Note: Descriptions are shown in the official language in which they were submitted.
CA 02692630 2010-02-09
REDUCED FOULING IN POLYMERIZATION REACTORS
FIELD OF THE INVENTION
The present invention relates to the reduction of fouling in reactors for the
polymerization of alpha-olefins in a fluid phase. More particularly the
present invention
relates to treating the internal surface of the reactor with a reducing agent,
particularly
hydrogen to reduce fouling.
BACKGROUND OF THE INVENTION
There is very little art in the field of treating fluid phase polymerization
reactors to
reduce the iron at the reactor surface to reduce fouling.
There are a number of patents relating to reducing static in gas phase
reactors
by incorporating chemicals such as "water add back" (U.S. patent 4,855,370 to
Chirillo
et al.,) controlling the voltage on the reactor wall where sheet formation is
likely to occur
(U.S. patent 4,532,311 to Fulks et al.,) and treating the internal reactor
surface with a
chrome containing compound prior to polymerization (U.S. patent 4,876,320 to
Fulks et
al.) These patents are all directed at reducing static electrical charges on
the wall of a
gas phase reactor.
United States patent 3,842,060 issued Oct. 15, 1974 to McDonald et at.,
assigned to Dart Industries Inc. teaches a method to reduce the gradual build
up of
polymer on the inside wall of a high pressure tubular reactor. The patent
teaches to
continuously add between about 10 and 150 ppm by volume of hydrogen to the
feed for
a high pressure tubular reactor. The patent does not suggest that the
treatment could
be carried out prior to the reaction. The reference teaches away from the
subject
matter of the present invention.
United States patent 5,501,878 issued March 26, 1996 to Barendregt et al,
assigned to Mannesmann Aktiengesellschaft and KTI Group B.V. teaches treating
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transfer line exchangers (TLE) made of boiler steel in a thermal cracking
process to
produce alpha olefins, with hydrogen to reduce Fe203 to Fe304 to reduce the
formation
of coke in the TLE. The Fe203 is believed to be a catalyst site for the
formation of coke
from ethylene or propylene. The composition of the steel is described as
"boiler" steel
in the abstract and at line 23 of Col. 1. However, the only composition
suggested in the
disclosure is 15Mo3 (Col. 1 line 30 and the examples). A 15 Mo steel would
contain 15
% molybdenum. This is well outside of the range of steels contemplated in the
present
invention. The reference teaches that the reaction is a gas phase cracking
reaction and
teaches away from a liquid phase polymerization reaction.
U.S. patent 7,056,399 issued June 6, 2006 to Cai et al., assigned to NOVA
Chemicals (International) S.A. also teaches treating transfer line exchangers.
The
transfer line exchanger is first cleaned (decoked), then reduced and then
further treated
with various chemicals such as sulphides, disulphides and disulfirams. This
teaches
away from the subject matter of the present invention in that the present
invention does
not require further treatment with the sulphur containing compounds.
While this art has been available for a number of years, as far as Applicant
can
determine no one has thought of applying the art to fluid, preferably liquid
polymerization reactions.
The present invention seeks to provide a process to reduce fouling in a fluid,
preferably liquid, phase alpha olefin polymerization reactor by reducing the
internal
surface of the reactor. It is known that Fe203 (Fen is a site for the
production of coke
in cracking reactors the presence of alpha olefins. Reducing the Fe203 removes
the
site for the alpha olefin to attach to the wall of the reactor. Deposits at
such sites may
slough off at various stages leading to a range of fouling from gels to black
specks.
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SUMMARY OF THE INVENTION
In one embodiment the present invention provides a process to reduce fouling
in
a fluid alpha olefin polymerization reactor having an internal steel surface
comprising
less than 10 weight % Mo, comprising prior to polymerization subjecting the
internal
steel surface to reduction by exposure to gaseous hydrogen at a temperature
greater
than 185 C for a time from 15 minutes to 30 hours, with shorter times at
higher
temperatures.
In a further embodiment the gas comprises not less than 70 volume % hydrogen
and the temperature is from 240 C to 360 C for a time from 10 hours to half an
hour
with shorter times at higher temperatures.
In a further embodiment the alpha olefin is ethylene alone or together with
one or
more alpha olefins selected from the group consisting of C3-8 alpha olefins.
In a further embodiment polymerization is conducted at a temperature from 75 C
to 330 C and at pressures from 0.78 MPa (110 psi) to 345 MPa (50,000 psi).
In a further embodiment the polymerization reactor has an internal steel
surface
comprising less than 0.4 weight % carbon, less than 1.5 weight % Mn, less than
2
weight % of Cr, less than 3 weight A of Ni, less than 1 weight % of Mo and
the balance
iron.
In a further embodiment the alpha olefins are dissolved or dispersed in not
less
than 30 weight % of a solvent or diluent.
In a further embodiment the alpha olefins are dispersed in not less than 50%
of
diluent and the reaction is conducted at a temperature from 82 C to 99 C and a
pressure from 0.760 MPa to 6.8 MPa.
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In a further embodiment the alpha olefins are dissolved in not less than 65%
of a
solvent and the reaction is conducted at a temperature from 120 C to 250 C and
a
pressure from 4 MPa to 31 MPa.
In a further embodiment there is less than 10 wt% of a solvent in the reactor
and
the reaction is conducted at a temperature from 150 C to 250 C and a pressure
from
138 MPa to 344 MPa.
In a further embodiment the alpha olefin is ethylene.
In a further embodiment the polymerization is carried out in the gas phase.
The above embodiments may be combined in whole or in part individually or in
combination with each other.
In a further embodiment of the present invention there is provided a process
to
reduce black specks in an ethylene polymer produced in a fluid, preferably
liquid, phase
reactor comprising treating the reactor according to the above embodiments.
BRIEF DESCRIPTION OF THE DRAWING
Figure 1 is a schematic drawing of the quartz reactor unit (QRU) used to
perform
the experiments.
Figure 2 is a plot of the time for reduction of iron on the steel surface
using 100
volume % hydrogen at various temperatures.
DETAILED DESCRIPTION
The present invention is applicable to any alpha olefin polymerization process
which takes place in the fluid phase, preferably liquid phase. Fluid phase
alpha olefin
polymerization reactions may take place over a broad range of temperature and
pressure conditions from 75 C to 330 C and at pressures from 0.78 MPa (110
psi) to
345 MPa (50,000 psi). This includes gas phase reactions, and preferably slurry
reactions, solution reactions and high pressure reactions.
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=
For gas phase, slurry and solutions polymers the reactions are catalyzed
typically with a catalyst selected from the group consisting of:
1. a chrome base catalyst, typically chromium oxide or a silyl chromate on
a silica
support, activated with an aluminum compound such as an aluminum alkyl
compound
which may be halogenated ( e.g. trimethyl aluminum, triethyl aluminum, diethyl
aluminum chloride etc.) or an alkyl aluminum alkoxide (e.g. diethyl aluminum
alkoxide);
2. a Ziegler natta catalyst typically a combination of a transition metal
compound
such as a halide (e.g. TiCI3, TiC14,) or a transition metal alkoxide
(Ti(OEt)4, etc.) a
magnesium compound (MgCl2, EtMgBu, etc) and an aluminum compound as noted
above; and
3. a single site catalyst such as a bridged or unbridged metallocene (e.g.
Cp2ZrX2
etc.) activated with a complex aluminum compound (MAO) or an ionic activator
(e.g. a
borate such as trityl borate) or a mixture thereof.
High pressure polymerizations may be conducted thermally without a catalyst or
in the present of a free radical initiator (e.g. a peroxide).
The polymerization may take place over a broad range of conditions at a
temperature from 75 C to 330 C and at pressures from 0.76 MPa (110 psi) to 345
MPa
(50,000 psi). This encompasses gas phase, slurry, solution and high pressure
polymerizations.
Gas phase reactions may take place over a range of temperatures and
pressures from about 75 C to abut 110 C, typically from 82 C to 99 C and at
pressures from 0.76 MPa (110 psi) to 3.4 MPa (about 500 psi). The gas phase
comprises the monomers, typically one or more C2-6 alpha olefins, such as
ethylene,
1-butene and 1-hexene, saturated gaseous hydrocarbons such as C2-6 alkanes,
inert
gases such as nitrogen, chain transfer agents such as hydrogen and optionally
from
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about 10 to 40, preferably about 13 to 35, most preferably from about 15 to 25
weight %
of a condensable alkane for heat removal from the reaction. The reaction
stream is
passed through a bed of catalyst and growing polymer particles at a pressure
and rate
sufficiently high to keep the bed fluidized. Generally there is an expanded
portion at the
top of the reactor to reduce the gas velocity and cause fines and polymer
particles to
fall back into the bed. The polymer is removed from the bed using a series of
pressure
lock valves and a receiving chamber and degassed. The unreacted monomers and
the
rest of the components in the gas phase leave the reactor and pass through a
compressor and a heat exchanger to compress and cool the gas and if present
cause a
condensable gas to form a suspended liquid phase (i.e. droplets). Additional
monomer
is added to the recycle stream and it enters the reactor beneath a disperser
plate and
again passes upward through the fluidized bed.
Slurry reactions may take place over a range of temperatures and pressures
from about 75 C to abut 110 C, typically from 82 C to 99 C and at pressures
from 0.76
MPa (110 psi) to 6.8 MPa (about 1000 psi). In a slurry reaction the olefins
and
monomers are typically dispersed in a hydrocarbon diluent such as mixture of
one or
more C4_8 saturated hydrocarbons and aromatic hydrocarbons. Typically the
reactor is
a loop reactor with one or more settling legs. The reactor contents, diluent,
monomer
and polymer (and chain transfer agent) circulate around the loop portion of
the reactor.
The diluent in the loop portion of the reactor is present in an amount of at
least 50
weight %, typically at least about 60 weight % of the reactor contents (i.e.
the polymer
may be up to about 40 weight % of the reactor contents in the main loop). In
the
settling leg of the reactor the diluent concentration may range from 40 to 50
weight %
and the polymer may be present in an amount from about 60 to 50 weight %.
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Solution reactions may take place over a range of temperatures and pressures
from about 120 C to about 250 C, typically from 180 C to 220 C and at
pressures from
4 MPa to 31 MPa (about 1900 to about 15,000 psi) typically from about 6 to 10
MPa
(about 2900 to about 4800 psi). The polymer concentration may be as high as
about
35 weight %, but typically is about 10 to 15 weight %. The concentration of
solvent in
the reactor is not less than about 65 weight %, typically from about 90 to 85
weight %.
High pressure reactions take place over a range of temperatures from about
150 C to 250 C, typically from about 180 C to about 230 C and at pressures
from 103
MPa (15000 psi) to 345 MPa (about 50,000 psi) typically from 138 MPa (20,000
psi) to
276 MPa (40,000 psi). The solvent or diluent if present is present in an
amount of less
than 10 weight %, preferably less than 3 weight %, most preferably less than
1.5 weight
%. Preferably a minimum amount or no solvent or diluent is present and the
liquid
phase is monomer.
Typically, apart from high pressure polymerization, the above polymerization
processes may be used to produce homopolymers and copolymers. The alpha olefin
is
selected from the group consisting of C2-8 alpha olefins. Typically the
copolymer will
generally contain less than about 15 weight %, preferably less than 10 weight
% of
comonomer selected from the group consisting of C3-8 alpha olefins. Some
typical
alpha olefin comonomers for ethylene include 1-butene, 1-hexene and 1-octene
(typically present only in solution processes).
The high pressure reaction is typically used to produce homopolymers of
ethylene, although in some cases where propylene is used as a chain transfer
agent
small amounts, typically less than 3 weight %, preferably less than 2 weight %
of
propylene may be incorporated into the polymer.
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The steel used to construct reactors for the above processes is typically a
carbon steel without a high chrome or nickel content. The steel typically
comprises not
less than about 90 weight % of Fe and less than about 10 weight % Mo,
preferably less
than 5 weight % Mo, most preferably less than 2 weight % Mo, desirably less
than 1
weight % Mo. The steel may have a composition comprising less than 0.4 weight
%
carbon, less than 1.5 weight % Mn, less than 2 weight % of Cr, less than 3
weight % of
Ni, less than 1 weight % of Mo and the balance iron. One suitable steel
comprises from
0.3 to 0.4, preferably from 0.3 to 0.38 weight % carbon, from 0.7 to 1.5
weight %,
preferably from 0.7 to 1 weight % of Mn, from 0.8 to 2, preferably from 0.8 to
1.2 weight
% of Cr, from 0.5 to 3 weight %, preferably from 0.5 to 2.5 weight % of Ni,
from 0.5 to 1
weight %, preferably from 0.5 to 0.65 weight % of Mo and the balance of iron.
The interior surface of the reactor is treated with a reducing gas typically
hydrogen, or a mixture comprising not less than 50 volume % hydrogen and up to
50
volume % of an inert gas such as helium, nitrogen and argon. Preferably, the
gas
mixture comprises not less than 75 volume %, preferably not less than 90
volume %, of
hydrogen and correspondingly up to 25 volume %, preferably up to 10 volume %
of one
or more inert gases selected from the group consisting of helium, nitrogen and
argon,
preferably nitrogen.
When the treatment gas comprises 100 volume % of hydrogen the reactor may
be treated for a time from 15 minutes to 30 hours at a temperature of at least
185 C,
with longer times at lower temperatures. Typically the temperature for
treatment will be
from 240 C to 360 C at times from about 10 hours to about 15 minutes
respectively.
For gas mixtures which are more dilute in hydrogen the times would be
proportionally
increased relative to the amount of dilution with the inert gas. When the
treatment gas
comprises not less than 70 volume % hydrogen and the temperature is from 240 C
to
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360 C the treatment time may be from 10 hours to half an hour with shorter
times at
higher temperatures. When the treatment gas comprises not less than 90 volume
% of
hydrogen and the temperature is from 240 C to 360 C the treatment time may be
from
8 hours to about 20 minutes with shorter times at higher temperatures.
Preferably, the treatment reduces the iron at interior the reactor surface to
Fe+2
without the formation of submicron iron powder or dust (e.g. iron particles
having a size
less than about 0.5, typically less than 0.2, most preferably less than about
0.15
microns).
The present invention will now be illustrated by the following example.
The experiments were carried out in two steps first using a muffle furnace to
oxidize and a quartz reactor unit (QRU) to reduce which is schematically shown
in
Figure 1.
Coupons of steel having the composition of that in NOVA Chemicals' high
pressure reactor were initially pre-oxidized for 4 hours in a muffle furnace
at 870 C in
an air flow to form hematite (Fe203). The degree of pre-oxidization was
calculated by
weight gain of the sample. For all samples the degree of oxidation was greater
than
95%.
The coupon was then transferred to the QRU where the coupon was placed in
the quartz tube of the QRU and brought to temperature. Dry hydrogen was flowed
through the unit and the weight loss of the sample was measured until there
was
theoretically up to 100% conversion of the hematite to magnetite (Fe304).
The QRU comprises a quartz tube 1 in a furnace 5. The quartz tube has an inlet
2 and an outlet 3 for gases. Coupons 4 are mounded inside the quartz tube 1
and a
stream of gas passes over the coupons at the desired temperature.
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A typical run of the QRU In the experiments the coupons 4 were placed in the
quartz tube 1. The quartz tube 1 was pressurized to test for leaks. If there
were no gas
leaks the quartz tube is purged with 500 standard cubic centimeters per minute
(sccm)
of N2 for one hour. The N2 flow rate is reduced to 250 sccm during heating.
When the
set temperature is reached the N2 flow rate is reduced to from 0 to 375 sccm.
A flow of
H2 is started at a rate from 500 to 125 sccm. This is continued at the set
temperature
for a period of time up 25 hours according to the experimental matrix. When
the run
time has been reached the reactor is cooled at a rate of 2 C per minute until
a
temperature of 100 C is reached and then the H2 flow is stopped and the N2
flow is set
at 500 sccm. The metal coupons 4, are removed from the quartz tube when the
reactor
cools to below 50 C.
The coupon was then removed from the test unit and a potassium thiocyanate
solution as an indicator for Fe(III) was applied to the surface of the sample.
If the
surface turned red it indicates the presence of Fe (III) indicating an
incomplete
reduction of the surface of the sample. If there is no color change all of the
Fe(III) had
been reduced to Fe(ll).
The coupons were treated at a number of temperatures and times and except for
the temperatures at 185 C there was complete reduction of the hematite to
magnetite.
The test results are set forth in Table 1.
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TABLE 1
Temperature Time Theoretical reduction of Fe(III)
Thiocyanate test
to Fe(II)
185 C 26 hr About 92 % Trace of red.
200 C 18 hr 100% No color change
210 C 14 hr 100% No color change
240 C 8 hr 100% No color change
270 C 2 hr 100% No color change
280 C 2 hr 100% No color change
295 C 1 hr 100% No color change
310 C 1 hr 100% No color change
335 C 30 min 100% No color change
355 C 20 min 100% No color change
_
The results of the tests are plotted in Figure 2. The results show it is
possible to
reduce Fe(III) on the internal wall of a reactor by treatment with hydrogen
gas
particularly at temperatures above 230 C for times of less than about 10
hours.
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