Patent Publication Number: US-6908423-B2

Title: Screw for a solid-bowl centrifuge and a method of extracting oil using the centrifuge

Description:
BACKGROUND AND SUMMARY OF THE INVENTION 
     The invention relates to a screw for a solid-bowl screw-type centrifuge and to a method of extracting oil by means of a solid-bowl screw-type centrifuge. 
     A method which has been particularly effective for the extraction of olive oil is known from European Patent Document EP 0 557 758. In this process, a two-phase separation is carried out in which the oil is separated directly from the solids/water mixture. 
     The efficiency of this known method is very good per se. 
     Nevertheless, it is desirable to lower the residual oil content in a rape in order to increase the economic efficiency of the oil extraction. 
     The present invention provides for, on the one hand, a screw for a solid-bowl screw-type centrifuge and a process for extracting oil from fruit or seed. 
     The present invention provides for a screw or kneading screw for a solid-bowl screw-type centrifuge which has at least one screw blade and at least one screw blade segment in a delivery path in sections or areas between adjacent screw spirals. In addition, the at least one screw blade is preferably provided with recesses which are constructed such that centrifuged material can flow through between adjacent screw spirals. 
     With respect to the method of separating or extracting oil, it was found to be particularly favorable for the oil, as a liquid phase, to be extracted directly in a two-phase separating process. The oil is extracted as a liquid or first phase from a second or mixed phase which may include a mixture of water and solids. Thus, seeds or reduced fruit, such as olives or avocados, are first guided into a solid-bowl screw-type centrifuge through a first portion of a separating zone having at least one screw blade with one or more screw spirals in a cylindrical section of the centrifuge. The at least one screw blade is preferably constructed without any recesses in a delivery path area between the screw spirals and, preferably, no blade segments are constructed in the delivery path. Subsequently, a passing takes place into a second portion of the separating zone in which recesses are constructed in the at least one screw blade, and blade segments are constructed the delivery path. Then the solids and the water are conveyed past a retarding plate or disk, which acts as a barrier to the oil, from the separating zone into a conically tapering section or dry zone of the screw and then out of the centrifuge. The oil is conveyed in an opposite direction out of the centrifuge. 
     Also, by use of the screw according to the present invention, a three-phase oil extraction process, which is still occasionally used, can be improved. In this case, oil is separated or extracted as a liquid or first phase, in a three-phase separating process, from a second phase comprising water and a third phase comprising solids. The process occurs as follows:
         the reduced fruit, such as olives or avocados or seeds are first guided into a solid-bowl screw-type centrifuge through a first portion of a separating zone having at least one screw blade with one or more screw spirals in a cylindrical section of the centrifuge. The at least one screw blade is preferably constructed without recesses, and preferably with no blade segments constructed in the delivery path between the screw spirals,   then, a passing takes place into a second portion of the separating zone, in which recesses are constructed in the at least one screw blade and blade segments are constructed in the delivery path,   then the three phases, water, solids and oil are guided/delivered out of the centrifuge essentially separately. The water and oil may exit at different levels toward a cylindrical end of the centrifuge and the solids may exit toward a conical end of the centrifuge.       

     By use of the screw according to the present invention, the economic efficiency of the oil extraction can be increased considerably. In this regard, reference is particularly made to tests explained herein and whose results are shown in  FIGS. 4 and 5 . The screw of the present invention can also be retrofitted without any problem into existing centrifuges. The screw according to the present invention is particularly suitable for an application in a process for extracting oil from fruit and seeds and for a better draining of water and/or separating of oil from mashes of organic materials (such as seed mash, fruit flesh mash, animal tissue, such as fish, egg, fatty tissue cells). 
     According to the present invention, a combination of recesses and blade segments are provided. The blade segments and the recesses preferably are constructed such in the axial direction that the recesses each form ducts extending in the axial direction (and/or at an angle or in a zigzag-type manner with respect to the center axis y), in which ducts the blade segments stand. 
     Also according to the present invention, the blade segments and the recesses may be constructed only in the cylindrical section of the screw body and a retarding disk may be provided in the conical section of the screw, particularly in the two-phase separation. 
     According to the prior art, solid-bowl screw-type centrifuges are known, in which recesses are provided in the screw blade. For example, see German Patent Document DE 41 32 693 A1. However, according to the present invention, the simple providing of such recesses is not sufficient to obtain a significant increase in efficiency. On the contrary, an increase in efficiency can be achieved when, in addition to recesses, provided in a center of the delivery path between adjacent screw spirals, the blade segments are also constructed. 
     It is also known to construct blade-segment-type screw spirals, as is shown, for example, in International Patent Document WO 97/23295. Those blade segments extend into the conical section, which is not favorable. In addition, those blade segments are distributed on the circumference of the screw body over its entire area, which was also found to be not favorable. In addition, it is not that additional blade segments are set up in the delivery path between the screw spirals, but the blade segments themselves form the screw spirals. Also, by use of this prior art screw, no satisfactory economic efficiency can be achieved when extracting olive oil. 
     According to the present invention, the blade segments in the delivery path may be constructed such that they extend into an area where solids are present, such as a solids area. However, there is an exterior area of, for example, approximately 25 mm that is preferably not reached by the blade segments, because relatively completely de-oiled solids and permanently discharged solids are already present in this exterior area. 
     Measuring results indicate that the screw according to the present invention leaves approximately 1 to 1.5% less oil in a discharged solids sludge. During an olive oil extraction campaign, this corresponds to a financial savings of approximately DM 300,000.00 to 500,000.00 per centrifuge machine. 
     The screw of the present invention may operate in an area of moist orujo or rape, because in that area a special separation of oil can be achieved by means of the blade segments. 
     By use of the present invention, a solids mash can be fed into a bowl or drum preferably by way of a rectangular tube. The rectangular tube must be so long that the entering mass or mash to be centrifuged is charged or forced through an oil layer while being protected in order not to mix with the oil layer at a later time. 
     In a filled centrifuge machine, an oil separation area may occur rather close to the screw body, for instance, at a distance of approximately 10, 20 . . . , to 40 to 50 mm. Fresh oil, as a distinct phase, can generally be recognized approximately in the range of 20 to 30 mm outside or away from the screw body. A distinct separating line usually exists here. The range of the oil separation area may vary with different centrifuges. 
     Charged solids, as part of a fed suspension, will therefore fill the centrifuge to such an extent that the latter is filled to the oil separation area (approximately 10-50 mm outside the screw body) with solids suspensions. The reason is that, as a rule, only so little water is in the orujo or rape mass that no water or only an extremely small layer of free water is formed between the oil and the solids suspension. In this case, the solids are dryer on the outside than on the inside or, in other words, a fraction of dry substance on the drum side is much higher than a fraction of dry substance toward the interior. 
     In the area of the recesses and blade segments, the solids suspension, just like the oil and an emulsion situated in-between, experiences three axial speeds particularly in a kneading area of the blade segments, from the screw body to an outside radial end of the blade segment. 
     Thus, a normal axial speed exists in the area of residual wall pieces or sections of the screw spirals. In contrast, in the area of the recesses, the axial speed is essentially zero. However, the axial speed in the area of the actual blade segments in the delivery path may amount to five times the normal speed. As a result, an elastoviscous sludge is deformed, compressed and relaxed in a standing solids area adjacent a surface of the drum. 
     In the area of the leading blade segments, for example x+1, x+2, x+3, x+4, the solids are additionally axially compressed. In the area of the recesses, they are then relaxed. This has the effect of pressure increases and relaxations. A setting-free or separation of the oil essentially takes place in a relaxation area and the extraction of oil is therefore more effective than without such relaxation areas. 
     In a rearward area, the screw body preferably has a cylindrical section and, in its adjoining forward section, a section which tapers essentially conically in a uniform or non-uniform—for example, stepped manner. The recesses and blade segments are constructed only in the area of the cylindrical section. 
     In the cylindrical section, the screw body preferably first has at least one screw spiral which is constructed without recesses as well as without blade segments and which is followed by additional screw spirals which are provided with the recesses and blade segments. 
     It is also conceivable that optional oil drainage ducts are constructed preferably in the first screw spiral. 
     The recesses preferably have a residual section of the screw blade on the circumference of the screw body. 
     Relative to one or several screw spirals, the blade segments may be uniformly, or may be non-uniformly, distributed on the circumference of the screw body. 
     The area of the recesses may amount to approximately 25-60%, preferably approximately 40-50% of the screw spiral area. 
     The recesses in the screw blades may be constructed such that they radially project at least beyond the solids area (for example, 70-95%, preferably 70-100% of the screw blade height). 
     The height of the blade segments may be approximately 0-30% lower than the height of the screw blade. 
     The blade segments may be constructed as rectangular metal plates. Trapezoidal, rounded elements and/or elements shaped to be tapering or widening and extending from the screw body radially outward or to the outside are also conceivable. 
     Other aspects and novel features of the present invention will become apparent from the following detail description of the invention when considered in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a screw, according to the present invention. 
         FIG. 2   a  is a top view of a section of a screw, according to the present invention. 
         FIG. 2   b  is a sectional view along Line IIb—IIb of  FIG. 2   a.    
         FIG. 3  is a perspective view of a solid-bowl screw-type centrifuge, for a two-phase extraction process, according to the present invention; 
         FIG. 4  is a graph representing a comparison, in a two-phase olive oil extraction process, of the improvement of efficiency of oil extraction as a function of throughput using a normal (known) screw versus a screw (special) according to the present invention. 
         FIG. 5  is a graph comparing the residual oil content in a rape during the extraction of olive oil by means of a solid-bowl screw-type centrifuge in the two-phase separating process using screws according to the invention and using screws according to the prior art. 
         FIG. 6   a  is a sectional view, along line VIa—VIa of  FIG. 6   b , of speed profiles in an area of a screw blade and blade segment, according to the present invention. 
         FIG. 6   b  is a sectional view, along the line VIb—VIb of  FIG. 6   a , of speed profiles in a screw spiral in an area of recesses, screw blades and blade segments for a screw, according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  illustrates a screw  1  for a solid-bowl screw-type centrifuge  50  (see FIG.  3 ), the screw  1  having a screw body  3  as well as, in this case, a screw blade  5  which surrounds the screw body  3  several times and forms several screw spirals (for example x, x+1, x+2, etc.). 
     A delivery path  7  for delivering/conveying a material to be centrifuged is formed between the screw spirals x, x+1, . . . . 
     In a rearward area  40  the screw body  3  has a cylindrical section  9  and, in an adjoining forward area  45 , the screw body  3  has a dry zone  33  or conically tapering section  11  that is essentially conical, uniformly or non-uniformly. 
     In a transition area  43  between the cylindrical section  9  and the conical section  11 , a retarder or disk  13  is placed on the screw body  3 . This placement was found to be successful particularly in a two-phase separation process separating material into an oil phase and a water/solids phase. Such placement may not be required in a three-phase separation process when separating material into oil, water and solids phases. The two phase separation process is shown in FIG.  3 . The three-phase separator process is not shown in the Figures. 
     The operation of a solid-bowl screw-type centrifuge  50  with screw  1 , among other components, is as follows. 
     As shown in  FIG. 3  material S to be centrifuged is fed or guided through a centrally arranged, adjustable inlet tube  14  into an inlet chamber  15 . From there material S may go through openings  17  into drum space  19 . Drum  21  surrounds the screw  1 . Inlet chambers  15  and openings  17 , or special distributors, may be arranged toward the rearward end or area  40  of the cylindrical section  3  (see FIG.  1 ). 
     In the drum space  19 , the material S to be centrifuged is accelerated to a rotational operating speed. Under the effect of the force of gravity, solids particles will be deposited on a wall of drum  21  within a very short time. 
     The screw  1  may rotate at a slightly lower or higher speed than the drum  21  and may deliver centrifuged solids F toward the conical section  11  out of the drum  21  to the solids discharge  23 . 
     In contrast, liquid L may flow to a larger drum diameter area at the rearward end or area  40  of the drum  21  and may be discharged at overflow  25 . In a two-phase extraction process, liquid L may represent the presence of oil. In a three-phase extraction process, liquid L may represent the presence of oil and water. 
     From a second screw spiral (x+1) to a fifth screw spiral (x+4), the screw  1  has recesses  27  in the screw blade  5 . 
     In the embodiment of  FIG. 1 , these recesses  27  are constructed such that one or more axial ducts  28  may be formed in an axial direction which may extend, for example, from a second to a fifth screw blade  5 . The ducts  28  may also be formed in a zigzag type manner or angularly with respect to a center axis y of the screw. An individual screw spiral x+1 etc. with recesses  27  and blade segments  29  is also conceivable. 
     In addition, blade segments  29  are arranged in the delivery path  7  formed between the screw spirals x+1, x+2 . . . of the screw blade  5 . Blade segments  29  may be constructed as metal strips and which may have a trapezoidal shape which widens radially from an outer circumference of the screw body  3 . 
     Blade segments  29  may be constructed during the cutting-off of material for forming the recesses  27 . The blade segments may be placed in the delivery path  7  and may be fastened in the path  7  by welding or by an equivalent means. 
     The cutting-off of the blade sections or segments  29  may take place such that the screw blade  5  is cut out to the circumference of the screw body  3 . However, as an alternative, a residual section  30  of the screw blade  5  may also remain standing at or on the circumference of the screw body  3 . If the cutting-out takes place essentially radially with respect to the drum  21  and screw axis y, trapezoidal blade segments  29  are obtained. The screw blades  5  may also be constructed as rectangular or rounded elements or be shaped as tapering or widening elements extending from the screw body  3  radially outward. 
     By a combination of recesses  27  and blade segments  29  in the delivery path  7 , the efficiency of some centrifugal separating processes can surprisingly be increased. 
     A screw  1  construction with recesses  27  and blade segments  29  has been particularly successful in the field of olive oil extraction. A two-phase separation process in which the oil is separated directly from a solids/water mixture, had been particularly successful in the extraction of olive oil. Such a process is described in European Patent Document EP 557 758. The efficiency of this already excellent process can be increased by using the screw  1  of the present invention, to (see FIGS.  1  and  3 ):
         separate the oil as a liquid or first phase directly in a two-phase separating process from a second phase mixture of water and solids,   reduced fruit, such as olives and avocados, are first guided in a solid-bowl screw-type centrifuge  50  through a first portion  31   a  of a separating zone  31  with one or several screw spirals x−1, x, . . . , in which the screw blade  5  has no recesses  27  and in which no blade segments  29  are formed in the delivery path  7 ,   then, in a second portion  31   b  of separating zone  31 , there is a passing through a screw area in which the recesses  27  are constructed in the screw blade  5  and the blade segments  29  are constructed in the delivery path  7 ,   then the solids and the water are conveyed past the retarding disk  13  out of the separating zone  31  into a conically tapering section  11  or dry zone  33  of the screw  1  and out of the centrifuge  50  at discharge  23 .       

     Comparisons of this two-phase separation process using a conventional or normal screw and the screw  1  of the present invention are illustrated in  FIGS. 4 and 5 . 
       FIG. 4  shows comparisons of the improvement of the efficiency of the oil extraction as a function of capacity or throughput (to/d, or tons/day).  FIG. 5  also shows that, when extracting olive oil by means of screw  1 , according to the present invention, the residual oil content in a rape could be lowered generally in the range of approximately 2% to 3%. The reduction shown in  FIG. 5  ranges from 2.6% to 2.9%. The economic efficiency of the oil extraction is therefore again considerably increased with respect to the already excellent prior art result of the two-phase separation of a) oil and b) water/solids. The modification or exchange of the conventional (prior art) screw by the screw  1 , according to the present invention, will therefore be beneficial within a short time. 
       FIGS. 6   a, b  show speed profiles in a screw spiral x, x+1 . . . in the area of the recesses  27  and blade segments  29 .  FIGS. 6   a  and  6   b  represent two views, 90° from each other, of the recesses  27 , blade segments  29  and screw blades  5 .  FIG. 6   a  shows that “in the shadow”  70  of the blade segment  29 , the speed (shown as an arrow or arrows ←) of the particles increases from the inside  71  toward the outside  72  of the blade segment  29 . At the upper edge  73  of the blade segment  29 , the maximal value V is reached which, according to  FIG. 6   b , is essentially constant at the upper blade segment edge  73 . At point O in  FIG. 6   b , the speed or velocity of the particles approaches its minimum, which could be zero. 
     Different dimensions as well as alignments and arrangements of the recesses  27  and of the blade segments  29  were found to be particularly successful in practice. By the variation of these parameters, the mixing effects between the screw spirals x, x+1 . . . can also be varied, which has a direct influence on the efficiency of the separating process. These parameters are described below with reference to  FIGS. 1 ,  2   a  and  2   b  as is the preferred position of the recesses  27  and the blade segments  29 . 
     For discussion, the screw  1 , as shown in  FIG. 1 , is viewed from the rearward area  40  of cylindrical section  9  toward the front area  45  of the conical section  11 . The screw  1  has several screw spirals, for example x−1, x, in first portion  31   a , and screw blades  5  are constructed to be continuous or free of recesses  27 . Preferably, one or more screw spirals x−1, x . . . are constructed to be continuous. In this area  31   a , no blade segments  29  are provided in the delivery path  7 . 
     This first portion  31   a  of the separation zone  31  zone is followed by a second portion  31   b  where, for example, several screw spirals x+1, x+2, . . . x+4 are provided with recesses  27  and in whose spaces or in whose delivery paths  7 , the blade segments  29  are in each case constructed or erected. The blade segments  29  may be welded on the screw body  3  or attached by other equivalent means. 
     The cylindrical section  9  extends maximally to a beginning of the conical section  11  of the screw  1 . In the transition area  43 , between the cylindrical section  9  and the conical section  11 , the retarding disk  13  is arranged. In the conical section  11 , the screw  1  may be constructed to be free of recesses  27  and no additional blade segments  29  may be arranged in the delivery path  7 . 
     For each screw spiral x+1, x+2 . . . in the cylindrical section  9 , there may be approximately 2-6, and preferably 4, recesses  27 . 
     Correspondingly, for each screw spiral x+1, x+2 . . . in the delivery path  7 , there may be approximately 2 to 6, and preferably 4, blade segments  29 . 
     The blade segments  29  are preferably distributed uniformly on the circumference of the screw body  3  but may be distributed non-uniformly. 
     Relative to the center axis or the axis of symmetry y of the screw  1 , the screw spirals x, x+1 . . . are each arranged at an angle or form an angle α with the center axis y (see  FIG. 2   a ). The magnitude of the angle α (measured at a lower edge  75  of the screw blade  5 ) is approximately between 60 and 85°, and preferably approximately 75 to 80°. 
     In contrast, as shown in  FIG. 2   a , the blade segments  29  enclose an angle δ with the center axis or axis of symmetry y, which may be smaller than angle α. The angle δ is approximately between 40 and 70°, and preferably approximately 50 to 55°. It is recommended to align, in the last screw spiral, for instance, x+5 . . . of the cylindrical section  9  before the retarding disk  13 , the blade segments  29  essentially parallel to the screw blade  5 . The maximal differential between angles α and δ, may be preferably approximately 10 to 11°. 
     The recesses  27  each have an area and the sum of those areas is a total recess area. The screw spirals x, x+1 . . . each have a surface area and the sum of those areas is a total screw spiral surface area. The total recess area of the recesses  27  may amount to approximately 25-60% of the total screw spiral surface area, and preferably 40-50%. 
     As shown in  FIG. 2   a , angle δ may be defined or determined such that a distance d (viewed as an axial extension of edges) between a blade segment edge  76  and a recess edge  77  is approximately 0 to 5 mm, and preferably approximately 2 to 3 mm. The distance d viewed from the screw body  3  becomes smaller with an increasing height of the blade segment. In the case of a trapezoidal shape of the blade segments  29 , the size of any distances “d” as measured from the screw body  3 , varies radially away from axis y and screw body  3  toward an outside position nearer a wall (not shown) of the drum  21 . Distance “d” becomes, for example, larger toward the outside position. 
     Furthermore, angle may be defined or determined such that a distance A (see  FIGS. 2   a ,  2   b ) viewed as an orthogonal extension of edges, between a longitudinal edge  78  of the screw blade  5  and the edge  77  of the recess  27  amounts to approximately 0 to 28%, and preferably 15 to 25%, of a distance z (See  FIG. 2   a ) between an adjacent pair of screw spirals, for example x+2 and x+3, preferably viewed at the low end of the screw (inside), as a function of the shape. 
     According to an embodiment of the present invention, the blade segment  29  may be arranged in the delivery path  7  such that center axis M (see  FIG. 2   a ) is situated precisely in the center of the delivery path  7 , as well as preferably also in the center of a connection line C, having segments C/2, of the apothem of the recesses  27  at a crossing point of opposite recess edges (not defined). 
     As an alternative, it is also possible to shift the center axis or center point M of the blade segments  29  with respect to the preferable position as stated above. 
     A height h (see  FIG. 2   b ) of the blade segments  29  (measured from the outer circumference of the screw body  3 ) is particularly decisive for the efficiency of the present invention. 
     According to the present invention, the height h of the blade segments  29  may be selected such that the segments  29  extend into an area where solids are present, or solids area  47 , during centrifugal separation. Correspondingly, the screw blades  5  should have recesses  27  which radially project at least above the area of the solids area  47 . 
     For example, in a case of centrifugal separation, solids are deposited relatively far to an outside or solids area  47  in the drum  21 . If the blade segments or paddles  29  do not at least extend into this solids area  47 , their efficiency remains low. A mixing effect of the recesses  27  and of the blade segments  29  in this solids  47  area clearly increases the efficiency of the centrifugal separation during the extraction of oil. 
     In practice, the height h (see  FIG. 2   b ) is selected to be approximately 0-30% lower than a screw blade height k. Thus, a radial course (not shown) of the recesses  27 , or, in effect, height h, amounts to approximately 70-100% of the height k. In addition, the screw blade  5  encloses an angle γ with a circumferential wall  79  of the screw body  3 , as shown in  FIG. 2   b . This angle γ is preferably smaller than an angle η which the blade segment  29  forms with the screw body  3 . 
     Although the present invention has been described and illustrated in detail, it is to be clearly understood that this is done by way of illustration and example only and is not to be taken by way of limitation. The spirit and scope of the present invention are to be limited only by the terms of the appended claims.