MXPA97002102A

MXPA97002102A - Procedure and apparatus for alfa-olefi gaseous phase polymerization

Info

Publication number: MXPA97002102A
Application number: MXPA/A/1997/002102A
Authority: MX
Inventors: Covezzi Massimo; Galli Paolo; Govoni Gabriele; Rinaldi Roberto
Original assignee: Montell Technology Company Bv
Priority date: 1995-07-20
Filing date: 1997-03-19
Publication date: 1997-08-01

Abstract

The present invention relates to a process for the gas phase polymerization carried out in two interconnected polymerization zones, to which one or more alpha-olefins CH2 = CHR are fed in the presence of catalyst under reaction conditions and from which the polymer product is discharged, the process is characterized in that the increasing polymer flows through a first polymerization zone under conditions of rapid fluidization, leaves the first zone and enters the second polymerization zone through which it flows in a densified form under the action of gravity leaves the second zone and is reintroduced to the first polymerization zone, thus establishing a circulation of polymer around the two polymerization zones: the novel procedure allows the olefins to be polymerized in the gas phase with high productivity per unit volume of the reactor without incurring problems of the lec ho fluidizado of the most advanced technique known

Description

PROCEDURE AND APPARATUS FOR ALFA-OLEFINAS GASEOUS PHASE POLYMERIZATION DESCRIPTIVE MEMORY The present invention relates to a process for the gas phase polymerization of defines carried out in two interconnected polymerization zones to which one or more α-olefin CH2 = CHR are fed in the presence of a catalyst under the polymerization conditions and from which the produced polymer is discharged. In the process of the present invention, the particles of the growing polymer flow through a first polymerization zone under conditions of rapid fluidization, leave said first zone and enter a second polymerization zone through which they flow in a form densified under the action of gravity, leave said second zone and are reintroduced in the first polymerization zone, in this way establishing a circulation of the polymer between the two polymerization zones. The development of catalysts with high activity and selectivity of the Ziegler-Natta type and, in more and more applications, of the metallocene type, has led to extensive use in an industrial scale of processes, where the polymerization of the olefins is carried in a gaseous medium in the presence of a solid catalyst. In comparison with the more conventional technology in the liquid suspension (of rnonómero or mixtures of onero ero / sol entre), this technology has the following advantages: a) operational flexibility: the reaction parameters can be optimized on the basis of the characteristics of the catalyst and the product and are not limited by the physicochemical properties of the liquid mixtures of the reaction components (generally, including hydrogen as a chain transfer agent); b) expanding the scale of the product: the swelling effects of the growing polymer particle and the solubilization of the polymer fractions in a liquid medium greatly reduce the production scale of all types of copolymers; c) minimize the downstream of the polymerization operations: the polymer is obtained directly from the reactor in the form of a dry solid and requires simple operations for the removal of the dissolved monomers and deactivation of the catalyst . All the technologies invented so far for the gas phase polymerization of the α-olefins provide for the maintenance of a polymer bed, through which the reaction gases flow; this bed is maintained in suspension both by means of mechanical agitation (stirred bed reactor) and by means of fluidization obtained by recirculating the reaction gases (fluid bed reactor). In both reactor types, the composition of the rnonomer around the polymer particle in the reaction is obtained sufficiently constant due to the induced stirring. These reactors are very close to the ideal behavior of the "continuous stirred tank reactor" (CSTR), making it relatively easy to control the reaction and thus ensuring the consistency of product quality when operating under steady-state conditions. The fluid reactor that operates under the conditions of "Bubbles" is, until now, the most established industrial technology. The polymer is limited in a vertical, cylindrical zone. The reaction gases leaving the reactor are absorbed by a centrifugal compressor, cooled and returned to the bottom of the bed through a distributor, together with monomers formed and with appropriate amounts of hydrogen. The entrance of the solid to the gas is limited by an appropriate dimension of the upper part of the reactor (dead work, that is, the space between the bed surface and the gas deviation), where the gas viscosity is reduced, and, in some designs, by the interposition of cyclones in the gas outlet line .. The flow velocity of the gas in circulation is established in order to ensure a fluidization velocity within a scale above the minimum fluidization velocity and below the " transport speed ".

The heat of reaction is removed exclusively by cooling the circulating gas. The catalyst components are fed continuously. The composition of the gas phase controls the composition of the polymer. The reactor is operated at a constant pressure, usually on the 1-3 MPa scale. The reaction kinetics is controlled by the addition of inert gases. A significant contribution to the reliability of fluid bed reactor technology in the polymerization of α-olefins was made by the introduction of a properly pre-treated spheroid catalyst of controlled dimensions and by the use of propane co or diluent (see UO 92/21706 ). Fluid bed technology has limits, some of which are discussed in more detail below. ñ) Removal of the reaction heat. The maximum fluidization velocity is subject to quite narrow limits (which already comprise the volumes of the reactor to be decoupled which are equal to or greater than those filled by the fluid bed). Depending on the heat of the reaction, the dimensions of the polymer and the density of the gas, a limit is reached for reactor productivity (expressed as output per hour per reactor cross-sectional unit), where operation with temperatures is desired of gas inlet greater than the dew point of the gas mixture. This limit can lead to reductions in the output of the plant, in particular in the copolyzing of ethylene with larger α-olefins (hexene, octene), which is carried out with conventional Ziegler-Natta catalysts, requiring gas compositions. rich in such defines. Many ways have been proposed to overcome the limits of traditional technology, in terms of heat removal, based on the partial condensation of the gases in circulation and the use of the latent heat of evaporation of the condensates to control the temperature in the inside the reactor (see EP-89691, USP 5,352,749, UO 94/28032). Although all the systems proposed to implement the principle technically deserve consideration, they make critical the operation of fluid reactors. In particular (and apart from the problems associated with the distribution of wet solids in the full chamber below the distribution grid), the technology used in EP-89691 and USP 5,352,749 patents has the turbulence generated by the grid for distribute the liquid on the polymer. The possible phenomenon of coalescence in the plenum can cause an uncontrollable phenomenon of poor distribution of liquid with the formation of agglomerates that can not be dispersed again, in particular in the case of polymers that have a tendency to stick. The criterion of discrimination given in USP 5,352,749 reflects situations under steady state conditions, but does not offer any possible guidance for situations of still a "reaction leakage" treatment, which can lead to an irreversible loss of fluidization, with a consequent collapse of the reactor. The method described in patent UO 94/28032 involves the separation of the condensates and their distribution on the grid by means of special nozzles properly located. In fact, condensates inevitably contain solids under reactive conditions, whose concentration can be very high at low amounts of condensate. In addition, the inherent difficulty of uniformly distributing a suspension over a number of nozzles can compromise the operability of some of them and a block in a nozzle adversely affects the distribution of the evaporating liquid in the relevant section of the reactor. It is also clear that the efficiency of the operation depends on a vigorous circulation of solids in the reactor and, below the injection points, this is reduced by an imbalance of the gas flow velocities caused by large amounts of condensates. further, any need for maintenance in a nozzle requires completely shutting down the reactor. B) Molecular weight distribution As already mentioned, a fluid bed shows a behavior directly comparable with an ideally mixed reactor (CSTR). It is generally known that, in the continuous polymerization of a-olefins in a single agitation step (which also involves the stable composition of the rnonomers and the chain transfer agent, usually hydrogen) with Ti catalysts of the Ziegler-Natta type , polyolefins having a relatively narrow molecular weight distribution are obtained. This feature is even more emphasized when rnetalocene catalysts are used. The amplitude of the molecular weight distribution has an influence on both the rheological behavior of the polymer (and thus the processability of the melt) as well as the final mechanical properties of the product, and it is a property that is particularly important for the (co) ethylene polymers. For the purpose of expanding the molecular weight distribution, the procedures based on several reactors in series have obtained industrial importance, in each of which it is possible to operate at least at different concentrations of hydrogen. A problem that typically also arises from these procedures is an insufficient homogeneity of the product. The homogeneity of the material is particularly critical in blow molding processes and in the production of thin films, where the presence of even small amounts of inhomogeneous material results in the presence of particles infused into the film ("fish eyes"). "). In patent application EP 574,821, a two-reactor system is proposed that operates under different polymerization conditions with mutual recirculation of the polymer between the two reactors. Even if the concept is adequate to solve the problem of product homogeneity, as shown by the experimental results, such a system involves investment costs and operational complexity. In other cases, the broad molecular weight distribution polymers are obtained by the use of mixtures of different Ziegler-Natta catalysts in a single-reactor, each catalyst being prepared in order to give a different response to hydrogen. It is clear that a mixture of granules, each with its own individuality, is obtained at the reactor outlet. It is difficult to obtain the homogeneity of the product by means of this route. C) Product discharge The technology to polymerize the α-olefins in gas phase reactors has developed rapidly in recent years, and the scale of the polymers obtainable in this way has been greatly expanded. In particular, apart from the ethylene and propylene copolymers, a wide range of copolymers can be produced industrially, for example: propylene / ethylene random copolymers, propylene / ethylene higher α-olefins and higher propylene α-olefins; low and very low density polyethylenes (LLDPE, VLDPE), modified with higher α-olefins containing from 4 to 8 carbon atoms; heterophasic copolymers of high impact force, obtained by the growth of the active centers of the catalyst, in successive stages, of one or more polymers listed above and of ethylene / propylene or ethylene / butene rubbers; and rubbers of EPR and EPDM. In short, in the polymers producible in the gas phase, the modulus of flexibility varies from 2300 MPa to values less than 100 MPa, and the fraction of soluble xylene varies from 1% to 80%. The flow, compactability and stickiness properties turn out to be extremely variable as a function of the degree of crystallinity, molecular weight and composition of the different phases of the polymer. Many of these products remain granulated and fluid (and thus processable) as long as they are maintained in a fluid or flowing state, which are conditions under which the static forces between the individual solid particles have no effect. More or less, they tend quickly to knead and form aggregates if they can be established or compact in stagnant areas; this phenomenon is particularly improved under reaction conditions where, due to the combined action of temperature and the large amount of dissolved hydrocarbons, the polymer is particularly soft, compressible and compactable, and sticky. The characterization of the soft and sticky polymers is effectively described in EP-348,907 or USP 4,958,006. The most direct solution for the discharge of polymer from the reactor consists of a direct discharge of the fluid bed through a controlled valve. This type of discharge combines simplicity with the advantage of not producing stagnant areas. Where a sufficiently low pressure of the discharge valve is maintained downstream (in the 0.5-3 gauge barium scale), the reaction is virtually stopped either by the reduction in temperature due to the evaporation of the monomers dissolved in the polymer or by the low partial pressure of the monomers in the gas: in this way, any risk in the downstream of the reactor receiving equipment is avoided. However, it is known that the amount of gas discharged with the polymer from a fluid bed through an orifice reduces very high values as a function of the reactor pressure, the fluidization rate, the density of the solids in the bed, etc. (See, for example: Maesi illa, "Flow properties of the fluidized dense phase" in "Fluidization" (Flow properties of the dense fluid phase, in Fluidization) p.651-676, eds Davidson R Harrison, Academic, New York, 1971). The high amounts of gas discharged with the polymer represent investment costs and operating costs, it being necessary to recompress this gas to recover the pressure of the reactor from the receiving pressure. In many industrial applications, discharge systems are discontinued in this way, with the interposition of at least two hoppers in alternating operation. For example, USP 4,621,952 discloses a discharge system in which the polymer is transferred intermittently and at high differential pressures from the reactor to the settling tank. The amount of polymer movement that, during the filling phase, first strikes the walls of the scrubber tank and then on the basis of the polymer, compact material that loses its properties of flowability. During the filling phase the pressure in the filling tank increases rapidly to the value of the reactor pressure and the temperature does not change significantly. The reaction will proceed adiabatically to high kinetics. With soft and viscous products, this easily leads to the formation of agglomerates that can not be granulated, with the consequent difficulties with the discharge to the receiver tank below. Similar observations apply to U.S. Patent 4,703,094. The limits of the intermittent system are clearly revealed by the proposals for complicated continuous systems. JP-A-58 032,634 provides the installation of an internal screw in the reactor to compact the polymer towards the discharge; U.S. Patent 4,958,006 proposes the installation of an extruder, the screws of which are supplied directly into the fluidized base reactor. Apart from the complication and the difficulty of the industrial application, the proposed systems are in any case completely unsuitable for feeding the polymer to a subsequent reaction stage. One of the polymerization processes has been discovered and this represents a first aspect of the present invention, which allows olefins to be polymerized in the gas phase with high hourly yield per unit volume of the reactor, without incurring the problems of the fluidized bed technologies of the state known in the art. A second aspect of the present invention relates to an apparatus for carrying out this method. The polymerization process is carried out in the gas phase of the present invention in a first and in a second interconnected zone of polymerization to which one or more α-olefins CH2 = CHR are supplied, wherein R is hydrogen or radical of hydrocarbon having 1-12 carbon atoms, in the presence of a catalyst under reaction conditions and from which the polymer produced is recharged. The process is characterized in that the particles of the polymer in development flow through the first in said polymerization zones under conditions of rapid fluidification, leave said first polymerization zone and enter the second of said polymerization zones through which they flow in a densified form under the action of gravity, leave said second polymerization zones and are reintroduced in said first polymerization zone thus establishing a circulation of the polymer between the two polymerization zones. As is known, the state of rapid fluidification is obtained when the velocity of the fluidizing gas is higher than the transport velocity, and is characterized in that the pressure gradient along the transport direction is a monotonic function of the amount of injected solid, for the flow velocity and equal density of fluidizing gas. Contrary to the present invention, in the fluidized bed technology of the state of the art, the velocity of the fluidising gas is maintained well below the transport speed in order to avoid phenomena of entrainment of solids and transfer of particles. The terms transport speed and rapid fluidization state are well known in the art; for a definition thereof, see, for example, "D. Geldart, Gas Fluidization Technology, page 155 et seq., 3. Uiley 8 Sons Ltd., 1986." In the second polymerization zone, where the polymer flows in densified form under the action of gravity, high values of the density of the solid are reached (solid density = kg of polymer per m 3 of the reactor occupied by the polymer), which they approximate the volumetric density of the polymer; a positive increase in the pressure in the flow direction can thus be obtained so that it becomes possible to reintroduce the polymer in the first reaction zone without the aid of special mechanical means. In this way, an "interlaced" circulation is established, which is defined by the equilibrium of the pressures and the two polymerization zones and by the loss d? load entered in the system. The invention is described with reference to the appended figures, which are given for illustrative purposes without limiting the invention, in which: Figure 1 is a schematic representation of the method according to the invention, Figure 2 is a schematic representation of a first embodiment of the method according to the invention, and FIG. 3 is a schematic representation of a second embodiment of the method according to the invention. Referring to Figure 1, the developing polymer flows through the first polymerization zone 1 under conditions of rapid fluidization in the direction of the arrow 14; in the second polymerization zone 2, the developing polymer flows in densified form under the action of gravity in the direction of the arrow 14 '. The two polymerization zones 1 and 2 are appropriately interconnected by sections 3 and 5. The equilibrium of the material is maintained by supplying monomers and catalysts and discharging the polymer. Generally the rapid fluidization condition is established in the first polymerization zone 1 by means of supplying a gas mixture consisting of 1 or more α-olefins CH2 = CHR (line 10) to said zone 1; preferably, the supply of the gas mixture below the point of reintroduction of the polymer to said first zone 1 is effected by the use, where appropriate, of gas distributing means, such as, for example, a distributor grid. The velocity of the transport gas to the first polymerization zone is higher than the transport velocity under operating conditions and is preferably between 2 and 15 rn / s, most preferably between 10 and 8 m / s. The control of the polymer circulating between the two polymerization zones can be effected by metering the amount of polymer leaving the second polymerization zone 2, using suitable means for controlling the flow of the solids, such as, for example, mechanical valves (sliding valve, V-ball valve, etc.) or non-mechanical valves (valve in L, valve in 3, counter-shutter, etc.). Generally, the polymer and gas mixture leaving the first polymerization zone 1 are led to a solid separation zone / gas 4. Solid separation / gas can be effected by using conventional separation means such as, for example , a separator of the inert type or preferably of the centrifugal type, or a combination of the two. The centrifugal separator (cyclone) can be of the axial, spiral, helical or tangential type. From the separation zone 4, the polymer enters the second polymerization zone 2. The gaseous mixture leaving the separation zone 4 is compressed, cooled and transferred, if appropriate with the addition of compositional and / or organic compounds. molecular weight regulators, the first polymerization zone 1. This transfer can be effected by means of a recirculation line 6 for the gaseous mixture, equipped with means for understanding 7 and cooling 8 and means for supplying the monorres and regulator of the molecular weight 13. Part of the gas mixture leaving the separation zone 4 can be transferred, after being compressed, to the connection zone 5 through the line 9, in order to facilitate the transfer of the polymer from the second to the first polymerization zone. Preferably, the various catalyst components are supplied to the first polymerization zone 1, at any point of said first polymerization zone 1. However, they can also be supplied at any point of said second polymerization zone 2. Any type of catalyst used in the polymerization of olefins can be used in the process of the present invention, since it is not important that it is in any particular physical state and catalysts can be used either in solid or liquid form, because, in contrast to the procedures in the gas phase of the known state of the art the process of the present invention does not necessarily require the use of catalysts in which at least one component is in granular form, but can be carried out with catalysts in which the various components are in solution. For example, catalysts based on titanium, chromium, vanadium or zirconium can be used in either supported or unsupported form. Examples of catalysts that can be used are described in USP 4,748,272, USP 4,302,566, USP 4,472,520 and USP 4,218,339. The catalysts of controlled morphology are particularly suitable, which are described in patents USP 4,399,054, USP 5,139,985, EP-395,083, EP-553,805, EP-553,806 and EP-601,525, and in general catalysts capable of giving polymers in the form of spheroidal particles having an average dimension of between 0.2 and 5 m, preferably between 0.5 and 3 mm. The process of the present invention is also particularly suitable for the use of metallocene catalysts, either in solution or supported. The various catalyst components can be introduced at the same point or at different points of the first polymerization zone. • The catalyst can be supplied without pretreatment or in the pre-polished form. In the cases in which other ascending flow polymerization stages are located, it is also possible to supply the polymerization zones of a catalyst dispersed in a polymer suspension coming from an upflow mass reactor, or a catalyst dispersed in a polymer dry coming from a gas phase reactor in upflow.

The polymer concentration in the reactive zones can be verified by the usual methods known in the state of the art, for example by measuring the differential pressure between two suitable points along the axis of the polymerization zones or by measuring the density by means of nuclear detectors (for example, t-rays). Operating parameters such as, for example, temperature, are those which are customary in gas phase olefin polymerization processes, for example between 50 ° C and 120 ° C. The process according to the present invention has many advantages. The interlocking configuration allows the adoption of relatively simple geometric configurations of the reactor. In practice, each reaction zone can be taught as a cylindrical reactor with a high aspect ratio (height / diameter ratio). From the point of view of construction, this particular geometric configuration of the reactor allows the adoption of high operating pressures, which are not economical in conventional fluidized base reactors. The process according to the present invention can thus be carried out at operating pressures between 0.5 and 10 MPa, preferably between 1.5 and 6 MPa. The consequent high density of the gas favors both the exchange of heat on an individual particle as well as the total elimination of the heat of reaction. It is therefore possible to choose the operating connections that improve the reaction kinetics.

In addition, the reactor through which the polymer flows under conditions of rapid fluidization (first polymerization zone) can be fully filled at polymer concentrations that can reach or exceed 200 km / m3. With the contribution of the second polymerization zone and taking into account the most favorable kinetic conditions that can be established, the process of the present invention makes it possible to obtain specific productivity (hourly output per unit volume of the reactor) which are much higher than those of the reactor. levels obtainable with the conventional technology of fluidized bases. It is thus possible to equal or even exceed the catalytic products of conventional gas phase processes, using polymerization equipment of much smaller dimensions, with a significant saving in the cost of construction of the plant. In the process according to the present invention, the entrainment of solids in the gaseous recirculation line at the outlet of the solid / gas separation zone and the possible presence of liquids leaving the cooler on the same line do not limit the efficiency of the first polymerization zone. Even when using gas distributing means such as, for example, a grid, the gas transport velocities in the plenum below the grid are still high and such as to ensure the entrainment of the small droplets of still considerable dimensions and of the polymer moistened, without static points. Since the transport gas comes into contact with the flow of the hot polymer arriving from the second polymerization zone, the vaporization of any liquid is virtually instantaneous. It is therefore possible to cool the gaseous mixture leaving the solid / gas separation zone at temperatures below the dew point in order to condense part of the gases. The gas / liquid mixture that is formed is then supplied to the first polymerization zone where it contributes to the elimination of heat without encountering the problems and limits of the known state of the art and without requiring the use of complicated devices proposed to avoid them. In addition to and / or replacing the partial condensation of the recirculating gases, the method of the invention opens a new path to the elimination of the heat of reaction. The characteristic geometric configuration (high surface / volume ratio) of the polymerization zone with rapid fluidization makes a significant external surface area available for direct heat exchange over this zone (and therefore with maximum heat transfer between the liquid cooling and the reaction system). Where it is convenient, additional or alternative surfaces for the exchange of heat inside the reactor may be present. The first polymerization zone can thus be cooled advantageously with external cooling means. The high turbulence associated with fast fluidization conditions and a high gas density ensure in each case a very high heat transfer coefficient. Any condensation on the inner walls is continuously eliminated by the strong radial and axial mixture of the polymer due to the conditions of rapid fluidification. In addition, this feature makes the proposed technology suitable for operation as a second stage supplied directly from an upflow mass reactor. It is also possible to supply part of the composition monomers in condensed form without any difficulty. As regards the elimination of the heat of reaction, the capacities offered by the method of the invention are superior to the known state of the art and the difficulties inherent in the previous technologies are solved. In addition, the volumetric ratios of the circulating gas do not necessarily depend on the heat exchange requirements. Advantageously, one or more inert gases are maintained in the polymerization zones, in quantities such that the sum of the partial pressures of the inert gases are preferably between 5 and 80% of the total gas pressure. The inert gas can be nitrogen or an aliphatic hydrocarbon having from 2 to 6 carbon atoms, preferably propane. The presence of the inert gas has numerous advantages, since it makes it possible to moderate the reaction kinetics while at the same time maintaining the total reaction pressures, which are sufficient to keep the height of the circulation compressor low and ensure a flow velocity of suitable mass for the exchange of heat on the particle in the bed and, through the cooler in the circulating gas mixture, for the elimination of the heat of reaction that has not been eliminated by the surfaces. In the process of the present invention, the presence of the inert gas has additional advantages, since it makes it possible to limit the temperature increase in the second polymerization zone, which operates in an essentially adiabatic mode, and also makes it possible to control the extension of the molecular weight distribution of the polymer, particularly in the polymerization of ethylene. This is because, as already indicated, the polymer flows vertically down through the second flow-through polymerization zone (packed flow period), surrounded by limited amounts of entrained gas. As is known, the molecular weight of the polymer in the polymerization of ethylene is controlled by the hydrogen / ethylene ratio in the gas phase and, to a lesser extent, by the temperature. In the presence of inert gases, since the reaction consumes ethylene but hydrogen only to a marginal degree, the ethylene / hydrogen ratio decreases along the polymer flow axis in the direction of motion, causing growth of polymer fractions in the same particle with decreasing molecular weights. The increase in temperature due to the reaction helps this effect. It is therefore possible, by means of an adequate balance of the gas composition and the residence times in the two polymerization zones, to effectively control the extension of the molecular weight distribution of the polymers while at the same time Maintains maximum product homogeneity. Conversely, if it is desired to produce polymers with a narrower molecular weight distribution, the mechanism described above can be restricted or avoided by appropriate selection of the reaction conditions, for example by limiting the amount of inert gas or by charging an adequate amount of gas. of reaction and / or formation monomer or monomers at suitable positions in the second polymerization zone. Advantageously, the gas that is charged to the second polymerization zone can be taken from the gaseous mixture leaving the solid / gas separation zone, after which it has been compressed. The amount of gas introduced is preferably set within values such that the relative velocity of the gas injected with respect to the flow velocity of the solid is maintained below the minimum fluidization velocity characteristic of the solid / gas system present in said second zone. polymerization. Under these conditions, the final polymer flow is not substantially altered. The operational flexibility of the methods of the invention is therefore total; the production of polymers of different molecular weight distribution is controllable by the gas composition, and if desired, by simple closing or opening of a valve in a gas line. Advantageously, the polymer can be discharged from areas where the density of solids is higher, for example, from suitable points in the second polymerization zone where large quantities of the densified fluid polymer are available, to minimize the amount of entrained gas. By inserting a control valve at a suitable point at the beginning of the exit region of the polymer from the second polymerization zone, the withdrawal of the produced polymer can be continuously controlled. The amount of gas that accompanies the polymer is extremely small and only slightly greater than that which can be achieved by means of the interposition device of a series of hoppers in alternating intermittent operation. In this way, all the limitations of the discharge systems of the known state of the art are overcome, with respect to the amount of gas entrained, as well as to the nature of the discharge products. As already indicated, the method of the present invention can be combined with conventional technologies in a sequential multi-stage process in which, in the initial part or in the final part of a polymerization section operated in accordance with the present invention, there are one or more polymerization steps using conventional technologies (in volume or in the gas phase, either in a fluidized bed or in a stirred bed). Also multi-step processes are possible where two or more steps are carried out with the method of the present invention. Furthermore, it is possible to combine the process according to the present invention with conventional gas-phase fluidized bed technologies by interposing between the two polymerization zones, as defined in the present invention, a polymerization zone using a bubble fluid bed , that is to say with gas fluidization velocities greater than the minimum fluidization velocity and lower than the transport velocity, while the closed circulation characteristic of the process of the present invention is always maintained. For example, a possible embodiment provides that the second polymerization zone consists of a first and a second section. In the first of said sections (with respect to the polymer flow in the final part) a fluidized bed is maintained by properly charging and distributing gases; in the second section, properly connected to the first, the polymer flows in a densified form by gravity. From the second section, the polymer is reintroduced in the first polymerization zone, keeping the circulation closed. With an adequate disengagement of the different zones, an enlargement of the molecular weight distribution of the polymer can be achieved, while retaining all the advantages described above. The above example is only one of the possible embodiments of the process of the invention which, in its general definition, comprises at least one zone of rapid fluidization interconnected with an area where the polymer flows in a densified form by gravity. The process of the present invention is applicable to the preparation of a large number of defined polymers without the disadvantages described above. Examples of polymers that can be obtained are: high density polyethylenes (HDPEs), having relative densities greater than 0.940), including ethylene copolymers and ethylene copolymers with α-olefins having from 3 to 12 carbon atoms; linear low density polyethylenes (LLDPEs) having relative densities less than 0.940) and very low density and ultra low density (VLDPEs and ULDPEs, which have lower relative densities of 0.920 to 0.880), which consist of ethylene copolymers with one or more α-olefins having from 3 to 12 carbon atoms; Ethylene and propylene elastomeric terpolymers with minor proportions of diene or ethylene and propylene elastomeric copolymers with a content of ethylene-derived units of between about 30 and 70% by weight; Isostatic polypropylene and crystalline copolymers of propylene and ethylene and / or other α-olefins having a content of propylene-derived units of more than 85% by weight; Heterophasic propylene polymers obtained by sequential polymerization of propylene and mixtures of propylene with ethylene and / or other α-olefins; Atactic polypropylene and amorphous copolymers of propylene and ethylene and / or other α-olefins containing more than 70% by weight of units derived from propylene; Poly-α-olefins, such as for example α1-butene-1, poly-4-ethyl-1-pentene; Polybutadiene and other polydiene rubbers. A further aspect of the present invention relates to an apparatus for the gas phase polymerization of α-olefins. The apparatus of the invention comprises a first vertical cylindrical reactor 20 equipped with a catalyst charging line 34, and a second vertical cylindrical reactor 30 equipped with a polymer discharge system 23 and characterized by: the upper region of the first reactor 20 it is connected by means of a first line 21 to a solid separator / gas 22 which in turn is connected to the upper region of the second reactor 30; the lower region of the second reactor 30 is connected by a second line 31 to the lower region of the first reactor 20; and the solid / gas separator 22 is connected by means of a recirculation line for the gas mixture 36 to the first reactor 20 in a region 37 at the bottom of said first reactor 20 below the entry point of the second line 31. Preferably, the first reactor 20 is equipped with gas distribution means 33, for example a grid, located between the entry point of the second line 31 and the region 37 at the bottom of this reactor. As an alternative, with reference to Figure 3, the gas distributor means in the first reactor 60 can be replaced by a cylindrical line 65, through which the gas flows at high speed and which is connected to the reactor 60 by means of a truncated cone section 62 whose inclination angle towards the vertical is preferably less than 45 ° and preferably between 30 and 10 °. Advantageously, both the catalyst (the complete line 66) and the polymer coming from the second reactor 70 through the line 77 can be transported through this truncated cone connection. A first valve 24 is generally inserted to control the flow rate of the polymer between the second reactor 30 and the second line 31. This valve can be mechanical or non-mechanical type. In the case where the gas distributing means 33 are present, part or all of the catalyst components can advantageously be injected through a third line 32 to said first reactor 20 at a point above the gas distributing means. Advantageously, the recirculation line for the gaseous mixture 36 is equipped with a compressor 26, a cooling system 27 and systems for introducing, together or separately, monomers 28 and molecular weight regulator 29. Two cooling systems can be present, one in the initial part and another in the final part of the compressor. Preferably, the first line 21 leaves the upper region of the first reactor laterally, observing that a lateral exit of the solid / gas mixture from the first reactor contributes substantially to the dynamic stability of the entire reaction system. The upper region of the first reactor 20 can have a cylindrical configuration with a diameter equal to that of the reactor or preferably it can be of truncated cone geometry with the wide end at the highest part. The first line 21 may be horizontal or may have a slope in the direction of gravity to facilitate the discharge of the polymer (see the configuration of line 71 in Figure 3). The second line 31 can be suitably tilted downward and can be connected (at a point immediately towards the end of the first valve 24) by a line 25 to the gas recirculation line 36 at a point towards the end of the compressor 26. From this In this manner, the flow of polymer is aided by the gas stream under pressure coming from the recirculation line, avoiding stagnant areas of polymer in the same line and in the point of introduction to the reactor 20. The connection system between The lower regions of the reactors can also be of the type described in Figure 3, in which the circulation of the polymer is obtained by means of a pneumatic valve L, 74, operated by the gas taken from the recirculation line through line 75. The valve L is connected to a line 77 leading to the first reactor 60, said line 77 is connected via the line 76 to the recirculation line 81. Through this line, the polymer is brought back into the reactor 60 by means of a suitable gas stream coming from line 76. The first reactor 20 can be advantageously equipped with external cooling means 35, such as wall heat exchangers. Two possible embodiments of the invention are illustrated in Figure 2 and Figure 3. These embodiments are for illustrative purposes only and do not limit the invention. With reference to Figure 2, 20 represents the first reactor operating under conditions of rapid fluidization and represents the second reactor through which the polymer flows in a densified form under the action of gravity; 21 and 31 are lines connecting the upper and lower regions of the two reactors; 34 is the catalyst charge line; 22 is a solid separator / gas; 23 is a polymer discharge system; 36 is the recirculation line for the gaseous mixture connecting said separator to a region 37 at the bottom of the first reactor; 24 is a control valve for controlling the flow rate of the polymer; 33 is a gas distributor device; 32 is a line for loading the catalyst; 26 is a compressor and 27 is a cooling system for the recirculation gas mixture; 28 and 29 are systems for monomer loading and molecular weight regulator; 25 is a line connecting the recirculation line 36 to line 31; 35 is the external cooling system of the first reactor 20. Referring to Figure 3, 60 represents the first reactor operating under conditions of rapid fluidization and 70 represents the second reactor through which the polymer flows in a densified form under the action of gravity; 71 and 70 are lines connecting the upper and lower regions of the two reactors; 66 is the catalyst feed line; 72 is a solid / gas separator; 73 is the polymer discharge system; 81 is the recirculation line for the gaseous mixture, which connects said separator 72 with a line 65 connected to the base of the first reactor 60 by a frusto-conical section 62; 74 is an L-shaped valve for controlling the flow rate of the polymer; 79 is a compressor and 80 is a cooling system for the gaseous recirculation mixture; 63 and 64 are feeding systems for onomer and molecular weight regulator; 75 is a line connecting the recirculation line 81 with the L-valve 74; 76 is a line connecting recirculation line 81 to line 77; 78 is a line connecting the recirculation line 81 to a region at the bottom of the second reactor 70; 61 is the external cooling system for the first reactor 20. The following examples will also illustrate the present invention without limiting its scope.

EXAMPLES General Polymerization Conditions. Polymerizations were carried out in a plant comprising a precontact section, where the various catalyst components were pre-mixed, a pre-pollination section and a gas phase polymerization section carried out in a reactor of the type described in the Figure 2. A solid catalyst component prepared according to the procedure described in Example 3 of EP-A-395083, triethylaluminum (TEAL) and a silane compound were previously contacted in hexane at 10 ° C for 10 minutes in the precontact container. The activated catalyst was fed to the prepolymerization section where the propylene was polyezed into a suspension using propane and dispersion media. Monomer feed and residence time were adjusted to obtain the desired prepolymerization yields, in terms of grams of polymer per gram of solid catalyst component. The prepolymer was continuously fed to the gas phase polymerization apparatus. The apparatus, which is described with reference to Figure 2, consists of two cylindrical reactors 20 and 30, connected by tubes 21 and 31. The reactor 20 is equipped with a heat exchanger 35. The rapid fluidization in the reactor 20 was achieved. recirculating gas from the gas / solid separator 22 to the bottom of the reactor 20, through a gas recirculation line 36. Gas distribution means were not used, the recirculation gases being directly fed to a region 37 in the bottom of the reactor 20, below the inlet point of the tube 31. The gas recirculation line was equipped with a compressor 26 and a heat exchanger 27. The prepolymer suspension reactor was fed to the reactor 20 at a point immediately above the entry point of the tube 31. The polymer recirculation was controlled through an L-valve 24 operated by a gas stream 25 turned from the recirculation line 36. The constituents were fed to the recirculation line 26. The polymer produced was continuously discharged from the reactor 30, through the tube 23. The total volume of the apparatus (ie, the reactors 20 and 30 in addition to the connection zones 21 and 31 ) was 250 1.

EXAMPLE 1 Polypropylene was prepared using a catalyst comprising dicyclopentyl-di-ethoxysilane (DCPMS) as the eilane compound. In the gaseous phase polymerization step, propane was used as the inert gas.

Main operating condition Precontact pass - TEAL / solid component (weight) 8 - TEAL / DCPMS (weight) 3 Pre-polymerization step - Performance (g / g) 100 Gas-phase polymerization - Temperature (° C) 85 - Pressure (barias mam.) 25 - Propylene (% mol) 91 - Propane (% mol) 8 - Hydrogen (% mol) 1 - Specific productivity (Kg / h 3) 140 Product characteristics - Volumetric Density (kg / 1) 0.45 EXAMPLE 2 Hexene modified LLDPE was prepared using a catalyst comprising cyclohexyl ethyl dirnetoxy silane (CMMS) as the silane compound. In the gas phase polymerization step, propane co or inert gas was used.

Main operating condition Precontact pass - TEAL / Ti (weight) 120 - TEAL / CMMS (weight) 20 Pre-polymerization step - Performance (g / g) 400 Gas-phase polymerization - Temperature (° C) 75 - Pressure (barias man ) 24 - Ethylene (% mol) 15 - 1-hexene (% mol) 1. 5 - Hydrogen (% mol) 3 - Propane (% mol) 80.5 - Specific productivity (g / h 3) 80 Product characteristics - molten bath index E (g / 10 min) 1.4 - Density (g / cm3) 0.908 The above-reported temperature was measured in the upper part of the reactor 30. The dew point of the gas mixture at the operating pressure is 66 ° C. The cooling fluid was recirculated in the heat exchanger 35 in such a way that a temperature of 63 ° C was obtained on the surface of the reactor 20. Under these conditions, the gas mixture partially condensed on the wall of the reactor, thus helping to remove the heat of reaction. There were no problems of errors during the operation.

Claims

NOVELTY OF THE INVENTION CLAIMS

1. - A process for the gas phase polymerization of α-olefins CH2 = CHR, wherein R is hydrogen or a hydrocarbon radical having 1-12 carbon atoms, carried out in a first and a second interconnected polymerization zone , to which one or more of said α-olefins are fed in the presence of a catalyst under reaction conditions and from which the polymer product is discharged, wherein the growing polymer particles flow through the first of said zones polymerization under conditions of rapid fluidization, leave said first polymerization zone and enter the second of the polymerization zones through which flow in a densified form under the action of gravity, leave said second polymerization zone and are introduced in the first polymerization zone, thus establishing a polymer circulation between the two polymerization zones.

2. A process according to claim 1, further characterized in that said fluidization conditions are stabilized by feeding a gaseous mixture comprising one or more of said α-olefins CH2 = CHR to said first polymerization zone.

3. - A method according to claim 2, further characterized in that said gaseous mixture is fed to the first polymerization zone in a region below the point of reintroduction of the polymer to said first polymerization zone.

4. A method according to claim 3, further characterized in that the supply of said gaseous mixture is carried out by means of gas distribution.

5. A process according to any of the preceding claims, further characterized in that the polymer and the gas mixture leaving said first polymerization zone are transported to a solid / gas separation zone and the polymer leaving said separation zone of solid / gas enters the second polymerization zone.

6. A method according to any of the preceding claims, further characterized in that the control of the polymer circulating between the two polymerization zones is affected by the dosage of the amount of the polymer leaving the second polymerization zone.

7. A process according to any of the preceding claims, further characterized in that the introduced polymer is continuously removed from the second polymerization zone. ß.- A method according to any of the preceding claims, further characterized in that the catalyst components are fed to the first polymerization zone. 9. A process according to any of the preceding claims, further characterized in that any of the reaction zones is fed by a catalyst in a polyered form. 10. A process according to any of the preceding claims, further characterized in that any of the reaction zone is fed with a catalyst dispersed in a polymer suspension. 11. A process according to any of the preceding claims, further characterized in that any of the reaction zone is fed with a catalyst dispersed in a dry polymer. 12. A process according to claim 5, further characterized in that the gaseous mixture leaving the solids / gas separation is compressed, cooled and transferred, if appropriate with the addition of consitution monomers, to the first polymerization zone . 13. A process according to claim 5, further characterized in that a part of the gaseous mixture leaving the solid / gas separation zone is used to transfer the polymer from the second zone to the first polymerization zone. 14. A method according to claim 5, further characterized in that a part of the gaseous mixture leaving said solid / gas separation zone is compressed and transferred to the second polymerization zone at the locations of the region where the polymer leaves the second zone. 15. A process according to claim 12, further characterized in that the gaseous mixture leaving the solid / gas separation zone is cooled to temperatures below the dew point. 16. A method according to any of the preceding claims, further characterized in that said first polymerization zone is cooled by external cooling means. 17. A process according to any of the preceding claims, further characterized in that the monomer or monomers of consitución are fed at least in a partially condensed form to the first polymerization zone. 1

8. A method according to claim 2, further characterized in that the velocity of the fluidizing gas to the first polymerization zone is between 2 and 15 m / s, preferably between 3 and 8 m / s. 1

9. A process according to claim 1, further characterized in that the polymer is in the form of spheroidal particles having average dimensions between 0.2 and 5 mm, preferably between 0.5 and 3 mm. 20. A method according to claim 1, further characterized in that the working pressure is between 0.5 and 10 MPa, preferably between 1.5 and 6 MPa. 21. A method according to claim 1, further characterized in that one or more inhertes gases are present in the polymerization zone at partial pressures of between 5 and 80% of the total gas pressure. 22. A process according to claim 21, further characterized in that the inert gas is nitrogen or an aliphatic hydrocarbon having 2-6 carbon atoms, preferably propane. 23. A method according to claim 1, further characterized in that an intermediate polymerization zone, which operates with a fluid bed, is interposed between the first and second polymerization zones. 24. An apparatus for the gas phase polymerization of α-olefins, comprising: a first vertical cylindrical reactor 20 equipped with a catalyst feed line 34; and a second vertical cylindrical reactor 30 equipped with a polymer discharge system 23; the upper region of said first reactor 20 being connected by a first line 21 to a solid / gas separator 22 which in turn is connected to the upper region of the reactor 30; the lower region of the second reactor 30 being connected by a second line 31 to the lower region of the first reactor 20; and the solid / gas separator 22 being connected by means of a recirculation line for the gas mixture 36 to the first reactor 20 in a region 37 at the bottom of the first reactor 20 below the entry point of the second line 31. 25 .- An appliance in accordance with the claim 24, further characterized in that the first reactor 20 is equipped with gas distributing means 33 located between the entry point of the second line 31 and the region 37 at the bottom of the first reactor 20. 26.- An apparatus according to the claim 24, further characterized in that a first control valve 24 for controlling the flow rate of the polyrnerator is interposed between the second reactor 30 and the second line 31. 27.- An apparatus according to the claim 25, further characterized in that said valve 24 is a mechanical valve. 28. An apparatus according to claim 25, further characterized in that said valve 24 is a non-mechanical valve. 29.- An appliance in accordance with the claim 25, further characterized in that said catalyst feed line 34 is connected through a third line 32 to the first reactor 20 at a point above said gas distributing means 33. 30.- An apparatus according to claim 24, further characterized in that said recirculation line for the gas mixture 36 is equipped with a compressor 26, a cooling system 27 and systems for introducing monomers 28 and molecular weight regulator 29. 31.- An apparatus in accordance with the claim 24, further characterized in that the first line 21 leaves the upper region of the first reactor 20 laterally. 32. An apparatus according to claim 24, further characterized in that T.a upper region of the first reactor is frusto-conical geometry with the wide end that is higher. 33.- An apparatus in accordance with the claim 30, further characterized in that said recirculation line for the gaseous mixture 36 is connected at a point downstream of the compressor 26, through a line 25 to the second line 31. 34.- An apparatus according to claim 24, characterized in addition because said first reactor 20 is equipped with external cooling means 35.