CA2051948C - High-temperature slurry polymerization of ethylene - Google Patents
High-temperature slurry polymerization of ethyleneInfo
- Publication number
- CA2051948C CA2051948C CA002051948A CA2051948A CA2051948C CA 2051948 C CA2051948 C CA 2051948C CA 002051948 A CA002051948 A CA 002051948A CA 2051948 A CA2051948 A CA 2051948A CA 2051948 C CA2051948 C CA 2051948C
- Authority
- CA
- Canada
- Prior art keywords
- process according
- temperature
- support
- polymerization
- silica
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
Links
Classifications
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F2/00—Processes of polymerisation
- C08F2/12—Polymerisation in non-solvents
- C08F2/16—Aqueous medium
- C08F2/18—Suspension polymerisation
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F210/00—Copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
- C08F210/16—Copolymers of ethene with alpha-alkenes, e.g. EP rubbers
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F10/00—Homopolymers and copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F110/00—Homopolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
- C08F110/02—Ethene
Abstract
This invention relates to a high-temperature slurry polymerization of ethylene where: the hexene is generated in situ; a chromium catalyst system 15 used; a trialkyl boron or a polyalkyl silane compound is used as a cocatalyst; the temperature of the reaction is above the foul curve temperature for a specific copolymer density; and where the polymerization takes place in a continuous loop reactor which has isobutane as a diluent. Additionally, hexene can be withdrawn, or added, to adjust the density.
Description
2 ~ 32926C~
HIGH-TEMPERATUAR~ SLURRY P~LY~ERIZATION ~F ETHYLENE
BACKGROVND O~ THE INVENTION
This invention relates -to a slurry polymerization of ethylene.
Chromium catalysts have long been known in the art. Supported chro~tum catalysts have been a domlnate factor in the commercial production of polyethylene. Originally, commerclal processes used solutlon polymerization techniques to produce polyolefins. However, it later became evident that a slurry polymeriza-tlon process was a more aconomical route to produce commercial grades and quantities of polymer.
Sl~rry polymerization is ul~like solutlon poly~erlYa-t:lon, in -that, the polymer, as it is formod during the slurry process, is largely insoLuble ln the inext carrylng diluent. Thifi makes i-t easier to recover -the polymer.
How~ver, certain control techniques which are easily carried out in a solutlon polymerization become nearly impossible in a slurry polymeri~ation. For example, in solution polymerization to make a polymer ~hat has a lower molecular weight, as well as higher meLt flow index, the te~perature can be raised, thus aecomplishing this desired result. However, in a slurry polymeri~ation there is a practical limit on increasing t~mperature because the point is quickly reached where the polymer goes into sol~ltion and fouls the reactor.
In general, reactor fouling occurs when polymer particLes, in a slurry polymerizatlon process, dissolve in-to the reactor's llquld phase.
SpcciELcally, the polymer particles, whilc d:issolving, lncrease the volume that -they occupy by se~eral orders of magnitude. This increase in volume causes the viscoslty to increase in -tho liquid phase of the raactor. IE the ;~:. '~
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...
2~19~18 32926C~
polymer particles continue to dissolve, thereby increasing -the viscosity, eventually a gel-like substance is formed which plugs the reactor. ~nplugging a fouled reactor is fl time-intensive and costly undertaking.
Over the years, those in the art have come up wi-th various rela-tionships to help reactor operators avoid a reactor fouling incident. One of the oldest relationships, in this art, is the densi-ty v.s. temperatllre foulcurvs. (See the figure). For many years, this relationship has been considered to be practically inviolable. This is in spite of the fact that it would be high]y desirable to operate a slurry polymerization process at a reactor temperature higher than the foul curve temperature for a ~pecific copolymer density.
SUMNARY OF THE INVENTION
It is an object of this invention to provlde an improved slurry polymerization process.
It is another object of this invention to provide a process to make a higher melt index polymer Erom fl slurry polymeri~ation process.
I-t is still another ob~ject of this invention to provide a slurry polymeri~ation process which as a byproduct produces hexene.
It is yet ~ fnrther object of this invention to provide A higher heat exchange efficiency for a polymerization reactor cooling system.
It is a further obJect of this invention to producq conditlons sultable to more efficLent ethylene-hexene copolymeriza-tion.
In accordance with -this invention, in a continuous loop reac-tor which has isobutane as a diluent, ethylene is contacted with a catalyst system made by (a) subjecting a supported chromium catalyst -to oxidizing conditions and thereafter to carbon monoxide under reducing conditions and (b) combining the catalyst of (a) with a cocatalyst selected from -trialkyl boron compounds and polyalkyl silane compounds, at a reactor temperature above the foul curve temperature for a copolymer of the density produced.
BRIEF DESCRIPTION OF THE FIGURE
The figure is a depictLon of the relationship between the density of a polymer and the temperature of the reac-tor system. Th:Ls fLguro is specLfically related to this relationship in the contex-t of a slurry polymerization of ethylene catalyized by fl chromium catalyst system, in which ~:
~ 9 ~ g 32926CA
hexene is generated in slt~l as a comonomer, and is depicted by line alpha which ls the Eoul curve.
D~TAILE~ DESCRIPTION OF THE INVENTION
With:in this description -the terms "cogel" and "cogel hydrogel" are arbitrarily used to describe co-gelled siliGa and titania. The term "tergel"
is used to describe -the product resulting from gela-tion togecher of silica, ti-tanis, and chromium. Hydrogel is defined as a support component containing water. "Xerogel" is a support componen-t which has been dried and is substAntially water-free.
'~ THE CATALYST SYST~M
One component of the catalyst sys-tem is the catalyst suppor-t. It is preferred that one or more refractory oxides comprise the cataly~t support.
Examples of refractory oxides include, but are not limited to, alumina, boria~
~ m~gnesia, silica, thoria, zirconia, or mixtures of two or more of these L compounds. Preferably the support is a predominantly silica support. By predominant3y silica ls meant either an essentially pure slllca support or a support comprising at least 90 weigh-t percent sil:Lca, the remainlng being primarily refractory oxides such as alumina, ~irconia, or titania. Nost ~! prefernbly the support contains 92 to 97 weight percen-t silica and 3 ~o 8 weight percent titania. The catalys-t support can be prepared in accordance with any method known in the art. For example, support preparation~ given in U.S. Patents 3,887,4~4; 3,900,457; 4,053,436; 4,151,122; 4,294,724; 4,392,990;
4,405,501; can be used to form the ca-talys-t support.
~ Another component of the catalyst system must ~e a chromium '~ compound. The chromium component can be combined with the support component ln any manner known in the art, such as forming a tergel. Alternatively, an aqueous solution of a water soluble chromium component can be added to a ~- hydrogel support~ Suitable chromium compounds include, but are not limited to, chromium acetate, chromium nitrate, chromium trioxide, or mixtures of two or more of -these types oE compounds. Additionally, a solution of a hydroc~rbon soluble chrom:Lum compound can be used to impregna-te a xerogel support. Suitable hydrocarbon soluble chromlum compounds include, bu-t are not limited to, tertiary butyl chromate, biscyclopentadienyl chrom:Lum II, chromium acetyl acetonate, or mlxtures of two or more of -these types of compounds.
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The chromlum component is used ln an amount sufficient to give about 0.05 to abollt 5, preferably about 0.5 to about 2 weigh-t percent chromium basedon the total weight of the chromium and support after activation. After the chromium component is placed on the support it is -then subjected to activation ln an o~ygen containing arnbient in the manner conven-tionally used in the art.
Because of economic reasons, the preferred oxygen-con-tainlng ambient is alr, preEerably dry air. The activation i~ carried out at an alevated temperature for about 30 minu-tcs to about 50 hours, preferably 2 -to 10 hours at a temperature within the r~nge of 400~C to 900~C. Under these condLtions at ~east a substantial portion Or any chromium in a lower valent state is converted to the hexavalent Eorm by this procedure.
The resulting supported catalys-t component is cooled and then subjected -to at least partial reduction of the hexavaleIIt chromium to a low~r valent state prior to combining with a cocatalyst. The reducing agent must be carbon monoxide. The carbon monoxide can be employed at temperatures between 300~ and 500~C. The partial pressure of -the reducing gas in the reduc-tion operation can be varied from subatmospheric pressur2s to relatively high pressures, but the simplest reducing operatlon is to utilize essentLally pure carbon monoxide at atmospheric pressure. Alternat:Lvely, a mixture of lO~n by volume oE carbon monoxide, in an inert ambient, such as nitrogen or argon, can also be used.
The reduction tlme can vary from a few minutes to several hours or more. Tlle extent of reduction can be followed by visual inspection oE the catalyst color. The color of the ini~ial oxygen activated catalyst is generally orange, indicatlng -the presence oE hexavalent chromium. The color of the reduced catalyst employed in -the invention is blue, indica-t:Lng that all or substantLally all of the initial hexavalent chromium has been reduced to a lower oxidation state. After reduc-tion, the reduced, supported ca-talyst component is gen~rally coo]ed, in an inert atmosphere~ such as argon or nltrogen, to flush out the reducing agent. After the flushing -treatment, the catalyst Ls kept away from contac-t wi-th either a reducing agent or an oxldiz:Lng ngent. Rxamples of these types of ca-talyst components can be found ln the art. For example, exemplary examples can be found Ln U.S. Patent 4,820,7~5.
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.
The cocatalysts can be either a trialkyl boron compound or a polyalkyl silane compound. Elowever, this is not meant to exclude join-t use ofthese compo~lnds in the snme copolymeriza-tion reactLon.
IE the cocatalyst chosen is a trialkyl boron compound the alkyl groups should have between l and 10 carbon atoms and preferflbly between 2 and 4 carbon atoms. Presen-tly the most preEerred compound :is -triethylborane. The trialkyl boron is used in an amount within the range of 0.5 -to lO weight percent based on the we:ight oE the chromium and support, with about l to B
weight p~rcent being preferred.
When using the trialkyl boron cocatalyst, the order oF addition of -the components can be impor-tant. It is preferred that the cocatalyst and the reduced supported catalyst be precontacted prior to its introduction to the ethy1ene monomer. In a sturry ot)eration this can be carried out by pretreating -the reduced supported catalyst with the coca-talys-t and then adding the resulting composition to the reactor. In -this manner the reduced supported catalyst and the cocatalyst can be introduced contLnuously in the polymerization reactlon. Examples of trialkyl boron used in conjunction wlth supported chromium catalysts can be found in U.S. Pfltent 4,820,7~5.
If the cocatalysts chosen is a poly~lkyl silane compound lt should ~~ be of the formula R4 m SiHm~ where m is an integer from 1 -to 4. R is ; indapendently selccted from any aliphatic and/or aromatic radlcal wlth one or more carbon atoms. Examples oE silana type compouncls -to use in this inventioninclude, but ara not limited to, e-thylsilane, diethylsilane, triethylsilane, phenylsilane, n-hexylsilane, diphenylsilane, triphenylsilane, and polyme-thylhydrosilane. Preferrad silane compounds include, bl2-t are not limlted to, diethylsilane, phenylsilane, n-hexylsilane, and mixtures of two or more of these type compounds. The amount of silane to be used is abou-t 0.1 to about 16 waight percent and preferably about 0.3 to about 8 weight percent based on the weight of the reduced supported catalyst. Most preferably abou-t 0.5 to about 4 weigh-t percent is used.
The silane compound can be contacted with -the reduced suppor-ted atAlyst prior -to the its use or it can be added to the reactor during -the copolymerizntlon. However, for max:lmum benefit, the reduced supported catalys-t preferably :Ls exposed to the silane compound prior to contacting the ethylene monomer. Tharefore, the silane and reduced supported catalyst more :~:
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2 ~ 329 26CA
preferably are precos~tActed prior -to introduction into the polymerization reactor.
When the silflne is added directly to the polymerization reactor, the silane usua]ly i6 added in a hydrocarbon solution, with the hydrocarbon usually being the same as the solvent contained in the reactor, but is not restricted to tha-t solvent. Dilute solutions, :i.e., about 0.005 to about I
weigh-t percent, are conveniently used when passing the silane solution in-to the reactor. If the silane flnd the reduced supported catalyst are precontacted prior to in-troduction to the polymeriza-tion reactor, a more concentrated silane solution can be used. After precontacting the reduced supported catalyst and silane, it is desirable to thorough]y mix the silane solution and the reduced supported catalyst.
REACTION CONDITIONS
The polymerization is carried ou-t under high-temperature slurry conditions. Such polymerization techniques are well known in the art and are disclosed, for exflmple, in U.S. Paten~ 3,248,179. It is essential for *his invention that a continuous loop reactor is used. Furthermore, the diluen-t utili~ed in this inventton must be isobutane.
The e~fect of using -this ca-talyst i9 to generate l-hexene in si-tu thus ~iving an ethylene/hexene copolymer from a pure ethylene feed.
In the figure3 the following designations are defined as:
Area One: is a daplction of the prior art area where safe non-fouling ~ opera-ting temperatures are found fot a given polymer density.
;~ Are8 Two: i8 a depict:Lon of the invention 8rea where safe non-fouling operating temperatures are found for a glven polymer density when using the catalyst sys-tem in t}n~s specification.
. . .
Line Alpha: is a depiction of the foul curva; this is the boundry between the prior art area of safe operating temperatures and -the new ; available operating temperatures of this invention; this line ~ is defIned by the followitlg equation:
; ~
T = 917.3 D-650.6 where ... , : :
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2 ~ 5 ~ 32926CA
(1) 0.930 < D < 0.960;
~2) T :Ls the temperature in degrees fahrenheit; and ~3) D is the density iII grams per cubic centimeter.
The preferred reactor temperature to use ,in this slurry process Ls described by the following boundrLes:
(l) (F.L.T.) < (O.T.); more preferably (2) ~F.L.T.) < (O.T.) < ~F.L.T.) t 20~F; And most preferably (3) (F.I..T.) ~ 5~F < (O.T.) < (F.L.T.) ~ 15~F;
~shere (F.L.T.) is the foul line temperature for fl certaln polymer density; and(~.T.) is the operatlng temperature of the reactor. The dif~erences ln the prefe~red ranges are a compromise be,tween th~ compet.ing factors of increased energy cost in performlng this process and the higher melt index polymers obtainable by this procsss.
Using this cata'1y~t system at these tempera-tures the amount of hexene generated in situ can be varied so as to adjust the dens.ity of the de~ired polymer by :increasing or decreasing the amount of cocatalyst use<l.
For example, increasing the amount oE cocatalyst will increase the amount of hexene generated. The dens:lty can also be controlled by removing unwanted hexene from the reactor's diluent thereby increasing the density. This removed hexene can be separa-ted from the diluent by any reasonable means known in the art. Additionally, -the density of the polymer can be controlled by aFEirmatively adding hexene -to the reactor.
EXAMPLE
A 600 gallon reactor was used to conduct this slurry polymerization of ethylene and l-he~ene. The reactor was similax to those disclosed in V.S.
Patent 3,248,179. Th~ reactor wfls being run con-tinuously at a production level of about 1000 lbs/hr polymer. The liqu,id phasa in the re~ctor was primarily the iso-butsne carrying diluent. The catalyst was chromium s~pported on a silica-titania support made in accordance with U.S. Patent 4,820,785. The cocatalyst selectecl was triethylborane. The monomer feed was essentially ethylene witll the hexene being generated in situ Eor incorporation into th~ copolymer. Although this was essentially an ethylene-hexene copolymerlzation, some other comonomers cou].d also have been generated in situ and subsequently copolymer.ized with the ethylene monomer. The polymerization .:
.
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was allowed to proceecl :Eor ~evera] clay~ before the tes-ting described iII Tflble 1 occurred. The following clata were collected.
Reactor Temperature Density in Elapsed Time in Degrees Grams Per in Minutes4 Fflhrenheit Cubic CentimeterS
0 211.5 0.9460 212.0 212.5 213.0 0.9~49 ' 60 213.5 214.0 lOS 214.5 0.9444 135 215.0 165 215.5 0.943 lg5 216.0 210 21~.5 '~ 225 217.0 o.94232 , ~ 255 217.5 270 218.0 285 218.5 0.94193 315 219.0 ' ~ 345 219.0 No reActor fouling yat.
:
Still no reactor fouling.
3Some signs of reactor stress but still no fouling. '~
~;-, 4A~ noted above, this polymerizat:Lon was continlled for several days ~' before this experiment was oonducted.
These densities are tbe densities of the polymer at the end of the production line. These polymers exited the reactor abou-t 1-3 hours ea--lier.
They represent fln average density made over -the preceeding hour~ ~' ~ .
''~ Comparln$ the densi-ty v.s. temperature relatL~nsh:lp in the Elgure with the data iLn the taSle it Ls apparent that the reactor was running above ~,; the foul curve for the reaction. It should Se notcd that the densities recorded are the densities of the polymer at the end of the production li.ne.
Taking lnto account the nearly linear decrease in *he polymer density it is apparent that -the polymer actually made at T=345 would be much lower in . .
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densit.y thfln the lnst .9418 density measurQment. ~11 in ~11, the r~actor was running at a tempera-ture of at least 5-10 degrees above the foul curve for about 3 ho~lrs. This ls a sign:Lficant increflSe i.n the allowed operat:ing area, thus giving additional advantages such as lmproved melt indexes.
Wh:Lle this invention has been described in detail. for purpose of il.lustratlon, i-t is not to be construed as l:Lmited thereby, bu-t ls lntended to cover all changes and modifi.cfl-tions withi.n the spirit and scope -thereof.
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HIGH-TEMPERATUAR~ SLURRY P~LY~ERIZATION ~F ETHYLENE
BACKGROVND O~ THE INVENTION
This invention relates -to a slurry polymerization of ethylene.
Chromium catalysts have long been known in the art. Supported chro~tum catalysts have been a domlnate factor in the commercial production of polyethylene. Originally, commerclal processes used solutlon polymerization techniques to produce polyolefins. However, it later became evident that a slurry polymeriza-tlon process was a more aconomical route to produce commercial grades and quantities of polymer.
Sl~rry polymerization is ul~like solutlon poly~erlYa-t:lon, in -that, the polymer, as it is formod during the slurry process, is largely insoLuble ln the inext carrylng diluent. Thifi makes i-t easier to recover -the polymer.
How~ver, certain control techniques which are easily carried out in a solutlon polymerization become nearly impossible in a slurry polymeri~ation. For example, in solution polymerization to make a polymer ~hat has a lower molecular weight, as well as higher meLt flow index, the te~perature can be raised, thus aecomplishing this desired result. However, in a slurry polymeri~ation there is a practical limit on increasing t~mperature because the point is quickly reached where the polymer goes into sol~ltion and fouls the reactor.
In general, reactor fouling occurs when polymer particLes, in a slurry polymerizatlon process, dissolve in-to the reactor's llquld phase.
SpcciELcally, the polymer particles, whilc d:issolving, lncrease the volume that -they occupy by se~eral orders of magnitude. This increase in volume causes the viscoslty to increase in -tho liquid phase of the raactor. IE the ;~:. '~
, .~
;;
...
2~19~18 32926C~
polymer particles continue to dissolve, thereby increasing -the viscosity, eventually a gel-like substance is formed which plugs the reactor. ~nplugging a fouled reactor is fl time-intensive and costly undertaking.
Over the years, those in the art have come up wi-th various rela-tionships to help reactor operators avoid a reactor fouling incident. One of the oldest relationships, in this art, is the densi-ty v.s. temperatllre foulcurvs. (See the figure). For many years, this relationship has been considered to be practically inviolable. This is in spite of the fact that it would be high]y desirable to operate a slurry polymerization process at a reactor temperature higher than the foul curve temperature for a ~pecific copolymer density.
SUMNARY OF THE INVENTION
It is an object of this invention to provlde an improved slurry polymerization process.
It is another object of this invention to provide a process to make a higher melt index polymer Erom fl slurry polymeri~ation process.
I-t is still another ob~ject of this invention to provide a slurry polymeri~ation process which as a byproduct produces hexene.
It is yet ~ fnrther object of this invention to provide A higher heat exchange efficiency for a polymerization reactor cooling system.
It is a further obJect of this invention to producq conditlons sultable to more efficLent ethylene-hexene copolymeriza-tion.
In accordance with -this invention, in a continuous loop reac-tor which has isobutane as a diluent, ethylene is contacted with a catalyst system made by (a) subjecting a supported chromium catalyst -to oxidizing conditions and thereafter to carbon monoxide under reducing conditions and (b) combining the catalyst of (a) with a cocatalyst selected from -trialkyl boron compounds and polyalkyl silane compounds, at a reactor temperature above the foul curve temperature for a copolymer of the density produced.
BRIEF DESCRIPTION OF THE FIGURE
The figure is a depictLon of the relationship between the density of a polymer and the temperature of the reac-tor system. Th:Ls fLguro is specLfically related to this relationship in the contex-t of a slurry polymerization of ethylene catalyized by fl chromium catalyst system, in which ~:
~ 9 ~ g 32926CA
hexene is generated in slt~l as a comonomer, and is depicted by line alpha which ls the Eoul curve.
D~TAILE~ DESCRIPTION OF THE INVENTION
With:in this description -the terms "cogel" and "cogel hydrogel" are arbitrarily used to describe co-gelled siliGa and titania. The term "tergel"
is used to describe -the product resulting from gela-tion togecher of silica, ti-tanis, and chromium. Hydrogel is defined as a support component containing water. "Xerogel" is a support componen-t which has been dried and is substAntially water-free.
'~ THE CATALYST SYST~M
One component of the catalyst sys-tem is the catalyst suppor-t. It is preferred that one or more refractory oxides comprise the cataly~t support.
Examples of refractory oxides include, but are not limited to, alumina, boria~
~ m~gnesia, silica, thoria, zirconia, or mixtures of two or more of these L compounds. Preferably the support is a predominantly silica support. By predominant3y silica ls meant either an essentially pure slllca support or a support comprising at least 90 weigh-t percent sil:Lca, the remainlng being primarily refractory oxides such as alumina, ~irconia, or titania. Nost ~! prefernbly the support contains 92 to 97 weight percen-t silica and 3 ~o 8 weight percent titania. The catalys-t support can be prepared in accordance with any method known in the art. For example, support preparation~ given in U.S. Patents 3,887,4~4; 3,900,457; 4,053,436; 4,151,122; 4,294,724; 4,392,990;
4,405,501; can be used to form the ca-talys-t support.
~ Another component of the catalyst system must ~e a chromium '~ compound. The chromium component can be combined with the support component ln any manner known in the art, such as forming a tergel. Alternatively, an aqueous solution of a water soluble chromium component can be added to a ~- hydrogel support~ Suitable chromium compounds include, but are not limited to, chromium acetate, chromium nitrate, chromium trioxide, or mixtures of two or more of -these types oE compounds. Additionally, a solution of a hydroc~rbon soluble chrom:Lum compound can be used to impregna-te a xerogel support. Suitable hydrocarbon soluble chromlum compounds include, bu-t are not limited to, tertiary butyl chromate, biscyclopentadienyl chrom:Lum II, chromium acetyl acetonate, or mlxtures of two or more of -these types of compounds.
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2 0 ~ 1 9 4 8 32926CA
The chromlum component is used ln an amount sufficient to give about 0.05 to abollt 5, preferably about 0.5 to about 2 weigh-t percent chromium basedon the total weight of the chromium and support after activation. After the chromium component is placed on the support it is -then subjected to activation ln an o~ygen containing arnbient in the manner conven-tionally used in the art.
Because of economic reasons, the preferred oxygen-con-tainlng ambient is alr, preEerably dry air. The activation i~ carried out at an alevated temperature for about 30 minu-tcs to about 50 hours, preferably 2 -to 10 hours at a temperature within the r~nge of 400~C to 900~C. Under these condLtions at ~east a substantial portion Or any chromium in a lower valent state is converted to the hexavalent Eorm by this procedure.
The resulting supported catalys-t component is cooled and then subjected -to at least partial reduction of the hexavaleIIt chromium to a low~r valent state prior to combining with a cocatalyst. The reducing agent must be carbon monoxide. The carbon monoxide can be employed at temperatures between 300~ and 500~C. The partial pressure of -the reducing gas in the reduc-tion operation can be varied from subatmospheric pressur2s to relatively high pressures, but the simplest reducing operatlon is to utilize essentLally pure carbon monoxide at atmospheric pressure. Alternat:Lvely, a mixture of lO~n by volume oE carbon monoxide, in an inert ambient, such as nitrogen or argon, can also be used.
The reduction tlme can vary from a few minutes to several hours or more. Tlle extent of reduction can be followed by visual inspection oE the catalyst color. The color of the ini~ial oxygen activated catalyst is generally orange, indicatlng -the presence oE hexavalent chromium. The color of the reduced catalyst employed in -the invention is blue, indica-t:Lng that all or substantLally all of the initial hexavalent chromium has been reduced to a lower oxidation state. After reduc-tion, the reduced, supported ca-talyst component is gen~rally coo]ed, in an inert atmosphere~ such as argon or nltrogen, to flush out the reducing agent. After the flushing -treatment, the catalyst Ls kept away from contac-t wi-th either a reducing agent or an oxldiz:Lng ngent. Rxamples of these types of ca-talyst components can be found ln the art. For example, exemplary examples can be found Ln U.S. Patent 4,820,7~5.
: ' , ~ ~ .
2 ~ ~ 1 9 ~ g 32~326C~
.
The cocatalysts can be either a trialkyl boron compound or a polyalkyl silane compound. Elowever, this is not meant to exclude join-t use ofthese compo~lnds in the snme copolymeriza-tion reactLon.
IE the cocatalyst chosen is a trialkyl boron compound the alkyl groups should have between l and 10 carbon atoms and preferflbly between 2 and 4 carbon atoms. Presen-tly the most preEerred compound :is -triethylborane. The trialkyl boron is used in an amount within the range of 0.5 -to lO weight percent based on the we:ight oE the chromium and support, with about l to B
weight p~rcent being preferred.
When using the trialkyl boron cocatalyst, the order oF addition of -the components can be impor-tant. It is preferred that the cocatalyst and the reduced supported catalyst be precontacted prior to its introduction to the ethy1ene monomer. In a sturry ot)eration this can be carried out by pretreating -the reduced supported catalyst with the coca-talys-t and then adding the resulting composition to the reactor. In -this manner the reduced supported catalyst and the cocatalyst can be introduced contLnuously in the polymerization reactlon. Examples of trialkyl boron used in conjunction wlth supported chromium catalysts can be found in U.S. Pfltent 4,820,7~5.
If the cocatalysts chosen is a poly~lkyl silane compound lt should ~~ be of the formula R4 m SiHm~ where m is an integer from 1 -to 4. R is ; indapendently selccted from any aliphatic and/or aromatic radlcal wlth one or more carbon atoms. Examples oE silana type compouncls -to use in this inventioninclude, but ara not limited to, e-thylsilane, diethylsilane, triethylsilane, phenylsilane, n-hexylsilane, diphenylsilane, triphenylsilane, and polyme-thylhydrosilane. Preferrad silane compounds include, bl2-t are not limlted to, diethylsilane, phenylsilane, n-hexylsilane, and mixtures of two or more of these type compounds. The amount of silane to be used is abou-t 0.1 to about 16 waight percent and preferably about 0.3 to about 8 weight percent based on the weight of the reduced supported catalyst. Most preferably abou-t 0.5 to about 4 weigh-t percent is used.
The silane compound can be contacted with -the reduced suppor-ted atAlyst prior -to the its use or it can be added to the reactor during -the copolymerizntlon. However, for max:lmum benefit, the reduced supported catalys-t preferably :Ls exposed to the silane compound prior to contacting the ethylene monomer. Tharefore, the silane and reduced supported catalyst more :~:
.
., ,,~:, - ~ .
2 ~ 329 26CA
preferably are precos~tActed prior -to introduction into the polymerization reactor.
When the silflne is added directly to the polymerization reactor, the silane usua]ly i6 added in a hydrocarbon solution, with the hydrocarbon usually being the same as the solvent contained in the reactor, but is not restricted to tha-t solvent. Dilute solutions, :i.e., about 0.005 to about I
weigh-t percent, are conveniently used when passing the silane solution in-to the reactor. If the silane flnd the reduced supported catalyst are precontacted prior to in-troduction to the polymeriza-tion reactor, a more concentrated silane solution can be used. After precontacting the reduced supported catalyst and silane, it is desirable to thorough]y mix the silane solution and the reduced supported catalyst.
REACTION CONDITIONS
The polymerization is carried ou-t under high-temperature slurry conditions. Such polymerization techniques are well known in the art and are disclosed, for exflmple, in U.S. Paten~ 3,248,179. It is essential for *his invention that a continuous loop reactor is used. Furthermore, the diluen-t utili~ed in this inventton must be isobutane.
The e~fect of using -this ca-talyst i9 to generate l-hexene in si-tu thus ~iving an ethylene/hexene copolymer from a pure ethylene feed.
In the figure3 the following designations are defined as:
Area One: is a daplction of the prior art area where safe non-fouling ~ opera-ting temperatures are found fot a given polymer density.
;~ Are8 Two: i8 a depict:Lon of the invention 8rea where safe non-fouling operating temperatures are found for a glven polymer density when using the catalyst sys-tem in t}n~s specification.
. . .
Line Alpha: is a depiction of the foul curva; this is the boundry between the prior art area of safe operating temperatures and -the new ; available operating temperatures of this invention; this line ~ is defIned by the followitlg equation:
; ~
T = 917.3 D-650.6 where ... , : :
," ~
'''~':. ' . :
2 ~ 5 ~ 32926CA
(1) 0.930 < D < 0.960;
~2) T :Ls the temperature in degrees fahrenheit; and ~3) D is the density iII grams per cubic centimeter.
The preferred reactor temperature to use ,in this slurry process Ls described by the following boundrLes:
(l) (F.L.T.) < (O.T.); more preferably (2) ~F.L.T.) < (O.T.) < ~F.L.T.) t 20~F; And most preferably (3) (F.I..T.) ~ 5~F < (O.T.) < (F.L.T.) ~ 15~F;
~shere (F.L.T.) is the foul line temperature for fl certaln polymer density; and(~.T.) is the operatlng temperature of the reactor. The dif~erences ln the prefe~red ranges are a compromise be,tween th~ compet.ing factors of increased energy cost in performlng this process and the higher melt index polymers obtainable by this procsss.
Using this cata'1y~t system at these tempera-tures the amount of hexene generated in situ can be varied so as to adjust the dens.ity of the de~ired polymer by :increasing or decreasing the amount of cocatalyst use<l.
For example, increasing the amount oE cocatalyst will increase the amount of hexene generated. The dens:lty can also be controlled by removing unwanted hexene from the reactor's diluent thereby increasing the density. This removed hexene can be separa-ted from the diluent by any reasonable means known in the art. Additionally, -the density of the polymer can be controlled by aFEirmatively adding hexene -to the reactor.
EXAMPLE
A 600 gallon reactor was used to conduct this slurry polymerization of ethylene and l-he~ene. The reactor was similax to those disclosed in V.S.
Patent 3,248,179. Th~ reactor wfls being run con-tinuously at a production level of about 1000 lbs/hr polymer. The liqu,id phasa in the re~ctor was primarily the iso-butsne carrying diluent. The catalyst was chromium s~pported on a silica-titania support made in accordance with U.S. Patent 4,820,785. The cocatalyst selectecl was triethylborane. The monomer feed was essentially ethylene witll the hexene being generated in situ Eor incorporation into th~ copolymer. Although this was essentially an ethylene-hexene copolymerlzation, some other comonomers cou].d also have been generated in situ and subsequently copolymer.ized with the ethylene monomer. The polymerization .:
.
::: ~:
was allowed to proceecl :Eor ~evera] clay~ before the tes-ting described iII Tflble 1 occurred. The following clata were collected.
Reactor Temperature Density in Elapsed Time in Degrees Grams Per in Minutes4 Fflhrenheit Cubic CentimeterS
0 211.5 0.9460 212.0 212.5 213.0 0.9~49 ' 60 213.5 214.0 lOS 214.5 0.9444 135 215.0 165 215.5 0.943 lg5 216.0 210 21~.5 '~ 225 217.0 o.94232 , ~ 255 217.5 270 218.0 285 218.5 0.94193 315 219.0 ' ~ 345 219.0 No reActor fouling yat.
:
Still no reactor fouling.
3Some signs of reactor stress but still no fouling. '~
~;-, 4A~ noted above, this polymerizat:Lon was continlled for several days ~' before this experiment was oonducted.
These densities are tbe densities of the polymer at the end of the production line. These polymers exited the reactor abou-t 1-3 hours ea--lier.
They represent fln average density made over -the preceeding hour~ ~' ~ .
''~ Comparln$ the densi-ty v.s. temperature relatL~nsh:lp in the Elgure with the data iLn the taSle it Ls apparent that the reactor was running above ~,; the foul curve for the reaction. It should Se notcd that the densities recorded are the densities of the polymer at the end of the production li.ne.
Taking lnto account the nearly linear decrease in *he polymer density it is apparent that -the polymer actually made at T=345 would be much lower in . .
'' '~
:. ~
~ g ~ ~) 32926C~
~ (~
densit.y thfln the lnst .9418 density measurQment. ~11 in ~11, the r~actor was running at a tempera-ture of at least 5-10 degrees above the foul curve for about 3 ho~lrs. This ls a sign:Lficant increflSe i.n the allowed operat:ing area, thus giving additional advantages such as lmproved melt indexes.
Wh:Lle this invention has been described in detail. for purpose of il.lustratlon, i-t is not to be construed as l:Lmited thereby, bu-t ls lntended to cover all changes and modifi.cfl-tions withi.n the spirit and scope -thereof.
::
.
:
, ~ ,, ::
,.,~
~' '~ ~
~, . .
,.~ .
, ~
! .' ~
~' ' . ' . ~
Claims (15)
1. A polymerization process comprising:
contacting, in a continuous loop reactor which has isobutane as a diluent, an ethylene feedstream with a catalyst system, which generates in situ hexene as a comonomer, made by a process comprising:
(a) subjecting a chromium catalyst component which is supported on a refractory oxide support to activation in an oxygen containing ambient at an elevated temperature sufficient to convert at least a portion of any chromium in a lower valent state to a hexavalent state;
(b) thereafter subjecting the thus activated composition of (a) to carbon monoxide under reducing conditions, and (c) thereafter contacting the thus reduced composition of (b) with a cocatalyst selected from trialkyl boron compounds and polyalkyl silane compounds;
under slurry polymerization conditions that include a polymerization temperature above the foul curve temperature to produce a copolymer with a density between 0.930 grams per cubic centimeter and 0.960 grams per cubic centimeter.
contacting, in a continuous loop reactor which has isobutane as a diluent, an ethylene feedstream with a catalyst system, which generates in situ hexene as a comonomer, made by a process comprising:
(a) subjecting a chromium catalyst component which is supported on a refractory oxide support to activation in an oxygen containing ambient at an elevated temperature sufficient to convert at least a portion of any chromium in a lower valent state to a hexavalent state;
(b) thereafter subjecting the thus activated composition of (a) to carbon monoxide under reducing conditions, and (c) thereafter contacting the thus reduced composition of (b) with a cocatalyst selected from trialkyl boron compounds and polyalkyl silane compounds;
under slurry polymerization conditions that include a polymerization temperature above the foul curve temperature to produce a copolymer with a density between 0.930 grams per cubic centimeter and 0.960 grams per cubic centimeter.
2. A process according to claim 1 wherein said ethylene feedstream consists essentially of pure polymerization grade ethylene.
3. A process according to claim 1 wherein said chromium catalyst component is about 0.05 to about 5 weight percent chromium based on the total weight of said chromium and support after activation.
4. A process according to claim 1 wherein said support is selected from the group consisting of alumina, boria, magnesia, silica, thoria, zirconia, and mixtures thereof.
5. A process according to claim 1 wherein said support is predominantly silica.
6. A process according to claim 5 wherein said predominantly silica support is about 92 to 97 weight percent silica the remainder being a titania component.
7. A process according to claim 5 wherein said predominantly silica support is a cogel of at least 90 weight percent silica the remainder being a titania component.
8. A process according to claim 5 wherein said predominantly silica support is essentially pure silica.
9. A process according to claim 1 wherein said activation is carried out in the air at a temperature within the range of 400° to 900°C.
10. A process according to claim 1 wherein said reducing conditions include a temperature within the range of 300° to 500°C.
11. A process according to claim 1 wherein said cocatalyst is triethylborane.
12. A process according to claim 1 wherein said silane cocatalyst is selected from the group consisting of ethylsilane, diethylsilane, triethylsilane, phenylsilane, diphenylsilane, triphenylsilane, n-hexylsilane, polymethylhydrosilane, and mixtures thereof.
13. A process according to claim 1 wherein said polymerization temperature is between 0° and 20°F above said foul curve temperature.
14. A process according to claim 1 where a portion of in situ hexene is removed from the process.
15. A polymerization process comprising:
contacting, in a continuous loop reactor which has isobutane as a diluent, an ethylene feedstream with a catalyst system, which generates in situ hexene as a comonomer, made by a process comprising:
(a) subjecting a chromium catalyst component which is supported on a silica/titania support to activation in air at a temperature between 400 to 900°C.
(b) thereafter subjecting the thus activated composition of (a) to carbon monoxide under reducing conditions; and (c) thereafter contacting the reduced composition of (b) with triethylborans;
under slurry polymerization conditions that include a polymerization temperature about 5° to about 15°F above the foul curve temperature to produce a copolymer with a density between 0.930 grams per cubic centimeter and 0.960 grams per cubic centimeter.
contacting, in a continuous loop reactor which has isobutane as a diluent, an ethylene feedstream with a catalyst system, which generates in situ hexene as a comonomer, made by a process comprising:
(a) subjecting a chromium catalyst component which is supported on a silica/titania support to activation in air at a temperature between 400 to 900°C.
(b) thereafter subjecting the thus activated composition of (a) to carbon monoxide under reducing conditions; and (c) thereafter contacting the reduced composition of (b) with triethylborans;
under slurry polymerization conditions that include a polymerization temperature about 5° to about 15°F above the foul curve temperature to produce a copolymer with a density between 0.930 grams per cubic centimeter and 0.960 grams per cubic centimeter.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US07/594,259 US5071927A (en) | 1990-10-09 | 1990-10-09 | High-temperature slurry polymerization of ethylene |
| US07/594,259 | 1990-10-09 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2051948A1 CA2051948A1 (en) | 1992-04-10 |
| CA2051948C true CA2051948C (en) | 1997-11-04 |
Family
ID=24378179
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002051948A Expired - Fee Related CA2051948C (en) | 1990-10-09 | 1991-09-20 | High-temperature slurry polymerization of ethylene |
Country Status (10)
| Country | Link |
|---|---|
| US (1) | US5071927A (en) |
| EP (1) | EP0480377B1 (en) |
| JP (1) | JPH04234412A (en) |
| KR (1) | KR0161989B1 (en) |
| CA (1) | CA2051948C (en) |
| DE (1) | DE69129216T2 (en) |
| ES (1) | ES2113866T3 (en) |
| HU (1) | HU211444B (en) |
| MX (1) | MX9101172A (en) |
| NO (1) | NO300141B1 (en) |
Families Citing this family (27)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US5137994A (en) * | 1991-05-23 | 1992-08-11 | Union Carbide Chemicals & Plastics Technology Corporation | Process for the simultaneous trimerization and copolymerization of ethylene |
| US5274056A (en) * | 1992-01-31 | 1993-12-28 | Phillips Petroleum Company | Linear, very low density polyethylene polymerization process and products thereof |
| US5208309A (en) * | 1992-01-31 | 1993-05-04 | Phillips Petroleum Company | Linear, very low density polyethylene polymerization process and products thereof |
| JPH1087518A (en) * | 1996-09-18 | 1998-04-07 | Mitsubishi Chem Corp | Production of 1-hexene |
| US6417305B2 (en) | 1996-12-17 | 2002-07-09 | E. I. Du Pont De Nemours And Company | Oligomerization of ethylene |
| US6432862B1 (en) | 1996-12-17 | 2002-08-13 | E. I. Du Pont De Nemours And Company | Cobalt catalysts for the polymerization of olefins |
| US6214761B1 (en) | 1996-12-17 | 2001-04-10 | E. I. Du Pont De Nemours And Company | Iron catalyst for the polymerization of olefins |
| US6423848B2 (en) | 1996-12-17 | 2002-07-23 | E. I. Du Pont De Nemours And Company | Tridentate ligand |
| AU5630198A (en) * | 1997-03-07 | 1998-09-10 | Phillips Petroleum Company | A process for polymerizing olefins |
| US5780698A (en) * | 1997-03-31 | 1998-07-14 | Chevron Chemical Company | Olefin oligomerization catalyst and process employing and preparing same |
| US6114483A (en) * | 1997-08-27 | 2000-09-05 | E. I. Du Pont De Nemours And Company | Polymerization of olefins |
| US6297338B1 (en) | 1998-03-30 | 2001-10-02 | E. I. Du Pont De Nemours And Company | Polymerization of olefins |
| US7906451B2 (en) | 1998-03-30 | 2011-03-15 | E.I. Du Pont De Nemours And Company | Mixed polymerization catalyst component for polymerizing olefins |
| EP0967234A1 (en) * | 1998-06-24 | 1999-12-29 | Fina Research S.A. | Production of polyethylene |
| US6201077B1 (en) * | 1998-12-01 | 2001-03-13 | Phillips Petroleum Company | Process that produces polymers |
| DE19912855A1 (en) * | 1999-03-22 | 2000-09-28 | Elenac Gmbh | Process for the production of ethylene copolymer |
| US6620895B1 (en) | 1999-03-22 | 2003-09-16 | E. I. Du Pont De Nemours And Company | Processing polyethylenes |
| AU7726800A (en) * | 1999-09-29 | 2001-04-30 | E.I. Du Pont De Nemours And Company | Manufacture of polyethylenes |
| AU7726900A (en) * | 1999-09-29 | 2001-04-30 | E.I. Du Pont De Nemours And Company | Preparation of curable polymers |
| US6407188B1 (en) | 1999-09-29 | 2002-06-18 | E. I. Du Pont De Nemours And Company | Polymerization of olefins |
| US6586541B2 (en) * | 2000-02-02 | 2003-07-01 | E. I. Du Pont De Nemours And Company | Process for production of polyolefins |
| WO2003106509A1 (en) * | 2002-06-17 | 2003-12-24 | Exxonmobil Chenical Patents Inc. | Polymerization process |
| US7459521B2 (en) * | 2004-08-06 | 2008-12-02 | E.I. Dupont De Nemours And Company | Heat-sealable polyolefins and articles made therefrom |
| US7910669B2 (en) * | 2009-03-17 | 2011-03-22 | Chevron Phillips Chemical Company Lp | Methods of preparing a polymerization catalyst |
| JP5749718B2 (en) | 2009-08-17 | 2015-07-15 | イー・アイ・デュポン・ドウ・ヌムール・アンド・カンパニーＥ．Ｉ．Ｄｕ Ｐｏｎｔ Ｄｅ Ｎｅｍｏｕｒｓ Ａｎｄ Ｃｏｍｐａｎｙ | Improvement of olefin polymerization method |
| US8916664B2 (en) | 2010-03-29 | 2014-12-23 | E I Du Pont De Nemours And Company | Olefin polymerization process |
| US8399580B2 (en) | 2010-08-11 | 2013-03-19 | Chevron Philips Chemical Company Lp | Additives to chromium catalyst mix tank |
Family Cites Families (14)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3248179A (en) * | 1962-02-26 | 1966-04-26 | Phillips Petroleum Co | Method and apparatus for the production of solid polymers of olefins |
| US3900457A (en) * | 1970-10-08 | 1975-08-19 | Phillips Petroleum Co | Olefin polymerization catalyst |
| US3887494A (en) * | 1970-11-12 | 1975-06-03 | Phillips Petroleum Co | Olefin polymerization catalyst |
| US3939137A (en) * | 1974-11-22 | 1976-02-17 | Phillips Petroleum Company | Silane adjuvant for chromium oxide catalyst |
| US4053436A (en) * | 1975-08-26 | 1977-10-11 | Phillips Petroleum Company | Spray dried titanium-containing catalyst for stress crack resistant polymer |
| US4151122A (en) * | 1977-12-05 | 1979-04-24 | Phillips Petroleum Company | Reduction and reoxidation of cogel or self-reduced catalyst |
| US4294724A (en) * | 1980-02-06 | 1981-10-13 | Phillips Petroleum Company | Titanium impregnated silica-chromium catalysts |
| US4405501A (en) * | 1982-01-20 | 1983-09-20 | Phillips Petroleum Company | Aging of chromium-containing gel at high pH |
| US4392990A (en) * | 1982-01-20 | 1983-07-12 | Phillips Petroleum Company | Heating silica gel in inert atmosphere before activation |
| US4818800A (en) * | 1985-07-29 | 1989-04-04 | Phillips Petroleum Company | Polymerization process utilizing a silica-supported chromium oxide catalyst and boron-containing adjuvant |
| US4820785A (en) * | 1986-06-16 | 1989-04-11 | Phillips Petroleum Company | In situ comonomer generation in olefin polymerization |
| US4816432A (en) * | 1987-05-28 | 1989-03-28 | Mobil Oil Corporation | Catalyst composition for polymerizing alpha olefins |
| CA1309801C (en) * | 1987-09-18 | 1992-11-03 | Elizabeth A. Boggs | Process for olefin polymerization |
| US4988657A (en) * | 1989-10-06 | 1991-01-29 | Phillips Petroleum Company | Process for olefin polymerization |
-
1990
- 1990-10-09 US US07/594,259 patent/US5071927A/en not_active Expired - Lifetime
-
1991
- 1991-09-18 KR KR1019910016275A patent/KR0161989B1/en not_active IP Right Cessation
- 1991-09-19 JP JP3239681A patent/JPH04234412A/en active Pending
- 1991-09-19 MX MX9101172A patent/MX9101172A/en unknown
- 1991-09-20 CA CA002051948A patent/CA2051948C/en not_active Expired - Fee Related
- 1991-10-08 EP EP91117139A patent/EP0480377B1/en not_active Expired - Lifetime
- 1991-10-08 ES ES91117139T patent/ES2113866T3/en not_active Expired - Lifetime
- 1991-10-08 NO NO913945A patent/NO300141B1/en not_active IP Right Cessation
- 1991-10-08 HU HU913177A patent/HU211444B/en not_active IP Right Cessation
- 1991-10-08 DE DE69129216T patent/DE69129216T2/en not_active Expired - Fee Related
Also Published As
| Publication number | Publication date |
|---|---|
| HUT59699A (en) | 1992-06-29 |
| HU211444B (en) | 1995-11-28 |
| MX9101172A (en) | 1992-06-05 |
| EP0480377B1 (en) | 1998-04-08 |
| NO913945D0 (en) | 1991-10-08 |
| KR0161989B1 (en) | 1999-01-15 |
| KR920008071A (en) | 1992-05-27 |
| JPH04234412A (en) | 1992-08-24 |
| CA2051948A1 (en) | 1992-04-10 |
| US5071927A (en) | 1991-12-10 |
| HU913177D0 (en) | 1992-01-28 |
| EP0480377A2 (en) | 1992-04-15 |
| ES2113866T3 (en) | 1998-05-16 |
| EP0480377A3 (en) | 1992-11-19 |
| NO913945L (en) | 1992-04-10 |
| DE69129216D1 (en) | 1998-05-14 |
| NO300141B1 (en) | 1997-04-14 |
| DE69129216T2 (en) | 1998-07-30 |
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