Patent Publication Number: US-8979356-B2

Title: Dual agitator mixer with sanitary tank

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to mixers of the type employed in sanitary processing of pharmaceuticals, cosmetics, or other products that are to be kept safe for human use, and is more specifically related to an improvement in dual agitator mixers that permits cleaning in place of all interior surfaces. 
     Coaxial, dual-agitator and single-agitator mixers are employed in industries such as pharmaceuticals, cosmetics, and food products where the ingredients to be mixed and the resulting product need to be kept sanitary or in some cases sterile. 
     In practice, these mixers are in the form of a large tank or similar vessel, which may have a capacity from about twenty gallons up to tens of thousands of gallons, usually formed of stainless steel, either with one agitator or with two internal agitators that rotate in opposite directions. Usually these are top-entering mixers, with coaxial vertical drive shafts that penetrate the center of the closed top of the vessel, with a lower-speed outer scraper agitator and a higher-speed inner agitator. The drive shafts are collocated with the axis of the tank or vessel, so that the agitators are centered in the vessel. The two agitators have respective drive shafts, driven by independent gearmotors, which can be electric or hydraulic, with an upper gearmotor driving the inner agitator and the lower gearmotor driving the outer or scraper agitator. 
     In mixers of this arrangement, the inner drive shaft that drives the inner agitator descends through a hollow outer shaft that drives the outer agitator. The outer shaft ends a short distance below the point where it enters the mixing vessel, with the inner shaft continuing on downward. This arrangement requires that there be two rotary seals: one seal where the outer shaft penetrates the top or head of the vessel, and a second rotary seal where the inner shaft exits the lower end of the hollow outer shaft. This design closes off and seals the hollow annular space between the inner shaft and the outer shaft. 
     A number of issues arise where the second seal, or annular space seal, resides within the tank or vessel. The inner seal is difficult to clean, especially when using clean-in-place or C.I.P. techniques. The seal may also fail, resulting in pieces of the seal falling into the product. Servicing the seal requires a person to enter the confined space inside the vessel, which leads to safety concerns for the user. In this typical arrangement the inner seal is often three feet or more away from the associated gearmotor, which makes the seal more susceptible to increased shaft run-out, carbon wear, which may contaminate the product, and premature seal failure. Also, any oil leakage from the upper gear motor will tend to run into the annular space where it may escape past the inner seal and enter the product in the vessel. 
     Where a clean-in-place or C.I.P. system is used to clean and sanitize the vessel and agitators between uses, only the agitators and the outer surfaces of the drive shafts can be exposed to the spray of the C.I.P. fluid, and it has not been possible to spray or flow C.I.P. fluid into the annular space. Even if an inner seal for the annular space is located outside the vessel, there is currently no satisfactory technique for cleaning the annular space and, despite that space being out of direct contact with the product itself, the annular space is continuously exposed to various vapors, and presents a significant cleaning concern. 
     Traditional C.I.P. cleaning of agitators and seal areas is carried out using in-tank spray devices. Some seals have provisions for flushing (which is not effective cleaning), it would be preferable to have a rotating spray of the C.I.P. fluid sprayed directly onto the seal faces and seal glands. Seal design is an industry concern, especially in terms of efforts to make the seals more sanitary. However, current modes of internal cleaning do not reach areas such as seals between inner and outer drive shafts, nor do they reach the annular spaces between inner and outer drive shafts. 
     According to an aspect of this invention, a sanitary mixer arrangement comprises a vessel with one or more agitators and associated drives. The vessel contains the ingredients that are to be mixed, and has a side wall and a closed top, with the mixer vessel having a vertical axis. In a dual-agitator mixer, an outer or scraper agitator and an inner agitator are driven respectively by an outer vertical tubular drive shaft and inner vertical drive shaft coaxial with the outer drive shaft. The drive shafts penetrate an opening at the closed top of the vessel and descent along the axis of the vessel. An outer seal is positioned at the opening in the closed top of vessel around the outer drive shaft and permitting rotation of the outer drive shaft. A lower drive arrangement rotates the outer drive shaft and is positioned above the closed top of the vessel. An upper drive arrangement drives the inner drive shaft and is positioned above the lower drive arrangement. 
     The outer drive shaft has an upper terminus beneath the upper drive arrangement. The inner drive shaft extends up above that terminus. The outer drive shaft and said inner drive shaft define an annular space between them that descends down from the terminus and through the closed top of the vessel, with the annular space being open into the interior of the vessel. The inner seal is mounted in said annular space at the terminus of said outer drive shaft, permitting rotation of the inner drive shaft relative to the outer drive shaft. 
     A central bore is drilled along the axis of the inner drive shaft from an upper end thereof down to a lower end position below the inner seal and above the closed top of the vessel. 
     A permanent rotary fitting at the upper end of the inner drive shaft permits the introduction of a cleaning fluid under pressure into the central bore. The cleaning fluid passes through the bore and the at least one radial passage into the annular space to clean accumulations from within the annular space and exhaust it into the interior of vessel. 
     Favorably, the outer agitator has a plurality of scraper vanes that sweep past the interior surface of the vessel side wall. 
     In a preferred embodiment, the inner drive shaft is solid below the lower end position of said central bore. 
     Several radial passages extend from the lower end position of the central bore to said annular space, to conduct the fluid from the central bore into the upper end of the annular space. That is, the upper part of the inner shaft is “rifle bored” with an axial passage, and at the bottom end of this passage the shaft is cross-bored so that there is a series of holes to create a rotary spray device with the cross-bored holes being strategically placed to wash the underside of the inner seal and to flush out the annular space. 
     A rotary union installed at the top end of the inner shaft allow a C.I.P. pipe to be permanently connected, so that the annular space can be flushed, automatically, every time the vessel C.I.P. system is run. The C.I.P. system can be hard-piped to the rotary union, so that the cleaning of the annular space occurs without any operator interaction. This results in the elimination of any cleaning concern about the annular space, and eliminates the potential for carbon dust to build up on the seal faces and fall to into the product over time. This arrangement also allows for service without need for an expensive confined-space entry. 
     In the described embodiment, the outer drive shaft has at least one cleaning slot extending therethough at a position below the closed top of the vessel and above a lower end of the outer drive shaft. An in-tank spray device within said vessel and near the top or head directs a spray of the C.I.P. fluid towards outer drive shaft to spray cleaning fluid at the cleaning slot or slots. This can provide additional cleaning of the lower part of the annular space and wash out any accumulated carbon or other possible contaminant. 
     The through-the-shaft-cleaning design can likewise be applied to a single-mixer design where the fluid is supplied into an axial bore in a single drive shaft to clean the shaft seal faces and also to clean the annular space around the shaft where it enters the tank head, especially in those cases where the space is long, e.g., where the tank head is insulated. 
     As is the conventional practice, the C.I.P. cleaning solution is made up as an aqueous solution of a caustic material, such as potassium hydroxide (KOH). At the end of the caustic phase of the cleaning process, the caustic solution and any entrained solids are discharged to a drain, and the solution is considered waste. A caustic recovery system may fitted to the C.I.P. system. 
     While the invention has been described with reference to a few selected embodiments, it should be recognized that the invention is not limited to those precise embodiments. Rather, many modifications and variations will be apparent to persons skilled in the art without departing from the scope and spirit of this invention, as defined in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is an elevation of the mixer and tank or vessel, according to one preferred embodiment of this invention. 
         FIG. 2  is a sectional elevation of the mixer. 
         FIG. 3  is a detailed sectional view thereof. 
         FIG. 4  is a partial sectional view of a single-shaft embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to the Drawing, and initially to  FIG. 1  thereof, a large industrial mixer  10  has a generally cylindrical tank or vessel  12  formed of stainless steel, with a top or head  14  closing off its upper end. The mixer also has an upper gear motor  16  positioned above a lower gear motor  18 , with a inlet/inspection port  20  at one side of the vessel head  14 . A support  22  for the lower gear motor  18  is affixed onto the head  14  of the vessel, with the gear motor  18  affixed onto that support  22 . A support  24  for the upper gearmotor  16  is secured to a top side of the lower gearmotor  18 , with the upper gearmotor  16  being affixed onto the top side of the support  24 . A larger diameter hollow outer drive shaft  26  extends up through the drive portion of the gearmotor  18 , while a smaller diameter inner drive shaft  28  coaxial with the outer drive shaft  26  passes up through the hollow core of the outer drive shaft  26 , then up through the support  24  to the drive portion of the upper gearmotor  16 . A rotary union  30  at the top of the upper gearmotor  16  is fitted onto the upper end of the smaller diameter inner drive shaft  28 . 
     A number of clean-in-place inlets  32  are shown installed on the top or head  14  of the tank. There is also a drain and associated valving at the base or lower end of the tank, but that is not shown in this view. 
     With reference now to  FIGS. 2 and 3 , on the inside of the tank or vessel  12  here is an outer scraper agitator, which is affixed onto the outer hollow drive shaft  26 . The agitator  24  has arms  36  that are welded onto the outer drive shaft  26  and these extend radially to vertical members  38 . A suitable number of scraper blades or vanes  40  are affixed onto the radially outer side of the vertical members  38 , and there may be additional mixer blades or vanes  42  on the radially inward side of the vertical members. There are also one or more inner agitators  44  attached at one or more locations along the inner drive shaft  28 , each of which has a plurality of vanes or blades. In practice the outer scraper agitator  34  and the inner agitators  44  rotate in opposite directions, with the inner agitators operating at a relatively high rotary speed and the outer scraper agitator operating at a much lower rotary speed. 
     The lower end of the inner drive shaft  28  may be fitted into a rotary support member, in some cases, which would be positioned at or near the center of the base of the vessel or tank  12 . 
     A lower rotary seal  46  is positioned in an associated seal flange  48  that is affixed along the tank axis at a penetration  50  at the center of the top or head  14  of the vessel, through which the coaxial drive shafts  26 ,  28  pass. This forms a seal where the shaft  26  enters the tank or vessel, i.e., into the space that needs to be clean and sanitary. An upper seal  52  is positioned at a top end  54  of the outer drive shaft  26  where the inner drive shaft  28  exits. There is an annular void or space  56  between the outer and inner drive shafts, extending from the upper end  54  of the outer drive shaft down into the vessel  12  to an open lower end  58  of the drive shaft  26 , where the inner drive shaft exits and continues downward. Here the lower end  58  is just below the location where the arms  36  of the outer agitator  34  are affixed onto the drive shaft  26  within the vessel. The upper seal  52  serves to close off the annular space  56  at the top end  54 , and permits the inner shaft  28  to rotate relative to the outer shaft  26 . 
     The upper portion of the inner drive shaft  28  is here configured as a through-the-shaft cleaning design, for cleaning the lower surface of the upper seal  56  as well as the surfaces of the outer and inner drive shafts that define the annular space  56 . Here an axial bore  60  extends from a top end  62  of the drive shaft  28  to a point  64  that is just below the location of the upper seal  52 . One or more cross-bore passages  66  extend radially outward from the axial bore  60  to the annular space  56 . The upper end of the bore  60  communicates with the rotary union  30  and piping is connected to the union  30  to supply a cleaning fluid, e.g., a caustic solution, and a subsequent water flush, and optionally sterilizing steam, into the annular space  56  between the drive shafts. This serves to clean and flush the entire length of the space  56 , and with the solution exiting out the open lower end  58  of the outer drive shaft. The cross-bore passages  66  act as a rotary sprayer, to remove any accumulations, including carbon particles, from the lower surface of the seal  52 . 
     The through-the-shaft cleaning system achieves a much higher degree of cleanliness than when the in-tank cleaning devices alone are used. 
     As shown in  FIG. 3 , there are perforations or openings  68  in the side wall of the outer drive shaft  26  at an intermediate position between the outer seal  46  and the agitator arms  36 . C.I.P. spray heads  70  are also present in the vessel  12 , each descending from an associated one of the C.I.P. inlets on the tank head  14 . One or more of these spray heads  70  are directed toward the location of these perforations or openings  68 . Thus, with this feature, during a C.I.P. cleaning operation, the C.I.P. solution enters through the openings  68  into the lower part of the annular void  56 , and then flushes out through the open lower end  58  of the shaft  26 . This ensures that there is additional flushing and cleaning of the lower end of the annular space, where accumulations are more likely to be present after a mixing operation. 
     In extremely sterile applications, this design permits use of live steam, either in lieu of cleaning solutions or subsequent to their use, for sterilizing these difficult-to-reach areas. 
     The tank  12  for this mixer arrangement may have a different design, with outer and inner agitators of any of a number of constructions, depending upon the mixer application. 
     Of course, the same principles as discussed above may be used for through-shaft cleaning of single-shaft mixers as well, as illustrated in  FIG. 4 . Here elements that correspond to features of the previously-described embodiment are identified with the same reference numbers, where possible, but raised by  100 . In tanks of the single-mixer design, it as always been a problem to clean the deep annular space  150  defined around the shaft  128  beneath the shaft seal  146  and around the supporting seal flange  148 , especially in the tanks where the tank head  114  includes a thickness of insulation  115 . 
     As in the previously-described embodiment, the shaft  128  is driven by a gearmotor  116  that is supported above the tank head, and has a rotary union  130  at its upper end to receive the C.I.P. fluid. Agitator arms  136  are shown here mounted on the rotary drive shaft  128 . The upper end of the shaft  128  has an axial bore  160  ending in one or more cross bores  166 , which allow the C.I.P. fluid to be sprayed onto the underside of the seal  146  and on the inner surfaces of the seal flange  148  and also to flood the deep annular space  150  between the shaft  128  an the insulation covering, with the fluid then draining out of the annular space into the tank. 
     While the invention has been explained with reference to a preferred embodiment, it should be understood that the invention is not limited to that precise embodiment. Rather many modifications and variations will present themselves to persons of skill in the art without departing from the scope and spirit of the invention, as defined in the appended claims.