GB2134121A - Linear low density polyethylene process - Google Patents

Abstract

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

SPECIFICATION Linear low density polyethylene process In the past, high pressure tubular reactors have been used extensively in free radical-initiated polymerization of ethylene for the production of conventional low density polyethylene (LPDE).

Commercial size reactors typically have tube diameters (ID) in the range from 0.5 to 3 inches (0.8 to 7.6 cm) and reactor lengths (including preheater sections) between 800 and 3000 ft. (244 and 914 m) or more. The reactors are usually operated at pressures in the range from 15,000 to 50,000 psi (103 to 345 MPa) 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 compressors, separator vessels and valves.

An important application for the low density polyethylene produced in high pressure tubular reactors has been in the manufacture of film, especially packaging films. Recently, however, linear low density polyethylene (LLDPE), which is a copolymer of ethylene and at least one C1-C18 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 (0.7 to 17 MPa) (100-2500 psi)) using vapor phase, liquid-phase slurry or liquidphase solution polymerization techniques and employing solid transition metal catalysts, such as magnesium halide supported titanium halide catalysts.However where high pressure tubular 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 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 experimental 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 the separator vessel. Unrestricted gas flow through the off-gas line is required for 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.

The present invention provides 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.

In accordance with the present invention, there is provided a continuous process for the production of linear low density polyethylene comprising: polymerizing ethylene and at least one C4-C18 alpha-olefin ethylene comonomer in the presence of a transition metal catalyst composition in an elongated tubular reaction zone at a pressure of from 69 to 345 MPa gauge (10,000 to 50,000 psig) and a temperature from 93 to 3430C (2000F to 6500 F); reducing the pressure of the reactor effluent containing polymer product and unreacted monomers to from 10.3 to 34.5 MPa gauge (1,500 to 5,000 psig);; passing the reactor effluent of reduced pressure to a high pressure separator vessel for at least partial separation of unreacted monomer from polymer, and adding from 5 to 200 ppm, preferably 10 to 100 ppm, based on the weight of the polymer, of an antifoaming agent to the reactor effluent upstream of the separator vessel. Any antifoaming agent known in the art could be used, but those of the silicone type are preferred.

It was most unexpected to find that an antifoaming agent would be effective in preventing plugging in the high pressure separator gas lines and safety devices. In polymerizations, antifoaming agents 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 formation of water. However, the successful use of the agents in the non-aqueous LLDPE copolymerization process could not have been deduced from the prior art teachings. The preferred antifoaming agent for use in the invention is a dimethyl silicone having a viscosity between 10-3 and 6x 1 o-2 m2/s (1000 and 60,000 centistokes), preferably between 4x 10-3 and 3 xl 0-2 m2/s (4,000 and 30,000 centistokes).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 comprises ethylene and at least one alpha-olefin having from 4 to 1 8 carbon atoms per molecule. Examples of preferred alpha-olefin comonomers are butene-1, pentene-1, hexene-1, 4-methyl-pentene-1, heptene-1 and octene-1, and mixtures of two or more of these. The ethylene concentration in the total olefin feed is usually maintained between 20 and 90 mol percent.

The polymer product generally contains from 87 to 98 wt% polymerized ethylene and from 1 3 to 2 wt% of comonomer-derived units. The polymer product has a density of from 0.910 to 0.935 g/cc.

The catalyst composition used in the process can be any one of the recently developed high activity 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 thus can be an alkyl aluminum having from 1 to 8 carbon atoms in the alkyl groups. It is advantageously selected fromtriaikyi aluminums, dialkylaluminum halides and mixtures thereof. 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, triisohexyl aluminum, tri-n-octyl aluminum, triisoctyl aluminum. The alkyl aluminum can, if desired, be complexed with an electron donor prior to introduction 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-pmethoxybenzoate, diethylcarbonate, ethyl acetate, dimethylmaleate, triethylborate, ethyl-o-chlorobenzoate, ethylnaphthenate, methyl-p-toluate, ethyl-pivalate, N,N,N' N,"tetramethylenediamine, 1,1,4- trimethylpiperazine and 2,5-dimethylpiperazine. The molar ratio of aluminum alkyl to electron donor should be limited to between 2/1 and 5/1. Solutions of the electron donor and the alkyl aluminum compound 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 preparation of the transition metal component of the catalyst composition, component (b), and any of the various techniques known in the art may be used. Typically, these techniques involve 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 respective 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 the inorganic and 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 alkylene diamines, e.g., N,N,N'N'-tetramethylethylene-diamine. The magnesium to electron donor molar ratio are equal to or higher than 1/1 and preferably between 2/1 and 10/1. Generally, the titanium content expressed as titanium metal is 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 form.

After compression to the operating pressure, the monomer feed is usually preheated to a temperature of from 93 to 2040C (2000 F to 4000 F) in a preheating 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 polymerization temperatures of from 930 to 3430C (2000F to 65O0F) in the reaction zone. The pressure shou!d normally be between 69 and 345 MPa gauge (10,000 psig and 50,000 psig), and preferably between 103 and 172 MPa gauge (15,000 psig and 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 30 seconds to 60 seconds and the duration of the pressure let-down in the bump cycle may be from 0.3 to 0.6 seconds. The bump cycle should cause a pressure reduction of from 3.45 to 34.5 MPa (500 psi to 5,000 psi).

The monomer feed is introduced at a rate to provide a residence time in the reactor of from 0.5 minutes to 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 10.3 to 34.5 MPa gauge (1,500 psig and 5,000 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 conventional 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 0.01 to 0.03 mole percent based on the total monomer feed.

Other additives which can be supplied to the processlnclude catalyst deactivators such as ethoxylated amines or glycols, antioxidants, lubricants, antistatic agents, slip agents, antiblock agents and heat and light stabilizers. The additives are provided in quantities known to be effective for their respective functions. In general the total concentration of these additives is from 0.01 to 5 percent based on the weight of the polymer product. These additives are suitably introduced together with the antifoaming agent.

Referring now to the drawing, which diagrammatically illustrates the LLDPE process of the invention, 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 1 3. 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 1 6a 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 a 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. Alternatively, the additives including catalyst deactivators are introduced by means of line 27a to block 24 (which in this case is the end of the reaction zone because of the addition of catalyst deactivator at block 24). High pressure separator 28 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 will open and release the gases by means of vent line 32 into a header (not shown). Comonomerfeed 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 34, 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 unreacted monomer is separated and removed in line 37.

The polymer in line 38 is fed to extruder-pelletizer 39 and then to storage 40.

The following Example illustrates the invention and the advantages desired therefrom.

Example Pilot plant polymerization of ethylene and butene-1 comonomer was carried out over an extended period of time in equipment arranged essentially as depicted in the drawing. The tubular reactor was about 125 m (410 feet) long containing 1.6 cm (5/8 inch) ID tubular segments connected in series by 25 blocks. However, since the additives, i.e. catalyst deactivator and antifoaming agent, were added with heptane as carrier in line 27a (alternative method shown by the dotted line in the figure) to the 22nd block, the length of the actual reaction zone was reduced to about 110 M (360 feet). The catalyst deactivator was an ethoxylated amine available under the tradename "Ethoduomeen T/1 3" from Armak Chemicals.

The antifoaming agent was dimethyl silicone "Viscasil" 50000 obtained from General Electric and having a viscosity of about 5x 10-3 m2/s (5,000 centistokes). The high efficiency solid titanium chloride/magnesium chloride catalyst contained about 3.4% Ti, 22.19/0, Mg, 74.5% Cl and was provided as a 10% slurry in "Primol" 355, a paraffin oil available from Exxon. The alkyl aluminum was triu-octyl aluminum (8% in heptane). The pertinent operating conditions at steady state conditions are shown in the Table. No problems with plugging were encountered during the experiment which lasted 2 months.

However, all previous experiments conducted at essentially the same operating conditions except that no antifoaming 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.

Table Reactor Pressure, (psig) MPa gauge (20,000) 138 Peak Temperature, (OF) OC (52+10) 270It5.5 Feed Temperature, (OF) (260) 127 Al/Ti Mole Ratio 20 Feed (Ibs/hr) (2000) 900 kg/hr Ethylene, mol % 60 Butene-1, mol % 40 Residence Time, Secs. 60 Additives, kg/kg polymer 0.0013 T/13,wt% 20.00 "Viscasil" 5000, wt % 1.25 Heptane, wt % 78.75 H.P. Separator Pressure, (psig) MPa gauge (3,800) 26 Conversion, wt % based on total feed 14 Production Rate (Ibs polymer/hr) kg polymer/hr (280) 85 Catalyst Productivity kg polymer/kg Ti Catalyst 2000 kg polymer/kg Ti 60,000 Polymer Density g/cc 0.92+ Melt Index 1.8

Claims (11)

Claims

1. A continuous process for the production of linear low density polyethylene comprising: polymerizing ethylene and at least one C4C1B alpha-olefin comonomer in the presence of a transition metal catalyst composition in an elongated tubular reaction zone at a pressure of from 69 to 345 MPa gauge (10,000 to 50,000 psig) and a temperature from 93 to 3430C (2O00F to 6500 F); reducing the pressure of the reactor effluent containing polymer product and unreacted monomers to from 10.3 to 34.5 MPa gauge (1,500 to 5,000 psig);; passing the reactor effluent of reduced pressure to a high pressure separator vessel for at least partial separation of unreacted monomer from polymer, and adding from 5 to 200 ppm, preferably 10 to 100 ppm, based on the weight of the polymer, of an antifoaming agent to the reactor effluent upstream of the separator vessel.

2. A process according to claim 1 , wherein the antifoaming agent is added in amount between 1 5 and 100 ppm based on the weight of the polymer.

3. A process according to claim 1 or 2, wherein the antifoaming agent is a dimethyl silicone having a viscosity between 1 0-3 and 6x 10-2 m2/s (1000 and 60,000 centistokes).

4. A process according to claim 3, wherein the viscosity is between 4x 1 0-3 and 3x 10-2 m2/s (4,000 and 30,000 centistokes).

5. A process according to any one of the preceding claims, wherein the antifoaming agent is added to the reactor effluent after the reduction in pressure.

6. A process according to any one of claims 1 to 4, wherein the antifoaming agent is added to the reactor effluent at the end of the polymerization zone.

7. A process according to any one of the preceding claims, wherein the catalyst composition comprises: (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. A process according to claim 7, wherein the halide of component (b) is chloride.

9. A process according to claim 7 or 8, wherein the alkyl aluminum compound is a trialkyl aluminum.

10. A process according to claim 9, wherein the trialkyl aluminum is tri-n-octyl aluminum.

11. A process according to claim 1 substantially as described with reference to the accompanying drawings.