The present invention relates to a method for spray drying of liquids containing solid materials in soluble or suspended form, according to which the liquid is atomized in a chamber while admitting a drying gas. Subsequently, this suspension of partially dried particles and drying gas is sucked through a movable perforated carrier surface where the partially dried particles or a substantial part of these particles settle before being subject to a final heating while the perforated carrier surface moves on in order to build up a coherent porous layer which is subsequently crushed to a desired particle size. This method is exceptional because of the generation of a whirling movement of the suspended partially dried particles in the drying gas in the lower part of the chamber or in a zone immediately over the moving perforated carrier surface.

According to the invention, it is appropriate to generate the above mentioned whirling movement by introducing a secondary turbulent gas stream near the part of the moving perforated carrier surface where the partially dried particles settle as a porous layer.

The invention relates furthermore to a spray drying apparatus designed to applicate the above described method and comprising a chamber, one or more atomizing devices placed in the upper part of the chamber for comminuting the liquid into droplets, devices for introducing a heated drying gas directed towards.the droplets, a movable perforated carrier surface placed at or under the lower part of the chamber for collecting partially dried particles of solid material, devices for sucking the gas stream with suspended partially dried solid particles towards and through the movable perforated carrier surface, and devices for a final

heating of the solid particles settled on the movable perforated surface, said apparatus being characterized, according to the invention, in that it is equipped with devices for the generation of a whirling movement of the suspended partially dried particles in the drying gas, placed at the lower part of the chamber or immediately over the part of the movable perforated surface where the solid particles settle.

Spray drying is widely used for drying various kinds of liquids such as solutions, suspensions, dispersions or emulsions containing widely differing products. The dried products come out as homogeneous dry powders.

The known kinds of such apparatus usually comprise a chamber or tower which can be either cylindric with a circular cross-section or consist of an upper cylindric section and a conical lower part or bottom. A liquid atomizer consisting of one or more nozzles or a quickly rotating disc is placed in the upper part of the chamber. The chamber is furthermore equipped with drying gas intake devices, usually placed in the upper part of the chamber, and exhaust devices for taking out the drying gas and the dried material in the lower part of the chamber.

The drying gas can be conducted through the chamber in counter-current or co-current. In the latter case the drying gas, such as heated atmospheric air, is introduced into the upper part of the chamber and thus dried to solid particles. The main part of the product is segregated as a dry powder in the lower part of the chamber while the drying gas with a content of evaporated liquid is sucked out through a suction opening which may e.g. be placed in the conical lower part, if such one is available. The powdered product accumulated in the bottom of the chamber can be removed

intermittently or continuously through sluices or by means of a screw conveyor. Pine particles which may occur in considerable quantities are led out of the chamber together with the exhaust gas and can be segregated from this by means of cyclones, filter sacks, electrofilters, washing columns or other per se known powder separators.

Instead of downwards directed nozzles, one can as mentioned above preferably use quickly rotating devices such as wheels or discs from which the liquid by the action of the centrifugal force is hurled out mostly horizontally as fine droplets. The drying gas is usually introduced in the upper part of the chamber, e.g. by means of distribution devices directed towards the atomizing zone and producing a rotating and turbulent movement of the drying gas which promotes a uniform distribution of the atomized droplets in the gas and ensures a good heat exchange.

Spray drying apparatus of the kind described usually procure a very fine-grained product. However, this is not always desirable. An aftertreatment of the powder by remoistening and agglomeration may therefore be necessary. Since the powder is subject to a total drying requiring a drying gas with a relatively high temperature, a thermal deterioration of the powder product may occur. This can be very disadvantageous when drying substances sensitive to heat such as foodstuffs and pharmaceutical products. Furthermore, the heat consumption is relatively big even if a part of the drying gas is reheated and recirculated.

Spray drying apparatus with a conical lower part are very bulky due to their considerable installation height. Chambers with a large volume also have a large starting and end volume causing a possible loss ~J £ a LIg

amount of product on interruptions of the production. In case of drying of inflammable substances, the presence of a big quantity of dried, comminuted powder suspended in hot oxygen-containing gas such as atmospheric air, of which a substantial part is led back into the drying chamber together with the recirculated gas, will cause a considerable explosion risk, especially in connection with start and stop.

Attempts have been made to overcome some of these disadvantages by incorporating an additional dryer in the bottom of the spray drying chamber so that the drying process takes place in two, three or more steps. A still moist powder is removed from the chamber and transferred to the additional dryer where the powder is preserved as a fluidized layer on the vibrating perforated plate through which a drying gas keeping the powder in motion is flowing from below. By means of this method, one obtains a better heat economy and a more gentle drying. One can also obtain a better particle size and size distribution because of the agglomeration taking place during the final drying. Subsequently, the dried powder can be removed, if desired after a subsequent cooling in the following stage.

However, the above described treatment does not solve all problems. It is in fact difficult to avoid a thermal denaturalization of substances sensitive to heat, and furthermore the agglomeration will not be efficient and the product will turn out uneven. Nor is it all types of powder which can be dried in this way. This applies to e.g. very fatty powders.

From the US patents No. 3,615,723 and 3,741,273 a spray drying apparatus is known working according to quite another drying principle. It comprises a drying tower or chamber where the liquid is introduced above and

atomized through a number of downwards directed nozzles into liquid drops which together with a laminar stream of a drying gas are conducted down through the chamber. The product is only partially dried in the chamber and collected as a layer on a wowen conveyor belt placed in the bottom of the chamber while the drying gas is sucked down through this. The still moist powder is conducted out of the chamber by means of the conveyor belt under coincident and/or subsequent drying.

The final drying and a possible cooling can take place in one or more separate sections or chambers where drying gas is conducted down through the conveyor belt. After the drying and cooling, the agglomerated product is conducted through a grinding device in which it is crushed to a wanted particle size and transported to collecting devices. Such a FILTERMAT ^® apparatus involves considerable advantes over the traditional spray drying apparatus, especially because one obtains an improved heat economy and a more gentle drying as well as any desired particle size and size distribution of the dried product.

In order to make drying apparatus of this kind work satisfactorily, the layer of powder settled on the conveyor belt must have an even thickness. In order to achieve this, the particles must be evenly distributed when settling. This is an aim which is hard to achieve since the powder is led downwards together with a more or less laminar stream of drying gas. This problem is particularly big if one uses cylindric drying chambers which a big diameter in which case the generated layer will be thickest in the middle with a decreasing thickness towards the edges of the conveyor belt even if this belt covers the entire bottom of the chamber.

In case of very big drying apparatus the use of a conveyor belt having a width which is considerably smaller than the diameter of the chamber has been required. In this case a funnel-shaped chamber bottom is necessary in order to conduct the powder towards the conveyor belt, resulting in a very uneven distribution of the powder when settling. Furthermore, there is a considerable risk of powder accumulation and adhesion on the walls of the chamber and the funnel-shaped bottom since the still moist powder will tend to agglomerate and adhere to the walls.

The object of the present invention is to avoid all the above mentioned disadvantages and to procure a spray drying apparatus in which the comminuted liquid drops are partially dried in a chamber and subsequently settle as evenly distributed still moist particles on a movable perforated carrier plate such as a conveyor belt while creating a layer of uniform thickness which is subject to a final drying in order to form a porous mass.

The special inventive effect is achieved by the generation of a whirling movement in the lower part of the drying chamber or in a zone over the moving perforated carrier surface such as a conveyor belt. This whirling movement which is to ensure a uniform distribution of the partially dried powder on the conveyor belt can be generated totally mechanically, e.g. by means of rotary arms or whipping devices. According to the preferred embodiment of the invention, a turbulent secondary stream of air is introduced into the bottom of the chamber immediately over the conveyor belt, e.g. through one or more nozzles or through an oblong narrow slot.

By designing the drying apparatus as described the following essential advantages are obtained:

- Practically no loss of powder or secondary powder when starting and stopping the apparatus.

- The powder quality is very high as a consequence of the gentle drying at the possible low temperature.

- The powder is collected on the filter belt in half- moist condition and is therefore directly converted into agglomerates without necessarily adding a separate agglomeration process.

- The exhaust gas is extensively free from powder and consequently the pollution risk is reduced and further gas filter installations such as filter sacks, electrofilters or extra cyclones are not necessary.

- The capacity is increased substantially since the heat transmission is increased due to the turbulence.

- The dimentions of the apparatus, especially the installation height, can be reduced.

- The operation is very simple and flexible as the operation conditions can be adjusted according to the properties of the product without any problems.

- The explosion risk is completely overcome as the drying gas does not contain suspended completely dry particles.

- The heat economy is extremely high as it is possible to run at a lower exhaust temperature since the relative air and powder humidity can be kept at a higher level.

Some of the stated advantages are also obtained by means of known apparatus, but is had not been anticipated that

the described combination gives all the advantages. It is especially surprising that one has been able to combine the turbulent stream of drying gas with suspended liquid drops with an accumulation of the partially dried particles directly on a band filter without operations stoppage. Due to the turbulence an efficient mixture of particles with drying gas is thus obtained and consequently a fast drying with a very small temperature difference between drying gas and particles.

A particular advantage is that it is possible to rebuild existing traditional spray drying apparatus in a simple way by mounting a bottom section comprising a conveyor belt made of a flexible filter material. The traditional spray drying apparatus generally have a cylindric chamber with a conical bottom section. It is possible to cut off the conical bottom section and mount the conveyor belt directly under the cylindric chamber. However , it is preferable to mount a transition unit with a quadratic or rectangular bottom cross-section of at least the same cross-sectional area as the cylindric section. It is thereby ensured that the powder accumulated in the bottom is distributed over the conveyor belt as a layer of uniform thickness. Moreover, the risk of substantial deposits of powder on the chamber wall is avoided. Such deposits often occur in the conical part of the traditional drying apparatus.

Specially preferred forms of embodiment of the invention are subject to several of the sub-claims.

The invention will be further illustrated by means of the drawing showing particularly appropriate forms of embodiment of the spray drying apparatus according to the invention.

Fig. 1 shows a vertical sectional view of the spray drying apparatus,

fig. 2 shows the spray drying apparatus as a partial sectional view seen from above,

fig. 3 shows a sectional view of the bottom section of a modified embodiment, and

fig. 4 shows the same bottom section as in fig. 3 as a partial sectional view seen from above.

As it appears from fig. 1, the apparatus consists of a cylindric drying chamber 1 with a liquid atomizer 2 placed in the upper part of the drying chamber. The liquid atomizer is shown as a rotary disc, but it can also consist of one or more nozzles. A certain number of tubes for the drying gas, 3, 4 is placed in the upper part of the chamber 1. The bottom of the chamber has the shape of a funnel 5 introduced at the bottom into an oblong sluice 6 which is connected to a subjacent chamber 7 placed over a perforated conveyor belt 8, e.g. made of wire netting or a wowen mat. The sluice 6 is surrounded by a frame 9 connected to a vibrating device 10 which transmits the vibrations or standing waves to the funnel 5. This can be made of steel or thick stretched plastic foil.

An inlet opening 11 for a secondary gas stream is placed under the conveyor belt 8 in order to create a turbulent zone 12 wherein the gas stream with suspended partially dried particles is mixed and evenly distributed. The particles will be led through the chamber 7 together with the gas stream and settle on the conveyor belt 8 while forming a layer 13. The drying gas for the final drying is conducted through a tube 14 over the conveyor belt and passes through the porous layer 13 towards the

exhaust funnel 15. The porous agglomerated and dried product 16 is disintegrated at the end of the conveyor belt 8 and if desired crushed to a wanted particle size in a (not shown) grinding device.

Figs. 3 and 4 show a modified embodiment of a bottom section. At the bottom the chamber 1 ends in a funnel 5 with an exhaust sluice 6. Under this sluice is placed a slot 11 for the introduction of a turbulent secondary gas stream.

In both of the embodiments shown in the figures the conveyor belt has a width corresponding to 0,5 - 0,6 of the diameter of the chamber. The conveyor belt can be wider if desired, it can e.g. have the same width as the diameter of the chamber or less. However, the chamber must in all cases be equipped with a bottom section with an opening inside the conveyor belt. This opening can be an oblong sluice placed with its longest dimension transverse to the moving direction of the conveyor belt as illustrated. However, the opening may also have any other form, e.g. circular, oval, triangular or similar.

Instead of a cylindric chamber one may choose a chamber with a quadratic or rectangular cross-section.

The bottom section may if desired consist only of a plastic foil which can be shaped as a funnel with a narrowing in order to create two chambers, one over and one under the narrowing. If the narrowing is equipped with a reinforcement or a rigid frame, this one may be connected to a vibrating device transmitting the vibrations or the standing waves to the entire bottom section and thus preventing adhesion of partially dried powder.

If the funnel-shaped bottom is made of steel, it can be connected to a hammering device transmitting the vibrations which propagate in the entire bottom section and thus efficiently prevent adhesion of material on the inside of the chamber and the bottom.