MXPA99011828A - Polymerisation process - Google Patents

Abstract

La presente invención se relaciona con un proceso continuo en lecho fluidificado por gas para la polimerización de uno o más monómeros olefínicos, en donde la corriente gaseosa de reciclo extraída del reactor se divide en dos corrientes (A y B) de manera que:(a) una primera corriente (A) que ha sido enfriada a una temperatura a la cual se condensa líquido, se vuelve a introducir directamente en el lecho fluidificado del reactor de tal manera que, en cualquier momento, dicho líquido condensado se introduce continuamente en dicho lecho a una velocidad mínima de 10 litros de líquido por m3 de material del lecho fluidificado por hora;y (b) una segunda corriente (B), la cual no ha sido sometida a la etapa de enfriamiento/condensación anterior, se pasa a través de un intercambiador y luego se vuelve a introducir entonces en el reactor. La introducción continúa de un líquido dentro del reactor e incluso elimina problemas de ensuciamiento que pueden encontrarse en los procesos convencionales de polimerización de olefinas en fase gaseosa.

Description

POLYMERIZATION PROCESS Field of the Invention The present invention relates to a continuous process for the polymerization of gas phase olefins in a fluidized bed reactor, which process has improved levels of productivity without fouling. The present invention also relates to a process for the start-up of a continuous process for the polymerization of olefins in the gas phase in a fluidized-bed reactor, whose process has improved levels of productivity without fouling. The present invention also relates to a process for controlling the incidents during a continuous process for the polymerization of olefins in the gas phase in a fluidized bed reactor, whose process has improved levels of productivity without fouling. STATE OF THE ART Processes for the homopolymerization and copolymerization of olefins in the gas phase are well known in the art. Said processes can be carried out, for example, by introducing the gaseous monomer into a stirred and / or fluidized bed comprising polyolefin and a catalyst for polymerization. In the polymerization of fluidized bed olefins, the polymerization is carried out in a fluidized bed reactor in which a bed of polymer particles is maintained in a fluidized state by means of an ascending gas stream comprising the gaseous reaction monomer. The start-up of said polymerization generally uses a bed of polymer particles similar to the polymer that it is desired to produce. In the course of the polymerization, new polymer is generated by the catalytic polymerization of the monomer and the product polymer is removed to maintain the bed in a more or less constant volume. An industrial acceptance process uses a fluidizing grid to distribute the fluidizing gas through the bed and to act as bed support when the gas supply is interrupted. The polymer produced is generally removed from the reactor via a discharge conduit disposed in the lower portion of the reactor, near the fluidizing grid. The fluidized bed comprises a bed of growing polymer particles, product polymer particles and catalyst particles. This bed is maintained in the fluidized state by the continuous upward flow from the base of the reactor of a fluidizing gas comprising recycle gas from the top of the reactor together with replacement feed. The fluidizing gas enters the bottom of the reactor and is passed, preferably through a fluidizing grid, into the fluidized bed. The polymerization of olefins is an exothermic reaction and, therefore, it is necessary to provide means for cooling the bed to dissipate the polymerization heat. In the absence of such cooling, the bed would increase in temperature until, for example, the catalyst becomes inactive or until the bed begins to melt. In the polymerization of fluidized bed olefins, the preferred method for dissipating the polymerization heat is to supply the gas polymerization reactor with a gas, preferably the fluidizing gas, which is at a temperature below the desired polymerization temperature, passing the gas through the fluidized bed to extract the polymerization heat, separate the gas from the reactor and cool it by passing it through an external heat exchanger, and recycle it to the bed. The temperature of the recycle gas can be adjusted in the heat exchanger to maintain the fluidized bed at the desired polymerization temperature. In this alpha-olefin polymerization method, the recycle gas generally comprises the monomeric olefin, optionally with, for example, an inert diluent gas such as nitrogen, and / or a gaseous chain transfer agent such as hydrogen. In this way, the recycle gas serves to supply the monomer to the bed, to fluidize the bed and to maintain the bed at the desired temperature. The monomers consumed by the polymerization reaction are usually replaced by adding replacement gas to the recycle gas stream. It is well known that the rate of production (ie, the space-time yield in terms of polymer weight produced per unit volume of the reactor space and per unit time) in commercial gas fluidized bed reactors of the type previously indicated, is limited by the maximum speed at which the heat can be dissipated from the reactor. The rate of heat dissipation can be increased, for example, by increasing the velocity of the recycle gas and / or by reducing the temperature of the recycle gas and / or by changing the thermal capacity of the recycle gas. However, there is a limit to the velocity of the recycle gas that can be used in commercial practice. Beyond this limit, the bed may become unstable and may even rise out of the reactor along with the gas stream, leading to blockage of the recycle line and damage to the recycle gas compressor or blower. There is also a limit on the degree to which recycle gas can be cooled in practice. This is mainly determined by economic considerations and, in practice, is generally determined by the temperature of the industrial cooling water available in situ. If desired, cooling can be used, but this increases production costs. Thus, in commercial practice, the use of cooled recycle gas as the sole means for dissipating the polymerization heat in the polymerization of olefins in a fluidized bed with gas has the drawback of limiting the maximum production rates obtainable.

The state of the art suggests a number of methods for increasing the heat dissipation capacity of the recycle stream. EP 89691 is related to a process to increase the space-time yield of continuous processes in fluidized bed with gas for the polymerization of fluid monomers, whose process comprises cooling part or all of the unreacted fluids to form a mixture of two phases of gas and entrained liquid, below the dew point, and reintroduce said two-phase mixture into the rea. The description of EP 89691 states that a primary limitation on the degree to which the recycle gas stream can be cooled below the dew point resides in the need to maintain the gas to liquid ratio at a level sufficient to maintain the liquid phase of the two-phase fluid mixture in a entrained or suspended state until the liquid vaporizes, further indicating that the quantity of liquid in the gas phase should not exceed about 20% by weight and preferably not exceed 10% by weight approximately, provided that the velocity of the recycle stream of two phases is high enough to maintain the liquid phase in suspension in the gas and to support the fluidized bed inside the rea. EP 89691 further discloses that it is possible to form a two-phase fluid stream within the rea at the point of injection by injecting gas and liquid separately under conditions that will produce a two-phase stream, but little advantage is seen by the the fact of operating in this way as a consequence of the added and unnecessary tax and the cost of separation of the gas and liquid phases after cooling. EP 173261 relates to a particular means for introducing a recycle stream into fluidized bed reas and, in particular, to a means for introducing a recycle stream comprising a mixture of two phases of gas and entrained liquid, such as it is described in EP 89691 (supra). WO 94/25495 describes a fluidized bed polymerization process comprising passing a gaseous stream comprising monomer through a fluidized bed rea in the presence of a catalyst, under reactive conditions, to produce a polymer product and a stream comprising unreacted monomer gases, compressing and cooling said stream, mixing said stream with feed components and returning a gaseous phase and a liquid phase to said rea, a method for determining stable operating conditions comprising: (a) observing density changes fluidized apparent in the rea associated with changes in the composition of the fluidizing medium; and (b) increasing the cooling capacity of the recycle stream by changing the composition without exceeding the level at which a reduction in the fluidized bulk density or a parameter indicative thereof would become irreversible. US 5,436,304 relates to a process for polymerizing one or more alpha-olefins in a gas phase rea having a fluidized bed and a fluidizing medium, wherein the fluidizing medium serves to control the cooling capacity of the rea and in where the apparent density function (Z) is maintained at a value equal to or greater than the calculated limit of the apparent density function. WO 94/28032, the content of which is incorporated herein for reference purposes only, relates to a continuous process in a fluidized bed with gas, wherein the gaseous recycle stream is cooled to a temperature sufficient to form a liquid and a gas. Separating the liquid from the gas and then feeding the liquid directly into the fluidized bed at or above the point at which the gaseous stream passing through the fluidized bed has practically reached the temperature of the gas stream being extracted from the rea, the total amount of liquid that can be introduced back into the fluidized bed polymerization reactor can be increased in order to cool the bed by evaporating the liquid, thereby improving the cooling level to achieve higher levels of productivity. The separated liquid can be suitably injected into the fluidized bed by means of one or more nozzles disposed therein. The nozzles can be gas atomizing nozzles where an atomizing gas is used to facilitate the injection of the liquid, or they can be spray nozzles of the liquid only type. The processes described above all contribute to increasing the levels of productivity that can be achieved in fluidized bed polymerization processes, which is also one of the objectives of the present invention. However, it is known in the art that a major problem encountered in such high productivity polymerization processes is the fouling phenomenon that can occur at any time in the reactor. Fouling of the reactor wall is a well-known phenomenon in gas phase polymerization. During the polymerization, fines can be adhered to the wall of the reactor forming agglomerates; this can sometimes be derived from the adhesion of the catalyst and polymer particles melting in the reactor wall. Their presence very often induces disturbances in fluidification that can lead to irreversible problems. For example, when these agglomerates become heavy, they can detach from the wall and block the fluidization grid and / or the polymer extraction system. In this way, the accumulation of fines and / or agglomerates on the wall of the reactor will be referred to as the fouling phenomenon. There are a number of descriptions in the prior art concerning the phenomenon of fouling, as well as many explanations and different theories as to their appearance. It has been said that the type of catalyst is responsible for fouling; static electricity has also been indicated as a cause of fouling; the operating conditions have also been considered as important in the appearance of soiling; in fact, the expert in the field has developed so many explanations and possible different solutions as cases of occurrence of said fouling. Therefore, it would be a breakthrough in the technique if the phenomenon of fouling could be considerably reduced or eliminated, regardless of what the explanation for this phenomenon may be. It has now surprisingly been found that when fouling problems occur they can be considerably reduced and even eliminated by using the process according to the present invention. SUMMARY OF THE INVENTION A process has now been found which is based on the continuous introduction of condensed liquid into the reactor, which has no adverse effect on the composition of the fluidized bed, which does not affect the fluidization conditions inside the reactor. and which considerably reduces or even eliminates the potential phenomenon of fouling inside the reactor. DETAILED DESCRIPTION OF THE INVENTION Therefore, and in accordance with the present invention, a continuous gas-fluidized bed process is provided for polymerizing an olefinic monomer selected from (a) ethylene, (b) propylene, (c) mixtures of ethylene and propylene and (d) mixtures of (a), (b) or (c) with one or more other alpha-olefins, in a fluidized bed reactor by the continuous recycle of a gas stream comprising at least part of the ethylene and / or propylene through the fluidized bed of said reactor, in the presence of a polymerization catalyst, under reactive conditions, characterized in that said gaseous recycle stream extracted from said reactor is divided into two streams (A and B) and because: (a) a first stream (A) which has been cooled to a temperature at which liquid condenses, is reintroduced directly into the fluidized bed of the reactor such that, at any time, said condensed liquid 3 is continuously introduced into said bed at a minimum speed of 10 liters of liquid per m of fluidized bed material per hour; and (b) a second stream (B), which has not been subjected to the previous cooling / condensing step, is passed through an exchanger and then re-introduced into the reactor. According to the present invention it is now possible to initially condense part of the recycle gas stream and introduce said condensed liquid directly into the fluidized bed at a very low production rate, or preferably before production begins. In this way, the control of the polymerization reaction is more easily maintained at a constant rate during the start-up of the process and the quantity of liquid entering the fluidized bed can be controlled more easily without disturbing the fluidization characteristics of the fluidized bed. process. One of the most interesting advantages found according to the present invention is the positive influence provided by the present process on the consecutive potential polymerization problems encountered with the known high productivity polymerization process., as demonstrated in the attached examples. In particular, it has now been found that the continuous introduction of the condensed liquid into the bed at a minimum speed of 10 liters of liquid per m of fluidized bed during the whole process, ie from the first moment and at any time in a row, is it results in a considerable reduction or even in the elimination of all the aforementioned fouling problems in the polymerization. In addition, it has been found that the presence of the second stream (B) and its passage through an exchanger is a mandatory requirement according to the present invention. In fact, the fact of operating with said second stream (B) of the present invention allows the process to satisfy the balance of both heat and mass. Preferably, the condensed liquid is introduced directly into the fluidized bed above the upper limit of the temperature gradient between the inlet fluidizing gas (the gaseous stream fed to the reactor) and the rest of the bed. According to the present invention, the quantity of liquid injected directly into the fluidized bed can be controlled by regulating the proportion of the gaseous stream that is cooled to form the biphasic mixture. Through the use of the process according to the present invention, the control of the reaction is maintained at a constant rate. Likewise, the start-up of the liquid injection can be carried out at a low production rate of the plant and the commutation can be carried out from a conventional operation to low capacities when the fluidized bed is not very active. According to a preferred embodiment of the present invention, the cooling / condensing step and the introduction of the condensed liquid into the reactor bed begins before the introduction of the active catalyst into the reactor and / or before the polymerization occurs.; under these start-up conditions, the second stream (B) is sufficiently heated by the exchanger to compensate for the increase in cooling resulting from the liquid injection, thereby maintaining the process heat balance.

The respective proportions of the currents (A) and (B), where the current (A) is subjected to the cooling / condensation stage and the current (B) passes through the exchanger, depend on the stage in which the find the process. The recycle gaseous stream drawn from the reactor generally comprises one or more unreacted gaseous monomers and optionally one or more inert hydrocarbons, inert gases such as nitrogen, one or more reaction activators or one or more moderators such as hydrogen, as well as particles entrained catalyst and / or polymer (hereinafter referred to as "fines"). A majority of these fines can be conveniently separated from the recycle gas stream by means of a cyclone. The recycle gas stream, fed to the reactor, also comprises replacement monomers sufficient to replace the monomers polymerized in the reactor. The process according to the present invention is suitable for the preparation of polyolefins in the gas phase by the polymerization of one or more olefins, of which at least one of them is ethylene or propylene. Preferred alpha-olefins to be used in the process of the present invention are those having from 3 to 8 carbon atoms. However, if desired, small amounts of alpha-olefins having more than 8 carbon atoms, for example 9 to 18 carbon atoms, may be employed. In this way, it is possible to produce ethylene or propylene homopolymers or ethylene or propylene copolymers with one or more alpha-olefins Qj-Cg. Preferred alpha-olefins are but-1-ene, pent-1-ene, hex-1-ene, 4-methylpent-1-ene and oct-1-ene. Examples of higher olefins that can be copolymerized with the main ethylene or propylene monomer, or as a partial substitute for the alpha-olefin comonomer t? -C, are dec-1-ene and ethylidene norbornene. When the process is used for the copolymerization of ethylene or propylene with alpha-olefins, ethylene or propylene is present as the main component of the monomers and is preferably present in an amount of at least 65% of the total monomers. The process according to the present invention can be used to prepare a wide variety of polymeric products, for example, linear low density polyethylene (LLDPE) based on ethylene copolymers with but-1-ene, 4-methylpent-l-ene or hex -1-ene, and high-density polyethylene (HDPE) which may be, for example, homopolyethylene or ethylene copolymers with a small portion of higher alpha-olefin, for example, but-1-ene, pent-1-ene , hex-1-ene or 4-methylpent-l-ene. The liquid that condenses from the recycle gaseous stream can be a condensable monomer, for example, but-1-ene, hex-1-ene, oct-1-ene, used co or comonomer for the production of LLDPE, or can be an inert condensable liquid, for example, a hydrocarbon or inert hydrocarbons, such as an alkane or alkanes or a cycloalkane or C ^ -Cg cycloalkanes, in particular butane, pentane or hexane. It is important that the liquid vaporizes inside the bed under the polymerization conditions used, so that the desired cooling effect is obtained and an important accumulation of liquid inside the bed is avoided. Suitably at least 95% by weight, preferably at least 98% by weight, and in particular virtually all of the liquid feed to the bed, is evaporated. In the case of liquid comonomers, part of the comonomer is polymerized in the bed and said polymerization can be from the liquid and from the gas phase. The presence of an associated olefinic monomer within the bed can be easily tolerated, provided that the amounts thereof do not adversely affect the fluidification characteristics of the bed.

The process is particularly suitable for the polymerization of olefins at an absolute pressure comprised between 0.5 and 6 MPa and at a temperature comprised between 30 and 130 ° C. For example, for the production of LLDPE, the temperature is suitably of the order of 70-90 ° C and, for the production of HDPE the temperature is generally 80-105 ° C, depending on the activity of the catalyst used. The polymerization reaction can be carried out in the presence of a catalyst system of the Ziegler-Natta type, consisting of a solid catalyst essentially comprising a transition metal compound and a cocatalyst comprising an organic compound of a metal (i.e. an organometallic compound, for example, an alkylaluminum compound). High activity catalytic systems have been known for several years and they are capable of producing large amounts of polymer in a relatively short time, thus making it possible to avoid a step of separating catalytic residues from the polymer. These high activity catalyst systems generally comprise a solid catalyst consisting essentially of transition metal, magnesium and halogen atoms. It is also possible to use a high activity catalyst consisting essentially of an activated chromium oxide by a heat treatment and associated with a granular support based on a refractory oxide. The process is also suitable for use with metallocene catalysts and with Ziegler catalysts supported on silica. The catalyst can suitably be used in the form of a prepolymer powder prepared previously during a prepolymerization step with the aid of a catalyst as described above. The prepolymerization can be carried out by any suitable process, for example, polymerization in a liquid hydrocarbon diluent or in the gas phase using a batch process, a semi-continuous process or a continuous process. The first stream (A) is cooled to a temperature such that the liquid condenses in the gaseous stream of recycle. This is preferably carried out by means of one or more heat exchangers. Suitable heat exchangers are already well known in the art. The second stream (B) passes through one or more exchangers. These exchangers can cool or heat the gas stream depending on the stage of the process. According to a preferred embodiment of the present invention, the condensed liquid, produced in the first stream (A) by the cooling / condensation step, is then separated from the gas mixture before its introduction into the bed. According to yet another embodiment of the present invention, the second stream (B) is cooled by an exchanger at a temperature at which liquid condenses, the condensed liquid being separated from the stream before it is introduced into the bed. Suitable means for separating the liquid are, for example, cyclone separators, large vessels that reduce the velocity of the gaseous stream to effect separation (drier drums), gas-liquid separators of the separator type of solid or liquid particles of the gases, and liquid washers, for example, venturi scrubbers. Such spacers are well known in the art. The use of a gas-liquid separator of the type of separator of solid or liquid particles of a gas is particularly advantageous in the process of the present invention. Another advantage derived from the use of a separator of solid or liquid particles of a gas is that the pressure drop inside the separator may be less than in other types of separators, thereby improving the efficiency of the overall process. A separator for solid or liquid particles of a gas, particularly suitable for use in the process of the present invention, is a vertical gas separator, commercially available and known as "Peerless" (Type DPV P8X). This type of separator uses the coalescence of droplets of liquid on a baffle device to separate the liquid from the gas. A large liquid reservoir is provided at the bottom of the separator to collect the liquid and inside which the condensable liquid is charged before starting the cooling of the gaseous recycle stream at a temperature at which the liquid condenses. The liquid reservoir allows the liquid to be stored, thereby providing control over the introduction of the liquid from the separator into the fluidized bed. This type of separator is very effective and provides a 100% separation of condensed liquid from the gas stream. The separated liquid washes the existing fines in the baffle device, thus avoiding the fouling of the baffles. The condensed liquid produced either directly from the cooling / condensing stage or from the separator (preferred embodiment) is then preferably introduced into the fluidized bed above the upper limit of the temperature gradient between the inlet fluidizing gas and the rest of the bed . The introduction of condensed liquid can be carried out at a plurality of points within this region of the fluidized bed and said points can be at different heights within this region. The point or points of introduction of the liquid are arranged so that the local concentration of liquid does not adversely affect the fluidization of the bed or the quality of the product, 3 'so that the liquid can disperse rapidly from each of the points and vaporizes in the bed to dissipate the polymerization heat of the exothermic reaction. In this way, the amount of liquid introduced for cooling purposes can very closely approximate the maximum load that can be tolerated without disturbing the fluidization characteristics of the bed and, therefore, offers the opportunity to achieve improved levels of productivity in the reactor. The liquid, if desired, can be introduced into the fluidized bed at different heights within the bed. Said technique can facilitate improved control over the incorporation of comonomers. The controlled dosing of fluid in the fluidized bed provides useful additional control over the temperature profile of the bed and, in the case that the liquid contains comonomer, provides useful control over the incorporation of comonomer into the copolymer. The liquid is preferably introduced into the lower part of the region of the fluidized bed above the upper limit of the temperature gradient between the inlet fluidizing gas and the rest of the bed. The commercial processes for the polymerization of olefins in fluidized bed and gas phase are generally carried out under conditions of constant, practically isothermal regime. However, although almost the entire fluidized bed is maintained at the practically desired isothermal polymerization temperature, there is usually a temperature gradient in the region of the bed immediately above the point of introduction of the cooled gas stream into the bed. The lower temperature limit of this region, where the temperature gradient exists, is the temperature of the incoming cold gaseous stream, and the upper limit is the temperature of the practically isothermal bed. In commercial reactors of the type that utilize a fluidizing grid, typically 10-15 m in height, this temperature gradient normally exists in a layer approximately 15 to 30 cm above the grid. In order to obtain the maximum benefit from the cooling of the condensed liquid, it is important that the liquid injection means is disposed in the bed above the region where this temperature gradient exists, that is, in the part of the bed that it has practically reached the temperature of the gas stream leaving the reactor. The point or points of introduction of the liquid into the fluidized bed can be, for example, 50-200 cm, preferably 50-70 cm above the fluidizing grid. In practice, the temperature profile within the fluidized bed can be determined first during the polymerization using, for example, thermocouples located in or on the walls of the reactor. The point or points of introduction of the liquid are then arranged to ensure that the liquid enters the region of the bed where the gaseous recycle stream has practically reached the temperature of the gaseous stream that is being withdrawn from the reactor. It is important to ensure that the temperature within the fluidized bed is maintained at a level below the sintering temperature of the polyolefin that constitutes the bed. The gas from the second stream (B) and the separator, if used, it is recycled to the bed, usually in the lower part of the reactor. In the case of using a fluidizing grid, said recycling is normally performed to the region below the grid and said grid facilitates the uniform distribution of the gas to fluidize the bed. The use of a fluidizing grid is preferred. The process of the present invention is carried out with a gas velocity in the fluidized bed that must be greater than or equal to that required to achieve a bubbling bed. The minimum gas velocity is generally about 6 cm / second, but the process of the present invention is preferably carried out using a gas velocity in the range of 30 to 100, more preferably 50 to 70 cm / second. The catalyst or prepolymer can be introduced, if desired, into the fluidized bed directly with the stream of condensed liquid. This technique can lead to improved dispersion of the catalyst or prepolymer in the bed. By injecting the condensed liquid in the fluidized bed in this way, any catalyst that is present in the liquid can benefit from the localized cooling effect of the liquid penetration around each of the injection means, which can prevent the appearance of points hot and the consequent agglomeration. If desired, liquid or liquid soluble additives, for example, activators, cocatalysts and the like, may be introduced into the bed, together with the condensed liquid stream, separated or not. In the event that the process of the present invention is used to prepare ethylene homo- or copolymers, replacement ethylene can be conveniently introduced, for example, to replace the ethylene consumed during the polymerization, at any point in the recycle stream. downstream of the heat exchanger for cooling / condensation (A) and before its introduction into the bed (for example, below the fluidizing grid, in the case that it is used). By adding replacement ethylene at that point, the amount of liquid that can be recovered from the heat exchanger (A) can be increased and productivity can be improved thereby. The condensed liquid can be introduced into the fluidized bed through suitably arranged injection means. A single injection medium may be used, or a plurality of injection means may be disposed within the fluidized bed.

A preferred arrangement is to provide a plurality of injection means spaced substantially equally in the fluidized bed in the region of liquid introduction. The number of injection means used is that number required to provide sufficient penetration and dispersion of the liquid in each injection medium to achieve good dispersion of the liquid through the bed. A preferred number of injection means is four. Each of the injection means may be supplied, if desired, with condensed liquid by means of a common conduit suitably disposed within the reactor. This can be achieved, for example, by means of a duct that passes upwardly through the center of the reactor. The injection means are preferably arranged so that they project substantially vertically into the fluidized bed, but can be arranged so that they protrude from the walls of the reactor in a substantially horizontal direction. The preferred injection means is a nozzle or a plurality of nozzles that include gas-induced atomization nozzles where a gas is used to facilitate the injection of the liquid, or nozzles of the liquid-only spray type. Suitable gas-induced atomization nozzles and liquid-only nozzles are as described in WO 94/28032 and WO 96/20780, the content of which is incorporated herein for reference purposes only. As already indicated, the present invention requires the continuous introduction of condensed liquid into the bed at a minimum speed of 10 liters of liquid 3 per m of fluidized bed material per hour. Preferably, said speed is greater than 40 liters of liquid per m of fluidized bed material per hour. The highest rate at which liquid can be introduced into the bed depends mainly on the degree of cooling desired in the bed and this, in turn, depends on the desired production rate of the bed. The production rates that can be obtained through commercial fluidized bed polymerization processes for the polymerization of olefins depend, inter alia, on the activity of the catalysts used and on the kinetics of such catalysts. Likewise, it has been found that the present invention is particularly useful for controlling incidents that may occur during a continuous polymerization process. The usual incidents encountered in a continuous polymerization process can be, for example, the interruption of the catalyst injection, the partial poisoning of the reaction or a mechanical failure. With the known high productivity (condensation) processes, these types of incidents translate into a loss of production and a period of operation in non-condensing mode. It has been observed that the periods of operation in the non-condensing mode are detrimental to the process and lead systematically to subsequent fouling problems. It has been found in a surprising manner that the present invention, which operates continuously in the condensation mode, provides a means by which fouling problems can be substantially reduced or eliminated altogether. According to another aspect of the present invention there is provided a process of starting a continuous process in gas-fluidized bed to polymerize an olefinic monomer selected from (a) ethylene, (b) propylene, (c) mixtures of ethylene and propylene and (d) mixtures of (a), (b) or (c) with one or more other alpha-olefins, in a fluidized bed reactor by the continuous recycle of a gas stream comprising at least part of the ethylene and / or propylene through the fluidized bed of said reactor, in the presence of a polymerization catalyst, under reactive conditions, characterized in that said gaseous recycle stream extracted from said reactor is divided into two streams (A and B) and because: (a) a first stream (A) which has been cooled to a temperature at which liquid is condensed, is reintroduced directly into the fluidized bed of the reactor in such a way that, at any time, said condensed liquid is removed. 3 continuously in said bed at a minimum speed of 10 liters of liquid per m of fluidized bed material per hour; and (b) a second stream (B), which has not been subjected to the previous cooling / condensing step, is passed through an exchanger and then re-introduced into the reactor. The start-up process according to the present invention begins before the introduction of the active catalyst into the reactor and / or before the polymerization occurs. Thus, and according to this preferred embodiment, the cooling / condensation step and the introduction of the condensed liquid into the reactor bed begins before the introduction of the active catalyst into the reactor and / or before the polymerization occurs. Under these start-up conditions, the second stream (B) is sufficiently heated by the exchanger to compensate for the increase in cooling resulting from the injection of liquid, thus maintaining the heat balance of the process. According to another preferred embodiment of the present invention, the catalyst or prepolymer is introduced into the fluidized bed directly with the stream of condensed liquid, separately or not. The advantages associated with this technique are those of an improved dispersion of the catalyst in a first phase of the process, which helps to avoid the formation of hot spots during the start-up process and therefore the subsequent agglomeration. Before initiating the introduction of liquid by using the process according to the present invention, gas phase polymerization and fluidized bed polymerization can be started by charging the bed with polymer particles and then initiating the flow of liquid gas through the bed. . Description of the Drawings The processes according to the present invention will now be illustrated with reference to the accompanying drawings. Figures 1-3 schematically show the processes according to the present invention. Figure 1 illustrates a gas-phase fluidized bed reactor essentially consisting of a reactor body (9) which is in general a vertical cylinder having a fluidizing grid located at its base. The reactor body comprises a fluidized bed (11) and a velocity reduction zone (12) which is generally of greater cross section compared to the fluidized bed. The gaseous reaction mixture exiting the upper part of the fluidized bed reactor constitutes the gaseous recycle stream and is passed through the line (13) to a cyclone (14) for the separation of most of the fines. The separated fines can be suitably returned to the fluidized bed. The gaseous recycle stream leaving the cyclone passes to a compressor (15). The gaseous recycle stream is then separated into a first stream (A) and a second stream (B). The stream (A) is passed through a heat exchanger (16) where it is cooled to a temperature at which liquid is condensed and then re-introduced directly into the fluidized bed of the reactor. The stream (B) is passed through an exchanger (18) and then introduced back into the reactor below the grid. Said gas is passed through the fluidizing grid to the bed, thereby ensuring that the latter is maintained in a fluidized state. A valve (17) is used to regulate the respective quantities of gaseous streams A and B. Catalyst or prepolymer is fed to the reactor via line (20) into the condensed liquid stream. The product polymer particles are extracted from the reactor by line (21). Figure 2 illustrates a preferred embodiment for carrying out the process of the present invention. In this arrangement, and after the cooling / condensing step in the heat exchanger (16), the resulting gas-liquid mixture is passed to the separator (22) where the liquid is separated from the gas. The separated liquid, coming from the separator (22), is introduced again directly into the reactor bed (9). A pump (23) is suitably located downstream of the separator (22). The gas leaving the separator is recycled to the lower part of the reactor (9).

Figure 2 illustrates another arrangement for carrying out the process of the present invention wherein the gas leaving the separator is reintroduced together with the gas stream (B) Figure 2 illustrates a further arrangement for carrying out the process of the present invention, according to which the compressor (15) is located after the separation of the gaseous recycle stream by the separator (22). This has the advantage that the compressor has to compress a smaller amount of gas and, therefore, can be of a reduced size, thus achieving an optimization and improved cost of the process. Figure 3 illustrates another embodiment for carrying out the process of the present invention. According to this arrangement, both recycle lines (A) and (B) are equipped with a gas / liquid separator (22, 24). Examples of the Invention The process according to the present invention will now be illustrated further with reference to the following Examples. Example 1 300 kg of an anhydrous polyethylene powder, such as a nucleation bed, was introduced into a 74 cm diameter fluidized bed reactor under nitrogen. A gaseous mixture heated to 90 ° C was then introduced into the reactor. The ascending speed was 38 cm / second. The components of the gas mixture and their respective partial pressures were as follows: - hydrogen: 0.35 MPa - ethylene: 0.5 MPa - pentane: 0.35 MPa - nitrogen: 0.8 MPa. schematic representation of the apparatus / process used in the present example. The valve on line A was regulated so that the gas velocity is 400 kg / hour (line A) which represents approximately 3.1% of the gas velocity of total recycle. The dew point of the gas mixture was 66 ° C. The temperature at the outlet of the exchanger located in the recycle line A was decreased in order to reach 65 ° C. Condensation occurred in the exchanger; the condensed liquid, ie pentane, was separated from the gas phase (as indicated in Figure 2, in separator 22) and reintroduced directly into the fluidized bed through a gas / liquid nozzle located at 0.6 m above the fluidizing grid. The liquid flow rate (pentane) was 10 liters per m fluidized bed per hour. Simultaneously, and in order to maintain the temperature inside the reactor at about 90 ° C, the temperature of the exchanger located in the recycle line B was correspondingly increased. In fact, said exchanger B needs to compensate for the usual thermal loss in the recycling line, as well as the cooling performed by the evaporation of liquid in the reactor. The injection of condensed liquid was maintained for approximately 30 minutes before the injection of the catalyst. A conventional Ziegler Natta catalyst was then introduced into the reactor at a rate of 20 g hour together with a triethylaluminum cocatalyst. The production increased progressively until reaching a constant production of 100 kg / hour of polyethylene. The outlet temperature of the heat exchanger located on line A and the gas flow through it were still regulated in order to obtain a flow velocity 3 of condensed liquid (pentane) of approximately 10 liters per m of fluidized bed per hour. The polymerization was carried out under stable conditions. No fouling of the reactor was observed. Comparative Example 2 The operation performed in this example was similar to that performed in the Example 1 except that the entire recycle gas flowed through line A and, thus, the by-pass line B was not used. In order to maintain the temperature of 90 ° C inside the reactor before the At the beginning of the polymerization, the temperature of the exchanger situated in said line A is consequently increased. Therefore, no condensation occurs in this exchanger. The catalyst was injected following the same procedure as in Example 1 except that no condensate was present in the recycle line during said start of the catalyst injection process. After approximately two hours of production, polymer crusts were found in the production. Detrimental fouling of the reactor was also observed. Comparative Example 3: simulation of incidents in the process A stable gas phase polymerization process was carried out in a 74 cm diameter reactor under the following conditions: the reactor contained 800 kg of an active polyethylene powder; the components of the gas mixture and their respective partial pressures were: - ethylene: 0.3 MPa - hydrogen: 0.21 MPa - pentane: 0.33 MPa - nitrogen: 0.76 MPa The dew point of the gas mixture was of 66 ° C. The rising gas velocity filé of 38 cm / second. A conventional Ziegler Natta catalyst was introduced into the reactor as a prepolymer at a rate of 1 kg hour; Triethylaluminum cocatalyst in pentane was continuously introduced at a rate of 600 ml / h.

The polyethylene production was approximately 200 kg / h. The polymerization temperature was 90 ° C. All the recycle gas flowed through line A; Line B was not used. Under these conditions, and in order to maintain the polymerization temperature of 90 ° C, the temperature of the exchanger (line A) was sufficiently cooled to approximately 62 ° C (ie, below the dew point of the gas mixture). . The condensed liquid (pentane) was separated from the recycle gas in a separator and reintroduced into the reactor through a gas / liquid nozzle located 60 cm above the fluidizing grid. The liquid injection speed was 1000 liters 3 per m fluidized bed per hour. In order to simulate mechanical failure, the catalytic prepolymer injection was stopped. Production decreased progressively. Therefore, the cooling requirement of the exchanger (line A) decreased until the temperature of said exchanger passed above the dew point of the gas mixture, so that no more condensed liquid was produced. In this stage, (no injection of condensed liquid into the bed), polyethylene production reached a value of approximately 100 kg / hour. Approximately 40 minutes after the injection of condensed liquid was stopped, hot spots were detected through thermocouples located on the wall. The polymerization was stopped. When the reactor was opened, it was observed that part of the bed had melted. It presented the appearance of a large agglomerate. Example 4: simulation of incidents in the process The conditions of the process were exactly the same as those used in Comparative Example 3. After the simulation of the incident, production decreased and the injection of condensed liquid (pentane) was also decreased, as in Comparative Example 3. When said flow rate of condensed liquid reached 40 liters per 3 m fluidized bed per hour, (which corresponds to a production of PE of 136 kg hour), part of the recycle gas was passed through the exchanger located on line B where the temperature was maintained at 72 ° C approximately (ie, about 5 ° C above the dew point of the gas mixture). Under these conditions it was possible to maintain the temperature at the outlet of the exchanger located on line A at 65 ° C approximately, that is, below the dew point of the gas mixture. The respective flow rates through lines A and B were regulated so that they were approximately 14.4% of the total flow velocity passing through line A, to maintain a liquid velocity. condensate 3 of around 40 liters per m of fluidized bed per hour. The temperature inside the reactor was maintained at 90 ° C. The production of polyethylene decreased progressively and correspondingly increased the temperature of the exchanger located in line B. No hot spots were recorded during the whole procedure and later no agglomerates were observed, so that high rates can be obtained without any problem of productivity.

Claims (9)

1. NOVELTY OF THE INVENTION Having described the present invention is considered as a novelty and, therefore, what is claimed as a novelty is contained in the following claims: 1.- A continuous process in fluidized bed by gas to polymerize an olefinic monomer selected from (a) ethylene, (b) propylene, (c) mixtures of ethylene and propylene and (d) mixtures of (a), (b) or (c) with one or more different alpha-olefins, in a fluidized bed reactor by recycle continuous of a gaseous stream comprising at least part of the ethylene and / or propylene through the fluidized bed of said reactor, in the presence of a polymerization catalyst, under reactive conditions, characterized in that said recycle gas stream extracted from said reactor is divided in two streams (A and B) and because: (a) a first stream (A) that has been cooled to a temperature at which liquid is condensed, is reintroduced directly into the fluidized bed of the reactor in such a way that, at any time, said condensed liquid is continuously introduced into said bed at a minimum speed of 10 liters of liquid per m of fluidized bed material per hour; and (b) a second stream (B), which has not been subjected to the previous cooling / condensing step, is passed through an exchanger and then re-introduced into the reactor.
2. 2. A process according to claim 1, characterized in that the condensed liquid is introduced directly into the fluidized bed above the upper limit of the temperature gradient between the inlet fluidizing gas and the rest of the bed.
3. 3. A process according to any of the preceding claims, characterized in that the second stream (B) is sufficiently heated by the exchanger to compensate for the increase in cooling resulting from the injection of liquid, thereby maintaining the heat balance of the process. .
4. 4 - A process according to any of claims 1 and 2, characterized in that the second stream (B) is cooled by the exchanger to a temperature at which liquid condenses, the condensed liquid being separated from the stream before its introduction into the bed.
5. 5. A process according to any of claims 1 to 3, characterized in that the condensed liquid is separated from the gas stream before its introduction into the bed.
6. 6. A process for the start-up of a gas-fired fluidized bed continuous process according to claim 1, for polymerizing an olefinic monomer selected from (a) ethylene, (b) propylene, (c) mixtures of ethylene and propylene and (d) mixtures of (a), (b) or (c) with one or more other alpha-olefins, in a fluidized bed reactor by continuously recycling a gas stream comprising at least part of the ethylene and / or propylene through the fluidized bed of said reactor, in the presence of a polymerization catalyst, under reactive conditions, characterized in that said gaseous recycle stream extracted from said reactor is divided into two streams (A and B) and because: (a) a first stream (A) that has been cooled to a temperature at which liquid condenses, is reintroduced directly into the fluidized bed of the reactor such that, at any time, said condensed liquid is continuously introduced into said bed at a minimum speed of 10 liters of 3 liquid per m fluidized bed material per hour; and (b) a second stream (B), which has not been subjected to the previous cooling / condensing step, is passed through an exchanger and then re-introduced into the reactor.
7. 7. A process according to claim 6, characterized in that the second stream (B) is heated by the exchanger to compensate for the increase in cooling resulting from the injection of liquid, thereby maintaining the heat balance of the process.
8. 8 - A process according to any of claims 6 or 7, characterized in that the polymerization catalyst is introduced into the fluidized bed directly with the stream of condensed liquid.
9. 9. A process according to any of claims 6 or 7, characterized in that the introduction of the condensed liquid in the reactor bed is initiated before the introduction of the active catalyst into the reactor and / or before the polymerization occurs.