Polymerization Process

The present invention relates to processes of producing olefine polymers in two or more stages.

According to the present invention there is provided a process for the production of an olefine polymer by effecting polymerization in at least two stages wherein, in one stage, polymerization is effected in a liquid medium which is at least one liquid olefine monomer which is being polymerized in the presence of an olefine polymerization catalyst, a product mixture of a solid olefine polymer and a liquid olefine monomeric phase is removed from the liquid polymerization stage, the liquid olefine monomeric phase is partially separated from the solid olefine polymer to give a concentrated mixture of solid olefine polymer and liquid olefine monomeric phase, which concentrated mixture has a higher content of solid olefine polymer than is in the mixture removed from the liquid polymerization stage and the concentrated mixture is passed to a subsequent polymerization stage in which polymerization is effected in the substantial absence of a liquid phase by contacting at least one gaseous olefine monomer with the solid olefine polymer in the presence of the olefine polymerization catalyst.

The concentrated mixture contains some of the liquid olefine monomeric phase and this concentrated mixture is passed to a polymerization stage in which one or more olefine monomers are polymerized in the gaseous phase in the substantial absence of a liquid phase. The liquid olefine monomeric phase present in the concentrated mixture may be vaporized before it is introduced into the subsequent polymerization stage, but it is preferred to introduce the concentrated mixture into the subsequent polymerization stage and to cause the liquid olefine monomeric phase to vaporize in the subsequent polymerization stage, the vaporization of the liquid monomer providing some cooling to the system.

The liquid olefine monomeric phase can be partially separated from the solid olefine polymer by any suitable technique and it is preferred to effect this partial separation by passing the mixture tangentially into a hydro-cyclone. Using a hydro-cyclone, the monomeric material separated can remain in the liquid phase and this can be recycled directly to the liquid olefine polymerization stage. By using a hydro-cyclone, a partial classification of the solid particles suspended in the liquid olefine monomeric phase can be effected. Some of the solid particles are relatively small and these particles can contain olefine polymerization catalyst which has effected only a small amount of polymerization. The small particles tend to remain suspended in the liquid olefine monomeric phase and thus are recycled to the liquid olefine polymerization stage. In this manner, the catalyst removed from the liquid olefine polymerization stage has, in general, produced an appreciable quantity of olefine polymer. A hydro-cyclone contains no moving parts and can be operated at high pressure. Thus, the use of a hydro-cyclone provides a convenient method of obtaining a partial separation of the liquid olefine monomeric phase from the solid olefine polymer under high pressure conditions.

It will be appreciated that the process of the present invention is applicable to olefine monomers which are volatile liquids under the polymerization conditions since some of the olefine monomer from the liquid olefine polymerization stage is vaporized either before, or after, the concentrated mixture is introduced into the subsequent polymerization stage. Any olefine monomers which are volatile liquids can be polymerized by the process of the present invention and suitable monomers which can be used as the liquid monomeric phase are propylene, butene-1and 4-methylpentene-1.The olefine monomer, or monomers, polymerized in the subsequent stage may be the same as used in the liquid olefine polymerization stage, but other gaseous olefine monomers may be used, for example ethylene. The present invention may be used to effect homopolymerization of a liquid olefine monomer in one stage and to effect copolymerization of the same olefine monomer, in the vaporized state, with ethylene in the subsequent stage.

Thus, in a particular embodiment of the present invention, liquid propylene is polymerized in one stage in the presence of an olefine polymerization catalyst, a mixture of liquid propylene monomer and solid polypropylene is partially separated and the concentrated mixture of solid polypropylene and liquid propylene monomer is passed to a subsequent polymerization stage in which the solid polypropylene is contacted with a gaseous mixture of ethylene and propylene in the presence of the olefine polymerization catalyst.

Since the amount of the liquid olefine monomeric phase present in the concentrated mixture can be appreciable, particularly when the olefine monomer has been vaporized, it may be preferred to effect a copolymerization process by contacting the solid olefine polymer with a gaseous olefine monomer, or monomers, in at least two stages.

Thus the concentrated mixture is passed to a stage in which the liquid olefine monomer is vaporized and the vaporized olefine monomer is polymerized, at least partially, and the mixture of solid olefine polymer and gaseous olefine monomer is passed to a further stage in which copolymerization is effected by contacting the polymer with a different gaseous olefine monomer, or a mixture of gaseous olefine monomers. Using this procedure, all of the olefine monomer introduced into the stage in which the concentrated mixture is introduced can be introduced as the liquid olefine monomer present in the concentrated mixture, but if desired additional olefine monomer, which may be different from that in the concentrated mixture, can also be introduced. The olefine monomer, or monomers, introduced into the further stage, and any olefine monomer introduced in addition to the concentrated mixture, can be introduced either in the liquid phase, and then vaporized, or in the gaseous phase. Since an additional cooling effect is generally obtained if liquid monomers are introduced and then vaporized, it is generally preferred to add the olefine monomers in the liquid phase.

In order to effect polymerization it is necessary that a polymerization catalyst is present in a catalytically active form at each stage of the polymerization. Depending on the nature of the polymerization catalyst, it may be necessary to introduce a quantity of fresh catalyst to each stage in the polymerization. However, in general, the catalysts can retain polymerization activity for a sufficient time to effect polymerization at all stages of the process, and are carried through the process with the solid olefine polymer as it is formed.

The polymerization conditions in the various stages are dependent on the olefine monomer, or monomers, used and the nature of the catalyst. Typically the temperature is at least 40.degree. C. and not more than 100.degree. C., particularly at least 50.degree. C. and not more than 80.degree. C. The pressure will be such as to maintain the olefine monomer in the liquid phase and when the monomer is propylene will be at least 225 psi gauge at 40.degree. C. and at least 630 psi gauge at 90.degree. C., it being appreciated that the critical temperature of propylene is 91.9.degree. C.

In the subsequent stage, in which polymerization is effected in the presence of a gaseous olefine monomer, at least a part of the gaseous olefine monomer will be an olefine monomer as used in the liquid form in the liquid olefine polymerization stage. In order to ensure that the olefine monomer as used in the liquid olefine polymerization stage is in the gaseous form in the subsequent stage, it is necessary to ensure either that the temperature of the subsequent stage is higher than that of the liquid olefine polymerization stage or, that the partial pressure of the olefine monomer as used in the subsequent polymerization stage is less than the partial pressure of that monomer in the liquid polymerization stage. Conveniently both stages are operated at essentially the same temperature, and the subsequent stage is operated at a lower pressure.

Thus, if propylene is being polymerized, the temperature in the subsequent stage is conveniently at least 40.degree. C. and may be as high as 100.degree. C., and the propylene partial pressure is correspondingly less than 220 psi gauge at 40.degree. C. and less than 620 psi gauge at 90.degree. C.

The total pressure in the subsequent stage is preferably less than the monomer partial pressure in the liquid polymerization stage but, if copolymerization is being effected, then the total pressure in the subsequent stage can be higher than the pressure in the liquid polymerization stage, without any condensation of the monomers occurring in the subsequent stage.

The process of the present invention is conveniently effected using a chain transfer agent in one or more stages of the polymerization, in order to control the molecular weight of the polymer obtained. A suitable chain transfer agent is hydrogen, and this is conveniently used in amounts of up to 10% molar relative to the olefine monomer, or monomers, the amount used being dependent on the nature of the desired polymer, the monomers used and the polymerization conditions, particularly the temperature.

As a further aspect, the present invention also provides a polymerization apparatus comprising a first polymerization vessel, means for withdrawing a liquid phase comprising a solid olefine polymer and a liquid olefine monomeric phase from the first polymerization vessel and for passing said liquid into a separation vessel, means for withdrawing, from the separation vessel, a concentrated mixture of solid olefine polymer and the liquid olefine monomeric phase, said concentrated mixture having an increased content of solid olefine polymer compared to the liquid phase introduced into the separation vessel, means for passing the concentrated mixture to a second polymerization vessel and means for maintaining the second polymerization vessel under conditions of temperature and pressure such that the olefine monomer forming the liquid olefine monomeric phase in the first polymerization vessel, is in the gaseous phase.

The separation vessel is preferably a hydro-cyclone and the hydro-cyclone is provided with means for the tangential introduction of the liquid phase from the first polymerization vessel.

The separation vessel is conveniently provided with a means for withdrawing a liquid phase, having a reduced content of solid olefine polymer, and returning this liquid phase directly to the first polymerization vessel.

The means for withdrawing the concentrated mixture from the separation vessel may include a valve whereby the concentrated mixture can be withdrawn intermittently from the separation vessel. However, it is preferred to arrange the separation vessel to operate in a manner such that there is a continuous withdrawal of the concentrated mixture from the separation vessel.

The polymerization apparatus can include a third polymerization vessel, means for withdrawing a mixture of a solid olefine polymer and a gaseous olefine monomeric phase from the second polymerization vessel and passing this mixture into the third polymerization vessel and means for passing into the third polymerization vessel at least one olefine monomer which is different from the monomer forming the liquid olefine monomeric phase withdrawn from the first polymerization vessel.

If desired, the polymerization apparatus may include more than three polymerization vessels, any further polymerization vessels being either subsequent to, but similar to, the third polymerization vessel, or preceding, and similar to, the first polymerization vessel. The terms "subsequent" and "preceding" are used herein with reference to the passage of solid olefine polymer through the polymerization apparatus.

The second, third and subsequent polymerization vessels should be suitable for effecting gas/solid reactions and can be fluidized bed reactor systems, stirred bed reactor systems or ribbon blender type reactor systems.

The first polymerization vessel is provided with means for introduction of a liquid olefine monomeric phase and for the introduction of a polymerization catalyst. However, it will be appreciated that it may be found convenient to introduce at least one catalyst component with the liquid olefine monomeric phase. Thus, it may be convenient to provide at least two means for the introduction of the liquid olefine monomeric phase, each stream of the liquid olefine monomeric phase containing, dissolved or suspended in it, a different component of the polymerization catalyst. If there is a vessel preceding the first polymerization vessel, then a liquid phase containing solid olefine polymer and a liquid olefine monomeric phase will be introduced into the first polymerization vessel from the preceding vessel. This liquid phase may contain sufficient of the polymerization catalyst and it will be unnecessary to provide a separate means for introducing the polymerization catalyst into the first polymerization vessel.

Polymerization is typically effected using a catalyst of the Ziegler type. Catalysts of this type are well-known and comprise a compound of a transition metal of Groups IVA to VIA of the Periodic Table together with an organo-compound of aluminum or of a non-transition metal of Groups IA to IIA of the Periodic Table. Using polymerization catalysts such as the Ziegler catalysts which include at least two components, it is possible either to mix together the catalyst components and introduce the mixture into the polymerization vessel, or to separately introduce the catalyst components into the polymerization vessel. It will be appreciated that Ziegler catalysts may contain more than two components and may include Lewis Base compounds. Catalysts containing more than two components may be found to be particularly useful in the process of the present invention.

In order that the present invention is more readily understood, one embodiment thereof will now be described with reference to the accompanying drawing which is a flow diagram of a polymerization apparatus containing three polymerization vessels.

Referring to the diagram, a polymerization vessel 1, having a stirrer 2, is provided with an inlet pipe 3 located at the base of the vessel 1 and an inlet pipe 4 located in the side wall of the vessel 1. The pipes 3 and 4 are provided with valves 5 and 6 respectively. The vessel 1 contains a reaction mixture 7 which consists of a solid olefine polymer and a liquid olefine monomer.

An outlet pipe 8 is located in the side wall of vessel 1, near the bottom thereof, and the pipe 8 forms a tangential inlet to a hydro-cyclone 9. An outlet pipe 10 at the bottom of the hydro-cyclone 9 is connected to a valve 11. From the valve 11 a connecting pipe 12 leads to a further polymerization vessel 13.

The hydro-cyclone 9 is provided with a liquid outlet 14 which leads to a pump 15 from which a return pipe 16 leads to the vessel 1.

The polymerization vessel 13 is provided with a stirrer 17 and contains a reaction mixture 18 which consists of a solid olefine polymer and a gaseous olefine monomer. The vessel 13 is also provided with an outlet pipe 19 which is connected to a valve 20. From the valve 20 a connecting pipe 21 leads to a further polymerization vessel 22.

The vessel 22 is provided with a stirrer 23 and contains a reaction mixture 24 which consists of a solid olefine polymer and a gaseous olefine monomer. At the base of the vessel 22 is located an inlet pipe 25 which is provided with a valve 26. An outlet pipe 27, provided with a valve 28, is located near the bottom of the side wall of vessel 22.

The stirrers 17 and 23 are of a type which will thoroughly agitate a bed of solid particle, for example anchor-type stirrers.

The illustrated apparatus can be used to prepare a propylene copolymer as hereafter described.

Liquid propylene monomer is introduced into the vessel 1 through inlet pipe 3, with stirrer 2 being rotated. At least one catalyst component is introduced through the inlet pipe 4. The liquid propylene monomer introduced through the pipe 3 conveniently contains an organo-aluminum compound, for example diethyl alluminum chloride, and a suspension of titanium trichloride in heptane is introduced through the pipe 4, the titanium trichloride and the organo-aluminum compound together forming the polymerization catalyst.

There is a continuous flow of liquid monomer, containing suspended solid, through the outlet pipe 8, into the hydro-cyclone 9, through outlet 14 and pump 15 and then along return pipe 16 back to the polymerization vessel 1.

Within the hydro-cyclone 9, the large particles of solid polymer are concentrated and collect at the bottom of the hydro-cyclone 9, and are continuously withdrawn from the hydro-cyclone 9 at a controlled rate through outlet pipe 10, valve 11 and connecting pipe 12 and passed into the vessel 13.

Most of the small particles contained in the liquid monomer passing into the hydro-cyclone 9 are retained in the liquid propylene monomer and returned to the vessel 1. These small particles are predominantly catalyst particles on which no appreciable amount of polymerization has occurred.

The liquid propylene monomer contained in the concentrated mixture passed into the vessel 13 vaporizes as it enters the vessel. The vaporized propylene is polymerized by the catalyst contained in the polymer particles.

From the vessel 13 there is a continuous controlled flow, of a mixture of solid polymer and gaseous propylene, through the outlet pipe 19, valve 20 and connecting pipe 21 to the vessel 22. Ethylene gas is introduced into the vessel 22 through the inlet pipe 25 and the ethylene and residual propylene are copolymerized onto the polymer particles. There is a continuous controlled flow of a polymer product, together with unpolymerized monomers, through outlet pipe 27 and valve 28.

The stirrers 17 and 23 are operated at a rate sufficient to provide an agitated bed of the solid polymer. The valves 11, 20, 26 and 28 are adjusted to maintain constant conditions within the vessels 1, 13 and 22 and, if desired, all of these valves, and the valve 5, are connected electrically to a control device (not shown) whereby the adjustment of the valves is continuously varied to control the conditions in the vessels 1, 13 and 22.

It will be appreciated that the vessels 1, 13 and 22 can be provided with outlets (not shown) for withdrawing gaseous monomer from the vessels, and these outlets may be connected to a recycle system (not shown) whereby the monomers are recycled in a known manner.

Using the apparatus illustrated to prepare a copolymer of propylene with ethylene, typical operating conditions are as follows;

Othervariations on the present invention will be apparent to the skilled worker and these may be effected without departing from the scope of the present invention as herein defined.