Method and device for producing spherical particles from a polymer melt

Abstract

The invention relates to a method and a device for the production of spherical particles, whereby a molten prepolymer or precondensate is transformed into droplets by means of a drip nozzle, the droplets are subjected to a countercurrent with a gas in a precipitation column until at least partial crystallization is achieved and are then subjected to an additional post-crystallization phase. In order to economically produce higher quality particles at a high flow rate, the molten prepolymer is transformed into droplets by means of a vibrating nozzle plate and/or direct vibration of the molten prepolymer or polymer and resulting droplets are subjected to an air and gas countercurrent.

What is claimed:  
     
         1 . A method of producing spherical particles from a polymer melt, particularly made of polyfunctional carboxylic acids and alcohols, such as PET or PBT pellets, a molten prepolymer and/or precondensate and/or non-stringy polymer being dripped into droplets using a drip nozzle, the droplets having a gas applied to them in counterstream in a fall tower for at least partial crystallization and then being transported to a further post-crystallization stage, characterized in that the molten polymer is dripped using a nozzle plate set into vibration and/or through direct vibration excitation of the molten prepolymer and/or polymer and the droplets thus formed have air applied to them as the gas in counterflow, the air being supplied to the fall tower at a temperature T 1 , which lies above the glass transition point of the prepolymer and/or polymer melt to be dripped and/or the air being supplied to the fall tower at a temperature such that the air is heated to at most a temperature T 2 ≦210° C. through thermal transfer from the droplets.  
     
     
         2 . The method according to  claim 1 , characterized in that the air is supplied to the fall tower at a temperature T 1  of T 1 ≦160°, particularly T 1 ≦120°.  
     
     
         3 . The method according to  claim 1  or  2 , characterized in that, for the production of PET pellets, the air is supplied to the fall tower at a temperature such that the air is heated to at most a temperature T 2 ≦160° C. through thermal transfer from the droplets.  
     
     
         4 . The method according to  claim 1  or  2 , characterized in that, for the production of PBT pellets, the air is supplied to the fall tower at a temperature such that the air is heated to at most a temperature T 2 ≦140° C. through thermal transfer from the droplets.  
     
     
         5 . The method according to  claim 1 , characterized in that, for the production of the prepolymer and/or precondensate, a catalyst based on titanium oxide, which increases the polycondensation speed, is added.  
     
     
         6 . The method according to  claim 1  or  2 , characterized in that the air is supplied to the fall tower in its lower region, particularly in the floor region.  
     
     
         7 . The method according to  claim 1  or  2 , characterized in that the air flows against the droplets in the lower region of the fall tower at a higher speed than in the upper region.  
     
     
         8 . The method according to  claim 1  or  2  characterized in that the fall tower is supplied air having a dew point temperature T t  of particularly −10° C.≦T t ≦−40° C.  
     
     
         9 . The method according to  claim 1  or  2 , characterized in that the air is guided in a first loop, which includes the fall tower, and a part of the air is removed from the first loop and supplied to a second loop, in which reaction substances such as ethylene glycol and/or butane diol, oligomers, and/or water are removed.  
     
     
         10 . The method according to  claim 9 , characterized in that the second loop is a spray loop, in which ethylene glycol and/or butane diol is sprayed, reaction substances condensed out in the second loop being supplied to an esterification or reesterification stage positioned upstream from the nozzle plate.  
     
     
         11 . The method according to  claim 10 , characterized in that the air removed from the first and/or the second loop, having reaction products contained therein which are not condensable in the second loop, such as acetaldehyde and/or tetrahydrofuran, is supplied to a heat transfer facility.  
     
     
         12 . The method according to  claim 1  or  2 , characterized in that the at least partially crystallized droplets are removed from the fall tower in the floor region of the fall tower via a slanted surface having openings which dry air flows through.  
     
     
         13 . The method according to  claim 12 , characterized in that the droplets, which are at least partially crystallized into the spheres, are transported floating and/or oscillating along the slanted surface and/or at least in the region of the openings present therein.  
     
     
         14 . The method according to  claim 12 , characterized in that the droplets, which are at least partially crystallized into the spheres, are classified after leaving the slanted surface.  
     
     
         15 . The method according to  claim 12 , characterized in that the droplets, which are at least partially crystallized into the spheres, reach an oversize separator from the slanted surface and particles separated there are supplied to a precondensation stage for the prepolymer.  
     
     
         16 . The method according to  claim 15 , characterized in that the particles are supplied to a precrystallization stage, which is positioned in a third loop which dry air flows through, after passing through the oversize separator.  
     
     
         17 . The method according to  claim 16 , characterized in that a portion is supplied to the second loop from the third loop, which includes the crystallization stage.  
     
     
         18 . A facility for producing spherical particles from polymer, particularly made of polyfunctional carboxylic acids and alcohols, particularly for producing PET or PBT pellets, including at least one nozzle device, which drips a molten prepolymer and/or polymer, a fall tower ( 20 ), positioned downstream thereto, which is positioned in a gas loop ( 40 ) via at least one gas intake opening at the floor and at least one outlet opening at the nozzle device, a transport device, positioned downstream from the fall tower, for the spherical particles at least precrystallized in the fall tower, and a crystallization stage, positioned downstream from the transport device, characterized in that the facility has a vibration exciter and a nozzle plate having nozzles which are distributed on an area, and the fall tower is positioned in the loop which guides the air and has a cross-section which is at least twice as large as the area.  
     
     
         19 . The facility according to  claim 18 , characterized in that the nozzles of the nozzle plate are distributed on an area which particularly corresponds to ¼ or ⅓ of the cross-section of the fall tower.  
     
     
         20 . The facility according to  claim 18 , characterized in that the nozzles are distributed on a circular area having a diameter D d , the fall tower has a circular cross-section having a diameter D f , and 1.5 D d ≦D f , particularly 2.0 D d ≦D f .  
     
     
         21 . The facility according to  claim 18 , characterized in that the nozzle plate and/or the molten prepolymer and/or polymer may be excited to vibration directly via the vibration exciter.  
     
     
         22 . The facility according to  claim 19 , characterized in that the diameter of the area formed by the nozzles of the nozzle plate has a ratio to the diameter of the fall tower like D d :D f  of approximately 1:2 to 1:10, particularly 1:5.  
     
     
         23 . The facility according to  claim 18 , characterized in that at least one device which increases the air intake speed is positioned in the fall tower in the region of its air intake opening.  
     
     
         24 . The facility according to  claim 18 , characterized in that the air outlet opening runs at a distance to the nozzle plate such that the particles dripped from the nozzle plate are subjected to an essentially laminar airflow directly after exiting the nozzle plate.  
     
     
         25 . The facility according to  claim 18 , characterized in that the transport device positioned in the floor region of the fall tower has a slanted surface, like a sieve or perforated plate, having openings which are permeated by dry air in such a way that the particles may be moved floating and/or oscillating along the surface.  
     
     
         26 . The facility according to  claim 25 , characterized in that the slanted surface ( 30 ) runs at a distance to the floor of the fall tower ( 20 ), and a first air intake opening is provided between the floor and the slanted surface and a second air intake opening of the air loop is provided above the slanted surface.  
     
     
         27 . The facility according to  claim 25 , characterized in that the slanted surface has an oversize separator positioned downstream, from which a crystallization stage operated using dry air is positioned downstream.  
     
     
         28 . The facility according to  claim 27 , characterized in that a line which guides oversize particles separated by the oversize separator leads to a precondensation stage positioned upstream from the nozzle plate.  
     
     
         29 . The facility according to  claim 18 , characterized in that a part of the air from the first loop, which includes the fall tower, may be supplied via a line to a purification stage forming a second loop.  
     
     
         30 . The facility according to  claim 29 , characterized in that the purification stage, a spray loop as the second loop, is connected via a line to an esterification stage positioned upstream from the nozzle plate.  
     
     
         31 . The facility according to  claim 29 , characterized in that a portion of the air having non-condensable substances may be supplied from the second loop to a heat transfer device via a line.  
     
     
         32 . The facility according to  claim 29 , characterized in that the crystallization stage is positioned in a third loop, which dry air flows through, connected to the second loop.  
     
     
         33 . The facility according to  claim 29 , characterized in that the second loop is connected via a line to the air intake openings or directly to the fall tower above the air intake opening, particularly above the device.  
     
     
         34 . The method according to  claim 9 , characterized in that the air removed from the first and/or the second loop, having reaction products contained therein which are not condensable in the second loop, such as acetaldehyde and/or tetrahydrofuran, is supplied to a heat transfer facility.