Note: Descriptions are shown in the official language in which they were submitted.
REDUCING FOULING IN HEAT EXCHANGERS
FIELD OF THE INVENTION
The present invention relates to a method of reducing the fouling of a heat
exchanger (heater) which comprises adding 0.01 to 100 weight ppm of a
polyoxyalkylene compound to a solution comprising 5 to 40 wt%, preferably 5-30
wt%,
of a polymer comprising one or more olefins and from 95 to 60, preferably 95
to 70 wt%
of solvent comprising one or more C4-8 saturated hydrocarbons before heating
the
solution in the heat exchanger.
BACKGROUND OF THE INVENTION
In petrochemical plants and for reaction process systems for organic compounds
such as olefin polymerization heat exchange is a unit process essential for
separation
systems. If fouling occurs in the heat exchanger, the efficiency of heat
exchange is
reduced, and the pressure drop across the heat exchanger increases. In extreme
cases, it is necessary to terminate production to remove the foulant. In any
event it is
good practice to seek to reduce build-up that reduces heat exchanger
efficiency and
could at some point "flake off" and contaminate downstream product(s).
United States patent 7,332,070 issued Feb. 19, 2008 to Nishida et al. assigned
to Mitsui Chemicals, Inc. teaches reducing fouling in a cooler by adding one
or more
nonionic surfactants to a solution containing olefin polymers which is cooled
to remove
heat of reaction. The patent does not disclose or suggest adding surfactant to
a
solution of containing olefin polymers which is to be heated to phase separate
solvent
from polymer.
The prevention of fouling of heat exchangers in petroleum refining facilities
is
illustrated by USP 4,200,518 A which discloses a method comprising adding 5 to
99
ppm polyalkylene amine to a hydrocarbon stream. However, the use of
polyalkylene
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amine may raise environmental concerns. JP-A No. 2004-43615 discloses a method
of
removing fouling materials by adding a dialkyl sulfide to raw oil, but its
influence on the
environment may also be a concern.
Polyolefins such as polyethylene, polypropylene, ethylene-.alpha.-olefin
copolymers and propylene-.alpha.-olefin copolymers are produced by a wide
variety of
processes such as a solution phase polymerization and gas-phase
polymerization.
Gas-phase polymerization process results in particulate (granular)
(co)polymers, and
unlike the solution phase polymerization process, does not need steps such as
separation of polymer from a solution.
In the solution phase polymerization process and gas-phase polymerization
process, polyolefins are produced by (co)polymerizing olefins in the presence
of a solid
catalyst such as a solid titanium-based Ziegler-Natta catalyst disclosed in,
for example,
USP 4,952,649 and JP-A No. 7-25946 or metallocene catalyst disclosed in JP-A
No.
2000-297114. In those processes, however, as the amount of the product is
increased,
heat of polymerization is usually increased. One method of removing heat in
solution
phase polymerization is by withdrawing the polymerizing solution once through
a pipe
etc. outside of the reactor, passing the solution through a heat exchanger to
cool it, and
returning it to the reactor. There is also employed a method wherein a part of
a gas
composed of hydrocarbons such as unreacted monomers (also referred to as
"hydrocarbon-containing gas") is withdrawn continuously from a gaseous phase
during
liquid phase polymerization or from the top of a reactor during gas-phase
polymerization, then the hydrocarbon-containing gas is cooled in a heat
exchanger to
remove heat of polymerization, and the gas (and a partially liquefied gas) is
returned as
polymerizable monomers to the polymerization reactor. However, fouling in the
heat
.. exchanger for polymer recovery is a problem.
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In short residence time solution processes (e.g. the residence time of the
solution in the reactor is less than about 15 minutes, preferably less than
about 10
minutes), the exothermic heat of reaction may be balanced somewhat by the
temperature of the reactants being fed to the reactor. The resulting polymer
solution
leaving the last reactor in a reactor train may be at a temperature from about
150 C to
225 C. However, in some instances it is desirable to heat the resulting
solution prior to
polymer recovery by devolatilizing/flashing the solvent. The step of heating
the solution
is typically conducted by passing the process stream through a heat exchanger.
This
may result in the above noted drawbacks. Additionally, as the heat exchanger
is hot,
any buildup of foulant in the heat exchanger may also result in charring of
the material
which could potentially flake off causing "black specks" in the resultant
polymer product.
Canadian Patent application 2598957, in the name of Cheluget et al, assigned
to
NOVA Chemicals, teaches adding a surface active agent selected from the group
consisting of carboxylate, sulfate, phosphate, phosphonate, and sulfonate
compounds
comprising a branched or un-branched, saturated or unsaturated alkyl group
comprising 6 to 30 carbon atoms, and mixtures thereof to a solution of
polyolefins prior
to subjecting the solution to flashing (devolatilization). The patent does not
disclose or
suggest the agents of the present invention.
Applicants have found a dearth of art in the field of reducing fouling of
heaters for
increasing the temperature of ex reactor solutions of polyethylene to assist
in polymer
recovery.
The present invention seeks to provide a process for reducing fouling in heat
exchangers for heating a solution of polymer in a solvent.
SUMMARY OF THE INVENTION
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The present invention provides in a process to increase the temperature of a
solution comprising from 5 to 40 wt% of a polymer comprising from 80 to 100
wt% of
ethylene and from 20 to 0 wt% of one or more C2-8 alpha olefins and from 95 to
60 wt%
of solvent comprising one or more C4-8 saturated hydrocarbons at an initial
temperature
from 150 C to 225 C and a pressure from 5 MPa to 18 MPa the improvement of
reducing heat exchanger fouling by adding to the solution from 0.01 to 100 ppm
by
weight of a polyoxyalkylene compound of the formula:
HO ¨ (CH2CH20)m[CH2CH(CH3)0]N(CH2CH20)pH
wherein M, N and P each represent the average number of repeating units and M
is
from 0 to 12, N is from 2 to 50 and P is from 0 to 12 provided that when M and
Pare 0
the polyoxyalkylene compound has a weight average molecular weight from 300 to
1500 and when one of N or M are present the polyoxyalkylene compound has a
weight
average molecular weight from 2500 to 3500, and passing the solution through a
heat
exchanger to increase its temperature by at least 20 C.
In a further embodiment, the polyoxyalkylene compound is a polypropylene
glycol.
In a further embodiment, in the polyoxyalkylene glycol the the sum of M+P is
from 4 to 20.
In a further embodiment, in the polyoxylakylene compound N is from 10 to 50.
In a further embodiment, the polyoxyalkylene compound is a catalyst
deactivator.
In a further embodiment, the polyoxyalkylene compound is used in conjunction
with an additional catalyst deactivator.
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In a further embodiment, the additional deactivator is selected from the group
consisting of C6-8 alkanoic carboxylic acids.
DETAILED DESCRIPTION
The efficiency of a heat exchanger is a major consideration when determining
the volume of polymer solution that may be adequately heated by a given heat
transfer
fluid. The overall amount of heat transfer depends on a number of factors,
including but
not limited to the materials used for construction of a heat exchanger, the
area of the
heat exchange surface (i.e. the number, length and diameter of tubes in the
tube sheet
of a shell and tube type heat exchanger), the rate of flow of polymer solution
and/or the
heat transfer fluid through the tube and shell sides of the heat exchanger
respectively,
whether the flows are parallel counter-current or parallel co-terminus, the
nature of fluid
flow (turbulent or Newtonian), and the nature and composition of the
exchanging fluids.
Optimization of heat transfer is most commonly addressed though the design
and construction of the associated heat exchanger equipment. As a result,
significant
capital investment may be required for making suitable upgrades such as the
installation of inserts to increase turbulent flow within the heat exchanger
tubes, the use
of larger heat exchangers or the use of heat exchangers with more heat
exchange
capacity. Alternatively, the heat transfer fluid may be heated to higher
temperatures,
but this requires significantly higher energy input.
However, all of the above goes for naught if the exchanger becomes fouled.
In the solution polymerization of ethylene with one or more comonomers,
typically 03-8, preferably C4-8 alpha olefins, the monomers are typically
dissolved in an
inert hydrocarbon solvent, typically a C5-12 hydrocarbon, which may be
unsubstituted or
substituted by a C1-4 alkyl group, such as pentane, methyl pentane, hexane,
heptane,
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octane, cyclohexane, methylcyclohexane and hydrogenated naphtha. An example of
a
suitable solvent that is commercially available is "Isoparn" E" (C8_12
aliphatic solvent,
Exxon Chemical Co.).
Catalyst and activators are also dissolved in the solvent or suspended in a
diluent miscible with the solvent at reaction conditions. Typically, the
catalyst may be a
Ziegler-Natta type catalyst or a single site type catalyst. Generally the
Ziegler-Natta
type catalysts comprise a transition metal halide, typically titanium, (e.g.
TiCI4), or a
titanium alkoxide (Ti(OR)4 where R is a lower C1-4 alkyl radical) on a
magnesium
support (e.g. MgCl2 or BEM (butyl ethyl magnesium) halogenated (with for
example
CCI4) to MgCl2) and an activator, typically an aluminum compound (AIX4 where X
is a
halide, typically chloride), a tri alkyl aluminum (e.g. AIR3 where R is a
lower C1-8 alkyl
radical (e.g. trimethyl aluminum), (RO)aAIX3_a where R is a lower C1-4 alkyl
radical, X is a
halide, typically chlorine, and a is an integer from 1 to 3 (e.g. diethoxide
aluminum
chloride), or an alkyl aluminum alkoxide (e.g. RaAl(OR)3-a where R is a lower
C1-4 alkyl
radical and a is as defined above (e.g. ethyl aluminum diethoxide). The
catalyst may
include an electron donor such as an ether (e.g. tetrahydrofuran etc.). There
is a large
amount of art disclosing these catalyst and the components and the sequence of
addition may be varied over broad ranges.
The catalyst may be a bulky ligand single site catalyst of the formula:
(L)¨M--(Y)p
wherein M is selected from the group consisting of Ti, Zr, and Hf; L is a
monoanionic
ligand independently selected from the group consisting of cyclopentadienyl-
type
ligands, and a bulky heteroatom ligand containing not less than five atoms in
total
(typically of which at least 20%, preferably at least 25% numerically are
carbon atoms)
and further containing at least one heteroatom selected from the group
consisting of
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boron, nitrogen, oxygen, phosphorus, sulfur and silicon, said bulky heteroatom
ligand
being sigma or pi-bonded to M; Y is independently selected from the group
consisting
of activatable ligands; n may be from 1 to 3; and p may be from 1 to 3,
provided that the
sum of n+p equals the valence state of M, and further provided that two L
ligands may
be bridged.
Non-limiting examples of bridging group include bridging groups containing at
least one Group 13 to 16 atom, often referred to a divalent moiety such as but
not
limited to at least one of a carbon, oxygen, nitrogen, silicon, boron,
germanium and tin
atom or a combination thereof. Preferably the bridging group contains a
carbon, silicon
or germanium atom, most preferably at least one silicon atom or at least one
carbon
atom. The bridging group may also contain substituent radicals as defined
above
including halogens.
Some bridging groups include but are not limited to a di C1_6 alkyl radical
(e.g.
alkylene radical for example an ethylene bridge), di C6-10 aryl radical (e.g.
a benzyl
radical having two bonding positions available), silicon or germanium radicals
substituted by one or more radicals selected from the group consisting of C1-6
alkyl, C6-
10 aryl, phosphine or amine radical which are unsubstituted or up to fully
substituted by
one or more C.1-6 alkyl or C6-10 aryl radicals, or a hydrocarbyl radical such
as a C1-6 alkyl
radical or a C6-10 arylene (e.g. divalent aryl radicals); divalent C1-6
alkoxide radicals (e.g.
-CH2CHOHCH2-) and the like.
Exemplary of the silyl species of bridging groups are dimethylsilyl,
nnethylphenylsilyl, diethylsilyl, ethylphenylsilyl or diphenylsilyl compounds.
Most
preferred of the bridged species are dimethylsilyl, diethylsilyl and
methylphenylsilyl
bridged compounds.
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Exemplary hydrocarbyl radicals for bridging groups include methylene,
ethylene,
propylene, butylene, phenylene and the like, with methylene being preferred.
Exemplary bridging amides include dimethylamide, diethylamide,
methylethylamide, di-t-butylamide, diisoproylamide and the like.
The term "cyclopentadienyl" refers to a 5-member carbon ring having
delocalized
bonding within the ring and typically being bound to the active catalyst site,
generally a
group 4 metal (M) through 115 - bonds. The cyclopentadienyl ligand may be
unsubstituted or up to fully substituted with one or more substituents
selected from the
group consisting of Ci-io hydrocarbyl radicals in which hydrocarbyl
substituents are
unsubstituted or further substituted by one or more substituents selected from
the group
consisting of a halogen atom and a C1-4 alkyl radical; a halogen atom; a C1-8
alkoxy
radical; a C6-10 aryl or aryloxy radical; an amido radical which is
unsubstituted or
substituted by up to two C1-8 alkyl radicals; a phosphido radical which is
unsubstituted
or substituted by up to two C1-8 alkyl radicals; silyl radicals of the formula
-Si-(R)3
wherein each R is independently selected from the group consisting of
hydrogen, a Ci.8
alkyl or alkoxy radical, and C6-10 aryl or aryloxy radicals; and germanyl
radicals of the
formula Ge-(R)3 wherein R is as defined above.
Typically the cyclopentadienyl-type ligand is selected from the group
consisting
of a cyclopentadienyl radical, an indenyl radical and a fluorenyl radical
where the
radicals are unsubstituted or up to fully substituted by one or more
substituents
selected from the group consisting of a fluorine atom, a chlorine atom; C1-4
alkyl
radicals; and a phenyl or benzyl radical which is unsubstituted or substituted
by one or
more fluorine atoms.
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If none of the L ligands is bulky heteroatom ligand then the catalyst could be
a
bis Cp catalyst (a traditional metallocene) or a bridged constrained geometry
type
catalyst or tris Cp catalyst.
If the catalyst contains one or more bulky heteroatom ligands the catalyst
would
have the formula:
(D)m
(L)n ¨ M ¨ (Y)p
wherein M is a transition metal selected from the group consisting of Ti, Hf
and Zr; D is
independently a bulky heteroatom ligand (as described below); L is a monoan
ionic
ligand selected from the group consisting of cyclopentadienyl-type ligands; Y
is
independently selected from the group consisting of activatable ligands; m is
1 or 2; n is
0, 1 or 2; p is an integer; and the sum of m+n+p equals the valence state of
M, provided
that when m is 2, D may be the same or different bulky heteroatom ligands.
For example, the catalyst may be a bis (phosphinimine), or a mixed
phosphinimine ketimide dichloride complex of titanium, zirconium or hafnium.
Alternately, the catalyst could contain one phosphinimine ligand or one
ketimide ligand,
one "L" ligand (which is most preferably a cyclopentadienyl-type ligand) and
two "Y"
ligands (which are preferably both chloride).
The preferred metals (M) are from Group 4 (especially titanium, hafnium or
zirconium) with titanium being most preferred. In one embodiment the catalysts
are
group 4 metal complexes in the highest oxidation state.
Bulky heteroatom ligands (D) include but are not limited to phosphinimine
ligands (PI) and ketimide (ketimine) ligands.
The phosphinimine ligand (PI) is defined by the formula:
R2-1
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R21 p = N _
R21
wherein each R21 is independently selected from the group consisting of a
hydrogen
atom; a halogen atom; C1-20, preferably C1-10 hydrocarbyl radicals which are
unsubstituted by or further substituted by a halogen atom; a C1-8 alkoxy
radical; a C6-10
aryl or aryloxy radical; an amido radical; a silyl radical of the formula:
¨Si¨(R22)3
wherein each R22 is independently selected from the group consisting of
hydrogen, a
01-8 alkyl or alkoxy radical, and C6-10 aryl or aryloxy radicals; and a
germanyl radical of
the formula:
¨Ge¨(R22)3
wherein R22 is as defined above.
The preferred phosphinimines are those in which each R21 is a hydrocarbyl
.. radical, preferably a 01-6 hydrocarbyl radical.
Suitable phosphinimine catalysts are Group 4 organometallic complexes which
contain one phosphinimine ligand (as described above) and one ligand L which
is either
a cyclopentadienyl-type ligand or a heteroatom ligand.
As used herein, the term "ketimide ligand" refers to a ligand which:
(a) is bonded to the transition metal via a metal¨nitrogen atom bond;
(b) has a single substituent on the nitrogen atom (where this single
substituent is a carbon atom which is doubly bonded to the N atom); and
(c) has two substituents Sub 1 and Sub 2 (described below) which are
bonded to the carbon atom.
Conditions a, b and c are illustrated below:
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Sub 1 Sub 2
\ /
metal
The substituents "Sub 1" and "Sub 2" may be the same or different and may be
further bonded together through a bridging group to form a ring. Exemplary
substituents include hydrocarbyls having from 1 to 20, preferably from 3 to 6,
carbon
atoms, silyl groups (as described below), amido groups (as described below)
and
phosphido groups (as described below). For reasons of cost and convenience it
is
preferred that these substituents both be hydrocarbyls, especially simple
alkyls and
most preferably tertiary butyl.
Suitable ketimide catalysts are Group 4 organometallic complexes which contain
one ketimide ligand (as described above) and one ligand L which is either a
cyclopentadienyl-type ligand or a heteroatom ligand.
The term bulky heteroatom ligand (D) is not limited to phosphinimine or
ketimide
ligands and includes ligands which contain at least one heteroatom selected
from the
group consisting of boron, nitrogen, oxygen, phosphorus, sulfur and silicon.
The
heteroatom ligand may be sigma or pi-bonded to the metal. Exemplary heteroatom
ligands include silicon-containing heteroatom ligands, annido ligands, alkoxy
ligands,
boron heterocyclic ligands and phosphole ligands, as all described below.
Silicon containing heteroatom ligands are defined by the formula:
¨ (Y)SiRxRyRz
wherein the ¨ denotes a bond to the transition metal and Y is sulfur or
oxygen. The
substituents on the Si atom, namely Rx, Ry and Rz, are required in order to
satisfy the
bonding orbital of the Si atom. The use of any particular substituent Rx, Ry
or Rz is not
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especially important to the success of this invention. It is preferred that
each of Rx, Ry
and Rz is a C1-2 hydrocarbyl group (i.e. methyl or ethyl) simply because such
materials
are readily synthesized from commercially available materials.
The term "amido" is meant to convey its broad, conventional meaning. Thus,
these ligands are characterized by (a) a metal-nitrogen bond; and (b) the
presence of
two substituents (which are typically simple alkyl or silyl groups) on the
nitrogen atom.
The terms "alkoxy" and "aryloxy" are also intended to convey their
conventional
meanings. Thus, these ligands are characterized by (a) a metal oxygen bond;
and (b)
the presence of a hydrocarbyl group bonded to the oxygen atom. The hydrocarbyl
group may be a Ci_io straight chained, branched or cyclic alkyl radical or a
C6-13
aromatic radical where the radicals are unsubstituted or further substituted
by one or
more C1-4 alkyl radicals (e.g. 2,6 di-tertiary butyl phenoxy).
Boron heterocyclic ligands are characterized by the presence of a boron atom
in
a closed ring ligand. This definition includes heterocyclic ligands which also
contain a
nitrogen atom in the ring. These ligands are well known to those skilled in
the art of
olefin polymerization and are fully described in the literature (see, for
example, U.S.
Patent's 5,637,659; 5,554,775; and the references cited therein).
The term "phosphole" is also meant to convey its conventional meaning.
"Phospholes" are cyclic dienyl structures having four carbon atoms and one
phosphorus atom in the closed ring. The simplest phosphole is C4PI-14 (which
is
analogous to cyclopentadiene with one carbon in the ring being replaced by
phosphorus). The phosphole ligands may be substituted with, for example, C1-20
hydrocarbyl radicals (which may, optionally, contain halogen substituents);
phosphido
radicals; amido radicals; or silyl or alkoxy radicals. Phosphole ligands are
also well
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known to those skilled in the art of olefin polymerization and are described
as such in
U.S. Patent 5,434,116 (Sone, to Tosoh).
The temperature of the reactor(s) in a high temperature solution process is
from
about 80 C to about 300 C, preferably from about 120 C to 250 C. The upper
temperature limit will be influenced by considerations that are well known to
those
skilled in the art, such as a desire to maximize operating temperature (so as
to reduce
solution viscosity), while still maintaining good polymer properties (as
increased
polymerization temperatures generally reduce the molecular weight of the
polymer). In
general, the upper polymerization temperature will preferably be between 200
and
300 C. The most preferred reaction process is a "medium pressure process",
meaning
that the pressure in the reactor(s) is preferably less than about 6,000 psi
(about 42,000
kiloPascals or kPa). Preferred pressures are from 10,000 to 40,000 kPa (1450-
5800
psi), most preferably from about 14,000-22,000kPa (2,000 psi to 3,000 psi).
The pressure in the reactor system should be high enough to maintain the
polymerization solution as a single phase solution and to provide the
necessary
upstream pressure to feed the polymer solution from the reactor system through
a heat
exchanger system and to a devolatilization system.
The solution polymerization process may be conducted in a stirred "reactor
system" comprising one or more stirred tank reactors or in one or more loop
reactors or
in a mixed loop and stirred tank reactor system. The reactors may be in tandem
or
parallel operation. In a dual tandem reactor system, the first polymerization
reactor
preferably operates at lower temperature. The residence time in each reactor
will
depend on the design and the capacity of the reactor. Generally the reactors
should be
operated under conditions to achieve a thorough mixing of the reactants. In
addition, it
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is preferred that from 20 to 60 wt% of the final polymer is polymerized in the
first
reactor, with the balance being polymerized in the second reactor.
On leaving the reactor the solution should comprise from 5 to 40, preferably 5
to
30, wt% of a polymer comprising one or more olefins and from 95 to 60,
preferably from
95 to 70, wt% of solvent at an initial temperature from 150 C to 225 C.
Generally the
solution on leaving the reactor will be at a pressure from 5 MPa (about 725
psi) to 18
MPa (2600 psi), preferably from 6 MPa (about 870 psi) to 10 MPa (about 1500
psi).
Typically the polymer will comprise from 80 to 100, preferably 85 to 100, most
preferably 90 to 100 wt% of ethylene and from 20 to 0, preferably not more
than 15,
.. most preferably not more than 10 wt% of one or more C2-8 alpha olefins. The
polyethylene prepared in accordance with the present invention may be LLDPE
having
a density from about 0.910 to 0.935 g/cc or (linear) high density polyethylene
having a
density above 0.935 g/cc. The present invention might also be useful to
prepare
polyethylene having a density below 0.910 g/cc (e.g. below 0.905 g/cc -the so-
called
very low and ultra low density polyethylenes).
From 0.01 to 100 ppm by weight of a polyoxyalkylene compound of the formula
HO-(CH2CH20)m[CH2CH(CH3)0]N(CH2CH20)pH
wherein M, N and P each represent the average number of repeating units and M
is
from 0 to 20, N is from 2 to 50 and P is from 0 to 20 is added to the solution
after the
last reactor. Generally, the polyoxyalkylene compound is added to the solution
in an
amount from 0.01 to 100 ppm (by weight), preferably from Ito 10 ppm (by
weight),
most preferably from 1 to 5 ppm (by weight based on the weight of the
solution).
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When M and P are zero, then the polyoxyalkylene compound is polypropylene
glycol. Suitable molecular weights for the polypropylene glycol range from not
less than
about 300 to not more than about 1,500, typically in the 425 to 800 range. N
may range
from about 5 to 25, preferably from 6 to about 18.
When M and P are integers their values individually may range from 1 to 20,
preferably from 1 to 12, and the sum of M+P may range from 4 to 20, preferably
from 4
to 10. In some embodiments the values for M and P are the same. N may range
from
to 50. In these embodiments the polyoxyalkylene compound is a block copolymer
of
ethylene glycol and propylene glycol. The copolymer may have a molecular
weight
10 from about 2500 up to about 3500.
In accordance with the present invention the polyoxyalkylene compound may
also act as a catalyst deactivator. In a further aspect of the present
invention the
polyoxyalkylene compound is used in conjunction with an additional
deactivator.
Typically the deactivator has a polar group. One suitable class of additional
deactivators comprises C4-10 alkanoic acids. Preferably the acids comprise
from 6 to 8
carbon atoms. Most preferably the deactivator is octanoic acid.
The carboxylic acid deactivator may be added to the solution of polyethylene
in
solvent in an amount from 1 to 100 ppm (by weight), preferably the compound is
added
to the solution in an amount from 1 to 10, most preferably from 1 to 5 ppm (by
weight).
On or shortly after exiting the last reactor the polyoxyalkylene compound,
optionally together with the carboxylic acid deactivator, is added to the
solution of
polyethylene. Typically the polyethylene is at a temperature from 150 C to 225
C and
a pressure from about 5 MPa to about 18 MPa, preferably from about 6 to about
10
MPa.
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The polyethylene solution is then passed through a heat exchanger to raise its
temperature by at least 20 C typically from 25 C to 50 C.
The heated solution then passes through a flash tank to remove a substantial
amount of the solvent. The solution/ polymer is then finished to produce
pellets
.. stripped of residual solvent.
The resulting polyethylene may be used in any number of applications. Low
density polyethylene may be used in film applications and higher density
polyethylene
may be used in injection and rotomolding.
In film applications, particularly, it is important that the polyoxyalkylene
.. compound not leave any detectable residue as determined by the naked eye.
EXAMPLES
The present invention will now be illustrated by the following non limiting
examples.
Example 1 - Screening Experiments
The solubilities of a number of potential candidates were tested in three
potential
delivery solvents: methyl pentane, cyclohexane, and xylene. Unless otherwise
indicated in the table below, sufficient polyoxyalkylene compound was added to
the
solvent to make a 2.5 wt% solution. The solution was heated to 50 C and
continuously
stirred for 30 minutes. At the end of 30 minutes, stirring was stopped and the
solution
.. was observed to see if the compound was immiscible or insoluble in the
potential
delivery solvent. The results are set forth in Table 1 below. In the table EO
is ethylene
oxide and PO is propylene oxide, misc is miscible, immisc is immiscible and RT
is room
temperature.
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TABLE .1
Polyoxyalkylene Molecular Methyl Cyclohexane Xylene
Compound Weight pentane
EO/PO/E0 1100 misc immisc. at RT, not tested
block copolymer miscible hot
EO/PO/E0 2000 misc. immisc. at RT, not tested
block copolymer miscible hot
EO/PO/E0 2750 misc at RT at misc at 10% misc at 10%
block copolymer 2%, clear only
at 50 C at
10%
EO/PO/E0 3800 immisc at misc not tested
block copolymer 2.5%
PO/PE/PO block 2150 misc. lmmisc at RT not tested
copolymer misc hot
PO/E0/P0 3600 insoluble at cloudy at RT, clear after
block copolymer RT, cloudy hot clear hot heating (10%)
(10%) (10%)
Dipropylene glycol immisc immisc not tested
Polypropylene 425 misc. misc. not tested
glycol
Polypropylene 1000 misc misc not tested
glycol
Trifunctional 600 immisc at RT immisc at RT
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polypropylene misc. hot but misc. hot but
glycol separates on separates on
cooling cooling
Example 2 ¨ "Hypovial' Test
A number of candidates from Example 1 were further tested using a "hypovial"
method. One ml of surfactant or a surfactant solution was added to 4 g of
methylpentane containing soluble residues of a Ziegler-Natta catalyst system.
The
catalyst residues were present in a concentration significant higher than
expected in the
heat exchanger (e.g. about 3500 ppm of TiCI4). If no precipitation initially
occurred,
the vials containing the surfactant and the catalyst residues were heated to
50 C and
allowed to cool to room temperature. The vials were then observed for
precipitation. If
no precipitation occurred the surfactant was believed worthy of further study.
The EO/PO/E0 block copolymer having a molecular weight of 2750 and the
PO/E0/P0 surfactant having a molecular weight of 3600 were tested as 10%
solutions
in xylene. All other candidates were tested neat.
Additionally 1,2-propanediol (not included in Example 1) was also tested in
this
manner.
The results are set forth in Table 2.
TABLE 2
Polyoxyalkylene Molecular Result
Compound Weight
EO/PO/E0 block copolymer 1100 Insoluble precipitate
EO/PO/E0 block copolymer 2000 Insoluble precipitate
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EO/PO/E0 block copolymer 2750 No precipitate
EO/PO/E0 block copolymer 3800 Insoluble precipitate
PO/E0/P0 block copolymer 2150 Insoluble precipitate
PO/E0/P0 block copolymer 3600 Cloudy, precipitate after heating
1,2- propanediol Cloudy even when hot
Polypropylene glycol 425 No precipitate
Polypropylene glycol 1000 No precipitate
Trifunctional 600 lmmisc cloudy
polypropylene glycol
Example 3 ¨ Catalyst Deactivation Unit and Continuous Polymerization Unit
Tests
Based on the above results EO/PO/ED block copolymer having an Mw of 2750
and polypropylene glycol having an Mw of 425 were tested in a catalyst
deactivation
unit and a continuous polymerization unit.
In the catalyst deactivation unit 10 ml of 10 wt% solution of the block
copolymer
dissolved in xylene or 10 ml of neat polypropylene glycol was added to 90 ml
of
methylpentane containing the soluble residues of a Ziegler-Natta catalyst
prepared in
accordance with the hypovial test, and the mixture was heated in a sealed
sample
cylinder to 200 C at a pressure from 1.5 -2 MPa (250 -300 psi) After 60
minutes at
200 C the cylinder was permitted to cool to room temperature before opening.
On
opening the methylpentane was evaluated for precipitation. No precipitation
was
observed for either sample.
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The continuous polymerization unit is a small scale continuous solution
polymerization reactor in which the solvent was cyclohexane and the catalyst
was a
Ziegler-Natta catalyst at about 18 ppm TiCI4. 100 mmo1/1 of surfactant (e.g.
about a 0.7
wt % solution) in cyclohexane was metered into the discharge from the reactor
at a rate
of 120 - 900 ppm. The solution passed through a block heater at a fixed
temperature
and was heated. The operation of the block heater was evaluated continuously
(inlet
temperature and outlet temperature) to evaluate the efficiency of the heater
over a 12
hour period. Little to no change in post heater temperature was observed. This
was a
"good" rating for each surfactant.
Example 4 - Short Pilot Plant Run
The polymerization was conducted in a solution phase continuously stirred tank
reactor (CSTR). The reactor pressure was about 16 MPa. The outlet temperature
from
the reactor was about 190 C. The reactor exit was fitted with a near Infrared
spectrometer probe as described in Canadian patent application 2,470887 laid
open for
.. public inspection December 14, 2005 in the name of Lacombe et al. assigned
to NOVA
Chemicals Corporation (corresponding to United States patent 7,566,571 issued
July
28, 2009 to Lacombe et al.). After exiting the reactor the polymer solution
passed
through a heat exchanger to raise its temperature to about 230-250 C.
Solutions of EO/PO/E0 block copolymer having an Mw of 2750 and
polypropylene glycol having an Mw of 425 were prepared at 2 wt% in methyl
pentane.
The reactor was operated to polymerize an ethylene octene copolymer in the
presence of a Ziegler-Natta catalyst at a concentration from 2.8 to 3.7 ppm
TiCI4. The
solutions were being tested as deactivators. The solutions were added
proximate to
the exit from the reactor. The amount of additive was increased stepwise over
time.
The temperature and conversions were monitored to determine the point at which
the
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additives deactivated the catalyst. Then the additive levels were increased by
two and
four times to determine if there were any unwanted effects. The baseline of
the NIR
detector (used as an indicator of fouling) was not increased over the baseline
for the
conventional deactivator. There was no increase in the isomerization of the
.. comonomer or impurities in the ethylene monomer. Both compounds were
effective as
deactivators.
Example 5 - Longer Term Pilot Plant trial
A longer term comparison was conducted comparing a 2 wt % solution of an
EO/PO/E0 block copolymer having a molecular weight of 2750 as a deactivator
with a
.. mixed solution (1 wt%) each of the EO/PO/E0 and the conventional
deactivator (C6-8
alkanolic acid). The reactor was as described in Example 4 and operated in the
same
manner. The additives were added proximate the exit of the second reactor in
an
amount to provide concentrations from 3.4 to 4.5 ppm of total deactivator at a
Ziegler-
Natta concentration ranging from 2.9 to 3.4 ppm TiC14. Solvent baselines were
measured before and after the trial to determine if there was any gross
fouling. No
gross fouling was detected.
The blended deactivator showed a better heat transfer than the EO/PO/E0 block
polymer on its own. The blended solution was at least as good as the
conventional
deactivator. No increase in heat exchanger fouling was observed in the
comparison.
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