The present invention relates to an apparatus and method for fluidizing a bed of a cohesive powder, situated in a container.

In the industrial processing technique, it is very impor¬ tant that chemical reactions are made to take place as economically as possible. In the case of a powdered and a gaseous reagent it can then be useful to carry out the reaction in a fluidized bed of the powder through which the gas flows in upward direction. It is herein of the utmost importance that the gas reaches all powder par¬ ticles, that there are no agglomerates of powder particles in the fluidization reactor, and that there are no cracks or channels in the powder bed, through which the gas could for instance flow through the bed without reacting. Thus, the aim is to obtain as even a fluidization of the in¬ dividual powder particles as possible.

This creates problems in cohesive powders (so-called C- powders) . A classification of powders into four classes, of which one class contains the C-powders, displaying cohesive properties, was proposed by D. Geldart in the article "Types of Gas Fluidization", in Powder Technology,

2, pages 285-292, (1972) . Because of the great mutual cohesion forces, the C-powder particles form agglomerates, and during gas flow cracks and channels occur in the bed, through which, at just a slight pressure drop over the total height of the bed, almost all the supplied gas escapes without having been active. In that situation, the bed does not fluidize, but remains static. Up to now, the following attemps have been made to improve the fluidization behaviour of cohesive powders.

It has, for instance, been attempted to solve the problem by mechanically stirring the powder bed, supplementary to supplying the upward gas flow. In doing this, fluidization is observed, but of larger and smaller aggregates. There is, however, no total disperse particle phase, and no effective mixing of solid state phase and gas phase oc¬ curs. Morever, channel formation still occurs at some distance from the stirring blades. Stirring, combined with a gas flow, therefore does not produce the desired result.

It has also been attempted to solve the problem by vibrating the powder bed, supplementary to supplying the upward gas flow. Although this does lead to fluidization, it does so only over a bed height of a few centimeters, which is not enough for most industrial purposes. Above this height, the bed is static once more and displays channel and crack formation. Vibrating, combined with a gas flow, therefore does not produce the desired result either.

For a long time, now, intensive research has been underway in many locations, both in industry and at universities at home and abroad, to try to fluidize cohesive powders. However, neither of the two methods discussed above, stirring or vibrating in combination with supplying a gas flow, had any useful results. Because of the bad results, experts in this particular field of study developed a prejudice against the use of these techniques for the fluidization of a C-powder bed.

It is an object of the present invention to solve the problems described above in fluidizing a cohesive powder and to provide an apparatus and method, by which a bed of a cohesive powder can be fluidized easily and at least almost completely.

For this purpose, the apparatus according to the invention

is provided with means for making a gas flow through the powder bed, means for stirring the powder bed, and means for vibrating the powder bed, and in the method according to the invention a gas flow is conducted through the powder bed, the powder bed is stirred and the powder bed is vibrated.

Is has now surprisingly been found, that exactly a com¬ bination of the measures which are known per se, conduc- ting a gas flow through the bed, stirring the bed, and vibrating the bed, produces an excellent fluidization of the C-powder bed, in which no agglomerates or cracks or channels are present, and that the formed disperse par¬ ticle phase expands over the whole of the bed. Fluidized bed heights in the order of 1 meter are hereby realized.

In an advantageous embodiment of the apparatus according to the invention, the stirring means comprise a vertical stirring shaft protruding into the bed, onto which stir- ring shaft at least one rotor is attached, the rotor comprising flat stirring blades, located in planes, paral¬ lel to the axis of the stirring shaft. In this embodiment, the bed can be fluidized surprisingly well. This contrasts with other (for instance inclined) stirring blade orien- tations, which turn out to in fact cause cracks in the powder bed.

It is furthermore advantageous if the apparatus is provided with several rotors, situated at mutual distances above each other in vertical direction, said mutual dis¬ tances being smaller than a maximum distance, the bottom rotor being situated as closely above the bottom of the bed as possible. The maximum distance is the distance, over which channel formation is just kept from occurring during stirring.

In a further advantageous embodiment, the container is a

cylindrical vessel, in which the stirring shaft substan¬ tially coincides with the central axis of the cylindrical vessel, and the stirring blades extend in horizontal direction and their horizontal blade length is ap- proximately equal to the cylinder radius of the vessel. Herein, a completely fluidized bed can easily and ef¬ ficiently be formed.

The optimum for the stirring speed can vary with the type of powder, but it typically lies at approximately 100 revolutions per minute, This is sufficient to make channel formation disappear, while higher stirring speeds would dissipate more energy.

An important parameter for fluidizing a bed of a cohesive powder according to the invention is the dimensionsless vibration number G of the vibration, which is defined as

ω ² A

G = (1)

wherein <X> is the angular frequency of the vibration, with <X> =2πf (in which f is the vibration frequency) , A is the amplitude of the vibration and g is the acceleration of gravity. The vibration number G of the vibration is preferably between 2 and 8. This leads to good fluidization results. The optimum may vary with the type of powder. A typical optimum value of the vibration number G is 3 , at a frequency of 30 Hz.

In a preferred embodiment the vibration means comprise the bottom of the container for transferring the vibration onto the lower boundary of the powder bed. The vibration means are preferably able to vibrate in vertical direc¬ tion. If the apparatus is provided with a horizontal gas distribution plate through which the gas flows in, on the

bottom side of the bed, it is advantageous that the vibration means comprise this gas distribution plate for transferring the vibration onto the powder bed. In this embodiment, the vibration can be transferred onto the powder bed in an advantageous manner.

In a further advantageous embodiment the apparatus is provided with means for adjusting the temperature and relative humidity of the gas, in balance with the moisture content of the powder grains. This stabilizes the moisture content in the powder grains themselves, and especially in the outer layers of the powder grains and the mutual cohesion forces between the grains related to this are also stabilized.

Furthermore, a highly advantageous embodiment of the invention is provided with grounded parts in the powder bed, and, when the container is a cylindrical vessel, the inner wall of the vessel may be provided with grounded copper sheets or strips, which preferably run parallel to the central axis of the cylindrical vessel, and the stir¬ ring means can be made of metal and be grounded. As a result, electrostatic charge, which can be formed in the powder bed due to frictional accumulation, and which can disturb the fluidization, is discharged.

Advantageous embodiments of the method according to the invention are the subject of the claims 17 - 24.

A method to characterize the cohesiveness of a powder is provided by determining the Hausner-ratio (see H.H. Haus- ner, Int. J. Powder Metal, 3_, page 7, (1967)) .

The Hausner ratio is defined as follows:

V,

HR = LPB ' TB ( 2 )

V. TB 'LPB

in which V _LPB is the loosely packed bulk volume, V _TB the tapped bulk volume, and p _LPB and p _TB are the loosely packed and tapped bulk densities of the characterized powder.

In general, powders are considered to be cohesive, and classified under the so-called cohesive powders (C-pow- ders) , when their Hausner ratio HR > 1.25 (see D. Geldart & A.C.Y. Wong, "Fluidization of Powders showing degrees of cohesiveness - I. Bed expansion", Chemical Engineering Science, 3_9, pages 1481-1488 (1984)) .

It is remarked that the cohesiveness depends on variable properties such as the moisture content of the powder and the temperature.

A number of fluidization experiments have been carried out with an embodiment of the fluidization apparatus, schematically represented in the accompanying figure, which will now be discussed.

The bed 2 of the cohesive powder is situated in a cylindrical, transparent perspex column 1 having a height of 1.35 m and an inner diameter of 288 mm. A gas distribution plate 3 is located on the bottom side of the column, said gas distribution plate being attached to the bottom of the column with a rubber ring and a number of flanges (not shown) . A porous plate of sintered steel is used as the gas distribution plate, said porous plate having a thickness of 2 mm. The pores in the plate are small enough to prevent the powder from clogging the plate (filter barrier of 3 μ ) .

Gas supplied through a supply line 4 ends up in the wind box 5 (a space below the gas distribution plate) , where the fluidization gas is evenly distributed over the gas distribution plate. The pressure drop across the plate, relative to the pressure drop across the powder bed, is sufficiently large (1:3) to distribute the gas evenly over the bed (independent of the bed height) . After flowing through the bed, the gas can flow away unhindered. Thus, the pressure above the powder bed remains atmospheric.

The vibration is generated by means of an hydraulic vibration system which sets a pin 6 vibrating. This vibration pin 6 is connected to the gas distribution plate 3, which is freely movable due to its flexible suspension. In the employed arrangement, the column 1 is fixed by means of a support and does not, therefore, vibrate it¬ self.

The employed vibration system can generate a vertical sinusoidal vibration and transfer it onto the gas distribution plate. This will cause the powder bed 2 to vibrate.

The vibration system we used comprises an oil pump 7 which provides the hydraulic pressure to control the vibration pin, and an electrically operated valve 8 for the hydraulic system, to equalize the position of the vibration pin 6 to an operation point provided by a signal generator 9. For this purpose, a feedback regulator 10 is present. Furthermore, a frequency meter 11 is present, from which the frequency set on the signal generator 9 can be read.

The vibration system is regulated such, that the instan- taneouε position of the vibration pen 6, and therefore also the position of the gas distribution plate 3, which is directly mechanically coupled to the vibration pen 6,

can be controlled relative to the base 12 of the apparatus to the instantaneous operation point of the signal generator 9. As a consequence of this arrangement, the shape, amplitude and frequency of the vibration are adjus- table.

The stirring system comprises a vertical stirring shaft 13 to which stirring blades 14 are attached. The stirring blades 14 are attached to a cylindrical ring (not shown) which can be slid round the stirring shaft and can be fixed thereto at a desired height by means of a locking screw connection. The stirring blades used are exchangeable.

The working of two types of stirring blades 14 herein was examined: blades which are at an acute angle with the axis of the stirring shaft, and blades which are parallel thereto. Stirring with the first type of blades turned out, after some tests, to cause cracks in the powder bed, stirring with parallel stirring blades did not. For that reason the latter were used. The stirring blades 14 can be rectangular and extend in their horizontal longitudinal direction over almost the entire radius of the perspex column 1 upto near the perspex column 1, with a stirred diameter of 26 cm, and have for instance a height of 3 cm and a thickness of 2 mm.

The stirring blades 14 form rotors which lie one above the other, each rotor comprising two stirring blades 14, which are longitudinally aligned. The stirring blades 14 of neighbouring rotors which are situated one above the other, are preferably arranged at straight angles to each other.

The stirring system further comprises a stirring motor (not shown), a speed regulator 15, and a power gauge 16 by which, by determining the torque applied to the shaft, the

torque which is absorbed by the powder bed under certain circumstances, can be determined.

The employed fluidization apparatus is further provided with a moistening system for the supplied gas. This comprises a moistening column 17, a gas flow meter and regulator 18, with which the flow of gas supplied to the powder bed can be controlled, a counter pressure regulator, which can for instance consist of a valve 20 controlled by a flow regulator 19, or which can for instance be a manually operated control valve, and a control valve 21 controlled by means of for instance a pneumatic converter (not shown) by a relative humidity meter and controller 22. With this moistening system, a chosen gas flow can be led through the powder bed at a desired temperature and relative humidity. Naturally, other embodiments of a moistening system with which this is possible can also be used in a fluidization apparatus according to the invention.

For diagnostic reasons, the fluidization apparatus is further provided with a number of pressure gauges 23 at different heights in the powder bed and with a pressure gauge 24 in the wind box.

Furthermore, the fluidization apparatus is provided with grounded copper strips, which have been adhered to the inside of the perspex column 1 in axial direction of the cylindrical column 1, for instance at mutual distances which are equal in circumferential direction. The stirring shaft 13 and the stirring blades 14 are also grounded. This prevents a build-up of static electric charge, which disturbs the fluidization.

Such a charge build-up is imputed to frictional ac¬ cumulation in the powder bed stirred inside the column 1. By maintaining, at the same time, a constant temperature

and humidity of the fluidization gas, in balance with the temperature and the moisture content of the grains of the powder, the moisture content in the outer layers of the powder grains can be stabilized at a sufficiently high value, so that the powder bed maintains a sufficiently high conductivity, and the built-up static electric char¬ ges can be effectively discharged through the grounded parts. The coagulation of the powder grains as a conse¬ quence of charge distributions present in the powder bed is then no longer important, as a consequence of which possible charge agglomeration can no longer disturb the fluidization.

Important fluidization experiments have been carried out with the apparatus described above, with a bed of cohesive starch powder having a moisture content of 10.2 % by weight on a dry basis, with a static (non-fluidized) bed height of approximately 76 cm. Air was used as fluidization gas, with a superficial speed between 0 and 10 mm/s through the powder bed, and with a temperature and controlled relative humidity, which were in balance with the temperature and the moisture content of the starch grains.

If only aeration took place then slugs, slowly ascending discs of static starch powder, would occur, pushed upwards by a disc of air below it. Upon tapping the column, these discs would fall apart and disc formation would start anew. This indicated a very bad mixing. Fluidization remained unattainable with aeration only.

The rotors of the stirring means had vertical mutual interspaces of 124 mm. When, apart from aeration, only stirring took place, conglomeration phenomena continued to exist in the powder bed. The powder maintained a flocculent appearance and did not fluidize well, certainly not up to grain level.

When, apart from aeration, only vibration took place, cracks and channels were observed in the powder bed fro some centimeters above the gas distribution plate and fro there the bed further remained static (no fluidization) .

When, apart from aeration, both stirring and vibratio took place, the starch powder bed would completely com into homogeneous, channel- and crack-free fluidizatio over its entire height of typically 100 cm. Visually, aggregates were no longer observable in the bed and th bed surface on the upper side of the fluid bed was flat.

The best results were obtained at a stirring speed o about 100 revolutions per minute, and a vibration number of the vibration between 2 and 8, the best fluidizatio result being obtained at G=3 with vibration amplitud A=0,83 mm, and vibration frequency f=30Hz.

Similar results were obtained in a fluidization experimen with a bed of cohesive melamine powder: during separat stirring or vibration, in addition to aeration, the be remained static as a consequence of agglomeration or th formation of cracks and channels, and fluidization did no take place. Combining aeration, stirring and vibration, however, did lead to good fluidization at a bed height o approximately 80 cm.

Experiments were also carried out with potato starc powder with a moisture content of 24 % by weight on a dr basis. Here too, the combination of aeration, stirring an vibration led to a significant improvement in th fluidization behaviour.

Except with starch and melamine, the apparatus and metho according to the present invention can of course also b used with any other cohesive powder, for instance mil powder, or cement.

It is clear, moreover, that other embodiments of the fluidization apparatus than those described above are also possible without moving outside the scope of the invention described in the claims, for instance embodiments provided with vibration means of a different kind.