CA1312172C - Method for reducing sheeting during polymerization of alpha-olefins - Google Patents
Method for reducing sheeting during polymerization of alpha-olefinsInfo
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- CA1312172C CA1312172C CA000582147A CA582147A CA1312172C CA 1312172 C CA1312172 C CA 1312172C CA 000582147 A CA000582147 A CA 000582147A CA 582147 A CA582147 A CA 582147A CA 1312172 C CA1312172 C CA 1312172C
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- ethylene
- olefins
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Abstract
METHOD FOR REDUCING SHEETING
DURING POLYMERIZATION OF ALPHA-OLEFINS
ABSTRACT
A method for reducing sheeting during polymerization of alpha-olefins in a low pressrue fluidized bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts wherein water is introduced water into said reactor in an amount sufficient to maintain the electrostatic levels at the site of possible sheet formation at levels which avoid sheeting without substantially altering the effectiveness of said catalysts.
DURING POLYMERIZATION OF ALPHA-OLEFINS
ABSTRACT
A method for reducing sheeting during polymerization of alpha-olefins in a low pressrue fluidized bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts wherein water is introduced water into said reactor in an amount sufficient to maintain the electrostatic levels at the site of possible sheet formation at levels which avoid sheeting without substantially altering the effectiveness of said catalysts.
Description
3~L2~ 72 METHOD FOR REDUCING SHEETING
~URING POLyMERIZATION OF ALPHA-OLEFINS
Field of the Invention Th~s invention relates to a method for reducing shseting during polymerization of alph~-olefins ~nd more particularly to a method for reducing sheeting during polymerization of polyethylene utilizing titanium b~sed catalysts or vanadium based catalysts with alkyl aluminum cocatalysts.
Summar~ o~ the Prior Art Conventional low density polyethylene has I been historically polymerized ~n heavy walled ; 15 autoclaves or tubular re~ctors at pressures as high as 50,000 psi and temperatures up to 300C or higher. The molecular structure of high pressure, low denslty palyethylene (HP-LDPE~ is high complex.
The permutations in the arra~gement of their simple building blocks are essentially ~nfinite. HP-LDPE's ~re characterized by an intricate long chain branched molecular architecture. These long chain branches have 8 drsmatic ef~ect on the melt rheology of these resins. ~P-LDPE's also possess a spectrum of shart chain branchesJ generally l to 6 carbo~
atoms in length. These short chain br~nches disrupt cryst~l ~ormation and depress resin ~ensity.
; ~ More recently, technology has been provided whereby low density polyethylene can be produced by ~luidized bed techniques at low pressures and ;' .
.
.: :
'"' ' ~312~ ~2 temperatures by copolymerizing ethylene with vsrious slpha-olefins. These low pressure LDFE (LP-LDPE) resins generally possess little, iE any, long chain branching and sre sometimes referred to as linear LDPE resins. They sre short chain branched with branch length and frequency controlled by the type and amount of comonomer used during polymerization.
As is well known to those sX~lled in the art, low pressure t high or low density polyethylenes can now be conventionally pro~ided by a fluidized bed process utilizing several families of catalysts to produce a full range of low density and high density products. The appropriate selection of catalysts to be utilized depends in part upon the type of end product desired, i.e., high density, low density, extrusion grflde, film grate resins and other criteria.
The various types of catalysts which may be used to produce polyethylenes in fluid bed reactors can generally be typed as follows:
TYPe I. The silyl chromate catalysts disclosed in U.S. Patent No. 3,324,101 to Baker and Carrick and U.S. Patent No. 3,324,095 to Carrick, Karapinks and Turbet. The silyl chromate catalysts are characterized by the presence therein of a group of the formula:
R O
Si 0-- Cr O
.
13~ 2172 wherein R is a hydrocsrbyl group having from i to 14 c~rbon ~toms. The preferred silyl chromste cat~lysts are th~ b~s(tri~rylsilyl) chromates and : more preferably bis~triphenylsilyl) chromate.
This catalyst i5 used on a support such as . silica, ~lumin~, thor~, zircon~ and the like, other supports such as carbon blacX, ~ : micro-crystalline cellulose, the non-sulfon~ted ion -~ exchange resins ~nd the like may be used.
TyPe II. The bis(cyclopentadienyl) chromium (II) compounds disclosed ~n U.S. Patent No. 3,879,368.
These bis(cyc~spent~dienyl) chromium ~II) compounds have the following formuls:
~R )~ )n"
Cr ~H)5~n' (~)S-n"
`~ wherein R' ~nd ~" may be the same or different C
to C20, inclusive, hydroc~rbon radicals, and n' and n" may be the same or different integers of 0 to 5, incluslve. The R' and R" hydrocarbon r~dicals may be s~tursted or unsatur~ted, ~nd can ~nclude : aliph~tic, al~cyclic snd aromatic rad~cals such as methyl, ethyl, propyI, butyl, pentyl, cyclopentyl, cyclohexyl, allyl, ~henyl ~nd naphthyl r~dicals.
These catalysts are used on a support as : heretofo~e described.
Type III. The catalysts 8S described in U.S. Patent No. 4,011,382. These cat~lysts contain chromium snd `" _ 4 1~ 72 based on the combined weight of the support and the chromium, titanium and fluorine, about 0.05 to 3.0, and preferably about 0.2 to 1.0 weight percent of chromium (calculated as ~r), about 1.5 to 9.0 and preferably about 4.0 to 7.0, weight percent of titanium (calculated as Ti), and 0.0 to about 2.5, and preferably about 0.1 to 1.0 weight percent of fluorine (calculated as F) The chromium compounds which may be used for the Type III catalysts include CrO3, or any compound of chromium which is oxidizable to CrO3 under the activation conditions employed. At least a portion of the chromium in the supported, activated catalyst must be in the hexavalent state.
Chromium compounds other than CrO3 which may be used are ~isclosed in U.S. Patent No. 2,825,721 and U.S. Patent No. 3,622,521 and include chromic acetyl acetonate, chromic nitrate, chromic acetate, chromic chloride, chromic sulfate, and ammonium chromate.
` The titanium compounds which may be used include all those which are oxidizable to TiO2 under the activation conditions employed, and include those disclosed in U.S. Patent No. 3,6Z2,521.
The fluorine compounds which may be used include HF, or any compound of 1uorine which will yield HF under the activation conditions employed.
The inorganic oxide materials which may be used as a support in the catalyst compositions are 1'~12~ 7~
The inorganic oxide materials which may be used as a support in the catalyst compositions are porous materisls having a high surface area, that i~, a surface area in the ran8e of about 50 to lOOO
S square meter~ per gram, and an average particle size of about 20 ~o 200 microns. The ~norganic oxides which may be used include silica, alumina, thoria, zirconia and vther romparable inorganic oxides, as well as mixtures of such oxides.
Type IV. The catalysts as described in U.S. Patent No. 4,302,566 in the names of F.~. Karol et al, and entitled, "Preparation o~ Ethylene Copolymers in Fluid Bcd Reactor" and ~ssigned to the same assignee ~ as the present application. These catalysts : 15 comprise at least one titanium compound, at least one magnesium compound, ~t least one electron donor compound, at least one activator compound and at least one inert carrier ma~erial.
The titanium compound has the structure 20 , Ti (OR)aXb wherein R is a Cl to C14 aliphatic or aromatic hydrocarbon radical, or COR' where R' is a Cl to C14 sliphatic or aromatic hydrocarbon radicali X
is Cl, Br, or I; a is O or l; b is 2 to 4 inclusive;
and a+b = 3 or 40 The titanium compounds can be used individually,or in combinstion thereof, ~nd would ~nclude TiC13, TiC14, Ti(OCH3~C13, Ti~oc6H5)cl3~ Ti(OCOCH3~)C13 and Ti(OC~c6Hs)cl3 The magnesium compound has the structure:
MgX2 -~-15407 . , wherein X is Cl, Br, or I. Such magnesium compounds can be used individually or -in combinations thereof and would include MgC12, M~Br2 and MgI2.
Anhydrous MgC12 is the preferred magnesium compound.
-The titanium compound and the magnesium compound sre generally used in a form which will facilitate thei~ dissolution in the electron donor compound.
The electron donor compound is an organic compound which is li~uid at 25C and in which the titanium compound and the magnesium compound are partially or completely soluble. The electron donor compounds are known as such or as Lew~s bases.
The electron donor compounds would include such compounds as alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ethers, cyclic ethers and aliphatic ketones.
The cstalyst may be modified with a boron halide compound hflving the structure:
BR X ' 3 wherein R is an aliphatic or aromatic hydrocarbon radical containing from 1 to 14 carbon atoms or OR', wherein R' is also an aliphatic or aromatic hydrocarbon radical containing from l to 14 carbon atoms; X' is selected from the group consisting of Cl and Br, or mixtures thereof, and; c is O vr 1 when R is an ~liphatic or aromatic hydrocarbon and O, 1 or 2 when R is OR'.
The boron halide compounds can be used individually or in combination thereof, and would include BC13, ~Br3, B~C2H5)C12.
. .
~ ~-B(oc2Hs)cl2~ B(C2H5) B(C6H5)C12~ B(OC6H5)Cl2' (C6H13)C12' B()C6Hl3)C12, and B(OC~H5)2Cl. Boron trichloride is the particularly prererred boron compound.
The ~ctivator compound has the structure:
Al(~") X'dH
where~n X' is Cl or ORl; Rl and R" are the same or di~ferent and are Cl to C14 saturated hydrocarbon radicals, d is 0 to 1.5, e is 1 or 0, and c~d+e = 3.
Such activator compounds can be used individually or in combinations thereof.
The carrier materials are solid~
particulate materials and would include inorganic materials such as oxides of silicon and aluminum and molecular sieves, ~nd organic materials such as ole~in polymers, e.g., polyethylene.
TYPe V. Vanadium based catalysts. These type catalysts generally include vanadium as the sctive ingredient, one such type catalyst generally comprises a supported precursor, a cocatalyst and a promoter. The supported precursor consists essentially of a vanadium compound and modifier impregnated on a solid, inert carrier. The vanadium compound in the precursor is the reaction product of a vanadium trihalide and an electron donor. The halogen in the vanadium trihalide is chlorine, bromine or iodine, or mixtures thereo~. A
particularly preferred vanadium trihalide is vanadium trichloride, VC13.
The electron donor is a liquid, organic Lewis base in which the vanadium trihalide is , ,.. .
~ ~, "~
~ ~3~ ~7~
soluble. The ~lectron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic scids, aliphatic esters, aliphatic ketones, aliphatic amines, aliphatic slcohols, alkyl and cyclo21kyl ethers, and mixtures thereof. Preferred eleotron donors are alkyl and eyeloalkyl ethers, including particularly tetrahydrofuran. Between about 1 to about 20, preferably between sbout 1 to about 10, and most preferably about 3 moles of the electron donor are complexed wi~h each mole o vanadium used.
The modifier used in the precursor has the formula:
MXa wherein:
M is either boron or AlR(3 a) and wherein each R is independently alkyl, provided that the total number of aLiphatic carbon stoms in any one R group may not exceed 14;
X is chlorine, bromine or iodine; and a is 0, l or 2, with the provision that when M ig boron a is 3.
Preferred modifiers include Cl to C6 alkyl aluminum mono- and ti- chlorides and boron trichloride. A particularly preferred modifier is diethyl aluminum chloride. About 0.1 to about 10, and pref erably abo~t 0.2 to about 2.5, moles of modifier are used per mole of electron donor.
The carrier is a solid, pa~tic~late porous material inert to the ~olymerizAtion. The carrier consists essentially of silica or alumina, i.e., oxides of silicon or aluminum or mixtures thereof.
: .
' . . ~,.
.
_ 9 Optionally, the carrier may contsin additional materials such as zirconia, ~thoria or other compounds chemically inert to the polymerization or mixtures thereof.
The carrier is u~ed as ~ dry powder having sn average p~rticle size of between about 10 to about 250, pre~erably about 20 to about 2~0, and most prefersbly about 30 to about 100, microns. The po~ous carrier has a ~urfsce sre~ of 8reater than or equal to about 3, and preferably gre~ter than or equ~1 to about SO, m /g. A preferred carrier is silica having pore sizes of greater than or equal to about 80, and preferably greater than or equal to about 100, angstroms. The carrier is predried by heating to remove water, preferably at a temperature of greater than or equal to about 600C.
The mount o~ carrier used is that which will provite a vanadium content of between about 0.05 to about 0.5 mmoles o~ vanadium per gram (mmole V/g), and preferably between about 0.2 to about 0.35 mmole V/8, ~nd most preferably about 0.29 mmole V/g.
l'he carrier is ordinarily free of prep~rative chemical treatment by reaction with an alkylaluminum compound prior to the formation of the supported precursor. Such treatment results in the formation o aluminum alkoxides chemically bonded to the carrier molecules. It~has been discovered that the use of such a treated carrier in the catalyst composition and process is not only nonessential, but instead results in undesirable agglomeration when used in the preparation o~ high density polyethylene .
~ D-15407 :~ `
' - i , , ' ' - ~ :
',. . .
(>0.94 ~Icc), resulting in a chunk-like, non-fr~ely flowing product,`~
The cocatalyst wh~ch can be employed for : the Type IY and Type V catalysts has the formula::~: 5 AlR3 ;~ wherein R is as previously de~ined in the definition o~ M. Preferred cocatalysts include C2 to C8 trialkylaluminum compounds. A particularly pre~erred cocatslyst ~s triiso~utyl alum~num, Between ~bout 5 to ~bout 500, and preferably between about ~0 to about 50, moles of cocatalyst are used per mole of vanad~um.
The promoter has the formula:
R'bCx (4_~) wherein:
R' is hydrogen or unsubstituted or h~losub~tituted lower, i,e,, up to sbout C6 containing, alkyl;
X' is halogen; snd b is 0, 1 or 2, Between about 0,1 to about 10, and prefersbly ~ between about 0,2 to about 2, moles of promoter are : uset per mole o~ cocatalyst, The catalyst is produced by first preparing the supported precursor, In one embodiment, the vanadium compound is prepared by dissolving the vanadium trihalide in the electron donor at : temperature between about 20CC up to the bo~ling point of the elect.on donor for:a few hours, ~, ~ 30 Preferably, mixing occurs:at about 65C for about 3 hours or more, The vanadium compound so produced is : then impregnated onto the carrier, Impregnation may .
~ D-15407 :~ ' .
, ~ .
`'.. : : ' . .
~2~7~
b~ effected by adding the carrier as a dry powder or 8S a slurry in the electron donor or other inert solvent. The liquid is removed by drying at less than about 100C for a few hours, preferably between about 45 to about-90C for about 3 to 6 hours. The modifier, dissolved in an inert solvent, such as a hydroearbon, is then mixed with the vanadium impregnated carrier. The liqu~d is removed by drying ~ temperatures of less than about 70C for a few hours, preferably between about 45 ~o about 65C f or ~hout 3 hours.
The cocatslyst and promoter are added to the supported precursor either before and/or during the polymerization reaction. The cocatalyst and promoter are add~d either together or sepsrately, and either simultaneously or sequentially during pol~merization. The cocatalyst and promoter are preferably added separately as solutions in inert solvent, such as isopentane, during polymerization.
In general, the above catalysts are introduced together with the polymerizable materials, into a reactor having an expanded section sbove a straight-sided section. Cycle gas enters the bottom of the reactor and passes upwsrd through a gas distributor plate into a fluidized bed located in the straight-sided section of the vessel. The gas distributor plate ser~es to ensure proper gas d~stribution snd to support the resin bed when gas flow is stopped.
G~s leaving the fluidized bed entrains resin particles. Most of these particles are d~sengaged as the gas passes through the expanded section where ~ts veloc~ty is reduced.
~3~ 7~
In order to satis~y certain end use appllcations for ethylene resins, such as ~or film, in~ection molding and roto-molding applications, c~t~lyst Types IV and V with fllkyl aluminum cocatalysts have been used. However, attempts to produce certa~n ethylene resins utilizing alkyl aluminum cocatalysts with the Type IV and V
catalysts supported on a porous silica substrate in certain fluid bed re ctors, hsve not been entirely satlsfactory from a practical commercial standpoint. This is prim~rily due to the formation of "sheets" in the reactor after a period of operat~on. The "sheets" can be characterized 8S
constituting a fused polymeric material.
It has been ~ound that a static mechanism is a contributor to the sheeting phenomena whereby catalyst and resin particles adhere to the reactor walls due to static ~orces. If allowed to reside long enough under a reactive environment, excess temperatures can re~ult in particle fusion.
N~merous causes for static ch~rge exist. Among them are generation due to frictionsl electrification of dissimilar materials, llmited static dissipation, introduction to the process of minute quantities of prostatic agents, excessive catalyst Pctivities, etc. Strong correlation exists between sheeting and the presence of excess static charges e~ther negative or positive. This is evidenced by sudden _hanges in static leYels followed closely by deviation in temperatures at the reactor wall.
These temperature deviations are either high or low. Low temperatures indicate particle adhesion ~ ~
- ~ , : . ..
' :.:
, 2~72 causing an insulating effect from the bed temperature. High deviations indicate reaction taking place in zones of limited heat transfer.
Following this, disruption in fluidization patterns ls generally evident, ~atalyst feed interruption can occur, product discharge system pluggage results, and thin fused agglomerates ~sheets) are noticed in the granular produc~.
The sheets vary widely in size, but are similar in most respects. They are usually about 1/4 to 1/2 inch thick and are from about one to five feet long, with a few specimens even longer. They have a width of about 3 inches to more than 18 inches. The sheets have e core composed of fused polymer which is oriented in the long direction of the sheets and their surfaces are covered with granular resin which has fused to the core. The edges of the sheets can have a hairy appearance from strands of fused polymer.
It is therefore an object o the present invention to provide a method for substantially reducing or eliminat~ng the amount of sheeting which `~ occurs during the low pressure fluidized bed polymerization of alpha-olefins utilizing titanium based compounts or vanadium based compounds as catalyst with alkyl aluminum as cocatalysts.
; Another obiect is to pro~ide a process for reducing sheeting in fluidized bed reactors utilized for the production of polyolefin resins wherein - 30 titanium or van~dium based catalysts with alkyl aluminum cocatalysts are employed.
;,", ',, . : , , :' "`" ` -...
.
17~
These and other ob~ects will become readily apparent from the following-descri~tion taken in con~unction with the accompanying drawing which generally indicates a typical gas phase fluidized bed polymerizAtion process for producing high den~ity ~nd low density polyolefins slightly modified to reflect the present invention.
~roadly contemplated, the present inYention provides a method or reducing sheet~ng during polymerization of alpha-olefins in a low pressure fluid~zed bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts which comprises introducing water into said reactor in an amount sufficient to maintain the electrostat~c levels at the site of possible sheet formation at levels ~hich a~oid sheeting without substantially altering the effectiveness of said catalysts.
The amount of water which is fed to the reactor depends on the stat~c voltage within the reactor and can generally range in an amount of 0.1 to about 2 ppm based on ethylene feed. In general, the nitrogen flow control is such as to permit a nitrogen flow of about 0 to about ll lbs/hr for an ethylene feed range of about 0 to about 50,000 lb/hr. The water cylinder temperature in the O'Brien box can generally be in a range of about 10C to about 40C. Nitrogen pressures can generally range from about 200 to 400 psig prefer~bly about 320 to about 370 psig.
The critical static voltage level for sheet ; formation is a complex ~unction of resin sintering .~
:, . .
.
. , .
~L31~7~
temperature, operating temperature, drag forces in the fluid bed, resin particle size distribution and recycle ~as compositlon. The static vol~age can be . reduced by a variety o~ techniques such as by ; 5 tresting the reactor surface to reduce static electric generation by inlection of an antistatic agent to inc~ease particl~ surface electrical . ~. conductivity thus p~omoting particle dischargin~; by installation of appropriate devices connected to the reactor walls which are designed to promote ~ electrical dischargin~ by creating areas of high : localized field stren~th, and by neutralization of charges by the in~ection or creation of ion pairs, ions or charged particles of the opposite polarity from the resin bed.
According to the present invention, the use of water add back to the gas phased low pressure polyethylene process will assist in the reduc~ion of agglomerate formation in the fluidized bed. This is accomplished by a reduction in the levels of positive. static voltage which lowers particle adhesive forces in the re~ction system.
Re~errin~ particulsrly to the sole figure of the drawing, a conventional fluidized bed reaction system for polymerizing alpha-olefins slightly modified to provide for water add back, includes a reactor 10 which consists o~ a reaction zone 12 and a velocity reduction zone 14.
The react$on zone 12 includes a bed of : 30 growing polymer particles, formed polymer particles :` and a minor amount of catalyst particles fluidized ~`i by the continuous flow of polymerizable and ~:
.
, ; - ~ ,. ; .. - .
' " :-, modifying gaseous components in the form of make-up feed and recycle gas through the reaction zone. To maintain a viable fluidized bed, the mass ~as flow rate through the bed is normally maintained above the minimum flow required for fluidization, and preferably from about 1.5 to about 10 times Gmf and more preferably from about 3 to ~bout 6 times Gmf. G~f ~s used in the accepted form as the abbrevi~tion for the minimum gas flow required to achieve fluidlzation, C.Y. W~n and Y.H. Yu, "Mechanics of Fluidization", Chemical Engineering Progress Symposium Series, Vol. 62, pg. 100-111 (1966).
It is highly desirable that the bed always contalns particles to prevent the formation of localized "hot spots" and to entrap and distribute the psrticulate catalyst throughout the reaction zone. On start up, the reactor is usually charged with a base of particulate polymer particles before g~s flow is initiated. Such particles may be identical in nature to the polymer to be formed or different therefrom. When different, they are withdrawn with the desired formed polymer particles as the first product. Eventually, a fluid~zed bed of the desired polymer particles supplants the start-up bed.
The appropriate catalyst used in the fluidized bed is preferably stored for service in a reservoir 16 under a blanket of a gas which is inert to the stored m terial, such as nitrogen or argon.
Fluidization is achieved by a high rate of gas r~cycle to and through the bed, typically in the order of about 50 times the rate of feed of make-up gas. The fluidized bed has~-the general appearance o~ ~ dense mass of viable particles in possible free-vortex flow es created by the percolation sf g8s through the bed. The pressure drop through the bed is equal to or slightly greater th~n the mass of the bed divided by the cross-s2ctional area. It is thus dependent on the geometry of the reactor.
Make-up gas ~s fed to the bed at a rate equ~l to the rate at which particulste polymer product is ~ithdrawn. The composition of the make-up g~s is determined by a gas analyzer 18 pos~tioned above the bed. The gas analyzer determines the composition of the ~as being recycled and the composition of the,make-up gas is adjusted accordingly to maintain An essentially steady stste gaseous composition within the rear.tion zone.
To insure complete fluidization, the recycle gas and, where desired, part or all of the ; 20 make-up gas ~re returned to the reactor at bsse 20 ~ b,elow the bed. Gas distribution plate 22 positioned ,, above the point of return ensures proper gas distribution and also supports the resin bed when ~ ~as flow is stopped.
;;~ 25 The port~on of the gas stream which does not resct in the bed constitutes the recycle gas which is removed from the polymeri~ation zone, preferably by psss~ng it into velocity reduction ~one 14 above the bed where entrained particles are , 30 given an opportunity to drop back into the bed.
The recycle gas is then compressed in a compressor 24 and thereafter passed through a heat ~' ~ ~ , ~, :
.
'~
., '' exchanger 26 wherein it is stripped of heat of reaction before it is returned to the bed. By constantly removing heat o~ reaction, no noticeable tempetatu2e gradient appears to exist within the upper portion 9~ the bed. A temperature gradient will exist in the bottom of the bed in 8 layer of about 6 to 12 tnches, between the temperature of the inlet gas and the temperature of the remainder of the bedO Thus, it has been observed that the bed acts to almost immediately ad~ust the temperature of the recycle gas above this bottom layer of the bed zone to make it conform to the temperature of the rem~inder of the bed thereby maintaining itself st ~n essentially constant temperature under steady conditlons. The recycle is then returned to the reactor at its base 20 and to the ~luidi~ed bed through distribution plate 22. The compressor 24 can also be placed downstream of heat exchanger 26.
Hydrogen msy be used as a chain transfer agent for conventional polymerization reactions of the types contemplated herein. In the case where ethylene is used as a monomer the ratio of hydrogen/ethylene employed will vary between 0 to ~ about 2.0 mo}es of hydrogen per mole of the monomer - 25 in the gas stream.
Any gas inert to the catalyst and reactants can also be present in the gas stream. The cocstalyst is ~dded to the gas recycle stream upstream of its connection ~ith the reactor ~s ~rom d$spenser 28 through line 30.
As is well known, it is essential to operate the fluid bed reactor at a temperature below ~2~2 _ ~9 the sintering temperature of the polymer particles.
Thus to-insure that sinteri~g will no~ occur, operating tem~eratures below sintering temperature ~re desired. For the production of ethylene polymers an operating temperature of rom about 90 to 100C ~s preferably used to prepare products hsving a density of about 0.94 to 0.97 while a temperature of about 75 to 95C is preferred for products having a density of about 0.91 to 0.94.
; 10 Norm~lly the fluid bed reactor is operated at pressures of up to about 80-110 psi for high density &nd 65-95 for low and medium density.
The catalyst is inJected ~nto the bed ~t a rate equal to its consumption at a point 32 which is abo~e ~he distribution plate 22. A gas which is inert to the catalyst such as nitrogen or argon is used to carry the catalyst into the bed. Injecting the catalyst at a po~nt above distribution plate 22 is an important feature. Since the catalysts normally used are highly active, in~ection into the area below the distribution plate may cause polymerizstion to begin there and eventually cause plugging of the distribution plate. In~ection into the viable bed, instead, aids in distributing the catalyst throughout the bed and tends to preclude the formation of localized spots of high catalyst concentration which may result in the formation of "hot spots".
Under a given set of operatin~ conditions, the fluidized bed is maintained at essentially a ; constant height by withdrawing a portion of the bed as product at 8 rate equal to the rate of ~ormation :, .... :
- .:
. . : -1~2~72 of the psrticulate polymer product. Since the rate of hest generation is directly related to product formation, a measurement of the temperature rise o~
the gas across the reactor (the difference between inlet g~s temperature and exit ~as temperature) is determ~native of the rate of particulate polymer ~ormation at a oonstant gas velocity.
The partic~late polymer product is prefer~bly withdr~wn at a point 34 at or close to distribution plate 22. The particulate polymer product is conven~ently snd preferably withdrawn through the sequential oper~tion of a pair of timed valves 36 ~nd 3S defining a segregation zone 40.
While Yalve 38 is closed, valve 36 is opened to emit a plug of gas ~nd product to the zone 40 between it and valve 36 which is then closed. Valve 38 is then opened to deliver the product to an external recovery zone and after delivery, valve 38 is then closed to await the next product recovery opera~ion.
Finally, the fluidized bed reactor is equipped with an adequate venting system to allow venting the bed during the start up and shut down.
The reactor does not require the use of stirring means andlor wall scraping means.
The re~ctor vessel is normally constr~cted of carbon steel and is designed for the operating conditions stated above.
In order to better illustrate the problems incident to the util$zation of the Type IV
catalysts, reference is again made to the drawing.
The titsnium based cata}yst (Type IV) is introduced into the reactor 10 at point 32. Under conventional ., ,-:~ , " ~3~ ~72 operations on certain resins, after a period of ~ime, sheets begin to form in reactor-10, at a site in the reactor proximate the wall of the resctor and located about a distance of one-h~lf the reactor diameter up ~rom the base of the fluid bed. The sheets of fused resin begin to appear in segregation zone 409 rapidly plugging the system, causing the reactor to be shut down. More characteristicslly the sheeting begins after production equivalent to 6 to 10 times the weight of the bed of resin in reactor 10.
Many possible causes were investigated in attempting to discover and eliminate the sheeting.
In the course of the investigation, thermocouples were ins~alled ~ust inside the reactor walls at elevations of 114 to }l2 reactor diameter above the gas dlstribution plate. Under conventionsl operationsl "skin" thermocouples indlcate temperatures equal to or slightly lower than the temperature of the fluidized bed. When sheeting occurs, these thermocouples m~y indicate tempersture excurslons of up to 20C ~bove the temperature of the fluidized bed thus providing reliable indication of the occurrence of sheeting. In addition, an electrostatic voltmeter was used to measure voltage on a 112 inch spherical electrode located ~n the fluid bed 1 inch radially from the reactor wall and usually 5 to 6 feet above the g~s distributor plste. The location was selected because sheet formation was observed to initiste in a band ranging ~rom 1/4 to 3l4 reactor dismeter in elevation sbove the ~ase of the fluid bed. As is well known for .
; ~
. , " ' ~ . .. .. . .
, 13 ~ rl ~
deep fluidized beds, this corresponds to the re~ion of least mixing intensity ne-ar the wall, i.e., a null zone where particle motion near the wall changes from generally upwsrd to generally downward. The possible c~uses investigated included factor~ affecting mixing in the fluidized bed, reactor operating conditions, c3talyst and resin pa~ticle size, part~cle size distribution, and others. A correlati~n was found between sheeting and buildup of static electric charge on the resin particles proximate the re~ctor walls. When the static voltage level of resin particles at particular sites proximate the reactor wall in a fluidized bed reactor is low, the reactor runs normally and no sheets are formed. When the static voltage level exceeds 8 critical level at those sites, uncontrolled sheeting occurs and the resctor must be shut down.
It was further discovered that sheeting could be substantially reduced and in some cases entirely eliminated by controlling static voltage in the ~luidized bed at a site proximate the reactor walls below the critical level for sheet formation.
This critical level for sheet formation is not a fixed v~lue, but is a complex function de~endent on variables including resin sintering temperature, operstin~ temperature, drag forces in the fluid bed, resin particle size distribution and recycle g8S
composition.
The critic~l voltage le~el Vc for sheeting ~; of ethylene homopolymers, ethylene-butene and ethylene-hexene copolyme~s is primarily a function : :`
7 ~
of the ~esin sintering *emperature, the reactor bed temperature and the concentration of hydrogen in the -recycle ~as.
The sintering temperature of the resin under reactor operating conditions ~s the te~perature at which ~ settled bed o~ resin in contact with a ~8S having the same composition as the reactor recycle gas used in producing the resin will sinter and form ~gglomerates when refluidization is attempted after allowing the ~ed to remsin settled for fifteen minutes. The sintering temperature is decreased by decreasing the resin denslty, by increasing the melt index and by increasing the amount of dissolved monomers and monomer type.
The constants in the equation were determined from data collected during reactor operation when the re~ctor ~ust began to exhibit sheeting symptoms through skin thermocouple temperature excursions above the bed temperature.
The voltage indicated on the voltage probe described earlier varies with time due to the random nature of a fluidized bed. Thus the crit~cal voltage, Vc, is expressed as A time averaged voltage. Voltage measurements are difficult to interpret because additional static electric charge is generated when sheet, formed because of a static chsrge, separates from the reactor wall~ In atdition, the sheeting phenomena can start ac a very local phenomenon and spread further clouding interpretation of voltage readings.
Although the sheeting mechanism is not fully understood, it is belieYed th t static ' ., ~ , , , ,. .. ~ :
.
.
~2~72 electricity generated in the fluid bed charges resin pArticles. When the charge ~n the par*icles reaches the level where the electrostatic forces trying to hold the chsrged particle near the reactor wall exceed the dr g forces in the bed trying to move the psrticle away from the w811, R layer o~ cstalyst contsining, polymerizing resln particles forms a non-fluidized layer nes~ the reactor wall. Heat removal from this layer is not sufficient to remove the heat of polymerization because the non-fluidized layer near the wall has less contact w~th ~he fluidizing gas than do particles in the fluidized portion of the bed. The heat of polymeriz~tion increased the temperature of the non-fluidized layer near the reactor wall unt~l the particles melt and fuse. At this point other particles from the fluidi~ed bed will stick to the fused layer and it will grow in size until it comes loose from the reactor wall. The separation of ~ dielectric from a conductor (the sheet from the reactor wall) is known to 8enerate additional static electricity thus accelerAting subsequent sheet ~ormstion.
The srt teaches various processes whereby ~ static voltage can be reduced or eliminated. These ; 25 comprise (1) reducing the rate of charge generation, (2~ increasing the rate of disch3rge of electrical chsrge, and (3) neutralization of electrical chsrge. Some processes suited for use in a fluidized bed comprise (1) use of an additive to inc~ease the conductivity of the particles thus providing a path for discharging, ~2) installation of grounding devices in a fluidized bed to provide D-154~7 . ., ~3~2~7~
additionQl area for disch~rging electrostatic chsrges to ~round, (3) ioniz- tion of g~s or p~rticles by electrical dischsrge to generate ions : to neutrslize electrostatic charges on the particles 9 ~nd (4) the use o~ r~tioactive sources to : produce radiation thAt ~ill create ions to neutralize elec~rostatic rhsrges on the particles.
The application of the~e techniques ~o a commercial scale, ~luidized bed, polymeriz~tion reactor m~y not be feasible or practical. Any ~dditive used must not act ~s 8 poison to the polymerizætion cat~lyst and must not ~dversely affect.the quality of the product. It had been previously thought that wster, the most widely used additi~e to reduce static on particles, c~nnot be used since it is 8 severe catalyst poison.
We h~ve found however that in certain specific reactions, i.e., when Type IV and ~
cat~lysts with alkyl aluminum cocatalysts are used in the fluidized polymerizstion process that the additlon of controlled minute quantities of water to the reactor drsmatically reduces the incidence of sheeting without producing severe detrimental effects to the catRlysts. The amount of water fed to the reactor depends on the static chsr~es present in the reactor.
Water addition csn be accomplished by a simple modification to the conventionsl procedures.
Thus referrin" ~gain to Fig. 1, ~n inert gas such as dry nitrogen from ~ nitrogen supply source, 41, is introduced into what is commonly referred to in the art an "O'Brien box" represented by Figure 42. The .~
.
.
. .
`` ~L312172 O'Brien box ~enerally contains one or more water tanks containing distilled w~ter and is equipped with temperat~re and flow control means all of which ~re not shown. The nitrogen is bubbled through one of the w~ter tanks which is a one liter st~inless steel cyllnder of distilled water in the temperature controlled housing. Water saturated nitrogen leaving the Q'Brien box 42, through line 44, is then flow controlled thro~gh heat traced tubing to enter the ole~in feed line such as ethylene feed line, 46, to the reaction cycle. Resultant water concentration in the ethylene is generally less than one part per million by volume. A moisture analy~er, 48, on the ethylene feed line 46 can be used for confirmation of water addition. R typical range for nitrogen flow control is about 0 to ll lbs/hr for sn ethylene ~eed range of 0 to 50,000 lbs/hr. With water cylinder temperature of 20C and : 350 psig nitrogen a ran8e of 0 to .3 ppm water isreali~ed. Ad~ustments in water temperature or nitrogen pressure can permit varying this range to desired levels.
Merely as illustrative the following information shows the water add back flow to the ; 25 reactor concentration calculations:
1. Determine vapor pressure of water at temperature in O!Brien box (PH20).
2. Determine flow of n~trogen with integrsl orifice (WN~).
~URING POLyMERIZATION OF ALPHA-OLEFINS
Field of the Invention Th~s invention relates to a method for reducing shseting during polymerization of alph~-olefins ~nd more particularly to a method for reducing sheeting during polymerization of polyethylene utilizing titanium b~sed catalysts or vanadium based catalysts with alkyl aluminum cocatalysts.
Summar~ o~ the Prior Art Conventional low density polyethylene has I been historically polymerized ~n heavy walled ; 15 autoclaves or tubular re~ctors at pressures as high as 50,000 psi and temperatures up to 300C or higher. The molecular structure of high pressure, low denslty palyethylene (HP-LDPE~ is high complex.
The permutations in the arra~gement of their simple building blocks are essentially ~nfinite. HP-LDPE's ~re characterized by an intricate long chain branched molecular architecture. These long chain branches have 8 drsmatic ef~ect on the melt rheology of these resins. ~P-LDPE's also possess a spectrum of shart chain branchesJ generally l to 6 carbo~
atoms in length. These short chain br~nches disrupt cryst~l ~ormation and depress resin ~ensity.
; ~ More recently, technology has been provided whereby low density polyethylene can be produced by ~luidized bed techniques at low pressures and ;' .
.
.: :
'"' ' ~312~ ~2 temperatures by copolymerizing ethylene with vsrious slpha-olefins. These low pressure LDFE (LP-LDPE) resins generally possess little, iE any, long chain branching and sre sometimes referred to as linear LDPE resins. They sre short chain branched with branch length and frequency controlled by the type and amount of comonomer used during polymerization.
As is well known to those sX~lled in the art, low pressure t high or low density polyethylenes can now be conventionally pro~ided by a fluidized bed process utilizing several families of catalysts to produce a full range of low density and high density products. The appropriate selection of catalysts to be utilized depends in part upon the type of end product desired, i.e., high density, low density, extrusion grflde, film grate resins and other criteria.
The various types of catalysts which may be used to produce polyethylenes in fluid bed reactors can generally be typed as follows:
TYPe I. The silyl chromate catalysts disclosed in U.S. Patent No. 3,324,101 to Baker and Carrick and U.S. Patent No. 3,324,095 to Carrick, Karapinks and Turbet. The silyl chromate catalysts are characterized by the presence therein of a group of the formula:
R O
Si 0-- Cr O
.
13~ 2172 wherein R is a hydrocsrbyl group having from i to 14 c~rbon ~toms. The preferred silyl chromste cat~lysts are th~ b~s(tri~rylsilyl) chromates and : more preferably bis~triphenylsilyl) chromate.
This catalyst i5 used on a support such as . silica, ~lumin~, thor~, zircon~ and the like, other supports such as carbon blacX, ~ : micro-crystalline cellulose, the non-sulfon~ted ion -~ exchange resins ~nd the like may be used.
TyPe II. The bis(cyclopentadienyl) chromium (II) compounds disclosed ~n U.S. Patent No. 3,879,368.
These bis(cyc~spent~dienyl) chromium ~II) compounds have the following formuls:
~R )~ )n"
Cr ~H)5~n' (~)S-n"
`~ wherein R' ~nd ~" may be the same or different C
to C20, inclusive, hydroc~rbon radicals, and n' and n" may be the same or different integers of 0 to 5, incluslve. The R' and R" hydrocarbon r~dicals may be s~tursted or unsatur~ted, ~nd can ~nclude : aliph~tic, al~cyclic snd aromatic rad~cals such as methyl, ethyl, propyI, butyl, pentyl, cyclopentyl, cyclohexyl, allyl, ~henyl ~nd naphthyl r~dicals.
These catalysts are used on a support as : heretofo~e described.
Type III. The catalysts 8S described in U.S. Patent No. 4,011,382. These cat~lysts contain chromium snd `" _ 4 1~ 72 based on the combined weight of the support and the chromium, titanium and fluorine, about 0.05 to 3.0, and preferably about 0.2 to 1.0 weight percent of chromium (calculated as ~r), about 1.5 to 9.0 and preferably about 4.0 to 7.0, weight percent of titanium (calculated as Ti), and 0.0 to about 2.5, and preferably about 0.1 to 1.0 weight percent of fluorine (calculated as F) The chromium compounds which may be used for the Type III catalysts include CrO3, or any compound of chromium which is oxidizable to CrO3 under the activation conditions employed. At least a portion of the chromium in the supported, activated catalyst must be in the hexavalent state.
Chromium compounds other than CrO3 which may be used are ~isclosed in U.S. Patent No. 2,825,721 and U.S. Patent No. 3,622,521 and include chromic acetyl acetonate, chromic nitrate, chromic acetate, chromic chloride, chromic sulfate, and ammonium chromate.
` The titanium compounds which may be used include all those which are oxidizable to TiO2 under the activation conditions employed, and include those disclosed in U.S. Patent No. 3,6Z2,521.
The fluorine compounds which may be used include HF, or any compound of 1uorine which will yield HF under the activation conditions employed.
The inorganic oxide materials which may be used as a support in the catalyst compositions are 1'~12~ 7~
The inorganic oxide materials which may be used as a support in the catalyst compositions are porous materisls having a high surface area, that i~, a surface area in the ran8e of about 50 to lOOO
S square meter~ per gram, and an average particle size of about 20 ~o 200 microns. The ~norganic oxides which may be used include silica, alumina, thoria, zirconia and vther romparable inorganic oxides, as well as mixtures of such oxides.
Type IV. The catalysts as described in U.S. Patent No. 4,302,566 in the names of F.~. Karol et al, and entitled, "Preparation o~ Ethylene Copolymers in Fluid Bcd Reactor" and ~ssigned to the same assignee ~ as the present application. These catalysts : 15 comprise at least one titanium compound, at least one magnesium compound, ~t least one electron donor compound, at least one activator compound and at least one inert carrier ma~erial.
The titanium compound has the structure 20 , Ti (OR)aXb wherein R is a Cl to C14 aliphatic or aromatic hydrocarbon radical, or COR' where R' is a Cl to C14 sliphatic or aromatic hydrocarbon radicali X
is Cl, Br, or I; a is O or l; b is 2 to 4 inclusive;
and a+b = 3 or 40 The titanium compounds can be used individually,or in combinstion thereof, ~nd would ~nclude TiC13, TiC14, Ti(OCH3~C13, Ti~oc6H5)cl3~ Ti(OCOCH3~)C13 and Ti(OC~c6Hs)cl3 The magnesium compound has the structure:
MgX2 -~-15407 . , wherein X is Cl, Br, or I. Such magnesium compounds can be used individually or -in combinations thereof and would include MgC12, M~Br2 and MgI2.
Anhydrous MgC12 is the preferred magnesium compound.
-The titanium compound and the magnesium compound sre generally used in a form which will facilitate thei~ dissolution in the electron donor compound.
The electron donor compound is an organic compound which is li~uid at 25C and in which the titanium compound and the magnesium compound are partially or completely soluble. The electron donor compounds are known as such or as Lew~s bases.
The electron donor compounds would include such compounds as alkyl esters of aliphatic and aromatic carboxylic acids, aliphatic ethers, cyclic ethers and aliphatic ketones.
The cstalyst may be modified with a boron halide compound hflving the structure:
BR X ' 3 wherein R is an aliphatic or aromatic hydrocarbon radical containing from 1 to 14 carbon atoms or OR', wherein R' is also an aliphatic or aromatic hydrocarbon radical containing from l to 14 carbon atoms; X' is selected from the group consisting of Cl and Br, or mixtures thereof, and; c is O vr 1 when R is an ~liphatic or aromatic hydrocarbon and O, 1 or 2 when R is OR'.
The boron halide compounds can be used individually or in combination thereof, and would include BC13, ~Br3, B~C2H5)C12.
. .
~ ~-B(oc2Hs)cl2~ B(C2H5) B(C6H5)C12~ B(OC6H5)Cl2' (C6H13)C12' B()C6Hl3)C12, and B(OC~H5)2Cl. Boron trichloride is the particularly prererred boron compound.
The ~ctivator compound has the structure:
Al(~") X'dH
where~n X' is Cl or ORl; Rl and R" are the same or di~ferent and are Cl to C14 saturated hydrocarbon radicals, d is 0 to 1.5, e is 1 or 0, and c~d+e = 3.
Such activator compounds can be used individually or in combinations thereof.
The carrier materials are solid~
particulate materials and would include inorganic materials such as oxides of silicon and aluminum and molecular sieves, ~nd organic materials such as ole~in polymers, e.g., polyethylene.
TYPe V. Vanadium based catalysts. These type catalysts generally include vanadium as the sctive ingredient, one such type catalyst generally comprises a supported precursor, a cocatalyst and a promoter. The supported precursor consists essentially of a vanadium compound and modifier impregnated on a solid, inert carrier. The vanadium compound in the precursor is the reaction product of a vanadium trihalide and an electron donor. The halogen in the vanadium trihalide is chlorine, bromine or iodine, or mixtures thereo~. A
particularly preferred vanadium trihalide is vanadium trichloride, VC13.
The electron donor is a liquid, organic Lewis base in which the vanadium trihalide is , ,.. .
~ ~, "~
~ ~3~ ~7~
soluble. The ~lectron donor is selected from the group consisting of alkyl esters of aliphatic and aromatic carboxylic scids, aliphatic esters, aliphatic ketones, aliphatic amines, aliphatic slcohols, alkyl and cyclo21kyl ethers, and mixtures thereof. Preferred eleotron donors are alkyl and eyeloalkyl ethers, including particularly tetrahydrofuran. Between about 1 to about 20, preferably between sbout 1 to about 10, and most preferably about 3 moles of the electron donor are complexed wi~h each mole o vanadium used.
The modifier used in the precursor has the formula:
MXa wherein:
M is either boron or AlR(3 a) and wherein each R is independently alkyl, provided that the total number of aLiphatic carbon stoms in any one R group may not exceed 14;
X is chlorine, bromine or iodine; and a is 0, l or 2, with the provision that when M ig boron a is 3.
Preferred modifiers include Cl to C6 alkyl aluminum mono- and ti- chlorides and boron trichloride. A particularly preferred modifier is diethyl aluminum chloride. About 0.1 to about 10, and pref erably abo~t 0.2 to about 2.5, moles of modifier are used per mole of electron donor.
The carrier is a solid, pa~tic~late porous material inert to the ~olymerizAtion. The carrier consists essentially of silica or alumina, i.e., oxides of silicon or aluminum or mixtures thereof.
: .
' . . ~,.
.
_ 9 Optionally, the carrier may contsin additional materials such as zirconia, ~thoria or other compounds chemically inert to the polymerization or mixtures thereof.
The carrier is u~ed as ~ dry powder having sn average p~rticle size of between about 10 to about 250, pre~erably about 20 to about 2~0, and most prefersbly about 30 to about 100, microns. The po~ous carrier has a ~urfsce sre~ of 8reater than or equal to about 3, and preferably gre~ter than or equ~1 to about SO, m /g. A preferred carrier is silica having pore sizes of greater than or equal to about 80, and preferably greater than or equal to about 100, angstroms. The carrier is predried by heating to remove water, preferably at a temperature of greater than or equal to about 600C.
The mount o~ carrier used is that which will provite a vanadium content of between about 0.05 to about 0.5 mmoles o~ vanadium per gram (mmole V/g), and preferably between about 0.2 to about 0.35 mmole V/8, ~nd most preferably about 0.29 mmole V/g.
l'he carrier is ordinarily free of prep~rative chemical treatment by reaction with an alkylaluminum compound prior to the formation of the supported precursor. Such treatment results in the formation o aluminum alkoxides chemically bonded to the carrier molecules. It~has been discovered that the use of such a treated carrier in the catalyst composition and process is not only nonessential, but instead results in undesirable agglomeration when used in the preparation o~ high density polyethylene .
~ D-15407 :~ `
' - i , , ' ' - ~ :
',. . .
(>0.94 ~Icc), resulting in a chunk-like, non-fr~ely flowing product,`~
The cocatalyst wh~ch can be employed for : the Type IY and Type V catalysts has the formula::~: 5 AlR3 ;~ wherein R is as previously de~ined in the definition o~ M. Preferred cocatalysts include C2 to C8 trialkylaluminum compounds. A particularly pre~erred cocatslyst ~s triiso~utyl alum~num, Between ~bout 5 to ~bout 500, and preferably between about ~0 to about 50, moles of cocatalyst are used per mole of vanad~um.
The promoter has the formula:
R'bCx (4_~) wherein:
R' is hydrogen or unsubstituted or h~losub~tituted lower, i,e,, up to sbout C6 containing, alkyl;
X' is halogen; snd b is 0, 1 or 2, Between about 0,1 to about 10, and prefersbly ~ between about 0,2 to about 2, moles of promoter are : uset per mole o~ cocatalyst, The catalyst is produced by first preparing the supported precursor, In one embodiment, the vanadium compound is prepared by dissolving the vanadium trihalide in the electron donor at : temperature between about 20CC up to the bo~ling point of the elect.on donor for:a few hours, ~, ~ 30 Preferably, mixing occurs:at about 65C for about 3 hours or more, The vanadium compound so produced is : then impregnated onto the carrier, Impregnation may .
~ D-15407 :~ ' .
, ~ .
`'.. : : ' . .
~2~7~
b~ effected by adding the carrier as a dry powder or 8S a slurry in the electron donor or other inert solvent. The liquid is removed by drying at less than about 100C for a few hours, preferably between about 45 to about-90C for about 3 to 6 hours. The modifier, dissolved in an inert solvent, such as a hydroearbon, is then mixed with the vanadium impregnated carrier. The liqu~d is removed by drying ~ temperatures of less than about 70C for a few hours, preferably between about 45 ~o about 65C f or ~hout 3 hours.
The cocatslyst and promoter are added to the supported precursor either before and/or during the polymerization reaction. The cocatalyst and promoter are add~d either together or sepsrately, and either simultaneously or sequentially during pol~merization. The cocatalyst and promoter are preferably added separately as solutions in inert solvent, such as isopentane, during polymerization.
In general, the above catalysts are introduced together with the polymerizable materials, into a reactor having an expanded section sbove a straight-sided section. Cycle gas enters the bottom of the reactor and passes upwsrd through a gas distributor plate into a fluidized bed located in the straight-sided section of the vessel. The gas distributor plate ser~es to ensure proper gas d~stribution snd to support the resin bed when gas flow is stopped.
G~s leaving the fluidized bed entrains resin particles. Most of these particles are d~sengaged as the gas passes through the expanded section where ~ts veloc~ty is reduced.
~3~ 7~
In order to satis~y certain end use appllcations for ethylene resins, such as ~or film, in~ection molding and roto-molding applications, c~t~lyst Types IV and V with fllkyl aluminum cocatalysts have been used. However, attempts to produce certa~n ethylene resins utilizing alkyl aluminum cocatalysts with the Type IV and V
catalysts supported on a porous silica substrate in certain fluid bed re ctors, hsve not been entirely satlsfactory from a practical commercial standpoint. This is prim~rily due to the formation of "sheets" in the reactor after a period of operat~on. The "sheets" can be characterized 8S
constituting a fused polymeric material.
It has been ~ound that a static mechanism is a contributor to the sheeting phenomena whereby catalyst and resin particles adhere to the reactor walls due to static ~orces. If allowed to reside long enough under a reactive environment, excess temperatures can re~ult in particle fusion.
N~merous causes for static ch~rge exist. Among them are generation due to frictionsl electrification of dissimilar materials, llmited static dissipation, introduction to the process of minute quantities of prostatic agents, excessive catalyst Pctivities, etc. Strong correlation exists between sheeting and the presence of excess static charges e~ther negative or positive. This is evidenced by sudden _hanges in static leYels followed closely by deviation in temperatures at the reactor wall.
These temperature deviations are either high or low. Low temperatures indicate particle adhesion ~ ~
- ~ , : . ..
' :.:
, 2~72 causing an insulating effect from the bed temperature. High deviations indicate reaction taking place in zones of limited heat transfer.
Following this, disruption in fluidization patterns ls generally evident, ~atalyst feed interruption can occur, product discharge system pluggage results, and thin fused agglomerates ~sheets) are noticed in the granular produc~.
The sheets vary widely in size, but are similar in most respects. They are usually about 1/4 to 1/2 inch thick and are from about one to five feet long, with a few specimens even longer. They have a width of about 3 inches to more than 18 inches. The sheets have e core composed of fused polymer which is oriented in the long direction of the sheets and their surfaces are covered with granular resin which has fused to the core. The edges of the sheets can have a hairy appearance from strands of fused polymer.
It is therefore an object o the present invention to provide a method for substantially reducing or eliminat~ng the amount of sheeting which `~ occurs during the low pressure fluidized bed polymerization of alpha-olefins utilizing titanium based compounts or vanadium based compounds as catalyst with alkyl aluminum as cocatalysts.
; Another obiect is to pro~ide a process for reducing sheeting in fluidized bed reactors utilized for the production of polyolefin resins wherein - 30 titanium or van~dium based catalysts with alkyl aluminum cocatalysts are employed.
;,", ',, . : , , :' "`" ` -...
.
17~
These and other ob~ects will become readily apparent from the following-descri~tion taken in con~unction with the accompanying drawing which generally indicates a typical gas phase fluidized bed polymerizAtion process for producing high den~ity ~nd low density polyolefins slightly modified to reflect the present invention.
~roadly contemplated, the present inYention provides a method or reducing sheet~ng during polymerization of alpha-olefins in a low pressure fluid~zed bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts which comprises introducing water into said reactor in an amount sufficient to maintain the electrostat~c levels at the site of possible sheet formation at levels ~hich a~oid sheeting without substantially altering the effectiveness of said catalysts.
The amount of water which is fed to the reactor depends on the stat~c voltage within the reactor and can generally range in an amount of 0.1 to about 2 ppm based on ethylene feed. In general, the nitrogen flow control is such as to permit a nitrogen flow of about 0 to about ll lbs/hr for an ethylene feed range of about 0 to about 50,000 lb/hr. The water cylinder temperature in the O'Brien box can generally be in a range of about 10C to about 40C. Nitrogen pressures can generally range from about 200 to 400 psig prefer~bly about 320 to about 370 psig.
The critical static voltage level for sheet ; formation is a complex ~unction of resin sintering .~
:, . .
.
. , .
~L31~7~
temperature, operating temperature, drag forces in the fluid bed, resin particle size distribution and recycle ~as compositlon. The static vol~age can be . reduced by a variety o~ techniques such as by ; 5 tresting the reactor surface to reduce static electric generation by inlection of an antistatic agent to inc~ease particl~ surface electrical . ~. conductivity thus p~omoting particle dischargin~; by installation of appropriate devices connected to the reactor walls which are designed to promote ~ electrical dischargin~ by creating areas of high : localized field stren~th, and by neutralization of charges by the in~ection or creation of ion pairs, ions or charged particles of the opposite polarity from the resin bed.
According to the present invention, the use of water add back to the gas phased low pressure polyethylene process will assist in the reduc~ion of agglomerate formation in the fluidized bed. This is accomplished by a reduction in the levels of positive. static voltage which lowers particle adhesive forces in the re~ction system.
Re~errin~ particulsrly to the sole figure of the drawing, a conventional fluidized bed reaction system for polymerizing alpha-olefins slightly modified to provide for water add back, includes a reactor 10 which consists o~ a reaction zone 12 and a velocity reduction zone 14.
The react$on zone 12 includes a bed of : 30 growing polymer particles, formed polymer particles :` and a minor amount of catalyst particles fluidized ~`i by the continuous flow of polymerizable and ~:
.
, ; - ~ ,. ; .. - .
' " :-, modifying gaseous components in the form of make-up feed and recycle gas through the reaction zone. To maintain a viable fluidized bed, the mass ~as flow rate through the bed is normally maintained above the minimum flow required for fluidization, and preferably from about 1.5 to about 10 times Gmf and more preferably from about 3 to ~bout 6 times Gmf. G~f ~s used in the accepted form as the abbrevi~tion for the minimum gas flow required to achieve fluidlzation, C.Y. W~n and Y.H. Yu, "Mechanics of Fluidization", Chemical Engineering Progress Symposium Series, Vol. 62, pg. 100-111 (1966).
It is highly desirable that the bed always contalns particles to prevent the formation of localized "hot spots" and to entrap and distribute the psrticulate catalyst throughout the reaction zone. On start up, the reactor is usually charged with a base of particulate polymer particles before g~s flow is initiated. Such particles may be identical in nature to the polymer to be formed or different therefrom. When different, they are withdrawn with the desired formed polymer particles as the first product. Eventually, a fluid~zed bed of the desired polymer particles supplants the start-up bed.
The appropriate catalyst used in the fluidized bed is preferably stored for service in a reservoir 16 under a blanket of a gas which is inert to the stored m terial, such as nitrogen or argon.
Fluidization is achieved by a high rate of gas r~cycle to and through the bed, typically in the order of about 50 times the rate of feed of make-up gas. The fluidized bed has~-the general appearance o~ ~ dense mass of viable particles in possible free-vortex flow es created by the percolation sf g8s through the bed. The pressure drop through the bed is equal to or slightly greater th~n the mass of the bed divided by the cross-s2ctional area. It is thus dependent on the geometry of the reactor.
Make-up gas ~s fed to the bed at a rate equ~l to the rate at which particulste polymer product is ~ithdrawn. The composition of the make-up g~s is determined by a gas analyzer 18 pos~tioned above the bed. The gas analyzer determines the composition of the ~as being recycled and the composition of the,make-up gas is adjusted accordingly to maintain An essentially steady stste gaseous composition within the rear.tion zone.
To insure complete fluidization, the recycle gas and, where desired, part or all of the ; 20 make-up gas ~re returned to the reactor at bsse 20 ~ b,elow the bed. Gas distribution plate 22 positioned ,, above the point of return ensures proper gas distribution and also supports the resin bed when ~ ~as flow is stopped.
;;~ 25 The port~on of the gas stream which does not resct in the bed constitutes the recycle gas which is removed from the polymeri~ation zone, preferably by psss~ng it into velocity reduction ~one 14 above the bed where entrained particles are , 30 given an opportunity to drop back into the bed.
The recycle gas is then compressed in a compressor 24 and thereafter passed through a heat ~' ~ ~ , ~, :
.
'~
., '' exchanger 26 wherein it is stripped of heat of reaction before it is returned to the bed. By constantly removing heat o~ reaction, no noticeable tempetatu2e gradient appears to exist within the upper portion 9~ the bed. A temperature gradient will exist in the bottom of the bed in 8 layer of about 6 to 12 tnches, between the temperature of the inlet gas and the temperature of the remainder of the bedO Thus, it has been observed that the bed acts to almost immediately ad~ust the temperature of the recycle gas above this bottom layer of the bed zone to make it conform to the temperature of the rem~inder of the bed thereby maintaining itself st ~n essentially constant temperature under steady conditlons. The recycle is then returned to the reactor at its base 20 and to the ~luidi~ed bed through distribution plate 22. The compressor 24 can also be placed downstream of heat exchanger 26.
Hydrogen msy be used as a chain transfer agent for conventional polymerization reactions of the types contemplated herein. In the case where ethylene is used as a monomer the ratio of hydrogen/ethylene employed will vary between 0 to ~ about 2.0 mo}es of hydrogen per mole of the monomer - 25 in the gas stream.
Any gas inert to the catalyst and reactants can also be present in the gas stream. The cocstalyst is ~dded to the gas recycle stream upstream of its connection ~ith the reactor ~s ~rom d$spenser 28 through line 30.
As is well known, it is essential to operate the fluid bed reactor at a temperature below ~2~2 _ ~9 the sintering temperature of the polymer particles.
Thus to-insure that sinteri~g will no~ occur, operating tem~eratures below sintering temperature ~re desired. For the production of ethylene polymers an operating temperature of rom about 90 to 100C ~s preferably used to prepare products hsving a density of about 0.94 to 0.97 while a temperature of about 75 to 95C is preferred for products having a density of about 0.91 to 0.94.
; 10 Norm~lly the fluid bed reactor is operated at pressures of up to about 80-110 psi for high density &nd 65-95 for low and medium density.
The catalyst is inJected ~nto the bed ~t a rate equal to its consumption at a point 32 which is abo~e ~he distribution plate 22. A gas which is inert to the catalyst such as nitrogen or argon is used to carry the catalyst into the bed. Injecting the catalyst at a po~nt above distribution plate 22 is an important feature. Since the catalysts normally used are highly active, in~ection into the area below the distribution plate may cause polymerizstion to begin there and eventually cause plugging of the distribution plate. In~ection into the viable bed, instead, aids in distributing the catalyst throughout the bed and tends to preclude the formation of localized spots of high catalyst concentration which may result in the formation of "hot spots".
Under a given set of operatin~ conditions, the fluidized bed is maintained at essentially a ; constant height by withdrawing a portion of the bed as product at 8 rate equal to the rate of ~ormation :, .... :
- .:
. . : -1~2~72 of the psrticulate polymer product. Since the rate of hest generation is directly related to product formation, a measurement of the temperature rise o~
the gas across the reactor (the difference between inlet g~s temperature and exit ~as temperature) is determ~native of the rate of particulate polymer ~ormation at a oonstant gas velocity.
The partic~late polymer product is prefer~bly withdr~wn at a point 34 at or close to distribution plate 22. The particulate polymer product is conven~ently snd preferably withdrawn through the sequential oper~tion of a pair of timed valves 36 ~nd 3S defining a segregation zone 40.
While Yalve 38 is closed, valve 36 is opened to emit a plug of gas ~nd product to the zone 40 between it and valve 36 which is then closed. Valve 38 is then opened to deliver the product to an external recovery zone and after delivery, valve 38 is then closed to await the next product recovery opera~ion.
Finally, the fluidized bed reactor is equipped with an adequate venting system to allow venting the bed during the start up and shut down.
The reactor does not require the use of stirring means andlor wall scraping means.
The re~ctor vessel is normally constr~cted of carbon steel and is designed for the operating conditions stated above.
In order to better illustrate the problems incident to the util$zation of the Type IV
catalysts, reference is again made to the drawing.
The titsnium based cata}yst (Type IV) is introduced into the reactor 10 at point 32. Under conventional ., ,-:~ , " ~3~ ~72 operations on certain resins, after a period of ~ime, sheets begin to form in reactor-10, at a site in the reactor proximate the wall of the resctor and located about a distance of one-h~lf the reactor diameter up ~rom the base of the fluid bed. The sheets of fused resin begin to appear in segregation zone 409 rapidly plugging the system, causing the reactor to be shut down. More characteristicslly the sheeting begins after production equivalent to 6 to 10 times the weight of the bed of resin in reactor 10.
Many possible causes were investigated in attempting to discover and eliminate the sheeting.
In the course of the investigation, thermocouples were ins~alled ~ust inside the reactor walls at elevations of 114 to }l2 reactor diameter above the gas dlstribution plate. Under conventionsl operationsl "skin" thermocouples indlcate temperatures equal to or slightly lower than the temperature of the fluidized bed. When sheeting occurs, these thermocouples m~y indicate tempersture excurslons of up to 20C ~bove the temperature of the fluidized bed thus providing reliable indication of the occurrence of sheeting. In addition, an electrostatic voltmeter was used to measure voltage on a 112 inch spherical electrode located ~n the fluid bed 1 inch radially from the reactor wall and usually 5 to 6 feet above the g~s distributor plste. The location was selected because sheet formation was observed to initiste in a band ranging ~rom 1/4 to 3l4 reactor dismeter in elevation sbove the ~ase of the fluid bed. As is well known for .
; ~
. , " ' ~ . .. .. . .
, 13 ~ rl ~
deep fluidized beds, this corresponds to the re~ion of least mixing intensity ne-ar the wall, i.e., a null zone where particle motion near the wall changes from generally upwsrd to generally downward. The possible c~uses investigated included factor~ affecting mixing in the fluidized bed, reactor operating conditions, c3talyst and resin pa~ticle size, part~cle size distribution, and others. A correlati~n was found between sheeting and buildup of static electric charge on the resin particles proximate the re~ctor walls. When the static voltage level of resin particles at particular sites proximate the reactor wall in a fluidized bed reactor is low, the reactor runs normally and no sheets are formed. When the static voltage level exceeds 8 critical level at those sites, uncontrolled sheeting occurs and the resctor must be shut down.
It was further discovered that sheeting could be substantially reduced and in some cases entirely eliminated by controlling static voltage in the ~luidized bed at a site proximate the reactor walls below the critical level for sheet formation.
This critical level for sheet formation is not a fixed v~lue, but is a complex function de~endent on variables including resin sintering temperature, operstin~ temperature, drag forces in the fluid bed, resin particle size distribution and recycle g8S
composition.
The critic~l voltage le~el Vc for sheeting ~; of ethylene homopolymers, ethylene-butene and ethylene-hexene copolyme~s is primarily a function : :`
7 ~
of the ~esin sintering *emperature, the reactor bed temperature and the concentration of hydrogen in the -recycle ~as.
The sintering temperature of the resin under reactor operating conditions ~s the te~perature at which ~ settled bed o~ resin in contact with a ~8S having the same composition as the reactor recycle gas used in producing the resin will sinter and form ~gglomerates when refluidization is attempted after allowing the ~ed to remsin settled for fifteen minutes. The sintering temperature is decreased by decreasing the resin denslty, by increasing the melt index and by increasing the amount of dissolved monomers and monomer type.
The constants in the equation were determined from data collected during reactor operation when the re~ctor ~ust began to exhibit sheeting symptoms through skin thermocouple temperature excursions above the bed temperature.
The voltage indicated on the voltage probe described earlier varies with time due to the random nature of a fluidized bed. Thus the crit~cal voltage, Vc, is expressed as A time averaged voltage. Voltage measurements are difficult to interpret because additional static electric charge is generated when sheet, formed because of a static chsrge, separates from the reactor wall~ In atdition, the sheeting phenomena can start ac a very local phenomenon and spread further clouding interpretation of voltage readings.
Although the sheeting mechanism is not fully understood, it is belieYed th t static ' ., ~ , , , ,. .. ~ :
.
.
~2~72 electricity generated in the fluid bed charges resin pArticles. When the charge ~n the par*icles reaches the level where the electrostatic forces trying to hold the chsrged particle near the reactor wall exceed the dr g forces in the bed trying to move the psrticle away from the w811, R layer o~ cstalyst contsining, polymerizing resln particles forms a non-fluidized layer nes~ the reactor wall. Heat removal from this layer is not sufficient to remove the heat of polymerization because the non-fluidized layer near the wall has less contact w~th ~he fluidizing gas than do particles in the fluidized portion of the bed. The heat of polymeriz~tion increased the temperature of the non-fluidized layer near the reactor wall unt~l the particles melt and fuse. At this point other particles from the fluidi~ed bed will stick to the fused layer and it will grow in size until it comes loose from the reactor wall. The separation of ~ dielectric from a conductor (the sheet from the reactor wall) is known to 8enerate additional static electricity thus accelerAting subsequent sheet ~ormstion.
The srt teaches various processes whereby ~ static voltage can be reduced or eliminated. These ; 25 comprise (1) reducing the rate of charge generation, (2~ increasing the rate of disch3rge of electrical chsrge, and (3) neutralization of electrical chsrge. Some processes suited for use in a fluidized bed comprise (1) use of an additive to inc~ease the conductivity of the particles thus providing a path for discharging, ~2) installation of grounding devices in a fluidized bed to provide D-154~7 . ., ~3~2~7~
additionQl area for disch~rging electrostatic chsrges to ~round, (3) ioniz- tion of g~s or p~rticles by electrical dischsrge to generate ions : to neutrslize electrostatic charges on the particles 9 ~nd (4) the use o~ r~tioactive sources to : produce radiation thAt ~ill create ions to neutralize elec~rostatic rhsrges on the particles.
The application of the~e techniques ~o a commercial scale, ~luidized bed, polymeriz~tion reactor m~y not be feasible or practical. Any ~dditive used must not act ~s 8 poison to the polymerizætion cat~lyst and must not ~dversely affect.the quality of the product. It had been previously thought that wster, the most widely used additi~e to reduce static on particles, c~nnot be used since it is 8 severe catalyst poison.
We h~ve found however that in certain specific reactions, i.e., when Type IV and ~
cat~lysts with alkyl aluminum cocatalysts are used in the fluidized polymerizstion process that the additlon of controlled minute quantities of water to the reactor drsmatically reduces the incidence of sheeting without producing severe detrimental effects to the catRlysts. The amount of water fed to the reactor depends on the static chsr~es present in the reactor.
Water addition csn be accomplished by a simple modification to the conventionsl procedures.
Thus referrin" ~gain to Fig. 1, ~n inert gas such as dry nitrogen from ~ nitrogen supply source, 41, is introduced into what is commonly referred to in the art an "O'Brien box" represented by Figure 42. The .~
.
.
. .
`` ~L312172 O'Brien box ~enerally contains one or more water tanks containing distilled w~ter and is equipped with temperat~re and flow control means all of which ~re not shown. The nitrogen is bubbled through one of the w~ter tanks which is a one liter st~inless steel cyllnder of distilled water in the temperature controlled housing. Water saturated nitrogen leaving the Q'Brien box 42, through line 44, is then flow controlled thro~gh heat traced tubing to enter the ole~in feed line such as ethylene feed line, 46, to the reaction cycle. Resultant water concentration in the ethylene is generally less than one part per million by volume. A moisture analy~er, 48, on the ethylene feed line 46 can be used for confirmation of water addition. R typical range for nitrogen flow control is about 0 to ll lbs/hr for sn ethylene ~eed range of 0 to 50,000 lbs/hr. With water cylinder temperature of 20C and : 350 psig nitrogen a ran8e of 0 to .3 ppm water isreali~ed. Ad~ustments in water temperature or nitrogen pressure can permit varying this range to desired levels.
Merely as illustrative the following information shows the water add back flow to the ; 25 reactor concentration calculations:
1. Determine vapor pressure of water at temperature in O!Brien box (PH20).
2. Determine flow of n~trogen with integrsl orifice (WN~).
3. Determine nitrogen pressure from reactor bottom head pressure (DN2) plus the pressure drop in the line.
:~
i: :
:~
i: :
4. Determine ethylene flow to reactor ,. (WC2H4~' 5. Assume nitrogen to be saturated with water:
P x WN2 ~ 18 = Water flow ~WH20) I0 WH20 x 28 x I~ = PPM H2O In ethyIene feed At 20C PH20 = 0.339 ps~a PN2 = 325 psi~ when reactor at 300 psig.
WNz - 0 to 11.36 ppm typically flow at 3 ppm Wc2~4 = 18,000 pph Water concentration = (.339) (3)_(10) =0.174 ppm 325 18,000 Static voltage in the reactor can be monitored near the reactor wall by one or more ststic voltage indicator~ 50 inserted into the reactor bed approxim~tely five feet above the distributor plate in the range of -15,000 to ~15,000 volts. With reaction in progress, changes in static voltage levels erom neutral to positive can be counteracted by feed oP the moisture laden nitrogen to the ethylene stream. I~ this is not performed, impending agglomerate formation will likely create a process upset. Care must be exercised to avoid excessive water levels which can result in unwanted negative st~tic vol*age levels.
The system is operated with various flow and check valves wh~ch are common ~n the srt and - hence not illustrated. In ~ddtion, line 44 is :
, .; ~
.
` ~ 31~72 preferably insulated ~nd steam traced prior to entering the gas feed line 4~. ~ -The polymers to which the present invention is primarily directet and which cause the sheeting problems aboYe reerred to ln the presence of titanium catalysts are linear homopolymers of ethylene or linear copolymers of a major mol percent (>90%) of ethylene, ~nd a~minor mol percent (<10~) of one or more C3 to C8 alpha olefins. The C3 to C8 alph~ olefins should not contain any brsnching on any of their carbon atoms which is closer than the fourth carbon atom. The preferred C3 to C8 alpha olefins are propylene, butene-l, pentene-l, hexene-l, 4-methylpentene-1, heptene-l, and octene-l~ This descript~on is not intended to exclude the use of this invention with alpha olefin homopolymer and copolymer resins in which ethylene is not a monomer.
; The homopolymers and copolymers have a density ran~ing from about 0.97 to 0.91~ The density of the copolymer, at a given melt index level is primarily regulated by the smount of the C3 to C8 comonomer which is copolymerized with the ethylene. Thus, the addition of progressively large~ amounts of the comonomers to the copolymers results in a progressive lowering of the density of the copolymer. The amount of each of the various C3 to C8 comonomers needed to ach~eve the same result will ~ary from monomer to monomeri under t~he same reaction conditions. In the absence of the comonomer, the ethylene would homopolymerize.
The melt index of a homopolymer or copolymer is a re~lection o~ its molecular weight.
, , :
:
, .
~3~2~2 - 29 ~
Polymers having 8 relPtively high molecular weight, have reiatively high viscosities flnd low melt index.
In a typical mode of ut~lizing the subject =-invention to reduce she~ting, a reactor vessel such as ~hown in Figure 1 and which is susceptible to sheeting problems by the polymerization of the above described materials utilizing Type IV and Type V
catalysts with an alkyl aluminum cocataly~t is part~lly filled with gr~nular polyethylene resin which is purged with a non-resctive gas such as nitrogen and is fluidi2ed by circulating said non-reacting gas through t~e reactor at a velocity aboYe the minimum fluidizing velocity (Gmf) of the granular polyethylene and preferably at 3 to 5 Gmf.
The reactor is brought up to operational temperatures by the gas and the resction is started by introducing the catalyst and cocatalyst to the resctor~ During reaction, static voltage levels approach those levels which cause sheeting, then the pressurel temperature and flow control in the O'Brien box are increased to permit nitrogen to become water saturated. The water s~turated nitrogen is then directed to the gas feed line and introduced into the reactor. The voltage levels in the reactor are monitored responsive to the water laden gas feed st~eam and the procedure is continued until the static ~oltage levels are brsught to levels of non-sheeting.
Having set ~orth the general nature of the invention, the fvllowing examples illustrate some specific embodlments of the 1nyention~ It is to be :
..
~ ~2~72 - 3~ -understood, howe~er, that this invent~on is not limited to the ex~m~les, si~ce the invention may be practiced by the use of various modifications.
Ex~mples 1 and 2 were conducted in a conventionsl bed reactor. The catalyst used was a Zi~gler type, ~it~n~um based eatalyst supported on porous silica produced as described earli~r as Type IV. Thè coc~talyst u~ed was triethyl aluminum. The products made in the examples were copolymers of ethylene and l-butene. HydrGgen was used as a chain transfer sgent to control the melt index of the polymer.
ExamPle 1 A fluidized bed reactor was started up at operat~ng condltions designed to produce a ilm gr~de low density ethylene copolymer product having a density of 0.918, a mel~ index of 1.0, and a sticking temperature of 104C. The reaction was ` 20 started by feeding catalyst to the reactor prech~rged ~ith a bed of granular resin similar to the p~otuct to be made. The cfltalyst was a mixture of S.S parts titanium tetrachloride, 8.5 parts magnesium c~loride and 14 parts tetrshydrofuran deposited on 100 parts Davison 8rade 955 silica ; which had been dehydrated at 600C ~nd treated with four parts triethylaluminum prior to deposit~on and was activated~with thirty-five psrts tri-n-hexyl ~aluminum subsequent to deposition. Prior to starting catalyst feed 9 the resctor and resin bed ~ere brought up to the operating tempersture of 85C, were purged o~ impurities by circulating nitrogen through the resin bed. Ethylene, butene . ' ., ` ~31~72 and hydro~en concentrat1ons were established at 53, 24, and~ , respectively. -Cocatalyst was fed at a r~te of 0.3 parts triethylaluminum per part of catalyst.
Reac~or star~-up was normal. After producing product for twenty-nine hours and equiYalent to 6-112 times the weight of the f}uidized bed, temp~rature excurs~ons of 1 to 2~C
~bove bed temperature were observed using thermocouples l~cated ~ust inside the reactor wall at an elevation of 1/2 reactor diameter ~bove the ~as distributor plate. Prior experience had shown that such temperature excursions ~re a positive indication that sheets of resin are being formed in the fluidized bed. Concurrently, bed voltsge (measured using an electrostatic voltmeter connected to a 112 inch diameter spherical electrode located one inch ~rom the reactor wall at an elevation of 1/2 reactor diameter Rbove the gas distributor plate) increased from a readin8 of approximately ~1500 to ~2000 volts to a'rea~ing of over +S000 volts and then dropped back to ~2000 volts over ~ 3 minute period. Temperature and voltage excursions continued for approxim8tely 12 hours and increased in frequency and magnitude. During this period, sheets of ~used polyethylene resin began to show up in the resin product. Evidence of sheeting became more severe, i.e., temperature excursions increased to ~s high as 20C above bed temperature and stayed high for extended periods of time and voltage ' excursions slso bec~me more ,frequent. The reactor was shut down because of the extent of sheeting.
.
' , ~ ' "" ; ' ~312172 ExamE~e 2 The fluldized bed reactor used in Exsmple 1 w~s stsrted up ~nd operated to produce a linear low density ethylene copolymer suitable for extrusion or rvtational molding and having a density of 0.934, a melt lndex of 5 and a sticking temperatu~e of ; 118C. The reaction W8S started by feeding catalyst similar to the catalyst in Example 1 except sctivated with 28 parts tri-n-hexylaluminum, to the resctor precharged with ~ bed of granular resin similar to the protuct to be made. Prior to starting catalyst feed the reactor and the resin bed were brought up to the operat~ng temperature o~
85C, and were purged of impurities with nitrogen.
The concentration of ethylene ~52~), butene (14~), hydrogen ~21%) were introduced into the reactor.
Cocatalyst triethylaluminum was fed at 0.3 parts per part of catalyst. The reactor was operated continuously for 48 hours and during that period produced resin equivalent to 9 times the amount of resin contained in the bed. After this 48 hour period of smooth operation9 sheets of fused resin be8an to come out of the reactor with the normal, granular ptoduct. At this time voltages ~easured l/Z reactor diameter ~bove the distributor plate averaged +2000 volts and ranged from 0 to ~10,000 volts, while the skin thermocouples at the same elevat~on indicated excursions of ~15C above the bed temperatu~. Two hours after the ~irst sheets were noted in the product ~rom the reactor, it was necessary to stop ~eeding catalyst and cocatalyst to the reactor to reduce the resin production rate , .
,,, : :
,; ..
`:` : ' ~31~172 because sheets were plugging the resin discharge system. One hour later, çatalyst ~nd cocatalyst feeds were restarted. The production of sheets continued and after two hours catalyst and cocatalyst feed were again stopped and the reaction WdS termin~ted by in~ecting carbon monoxide. The voltage at this ~ime was > ~12,000 ~olts and the thermocouple excursions continued until the poison was ~n~ected. In totsl, the re~ctor was op~rated for 53 hours and produced 10-1/2 bed volum~s of resin before the reaction was stopped due to sheeting.
The following Example illustrstes the prevention of sheeting by adding water to the gas feed during periods of high volta8e in the reactor.
ExamPle 3 The reac~or of Examples 1 and 2 were modified AS shown ~n Figure 1 ~nd a high density film grade polyethylene resin of 0.946 density, 7.5 flow ind~x, and sticking temperature of 124C was continuously produced. The product was an ethylene-hexene copolymer utilizing a vanadium based c~talyst with an aluminum alkyL cocatalyst and halogen promoter for polymerization.
The catalyst contained 0.29 millimoles vanadium per gram of precursor and 1.2~ aluminum added in the form of diethylaluminum chloride on a Davison silica support o~ 30-130 micron s~ze. Reaction proceeded with a bed temperature of 98C, 7~% ethylene t 1.670 hydrogen, lq2~ hexene, and the remaining concentration inert gases of nitrogen, methane, .
' ~-~5407 ' ', ~ '' ~ ' " , ' , ~ 3~2~7~
isopentane, etc. under a reactor pressure of 315 psia. Cocatalyst was controlled by feeding - -tr~ethylaluminum to m~intain 200 ppmv in the resin produced. Freon was fed AS a promoter to maintain a r&tio o 0.7 moles freon to each mole of teal.
Production r~te was su~tained at approximately 20,000 pph or a space time yield of 5 mlbs/hr/~t3 o~ bed volume.
~uring production gradual incrPasP in sta~ic voltage levels measured 5 ft above the distributor plate at the reactor wall began about 18 hours a~ter st~ble production was ~chieved~ Voltage build up appeared with small static spikes from 0 to ~00-300 volts every 1 to 5 minutes. The trend cont~nuet upwsrd with a ~ase line shift ~rom 0 to 1000-5000 volts ant static spikes to 10,000-15,000 volts with an increasing frequency. Associated with the stQtic were deviations in skin temperatures measuret 3 to 6 ft above the distributor plate at the reactor wall. These deviations were generally negatiYe indicating an insulating effect due to resin accumulation adhered to the wall. If sllowed to continue sheet formation will occur eventually leading to a reactor shutdown from plugged discharge systems or blockage at the distributor plate resulting in 8 large agglomerate formation due to loss o 1uidization.
At this point water was added by establishing approximately 5 pounds per hour nitrogen feed through the cylinder containing ;~ distilled water at 20~C and 350 psig to the ethylene feed line t~ the reactor. Resultant water -,. :
131 2~72 concentration in the ethylene feed was 0.2 ppmv.
Adiustments were msde to control the static level ne~r zero. Care was exercised to avoid excessive water ~eed which can result in unw~nted negative ~tatic excursions which can also lead to a sheeting ineldent.
The static le~el was brought to control near zero ~nd sheet formation was avoided and stable reactor operation was maintained w~thout unwanted shutdowns from sheeting incidents.
Example 4 The-mod~fied reactor of Exampl~ 3 was utilized to produce a linear low density film resin. ~he resin produced was a 0.917 density, 2.7 melt index ethylene-hexene copolymer with a sticking temperature of 102~C. The catalyst used was a titanium based on a silica support. Loading o~
titanium was 0.25 millimoles per gram of precursor.
Magnesium chloride, diethylaluminum chloride, and tri-normal hexylaluminum were added in molar ratios of 3, .02, .02 respectively to the titanium content. Silica support was Davison 955 with a micron si2e range o~ 10-80. Reaction is proceeded with a bed temperatur~ of 76C, 29~ ethylene, 11%
hydrogen, 5% l-hexene, and the remsining concentrations inert nitrogen, ethane, methane, snd isopentane. Cocatalyst was fed to control 300 ppmw triethylaluminum in the resin. Catalyst productivity under these conditions was 2203 pounds o~ polyethylene produced per pound of catslyst.
Production r~tes were 18,000 pounds per hour or 4.5 space time yield.
:, ~3~2~
- 3~ - -A sudden incre~se in ethylene concentration resulted in a pronounced increase in catalyst ~; activity. Static voltage near the reactor w811 ; ~ncreased from near zero to 6000 volts over a ten : 5 minute period. Sk~n temperature-e at the wall showan ~ncrease indic~ting the sudden ~ormat~on of polymer sheets along the wall of the reactor at the 6 ft leve} above the dist~ibution plate. If allowed to oon~inue, 8 reactor shutdown was ~mminent due to plugged product discharge systems.
. Water add b~ck was beg~n with nitrogen flow of 4 pph through the water cylinder in the temperature controlled housing at 20C. Resultant water concentration in the ethylene was less thsn 0.2 ppm. Static voltage quickly returned to near zero. Reactor skin temperature deviations subsided within ten minutes and normal reactor production resumed.
: `
~ :, ~, i
P x WN2 ~ 18 = Water flow ~WH20) I0 WH20 x 28 x I~ = PPM H2O In ethyIene feed At 20C PH20 = 0.339 ps~a PN2 = 325 psi~ when reactor at 300 psig.
WNz - 0 to 11.36 ppm typically flow at 3 ppm Wc2~4 = 18,000 pph Water concentration = (.339) (3)_(10) =0.174 ppm 325 18,000 Static voltage in the reactor can be monitored near the reactor wall by one or more ststic voltage indicator~ 50 inserted into the reactor bed approxim~tely five feet above the distributor plate in the range of -15,000 to ~15,000 volts. With reaction in progress, changes in static voltage levels erom neutral to positive can be counteracted by feed oP the moisture laden nitrogen to the ethylene stream. I~ this is not performed, impending agglomerate formation will likely create a process upset. Care must be exercised to avoid excessive water levels which can result in unwanted negative st~tic vol*age levels.
The system is operated with various flow and check valves wh~ch are common ~n the srt and - hence not illustrated. In ~ddtion, line 44 is :
, .; ~
.
` ~ 31~72 preferably insulated ~nd steam traced prior to entering the gas feed line 4~. ~ -The polymers to which the present invention is primarily directet and which cause the sheeting problems aboYe reerred to ln the presence of titanium catalysts are linear homopolymers of ethylene or linear copolymers of a major mol percent (>90%) of ethylene, ~nd a~minor mol percent (<10~) of one or more C3 to C8 alpha olefins. The C3 to C8 alph~ olefins should not contain any brsnching on any of their carbon atoms which is closer than the fourth carbon atom. The preferred C3 to C8 alpha olefins are propylene, butene-l, pentene-l, hexene-l, 4-methylpentene-1, heptene-l, and octene-l~ This descript~on is not intended to exclude the use of this invention with alpha olefin homopolymer and copolymer resins in which ethylene is not a monomer.
; The homopolymers and copolymers have a density ran~ing from about 0.97 to 0.91~ The density of the copolymer, at a given melt index level is primarily regulated by the smount of the C3 to C8 comonomer which is copolymerized with the ethylene. Thus, the addition of progressively large~ amounts of the comonomers to the copolymers results in a progressive lowering of the density of the copolymer. The amount of each of the various C3 to C8 comonomers needed to ach~eve the same result will ~ary from monomer to monomeri under t~he same reaction conditions. In the absence of the comonomer, the ethylene would homopolymerize.
The melt index of a homopolymer or copolymer is a re~lection o~ its molecular weight.
, , :
:
, .
~3~2~2 - 29 ~
Polymers having 8 relPtively high molecular weight, have reiatively high viscosities flnd low melt index.
In a typical mode of ut~lizing the subject =-invention to reduce she~ting, a reactor vessel such as ~hown in Figure 1 and which is susceptible to sheeting problems by the polymerization of the above described materials utilizing Type IV and Type V
catalysts with an alkyl aluminum cocataly~t is part~lly filled with gr~nular polyethylene resin which is purged with a non-resctive gas such as nitrogen and is fluidi2ed by circulating said non-reacting gas through t~e reactor at a velocity aboYe the minimum fluidizing velocity (Gmf) of the granular polyethylene and preferably at 3 to 5 Gmf.
The reactor is brought up to operational temperatures by the gas and the resction is started by introducing the catalyst and cocatalyst to the resctor~ During reaction, static voltage levels approach those levels which cause sheeting, then the pressurel temperature and flow control in the O'Brien box are increased to permit nitrogen to become water saturated. The water s~turated nitrogen is then directed to the gas feed line and introduced into the reactor. The voltage levels in the reactor are monitored responsive to the water laden gas feed st~eam and the procedure is continued until the static ~oltage levels are brsught to levels of non-sheeting.
Having set ~orth the general nature of the invention, the fvllowing examples illustrate some specific embodlments of the 1nyention~ It is to be :
..
~ ~2~72 - 3~ -understood, howe~er, that this invent~on is not limited to the ex~m~les, si~ce the invention may be practiced by the use of various modifications.
Ex~mples 1 and 2 were conducted in a conventionsl bed reactor. The catalyst used was a Zi~gler type, ~it~n~um based eatalyst supported on porous silica produced as described earli~r as Type IV. Thè coc~talyst u~ed was triethyl aluminum. The products made in the examples were copolymers of ethylene and l-butene. HydrGgen was used as a chain transfer sgent to control the melt index of the polymer.
ExamPle 1 A fluidized bed reactor was started up at operat~ng condltions designed to produce a ilm gr~de low density ethylene copolymer product having a density of 0.918, a mel~ index of 1.0, and a sticking temperature of 104C. The reaction was ` 20 started by feeding catalyst to the reactor prech~rged ~ith a bed of granular resin similar to the p~otuct to be made. The cfltalyst was a mixture of S.S parts titanium tetrachloride, 8.5 parts magnesium c~loride and 14 parts tetrshydrofuran deposited on 100 parts Davison 8rade 955 silica ; which had been dehydrated at 600C ~nd treated with four parts triethylaluminum prior to deposit~on and was activated~with thirty-five psrts tri-n-hexyl ~aluminum subsequent to deposition. Prior to starting catalyst feed 9 the resctor and resin bed ~ere brought up to the operating tempersture of 85C, were purged o~ impurities by circulating nitrogen through the resin bed. Ethylene, butene . ' ., ` ~31~72 and hydro~en concentrat1ons were established at 53, 24, and~ , respectively. -Cocatalyst was fed at a r~te of 0.3 parts triethylaluminum per part of catalyst.
Reac~or star~-up was normal. After producing product for twenty-nine hours and equiYalent to 6-112 times the weight of the f}uidized bed, temp~rature excurs~ons of 1 to 2~C
~bove bed temperature were observed using thermocouples l~cated ~ust inside the reactor wall at an elevation of 1/2 reactor diameter ~bove the ~as distributor plate. Prior experience had shown that such temperature excursions ~re a positive indication that sheets of resin are being formed in the fluidized bed. Concurrently, bed voltsge (measured using an electrostatic voltmeter connected to a 112 inch diameter spherical electrode located one inch ~rom the reactor wall at an elevation of 1/2 reactor diameter Rbove the gas distributor plate) increased from a readin8 of approximately ~1500 to ~2000 volts to a'rea~ing of over +S000 volts and then dropped back to ~2000 volts over ~ 3 minute period. Temperature and voltage excursions continued for approxim8tely 12 hours and increased in frequency and magnitude. During this period, sheets of ~used polyethylene resin began to show up in the resin product. Evidence of sheeting became more severe, i.e., temperature excursions increased to ~s high as 20C above bed temperature and stayed high for extended periods of time and voltage ' excursions slso bec~me more ,frequent. The reactor was shut down because of the extent of sheeting.
.
' , ~ ' "" ; ' ~312172 ExamE~e 2 The fluldized bed reactor used in Exsmple 1 w~s stsrted up ~nd operated to produce a linear low density ethylene copolymer suitable for extrusion or rvtational molding and having a density of 0.934, a melt lndex of 5 and a sticking temperatu~e of ; 118C. The reaction W8S started by feeding catalyst similar to the catalyst in Example 1 except sctivated with 28 parts tri-n-hexylaluminum, to the resctor precharged with ~ bed of granular resin similar to the protuct to be made. Prior to starting catalyst feed the reactor and the resin bed were brought up to the operat~ng temperature o~
85C, and were purged of impurities with nitrogen.
The concentration of ethylene ~52~), butene (14~), hydrogen ~21%) were introduced into the reactor.
Cocatalyst triethylaluminum was fed at 0.3 parts per part of catalyst. The reactor was operated continuously for 48 hours and during that period produced resin equivalent to 9 times the amount of resin contained in the bed. After this 48 hour period of smooth operation9 sheets of fused resin be8an to come out of the reactor with the normal, granular ptoduct. At this time voltages ~easured l/Z reactor diameter ~bove the distributor plate averaged +2000 volts and ranged from 0 to ~10,000 volts, while the skin thermocouples at the same elevat~on indicated excursions of ~15C above the bed temperatu~. Two hours after the ~irst sheets were noted in the product ~rom the reactor, it was necessary to stop ~eeding catalyst and cocatalyst to the reactor to reduce the resin production rate , .
,,, : :
,; ..
`:` : ' ~31~172 because sheets were plugging the resin discharge system. One hour later, çatalyst ~nd cocatalyst feeds were restarted. The production of sheets continued and after two hours catalyst and cocatalyst feed were again stopped and the reaction WdS termin~ted by in~ecting carbon monoxide. The voltage at this ~ime was > ~12,000 ~olts and the thermocouple excursions continued until the poison was ~n~ected. In totsl, the re~ctor was op~rated for 53 hours and produced 10-1/2 bed volum~s of resin before the reaction was stopped due to sheeting.
The following Example illustrstes the prevention of sheeting by adding water to the gas feed during periods of high volta8e in the reactor.
ExamPle 3 The reac~or of Examples 1 and 2 were modified AS shown ~n Figure 1 ~nd a high density film grade polyethylene resin of 0.946 density, 7.5 flow ind~x, and sticking temperature of 124C was continuously produced. The product was an ethylene-hexene copolymer utilizing a vanadium based c~talyst with an aluminum alkyL cocatalyst and halogen promoter for polymerization.
The catalyst contained 0.29 millimoles vanadium per gram of precursor and 1.2~ aluminum added in the form of diethylaluminum chloride on a Davison silica support o~ 30-130 micron s~ze. Reaction proceeded with a bed temperature of 98C, 7~% ethylene t 1.670 hydrogen, lq2~ hexene, and the remaining concentration inert gases of nitrogen, methane, .
' ~-~5407 ' ', ~ '' ~ ' " , ' , ~ 3~2~7~
isopentane, etc. under a reactor pressure of 315 psia. Cocatalyst was controlled by feeding - -tr~ethylaluminum to m~intain 200 ppmv in the resin produced. Freon was fed AS a promoter to maintain a r&tio o 0.7 moles freon to each mole of teal.
Production r~te was su~tained at approximately 20,000 pph or a space time yield of 5 mlbs/hr/~t3 o~ bed volume.
~uring production gradual incrPasP in sta~ic voltage levels measured 5 ft above the distributor plate at the reactor wall began about 18 hours a~ter st~ble production was ~chieved~ Voltage build up appeared with small static spikes from 0 to ~00-300 volts every 1 to 5 minutes. The trend cont~nuet upwsrd with a ~ase line shift ~rom 0 to 1000-5000 volts ant static spikes to 10,000-15,000 volts with an increasing frequency. Associated with the stQtic were deviations in skin temperatures measuret 3 to 6 ft above the distributor plate at the reactor wall. These deviations were generally negatiYe indicating an insulating effect due to resin accumulation adhered to the wall. If sllowed to continue sheet formation will occur eventually leading to a reactor shutdown from plugged discharge systems or blockage at the distributor plate resulting in 8 large agglomerate formation due to loss o 1uidization.
At this point water was added by establishing approximately 5 pounds per hour nitrogen feed through the cylinder containing ;~ distilled water at 20~C and 350 psig to the ethylene feed line t~ the reactor. Resultant water -,. :
131 2~72 concentration in the ethylene feed was 0.2 ppmv.
Adiustments were msde to control the static level ne~r zero. Care was exercised to avoid excessive water ~eed which can result in unw~nted negative ~tatic excursions which can also lead to a sheeting ineldent.
The static le~el was brought to control near zero ~nd sheet formation was avoided and stable reactor operation was maintained w~thout unwanted shutdowns from sheeting incidents.
Example 4 The-mod~fied reactor of Exampl~ 3 was utilized to produce a linear low density film resin. ~he resin produced was a 0.917 density, 2.7 melt index ethylene-hexene copolymer with a sticking temperature of 102~C. The catalyst used was a titanium based on a silica support. Loading o~
titanium was 0.25 millimoles per gram of precursor.
Magnesium chloride, diethylaluminum chloride, and tri-normal hexylaluminum were added in molar ratios of 3, .02, .02 respectively to the titanium content. Silica support was Davison 955 with a micron si2e range o~ 10-80. Reaction is proceeded with a bed temperatur~ of 76C, 29~ ethylene, 11%
hydrogen, 5% l-hexene, and the remsining concentrations inert nitrogen, ethane, methane, snd isopentane. Cocatalyst was fed to control 300 ppmw triethylaluminum in the resin. Catalyst productivity under these conditions was 2203 pounds o~ polyethylene produced per pound of catslyst.
Production r~tes were 18,000 pounds per hour or 4.5 space time yield.
:, ~3~2~
- 3~ - -A sudden incre~se in ethylene concentration resulted in a pronounced increase in catalyst ~; activity. Static voltage near the reactor w811 ; ~ncreased from near zero to 6000 volts over a ten : 5 minute period. Sk~n temperature-e at the wall showan ~ncrease indic~ting the sudden ~ormat~on of polymer sheets along the wall of the reactor at the 6 ft leve} above the dist~ibution plate. If allowed to oon~inue, 8 reactor shutdown was ~mminent due to plugged product discharge systems.
. Water add b~ck was beg~n with nitrogen flow of 4 pph through the water cylinder in the temperature controlled housing at 20C. Resultant water concentration in the ethylene was less thsn 0.2 ppm. Static voltage quickly returned to near zero. Reactor skin temperature deviations subsided within ten minutes and normal reactor production resumed.
: `
~ :, ~, i
Claims (22)
1. A method for reducing sheeting during polymerization of alpha-olefins in a low pressure fluidized bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts which comprises introducing water into said reactor in an amount sufficient to maintain the electrostatic levels at the site of possible sheet formation at levels which avoid sheeting without substantially altering the effectiveness of said catalysts.
2. A method according to claim 1, wherein one of said alpha-olefins is ethylene.
3. A method according to claim 2 wherein said water is introduced into said reactor by passing a pressurized inert gas at a controlled rate of flow through a temperature controlled container containing water to add water to said inert gas, directing said inert gas containing water from said temperature controlled container into admixture with said ethylene and thereafter introducing said admixture into said reactor.
4. A method according to claim 3 wherein said inert gas is nitrogen.
5. A method according to claim 4 wherein the flow rate of said nitrogen gas the flow rate of said ethylene, and the temperature of said water in said container are controlled and are determined responsive to static levels in said reactor.
6. A method according to claim 5 wherein the water content in said admixture entering said reactor is less than one part per million by volume based on said ethylene feed.
7. A method according to claim 5 wherein the water content in said admixture entering said reactor is about 0.1 to about 2 parts per million by volume based on said ethylene feed.
8. A method according to claim 5 wherein the flow rate of said nitrogen is varied between 0 to 11 lbs/hr for an ethylene feed range of about 0 to 50,000 lbs/hr.
9. A method for reducing sheeting during production of polyolefins by polymerization of alpha-olefins in a low pressure fluidized bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts which comprises introducing water into said reactor said water being introduced into said reactor by passing a pressurized inert gas at a controlled rate of flow through a temperature controlled container containing water to add water to said inert gas, directing said inert gas containing water from said temperature controlled container into admixture with said alpha-olefins and thereafter introducing said admixture into said reactor, said water being introduced into said reactor in an amount sufficient to maintain the electrostatic levels at the site of possible sheet formation at levels which avoid sheeting without substantially altering the effectiveness of said catalysts.
10. A method according to claim 9 wherein said inert gas is nitrogen.
11. A method according to claim 10 wherein the flow rate of said nitrogen gas, the flow rate of said alpha-olefins and the temperature of said water in said container are controlled and are adjusted responsive to static levels in said reactor.
12. A method according to claim 9 wherein the water content in said admixture entering said reactor is less than one part per million by volume based on said ethylene feed.
13. A method according to claim 9 wherein the water content in said admixture entering said reactor is about 0.1 to 2 parts per million by volume based on said ethylene feed.
14. A method according to claim 10 wherein the flow rate of said nitrogen is varied between about 0 to 11 lbs/hr for an alpha-olefin feed range of about 0 to 50,000 lbs/hr.
15. A method according to claim 9 wherein the temperature of said water in said container is controlled within the range of about 10°C to 40°C.
16. A method according to claim 9 wherein said polyolefins are linear homopolymers of ethylene or linear copolymers of a major mole percent (? 90%) of ethylene, and a minor mole percent (?10%) of one or more C3 to C8 alpha-olefins.
17. A method according to claim 16 wherein said polyolefins are homopolymers or copolymers of propylene, butene-1, pentene-1, hexene-1, 4-methylpentene-1, heptene-1, or octene-1.
18. A method for reducing sheeting during production of polyolefins by polymerization of alpha-olefins in a low pressure fluidized bed reactor utilizing titanium or vanadium based compounds as catalysts together with alkyl aluminum cocatalysts which comprises introducing water into said reactor said water being introduced into said reactor by passing a pressurized nitrogen gas at a flow rate of about 0 to 11 lbs/hr. for an alpha-olefin feed rate of about 0 to 50,000 lbs/hr.
through a temperature controlled container containing water at a temperature of about 10°C to 40°C to add water to said nitrogen gas, directing said nitrogen gas containing water from said temperature controlled container into admixture with said alpha-olefins and thereafter introducing said admixture into said reactor, said water being introduced into said reactor in an amount of about less than one part per million by volume based on said ethylene feed.
through a temperature controlled container containing water at a temperature of about 10°C to 40°C to add water to said nitrogen gas, directing said nitrogen gas containing water from said temperature controlled container into admixture with said alpha-olefins and thereafter introducing said admixture into said reactor, said water being introduced into said reactor in an amount of about less than one part per million by volume based on said ethylene feed.
19. A method according to claim 18 wherein the flow rate of said nitrogen gas, the flow rate of said alpha-olefins and the temperature of said water in said container are controlled and are adjusted responsive to static levels in said reactor.
20. A method according to claim 18 wherein the water content in said admixture entering said reactor is not less than about 0.1 part per million by volume based on said ethylene feed.
21. A method according to claim 18 wherein said polyolefins are linear homopolymers of ethylene or linear copolymers of a major mole percent (? 90%) of ethylene, and a minor mole percent (? 10%) of one or more C3 to C8 alpha-olefins.
22. A method according to claim 21 wherein said polyolefins are homopolymers or copolymers of propylene, butene-1, pentene-1, hexene-1, 4-methyl-pentene-1, heptene-1, or octene-1.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000582147A CA1312172C (en) | 1988-11-03 | 1988-11-03 | Method for reducing sheeting during polymerization of alpha-olefins |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000582147A CA1312172C (en) | 1988-11-03 | 1988-11-03 | Method for reducing sheeting during polymerization of alpha-olefins |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1312172C true CA1312172C (en) | 1992-12-29 |
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Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000582147A Expired - Lifetime CA1312172C (en) | 1988-11-03 | 1988-11-03 | Method for reducing sheeting during polymerization of alpha-olefins |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1312172C (en) |
-
1988
- 1988-11-03 CA CA000582147A patent/CA1312172C/en not_active Expired - Lifetime
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