Decanter centrifuge and associated method for producing cake with reduced moisture content and high throughput

Abstract: A decanter centrifuge comprises a bowl (12) rotatable about a longitudinal axis, the bowl being provided with a cake discharge opening (18) at one end and a liquid phase discharge opening (20). The bowl has a cylindrical portion (28) and a beach portion (42) disposed between the cylindrical portion and the cake discharge opening. A beach area is provided on an inner surface of the bowl at the beach portion of the bowl, the beach area including a first section and a second section with the second section located between the first section and the cake discharge opening. The second section of the beach area has a less steep or smaller slope than the first section. A conveyor (14) is at least partially disposed inside the bowl for rotation about the longitudinal axis at an angular speed different from an angular rotational speed of the bowl. The conveyor includes a helical screw (24) disposed inside the bowl for scrolling a deposited solids cake layer along the inner surface of the bowl towards the cake discharge opening. A feed element (10) extends into the bowl and the conveyor for delivering a feed slurry into a pool (11) inside the bowl. A flow control structure (30,34,36,40) is provided in the bowl proximate to the second section of the beach area for impeding a flow of cake along the bowl towards the cake discharge opening.

Description: The invention relates to a decanter centrifuge according to the pre-characterizing portion of claim 1 and to a method for operating a decanter type centrifuge according to the pre-characterizing portion of claim 7. Decanter centrifuges in general are well known from the state of the art, for instance from DE 40 33 012 A1 and from EP 0 747 127 A2, which EP 0 747 127 A2 is state of the art according to Article 54(3) EPC. Said decanter centrifuges comprise a bowl rotatable about a longitudinal axis, said bowl having a cake discharge opening at one end and a liquid phase discharge opening, said bowl having a cylindircal portion and a beach portion between said cylindrical portion and said cake discharge opening, a beach area being provided on an inner surface of said bowl at said beach portion, said beach area including a first section of a steep slope and a second section of a less steep slope, said second section being located between said first section and said cake discarge opening; a conveyor having at least a portion disposed inside said bowl for rotation about said longitudinal axis at an angular speed different from an angular rotational speed of said bowl, said conveyor including a helical screw disposed at a helix angle inside said bowl for scrolling a deposited solids cake layer along said inner surfae of said bowl towards said cake discharge opening; a feed element extending into said bowl and said conveyor for delivering a feed slurry into a pool inside said bowl; and a flow control structure provided in said beach area proximate to said cake discharge opening for impeding a flow of cake along said beach area towards said cake discharge opening. EP 0 565 268 A2 discloses a decanter centrifuge including a discontinuous screw conveyor having flights only within the generally cylindircal portion of the centrifuge bowl. A restricting disc is provided adjacent to the heavy phase material discharge ports such that the restriction contacts the build-up of heavy phase within the portion of the bowl where the flights are discontinuous. A decanter centrifuge according to EP 0 600 628 A2 comprises a bowl which rotates about a horizontal or vertical axis and contains a helical screw conveyor for separating a slurry fed to the bowl into its constituent solids and liquid. The scroll is arranged to rotate at a differential speed within the bowl. At least some of the flights of the scroll conveyor are inclined backwards relative to the solids discharge end of the bowl. JP 58043253 A discloses a screw type continuous centrifugal settler of double cant type. The purpose of said settler is to convey settling solid particles smoothly from the inside of liquid onto a liquid surface by providing a stagnation section in the part right under the liquid suface between a scraping section and a dehydrating section. Thereby a stagnating section is provided in the part right under a liquid surface between a scraping section and a dehydrating section. The settling solid particles conveyed in the scraping section are conveyed onto the surface after stagnation once in the section. The revers flow phenomenon into the liquid during the conveyance of the settling solid particles onto the surface is prevented, and the solid particles can be conveyed very smoothly on the surface. In GB 2 064 997 A a screen bowl decanter centrifuge is described, which screen bowl decanter centrifuge has a horizontally rotatable bowl within which a scroll conveyor is coaxially journalled for rotation at a slightly different speed for conveying solids towards a solids discharge end of the bowl. The other end of the bowl has a liquids outlet. The bowl has an imperforate frusto-conical portion which converges towards the solids discharge end and a perforate screen portion downstream of the imperforate frusto-conical portion in the direction of solids discharge for assisting dewatering of the solids. A decanter centrifuge according to US,934,792 has a screw conveyor within an imperforate bowl defining a helical chamber. The conveyor is provided with a baffle extending in generally axial direction between the screw flights, thereby deviding the helical chamber within the bowl into a separating zone and a heavy phase discharging zone. The outer portion of the baffle defines with the bowl a restricted passageway which permits the underflow of separated heavy phase material from the separating zone to the heavy phase discharging zone. Pressurizing means, preferably means for mainaining the inner surface of the separated light phase layer inwardly of the weir surfae of the heavy phase discharge port, helps advance the separated heavy phase discharge port, helps advance the weparated heavy material through the restricted passageway and toward the port through which heavy phase material is dicharged. A sedimentation centrifuge according to US 5,310,399 includes a bowl which is supplied with a feed mixture, i.e., a solid-liquid mixture, from a supply pipe. When the bowl rotates at high speed, heavy solids are displaced radially outwardly toward and sedimented on the inner wall surface of the bowl. The solid particles are then discharged through a discharge hole of the bowl and an outlet passage by a screw conveyor in the bowl. During rotation of the bowl, fins on the screw conveyor assist in separating the liquid into heavy and light liquid phases with increased separation efficiency. The screw conveyor has screw flights with an inner portion which is inclined with respect to the axis of rotation of the bowl and an outer portion which is normal to an inner wall surface of the bowl. The separated heavy and light liquids are separately dicharged from the bowl through respective outlet passages. A further decanter centrifuge is known from WO 93/22062 A1. The decanter centrifuge according to WO 93/22062 A1 serves for special slurry centrifuging, and consists of an inlet pipe with a mixer chamber and a drum with a spiral screw for the feeding of slurry. Furthermore, it comprises a slurry and reject outlet. The angle of the spiral screw is greater than 90° in the direction of transport. This implies that the blade of the spiral screw sweeps backwards and the angle of the blade to a plane at right angles to the lengthwise axis is approximately 20 to 45° in the first part of the centrifuge. It is successively diminished to approximately 10° in the outlet end of the centrifuge. As it will be clear from the foregoing description of the state of the art, a decanter centrifuge generally includes an outer bowl, an inner hub carrying a worm conveyor, a feed arrangement for slurry to be processed, and discharge ports of cake solids and clarified liquid. The bowl includes a cylindrical section and a conical beach section. The bowl and the hub are rotated at high, yet slightly different angular speeds so that heavier solid paritcles of a slurry introduced into the bowl are forced by centrifugation into a layer along the inner surface thereof. By differential rotation of the worm conveyor and the bowl, the sediment is conveyed or scrolled to a cake discharge opening at the smaller, conical end of the bowl. Additional discharge openings are provided in the bowl, as usually at an end opposite of the conical section for discharging a liquid phase separated from the solid particles in the centrifuge apparatus. One of the goals in centrifuge operation is to produce cakes with a low moisture content. One proposed method, published in Research Disclosure, March 1993, Number 347, for reducing cake moisture content entails the disposition of a flow control structure proximate to the cake discharge port to reduce the volume flow rate of the cake by 25% to 75%. The flow control structure could be a ring shaped dam extending radially outwardly from the axis of the bowl, a dam disposed between two turns or wraps of the conveyor, an increased beach climb angle, an increased conveyor blade thickness, or an increased or decreased conveyor helix angle. It was asserted that by decreasing the volume flow rate of the solids by about one-half, or between 25% and 75%, the velocity at the interface between the liquids and the sedimented solids is in the reverse direction, i.e., towards the pool and away from the cake discharge port. Liquid from the pool and liquid expressed from the cake layer are drained back into the pool rather than carried out of the bowl with the sedimented solids. Although a drier cake is obtainable by the published technique discussed above, the problem generated by such a cake flow control solution is that the cake production rate or throughput is reduced, thus increasing costs and reducing efficiency. It is also known to form a dip weir along the outer surface of the conveyor hub, at or about the location of the junction between the cylindrical and conical sections of the bowl, to serve in selecting the driest portion of the cake at the discharge end of the bowl. The dip weir blocks the transport of the sludge cake in such a manner that the most compacted part of the cake passes under the dip weir and reaches the cake discharge opening. The dip weir also acts to provide the appropriate resistance to cake flow so as to maintain a large cake thickness upstream of the weir, creating high compacting pressure and long residence time. In conventional practice, the dip weir is fixed to the hub so that the radial gap between the outer edge of the dip weir and the inner surface of the bowl is constant or fixed. The designer must position and dimension the weir to minimize cake moisture content while not excessively increasing cake transport resistance through the gap so as to unduly limit the solids capacity of the machine. The optimal gap height depends on the nature of the cake, the G level, and the cake flow rate or solids throughput. The designer is forced to guess at the correct gap height, guided somewhat by past experience. It is an object of the present invention to provide a decanter centrifuge and a method for operating it, which centrifuge and method provide reduction in the moisture content of a discharged cake in comparison with the state of the art, while maintaining a relatively high cake throughput rate. According to the invention this object is achieved by a decanter centrifuge according to claim 1 and by a method for operating a decanter type centrifuge according to claim 7. In a decanter type centrifuge in accordance with the present invention, a drier cake product is obtained with a higher cake throughput than in the prior art. Preferred and advantageous embodiments of the decanter centrifuge according to the invention are subject matter of claims 2 to 6. Preferred and advantageous developments of the method according to the invention are subject matter of claims 8 to 10. Embodiments of the invention will now be described with respect to the accompinying drawings in which only Fig. 19A and 19B belong directly to the invention, with the remaining Figs. being introduced for explanation purposes only. Fig. 1 is a diagram of a decanter centrifuge with an adjustable gate for moisture content control. Fig. 2 is a schematic partial longitudinal cross-sectional view of a specific embodiment of a decanter centrifuge according to Fig. 1. Fig. 3 is a schematic front elevational view of a gating element and a particular embodiment of an associated actuator and locking mechanism shown in Fig. 2. Fig. 4 is a schematic side view of the gating element and associated cam actuator and locking mechanism of Fig. 3. Fig. 5 is a schematic side elevational view of another gating element and associated fluid actuator and locking mechanism for implementing the decanter centrifuge of Fig. 2. Fig. 6 is a schematic front elevational view of yet another gating element and associated actuator and locking mechanism for implementing the decanter centrifuge of Fig. 2. Fig. 7 is a schematic partial longitudinal cross-sectional view of another embodiment of a decanter centrifuge according to Fig. 1. Fig. 8 is a view similar to Fig. 7, showing a modification of the decanter centrifuge of that drawing figure. Fig. 9 is a schematic partial longitudinal cross-sectional view of a baffle bolted onto a mounting bracket which bridges across adjacent screw wraps. Fig. 10 is a baffle plate or gating element showing a difference in heights between clarified liquid on one side and cake on an opposite side of the baffle plate. Fig. 11 is a schematic partial longitudinal cross-sectional view of a decanter centrifuge with a moisture control gating element, depicting use of the gating element to facilitate a three-phase separation process. Fig. 12 is a diagram, looking down on an inner surface of a flattened bowl of a decanter centrifuge, for discussing motion of a cake layer between adjacent vanes and over the bowl surface. Fig. 13 is a diagram, essentially looking along a helical cut, parallel to a conveyor vane, showing a cake layer on a beach surface of a bowl of a decanter centrifuge. Fig. 14 is a diagram similar to Fig. 13, showing velocities and flow directions of cake sludge particles as they are conveyed upwardly, in opposition to the centrifugal force, along the beach surface. Fig. 15 is a diagram similar to Figs. 13 and 14, showing a cake profile and cake particle flow directions along a simple beach section of a decanter centrifuge. Fig. 16 is a diagram similar to Figs. 13-15, showing a cake profile and cake particle flow directions along a compound beach. Fig. 17 is a diagram similar to Fig. 16, showing a cake profile along a compound beach section provided at a cake discharge port with a flow-control structure such as a gate. Fig. 18A is a diagram similar to Fig. 16, where a second section of the compound beach has a zero climb angle. Fig. 18B is a diagram similar to Fig. 16, where a second section of the compound beach has a negative climb angle. Fig. 18C is a diagram similar to Fig. 18B, where a second section of the compound beach is more negatively sloped. Fig. 19A is a schematic partial longitudinal cross-sectional view of a decanter centrifuge employing a flow-control structure in conjunction with a compound beach, in accordance with the present invention. Fig. 19B is a view similar to Fig. 12, taken in the direction A-A in Fig. 19A. Fig. 20 is a graph illustrating cake dryness and solids throughput for different machines. Fig. 21 is a view similar to Fig. 19A, showing a decanter centrifuge employing another flow-control structure in conjunction with a compound beach. Fig. 22 is a view similar to Figs. 19A and 21, showing a decanter centrifuge employing yet another flow-control structure in conjunction with a compound beach. Fig. 23 is a view similar to Figs. 12 and 19B, showing a further flow-control structure for use in conjunction with a compound beach of a decanter type centrifuge. Fig. 24 is a view similar to Fig. 23, showing an additional flow-control structure for use in conjunction with a compound beach of a decanter type centrifuge. Fig. 25 is a schematic partial longitudinal cross-sectional view of a compound beach. Like reference numerals in the drawings designate the same structural elements. Figs. 1-11 relate to a gating element for controlling the moisture content of cake exiting a decanter centrifuge. The remaining drawing figures relate to improvements which result in an especially low cake moisture content, without substantially reducing the rate of cake output and even increasing the rate of cake output in certain configurations of the centrifuge. Fig. 1 diagrammatically illustrates the lower half of a decanter type centrifuge comprising a solid or perforated bowl 12, a worm or screw type conveyor 14, and a slurry feed arrangement that includes a feed pipe 10, a feed compartment (not shown) and one or more openings (not shown) in the conveyor hub 22 to allow slurry to pass from the feed compartment to a liquid pool 11 in the bowl. Bowl 12 is rotatable about a longitudinal axis 16 and has a cake discharge opening 18 at one end and a liquid phase discharge opening 20 at an opposite end. Conveyor hub 22 has at least a portion disposed inside bowl 12 for rotation about longitudinal axis 16 at an angular speed different from an angular rotational speed of bowl 12. Conveyor 14 further includes a helical screw or worm 24 attached to conveyor hub 22 and disposed inside bowl 12 for scrolling a cake layer 26 along an inner surface 28 of bowl 12 towards cake discharge opening 18. An adjustable component 30 on conveyor hub 22 forms a gap 32 between the hub and inner surface 28 of bowl 12 so that the gap has a size adjustable independently of hub rotation speed. Adjustable gap 32 enables an optimization of the moisture content of cake exiting bowl 12 at cake discharge opening 18 or other performance parameters. Preferably, adjustable component 30 includes a gating element 34 movably mounted to hub 22 and locking hardware 36 for maintaining the gating element at a predeterminable location relative to the hub. Gap 32 is defined by an edge 38 of gating element 34 and the inner surface 28 of bowl 12. The magnitude of gap 32 is adjustable by shifting gating element 34 towards or away from inner surface 28. Preferably, gating element 34 is operatively connected to an actuator 40 which is disposed inside hub 22 and bowl 12, but may be disposed outside of those components. Actuator 40 is located so that the position of gating element 34 may be adjusted without significant disassembly of the decanter centrifuge. Generally, gating element 34 is juxtaposed to a beach section 42 of bowl 12 and cooperates therewith in defining gap 32. Gating element 34 may be disposed between a pair of adjacent wraps 44 and 46 of conveyor screw 24, as shown in Figs. 1 and 2. Alternatively, gating element 34 may be disposed downstream of the last wrap 44 of conveyor screw 24, as discussed hereinafter with reference to Figs. 7 and 8. As illustrated in Fig. 2, gating element 34 may take the form of a baffle plate 48 disposed between adjacent wraps 44 and 46 of screw 24. Baffle plate 48 is disposed approximately perpendicularly to wraps 44 and 46 and may be guided in grooves 92 (see Fig. 6) provided therein. The functions of actuator 40 and locking mechanism 36 may be combined in a single hardware assembly or mechanism 50. As discussed above, mechanism 50 may serve to enable manual or, alternatively, automatic adjustment of the gap 32 between inner surface 28 of bowl 12, on the one hand, and conveyor hub 22 or, more particularly, baffle plate 48, on the other hand. In the case of manual adjustment, mechanism 50 is at least partially mounted to conveyor hub 22 and is operatively connected to baffle plate 48 for enabling a manual adjustment. Manual adjustment may require centrifuge stoppage, followed by either partial disassembly of the decanter centrifuge or by accessing the locking mechanism 36 through an access opening 43 provided in beach section 42 of bowl 12. Alternatively, a coupling or linkage mechanism (not shown) may be provided for enabling manual adjustment even during operation of the centrifuge. For instance, where adjusting and locking hardware 50 is hydraulic (Fig. 5), slippage couplings (not shown) are provided for connecting stationary and rotating portions of the hydraulic circuit. The reservoir 70 of pressurization fluid (see Fig. 5) may be fixed or rotating with conveyor hub 22. The position of baffle plate 48, and accordingly the gap 32 between the baffle plate and inner bowl surface 28, may be automatically varied in accordance with feedback from a sensor (not shown) monitoring cake moisture content. A microprocessor programmer (not shown) may be provided for controlling the position of baffle plate 48 pursuant to such input instructions and such variables as the nature of the cake, the G level and the cake flow rate. Figs. 3 and 4 illustrate a specific embodiment of actuator and locking mechanism 50. A radially inner edge 52 of baffle plate 48 is held in engagement with a camming element 54 by means of one or more biasing springs 56 and 58 coupled at their inner ends to a plate 23 fixed to conveyor hub 22. As camming element 54 is turned or pivoted about an eccentric axis of rotation 60 via a non-illustrated linkage mechanism, baffle plate 48 reciprocates in a radial direction, thereby modifying the size of gap 32. Camming element 54 and springs 56 and 58 are housed inside conveyor hub 22 to prevent solids from jamming the mechanism. Conveyor wrap 44 can be provided with a window 62 traversed by the linkage mechanism (not illustrated). Baffle plate 48 may be located in a plane which is approximately parallel to the common longitudinal axis 16 (Fig. 1) of rotation of bowl 12 and conveyor hub 22. This orientation is not critical, however, and the baffle plate 48 may be disposed in a plane oriented at an angle relative to rotation axis 16. Moreover, a second baffle plate (not shown) may be provided on conveyor hub 22 in diametric opposition to baffle plate 48. Gating element 34 and, more particularly, baffle plate 48 serves to control the solids concentration admitted for discharge at opening 18. Baffle plate(s) 48 divides the annular space between bowl 12 and conveyor hub 22 into two regions with a distinct difference in liquid pool and solids level across the baffle plate. Upstream of baffle plate 48, in a direction opposite to the flow of cake layer 26, the pool and solids level is deeper as set by the centrate weir. The deeper pool enhances clarification and a build-up of a thicker cake layer 26 for compaction and dewatering and also provides buoyancy to reduce conveyance torque. Downstream of baffle plate 48, the solids level is controlled by the spillover point of beach section 42. There cake layer 26 is strongly affected by the centrifugal field such that the surface of the cake layer is roughly parallel to rotation axis 16 and is approximately at the radius of the spillover. The baffle plate 48 skims off the driest solids adjacent to bowl inner surface 28. Cake solids in gap 32, which is generally between 0.25 and 1.5 inches wide, depending on the process, the size of the machine and the throughput, form a "plug" to seal the deep pool 11 on the upstream side of the machine (right side in Figs. 1 and 2) from the shallower pool with concentrated solids on the downstream side of the machine (beach discharge end at the left side in Figs. 1 and 2). The position of baffle plate 48 relative to wraps 44 and 46 should be adjusted to change the size of gap 32 as needed by the process, specifically to skim off the driest solids near the bowl wall or to reduce instability caused by washout of the plug. It is desirable to have the size of gap 32 adjustable while the machine is running. However, it is satisfactory when the position of baffle plate 48 can be adjusted without disassembling the machine, for instance through access opening 43 under cover plate 45, while the centrifuge is stationary. As illustrated in Fig. 5, another specific embodiment of actuator and locking mechanism 50 includes a pair of pistons 64 and 66 connected in a hydraulic circuit 68 to a pressurized oil reservoir 70 via a closed-loop hydraulic switch or valve 72 which is remotely controlled via an electro-mechanical control 74 external to bowl 12. The linkage mechanism for turning camming element 54 (Figs. 3 and 4) or a connection 76 from electro-mechanical control 74 (Fig. 5) may rotate with conveyor hub 22. To effectuate an adjustment in the position of baffle plate 48, slippage couplings (not shown) are provided for connecting stationary and rotating portions of actuator and locking mechanism 50. In this case, baffle plate 48 can be adjusted while the machine is running. Fig. 6 depicts yet another embodiment of actuator and locking mechanism 50 which includes a rocker-arm lever 78 pivotably connected to hub 22 via a fulcrum post 80 and pivotably linked at one end to a stub 82 of baffle plate 48. At an opposite end, the orientation of rocker-arm lever 78 is controlled by a stud 84 threaded to the conveyor hub 22 by a locknut 86 during centrifuge operation. A cover 88 is provided on hub 22 over an access aperture 90. Retainers such as brazed jam nuts 87 are provided on opposite sides of lever arm 78 for suitably securing stud 84 thereto. Lever arm 78 is further furnished with a swivel 89 having a throughhole for providing a rotating fit for stud 84. Baffle plate 48 is preferably made of titanium with a ceramic wear surface and is slidably arranged between two fixed plates 91 and in grooves 92 provided in conveyor worm wraps 44 and 46. Baffle plate 48 may be maintained in position partially by virtue of centrifugal force. Where only one baffle plate 48 is provided, conveyor hub 22 is balanced with the baffle plate installed and positioned centrally with respect to its range. Any further minor changes may be counterbalanced with a large-diameter set screw and locking nut (not shown) 180° opposite in the end of the conveyor hub 22. In another specific configuration of the decanter centrifuge, illustrated in Fig. 7, bowl 12 has a cylindrical portion 100 and a conical portion 102 defining beach section 42 along its inner surface. Gating element 34 takes the form of an annular dip weir 104 disposable at different longitudinal positions along conveyor hub 22. Dip weir 104 is provided with an annular rod 106 extending outside of centrifuge bowl 12 for enabling a manual repositioning of weir 104, as indicated by phantom lines 108, to change the size of gap 32 between dip weir 104 and beach section or surface 42. Rod 106 enables weir position adjustment from outside the machine, without disassembly. Moreover, as discussed hereinabove, this adjustment may be implemented while the machine is running, in the event that slippage couplings (not shown) are provided for connecting stationary and rotating portions of rod 106. Alternatively, the position of dip weir 104 may be adjusted by shutting down the machine, reaching in through an access opening 43 under cover plate 45 in bowl 12, manually unlocking the dip weir, and sliding it axially to another position. Dip weir 104 is then fixed in the new position relative to hub 22 by locking hardware or mechanism 36 (Fig. 1). It is to be noted that for compactible cake solids, decanter centrifuges generally run with "superpool": the pool level (set by effluent weirs) is radially inward of the radial position of cake discharge opening 19. All the cake 26 is therefore acted upon by buoyancy and, in addition, "hydraulic assist" due to the superpool head forces the cake toward cake discharge opening(s) 18. With the design of Fig. 7, the amount of superpool must be set large enough so that cake layer 26 is transported to cake discharge opening(s) 19 even though part of beach section 42 is without a conveyor. As illustrated in Fig. 8, the embodiment of Fig. 7 may be modified by dividing beach section 42 into two portions or areas 110 and 112 with different slopes. Dip weir 104 is positionable along beach portion 112 which has a smaller slope than beach area 110, thereby providing a greater degree of adjustability in the size of gap 32. The increased .amount of superpool head required by the conveyor-free portion 112 of beach section 42 may be used to further advantage in the configuration of Fig. 8. Here, beach portion 110 is provided with conveyor wraps 114 and is steeper than beach portion 112. This allows the conveyor-free beach portion 112 to be longer, without changing the overall length. In the embodiments of Figs. 7 and 8, dip weir 104 has an outer diameter which decreases in a direction of cake advancement, towards discharge opening 18. In a modified configuration, dip weir 104 may have an external diameter which increases from left to right in Figs. 7 and 8. As depicted in Fig. 9, a modified decanter centrifuge includes a cake gating or metering mechanism in the form of a baffle plate 116 attached via bolts 118 to a bracket 120 which in turn extends between and is connected to adjacent wraps 122 and 124 of conveyor 14. To adjust gap 32 between baffle plate 116 and beach section 42 of bowl 12, cover plate 45 is removed to allow access to the baffle plate through opening 43. Bolts 118 are loosened and baffle plate 116 shifted relative to bracket 120. Another purpose of having an adjustable baffle/gating element is to foster a deep pool operation (which is beneficial as discussed above) such that the pool level is very much above the spill-over point (super-pool) as indicated schematically by the distance H in Fig. 10 between the height of cake 26 at an outlet side of baffle or gating element 34 and the height of pool 11. How much the pool level increments across baffle or gating element 34 depends on the flow resistance, which in turn depends on the solids rate, the size of gap 32 and the rheological properties of the cake. Gap 32 is usually between 6.35 mm (0.25 inch) and 3.81 cm (1.5 inch). For a high solids rate, gap 32 can have moderate width. For a low solids rate, the gap needs to be smaller to provide the same resistance. For raw mixed sludge with primary sludge that has fiber and substrate materials, the width of gap 32 should be moderate, whereas for waste activated sludge or digested sludge without fibrous materials, the gap needs to be smaller. Fig. 11 illustrates use of an adjustably positioned gating element 124 as described hereinabove to facilitate a three-phase separation process to prevent a lightest phase such as oil 126 from being entrained by a cake or solid phase 128 as the latter emerges from an oil-water pool 130 at a conical section 132 of a decanter centrifuge (not designated). Gating element 124 may take the form of a dip weir which is placed upstream of a solids emergence zone 134 so as to reduce entrainment of oil phase 126 by cake or solid phase 128. An outer edge 136 of dip weir 124 must penetrate beyond an oil-water interface 138 to be effective. A dip weir with a tight opening would be ideal if not for the fact that it might run into cake solids layer 128, which for granular solids can generate undesirable high torque. Given that the location of oil-water interface 138 and a water-solid interface 140 are not known, the centrifuge has to be operated with close monitoring of the oil discharged with the cake solids 128 and the torque level experienced by the machine. The adjustable gap enables optimization in response to the monitoring. A decanter centrifuge with an adjustable gating element as disclosed above with reference to Figs. 1-11 demonstrates certain advantages with respect to the classification of fine solids. However, although the moisture content of the cake is controllable to a substantial extent, large reductions in moisture content are not possible without compromising the production rate. As discussed below, cake moisture content may be reduced dramatically, without substantially reducing the cake production rate, by using a gating element or, more generally, a cake flow control structure, in conjunction with a compound beach. Results are optimized when the pool level and the junction between a first beach section and a less steep downstream beach section are located at approximately the same distance from the centrifuge rotation axis. Conceptual Considerations The concept of a flow control structure in the beach zone arises as a consequence of far-reaching theoretical analyses, followed by extensive confirmatory laboratory tests of models of the beach zone. As background for understanding the rationale of the present inventions, the underlying theoretical considerations are summarized here. Development on a Plane The inner surface of the bowl may be developed on a plane. Since the thickness of the sludge layer on the beach is generally small compared with the bowl radius, one may envisage the flow as occurring on that planar surface, tilted at the beach angle β (see Figs. 3 and 5) to the axial direction. Fig. 12 is a schematic view of that plane, viewed in the direction of the centrifugal field. The helical conveyor appears as a series of parallel vanes 210 inclined at the helix angle α to the direction of rotation 212, a direction normal to the centrifuge rotation axis 16 (Fig. 1 et seq.). Each pair of adjacent vanes 210 forms a channel 214 along which the sludge cake is guided and transported (as at 216) toward a cake discharge plane 218. Within channel 214, the sludge cake can occupy up to a maximum width W equal to the distance between the adjacent vane surfaces 214a and 214b that form the channel and extends above the inner surface of the bowl by the cake height h (Fig. 13). Reference Frame of the Conveyor Consider the motions as seen by an observer who moves at the same angular speed as the conveyor. In this reference frame, conveyor vanes 210 are stationary, while the plane representing the bowl wall (plane of the paper in Fig. 12) slides past them, in a direction 212 normal to the centrifuge rotation axis 16 (Figs. 1 et seq.), with a speed equal to the bowl wall radius R multiplied by the differential angular speed between the bowl and the conveyor, ΔΩ. As a result of one component of the frictional force, the sliding of the bowl wall past the conveyor vanes tends to drag the cake against the driving face 214a of each vane. Even more importantly for conveyance, the other and larger component of the frictional force exerted by the bowl wall acts to drag the cake along the channel 214. The cake is transported "uphill" against the component of centrifugal force that acts in the "downhill" direction on the beach. Thus, the mechanism of cake transport may be summarized as follows: by reason of the relative motion, R x ΔΩ, between the bowl and the conveyor vanes, the bowl drags the cake to the solids discharge end through the channels formed by the conveyor vanes, overcoming a component of the centrifugal force as well as the frictional force exerted by the vanes against the direction 216 of the cake flow. The Belt Analog Fig. 13 shows an analog that contains the important features of the process described above. A belt 220 representing the bowl wall is inclined at a "climb angle" γ to the "horizontal" 222, which is normal to the centrifugal field G. Belt 220 moves in an uphill direction with a relative speed U equal to the triple product of the bowl inner surface radius R, the differential angular speed ΔΩ, and cos(α), where α is the helix angle (Fig. 12). For all practical purposes, U = (R x ΔΩ) inasmuch as α is generally less than 15 degrees. The frictional force applied by the belt drags the sludge cake lying on the surface of the belt uphill against a component of the centrifugal force acting on the mass of the cake. The climb angle γ is the effective uphill angle the sludge cake has to overcome. To a good approximation, the climb angle γ (in radians) is the product of the helix angle α (in radians), and the beach angle β (in radians). In the cylindrical clarifier section, where the beach angle is zero, the climb angle is of course also equal to zero. In practice, the climb angle of the beach is quite small, of the order of 1°. In order that details may be seen more easily, therefore, Fig. 13 as well as other figures to follow, has been drawn with a greatly enlarged vertical scale. In Fig. 13, the sedimented sludge cake is overlain by the liquid slurry in a pool 224. The liquid slurry itself has comparatively small motion, and its main effect as regards sludge cake 226 is that it provides a buoyancy force that facilitates the conveyance of the sludge cake uphill. The Velocity Profiles It is assumed that the rheology of the sludge cake is such that it behaves somewhat as a liquid and that it flows under the influence of viscous stresses. With reference to Fig. 14, viscosity causes the portion of the cake sludge layer 226 immediately adjacent to the moving belt 220 (Figs. 13 and 14) to be dragged forward with the speed U of the belt. That layer in turn exerts a viscous force on the next adjacent layer, causing it also to move uphill, but at a slightly lesser speed. This scenario is repeated, layer by layer, in chain-like fashion from the surface of the belt to the surface of the cake. Thus the sludge cake moves forward not uniformly as a solid plug or body but with a respective velocity profile VP 1 , VP 2 , VP 3 , etc., and a respective thickness profile h 1 , h 2 , h 3 , etc., depending on the position x 1 , x 2 , x 3 along belt 220. In Fig. 14, arrows 228 extending to the velocity profile curves VP 1 , VP 2 , VP 3 signify the speed of cake sludge particles at different distances from belt 220. Given particular values of the cake flow rate, of the climb angle γ, and given the properties of the material forming the cake, the shapes of the velocity profiles VP 1 , VP 2 , VP 3 , etc., depend upon cake height h (Fig. 13). Fig. 14 shows, for the same flow rate, velocity profiles VP 1 , VP 2 , VP 3 at three different positions x 1 , x 2 , x 3 where the respective cake heights h 1 , h 2 , h 3 are different from each other. For illustrative purposes, the cake height is assumed to increase from position X 1 to position x 2 to position x 3 (h 1 less than h 2 less than h 3 ). Since the flow rate is the same at the three positions x 1 , x 2 , x 3 , the areas lying between the three velocity profiles VP 1 , VP 2 , VP 3 and the respective heights h 1 , h 2 , h 3 are all the same, even though the shapes are quite different from each other. At position x 1 , the respective profile VP 1 is relatively uniform, and the speed at the cake-pool interface is in the forward direction, as indicated by an arrow 230. At position x 2 , the respective profile VP 2 is less uniform, and the speed drops to zero at the interface between the cake sludge layer 226 and the slurry pool 224, at a point 231 (height h 2 above belt 220). At position x 3 , where cake height h 3 is largest, the respective velocity profile VP 3 indicates forward flow near belt 220, but rearward flow near the cake-pool interface, as indicated by an arrow 232. The total downhill component of the centrifugal field that acts upon cake layer 226 at any particular location is proportional to the mass of cake, and thus to the cake height h (as generically labeled in Fig. 13). With a thin layer of cake, as at position x 1 , the frictional force applied by belt 220 is sufficient to carry the whole cake layer forward. At position x 2 , where the mass of cake is larger, the belt friction is just barely able to support the entire cake thickness in the forward direction. When the mass of cake is even larger, as at position x 3 , the belt friction is not sufficient to transport the entire cake thickness forward, with the result that the outer layer-of cake slips rearward. Backflow A zone 234 of cake backflow in Fig. 14 is shown stippled. A curve 236 divides rearward-flow zone 234 from a zone 238 of forward flow. From a point 240 to a point 242 along a streamline 244a, cake particle motion is rearward (away from the cake discharge opening 18); at point 242, the flow turns around, and cake particle motion is forward (toward the cake discharge opening 18) between point 242 and any subsequent point 246 of streamline 244b. At the interface between flowing sludge cake 226 and overlying pool 224 of slurry liquid, the cake motion is forward upstream of point 231 but rearward downstream of point 231. This pattern, emphasized by the arrowheads placed on the interface in Fig. 14, is highly significant, as explained below. A Conventional Cake Profile Fig. 15 shows, by means of the belt analog, a cake profile 248a and 248b and an associated flow pattern for a conventional centrifuge with a beach 250 of uniform angle. Fig. 15 also shows a below-pool zone 252 with cake profile 248a, and an above-pool zone 254 with cake profile 248b, or so-called "dry beach." The purpose of the dry beach 254 is to provide a drying-out area where liquid can be expressed from cake 226 without interference from an overlying pool of liquid. The cake leaves the clarifier 256, enters the below-pool zone 252 of the beach, is transported up beach 250, and finally leaves the machine at a cake discharge port 256. The effective density of the cake experiences a jump when the cake passes from below-pool zone 252 to the above-pool zone 254, because the buoyancy provided by liquid in pool 224 is lost. It has been found that this gives rise to the cake profile 248a and 248b. From a first point 258 to a second point 260 of profile 248a, cake height h increases and the interface motion is forward, as indicated by an arrow 262. From second point 260 to a third point 264 along cake profile 248a, the cake height continues to increase, but the interface motion is rearward, as indicated by arrow 266. The cake emerges from pool 224 at point 264. From point 264 to a fourth point 268 on cake profile 248b, the cake height decreases, and the interface speed is rearward. Finally, from point 268 to a cake discharge point 270, the cake height remains nearly constant and the interface motion is forward. Within a triangular zone 272 defined by points 260, 264 and 268 is a trapped, recirculating vortex-like area of cake. Along the cake profile 248a between points 260 and 268, the rearward motion of the interface prevents pool liquid from being entrained by the cake 226 as it emerges from pool 224. This is good, but on the other hand the interface motion between points 268 and 270 is forward. This means that when liquid is expressed from the cake in dry-beach zone 254, some part of the expressed liquid is carried forward instead of draining back into the pool. The purpose of the dry beach in expression and drainage of additional moisture from the cake is thus at least partially negated. Conventional Compound Beach Fig. 16 shows a compound beach 274, with a relatively large initial climb angle γ 1 in below-pool zone 252 (where buoyancy provides assist), and a relatively small climb angle γ 2 in the above-pool zone 254 (where the assisting effect of buoyancy has been lost). The cake profile, and the pattern of interface motions, are respectively similar to those in the uniform beach case, Fig. 15. Similar features are labeled with the same reference numerals in Figs. 15 and 16, As in the single beach case, the surface of the cake moves forward in the dry beach area, carrying expressed liquid to the solids discharge end and thereby resulting in wetter cake. Compound Beach with Flow Impedance The geometric configuration of Fig. 17 is like that of Fig. 16 (same compound beach 274), but now the cake flow is impeded by a flow-control structure 276 proximate to cake discharge port 256. Flow-control structure 276 may take the particular form of a gate, dam or weir that constricts the flow area between the gate and the inner surface of the bowl at discharge port 256. Flow-control structure 276 can assume other forms, as discussed below. When the cake flow is blocked so as to be reduced to about half the unimpeded rate, an extended recirculating zone 280 is established. Along a portion of a cake profile 282a between points 284 and 286, the interface motion is rearward, thus preventing pool liquid from being carried forward with the sludge cake 226. Perhaps more importantly, the interface motion of cake profile 282b is rearward between points 286 and 288 thus signifying that liquid expressed from the cake beyond the point of pool emergence at 286 can not be carried forward with the cake to the cake discharge port. Thus, the flow impedance imposed by flow-control structure 276 acts to enhance the cake dryness. This geometry combines the benefit of using the flow-control structure to get drier cake and the benefit of a compound beach to avoid excessive reduction of solids throughput capacity. Compound Beach with Zero Second Angle and Flow Impedance Fig. 18 depicts the limiting form 290 of the compound beach where a second beach section 292 has a climb angle equal to zero. This geometery has special advantages. It provides higher cake flow capacity as compared to Fig. 17 where the second beach angle is small but nonzero and at the same time produces dry cake as with all other designs utilizing the cake-flow control structure. Pool 224 has a level or surface 294 set very close to the level of second beach section 292, and must be adjusted carefully. Alternatively stated, pool surface 294 is approximately at the same distance from the centrifuge axis as the second beach section 292. This common distance is implemented by having the liquid discharge port at approximately the same distance from the centrifuge axis (conveyor and bowl rotation axis) as the junction 296 between a first beach section 298 and second beach section 292. Because buoyancy eases the task of lifting the sludge cake 226 against the force of the centrifugal field G, the first beach section 298 may have a relatively large beach angle, and therefore may be relatively short. The savings in length over the conventional design of Fig. 15 makes available the length required for the section beach section 292. In an actual decanter centrifuge, a non-zero beach angle has the effect of creating a variation of cake thickness over the distance from one vane surface to the adjacent one forming the helical channel. The cake thickness is deeper at the driving face 214a of the conveyor vane and shallower toward the trailing face 214b of the adjacent conveyor vane. However, if the climb angle in the second part of the compound beach in an actual decanter is zero, the cake thickness is uniform across the helical channel formed by adjacent vanes or wraps; that is, the cross-section of the cake is rectangular, with its surface parallel to the straight beach. This is found to be advantageous to deliquoring, hence the configuration of Fig. 18 is preferred. In some applications, it may be advantageous to provide centrifuge bowl 400 with a compound beach comprising a first beach section 402 and a second beach section 404, the latter angled slightly downward (with respect to the horizontal) towards the solids discharge opening 406 so that both the beach angle β 2 and the climb angle γ 2 become negative, as illustrated in Fig. 18B. Cake 408 is dewatered in second beach section 404 under increasing G-force (arrow 410). A conveyor screw 412 also conforms to the geometry of bowl 400, including first beach section 402 and second beach section 404. In another design shown in Fig. 18C, the climb angle β 2 of a second beach section 424 of a compound beach 426 of a centrifuge bowl 438 has a comparatively large negative value, while a conveyor screw 428 terminates at a junction 430 between a first beach section 432 and second beach section 424. In this configuration, conveyance of cake 434 on the second beach 424 is effected by means of the centrifugal field (arrow 436). In some applications, a large negative beach angle β 2 , with its associated increase of G-force 436 toward a cake discharge opening 440, enhances further cake dewatering. Description of the Preferred Embodiment A decanter centrifuge may include more than one type of flow-control structure 276 to impede the cake flow as discussed above with reference to Figs. 17 and 18. The flow control structures, located proximate to cake or heavy-phase discharge port 256, impede the volume flow rate of cake solids 226 conveyed out of the bowl of a decanter centrifuge. It has been found that by reducing the solids volume flow rate by about one-half, or more generally between 25% and 75% of the otherwise unimpeded solids volume flow rate, the velocity of cake particles at an upper surface of the cake 226 is in the reverse direction, that is, back towards pool 224, over substantially the entire length of an above-pool zone 300, 302 of beach 274, 298, as well as the point (286 in Fig. 17) where the solids emerge from the pool. Liquid from pool 224 and liquid expressed from solids within the above-pool zone 300, 302 are thus rejected and drained back into the pool 224 rather than carried out of the centrifuge bowl with the sedimented cake 226. As a result, a decanter centrifuge incorporating a compound beach 274 or 298 together with an associated flow-control structure 276 produces a drier cake since less liquid reaches cake discharge port 256. Flow-control structures as described hereinabove with reference to Figs. 1-7 and 9 result in drier cake product. However, the drier cake is obtained at the expense of reduced cake flow capacity. In order to improve cake dryness, without the loss of cake flow capacity, the preferred geometry has a zero-degree beach in accordance with Fig. 18. It is noted that the amount of reduction of the solids volume flow rate produced by flow control structure 276 depends on the type and consistency of the feed slurry, as well as on the dimensions and operating conditions of the centrifuge. Although reducing the solids volume flow rate by about one-half is the optimal amount of reduction when the mixture behaves substantially as a Newtonian fluid, the best way to determine the optimal amount of reduction is through empirical tests. It is also noted that the preferred compound beach geometry with the second beach angle at zero degrees and with a flow-control structure produces drier cake and higher throughput in comparison with conventional single-beach geometries whether with flow control, which suffers from lower throughput, or without flow control, which results in wetter cake and somewhat lower throughput as compared to the preferred geometry discussed above. Fig. 19A shows a partial cross-sectional view of the solids end of a decanter centrifuge 304. Centrifuge 304 includes a screw-type conveyor 306 mounted within a bowl 308 having a generally cylindrical clarifier section 310, a tapered compound beach 312, and at least one heavy-phase or cake discharge port 314 communicating with the tapered beach section. Conveyor 306 includes a conveyor hub 316 and a generally helical conveyor blade or screw 318 having a plurality of turns or wraps (not separately designated) disposed about the hub 316. Bowl 308 and conveyor 306 rotate at high speeds via a driving mechanism (not shown), but at slightly different angular velocities, about an axis 322. A slurry feed of solid/liquid mixture is introduced into the decanter centrifuge 304 through a feed pipe 324 having at least one opening 326 which allows the feed slurry to enter bowl 308 through at least one feed port 328 formed in the conveyor hub 316 and which acts as a feed accelerator. A centrifugal force field generated by the rotating pool of liquid (not shown) in rotating bowl 308 causes suspended solids in the slurry mixture to sediment on an inner surface 330 of bowl 308. The effluent liquid leaves the decanter centrifuge 304 through at least one effluent liquid discharge port (not shown) at the effluent end of the clarifier section 310. The radial location of the discharge port (which may be annular) establishes the radial level 294 (Fig. 18) of the liquid pool 224 (Fig. 18). The surface 294 of the pool 224 is substantially cylindrical. Bowl 308 includes a tapered beach 312 including a first beach section 334 having a respective beach angle β 1 and a second beach section 336 having a respective beach angle β 2 . Beach angle β 2 of section beach section 336 is less than beach angle β 1 of first beach section 334. Preferably, beach angle β 2 is approximately zero degrees. Conveyance of the solids up beach 312, radially inward toward the axis 322, and against the counterposing outward radial force of the centrifugal field, is effected by virtue of the difference in angular velocities between bowl 308 and the conveyor 12. This differential allows the conveyor 306, having a helix angle α, to cooperate with bowl 308 so as to transport the sedimented solids toward the discharge port 314. The practical realization of flow-control structure 276 described above in connection with Figs. 17 and 18 takes the form here of a dam-like structure such as a baffle or gate 338, near the exit plane of conveyor 306, that spans between two adjacent wraps 340 of helical conveyor screw 318. Fig. 19B is a view of the same gate or baffle 338 as seen looking in the radial direction A-A in Fig. 19A. Helical conveyor screw 318, particularly adjacent wraps 340, appears as a series of parallel vanes inclined at the helix angle α to the direction of rotation 342, a direction normal to the centrifuge rotation axis 322. Adjacent wraps 340 form a channel 344 along which the sludge cake is guided and transported (as indicated by arrow 346) toward a cake discharge plane 348. In order to reach cake discharge port 314 in discharge plane 348, the flow must pass through a space between the bowl wall and the most radially-outward part of gate 338. Because of the constriction of cake height as the cake passes through the gate area, the flow is impeded, in accord with the principle illustrated by Fig. 18. An 46 cm (18-inch) diameter by 71 cm (28-inch) length solid bowl centrifuge 304 in accordance with the preferred geometry of Figs. 19A and 19B was built and tested on fine particle calcium carbonate slurry with 5-micron mean particles. The built centrifuge 304 has a short cylindrical clarifier 310, a first beach section 334 inclined at a 15-degree angle β 1 , and a second beach section 336 inclined at a zero-degree angle β 2 . Two approximately axially oriented baffles similar to baffle or gate 338 in Figs. 19A and 19B are positioned (one at each helix in a double-helix conveyor 306) at the exit of zero-degree beach section 336 where the dry cake discharges from the machine. The pool was set close to an intersection or junction 341 between the two beaches 334 and 336. The bowl was rotated at a speed of 2000 revolution/min generating 1000x gravity at the clarifier bowl wall and about 800x gravity at the zero-degree beach 336. Various radial gap widths, i.e., extent of flow control, have been tested. In Fig. 20, the results are compared with those obtained for a similar size decanter (46 cm (18'') diameter by 71 cm (28'') long) but with conventional single beach geometry under identical rotational speed. Curve 1-1 of Fig. 20 shows the cake dry solids percent obtained from the conventional decanter under different rates up to 417 kg/h (920 1b/hr) (dry basis). The results are compared with those obtained from the preferred geometry having a compound beach but with different extents of flow control - (curve 2-2) no control and large gap; (curve 3-3) some control with 1.27 cm (0.5-in) gap; and curve (4-4) tight control with 6.35 mm (0.25-in) gap. The compound beach configurations all have much higher capacity and greater cake dryness than the conventional decanter (curve 1-1). In all cases, the cake solids obtained by the preferred geometry were about 3-4% drier as compared to those obtained with the conventional decanter. Up to 635 kg/h (1400 1b/hr) solids (dry basis) was processed at 76% cake for the preferred geometry with 1.27 cm (0.5-in) gap versus 417 kg/h (920 1b/hr) solids (dry basis) processed with the convetional decanter and at a much lower cake solids of 72.5%. Although gate 338, in spanning the space between successive vane wraps 340, is shown in Fig. 19B as being oriented in the axial direction, it may lie at any orientation relative to the vane direction. For instance, it might be oriented to be perpendicular to the vane surfaces. Since the optimum baffle opening is not known exactly in advance, and since it will in any event depend upon the particular rheology of the sludge cake, it is highly advantageous for the baffle position to be adjustable, even more so if the position can be adjusted on the fly, as it were. Various techniques for gating adjustability are discussed above with reference to Figs. 1-11. The guiding concept of the invention, namely, impeding the flow rate of cake by an appropriate amount, may be realized practically in ways other than by the structure of Fig. 19A. For example, Fig. 21 shows a configuration in which the flow-impeding structure is an annular ring-shaped disk 350 attached to the conveyor hub 316. Alternatively, Fig. 22 shows a flow-impeding structure in the form of an annular ring-shaped disk 352 attached to the wall of bowl 308. In Figs. 21 and 22, the same structures as in Fig. 19A are designated by the same reference numerals. While the flow-impeding structures of Figs. 19A, 21, and 22 are shown as adjacent to the exit plane 348 (Fig.19B) of conveyor 306, they may also be situated further upstream. Fig. 23 represents the development on a plane of a conveyor screw 354. In a flow-control zone 256 near a cake discharge port (not shown), the helix angle of the conveyor 354 is reduced from a first value α 1 to a smaller value α 2 . This change in the helix angle reduces the flow area through the channel formed by adjacent wraps 358 of conveyor screw 354 and thus establishes an impedance to the cake flow. Each pair of adjacent vanes or wraps 358 forms a channel 360 along which the sludge cake is guided and transported, as indicated by an arrow 364, toward a cake discharge plane 362. A further embodiment of the flow-control concept is shown in Fig. 24, which is also a representation of a conveyor screw 366 developed on a plane. Here, in a flow-control zone 368 adjacent a cake discharge port, the thickness of the conveyor screw vane or wrap 370 is increased from the relatively small value t 1 typical of conventional practice to a relatively large value t 2 in the flow-control zone 368. By this means the cross-sectional area for cake flow through a channel 372 formed by adjacent wraps 370 of the conveyor screw 366 is decreased from w 1 to the smaller value w 2 in flow-control zone 368, thereby providing an impedance to the flow of cake towards a cake discharge plane 374. Although the several embodiments of the flow-control concept shown in Figs. 19A, 21, 22, 23 and 24 have been shown in the context of a compound beach 312 in which the second beach section 336 has a zero beach angle β 2 , these embodiments may also be applied to a compound beach in which the second beach section 338 has a non-zero beach angle. Under certain circumstances, they may also advantageously be applied to a beach with a uniform beach angle. Another beach geometry incorporating the flow-control concept is depicted schematically in Fig. 25. A beach 376 has three sections: a below-pool zone 378 with a relatively large beach angle β 3 ; an above-pool zone 380 with a relatively small or a zero beach angle β 4 ; and a flow-control zone 382 having a beach angle β 5 larger than that of the second beach section 380. The last beach section 382 provides the flow impedance that results in the flow pattern illustrated by Fig. 18. Although the invention in its various forms has been described in the context of separating the solid and liquid components of a feed slurry, it is equally applicable to the separation of a heavier-phase liquid from a lighter-phase liquid.

Claims: A decanter centrifuge comprising a bowl (308) rotatable about a longitudinal axis (322), said bowl (308) having a cake discharge opening (314) at one end and a liquid phase discharge opening, said bowl (308) having a cylindrical portion (310) and a beach portion (312) between said cylindrical portion (310) and said cake discharge opening (314), a beach area (312) being provided on an inner surface (330) of said bowl (308) at said beach portion (312), said beach area (312) including a first section (334) of a steep slope and a second section (336) of a less steep slope, said second section (336) being located between said first section (334) and said cake discharge opening (314), the centrifuge further comprising a conveyor (306) having at least a portion disposed inside said bowl (308) for rotation about said longitudinal axis (322) at an angular speed different from an angular rotational speed of said bowl (308), said conveyor (306) including a helical screw (318) disposed at a helix angle inside said bowl (308) for scrolling a deposited solids cake layer along said inner surface (330) of said bowl (308) towards said cake discharge opening (314), the centrifuge additionally comprising a feed element (324) extending into said bowl (308) and said conveyor (306) for delivering a feed slurry into a pool inside said bowl (308), said centrifuge also comprising a flow control structure (338) provided in said beach area (312) proximate to said cake discharge opening (314) for impeding a flow of cake along said beach area (312) towards said cake discharge opening (314), characterized in that said second section (336) of said beach area (312) is imperforate and said flow control structure includes a gating element or baffle plate (338) extending substantially perpendicularly to adjacent screw wraps (340) of said conveyor (306) and having a radially outer edge oriented substantially parallel to said inner surface (330) of said bowl (308) in all states of adjustment of said gating element or baffle plate (338), said gating element or baffle plate (338) being spaced a variable and adjustable predeterminable distance from said inner surface (330) of said bowl (308), said first section (334) and said second section (336) of said beach area (312) serving to maintain a high throughput of cake towards said cake discharge opening (314) in opposition to the impedance provided by said flow control structure (338). The centrifuge defined in claim 1 wherein said flow control structure (338) increases a cake flow cross-section upstream of said flow control structure (338) and is disposed along said second section (336) of said beach area (312). The centrifuge defined in claim 1 wherein said first section (334) and said second section (336) of said beach area (312) are contiguous with one another along a junction (341), said liquid phase discharge opening being disposed at a predetermined distance from said longitudinal axis (322), said junction being (341) disposed at approximately said predetermined distance from said longitudinal axis (322), whereby said pool is approximately coextensive with said cylindrical portion (310) and said first section (334) of said beach area (312) and whereby said second section (336) of said beach area (312) is disposed outside of said pool. The centrifuge defined in claim 1 wherein said second section (336) of said beach (312) has a slope of approximately 0. The centrifuge defined in claim 1 wherein said second section (424) of said beach area (426) has a negative slope so that said beach diverges from a hub of said conveyor (428) towards said cake discharge opening (440). The centrifuge defined in claim 5 wherein said negative slope is substantial and said screw conveyor (428) terminates at a longitudinal location upstream of said second section (424) of said beach area (426). A method for operating a decanter type centrifuge, comprising rotating a bowl (308) about a longitudinal axis (322) at a first rate of rotation, said bowl (308) having a cake discharge opening (314) at one end and a liquid phase discharge opening, said bowl (308) having a cylindrical portion (310) and a beach portion (312) between said cylindrical portion (310) and said cake discharge opening (314), a beach area (312) being provided on an inner surface (330) of said bowl (330) at said beach portion (312), said beach area (312) including a first section (334) of a steep slope and a second section (336) of a less steep slope, said second section (336) being located between said first section (334) and said cake discharge opening (314), the method further comprising delivering a feed slurry to a pool in said bowl (308) during the rotating thereof, the method additionally comprising maintaining said pool in said bowl (308) during the rotating of said bowl (308) so that said pool is substantially coextensive with said cylindrical portion (310) and said first section (334) of said beach area (312) and so that said second section (336) of said beach area is disposed outside of said pool, the method also comprising rotating a screw conveyor (306) having a helix angle about said longitudinal axis (322) at a second rate of rotation different from said first rate of rotation, the method further comprising scrolling a cake layer via said screw conveyor (306) along an inner surface (330) of said bowl (308) towards said cake discharge opening (314) and, in a portion of said beach area (312), impeding flow of said cake layer along said inner surface (330) and discharging cake through said cake discharge opening (314) and a liquid phase through said liquid phase discharge opening in said bowl (308), characterized in that said second section (336) of said beach area (312) is imperforate so that all of the liquid expressed from said cake in said second section (336) of said beach area (312) flows along said second section (336) away from said longitudinal axis (322); the impeding of the flow of said cake layer is adjusted by repositioning a gating element or baffle plate (338) which extends substantially perpendicularly to adjacent screw wraps (340) of said conveyor (306), the gating element or baffle plate (338) having a radially outer edge oriented substantially parallel to said inner surface (330) of said bowl (308) in all states of adjustment of said gating element or baffle plate (338); and a high throughput of cake towards said cake discharge opening (314) is maintained by said first section (334) and said second section (336) of said beach area (312) in opposition to the impedance provided by said gating element or baffle plate (338). The method defined in claim 7 wherein impeding the flow of said cake layer includes increasing cake flow cross-section along said second section (336) of said beach area (312) upstream of said flow control structure (338). The method defined in claim 7 wherein said second section (424) of said beach (426) has a negative slope so that a radius of said second beach section (424) at a downstream end thereof is larger than a radius of said second beach section (424) at an upstream end thereof. The method defined in claim 9 wherein said negative slope is substantial and said screw conveyor (306) terminates at a longitudinal location upstream of said second beach section (424).

Inventors: LEUNG WOON-FONG, SHAPIRO ASCHER H, LEUNG, WOON-FONG, SHAPIRO, ASCHER H.
Applicants: BAKER HUGHES INC, BAKER HUGHES INCORPORATED
Application year: 1997
Main classification: B04B1
IPC codes: B04B1