Patent Application
20100065669
The present invention relates to milling and drying apparatus which incorporates a cyclone, and to methods of operation of such apparatus.
The use of a cyclone to separate, mill, or dry material is known, and various applications of cyclones have been described in a number of prior art specifications. For example, U.S. Pat. No. 5,236,132 (Rowley) discloses a comminutor/dehydrator which incorporates a cyclone, as does U.S. Pat. No. 4,390,131 (Pickrel). U.S. Pat. No. 4,743,364 and No. 6,206,202 both disclose classifying/separating apparatus incorporating a cyclone. However, the prior art designs in general fail to provide fine control of processing conditions within the cyclone. This in turn limits the range of products which can be processed, and also limits the quality of the output product. Further, most if not all of the known comminuting/dehydrating cyclones operate only batch processes.
It is an object of the present invention to provide apparatus which incorporates a cyclone and which is capable of continuously milling and/or drying a large range of different products with fine control over the particle size/moisture content of the output product.
The present invention provides a cyclone comprising: an upper cylindrical portion which opens into the wider end of a lower frustro-conical portion, with the longitudinal axes of said upper and lower portions aligned and substantially vertical; a primary air inlet into the cyclone arranged such that the inlet air is substantially tangential to the circumference of the cyclone; an exhaust outlet at or adjacent the top of the cylindrical portion; a control valve associated with said exhaust outlet and capable of partially or completely shutting off said exhaust outlet; a secondary air inlet associated with the narrow end of the frustro-conical portion and provided with an airflow stabilising device which is adapted to admit a stream of air substantially along or spirally around the longitudinal axis of the cyclone; means for removing processed product from the cyclone; means for moving said airflow stabilising device into and out of the narrow end of the frustro-conical portion during product processing and/or between product processing.
Preferably, said airflow stabilising device has an outer wall which is frustro-conical in shape and an interior tube through which air is supplied in use; said airflow stabilising device being dimensioned and arranged such that the narrow end of said frustro-conical outer wall is insertable in the narrow end of said frustro-conical portion of the cyclone.
Preferably, the interior tube is arranged to be movable into and out of the narrow end of the frustro-conical portion either together with and/or independently of the frustro-conical exterior of the airflow stabilising device. Movement of the airflow stabilising device into and out of the narrow end of the frustro-conical portion of the cyclone, and movement of the interior tube relative to the frustro-conical portion, can be achieved by any of a wide variety of suitable devices, e.g. manual adjustment, electric motors, electric stepper motors, hydraulic rams, pneumatic rams; a person skilled in the art will appreciate that still further devices could be used for these movements, to suit particular applications.
The means of removing the processed product may be an annular gap at the narrow end of the frustro-conical portion between the wall of the frustro-conical portion and the airflow stablising device through which product is emitted in use. However, another possibility is that means of removing processed product are provided in the form of one or more outlets formed in the wall of the frustro-conical portion of the cyclone, through which product may be withdrawn.
Preferably, the cyclone further comprises a cylindrical core mounted within the upper cylindrical portion of the cyclone, with the longitudinal axis of the cylindrical core parallel to, or coincident with, the longitudinal axis of said upper cylindrical portion.
The present invention further provides a method of operating a cyclone as described above, wherein the airflow stabilising device is supplied with air from a blower or fan or, in the alternative, the airflow stabilising device is supplied with air simply by permitting air at ambient pressure to pass into said device. The air may be heated or unheated.
By way of example only, preferred embodiments of the present invention are described in detail with reference to the accompanying drawings, in which:
a is a view similar to
Referring in particular to
A cylindrical core 5 is mounted in the top of the cylindrical portion 3, with the longitudinal axis of the core 5 coaxial with the longitudinal axis of the portion 3. The upper end of the core 5 projects from the top of the cylindrical portion 3, which is otherwise closed. The lower end of the core 5 is formed with a flared portion 6, the length of which is adjustable. The distance by which the core 5 projects into the cylindrical portion 3 can be adjusted using any suitable known means, (e.g. screw adjusters or hydraulic rams (not shown)).
When the cyclone is in operation, the core 5 physically separates the relatively hot, dry exhaust gases from the relatively cool and wet inlet air and entrained product. In addition, the core 5 acts as a heat exchanger:—the core is heated by the exhaust gases, and this is transferred to the relatively cool inlet air by conduction, convection and radiation. This effect is particularly marked at relatively low inlet air velocities.
The more the core 5 is lowered down the cylindrical portion 3, the greater the volume of air and entrained material in the area between the top of the portion 3 and the flange 6. This gives an increase in dwell time which can be useful for ensuring complete processing, especially when the inlet air through the duct 10 has a relatively low velocity and/or when very fine materials are being processed. The above described retention effect is increased by enlarging the outer diameter of the flange 6.
A conical valve 7 is mounted at the top end of the cylindrical core 5 and can be raised or lowered in the direction of arrows A to partially or completely close off the top of the core 5. The more the top of the core 5 is closed off, the greater the backpressure in the cyclone and in particular, the greater the pressure in the inner vortex, as hereinafter described.
The top end of the cylindrical core 5 opens into an exhaust duct 8, the other end of which may be vented to atmosphere and/or connected to the inlet of a blower or fan 9, as more particularly described with reference to
The delivery side of a product inlet device 11 opens into the air inlet duct 10. The device 11 may be of any suitable known type, (e.g. a rotary valve for solids or an injection nozzle for liquids) and is in communication with a source of the product to be processed in the cyclone, such as a feed hopper (not shown in
The air and entrained product coming into the cyclone from the duct 10 is admitted approximately tangentially to the circumference of the cylindrical portion 3, and preferably as close to the top of the cylindrical portion 3 as possible, so that product has a maximum dwell time within the cyclone. Once inside the cyclone, the air and entrained product initially follow a spiral path around the inner walls of the cyclone, as indicated by arrows C, spiralling around the cyclone down towards the narrow end of the frustro-conical portion 4. This forms a relatively high-pressure first vortex adjacent the walls of the cyclone. Adjacent the narrow end of the frustro-conical portion 4, a reverse spiral flow forms a second vortex (as indicated by arrows D) which extends from point adjacent the lower end of the cyclone to the top of the cyclone, approximately along the longitudinal axis of the cyclone.
This pattern of airflow within the cyclone produces a relatively stable pattern of velocity and pressure variations across the width of the cyclone, i.e. in a substantially horizontal plane. The air velocity varies inversely with the air pressure. It will be appreciated that the actual air velocities and pressure at any given point depend upon the air inlet velocity and pressure and the dimensions of the cyclone, but once the cyclone is in operation and the pattern of airflow is established, there is a consistent horizontal pattern of a low velocity/high-pressure zone immediately adjacent the cyclone walls, then the area of the first vortex, which is high velocity and correspondingly low pressure, then a transition zone between the first and second vortices, in which the air velocity gradually drops, reaching zero at the interface between the two vortices, and then increases (reversed in direction) towards the core of the second vortex, with the pressure varying inversely to the velocity.
The entrained product does not move in a smooth spiral around the cyclone:—the particles of the product impact upon each other and upon the walls of the cyclone; this has the effect of comminuting/milling the product, and is the main comminuting effect if the product being processed is noncellular. However, if the product is cellular, (e.g. fruit, vegetables, cereals, clays) then the main comminuting/milling effect is caused by the movement of the product between the high and low pressure in areas described above:—as the cellular particles move from a high pressure area to a lower pressure area, the material on the outside of the particle tends to spall under the pressure differential. Further, any water contained in the particles evaporates rapidly as the particle moves to a lower pressure zone; this evaporation may be sufficiently rapid to “explode” the particle. As the particles break down, more of the particle surface is exposed, and this of course facilitates further evaporation.
The final particle size of the product depends upon the inlet velocity of the air into the cyclone, the dwell time of the product in the cyclone, and the nature of the product itself:—obviously, some products are more brittle than others and fracture more readily under impact.
The product is dried by tumbling in the air stream, causing evaporation both of surface moisture and of moisture contained within the product, as described above. The rate of drying is governed by the air temperature and humidity and by the rate at which the product is comminuted:—a product which breaks up rapidly into small particles is dried more rapidly, since the drying air can contact the greater surface area of the product.
Although hot air obviously will dry more effectively than cooler air, for a majority of organic products it is advantageous to keep the temperature of the product as low as possible, preferably no higher than 50° Centigrade. For such products, although the inlet air temperature is typically in the range 70-85° Centigrade, evaporative cooling of the product plus the very short dwell time in the cyclone (typically 0.1 second for relatively dry product up to about three or four seconds for very wet product) helps to keep the heating of the product to a minimum:—typically, the exit temperature of the product is about 35° Centigrade.
However, for products which will tolerate a higher drying temperature, using inlet air at a higher temperature improves the efficiency of the apparatus:—for example, an air inlet temperature above 75° Centigrade gives extremely efficient operation and inlet air temperatures up to 150° C. have been used successfully for some products. It will be appreciated that it is important that the temperature of the circulating air mass in the cyclone is maintained at a temperature which is not damaging to the product being dried, and the optimum inlet air temperature for each particular type of product is determined by experimentation since the temperature of the circulating air mass in the cyclone is not dependent only upon the inlet air temperature:—it is affected also by the temperature of the material to be dried, the type of material to be dried and the volume of material being dried relative to the volume of inlet air.
Temperature sensors marked by * in
The temperature of the exhaust air generally is higher than the inlet air temperature; due to the use of the cylindrical core 5 as a heat exchanger, this temperature differential is used to heat the inlet air, resulting in a high efficiency operation. It is believed that a possible explanation for the heating of the exhaust air is that water vapour evaporated from the product may be moved to the higher pressure areas of the cyclone due to the water vapour activity gradient. Effectively, such water vapour may be considered super cooled and if nucleation sites are present (provided for example by fine particles in the exhaust air), the water vapour will condense, releasing its heat of evaporation which heats the surrounding air. It seems probable that this mechanism typically would occur inside the cylindrical core 5.
In conventional designs of cyclone, the position in the cyclone of the first and second vortices, and the level in the cyclone at which the airflow from the first vortex reverses to form the second vortex, tend to vary substantially during the period of operation of the cyclone:—the patterns of air movement are not stable, and the vortices precess about their average positions. However, for the cyclone to operate reliably and consistently, it is important that the vortices are as stable as possible, since their position governs the levels at which particles are deposited on the cyclone wall by the air stream, and also the size of particle which is deposited. Further, if the second vortex moves too close to the wall of the cyclone, it entrains some of the processed material which has been deposited there, and draws it into the exhaust system. This wastes processed material and also contaminates the exhaust gases.
It has been discovered that it is possible to stabilise the vortices by introducing a secondary flow of air into the lower end of the cyclone, using an externally-mounted airflow stabilising device 13 (which is shown on an enlarged scale in
The airflow stabilising device 13 has a partly frustro-conical exterior 14 and a central cylindrical bore 15 which receives a tube 15a as a sliding fit or as a screw-threaded connection as indicated by reference 15b. The longitudinal axis of the bore 15 is aligned with the longitudinal axis of the cyclone 2. In an alternative construction (not shown), the bore 15 and tube 15a may be flared to produce a Venturi effect. The exterior 14 and the tube 15a can be advanced into or withdrawn from the end of the cyclone as indicated by arrows E, either together or independently of each other. An annular gap X is formed between the exterior wall of the frustro-conical portion 14 of the device 13 and the lower end of the cyclone. The size of the gap X may be varied by moving the device 13 towards or away from the cyclone.
The airflow stabilising device 13 is mounted on the exterior of the apparatus, immediately below the lower end of the cyclone. The device 13 is supported by means of a frame work 40 consisting of a first stationary cross bar 41 which is rigidly secured to the cyclone a short distance above the lower end of the cyclone and is substantially horizontal. Two vertical screw-threaded rods 41,42 are mounted one at each end of the cross bar 41 so as to extend downwards below the lower end of the cyclone. The second cross bar is split into two aligned sections 43,44; one of the sections 43 is mounted adjacent one end on the corresponding threaded rod 41 and a portion of the remainder is rigidly secured across part of the diameter of the exterior 14, along its lower edge. The other cross bar 44 is arranged in a similar manner, with one end engaged with the threaded rod 42 and at least part of the remainder of the cross piece 44 secured across a diameter of the lower edge of the exterior 14. The cross pieces 43 and 44 thus reinforce the exterior 14 along its lower edge and also mount the device 13 for movement in the direction of arrows E. Rotation of the rods 41,42 moves the device 13 and the cross pieces 43,44 up or down relative to the cyclone, depending upon the direction of rotation of the rods 41,42. The rotation of the rods 41,42 can be achieved manually, simply by rotating one or both ends of each rod, or, as shown in
The tube 15a is connected to the exterior 14 by a screw-threaded connection 15b between the bore 15 and the exterior of the tube 15a. This screw-threaded connection may be only a short section as indicated by reference numeral 15b, or may be a longer section or even the full length of the tube if necessary. To move the tube 15a relative to the exterior 14, the tube 15a is rotated by a motor 47 and drive 48 to move up or down in the direction of arrows E, depending upon the direction of rotation of the tube. The motor 47 and drive 48 are supported by one or both of the cross pieces 43,44 but the connection is omitted from
In another arrangement, the screw-threaded connection 15b, motor 47 and drive 48, are omitted and the tube 15a is simply a sliding fit in bore 15. In this case, the tube 15a can be moved in the direction of arrows E with the exterior 14 as described above, and if it is necessary to move the tube 15a relative to the exterior 14, the position of the tube 15a relative to the exterior 14 is adjusted manually, or by hydraulic or pneumatic rams (see below).
A further possible construction is shown in
As in the embodiment described with reference to
A second crossbar is split into two aligned sections, 53,54; one of the sections 53 is rigidly secured across part of the diameter of the exterior 14, along its lower edge, and extends beyond the outer edge of the exterior 14 to receive the outer end of the rod 55 of the ram 50. The other crossbar 54 is rigidly secured across part of the diameter of the exterior 14 along its lower edge and is aligned with the crossbar 53 in a horizontal plane. The crossbar 54 extends beyond the outer edge of the exterior 14 to receive the outer end of the rod 56 associated with the ram 50a. Operation of the rams 50,50a to reciprocate the corresponding rods 55,56 thus moves the crossbar 53,54 in the direction of arrows E and thus moves the airflow stabilising device into or out of the lower end of the cyclone. The construction is stabilised by two spaced vertical guide rods 60, each of which extends through the crossbar 52 in a bearing 61, to allow the rods 60 to slide relative to the crossbar 52 when the rams 50,50a, are operated.
In the embodiment shown in
Movement to the tube 15a may also be achieved using a screw threaded connection between the exterior of the tube 15a and the interior of the bore 15 through the exterior 14, using the same arrangement as described with reference to
It will be appreciated that the arrangement shown in
a shows two rams 50,50a, and a single ram 51. However, a single ram, or multiple rams, may be used instead.
The object of the airflow stabilising device 13 is to stabilise the vortices, particularly the second vortex, so that it does not substantially vary in position within the cyclone. This means that the second vortex will reliably pick up under-processed material from higher up the cyclone, but will not disturb the adequately processed material which has been deposited in the lower part of the cyclone. The natural patterns of airflow in the cyclone, as shown in
Without the airflow stabilising device 13, the material leaving the cyclone through the gap X tends to be very mixed in particle size, since the precessing of the second vortex means that some particles are over processed and some particles which require further processing fail to be re-entrained and end up in the dead zone.
The use of the airflow stabilising device 13 not only makes the establishment of the vortices much more reliable, but also makes the position of the second vortex adjustable:—the more the tube 15a is advanced into the base of the cyclone, the more the lower end of the second vortex is raised, and the larger the dead zone 30. Since the particles in the dead zone eventually will pass out of the gap X, this means that the particle size of the processed material is increased by advancing the tube 15a into the base of the cyclone. Conversely, the more the tube 15a is withdrawn towards the position of
The airflow stabilising device and/or the position of the tube 15a relative to the exterior 14 can be moved relative to the base of the cyclone during a processing run, but in general would be set up for recovery of a particular particle size at the start of a run.
Advancing the frustro-conical portion 14 of the device 13 further into the end of the cyclone will reduce the size of the annular gap X and thus slow the flow of product from the cyclone; withdrawing the frustro-conical portion 14 will increase the rate of flow of product from the cyclone. In operation, product tends to leave the annular gap X in spurts or batches due to the natural pulsing action of the cyclone. The size of the gap X is adjusted for the required particle size. Product leaving through gap X falls onto an angled chute 13a (shown diagrammatically in
In general, it has been found that there is some airflow into the base of the cyclone through the gap X, causing some re-entrainment of product from the dead zone 30, but that this airflow is sufficiently low that the re-entrainment effect is not significant in practice.
For the apparatus to be used to maximum efficiency, and to enable a large variety of products to be processed under optimum conditions, it is necessary to be able to control the following variables accurately:—
In general, for given operating conditions, there is a fixed relationship between the particle size of the product after processing and its moisture content. However, if a higher moisture content is required without a change in the particle size, this can be achieved by closing down the conical valve 7 to reduce the amount of air vented to exhaust.
Referring to
Heat from the heat exchanger 25 can be supplied to either or both of the air heaters 26, 27 which can be used to heat respectively the inlet air supplied by the blower 9 to the inlet duct 10 and the air supplied by the blower 9 to the airflow stabilising device 13.
Sensors (not shown) inside the cyclone 2 record the pressure and humidity in the operating zones of the cyclone.
The blower 9 has separate outputs for the inlet duct 10 and the airflow stabilising device 13, to allow air to be supplied at different temperatures and velocities if necessary. However, for many products, air is supplied at the same velocity and pressure to both the inlet duct 10 and the stabilising device 13, in which case the blower may be connected to a single heater which supplies both the duct 10 and the device 13. Alternatively, the atmospheric air supplied to the blower 9 may be preheated by a heater 31.
Another possible configuration is to supply uncontrolled air to the airflow stabilising device 13 from ambient air, at ambient temperature and pressure. In this configuration, the airflow stabilising device 13 is not connected to the blower 9, and air is drawn through the tube 15a of the device 13 by the upward movement of air forming the second vortex.
The length of the tube 15a of the airflow stabilising device is such that air entering the base of the cyclone through this tube from atmosphere has its turbulence markedly reduced, if not eliminated, by the time it reaches the inner end of the tube.
The operator makes a decision as to whether to supply air to the airflow stabilising device from atmosphere or via the blower 9 based on the properties of the ambient air at the time of processing, the type of product which is being processed, and the available adjustment of the primary inlet airflow and the back pressure applied by the adjustment of the exhaust valve 7.
The general sequence of operation of the apparatus, from start-up, is as follows:—first, the setting of the conical valve 7 and the stabilising device 13 are adjusted to suitable settings for the product to be processed, and a suitable temperature for the cyclone inlet air is selected, based on data acquired from previous processing runs for that product.
Initially, the blower 9 is started to duct air to the inlet duct 10 and to the airflow stabilising device 13; if necessary, one or both streams of air are heated using the air heaters 26 and /or 27, or the heater 31. When the temperature monitors inside the cyclone indicate that the cyclone has reached the desired operating temperature, the product to be processed is fed into the inlet duct air stream through the device 11. At first, a slow feed rate is used, and as product starts to leave the cyclone through the gap X, the feed rate is gradually increased to the normal processing rate for that product.
The product being processed is swept into the cyclone by the stream of the air through the inlet duct 10, and travels in a substantially spiral path around the interior of the cyclone, as described above. The fully processed product leaves the cyclone through the gap X.
The drawings illustrate a single pass through a single cyclone only, but it will be appreciated that multiple passes can be made through a single cyclone, simply by returning the processed products from collection point 28 to product supply 29. Alternatively, two or more cyclones (of the same or different specification) may be used in series and/or in parallel.
The above described apparatus may be varied in a number of ways:—
This method is particularly suitable for the processing of small experimental amounts of product.
The dimensions and proportions of the cyclone and other apparatus may be varied widely, to suit the type and volume of product to be processed. Typical dimensions of a cyclone to be used for processing foodstuffs and other organic materials, including sawdust, at a rate in the range 50-400 kilograms of water evaporated per hour are as follows:—
The operating conditions for a cyclone of the above described dimensions would of course vary with the product to be processed, but typically would be as follows:
Velocity of inlet air through duct 10 and through the stabilising device 13: 35 m per second-120 m per second. Even higher velocities may be used for some product or to clean out the interior of the cyclone. However, the preferred velocity range for most product is 65-85 m per second.
Pressure of the inlet air—up to 1.8 bars above atmospheric pressure.
Temperature of the inlet air—in the range ambient −80° Centigrade.
The above-described apparatus has been found suitable for processing a very large range of materials, including the following:—marine products such as shellfish meat and shellfish shells, fish waste, fish and seaweed;
Cereal products such as wheat, maize, barley, brewers spent grain, stillage, gluten and flour; vegetables and herbs; fruit and nuts; wastes and nonbiological materials such as sawdust, newsprint, straws, bark, coal, concrete, feldspar, glass, clay and stone; animal products such as antlers, antler velvet, bone, bone marrow, cartilage and eggs. Liquid or semi liquid products such as egg white or gluten also can be processed successfully.
Examples of processing conditions for specific products:—
| Number | Date | Country | Kind |
|---|---|---|---|
| 506609 | Aug 2000 | NZ | national |
This is a continuation-in-part of U.S. application Ser. No. 11/258,753 filed Oct. 25, 2005.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 10362408 | Feb 2003 | US |
| Child | 11258753 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11258753 | Oct 2005 | US |
| Child | 12606509 | US |
