Apparatus and process for introducing a suspension into a reactor

Abstract: The present invention relates to an apparatus for introducing a predetermined volume of suspension into a reactor from a storage vessel. According to the invention the storage vessel 1 is connected via a first conduit 4 to an intermediate vessel 2 which is in turn connected to a feed vessel 3 via a second conduit 2, the feed vessel 3 is connected via a third conduit 8 to the reactor 11, each of the first, second and third conduits 4, 6, 8 , is provided with a shut-off valve 5, 7, 9, the vessels, conduits and valves being so arranged that the suspension can be transferred by gravity from the storage vessel 1 to the intermediate vessel 2 and from the intermediate vessel 2 to the feed vessel 3 and can be transferred from the feed vessel 3 to the reactor 11 by gravity and/or the effect of pressure difference between the feed vessel 3 and the reactor 11. The present invention also relates to a process for introducing a predetermined volume of suspension into a reactor using the apparatus.

Description: The present invention relates to an apparatus for introducing a suspension into a reactor, in particular for introducing a suspension into a gas-phase alpha-olefin polymerisation reactor, and to a process using the apparatus of the invention. A fluidized-bed polymerization reactor system generally comprises an upright elongate reactor in which the particles of the polymer being formed are kept in the fluidized state by an upwardly flowing fluidization gas stream containing the alpha-olefin(s) to be polymerized. In the lower part of the reactor, there is usually a perforated or porous plate, commonly known as a fluidization grid, which enables the fluidization gas stream, which enters the reactor below the fluidization grid, to be distributed into the fluidized bed to keep the polymer particles in the fluidized state above the grid. The reactor system also usually comprises a recycle line connecting the top of the reactor to the bottom of the reactor below the fluidization grid. This recycle line generally comprises a compressor for circulating the gas mixture leaving the top of the reactor and at least one heat exchanger for cooling the gas mixture to remove heat of reaction before the gas mixture is returned to the bottom of the reactor. The reactor is provided with means for introducing the alpha-olefin(s), catalyst and other components of the reaction system, as well as means for removing the polymer product. Catalysts suitable for polymerising alpha-olefins in gas phase fluidized-bed reactors are well known in the art. The catalyst can be introduced into the reactor as a suspension in a liquid. Japanese Patent Application published under n°58-79533 discloses apparatus for feeding a catalyst suspension into a reactor, which apparatus comprises a storage vessel equipped with an external recycle line by which the suspension can be withdrawn from and returned to the storage vessel. A feed line connects the recycle line to the reactor. With this apparatus, the suspension can be continuously withdrawn from the storage vessel and fed either to the reactor or returned to the storage vessel. Thus, the suspension can be fed intermittently to the reactor even though it is continuously withdrawn from the storage vessel. The storage vessel is equipped with a mechanical stirrer and the recycle line has a pump. The use of the pump is likely to give rise to the appearance of very fine catalyst particles due to the grinding or attrition of the latter. Fine catalysts particles can cause problems, particularly in the operation of a gas-phase polymerisation reactor. Fine particles present in a fluidised bed reactor can be entrained out of the reactor with the fluidising gas and may cause blockage of equipment in the recycle conduit or downstream of the reactor. US 4398852 describes apparatus for adding solid particles directly from a container into a reactor by gravity. Such an introduction does not use a liquid and cannot be effected by a pressure difference between the container and the reactor since the container is pressurised to substantially the same pressure as the reactor. GB 1333738 describes a process for introducing solid catalytic particles into a fluidised bed reactor by use of a metering valve used to introduce sequentially small quantities of particles into the reactor. The introduction of solid particles into a reactor requires less precision than the introduction of solid particles in a suspension. A process and apparatus have now been found for introducing solid particles in suspension in liquid into a reactor, while overcoming or at least mitigating the problem referred to above. More particularly the apparatus according to the invention does not require the use of a pump. The suspension is introduced into the reactor by gravity and.or the action of a gas pressure. Furthermore, the process of the invention can be used to transfer a suspension from a storage vessel to a reactor maintained at a pressure which is higher than that prevailing in the storage vessel. Thus according to the present invention apparatus for introducing a predetermined volume of suspension into a reactor comprises (a) a storage vessel maintained at a pressure P1, (b) an intermediate vessel connected to the storage vessel via a first conduit provided with a shut-off valve, having an internal capacity substantially equal to the predetermined volume of suspension, the shut-off value, the conduit and the intermediate vessel being so arranged that the volume of suspension can be transferred by gravity from the storage vessel to the intermediate vessel, (c) a feed vessel connected to the intermediate vessel via a second conduit provided with a shut-off valve so arranged that the predetermined volume of suspension can be transferred by gravity from the intermediate vessel to the feed vessel and connected to the reactor maintained at a pressue P2 higher than P1, via a third conduit arranged at or close to the lowest part of the feed vessel and provided with an entry shut-off valve, the feed vessel is provided with a gas feed conduit and has a capacity sufficient to contain a volume of suspension and a quantity of gas under pressue such that the pressure difference between the feed vessel and the reactor enables the predetermined volume of suspension to be introduced into the reactor The vessels can have any cross sectional shape but commonly have circular cross-sections. The storage vessel is generally cylindrical and advantageously has a sufficient capacity to be capable of feeding the reactor with suspension for a period ranging from 1 to 10 days. Thus, for an industrial scale reactor, the storage vessel can have a volume between 0.5 and 10 m³. The storage vessel can be provided with a stirring system intended to homogenise the suspension and/or with systems enabling a minimum or maximum height of suspension to be detected. It can be provided with a gas feed conduit and with a gas inlet conduit. In this case, the storage vessel can be provided with a gas pressure control system intended to maintain a constant pressure P₁ by introducing and/or releasing gas. The intermediate vessel generally consists of a cylindrical tube preferably arranged vertically so as to transfer by gravity its content into the feed vessel more easily. Its total internal capacity is substantially equal to the predetermined volume of suspension to the introduced into the reactor. In this last case the intermediate vessel is generally used for measuring the predetermined volume of suspension. In industrial plants the capacity of the intermediate vessel can typically vary between 0.1 and 10 litres. The intermediate vessel, is connected via the first conduit, to the storage vessel, in particular to the side wall or to the wall forming the lowest part of the storage vessel. Furthermore it is also connected, via the second conduit, to the feed vessel, in particular to the top of the feed vessel so as to facilitate the introduction of suspension into this vessel. According to one embodiment an additional feed conduit can open into the intermediate vessel. This additional feed conduit can be employed for introducing another component preferably a liquid into the intermediate vessel so that it can afterwards be fed to the reactor. The feed vessel is connected to the reactor via a third conduit, called the entry conduit. In order to facilitate the introduction of suspension into the reactor, the entry conduit is arranged at or close to the lowest part of the feed vessel. The capacity of the feed vessel is generally greater than that of the intermediate vessel. The feed vessel can conveniently comprise a cylindrical upper part and a conical lower part. In this case the entry conduit is preferably connected to the apex of the cone. When the predetermined volume of suspension is introduced from the feed vessel into the reactor by the effect of a pressure difference between the feed vessel and the reactor, the feed vessel is provided with a gas feed conduit and optionally with a purge valve, which can be arranged on upper part of the vessel. The purge valve can be used in order to obtain in the feed vessel a pressure below the pressure of the reactor. The obtained pressure can be substantially equal or similar to that of the storage vessel. The capacity of the feed vessel is sufficient to contain a volume of supension and a quantity of gas under pressure such that the pressure difference between the vessel and the reactor enables the predetermined volume of suspension to be introduced into the reactor. The entry conduit is provided with an entry valve. In order to limitate the introduction of gas into the reactor, when pressure difference is used, the entry conduit is advantageously provided with an entry valve which is ultrafast in operation. It can also be provided with a safety valve arranged between the entry valve and the reactor for example in the neighbourhood of the end of the entry conduit which opens into the reactor. The entry conduit preferably has a small diameter and a length which is as short as possible so as to contain only a small volume of suspension. It preferably opens into a wall of the reactor, that means that the conduit does not extend into the reactor which could disturb its operation. Furthermore, in order to promote the dispersion of the suspension within the reactor, it is recommended that the entry conduit should open perpendicularly to a vertical wall of the reactor or advantageously in a direction pointing towards the bottom of the reactor, having a slope measured in relation to the horizontal plane which can be up to 60 º and in particular between 10 and 50 º . The invention includes a process for introducing a predetermined volume of suspension contained in a storage vessel maintained at a pressure P1, into a reactor maintained at a higher pressure P2, comprising a sequence of steps which consist in (a) transferring by gravity the predetermined volume of suspension from the storage vessel to an intermediate vessel by opening and closing a valve in a conduit connecting the storage vessel and the intermediate vessel, (b) transferring by gravity the volume of suspension to a feed vessel by opening and closing a valve in a conduit connecting the intermediate vessel and the feed vessel, (c) introducing a gas into the feed vessel by means of a gas feed conduit to obtain a pressure P3 which is greater than P2 and such that the difference P3 - P2 is able to introduce the volume of suspension into the reactor during step (d) and (d) transferring the suspension to the reactor by a rapid opening and closing of a valve in a conduit connected the feed vessel and the reactor, the pressure P4 after completion of the transfer and after the closing of the valve still being greater than P2. Throughout the description, which follows and the example the pressures are absolute pressure and are expressed in MPa. The process of the invention can be carried out with the apparatus according to the invention with the proviso that the feed vessel is provided with a gas feed conduit and optionally with a purge valve. According to the process of the invention, a reactor in which a pressure P2 prevails is fed with a suspension contained in a storage vessel in which a pressure P1 prevails. Preferably the suspension is stored under a gas which is inert towards the particles contained in the suspension such as nitrogen. The pressure P₁ is in most case equal or slighly higher than atmospheric pressure. For example P₁ can be comprised between 0,1 and 0,2 MPa. Furthermore, the pressure P₁ can be kept constant with the aid of a pressure control system. In a preliminary step of the process, the feed vessel can be purged to the atmosphere by opening the purge valve when it is at a pressure higher than the pressure P₁ in particular with a view to obtaining a pressure P₅ which is no greater than pressure P₁. During this operation the valve arranged between the intermediate vessel and the feed vessel can be also be opened so as to purge the intermediate vessel as well when P₁ is higher than atmospheric pressure. The purging operation is performed in order to obtain pressures of the same order of magnitude between the storage vessel and the feed vessel, which makes it possible to facilitate the use of the apparatus especially by avoiding movement of gas from the feed vessel towards the storage vessel. In a first step of the process a volume of suspension is introduced into the intermediate vessel by opening and then closing the valve connecting the storage vessel to the intermediate vessel. This introduction is made easier when the pressure P₁ is greater than atmospheric pressure. In most cases the intermediate vessel is completely filled with the suspension because its capacity corresponds to the volume of suspension to be introduced into the reactor. However, it can also be filled with a smaller volume of suspension by limiting the time of opening of the valve which connects it to the storage vessel. In a second step of the process a transfer of the volume of suspension from the intermediate vessel towards the feed vessel is carried out by opening and then closing the valve connecting the intermediate vessel to the feed vessel. In a third step of the process the actual introduction of the volume of suspension into the reactor is performed. To do this, a gas which is preferably inert towards the particles present in the suspension, such as nitrogen, is introduced into the feed vessel, so as to obtain a pressure P₃ which is higher than the pressure P₂ prevailing in the reactor. It is essential that pressure difference P₃ - P₂ should be able to introduce the volume of suspension into the reactor. This means that the gas pressure prevailing-in the feed vessel must always be higher than the pressure P₂ even when all the suspension has been introduced into the reactor. Futhermore P₃ is advantageously choosen in relation with the capacity of the feed vessel in order to avoid disturbance of the working of the reactor and for having a good dispersion of the suspension in the reactor. After the introduction of gas, the entry valve is opened in order to introduce the volume of the suspension into the reactor closing of this valve takes place after the introduction of the volume of suspension and preferably very fast, so as to limit the quantity of gas injected into the reactor. In all cases it is preferable to close the entry valve so as to obtain in the feed vessel a pressure P₄ which is higher than P₂ so as to avoid a gas flow of gas from the reactor towards the feed vessel. In practice the suspension is introduced into the reactor preferably by opening the entry valve and then closing it as soon as the pressure prevailing in the feed vessel reaches a predetermined valve P₄ which should indicate when all the suspension has been introduced into the reactor. According to an advantageous embodiment of the process, when the entry conduit is provided with a safety valve in addition to the entry valve, the actual introduction of suspension into the reactor takes place after the introduction of gas into the entry vessel, by opening the safety valve and then the entry valve and then by closing the entry valve and then the safety valve. The steps of the process form a sequence which makes it possible to introduce a predetermined single volume of suspension into the reactor. The sequence can advantageously last from 1 to 500 seconds and preferably from 60 to 180 seconds. In order to introduce a suspension sequentially into a reactor the introduction sequence of the process may be repeated a number of times and preferably cyclically. In particular, from 1 to 20 introduction sequences may be performed per hour. Typically, from 1 to 20 litres of suspension per hour may be introduced in this way into a reactor of an industrial plant. This introduced quantity depends, of course, on a number of factors and in particular on the capacity of the intermediate chamber, on the length of an introduction sequence and on the number of sequences per hour. When the intermediate vessel is provided with a liquid feed conduit, this conduit can be used to clean the introduction apparatus in order to avoid blockage, especially in the entry conduit. For example, during a sequential introduction of suspension into a reactor one or more introductions of liquid can be interposed between two introductions of suspension. The process of the invention is particularly useful for introducing particles with catalytic activity suspended in a liquid into a reactor with a fluidised and/or mechanically stirred bed, in which a gas-phase polymerisation or copolymerisation of alpha-olefins containing from 2 to 12 carbon atoms is carried out. In this case the pressure P₂ is in most cases between 0.1 and 5 MPa. The gas-phase polymerisation reactor may advantageously consist of equipment such as that described in French Patent No. 2,207,145 or French Patent No. 2,335,526. This equipement comprises essentially a fluidised-bed polymerisation reactor comprising a vertical cylinder equipped with a fluidisation grid and supporting a disengagement chamber and a conduit for recycling the gaseous reaction mixture, connecting the top of the disengagement chamber to the base of the reactor. The particles with catalytic activity may be particles of a solid catalyst containing at least one transition metal of groups IV, V and VI of the Periodic Classification of the elements. This solid catalyst can be either a catalyst of the Ziegler-Natta type, comprising especially a titanium and/or vanadium and/or zirconium halide, preferably a magnesium compound and/or refractory oxide, or a catalyst based on chromium oxide supported on a refractory oxide such as silica or alumina, which catalyst is thermally activated in a non-reducing atmosphere. The catalyst particles to be introduced into the reactor can have a mass-average diameter Dm of between 50 and 150 microns, preferably between 70 and 120 microns, and a particle size distribution such that the ratio of Dm the number-average diameter Dn of the said particles is lower than 4, preferably lower than 3. The particles with catalytic activity to be introduced into the reactor can also be alpha-olefin prepolymer obtained by bringing one or more alpha-olefins into contact with a catalyst as described above and with at least one organometallic compound of a metal of groups II and III of Periodic Table of Elements, in particular an organoaluminium, organozinc or organomagnesium compound. The prepolymer may have a molar ratio of the metal of the organometallic compound to the transition metal of the catalyst of between 0.1 and 20, preferably between 0.2 and 10. It may have a relative density of between 0.930 and 0.970, and may contain from 2 x 10⁻³ to 1 millimole of transition metal per gram. The particles of prepolymer with catalytic activity can have a mass-average diameter Dm ranging from 80 to 400 microns, preferably from 100 to 300 microns, and a particle size distribution such that the ratio Dm/Dn of the said particles is lower than 4, preferably lower than 3. The particles with catalytic activity to be introduced in suspension into the reactor can also consist of an alpha-olefin polymer or copolymer prepared beforehand in another polymerisation reactor by bringing one or more alpha-olefins into contact with an optionally prepolymerised catalyst and with at least one organo-metallic compound such those described above. The polymer or copolymer can have a molar ratio of the metal of the organometallic compound to the transition metal of the catalyst of between 0.1 and 50, preferably between 0.2 and 20, and more particularly between 0.5 and 10. The prepolymer or copolymer can have a relative density ranging from 0.880 to 0.970 and can contain less than 2 x 10⁻³ millimoles of transition metal per gram. The particles of polymer or copolymer with catalytic activity can have a mass-average diameter Dm larger than 400 microns and smaller than 1000 microns, and a particle size distribution such that the ratio Dm/Dn of the said particles is lower than 4, preferably lower than 3. The liquid in which the particles are in suspension is generally a solvent which is inert towards the particles, such as a saturated hydrocarbon containing from 3 to 10 carbon atoms, and in particular n-hexane or n-heptane. However, in some cases the liquid of the suspension can be a reactive solvent such as one or more alpha-olefins. The suspension to be introduced into the reactor can contain from 5 to 500 g of particles per litre of liquid. More particularly it can contain from 200 to 300 g of prepolymer per litre of the liquid. The characteristics of the invention are illustrated by means of Figures 1 and 2. Figure 1 shows an apparatus according to the present invention. Figure 2 shows the changes in pressure in the feed vessel of the apparatus during an injection sequence performed according to the process of the invention. Figure 1 shows diagrammatically an apparatus for introducing suspension into a reactor. It comprises a storage vessel (1), an intermediate vessel (2) a feed vessel (3). The storage vessel (1) is connected to the intermediate vessel (2) via a first conduit (4) provided with a shut-off valve (5), the intermediate vessel (2) is connected to the feed vessel (3) via a second conduit (6) provided with a shut-off valve (7), the feed vessel (3) is connected to the reactor (11) via a third conduit (8) provided with an entry valve (9) and with a safety valve (10). The storage vessel (1) is fed with suspension by the conduit (12). The intermediate vessel (2) consists of a vertical tube (2), and is provided with a liquid feed conduit (13). The feed vessel (3) consists of a cylindrical upper part and of a lower part in the form of a cone. This feed vessel (3) is provided with a purge valve (16) and with a gas delivery conduit (14) equipped with a valve (15). Figure 2 shows diagrammatically the changes in pressure in the feed vessel (3) during a sequence of introduction of suspension into the reactor (11). The abscissa axis represents time and the ordinate axis the pressure. P₁ and P₂ denote the pressure in the storage vessel (1) and in the reactor (11) respectively. T₁ denotes the period of opening of the valve (15), T₃ the period of opening of the valve (9) and T₅ shows the period of opening of the purge valve (16). The present invention is illustrated below with reference 1. Example 1 : The operation is carried out in a device as shown in Figure 1 comprising (a) a cylindrical 1.5 m³ storage vessel (1) (b) an intermediate vessel consisting of a vertical tube 4.6 cm in internal diameter and with an internal capacity of exactly 0.5 litres, (c) a feed vessel with a total capacity of 19 l, consisting of a cylindrical part 25 cm in diameter and with a height of 43 cm, and of a conical part which has a large height of 31,cm, (d) of a entry conduit (8) which has a diameter of 1.3 cm and a total length of 2 m and whose end part (17) opens into the reactor in a direction pointing downwards and at a slope of 30° relative to the horizontal plane. The storage vessel initially contains 1 m³ of a suspension containing 210 g per litre of particles in n-hexane. The particles comprised ethylene prepolymer prepared from catalyst as described in Example 1 of French Patent N° 2 405 961, containing 40 g of polyethylene per millimole of titanium. This storage vessel (1) is kept at a pressure P₁ of 0.150 MPa. The introduction of a volume of 0.5 l of suspension into the fluidised-bed reactor (11) where a gas-phase polymerisation reaction of ethylene is carried out at a pressure of 2.0 MPa is performed by carrying out the following operations : a) the entry vessel (3) is purged by opening the valve (16) until a pressure P₅ equal to 0.105 MPa is obtained. b) 0.5 l of suspension are introduced into the intermediate chamber (2) by opening the valve (5) and then closing it again. c) 0.5 l of suspension are transferred from the intermediate vessel (2) towards the entry vessel (3) by opening the valve (7) and then closing it again. d) 0.5 l of suspension in the feed vessel (3) are introduced into the reactor (11) in a first step by introducing nitrogen by means of the conduit (14) until a pressure of 2.15 MPa is obtained and then, in a second step, by opening the valve (10) and then the valve (9) until a pressure of 2.05 MPa is obtained and to close the valve (9) and then the valve (10). This introduction sequence is repeated 15 times in 1 hour.

Claims: An apparatus for introducing a predetermined volume of suspension into a reactor (11) comprising : (a) a storage vessel (1) maintained at a pressure P1, (b) an intermediate vessel (2) connected to the storage vessel (1) via a first conduit (4) provided with a shut-off valve (5), having an internal capacity substantally equal to the predetermined volume of suspension, the shut-off valve (5), the conduit (4) and the intermediate vessel (2) being so arranged that the volume of suspension can be transferred by gravity from the storage vessel (1) to the intermediate vessel (2), (c) a feed vessel (3) connected to the intermediate vessel (2) via a second conduit (6) provided with a shut-off valve (7) so arranged that the predetermined volume of suspension can be transferred by gravity from the intermediate vessel (2) to the feed vessel (3) and connected to the reactor (11) maintained at a pressure P2 higher than P1, via a third conduit (8) arranged at or close to the lowest part of the feed vessel (3) and provided with an entry shut-off valve (9), the feed vessel (3) is provided with a gas feed conduit (14) and has a capacity sufficient to contain a volume of suspension and a quantity of of gas under pressure such that the pressure difference between the feed vessel (3) and the reactor (11) enables the predetermined volume of suspension to be introduced into the reactor (11). An apparatus according to Claim 1, characterised in that the intermediate vessel (2) is a cylindical tube arranged vertically. An apparatus according to Claim 1, characterised in that an additional feed conduit (13) opens into the intermediate vessel (2). An apparatus according to Claim 1, characterised in that the shape of the feed vessel (3) comprises a cylindrical upper part and a lower part which is a cone. An apparatus according to Claim 1, characterised in that the third conduit (8) is provided with a safety valve (10) arranged between the entry valve (9) and the reactor (11). An apparatus according to Claim 1, characterised in that the feed vessel (3) is provided with a purge valve (16). A process for introducing a predetermined volume of suspension contained in a storage vessel maintained at a pressure P1, into a reactor maintained at a higher pressure P2, comprising a sequence of steps which consist in : (a) transferring by gravity the predetermined volume of suspension from the storage vessel to an intermediate vessel by opening and closing a valve in a conduit connecting the storage vessel an the intermediate vessel, (b) transferring by gravity the volume of suspension to a feed vessel by opening and closing a valve in a conduit connecting the intermediate vessel and the feed vessel, (c) introducing a gas into the feed vessel by means of a gas feed conduit to obtain a pressure P3 which is greater than P2 and such that the difference P3 - P2 is able to introduce the volume of suspension into the reactor during step (d). (d) transferring the suspension to the reactor by a rapid opening and closing of a valve in a conduit connected the feed vessel and the reactor, the pressure P4 after completion of the transfer and after the closing of the valve still being greater than P2. A process according to Claim 7, characterised in that the sequence comprises a preliminary step during which the feed vessel (3) is purged by opening a purge valve. A process according to Claim 8, characterised in that during the preliminary step the valve arranged between the intermediate vessel and the feed vessel is opened in order to purge the intermediate vessel. A process according to Claim 7, characterised in that the sequence of steps is repeated a number of times. A process according to Claim 10, characterised in that a liquid is introduced into the intermediate vessel by means of a liquid feed conduit which opens into the intermediate vessel and then into the reactor.

Inventors: PES ROBERT BP CHEMICALS S N C, PES, ROBERT, BP CHEMICALS S.N.C.
Applicants: BP CHEM INT LTD, BP CHEMICALS SNC, BP CHEMICALS LIMITED, BP CHEMICALS S.N.C.
Application year: 1991
Main classification: B01J3
IPC codes: C08F4, G01F11, C08F2, B01J8, C08F10, B01J3