WO1995021697A1

WO1995021697A1 - Centrifugal separations apparatus

Info

Publication number: WO1995021697A1
Application number: PCT/AU1995/000055
Authority: WO
Inventors: Christopher Roy Greig; Peter James Tait; Russell Kirk
Original assignee: Stg Holdings Pty Ltd
Priority date: 1994-02-08
Filing date: 1995-02-02
Publication date: 1995-08-17
Also published as: CU22472A3; MX9603283A; ZA95787B; AUPM376094A0; BR9506766A; CN1143337A; EP0742740A1; JPH09508574A; EP0742740A4

Abstract

Centrifugal separations apparatus (10), for example for separating sugar from massecuite, including centrifuge means (11) having an inlet (32), a liquids discharge (31), a lip, a solids discharge (18), and a deflector (23) operatively associated with the solids discharge, wherein: a) the lip is adapted to operatively discharge solids being discharged from the centrifuge means (11) at a trajectory and the deflector (23) is adapted to interrupt the trajectory to decelerate and change the direction of travel of the solids, to prevent them impacting the walls of the chamber (24); or b) the deflector (23) is operatively associated with the lip of the centrifuge means (11) and cooperable with solids being discharged from the solids discharge whereby build-up of solids on the deflector is substantially prevented; or c) the lip configuration causes the trajectories of the discharging crystals to be relatively independent of their size and/or their residual syrup/moisture content; or d) including support means, such as a spider-like construction having one or more webs substantially parallel to the angular motion of the solids being discharged from the solids discharge, operatively supporting the centrifuge means whereby the impact area for solids discharged from the solids discharge may be minimized; or e) including blending means operatively associated with the solids discharge for blending solids discharged from the centrifugal separations apparatus.

Description

CENTRIFUGAL SEPARATIONS APPARATUS This invention relates to centrifugal separations apparatus.

This invention has particular hut not exclusive application to centrifugal separations apparatus for separating sugar crystals from a sugar solution, and for illustrative purposes, reference will be made to such application. In particular, this invention is directed to separations requiring highly washed sugar crystals for human consumption and/or for feedstock for a sugar refinery where high quality crystals are required. However, it is to be understood that this invention could be used in other applications, such as centrifugal separation of other liquids and solids and in particular where the solids once separated from the liquid are mobile.

In the centrifugation of solids from a liquid containing such solids, there may be defined two main separation mechanisms. The filtration mechanism occurs where the volume fraction of liquid in the mixture exceeds the voidage between the solids in the mixture. The drainage mechanism occurs where the volume fraction of liquid is less than the voidage between the solids in the mixture. Where the volume fraction of liquid is equal to the voidage, this is known as the drainage transition. Description of the Prior Art

In the processing of sugar crystals, a filtration centrifugation separation process is frequently employed to separate the sugar crystals from a sugar solution. The filtration centrifugation separation process is often batch operated and the equipment is emptied and cleaned after each batch. This is especially so for highly pure crystal sugar for consumption or refining where crystal quality is important. The batch operation imposes a considerable down time for the filtration centrifugation and other associated plant equipment requiring a larger number of filtration centrifugation units to satisfy the throughput of a sugar crystallization plant.

Attempts have been made to provide continuous filtration centrifugation equipment for high quality sugar production, however, such equipment often leads to breakage of the sugar crystals as a result of collisions of the sugar crystals with the walls of the centrifuge casing. Such equipment often does not provide adequate washing of the crystals. Also, such equipment often clogs with hard sugar and/or agglomerated lumps of sugar are produced in the product.

In most industrial instances, the separation of product sugar from massecuite and washings is carried out in batch machines. In a batch centrifuge, a predetermined charge of massecuite is delivered to a centrifuge basket. The basket is then accelerated to a predetermined angular velocity and held for a predetermined spin time to achieve a desired level of syrup removal before wash water and/or an alternative liquid is sprayed onto the exposed crystal layer for a predetermined "wash time". The basket is then held at the aforementioned, or an alternative, angular velocity before decelerating the basket and discharging the washed crystals therefrom.

Some advantages of the batch machine are that crystal size and shape are preserved since the crystals do not undergo any high velocity impacts and that by appropriate selection of spin times and wash times, the required level of residual impurities can be controlled. However these machines require time to be charged with feedstock and emptied of product and also require time to accelerate and decelerate thus reducing their effective capacity.

Maintenance requirements are significant due to the cyclic operation and power consumption is increased due to the inefficiency of the cyclic operation. Further disadvantages arise in the increased size of plant both upstream and downstream of the centrifuge due to intermittent charging and emptying.

Continuous centrifuges are widely accepted in "low purity" applications which have less stringent specifications on residual impurities and crystal size distribution.

Both batch and continuous types of centrifuge are currently used. Continuous centrifuges have not been widely accepted for high purity product sugar separation due to such deficiencies as an inability to produce consistent high purity sugar (low levels of residual impurities) .

Another problem with current continuous centrifuges is crystal breakage which occurs as a result of high speed impacts between the crystals leaving the conical screen and the sugar chamber walls and also crystal impacts with other crystals. Additionally, wet lumps of sugar may form as a result of build-up on the walls of the sugar chamber including the conical discharge hopper. Agglomerates of sugar adversely affect sugar crystal quality. The sugar chamber may also include a collecting wall or deflector.

The present invention aims to alleviate one or more of the above disadvantages and to provide centrifugation separations apparatus for separating solids and liquids from a mixture of solids and liquids which will be reliable and efficient in use. Broad Description of the Invention

With the foregoing in view, this invention in one aspect resides broadly in centrifugation separations apparatus for separating solids from a liquid, said apparatus including:- centrifugation means having an inlet, a liquids discharge and a solids discharge; a deflector operatively associated with the solids discharge, the deflector being adapted to interrupt the trajectory of the solids leaving the solids discharge to decelerate and change the direction of travel of the solids, and wherein the deflector is so formed and arranged that, in use, solids make a plurality of impacts with the deflector. The configuration of the deflector is such that the impact of the solids with the deflector is sufficient to substantially dissipate the radial velocity of the solids. Preferably, the deflector is so formed and arranged that, in use, the solids impact from three (3) to twenty (20) times with the deflector. More preferably, the deflector is so formed and arranged that, in use, the solids slide across the surface of the deflector.

In another aspect, this invention resides in centrifugation separations apparatus including:- centrifugation means having an inlet, a liquids discharge and a solids discharge; a deflector operatively associated with the solids discharge; wherein the solids discharge includes a lip configuration which causes the trajectories of the discharging crystals to be relatively independent of their size and/or their residual syrup/moisture content.

The solids discharge also preferbaly includes a curved edge cooperable with the deflector whereby solids of various sizes and/or residual liquid content make their initial impact at substantially the same location on the deflector. It is also preferred that the initial impact of the solids with the deflector surface is at an angle less than 15^c. The centrifugation means may be a filtering centrifuge but preferably, the centrifugation means includes a conical filtering centrifuge.

In another aspect, this invention resides in centrifugation separations apparatus including:- centrifugation means having an inlet, a liquids discharge and a solids discharge; a deflector operatively associated with the solids discharge, and wherein the solids discharge includes a lip configuration which causes the trajectories of the discharging crystals to be relatively independent of their size and/or their residual syrup/moisture content.

In another aspect, this invention resides broadly in centrifugation separations apparatus as claimed in any one of the preceding claims, and including blending means operatively associated with the solids discharge for blending and massaging the solids discharged from the centrifugation separations apparatus whereby the discharged solids have selected characteristics which are substantially uniform. Alternatively, the centrifugation separations apparatus includes:- centrifugation means having an inlet, a liquids discharge and a solids discharge; a deflector operatively associated with the solids discharge, and blending means operatively associated with the solids discharge for blending and massaging the solids discharged from the centrifugation separations apparatus whereby the discharged solids have selected characteristics which are substantially uniform. Preferably, the blending means includes a screen and at least one arm rotatable in a plane spaced above the screen. Preferably, the centrifugation means includes a basket having an upper section and a lower section, each respective section including perforated respective upper and lower screen portions, and wherein the lower section includes a cone half angle and screen characteristics chosen such that a drainage transition of solids operatively passing therethrough is located in the upper section. More preferably, the upper screen portion is selected for operation with a desired size range for the solids, and the lower screen portion is selected for a desired filtration rate of liquid therethrough. Also, the centrifugation means includes two sections comprising a first part or lower section having a cone half angle from the axis less than 25° and a second part or upper section having a cone half angle from the axis larger than that of the lower section, but more preferably, the cone half angle for the lower section is in the range 0° to 15° and the cone half angle for the upper section is in the range 24° to 26°. Suitably, the solids include sugar crystals, and the liquid includes an aqueous sugar solution. In another aspect, this invention resides broadly in a continuous process for the centrifugal separation of sugar from a massecuite comprising crystals and a sugar solution syrup, said the process including:- providing centrifugation separations apparatus including centrifugation means having an inlet, a liquids discharge and a solids discharge; providing a deflector operatively associated with the solids discharge; continuously feeding the massecuite to the inlet; operating the centrifugation means to separate the crystals from the syrup, and collecting the crystals from the centrifugation means by impaction against the deflector whereby at least some of the crystals impact with the deflector more than once.

In another aspect, this invention resides broadly in a continuous process for the centrifugal separation of sugar from a massecuite comprising crystals and a sugar solution syrup, the process including:- providing centrifugation separations apparatus including centrifugation means having an inlet, a liquids discharge and a solids discharge; providing a deflector operatively associated with the solids discharge; continuously feeding the massecuite to the inlet; operating the centrifugation means to separate the crystals from the syrup, and collecting the crystals from the centrifugation means by impaction against the deflector whereby at least some of the crystals impact are collected at a trajectory interrupted by the deflector causing the crystals to decelerate and to alter their direction and wherein the crystals are subjected to a plurality of impacts with the deflector. Additionally, the crystals may be caused to slide across at least some of the surface of the deflector, but preferably, solids are sugar crystals and the crystals impact from three (3) to twenty (20) times with the deflector, but have an initial impact of the crystals with the deflector surface is at an angle less than 15°. Additionally, the process may include lubricating the deflector surface. Preferably, the centrifugation means includes two sections comprising a first part or lower section having a cone half angle from the axis less than 25° and an second part or upper section having a cone half angle from the axis larger than in the lower section and transition from a filtration mechanism to a drainage mechanism for separation of the syrup from the crystals does not occur in the lower section. More preferably, the process includes blending the crystals in blending means operatively associated with the solids discharge for blending and massaging the solids discharged from the centrifugation separations apparatus whereby the discharged solids have substantially uniform characteristics. Description of Several Embodiments

The solids may be any mobile solids, such as sand, minerals, foods or such like. For example, the solids may be sugar crystals, and the liquid may be an aqueous sugar solution.

The centrifuge means may be a continuous conical filtering centrifuge and the inlet may be fed with a mixture of sugar solution and sugar crystals into the base of the continuous conical centrifuge, the wall of which includes a filtering screen.

In use the mixture forms a thin layer which flows up the conical screen (which could consist of multiple conical sections possibly having different cone angles and/or screen types). The centrifugal force causes the sugar solution to filter through the screen where it is collected in and discharges from a liquid collection chamber, such as, in the case of sugar separation, a syrup chamber. A solids layer, such as sugar crystals, flows outwardly up the conical screen during which the layer may be further washed to remove residual liquid from the solids. In the case of sugar separation, sugar syrup or water may be used to wash the sugar crystals flowing upward along the filtering screen of the centrifuge basket. The washing of the crystals may be achieved by providing an arrangement of wash sprays and a control system which varies the rate of spray water application with changing feed rates to ensure consistency in crystal purity.

The drained and/or washed crystals discharge substantially tangentially from the top of the centrifuge basket which may incorporate an upper edge having a curved geometry to ensure that crystals of various sizes and/or residual syrup and moisture content will have the same or close to the same trajectory as they enter the sugar chamber.

The deflector may be of any configuration which substantially dissipates the radial velocity of the sugar crystals. For example, the deflector may include a static or rotating surface such that the trajectory of the crystals passing into the sugar chamber is interrupted by the static or rotating surface causing the crystals to decelerate and to alter their direction and wherein the crystals may be subjected to a plurality of impacts with the surface.

The crystals may impact from three (.3) to twenty (20) times or more with the surface. In a preferred form, the crystals impact at least six (6) times with the surface, and possibly ten (10) time with the surface. Additionally, in use, the crystals, having struck the surface of the deflector, may continue to strike against or slide along the surface until they pass beyond its extremity whereupon the crystals will have substantially lost their radial velocity. Thus, where the deflector is operatively associated with the solids discharge and cooperable with solids being discharged from the solids discharge, the deflector may be cooperable with centrifuge means having an edge with a curved geometry whereby solids of various sizes and/or residual liquid content are discharged from the solids discharge at substantially the same trajectory. Suitably, the curved edge and deflector act synergistically to substantially prevent a buildup of solids on the deflector.

The crystals may then pass through the lower part of the solids collection chamber or sugar chamber which would usually include a conical discharge hopper. The bottom of the sugar chamber (usually within the conical discharge hopper) may include a blending device which massages and blends the product sugar to evenly distribute moisture, to break up and blend any agglomerates of wet sugar which may form and which may also serve to provide a uniform rate of discharge of product sugar from the sugar chamber.

As hereinbefore defined, the solids discharge may include a lip configuration which causes the trajectories of the discharging crystals to be relatively independent of their size and/or their residual syrup/moisture content.

The configuration may be a curve, reducing radius or such like. In one embodiment, the lip configuration is a lip radius.

The magnitude of the lip radius is preferably larger than or equal to that calculated by the formula:

^■ - = 9.8 cosθ + R w² sinθ

where V is the relative velocity between the crystals and the basket; θ is the angle from the horizontal at which free crystals leave the basket (preferably < 2°); R is the basket radius at θ; w is the basket rotation speed (rad sec" ¹ ), and δ is the radius of curvature at the top of the basket. V is difficult to determine but is typically in the range 0.5 to 5 HIS"¹ and for high grade sugar applications close to 1 ms"¹. Typically, θ would be selected at or near 1° . Refer to Figure 5.

Additionally, the discharging crystals may impact with a static or moving deflector in which the initial impact of the crystals with the deflector surface is at an angle smaller than 90^β and preferably less than IS". The profile of the deflector may be such that subsequent impacts are very close together with the result that at least 10 impacts are achieved and that the contact with the deflector is effectively "sliding" contact.

Preferably, the deflector includes an inwardly curving portion and an outwardly curving portion and the initial impact of the solids on the deflector is located close to the junction of the inwardly and outwardly curving portions. More preferably, the deflector has a reducing radius towards its lower end. In a further preferred embodiment, the shape of the deflector may be such that the path taken by crystals after they have left the deflector is of as great a distance as possible prior to the next impact with the sugar chamber or monitor casing. This may be achieved in practice by use of a deflector shape which has a reducing radius towards its lower end. The contact between the crystals and the deflector is believed to cause a film of sugar and/or syrup to form which can then initiate the build up of a significant layer of sugar and/or syrup. This may be avoided by lubricating the deflector surface by applying a fine spray of water or steam near to the location where the crystals leave the basket- either onto the crystals or the basket; or by adding the water spray or steam directly onto the deflector surface.

The amount of lubricant that needs to be added for a given quality of sugar is mainly dependent on the perimeter of the first impact of the deflector. Therefore in the preferred embodiment of the invention, the mutual impact radius is kept as small as possible, the limitation being to provide sufficient gap to allow any foreign object that is likely to be contained in the massecuite to pass through the gap.

In a process using the apparatus of the invention, the solids may be discharged from the centrifugation means within a range of trajectories. Preferably however, the solids are discharged from the centrifugation means at substantially the same trajectory and the trajectory is interrupted by the deflector to decelerate and change the direction of travel of the solids and the solids impact with the deflector sufficiently to substantially dissipate radial velocity of the solids. In particular, the process may include the following steps:

The mixture is fed into the bottom "loading zone" of the rotating centrifuge basket where the feed mixture forms a thin layer that is presented to the conical screen supported by the centrifuge basket, and wherein the loading zone may be perforated and lined with a screen to provide early removal of syrup.

As a consequence of the centrifugal force applied to the mixture, a thin layer of the mixture is formed which flows up the conical screen with continuous progressive removal of syrup from the mixture through the screen.

A liquid, usually water or other liquid containing less impurities than the syrup, may then be sprayed onto the layer to aid the syrup removal.

Steam may also be added adjacent the layer.

The removed syrup and washings are collected in a chamber and drained from the centrifuge, or collected and drained separately from the centrifuge. The filtering and motion of the layer on the screen leads to the classification of crystals with the finer crystals moving through the layer to the screen.

It is believed that the purged crystals discharge from the top rim of the conical basket on a tangential path with a velocity component in each of the vertical and the radial directions somewhat smaller than the velocity component in the circumferential direction. Additionally, the vertical component of the crystal velocity is typically very small.

The classification of crystals in the layer on the screen may be expected to result in the purged crystals taking varying trajectories upon leaving the basket.

Specifically, the fine crystals will tend to have retained some residual liquid and as a result may be expected to follow the top edge of the basket with very little vertical velocity. The larger dryer crystals may be expected to leave with a vertical velocity component similar to that which they had while travelling up the screen. However, it is believed that the lip radius configuration for solids discharge described herein causes all the solids to take a similar trajectory despite the classification process.

Having left the basket, the purged crystals then travel through the sugar chamber at a small angle to the surface of the deflector such that the impact energy is low enough so as to prevent the crystals from breaking but which may lead to a reduction in velocity or a change in direction. The crystals and/or the impact surface may be wetted to assist the continued motion of the crystals after impact with the deflector. In the preferred embodiment, the crystals will generally have a number of subsequent impacts with the deflector before passing through the annular sugar chamber and thence impacting the wall of the sugar chamber some distance below the end of the deflector.

Prior to leaving the sugar chamber, the product sugar which might contain variations in crystal size and moisture content and might contain lumps (usually of wet sugar crystals), may be mixed under gentle conditions of shear so as to provide a well mixed crystal product of relatively steady moisture content and size distribution and which is free of lumps.

In one preferred design, the basket has two sections. The first part or lower section has a cone half angle from the axis less than 25° and preferably in the range 0^β to 15^β and more preferably in the range of from 9° to 12°. The screen length of this section in combination with the screen type (mainly filtration resistance as determined by parameters such as aperture size, open area and thickness) are selected so as to achieve only sufficient removal of the syrup such that the transition from filtration mechanism to drainage mechanism does not exist in the lower section. For very low viscosity syrups, it may be preferable for the screen in the lower section to be such that effectively no syrup is removed.

The upper section has a cone half-angle from the axis which is preferably larger than in the lower section, and which may be greater than 18°, depending upon the size and other characteristics of the solids being centrifuged. For sugar crystals having an equivalent diameter greater than 600 μm, the preferred cone half angle is in the range 24^β to 26°, and preferably 25°. This part of the basket has the screen such that syrup is removed rapidly and that the greater part of the screen is above the drainage transition and thus dedicated to removing residual syrup by the mechanism of drainage and usually aided by washing.

The deflector shape and location are determined by the requirements to have the impacting crystals approach the impact point on the deflector surface at a small angle and then for there to occur a number of subsequent impacts (at least 6 to 10) at short intervals and then after leaving the deflector to have a long flight path before impact with the casing of the centrifugal separations apparatus. Thus, there are three design rationales to which the shape and location of the deflector are directed - firstly, to minimize the initial impact angle; secondly, to have many impacts with the deflector surface; and thirdly, to have as long a flight path as possible after leaving the deflector. Thus, for the first rationale, to minimise the approach angle of the solids to the initial impact point, the deflector shape and location are such that a tangent drawn in a sectional elevation of the deflector through the impact point will be small as shown in Figure 11. The angle of impact in sectional elevation is necessary, but not sufficient to minimize the angle in general. A second requirement to minimise the impact approach angle in plan view, such that the radius of the impact point is also minimised as shown in Figure 12. This latter requirements must be compromised by the need to leave adequate radial clearance between the basket lip and the deflector to allow for movement and the passing of foreign objects and lumps.

For the second rationale, to design a deflector which ensures several impacts at short intervals, use is made of a three dimensional model which was developed to accurately predict the trajectory of sugar crystals upon leaving a centrifuge and impacting deflectors of various shapes.

This model also predicts the path of crystals after leaving the deflector so that the third rationale of achieving a long flight path before further impacts with the casing can be assessed.

A computer model has been developed to predict the flight path to, along and after the deflector which allows best geometry and configuration to be selected. A computer program may be used to calculate the crystal trajectory profile according to the model described in the above equation.

The abovementioned computer model was developed to accurately predict the trajectory of sugar crystals upon leaving a centrifuge and impact with deflectors of various shapes. Such computer programs are capable of handling a variety of shapes of deflector including multiple cones, continuous curves, combinations of cylinders and such like to determine the crystal path.

The programs follow the trajectory of individual crystals as they leave the centrifuge basket and impact many times with the deflector, the casing and the discharge cone. Many variables are included to make the model universally applicable, such as the vertical velocity up the basket, the distance to the deflector, the shape of the deflector, geometry of the casing and discharge cone, and the impact efficiency.

The programs have been used to design a self cleaning deflector of the present invention in which the crystal impacts many times with the deflector - commonly more than 12. It is believed that the crystal should remain substantially close to the surface between impacts to avoid the build up of deposits. Trajectory models can handle a variety of shapes of deflectors including multiple cones, continuous curves, combinations of cylinders or such like to determine the crystal path. One preferred embodiment is given when the curve representing the deflector shape is given by an equation of the form

^R _rβ - R - (Kl/Z)^κ2 + K3 Z^κ4

where the variables R and Z are defined in Figure 8 and the parameters R_{r β t} , Kl, K2, K3 and K4 have the following values:

R_ref = 0.7 to 1.2 and preferably about 0.85

Kl = 0.0005 to 0.002 and preferably about 0.001

K2 = 0.4 to 0.66 and preferably about 0.5

K3 = 0 to 3 and preferably about 2.5

K4 = 0 to 4 and preferably about 3.5 It will be appreciated that other shapes may be calculated to satisfy the three rationales set forth above. However, the preferred parameters above provide a deflector shape close to the one shown in Figures 6 and 7 which has been found to achieve the desired physical and process objectives in practice. The crystal bounce profile in Figure 8 is based upon the deflector defined by the above listed parameters when substituted into the above deflector curve, and having an assumed bounce efficiency of one (1). In practice, the bounce efficiency is less than one (1) and this would alter the bounce profile slightly. Observation of operating deflectors in accordance with the invention, and in particular, in accordance with the abovelisted parameters, suggests that the crystals in fact remain very close to the deflector surface and even appear to slide across the deflector.

It is believed that the lip radius incorporated in the top of the basket provides a narrow impact zone around the deflector for substantially all crystals and the shape of the deflector described above leads to sliding motion, that is, many impacts.

In one embodiment, a direct drive arrangement may be used to spin the basket as this obviates the need for vee belts and the associated guarding which provides a site for crystals to impact and for lumps to form. Using the direct drive arrangement, the rotating gear and the syrup chamber may be supported on the frame by a structural "spider" with webs which may be substantially parallel to the angular motion of the crystals which have left the deflector in order to reduce the area for crystal impacts and lump formation.

Additionally, the sugar produced by the apparatus of this invention may be subjected to relatively gentle conditions of shear which mixes the sugar to produce a crystal product that is consistent in terms of crystal size, moisture and residual syrup or impurity levels. In one embodiment this is achieved by shearing the sugar between two surfaces where the maximum differential linear speed is less than 30 ms"¹ and greater than 0.3 ms"¹ but preferably in the range 3 to 10 ms" . Furthermore, one of the surfaces may be perforated such that the blended sugar crystals discharge from the machine through the perforations. The perforated surface may be stationery and the moving surface may be a rotor having arms which extend out to the diameter of the stationery surface, continually sweeping the surface.

The gap between the surfaces may be between 1 mm and 50 mm but preferably is between 3 mm and 12 mm. Similarly, in the configuration where one of surfaces is perforated, the size of the openings may be between 1 mm and 50 mm but preferably is between 3 mm and 12 mm or more preferably between 5 mm and 8 mm.

For convenience in terms of equipment maintenance, the shearing device may be located at the point of discharge of the product sugar leaving the sugar chamber. This will usually be at the bottom of the conical discharge hopper. However the shearing device may be located at any stage in the trajectory of the sugar crystals. In one embodiment, the shearing device is located at a position after the sugar crystals have left the deflector where such is used, or may be located after the first impact with the sugar chamber wall but preferably at a location at which the crystals have a downward velocity component due to the action of gravity. Description of the Figures In order that this invention may be more readily understood and put into practical effect, reference will now be made to the accompanying drawings which illustrate a typical embodiment of the invention and wherein:

FIG..1 is a partly cut away view of a centrifugal separations apparatus;

FIG. 2 is a diagrammatic representation of a centrifuge basket for the centrifugal separations apparatus of FIG.

1;

FIG. 3 is a diagrammatic representation of a portion of a centrifuge basket showing the disposition of feedstock thereon;

FIG. 4 is a diagrammatic representation of a portion of a centrifuge basket showing the shape of the basket lip; FIG. 5 is a diagrammatic detail portion of the basket lip of FIG. 4 and showing the trajectory of solids discharged therefrom;

FIG. 6 is a sectional view of a deflector for the centrifugal separations apparatus of FIG. 1;

FIG. 7 diagrammatic sectional view of a portion of the deflector of FIG. 6;

FIG. 8 is a diagrammatic representation of a crystal bounce profile for a deflector of the present invention; FIGS. 9 and 10 respectively show a blender assembly in plan and elevation, and

FIGS. 11 and 12 respectively show the solids initial impact angle of solids with the deflector.

Referring to Figure 1, a centrifuge apparatus 10 includes a centrifuge basket 11 operatively associated with a feed inlet 32, a syrup discharge port 31 operatively associated with the filtrate side of the centrifuge basket 11 and a solid discharge 18 operatively associated with a sugar chamber 24. The sugar chamber is separated from the filtrate side of the centrifuge basket 11 by a syrup chamber wall 19.

The centrifuge basket 11 includes a top section 12 and a bottom section 13 as shown. Additionally, the deflector 23 is provided within a monitor casing 28 which connects to the wall of the sugar chamber 24 as shown. The discharge cone

25 is connected to the base of the sugar chamber 24.

A syrup chamber 30 is also provided and has fluid connection with a syrup discharge port 31, the feed stream represented by an arrow 14 enters a feed inlet 32. Additionally, three water spray lines 33 are provided together with a steam lance 34 into the centrifuge apparatus 10. The sugar chamber 24 is provided with an inspection hatch 35.

The centrifuge basket 11 is rotated by a centrifuge motor 37 and the centrifuge apparatus 10 is supported on four centrifuge mounts 38 (two of which are shown). It will be appreciated that the motor 37 may under drive or over drive the basket by electric power or hydraulic as desired, ύ or other drive power sources fa ybe utilized for this purpose. Preferably, the hot water spray lines 33 are supplied with water under the operation of control valves.

Referring to Figure 2, the centrifuge basket 11 includes a top section 12 and a bottom section 13. The bottom section 13 has a nominal pitch of 12° from the vertical and the top section 12 has a nominal pitch of 25° from the vertical.

Referring to Figures 2 and 3, a filtration zone 16 extends through the bottom section 13 and partly into the top section 12 and a drainage zone 15 extends the remainder of the way through the top section 12, The transition between the drainage zone 15 should be above the transition between the top section 12 and the bottom section 13. That is, the drainage zone 15 needs to be where the basket angle is preferably greater than or equal to 24°. When the viscosity is low, it may be that the filtration zone 15 is entirely in the top section. In other words, the bottom section 13 would have a screen of very high filtration resistance or no screen at all (not perforated). The wall of the centrifuge basket 11 is preferably perforated by drilling, and includes a supporting πiesh having a relatively high transverse permeability supporting a filter screen of an aperture size selected to retain crystals above a desired size. The length of the drainage zone 15 is preferably from 5 to 20 times the length of the filtration zone 16. The length of the drainage zone 15 in proportion to the filtration zone increases with the efficiency of removal of impurities from the solids being filtered, and also as the viscosity of the syrup increases and as the particle size decreases.

Referring to Figures 4 and 5, the centrifuge basket 11, a portion of which is shown diagrammatically, is rotatable in the direction of a rotation arrow 17 and includes a lip 42, shown in a lip portion detail 47 more particularly in Figure 4.

The centrifuge basket 11 has a centrifuge radius 40 represented by the symbol R, and the lip portion detail 47 shows that the lip 42 has a lip radius 41 represented by the symbol δ. Sugar crystals leave the lip 42 at the centrifuge radius 40 to follow a crystal trajectory 43. The crystal trajectory 43 is at a trajectory angle 44, represented by the symbol θ, with respect to a horizontal 45. By mensuration, the trajectory angle 44 is complementarily identical with respect to a vertical 46.

Referring to Figures 6 and 7, the deflector 23 includes an inwardly curving portion 61 and an outwardly curving portion 62. Of particular note is the reducing radius of the deflector near its lower extremity 65 whereby the radius of the deflector at its lower end is smaller than that of the sugar chamber 24 and the lower end is radially inwardly spaced from the wall of the sugar chamber 24. This feature is the primary cause for ensuring that the crystals have a flight path which is maximized prior to subsequent impact with the casing.

Referring to Figure 7 in particular, the deflector 23 is in a preferred embodiment formed having arcs A, B and C having the radii set forth in the below.

ARC A B C

START X 760.000 759.252 633.846

Y 0.000 270.672 375.081

CENTRE X 326.226 616.014 642.168

Y 132.373 226.145 444.585

END X 759.252 633.846 572.168

Y 270.672 375.081 444.585

ARC RADIUS 460.00 150.00 70.00

ANGLE 34.22 65.9 83.17

Referring to Figure 8, a crystal bounce profile 20 shows a "two dimensionalized" representation of a trajectory 21 of a crystal discharge from a discharge point 22 as it exits the centrifuge basket 11 of Figures 1 and 2 and strikes a deflector 23 before entering a sugar chamber 24 and discharge cone 25. It will be appreciated that the crystal bounce profile 20 is in the form of a spiral, and is shown in two dimensions by disregarding the circumferential or tangential position of a discharged crystal. Additionally, the profile depicted in Figure 8 is diagrammatic and shows five impacts for clarity, whereas the number of impacts for this shape of deflector predicted by the model is in fact more than twelve with a concentration of bounces near the initial impact point.

The deflector 23 is at the top 26 of the centrifuge apparatus 10 of Figures 1 and 2 and the discharge cone 25 is at the bottom 27 of the centrifuge apparatus 10 shown in Figure 1.

Referring to Figures 9 and 10, the sugar crystals after leaving the deflector impact with the casing and eventually enter the conical discharge hopper where they slide down the wall of the discharge hopper and into a blending apparatus 80. The solids first fall onto a screen 90, and some of the solids pass through the screen 90 and the remainder rests on the screen 90 and is massaged between one of four rotor arms 94 of a rotor 93 and the screen 90. Some of the sugar may also travel with the rotor arms 94 as they rotate in the direction of arrow 92

In the case of the solids being sugar crystals, the massaging disperses the product sugar into a relatively free flowing granular material which is free from lumps. The screen 90 is comprised of a woven mesh having square apertures approximately 7 mm on each side so the blended product passes easily through the apertures under the gentle pressure of the rotor.

In the embodiment illustrated, the rotor arms 94 which are made from 20 mm to 40 mm diameter round bar. The clearance between the bottom of the rotor arms 94 and the screen 90 is about 8 mm. The outside diameter of the rotor arms is the same as the screen deck diameter and is about 1000 mm. The rotational speed of the rotor is about 150 rpm. Under these conditions, effective blending and dispersion is achieved substantially without breakage of the individual crystals.

Referring to Figures 11, the solids leave the lip 42 at a trajectory 110 strike the deflector 23 at an initial impact point 111 at angle having a vertical component α. which is minimized, and preferably less than 15°. Referring to Figure 12, the trajectory 110 is shown intersecting with a close deflector 120 and a distant deflector 121, and it will be seen that the horizontal component of the initial contact angle α-^ for the close deflector 120 is smaller than the contact angle α₂ for the distant deflector 121. Thus, it will be seen that, firstly, the position of the deflector in the vertical direction is such that the first impact 111 is close to the point of inflection on the deflector, that is, close to the junction of the inwardly curving portion 61 and the outwardly curving portion 62, whereby slight changes in the point of contact will have minimal affect on the contact angle α_z , and that, secondly, the deflector 23 is as close as is practicable to the point of discharge from the lip 42.

In use, the centrifuge apparatus 10 may be used to separate sugar crystals from massecuite by feeding a feed stream 14 containing a mixture of syrup and sugar crystals (massecuite) into the feed inlet 32 of the centrifuge apparatus 10 and rotating the centrifuge basket 11 by activating the centrifuge motor 37. The feed stream of massecuite enters the centrifuge basket 11 in the bottom section 13 and flows upwardly and outwardly to the top section 12 by the centrifugal action of the rotating centrifuge basket 11.

The syrup which filters through the centrifuge basket 11 passes into the syrup chamber 30 and is discharged through the syrup discharge port 31 and the sugar crystals separated from the syrup pass out of the centrifuge basket 11 and into the sugar chamber 24 after contacting the deflector 23 a number of times. The sugar crystals from the sugar chamber 24 flow through the discharge cone 25. The rotational speed of the basket is such that the tangential velocity of the sugar crystals at the discharge is no greater than 80 ms" ^x and preferably less than 60 ms" -¹.

From the point of discharge, the separated crystal product is agitated or blended by massaging the product between a rotating rotor and a static screen. The blended sugar passes through the screen apertures. It will of course be realised that the above has been given only by way of illustrative example of the invention and that all such modifications and variations thereto as would be apparent to persons skilled in the art are deemed to fall within the broad scope and ambit of the invention as is claimed in the following claims.

Claims

THE CLAIMS DEFINING THE INVENTION ARE AS FOLLOWS:-

1. Centrifugation separations apparatus for separating solids from a liquid, said apparatus including:- centrifugation means having an inlet, a liquids discharge and a solids discharge; a deflector operatively associated with said solids discharge, said deflector being adapted to interrupt the trajectory of the solids leaving said solids discharge to decelerate and change the direction of travel of the solids, and wherein said deflector is so formed and arranged that, in use, solids make a plurality of impacts with said deflector.

2. Centrifugation separations apparatus as claimed in claim 1, wherein the deflector is so formed and arranged that, in use, the solids impact from three (3) to twenty (20) times with said deflector.

3. Centrifugation separations apparatus as claimed in claim 1 or claim 2, wherein the deflector is so formed and arranged that, in use, the solids slide across the surface of said deflector.

4. Centrifugation separations apparatus including:- centrifugation means having an inlet, a liquids discharge and a solids discharge; a deflector operatively associated with said solids discharge; wherein said solids discharge includes a lip configuration which causes the trajectories of the discharging crystals to be relatively independent of their size and/or their residual syrup/moisture content.

5. Centrifugation separations apparatus as claimed in any one of the preceding claims, wherein said solids discharge includes a curved edge cooperable with said deflector whereby solids of various sizes and/or residual liquid content make their initial impact at substantially the same location on said deflector.

6. Centrifugation separations apparatus as claimed in claim 5, wherein said initial impact of the solids with said deflector surface is at an angle less than 15°.

7. Centrifugation separations apparatus as claimed in claim 5 or claim 6, wherein said centrifugation means includes a conical filtering centrifuge and said curved edge includes a lip radius and the magnitude of the lip radius is larger than or equal to that calculated by the formula:

-^ = 9.8 cosθ + R w² sinθ

where V is the relative velocity between the crystals and the centrifuge; θ is the angle from the horizontal at which free crystals leave the centrifuge and is less than or equal to 2°;

R is the basket radius of the centrifuge at θ; w is the rotation speed in radians per second of the centrifuge, and δ is the lip radius at the top of the centrifuge.

8. Centrifugation separations apparatus as claimed in claim 7, wherein, the solids are sugar crystals and the liquid is an aqueous sugar solution and V is at or near 1 metre per second and θ is at or less than 1° .

9. Centrifugation separations apparatus as claimed in any one of the preceding claims, wherein said deflector includes an inwardly curving portion and an outwardly curving portion.

10. Centrifugation separations apparatus as claimed in claim 9, wherein the initial impact of the solids on the deflector is located close to the junction of said inwardly and outwardly curving portions.

11. Centrifugation separations apparatus as claimed in any one of the preceding claims, wherein said deflector has a reduced radius at its lower end.

12. Centrifugation separations apparatus as claimed in any one of the preceding claims, and including blending means operatively associated with said solids discharge foϋ blending and massaging the solids discharged from the centrifugation separations apparatus whereby the discharged solids have selected characteristics which are substantially uniform.

13. Centrifugation separations apparatus including:- centrifugation means having an inlet, a liquids discharge and a solids discharge; a deflector operatively associated with said solids discharge, and blending means operatively associated with said solids discharge for blending and massaging the solids discharged from the centrifugation separations apparatus whereby the discharged solids have selected characteristics which are substantially uniform.

14. Centrifugation separations apparatus as claimed in claim 12 or claim 13, wherein blending means includes a screen and at least one arm rotatable in a plane spaced above said screen.

15. Centrifugation separations apparatus as claimed in any one of the preceding claims, wherein said centrifugation means includes a basket having an upper section and a lower section, each respective section including perforated respective upper and lower screen portions, and wherein said lower section includes a cone half angle and screen characteristics chosen such that a drainage transition of solids operatively passing therethrough is located in said upper section. 15. Centrifugation separations apparatus as claimed in claim 14, wherein said upper screen portion is selected for operation with a desired size range for the solids, and said lower screen portion is selected for a desired filtration rate of liquid therethrough.

16. Centrifugation separations apparatus as claimed in any one of the preceding claims, wherein said centrifugation means includes two sections comprising a first part or lower section having a cone half angle from the axis less than 25° and a second part or upper section having a cone half angle from the axis larger than that of said lower section.

17. Centrifugation separations apparatus as claimed in claim 16, wherein the cone half angle for the lower section is in the range 0° to 15" and the cone half angle for the upper section is in the range 24° to 26°.

18. Centrifugation separations apparatus as claimed in any one of the preceding claims, wherein the solids include sugar crystals, and the liquid includes an aqueous sugar solution.

19. A continuous process for the centrifugal separation of sugar from a massecuite comprising crystals and a sugar solution syrup, said process including:- providing centrifugation separations apparatus including centrifugation means having an inlet, a liquids discharge and a solids discharge; providing a deflector operatively associated with said solids discharge; continuously feeding the massecuite to said inlet; operating said centrifugation means to separate said crystals from said syrup, and collecting said crystals from said centrifugation means by impaction against said deflector whereby at least some of the crystals impact are collected at a trajectory interrupted by said deflector causing the crystals to decelerate and to alter their direction and wherein the crystals are subjected to a plurality of impacts with the deflector.

20. A process as claimed in claim 19, and including causing the crystals to slide across at least some of the surface of said deflector.

21. A process as claimed in claim 19 or claim 20, wherein the crystals impact from three (3) to twenty (20) times with said deflector.

22. A process as claimed in claim 21, wherein the initial impact of the crystals with said deflector surface is at an angle less than 15°.

23. A process as claimed in any one of claim 19 to 22, and including lubricating said deflector surface.

24. A process as claimed in any one of claims 19 to 23, wherein said centrifugation means includes two sections comprising a first part or lower section having a cone half angle from the axis less than 25° and an second part or upper section having a cone half angle from the axis larger than in the lower section and transition from a filtration mechanism to a drainage mechanism for separation of the syrup from the crystals does not occur in the lower section.

25. A process as claimed in any one of claims 19 to 24, wherein and including blending said crystals in blending means operatively associated with said solids discharge for blending and massaging the solids discharged from the centrifugation separations apparatus whereby the discharged solids have substantially uniform characteristics.