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Description  |
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BACKGROUND OF THE INVENTION
This invention relates to an improved liquid-gas condensation tower and
particularly relates to a recycle gas scrubbing tower useful in a gas
phase olefin polymerization system.
A conventional recycle gas scrubber used in a gas phase olefin
polymerization system such as used in U.S. Pat. No. 3,971,768,
incorporated herein by reference, comprises a condensation tower designed
to separate quench liquid, monomer, hydrogen and polymer fines. A typical
tower comprises a vertical cylindrical chamber in which unseparated
off-gas material from a gas-phase olefin polymerization reactor is
introduced, and condensor trays positioned at various heights along the
chamber. Gaseous material is removed from the upper portion of the tower
while residual liquid material, including polymer fines, is removed from
the bottom of the tower. It has been observed that in using a conventional
scrubber apparatus in a gas phase olefin polymerization system, lumps of
polymer formed in the condensation tower and caused fouling and pluggage
in downstream equipment. An improved scrubber apparatus which avoids
formation of polymer lumps would prevent many unplanned unit shutdowns.
SUMMARY OF THE INVENTION
In an apparatus, such as a liquid-gas condensation tower, in which a liquid
phase is withdrawn rapidly through an exit port from a chamber, an
improvement which assures complete agitation throughout the liquid phase
comprises an array of liquid agitation devices positioned within the
liquid and above a cone-shaped vortex breaker structure which is placed
above the exit port.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a view of a condensation tower system showing the apparatus of
this invention.
FIG. 2 shows in more detail a side view of the vortex breaker and eductor
array of this invention.
FIG. 3 is a top view of the vortex breaker and eductor array of this
invention.
FIG. 4 shows an eductor useful in this invention.
BRIEF DESCRIPTION OF THE INVENTION
The invention described herein is an apparatus useful in separating
components of off-gas from a gas phase olefin polymerization system. Such
apparatus of this invention, commonly called a recycle scrubbing tower,
comprises a condensation tower designed to separate reactor off-gas
components in which is positioned a cone-shaped vortex breaker and a
series of liquid mixing devices. In this invention the cone-shaped vortex
breaker is placed in the bottom of the condensation unit to prevent vortex
formation, prevent active polyolefin powder accumulation, and improve
liquid flow patterns in the bottom of the tower. Mixing devices, such as
mixing eductors, are placed below the normal liquid level in the tower,
cooperatively positioned above the cone-shaped vortex breaker and oriented
such that liquid circulation prevents polyolefin accumulation. The action
of the mixing devices together with the cone-shaped vortex breaker is to
reduce static areas in the tower where polymer accumulations may form.
In greater detail the vortex breaker used in this invention is a
cone-shaped structure typically attached to the bottom of the tower. By
the term "cone-shaped" we mean a geometric structure with a round or
multi-sided base tapering upward to a peak. The top of the structure
typically contains a purge hole so that the "cone-shaped" structure
appears slightly truncated. Purge holes usually are placed at the top and
bottom of the structure for pressure equalization to maintain structural
integrity of the cone-shaped device. However, the structure should not
have any substantial horizontal areas where polymer powder could
accumulate. A pyramidal structure with a base other than a circle, ellipse
or the like is considered within our invention.
The mixing devices useful in this invention are capable of moving liquid in
the tower bottom such that accumulation of polymer powder is prevented.
The preferred mixing device is a mixing eductor in which liquid under
pressure is passed through a nozzle which entrains liquid from the tower
bottom and discharges the mixed liquids from the eductor. Such devices are
especially suitable for mixing liquids containing powdered solids.
Gas phase reactor systems in which this invention is useful include both
stirred bed reactors and fluidized bed reactor systems. Examples of such
reactor systems are described in U.S. Pat. Nos. 3,957,448, 3,965,083,
3,971,768, 3,970,611, 4,129,701, 4,101,289, 3,652,527, and 4,003,712, all
incorporated by reference herein.
The gas phase olefin polymerization reactor systems in which the present
invention is useful comprise a reactor vessel to which olefin monomer and
catalyst components can be added and which contain a bed of forming
polymer particles. Typically, catalyst components are added together or
separately through one or more valve-controlled ports in the reactor
vessel. Olefin monomer, typically, is provided to the reactor through a
recycle gas system in which unreacted monomer removed as off-gas and fresh
feed monomer are mixed and injected into the reactor vessel.
A quench liquid can be added to polymerizing olefin in order to control
temperature. In propylene polymerization, the quench liquid can be liquid
propylene. In other olefin polymerization reactions, quench liquid can be
a liquid hydrocarbon such as propane, butane, pentane or hexane,
preferably, isobutane or isopentane. Depending on the specific reactor
system used, quench liquid can be injected into the reactor vessel above
or within the bed of polymer particles.
The polyolefins produced in a gas phase olefin polymerization system using
this invention include polymers of alpha-olefins and substituted
alpha-olefins, ethylene polymer, propylene polymer, copolymers of ethylene
and propylene, and copolymers of ethylene or propylene with other
copolymerizable alpha-olefins. Many such polyolefins typically are
prepared by contacting an olefin monomer with a catalyst comprising an
organoaluminum compound and a transition metal compound.
Useful organoaluminum compounds include trialkylaluminum, dialkylaluminum
halides, mixtures of trialkylaluminum with dialkylaluminum halides and
mixtures of trialkylaluminum with alkylaluminum dihalides. Also
catalytically effective amounts of mixtures of trialkylaluminum and
dialkylaluminum halides can be used in conjunction with alkylaluminum
dihalides. Useful halides include bromides and chlorides and useful alkyl
radicals contain from two to about six carbon atoms. The preferable halide
is chloride and the preferable alkyl radical is ethyl. Diethylaluminum
chloride (DEAC) is most preferable in propylene polymerizations. In a
trialkylaluminum-dialkylaluminum halide mixture, the preferred amount of
trialkylaluminum is about 20 to 50 mol percent. In a
trialkylaluminum-alkylaluminum dihalide mixture, the preferred amount of
trialkylaluminum is about 30 to 70 mol percent and most preferably about
40 to 60 mol percent.
The transition metal compounds useful as a component in the catalyst system
of this invention are compounds of transition metals of Groups IVB, VB and
VIB of the Periodic Table. Preferably, the transition metal compound is a
halide of titanium, vanadium, chromium or zirconium. Most preferably,
titanium trichloride and especially activated titanium trichloride is used
for propylene polymerizations. Titanium trichloride can be activated to a
high degree of polymerization activity by chemical and physical means. One
activated titanium trichloride has an approximate stoichiometric formula
of TiCl.sub.3. 1/3AlCl.sub.3 and has been comminuted. Further, titanium
trichloride can be activated by forming adducts with Lewis bases such as
ethers or by supporting the titanium trichloride on a catalytically inert
substance such as a metal oxide or salt. One suitable titanium trichloride
is described in U.S. Pat. No. 3,984,350 incorporated by reference herein.
Other suitable transition metal compounds are halides, oxyhalides,
alkyloxyhalides, aryloxyhalides, alkoxides, or aryloxides of a Group IVB
or VB transition metal, preferably vanadium or zirconium, and most
preferably titanium. Preferable compounds for ethylene polymerization
include titanium tetrachloride, titanium tetrabromide, butoxytitanium
trichloride, dibutoxytitanium dichloride, tetrabutyl titanate, vanadium
tetrachloride and zirconium tetrachloride. Also useful are supported and
unsupported Group VIB oxides, such as chromia or chromia on alumina.
The molar ratio of transition metal halide to organoaluminum compound in a
catalyst system can range from about one-tenth to about 10, typically is
about 1 to 3 and preferably is about 2. The amount of catalyst in a
polymerization depends on the reactor size and type and on the amount and
type of olefin monomer and is known to the skilled artisan.
Catalyst additives can be added in minor amounts such as disclosed in U.S.
Pat. Nos. 3,950,268 and 4,072,809 incorporated herein by reference.
In gas-phase olefin polymerization systems useful in this invention,
typically off-gas leaving the polymerization reactor, together with some
entrained polymer fines, are fed to a recycle scrubber tower. Contact with
circulating quench liquid, or liquid monomer, partially condense the
reactor off-gas. After condensate is circulated and cooled, it can be
returned to the upper portion of the scrubber or returned to the reactor
as quench liquid. Overhead gas from the scrubber tower can be separated
further and can be compressed for return to the reactors in a regulated
stream. Such recycle scrubber typically is a high flow unit in which
substantial quantities of off-gas and condensate are circulated.
Condensate rapidly leaving the scrubber through an exit port positioned in
the bottom of the scrubber tower may form a vortex which will create
accumulations of polymer fines in areas of the tower with inadequate
agitation. It was found that baffles placed in the tower bottom as vortex
breakers did not prevent polymer accumulations.
The cone-shaped vortex breaker described in this invention is placed above
the liquid exit port located in the lower portion of a gas-liquid
condensation tower. The base of the cone-shaped structure should be at
least as large as the liquid exit port and preferably the diameter or
width of the base is fifty to one hundred percent larger than the exit
port. The slope angle of the cone-shaped structure should be steep enough
such that solids will not accumulate on its surface. A 60.degree. cone is
found adequate. Typically the vortex breaker is supported by legs attached
to the tower bottom.
Above and in proximity to the cone-shaped vortex breaker and below the
minimum liquid level in the condensor tower is placed a regular array of
mixing devices, the effluent of which is directed generally downward such
that areas of inadequate circulation in the bottom of the tower are
avoided. The downward angle between the effluent direction and a
horizontal plane can be about 0.degree. to about 90.degree., preferably is
about 20.degree. to about 60.degree., and most preferably about
45.degree.. Any number of mixing devices, preferably eductors, can be
spaced in a regular array such that no liquid volume in the tower bottom
is not sufficiently agitated. Four to six eductors are adequate. The
amount of liquid ejected by the eductors should be sufficient for adequate
circulation. It is found that circulation to the eductors of about 15% of
the total circulation of liquid through the condensor system is adequate
for proper agitation. A typical eductor useful in this invention entrains
about three liters of suction liquid for each liter of pressure liquid
passed through it.
The eductor array typically is connected to a liquid supply manifold which
is secured above the vortex breaker by struts attached to the tower sides
or legs attached to the tower bottom.
The vortex breaker described in this invention is placed at a distance
above the exit port such that liquid can readily flow around the vortex
breaker and into the exit port but not so high as to permit a vortex to
form. Typically the height of the vortex breaker structure above the exit
port is about 0.5 to about 1.5, preferably about 0.75 to about 1.0, times
the diameter of the exit port. The eductor array is placed above the
vortex breaker structure at a distance such that substantially complete
agitation of the tower bottom is achieved.
The condensation liquid circulation system described herein is equipped
with suitable valves and flow controllers designed to regulate circulation
as required by process conditions.
Our invention is illustrated by reference to the drawings. Within
condensation vessel 1 are placed a series of condensor trays 3 which
typically are semicircular plates positioned at about a 5.degree.-angle
downward from horizontal and preferably have a slot or hole in the rear
section of the tray to aid circulation of fluid. Condensation liquid is
injected through port 7 and partially condenses vapor containing a
condensable vapor which enters through port 5 above liquid level 6.
Uncondensed gas exits through port 8 after passing through demister pad 10
designed to prevent solids carryover. Liquid leaves the vessel through
exit port 9 and is recirculated by pump 11 through line 13. A portion of
the liquid is cooled in heat exchanger unit 15 and recirculated to the
condensor vessel by line 17. A portion of the liquid is taken by line 19
to a reactor vessel. A portion of the recirculated liquid is transferred
by line 21 through port 23 to a manifold 25 feeding an array of eductors
27 through pipe stems 28. Said manifold and eductor array supported by
struts 29 are situated above a cone-shaped structure 31 supported by legs
33. The cone-shaped structure 31 contains a top purge hole 35 and a bottom
purge hole 36.
Eductor 27 contains a pressure liquid port 41 and nozzle 43 through which
pressure liquid is ejected and mixes in chamber 45 with suction liquid
entering through ports 47. Mixed liquid is ejected through mouth 49.
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