Patent Publication Number: US-2005141342-A1

Title: Mixer for aseptic liquids

Description:
This invention relates to a mixer for aseptic liquids.  
      Magnetically driven mixers have heretofore been provided. They have been commercially available from Vestec-Moritz Ltd., Lightnin and Apco Technologies as well as others. These commercially available magnetic mixers share a number of shortcomings. Typically such mixers have relatively large diameter weldments which require a large hole in the bottom wall of the tank. This large hole increases the lack of clearance in a typically already crowded region of the tank. In addition this large diameter opening increases the tendency of warpage in the tank bottom wall during contraction which occurs during welding of the weldment. This tendency for increased warpage causes variations in the assembly of the mixer and often causes weakening of the magnetic coupling in the mixer, resulting in inconsistent mixing. Also commercially available mixers are often difficult to clean in place. There is therefore a need for a new and improved mixer for aseptic liquids which overcomes these difficulties.  
      In general, it is an object of the present invention to provide a mixer for aseptic liquids in which closer and more reliable tolerances are achieved to provide improved magnetic coupling, making it possible to deliver higher torque.  
      Another object of the invention is to provide a mixer of the above character in which a weldment is provided for the tank in which the mixer is located that can accommodate a thick rigid weld to inhibit or prevent warpage of the tank bottom wall.  
      Another object of the invention is to provide a mixer of the above character having driven magnets which are caged to provide a rigid and permanent mounting for the magnets.  
      Another object of the invention is to provide a mixer of the above character which is designed to facilitate cleaning.  
      Another object of the invention is to provide a mixer of the above character which has capabilities for retaining the liquid within the tank in motion during draining of the tank.  
      Another object of the invention is to provide a mixer of the above character in which capability is provided for measuring the rotation of the hub from outside of the tank. 
    
    
      Additional objects and features of the invention will appear from the following description in which the preferred embodiments are set forth in detail in conjunction with the accompanying drawings.  
       FIG. 1  is a perspective view of a mixer for an aseptic liquid incorporating the present invention shown mounted in the bottom wall of a tank.  
       FIG. 2  is a cross sectional view of the mixer shown in  FIG. 1 .  
       FIG. 3  is a cross sectional view taken along the line  3 - 3  of  FIG. 2 .  
       FIG. 4  is a cross sectional view taken along the line  4 - 4  of  FIG. 2 .  
       FIG. 5  is a side elevational view of the weldment post used in the mixer shown in  FIGS. 1 and 2 .  
       FIG. 6  is a cross sectional view taken along the line  5 - 5  of  FIG. 2 . 
    
    
      A mixer for an aseptic liquid in a vessel having a wall and having an opening therein and containing the aseptic liquid to be mixed comprises a weldment post extending through the opening in the wall and being adapted to be welded into the wall to form a liquid tight seal with the wall of the vessel. The weldment post has a distal extremity inside the wall and has a proximal extremity outside the wall. The weldment post has a bore therein extending out through the proximal extremity of the weldment post. A magnet drive assembly is disposed in the bore in the weldment post. A drive motor is carried by the weldment post for driving the magnet drive assembly. A hub is rotatably mounted on the weldment post. Impellers are mounted on the hub. A magnet driven assembly is mounted in the hub and is disposed in close proximity to the magnet drive assembly but is separated therefrom by a liquid-tight seal. The magnet drive assembly includes a plurality of circumferentially spaced apart driven permanent magnets. Bars cage the driven permanent magnets to prevent movement of the driven permanent magnets with respect to the hub.  
      More particularly as shown in  FIGS. 1 through 6  of the drawings, the mixer  11  is shown mounted in the bottom wall  12  of a vessel or tank  13  and which is adapted to receive an aseptic liquid  14  to be stirred or mixed in the tank  13 .  
      The mixer  11  consists of a weldment post  16  which is welded into the bottom wall  12  of the tank  13 . As shown in  FIGS. 2 and 4 , the weldment post  16  is substantially cylindrical in shape and is formed of suitable non-ferrous metal as for example a low carbon stainless steel such as 316L so that it is transparent to magnetic flux. The weldment post  16  is provided with a distal cylindrical extremity  18  disposed within the vessel or tank  13  and extends upwardly from the bottom wall  12 . It is also provided with a proximal extremity  19  which extends downwardly from the bottom wall  12  of the vessel or tank  13 .  
      The weldment post  16  is provided with an axially extending bore  21  extending from the distal extremity and opening through the proximal extremity  19  so that the distal extremity  18  is provided with a thin side wall  22 . This thin side wall  22  should be as thin as possible while providing the necessary rigidity and strength to maximize magnetic flux transfer as hereinafter described. In accordance with the present invention, this wall thickness is preferably approximately 0.085″±5%.  
      The uppermost portion  23  of the proximal extremity  19  is of thickness extending radially outwardly from the bore  31 . It can have a thickness ranging from ¼″ to ½″ and preferably is approximately {fraction (3/8)}″ in thickness. It has a suitable length as for example 1″ to make it possible to accommodate different wall thicknesses for the bottom wall  12  of the vessel or tank  13 . That makes it possible to provide thick rigid welds as for example thick rigid upper and lower fillet welds  24  and  26  (see  FIG. 2 ) which can be utilized to inhibit or prevent warpage of the bottom tank wall  12 . The lowermost portion  27  is of reduced thickness and has provided thereon at its lowermost extremity a radially extending clamping flange  28 . An L-shaped slot  29  extends upwardly through the flange  28  and then extends circumferentially in the lowermost portion  27 . The slot  29  is provided for a purpose hereinafter described.  
      A magnet drive assembly  31  is rotatably mounted in the bore  21 . The magnet drive assembly  31  consists of a drive shaft  32  which through cooperative coupling means  33  is coupled to a drive shaft  36  of an electric motor  37 . The electric motor  37  can be of a suitable size as for example ½ HP up to 30 HP and can be AC or DC. Also alternatively it can be an air motor driven by compressed air.  
      The motor  37  is supported by the weldment post  16  by use of an adapter plate  41  which is adapted to fit NEMA frame motors as for example a 36C NEMA frame. The adapter plate  41  is provided with a plurality of bolt holes  42  as for example four equally spaced apart circumferentially of the adapter plate  41 . The adapter plate  41  is in the form of a circular flat plate that overlies the motor  37  and is secured thereto by bolts  43  extending through the holes  42  and threaded into the motor  37 . The adapter plate  41  is provided with a centrally disposed upstanding collar  46  which has a bore  47  extending therethrough. The collar  46  is provided with a radially extending clamping flange  51  on the outer surface thereof intermediate the upper and lower extremities of the collar  46  and is sized so that it can mate with the clamping flange  22  carried by the proximal extremity  19  of the weldment post  16 . A radially extending cylindrical pin  53  is mounted on the collar  46  above the flange  51  and is sized so that it can be moved up into the L-shaped slot  29  provided on the weldment post  16  and then rotated to provide a temporary support for the motor  37 . A sanitary clamp  56  of a conventional type is mounted over the mating flanges  28  and  51  to secure the flange  51  to the flange  22  and to thereby provide support for the adapter plate  41  and the motor  37  carried thereby.  
      A cylindrical shaft adapter  61  which is provided with a bore  62  for receiving the shaft  36  of the motor  37  and is secured to the shaft  36  by a key  63  so that it will rotate therewith. A shaft adapter top  66  is secured to the shaft adapter  61  by suitable means such as welding. The shaft adapter top  66  is provided with a centrally disposed male-type bayonet type fitting  67  forming the male portion of the cooperative coupling means  33  which is formed by spaced apart tapered side walls  68  adjoining spaced-apart vertically extending walls  69 . The magnet drive shaft  32  is formed of a suitable material such as 1018 stainless steel. The drive shaft  32  is provided with a female-type bayonet-type recess  71  which serves as the female portion of the cooperative coupling means  33 .  
      The drive shaft  32  is generally cylindrical in configuration and has an outer cylindrical surface  72  that is provided with a plurality as for example 12 axially extending recesses  76  as shown in  FIG. 5  which are generally rectangular in cross section and which open through the outer cylindrical surface  72 . A permanent drive magnet  77  is mounted in each of the recesses  76  and extends the length of the recess. The drive magnets  77  are typically of the rare earth type that generate a strong magnetic flux. For example one suitable rare earth material is samarium cobalt. It has very good magnetic qualities; however, it is relatively brittle and is also relatively expensive. The drive magnets  77  are generally rectangular in shape and in cross section fit closely within the rectangular recesses  76 . They are held in place in a suitable manner such as by epoxy (not shown) so that the outer curved surfaces  78  of the magnets  77  are generally flush with the outer cylindrical surface  72  of the magnet drive shaft  71 . The magnets  77  have longitudinal axes which are parallel to the axis of the shaft  32 . The permanent drive magnets  77  have north and south poles which are perpendicular to the longitudinal axes. The outer surfaces  78  of the drive magnets  77  have north and south poles alternately spaced apart circumferentially around the cylindrical surface  72  of the drive shaft  36 . The clearance between the outer surfaces or faces  78  of the drive magnets  77  and the inside surface of the wall  22  of the weldment post  16  may be from 0.150″ to 0.20″ and preferably about 0.185″. It should be appreciated that it is desirable to have the faces of the magnets  77  travel in as close proximity as possible to the inner surface of the very thin stainless steel wall  22  of the weldment post  16  so there is a maximum transfer of magnetic flux from the drive magnets  77  through the wall  22  of the distal extremity  18  of the weldment post  16 .  
      Means is provided for supporting the drive shaft  36  so it is rigidly and precisely maintained in its rotational position within the weldment post  16 . Upper and lower double sided ball bearings  81  and  82  of a conventional type serve to mount the drive shaft  32  within the weldment post  16  and also serve to center the magnet drive shaft  32  and to hold it rigidly in position. The lower ball bearing  82  is secured in the bore  21  in a suitable manner such as by use of an epoxy. The upper ball bearing  81  is held in place by a snap ring  83  engaging the shaft  32 . As can be seen, the drive magnets  77  are retained in their axial positions by the ball bearings  81  and  82  as well as being epoxied into place as hereinbefore described.  
      In case of disassembly, the magnet drive assembly  31  can be removed by threading a bolt (not shown) into a threaded opening (not shown) provided in the upper extremity of the drive shaft  32 .  
      An impeller drive assembly  86  is mounted over the uppermost portion  23  of the distal extremity  18  of the weldment post  16 . The impeller drive assembly  86  consists of hub  87  also formed of a non-ferrous material such as stainless steel 1013. The hub  87  is provided with a lower cylindrical extremity  88  that has a radially outwardly extending flange  89 . A bore  91  extends through the hub  87  and is of varying diameters. In the lower cylindrical extremity  88 , the bore  91  has a size so that a thin wall  92  is provided having a thickness which generally corresponds to the thickness of the wall  22  of the weldment post  16 . The hub  87  is also provided with an intermediate flared portion  93  which forms a shoulder  94  which is generally parallel but spaced apart from the radially extending flange  89  with the thin wall  92  extending therebetween. The hub  87  is also provided with a cylindrical upper extremity  96 . The lower cylindrical extremity  88  in conjunction with the flange  89  and the shoulder  94  forms a part of a magnet cage assembly  97 . The magnet cage assembly  97  includes a plurality of circumferentially spaced apart elongate driven magnets  98  which also are rectangular in shape and cross section and have longitudinal axes which are parallel to the axis of the shaft  32 . The magnets  98  have inner curved surfaces or faces  99  which are disposed in very close proximity to the inner surface of the cylindrical wall.  
      These driven magnets  98  are provided with north and south poles which face in directions perpendicular to the longitudinal or vertical axes of the driven magnets  98 . Alternate circumferentially spaced apart faces of the driven magnets  98  are alternatively north and south poles. These driven magnets  98  are of the permanent type and can be formed of the same material as the drive magnets  77 .  
      These driven permanent magnets  98  are caged within a cage  101  formed of mild steel-bars generally rectangular in cross section surrounding the driven magnets in conjunction with the wall  98  (see  FIG. 5 ). Three bars are provided for each magnet with two side bars  102  and  103  being disposed on opposite sides of the driven magnet  09  and the third or outer bar  104  being disposed on the outer surface of the driven magnet  98  opposite the face or inner surface  99  to encompass three of the four sides of each driven permanent magnet  98 . These bars  102 ,  103  and  104  are tack welded to each other and to the wall  98  of the lower cylindrical extremity  88  of the hub  87  to thereby encase the driven magnets  98  to prevent movement of the driven permanent magnets  98  with respect to the hub  87 . The upper and lower extremities of the driven magnets  91  are also caged by two upper and two lower semicircular magnet rings  106  and  107  which are also tack welded to the bars  102 ,  103  and  104  at opposite ends thereof. The lower magnet rings  107  are seated upon the outwardly extending radial flange  89  and are also tack welded to it. The upper magnet rings  106  are also tack welded to the shoulder  94 .  
      A hub sleeve  108  is seated over the cage  101  and is welded to the hub  87  to provide a fluid-tight enclosure for the magnet cage assembly  97  so that the space containing the magnet cage assembly  97  cannot be entered by any liquid within the vessel or tank  13 .  
      In order to provide a pumping action for lower levels of liquid within the tank  13 , the hub sleeve  108  is provided with a plurality of circumferentially spaced apart notches  109  (see  FIG. 7 ) as for example three which are spaced apart 120 degrees opening through the lower extremity of the hub sleeve  108  and facing outwardly from the hub sleeve  108 . Each of the notches  109  is formed with a surface which  111  is inclined inwardly at a relatively small angle as for example 30° which adjoins another surface  112  which extends outwardly to provide a notch  109  which is generally L-shaped in configuration. Each of the notches  109  can have a suitable length as for example ½″ and have a height of approximately ¼″. The hub sleeve  108  is provided with an upper inclined surface  113  which mates with and adjoins an outer surface  114  of the intermediate flared portion  93  of the hub  87  to form a continuous sloping surface between the surfaces  113  and  114 .  
      As shown particularly in  FIG. 2  the magnet cage assembly  97  carried by the thin walled lower cylindrical extremity of the hub  87  fits over in relatively close proximity to the thin walled distal extremity  18  of the weldment post  16 . With thin walls  22  and  98  being immediately adjacent to each other, the magnetic flux interaction between the drive magnets  77  and the driven magnets  98  is maximized.  
      Means is provided for rotatably supporting the hub  87  on the weldment post  16  and consists of a male bearing  116  which is generally L-shaped in cross section and a cylindrical female bearing  121 , a large lower o-ring  122  and a small o-ring  123 . The large o-ring  122  is first placed on the uppermost portion  23  of the distal extremity  18  of the weldment post  16  followed by the male bearing  116  which is secured to the weldment post  16  by a suitable means such as welding so that it remains stationary with the weldment post  16 . Thereafter, the female bearing  121  is mounted on the male bearing  116  to mate therewith for rotation thereon. The small o-ring  123  is seated over the female bearing  121 . The male bearing  116  and the female bearing  121  are formed of a suitable material such as a tungsten carbide with a 12% nickel binder. Alternatively and also for use in the biopharmaceutical industry a silicon carbide can be used. The o-rings  122  and  123  are conventional TEF-steel o-rings which are very desirable in the present use because they are long wearing and do not deform and provide a good liquid-tight seal.  
      A bearing shaft  131  is mounted on top of the uppermost portion  23  of the distal extremity  18  of the weldment post  16 . The bearing shaft  131  is provided with threads  132  threaded into a threaded bore  133  in the uppermost portion  23 . The bearing shaft  131  has a cylindrical portion  134  that fits into the male bearing  116  and permitting rotation of the female bearing  121  thereon. The bearing shaft  131  is provided with a pair of flats  136  to facilitate insertion and removal of the bearing shaft  131  with a tool (not shown).  
      A plurality of inclined holes  146  are provided in the hub and extend into the bore  141  provided in the hub  87 . These holes serve to create low pressure areas on the outside of each hole in the hub so that during rotation of the hub these low pressure areas act as a pump to accelerate the flow of liquids up through the center of the impeller through the bore  141 . This flow of liquid through these holes has two functions. The liquid serves to lubricate the bearings  114  and  116  and also takes away any heat which is created by friction in movement of the bearings  114  and  116  to thereby distribute the friction-created heat throughout the liquid in the vessel or tank  13 . The inclined holes  146  are inclined at a suitable angle ranging from 5 to 15 degrees. As shown in  FIG. 1 , the holes are provided adjacent the very top edge of the female bearing  121 . The bearing shaft  131  is provided with inclined surfaces  148  to provide smooth surfaces for receiving the inflow of liquid from the holes  141  and passing upwardly through the bore  142 .  
      A plurality of impellers  151  as for example three as shown in the drawings are mounted on the hub  87 . As desired by the customer, a wide range of different types of impellers or blades can be provided to achieve the desired mixing in the vessel or tank  13 .  
      In order to measure the speed of rotation of the hub carrying the impellers  151 , a strong permanent magnet  156  serving as a sensor element is mounted in the proximal extremity or base  94  of the hub  95 . Sensing means  157  is mounted below the bottom wall  12  on the sanitary clamp  56  for sensing each rotation of the sensor element  156  and therefore directly measures the speed of rotation of the impellers  151  inside the tank  13  and not the speed of the drive motor  37 .  
      A conventional controller  161  is provided as a part of the mixer  11  for operating the motor  37 . It is provided with an on-off toggle switch  162  and a speed control knob  163  as well as other features.  
      Operation and use of the mixer  11  of the present invention may now be briefly described as follows. The mixer  11  can be installed by cutting a small hole as for example a two diameter hole in the bottom wall  12  of the tank if one is not already present and then welding the weldment post  16  into place to provide a weldment post  16  which extends upwardly into the interior of the vessel or tank  13 . The hub  87  carrying the impellers  151  and the impeller drive assembly  86  is lowered onto the weldment post  16 .  
      As soon as this has been accomplished, the magnet drive assembly  31  can be inserted from outside of the vessel or tank  13  into the bore  21  of the weldment post  16 . When the drive assembly  31  is approximately one half way toward the home position, the magnetic forces of the drive magnets  77  coacting with the magnetic forces from the driven magnets  91  cause the drive assembly  31  to snap into place and be automatically aligned circumferentially. As soon as this has been accomplished, the adapter plate  41  carrying the motor  37  can be moved into place to cause the pin  53  to be seated in the bayonet type recess  71  of the weldment post  16 . Thereafter, the sanitary clamp  56  can be affixed to secure the adapter plate  41  and the motor  37  carried thereby in a fixed position.  
      As soon as the assembly of the mixer  11  has been completed, the vessel or tank  13  is ready to be used in a process. When the vessel has been filled to the desired level with a liquid to be mixed in the tank  13 , the mixer can be placed in operation to cause agitation and mixing of the liquid in the tank in a conventional manner. During the mixing of the liquid, liquid will pass through the holes  141  which are inclined in a backward direction with respect to the direction of rotation of the impeller blades  151  to cause liquid to enter through the holes  141  and to cool the male and female bearings  116  and  121  and also collect any heat which may be generated by the bearings and distributing this heat throughout the liquid in the tank or vessel. By this continuous mixing, the cells in the liquid are kept in motion. No cells are trapped within the mixer where they could die. By this continuous mixing in this manner it is possible to keep the cells alive by maintaining them in contact with oxygen bubbles in the liquid.  
      The holes  141  also serve an additional function during cleaning of the tank. For example cleaning may be accomplished with the mixer assembly in place by flooding it with a cleaning liquid to actually immerse the impeller blades  151  while they are rotating in the cleaning liquid followed by cleansing the same in water. Alternatively the tank can be drained of liquid and then a cleaning fluid introduced by the use of a spray ball (not shown) mounted near the top of the tank to shower the mixer assembly with the cleaning fluid. The holes  141  facilitate movement of the cleaning liquid through to clean the mixer assembly. As the liquid in the tank is being drained from the tank it falls below the level of the impeller blades  151 , mixing of the liquid in the tank below this level normally would be radically eliminated but for the fact that in the present invention, notches  117  have been provided at the bottommost surface of the hub  87  to cause agitation and mixing of the liquid in the lowermost levels of the tank. Thus, substantially continuous mixing of the liquid in the tank occurs even when the liquid is being drained from the tank. By the use of the strong magnet on the bottom of the hub, it is possible to measure true impeller rpm rather than the speed of the motor driving the impeller hub.  
      Thus it can be seen that the mixer  11  of the present invention has a number of important features. As pointed out above, true impeller rpm is measured to make possible timed mixing cycles. The construction of the mixer  11  is very robust and can be readily installed without warpage of the tank wall. Also it has a relatively small footprint. A variety of agitation blades can be provided on the impeller hub. By installation of the weldment posts of the type hereinbefore described in a plurality of tanks, it is possible to move the motor drives and impeller hubs from tank to tank as needed. The construction of the hub makes it possible to agitate and mix the liquids in the tank during drainage of the tank until the tank is completely drained. The use of the weldment post of the present invention provides a rigid cylindrical portion of substantial height for welding into a small diameter hole in the tank bottom without deformation of the tank bottom.  
      Since the sensing element is mounted in the impeller hub  87  it is possible to measure the rotation of the hub  87  from outside the tank. The use of the weldment post of the present invention makes it possible to work with a small hole in the bottom wall of the tank. By providing thick rigid welds between the weldment post and the tank, warpage is reduced to a minimum. By utilizing close tolerances and thin walls separating the drive assembly from the driven assembly, it is possible to provide a much closer magnetic coupling between the two assemblies and thereby deliver a higher torque. By providing a drive unit which is centered between the upper and lower ball bearings it is possible to maintain a rigid position for mounting of the magnet drive assembly. By mounting the driven magnets in cages it is possible to keep the magnets locked in welded cages so they cannot move, preventing wobbling and degeneration of the magnetic coupling between the drive assembly and the driven assembly. The large backwardly facing holes in the rotating impeller hub inside the tank create low pressure areas which during rotation act as a pump during normal operation of the mixer and also to pull cleaning fluid through the inside of the hub during cleaning operations. The large diameter bore in the impeller hub permits liquids to flow therein without substantial interference. The parts used in the mixer have been designed with radiused edges to reduce the retention of a meniscus on the bearings and thereby supports the flow of aseptic liquids and cleaning fluid. The large bore in the impeller hub can accommodate the use of an extension shaft (not shown) for delivering the desired agitation to all areas of the tank, thereby combining the abilities of top mounted and bottom mounted mixers.