IE44841B1 - Improvements in or relating to the discharge of cohesive particulate materials - Google Patents

Abstract

In order to be able to unload the powdery product (5), for example whole-milk powder, from a silo (4) through an unloading aperture connected to an outlet pipe (8) and arranged in a filter-like plate (6), the product (5) is aerated through the plate (6) and, at the same time, the plate (6) is made to vibrate vertically by a vibrator (26). In this case, it is important that the aerating and the vibration regions are spatially not or hardly separated from one another, and that the joint action of the aeration and the vibrations extends up to the unloading aperture. Such a method makes it possible to cause adherent powdery products to flow continuously, it being possible for the outflowing quantity to be influenced by the quantity of air supplied.

Description

This invention relates to a method of varying the rate of discharge of a cohesive particulate material from a bulk mass of the material, for example in a bunker or --silo.

Practice has shown that the controllable discharge of cohesive particulate material from a bunker or silo involves great difficulties. Discharge by gravity is often inadequate, so that auxiliary means must be used to empty the silo. For this purpose a large variety of silo dis10 charge systems for cohesive materials has been conceived and marketed. The greatly varying nature of these systems, shows that they are not universally applicable. Most of the systems are only suitable for certain powders or particular combinations of a certain powder and a certain silo, and even then are used with greatly varying success.

A general drawback of current discharge systems is that the auxiliary means is often directed towards activating the flow in the direct vicinity of the outlet opening. In particular for perishable materials, for example, whole20 milk powder/ it is essential that during the withdrawal of these materials from the bunker or silo the entire body of t' powder therein is activated, or in other words, each unit volume of material has the same average residence time.

This mean^ that there may be no dead corners in which i powdered material could remain and be spoiled. With current discharge systems, in order for the silo to be completely activated and to be emptied, it must be equipped with a generally steep outlet funnel, which decreases the available storage capacity and increases the risk of the formation of bridges.

Among the conventional systems for discharging cohesive materials from silos are mechanical systems, ranging from conveyor belts, conveyor screws and stirrers (e.g. the Nauta mixer, With which the product can be transported from the silo and moreover the contents of the silo can be maintained in loose condition) to scrapers and chains. A general disadvantage of mechanical systems is that they are rather expensive in maintenance, while the mass flov? is mostly not controllable or is controllable to a limited extent only.

In some instances, in order to prevent the formation of bridges in the conical outlet, inflatable cushions are provided in the wall, which can be caused to pulsate. Thus, among other possibilities, a rubber membrane is maintained in motion by a continuous stream of air, with the air simultaneously effecting an increase in porosity. In other instances, incidental aeration is used, for example, with a so-called air gun, with which a bridge formed is broken by means of air pulses released in the powder.

A very important category of auxiliary means comprises vibratory apparatus. Vibrating the powder at the silo wall or on the silo bottom in the area around the opening can promote discharge. Thus, among other possibilities, a bottom or outlet cone movably suspended relative to the remainder of the bunker is vibrated by means of a vibrator or out-of-balance motor to prevent bridge formation and rat30 holing. Often, so-oalled bunker beaters and vibrators, including ultrasonic vibrators, are attached to the wall or to the bottom of the silo (often the cone), among other 4S841 - 4 purposes for reducing wall friction.

Furthermore, in a number of cases, the discharge can be controlled by means of a vibrated conical body or a-“vibrated lattice of slats disposed just over or within' the outlet opening. Power consumption and the cost of the vibratory apparatus used in such systems may sometimes be considerable.

The use of vibrations may have an adverse effect-if the contents of the silo are consolidated or compacted thereby.

Thus a reduction in wall friction gives rise to a higher . consolidation pressure in the bottom region of the silo, whereby the material is additionally densified. Owing to the increase of the resulting forces- between the particles, the very effect may be that discharge is rendered more difficult, the public is cautioned against subjecting silos to vibration, as it may densify the contents thereof (E.E.U.A. Handbook, 1963, No. 15,pp. 11-93, in particular page 93). Naturally, the risk of densification will increase as the material is more cohesive.

A method of discharge much used in practice for powders which are not cohesive or are only slightly cohesive is the so-called pneumatic discharge, in which the material in the silo is aerated, mostly at the wall or on the bottom around the outlet opening only, so that it locally acquires a more or less homogeneous fluidized state.

The place at which, and the manner in which, aeration is effected may vary greatly. In a number of cases air is also blown into the silo during storage, when there is no discharge, for the sole purpose of keeping the bulk naterial in a loose condition, and preventing consolidation in the bottom region of the silo. Generally speaking, the mass rate can be properly controlled with the amount of gas - 5 supplied via aeration units in the bottom part of the silo.

In this method, the powder around the outlet opening would have a more or less constant mass density which, for noncohesive substances, would be approximately equal to that with minimum fluidization. This enhances regular discharge.

The possibilities of pneumatically discharging poorly running, cohesive powders from bunkers or silos, however, have been rather limited so-far. It is well-known that for proper pneumatic discharge, the powder concerned must be capable of being fluidized fairly readily (E.S.U.A.-Handbook. 1963, No. 15, pp. 11-93, in particular page IS). The method has therefore proved unsuitable for strongly cohesive material, the cohesive forces between the particles ι being so great as to prevent fluidization of the material.

By modulating the air with acoustic waves (300-400 Herts; cf., e.g., S. Medcraft, Chemical and Process Engineering, April 1971, Sonic activation of powders) before it is blown into the powder, the flow characteristics can be improved. In a number of cases the pneumatic method has thus been rendered suitable for cohesive material.

Furthermore, it is well-known that, in reactors, the fluidizatioii characteristics of a powdered material can in principle be improved by means of mechanical vibrators, in which cases, however, the entire column is brought into vibration. The problems which then arise when the system is applied on a large scale have barred general use in practice.

It has now surprisingly been found that even highly cohesive powdered materials, such as moist, whole-milk powder, which cannot be withdrawn from a silo or bunker in a sufficiently controllable manner by aeration or vibration alone, can nevertheless be discharged from the silo or bunker with a properly controllable mass rate and in a - 6 fully activated condition by applying simultaneous aeration and vibration to these powders in a manner to be described in more detail below.

According to the present invention, there is provided a method of varying the rate of discharge of a cohesive particulate material from a bulk mass of the material through an outlet, the method comprising locally fluidizing the material by injecting air into the material and locally vibra ting the material with a vertical vibratory component, such that vibrations are propagated through the material to the place where the air is injected and such that the material in a region adjacent the outlet is simultaneously fluidised and vibrated, variation in the rate of discharge being effected by adjusting the rate of air flow into the material and/ or the amplitude and/or the frequency of the vibration.

In use of the method according to this invention, in which a combination of aeration and vibration is applied, the flowability of poorly running powders is considerably improved so that it has proved possible to withdraw cohe20 sive powdered material from a bunker or silo at a controllable variable flow rate. The fluidization characteristics of the material on the bottom of the bulk mass are greatly improved, so that the product at that position assumes [ i liquid-like properties.

The fluidization and vibration must take place at coincident or adjacent locations. Of crucial importance in this connection is that the vibrations propagating through the material can reach the place where the air is injected. This distance is naturally partly determined by the nature of the powdered material in question, and, for example, in the case of whole-milk powder, must be no more than approxi4 4 8 41 - 7 mately a few centimeters. Preferably, the air is injected by an aeration element which is in contact with the powdered material ahd is vibrated.

Th'e mass rate may be controlled by adjusting the flow rate of air supplied to the bottom region of the silo, and, to a more limited extent, by adjusting the intensity of vibration. When the supply of air and the vibration stops, the outflow stops. In the case of greatly cohesive solids, such as whole-milk powder, this happens virtually immediately. In other cases, for example, potato flour, the material may continue to flow out of the silo for some time.

It is found in practice that, by using a method in accordance with the present invention, a silo can always be emptied readily and virtually entirely, while the silo contents are completely activated. In the case of less cohesive powders, which may also be withdrawn from the silo using aeration alone, it is found that discharge rates can be Increased while maintaining controllability when vibration is also used.

For a better understanding of the present invention and to show hdw it,may be carried into effect, reference will now be made, by way of example, to the following Examples and to the accompanying drawings, in which: Figure 1 shows diagrammatically an arrangement for discharging a cohesive particulate material from a bunker; Figure 2 shows part of the arrangement of Figure 1 on a larger scale; Figure 3 corresponds to Figure 2 but shows an alternative arrangement; Figure 4 is a graph representing the performance of 4841 - 8 the arrangement of Figures 1 and 2; and Figure 5 is a graph representing the performance of the arrangement of Figure 3.

Example I ' Spray-dried whole-milk powder having a moisture content of 3% and a f^t content of 26% was discharged from a silo.

The test was conducted using the arrangement of Figure 1.

In Figure 1, there is shown a storage bunker 1 for the powder, which is equipped with a dust collector 2 and a terminal 3 for connection to a source of compressed air.

The storage bunker 1 discharges into a silo 4.

The silo' 4 comprises a cylindrical perspex tube, 1.3 m long and 0.14 m xn dxameter, and having a bottom comprising a porous stainless steel plate 6 having an aperture 7 of diameter 7 mm. The powder is designated by reference numeral 5.

In Figure 2-, the silo is shown diagrammatically in cross-section. In the construction illustrated in Figure 2, the plate 6 itself was vertically vibrated. An outlet pipe S passes through an air inlet cavity 9 in a flexible manner and is connected to the vibrating table of an electromagnetic exciter 26. Beneath the outflow pipe 8 there are a collecting vessel 10 and a weighing table 11 which are not sub25 jected to the vibration of the exciter 26. The plate 6 could thus be vibrated by the exciter 26 at a varying frequency and amplitude. The plate 6 is connected in a flexible manner to the silo wall by being glued to a strip of neoprene rubber 12, which in turn is clamped between flanges 13 of the silo wall. The silo 4 was suspended in a v rigid manner as free from vibration as possible. The dia4 48di meter of the- outlet tube 8, 20 mm. is considerably greater than that of the outlet aperture 7, in order to create a.slight drop in pressure across the tube. Measurements were made with a closed silo with variable bed height. After each test the column was replenished. The air was supplied via a ball valve 20, an air filter 21, a pressure regulator 22, a needle valve 23 and,after the rate of flow had been measured with a rotameter 24, to the bottom of the column, and supplied uniformly over the column diameter by means of the fine porous plate 6. In order to prevent a decrease in air pressure at the top of the column during the outflow of the powder, so that the bed would tend to hang in the column as a consequence of the resulting pressure drop, some of the air was supplied to the top via a pressure equalizing conduit 14. The pressure in the top of the bed is thus equal to the pressure below the filter, and was measured by means of a water-filled manometer 25. All air leaves the column through the outlet aperture 7. The flow rate of the powder was measured with the electronic weighing platform 1\1, connected to a power unit 32 and provided with a zero setting 30. The amount of discharged powder could be noted or recorded as desired. The vibrations were analyzed using a piezo-electric acceleration pick-up transducer 38. The signal from the transducer 38 was amplified and supplied to a tube volt meter 36 and an oscilloscope 37, possibly after being integrated once or twice in a vibration amplifier-integrator unit 34 and then via a filter 35» A power unit is indicated at 33.

Using the above equipment, the measurements of the mass rate were invariably made under'such conditions with respect to the intensity of combined aeration and vibration that there was virtually uniform outflow of the whole-milk a j10 powder. The mass rate itself could then he reasonably accurately determined from the inclination of the straight line found on the paper of recorder 31.

In this test the rate at which the model silo was emptied was plotted against the total quantity of air supplied to ,the column. The frequency of the vibrations, which were generated with an RC generator 29 and supplied via an amplifier 28 and an ammeter 27 to the exciter 26, was varied between 30 and 100 cycles, and the amplitude of the vibration was controlled by altering the input current of the exciter.

The input voltage was 25 V. Without a powder in the column, the filter plate vibrated sinusoidally. When the column contained a powder, the vibration was complex. As soon as the powder was contacted by the plate, components of higher frequency were superimposed upon the ground tone, which affected flow behaviour. The vibrations proved to be more effective as the deviation from the sinusoidal form was greater.

The measurements proved that in the entire frequency range examined, a uniform but variable discharge could be obtained if the amplitude of the vibration was sufficiently large.

Figure 4, is a graph showing the discharge rate of the powder plotted along the vertical axis in kg/min. against the air rate in litres/min. Figure 4 shows that the flow rat$ of the powder increases regularly with the total air rate. The variation in flow rate which can be achieved by adjusting the generator frequency and the amplitude of the vibratioii is much more limited. It is true that the flow rate decreases as the vibration acceleration is lowered, \ ·' 4 8 41 - 11 but the outflow tends to become irregular. It was impossible for the corresponding points, which may be considerably below the curve shown, to be determined in a sufficiently reliable manner, and these are not shown in Figure 4.

The first series of measurements proved that a regular and properly controllable outflow could be achieved throughout the range of air rates used when the most favourable generator frequency is selected, namely, between 60 and 80 Hertz, and in addition when the input current intensity of the exciter was at least 1 Amp. The overall levels of the peak value and of the average value of the vibration acceleration, as measured by the tube volt meter, were then 9 g and 5g on average. Both figures were rather susceptible to fluctuation. The associated amplitude of the vibration was less than 0.5 mm and the power to be supplied to the vibrator was approximately 20 W. Viith an air rate of 3 m /h. the rate of powder discharged from the silo was approximately 0.5 ton /h.

With frequencies lower than 40 cycles, outflow was more irregulafi, and the bulk material tended to stick to the walls.

Example II Unlike Example I, in which ths places of aeration and vibration coincided, in the test described in this example use was mide Of a vibrator arranged so that aeration and vibratign were effected at separate locations. The vibrator part of the equipment used in this test is shown in Figure 3. The distributer plate 6 is fixed in this instance, and the diameter 7 gf the outflow apparatus has been increased to 12 mm. Disposed just over the plate 6 is a cruciform member 15 comprising two interconnected square tubes 16, The member 15 was vibrated from below by means of a vibratory pin 17, passed through the outflow pipe 8, which pin was connected to the vibrator and had an external diameter of 6 mm.

It turned out that the silo could be emptied only if the member 15. were less than a few centimetres from the plate 6. The test was run using a generator frequency of 167 cycles, at which the vibrating system resonated. Here 'again, the rate at which the silo was emptied increased uniformly with the total amount of air supplied to the column (Figure 5). At the vibration accelerations used, there was no regular outflow at air rates less than 5 1/min At higher air rates the required input current intensity of the vibrator was invariably less than 1 Amp. The power input of the vibrator was between 10 and 20 W. The maximum measured peak value of the vibration acceleration was 10 g. which is considerably higher than with the vibrated plate. The amplitude of the vibration was invariably less than 1 mm. ' Example III 1 Some tentative measurements were made using the equipment described in Example I, and applying aeration only. The measurements were made with spray-dried whole25 milk powder.(moisture content 3%, fat content 26%) and with potato flour (moisture content 18%) kaolin and ptoluene sulphonamide.

It turned out that the three cohesive powders last mentioned could be withdrawn from these silo in slightly consolidated condition using air alone. With the highly cohesive milk powder, this did not succeed at all.

Within the scope of the method according to the present invention, many other embodiments are conceivable in which the principle of local and simultaneous aeration and vibration of the cohesive powdered material at coinci5 dent, or almost coincident, locations can be realized.

Thus the method can be carried out using a system in which vibrated aeration units, in the form, for example, of a system of' porous pipes, are used to blow air into the bottom of the silo and in addition to set the product in vibration at the same place. By rotating one or a few pipes, the entire silo contents can be activated even if the latter has no outlet funnel. Also, the required vibratory equipment may be limited.

Claims (4)

1. A method of varying the rate of discharge of a cohesive particulate material from a bulk mass of the material through an outlet, the method comprising locally 5 fluidizing the material by injecting air into the material and locally vibrating the material with a vertical vibratory component, such that vibrations are propagated through the material to the place where the air is injected and. such that the material in a region adjacent the outlet, is 10 simultaneously fluidized and vibrated, variation in the rate of discharge being effected by adjusting the rate of air flow into the material and/or the amplitude and/or the frequency of the vibration.

2. A method as claimed in claim 1, in which the air is 15 injected into the material through an aeration element which is vibrated to vibrate the material.

3. A method as claimed in claim 1 or 2, in which the variation in the rate of discharge is effected by adjusting the rate of air flow. 20

4. A method as claimed in claim 1 and substantially as described herein with reference to the accompanying drawings. .