Classifications

• • C—CHEMISTRY; METALLURGY
  • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
  • C08F—MACROMOLECULAR COMPOUNDS OBTAINED BY REACTIONS ONLY INVOLVING CARBON-TO-CARBON UNSATURATED BONDS
  • C08F10/00—Homopolymers and copolymers of unsaturated aliphatic hydrocarbons having only one carbon-to-carbon double bond
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
  • B01J—CHEMICAL OR PHYSICAL PROCESSES, e.g. CATALYSIS OR COLLOID CHEMISTRY; THEIR RELEVANT APPARATUS
  • B01J8/00—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes
  • B01J8/18—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles
  • B01J8/24—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles according to "fluidised-bed" technique
  • B01J8/40—Chemical or physical processes in general, conducted in the presence of fluids and solid particles; Apparatus for such processes with fluidised particles according to "fluidised-bed" technique with fluidised bed subjected to vibrations or pulsations
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B08—CLEANING
  • B08B—CLEANING IN GENERAL; PREVENTION OF FOULING IN GENERAL
  • B08B7/00—Cleaning by methods not provided for in a single other subclass or a single group in this subclass
  • B08B7/02—Cleaning by methods not provided for in a single other subclass or a single group in this subclass by distortion, beating, or vibration of the surface to be cleaned

Definitions

• the present invention relates to a process for treating the internal wall of a polymerization reactor More particularly, the present invention relates to a process for treating the internal wall of a fluidized bed reactor for gas phase polymerization by means of vibrations. It is known to polymerize one or more monomers in gas phase at a pressure which is higher than atmospheric pressure in a fluidized bed reactor where polymer particles being formed are maintained in the fluidized state by virtue of a reaction gas mixture containing the monomer(s) to be polymerized and travelling in an upward stream. The polymer thus manufactured in powder form is generally drawn off from the reactor in order to maintain the bed at a more or less constant volume.
• a process which is preferred on the industrial scale employs a fluidization grid which distributes the reaction gas mixture through the bed and which acts as support for the bed in the event of a cut in the upward gas flow.
• the reaction gas mixture leaving via the top of the fluidized bed reactor is recycled to the base of the latter under the fluidization grid through the intermediacy of an external circulation conduit fitted with a compressor.
• the fluidized bed reactors according to the present invention can be represented by a volume whose enclosure (wall) consists of at least one surface of revolution generated by the rotation of a rectilinear and/or curvilinear segment about a vertical axis known as an axis of revolution, and of a disengagement vessel which is above the said surface.
• the wall of the reactor is therefore a surface of revolution comprising a vertical axis of revolution above which is the enclosure of a disengagement vessel.
• Conventional fluidized bed reactors employed for the gas phase polymerization of olef ⁇ n(s) usually consist of a cylinder (1) with a vertical axis, above which is a disengagement vessel (3), in accordance with Figure 2, which shows diagrammatically a preferred apparatus for gas phase polymerization according to the present invention.
• the cylindrical part of the reactor is usually characterised by a height/diameter ratio (H/D) which is comprised between 1 and 15, preferably between 2 and 8, D representing the internal diameter of the reactor.
• the disengagement vessel which is above the cylinder capable of containing the fluidized bed has, in principle, a cross-section which is larger than that of the cylinder.
• a bulb consisting essentially of a conical frustum of revolution with a vertical axis coinciding with the axis of the cylinder, with an apex pointing downwards with an angle preferably of between 10° and 60° and having above it a dome of substantially hemispherical shape.
• the small base of this conical frustum coincides with the upper end of the cylinder of the reactor, and its large base coincides with the base of the dome.
• It may also consist of a vertical cylinder connected to the cylinder capable of containing the fluidized bed by a connecting surface in the shape of a flared conduit. In this case this cylinder has a vertical axis coinciding with the axis of the cylinder capable of containing the fluidized bed and a roof generally of substantially hemispherical shape.
• the essential purpose of the disengagement vessel is to slow down the upward gas stream which, after having passed through the fluidized bed, can entrain relatively large quantities of solid particles. As a result of this, most of the entrained solid particles return directly into the fluidized bed. Only the finest particles can be entrained out of the reactor.
• the presence of fines in the reactor can affect the properties of the polymer by increasing the gel content of the finished products such as plastic films and receptacles.
• another phenomenon can also arise during the polymerization, namely the formation of agglomerates on the internal wall of the reactor.
• This formation of agglomerates corresponds to the adhesion of the molten particles of catalyst and of polymer to the wall of the reactor, inter alia in the disengagement vessel.
• these agglomerates become heavy they can separate from the wall and thus block the fluidization grid and/or the system for drawing off the polymer.
• the accumulation of the fines and of the agglomerates on the wall of the reactor will hereinafter be called, generally, fouling of the reactor.
• the reactor is stopped at regular intervals, in order to clean its wall and to extract the agglomerates from it. This can be done by means of water or of nitrogen under pressure. Cleaning of this type brings about the entries of poison into the reactor, and this automatically involves a purge of the reactor and a drying operation in order to remove these poisons. This procedure takes time and is not very economical.
• US-5,461,123 claims a method for polymerizing in a gas phase fluidized bed reactor, comprising generating a low frequency, high pressure sound wave inside the reactor system, which wave has sufficient frequency and pressure to prevent or remove solid particle build-up on interior surfaces of the reactor system. While interesting on a scientific point of view, the use of high power loudspeakers inside fluidised bed reactors as suggested in US-5,461,123 would appear to be difficult to implement on an industrial plant.
• the present invention consequently consists of a process for treating the internal wall of a fluidized bed reactor for gas phase polymerization, characterized in that the external wall of the reactor is subjected to at least one vibration which is generated by at least one exciter of a mechanical, hydraulic, pneumatic, electrical or electromechanical type or with active materials, which is either placed directly on the external wall of the reactor or connected to this wall by means of at least one device for transmitting the excitation.
• the present invention is particularly appropriate to the polymerization reactions of one or more of the monomers such as olefins, polar vinyl monomers, dienes, acetylenes and aldehydes.
• the process according to the present invention preferably applies to the polymerization of one or more olefinic monomers such as ethylene, propylene, 1-butene, 1-pentene, 4-methyl- 1 -pentene, 1-hexene and 1-octene.
• olefinic monomers such as ethylene, propylene, 1-butene, 1-pentene, 4-methyl- 1 -pentene, 1-hexene and 1-octene.
• a proper vibration mode is characterized by a proper frequency of vibration and by its corresponding elastic mode line.
• the study of these elastic mode lines makes it possible to identify vibrational nodes corresponding to a zero amplitude of deformation of the structure and vibration antinodes corresponding to a maximum amplitude of deformation of the structure.
• the polymerization reactor will be characterized by proper vibrational modes and therefore by proper frequencies and elastic mode lines. These vibrational modes are specific to the construction, the geometry and the securing of the peripheral lines and circuits of the reactor.
• the vibration to which the wall of the reactor is subjected is characterized by a frequency or by the combination of two or more frequencies of between 0.85 and 1.15 times the proper frequency or frequencies of the proper mode(s) of vibration of the said wall.
• This frequency or these frequencies are preferably between 0.90 and 1.10, more preferably between 0.95 and 1.05 times the proper frequency or frequencies of a proper mode or of the proper modes of vibration of the said wall. It is actually preferred to find and adopt frequencies close to those of the proper modes of vibration, because this allows the wall of the reactor to be appropriately excited by means of a limited force.
• These proper modes of vibration are preferably chosen such that the corresponding elastic mode lines produce a deformation of the cross-sections of the reactor while keeping the vertical axis of the reactor stationary.
• These proper modes of vibration are preferably also characterized by elastic mode lines whose main direction of deformation has a predominant component which is perpendicular to the vertical axis of the reactor.
• a "transverse” vibration ( Figure 4 (a) consists in a periodic bending deformation of the whole tube structure, including its longitudinal axis, thesaid deformation comprising vibration nodes at which the transverse displacement amplitude of the tube is zero, and vibration antinodes at which the transverse displacement amplitude is at a maximum.
• the "transverse” vibration is a type of vibration wherein the longitudinal axis of the tube is subjected to a periodic deformation.
• a "torsion" vibration results from a periodic act of twisting or turning one end of the tube in one direction, the other end being held motionless or twisted in the opposite direction ; in a "torsion” vibration, the longitudinal axis of the tube is not shifted and therefore remains motionless.
• a "section” vibration consists in a periodic deformation of the transverse section of the tube which : (ci) ( Figure 4(c ) either remains circular but with a periodic change of the diameter : the vibration is also called “axial” vibration or “compression” vibration, and may result from a periodic act of compressing one end of the tube in one direction parallel to the longitudinal axis of the tube, the other end being held motionless or compressed in the opposite direction ; in this vibration, the longitudinal axis of the tube is not shifted and remains motionless, while the diameter of the circular transverse section of the tube periodically varies, (c 2 ) ( Figure 4(c 2 )) or periodically changes from a circular section into a non- circular section, while keeping motionless the longitudinal axis of the tube.
• a "skin" vibration consists in a propagation of a distorting wave of the surface of the wall of the tube along a limited portion of thesaid wall, while the longitudinal axis is not shifted and remains motionless.
• the vibration generated in the presently claimed process are section vibrations, preferably axial or compression vibrations. Transverse and torsion vibrations are obviously preferably excluded from the presently claimed process.
• the Applicant Company has quite unexpectedly found that not only did the process according to the invention entail no risk of rupture of the reactor and/or of its attachments, but also that the problems of fouling were solved in an efficacious manner that is easy to use and economical.
• the vibration to which the wall of the reactor is subjected is generated by at least one exciter of a mechanical, hydraulic, pneumatic, electrical or electromechanical type or with active materials.
• Exciters which may be mentioned by way of examples without any limitation being implied are a vibrator with rotating masses or an electromagnetic vibrator as exciters of a mechanical type, a servo-controlled hydraulic jack as exciter of a hydraulic type, a servo-controlled pneumatic jack as exciter of a pneumatic type, a solenoid core as exciter of an electrical type, an electrical motor unbalance as exciter of an electromechanical type, and a jack consisting of multilayer piezoelectric ceramics as exciter of a type with active materials.
• the vibration to which the wall of the reactor is subjected is generated by at least one exciter of a mechanical type, preferably an electromagnetic vibrator.
• This exciter preferably communicates a periodic force or a periodic movement applied at one or more points of the wall of the reactor. As indicated above, this exciter is either placed directly on the external wall of the reactor or connected to this wall by means of a device for transmitting the excitation. In the case of the use of a device for transmitting the excitation, it is important that this device should be connected to a vibration antinode of the chosen mode of vibration in order to reduce the forces and hence the energy required. This is why a rigid device for transmitting the excitation is preferably employed, in order to guarantee a transmission of the excitation without any phase change.
• the fundamental frequency of the forces or movements applied by the exciter is preferably between 0.85 and 1.15 times the frequency of the chosen proper mode of vibration (condition A). This frequency is preferably between 0.90 and 1.10, more preferably between 0.95 and 1.05 times the proper frequency of a selected proper mode of vibration of the said wall.
• the periodic forces or the movements applied by the exciter are preferably applied at points which are sufficiently far from the vibration nodes and/or close to the vibration antinodes of the chosen -proper mode of vibration (condition B).
• the directions of the periodic forces or the movements applied by the exciter are preferably colinear with the direction of the deformation mode at this point (condition C).
• condition D the amplitude of the periodic forces or of the movements applied by the exciter is such that the vibration generated does not present any risks to the mechanical firmness of the structure (condition D).
• a number of exciters are installed which impart, through mechanical transmissions, periodic forces or periodic movements applied at various points of the external wall.
• the conditions (A), (B), (C) and (D) described above are obeyed for each of the excitations, and forces or movements are applied which accompany the direction of the deformations given by the elastic mode line of the chosen proper mode of vibration.
• the effectiveness of the excitation would be low or even zero.
• the vibration to which the wall of the reactor is subjected is generated by at least two exciters placed directly on the external wall of the reactor at a height of application which corresponds to the points where the section of the reactor exhibits the largest deformations with the chosen mode of vibration.
• the points of application of the excitation are close to the points subjected to the largest movements, with the chosen mode, on the periphery of the section of the reactor.
• the direction of the excitation is perpendicular to the vertical axis of revolution of the reactor and preferably intersects this axis. These points of application are preferably diametrically opposed in relation to the vertical axis of revolution of the reactor.
• At least two devices for transmitting the excitation will preferably be employed, placed directly on the external wall of the reactor at a height of application which corresponds to the points where the section of the reactor exhibits the largest deformations with the chosen mode of vibration.
• FIG. 2 shows diagrammatically an illustration of an apparatus for gas phase polymerization of olefin(s) according to the present invention.
• the apparatus includes:
• a fluidized bed reactor (1) fitted with a top (2) and a base comprizing a fluidization grid (4), and consisting of a cylinder with a vertical side wall and a disengagement or desurging chamber (3) above the said cylinder, the top of the chamber (3) forming the top (2) of the reactor,
• an entry chamber (9) for a reaction gas mixture situated under the grid (4) and communicating with the reactor (1) through the intermediacy of the grid (4), and
• an external circulation conduit (5) for the reaction gas mixture connecting the top (2) of the reactor to the entry chamber (9) for the reaction gas mixture and including a compressor (8) and at least one heat exchanger (6, 7),
• conduits (10) for feeding the reaction gas mixture with constituents such as one or more olefins, for example ethylene or propylene or C4 to CIO alpha-olefins, one or more, preferably unconjugated, dienes, hydrogen and one or a number of inert gases such as nitrogen or Cl to C6, preferably C2 to C5, alkanes may open into the external circulation conduit (5).
• constituents such as one or more olefins, for example ethylene or propylene or C4 to CIO alpha-olefins, one or more, preferably unconjugated, dienes, hydrogen and one or a number of inert gases such as nitrogen or Cl to C6, preferably C2 to C5, alkanes may open into the external circulation conduit (5).
• Another subject-matter of the present invention is a process for gas phase continuous polymerization of olefin(s) in a reactor containing a fluidized and optionally mechanically stirred bed, consisting of a cylinder with a vertical side wall and a disengagement or desurging chamber above the said cylinder, at an absolute pressure higher than the atmospheric pressure, by continuous or intermittent introduction of a catalyst into the reactor, continuous introduction of olefin(s) into a reaction gas mixture passing through the reactor according to an upward stream, removal of the heat of polymerization by cooling the recycled reaction gas mixture, and drawing off the polymer manufactured, which process is characterized in that the external wall of the reactor is subjected to at least one vibration which is generated by at least one exciter of a mechanical, hydraulic, pneumatic, electrical or electromechanical type or with active materials, which is either placed directly on the external wall of the reactor or connected to this wall by means of at least one device for transmitting the excitation
• the process of the invention is very particularly suitable for polyolefin powders, especially of high or linear low density polyethylene, for example of relative density ranging from 0.87 to 0.97, or of polypropylene.
• the polymers manufactured according to the present process may in particular be powders corresponding essentially to the B type and sometimes to the A and B types, according to the classification given by D. Geldart in "Gas Fluidization Technology” published in “A. Wiley-Interscience Publication” by John-Wiley & Sons (1986), pages 33 to 46.
• the polymers may consist of particles which have a mass-average diameter ranging from 300 to 2000, preferably from 500 to 1500 ⁇ m.
• the process for gas phase continuous polymerization of olefin(s) is carried out in a reactor containing a fluidized and optionally mechanically stirred bed maintained at an absolute pressure PI which can range from 0.5 to 6, preferably from 1 to 4 MPa.
• the temperature of the fluidized bed may be maintained at a value ranging from 30 to 130°C, preferably from 50 to 1 10°C.
• a gas reaction mixture passes through the reactor at an upward speed which may range from 0.3 to 1 m/s, preferably from 0.4 to 0.8 m/s.
• the reaction gas mixture may contain one or more olefins, especially from C2 to CIO, preferably from C2 to C8, for example ethylene or propylene, or a mixture of ethylene with at least one C3 to CIO, preferably C3 to C8, olefin, for example propylene, 1-butene, 1-hexene, 4-methyl- 1-pentene or 1-octene and/or else with at least one diene, for example an unconjugated diene. It may also contain hydrogen and/or an inert gas such as nitrogen or an alkane, for example from Cl to C6, preferably from C2 to C5.
• the polymerization process may in particular be carried out according to the process described in Patent Application PCT No. 94/28032.
• a catalyst comprising at least one transition metal belonging to groups 4, 5 or 6 of the Periodic Classification of the elements (approved by the Nomenclature Committee of the "American Chemical Society", see “Encyclopedia of Inorganic Chemistry", editor R. Bruce King, published by John Wiley & Sons (1994)).
• a catalyst system of the Ziegler-Natta type may be employed, including a solid catalyst comprising a compound of a transition metal such as those mentioned above and a cocatalyst comprising an organometallic compound of a metal belonging to groups 1, 2 or 3 of the Periodic Classification of the elements, for example an organoaluminium compound.
• High activity catalyst systems have already been known for many years and are capable of producing large quantities of polymer in a relatively short time, with the result that it is possible to avoid the stage of removal of the catalyst residues present in the polymer.
• These high activity catalyst systems generally include a solid catalyst comprising essentially transition metal, magnesium and halogen atoms. It is also possible to employ a high activity catalyst essentially comprising a chromium oxide activated by a heat treatment and used in combination with a granular support based on refractory oxide.
• the polymerization process is very particularly suitable for being employed with metallocene catalysts such as zirconocene, hafnocene, titanocene or chromocene, or Ziegler catalysts supported on silica, for example based on titanium or vanadium.
• metallocene catalysts such as zirconocene, hafnocene, titanocene or chromocene, or Ziegler catalysts supported on silica, for example based on titanium or vanadium.
• the abovementioned catalysts or catalyst systems may be employed as they are directly in the fluidized bed reactor or may be converted beforehand into olefin prepolymer, in particular in the course of a prepolymerization bringing the catalyst or catalyst system into contact with one or more olefins such as those referred to above, in a hydrocarbon liquid medium or in gaseous phase, according, for example, to a noncontinuous or continuous process.
• the process is very particularly suitable for manufacturing polyolefins in powder form, in particular of high or linear low density polyethylene, of relative density ranging, for example, from 0.87 to 0.97, or of polypropylene, or of copolymers of propylene with ethylene and/or C4 to C8 olefins, or of elastomeric copolymers of propylene with ethylene and optionally at least one unconjugated diene, of relative density ranging, for example, from 0.85 to 0.87.
• high or linear low density polyethylene of relative density ranging, for example, from 0.87 to 0.97, or of polypropylene, or of copolymers of propylene with ethylene and/or C4 to C8 olefins, or of elastomeric copolymers of propylene with ethylene and optionally at least one unconjugated diene, of relative density ranging, for example, from 0.85 to 0.87.
• the process is not only simple, reliable and easy to implement, but it also makes it possible to minimize and/or radically eliminate the problems of fouling of the wall of the reactor.
• the present invention also consists of a process for treating the internal wall of a fluidized bed reactor for gas phase polymerization, characterized in that the appropriate vibrational states of the reactor are identified and that the external wall of the reactor is subjected to at least one vibration whose characteristics correspond to those of at least one of the identified appropriate vibrational states and whose effects produce the reduction and/or the elimination of the fouling of the internal wall of the reactor.
• the said vibration is preferably generated by at least one mechanical, hydraulic, pneumatic, electrical or electromechanical exciter or an exciter with active materials, which is either placed directly on the external wall of the reactor or connected to this wall by means of at least one device for transmitting the excitation.
• the examples of preferred exciters and the preferred characteristics of the said vibration are in accordance with what has already been described above in the present invention.
• the appropriate vibrational state will preferably be characterized by one or several proper modes of vibration of the reactor which are chosen such that the combination of the corresponding elastic mode lines entails a deformation of the cross-sections of the reactor while keeping the vertical axis of the reactor stationary, particularly sections in the case of which there is a risk of attachment of product to the wall.
• These proper modes of vibration are also preferably characterized by elastic mode lines whose main direction of deformation has a predominant component which is perpendicular to the vertical axis of the reactor. Any suitable method may be employed in order to identify the vibrational states of the reactor.
• a fluidized bed reactor for olefin polymerization was modelled using finite elements of shells representing the mean surface of the reactor enclosure.
• the latter consists of a cylinder with a vertical axis, with a disengagement vessel above.
• the cylinder has a diameter of 700 mm and a height of 7000 mm above the fluidization grid.
• the disengagement vessel is a bulb consisting of a conical frustum of revolution with a vertical axis coinciding with the axis of the cylinder, with an apex pointing downwards with an angle of 12° and having above it a dome of hemispherical shape.
• the whole is made of carbon steel of 12 mm thickness.
• Figure 1 shows a graphical representation of the reactor and the display of its mesh network.
• the following table shows the first vibrational modes of the reactor, namely those corresponding to a frequency lower than 100 Hz.
• Figure 3 shows the elastic mode lines of the reactor for the proper frequencies lower than 100 Hz.
• the reactor should therefore be excited in vibration in the vicinity of a proper mode whose elastic mode line exhibits the largest amplitudes in the vicinity of the cap and stresses as little as possible the reactor supports and the pipes which are connected there.
• the reactor should therefore be excited in vibration in the vicinity of a proper mode whose elastic mode line exhibits the largest amplitudes in the vicinity of the cylindrical part of the reactor and stresses as little as possible the reactor supports and the pipes which are connected there.
• the first proper mode is expressed as a deformation of the whole reactor, which can result in a considerable stressing of the supports. This mode is therefore inappropriate.
• the mode No. 3 at 90.9 Hz appears to be the most appropriate one because it minimizes the stressing of the main support and optimizes the amplitude of deformation in the vicinity of the disengagement vessel.
• the amplitude of deformation is highest at the connection between the cylindrical body (barrel) and the disengagement vessel (bulb).
• the mode No. 2 at 70 Hz also appears appropriate because it minimizes the stressing of the main support and optimizes the amplitude of deformation in the vicinity of the vertical wall of the lower cylindrical part of the reactor.
• Two variable-frequency electromagnetic vibrators connected to a 220 V single-phase alternating current supply were employed as an exciter. These vibrators were placed on two supports bearing perpendicularly on the vertical axis of the reactor, at the weld joining the lower conical part of the bulb to the cylinder of the reactor, that is exactly at 7000 mm above the fluidization grid. These supports are diametrically opposed.
• the two vibrators operate in phase. Each of them is provided with an inert weight of the same mass.
• the alternating rectilinear motion of these inert weights, at a frequency of 91 Hz, imparts to the wall, through the intermediacy of the rigid support of the exciter, a harmonic force of the same frequency which maintains the chosen mode of vibration of the reactor.
• the maximum amplitudes of the wall are of the order of 10 microns; the study of the fatigue of the reactor wall shows that these amplitudes are devoid of inconvenience for the mechanical firmness of the reactor.
• the vibrators operate as soon as the test begins (before any pouring of the bed) at a frequency of 91 Hz, that is to say at the proper frequency of contraction and of expansion of the bulb, calculated and then measured on the reactor.
• the reactor, its barrel and its bulb are initially clean after blasting the walls with nitrogen.
• the deposit at the wall of the bulb and of the upper barrel does not exceed 1 mm in thickness.
• An identical test was carried out without subjecting the reactor to a forced vibration. This resulted in deposits whose thicknesses are between 3 and 10 mm in the case of all the catalyst systems employed in the study.

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Citations (4)

• 1996
  • 1996-12-27 FR FR9616341A patent/FR2757784B1/fr not_active Expired - Fee Related
• 1997
  • 1997-12-19 JP JP52973898A patent/JP2001507277A/ja active Pending
  • 1997-12-19 AU AU53300/98A patent/AU5330098A/en not_active Abandoned
  • 1997-12-19 WO PCT/GB1997/003500 patent/WO1998029186A1/en not_active Application Discontinuation
  • 1997-12-19 EP EP97950290A patent/EP0948401A1/en not_active Withdrawn
• 1999
  • 1999-06-25 NO NO993165A patent/NO993165L/no not_active Application Discontinuation

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