Note: Descriptions are shown in the official language in which they were submitted.
D-17427-1 21931 4~
IMPROVEMENT IN FLUIDIZED BED REACTION SYSTEMS
USING UNSUPPORTED CATALYSTS
FIELD OF THE INVENTION
This invention relates to a method for improving the space time
yield of an exothermic polymerization reaction conducted in a fluidized
bed reactor employing an unsupported transition metal catalyst, by
increasing the removal of the heat of polymerization from the reactor
by cooling gases, continuously removed from the reactor, to a
temperature below the dew point temperature of such gases and
returning the resultant two phase fluid mixture into the reactor to
maintain the temperature of the fluidized bed at the desired level.
BACKGROUND OF THE INVENTION
The use of a condensing mode of operation in a continuous gas
fluidized bed polymerization was taught in U.S. Patent Nos. 4,543,399
and 4,588,790. That process provided cooling part or all of the recycle
stream to form a mixture comprising both a gas phase and a liquid
phase prior to reintroducing said stream into the reactor, where the
liquid portion of the recycle stream was vaporized. This invention
greatly improved the production rate and cooling capacity of the gas
phase process.
Recently, U.S. Patent Nos. 5,352,749 and 5,436,304 have
disclosed variations on the condensing mode operation.
In U.S. Patent No. 5,317,036 the use of condensing mode was
taught for unsupported soluble catalysts. Since that time, condensing
mode has also been disclosed for use with supported metallocene
catalysts in U.S. Patent Nos. 5,405,922 and 5,462,999.
It has now been found that the use of condensing mode with
unsupported soluble catalysts assists the migration of the unsupported
catalyst components in a m~nn~r not contemplated by the prior art.
219~147
D-17427-1
SUMMARY OF THE INVENTION
The present invention provides a process for increasing the
space time yield of polymer production in a fluidized bed reactor
employing an exothermic polymerization reaction by cooling the recycle
stream to below its dew point and returning the resultant two-phase
fluid stream to the reactor to maintain the fluidized bed at a desired
temperature above the dew point of the recycle stream. The cooling
capacity of the recycle stream is increased both due to the greater
temperature differential between the entering recycle stream and the
reactor and by the vaporization of the condensed liquids entrained in
the recycle stream.
The amount of condensation, and thus the increase in production
rate, can be further enhanced by altering the process conditions so as
to increase the dew point of the recycle stream.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention there is provided a
process for conducting condensing mode with unsupported catalysts.
While not limited to any particular type or kind of polymerization
reaction (so long as the reaction is of an exothermic nature), this
invention is particularly suited to polymerization reactions involving
the polymerization of one or more of the monomers listed below:
I. Olefin type: ethylene, propylene, butene-1, pentene-1, 4-
methylpentene-1, hexene-1, styrene.
II. Polar vinyl monomer type: vinyl chloride, vinyl acetate;
vinyl acrylate, methyl methacrylate, tetrafluoroethylene, vinyl ether,
acrylonitrile.
III. Diene type (conjugated and non-conjugated): butadiene,
1,4-hexadiene, isoprene, ethylidene norbornene.
IV. Acetylene type: acetylene, substituted acetylene, such as
methyl acetylene.
V. Aldehyde type: formaldehyde.
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D-17427-1
It is to be noted that the unsupported catalysts employable in
the fluidized bed polymerization of the above monomer types would,
respectively, most usually be as follows:
I. Coordinated anionic catalyst
II. Cationic catalyst for copolymers with ethylene only;
others of this type require a free-radical catalyst
III. Either a free-radical catalyst or a coordinated anionic
catalyst
IV. A coordinated anionic catalyst
V. An anionic catalyst
Preferably, the catalyst is of the type disclosed in U.S. Patent
Nos. 5,317,036; 5,405,922; 5,462,999 and U.S. Serial No. 08/412,964
filed March 29, 1995. Most preferably, the catalyst is the reaction
product of methylaluminoxane or modified methylaluminoxane and a
cyclopentadienyl or substituted-cyclopentadienyl zirconium tris alkyl
carbamate or carboxylate.
Although this invention is not limited to any specific type of
polymerization reaction, the following discussions of the operation of
the process are directed to polymerizations of olefin-type monomers
where the invention has been found to be especially advantageous.
In very general terms, a conventional fluidized bed process for
producing resins, particularly polymers produced from monomers, is
practiced by passing a gaseous stream cont~ining one or more
monomers continuously through a fluidized bed reactor under reactive
conditions and in the presence of a catalyst. The gaseous stream
cont~ining unreacted gaseous monomer is withdrawn from the reactor
continuously, compressed, cooled and recycled into the reactor.
Product is withdrawn from the reactor. Make-up monomer is added to
the recycle stream.
The polymer-forming reaction is exothermic, m~king it
necessary to maintain in some fashion the temperature of the gas
stream inside the reactor at a temperature not only below the resin
2193147
D-17427-1
and catalyst degradation temperatures, but at a temperature below the
fusion or st.il~king temperature of resin particles produced during the
polymerization reaction. This is necessary to prevent plugging of the
reactor due to rapid growth of polymer chunks which cannot be
removed in a continuous fashion as product. It will be understood,
therefore, that the amount of polymer that can be produced in a
fluidized bed reactor of a given size in a specified time period is directly
related to the amount of heat which can be withdrawn from the
fluidized bed.
In accordance with this invention the recycle gas stream is
intentionally cooled to a temperature below the dew point of the recycle
gas stream to produce a two-phase, gas-liquid mixture under
conditions such that the liquid phase of said mixture will remain
entrained in the gas phase of said mixture at least from the point of
entry into the fluidized bed reactor until volatilized or until passage
into the fluidized bed. A substantial increase in space time yield
results from the practice of this invention with little or no change in
product properties or quality. When practiced as described herein the
overall process proceeds continuously and smoothly and without
unusual operational difficulties.
It may be desirable in some instances to raise the dew point of
the recycle gas stream to further increase heat removal. The recycle
stream dew point can be increased by: (1) raising the operating
pressure of the reaction system; (2) increasing the concentration of
condensible fluids in the recycle stream; and/or (3) reducing the
concentration of non-condensible gases in the recycle stream. In one
embodiment of this invention, the dew point of the recycle stream may
be increased by the addition of a condensible fluid to the recycle stream
which is inert to the catalyst, reactants, and the products of the
polymerization reaction. The fluid can be introduced into the recycle
stream with the make-up fluid or by any other means or at any other
D-17427-1 21931~7
- 5-
point in the system. Examples of such fluids are saturated
hydrocarbons, such as butanes, pentanes or hexanes.
A primary limitation on the extent of condensing is the solubility
and softening effect of the condensible component on the polymer
particles. This is affected by the choice of condensing agent, its
concentration, the cycle gas composition reaction conditions such as
temperature and pressure, and the molecular weight, density and
chain br~nching distribution of the polymer. Stable condensing
operation has been achieved at low and high fluidized bulk densities
despite the teachings of U.S. Patent No. 5,352,749.
In practical experience, condensing levels of 20 wt. % and above
have been demonstrated while not exceeding the softening limit of the
polymer.
The entry point for the two-phase, recycle stream preferably is
below the fluidized bed (polymerization zone) to ensure uniformity of
the upwardly flowing gas stream and to maintain the bed in a
suspended condition. The recycle stream cont~ining entrained liquid is
introduced into the reactor at a point in the lower region of the reactor
and most preferably at the very bottom of the reactor to ensure
uniformity of the fluid stream passing upwardly through the fluidized
bed. Other points of entry include any number of ports along the side
of the reactor directly into the fluidized bed, with release being at the
wall or through a pipe or tube into the body of the bed. Likewise, the
entry point can be into the bed from a pipe extending through the
distributor plate. Atomization or spray nozzles can be used to
distribute the flow into the bed.
A baffle or simil~r means for preventing regions of low gas
velocity in the vicinity of the recycle stream entry point may be
provided to keep solids and liquids entrained in the upwardly flowing
recycle stream. One such means includes the annular disk as taught
in U.S. Patent Nos. 4,877,587 and 4,933,149.
21931~7
D-17427-1
Although there is no apparent advantage in doing so, the two-
phase, recycle stream can be divided into two or more separate streams
one or more of which can be introduced directly into the polymerization
zone provided that sufficient gas velocity below and through the bed is
provided to keep the bed suspended. In all cases the composition of the
gas stream is kept essentially uniform and flowing in a manner such
that there are no dead spaces in the bed where unremovable solids can
form.
It will be apparent that if desired, it is possible to form a two-
phase fluid stream within the reactor at the point of injection by
separately injecting gas and liquid under conditions which will produce
a two-phase stream. Little advantage is seen in operating in this
fashion due to the added and unnecessary burden and cost of
separating the gas and liquid phases after cooling. In WO 94/28032
published December 8, 1994, a contrary view is expressed as to the
advantage gained through this mode of operation. It may however, be
desirable to inject make-up monomer into the reactor in this fashion.
The injection of liquid or gaseous make-up monomer at the point of
entry of the two-phase recycle stream or elsewhere in the reactor or in
the recycle stream is contemplated by this invention.
The advantages of this invention are not limited to the
production of polyolefin resins. This invention can be practiced in
connection with any exothermic polymerization process carried out in a
gas phase fluidized bed. The advantages of this invention over
conventional processes will generally increase in direct relation to the
nearness of the dew point temperature of the recycle stream to the
reaction temperature within the interior of the fluid bed.
The applicability of this invention to the production of any given
polymer can be determined by the use of the following formula:
X = P H r~n
GmaSs Gpgas (Trxn - T limit)
21931~7
~ ~ D-17427-1
P = desired polymer production rate; constrained to rates
giving X less than 1.0 without subject invention.
Hrxn = heat of polymerization of specific polymer being
produced.
Gmass = mass flow rate of recycle stream; limited to a minimum
value by the need for adequate fluidization and mixing
in the bed and to a m~ximum value by entrainment of
solids. Specific minimz~ and mzlxim~ depend on
numerous factors known to those skilled in the art.
CPgas = heat capacity of the recycle stream.
Trxn = temperature of the reaction zone (fluid bed); has
m~ximum value depending on the sti(~king
temperature of the polymer at the pressure of the
recycle stream and/or the catalyst performance, and a
minimum value which is dependent on catalyst
performance.
Tlimit = minimum temperature of the recycle stream entering
the reaction zone. This temperature is either the dew
point of the recycle stream or the cooling limit of the
heat ex~h~nge zone, whichever is higher. If Tlimit is
the recycle stream dew point, the invention is
practiced by simply cooling the stream to a
temperature below its dew point. If Tlimit is
controlled by the heat exch~n~e zone, the invention is
practiced by adding a condensable fluid to increase the
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D-17427-1
dew point of the recycle stream to a temperature above
the cooling limit of the heat exchange zone.
Whenever the value of X is greater than 1, the use of this
invention will afford a benefit and as the value of X increases, the
greater are the benefits which can result from this invention.
In general, the height to diameter ratio of the reaction zone
within a reactor varies within the range of about 2.7:1 to about 4.6:1.
The range, of course, can vary to larger or smaller ratios depending
upon the desired production capacity. The cross-sectional area of the
velocity reduction zone is typically within the range of about 2.6 to
about 2.8 multiplied -by the cross-sectional area of the reaction zone.
The reaction zone includes a bed of growing polymer particles,
formed polymer particles and a minor amount of catalyst particles
fluidized by the continuous flow of polymerizable and modifying
gaseous components in the form of make-up feed and recycle fluid
through the reaction zone. To maintain a viable fluidized bed, the
superficial gas velocity through the bed must exceed the minimum flow
required for fluidization, and preferably is at least 0.2 ft/sec above
minimum flow. Ordinarily, the superficial gas velocity does not exceed
5.0 ft/sec and usually no more than 2.6 ft/sec is sufficient.
It is essential that the bed always contain particles to prevent
the formation of localized "hot spots" and to entrap and distribute the
particulate catalyst throughout the reaction zone. On start up, the
reactor is usually charged with a base of particulate polymer particles
before gas flow is initiated. Such particles may be identical in nature
to the polymer to be formed or different therefrom. When different,
they are withdrawn with the desired formed polymer particles as the
first product. Eventually, a fluidized bed of desired polymer particles
supplants the start-up bed.
The partially or totally activated precursor composition and/or
catalyst used in the fluidized bed is preferably stored for service in a
~ `~ D-17427-1 21931~7
g
reservoir under a blanket of a gas which is inert to the stored material,
such as nitrogen or argon.
Fluidization is achieved by a high rate of fluid recycle to and
through the bed, typically in the order of about 50 times the rate of
feed of make-up fluid. The fluidized bed has the general appearance of
a dense mass of individually moving particles as created by the
percolation of gas through the bed. The pressure drop through the bed
is equal to or slightly greater than the weight of the bed divided by the
cross-sectional area. It is thus dependent on the geometry of the
reactor.
Make-up fluid can be fed directly to the bed but more often is fed
to the recycle line. When fed to the recycle line it is generally fed
before or after either the heat exchanger or cycle gas cooler. The
composition of the make-up stream is determined by a gas analyzer.
The gas analyzer determines the composition of recycle stream and the
composition of the make-up stream adjusted accordingly to maintain
an essentially steady state gaseous composition within the reaction
zone.
The gas analyzer can be a conventional gas analyzer which rates
in conventional m~qnner to indicate recycle stream composition and
which is adapted to regulate the feed and is commercially available
from a wide variety of sources. The gas analyzer may also be one of the
more sophisticated, rapid gas analyzers as disclosed in U.S. Patent No.
5,437,179. Generally, the gas analyzer can be positioned so as to
receive gas from a point between the velocity reduction zone and heat
ex~h~nger.
To ensure complete fluidization, the recycle stream and, where
desired, part of the make-up stream are returned through the recycle
line to the reactor below the bed. There may be preferably a gas
distributor plate above the point of return to aid in fluidizing the bed.
In passing through the bed, the recycle stream absorbs the heat of
reaction generated by the polymerization reaction.
~ ~ D-17427-1 21931~7
- 10-
At times, the use of aluminoxane has appeared to contribute to
the generation of static electricity. Both induced and natural
condensing mode operation has been shown to control static. It is
believed that this static "dissipation" results primarily from the
instantaneous feed rate of the condensing medium, such as,
isopentane, to the reactor rather than from the condensation of the
isopentane already in the reactor. Thus, changes in the feed rate of the
condensing medium can dramatically affect the reactor static. By
feeding a spray of the condensing medium directly to the bottom of the
distributor plate more liquid hits the plate at a given overall reactor
condensing medium concentration than would hit the plate if the
condensing medium were fed to the bottom head or recycle line. The
advantage of doing this is that a lower amount of condensing medium
is required and thus it is easier to avoid flooding the fluidized bed.
The portion of the fluidizing stream which does not act in the
bed constitutes the recycle stream which is removed from the
polymerization zone, preferably by passing it into velocity reduction
zone above the bed where entrained particles are given an opportunity
to drop back into the bed.
The recycle stream is then compressed in a compressor and then
passed through a heat e~ch~nge zone wherein the heat of reaction is
removed before it is returned to the bed. The heat exchange zone is
typically a heat e~chz~nger which can be of the horizontal or vertical
type. The recycle stream is then returned to the reactor at its base and
to the fluidized bed through a gas distributor plate. A gas deflector is
preferably installed at the inlet to the reactor to prevent contained
polymer particles from settling out and agglomerating into a solid
mass. The annular disk referred to earlier is one means of
accompli~hing this.
The temperature of the bed is controlled at an essentially
constant temperature under steady state conditions by constantly
removing the heat of reaction. No noticeable temperature gradient
` ~ D-17427-1 ~19~117
appears to exist within the upper portion of the bed in polyethylene. In
polypropylene a small temperature gradient across the upper bed of
about 1 to 2C is not unusual. A temperature gradient will exist in the
bottom of the bed in a layer of about 6 to 12 inches, between the
temperature of the inlet fluid and the temperature of the remainder of
the bed.
Good gas distribution plays an important role in the operation of
the reactor. The fluidized bed contains growing and formed particulate
polymer particles, as well as catalyst particles. As the polymer
particles are hot and possibly active, they must be prevented from
settling, for if a quiescent mass is allowed to exist; any active catalyst
contained therein may continue to react and cause fusion. Diffusing
recycle fluid through the bed at a rate sufficient to maintain
fluidization through the bed is, therefore, important.
A gas distribution plate is a preferred means for achieving good
gas distribution and may be a screen; slotted plate, perforated plate, a
plate of the bubble-cap type and the like. The elements of the plate
may all be stationary, or the plate may be of the mobile type disclosed
in U.S. 3,298,192. Whatever its design, it must diffuse the recycle fluid
through the particles at the base of the bed to keep the bed in a
fluidized condition, and also serve to support a quiescent bed of resin
particles when the reactor is not in operation.
The preferred type gas distributor plate is generally of the type
which is fabricated from metal and which has holes distributed across
its surface. The holes are normally of a diameter of about 1/2 inch.
The holes extend through the plate, and over each hole there is
positioned a triangular angle iron which is fixedly mounted to plate.
The angle irons serve to distribute the flow of fluid along the surface of
the plate so as to avoid st~gn~nt zones of solids. In addition they
prevent the resin from flowing through the holes when the bed is
settled.
D-17427-1 219~147
- 12-
Any fluid inert to the catalyst and reactants can also be present
in the recycle stream. An activator compound, if utilized, is preferably
added to the reaction system downstream from heat exch~nger or
directly into the fluid bed, possibly with a carrier fluid such as the
condensing agent or a liquid monomer.
It is essential to operate the fluid-bed reactor at a temperature
below the sintering temperature of the polymer particles to ensure that
sintering will not occur. The sintering temperature is a function of
resin density. In general, polyethylene low-density resins, for example,
have a low sintering temperature and polyethylene high-density
resins, for example, have a higher sintering temperature. For
example, temperatures of from about 75_C to about 95_C are used to
prepare ethylene copolymers having a density of from about 0.91 g/cm3
to about 0.95 g/cm3, while temperatures of from about 100' C to about
115_C are used to prepare ethylene copolymers or homopolymers
having a density of from about 0.95 g/cm3 to about 0.97 g/cm3.
The fluid-bed reactor may be operated at pressures of up to
about 1000 psi, and is for polyolefin resin production preferably
operated at a pressure of from about 100 psi to about 100 psi, with
operation at the higher pressures in such ranges favoring heat transfer
since an increase in pressure increases the unit volume heat capacity
of the gas.
The partially or totally activated precursor composition and/or
catalyst (hereinafter collectively referred to as catalyst) is injected into
the bed at a rate equal to its consumption. Preferably, the catalyst is
injected at a point in the bed where good mixing of polymer particles
occurs. Injecting the catalyst at a point above the distribution plate is
an important feature for satisfactory operation of a fluidized bed
polymerization reactor. Since catalysts are highly active, injection of
the catalyst into the area below the distributor plate may cause
polymerization to begin there and eventually cause plugging of the
distributor plate. Injection into the fluidized bed, instead, aids in
- ~ D-17427-1 2193147
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distributing the catalyst throughout the bed and tends to preclude the
formation of localized spots of high catalyst concentration which may
result in the formation of "hot spots." Injection of the catalyst into the
reactor above the bed may result in excessive catalyst carryover into
the recycle line where polymerization may begin and plugging of the
line and heat exch~nger may eventually occur.
The catalyst can be injected into the reactor by various
techniques. Not all modes of spraying the unsupported catalysts were
found to be equally effective. In some operational modes spraying
catalyst directly into the bed coated the resin particles and caused
them to grow uncontrollably. Several modes of preferred operation
were found. One mode involved separately feeding the organometallic
complex and the activator. Yet another preferred mode resulted from
feeding the combined components into the reactor but allowing
sufficient time for the sprayed particles to begin evaporation before
contacting the bed particles. In yet another preferred mode the two
components were fed in a carrier that allowed them to begin losing
solubility while they were being fed and resulted in nucleating active
catalysts as the feed was entering the reactor. It is believed that the
use of an atomization nozzle into the bed with a large oversized
support tube flow tends to prevent fouling. Similarly, the use of an
atomization nozzle through the distributor plate so that catalyst is fed
just above the plate also tends to prevent fouling. At this stage it is
not clear which is the best.
One problem encountered with soluble catalyst feed is that the
catalyst droplets collide with polymer powder in the fluid bed and coat
the polymer particles. Subsequent polymerization leads to powder
particles of larger and larger size. This effect is greatly attenuated and
m~n~ged by forming a solid catalyst particle from the reaction of the
metallocene compound and aluminoxane compound as they are being
fed to the reactor.
- ~ D-17427-1 21931~7
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The metallocene catalyst, is often provided as a dilute solution
in aliphatic or aromatic hydrocarbon such as toluene or isopentane.
The aluminoxane can also be in an aliphatic or aromatic hydrocarbon
such as iisopentane or toluene. Each are continuously added to the
reactor (or intermittently), and are premixed prior to entering with a
carrier fluid of isopentane or other inert hydrocarbon. Additionally, an
inert gas such as nitrogen can be added to act as a carrier gas for
spraying or dispersing the catalyst into the reactor vessel. The
metallocene compound and the aluminoxane compound react under
this scenario to form the active catalyst species which may be insoluble
in the isopentane carrier. This precipitates as a fine solid of only a few
microns in diameter (or less) which is conveyed the rest of the way into
the reactor as a slurry in isopentane. The reaction and subsequent
precipitation appears to be very fast. The adduct is soluble in toluene,
and although there may be toluene present, there is much more
isopentant so that it precipitates. This improves the morphology of
polymer prepared by the soluble metallocene catalysts feed to gas
phase reactors. The catalyst and aluminoxane compound can react to
form a particle insoluble in the carrier fluid, and this particle acts as
the template for polymerization leading to polymer powder having good
morphology, good resin bulk density and good flow characteristics.
Fouling of the reactor is also reduced. Use of this templated liquid feed
catalyst reduces fouling when operating over the full range of
condensing-mode .
Similarly, aluminoxane or other activator in a toluene solution
may be used and can be precipitated by contacting with the carrier
isopentane. This can be done prior to where the catalyst mixes with
the isopentane carrier, at the same location the catalyst is added the
carrier, or downstream of where the catalyst is injected into the carrier
isopentane. The catalyst particles can be sprayed into the reactor
using an atomization nozzle with isopentane and nitrogen as the
carrier fluids. The spray can be directed upwards from the bottom of
- ~ D-17427 1 21931~
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the reactor with no distributor plate in place. Therefore, there is only a
small distance that the particles must travel before impacting the bed.
In this time, the carrier fluid has evaporated leaving the catalyst
particles precipitated in clusters. Even if the carrier does not
evaporate, the precipitated catalyst particles tend to disperse. Feeding
catalyst in this m~nner is not a criteria for this invention. If can be
feed also directly to the bed if provisions are made to properly disperse
the catalyst at the polymer tip.
Alternatively, at least some organometallic complexes and
aluminoxane activators need not be precontacted to give good catalyst
productivity and resin or good morphology. Specifically what has been
demonstrated is that via specialized feeding the catalyst components
into the reactor new discrete resin particles are formed in the fluidized
bed without run-away resin particle agglomeration. Operation in
condensing mode, or even with high levels of condensable components
while not in condensing, is beneficial in that it allows the solvation and
migration of the catalyst precursors throughout the polymer being
formed. (The precursors must find each other to make the active
catalyst for polymerization). A dramatic increase in catalyst
productivity can be obtained as the isopentane concentration is
increased so that the cycle gas dew point increases from 20 to 33C.
This demonstrates the benefit of the invention at higher isopentane
levels, particularly when operating in condensing mode. Very high
levels of condensing, say above 25 wt% may be of further benefit in
assisting catalyst precursor migration and the establishment of the
active site.
As noted above, reaction of the catalyst precursors to form an
insoluble adduct in the condensing component may provide the
template for polymer growth and particle replication. Experience
shows that the primary resin particles are generally spherical in shape
and solid with a diameter r~nging from about 10 to 100 microns.
These particles may be agglomerated into larger particles. One
- D-17427 1 2193147
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advantage of the spraying of the catalyst into the reactor is the ability
to control the size of the particles generated. Via control of the spray
nozzle design and the velocity of the carrier or carriers a large range of
particle sizes can be achieved. This is especially advantageous when
b~l~nçing catalyst productivity needs with thermal sensitivity aspects
of catalysts. It is also beneficial to increase the size of polymer
particles when lowering the bed volume when transitioning between
products during production runs.
There are several additional advantages for adducts that are
soluble. One is that as the sprayed particle's solvent evaporates the
desolubilized components can form a template which can lead to
improved morphology. Additionally, the initial polymerization tends to
form a rigid framework that can also serve as a template for the resin
particle.
Unsupported catalysts are not required to be bound to a non-
mobile attachment like a supported catalyst. The mobility of the
unsupported catalyst is enhanced by operating in the condensed mode
as the liquid helps disperse the catalyst components allowing improved
mixing of the components and high efficiency of catalyst formation.
Further diffusion of catalyst within the particle is aided by swelling of
the polymer by the condensing agent. The organometallic complex and
activator are more uniformly dispersed throughout the resin particles,
resulting in improved catalyst productivity and product uniformity
(homogeneity). This concept of swelling the polymer aiding
catalyst/activator dispersion also applies to an insoluble adduct
catalyst if the individual components that must react to form the
insoluble catalyst are themselves soluble in the condensing media.
Another way to achieve the benefit of this invention is to use an
activator that precipitates or gellates in the polymerization vessel (or
prior to placement in the reactor) prior to reaction with the procatalyst.
This precipitation can be caused by formation of a discontinuous phase
by the condensing media. For example, regular aluminoxane is often
21931~7
D-17427-1
provided in toluene and is insoluble in isopentane. It can be
precipitated in the feed line by mi~ing with, for example, and
isopentane stream. It may also precipitate in the reactor as the
toluene is dispersed via mi~ing with the condensing agent which is
often isopentane. Operation in condensing mode will lead to the
aluminoxane's precipitation. Other ways to precipitate the
aluminoxane include reaction with a flocculating agent such as a
disfunctional molecule, a polar substance (e.g. MgCl2) or a surfactant.
The precipitated aluminoxane will then act as an excellent template
for polymer replication and growth. Operation at high condensing
levels aids in swelling the polymer and the dispersion of the
metallocene procatalyst across the aluminoxane particle.
Modified aluminoxane is soluble to some degree in isopentane,
yet it may lose solubility rapidly in the absence of the condensing
medium, such as liquid isopentane. This condition may exist in the
fluid bed of polymer particles even during condensing mode operation.
The condensed liquid does not necessarily penetrate the entire height
of the bed, so that there is a "dry" region at the top and some length
down into the bed. The constant movement of the procatalyst,
activator and newly formed resin particle into and out of "dry" and
"wet" zones may improve dispersion of the catalyst components
throughout the particle and also provide a stable template for particle
growth without runaway agglomeration. In theory, the extent of
condensing can be used to control the resin particle size and
morphology.
Addition to a "dry" zone should provide rapid nucleation and
better particle characteristics. Addition to a "wet" zone should provide
improved catalyst productivity, cooling of the particle to prevent hot
spotting, fouling reduction and control of resin agglomeration.
The aluminoxane may also be provided in supercritical solvent
such as ethylene or ethane (there is an obvious advantage for using
21931~7
D-17427-1
- 18-
ethylene since it is a monomer). The aluminoxane would precipitate
very quickly forming a template for polymer replication.
The aluminoxane can be first added through an atomization
nozzle to the bottom cone with the distributor plate removed, and the
metallocene cocatalyst can be added by an injection tube through the
side of the reactor to the bed. The aluminoxane feed can also be moved
directly to the fluid bed with a distributor plate in place. The catalyst
feed tube is preferably fed into the bed about six inches below the
aluminoxane feed tube, but their locations can be reversed or moved
further apart. These can be reversed, separated further or brought
closer together to effect the process or catalyst advantages. Simple
necked down injection tubes can also be used with appropriate carrier
liquid and gas flows to each. It is important to note that precontacted
procatalyst and alllminox~ne often leads to massive fouling of such
injection tubes when inserted directly into the fluid bed.
Additionally, aluminoxane might be added directly to the cycle
line before the distributor plate. Condensing operation may help in
flushing the aluminoxane into the reactor if added to the cycle line.
Solution feeding of homogeneous polymerization catalysts to a
fluidized bed can be complicated by coating of existing particles with
new polymer growth, or by "skinning over" of droplets which leads to
lower activity. This can be avoided by segregating the catalytically
active sites into discrete domains, separated by regions that are
catalytically inactive. This results in m~ximum surface-to-volume
ratio for the incipient catalyst sites which leads to m~ximum rates.
Catalyst segregation can be achieved in several ways. One way
is to use emulsions of a given catalyst component, such as, an emulsion
of methylaluminoxane in a saturated hydrocarbon. The emulsions can
be created and stabilized by a number of techniques in order to
favorably influence the nature of the ensuing polymerization. An
example of emulsion creation would be to add mineral oil to a toluene
solution of aluminoxane and then strip the toluene.
21931~7
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Another way to achieve catalyst segregation would be to premix
the catalyst components and then add a co-solvent in which the
resulting mixture is insoluble. An example of this is where the
metallocene is mixed with aluminoxane/toluene and mineral oil.
Another way to achieve catalyst segregation is to add a
component that reacts with one of the catalyst components and causes
them to become insoluble. An example of such a component might be a
di- or tri-functional molecule such as ethylene glycol, which would lead
to cross-linking and subsequent insolubility of the aluminoxane.
Another way to achieve catalyst segregation would be to add
small amounts of an attractant component, which through favorable
intermolecular interactions causes one or more catalyst components to
cluster about this component.
The catalyst is preferably fed into the reactor at a point from
about 20 to 50 percent of the reactor diameter away from the reactor
wall and below the fluidized bed or at the bed at a height of up to about
50 percent of the height of the bed. The catalyst may li_ewise be fed at
or right above the distributor plate at a point from about 20 to 50
percent of the reactor diameter away from the reactor wall and at any
of a multiple of locations in or right above the distributor plate.
A gas which is inert to the catalysts such as ethane, nitrogen or
argon, is preferably used to carry the catalyst into the bed.
The rate of polymer production in the bed depends on, among
other things, the rate of catalyst injection, the amount of condensing
medium and the concentration of monomer(s) in the recycle stream.
The production rate is conveniently controlled by simply adjusting the
rate of catalyst injection.
Since any change in the rate of catalyst injection will change the
reaction rate and hence rate of generation of the heat of reaction, the
temperature of the recycle stream entering the reactor is adjusted
upwards and downwards to accommodate any change in the rate of
heat generation. This ensures the maintenance of an essentially
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constant temperature in the bed. Complete instrumentation of both
the fluidized bed and the recycle stream cooling system is, of course,
useful to detect any temperature change in the bed so as to enable
either the operator or a conventional automatic control system to make
a suitable adjustment in the temperature of the recycle stream.
Under a given set of operating conditions, the fluidized bed is
maintained at essentially a constant height or weight by withdrawing
a portion of the bed as product at the rate of formation of the
particulate polymer product. Since the rate of heat generation is
directly related to the rate of product formation a measurement of the
temperature rise of the fluid across the reactor (the difference between
inlet fluid temperature and exit fluid temperature) is indicative of the
rate of particulate polymer formation at a constant fluid velocity if no
vaporizable liquid is present in the inlet fluid.
On discharge of particulate polymer product from the reactor, it
is desirable and preferable to separate fluid from the product and to
return the fluid to the recycle line. There are numerous ways known to
the art to accomplish this. See U.S. Patent No. 4,543,399. Another
preferred product discharge system which may be alternatively
employed is that disclosed in U. S. Patent No. 4,621,952. Such a
system employs at least one (parallel) pair of tanks comprising a
settling tank and a transfer tank arranged in series and having the
separated gas phase returned from the top of the settling tank to a
point in the reactor near the top of the fluidized bed.
The fluidized-bed reactor is equipped with an adequate venting
system to allow venting the bed during start up and shut down. The
reactor does not require the use of stirring and/or wall scraping. The
recycle line and the elements therein should be smooth surfaced and
devoid of unnecessary obstructions so as not to impede the flow of
recycle fluid or entrained particles.
Among the polymers which may be produced in the process of
the present invention are homopolymers of ethylene, propylene, butene
D-17427-1 21 93147
or copolymers of a major mole percent of ethylene- propylene or butene
and a minor mole percent of one or more C2 to C8 alpha-olefins. The
C2 to C8 alpha-olefins preferably should not contain any br~nching on
any of their carbon atoms which is closer than the fourth carbon atom.
The preferred C2 to C8 alpha-olefins are ethylene, propylene, butene-l,
pentene-l, hexene-l, 4-methylpentene-1 and octene-l.
The ethylene polymers, for example, have a melt flow ratio of
over about 22. The melt flow ratio value is another means of
indicating the molecular weight distribution of a polymer. A melt flow
ratio (MFR) of 22 thus, for example, corresponds to a Mw/Mn value (as
determined by conventional size exclusion chromatography) of about
2.7.
The ethylene homopolymers have a density of about > 0.958 to <
0.972 gm/cc.
The ethylene copolymers have a density less than about 0.96
gm/cc. The density of the ethylene copolymer, at a given melt index
level for the copolymer, is primarily regulated by the amount of the C3
to C8 comonomer which is copolymerized with the ethylene. In the
absence of the comonomer, the ethylene would homopolymerize to
provide polymers having a density of about <0.96. Thus, the addition
of progressively larger amounts of the comonomers to the copolymers
results in a progressive lowering of the density of the copolymer. The
amount of each of the various C3 to C8 comonomers needed to achieve
the same result will vary from monomer to monomer, under the same
reaction conditions.
Thus, to produce binary copolymers of ethylene with the same
density and melt index, larger molar amounts of the different
comonomers would be needed in the order of C3>C4>Cs>C6>C7>Cg.
When made in the fluid-bed process described herein, ethylene
polymers are granular materials which have a settled bulk density of
about 15 to 32 pounds per cubic foot and an average particle size of the
order of about 0.005 to about 0.10 inches preferably about 0.06 to 0.10
21931~1
~ D-17427-1
inches. Particle size is important for the purposes of readily fluidizing
the polymer particles in the fluid-bed reactor, as herein described.
Whereas the exact scope of the instant invention is set forth in
the appended claims, the following specific examples illustrate certain
aspects of the present invention and, more particularly, point out
methods of evaluating the same. However, the examples are set forth
for illustration only and are not to be construed as limitations on the
present invention except as set forth in the appended claims. All parts
and percentages are by weight unless otherwise specified.
Example 1 and lA
Catalyst
rl5-indenyl zirconium tris(diethyl carbamate) catalyst as a 0.025
Molar solution in toluene. Approximately 1 liter of catalyst is prepared
and placed in a continuously stirred vessel under a purified nitrogen
blanket at 400 psi pressure. Catalyst is withdrawn from the vessel
and added to the reactor continuously using a high pressure syringe
pump.
The catalyst feed rate of 7.5 cc/hr of catalyst solution is
controlled by the syringe pump. Flows of isopentane and nitrogen at
rates of 0.8 lb/hr and 7.0 lb/hr respectively carry the catalyst into the
reactor.
The catalyst and carriers entered the reactor through a 1/8 inch
injection tube with the tip diameter reduced to 0.041 inch to aid in
atomization and dispersion. The injection tube is inserted 2.0 feet
above the plate and about 3 inches into the bed.
Activator
Modified methyl aluminoxane (MMAO-3A) as a 13 wt% solution
in isopentane (Akzo Nobel, diluted from a 26 wt% solution) is added at
a rate of 100 cc/hr to the reactor through a 1/8 inch injection tube with
the tip diameter reduced to 0.041 inch. Isopentane carrier at a rate of
- ~ D-17427-1 21931~7
- 23-
0.8 lb/hr and nitrogen carrier at a rate of 5.0 lb/hr aided the MMAO-
3A's dispersion and atomization in the reactor. The MMA0-3A is
added to the bed 1.5 ft above the distributor plate (6 inches below the
catalyst feed tube) and about 4 inches into the bed.
Reactor
The 14 inch diameter reactor is used with an 85 lb bed weight
and an approximate bed height of 8 ft. Monomers and other cycle gas
components are added to the reactor at the compressor case behind the
impeller. An additional isopentane allows the control of the cycle gas
dew point independently of the catalyst and activator carrier flows.
Start-Up
The reactor is opened prior to the experiment. To prepare it for
polymerization, the reactor and a LLDPE seed bed are dried to about
10 ppm water in the cycle gas using hot nitrogen. About 500 cc of 5
wt% TiBA in isopentane is circulated for an hour and the reactor
vented to flare prior to introducing monomer. MMAO-3A addition is
started about 30 minutes before catalyst feed begins.
Conditions
Total Pressure, psig 375
Ethylene Partial Pressure, psi 180
Hydrogen Partial Pressure, psi 0
Hexene Partial Pressure, psi 5.9
Isopentane Partial Pressure, psi22.0 (5.65 mole %)
Nitrogen Partial Pressure, psi 182
H2/C2 Mole Ratio 0.0
C6/C2 Mole Ratio 0.015
Bed Temperature, C 70
Inlet Gas Temperature, C 69.5
Cycle Gas Dew Point, C 41.2
. ~ D-17427-1 2~3147
- 24-
Cycle Gas Velocity, ft/sec 1.20
Production Rate, lb/hr 16
Average Residence Time, hr 5.3
STY, lb/hr/ft3 2.0
Veratrole (1,2-dimethoxy benzene) is added to the cycle gas line
at the compressor suction to reduce fouling of the cycle gas line,
compressor and cycle gas cooler. The feed rate is about 50 cc/hr of a
0.01 wt % solution in isopentane.
Resin
A polymer is produced with an I2 melt index of 0.86 dg/min, an
I21 flow index of 16.2 dg/min, and a density of 0.9265 g/cc. The resin
particle size is 0.075 inch, the resin bulk density is 18.5 lb/ft3 and the
fluidized bulk density is 9.5 lb/ft3.
The comparative commercial performance at a 30 C inlet gas
temperature corresponds to a condensing level of 8.7 wt%.
Condensing levels for a range of inlet gas temperatures are
s-lmm~rized below:
Inlet Gas Temp. C Liquid Condensed, wt%
6.8
8.7
12.0
-10 19.3
Increasing the isopentane concentration from 5.65 mole% (22
psi) to 8 mole% (31.2 psi) allows for 20.4 wt% condensing with a 10 C
inlet gas temperature.
Further increasing the isopentane concentration to 11 mole%
(42.9 psi) achieves 22.6 wt% condensing with a 25 C inlet gas
. `~ D-17427-1 2193147
- 25-
temperature. This temperature is well within the capability of a
commercial UNIPOL(~ plant with refrigerated cooling.
For conditions used in both examples above, the level of nitrogen
is decreased to accommodate the additional isopentane, resin bulk
properties changes slightly and the dew point is higher.
