U.S. patent application number 10/066012 was filed with the patent office on 2003-06-12 for solids concentration in slurry polymerization.
Invention is credited to Kufeld, Scott E., Marek, Garry A., Obath, George O., Qualls, Wesley R., Tait, John H..
Application Number | 20030109651 10/066012 |
Document ID | / |
Family ID | 23801589 |
Filed Date | 2003-06-12 |
United States Patent
Application |
20030109651 |
Kind Code |
A1 |
Kufeld, Scott E. ; et
al. |
June 12, 2003 |
Solids concentration in slurry polymerization
Abstract
An olefin polymerization apparatus is disclosed wherein monomer,
diluent and catalyst are circulated in a continuous pipe loop
reactor and an intermediate product slurry is recovered by a
continuous product take off means. The intermediate product slurry
is passed to a hydrocyclone to stratify polymer particles based on
size. A slurry containing predominantly fines is withdrawn overhead
and returned to the reaction zone. The slurry containing
predominantly fines is preferably returned to the polymerization
zone just upstream of the circulation pump and in a preferred
embodiment this pressure differential is the sole driving force for
the hydrocyclone separation. In another embodiment the product
slurry from the bottom of the hydrocyclone is passed to a
solids-liquid separator where the solids settle to the bottom and
essentially polymer free diluent is recycled. In another
embodiment, the intermediate product slurry is withdrawn via a
conical settling leg. Because of the converging nature of the leg,
as polymer is withdrawn the velocity increases progressively going
from the entry to the exit of the leg thus avoiding polymer
buildup.
Inventors: |
Kufeld, Scott E.; (Houston,
TX) ; Tait, John H.; (Stafford, TX) ; Obath,
George O.; (Houston, TX) ; Qualls, Wesley R.;
(Houston, TX) ; Marek, Garry A.; (Dayton,
TX) |
Correspondence
Address: |
CHEVRON PHILLIPS CHEMICAL COMPANY LP
LAW DEPARTMENT - IP
P.O BOX 4910
THE WOODLANDS
TX
77387-4910
US
|
Family ID: |
23801589 |
Appl. No.: |
10/066012 |
Filed: |
January 31, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10066012 |
Jan 31, 2002 |
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09453675 |
Dec 3, 1999 |
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6420497 |
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Current U.S.
Class: |
526/73 ;
526/88 |
Current CPC
Class: |
C08F 210/16 20130101;
C08F 210/14 20130101; C08F 4/24 20130101; C08F 2/14 20130101; C08F
210/16 20130101; C08F 210/16 20130101; C08F 210/16 20130101 |
Class at
Publication: |
526/73 ;
526/88 |
International
Class: |
C08F 002/00 |
Claims
That which is claimed is:
1. A polymerization process comprising: introducing into a
continuous elongated polymerization zone at least one monomer,
catalyst and a diluent under polymerization conditions which
include sufficient pressure to maintain said diluent in the liquid
state, thus producing a slurry of polymer particles in said
diluent; creating a zone of lower pressure and a zone of higher
pressure to circulate said slurry through said elongated
polymerization zone; withdrawing a portion of said slurry, said
portion comprising withdrawn liquid diluent and withdrawn solid
polymer particles comprising fines and larger particles as an
intermediate product of said process; introducing said withdrawn
liquid diluent and withdrawn solid polymer particles into a
separation zone; withdrawing a recycle slurry concentrated in said
fines from a top portion of said separation zone and passing said
recycle slurry back to said polymerization zone; and withdrawing a
product slurry concentrated in said larger particles from a bottom
portion of said separation zone; and passing all of said product
slurry to downstream processing.
2. A process according to claim 1 wherein said at least one monomer
is at least one olefin monomer and wherein said withdrawing a
portion of said slurry is done on a continuous basis.
3. A process according to claim 2 wherein said recycle slurry is
passed to said zone of lower pressure, wherein said withdrawing a
portion of said slurry is done from said zone of higher pressure,
wherein said withdrawn liquid diluent and withdrawn solid polymer
particles are introduced into said separation zone tangentially and
wherein the pressure differential is the sole driving force for
said separation.
4. A process according to claim 2 wherein said at least one monomer
comprises ethylene and 0.01-5 weight per cent of a higher 1-olefin,
said diluent comprises isobutane, said catalyst comprises chromium
oxide, said fines have a particle size of less than 150 microns and
wherein said product slurry has at least 50 per cent lower fines as
compared with said intermediate product of said process.
5. A process according to claim 2 wherein said recycle slurry and
said product slurry are controlled so as to give a recycle
comprising 20-40 weight per cent of said withdrawn solid polymer
particles introduced into said separation zone.
6. A process according to claim 2 wherein said product slurry
concentrated in said larger particles is passed to a solids-liquid
separation zone and wherein liquid diluent essentially free of
polymer is withdrawn from an upper zone of said solids-liquid
separation zone and recycled to said polymerization zone and
wherein a concentrated product slurry is withdrawn from a bottom
zone of said solid-liquid separation zone.
7. A process according to claim 6 wherein said concentrated product
slurry is passed to a heated zone.
8. A process according to claim 7 wherein said slurry of polymer
particles in diluent in said polymerization zone is maintained at a
concentration of at least 40 weight per cent solid olefin polymer
particles based on the weight of said polymer particles and the
weight of said diluent.
9. A process according to claim 7 wherein said concentrated product
slurry is heated in said heated zone to give a heated concentrated
product slurry which is passed to a single flash zone wherein a
major portion of diluent therein is vaporized and thus separated
from solid polymer particles in said heated concentrated product
slurry, the thus separated diluent thereafter being condensed for
recycle, without any compression, by heat exchange.
10. A process according to claim 7 wherein said concentrated
product slurry is heated in said heated zone to give a heated
concentrated product slurry, said process comprising in addition:
passing said heated concentrated product slurry to a high pressure
flash zone where it is exposed to a pressure drop such that a major
portion of diluent therein is vaporized thus leaving solid polymer
particles and entrained diluent; withdrawing said solid polymer
particles and entrained diluent from a bottom zone of said high
pressure flash zone; and passing the thus withdrawn solid polymer
particles and entrained diluent to a low pressure flash zone.
11. A polymerization process comprising: introducing into a
continuous elongated polymerization zone at least one monomer,
catalyst and diluent under polymerization conditions, thus
producing a slurry of polymer particles in said diluent;
circulating said slurry through said elongated polymerization zone;
withdrawing a portion of said slurry, said portion comprising
withdrawn diluent and withdrawn solid polymer particles comprising
fines and larger particles as an intermediate product of said
process; introducing said withdrawn diluent and said withdrawn
solid polymer particles into a separation zone; withdrawing a
recycle slurry concentrated in said fines from a top portion of
said separation zone passing said recycle slurry back to said
polymerization zone; withdrawing a product slurry concentrated in
said larger particles from a bottom portion of said separation
zone; passing said product slurry concentrated in said larger
particles to a solids-liquid separation zone; withdrawing diluent
essentially free of polymer from an upper zone of said
solids-liquid separation zone and recycling the thus withdrawn
diluent to said polymerization zone; and withdrawing a concentrated
product slurry from a bottom zone of said solids-liquid separation
zone.
12. A process according to claim 11 wherein said monomer comprises
ethylene and 0.01-5 weight percent 1-hexene based on the total
weight of said ethylene and said hexene, wherein said diluent is
isobutane and wherein said catalyst comprises chromium oxide.
13. A process according to claim 12 wherein said polymerization
zone is maintained liquid full.
14. A process according to claim 13 wherein said withdrawing a
portion of said slurry is done on a continuous basis.
15. A polymerization process comprising: polymerizing, in a loop
reaction zone, at least one olefin monomer in a liquid diluent in
the presence of catalyst to produce a fluid slurry comprising
liquid diluent and solid olefin polymer particles; maintaining a
concentration of said solid olefin polymer particles in said fluid
slurry in said reaction zone of greater than 40 weight per cent
based on the weight of said polymer particles and the weight of
said liquid diluent; providing a conical settling zone having an
upper zone in open communication with said fluid slurry and a lower
settling zone; periodically opening communication between said
lower settling zone and a downstream processing zone to thus
withdraw the resulting product slurry.
16. A loop reactor apparatus comprising: a plurality of vertical
pipe segments; a plurality of upper lateral pipe segments; a
plurality of lower lateral pipe segments; wherein each of said
vertical pipe segments is connected at an upper end thereof to one
of said upper lateral pipe segments, and is connected at a lower
end thereof to one of said lower lateral pipe segments thus
defining a continuous flow path adapted to convey a fluid slurry,
said reactor being substantially free from internal obstructions;
means for introducing monomer reactant, polymerization catalyst and
diluent into said reactor; impeller means for continuously moving
said slurry along said flow path; at least one hollow appendage for
withdrawing product slurry; a hydrocyclone having an inlet in an
upper portion thereof, a recycle outlet in a top portion thereof
and a product slurry withdrawal line in a bottom portion thereof; a
heated flash line; a first conduit means connecting said at least
one hollow appendage with said inlet of said hydrocyclone adapted
to convey product slurry to said hydrocyclone; a second conduit
means adapted to convey recycle from said recycle outlet back to
said reactor at a location just upstream of said impeller; and a
third conduit means adapted to convey bottoms slurry from said
bottom portion to said heated flash line.
17. An apparatus according to claim 16 wherein said at least one
hollow appendage is an elongated conduit adapted to continuously
withdraw product slurry.
18. An apparatus according to claim 16 wherein said at least one
hollow appendage is cone shaped.
19. A loop reactor apparatus comprising: a plurality of vertical
pipe segments; a plurality of upper lateral pipe segments; a
plurality of lower lateral pipe segments; wherein each of said
vertical pipe segments is connected at an upper end thereof to one
of said upper lateral pipe segments, and is connected at a lower
end thereof to one of said lower lateral pipe segments thus
defining a continuous flow path adapted to convey a fluid slurry,
said reactor being substantially free from internal obstructions;
means for introducing monomer reactant, polymerization catalyst and
diluent into said reactor; impeller means for continuously moving
slurry along said flow path; at least one hollow appendage for
withdrawing product slurry; a hydrocyclone having an inlet in an
upper portion thereof, a recycle outlet in a top portion thereof
and a product slurry withdrawal line in a bottom portion thereof; a
solid-liquid separator having an inlet, a diluent recycle outlet in
an upper portion thereof and a product outlet in a bottom portion
thereof; a first conduit means connecting said at least one hollow
appendage with said inlet of said hydrocyclone adapted to convey
product slurry to said hydrocyclone; a second conduit means adapted
to convey recycle from said recycle outlet back to said reactor at
a location just upstream of said impeller; and a third conduit
means adapted to convey bottoms slurry from said bottom portion of
said hydrocyclone to said inlet of said solid-liquid separator; and
a forth conduit means adapted to convey separated diluent from said
diluent recycle outlet back to said reactor.
20. An apparatus according to claim 19 comprising in addition: a
heated flash line; and a fifth conduit adapted to convey product
slurry from said product outlet of said solid-liquid separator to
said heated flash line.
Description
BACKGROUND OF THE INVENTION
[0001] This invention relates to withdrawing a slurry of a solid in
a liquid from a flowing stream of the slurry.
[0002] 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.
[0003] One problem presented by this technique relates to the
matter of "fines." The produced polymer is in the form of particles
of varying sizes suspended in the diluent. The smaller particles
are referred to as fines. The fines may be the result of many
factors. Some polymer particles form during a single pass through
the reactor loop and exit the first time they come to a take off
point. Such particles may be smaller because of their shorter time
in the reaction zone. Other particles make varying numbers of loops
before being withdrawn. For these particles, some may be physically
broken up by contact with the pump impeller. Others may be in
diluent which is heated by friction at the impeller surface to the
point that bubbles are formed, which bubbles later collapse and
fracture polymer particles. Other fines may be the result of
smaller catalyst particles.
[0004] The mechanism by which the polymer is formed involves a
catalyst site adding together literally thousands of monomer units
in a fraction of a second and thereafter terminating the polymer
chain and starting another. Hence, the small particles do not
represent lower molecular weight material--the molecular weight is
controlled by well known process variables. Thus the polymer
molecules making up large particles and small particles are of
essentially the same molecular weight. Rather, the fines are
undesirable because they interfere with downstream polymer
finishing.
[0005] All of this is to be distinguished from fines that may
result downstream in the polymer finishing operation where diluent
containing a small amount of low molecular weight (and hence
soluble) polymer is flashed with the result that the low molecular
weight material precipitates out as low molecular weight fines.
Because they are in admixture with vapor these low molecular weight
fines can be separated from the vapor with a cyclone separator.
[0006] Cyclone separators which separate solids from vapors are to
be distinguished from hydrocyclone separators, sometimes referred
to as liquid separators. These hydrocyclone separators stratify
solid particles in a liquid slurry based on particle size.
[0007] The settling legs themselves also can present problems.
First, they represent the imposition of a "batch" technique onto a
basically 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, conventional
settling legs have sections in which polymer can collect while
waiting for the next dump cycle and such collected polymer can melt
over time and deposit on the inside walls of the settling leg.
[0008] In spite of these limitations, settling legs continue to be
employed. 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.
[0009] Slurry can be withdrawn on a continuous basis by taking
advantage of the fact that without the periodic upsets caused by
settling leg firings, a higher overall reactor concentration can be
achieved.
SUMMARY OF THE INVENTION
[0010] It is an object of one aspect of this invention to separate
fines from a slurry reactor effluent by means of a hydrocyclone and
recycle the fines to the reaction zone;
[0011] It is another object of this invention to reduce the energy
required to separate diluent from product polymer solids; and
[0012] It is yet another object of this invention to avoid polymer
build-up in the take off means;
[0013] In accordance with one embodiment of this invention, slurry
comprising polymer and diluent is withdrawn from a flowing stream
and passed to a hydrocyclone where fines and a portion of the
diluent are separated from the remaining slurry as overhead and
recycled to the reaction zone. In accordance with one aspect of
this embodiment, the hydrocyclone bottoms slurry is passed to a
solid-liquid separator where additional diluent is separated from
the polymer. In accordance with another aspect of this embodiment,
the hydrocyclone bottoms slurry is passed through a heated flash
line prior to further downstream processing.
[0014] In accordance with another embodiment of this invention, an
olefin polymerization is carried out in a loop reaction zone under
conditions of high solids concentration and a portion of the
circulating slurry is withdrawn through a tapered settling leg.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] In the drawings, forming a part hereof,
[0016] FIG. 1 is a schematic perspective view of a loop reactor
having a take off means and a downstream polymer recovery
system;
[0017] FIG. 2 is a cross section of the hydrocyclone of FIG. 1;
[0018] FIG. 3 is a cross section of the solid-liquid separator of
FIG. 1;
[0019] FIG. 4 is a partial cross section of the circulation pump
area of the reactor showing the impeller;
[0020] FIG. 5 is a side view partly in section of a portion of a
reactor loop of FIG. 1 showing one embodiment of a continuous take
off mechanism in greater detail;
[0021] FIG. 5A is a view similar to FIG. 5 except the take off
mechanism comes off from a vertical leg of the reactor;
[0022] FIG. 6 is a side view showing a conventional settling
leg;
[0023] FIG. 7 is a side view showing a tapered take off means;
[0024] FIG. 8 is schematic side view of a reactor leg showing
multiple take off means; and
[0025] FIG. 9 is a schematic side view showing a solid-liquid
separator which can be used in a preferred embodiment of the
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0026] Surprisingly in accordance with this invention the
imposition of a hydrocyclone between an effluent take off and
downstream solids-diluent separation results in better quality
product (because of drastically reduced fines) with little or no
increase in energy usage because the reactor pressure differential
between the take off point and the pump suction drives this
step.
[0027] By fines is meant any arbitrary delineation between larger
particles which are desired for final processing and smaller
particles which it is desired to recycle back to the reactor.
Generally, fines are those particles which have a diameter of
100-200 microns or less. One common definition is those particles
which will go through a 100 mesh screen, i.e. less than about 150
microns (5.9 mils) in diameter. The operational set point of the
hydrocyclone can be based on the percentage of polymer going
overhead and the percentage taken out the bottom of the
hydrocyclone. For instance the recycle diluent can carry from about
1-75, preferably 20-50, more preferably 20-40 weight per cent of
the polymer, the remaining portion going out in the diluent removed
from the bottom. Alternatively a suitable set point is that at
which there is a 50/50 weight per cent split in particles of an
arbitrary size within, say, the 100 to 200 micron range, between
fines going overhead and the larger particles going out the bottom.
For instance, 150 microns could be chosen such that 50 weight per
cent of particles of that size went overhead and 50 weight per cent
went out the bottom with the product slurry. At sizes progressively
smaller than that arbitrary set point a progressively greater than
50 weight per cent proportion goes overhead. Similarly, at
progressively larger sizes, a progressively greater proportion goes
out the bottom in the product slurry. Thus the recycle diluent is
concentrated in fines and the product slurry concentrated in larger
particles.
[0028] By "concentrated in fines" is meant having a significantly
higher concentration of fines (at least about 5 wt. per cent
higher, generally at least 50 wt. per cent higher) than was present
in the feed to the hydrocyclone. By "concentrated in larger
particles" is meant having a significantly lower concentration of
fines (at least about 5 wt. per cent lower, preferably at least 25
wt. per cent lower, generally at least 50 per cent lower) than was
present in the feed to the hydrocyclone.
[0029] Referring now to the drawings, there is shown in FIG. 1 a
loop reactor 10 having vertical pipe segments 12, upper lateral
pipe segments 14 and lower lateral pipe segments 16. These upper
and lower lateral pipe segments define upper and lower zones of
horizontal or generally lateral (as opposed to straight vertical)
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.
As shown here, all of the upper segments and two of the lower
segments are continuously curved and the remaining two lower
segments are straight pipes connected at each end to a vertical
segment by the smooth bend or elbow. The continuously curved
segments can be simply two elbows connected together. Reference
herein to lateral pipe segments is meant to include two 90 degree
elbows affixed together, a smoothly curved segment or a straight
pipe connected at each end by an elbow to a vertical pipe. The
polymerization mixture is circulated by means of impeller 22 (shown
in FIG. 4) 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 (take off means) for taking off an intermediate
product slurry is designated broadly by reference character 34.
[0030] Withdrawn slurry effluent is conveyed via effluent
withdrawal line 36 and preferably introduced into hydrocyclone 38,
valve 58 thus being open and valve 62 closed. Diluent carrying
smaller particles of polymer (fines) passes via fines recycle line
40 through valve 60 to a zone of reduced pressure within the
reactor caused by the suction of the pump as described in greater
detail hereinafter in the description of FIG. 2 and FIG. 4. Slurry
carrying the larger particles is withdrawn through open valve 44
via product line 42.
[0031] At this point this product slurry can pass directly to
downstream processing via line 50, valve 52 being open and valve 46
being closed. Alternatively the product slurry can pass to
solid-liquid separator 48, valve 46 being open and valve 52 closed.
Generally, all of the bottoms slurry will either be passed to
solid-liquid separator 48 or all of it will be passed directly to
downstream processing via line 54. However, in a less preferred
option a portion could be passed to separator 48 and a portion via
conduits 54 and 56 directly to flash line heater 72.
[0032] In solid-liquid separator 48, the solids settle to the
bottom as described in greater detail in the description of FIG. 3.
Liquid (essentially diluent) is pumped out by means of recycle line
pump 66 and recycled to the reactor via separator liquid recycle
line 64. The thus concentrated slurry is then passed via
concentrated slurry line 68 through valve 70 to heated flash line
56 for diluent separation and polymer recovery as will be described
in detail hereinafter.
[0033] FIG. 2 shows hydrocyclone 38 in greater detail.
Hydrocyclones are well known in the art and can be readily obtained
from commercial sources. A hydrocyclone with an internal diameter
of 25-30 inches is satisfactory, for instance. The effluent slurry
withdrawn can be introduced directly into the hydrocyclone via
entry port 92 which is preferably tangential. The entry port is in
the upper portion of the hydrocyclone, i.e. above the mid point,
but below the actual top portion of the hydrocyclone. The inertia
carries the larger particles in a downward spiral path
predominantly adjacent to the outer walls and into the lower
section, which is generally conical as shown, where they collect
and are withdrawn off the bottom as a product slurry via line 42.
The smaller particles are pulled into central vortex finder 94 and
exit the top as a slurry in the diluent withdrawn via fines recycle
line 40. There are no moving parts in the hydrocyclone and
generally the separation is entirely driven by the pressure
differential between the pressure in the reactor at the point of
the take off means 34 and the suction zone of the circulating pump.
If necessary a small pump could be used as a minor supplement to
drive the fines recycle stream. If desired, diluent could be added
via line 37 (FIG. 1) to adjust the slurry concentration to the
optimum level for the hydrocyclone, but preferably this is not
done.
[0034] FIG. 3 shows in detail the solid-liquid separator.
Solid-liquid separators are well known in the art and can be
readily obtained from commercial sources. Slurry carrying
predominantly larger polymer particles and diluent is introduced
via product line 42 through entry 95 which is preferably a
tangential entry as shown. As the mass of slurry flowing into the
separator reaches the larger diameter portion it slows down and the
solids settle to the bottom where they are withdrawn via
concentrated slurry line 68. Liquid largely free of solids is
pumped out by pump 66 and returned to the reactor via separator
recycle line 64 which is located in an upper portion of the
solid-liquid separator, i.e. above the mid point but below the top
and below the entry port.
[0035] FIG. 4 shows in greater detail the reactor circulating pump
means for continuously moving the slurry along its flow path. As
can be seen in this embodiment the impeller 22 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 12 psi, generally
about 12.5 psi. As much as 50 psi 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. This figure shows fines
recycle line 40 entering at the suction end of the pump.
[0036] 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
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. The units for the
pressure are ft/ft which cancel out. This assumes the density of
the slurry which generally is about 0.45-0.6 g/cc.
[0037] The vertical segments are generally 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 (or generally lateral) segments
25-30 feet in flow path length. Any number of loops can be employed
in addition to the eight depicted 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.
[0038] Alternatively the longer axis can be disposed
horizontally.
[0039] 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.
[0040] Reactor slurry flow rate is generally within the range of
10,000 to 40,000, preferably 25,000 to 35,000 gallons/minute. The
average time for the slurry to make one complete pass through the
reaction zone is generally within the range of 20 to 90, preferably
30 to 60 seconds.
[0041] Referring now to FIG. 5, there is shown the smooth curve of
lower pipe segment 16 having associated therewith a take off
mechanism 34 in the form of a continuous take off. As shown, the
mechanism comprises a take off cylinder 96 attached, in this
instance, at a tangent to the outer surface of curved pipe segment
16. Coming off cylinder 96 is slurry withdrawal line 36. Disposed
within the take off cylinder 96 is a ram valve 97 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.
Emergency shut off valve 98 allows for an emergency shut down and
proportional motor valve 99 regulates flow. Take off cylinder 96
can have an internal diameter in the range of about 1-8 inches,
preferably 1.5-3 inches, most preferably about 3 inches.
[0042] It is noted that there are orientation concepts here. One is
the attachment angle of the take off cylinder, i.e. tangential as
in FIG. 5 or perpendicular as in FIG. 5A and FIG. 8 or any angle
between these two limits of 0 and 90 degrees.
[0043] Another is the radial angle from the center plane of the
longitudinal segment. This angle is preferably 0 or about 0.
[0044] Commercial production of predominantly ethylene polymers in
isobutane diluent using settling legs has historically been limited
to a maximum solids concentration in the reactor of 37-40 weight
percent for high 0.936-0.970 (more typically 0.945-0.960) density
ethylene polymers with values as high as 42-46 weight per cent
possible with maximized process enhancements. With lower
(0.900-0.935 more typically 0.920-0.935) density polymers values as
high as 36-39 are possible with process enhancements (but still
using settling legs). Whatever the maximum for a given set of
process conditions, improvement in solids concentration is possible
simply by taking the slurry off continuously.
[0045] It must be emphasized that in a commercial operation as
little as a one percentage point increase in solids concentration
is of major significance. With lower density ethylene polymers
where the starting point is 36-39 weight per cent solids in the
reactor, similar increases (i.e. at least 10, or even 15 percentage
points) can be achieved.
[0046] FIG. 5A shows a take off in the form of a continuous take
off attached laterally at a 90 degree angle. The take off location
is just downstream of the circulation means and the recycle return
just upstream so as to give the greatest possible pressure
differential to drive the hydrocyclone.
[0047] FIG. 6 shows a conventional settling leg 34A coming off a
lower pipe section 16. Valves 33A and 35A operate as follows. Block
valve 33A is open continuously except when closed for settling leg
maintenance. Polymer take off valve 35A is closed for a period of
time, generally about 0.25-3 minutes, preferably about 0.5 to 1.5
minutes, more preferably about 1 minute. Then valve 35A is opened
to allow the accumulated slurry to exit. Such settling legs can
vary somewhat in size but are generally about 6-10 inches in
internal diameter, preferably about 8 inches, with the block valve
being of similar size. Polymer take off valve 35A is generally
about 3 inches in internal diameter.
[0048] FIG. 7 shows a take off means 34B located at the bottom of a
downflow loop segment 12 although it could be located anywhere
along a lower pipe segment 16. Valves 33B and 35B operate in the
same manner as valves 33A and 35A. However the take off means is in
the shape of a cone with the upper zone in open communication with
the fluid slurry in the reaction zone and a lower settling zone
from which polymer slurry can be removed periodically. In this way
as the exit valve 35A is opened and the slurry contained in the
cone shaped settling leg exits it flows down the tapered walls and
increases in velocity thus inhibiting build up of polymer on the
walls.
[0049] The tapered take off legs can have an upper internal
diameter within the range of 4-24 inches, preferably 6-10 inches,
more preferably about 8 inches. Alternatively the size can be
defined relative to the internal diameter of the reactor pipes with
the upper diameter of the tapered take off thus being from about
25-50 per cent of the reactor pipe internal diameter. In all
instances the internal diameter of the upper end of the tapered
take off means 34B is at least twice, generally about four times,
the internal diameter of the lower end. The lower end of the
tapered take off can have an internal diameter within the range of
1-8, preferably 1.5-3, more preferably about 2-3 inches.
[0050] FIG. 8 shows a lower pipe section 16 having a plurality of
take off means 34 C. These can be continuous take off means as
depicted in FIG. 5. In an alternative embodiment of the invention,
each of these take off means can be a conventional settling leg as
shown in FIG. 6, or, preferably a tapered settling leg as shown in
FIG. 7. With a plurality of intermittent take off means the flow to
the hydrocyclone can be essentially continuous.
[0051] In another embodiment of the invention, a continuous take
off means can utilize the cone shaped shape of FIG. 7 and valves
33A and 35A are simply left open except during a total shutdown of
the take off means. In this embodiment of this invention the
conical leg takes the place of the take off cylinder 96 of FIG. 5
(except in this embodiment it would generally be disposed
vertically). By simply taking a product slurry effluent stream off
continuously, a small but significant increase in reactor solids
concentration (at least 1 wt. per cent) is made possible because
the absence of upsets in the flowing slurry stream caused by the
periodic "firing" of a batch settling leg. This absence of upsets
also allows operating at higher circulation velocities which gives
an additional small, but significant, increase reactor solids
concentration.
[0052] This embodiment is less preferred because with a continuous
take off, polymer build up is not a significant problem.
[0053] FIG. 9 shows a second solid-liquid separator, 41, which can
be used in an alternative embodiment of the invention. In this
embodiment, fines recycle line 40 can go to this separator instead
of going directly back to the reactor. In separator 41 the fines
are separated from the diluent with the diluent being taken
overhead via line 43. Line 43 can go directly back to the reactor
via valve 60 for instance or can connect with any downstream line
such as 42, 64, or 56. In this way fines can be separately
collected via line 45 and passed to any conventional finishing
operation. Such a fines removal line is not generally needed,
however since most of the fines have active catalyst cites and, on
reintroduction into the reactor grow to normal size. Hence there is
no need to separately remove fines to prevent gradual build up of
an unacceptably high level of fines in the reactor.
[0054] Referring now back to FIG. 1, the slurry in conduit 56 is
passed to a high pressure flash chamber 76. Conduit 56 includes a
surrounding conduit 72 which is provided with a heated fluid which
provides indirect heating to the slurry material in flash line
conduit 56 This constitutes a flash line heater. The high pressure
flash chamber (zone) can be operated at a pressure within the range
of 100-1500 psia (7-105 kg/cm.sup.2), preferably 100-275 psia (7-19
kg/cm.sup.2), more preferably 125-200 psia (8.8-14 kg/cm.sup.2).
The high pressure flash chamber (zone) can be operated at a
temperature within the range of 100-250 F. (37.8-121 C.),
preferably 130-230 F. (54.4-110 C.), more preferably 150-210 F.
(65.6-98.9 C.). The narrower ranges are particularly suitable for
polymerizations using 1-hexene comonomer and isobutane diluent,
with the broader ranges being suitable for higher 1-olefin
comonomers and hydrocarbon diluents in general.
[0055] The low pressure flash chamber (zone) can be operated at a
pressure within the range of 15-65 psia (1.05-4.57 kg/cm.sup.2),
preferably 16-45 psia (1.12-3.16 kg/cm.sup.2) more preferably 18-25
psia (1.27-1.76 kg/cm.sup.2). The low pressure flash chamber (zone)
can be operated at a temperature within the range of 100-250 F.
(37.8-121 C.), preferably 130-230 F. (54.4-110 C.), more preferably
150-210 F. (65.6-98.9 C.). Generally the temperature in the low
pressure flash chamber zone will be the same or 1-20 F. (0.6-11 C.)
below that of the high pressure flash chamber zone although
operating at a higher temperature is possible. The narrower ranges
are particularly suitable for polymerizations using 1-hexene
comonomer and isobutane diluent, with the broader ranges being
suitable for higher 1-olefin comonomers and hydrocarbon diluents in
general.
[0056] Vaporized diluent exits the flash chamber 76 via conduit 78
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 utilize 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 and entrained liquid
are withdrawn from high pressure flash chamber 76 via line 82 for
further processing using techniques known in the art. Preferably
they are passed to low pressure flash chamber 84 and thereafter
recovered as polymer product via line 86. The entrained liquid
(primarily diluent) flashes overhead and passes through compressor
88 to line 78 thus forming combined line 90. This high pressure/low
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.
[0057] Thus in accordance with one embodiment of the invention, a
hydrocyclone is positioned between a continuous take off and a
flash line heater.
[0058] In accordance with more specific embodiments of this
invention, the continuous take off and hydrocyclone are operated in
conjunction with a high pressure/low pressure flash system. The
continuous take off not only allows for higher solids concentration
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. 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 subsequent pressure drop is more efficient because of the
continuous flow thus giving better cooling. Because of the
hydrocyclone, the solids are not only more concentrated but are
also are predominantly large particle size.
[0059] Alternatively, the reactor effluent can be passed directly
to the low pressure flash chamber 84 via line 85. When operating
with both flash chambers, valve 87 is closed and valves 74, 79 and
89 are open. However in accordance with this alternative, valves
74, 79 and 89 are closed and valve 87 is open or else no high
pressure flash chamber is present at all.
[0060] The use of the hydrocyclone in combination with the
solid-liquid separator lends itself especially well to going
directly to the low pressure flash and compress the small amount of
diluent present. In this single flash embodiment, the flash line
heater formed by conduit 72 can be eliminated; if desired, however,
the flash line heater can be used in conjunction with a single
flash chamber (i.e. flash chamber 84) which can be operated at the
typical pressure for the low pressure zone.
[0061] In accordance with another embodiment of the invention, take
off means 34 is tapered and the slurry periodically withdrawn is
passed directly through the flash line heater via lines 36 and 54.
In this embodiment, solids concentration in the reactor is
maintained at greater than 40 weight per cent. The high solids
concentration makes the process more efficient and the tapered
shape of the settling leg inhibits polymer build up.
[0062] 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 99
being used to control the rate of continuous withdrawal to maintain
the total reactor pressure within designated set points.
[0063] Throughout this application, the weight of catalyst is
disregarded since the productivity, particularly with chromium
oxide on silica, is extremely high.
[0064] The invention is of primary utility in olefin
polymerizations 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 20, 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.
[0065] Suitable diluents (as opposed to solvents or monomers) are
well known in the art and include hydrocarbons which are inert or
at least essentially inert and liquid under reaction conditions.
Suitable hydrocarbons include isobutane, n-butane, propane,
n-pentane, i-pentane, neopentane and n-hexane, with isobutane being
especially preferred.
[0066] 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. Also suitable are organometal catalysts
including those known in the art as "Ziegler" or "Ziegler-Natta"
catalysts.
[0067] While this invention has been described in detail for the
purpose of illustration, it is not to be construed as limited
thereby, but is intended to cover all changes within the spirit and
scope thereof.
* * * * *