U.S. patent application number 15/126792 was filed with the patent office on 2017-04-13 for self cleaning reactor system.
This patent application is currently assigned to NOVA Chemicals (International) S.A.. The applicant listed for this patent is NOVA Chemicals (International) S.A.. Invention is credited to Stephen Brown, Eric Clavelle, Peter Zoricak.
Application Number | 20170101354 15/126792 |
Document ID | / |
Family ID | 52649075 |
Filed Date | 2017-04-13 |
United States Patent
Application |
20170101354 |
Kind Code |
A1 |
Brown; Stephen ; et
al. |
April 13, 2017 |
SELF CLEANING REACTOR SYSTEM
Abstract
This invention relates to a self cleaning reactor and to a
process for the oligomerization of ethylene that employs a
self-cleaning reactor. The reactor includes a mass of inert,
particulate cleaning bodies that are entrained by the liquid in the
reactor and scour the internal surfaces of the reactor during
normal operation. This scouring action reduces the level of fouling
on the reactor surfaces. Foulant material (polyethylene) is removed
from the process on a continuous basis but the cleaning bodies
remain within the reactor.
Inventors: |
Brown; Stephen; (Calgary,
CA) ; Zoricak; Peter; (Calgary, CA) ;
Clavelle; Eric; (Calgary, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NOVA Chemicals (International) S.A. |
Fribourg |
|
CH |
|
|
Assignee: |
NOVA Chemicals (International)
S.A.
Fribourg
CH
|
Family ID: |
52649075 |
Appl. No.: |
15/126792 |
Filed: |
February 2, 2015 |
PCT Filed: |
February 2, 2015 |
PCT NO: |
PCT/IB2015/050785 |
371 Date: |
September 16, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C08F 10/02 20130101;
B01J 19/002 20130101; B01J 19/18 20130101; C08F 210/16 20130101;
C07C 2531/34 20130101; C07C 2531/22 20130101; C07C 2531/14
20130101; C07C 2531/24 20130101; C07C 2/36 20130101; C07C 11/107
20130101; C07C 11/02 20130101; B01J 2219/00252 20130101; B01J
19/0066 20130101; C07C 2/36 20130101; B01J 8/10 20130101; B01J
2208/00867 20130101; C08F 2/01 20130101; B01J 2219/00247 20130101;
C08F 2/002 20130101; B01J 2219/00033 20130101; C08F 2500/02
20130101; B01J 2219/0099 20130101; B01J 14/00 20130101; C07C 2/36
20130101; C07C 2/32 20130101; C08F 210/14 20130101; C08F 10/02
20130101 |
International
Class: |
C07C 2/32 20060101
C07C002/32; B01J 19/00 20060101 B01J019/00; B01J 14/00 20060101
B01J014/00 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 27, 2014 |
CA |
2847814 |
Claims
1. A self cleaning, continuous flow reactor, said reactor
comprising 1) at least one inlet line for liquid reactants; 2) at
least one outlet line for liquid products; 3) a mixing system to
mix liquid contained within said reactor; 4) a mass of cleaning
bodies contained within said reactor, with the provisos that a)
said mixing system provides sufficient liquid velocity to suspend
said cleaning bodies within said reactor; b) said cleaning bodies
have a particle size of from 2 millimeters to 2 centimeters; and c)
said cleaning bodies are retained within said reactor during
operation of the reactor.
2. The reactor of claim 1 wherein said reactor is a continuously
stirred tank reactor.
3. The reactor of claim 1 wherein said cleaning bodies have a
particle size of from about 2 millimeters to about 2 centimeters
and a specific gravity that is greater than the specific gravity of
said liquid contained within said reactor.
4. A process for the removal of by-product polyethylene from a
continuous flow, mixed, oligomerization reactor, said process
comprising: a) providing input flows comprising ethylene, solvent,
and an oligomerization catalyst system to said reactor; b)
oligomerizing ethylene under continuous flow conditions within said
reactor; and c) providing a discharge stream from said reactor
comprising solvent, oligomer product and polyethylene by-product;
characterized in that said process is conducted in the presence of
a mass of reactor cleaning bodies, with the proviso that
substantially all of said cleaning bodies remain within said
reactor during said process.
5. The process of claim 4 wherein said reactor is a continuously
stirred tank reactor.
6. The process of claim 4 wherein said continuously stirred tank
reactor is operated under stirring conditions that are sufficient
to suspend a portion of said cleaning bodies.
7. The process of claim 4 wherein said cleaning bodies have a
particle size of from about 2 millimeters to about 2 centimeters
and a specific gravity that is greater than the specific gravity of
said liquid contained within said reactor.
8. The process of claim 4 wherein said oligomerization catalyst
system comprises a source of active chromium, an activator and a
diphosphine ligand defined by the formula
(R.sup.1)(R.sup.2)--P.sup.1-bridge-P.sup.2(R.sup.3)(R.sup.4)
wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently
selected from the group consisting of hydrocarbyl and
heterohydrocarbyl and said bridge is a moiety that is bonded to
both phosphorus atoms.
9. The process of claim 4 wherein said solvent is an aliphatic
solvent.
10. The process of claim 4 wherein the total rate of polymer
deposition upon the walls of said oligomerization reactor is less
than 1000 ppm per hour, based on [the total amount of oligomer
product].
11. The process of claim 4 wherein said cleaning bodies comprise a
variety of cleaning bodies having different particle sizes.
Description
TECHNICAL FIELD
[0001] This invention relates to the oligomerization of ethylene
using a self cleaning reactor.
BACKGROUND ART
[0002] Many chemical reactions cause reactor fouling. The reactor
of this invention is intended for use in a process that produces a
desired liquid hydrocarbon product (ethylene oligomers) and an
undesired polymeric by-product (polyethylene).
[0003] The by-product polymer causes fouling of internal reactor
surfaces such as reactor walls, baffle surfaces and impeller shafts
and blades. Over the course of time, the fouling becomes severe
enough that the reactor must be shut down and cleaned. Examples of
commonly used cleaning methods include chemical cleaning (using a
hot solvent, detergent or a combination of the two); mechanical
cleaning (using high pressure water or solvent; brushes, and/or
cutting techniques) and combination of chemical and mechanical
cleaning techniques.
[0004] This invention mitigates reactor fouling problems with a
self-cleaning function that performs during the normal operation of
the reactor. The reactor uses a large number of inert, particulate
cleaning bodies. These cleaning bodies are entrained by the reactor
liquid and are directed against the internal reactor surfaces. The
motive force for the reactor liquid is also useful for mixing the
liquid reactor contents. Contact between the cleaning bodies and
the reactor surfaces provides an abrasive action that cleans the
reactor surfaces, and/or prevents the polyethylene from sticking to
the surfaces.
[0005] A prior art system that provides in-situ cleaning of heat
exchangers is sold by the Taprogge company of Wetter, Germany. The
Taprogge heat exchanger cleaning system uses a plurality of
polymeric spheres that are forced through the tube side of a water
cooled, shell and tube exchanger. The cleaning balls scour the
inside of the tubes and thereby remove foulants (such as sediment)
from the cooling water that flows through the tubes. However, it
will be appreciated that the size of the cleaning spheres must be
closely matched to the internal tube diameter. Abrasion of cleaning
spheres (or loss of elasticity, in the case of rubber spheres) can
limit the service life of the cleaning balls and frequent
replacement of the cleaning spheres can be necessary. For example,
it has been reported that cleaning spheres made from natural rubber
typically require replacement after four weeks.
[0006] The present invention uses a much larger number of smaller
cleaning bodies. These cleaning bodies do not need to be "size
matched" to any particular surface dimension (whereas the diameter
of the Taprogge cleaning bodies is size matched to the exchanger
tube diameter). In general, the cleaning bodies of this invention
are simpler, more robust and easier to maintain than the cleaning
spheres of the Taprogge technology.
DISCLOSURE OF INVENTION
[0007] In one embodiment, this invention provides a self cleaning,
continuous flow reactor, said reactor comprising:
[0008] 1) at least one inlet line for liquid reactants;
[0009] 2) at least one outlet line for liquid products;
[0010] 3) a mixing system to mix liquid contained within said
reactor;
[0011] 4) a mass of cleaning bodies contained within said reactor,
with the provisos that [0012] a) said mixing system provides
sufficient liquid velocity to suspend said cleaning bodies within
said reactor; [0013] b) said cleaning bodies have a particle size
of from 2 millimeters to 2 centimeters; and [0014] c) said cleaning
bodies are retained within said reactor during operation of the
reactor.
[0015] In another embodiment, the present invention provides:
a process for the removal of by-product polyethylene from a
continuous flow, mixed, oligomerization reactor, said process
comprising:
[0016] a) providing input flows comprising ethylene, solvent, and
an oligomerization catalyst system to said reactor;
[0017] b) oligomerizing ethylene under continuous flow conditions
within said reactor; and
[0018] c) providing a discharge stream from said reactor comprising
solvent, oligomer product and polyethylene by-product;
characterized in that said process is conducted in the presence of
a mass of reactor cleaning bodies, with the proviso that
substantially all of said cleaning bodies remain within said
reactor during said process.
BEST MODE FOR CARRYING OUT THE INVENTION
Cleaning Bodies
[0019] The cleaning bodies can be of any shape and of any material
that can be suspended and circulated by the agitation system used
in the reactor. Cleaning body material selection will also be
dictated by compatibility with the process and the reactor system
components such as agitators, draft tube, shaft(s), baffles,
injectors and the vessel wall.
[0020] The cleaning bodies are preferably made from a material that
is less hard than the hardness of the materials used to construct
the reactor system components to avoid undue abrasion/erosion of
the reactor system components. The term "hardness" is meant to
convey its conventional meaning in the context of the well-known
Mohs hardness scale. For example, silica is known to be hard (and
it has a high Mohs hardness number) and the use of silica could
lead to the abrasion of the reactor system components.
[0021] The cleaning bodies should also be "inert"--which, in the
context of this invention, is intended to mean that the cleaning
bodies do not adversely affect the catalyst system that is used in
the oligomerization reactor.
[0022] The cleaning bodies should also be "suspended" in the
reaction medium--i.e. the particles should not remain on the bottom
of the reactor during the process of this invention. Persons
skilled in the art will recognize that several factors will affect
the ability of a solid particle to become suspended in a mixed
liquid, including the particle size and density; the type and speed
of agitation and the fluid viscosity. One correlation that may be
used to estimate the "just suspended speed" for an agitated reactor
was developed by Zweitering:
Njs = S ( .mu. .rho. ) 0.1 [ g ( .rho. p - .rho. ) .rho. ] - 0.45 X
0.13 d p 0.2 D - 0.85 " - Correlation 1 - " ##EQU00001##
where the terms of the correlation are defined as follows (with
units in parenthesis): Njs is the just suspended speed
(rad/second), S a coefficient specific to a particular agitation
system (dimensionless), .mu. the liquid viscosity
(Pascals/seconds), .rho. the liquid density (kilogram/meter.sup.3),
g the gravitational constant (9.81 meters/second.sup.2), .rho.p the
cleaning body density (kilogram/meter.sup.3), X the mass ratio of
suspended solids to liquid.times.1000 (dimensionless), dp the
cleaning body characteristic diameter (meters), and D the agitator
characteristic diameter (meters).
[0023] The agitator speed N must typically be larger than Njs speed
to ensure that the cleaning bodies do not settle on the floor of
the reactor vessel or other horizontal surface. In general, the
agitator needs to operate at a multiple of this speed to ensure
good circulation and thus good, sufficient cleaning action
throughout the vessel. The multiple will depend on the agitation
system. For example, a draft tube and agitator system will require
a lower multiple than a system with an agitator and no draft tube
or an agitator and no baffles for example.
[0024] The cleaning bodies do not need to be of equal diameter. A
distribution of diameters can be advantageous in increasing the
cleaning effectiveness. In addition, binary mixtures can be used to
lower the liquid velocity that is required to suspend the cleaning
bodies ("Njs" in the correlation, below).
[0025] The agitation system can be an agitator or agitators in a
baffled or unbaffled tank. For vessels with internal height to
internal diameter ratios over 1.25, it may be necessary to have
multiple agitators to ensure circulation throughout the vessel. The
use of a draft tube is another option; and the combination of
multiple agitators with a draft tube may be optimum. The agitation
system is not limited to a system with rotating impellers--for
example, a series of jets may provide the required mixing.
[0026] The correlation described above may be used for reactions
equipped with an agitator.
[0027] The following correlation is more suitable for a reactor
that is equipped with mixing jets:
Vjs = 2 ( ( .rho. p - .rho. ) .rho. ) 2.08 ( .mu. .rho. ) 0.16 g
0.42 T 1.16 d p 0.1 C w 0.24 D J ##EQU00002##
where the terms of the equation are defined as follows (with units
in parenthesis): Vjs is the just suspended jet speed, .mu. is the
liquid viscosity (Pascals/seconds), .rho. is the liquid density
(kilogram/meter.sup.3), G is the gravitational constant (9.81
meters/second.sup.2), .rho.p is the cleaning body density
(kilogram/meter.sup.3), T is the tank diameter, dp is the cleaning
body characteristic diameter (meters), Cw is the percent weight
fraction of solids (based on the weight of the solids/weight of
solids+reaction medium), and Dj is the jet diameter.
[0028] The coefficient s has a value of typically between 3.0 and
8.0 depending on the reactor geometry and agitator type and number
of agitators for (continuously stirred reactors).
[0029] The correlations described above are discussed in further
detail in Handbook of Industrial Mixing--Science and Practice,
Edited by Edward L. Paul, Victor A. Atiemo-Obeng, Suzanne M.
Kresta, Wiley-Interscience, 2004, at pages 558-564.
[0030] These correlations provide a useful starting point for
estimating the agitator speed that is required to suspend a given
mass of cleaning bodies having a known particle size and density.
In the alternative, for a given (known) agitator speed in a known
reactor volume, the maximum particle size and/or density of a
potential cleaning body can be estimated.
[0031] The particle size of the cleaning bodies is preferably from
about 2 millimeters to about 2 centimeters. While not wishing to be
bound by theory, it is believed that the cleaning bodies remove
foulant (polyethylene) from the reactor walls by a physical
scrubbing/scouring action. Accordingly, it is believed that
momentum transfer (from the cleaning body to the foulant) is
required for successful cleaning and it is for this reason that a
minimum particle size of 2 mm is preferred. Particle sizes of
greater than 2 cm may be useful, but it becomes more difficult to
"suspend" a cleaning body of a greater density as the particle size
increases.
[0032] The density of the cleaning bodies is preferably greater
than the density of the oligomerization medium (so that the
cleaning bodies do not float).
[0033] The upper limit on the density of the cleaning bodies is
limited by the ability of the agitator system to suspend the
cleaning bodies. The use of higher densities can make it difficult
to suspend the cleaning bodies at reasonable agitator speeds in the
small reactors. However, as will be clear to persons of skill in
the art, this problem diminishes with larger reactors--i.e. the
fluid velocity that is provided by the larger agitators in larger
reactors is sufficient to suspend cleaning bodies having a higher
density. Thus, although the accompanying examples illustrate the
use of cleaning bodies having a relatively low density (in a small
reactor), the correlations given above indicate that cleaning
bodies having a higher density/specific gravity are suitable for
use in larger reactors. The use of cleaning bodies having a higher
density can be desirable (provided that they are suspended) because
they can provide more momentum/cleaning action as they come in
contact with the foulant on the internal surfaces of the
reactor.
[0034] The upper limit on the density of the cleaning bodies may be
calculated/estimated from correlation 1 above--and is a function of
the particle size of the cleaning bodies; the mass of cleaning
bodies; the rate of agitation (fluid velocity) and fluid viscosity.
As a practical matter, it is preferred to use a cleaning body
having a density of less than 7 grams per cubic centimeter
(especially less than 3 g/cc) given the power requirements that
would be necessary to suspend denser cleaning bodies.
[0035] It is important to note that the cleaning bodies remain
within the reactor, but the foulant (polyethylene) is removed from
the reactor during the process of this invention.
[0036] This allows long reaction "runtimes" without severe reactor
fouling. The foulant is removed from the reactor with the product
stream and may be observed in the product stream as small particles
or flakes.
Catalyst System for Oligomerization Process
[0037] The catalyst system used in the process of the present
invention must contain three essential components, namely: [0038]
(i) a source of chromium; [0039] (ii) a diphosphine ligand; and
[0040] (iii) an activator. Preferred forms of each of these
components are discussed below.
Chromium Source
[0041] Any source of chromium that is soluble in the process
solvent and which allows the oligomerization process of the present
invention to proceed may be used. Preferred chromium sources
include chromium trichloride; chromium (III) 2-ethylhexanoate;
chromium (III) acetylacetonate and chromium carbonyl complexes such
as chromium hexacarbonyl. It is preferred to use very high purity
chromium compounds as these should generally be expected to
minimize undesirable side reactions. For example, chromium
acetylacetonate having a purity of higher than 99% is commercially
available (or may be readily produced from 97% purity
material--using recrystallization techniques that are well known to
those skilled in the art). The present process preferably operates
at a temperature of from 30 to 50.degree. C. We have observed that
very low Cr concentrations in the reactor are optimum for this
temperature--with a range of 0.1 to 3.times.10.sup.-6 molar being
suitable and from 0.3 to 0.8.times.10.sup.-6 being optimum.
Diphosphine Ligand Used in the Oligomerization Process
[0042] In general, the ligand used in the process of this invention
is defined by the formula
(R.sup.1)(R.sup.2)--P.sup.1-bridge-P.sup.2(R.sup.3)(R.sup.4)
wherein R.sup.1, R.sup.2, R.sup.3 and R.sup.4 are independently
selected from the group consisting of hydrocarbyl and
heterohydrocarbyl and said bridge is a moiety that is bonded to
both phosphorus atoms.
[0043] The term hydrocarbyl as used herein is intended to convey
its conventional meaning--i.e. a moiety that contains only carbon
and hydrogen atoms. The hydrocarbyl moiety may be a straight chain;
it may be branched (and it will be recognized by those skilled in
the art that branched groups are sometimes referred to as
"substituted"); it may be saturated or contain unsaturation and it
may be cyclic. Preferred hydrocarbyl groups contain from 1 to 20
carbon atoms. Aromatic groups--especially phenyl groups--are
especially preferred. The phenyl may be unsubstituted (i.e. a
simple C.sub.6H.sub.5 moiety) or contain substituents, particularly
at an ortho (or "o") position.
[0044] Similarly, the term heterohydrocarbyl as used herein is
intended to convey its conventional meaning--more particularly, a
moiety that contains carbon, hydrogen and at least one heteroatom
(such as O, N, R and S). The heterohydrocarbyl groups may be
straight chain, branched or cyclic structures. They may be
saturated or contain unsaturation. Preferred heterohydrocarbyl
groups contain a total of from 2 to 20 carbon+heteroatoms (for
clarity, a hypothetical group that contains 2 carbon atoms and one
nitrogen atom has a total of 3 carbon+heteroatoms).
[0045] It is preferred that each of R.sup.1, R.sup.2, R.sup.3 and
R.sup.4 is a phenyl group (with an optional substituent in an ortho
position on one or more of the phenyl groups).
[0046] Highly preferred ligands are those in which R.sup.1 to
R.sup.4 are independently selected from the group consisting of
phenyl and o-fluorophenyl. The resulting ligands are useful for the
selective tetramerization of ethylene to octene-1 with some co
product hexene also being produced.
[0047] The term "bridge" as used herein with respect to the ligand
refers to a moiety that is bonded to both of the phosphorus atoms
in the ligand--in other words, the "bridge" forms a link between
P.sup.1 and P.sup.2. Suitable groups for the bridge include
hydrocarbyl and an inorganic moiety selected from the group
consisting of N(CH.sub.3)--N(CH.sub.3)--, --B(R.sup.6)--,
--Si(R.sup.6).sub.2--, --P(R.sup.6)-- or --N(R.sup.6)-- where
R.sup.6 is selected from the group consisting of hydrogen,
hydrocarbyl and halogen.
[0048] It is especially preferred that the bridge is --N(R.sup.5)--
wherein R.sup.5 is selected from the group consisting of hydrogen,
alkyl, substituted alkyl, aryl, substituted aryl, aryloxy,
substituted aryloxy, halogen, alkoxycarbonyl, carbonyloxy, alkoxy,
aminocarbonyl, carbonylamino, dialkylamino, silyl groups or
derivatives thereof and an aryl group substituted with any of these
substituents. Highly preferred bridges are those in which R.sup.5
is a C.sub.1 to C.sub.12 alkyl--especially isopropyl (i.e. when
R.sup.5 is isopropyl).
Activator (or "Co-Catalyst")
[0049] The activator may be any compound that generates an active
catalyst for ethylene oligomerization. Mixtures of activators may
also be used. Suitable compounds include organoaluminum compounds,
organoboron compounds and inorganic acids and salts, such as
tetrafluoroboric acid etherate, silver tetrafluoroborate, sodium
hexafluoroantimonate and the like. Suitable organoaluminium
compounds include compounds of the formula AlR3, where each R is
independently C.sub.1-C.sub.12 alkyl, oxygen or halide, and
compounds such as LiAIH.sub.4 and the like. Examples include
trimethylaluminium (TMA), triethylaluminium (TEAL),
tri-isobutylaluminium (TIBA), tri-n-octylaluminium, methylaluminium
dichloride, ethylaluminium dichloride, dimethylaluminium chloride,
diethylaluminium chloride, ethylaluminiumsesquichloride,
methylaluminiumsesquichloride, and alumoxanes (also referred to as
aluminoxanes). Alumoxanes are well known in the art as typically
oligomeric compounds which can be prepared by the controlled
addition of water to an alkylaluminium compound, for example
trimethylaluminium. Such compounds can be linear, cyclic, cages or
mixtures thereof. Commercially available alumoxanes are generally
believed to be mixtures of linear and cyclic compounds. The cyclic
alumoxanes can be represented by the formula [R.sup.6AlO].sub.s and
the linear alumoxanes by the formula R.sup.7(R.sup.8AlO).sub.s
wherein s is a number from about 2 to 50, and wherein R.sup.6,
R.sup.7, and R.sup.8 represent hydrocarbyl groups, preferably
C.sub.1 to C.sub.6 alkyl groups, for example methyl, ethyl or butyl
groups. Alkylalumoxanes especially methylalumoxane (MAO) are
preferred.
[0050] It will be recognized by those skilled in the art that
commercially available alkylalumoxanes may contain a proportion of
trialkylaluminium. For instance, some commercial MAO contains up to
35 weight % trimethylaluminium (TMA), and commercial "modified MAO"
(or "MMAO") contains both TMA and TIBA. Quantities of
alkylalumoxane are generally quoted herein on a molar basis of
aluminium (and include such "free" trialkylaluminium).
[0051] A combination of a MAO with additional TEAL is preferred for
this invention. The combined use of MAO and TEAL can provide a cost
effective cocatalyst system.
[0052] In the preparation of the catalyst systems used in the
present invention, the quantity of activating compound to be
employed is easily determined by simple testing, for example, by
the preparation of small test samples which can be used to
oligimerize small quantities of ethylene and thus to determine the
activity of the produced catalyst. It is generally found that the
quantity employed is sufficient to provide 500 to 5,000 moles of
aluminium per mole of chromium. A mix of MAO and TEAL is preferred
with the moles of aluminum from the MAO that are provided being
about 40 to 60 mole % of the total moles of aluminum in the
activator. Molar Al/Cr ratios of from 1000/1 to 3500/1 are
preferred. Additional TEAL increases the total Al/Cr ratio but may
actually reduce overall costs as TEAL is much less expensive than
MAO. The use of a combined MAO+TEAL cocatalyst system is shown in
the examples. We have also found that the overall concentration of
aluminum in the reactor should be from 1000 to 3000.times.10.sup.-6
molar.
Part B Hydrogen
[0053] The use of hydrogen is highly preferred in the process of
the present invention. Optimum ethylene:hydrogen ratios
(weight:weight in the feed) are believed to be from 150/1 to
800/1.
Part C Catalyst: Ratios and Preparation
[0054] For oligomerizations at temperatures higher than 50.degree.
C., the chromium and ligand may be present in almost any molar
ratio in which the ligand is provided in a molar excess to the
chromium. Stated alternatively: a molar equivalent of ligand and
chromium provides an active catalyst and excess ligand (though not
necessary) does not generally have an adverse impact upon activity
at high temperature. At lower temperatures, we have observed a
negative impact upon catalyst activity when a molar excess of
ligand (to Cr) is used. The optimum ligand: Cr ratio has been
observed to be from 0.7/1 to 1.0/1, especially from 0.75/1 to
0.85/1, at temperatures of from 30 to 50.degree. C.
[0055] A variety of methods are known to purify solvents used to
prepare the catalysts including use of molecular sieves (3A),
adsorbent alumina and supported de-oxo copper catalyst. Several
configurations for the purifier system are known and depend on the
nature of the impurities to be removed, the purification efficiency
required and the compatibility of the purifier material and the
process solvent. In some configurations, the process solvent is
first contacted with molecular sieves, followed by adsorbent
alumina, then followed by supported de-oxo copper catalyst and
finally followed by molecular sieves. In other configurations, the
process solvent is first contacted with molecular sieves, followed
by adsorbent alumina and finally followed by molecular sieves. In
yet another configuration, the process solvent is contacted with
adsorbent alumina. One preferred purifier system consists of
molecular sieves, followed by adsorbent alumina and finally
followed by another set of molecular sieves.
Part D Reaction Conditions (General)
[0056] Irrespective of the process conditions employed, the
oligomerization is typically carried out under conditions that
substantially exclude oxygen, water, and other materials that act
as catalyst poisons. In addition, the reactor is preferably purged
with a nonreactive gas (such as nitrogen or argon) prior to the
introduction of catalyst. A purge with a solution of MAO and/or
aluminum alkyl may also be employed to lower the initial level of
catalyst poisons. Also, oligomerizations can be carried out in the
presence of additives to control selectivity, enhance activity and
reduce the amount of polymer formed in oligomerization
processes.
[0057] The process of this invention requires the use of a solvent
or diluent because the undesirable formation of C.sub.10.sub.+
oligomers has been observed to increase under continuous flow
oligomerization conditions when the concentration of octene in the
reactor increases. The addition of a solvent mitigates this
problem. Suitable solvents include saturated C.sub.6 to C.sub.20
aliphatics (such as hexane, heptane, etc.) and saturated
cycloaliphatics (such as cyclohexane or methyl cyclohexane).
Unsaturated aliphatics (especially 1-olefins such as 1-hexene;
1-heptene and 1-octene) should be avoided because the use of such
unsaturates has been observed to lead to the undesired formation of
higher oligomers.
[0058] Mixtures of inert diluents or solvents also could be
employed. The preferred solvents are aromatic hydrocarbons or
saturated aliphatics such as, for example, isobutane, pentane,
toluene, xylene, ethylbenzene, cumene, mesitylene, heptane,
cyclohexane, methylcyclohexane, chlorobenzene, dichlorobenzene, and
mixtures of aliphatics sold under the trademark Isopar.RTM..
Cyclohexane and linear C.sub.6 to C.sub.10 saturated aliphatics are
especially preferred. Heptane is an especially preferred linear
aliphatic because it is readily separated from the oligomers
produced by this reaction using conventional distillation
techniques.
[0059] The ethylene feedstock for the oligomerization may be
substantially pure or may contain other olefinic impurities and/or
ethane.
[0060] The feedstock is preferably treated to remove catalyst
poisons (such as oxygen, water and polar species) using techniques
that are well known to those skilled in the art. The technology
used to treat feedstocks for polymerizations is suitable for use in
the present invention and includes the molecular sieves, alumina
and de-oxo catalysts described above for analogous treatment of the
process solvent.
Reactor
[0061] The present invention must be conducted under continuous
flow conditions using a mixed reactor.
[0062] The term "continuous flow" is meant to convey its
conventional meaning--i.e. reactants are continuously added to the
reactor and product is continuously withdrawn.
[0063] Similarly, the term "mixed reactor" is meant to convey its
conventional meaning--i.e. a reactor that contains an agitator or
mixing system. A continuously stirred tank reactor ("CSTR") is
generally preferred. However, a loop reactor in which mixing is
provided by a circulating pump is also suitable (and such reactors
are well known to those skilled in the art and are in commercial
use).
[0064] The use of a CSTR is generally preferred as it is desirable
to maintain essentially homogenous reactor conditions--i.e. as will
be appreciated by those skilled in the art, a well mixed CSTR will
provide homogenous reactor conditions (in contrast to a plug flow,
or tubular reactor, in which the reactor conditions are typically
very different at the inlet and discharge). More than one CSTR may
be used.
[0065] The reactor also contains a large number of cleaning bodies.
The cleaning bodies remain within the reactor during the process of
this invention. The reactor is operated on a continuous flow
basis--i.e. reactants are added to the reactor and products are
removed from the reactor during the operation of the process.
Accordingly, it is necessary to design the reactor outlet/product
lines to ensure that the cleaning bodies remain within the reactor.
A simple screen at the mouth of the product discharge line is one
simple design option that may be used to retain the cleaning bodies
within the reactor. The use of a hydrocyclone to remove the
cleaning bodies from the product stream with centrifugal force is
also contemplated.
Specific Process Conditions
[0066] The process of the present invention specifically requires a
solvent and typically uses a catalyst concentration of from 0.1 to
3.times.10.sup.-6 moles of Cr per litre (micromolar).
[0067] The reactor temperature is from about 30 to about
130.degree. C., especially from about 35 to about 75.degree. C. and
most especially from about 35 to 45.degree. C. In general, lower
temperatures have been observed to improve selectivity (when other
reaction variables are held constant).
[0068] Optimum chromium concentrations have been observed to be
from 0.1 to 3 micromolar especially 0.3 to 0.8. Reactor Hold up
times (HUT where HUT=reactor volume/flow to reactor) are from 40 to
180 minutes, especially 60 to 90 minutes.
[0069] Another preferred element of the present invention is the
use of ethylene concentrations of 3 to 15 weight %, especially from
5 to 10 weight %.
[0070] The total operating pressure of the process is a function of
ethylene concentration, hydrogen concentration, solvent choice and
temperature. The use of comparatively low temperature means that a
higher ethylene concentration may be achieved at a given pressure
(as ethylene solubility increases at lower temperatures). Preferred
operating pressures are from 2 to 20 Mega Pascals (MPa) especially
from 4 to 10 MPa.
Part E Reactor Control
[0071] The control systems required for the operation of agitated
reactors are well known to those skilled in the art and do not
represent a novel feature of the present invention. In general,
temperature, pressure and flow rate readings will provide the basis
for most conventional control operations. The increase in process
temperature (together with reactor flow rates and the known
enthalpy of reaction) may be used to monitor ethylene conversion
rates. The amount of catalyst added to the reactor may be increased
to increase the ethylene conversion (or conversely, decreased to
decrease ethylene conversion) within desired ranges. Thus, basic
process control may be derived from simple measurements of
temperature, pressure and flow rates using conventional
thermocouples, pressure meters and flow meters. Advanced process
control (for example, for the purpose of monitoring product
selectivity or for the purpose of monitoring process fouling
factors) may be undertaken by monitoring additional process
parameters with more advanced instrumentation. Known/existing
instrumentation that may be employed include in-line/on-line
instruments such as NIR infrared, Fourier Transform Infrared
(FTIR), Raman, mid-infrared, ultra violet (UV) spectrometry, gas
chromatography (GC) analyzer, refractive index, on-line
densitometer or viscometer. The use of NIR or GC to measure the
composition of the oligomerization reactor and final product
composition is especially preferred. A GC analyzer was used to
measure the composition of the reactor discharge in the
accompanying examples.
[0072] The measurement may be used to monitor and control the
reaction to achieve the targeted stream properties including but
not limited to concentration, viscosity, temperature, pressure,
flows, flow ratios, density, chemical composition, phase and phase
transition, degree of reaction, polymer content, selectivity.
[0073] The control method may include the use of the measurement to
calculate a new control set point. The control of the process will
include the use of any process control algorithms, which include,
but are not limited to the use of PID, neural networks, feedback
loop control, forward loop control and adaptive control.
Catalyst Deactivation, Catalyst Removal and Polymer Separation
[0074] In general, the oligomerization catalyst is preferably
deactivated immediately downstream of the reactor as the product
exits the reaction system. This is to prevent polymer formation and
potential build up downstream of the reactor and to prevent
isomerisation of the 1-olefin product to the undesired internal
olefins. It is generally preferred to flash and recover unreacted
ethylene before deactivation. However, the option of deactivating
the reactor contents prior to flashing and recovering ethylene is
also acceptable. The flashing of ethylene is endothermic and may be
used as a cooling source.
[0075] In general, many polar compounds (such as water, alcohols
and carboxylic acids) will deactivate the catalyst. The use of
alcohols, amines and/or carboxylic acids is preferred--and
combinations of these are contemplated.
[0076] The deactivator may be added to the oligomerization product
stream before or after the volatile unreacted reagents/diluents and
product components are separated. In the event of a runaway
reaction (e.g. rapid temperature rise) the deactivator can be
immediately fed to the oligomerization reactor to terminate the
reaction. The deactivation system may also include a basic compound
(such as sodium hydroxide) to minimize isomerization of the
products (as activator conditions may facilitate the isomerization
of desirable alpha olefins to undesired internal olefins).
[0077] The process of this invention causes polymer to flow out of
the oligomerization reactor. The polymer exits the reactor with
solvent and the oligomer product. This polymer is then separated
from the oligomer product.
[0078] Polymer separation preferably follows catalyst deactivation.
Two "types" of polymer may exist, namely polymer that is dissolved
in the process solvent and non-dissolved polymer that is present as
a solid or "slurry".
[0079] Solid/non-dissolved polymer may be separated using one or
more of the following types of equipment: centrifuge; cyclone (or
hydrocyclone), a decanter equipped with a skimmer or a filter.
Preferred equipment include so called "self-cleaning filters" sold
under the name V-auto strainers, self-cleaning screens such as
those sold by Johnson Screens Inc. of New Brighton, Minn. and
centrifuges such as those sold by Alfa Laval Inc. of Richmond, Va.
(including those sold under the trademark Sharples.RTM.). The Pall
Filter Company also sells filters that are suitable for removing
solid polymer from the liquid process stream of this invention.
[0080] Soluble polymer may be separated from the final product by
two distinct operations. Firstly, low molecular weight polymer that
remains soluble in the heaviest product fraction (C.sub.20+) may be
left in that fraction. This fraction will be recovered as "bottoms"
from the distillation operations (described below). This solution
may be used as a fuel for a power generation system.
[0081] An alternative polymer separation comprises polymer
precipitation caused by the removal of the solvent from the
solution, followed by recovery of the precipitated polymer using a
conventional extruder. The technology required for such
separation/recovery is well known to those skilled in the art of
solution polymerization and is widely disclosed in the
literature.
[0082] In another embodiment, the residual catalyst is treated with
an additive that causes some or all of the catalyst to precipitate.
The precipitated catalyst is preferably removed from the product at
the same time as by-product polymer is removed (and using the same
equipment). Many of the catalyst deactivators listed above will
also cause catalyst precipitation. In a preferred embodiment, a
solid sorbent (such as clay, silica or alumina) is added to the
deactivation operation to facilitate removal of the deactivated
catalyst by filtration or centrifugation.
Product Work Up/Distillation
[0083] In one embodiment of the present invention, the
oligomerization product produced from this invention is added to a
product stream from another alpha olefins manufacturing process for
separation into different alpha olefins. As previously discussed,
"conventional alpha olefin plants" (wherein the term includes i)
those processes which produce alpha olefins by a chain growth
process using an aluminum alkyl catalyst; ii) the aforementioned
"SHOP" process; and iii) the production of olefins from synthesis
gas using the so called Lurgi process) have a series of
distillation columns to separate the "crude alpha product" (i.e. a
mixture of alpha olefins) into alpha olefins (such as butene-1,
hexene-1 and octene-1). The mixed hexene-octene product which is
preferably produced in accordance with the present invention is
highly suitable for addition/mixing with a crude alpha olefin
product from an existing alpha olefin plant (or a "cut" or fraction
of the product from such a plant) because the mixed hexene-octene
product produced in accordance with the present invention can have
very low levels of internal olefins. Thus, the hexene-octene
product of the present invention can be readily separated in the
existing distillation columns of alpha olefin plants (without
causing the large burden on the operation of these distillation
columns which would otherwise exist if the present hexene-octene
product stream contained large quantities of internal olefins). As
used herein, the term "liquid product" is meant to refer to the
oligomers produced by the process of the present invention which
have from 4 to (about) 20 carbon atoms.
[0084] In another embodiment, the distillation operation for the
oligomerization product is integrated with the distillation system
of a solution polymerization plant (as disclosed in Canadian Patent
Application No. 2,708,011, Krzywicki et al.).
[0085] It will be appreciated that the process solvent must also be
separated from the liquid product. This may be done, for example,
using distillation. It is highly preferred to recycle the separated
solvent back to the oligomerization reactor after it has been
distilled/purified.
EXAMPLES
Continuous Operation--General Conditions
[0086] A continuously stirred tank reactor (CSTR) having a nominal
volume of two liters was used for these experiments.
[0087] The CSTR was fitted with external jacket for
heating/cooling.
[0088] The chromium source for the catalyst was chromium
tri(acetylacetonate), or Cr(acac).sub.3. The ligand was a P--N--P
ligand in which the nitrogen bridging atom was substituted with an
isopropyl group and each P atom was substituted with two
ortho-fluoro phenyl groups. This ligand and its synthesis are known
to those skilled in the art. Further details are provided in U.S.
Pat. No. 8,252,956 (Gao et al.).
[0089] The cocatalyst was a combination of modified MAO (MMAO-3A)
and TEAL.
[0090] MMAO-3A was purchased as a solution of methylaluminoxine (7
weight % Al in isopentane) from Akzo-Nobel.
[0091] TEAL was purchased as a 25 wt % TEAL solution in heptane
from Akzo-Nobel. Catalyst, ligand and co-catalyst were added to the
reactor (i.e. "in situ" catalyst formation).
[0092] The reactor was operated in a continuous manner--i.e.
product was removed from the reactor during the reaction and feed
(ethylene, hydrogen, solvent and catalyst) was added continuously.
Ethylene and hydrogen were added to the solvent outside of the
reactor and then directed to the reactor via a common feed line.
Cyclohexane was used as the solvent in all examples.
[0093] The use of two different types of cleaning bodies is
reported in Table 1, namely polyethylene and polypropylene
pellets.
[0094] Using correlation (1)--as described earlier--it was
estimated that an agitator speed of 1000 revolutions per minute
would be sufficient to suspend the cleaning bodies used in these
examples (which had a particle size of about 3 millimeters and a
specific gravity of about 1). By way of further explanation, it
should be acknowledged that correlation 1 should be regarded as a
tool to calculate an estimated value/starting point for the fluid
velocity that is required to suspend the cleaning bodies. The value
for the fluid velocity that is calculated from correlation 1 can
then be verified/confirmed by simple experimentation.
[0095] A series of exploratory investigations on the use of
different cleaning bodies was undertaken using a mixed vessel (a
proxy for a reactor) having transparent plastic walls. The vessel
was equipped with different types of agitators and different types
of internal components (baffles and draft tubes). These results
from the investigations are consistent with the predictions made by
the correlations referred to above--in particular, the use of
polyethylene (PE) and polypropylene (PP) particles having a density
of about 1 gram per cubic centimeter and a particle size of about 3
millimeters was observed to produce a "suspended" mass of cleaning
particles at the agitator speeds of the small vessel.
[0096] Cleaning bodies were added to the reactor prior to start of
each run for each of the invention examples, in the amounts
indicated in Table 1.
[0097] Cleaning bodies were not used in the comparative examples,
which are indicated with a "C" in Table 1--e.g. experiments 1C and
5C. The internal reactor surfaces from these experiments were
observed to be completely covered--with the thickness of the
polymer coating being quite extreme in some cases.
[0098] After each run was completed, the reactor was opened up to
remove the cleaning bodies and to determine their effectiveness.
The effectiveness of the cleaning bodies was determined by
estimating percent of reactor surface area which remained free of
polymer deposits. One essential feature of this invention is that
foulant is removed from the reactor during operation--i.e. it does
not simply accumulate within the reactor and/or on the cleaning
bodies. This was confirmed by observing the presence of
polyethylene "flakes" in the liquid product that was discharged
from the reactor.
[0099] The oligomer product that was produced in these examples was
typical of that disclosed in U.S. Pat. No. 8,252,956 (Gao et al.)
and comprised a combination of hexene and octene oligomers with a
minor amount of C.sub.10.sub.+ oligomers. The amount of reactor
fouling in the inventive examples was low--less than 1000 parts per
million (by weight) of polymer was deposited on the reactor
material surfaces, based on the total amount of ethylene
consumed.
TABLE-US-00001 TABLE 1 Estimated Percent of Cleaning Reactor Run
Cleaning Bodies Reactor [Cr] Average Stirring Surface Area Length
Body Mass Concentration Al:Cr Productivity Speed Free of Run # (hr)
Type (g) (mM) (mol:mol) (gProduct/gCr) (rpm) Polymer [Cr] = 0.67
mM, Al:Cr = 3000, 100 g PP cleaning bodies 1C 18.7 none -- 0.66
3000 4886447 1000 0 2 9 PP 100 0.67 3000 4791209 1000 95 3 18.3 PP
100 0.67 3000 4578755 1000 55 4 22.9 PP 100 0.67 3000 4857143 850
27 5C 24.1 none -- 0.67 3000 4688645 1000 0 [Cr] = 0.34 mM, Al:Cr =
4000, 200 g PP or PE cleaning bodies 6C 18.2 none -- 0.32 4000
11120879 1000 0 7 9.3 PP 200 0.34 4000 8849817 1000 75 8 20.5 PP
200 0.34 4000 9238095 1225 65 9 18.7 PE 200 0.32 4000 11106227 1000
30 [Cr] = 1.1-1.2 mM, Al:Cr = 2000 or 3957, 50 or 200 g PE cleaning
bodies 10C 19.4 none -- 1.19 2012 2620879 1000 0 11 14.9 PE 50 1.1
2000 2410256 1000 18 12 17.4 PE 200 1.16 3957 3343968 1000 95 [Cr]
= 0.33 mM, Al:Cr = 2000, 200 g PP cleaning bodies, stirring speed =
1000 or 1450 13C 18.2 none -- 0.32 4000 11120879 1000 0 14 18.2 PP
200 0.33 2000 9479853 1000 25 15 18.9 PP 200 0.33 2000 9992674 1450
55 Productivity = grams of oligomer product (hexene + octene) per
gram of chromium.
Example 1
[0100] Runs 1-5 show the effect of run length on the percent of
surface area which remains free of polymer deposits. These runs
were done at the same reactor Cr concentration and Al:Cr ratio,
both of which have a significant impact on rate of fouling.
Example 2
[0101] Runs 6-9 show the similar effect as example 1; however, at
lower reactor Cr concentration which shows higher level of fouling
even when more cleaning bodies are used.
Example 3
[0102] Runs 10-12 show the effect of the mass of cleaning bodies
added to the reactor. The reactor was observed to be less fouled as
the mass of cleaning bodies is increased.
Example 4
[0103] Runs 13-15 show the effect of agitator stirring speed. As
the stirring speed is increased from 1000 to 1450 rpm the level of
fouling in the reactor was observed to decrease.
Comparative Example
[0104] The process of this invention requires that the cleaning
particles are suspended in the reaction fluid. As described above,
the "just suspended speed" can be calculated/estimated using
correlation 1 (and experiments to confirm the calculation may be
easily completed).
[0105] A comparative experiment was conducted using conditions that
were insufficient to provide the just suspended speed. This was
done using a dense polymer (a fluoropolymer, designated PFA) and an
agitator speed that was too low to suspend the PFA particles. An
oligomerization reaction was then completed using oligomerization
conditions (temperature, pressure, catalyst and ethylene films,
etc.) that were similar to those of experiment 11. At the end of
this oligomerization experiment, the reactor was opened and
observed to be clean at the bottom of the reactor but heavily
fouled elsewhere. This comparative experiment provides further
evidence that serves to confirm that:
[0106] 1) the scouring action of the cleaning bodies is important
(this observation is consistent with the observation that flakes of
polymer exit the reactor in the liquid product stream. In contrast,
if the polymer foulant was simply being deposited upon the cleaning
bodies, then the use of suspended cleaning bodies would be less
important); and
[0107] 2) the reactor must be operated at a fluid velocity that is
at or above the "just suspended speed" in order to achieve the best
results.
[0108] It should also be noted that the PFA cleaning bodies would
also be suitable for use in the present invention if the agitator
speed was sufficient to provide the just suspended speed in the
reaction liquid.
INDUSTRIAL APPLICABILITY
[0109] The technology of this invention improves the efficiency of
an oligomerization process by reducing the rate at which the
oligomerization reactor becomes fouled. The oligomers that are
prepared by this process may be used as comonomers for the
preparation of ethylene copolymers.
* * * * *