Abstract:
A centrifugal separator is provided which maximizes solids recovery yield and separated solids dryness with minimal user intervention. The centrifugal separator includes a rotatable separator bowl, which is selectively rotatable and supported by a hollow shaft spindle. A scraper and feed assembly including a plurality of scraper blades is supported by the shaft spindle within the separator bowl. A feed liquid is supplied to the separator bowl from the scraper blades at substantially the interior surface of the separator bowl so that the feed liquid is gradually accelerated upon entering the separator bowl. High speed operation of the centrifugal separator is performed to generate separation forces as high as 30,000 g&#39;s at the interior surface of the separator bowl. This allows the feed liquid to be safely and effectively separated at lower stress levels within the separator bowl.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims priority under 35 U.S.C. 517 119(e) to provisional patent application Ser. No. 60/223,409 filed Aug. 4, 2000, the disclosure of which is hereby incorporated by reference. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention generally relates to centrifuges and in particular to a centrifuge enabling automatic discharge of solids from a separated centrate.  
           [0003]    Many different types of centrifugal separators are known for separating heterogeneous mixtures according to “their specific gravities components”. A heterogeneous mixture, which may also be referred to as feed material or liquid feed, is injected into a rotating bowl of the separator. The bowl rotates at high speeds and forces particles of the mixture to separate from the liquid centrate. As a result, a dense solids cake compresses tightly against the surface of the bowl and the liquid centrate forms radially inward from the solids cake.  
           [0004]    The bowl may rotate at speeds sufficient to produce 20,000 g&#39;s so that the solids may be separated from the centrate. Typically, the liquid feed travels at a relatively slow speed before being introduced through feed holes to the rotating bowl where the liquid feed is instantaneously accelerated to the angular speed of the rotating bowl. However, introducing the liquid feed to the bowl at such high speeds creates shear forces that often destroy a large amount of the solid component of the liquid feed before separation.  
           [0005]    While the solids accumulate along the wall of the bowl, the centrate is drained. Once it is determined that a desired amount of the solids has been accumulated, the separator is placed in a discharge mode. In one such discharge mode, a scraper blade extending the length of the rotating bowl is placed in a scraping position against the separator wall and the bowl is rotated at a low scraping speed. Then, the solids are scraped from the sides of the bowl and fall toward a solids collecting outlet. However, such scraping systems do not effectively remove wet or sticky solids which may have the consistency of peanut butter. In such instances, the sticky solids remain stuck on the separator wall and scraper blades or fall from the wall and then reattach to the blades before reaching the collecting outlet. As a result, the solids recovery yield is reduced and the remaining solids undesirably contaminate the separator.  
         BRIEF SUMMARY OF THE INVENTION  
         [0006]    In accordance with the present invention, a centrifugal separator is provided which automatically discharges solids and maximizes the amount of solids recovery yield and the dryness of the separated solids with minimal user intervention. Full hermetic containment of the separation process is achieved by this centrifugal separator so that automatic “clean in place” (C.I.P.) and “sterilization in place” (S.I.P.) operations may be performed. Accordingly, the centrifugal separator of the present invention is able to perform a wide range of liquid/solid and liquid/liquid separations in biotechnology, pharmaceutical, chemical, food and beverage, and other industrial processes.  
           [0007]    The centrifugal separator includes a rotatable separator bowl supported by a hollow shaft spindle. The shaft spindle also supports and positions a scraper and feed assembly for axially moving within the separator bowl. The scraper and feed assembly includes a plurality of scraper blades having a small surface area that extend to substantially the interior surface of the separator bowl. A feed liquid is supplied to the separator bowl by liquid feed passages which pass through the shaft spindle to the scraper and feed assembly so that the feed liquid exits proximate the ends of the scraper blades at substantially the interior surface of the separator bowl. This prevents the feed liquid from being instantaneously over-accelerated due to the angular velocity of the separator bowl. As a result, the feed liquid is subjected to shear forces that are greatly reduced and the feed liquid is less likely to be harmed as compared to the prior art.  
           [0008]    The separator bowl is preferably a tubular bowl having a relatively small diameter and a long length. By the use of such tubular separator bowls, high speed operations of the centrifugal separator may be performed to generate separation forces as high as 30,000 g&#39;s at the interior surface of the separator bowl. This allows the feed liquid to be safely and effectively separated at lower stress levels within the separator bowl.  
           [0009]    As a result of the high speed operation, the centrifugal separator is able to more effectively separate the solids from the residual liquid so that the dryness of the accumulated solids cake is increased. Even though the scraper blades have a relatively small surface area, the solids from the walls of the separator bowl may be more easily and effectively scraped. To scrape and discharge all of the accumulated solids, the scraper and feed assembly is slowly raised then lowered while the separator bowl is slowly rotated. By the combination of the accumulated solids being drier and the scraper blades having a small scraping surface area, the amount of the discharged solids is greatly increased. The centrifugal separator according to the present invention may thereby be operated aseptically and provide C.I.P. or S.I.P. operations. 
       
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
       [0010]    The invention will be more fully understood by reference to the following detailed description of the invention in conjunction with the drawings, of which:  
         [0011]    [0011]FIG. 1 illustrates a centrifugal separator according to an embodiment of the present invention;  
         [0012]    [0012]FIG. 2 is a transparent view of a scraper and feed assembly according to an embodiment of the present invention;  
         [0013]    [0013]FIG. 3 illustrates the operation of a centrifugal separator in a feed mode according to an embodiment of the present invention;  
         [0014]    [0014]FIG. 4 illustrates the operation of a centrifugal separator in a drain mode according to an embodiment of the present invention;  
         [0015]    [0015]FIG. 5 illustrates the operation of a centrifugal separator in a scrape mode according to an embodiment of the present invention; and  
         [0016]    [0016]FIGS. 6A and 6B illustrate a centrifugal separator utilizing a feed cone in another embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]    A centrifugal separator  100  according to one embodiment of the present invention is illustrated in FIG. 1. The centrifugal separator  100  includes a cylindrical separator bowl  110 , preferably a conventional tubular type bowl having a relatively small diameter D and a long length L such that the ratio of L/D is approximately 5/1. For example, a separator bowl  110  having a bowl diameter up to 500 mm and a flow capacity up to 100l/min. may be used so that sufficient rotational speeds may be achieved at the interior surface of the separator bowl  110  to generate separation forces from 20,000 g&#39;s to 30,000 g&#39;s. Tubular type bowls provide cost and performance advantages over other known cylindrical bowls, such as “basket” type centrifuge bowls, for similar pool areas and gravitational forces. For instance, because the radius of the tubular bowls are much smaller, lower peripheral velocity results which reduces windage, friction and heat generation. Also, the longer length of the tubular bowl provides better liquid stability because axial liquid waves are damped out.  
         [0018]    A scraper and feed assembly  120  is operatively connected to a hollow scraper shaft spindle  130  within the separator bowl  110 . The scraper shaft spindle  130  extends out from the separator bowl  110  to a feed pipe  140 . A scraper shaft seal  132  is positioned where the scraper shaft  130  extends from the separator bowl  110  to prevent liquids and solids from escaping the separator bowl  110 . A rotary union  142  connects the feed pipe  140  to the scraper shaft spindle  130  so that the liquid feed may be injected into the separator bowl  110 .  
         [0019]    A variable speed drive motor  150  is connected to a main bearing assembly  134  of the scraper shaft spindle  130  by a drive belt  152 . The drive motor  150  is controllably operated in conjunction with a scraper shaft clutch  136  to rotate the separator bowl  110  at the desired speeds for separating the liquid feed. A scraper actuator piston  126  is also operatively connected to the scraper shaft spindle  130  in combination with the scraper shaft clutch  136  for raising and lowering the scraper and feed assembly  120  within the separator bowl  110 . In a discharge mode, the scraper shaft clutch  136  is engaged for holding the scraper shaft spindle  130  stationary and slowly rotating the separator bowl  110  at a low scraping speed so that scraper blades maintain contact and scrape solids from the walls of the separator bowl  110 . In other operating modes, the scraper shaft clutch  136  is disengaged so that the scraper and feed assembly  120  rotates at the same speed and in the same direction as the separator bowl  110  (i.e., the scraper and feed assembly  120  is stationary relative to the separator bowl  110 ).  
         [0020]    A more detailed view of the scraper and feed assembly  120  is shown in FIG. 2. FIG. 2 illustrates three axial scraper blades  122  attached to the scraper and feed assembly  120 . It should be appreciated that the scraper and feed assembly  120  may be designed with a varying number of scraper blades  122  depending on the surface area of the separator bowl  110  that is desired to be scraped while maintaining a stable and high speed rotation.  
         [0021]    The scraper and feed assembly  120  includes liquid feed passages  124  that channel the feed liquid from the scraper shaft spindle  130  and through the scraper and feed assembly  120  to first and second outer feed holes  126  and  128  on the scraper blades  122  so that the liquid feed is ejected at the surface of the separator bowl  110 . The coriolis force due to the rotation of the scraper and feed assembly  120  causes the feed liquid to accelerate towards the first outer feed hole  126  at the surface of the separator bowl  110 . If the feed liquid is prevented from exiting at the first outer feed hole  126  due to an accumulation of solids or other means, the liquid may exit at the second outer feed hole  128  with substantial acceleration towards the surface of the separator bowl  110 . By ejecting the feed liquid away from the scraper and feed assembly  120  and towards the surface of the separator bowl  110 , the liquid is more gradually accelerated and is prevented from being instantaneously accelerated due to the angular velocity at which the bowl is rotating. Thereby, the shear forces to which the feed liquid are subjected are greatly reduced so that the feed liquid is less likely to be damaged.  
         [0022]    It is to be noted that in the illustrated embodiment of FIG. 2, drill holes formed on the surface of the scraper and feed assembly  120  during the creation of the feed passages  124  are subsequently filled. Other fabrication techniques may obviate the need for drilling and filling these surface holes.  
         [0023]    In accordance with the operation of the centrifugal separator  100 , a feed mode for the liquid feed will be described with reference to FIG. 3. In the feed mode, the feed liquid is introduced through the feed pipe  140 . The scraper clutch  136  is disengaged so that the scraper shaft spindle  130  is free to rotate with the separator bowl  110 . The feed liquid flows from the feed pipe  140  through the scraper shaft seal  132  to the scraper shaft spindle  130  in the direction shown by the arrows. The feed liquid continues through the feed passages  124  of the scraper and feed assembly  120  and enters the separator bowl  110  at its outer surface. Due to the centrifugal force, the liquid flows up the pool surface of the separator bowl  110 . Any overflow feed liquid decants over a weir  182  as clarified liquid (centrate) at the top of the separator bowl  110  and then flows into a centrate case  180 . As the liquid flows through the separator bowl  110 , it is clarified of entrained solid particles by the high centrifugal force acting upon the liquid. The solids are forced to settle on the inside wall of the separator bowl  110  and collect as a compressed solids cake as a result of the centrifugal force.  
         [0024]    Because the scraper clutch  136  is not engaged, the separator bowl  110  and the scraper and feed assembly  120  rotate together in the same direction at a high speed, for example in a clockwise direction as indicated by the arrow. Accordingly, the liquid feed passing through the scraper shaft  130  is gradually accelerated through the feed passages  124  to the angular velocity of the scraper and feed assembly  120 . As the separator bowl  110  rotates, solids  184  collect along the surface of the separator bowl  110  and a rotating liquid pool  186  forms inward from the solids  184 .  
         [0025]    Next, the centrifugal separator  100  is placed in a bowl drain mode as shown in FIG. 4 when the separator bowl  110  has been determined to be sufficiently full of solids, usually by the turbidity of the centrate. The liquid feed is shut off and then the bowl driver electronically brakes the separator bowl  110  to a full stop. The residual liquid in the separator bowl  110  drains into a residual liquid cup  160  while the solids remain on the surface of the separator bowl  110 . The residual liquid cup  160  is preferably provided with a shaped bottom surface for channeling the residual liquid to a residual liquid discharge port  162  located at the bottom of the residual liquid cup  160  for transport of the residual liquid back to liquid feed storage (not shown). The bowl drain mode may also include a step of rotating the separator bowl  110  briefly at a high speed to further drain liquid from the accumulated solids. After this optional spinning step, the solids become drier which improves the efficiency of the subsequent scraping steps.  
         [0026]    When the separator bowl  110  has been completely drained of residual liquid, the centrifugal separator  100  as shown in FIG. 5 enters a scrape mode. The residual liquid cup  160  swings away from the bottom of the separator bowl  110  so that a solids discharge port  170  is positioned beneath the bowl  110  to collect falling solids without mixing with the residual liquid.  
         [0027]    The scraper shaft  130  is engaged by the scraper clutch  136  to prevent the scraper shaft  130  from rotating. The separator bowl  110  rotates slowly in an opposite direction from the feed mode (in a counter clockwise direction as shown by the arrow in FIG. 5). Then, the scraper actuator  126  slowly draws up the scraper shaft  130  and the scraper and feed assembly  120  up towards the top of the separator bowl  110  as indicated by the arrows. The solids cake is scraped from the walls of the separator bowl  110  and towards the center of the separator bowl  110  so that the scraped solids are free to fall out of the discharge port  170  and into a receiving container (not shown). After the scraper and feed assembly  120  reaches the reversing point near the top of the separator bowl  110 , the scraper actuator  126  reverses in direction so that the scraper shaft  130  and the scraper and feed assembly  120  descend toward the bottom of the separator bowl  110 . The scraping process continues until the stopping point near the bottom of the separator bowl  110  is reached. It is appreciated that the solids scraping from the separator bowl  110  can be performed in either direction (both counter clockwise and clockwise).  
         [0028]    In another embodiment of the invention, a centrifugal separator  200  having an alternative liquid feed path is shown in FIGS. 6A and 6B. A feed cone  200  positioned at the bottom of the separator bowl  110  is used to feed liquid up into the separator bowl  110 . The feed cone  200  is caused to rotate by plastic pins  204  on the feed cone  200  and metal vanes  202  on the separator bowl  110 . This method of rotating the feed cone  200  with the separator bowl  110  allows the separator bowl  110  to go through mild oscillations; the separator bowl  110  maintains its center of rotation while being filled with liquid and is not restricted by the feed cone  200 . The feed liquid is injected through a feed port  230  when the feed cone  200  is positioned in an upper connect position to the separator bowl  110  for a feed mode. A positioning mechanism  220 , including bearings, shaft seals, and an actuator piston, is used to raise and lower the feed cone  200  between the feed mode as illustrated in FIG. 6A and a liquid drain mode as illustrated in FIG. 6B. In the drain mode the feed cone  200  is lowered by the positioning mechanism so that residual liquid may drain down from the separator bowl  110  through a residual liquid port  240 . Subsequently, the feed cone  200  is pivoted from beneath the separator bowl  110  to enable scraped solids to fall into the solids discharge port  170 .  
         [0029]    The liquid feed apparatus of FIGS.  1 - 5  or of FIGS. 6A and 6B can also be used for the purpose of cleaning the centrifuge and associated elements through the introduction of appropriate liquid cleaning agents in the liquid feed path.  
         [0030]    In a preferred embodiment, all of the separating, draining and scraping operations take place in a sealed environment, enabling operation at various pressures and temperatures. Contamination is thereby minimized.  
         [0031]    It is understood that a variety of control mechanisms with suitable human and/or computer interfaces are preferably provided for the purpose of automating the filling, draining and scraping operations. Manual operation may be alternately enabled through the provision of various actuators.  
         [0032]    It will be apparent to those skilled in the art that other modifications to and variations of the above-described techniques are possible without departing from the inventive concepts disclosed herein. Accordingly, the invention should be viewed as limited solely by the scope and spirit of the appended claims.