U.S. patent application number 10/138627 was filed with the patent office on 2002-10-24 for raman analysis system for olefin polymerization control.
Invention is credited to Bartel, Paul A.., Long, Robert L., Young, Robert Earl.
Application Number | 20020156205 10/138627 |
Document ID | / |
Family ID | 26844116 |
Filed Date | 2002-10-24 |
United States Patent
Application |
20020156205 |
Kind Code |
A1 |
Long, Robert L. ; et
al. |
October 24, 2002 |
RAMAN ANALYSIS SYSTEM FOR OLEFIN POLYMERIZATION CONTROL
Abstract
Method of olefin polymerization in a reactor, such as a slurry
loop olefin polymerization reactor, is provided. The method
includes conducting in-situ, real time spectroscopic analysis of
one or more reactor constituents. The reactor constituents analyzed
may be present in the reactor in either the liquid phase or the
solid phase, or both. In response to the measured in-situ values,
one or more reactor constituents may be metered into the
reactor.
Inventors: |
Long, Robert L.; (Houston,
TX) ; Young, Robert Earl; (Fountain Valley, CA)
; Bartel, Paul A..; (Baton Rouge, LA) |
Correspondence
Address: |
Kevin M. Faulkner
ExxonMobil Chemical Company
P O Box 2149
Baytown
TX
77522-2149
US
|
Family ID: |
26844116 |
Appl. No.: |
10/138627 |
Filed: |
May 3, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10138627 |
May 3, 2002 |
|
|
|
09627498 |
Jul 28, 2000 |
|
|
|
60146632 |
Jul 30, 1999 |
|
|
|
Current U.S.
Class: |
526/60 ; 526/59;
526/64 |
Current CPC
Class: |
C08F 10/00 20130101;
Y10S 526/905 20130101; C08F 2400/02 20130101; C08F 2500/12
20130101; C08F 2/00 20130101; C08F 210/14 20130101; C08F 210/16
20130101; C08F 210/16 20130101; C08F 10/00 20130101 |
Class at
Publication: |
526/60 ; 526/59;
526/64 |
International
Class: |
C08F 002/00 |
Claims
1. A method of olefin polymerization in a slurry reactor containing
reactor constituents having a liquid phase comprising: irradiating
in-situ the reactor constituents; measuring scattered energy from
the irradiated reactor constituents; determining a concentration of
one or more reactor constituents; and metering a flow of at least
one reactor constituent into the reactor in response to the
determined concentration.
2. The method of claim 1 wherein the determining step is performed
on at least one reactor constituent in the liquid phase.
3. The method of claim 2 wherein at least one of the reactor
constituents in the liquid phase is hydrogen.
4. The method of claim 1 wherein the reactor constituents are
circulated in the reactor.
5. A method of producing a polyolefin in a slurry reactor
containing reactor constituents comprising: irradiating in-situ a
slurry; measuring scattered energy from the slurry; determining
from the measured scattered energy a concentration of one or more
reactor constituents; comparing the concentration of one or more
reactor constituents with one or more values that correlate to one
or more selected physical properties of the polyolefin; and
metering, in response to the comparing step, a flow of one or more
reactor constituents into the reactor.
6. The method of claim 5 wherein the determining step is performed
on at least one reactor constituent in a liquid phase of the
slurry.
7. The method of claim 6 wherein at least one of the reactor
constituents in the liquid phase is hydrogen.
8. The method of claim 5 wherein at least one of the selected
physical properties of the polyolefin is a melt flow rate.
9. The method of claim 8 wherein the flow of one or more of the
reactor constituents into the reactor is metered such that the
polyolefin produced may be defined, in part, by a melt flow rate
value within a selected melt flow rate range.
10. A method of producing a polyolefin in a reactor containing
reactor constituents in liquid phase comprising: irradiating
in-situ the liquid phase; measuring the frequencies scattered by
the irradiated liquid phase; determining from the measured
frequencies a concentration of one or more reactor constituents;
comparing the concentration of one or more reactor constituents
with one or more values that correlate to one or more selected
physical properties of the polyolefin; and metering, in response to
the correlating step, a flow of one or more reactor constituents
into the reactor.
11. The method of claim 10 wherein one of the selected physical
properties of the polyolefin is a melt flow rate value within a
selected melt flow rate range and wherein the flow of one or more
reactor constituents into the reactor is metered such that the
polyolefin produced may be defined, in part, by a melt flow rate
value within the melt flow rate range.
12. A method of producing a polyolefin in a slurry loop reactor
containing reactor constituents, including hydrogen, in a liquid
phase comprising: irradiating in-situ the liquid phase; measuring
the frequency scattered by the hydrogen in the liquid phase;
determining the concentration of hydrogen in the liquid phase from
the measured frequency; comparing the concentration of hydrogen to
a hydrogen concentration value that correlates to a melt flow rate
value within a selected melt flow rate range; and metering, in
response to the concentration of hydrogen measured, a flow of the
hydrogen into the reactor such that the polyolefin produced may be
defined, in part, by a melt flow rate value within the selected
melt flow rate range.
13. A method of producing a polyolefin in a slurry reactor
containing reactor constituents comprising a liquid phase and a
solid phase forming a slurry reaction mixture, said method
comprising: irradiating in-situ at least a portion of the slurry
reaction mixture; measuring scattered energy from the slurry;
determining from the measured scattered energy a concentration of
one or more reactor constituents; controlling the flow of one or
more reactor constituents into the reactor based on the
concentration of one or more reactor constituents.
14. The method of claim 13 wherein the one or more reactor
constituents in said determining step are the same as said one or
more reactor constituents in the controlling step.
15. The method of claim 13 wherein at least one of the one or more
reactor constituents in said determining step is different from
said one or more reactor constituents in the controlling step.
16. The method of claim 13 wherein the concentration determined is
that of ethylene in the liquid phase of a slurry reaction
mixture.
17. The method of claim 13 wherein the measured concentration of
ethylene and at least one other alpha-olefin copolymer in the
liquid phase of a slurry reaction mixture are used to control the
ratio of one or more reactor constituents into the reactor.
18. The method of claim 13 wherein the measured concentration of
polymer in the solid phase of a slurry reaction mixture is used to
control the flow of one or more reactor constituents into the
reactor.
19. The method of claim 13 wherein the flow controlled is diluent.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation-in-part of U.S. patent
application Ser. No. 09/627,498 filed Jul. 28, 2000, which claims
priority to U.S. Provisional Patent Application No. 60/146,632
filed Jul. 30, 1999, the entire disclosures of which are
incorporated herein by reference.
FIELD OF INVENTION
[0002] This invention relates to spectroscopic in-situ analysis of
constituents in a chemical reaction. More particularly, this
invention relates to spectroscopic in-situ analysis of constituents
in a slurry loop polymerization reactor.
BACKGROUND OF THE INVENTION
[0003] Spectroscopic analysis is a branch of analytical chemistry
devoted to identification of elements and elucidation of atomic and
molecular structure. Generally, the identification of elements and
elucidation of atomic and molecular structure is accomplished by
illuminating or irradiating the substance under examination and
then measuring the radiant energy absorbed or emitted by the
substance. The energy absorbed or emitted may be in any of the
wavelengths of the electromagnetic spectrum. By comparing and/or
correlating the measured wavelengths absorbed or emitted by the
sample with wavelengths emitted or absorbed from known elements or
molecules, information about a sample may be determined.
[0004] More particularly, spectroscopic analysis generally requires
isolating a portion of the substance under investigation. The
isolated portion is then prepared for illuminating or irradiating
by an energy source. After irradiation, the energy absorbed or
emitted by the isolated portion is measured and correlated to
values derived from known materials measured under similar
conditions.
[0005] Spectroscopic analysis is a common tool used in laboratories
and industrial processes. Its uses include determining the
molecular identity and properties of a chemical composition as well
as monitoring the progress of a reaction. Whether conducting a
laboratory exercise or industrial process, this type of information
is desirable. This is so because, for example, data derived from
spectroscopic analysis may be used to identify the final product of
these reactions and determine the consumption and/or identity of
intermediates produced at selected stages in a multistage
process.
[0006] For industrial processes and particularly industrial
chemical reactions, in-situ identification and monitoring of (i)
the reaction constituents, (ii) the reaction intermediates, (iii)
the consumption rate of the starting materials, and (iv) the final
product are desirable. In-situ analysis is desirable generally
because the analysis environment is the reaction environment within
the reaction vessel. In this way, the isolation and preparation of
a portion of the substance under investigation prior to irradiation
is avoided. And still more desirable is the acquisition and
assimilation of analysis information after the passage of a
relatively short period of time from the moment the analysis
process is initiated, otherwise referred to as "real time
analysis".
[0007] However, there remain many industrial processes, and
particularly industrial chemical reaction environments, for which
spectroscopic analysis techniques do not offer an investigator the
option of conducting reliable, in-situ, real time analysis. As
such, there exists a need for further development in the field of
in-situ, real time spectroscopic analysis and the application
thereof in industrial processes.
SUMMARY OF THE INVENTION
[0008] The present invention provides both apparatus and methods
for conducting in-situ, real time spectroscopic analysis of one or
more reaction constituents present in a reactor, particularly a
slurry olefin polymerization reactor and more particularly, a
slurry loop olefin polymerization reactor. Examples of reaction
constituents include polymerized and polymerizable olefins.
Examples of polymerized olefins include, but are not limited to
polypropylene, polyethylene, polyisobutylene, and homopolymers and
copolymers thereof. Other examples of reactor constituents include,
but are not limited to hydrogen, propane, ethane, butane monomers,
and comonomers. Examples of monomers and comonomers include, but
are not limited to ethylene, propylene, butene, hexene, octene,
isobutylene, styrene, norbornene and the like.
[0009] Without limiting the present invention to any particular
spectroscopic analysis technique, the inventors have observed in a
slurry reaction environment a correlation between in-situ collected
Raman spectra (a product of Raman spectroscopy) from the liquid
phase of the reaction environment and the concentration of at least
one reactor constituent. Furthermore, the inventors have discovered
that this correlation, in combination with in-situ, real time
analysis of at least one reactor constituent in such a reactor will
allow for improved control of the final product properties, such as
melt flow rate. Improved control of the final product properties is
achieved by metering the flow of at least one reactor constituent
into the slurry reactor in response to the in-situ measured
concentration of at least one reactor constituent.
[0010] In one embodiment, a method of olefin polymerization in a
reactor having reactor constituents in a liquid phase is provided.
The method steps include measuring in-situ a first reactor
constituent and metering the flow of a second reactor constituent
into the reactor in response to the measuring step. The first and
second reactor constituents may be the same constituent or they may
be different constituents.
[0011] In another embodiment, another method of olefin
polymerization in a multi-phase reactor containing reactor
constituents is provided. The method steps include irradiating
in-situ the reactor constituents, measuring scattered or reflected
energy from the irradiated reactor constituents, determining from
the measured scattered or reflected energy a concentration of at
least one reactor constituent, and metering the flow of at least
one reactor constituent into the reactor in response to the
determining step.
[0012] In another embodiment, a method of olefin polymerization in
a reactor containing reactor constituents in a liquid phase is
provided. These method steps include irradiating in-situ the liquid
phase, measuring the frequencies scattered or reflected by the
irradiated liquid phase, correlating at least one measured
frequency with the concentration of a first reactor constituent,
and metering, in response to the correlating step, a flow of the
first reactor constituent into the reactor.
[0013] In another embodiment, another method of producing a
polyolefin in a reactor containing reactor constituents in a liquid
phase is provided. These method steps include irradiating in-situ
the liquid phase, measuring the frequencies scattered by the
irradiated liquid phase, determining from the measured frequencies
a concentration of one or more reactor constituents, comparing the
concentration of one or more reactor constituents with one or more
values that correlate to one or more selected physical properties
of the polyolefin, and metering, in response to the correlating
step, the flow of one or more reactor constituents into the
reactor. One of the selected physical properties of the polyolefin
may be melt flow rate. Additionally, the metered flow of one or
more reactor constituents into the reactor may be controlled such
that the polyolefin produced may be defined, in part, by a melt
flow rate value within a selected melt flow rate range.
[0014] In another embodiment, a method of olefin polymerization in
a slurry reactor containing reactor constituents, including
hydrogen, in a liquid phase is provided. These method steps
include, irradiating in-situ the liquid phase, measuring the
frequency scattered or reflected by the hydrogen in the liquid
phase, determining the concentration of hydrogen in the liquid
phase from the measured frequency, and metering, in response to the
concentration of hydrogen measured, the flow of the hydrogen into
the reactor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 is a block diagram of a slurry loop reactor plant
schematically illustrating an in-situ Raman spectroscopic system
and data feed from the same into the slurry loop reactor plant
control system.
[0016] FIG. 2 is a schematic view of the fiber optic probe
assembly.
[0017] FIG. 3 is an enlarged, fragmented, cross sectional view of a
fiber optic probe tip.
[0018] FIG. 4 is a schematic illustration of a laboratory slurry
reactor.
[0019] FIG. 5a is an illustration of Raman spectra of a
pentane/polypropylene slurry.
[0020] FIG. 5b is an expanded view of a portion of the spectra of
FIG. 5a.
[0021] FIG. 6 is a plot of hydrogen pressure vs. hydrogen
prediction by Raman.
[0022] FIG. 7 is an expanded Raman spectra of acetonitrile in
pentane.
[0023] FIG. 8 is an illustration of Raman spectrum of polypropylene
granules.
[0024] FIG. 9 is a plot of predicted MFR from Raman analysis vs.
known MFR.
[0025] FIG. 10 is a plot of 1-hexene and ethylene Raman
spectra.
[0026] FIG. 11 is a plot of a relationship between the peak height
of 1-hexene vs. 1-hexene concentration.
[0027] FIG. 12 is a plot of a relationship between the peak height
of ethylene vs. ethylene concentration.
[0028] FIG. 13 is a plot of an expanded Raman spectra for the
slurry described in Example 5.
[0029] FIG. 14 is a plot of a relationship between the ethylene
peak ratio and the gas chromatograph ethylene concentration
measurement.
[0030] FIG. 15 is a plot of a correlation between the Raman
polyethylene copolymer peak ratio and the slurry
density/polyethylene copolymer concentration in the slurry loop
reactor described in Example 5.
[0031] FIG. 16 is a schematic illustration of an imaging probe
assembly.
DETAILED DESCRIPTION OF THE INVENTION
[0032] The present invention provides both apparatus and methods
for conducting in-situ, real time spectroscopic analysis of one or
more reactor constituents present in a reactor, particularly a
slurry olefin polymerization reactor and more particularly, a
slurry loop olefin polymerization reactor. Examples of reaction
constituents include, but are not limited to polymerized and
polymerizable olefins. Generally, polymerizable olefins include,
but are not limited to C.sub.2, C.sub.3-C.sub.20, alpha olefins,
C.sub.4-C.sub.20 diolefins, C.sub.5-C.sub.20 cyclic olefins,
C.sub.7-C.sub.20 vinyl aromatic monomers and C.sub.4-C.sub.20
geminally disubstituted olefins. More specific examples of
polymerizable olefins include, but are not limited to propylene,
ethylene, 1-butene, 1-hexene, 1-octene, isobutylene, 1,4-hexadiene,
dicyclopentadiene, norbornene, ethylidene norbornene, vinyl
norbornene and styrene, and products thereof, such as polyolefin
copolymer elastomers and engineering olefin copolymers.
[0033] A slurry loop olefin polymerization reactor can generally be
described as a loop-shaped continuous tube. In some instances, the
reactor design may be generally "O" shaped. One or more fluid
circulating devices, such as an axial pump, urge the reactor
constituents within the tube in a desired direction so as to create
a circulating current or flow of the reactor constituents within
the tube. Desirably, the fluid circulating devices are designed to
provide high velocity of motion and a very intensive and
well-defined mixing pattern of the reactor constituents. The
reactor may be totally or partially jacketed with cooling water in
order to remove heat generated by polymer polymerization.
[0034] In the slurry loop olefin polymerization reactor, the
polymerization medium may include reactor constituents, such as a
liquid monomer, like propylene and/or a hydrocarbon solvent or
diluent, advantageously aliphatic paraffin such as propane,
isobutane, hexane, heptane, cyclohexane and/or an aromatic diluent
such as toluene. The polymerization temperatures may be those
considered low, e.g., less than 50.degree. C., desirably 0.degree.
C.-30.degree. C., or may be in a higher range, such as up to about
150.degree. C. desirably from about 50.degree. C. up to about
80.degree. C. or at any range between the end points indicated.
Pressures can vary from about 100 to about 700 psia (0.69-4.8 MPa).
Additional description is given in U.S. Pat. Nos. 5,274,056 and
4,182,810 and WO 94/21962 which are each fully incorporated by
reference. As such, the reactor constituents generally are a
combination of both solids, such as for example catalysts, catalyst
supports, polymerized olefins, and the like as understood by those
skilled in the art and liquids, such as those described above. The
percentage of solids within the reactor constituents may be as high
as 60 weight percent (wt %) of the reactor constituents. Desirably,
the weight percent of solids is in the range of 45 wt % to 50 wt
%.
[0035] The slurry loop olefin polymerization reactor may be
operated in a single stage process or in multistage processes. In
multistage processing, the polymerization of olefins is carried out
in two or more reactors. These reactors may be configured in series
or in parallel or a combination thereof. Examples of other olefin
polymerization reactors suitable for multistage processing with
slurry loop olefin polymerization reactors include slurry and
slurry loop olefin polymerization reactors, gas phase olefin
polymerization reactors, and other moving-bed, fixed-bed, or
fluid-bed reactors.
[0036] Without limiting the present invention to any particular
spectroscopic analysis technique, the present invention employs
Raman spectroscopic techniques to determine the in-situ
concentration of at least one reactor constituent, such as for
example, hydrogen and desirably dissolved hydrogen present in the
liquid phase. Examples of other measurable reactor constituents
include, but are not limited to diluents, monomers, comonomers, the
identity of reaction intermediates, and final polymer properties,
such as melt flow rate, comonomer content, crystallinity, melt
index, viscosity index, polymer melt viscosity, density and percent
unsaturation, and the like.
[0037] Raman spectroscopy analysis begins by irradiating a material
under investigation with energy, such as electromagnetic energy for
example in the visible or near infrared wavelength regions. The
radiation is scattered upon impact with the material. The scattered
radiation may be classified as elastically scattered and
inelastically scattered radiation. The inelastically scattered
radiation is referred to as Raman scatter. The wavelengths and
intensities of the Raman scatter make up the Raman spectrum. It is
the Raman spectrum that provides chemical, structural and other
information about the irradiated material.
[0038] The present invention utilizes data derived from in-situ
Raman sampling of reactor constituents in the liquid phase of a
slurry olefin polymerization reactor. Such data includes properties
and concentrations of reactor constituents. This data is used to
control the polymerization reaction and final polymer product
properties, such as melt flow rate, comonomer content, and the
like. The polymerization reaction control is achieved, for example,
by metering the flow of reactor constituents into the reactor in
response to the Raman sampling data. These and other details of the
present invention will be more fully described by reference to the
accompanying Figures and the following discussion.
[0039] Turning now to FIG. 1, a slurry loop reactor plant 100 is
schematically illustrated. The slurry loop reactor plant 100
includes a slurry loop reactor vessel, portions of which are
designated by reference numbers 102a and 102b, an analyzer system
104, a reactor control system 106 and a reactor constituent feed
source 108. An example of a suitable reactor control system 106 is
more fully described in U.S. Pat. No. 5,682,309 which is
incorporated by reference herein in its entirety.
[0040] Briefly, the reactor control system 106 controls the slurry
loop reactor plant processes. These processes include (i)
manipulated variables, such as for example, hydrogen feed flow
rate, total feed rate and catalyst flow rate and (ii) control
variables, such as, for example, melt flow ratio, ethylene content,
and product rate. The reactor control system 106 includes a
processor, sensors and sensor circuitry (not shown). The sensors
and sensor circuitry provide data, such as measures of the control
variables. The processor provides memory for storing data, such as
correction time constants, upper and lower limits for control
variables and generates signals responsive to sensor data and
limits data. Such signals generated by the reactor control system
106 and conveyed by conduit 105 to the reactor constituent feed
source 108 can influence the metering of reactor constituents from
the reactor constituent feed source 108 through conduit 107 and
into the slurry loop reactor vessel 102b.
[0041] The analyzer system 104 includes an in-situ probe 139, such
as a fiber optic probe, secured to the reactor vessel 102a, a
radiation source 112, such as a laser, connected to the probe 139
by a conduit 114, such as a fiber optic cable. Another conduit 116,
such as a fiber optic cable, connects the probe 139 to a spectrum
converter 118, such as a Raman spectrum converter. The spectrum
converter 118 is connected via conduit 120 to an analyzer 122, such
as a Raman analyzer. The analyzer 122 is connected via conduit 124
to the reactor control system 106.
[0042] The probe 139 includes a center radiation transmission
conduit (not shown), such as a fiber optic cable, for conducting
radiation energy from the radiation source 112 into the reactor
vessel 102 and ultimately for irradiating one or more reactor
constituents. Surrounding the center conduit is a plurality of
receiving conduits (not shown), such as a plurality of fiber optic
cables, for receiving radiation scattered by at least one of the
irradiated reactor constituents. The receiving conduits also convey
the scattered radiation to the spectrum converter 118.
[0043] In the operation of the present invention, laser light from
irradiation source 112 is delivered via an optical fiber within
conduit 114 to an optical fiber within the center radiation
transmission conduit which is in communication with the reactor
constituents. Irradiation of the reactor constituents generates
scattered radiation, a portion of which is collected by one or more
optical fibers that form the receiving conduits. The collected
scattered radiation is conveyed from the receiving conduits to the
spectrum converter 118 by one or more optical fibers within conduit
116. In the spectrum converter 118, the scattered radiation is
filtered by a holographic notch filter to remove unshifted
radiation. A CCD (Charged Couple Device) camera records radiation
intensity over a range of selected wavelengths. The selection of
wavelengths is dependent, in part, on the wavelength of the laser
light irradiating the reactor constituents and the reactor
constituents being investigated. The analyzer 122 receives the
wavelength data from the spectrum converter 118 via conduit 120.
The analyzer may be preprogrammed to examine selected wavelengths
corresponding to reactor constituents that the reactor operator may
desire to monitor and/or examine. For example, the wavelength shift
of 4140 cm.sup.-1 corresponds to the dissolved hydrogen in the
slurry loop reactor vessel 102. The wavelength shifts for other
reactor constituents may be found in "The Handbook of Infrared And
Raman Characteristic Frequencies of Organic Molecules," Daimay
Lin-Vien, et al, (1991). The recorded intensities may be plotted as
peaks of varying heights as a function of wavelength. Information,
such as the concentration of the reactor constituent, may be
estimated by calculating the area under or the height of one or
more of the associated peaks. Alternatively, multivariate
statistical methods, such as principle component regression or
partial-least squares regressions can also be used to correlate the
concentration of the reactor constituents or polymer properties to
the spectral intensities. The above described irradiation/analysis
cycle may be repeated between every 5 seconds to 1,000 seconds or
as otherwise desired.
[0044] Data from the analyzer 122, such as the concentration of
hydrogen in the liquid phase (which may also be referred to as the
"dissolved hydrogen concentration"), may be conveyed via conduit
124 to the reactor control system 106. As previously described, the
processor in the control system 106 can compare the preset upper
and lower limits for reactor constituent concentrations and the
flow of these reactor constituents into the reactor vessel 102 with
the data from the analyzer 122 and adjust or meter the flow thereof
accordingly.
[0045] For example, in the case of an olefin slurry loop
polymerization reactor, hydrogen can serve as a polymer chain
transfer agent. In this way, the molecular weight of the polymer
product can be controlled. Additionally, varying the hydrogen
concentration in olefin polymerization reactors can also vary the
polymer melt flow rate (MFR). In some instances, customers may
specify a very narrow polymer MFR range for their product(s). The
present invention allows the polymer manufacture to produce polymer
having a selected MFR range. This is accomplished by knowing the
relationship between hydrogen concentration and the MFR of polymers
produced by a specific reactor and programming the target MFR or
MFR range into the control system 106 processor. By monitoring the
hydrogen concentration data generated by the analyzer system 104
and comparing this data to the upper and lower limits of the target
MFR range, the flow of hydrogen into the reactor vessel 102 may be
metered so that the MFR range of the polymer product may remain
compliant with the target MFR range.
[0046] While the above example is specific to hydrogen
concentration and polymer MFR, it will be understood by those
skilled in the art that other reactor constituent properties and
reactor constituent concentrations measured in the reactor vessel
may also be correlated to final polymer properties. In a similar
way as described above, the final polymer properties may be
achieved by controlled metering of these reactor constituents into
the reactor vessel 102 in response to data generated by the
analyzer system 104 in concert with an appropriately programmed
processor (programs which are readily available or which are known
to or can be created by those skilled in the art). For example, in
the article titled, "Modelling Of The Liquid Phase Polymerization
Of Olefins In Loop Reactors" by Zacca and Ray which appears in
Chemical Engineering Science, Vol. 48, No. 22, page 3743, and the
article entitled "Moving-Horizon State Estimation Applied to an
Industrial Polymerization Process" by Louis P. Russo and Robert E.
Young which appears in the American Control Conference Proceedings,
1999, San Diego, Calif., both of which are incorporated by
reference herein in their entirety, mathematical models are
provided for describing the dynamics of the polymerization of
olefins in a slurry loop reactor. One skilled in the art will
recognize that the teachings in these articles are not catalyst
specific and are applicable for use with the present invention as
well as for use in describing and/or understanding the dynamics of
a slurry loop reactor employing, for example, one or more
metallocene catalysts systems.
[0047] Turning now to FIG. 2, a fiber optic probe assembly 126
secured to the reactor vessel wall 128 by engaging flanges 129 is
illustrated. The fiber optic probe assembly 126 extends for a
distance into the interior 130 of the reactor vessel 102a. The
fiber optic probe assembly 126 includes a probe housing 132 having
a first end 134 and second end 136 and portions defining a probe
channel 133 (illustrated in ghost) sized for slidably receiving a
probe 139, desirably formed from stainless steel. The probe 139
includes a fiber optic channel 156 (FIG. 3) sized for receiving the
center fiber optic transmission conduit and the fiber optic
receiving conduits described above. The transmission and receiving
fiber optic conduits are generically illustrated in FIG. 2 by the
structure identified by the reference number 143. The center fiber
optic transmission conduit and the fiber optic receiving conduits
143 terminate at the probe tip 152 (FIG. 3). The probe channel 133
extends substantially the length of the probe housing 132 between
the first and second ends 134 and 136, respectively. Between the
first and second ends 134 and 136, respectively, a pair of ball
valves 140a and 140b are secured to the probe housing 132 and are
aligned with the probe channel 133. The probe 139 enters the probe
housing 132 through a re-sealable fitting 141. The probe 139 may be
extended, retracted or selectively positioned within the probe
channel 133 by a probe insertion assembly 142.
[0048] The probe insertion assembly 142 is secured to the second
end 136 of the probe housing 132. The probe insertion assembly 142
includes a guide rod 144, a plate 146 moveably secured to the guide
rod 144 and to a threaded rod 148 for selectively positioning and
securing the plate 146 along the length of the guide rod 144. A
connection 150 secures one end of the probe 139 to the plate
146.
[0049] While the analyzer system 104 does not require continuous
and/or simultaneous correlation or calibration of the reactor
constituent data with data obtained from an irradiated reference
material, calibration of the analyzer system, from time to time,
may be desirable. Calibration of the analyzer system 104 may be
performed by positioning the probe tip 152 (FIG. 3) between ball
valve 140b and the second end 136 of the probe housing 132. In this
way, the ball valves 140a and 140b may be rotated to interrupt
contact between the reactor constituents and the irradiating and
collecting ends of the transmission and receiving conduits 143. The
ball valves 156a and 156b may be rotated to isolate a flow of
purging fluid, such as liquid propylene, in a conduit 158 from
entering a calibration material conduit 160. The calibration
material conduit 160 communicates with a calibration material
source 162 and a portion of the probe channel 133 that is defined
by the portion of the probe housing 132 between the ball valve 140b
and the second end 136. In this way, a reference material may be
segregated from the reactor constituents when the analyzer system
104 is being calibrated or its accuracy checked.
[0050] Calibrating the analyzer system 104 may be performed by
contacting the irradiating and collecting ends of the transmission
and receiving conduits 143 at the probe tip 152 (FIG. 3) with a
quantity of the reference material. The reference material is
irradiated and the energy scattered by the reference material is
collected. The reference material data is processed in the same
manner as the reactor constituent data except that the reference
data obtained during calibration is compared to known data for the
reference material.
[0051] Selection of the reference material for calibration may, in
some instances, depend upon the reactor constituent(s) being
investigated and/or monitored. For example, as described above,
when monitoring the concentration of hydrogen in the reactor vessel
for purposes of controlling the MFR of polymer product, it may be
desirable to select reactor grade hydrogen as one of the reference
materials to calibrate the analyzer system 104.
[0052] Referring now to FIG. 3, an enlarged view of a portion of
the probe 139 which rests in the probe channel 133 adjacent the
first end 134 is illustrated. Slightly rearward of the probe tip
152 in a direction towards the second end 136 (not shown), the
cross sectional area of the probe 139 increases in a flared section
154. The flared section 154 facilitates the sealing of the probe
139 within the probe channel 133. The fiber optic channel 156
extends the length of the probe 139 and is sized for receiving the
fiber optic transmitting and receiving conduits 143.
[0053] When the analyzer system 104 is used for investigating
and/or monitoring the concentration of hydrogen in a slurry loop,
propylene polymerization reactor, specific, non-limiting examples
of suitable analyzer system components include: a Kaiser HoloProbe
Process Raman Analyzer, manufactured by Kaiser Optical Systems,
Inc. of Ann Arbor, Mich., a Visible 400 mW, 532 nm solid state
Diode-pumped frequency YAG laser, manufactured by Coherent, Inc.
and supplied by Kaiser Optical Systems, and a Visionex Captron
Probe, manufactured by Visionex, Atlanta Ga.
[0054] Additionally, it is desirable, but not necessary, that the
fiber optic probe assembly 126 meet the following minimum
particular specifications: operating conditions of 600 psig (41.36
bars) and 165.degree. F. (73.9.degree. C.), design conditions of
700 psig (48.25 bars) and temperatures in the range of from
-49.degree. F. (35.degree. C.) to 302.degree. F. (150.degree. C.).
Furthermore, it is desirable that the optic probe assembly 126,
including fiber optics, epoxy and related components, sustain
without loss of integrity: (i) exposure to light hydrocarbons and
TEAL ((C.sub.2H.sub.5).sub.3Al) at concentrations in the range of
from 0.01 to 500 ppm; (ii) thermal cycling from 0.degree. C. to
100.degree. C. over a one hour period. It is also desirable that
the fiber optic cables (the transmission conduit and the receiving
conduits) be secured within and throughout the length of the probe
139.
[0055] The above description illustrates the use of an analyzer
system employing a single probe for analyzing one or more reactor
constituents. It will be understood by those skilled in the art
that the analyzer system may be configured to include more than one
probe which may be located at one or more locations along the
reactor vessel. Additionally, in the case of multistage reactors,
the analyzer system may be configured to include probes located at
one or more locations along one or more reactors. In this way, one
or more reactor constituents may be analyzed at one or more
locations within the overall process and particularly, the overall
olefin polymerization process.
EXAMPLES
[0056] The following examples are presented to illustrate the
foregoing discussion. Although the examples may be directed to
certain embodiments of the present invention, they are not to be
viewed as limiting the invention in any specific respect. The
equipment used and the experimental procedure employed to obtain
the data in the following tables and figures are outlined
below.
[0057] Equipment
[0058] The apparatus used in the hydrogen concentration and polymer
properties experiments is schematically illustrated in FIG. 4. The
equipment consisted of the following major components:
[0059] A Kaiser Holoprobe 532 Raman Spectrometer
[0060] A 200 mW 532 nm Solid State YAG Laser
[0061] Visionex Captron Probe with integral 532 nm notch filter
[0062] Low-Hydroxyl Silica Fiber Optic Cables
[0063] A Gateway GS-400 Computer used to control spectrometer and
record spectra.
[0064] Stirred Vessel with Pressure Measurement/Control
[0065] Experimental Procedure
[0066] The hydrogen concentration experiment was designed to
provide a simulation of slurry loop reactor conditions. Pentane was
substituted for propylene because of simpler handling requirements.
Hydrogen concentration experiments were conducted in order to
determine the sensitivity level of the hydrogen measurement. The
polymer properties experiment simply consisted of acquiring Raman
spectra of polypropylene granules. They are described in more
detail below.
[0067] Weighed amounts of polypropylene granules and pentane were
added to the vessel to create a slurry of known concentration. The
vessel was then sealed, purged with nitrogen, and agitation was
established. Hydrogen was then added to the system to achieve
desired system pressure. At each pressure level, Raman spectra were
collected and recorded in a manner similar as described above. The
system pressure was used to estimate hydrogen concentration. This
was repeated for several slurry concentration levels.
[0068] The polymer property experiment utilized the Raman probe,
spectrometer and related equipment, but consisted of simply placing
the Raman probe in a plastic bag containing polypropylene granules
and acquiring spectra.
Example 1
[0069] Hydrogen Concentration
[0070] Hydrogen exhibits a peak at a Raman shift of 4140 cm.sup.-1.
This hydrogen peak along with two peaks from the pentane solvent
(at 1361 cm.sup.-1 and 2735 cm.sup.-1) were used for the hydrogen
concentration measurement. These are shown in FIGS. 5a and 5b.
[0071] The pentane peaks were used in order to establish the
hydrogen measurement as a ratio against other major components.
This method provides a means of correcting the hydrogen prediction
for changes in scattering intensity caused for example, by changing
polymer concentration. Desirably, in polyolefin field reactors,
measurement of hydrogen will also use one or more bands from the
monomer and polymer. The area under each band (or peak) was
integrated using Grams 32 data analysis software. The peak areas
were used to develop a hydrogen concentration prediction equation
of the form:
H.sub.2(ppm)=a.sub.1*A.sub.4138+a.sub.2*(A.sub.4138/A.sub.1361)+a.sub.3*(A-
.sub.4138/A.sub.2735)+C
[0072] where:
[0073] A.sub.n are the Peak Areas
[0074] a.sub.1 . . . a.sub.3 are the regression coefficients
[0075] C is a constant
[0076] A plot of estimated hydrogen concentration versus Raman
predicted hydrogen is shown in FIG. 6.
Example 2
[0077] Hydrogen Measurement Sensitivity
[0078] The determination of hydrogen measurement sensitivity was
conducted using a certified blend of hydrogen in propylene liquid.
Several Raman spectra were collected, and the repeatability of the
analysis was used to establish the lower limit of hydrogen
measurement sensitivity. This value is assumed to be the upper
level of sensitivity to hydrogen as the presence of polymer
granules will have a negative impact on the measurement. The impact
of polymer granules on sensitivity to hydrogen was evaluated by
observing the impact on a surrogate compound (acetonitrile) in a
pentane/polypropylene slurry. A surrogate in pentane was used to
simplify handling in the laboratory. It is believed that the
results for hydrogen in propylene will be similar. The calculations
are shown below:
[0079] Calculation of Measurement Sensitivity/Repeatability
[0080] Table 1 reports the results from 240 parts per million
("ppm") hydrogen in propylene repeatability test.
1 TABLE 1 No. Peak Ht. 1 101.05 2 103.39 3 108.54 4 110.18 5 112.24
6 114.84 7 108.46 Avg. 108.39 Stdev. 4.81
[0081] An estimate of the repeatability in ppm H.sub.2 (also the
standard deviation in ppm of H.sub.2) of the instrument can be made
using the standard deviation of the seven sequential
measurements.
Repeatability (ppm H.sub.2 )=(4.81/108.39).times.240 ppm=10.6 ppm
H.sub.2
[0082] The minimum detectable limit can be estimated at three times
the standard deviation in ppm of H.sub.2 of the measurement.
Minimum Detectable=.about.3.times. Std. Dev. in ppm of H.sub.2
[0083] As such, the minimum detectable limit in propylene liquid
using the above equipment configuration equals approximately 33 ppm
H.sub.2
[0084] Impact of Polymer Slurry on H2 Measurement
Sensitivity/Repeatabilit- y
[0085] FIG. 7 illustrates the expanded spectra of acetonitrile in a
polypropylene/pentane slurry at several polymer concentrations. As
can be seen from FIG. 7 above, the acetonitrile peak at 50%
polypropylene slurry is approximately 50% the size of the peak with
no polymer present. Assuming that a 50% slurry similarly depresses
the hydrogen peak size by 50%, then the estimated hydrogen
sensitivity is as follows:
[0086] Taken from above that hydrogen measurement
repeatability/sensitivit- y in propylene liquid is 10.6/33 ppm
H.sub.2, then:
[0087] H.sub.2 Repeatability (at 50% polymer)=10.6 ppm/0.5=22 ppm
H.sub.2
[0088] H.sub.2 Sensitivity (at 50% polymer)=33 ppm/0.5=66 ppm
H.sub.2
Example 3
[0089] Polymer Properties Measurement
[0090] The measurement of polypropylene properties such as MFR was
based on the correlation of Raman spectra collected from
polypropylenes with known (as determined by a primary method such
as NMR or a Rheometer) properties.
[0091] Spectral Modeling for MFR
[0092] Each individual spectra taken is represented by an array of
approximately 4400 frequency vs. intensity values. (1 row with 4400
columns). For a model set with 20 samples, this produces a data
array with a dimension of 20 rows by 4400 columns.
[0093] To successfully produce a prediction model from this data
array, it is necessary to first reduce the size of the data set.
This was accomplished using Principal Components Analysis (PCA).
PCA reduces this large data set to a number of covariant orthogonal
vectors referred to as Principal Components or PCs. Each PC
contains the covariant (correlated) data contained in the data set.
The first PC represents the highest valued covariant behavior, with
each PC in descending order representing lower intensity (intensity
value) information.
[0094] Each spectra is then assigned a "score" for each PC. The
score is the amount of each principal component found in the
spectra. The scores represent the independent variables that are
regressed against properties of interest in order to produce a
prediction model. The regression of the scores produced a
"regression vector" with a coefficient for each intensity value
utilized.
[0095] The form of the predictive equation is:
predicted
value=k.sub..lambda..sub..sub.1A.sub..lambda..sub..sub.1+K.sub..-
lambda..sub..sub.2+. . .
+k.sub..lambda..sub..sub.nA.sub..lambda..sub..sub- .n
[0096] where
[0097] A.sub..lambda.n is the absorbance at the n.sup.th
frequency
[0098] k.sub..lambda.n is the regression coefficient for the
n.sup.th frequency
[0099] MFR Prediction
[0100] MFR prediction models were developed on polypropylene
granules samples previously characterized in the lab in accordance
with ASTM D-1238-95 Procedure B. It has been found that it is
necessary to create separate prediction models for homopolymers and
copolymers.
[0101] Both the homopolymers and copolymer models had the following
characteristics: (i) 271 cm.sup.-1 to 1913 cm.sup.-1 Raman shift,
(ii) Developed by PCA/PLS and (iii) 4 Principal Components used in
regression.
[0102] A Raman spectrum of polypropylene granules is shown below in
FIG. 8. The frequency region used is illustrated on the plot.
[0103] FIG. 9 illustrates a parity plot of Raman predicted MFR
versus Lab MFR. The Standard Error of Cross Validation (SEV) is as
follows:
[0104] Homo-polymers: 0.32 MFR
[0105] Copolymers: 0.41 MFR
[0106] As can be seen from the FIG. 9, spectroscopic analysis, and
particularly Raman analysis can be used to predict MFR of
polypropylene granules.
[0107] While the present invention has been described and
illustrated by reference to particular embodiments, it will be
appreciated by those of ordinary skill in the art that the
invention lends itself to many different variations not illustrated
herein. For these reasons, then, reference should be made solely to
the appended claims for purposes of determining the true scope of
the present invention.
Example 4
[0108] Raman Stectroscopy Sensitivity to Ethylene and 1-Hexene in a
Simulated Slurry Loop Reactor
[0109] The apparatus used in this simulation included the following
major components:
[0110] A Kaiser HoloLab 5000 Raman Spectrometer
[0111] A NIR 785 nm/250 mW External Cavity Wavelength Stabilized
Laser Diode
[0112] A imaging style optic, immersion probe with holographic
probe head filter (FIG. 16), available from Kaiser Optical Systems,
probe head model number HFPH-FC-S-785, immersion optic model number
IMO-H-0.1
[0113] A fiber optic cable (4 fibers)
[0114] A personal computer with software to control spectrometer
and collect/analyze spectra
[0115] A stirred vessel with pressure measurement
[0116] Experimental Procedure
[0117] The experiment simulated slurry loop reactor conditions.
Isopentane was substituted for the isobutane diluent for ease of
handling. The vessel was first charged with isopentane and
high-density polyethylene granules at a ratio similar to slurry
loop reactor conditions. The vessel was then sealed and stirred
throughout the experiment. To this slurry, aliquots of 1-hexene
were added volumetrically. Spectra were collected before and after
each addition of hexene.
[0118] After completing the 1 -hexene additions, ethylene was then
added to the slurry. Ethylene gas was fed to the vessel and
quantitated by weighing the ethylene delivery cylinder. The
ethylene gas pressurized the reaction vessel and was forced into
solution by pressure and stirring agitation.
[0119] 1-hexene exhibits a peak at a Raman shift of 1640 cm.sup.-1.
Ethylene exhibits a peak at a Raman shift of 1620 cm.sup.-1. FIG.
10 illustrates the expanded spectra of these peaks in the slurry
system. These peaks were used for the concentration measurements.
The relationship between the peak height of the 1-hexene peak
versus hexene concentration is shown in FIG. 11. The relationship
between the peak height of the ethylene peak versus ethylene
concentration is shown in FIG. 12.
[0120] It is clear, in view of the FIGS. 10-12 and the above
discussion, that ethylene and 1-hexene concentrations in a typical
slurry loop reactor environment can be determined by Raman
spectroscopy.
Example 5
[0121] On-Line Validation of Raman Spectroscopy for Ethylene and
Polyethylene Copolymer. Prophetic Example of Hexene Concentration
Measurements
[0122] The apparatus used in Example 5 included the following major
components:
[0123] A Kaiser HoloLab 5000 Raman Spectrometer
[0124] A NIR 785 nm/250 mW External Cavity Wavelength Stabilized
Laser Diode
[0125] A imaging style optic, immersion probe with holographic
probe head filter, available from Kaiser Optical Systems, probe
head model number HFPH-FC-S-785, immersion optic model number
IMO-H-0.1
[0126] A 100 meter, jacketed fiber optic cable (4 fibers)
[0127] A personal computer with software to control spectrometer
and collect/analyze spectra
[0128] A commercial slurry-loop, polyethylene reactor system
[0129] Experimental Procedure
[0130] The imaging probe was inserted directly into the slurry-loop
reactor. Spectrum was collected as the reacting slurry flowed past
the imaging probe tip. FIG. 13 exhibits the expanded Raman spectra
collected from the slurry polyethylene copolymer reaction
system.
[0131] Ethylene Concentration Measurements
[0132] Ethylene concentrations were measured during a typical
commercial polyethylene production run in the above commercial
slurry loop reactor system operated at 550 psi, approximately 100
degrees C., with ethylene as the primary monomer and 1-hexene as
the comonomer. For the analysis, the area of the known peak for the
ethylene was divided by the area of an isolated isobutane peak (the
reference peak). The result is the ethylene peak ratio. The peak
ratio results were then compared to the results from the current
ethylene measurement produced by a downstream gas chromatograph.
FIG. 14 shows the relationship between the ethylene peak ratio and
the gas chromatograph ethylene concentration measurement.
[0133] Based upon the peak ratio technique, the ethylene
concentration prediction equation is as follows:
Ethylene (wt %)=X*(A.sub.1620/A.sub.796)+C
[0134] X=slope of linear regression
[0135] A.sub.n=peak areas
[0136] C is a constant
[0137] It is clear, in view of FIGS. 13 and 14 and the above
discussion, that the ethylene concentration may be predicted within
acceptable variations within a commercial loop slurry reactor.
[0138] Hexene Concentration Measurements
[0139] Due to reaction conditions, the 1-hexene peak at a Raman
shift of 1640 cm.sup.-1 is generally not acceptable for the peak
ratio technique. However, spectral modeling can offer a solution.
For example, each spectra may be treated as an array of Raman shift
(frequency) versus intensity values. The arrays of values from
multiple spectra can be assembled to produce the matrix for
chemometric analysis. Partial Components Analysis (PCA) and Partial
Least Squares (PLS) can be used for the model creation. Using the
information in the spectral range from 1580 cm.sup.-1 to 1700
cm.sup.-1, an effective model for hexene concentration (as
indicated by the downstream gas chromatograph) can be produced.
[0140] Polyethylene Copolymer Concentration Measurements
[0141] Polyethylene copolymer concentrations were measured during a
typical commercial polyethylene production run in the above
commercial slurry loop reactor system operated at 550 psi,
approximately 100 degrees C, with ethylene as the primary monomer
and 1 -hexene as the comonomer. FIG. 15 illustrates a correlation
between the Raman polyethylene copolymer peak ratio and the slurry
density/polyethylene copolymer concentration in the commercial
slurry loop system.
[0142] Based upon the peak ratio technique, the polyethylene
copolymer concentration at the probe tip can be calculated. This
prediction can be compared to the results of the reactor's on-line
nuclear density analysis instrumentation. The prediction equation
is as follows:
Slurry Density (polymer
concentration)=X*(A.sub.1295/A.sub.796)+C
[0143] X=slope of linear regression
[0144] A.sub.n=peak areas
[0145] C is a constant
[0146] While the present invention has been described and
illustrated by reference to particular embodiments, it will be
appreciated by those of ordinary skill in the art that the
invention lends itself to many different variations not illustrated
herein. For these reasons, then, reference should be made solely to
the appended claims for purposes of determining the true scope of
the present invention.
[0147] Although the appendant claims have single appendencies in
accordance with U.S. patent practice, each of the features in any
of the appendant claims can be combined with each of the features
of other appendant claims or the main claim.
* * * * *