CA1221197A

CA1221197A - Linear low density polyethylene process

Info

Publication number: CA1221197A
Application number: CA000440535A
Authority: CA
Inventors: Charles A. Trischman; Kryspin P. Maciejewski
Original assignee: El Paso Polyolefins Co
Current assignee: El Paso Polyolefins Co
Priority date: 1983-02-04
Filing date: 1983-11-07
Publication date: 1987-04-28
Also published as: FR2540502A1; GB2134121B; IT1174473B; DE3401614A1; GB2134121A; JPS59145209A; GB8400034D0; IT8419200A0

Abstract

ABSTRACT OF THE DISCLOSURE

In the production of linear low density polyethylene in a high pressure tubular reaction zone using a solid supported transition metal catalyst, plugging of equipment downstream of the reaction zone is prevented by the addition of an antifoam agent to the reaction zone effluent at a location upstream of the high pressure separation zone employed in separating unreacted monomer from the polymer.

Description

BACKGROUND OF THE INVENTION

In the past, high pressure tubular reactors have been used extensively in free radical-initiated polymerization of ethylene for the production of con-ventional low density polyethylene tLPDE). Commercial size reactors typically have tube diameters (ID) in the range from 0 r 5 to 3 inches and reactor lengths (including preheater sections) between about 300 and 3000 ft. or more. The reactors are usually operated at pressures in the range from about 15,000 to about 50,000 psi or even higher. Because of the high pressure, the investment costs have been high for commercial plants including the aforementioned tubular reactors and other necessary process equipment such as com-pressors~ separator vessels, valves, etcu An important application for the low density polyethylene produced in the high pressure tubular reactors has been in the manufacture of film, es-pecially packaging films. Recently, however, linearlow density polyethylene (LLDPE), which is a copolymer of ethylene and at least one C4-Cl8 alpha-olefin, has captured a substantial portion of this market.
Because of inherent lower capital costs, new capacity commercial installations for the production of LLDPE
resins are usually designed for low to medium pressure - operations (100-2500 psi) using vapor phase, liquid-phase slurry or liquid-phase solution polymerization tech-niques and employing solid transition metal catalysts, such as magnesium halide supported titanium halide catalysts.
However where high pressure tubulax reactor process equipment is already available, relatively minor modifications with small incremental capital investment are required for conversion to a high pressure process for the production of LLDPE resins using solid transition ~r~
s' metal catalysts. The products obtained from such a process are as good or even better than those from low to medium pressure LLDPE processes.
An unexpected operational problem was noted during the high pressure tubular reactor LLDPE experi-mental work leading up to the present invention.
Specifically, severe plugging occurred in the high pressure separator vessel off-gas line, as well as in safety pressure relief valve and in the rupture disc port on this separator vessel. Unrestricted gas flow through the off-ga~ line is required ~or normal pressure control of the process. The relief valve and rupture disc ports must remain open to allow these safety devices to function as over-pressure protection for the high pressure vessel.
It is therefore an object of the present invention to provide a process for the production of linear low density polyethylene (LLDPE) in a high pressure process employing a tubular reaction zone and a solid transition metal catalyst composition in which the above-mentioned problems with plugging are not encountered.

BRIEF DESCRIPTION OF THE DRAWING
The drawing diagramatically illustrates a linear low density polyethylene (LLDPE) process including the improvement of the invention.

THE INVENTION

In accordance with the present invention, there is provided an improved continuous process for the production of linear low density polyethylene, in which process ethylene and at least one C4-C18 alpha-olefin 9~7 comonomer are polymerized in the presence of a transition metal catalyst composition in an elongated tubular reaction æone at a pressure in the range from about 10,000 to about 50,000 psig and temperatures from about 200F to about 650F, reactor effluent is lowered in pressure to a value in the range from about l,500 psig to about 5000 psig and passed to a high pressure separator vessel for separation of a stream of unreac~ed monomer from polymer. From about 5 to about 200 ppm, preferably from about lO to about 100 ppm, based on the polymer weight of an anti-foaming agent is added to the reaction zone effluen~
upstream of the high pressure separator vessel~ Any antifoaming agent known in the art could be used, however, those of the silicone type are preferred.
It was most unexpected to find that an anti-foaming agent would be effective in preventing plugging in the high pressure separator gas lines and safety devices. In polymerizations, antifoaming agents ~0 have previously only been employed to prevent foaming in aqueous systems such as emulsion or suspension polymerization systems. They have also been known to be useful in polymerizations accompanied by the form-ation of water. However, the successful use of the agents in the nonraqueous LLDPE copolymerization process could not be deduced from the prior art teach-ings.
The preferred antifoaming agent for use in the invention is a dimethyl silicone having a viscosity in the range between about lO00 and about 60,000 centistokes, preferably between about 4,000 and about 30,000 centristokes. Since these agents are liquid they are easily added to the reaction zone effluent by means of a metering pump. The addition should be made to the effluent upstream of the high pressure separator vessel. Preferably, the point of addition should be some distance away from the high pressure separator to permit thorough mixing of the antifoaming agent with the effluent. Most preferably, the addition is made at the end of the polymerization zone.
The polymerization feed is comprised of ethylene and at least one alpha-olefin having from 4 to 18 carbon atoms per molecule. Examples of preferred alpha-olefin comonomers are butene-l, pentene-l, hexene-l, 4-methyl pentene-l, heptene-l and octene-l, and mixtures thereof. The ethylene concentration in the to-tal olefin feed is usually maintained between about 20 and about 90 mol percent.
The polymer product qenerally contains from about 87 to about 98 wt % polymerized ethylene and from about 2 to about 13 wt ~ of comonomer-devised units. The polymer product has a density in the range from about 0.910 to about 0~935.
The catalyst composition used in the process can be any one of the recently developed high activ-ity titanium halide/magnesium compound catalyst components and organo aluminum cocatalyst components disclosed, e.g., in U.S. Patents Nos. 3,803,105, 3,953,414, 4,298,718 and 4,315,911.
One component, component (a~ of the catalyst composition is an alkyl aluminum having from 1 to 8 carbon atoms in the alkyl groups. It is advantageously selected from trialkyl aluminums, dialkylaluminum halides or mixtures thereo~. The preferred halide is chloride. Examples of suitable alkyl aluminums are diethylaluminum chloride, di-n-butylaluminum chloride, triethyl aluminum, trimethyl aluminum, tri-n-butyl aluminum, tri-isobutyl aluminum, --5~
triisohexyl aluminum, tri-n~octyl aluminum, triiso-octyl aluminum. The alkyl aluminum can, if desired, be complexed with an electron donor prior to intro-duction into the polymerization reactor. Preferably, the donors are selected from diamines or esters or carboxylic acids, particularly esters of aromatic acids.
Some typical examples of such compounds are methyl- and ethylbenzoate, methyl- and ethyl-p-methoxy-benzoate, diethylcarbonate, ethylacetate, dimethyl-maleate, triethylborate, ethyl-o-chlorobenzoate, ethylnapnthenate, methyl-p-toluate, ethyl-pivalate, N,N,N',N'-tetramethylenediamine, 1,1,4,-trimethyl-piperazine, 2,5-dimethylpiperazine and the like~
The molar ratio of aluminum alkyl to electron donor should be limited to a range between about 2 and about 5. Solutions of the electron donor and the alkyl aluminum compounds in a hydrocarbon such as hexane or heptane are preferably prereacted for a certain period of time generally less than 1 hour prior to feeding the mixture into the polymerization reaction zone.
It is not critical to the process of the present invention what method is used in the prep-aration of the transition metal component of thecatalyst composition, component (b), and any of the various techniques known in the art may be used.
Typically, these techniques in~olve the reaction of a titanium compound, e.g., a titanium halide or a titanium oxyhalide with a magnesium compound such as a halide. The product may be treated if desired, with an electron donor compound.
The halogen in the respecti~e halides can be chlorine, bromine or iodine, the preferred halogen being chlorine. The electron donor, if it is used, is suitably selected from the esters of inorganic and '~

'7 organic oxygenated acids and the polyamines. Examples of such compounds are the esters of aromatic carboxylic acids, such as benzoic acid, p-methoxybenzoic acid and p-toluic acids and particularly the alkyl esters of said acids; the aklylene diamines, e.g., N, N, N', N'-tetramethylethylene-diamine. The magnesium to electron donor molar ratio are equal to or higher than 1 and preferably between 2 and 10. Generally, the titanium content expressed as titanium metal ranges between 0.1 and 20 wt % in the supported catalyst component. Treatment steps may also be included in the preparation in order to obtain component (b) in spherical or spheroidal for~.
~Eter compression to the operating pressure, the monomer feed is usually preheated to a temperature in the range from about 200F to about 400~F in a pre-heating zone by indirect heat exchange with super heated steam.
The feed is then introduced to the inlet end of the tubular reaction zone, where it is contacted with the catalyst also fed to the inlet of the reaction zone. The catalyst is fed at a rate to provide poly-merization temperatures in the range from about 200F
to about 650F in the reaction zone. The pressure should normally range between about 10,000 psig and about 50,000 psig, and preferably between about 15,000 psig and about 25,000 psig. These pressure ranges include periodic pressure changes purposely employed to prevent accumulation of polymer on the interior walls of the reactor tube. These pressure changes are known as 'bump cycles' and are being effected by the operation of 'let down' valves at the outlet of the reactor tube. The interval between two sequential bump cycles may be from about 30 seconds to about 60 seconds and the duration of the pressure let-down in the bump cycle may be from about 0.3 to about 0.6 seconds. The bump cycle should cause a pressure reduction of from about 500 psi to about 5,000 psi.
The monomer feed is introduced at a rate to provide a residence time in the reactor of from about 0.5 minutes to about 2 minutes.
After addition of the antifoaming agent to the reaction zone effluent, the mixture is conducted to a high pressure separator vessel which is usually maintained between about 1,500 psig and about S,000 psig. Here a major portion of the unreacted monomer is separated from the molten polymer product, which is subsequently passed to a low pressure separator for further separation of gaseous monomer. The molten polymer is suitably extruded and pelletized.
The high pressure separator off-gases are freed of entrained catalyst and low molecular wax by any conven-tional means and recycled to the system after compression.
Hydrogen can be employed in the polymerization for molecular weight control, and if used, in concentration from about 0,01 to about 0~03 mole percent based on th~ total monomer feed.
Other additives which can be supplied to the process include catalyst deactivators such as ethoxy-lated amines or glycols, antioxidants, lubricants, antistatic agents, slip agents, antiblock agents, heat and light stabilizers and others. The additives are provided in quantities known to be effective for their respective functions. In general the total concentration of these additives range from about 0.01 to about 5 percent based on the weight of the polymer product. These additives are suitably intro-duced together with the antifoaming agent.
Referring now to the drawing there is shown a partially compressed ethylene stream from source 10, " '''~.~

being mixed with recycle stream 11 and compressed to the desired pressure in the final stages 12 of a high pressure compressor and then preheated in steam jacket heater 13. Alkyl aluminum cocatalyst is added via conduit 14 in sufficient quantities to provide the required Al-Ti ratio in the polymerization zone and also, if desired, to scavenge impurities from the monomer feed. The mixed monomer/alkyl aluminum stream in conduit 15 is passed to the inlet of a tubular reactor consisting of a plurality of jacketed (not shown) tubular sections 16a to 16z.
These tubular sections are connected in series by blocks, such as blocks 17 to 25. A slurry of a high efficiency supported titanium catalyst component in a suitable inert diluent is pumped to the inlet (at block 17) of the tubular reactor and the reactor effluent is passed through the high pressure letdown valve 26, which provides for a cyclical pressure reduction within the reactor. Antifoaming agent and usually other additives such as antioxidant lubricants and catalyst deactivator in a suitable inert diluent are added by means of line 27 to the reactor effluent upstream of high pressure separator vessel 28.
Alternately, the additives including catalyst deactiv-ators are introduced by means of line 27a to block 24(which in this case is the end of the reaction ~one because of the addition oE catalyst deactivator at block 24). High pressure separator 23 is equipped with a rupture disc 29. In case of disc failure due to pressure overload, the gases are vented into a header (not shown). Also safety pressure relief valve 31 in off-gas line 30 ~ill open and release the gases by means of vent line 32 into a header (not shown).
Comonomer feed is provided in line 33 and mixed with the gas stream in line 30. After various catalyst and wax removal steps (only one shown) at 35, the gas is fed in line 11 to compressor stages 12 completing the loop. Molten polymer is passed in line 35 to low pressure separator 36, wherein additional unre-acted monomer is separated and removed in line 37.The polymer in line 38 is fed to extruder-pelletizer 39 and then to storage ~0.
The following examples illustrate the invention and the advantages desired therefrom.
EXAMPLES 1 and 2 Pilot plant polymerization of ethylene and butene-l comonomer was carried out over an extended period of time in equipment arranged essentially as depicted in the drawing. The tubular reactor was about 410 feet long containing 5/8 inch ID tubular segments connected in series by 25 blocks. However, since the additives, i.eO, catalyst deactivator and antifoaming agent were added with heptane as carrier in line 27a (alternate method shown in the figure) to the 22nd block, the length of the actual reaction zone was reduced to about 360 feet. The catalyst deactivator was an ethoxylated amine available under the tradename Ethoduomeen MT/13 from Armak Chemicals.
The antifoaming agent was a dimethyl silicone ViscasilTM5000 obtained from General Electric and having a viscosity of about 5,000 centistokes The high efficiency solid titanium chloride/magnesium chloride catalyst contained about 3.4% Ti, 22.1%
Mg, 74.5% Cl and was provided as a 10% slurry in Primol 355, a paraffin oil available from Exxon.
The alkyl aluminum was tri-n-octyl aluminum ~8%
in heptane). The pertinent operating conditions at ~2~ 3 7 steady state conditions are shown in Table 1. No problems with plugging were encountered during the experimentation which lasted 2 months. However, all previous experimentations conducted at essentially the same operating conditions except that no anti-foaming agent was added, resulted in severe plugging of the high pressure separator off-gas line or of safety equipment associated with the high pressure separator, i.e., the pressure relief valve and/or the rupture disc port.
It is obvious to those skilled in the art that many variations and modifications can be made to the process of this invention. All such departures from the foregoing specification are considered within the scope of this invention as defined by this spec-ification and appended claims.

3~7 TABLE I
Reactor Pressure, psig 20,000 Peak Temperature~ F 520~10 Feed Temperature, ~F 260 Al/Ti Mole Ratio 20 Feed, lbs/hr 2000 Ethylene, mol % 60 Butene-1, mol % 40 Residence Time, Secs. 60 Additives, lbs/lb polymer 0.0013 T~13, wt % 20.00 Viscasil 5000, wt g 1.25 Heptane, wt % 78.75 H~P. Separator Pressure, psig 3,800 Conversion, wt % based on total feed 14 Production Rate lbs polymer/hr 280 Catalyst Productivity lbs polymer/lb Ti Catalyst 2000 lbs polymer/lb Ti 60,000 Polymer Density 0.92+
Melt Index 1~8

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A continuous process for the production of linear low density polyethylene comprising:
polymerizing ethylene and at least one C4-C18 alpha-olefin comonomer in the presence of a trans-ition metal catalyst composition in an elongated tubular reaction zone at a pressure in the range from about 10,000 to about 50,000 psig and a temperature from about 200°F to about 650°F;
reducing the pressure of the reactor effluent containing polymer product and unreacted monomers to a value in the range from about 1,500 to about 5,000 psig;
passing the reactor effluent of reduced press-ure to a high pressure separator vessel for at least partial separation of unreacted monomer from polymer and adding from about 5 to about 200 ppm based on the weight of the polymer of an antifoaming agent to the reactor effluent upstream of the reactor vessel.

2. The process of claim 1, wherein the anti-foaming agent is added in amount between about 15 and about 100 ppm based on the weight of the polymer.

3. The process of claim 1, wherein the anti-foaming agent is a dimethyl silicone having a viscosity in the range between about 1000 and about 60,000 centistokes.

4. The process of claim 1, wherein the viscosity is in the range between about 4,000 and about 30,000 centistokes.

5. The process of claim 1, wherein the anti-foaming agent is added to the reactor effluent after the reduction in pressure.

6. The process of claim 1, wherein the antifoaming agent is added to the reactor effluent at the end of the polymerization zone.

7. The process of claim 1, wherein the catalyst composition is comprised of:
(a) an alkyl aluminum compound having from 1 to 8 carbon atoms per alkyl group and, (b) a titanium halide supported on a magnesium halide.

8. The process of claim 7, wherein the halide of component (b) is chloride.

9. The process of claim 7, wherein the alkyl aluminum compound is a trialkyl aluminum.

10. The process of claim 9, wherein the tri-alkyl aluminum is tri-n-octyl aluminum.