CA1085095A - Process for achieving high conversions in the production of polyethylene - Google Patents
Process for achieving high conversions in the production of polyethyleneInfo
- Publication number
- CA1085095A CA1085095A CA289,241A CA289241A CA1085095A CA 1085095 A CA1085095 A CA 1085095A CA 289241 A CA289241 A CA 289241A CA 1085095 A CA1085095 A CA 1085095A
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- Prior art keywords
- reaction
- zones
- zone
- cooling
- psi
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- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J19/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
- B01J19/24—Stationary reactors without moving elements inside
- B01J19/2415—Tubular reactors
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J3/00—Processes of utilising sub-atmospheric or super-atmospheric pressure to effect chemical or physical change of matter; Apparatus therefor
- B01J3/04—Pressure vessels, e.g. autoclaves
- B01J3/042—Pressure vessels, e.g. autoclaves in the form of a tube
-
- C—CHEMISTRY; METALLURGY
- C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
- C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
- C08F10/00—Homopolymers and copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
- C08F10/02—Ethene
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
- B01J2219/00—Chemical, physical or physico-chemical processes in general; Their relevant apparatus
- B01J2219/00049—Controlling or regulating processes
- B01J2219/00051—Controlling the temperature
- B01J2219/00121—Controlling the temperature by direct heating or cooling
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- Chemical & Material Sciences (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Health & Medical Sciences (AREA)
- Medicinal Chemistry (AREA)
- Polymers & Plastics (AREA)
- Addition Polymer Or Copolymer, Post-Treatments, Or Chemical Modifications (AREA)
- Polymerisation Methods In General (AREA)
Abstract
ABSTRACT OF THE DISCLOSURE
Polyethylene is produced by polymerization of ethylene alone or with comonomers and/or telogens (modifiers) in an elongated tubular reactor having an inlet and outlet and preferably containing four reaction zones. Conversions of up to 40% are achieved without loss of optical or physical product quality.
Polyethylene is produced by polymerization of ethylene alone or with comonomers and/or telogens (modifiers) in an elongated tubular reactor having an inlet and outlet and preferably containing four reaction zones. Conversions of up to 40% are achieved without loss of optical or physical product quality.
Description
BACRGROUND OF T~E INVf~Nl'ION
2 The instant invention relates to a process for
3 the polymerization of ethylene alone or with comonomers
4 and/or telogens (modifiers) at elevated temperatures and pressures in an elongated tubular reac-tor wherein 6 relatively high conversions are obtained without loss 7 of optical or physical product quality.
8 The polymerization of ethylene to solid poly-9 ethylene in an elongated tubular reactor at elevated temperatures and pressures in the presence of free 11 radical or free oxygen producing initiators that decom-12 pose at or below the polymerization temperature to 13 generate free radicals is well known in the art. Various 14 types of tubular reactor systems have been developed and are in commercial use. One of the more basic 16 systems onaom~a&e~ feeding a pressurized stream consisting 17 of ethylene, initia-tor and optionally a modifier into 18 one end of a tubular reactor consisting o~ one reaction 19 zone and one cooling zone. Owing to the exothermic nature of the reaction, the temperature increases as 21 the reaction proceeds to a maxi~um or peak temperature 22 and considerable heat is evolved. Heat control methods 23 must be employed to prevent violent runaway reactions 24 and explosions as well as to increase conversions.
One method for achieving heat control involves the use 26 of a long tubular reactor wherein some measure of tempera-27 ture control is obtained from the relatively large 28 ratio of heat dissipating surface to reactor volume.
29 Thus, heat control is also accomplished by providing the reactor with a coo]ing jacket.
31 It has also been discovered that effective control 32 of the polymeriza-tiol~ reaction can be main-tained together . :. ~ - ..
:' '- , . ~ ' ~
. , ~
with an increase in conversion if instead of introducing ethyl-ene at one point in the reactor, it is injected as sidestreams at one or more additional points downstream in the reaction tube.
The injected ethylene is made to serve both as a coolant and as a monomer for further polymerization. See U.S. 3,725,378.
Another process using multiple sidestreams is disclosed in U.S. 3,628,918. This patent discloses three reaction zones, three cooling zones and two monomer sidestreams; one introduced prior to the second reaction zone and the other introduced prior to the third reaction zone in a tubular reactor having a larger diameter for the cooling zones than that of the reaction zones.
Generally, all of the conventional polymerization systems attempt to achieve the highest level of conversion without loss of optical and physical properties of the resulting polymer. For example, the two reaction zone and three reaction zone processes discussed above can achieve about 20 to 25 per-cent conversion. It has heretofore been assumed and observed by those skilled in the art that when conversion levels about 25 percent are achieved, both optical and physical properties rapidly deteriorate regardless of the pressure and temperature histories achieved.
The quality of polyethylene particularly haze is affected by a number of mechanisms. Haze increases with increas-ing high molecular weight components. By maintaining a high flow number and preferably a flow number greater than 3.3 ft /sec.
one obtains the greatest percent of turbulent flow one thereby minimizes long straight chains which are formed dominantly during the .:
- ~ '' : :
; ~
~ 10~35095 1 laminar flow. On the other hand increase in reaction 2 zone average pressure improves haze by decreasing the 3 amount of long chain branching th~ereby decreasing the 4 high molecular weight component in the molecular weight distribution. An increase in reaction zone average or 6 peak temperatures increases long chain branching of 7 the polymerization reaction. The increasedhigh molecular 8 weight component in the molecular weight distribution 9 results in increased haze. Additionally, the increase in polymer concentration obtained through higher conver-11 sions was known to generally cause an increase in the 12 opportunity for the long chain branches to form on 13 existing polymer molecules. Thus, it has generally 14 been accepted that high conversions lead to greater long chain branches and hence reduce haze quality, 16 i.e., higher haze.
18 In accordance with this invention it has been 19 unexpectedly discovered that one may obtain a signifi-cant increase in conversion while maintaining high 21 quality haze or an even greater increase in conversion 22 with a slight increase in haze value. More particularly, 23 it has been unexpectedly discovered that the pressure 24 and temperature history of the reaction mass during the polymerization reaction in a tubular reactor is more 26 significant in determining product quality than the 27 absolute level of conversion obtained. It is therefore 28 possible to obtain high quality product while obtaining 29 greater than 25~ conversion of the monomer reaction mass to polymer. That one could obtain high quality 31 at greater than 25% conversion is contrary to that 32 previously conceived by those skilled in the art.
, .
85~95 1 These high conversions are achieved by use of a -2 tubular reactor comprised of a plurality of reaction 3 zones followed by cooling zones wherein a monomer ~ sidestream is introduced at least after first and second reaction zones while still maintaining a monomer 6 feed stxeam through the inlet of the reactor and in 7 accordance with the invention the use of an additional 8 reaction and cooling zone accompanied by a decrease in 9 the operating temperatures in the previous reaction zones.
It is preferred that the flow number be kept greater 11 than about 3.3 ft2/sec. in the reaction zones and that . -12 the pressure drop in the tubular reactor, between the 13 inlet of the first reaction zone and the outlet of 14 the last reaction zone, not to exceed about 6000 psi when operating at preassures between 25,000 and 50,000 16 psi at the inlet of the .first reaction zone. The 17 flow number as used herein means the bulk fluid velocity 18 in ft/sec. times the diameter in feet.
The present invention will be further illustrated 21 by reference to the drawings in which: .
22 Figure 1 is an illustration of a tubular reactor 23 having four reaction zones in accordance 24 with an aspect of the invention and also provided with sidestreams and different diameter 26 tubes to provide high conversion, reaction 27 zone boundary layer control and pressure 28 drop control.
3~ In accordance with this invention there is provided 31 a process for producing polyethylene or copolymers of 32 ethylene in an elongated tubular reactor having an -- 1~385095 1 inlet and an outlet, at a pressure of about 20,000 2 psi to about 60,000 psi comprising passing the reaction 3 mass through a plurali-ty of reaction zones at a flow 4 number of at least about 3.3 ft2/sec., each reaction zone followed by a cooling/repreparation zone, and wherein 6 free radical producing substance is injected at the 7 beginning of each reaction zone, the improvement com-~
8 prising adding at least one additional final reaction 9 zone and cooling zone and decreasing the operating temperatures in the previous reaction zones whereby 11 there is obtained at least 28% conversion of ethylene 12 to polyethylene while maintaining product haze value.
13 Preferably, the reactor comprises three or four 14 reaction zones with or without the use of cold gas injection to facilitate cooling. However, in accor-16 dance with this invention four or more reaction zones 17 may be utilized. The unexpected aspect oE this inven-18 tion is that one may add additional reaction zones 19 to the prior art processes and obtain increased conver-sion of monomer to polymer without deterioration of 21 product quality by decreasing the operating temperature 22 ranges in the previous reaction zones.
23 More specifically, a conventional tubular reactor 24 comprising two reaction zones may be converted to a three reaction zone process with a 6 to 10% conversion 26 increase and the maintenance of haze value b~ aading a 27 third reaction zone; lowering the operating temperature in the 28 first two reaction ~ones by about 40F to about 70F
29 or approximately 15 to 20%~T reduction; However in accordance with this invention greater or lesser reduction may be employed; and 31 minlmizing pressure differences between reaction zone, i.e. main- :
32 taining the least amount of pressure drop so as to operate at a 33 maximum average pressure. More preferably, ~j.,,"4 - -~ ~ lQ85~)95 a conventional three reaction zone tubular reactor containing one or two cold side streams can be converted to a four reaction zone tubular reactor with two cold gas cooling streams to obtain 28% or greater conversion while maintaining high quality product, more particularly high quality haze by lowering the operating aT in the first three reaction zones from about 60F to about 90 F or approximately 15 to 20% ~T reduction.
In the event haze is not a limiting criteria, even greater conversion increases may be obtained (up to 10 to 12~
conversion increases) due to the less restrictive temperature reductions in the early sections of the reactor and the positive effects of having full use of the additional reàction zone for increase in conversion. In all cases, however, pressure dropped minimization is employed for maximiZing uniformity of the product made in each subsequent zone.
The present invention is not limited to any specific tubular reactor design, modifler, comonomer system, operating pressure or temperature variables, or initiator system, but it is preferred that the pressure drop throughout the reactor is kept below about 6,000 psi from the inlet of the first reaction zone to the outlet of the last reaction zone and the flow number kept above about 3.3 ft /sec. as disclosed in Canadian Patent No.
993,598.
The tubular reactor may be an elongated jacketed tube or pipe, usually in section or blocks, of suitable strength and having inside diameter of about 0.50 to 3 inches, preferably about 1.0 to 2.5 inches. The tubular reactor usually has a length to diameter ratio of about ' ', . ' ' ' ' ~' :. - :. ' : ~ ~
:
1085~95 1 1000 to 1, to ~0,000 to 1. The specific len~th employed 2 depends upon the specific tubing configuration used in 3 the final application of the design, as required for ~ sufficient heat removal.
The tubular reactor is operated at pressures from 6 about 20,000 to 60,000 psi. Althou~h pressures higher 7 than 60,000 psi can be used, it is preferred that the 8 pressure be about 30,000 to 50,000 psi.
9 The temperature maintained in the reactor is variable and is primarily controlled by and dependent 11 on the specific initiator system employed. Temperatures 12 are usually within the range of about 150 to 350~C or -~
13 higher, preferably about 160 to 327C or higher, more v~ry D 14 pre'erably about 165 to 315C and ~.. rie~ in the different reaction zonès.
16 Initiators suitable for use in t~e instant inven-s ~ c e ~
17 tion are free radical producing sub~tanc~. Non-limiting :~-18 .examples of such free radical producing substances 19 include oxygen; peroxide compounds such as hydrogen peroxide, decanoyl peroxide, t-butyl peroxy neodecanoate, 21 t-butyl peroxypivalate, 3,5,5-trimethyl hexanoyl peroXide, .:
22 diethyl peroxide, t-butyl peroxy-2-ethyl hexanoate, t-buty].
23 peroxy isobutyrate, benzoyl peroxide, t-butyl peroxy 24 acetate, t-butyl peroxy benzoate, di-t-butyl peroxide, and 1,1,3,3-tetramethyl butyl hydroperoxide; alkali 26 metal persulfates, perborates and percarbonates; and 27 azo compounds such as azo bis isobutyronitrite. Pre-28 ferred are organic peroxides.
29 It is understood that mixtures of the ahove initiators may be .injected into the various reaction 31 zones. It is also understood tha-t the initiator or 32 initiators may be introduced directly into the main ~thyle~e ~ ~ ' -- 1085~)95 s~ream or in conjunction with a side ethylene stream or 2 at such poi.nts where the reaction mixture is at the 3 proper temperature for initiation or to reinitiate the 4 polymerization reaction.
The feedstock employed in the present invention 6 may be ethylene or predominantly ethylene together with 7 a telogen tmodifier) or comonomer. Known telogens or 8 modifiers, as the term is used herein, are illustrated 9 by the saturated aliphatic aldehydes, such as ormal-dehyde, acetaldehyde and the like, the saturate aliphatic 11 ketones, such as acetone, diethyl ketone, diamyl ketone, 12 and the like, the saturated aliphatic alcohols, such as 13 methanol, ethanol, propanol, and the like, paraffins 14 or cycloparafins such as pentane, hexane, cyclohexane, and the like, aromatic compounds such as toluene, 16 diethylbenzene, xylene, and the like, and other compounds 17 which ac~ as chain terminating agents such as carbon :
18 tetrachloride, chloroform, etc.
19 The process of the present invention may also be used to produce copolymers of ethylene with one 21 or more polymerizable ethylenically unsaturated monomers .:~
22 having a 23 CH2=C
24 group and which under~o addition polymerization.
These copolymers may be produced with or without 26 modifiers present. Polymerizable e-thylenically unsaturated 27 monomers having a 28 CH2=C~
29 group and which undergo addition polymerization are, for example, alpha monoolefins, such as propylene, butenes, 31 pentenes, etc., the acrylic, haloacrylic and methacrylic 32 acids, esters, nitri.les, and ar~lides such as acrylic acid, --` 1085095 Jnef ~c~i~e 1 chloroacrylic acid, m~thacrylic acid, cyclohexyl mctha~
2 methyl acrylate, acrylonitrile, acr~lamide; the vinyl and 3 vinyl-idene halides; the N-vinyl amides; the vin~l 4 carboxylates, such as vinyl acetate; the N-vinyl aryls, such as styrene; the vinyl eth~rs, ketones or other 6 compounds, such as vinyl pyridine, and the like. Co-7 monomers and telogens or modifiers are used to modify~
8 the properties of the ethylene polymer produced.
9 Accordingly, the term polyethylene, as used herein is e~.~e~ e meant to include the so modified thylel.e polymers as 11 well as homopolyethylene.
12 llaze is a product property which varies depending 13 on the reactor design and operating conditions. For 14 high clarity polyethylene film applications a haze in the range of about 3.5 to 5.0 percent is generally 16 required. For medium clarity film applications a 17 haze in the range of about 5.0 to 7.5 is generally 18 required. Haze properties as heretofore explained -19 normally deteriorate with an increase in operating 20 temperature, a decrease in operating pressure, and/or ~ -21 an increase in polymer conversion. Surprisingly, the 22 instant invention leads to conversions hi~her than that 23 thou~ht possible in the art while maintaining equivalent 2~ or superior haze properties over that of conventional lo~er conversion processes. The higher conversions 26 are possible for example by converting a three reaction 27 zone tubular reactor to a four reac-tion zone tubular 28 reactor with the judicious selection of the operating 29 pressure and temperature ranges, i.e. minimization o~
pxessure drop and decreasincJ the operational pea~
31 temperatures of the first three reac-tion zones. ~enerally, 32 as the fluid moves through the reactor the resulting product ohtained 33 has a higher and less :' :
lC85095 1 acceptable haze value owing to the pressure decrease residence time at 2 high temperature and the free radical attack of the existing polymer at 3 each additional initiator injector point; this leads to 4 side branching reactions, thus leading to an increase in haze. Therefore, it would be unexpected to obtain, 6 at high conversions and in a four reaction zone reactor, 7 an ethylene polymer with acceptable optical and physical 8 properties for clear film applications. The term haze 9 as used in the instant specification and claims is the percentage of transmitted light which in passing through 11 a polymer specimen herein deviates from the incident 12 beam by forward scattering as measured by ASTM-D-1003-61.
13 Acceptable physical properties of the polymers 14 produced by the instant invention include a density of about .91 to .935, preferably about .92 to .93 and a 16 melt index of about .1 to 40, preferably about .3 to 30.
17 The melt index of the polymer herein is determined 18 as the weight of resin, expressed in grams; that is, 19 extruded in ten minutes at 180C through a standard orifice when a standard load is applied on the molten 21 polymer. See ASTM-D-123~-57T.
22 To illustrate the present invention in all of its 23 aspects, a tubular reactor for the production of 24 polyethylene is illustrated in Figure 1. The ~eed gases are introduced through line A to a preheater 26 which heats the feed to its ini-tiation temperature.
27 The tubular reactor c has eight zones having an internal 28 diameter from 0.5 to 3.0 inches. Four zones (1, 3, 5 and 7) 29 are reaction zone and are smaller in diameter by approxi-mately 0.25 inches than the four coolin~ zones 2, 4, 6 and 31 Initiator is introduced after the preheater and also 32 at the end of coolin~J zones 2, 4 and 6 a~ter a minimum :
1~851~5 1 init:iation temperature i5 rcached. Monomer sidestream 2 feeds for cooling and reducing pressure drop are also 3 introduced toward the end of cooling zones 2 and 4.
4 Table I sets forth the internal diameter of the tubes~
S in each zone, the calculated bulk fluid velocities of 6 each zone and the calcula-ted flow number in each reactor 7 zone for a particular design. In any particular design, 8 the cdiameters of the various zones will vary within the 9 tube size ran~es previously mentioned.
T~BLE I
11 Internal ~ulk Fluid :
12 Diameter Velocity Flow 13 (inches) (ft/sec) _ Number 14 Zone 1 1.25 46 4.85 Zone 2 1.50 32 4.04 ;:
16 Zone 3 1.50 48 6.10 17 Zone 4 1.75 36 5.20 ::
18 Zone 5 1.75 48 6.90 :~
19 Zone 6 2.00 36 6.0 Zone 7 1.75 ~8 . 6.90 21 Zone 8 2.00 36 ~.00 22 The ratio of injected sidestream monomer feed to 23 inlet monomer feed mus-t be regulated to provide maximum 24 utilization of the cooling available in the feed stream while at the same time effecting the desired heat distri- `
26 bution to mai.ntai~n the polymerization reaction at clesired 27 operating temperatures and polymeri.~e some OL the 28 incoming monomer. With too large a ratio of sidestream 29 injected .into the inlet monomer, for example, the tempera-ture after mixing may be well below the initiation 31 temperature of certain initiators and in such cases 32 additional heat would bc needed to bring the mixture 33 back up to ini-tiation temperature range.
.
.
.
1~35~95 1 The tcmperature of the injected monomer d~pends 2 upon the ratio oE the latter to the inlet monomer and 3 should be kept as low as practical to secure optimum 4 cooling. ~lonomer cooled below ambient temperature may be used to provide a large heat reservoir. ~lowever, 6 the lowest temperature to which the injected monomer 7 feed can be cooled with any given number of injection 8 points is governed also by the resultant temperature 9 of the reaction mixture after injection, which in turn depends to some extent upon the initiator system employed.
11 Thus, the specific formula for optimum operating condi-12 tions is different in every design case and varies for 13 different product property requirements. The flow rate, 14 flow split between injected and inlet monomer, inJected lS monomer feed temperature, gas and injection point locations 16 initiator must be related to the particular design 17 and initiator system used for a given set of desired 18 polymerization temperature and pressure conditions.
19 These are readily subject to calculation by those skilled in the art.
8 The polymerization of ethylene to solid poly-9 ethylene in an elongated tubular reactor at elevated temperatures and pressures in the presence of free 11 radical or free oxygen producing initiators that decom-12 pose at or below the polymerization temperature to 13 generate free radicals is well known in the art. Various 14 types of tubular reactor systems have been developed and are in commercial use. One of the more basic 16 systems onaom~a&e~ feeding a pressurized stream consisting 17 of ethylene, initia-tor and optionally a modifier into 18 one end of a tubular reactor consisting o~ one reaction 19 zone and one cooling zone. Owing to the exothermic nature of the reaction, the temperature increases as 21 the reaction proceeds to a maxi~um or peak temperature 22 and considerable heat is evolved. Heat control methods 23 must be employed to prevent violent runaway reactions 24 and explosions as well as to increase conversions.
One method for achieving heat control involves the use 26 of a long tubular reactor wherein some measure of tempera-27 ture control is obtained from the relatively large 28 ratio of heat dissipating surface to reactor volume.
29 Thus, heat control is also accomplished by providing the reactor with a coo]ing jacket.
31 It has also been discovered that effective control 32 of the polymeriza-tiol~ reaction can be main-tained together . :. ~ - ..
:' '- , . ~ ' ~
. , ~
with an increase in conversion if instead of introducing ethyl-ene at one point in the reactor, it is injected as sidestreams at one or more additional points downstream in the reaction tube.
The injected ethylene is made to serve both as a coolant and as a monomer for further polymerization. See U.S. 3,725,378.
Another process using multiple sidestreams is disclosed in U.S. 3,628,918. This patent discloses three reaction zones, three cooling zones and two monomer sidestreams; one introduced prior to the second reaction zone and the other introduced prior to the third reaction zone in a tubular reactor having a larger diameter for the cooling zones than that of the reaction zones.
Generally, all of the conventional polymerization systems attempt to achieve the highest level of conversion without loss of optical and physical properties of the resulting polymer. For example, the two reaction zone and three reaction zone processes discussed above can achieve about 20 to 25 per-cent conversion. It has heretofore been assumed and observed by those skilled in the art that when conversion levels about 25 percent are achieved, both optical and physical properties rapidly deteriorate regardless of the pressure and temperature histories achieved.
The quality of polyethylene particularly haze is affected by a number of mechanisms. Haze increases with increas-ing high molecular weight components. By maintaining a high flow number and preferably a flow number greater than 3.3 ft /sec.
one obtains the greatest percent of turbulent flow one thereby minimizes long straight chains which are formed dominantly during the .:
- ~ '' : :
; ~
~ 10~35095 1 laminar flow. On the other hand increase in reaction 2 zone average pressure improves haze by decreasing the 3 amount of long chain branching th~ereby decreasing the 4 high molecular weight component in the molecular weight distribution. An increase in reaction zone average or 6 peak temperatures increases long chain branching of 7 the polymerization reaction. The increasedhigh molecular 8 weight component in the molecular weight distribution 9 results in increased haze. Additionally, the increase in polymer concentration obtained through higher conver-11 sions was known to generally cause an increase in the 12 opportunity for the long chain branches to form on 13 existing polymer molecules. Thus, it has generally 14 been accepted that high conversions lead to greater long chain branches and hence reduce haze quality, 16 i.e., higher haze.
18 In accordance with this invention it has been 19 unexpectedly discovered that one may obtain a signifi-cant increase in conversion while maintaining high 21 quality haze or an even greater increase in conversion 22 with a slight increase in haze value. More particularly, 23 it has been unexpectedly discovered that the pressure 24 and temperature history of the reaction mass during the polymerization reaction in a tubular reactor is more 26 significant in determining product quality than the 27 absolute level of conversion obtained. It is therefore 28 possible to obtain high quality product while obtaining 29 greater than 25~ conversion of the monomer reaction mass to polymer. That one could obtain high quality 31 at greater than 25% conversion is contrary to that 32 previously conceived by those skilled in the art.
, .
85~95 1 These high conversions are achieved by use of a -2 tubular reactor comprised of a plurality of reaction 3 zones followed by cooling zones wherein a monomer ~ sidestream is introduced at least after first and second reaction zones while still maintaining a monomer 6 feed stxeam through the inlet of the reactor and in 7 accordance with the invention the use of an additional 8 reaction and cooling zone accompanied by a decrease in 9 the operating temperatures in the previous reaction zones.
It is preferred that the flow number be kept greater 11 than about 3.3 ft2/sec. in the reaction zones and that . -12 the pressure drop in the tubular reactor, between the 13 inlet of the first reaction zone and the outlet of 14 the last reaction zone, not to exceed about 6000 psi when operating at preassures between 25,000 and 50,000 16 psi at the inlet of the .first reaction zone. The 17 flow number as used herein means the bulk fluid velocity 18 in ft/sec. times the diameter in feet.
The present invention will be further illustrated 21 by reference to the drawings in which: .
22 Figure 1 is an illustration of a tubular reactor 23 having four reaction zones in accordance 24 with an aspect of the invention and also provided with sidestreams and different diameter 26 tubes to provide high conversion, reaction 27 zone boundary layer control and pressure 28 drop control.
3~ In accordance with this invention there is provided 31 a process for producing polyethylene or copolymers of 32 ethylene in an elongated tubular reactor having an -- 1~385095 1 inlet and an outlet, at a pressure of about 20,000 2 psi to about 60,000 psi comprising passing the reaction 3 mass through a plurali-ty of reaction zones at a flow 4 number of at least about 3.3 ft2/sec., each reaction zone followed by a cooling/repreparation zone, and wherein 6 free radical producing substance is injected at the 7 beginning of each reaction zone, the improvement com-~
8 prising adding at least one additional final reaction 9 zone and cooling zone and decreasing the operating temperatures in the previous reaction zones whereby 11 there is obtained at least 28% conversion of ethylene 12 to polyethylene while maintaining product haze value.
13 Preferably, the reactor comprises three or four 14 reaction zones with or without the use of cold gas injection to facilitate cooling. However, in accor-16 dance with this invention four or more reaction zones 17 may be utilized. The unexpected aspect oE this inven-18 tion is that one may add additional reaction zones 19 to the prior art processes and obtain increased conver-sion of monomer to polymer without deterioration of 21 product quality by decreasing the operating temperature 22 ranges in the previous reaction zones.
23 More specifically, a conventional tubular reactor 24 comprising two reaction zones may be converted to a three reaction zone process with a 6 to 10% conversion 26 increase and the maintenance of haze value b~ aading a 27 third reaction zone; lowering the operating temperature in the 28 first two reaction ~ones by about 40F to about 70F
29 or approximately 15 to 20%~T reduction; However in accordance with this invention greater or lesser reduction may be employed; and 31 minlmizing pressure differences between reaction zone, i.e. main- :
32 taining the least amount of pressure drop so as to operate at a 33 maximum average pressure. More preferably, ~j.,,"4 - -~ ~ lQ85~)95 a conventional three reaction zone tubular reactor containing one or two cold side streams can be converted to a four reaction zone tubular reactor with two cold gas cooling streams to obtain 28% or greater conversion while maintaining high quality product, more particularly high quality haze by lowering the operating aT in the first three reaction zones from about 60F to about 90 F or approximately 15 to 20% ~T reduction.
In the event haze is not a limiting criteria, even greater conversion increases may be obtained (up to 10 to 12~
conversion increases) due to the less restrictive temperature reductions in the early sections of the reactor and the positive effects of having full use of the additional reàction zone for increase in conversion. In all cases, however, pressure dropped minimization is employed for maximiZing uniformity of the product made in each subsequent zone.
The present invention is not limited to any specific tubular reactor design, modifler, comonomer system, operating pressure or temperature variables, or initiator system, but it is preferred that the pressure drop throughout the reactor is kept below about 6,000 psi from the inlet of the first reaction zone to the outlet of the last reaction zone and the flow number kept above about 3.3 ft /sec. as disclosed in Canadian Patent No.
993,598.
The tubular reactor may be an elongated jacketed tube or pipe, usually in section or blocks, of suitable strength and having inside diameter of about 0.50 to 3 inches, preferably about 1.0 to 2.5 inches. The tubular reactor usually has a length to diameter ratio of about ' ', . ' ' ' ' ~' :. - :. ' : ~ ~
:
1085~95 1 1000 to 1, to ~0,000 to 1. The specific len~th employed 2 depends upon the specific tubing configuration used in 3 the final application of the design, as required for ~ sufficient heat removal.
The tubular reactor is operated at pressures from 6 about 20,000 to 60,000 psi. Althou~h pressures higher 7 than 60,000 psi can be used, it is preferred that the 8 pressure be about 30,000 to 50,000 psi.
9 The temperature maintained in the reactor is variable and is primarily controlled by and dependent 11 on the specific initiator system employed. Temperatures 12 are usually within the range of about 150 to 350~C or -~
13 higher, preferably about 160 to 327C or higher, more v~ry D 14 pre'erably about 165 to 315C and ~.. rie~ in the different reaction zonès.
16 Initiators suitable for use in t~e instant inven-s ~ c e ~
17 tion are free radical producing sub~tanc~. Non-limiting :~-18 .examples of such free radical producing substances 19 include oxygen; peroxide compounds such as hydrogen peroxide, decanoyl peroxide, t-butyl peroxy neodecanoate, 21 t-butyl peroxypivalate, 3,5,5-trimethyl hexanoyl peroXide, .:
22 diethyl peroxide, t-butyl peroxy-2-ethyl hexanoate, t-buty].
23 peroxy isobutyrate, benzoyl peroxide, t-butyl peroxy 24 acetate, t-butyl peroxy benzoate, di-t-butyl peroxide, and 1,1,3,3-tetramethyl butyl hydroperoxide; alkali 26 metal persulfates, perborates and percarbonates; and 27 azo compounds such as azo bis isobutyronitrite. Pre-28 ferred are organic peroxides.
29 It is understood that mixtures of the ahove initiators may be .injected into the various reaction 31 zones. It is also understood tha-t the initiator or 32 initiators may be introduced directly into the main ~thyle~e ~ ~ ' -- 1085~)95 s~ream or in conjunction with a side ethylene stream or 2 at such poi.nts where the reaction mixture is at the 3 proper temperature for initiation or to reinitiate the 4 polymerization reaction.
The feedstock employed in the present invention 6 may be ethylene or predominantly ethylene together with 7 a telogen tmodifier) or comonomer. Known telogens or 8 modifiers, as the term is used herein, are illustrated 9 by the saturated aliphatic aldehydes, such as ormal-dehyde, acetaldehyde and the like, the saturate aliphatic 11 ketones, such as acetone, diethyl ketone, diamyl ketone, 12 and the like, the saturated aliphatic alcohols, such as 13 methanol, ethanol, propanol, and the like, paraffins 14 or cycloparafins such as pentane, hexane, cyclohexane, and the like, aromatic compounds such as toluene, 16 diethylbenzene, xylene, and the like, and other compounds 17 which ac~ as chain terminating agents such as carbon :
18 tetrachloride, chloroform, etc.
19 The process of the present invention may also be used to produce copolymers of ethylene with one 21 or more polymerizable ethylenically unsaturated monomers .:~
22 having a 23 CH2=C
24 group and which under~o addition polymerization.
These copolymers may be produced with or without 26 modifiers present. Polymerizable e-thylenically unsaturated 27 monomers having a 28 CH2=C~
29 group and which undergo addition polymerization are, for example, alpha monoolefins, such as propylene, butenes, 31 pentenes, etc., the acrylic, haloacrylic and methacrylic 32 acids, esters, nitri.les, and ar~lides such as acrylic acid, --` 1085095 Jnef ~c~i~e 1 chloroacrylic acid, m~thacrylic acid, cyclohexyl mctha~
2 methyl acrylate, acrylonitrile, acr~lamide; the vinyl and 3 vinyl-idene halides; the N-vinyl amides; the vin~l 4 carboxylates, such as vinyl acetate; the N-vinyl aryls, such as styrene; the vinyl eth~rs, ketones or other 6 compounds, such as vinyl pyridine, and the like. Co-7 monomers and telogens or modifiers are used to modify~
8 the properties of the ethylene polymer produced.
9 Accordingly, the term polyethylene, as used herein is e~.~e~ e meant to include the so modified thylel.e polymers as 11 well as homopolyethylene.
12 llaze is a product property which varies depending 13 on the reactor design and operating conditions. For 14 high clarity polyethylene film applications a haze in the range of about 3.5 to 5.0 percent is generally 16 required. For medium clarity film applications a 17 haze in the range of about 5.0 to 7.5 is generally 18 required. Haze properties as heretofore explained -19 normally deteriorate with an increase in operating 20 temperature, a decrease in operating pressure, and/or ~ -21 an increase in polymer conversion. Surprisingly, the 22 instant invention leads to conversions hi~her than that 23 thou~ht possible in the art while maintaining equivalent 2~ or superior haze properties over that of conventional lo~er conversion processes. The higher conversions 26 are possible for example by converting a three reaction 27 zone tubular reactor to a four reac-tion zone tubular 28 reactor with the judicious selection of the operating 29 pressure and temperature ranges, i.e. minimization o~
pxessure drop and decreasincJ the operational pea~
31 temperatures of the first three reac-tion zones. ~enerally, 32 as the fluid moves through the reactor the resulting product ohtained 33 has a higher and less :' :
lC85095 1 acceptable haze value owing to the pressure decrease residence time at 2 high temperature and the free radical attack of the existing polymer at 3 each additional initiator injector point; this leads to 4 side branching reactions, thus leading to an increase in haze. Therefore, it would be unexpected to obtain, 6 at high conversions and in a four reaction zone reactor, 7 an ethylene polymer with acceptable optical and physical 8 properties for clear film applications. The term haze 9 as used in the instant specification and claims is the percentage of transmitted light which in passing through 11 a polymer specimen herein deviates from the incident 12 beam by forward scattering as measured by ASTM-D-1003-61.
13 Acceptable physical properties of the polymers 14 produced by the instant invention include a density of about .91 to .935, preferably about .92 to .93 and a 16 melt index of about .1 to 40, preferably about .3 to 30.
17 The melt index of the polymer herein is determined 18 as the weight of resin, expressed in grams; that is, 19 extruded in ten minutes at 180C through a standard orifice when a standard load is applied on the molten 21 polymer. See ASTM-D-123~-57T.
22 To illustrate the present invention in all of its 23 aspects, a tubular reactor for the production of 24 polyethylene is illustrated in Figure 1. The ~eed gases are introduced through line A to a preheater 26 which heats the feed to its ini-tiation temperature.
27 The tubular reactor c has eight zones having an internal 28 diameter from 0.5 to 3.0 inches. Four zones (1, 3, 5 and 7) 29 are reaction zone and are smaller in diameter by approxi-mately 0.25 inches than the four coolin~ zones 2, 4, 6 and 31 Initiator is introduced after the preheater and also 32 at the end of coolin~J zones 2, 4 and 6 a~ter a minimum :
1~851~5 1 init:iation temperature i5 rcached. Monomer sidestream 2 feeds for cooling and reducing pressure drop are also 3 introduced toward the end of cooling zones 2 and 4.
4 Table I sets forth the internal diameter of the tubes~
S in each zone, the calculated bulk fluid velocities of 6 each zone and the calcula-ted flow number in each reactor 7 zone for a particular design. In any particular design, 8 the cdiameters of the various zones will vary within the 9 tube size ran~es previously mentioned.
T~BLE I
11 Internal ~ulk Fluid :
12 Diameter Velocity Flow 13 (inches) (ft/sec) _ Number 14 Zone 1 1.25 46 4.85 Zone 2 1.50 32 4.04 ;:
16 Zone 3 1.50 48 6.10 17 Zone 4 1.75 36 5.20 ::
18 Zone 5 1.75 48 6.90 :~
19 Zone 6 2.00 36 6.0 Zone 7 1.75 ~8 . 6.90 21 Zone 8 2.00 36 ~.00 22 The ratio of injected sidestream monomer feed to 23 inlet monomer feed mus-t be regulated to provide maximum 24 utilization of the cooling available in the feed stream while at the same time effecting the desired heat distri- `
26 bution to mai.ntai~n the polymerization reaction at clesired 27 operating temperatures and polymeri.~e some OL the 28 incoming monomer. With too large a ratio of sidestream 29 injected .into the inlet monomer, for example, the tempera-ture after mixing may be well below the initiation 31 temperature of certain initiators and in such cases 32 additional heat would bc needed to bring the mixture 33 back up to ini-tiation temperature range.
.
.
.
1~35~95 1 The tcmperature of the injected monomer d~pends 2 upon the ratio oE the latter to the inlet monomer and 3 should be kept as low as practical to secure optimum 4 cooling. ~lonomer cooled below ambient temperature may be used to provide a large heat reservoir. ~lowever, 6 the lowest temperature to which the injected monomer 7 feed can be cooled with any given number of injection 8 points is governed also by the resultant temperature 9 of the reaction mixture after injection, which in turn depends to some extent upon the initiator system employed.
11 Thus, the specific formula for optimum operating condi-12 tions is different in every design case and varies for 13 different product property requirements. The flow rate, 14 flow split between injected and inlet monomer, inJected lS monomer feed temperature, gas and injection point locations 16 initiator must be related to the particular design 17 and initiator system used for a given set of desired 18 polymerization temperature and pressure conditions.
19 These are readily subject to calculation by those skilled in the art.
Claims (10)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. In a process for producing polyethylene in an elongated tubular reactor having an inlet and an outlet, at a pressure of about 20,000 psi to about 60,000 psi comprising passing the reaction mass through a plurality of reaction zones at a flow number of at least about 3.3 ft2/sec., each reaction zone followed by a cooling/
repreparation zone, and wherein free radical producing substance is injected at the beginning of each reaction zone, the improvement comprising adding at least one additional final reaction zone and cooling zone and decreasing the operating temperatures in the previous reaction zones whereby there is obtained at least a 28% conversion of ethylene to polyethylene while main-taining product haze value.
repreparation zone, and wherein free radical producing substance is injected at the beginning of each reaction zone, the improvement comprising adding at least one additional final reaction zone and cooling zone and decreasing the operating temperatures in the previous reaction zones whereby there is obtained at least a 28% conversion of ethylene to polyethylene while main-taining product haze value.
2. The process of claim 1 wherein a sidestream of ethylene is injected into the first and second cooling zones.
3. The process of claim 1 wherein the operating pressures can be between 25,000 psi and 50,000 psi.
4. The process of claim 1 wherein the pressure drop across the length of the reactor does not exceed 6,000 psi.
5. The process of claim 1 wherein the flow number is at least 4.0 ft2/sec.
6. The process of claim 1 wherein the haze value is about 3.5 to about 7.5.
7. The process of claim 1 wherein the decrease in operating temperature range is about 15 to 20%.
8. The process of claim 2 wherein the tubular reactor comprises four reaction zones and cooling/repreparation zones.
9. The process of claim 1 wherein the decrease in operating temperature ranges is about 5 to 30%.
10. The process of claim 1 wherein the tubular reactor comprises three or four reaction zone and three or four cooling/repreparation zones and no ethylene side streams.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US73651376A | 1976-10-28 | 1976-10-28 | |
| US05/840,870 US4135044A (en) | 1977-08-08 | 1977-10-11 | Process for achieving high conversions in the production of polyethylene |
| US840,870 | 1992-02-25 | ||
| US736,513 | 1996-10-24 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1085095A true CA1085095A (en) | 1980-09-02 |
Family
ID=27113056
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA289,241A Expired CA1085095A (en) | 1976-10-28 | 1977-10-21 | Process for achieving high conversions in the production of polyethylene |
Country Status (7)
| Country | Link |
|---|---|
| AU (1) | AU508107B2 (en) |
| CA (1) | CA1085095A (en) |
| DE (1) | DE2748263A1 (en) |
| FR (1) | FR2369300A1 (en) |
| GB (1) | GB1593661A (en) |
| IT (1) | IT1114443B (en) |
| NL (1) | NL7711881A (en) |
Families Citing this family (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| DE3912975A1 (en) * | 1989-04-20 | 1990-11-08 | Basf Ag | POLYETHYLENE AND COPOLYMERISATES FROM MAJOR CONTENTS OF ETHYLENE |
| SE529006C2 (en) * | 2004-11-15 | 2007-04-03 | Chematur Eng Ab | Process and system for supercritical water oxidation of a stream containing oxidizable material |
| CN102256697B (en) * | 2008-12-18 | 2014-11-12 | 埃克森美孚化学专利公司 | High pressure polymerization process |
| US20120240960A1 (en) | 2009-12-22 | 2012-09-27 | Philip Henri Cornelissen | Separation Vessels For Use In Polymerization Processes And Methods For Cleaning Same |
Family Cites Families (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US3628918A (en) * | 1970-05-15 | 1971-12-21 | Exxon Research Engineering Co | Apparatus for production of polyethylene |
-
1977
- 1977-10-21 CA CA289,241A patent/CA1085095A/en not_active Expired
- 1977-10-26 AU AU30063/77A patent/AU508107B2/en not_active Expired
- 1977-10-27 IT IT2905677A patent/IT1114443B/en active
- 1977-10-27 DE DE19772748263 patent/DE2748263A1/en active Granted
- 1977-10-27 FR FR7732419A patent/FR2369300A1/en active Granted
- 1977-10-28 GB GB4497577A patent/GB1593661A/en not_active Expired
- 1977-10-28 NL NL7711881A patent/NL7711881A/en active Search and Examination
Also Published As
| Publication number | Publication date |
|---|---|
| FR2369300B1 (en) | 1984-06-15 |
| IT1114443B (en) | 1986-01-27 |
| FR2369300A1 (en) | 1978-05-26 |
| AU3006377A (en) | 1979-05-03 |
| GB1593661A (en) | 1981-07-22 |
| DE2748263C2 (en) | 1993-07-01 |
| DE2748263A1 (en) | 1978-05-03 |
| AU508107B2 (en) | 1980-03-06 |
| NL7711881A (en) | 1978-05-03 |
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