Patent Document

BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to the removal of molten polymer from a polymer mixer. More specifically, this invention relates to the removal of molten polyethylene from a continuous polymer mixer. 
         [0003]    2. Description of the Prior Art 
         [0004]    Although, for sake of clarity and brevity, this invention is described in terms of conveying molten polyethylene, this invention is not limited to that type of polymer. 
         [0005]    Ethylene is polymerized to polyethylene homopolymers and co-polymers by a number of different processes to make different polymeric products such as low density polyethylene, high density polyethylene, and linear low density polyethylene which exhibits favorable characteristics found in both low density and high density polyethylene. For sake of example only, this invention is described herein primarily in terms of a slurry phase (suspension) polymerization process for making high density polyethylene (HDPE). 
         [0006]    The slurry polymerization process typically takes place in a closed loop (horizontal or vertical) reactor using a hydrocarbonaceous solvent such as n-hexane, isobutane, isopentane, and the like. The essentially liquid feed mixture of ethylene, co-monomer(s), if any, catalyst, and any additives is continuously pumped in a loop while the polymerization reaction takes place. 
         [0007]    The process can employ known catalyst systems such as a silica-supported chromium/aluminum catalyst with or without a co-catalyst such as triethyleborane, or Ziegler-Natta catalyst systems comprised of titanium tetrachloride/trialkyl aluminum, or other transition metals such as zirconium and vanadium in place of the titanium. These catalyst systems are well known in the art and more detail is not necessary to inform one skilled in the art. 
         [0008]    While the aforesaid feed mixture is continuously circulated in the loop reactor, polymerization takes place at temperatures below the melting point of the polyethylene formed thereby producing a slurry of solid polyethylene particles in the liquid feed mixture. The reaction typically takes place at a temperature of from about 185 to about 220 degrees Fahrenheit (F.) at a pressure of from about 500 to about 650 psig. A slurry containing, among other things, HDPE and solvent is drawn off from the reactor either continuously or intermittently, as desired. 
         [0009]    The loop reactors are normally formed from large diameter pipes, e.g., from about 10 to about 30 inches in inside diameter, and can be about 50 feet across with lengths of from about 250 to about 300 feet in length. 
         [0010]    The slurry withdrawn from the reactor is processed for the removal of solvent for re-use in the reactor. The remaining solid polyethylene particles are then passed to a drying, mixing, extruding, and pelletizing system wherein the particles are converted to solid polyethylene pellets. The pellets are packaged and marketed as a product of the polyethylene production plant in which the foregoing process was carried out. 
         [0011]    A high shear mixer such as a commercially available fifteen inch Farrell continuous mixer has been employed in this system. The operation of the mixer unit is to receive solid polyethylene powder at a temperature of from about ambient to about 140 F, and to mix this powder until the mixing action raises the temperature of the powder to a temperature of from about 360 to about 420 F, thereby melting the powder and forming a stream of molten HDPE. 
         [0012]    The molten polymer is removed through an outlet orifice gate carried by the mixer and transferred through a chute/hopper conduit combination to an extruder unit in which the molten polymer is extruded as a first step toward making solid polymer pellets suitable for storage, packaging, and the like. 
         [0013]    It is the orifice gate of the mixer in the drying, mixing, extruding, and pelletizing system to which this invention is directed. The extent of the mixing undergone by the polymer in the mixer determines the temperature of the polymer when it leaves the mixer and enters the outlet gate orifice. 
         [0014]    Heretofore, the hinged gate inside the outlet orifice of the mixer carried a clevis at its lower end. This clevis was internally threaded and thereby connected to a threaded shaft end, the opposing end of this shaft being connected to an actuator. 
         [0015]    Operation of the actuator moves the hinged orifice gate backward or forward, as desired. 
         [0016]    Movement of the gate backward gradually opens the orifice further, thereby allowing a greater volume of polymer to leave the mixer through the outlet orifice. This shortens the residence time for the polymer in the mixer, and lowers the temperature of the polymer exiting the outlet orifice. 
         [0017]    Movement of the gate forward gradually closes the orifice further, thereby allowing a lesser volume of polymer to leave the mixer through the outlet orifice. This retains the polymer in the mixer for a longer mixing time, and thereby raises the temperature of the polymer exiting the outlet orifice. 
         [0018]    Thus, the orifice gate of the mixer is used to vary, as and when desired, the melt temperature of the polymer leaving the mixer. The orifice gate/actuator combination affords an infinite number of gate settings that control the amount of molten polymer leaving the outlet orifice in which the gate is movably carried. Accordingly, great flexibility is available in achieving the desired melt temperature of the polymer exiting the outlet gate orifice of the mixer. 
         [0019]    In actual operation, frequent failure of the actuator shaft at its threaded end was experienced. This failure required that the polymer mixer be shut down, the mixer opened, the broken threaded portion in the lower part of the gate drilled out, and a new threaded shaft installed in place of the failed shaft. This repair work usually translated into 14 to 24 hours of mixer downtime and lost mixer production, an expensive loss. 
         [0020]    Surprisingly, it was found that even though the actuator shaft was carried essentially horizontally between the actuator and the orifice gate, when the actuator was operated to move the gate forward, a net downward force was exerted on the threaded end of that shaft which caused the frequent failures of this type of shaft. 
         [0021]    This invention addresses and corrects the failure mode of the aforesaid threaded actuator shafts. 
       SUMMARY OF THE INVENTION 
       [0022]    Pursuant to this invention, there is provided a method and apparatus for operating a polymer mixer orifice gate which employs a flanged clevis that has a depression therein and a flanged actuator shaft that carries a matching protuberance so that when the flanges are fixed to one another, the protuberance slidably fits into the depression. 
         [0023]    This arrangement was found to eliminate the net downward force on the clevis and the failure mode of the prior art threaded actuator shafts. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0024]      FIG. 1  shows a partial cross-section of a conventional polymer mixer and its relation with its outlet orifice gate assembly and actuator. 
           [0025]      FIG. 2  shows a close-up of the prior art threaded actuator/shaft/clevis assembly. 
           [0026]      FIG. 3  shows an exploded view of the assembly of  FIG. 2 . 
           [0027]      FIG. 4  shows an end view of the prior art clevis of  FIGS. 1-3 . 
           [0028]      FIG. 5  shows one embodiment of the flanged depression/protuberance assembly of this invention. 
           [0029]      FIG. 6  shows the sliding mating of the protuberance and depression of the apparatus of  FIG. 5  when assembled for operation by an actuator and its shaft. 
           [0030]      FIG. 7  shows the terminal end of the actuator shaft flange of  FIG. 6 . 
           [0031]      FIG. 8  shows the terminal end of the clevis flange of  FIG. 6 . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]      FIG. 1  shows a partial section of a typical prior art polymer mixer  1 . Mixer  1  has an enclosed body  2  in which is carried a pair of vertically opposed mixing rollers, the lower of which is shown, in part, as element  3  in  FIG. 1 . This pair of rollers, upon rotation by a motor, not shown, imparts high shear mixing to the polymer carried in the inner volume  4  between body  2  and roller  3  in well known manner. Lower roller  3  is carried at its non-motor end by a bearing assembly  5 . Cooling fluid is circulated through the interior of roller  3  by way of piping  6  and  7  as shown by arrows  8  and  9 . For more details on mixer  1  as a whole and its mixing rollers see U.S. Pat. No. 7,392,988. 
         [0033]    Rotation of roller  3  moves the polymer in the direction shown by arrow  10  toward an outlet window  15 . Below window  15  an outlet orifice  16  is carried in a fluid communication manner with that window and interior  4  as shown by arrow  17 . 
         [0034]    Outlet orifice  16  has an upper polymer inlet at window  15  and a lower polymer outlet at  18 . Between opposed inlet and outlets  15  and  18  extends an elongate, essentially closed hollow orifice body  19 . Body  19  can have any desired cross-sectional configuration from circular to square and anything in between, but is normally square or rectangular. Whatever its cross-sectional configuration, body  19  has enclosing, opposing elongate sides. In  FIG. 1  the cross-sectional configuration is, as an example, square so that opposing elongate sides  20  and  21  make up part of body  19 . 
         [0035]    An orifice gate  25  is carried essentially diagonally internally of body  16   
         [0036]    The upper end  27  of orifice gate  25  is hinged at  26  to body  2  so that the lower end  28  of gate  25  can be moved toward or away from elongate side  21 . 
         [0037]    The lower end  28  of gate  25  carries a laterally extending member  29 . Member  29  extends essentially laterally toward elongate side  20 . Side  20  has an aperture  30  therein through which member  29  can be moved. 
         [0038]    Member  29  carries at its terminal (free), distal end a hinged clevis  31 . 
         [0039]    Clevis  31  threadably engages one end of shaft  32  which shaft is carried by actuator  33  at its opposing end. Actuator  33  can be pneumatic, hydraulic or the like as desired. By operation of actuator  33 , shaft  32 , and, therefore, clevis  31  and lower end  28 , can be moved toward or away from side  21  at will as shown by arrows  34  and  35 . 
         [0040]    In  FIG. 1  shaft  32  is shown in an extended mode in the direction of arrow  35  thereby moving gate  25  to an essentially closed position which allows essentially no polymer to flow (arrow  36 ) to outlet  18 . This is not a normal setting when the mixer is in operation. Typically, in operation, orifice gate  25  is maintained more than halfway open. The extent of the gate opening is regularly varied in operation in order to vary the melt temperature of outlet polymer  36 . To open the orifice to allow for a greater volume of polymer  36  flow, actuator  33  is operated to move shaft  32  in the direction of arrow  34 . 
         [0041]    The cyclic movement of shaft  32  forward against molten, viscous polymer  17 , and then backward, together with the unobvious net downward force on the shaft end that is threaded into clevis  31  resulted in stress cycling (cyclic fatigue) that caused the threaded end, element  48  in  FIG. 3 , to fail prematurely and regularly in the prior art equipment. 
         [0042]      FIG. 2  shows an enlarged view of the actuator  33 /shaft  32 /clevis  31 /member  29  assembly, and better shows that clevis  31  is hinged to member  29  at  40 .  FIG. 2  also better shows that the distal end of shaft  32  is threaded into an interior threaded recess  41  of clevis  31 . 
         [0043]      FIG. 3  shows the assembly of  FIG. 2  when disassembled.  FIG. 3  shows distal end  45  of member  29  carrying hinge pin  40 . Clevis  31  is shown to carry an aperture  46  that matches and receives pin  40  when assembled as shown in  FIG. 2 . The opposing end  47  of clevis  31  carries an internally threaded opening  41 . Shaft  32  is shown to have an externally threaded shaft end  48  that is sized to threadably engage opening  41  until shaft face  49  abuts clevis face  47  as shown in  FIG. 2 . It is this threaded end  48  that failed in the prior art equipment due to stress cycling, and it is this type of failure that Applicant&#39;s invention eliminates. 
         [0044]      FIG. 4  shows an end view of clevis  31  with its slot  50  that is adapted to receive clevis  31  and allow aperture  46 ,  FIG. 3 , to align with and receive pin  40 . 
         [0045]      FIG. 5  shows one embodiment of this invention wherein the threaded clevis of  FIGS. 1-3  is replaced with a flanged clevis  55 , and the threaded actuator shaft  32  of  FIGS. 1-3  is replaced with a flanged actuator shaft  56 . 
         [0046]    More specifically clevis  55  is shown to have a normal clevis member  57  with an aperture  58  that aligns with and receives pin  40  as in  FIGS. 1-3 . However, clevis  55  carries at its opposing, shaft meeting end, flange  59 . The terminal end  60  of flange  59  carries a depression  61  of finite depth  62 . 
         [0047]    Actuator shaft  65  carries at its distal end from actuator  33  a flange  66 . Flange  66  carries at its terminal end  67  a protuberance (projection)  68  of finite depth  69 . 
         [0048]    Protuberance  68  is of a cross-section that allows sliding, mating bossed, movement into the interior of depression  61  up to its depth  69 . Depth  69  can be equal to or somewhat less than depth  62 . The cross-section of depression  61  and protuberance  68  can vary so long as a sliding fit is provided as faces  60  and  67  are brought into abutment and bolted together by way of holes  70  and  71 . The cross-section can vary from circular to square or rectangular as desired. 
         [0049]      FIG. 6  shows terminal face  67  of flange  66 , and further shows that in this particular example protuberance  68  is essentially circular in configuration. 
         [0050]      FIG. 7  shows terminal face  60  of flange  59 , and further shows that in this particular example depression  61  has a matching circular configuration that is just slightly larger in diameter than protuberance  68 . 
         [0051]      FIG. 8  shows the flanged shaft  56  and clevis  55  of  FIG. 5  when flange faces  60  and  67  are brought into contact and fixed in that position for operation of mixer  1 . In this operating configuration, protuberance  68  has slid into the interior of depression  61  up to its depth  69 ,  FIG. 5 , which positioning is maintained by flanges  59  and  66  being fixed to one another. 
         [0052]    In  FIG. 8 , the transverse clearance  72  between an exterior side of protuberance  68  and an opposing internal wall of depression  61  can vary so long as stress cycling failure is eliminated, and can be in the range of from about one thousandth to about one tenth of an inch. The clearance  75  between end face  73 ,  FIG. 5 , of protuberance  68  and bottom face  74  of depression  61  can vary from essential abutment up to a finite clearance within the range of transverse clearance  72 . 
         [0053]    It was surprisingly found that even though flanges  59  and  66  are bolted to one another, there is still sufficient movement allowed between protuberance  68  and depression  61  to eliminate the net downward force on the assembly caused by movement of the actuator shaft, and thereby eliminate the problem of stress cycling failure at the point where the actuator shaft is fixed to the clevis. 
         [0054]    It has also been surprisingly found that this invention increases the load carrying capability of the actuator shaft. 
         [0055]    A substantial advantage for this invention is that the mixer need not be shut down should it become necessary to repair the actuator because the flanges allow the gate to be clamped in an open position.

Technology Category: 7