CH619903A5 - Method for metering the unloading of adherent powdery products from a silo or hopper - Google Patents

Description

The invention relates to a method according to the preamble of claim 1.

In practice, it has been found that the controlled metering of cohesive powdery material from a silo or bunker presents considerable difficulties, since the gravity of the material alone is not sufficient to empty the silo or bunker, so that an auxiliary device is required for this. Different systems for unloading cohesive powdery material from silos or the like are known on the market. The different types of such systems show that they are generally only suitable for a certain powdery good or a certain combination of powdery good and silo, and even then they can only be used with greatly varying success.

In addition, these known systems also have a general disadvantage in that they are aimed only at activating the flow directly around the discharge opening. However, especially for perishable goods or materials, e.g. Whole milk powder, it is essential that during the withdrawal of these materials from the bunker or silo all the powder mass present in them is activated, in other words, each volume unit of the material has the same average residence time. This means that there are no blind spots where the powdery material could spoil. In order to be able to fully activate and empty the silo with the common unloading systems, it must then be provided with a mostly steep discharge funnel, which reduces the available storage space and increases the risk of bridging.

Among the known unloading systems for extinguishing cohesive powder from silos can be Calculate mechanical systems that vary from conveyor belts, screw conveyors and agitators (e.g. the Nauta mixer, with which the product can be conveyed out of the silo and the silo content can also be kept loose) to scrapers and chains. Mechanical systems have the general disadvantage that they are rather expensive to maintain, while the mass flow can usually not be regulated or can only be regulated to a limited extent. In order to avoid bridging in the discharge cone, inflatable cushions are sometimes provided in the wall, which can be pulsed. Among other things, a rubber membrane is kept in motion by a continuous flow of air, the air simultaneously increasing the porosity; occasional ventilation also takes place, e.g. by a so-called air cannon, whereby a bridge formed is broken up by air blows which are directed into the powder.

A particularly important category of aids includes vibrators. The outflow can be promoted in that the powder is vibrated on the silo wall or on the silo bottom in the area around the opening. Among other things, a floor or outflow cone that is movably suspended relative to the rest of the bunker is set in motion by a vibrator or unbalance motor in order to avoid bridge and io gear formation ("rat-holing"). Often, "bunker knockers" and vibrators, among which ultrasound, are attached to the wall or on the bottom of the silo (often the cone), among other things. to reduce wall friction.

Furthermore, it is shown in some cases that the outflow can be regulated with the aid of a conical body which is excited to vibrate or a lamellar grid which is excited to vibrate and which is arranged directly above or in the outflow opening. The energy consumption and the cost of the vibrators used can sometimes be considerable.

The use of vibrations can be inappropriate if the content of the silo is consolidated or compacted. A reduction in the wall friction causes a higher consolidation pressure in the lower part of the 25 silo, which also further compresses the bulk material. The outflow can be made more difficult by the increase in the forces between the particles, which are the result. It is therefore also warned against subjecting silos to vibrations because their powdery contents can condense (E.E.UA.-Handbook, 1963, No. 15, pages 11-93, especially page 93). The risk of compaction will understandably increase, depending on the cohesive material involved.

One method that is frequently used in practice for non-and slightly cohesive powders 35 for emptying is the so-called pneumatic emptying, in which the material in the silo, mostly only on the wall or on the floor around the outflow opening, is ventilated so that it is local gets into a more or less homogeneous fluidized state. 40 Where and how the air is blown can vary widely. In some cases, air is blown into the silo even during storage when there is no discharge, with the sole purpose of keeping the bulk loose and avoiding consolidation in the lower part of the silo. The mass flow can generally be regulated well with the amount of gas that is supplied via ventilation units in the lower part of the silo. With this method, the powder around the outflow opening would have a more or less constant mass density, which for non-cohesive substances would be approximately equal to the mass density with minimum fluidization. This benefits the uniformity of the outflow.

The options for pneumatically emptying poorly flowing, cohesive powders from bunkers or silos have so far been limited. It is generally known that the powder in question must be able to be fluidized fairly easily for good pneumatic emptying (E.E.UA.Handbook, 1963, No. 15, pages 11-93, especially page 15). The method has proven unsuitable for highly cohesive material because the cohesive forces between the particles are then so great that fluidization of the material is thereby prevented. By modulating the air with sound waves (300-400 Hz; see e.g. S. Medcraft, chemical and process engineering, April 1971, "Sonic activation of powders") before it is blown into the powder, the flow properties can be improved. In some cases, the pneumatic method is suitable for cohesive material.

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619 903

It is also known that the fluidization properties of a powdery material in reactors can in principle be improved with the aid of mechanical vibrations, in which cases, however, the entire column is set in vibration. The problems that then arise with larger dimensions have stood in the way of general practical application.

Surprisingly, it has been shown that even very cohesive powdery goods, such as moist whole milk powder, which cannot be removed from a silo or bunker or cannot be removed sufficiently or only controllably by ventilation or vibration, are still metered out of the silo with a fully controllable mass flow in a fully activated state can, namely that such powders are simultaneously aerated and vibrated in a manner to be specified.

According to the invention, a method was developed which is characterized by the features specified in the characterizing part of claim 1.

By using the method according to the invention, using a combination of aerating and vibrating, the outflow of heavy-flowing powders is considerably improved, in that it has been shown to be possible to remove cohesive powdery material with a constant and adjustable mass flow rate from a bunker or silo . According to the invention, the fluidization properties of the goods on the bottom of the bunker or silo are greatly improved, so that the goods take on “liquid-like properties” there.

It is essential in the method according to the invention that the combined ventilation and vibrating is carried out at spatially almost non-separate locations. It is of crucial importance that the vibrations propagating through the powder can reach the place where ventilation takes place. This distance is of course determined by the type of powdered product and, for example in the case of whole milk powder, is not more than about a few centimeters. It is preferred to vibrate the ventilation element in contact with the powdery material.

The mass flow can be regulated with the amount of air supplied in the lower part of the silo and to a lesser extent with the vibration intensity. As soon as either the air supply or the vibration is stopped, the outflow stops. With strongly cohesive solids, such as whole milk powder, this happens almost immediately. In other cases, e.g. with potato flour, the good will continue to flow for some time.

In practice, it turns out that with the method according to the invention, a silo can always be emptied easily and almost completely, while the silo content is fully activated (“mass flow”). In the case of less cohesive powders, which can also be removed from the silo with air alone, it is found that the load on the application of vibrations can be increased while maintaining the controllability.

In addition to the advantages of the method according to the invention, there is a lower risk of bridging and rat-holing, a more constant metering of mass and volume and the avoidance of clumping. The latter can be of particular importance when dispensing in packaging material and reactor vessels.

The invention is illustrated by the examples below.

Example I

A test was carried out with regard to the meterability from a silo with spray-dried whole milk powder, the moisture content of which was 3% and the fat content was 26%.

The test was carried out using the apparatus shown schematically in FIG. 1.

In Fig. 1, 1 is a storage bunker for the powdery material - in this example, the whole milk powder 5 mentioned above - provided with a dust catcher 2 and a compressed air connection 3.

4 with a silo is referred to, consisting of a cylindrical tube made of Per-spex (L = 1.3 m, D = 0.14 m), provided with a bottom 6 made of a permeable io poral plate made of stainless steel, in which plate an opening 7 (Fig. 2) is arranged (D = 7 mm). The powdery material is indicated by 5.

In Fig. 2 the silo is shown schematically in section. In the construction shown in FIG. 2, the gas distribution plate 6 itself was made to vibrate vertically. The outflow pipe 8 is guided in a flexible manner through the air inlet space 9 and, in addition to a collecting container 10 and a weighing surface 11, is connected to a vibrating table of an electromagnetic vibration generator 26. 2o The distribution plate 6 could thereby be set in vibration with varying frequency and amplitude. The plate 6 is flexibly connected to the silo wall by gluing with a strip 12 of neoprene rubber, which in turn is clamped 25 between the flanges 13 of the silo wall. The silo 4 was as rigid as possible and hung free of vibrations. The diameter of the outflow pipe 8 (D = 20 mm) was chosen to be considerably larger than the diameter of the outflow opening 7, so that the pressure drop across the pipe would remain small. It was measured with a closed silo at 3o changed bed height. The column was refilled after each attempt. The air was introduced through a ball valve 20, an air filter 21, a pressure regulator 22, a needle valve 23 and, after the flow had been measured with a rotary meter 24, partially introduced into the column 35 ne below and by means of the fine poral filter 6 uniformly distributed over the column cross-section. In order to avoid,

that during the outflow of the powder the air pressure at the top of the column would drop and the bed would tend to get stuck in the column 40 as a result of the pressure drop which occurred, some of the air was introduced from above via the pressure compensation line 14. The pressure above the bed was equal to the pressure below the filter and was measured using a water-filled monometer 25. The air left the column exclusively through the out-45 flow opening 7. The mass flow of the powder was measured with the electronic weighing plate 11, connected to the feed unit 32 and provided with the zero adjustment device 30. The amount of powder emitted could be written down or registered as required. The vibrations were analyzed using a piezoelectric accelerometer 38. The signal was amplified and then optionally integrated once or twice in a vibration amplifier integrator unit 34 and then fed via a filter 35 to a tube voltmeter and a 55 oscilloscope. A feed unit is shown at 33.

Using the device described above, the mass flow measurements were always made under such conditions in terms of the intensity at which ventilation and vibration were used in combination that 6o a practically uniform outflow of the whole milk powder was mentioned. The mass flow rate itself could then be determined almost exactly from the steepness of the straight lines found on the paper of the recorder 31.

In this experiment, the speed at which the model silo flowed empty was tested as a function of the total amount of air supplied to the column. The frequency of the vibrations generated with an RC generator 29, the vibration via an amplifier 28 and an ammeter 27

619 903

The generator 26 were supplied was varied between 30 Hz and 100 Hz and the vibration amplitude was controlled with the input current of the vibration generator. The input voltage was 25 V. Without powder in the column, the filter plate swung sinusoidally. The vibration was complex in a powder-containing column. As soon as the powder was pushed through the plate, components of higher frequency were superimposed on the basic tone, which influenced the flow behavior. The vibrations proved to be more effective the greater the deviation from the sinus shape.

The measurements showed that a uniform and controllable outflow could be achieved in the entire frequency range tested if the amplitude of the oscillation was chosen large enough.

4 shows the mass flow of the powder in kg / min against the air flow in 1 / min along the vertical axis. 4 shows that the mass flow of the powder increases uniformly with the total air flow. The controllability with the generator frequency and amplitude is much more limited. The mass flow rate decreases when the vibration acceleration is reduced, but the outflow becomes uneven. The corresponding points, which can lie considerably below the drawn curve, could not be determined reliably enough and are therefore not included in FIG. 4.

The first series of measurements showed that a uniform and easily controllable outflow over the entire range of applied air flows could be achieved if the favorable generator frequency was selected, namely between 60 Hz and 80 Hz, and the input current strength of the vibration generator was at least 1 ampere. The “overall” levels of the peak value and the average value of the vibration acceleration measured by the tube voltmeter were then 9 g and 5 g on average. Both were subject to fairly high fluctuations. The associated amplitude of the vibration was less than 0.5 mm and the power supplied to the vibrator was about 20 W. With an air flow of 3 ms / hour, about 0.5 tons of powder flowed out of the silo per hour (diameter opening 7 mm).

At frequencies below 40 Hz the outflow was more uneven and the bulk material tended to settle.

Example II

In a departure from Example I, in which the points of ventilation and vibration coincided, the experiment described in this example was carried out with a vibrator in which the points of ventilation and vibration did not coincide. The vibrator part of the device used in this experiment is shown in FIG. 3. The distribution plate 6 is fixed and the diameter of the outflow opening is increased by 7 to 12 mm. A cruciform member 15 is arranged directly above the filter plate, which comprises two square tubes 16 which engage in one another. The organ 15 was driven from below by means of a vibrating pin 17, which was guided through the outflow pipe 8 and was connected to the vibrator and whose inside diameter was 6 mm.

It turned out that the silo could only be emptied if the cross was placed less than a few centimeters above the gas distribution plate. The test was carried out at a generator frequency of 167 Hz, at which the vibration system resonated. The speed at which the silo flowed empty also increased evenly with the total amount of air introduced into the column (FIG. 5). With the vibration accelerations applied, there was no uniform outflow under an air flow of 5 l / min. With higher air flows, the required input current strength of the vibrator was always less than 1 ampere. The power consumed by the vibrator was between 10 and 20 W. The measured peak value of the vibration acceleration was a maximum of 100 g; this is much higher than with the driven plate. The vibration amplitude was always less than 1 mm.

Example III

With the apparatus discussed in Example I, some orienting measurements were carried out for comparison purposes, with only ventilation. The measurements were carried out on the spray-dried whole milk powder already mentioned (moisture content 3%, fat content 26%), potato flour (moisture content 18%) as well as kaolin and p-toluenesulfonamide.

It was found that the three last-mentioned cohesive powders, in a slightly consolidated state, could still be extracted with air alone in the silo. With the very cohesive milk powder, this did not work at all.

Within the scope of the method according to the invention, many embodiments are still possible in which the principle of locally ventilating and vibrating the cohesive powdery material locally at almost non-separate locations can be implemented. For example, one can think of a system in which ventilation units excited in the form of vibrations, e.g. a system of porous tubes,

Air (below) is blown into the silo and also causes the product to vibrate at the same point. By rotating one or a few pipes, if necessary, the entire silo content can be activated even if no discharge funnel is attached. The required vibrating apparatus can also be kept to a limited extent.

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2 sheets of drawings

Claims (3)

619 903 1. A method for dosing the discharge of mutually adhering pulverulent material from a silo or bunker having an unloading opening, characterized in that the material is locally ventilated and at the same time is directly exposed to vibrations with vertically directed components, the ventilation and the vibration area not spatially or are almost not separated from each other, so that the vibrations penetrate into the area directly at the discharge opening in which the ventilation is effective, so that the combined effect of the vibrations and the ventilation extends as far as the discharge opening. 2. The method according to claim 1, characterized in that a ventilation element (6) in contact with the material is vibrated. 2nd PATENT CLAIMS 3. The method according to claim 1 or 2, characterized in that the flow rate of the outflowing material is influenced by the amount of air.