the first and second pressures.
18. The method of claim 17 in which the first pressure is above 600
psia and the third pressure in the intermediate flash vessel is
above 180 psia, and the second pressure in the primary flash vessel
is below 25 psia.
19. A method of continuously obtaining polymer product from an
olefin polymerization reactor comprising an endless loop of pipe,
the method comprising: circulating within the loop a slurry of
polymer particles and liquids while maintaining the reactor at a
first pressure above 400 psia; continuously conveying an amount of
the polymer particles from the reactor first through a discharge
means located below a horizontal midline of a cross-section of the
reactor pipe and then through a transfer line; receiving the
polymer particles at the inlet of a non-cyclonic primary flash
vessel having vertical sidewalls and a conical bottom and
maintained at a second pressure less than 25 psia, whereupon the
particles settle to the bottom of the flash vessel; removing a
portion of the polymer particles via an outlet at the bottom of the
flash vessel while retaining an amount of particles sufficient to
maintain a dynamic seal between the inlet and the outlet of the
vessel and maintaining a level which fills the conical bottom of
the vessel.
20. A method of continuously obtaining polymer product from an
olefin polymerization reactor comprising an endless loop of pipe,
the method comprising: circulating within the loop a slurry of
polymer particles and liquids while maintaining the reactor at a
first pressure above 400 psia; continuously conveying an amount of
the polymer particles from the reactor first through a discharge
means located below a horizontal midline of a cross-section of the
reactor pipe and then through a transfer line; and receiving the
polymer particles at the inlet of a non-cyclonic first flash vessel
having vertical sidewalls and a conical bottom and maintained at a
second pressure less than 25 psia, whereupon the particles settle
to the bottom of the first flash vessel.
21. The method of claim 20 in which the reactor is a horizontal
loop reactor.
22. The method of claim 21 in which the discharge means is located
upstream of a reactor-circulating pump in the reactor.
23. The method of claim 20 in which the first pressure is above 600
psia.
24. The method of claim 21 in which the first pressure is between
635 to 675 psia and the liquids include isobutane and ethylene.
25. The method of claim 21 in which the first pressure is about 650
psia and the liquids include isobutane and ethylene.
26. The method of claim 20 including controlling the flow through
the discharge means in response to the pressure of the reactor, and
adding one or more input streams to the reactor at a constant
rate.
27. The method of claim 26 in which the reactor is a horizontal
loop reactor.
28. The method of claim 26 in which the reactor is a vertical loop
reactor.
29. The method of claim 20 in which the particles enter the
non-cyclonic first flash vessel at a tangent to the vertical
sidewall in the upper half of the vessel.
30. The method of claim 20 further comprising removing a portion of
the polymer particles via an outlet at the bottom of the first
flash vessel while retaining an amount of particles sufficient to
maintain a dynamic seal between the inlet and the outlet of the
vessel.
31. The method of claim 20 in which the reactor is a vertical loop
reactor.
32. The method of claim 31 in which the particles enter the
non-cyclonic first flash vessel at a tangent to the vertical
sidewall in the upper half of the vessel.
33. The method of claim 31 further comprising removing a portion of
the polymer particles via an outlet at the bottom of the first
flash vessel while retaining an amount of particles sufficient to
maintain a dynamic seal between the inlet and the outlet of the
vessel.
34. The method of claim 33 in which polymer particles are removed
while maintaining a level which fills the conical bottom of the
vessel.
35. The method of claim 31 including the additional step, before
the polymer particles enter the first flash vessel, of receiving
the polymer particles at the inlet of a second non-cyclonic flash
vessel with an upper section having vertical sidewalls and the
inlet being tangential to the sidewall, a conical bottom with an
outlet therein, and operated at a third pressure intermediate
between the first and second pressures.
36. The method of claim 35 in which the first pressure is above 600
psia and the third pressure in the second flash vessel is above 180
psia, and the second pressure in the first flash vessel is below 25
psia.
37. A method of continuously obtaining polymer product from an
olefin polymerization reactor comprising an endless loop of pipe,
the method comprising: circulating within the loop a slurry of
polymer particles and liquids while maintaining the reactor at a
first pressure above 400 psia; continuously conveying an amount of
the polymer particles from the reactor first through a discharge
means located below a horizontal midline of a cross-section of the
reactor pipe and then through a transfer line; receiving the
polymer particles at the inlet of a non-cyclonic first flash vessel
having vertical sidewalls and a conical bottom and maintained at a
second pressure less than 25 psia, whereupon the particles settle
to the bottom of the first flash vessel; and including the
additional step, before the polymer particles enter the first flash
vessel, of receiving the polymer particles at the inlet of a second
non-cyclonic flash vessel with an upper section having vertical
sidewalls and the inlet being tangential to the sidewall, a conical
bottom with an outlet therein, and operated at a third pressure
intermediate between the first and second pressures.
38. The method of claim 37 in which the first pressure is above 600
psia and the third pressure in the second flash vessel is above 180
psia, and the second pressure in the first flash vessel is below 25
psia.
39. A method of continuously obtaining polymer product from an
olefin polymerization reactor comprising an endless loop of pipe,
the method comprising: circulating within the loop a slurry of
polymer particles and liquids while maintaining the reactor at a
first pressure above 400 psia; continuously conveying an amount of
the polymer particles from the reactor first through a discharge
means located below a horizontal midline of a cross-section of the
reactor pipe and then through a transfer line; receiving the
polymer particles at the inlet of a non-cyclonic first flash vessel
having vertical sidewalls and a conical bottom and maintained at a
second pressure less than 25 psia, whereupon the particles settle
to the bottom of the first flash vessel; removing a portion of the
polymer particles via an outlet at the bottom of the first flash
vessel while retaining an amount of particles sufficient to
maintain a dynamic seal between the inlet and the outlet of the
vessel and maintaining a level which fills the conical bottom of
the vessel.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a continuation of application Ser. No.
10/176,289, filed Jun. 20, 2002, now pending, which is a
continuation of application Ser. No. 09/586,370, filed Jun. 2,
2000, which is a divisional of application Ser. No. 08/893,200,
filed on Jul. 15, 1997, which issued as U.S. Pat. No. 6,239,235, on
May 29, 2001.
BACKGROUND OF THE INVENTION
[0002] This invention relates to the polymerization of olefin
monomers in a liquid diluent.
[0003] Addition polymerizations are frequently carried out in a
liquid which is a solvent for the resulting polymer. When high
density (linear) ethylene polymers first became commercially
available in the 1950's this was the method used. It was soon
discovered that a more efficient way to produce such polymers was
to carry out the polymerization under slurry conditions. More
specifically, the polymerization technique of choice became
continuous slurry polymerization in a pipe loop reactor with the
product being taken off by means of settling legs which operated on
a batch principle to recover product. This technique has enjoyed
international success with billions of pounds of ethylene polymers
being so produced annually. With this success has come the
desirability of building a smaller number of large reactors as
opposed to a larger number of small reactors for a given plant
capacity.
[0004] Settling legs, however, do present two problems. First, they
represent the imposition of a "batch" technique onto a basic
continuous process. Each time a settling leg reaches the stage
where it "dumps" or "fires" accumulated polymer slurry it causes an
interference with the flow of slurry in the loop reactor upstream
and the recovery system downstream. Also the valve mechanism
essential to periodically seal off the settling legs from the
reactor upstream and the recovery system downstream requires
frequent maintenance due to the difficulty in maintaining a tight
seal with the large diameter valves needed for sealing the
legs.
[0005] Secondly, as reactors have gotten larger, logistic problems
are presented by the settling legs. If a pipe diameter is doubled
the volume of the reactor goes up four-fold. However, because of
the valve mechanisms involved, the size of the settling legs cannot
easily be increased further. Hence the number of legs required
begins to exceed the physical space available.
[0006] In spite of these limitations, settling legs have continued
to be employed where olefin polymers are formed as a slurry in a
liquid diluent. This is because, unlike bulk slurry polymerizations
(i.e. where the monomer is the diluent) where solids concentrations
of better than 60 percent are routinely obtained, olefin polymer
slurries in a diluent are generally limited to no more than 37 to
40 weight percent solids. Hence settling legs have been believed to
be necessary to give a final slurry product at the exit to the
settling legs of greater than 37-40 percent. This is because, as
the name implies, settling occurs in the legs to thus increase the
solids concentration of the slurry finally recovered as product
slurry.
[0007] Another factor affecting maximum practical reactor solids is
circulation velocity, with a higher velocity for a given reactor
diameter allowing for higher solids since a limiting factor in the
operation is reactor fouling due to polymer build up in the
reactor.
SUMMARY OF THE INVENTION
[0008] It is an object of this invention to produce olefin polymers
as a slurry in a liquid diluent utilizing continuous product slurry
takeoff;
[0009] It is a further object of this invention to operate a slurry
olefin polymerization process in a diluent at a reactor solids
concentration high enough to make direct continuous product takeoff
commercially viable;
[0010] It is a further object of this invention to operate a slurry
olefin polymerization process in a diluent at higher circulation
velocities.
[0011] It is yet a further object of this invention to operate a
slurry olefin polymerization process in a diluent in a reaction
zone of greater than 30,000 gallons; and
[0012] It is still yet a further object of this invention to
provide a loop reactor apparatus having a capacity of greater than
30,000 gallons and having a continuous take off means.
[0013] In accordance with one aspect of this invention, an olefin
polymerization process is carried out at a higher reactor solids
concentration by means of continuous withdrawal of product
slurry.
[0014] In accordance with another aspect of this invention, a loop
reactor olefin polymerization process is carried out by operating
at a higher circulation velocity for a given reactor pipe
diameter.
[0015] In accordance with another aspect of this invention, a loop
polymerization apparatus is provided having an elongated hollow
appendage at a downstream end of one of the longitudinal segments
of the loop, the hollow appendage being in direct fluid
communication with a heated flash line and thus being adapted for
continuous removal of product slurry.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] In the drawings, forming a part hereof,
[0017] FIG. 1 is a schematic perspective view of a loop reactor and
polymer recovery system;
[0018] FIG. 2 is cross section along line 2--2 of FIG. 1 showing a
continuous take off appendage;
[0019] FIG. 3 is a cross section along line 3--3 of FIG. 2 showing
a ram valve arrangement in the continuous take off assembly;
[0020] FIG. 4 is a cross section of a tangential location for the
continuous take off assembly;
[0021] FIG. 5 is a side view of an elbow of the loop reactor
showing both a settling let and continuous take off assemblies;
[0022] FIG. 6 is a cross section across line 6--6 of FIG. 5 showing
the orientation of two of the continuous take off assemblies; FIG.
7 is a side view showing another orientation for the continuous
take off assembly;
[0023] FIG. 8 is a cross sectional view of the impeller
mechanism;
[0024] FIG. 9 is a schematic view showing another configuration for
the loops wherein the upper segments 14a are 180 degree half
circles and wherein the vertical segments are at least twice as
long as the horizontal segments and
[0025] FIG. 10 is a schematic view showing the longer axis disposed
horizontally.
[0026] FIG. 11 is a schematic diagram illustrating a process for
separating polymer from diluent in accordance with the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0027] Surprisingly, it has been found that continuous take off of
product slurry in an olefin polymerization reaction carried out in
a loop reactor in the presence of an inert diluent allows operation
of the reactor at a much higher solids concentration. Commercial
production of predominantly ethylene polymers in isobutane diluent
has generally been limited to a maximum solids concentration in the
reactor of 37-40 weight percent. However, the continuous take off
has been found to allow significant increases in solids
concentration. Furthermore, the continuous take off itself brings
about some additional increase in solids content as compared with
the content in the reactor from which it takes off product because
of the placement of the continuous take off appendage which
selectively removes a slurry from a stratum where the solids are
more concentrated. Hence concentrations of greater than 40 weight
percent are possible in accordance with this invention.
[0028] Throughout this application, the weight of catalyst is
disregarded since the productivity, particularly with chromium
oxide on silica, is extremely high.
[0029] Also surprisingly, it has been found that more aggressive
circulation (with its attendant higher solids concentration) can be
employed. Indeed more aggressive circulation in combination with
the continuous take off, solids concentrations of greater than 50
weight percent can be removed from the reactor by the continuous
take off. For instance, the continuous take off can easily allow
operating at 5-6 percentage points higher; i.e., the reactor can be
adjusted to easily raise solids by 10 percent; and the more
aggressive circulation can easily add another 7-9 percentage points
which puts the reactor above 50 percent. But, because the
continuous take off is positioned to take off slurry from a stratum
in the stream which has a higher than average concentration of
solids, the product actually recovered has about 3 percentage
points (or greater) higher concentration than the reactor slurry
average. Thus the operation can approach an effective slurry
concentration of 55 weight percent or more, i.e. 52 percent average
in the reactor and the removal of a component which is actually 55
percent (i.e. 3 percentage points) higher.
[0030] It must be emphasized that in a commercial operation as
little as a one percentage point increase in solids concentration
is of major significance. Therefore going from 37-40 average
percent solids concentration in the reactor to even 41 is
important; thus going to greater than 50 is truly remarkable.
[0031] The present invention is applicable to any olefin
polymerization in a loop reactor utilizing a diluent so as to
produce a product slurry of polymer and diluent. Suitable olefin
monomers are 1-olefins having up to 8 carbon atoms per molecule and
no branching nearer the double bond than the 4-position. The
invention is particularly suitable for the homopolymerization of
ethylene and the copolymerization of ethylene and a higher 1-olefin
such as butene, 1-pentene, 1-hexene, 1-octene or 1-decene.
Especially preferred is ethylene and 0.01 to 10, preferably 0.01 to
5, most preferably 0.1 to 4 weight percent higher olefin based on
the total weight of ethylene and comonomer. Alternatively
sufficient comonomer can be used to give the above-described
amounts of comonomer incorporation in the polymer.
[0032] Suitable diluents (as opposed to solvents or monomers) are
well known in the art and include hydrocarbons which are inert and
liquid under reaction conditions. Suitable hydrocarbons include
isobutane, propane, n-pentane, i-pentane, neopentane and n-hexane,
with isobutane being especially preferred.
[0033] Suitable catalysts are well known in the art. Particularly
suitable is chromium oxide on a support such as silica as broadly
disclosed, for instance, in Hogan and Banks, U.S. Pat. No.
2,285,721 (March 1958), the disclosure of which is hereby
incorporated by reference.
[0034] Referring now to the drawings, there is shown in FIG. 1 a
loop reactor 10 having vertical segments 12, upper horizontal
segments 14 and lower horizontal segments 16. These upper and lower
horizontal segments define upper and lower zones of horizontal
flow. The reactor is cooled by means of two pipe heat exchangers
formed by pipe 12 and jacket 18. Each segment is connected to the
next segment by a smooth bend or elbow 20 thus providing a
continuous flow path substantially free from internal obstructions.
The polymerization mixture is circulated by means of impeller 22
(shown in FIG. 8) driven by motor 24. Monomer, comonomer, if any,
and make up diluent are introduced via lines 26 and 28 respectively
which can enter the reactor directly at one or a plurality of
locations or can combine with condensed diluent recycle line 30 as
shown. Catalyst is introduced via catalyst introduction means 32
which provides a zone (location) for catalyst introduction. The
elongated hollow appendage for continuously taking off an
intermediate product slurry is designated broadly by reference
character 34. Continuous take off mechanism 34 is located in or
adjacent to a downstream end of one of the lower horizontal reactor
loop sections 16 and adjacent or on a connecting elbow 20.
[0035] The continuous take off appendage is shown at the downstream
end of a lower horizontal segment of the loop reactor which is the
preferred location. The location can be in an area near the last
point in the loop where flow turns upward before the catalyst
introduction point so as to allow fresh catalyst the maximum
possible time in the reactor before it first passes a take off
point. However, the continuous take off appendage can be located on
any segment or any elbow.
[0036] Also, the segment of the reactor to which the continuous
take off appendage is attached can be of larger diameter to slow
down the flow and hence further allow stratification of the flow so
that the product coming off can have an even greater concentration
of solids.
[0037] The continuously withdrawn intermediate product slurry is
passed via conduit 36 into a high pressure flash chamber 38.
Conduit 36 includes a surrounding conduit 40 which is provided with
a heated fluid which provides indirect heating to the slurry
material in flash line conduit 36. Vaporized diluent exits the
flash chamber 38 via conduit 42 for further processing which
includes condensation by simple heat exchange using recycle
condenser 50, and return to the system, without the necessity for
compression, via recycle diluent line 30. Recycle condenser 50 can
utilized any suitable heat exchange fluid known in the art under
any conditions known in the art. However preferably a fluid at a
temperature that can be economically provided is used. A suitable
temperature range for this fluid is 40 degrees F. to 130 degrees F.
Polymer particles are withdrawn from high pressure flash chamber 38
via line 44 for further processing using techniques known in the
art. Preferably they are passed to low pressure flash chamber 46
and thereafter recovered as polymer product via line 48. Separated
diluent passes through compressor 47 to line 42. This high pressure
flash design is broadly disclosed in Hanson and Sherk, U.S. Pat.
No. 4,424,341 (Jan. 3, 1984), the disclosure of which is hereby
incorporated by reference. Surprisingly, it has been found that the
continuous take off not only allows for higher solids concentration
upstream in the reactor, but also allows better operation of the
high pressure flash, thus allowing the majority of the withdrawn
diluent to be flashed off and recycled with no compression. Indeed,
70 to 90 percent of the diluent can generally be recovered in this
manner. This is because of several factors. First of all, because
the flow is continuous instead of intermittent, the flash line
heaters work better. Also, the pressure drop after the proportional
control valve that regulates the rate of continuous flow out of the
reactor has a lower pressure which means when it flashes it drops
the temperature lower thus further giving more efficient use of the
flash line heaters.
[0038] Referring now to FIG. 2, there is shown elbow 20 with
continuous take off mechanism 34 in greater detail. The continuous
take off mechanism comprises a take off cylinder 52, a slurry
withdrawal line 54, an emergency shut off valve 55, a proportional
motor valve 58 to regulate flow and a flush line 60. The reactor is
run "liquid" full. Because of dissolved monomer the liquid has
slight compressibility, thus allowing pressure control of the
liquid full system with a valve. Diluent input is generally held
constant, the proportional motor valve 58 being used to control the
rate of continuous withdrawal to maintain the total reactor
pressure within designated set points.
[0039] Referring now to FIG. 3, which is taken along section line
3-3 of FIG. 2, there is shown the smooth curve or elbow 20 having
associated therewith the continuous take off mechanism 34 in
greater detail, the elbow 20 thus being an appendage-carrying
elbow. As shown, the mechanism comprises a take off cylinder 52
attached, in this instance, at a right angle to a tangent to the
outer surface of the elbow. Coming off cylinder 52 is slurry
withdrawal line 54. Disposed within the take off cylinder 52 is a
ram valve 62 which serves two purposes. First it provides a simple
and reliable clean-out mechanism for the take off cylinder if it
should ever become fouled with polymer. Second, it can serve as a
simple and reliable shut-off valve for the entire continuous take
off assembly.
[0040] FIG. 4 shows a preferred attachment orientation for the take
off cylinder 52 wherein it is affixed tangentially to the curvature
of elbow 20 and at a point just prior to the slurry flow turning
upward. This opening is elliptical to the inside surface. Further
enlargement could be done to improve solids take off.
[0041] FIG. 5 shows four things. First, it shows an angled
orientation of the take off cylinder 52. The take off cylinder is
shown at an angle, alpha, to a plane that is (1) perpendicular to
the centerline of the horizontal segment and (2) located at the
downstream end of the horizontal segment 16. The angle with this
plane is taken in the downstream direction from the plane. The apex
for the angle is the center point of the elbow radius as shown in
FIG. 5. The plane can be described as the horizontal segment cross
sectional plane. Here the angle depicted is about 24 degrees.
Second, it shows a plurality of continuous take off appendages, 34
and 34a. Third, it shows one appendage, 34 oriented on a vertical
center line plane of lower segment 16, and the other, 34a, located
at an angle to such a plane as will be shown in more detail in FIG.
6. Finally, it shows the combination of continuous take off
appendages 34 and a conventional settling leg 64 for batch removal,
if desired.
[0042] As can be seen from the relative sizes, the continuous take
off cylinders are much smaller than the conventional settling legs.
Yet three 2-inch ID continuous take off appendages can remove as
much product slurry as 14 8-inch ID settling legs. This is
significant because with current large commercial loop reactors of
15,000-18000 gallon capacity, six eight inch settling legs are
required. It is not desirable to increase the size of the settling
legs because of the difficulty of making reliable valves for larger
diameters. As noted previously, doubling the diameter of the pipe
increases the volume four-fold and there simply in not enough room
for four times as many settling legs to be easily positioned. Hence
the invention makes feasible the operation of larger, more
efficient reactors. Reactors of 30,000 gallons or greater are made
possible by this invention. Generally the continuous take off
cylinders will have a nominal internal diameter within the range of
1 inch to less than 8 inches. Preferably they will be about 2-3
inches internal diameter.
[0043] FIG. 6 is taken along section line 6-6 of FIG. 5 and shows
take off cylinder 34a attached at a place that is oriented at an
angle, beta, to a vertical plane containing the center line of the
reactor. This plane can be referred to as the vertical center plane
of the reactor. This angle can be taken from either side of the
plane or from both sides if it is not zero. The apex of the angle
is located at the reactor center line. The angle is contained in a
plane perpendicular to the reactor center line as shown in FIG.
6.
[0044] It is noted that there are three orientation concepts here.
First is the attachment orientation, i.e. tangential as in FIG. 4
and perpendicular as in FIG. 2 or 7 or any angle between these two
limits of 0 and 90 degrees. Second is the orientation relative to
how far up the curve of the elbow the attachment is as represented
by angle alpha (FIG. 5). This can be anything from 0 to 60 degrees
but is preferably 0 to 40 degrees, more preferably 0 to 20 degrees.
Third is the angle, beta, from the center plane of the longitudinal
segment (FIG. 6). This angle can be from 0 to 60 degrees,
preferably 0 to 45 degrees, more preferably 0-20 degrees.
[0045] FIG. 7 shows an embodiment where the continuous take off
cylinder 52 has an attachment orientation of perpendicular, an
alpha orientation of 0 (inherent since it is at the end, but still
on, the straight section), and a beta orientation of 0, i.e. it is
right on the vertical centerline plane of the lower horizontal
segment 16.
[0046] FIG. 8 shows in detail the impeller means 22 for
continuously moving the slurry along its flow path. As can be seen
in this embodiment the impeller is in a slightly enlarged section
of pipe which serves as the propulsion zone for the circulating
reactants. Preferably the system is operated so as to generate a
pressure differential of at least 18 psig preferably at least 20
psig, more preferably at least 22 psig between the upstream and
downstream ends of the propulsion zone in a nominal two foot
diameter reactor with total flow path length of about 950 feet
using isobutane to make predominantly ethylene polymers. As much as
50 psig or more is possible. This can be done by controlling the
speed of rotation of the impeller, reducing the clearance between
the impeller and the inside wall of the pump housing or by using a
more aggressive impeller design as is known in the art. This higher
pressure differential can also be produced by the use of at least
one additional pump.
[0047] Generally the system is operated so as to generate a
pressure differential, expressed as a loss of pressure per unit
length of reactor, of at least 0.07, generally 0.07 to 0.15 foot
slurry height pressure drop per foot of reactor length for a
nominal 24 inch diameter reactor. Preferably, this pressure drop
per unit length is 0.09 to 0.11 for a 24 inch diameter reactor. For
larger diameters, a higher slurry velocity and a higher pressure
drop per unit length of reactor is needed. This assumes the density
of the slurry which generally is about 0.5-0.6.
[0048] Referring now to FIG. 9 the upper segments are shown as 180
degree half circles which is the which is the preferred
configuration. The vertical segments are at least twice the length,
generally about seven to eight times the length of the horizontal
segments. For instance, the vertical flow path can be 190-225 feet
and the horizontal segments 25-30 feet in flow path length. Any
number of loops can be employed in addition to the four depicted
here and the eight depicted in FIG. 1, but generally four or six
are used. Reference to nominal two foot diameter means an internal
diameter of about 21.9 inches. Flow length is generally greater
than 500 feet, generally greater than 900 feet, with about 940 to
1,350 feet being quite satisfactory.
[0049] Commercial pumps for utilities such as circulating the
reactants in a closed loop reactor are routinely tested by their
manufacturers and the necessary pressures to avoid cavitation are
easily and routinely determined.
EXAMPLES
[0050] A four vertical leg polymerization reactor using a 26 inch
Lawrence Pumps Inc. pump impeller D51795/81-281 in a M51879/FAB
casing was used to polymerize ethylene and hexene-1. This pump was
compared with a 24 inch pump which gave less aggressive circulation
(0.66 ft of pressure drop vs 0.98). This was then compared with the
same more aggressive circulation and a continuous take off assembly
of the type shown by reference character 34 of FIG. 5. The results
are shown below.
1 DATA TABLE Description 24 in Pump 26 in Pump 26 in Pump + CTO
Date of Operation Oct 4-9, 1994 May 24-28, 1995 Nov 15-18, 1996
Avg. Reactor Solids 39 45 53 Concentration, wt % Polymer Production
40.1 40.7 39.9 Rate, mlbs/hr Reactor Circulation 430 691 753 Pump
Power, kw Circulation Pump 14.3 22.4 23.7 Pressure Diff, psi
Circulation Pump 61.8 92.5 92.4 Head, ft Reactor Slurry Flow 39 46
45 Rate, mGPM Reactor Slurry Density, gm/cc 0.534 0.558 0.592
Reactor Temperature, F. 215.6 218.3 217.0 Ethylene 4.43 3.67 4.9
Concentration, wt % Hexene-1 0.22 0.17 0.14 Concentration, wt %
Reactor Heat Transfer 270 262 241 Coefficient Reactor Inside
22.0625 22.0625 22.0625 Diameter, inches Reactor Volume, gal 18700
18700 18700 Reactor Length, ft 941 941 941 Pressure Drop per Foot
0.066 0.098 0.098 of Reactor, ft/ft
[0051] As noted above, a flash vessel design which may be used in
conjunction with the continuous take off techniques discussed
herein is disclosed in U.S. Pat. No. 4,424,341 to Hanson and Sherk,
which is incorporated by reference. For the convenience of the
reader, aspects of the Hanson and Sherk technique applicable to the
present continuous take off techniques are reproduced below.
[0052] While the present invention is applicable to any mixture
which comprises a slurry of polymer solid and diluent, it is
particularly applicable to the slurries resulting from olefin
polymerizations. The olefin monomers generally employed in such
reactions are 1-olefins having up to 8 carbon atoms per molecule
and no branching nearer the double bond than the 4-position.
Typical examples include ethylene, propylene, butene-1,1-pentene,
and 1,3-butadiene.
[0053] Typical diluents employed in such olefin polymerizations
include hydrocarbons having 3 to 12, preferably 3 to 8 carbon atoms
per molecule, such as propane, propylene, n-butane, n-pentane,
isopentane, n-hexane, toluene, isooctane, isobutane, 1-butene, and
the like. In some cases, naphthene hydrocarbons having 5 to 6
carbon atoms in the naphthenic ring are also used. Examples of such
naphthenic hydrocarbons include cyclohexane, cyclopentane,
methylcyclopentane, ethylcyclohexane, and the like.
[0054] The temperature to which the slurry is heated for
vaporization will vary of course depending upon the nature of the
diluent, the nature of the polymer, and the temperature of the heat
exchange fluid that is used to condense the vaporized diluent.
Obviously, the temperature must be raised above the dew point of
the diluent at the flashing pressure. Further the temperature
should be below that of the melting point of the polymer to
preclude accumulation of polymer in the process vessels and to
preclude agglomeration of the polymer particles.
[0055] The pressure for the first flash step will likewise vary
depending upon the nature of the diluent and the temperature
selected. Typically, pressures in the range of about 30 to about
300 psia can be employed, preferably about 150 to 250 psia.
[0056] The heat exchanging fluid used to condense the vapor from
the first flash step is, as indicated above, at a temperature in
the range of about 40.degree. F. to 130.degree. F. A particularly
preferred embodiment uses a heat exchange fluid at a temperature of
moderate ambient conditions, for example, temperatures in the range
of 60.degree. to 100.degree. F., more preferably 86.degree. to 960
F.
[0057] A further understanding of the present invention will be
provided by referring to FIG. 11 which illustrates a system
comprising an embodiment of the invention.
[0058] In the embodiment illustrated in FIG. 11, the polymerization
is carried out in a loop reactor 110. The polymerization mixture is
circulated by agitator 111. Monomer and diluent are introduced
through conduits 114 and 116, respectively, connected to conduit
113. Catalyst is added through conduit 117. Normally catalyst is
introduced as a suspension in a hydrocarbon diluent.
[0059] Polymer slurry is removed from the loop to a settling leg
118. The slurry passes from settling leg 118 to conduit 119 and
into flash chamber 120. Conduit 119 has an indirect heat exchange
means such as a flash line heater 121. The flash chamber 120 as
illustrated includes in its lower end a gas distribution plate 122.
Heated diluent vapor provided via conduit 123 is passed into the
flash chamber 120 and through the distributor plate 122 in such a
fashion as to cause a fluidized bed of polymer solids to occur in
the flash chamber.
[0060] Vaporized diluent exits the flash chamber 120 via conduit
124 through which it is passed into a cyclone 125 which separates
entrained polymer particles from the vapor. Polymer particles
separated by the cyclone are passed via line 126 to a lower
pressure flash chamber 127.
[0061] The polymer particles in the fluidized bed are withdrawn via
conduit 128 and also passed into the lower pressure flash chamber
127. In flash chamber 127 substantially all the diluent still
associated with the polymer is vaporized and taken overhead via
conduit 129 to a second cyclone 130.
[0062] The major portion of the diluent associated with the polymer
solids as they leave settling leg 118 will have been taken to
cyclone 125 as vapor via conduit 124. The vapor after having a
substantial part of any entrained solids removed is passed via line
131 through a filter capable of removing any remaining polymer
fines. The vapor stream is then split. One portion is passed via
conduit 133 through a heat exchanger 134 wherein the vapor is
condensed by indirect heat exchange with a heat exchange fluid. The
condensed diluent is then passed to an accumulator 135 via conduit
136. Any uncondensed vapors and gases can be removed overhead from
the accumulator 135. A pump 137 is provided for conveying the
condensed diluent back to the polymerization zone.
[0063] The other portion of the diluent vapor is passed via line
138 through a blower 139 which forces the vapor into conduit 123 to
provide at least part of the diluent vapor needed to provide the
fluidized bed in flash chamber 120. The vapor that is passed into
conduit 123 is first passed through a heat exchange zone 140
wherein the vapor is heated if desired to provide part or all of
the heat needed for heating the polymer slurry provided by conduit
119.
[0064] The polymer solids in the lower pressure flash tank are
passed via line 141 to a conventional conveyor dryer 142 from which
the polymer can be packaged or otherwise handled while in contact
with the atmosphere.
[0065] The vapors exit the secondary cyclone 130 via line 143 to a
filter 144 such as a bag filter capable of removing any substantial
amounts of polymer fines. The filter vapor is then passed to a
compressor 145 and the compressed vapors are passed through conduit
146 to an air-fin cooler 147 wherein a portion of the compressed
vapors are condensed. The remaining vapors are passed through
conduit 148 to a condenser 149 where most of the remaining vapors
are condensed and the condensate is passed through conduit 150 to
knockout drum 151 or a fractionator. The condensed diluent can then
be removed via conduit 152 and recycled to the polymerization
process. Since the major portion of the diluent is recovered from
the intermediate pressure flash chamber, the load on compressor 145
is much lower than in prior art techniques of the type illustrated
in U.S. Pat. No. 3,152,872.
[0066] It is important to note that there are many variations of
the illustrated embodiment which fall within the scope of the
present invention. For example, it is within the scope of the
present invention to eliminate the flash line heater 121 and to
have all the heat supplied by the heated diluent vapor that is used
to provide a fluidized bed in flash chamber 120. Further, in some
instances, it may be desirable to have the cyclone 125 actually
present in the flash chamber rather than being connected to it by a
conduit. Still further, it is within the scope of the present
invention to eliminate the fluidized bed concept and to supply all
the heat needed by other means such as the flash line heater 121.
In such a modification, there obviously would no longer be a need
for the gas distributor plate 122.
[0067] It is noted that when recycled diluent vapor from the first
flash step is used as the fluidizing medium in the first flash
step, it can sometimes lead to alterations in the properties of the
polymer since it often will contain monomer that could react in the
flash step. Under such circumstances, it is thus preferred to use a
substantially pure heated diluent as the fluidizing medium or to
eliminate the fluidized bed concept and use flash line heaters to
provide all the necessary heat.
[0068] In regard to embodiments employing the fluidized bed
concept, experiments were conducted to determine the conditions
that would be most suitable for producing a fluidized bed of the
polymer particles. The particles employed were polyethylene
particles having sphericities in the range of about 0.55 to 0.60 as
determined by the Ergun equation as disclosed in Zenz, F. and D.
Othmer, Fluidization and Fluid-Particle Systems, New York;
Reinhold, 1960, p. 75. The Ergun equation is 1 P L gc = 150 ( 1 -
Em ) 2 Em 3 U O ( dp ) 2
[0069] where:
[0070] .DELTA.P=pressure drop over the bed length.
[0071] L=bed length.
[0072] gc=dimensional constant when units of force such as
lbs-force or Kg-force are used.
[0073] Em=porosity of packed bed.
[0074] .mu.=viscosity of flowing gas.
[0075] U.sub.o=gas superficial velocity (based on bed
cross-sectional area).
[0076] .phi..sub.s=sphericity of the particles.
[0077] dp=mean particle diameter for mixture.
[0078] For the polyethylene fluff particles having sphericities in
the range of about 0.55 to 0.60, it was determined that good
fluidization was obtained with the superficial velocity of the
fluidizing gas being in the range of about 0.4 to 0.8 ft/sec. It
was further noted that slugging of the bed was a problem when the
height of the bed was allowed to be more than about 3 times its
diameter. Generally, it would be preferable for the bed height to
be no greater than two times its diameter.
[0079] The preferred bed diameter and rate of feeding such a
polymer slurry can be calculated by the formula: 2 t = 750 D 3
W
[0080] where:
[0081] t=Residence time in minutes necessary for desired level of
diluent separation.
[0082] D=Bed diameter, ft.
[0083] W=Fluff feed rate, lb/hr.
[0084] Rate data obtained during measurement of equilibrium
isobutane absorption on polymer fluff indicated that 2 to 3 minutes
should be adequate for such polyethylene fluff. Thus, for a pilot
plant scale process producing 22 pounds per hour of fluff, a bed
diameter of at least about 4 inch would be preferred. For a
commercial process producing 17,500 pounds of fluff per hour, a bed
diameter of at least about 4 feet would be preferred. Residence
times greater than 10 minutes generally should not be
necessary.
[0085] The following example sets forth typical conditions that can
be used in a commercial scale process in employing the present
invention.
EXAMPLE
[0086] A typical ethylene homopolymerization process would be the
polymerization conducted at a pressure of about 650 psia and a
temperature of about 225.degree. F. The settling leg would be
operated to accumulate and discharge about 55 weight percent
solids. An example of such a process would result in a polymer
slurry product containing about 17,500 pounds per hour of
polyethylene and about 14,318 pounds per hour of isobutane diluent.
This slurry would then be flashed to 180 psia and 180.degree. F. to
vaporize the major portion of the diluent. The auxiliary heat
necessary to cause the effluent to be at 180.degree. F. after the
pressure drop to 180 psia can be supplied by preheating the
effluent, by heating recycled fluidizing diluent, or by a
combination of the two methods. About 90 percent of the diluent is
taken overhead from flash zone 120 at 180 psia. Even assuming that
there would be a further pressure drop between flash zone 120 and
accumulator 135, the isobutane diluent could readily be condensed
against 60.degree. to 80.degree. F. cooling water without
compression. The remaining 10 percent of the diluent and the fluff
are then passed into a lower pressure flash tank wherein they are
exposed to a pressure in the range of about 20 to 30 psia. The
diluent vapor from the lower pressure flash tank can then be
condensed using compression and cooling. The use of the preliminary
higher pressure tank results in a significantly lower compression
load than was required in the conventional process in which slurry
was immediately flashed to a pressure in the range of 20 to 30
psia.
* * * * *