GB2043474A - Method and apparatus for drying a pumpable substance containing a liquid - Google Patents

Abstract

Inside a housing (1), a drying chamber (3) is defined by a cylindrical, gas-permeable wall (2). The annular space between the housing (1) and the wall (2) is divided into a plurality of superposed annular chambers (4a, 4b, 4c, 4d) divided by horizontal annular plates (5a, 5b, 5c) which are charged with preheated air through ducts (10 to 13) which enter respective annular chambers (4a, 4b, 4c, 4d) tangentially. Openings (7, 29) provided in the wall (2) are directed partially radially and partially obliquely so that a rotating air cushion (30) is formed at the inner face of the wall (2) which prevents adhesion of the solid substance thereto. The air travels along a helical path inside the drying chamber (line S) and therefore a long dwell time results and hence intensive drying. The substance to be dried is introduced at spray head (27), and the dried substance leaves at opening (28). <IMAGE>

Description

SPECIFICATION Method and apparatus for drying a pumpable substance containing a liquid The invention relates to a method and a device for drying a pumpable substance containing a liquid which is sprayed into a chamber and is freed, at least mainly, from the liquid in contact with a preheated, flowing gas, preferably hot air.

It is known to spray a liquid laden with solids into a stream of hot air so as to evaporate the liquid or to dry the solid substances.

In a known installation of this kind, a spray nozzle with its mouth directed downwards is used, through which the substance to be dried is sprayed into the hot air. In order to prevent caking of the still moist product On the inner wall of the installation, this must have a great structural height; only so is the necessary dwell time achieved which can ensure adequate drying.

If a deflector or a spray wheel is used instead of a spray nozzle, then for the same reasons, the installation must have a relatively large diameter, that is to say a considerable dimension in width.

All the known methods and installations for spray drying thus have the disadvatage that they have very large dimensions and so need a considerable amount of space, if a satisfactory drying is to be achieved.

Thus it is the object of the present invention to propose a method and a device for spray drying which enable an optimum degree of drying to be achieved with a compact form of construction and which avoid with certainty the unwanted adhesion of the solid substances to the inner wall of the drying chamber. Because of its compactness, the device according to the invention further favours its combination with existing installations, particularly dust-arresting installations and fluidized beds.

This problem is solved with reference to the present invention by the combinations of features which are defined in the independent claims.

One example of an embodiment of the device according to the invention and two variants of use are explained below with reference to the accompanying drawing.

Figure 1 is a simplified vertical section through a device for spray drying, Figure 2 is a section on the line Il-Il in Fig.

1, and Figures 3 and 4 are diagrammatic vertical sections of two examples of use.

The device designated as a whole by Z comprises, inside a circular cylindrical housing 1, coaxial with this, a gas-permeable circular cylindrical wall 2 which bounds a drying chamber 3. The gap between the housing 1 and the wall 2 is divided by annular plates 5a/5b/5c into four annular chambers 4a, 4b, 4c, 4d which lie one above the other like stories. Thus the plates 5a/5b/5c only extend over the width of the annular chamber and leave the central drying chamber 3 free throughout over its whole height.

At the top, the chamber 3 is bounded by a cover 6 which is likewise gas-permeable so that a hot-gas receiving chamber 8 is formed between this cover and the upper horizontal wall of the housing 1.

The wall 2 surrounding the chamber 3 consists of a material which is provided with numerous small openings or pores 7/29 and so is gas-permeable over its entire area. Perforated sheet metal, dry powdered metals and sintered plastics plates are suitable for this for example. In addition to the fine openings 29 which, with a perforated sheet metal extend substantially in the radial direction through the wall 2, further inflow openings 7 are provided (Fig. 2), which end obliquely in the inner face of the wall 2 so as to achieve a rotating movement of the stream of air entering.

The inclination of the inlets 7 can be varied taking into consideration the entry speed and the required dwell time of the substance.

Leading into the top chamber 8 disposed above the cover 6 and into each annular chamber 4a to 4d is a hot-air pipe 9 to 13, in each of which an air heater 14 to 18 is installed. The pipes provided with regulating valves 19 to 23 are connected to a duct 24 which is charged with air from a fan 25.

Projecting into the chamber 3 from below is a pipe 26 which comprises a spray.nozzle 27 at its upper end. Instead of the spray nozzle, another spraying device, for example a deflector or a spray wheel could also be used. The chamber wall 2 becomes narrower downwards in a taper to an outlet 28.

When the device described is in operation, a liquid containing a solid, for example tomato pulp, is sprayed into the upper portion of the chamber 3 through the nozzle 27. At the same time, hot air is blown in through the pipes 9 to 13 and rotates in the annular chambers 4a to 4d as a result of the obliquely directed mouths of these pipes (see Fig. 2).

The hot air penetrates on the one hand through the radial openings or pores 29 (Fig.

2), and on the other hand through the obliquely disposed openings 7 into the chamber 3. In the course of this a rotating air cushion 30 forms at the inner face of the wall 2 and at the chamber cover 6, as a result of which the solid components contained in the drying substance are reliably prevented from coming into contact with the wall 2 and adhering there.

The substance emerging from the spray nozzle 27 is caught by the rotating stream of warm air and conveyed slowly downwards from stage to stage along the helical line S indicated, so that as a result of the long helical path to be covered, there is a considerable dwell time inside the drying chamber.

The hot air entering the annular chambers 4a to 4d is preferably preheated by the air heaters so that its temperature on entry into the annular chamber 4a amounts to about 300 C annular chamber 4b amounts to about 200 C annular chamber 4c amounts to about 100 C.

In the bottom annular chamber 4d, the temperature of the hot air may likewise amount to 100 C or be below this value, and according to the operating conditions, more intensive cooling is also possible. The air entraining the dried product leaves the chamber through the opening 28 and is then conveyed to a filter where the product is separated from the air in known manner.

The hot air introduced into the top chamber 8 through the pipe 9 can have an entry temperature of about 100 C within the scope of the example described.

According to one form of embodiment of the device described, all the pipes 9 to 13 lead into the annular chambers 4a to 4d in the same manner, as shown in Fig. 2. A modification of this device provides, however, that the tangential openings of the pipes 9 to 13 lead alternately to right and left into the annular chambers so that the hot air rotates in opposite directions in adjacent annular chambers. In some circumstances the dwell time of the sprayed substance inside the device can be influenced by this means.

Figs. 3 and 4 show two special applications of the device described, the reference numerals used already with reference to Figs.

1 and 2 being retained.

According to Fig. 3 an atomizing drier Z is disposed inside a dust-arresting installation.

This dust-arresting installation comprises a substantially cylindrical housing 31 which is tapered downwards and in the upper portion of which the known sack-shaped dust filters 32 are disposed hanging. Air is drawn out of the housing 31 through the filter bags 32 by a suction fan 33 and the solids entrained in the air remain behind on the outer surface of the filter bags 32 and from time to time are shaken off or blown downwards by a fan 34.

The device Z here works otherwise as described with reference to Figs. 1 and 2. While the air saturated with moisture escapes upwards in the direction of the arrows, the dried solids leave the installation through a lock 35.

As an auxiliary device, a fluidized bed 44 may be provided in this installation, to which warm air is supplied through an annular passage 45.

In the installation illustrated in Fig. 4, the solid substances are only dread to a certain moisture content inside the device Z and then enter a fluidized bed 36 disposed underneath, in the direction of the arrows, the fluidized bed being charged with warm air from below through a pipe 37 in known manner. Following on the finish-drying in the fluidized bed 36, the dry solids pass through an opening 38 and an overflow 39 into a pipe 40 and can be removed from this through a lock 41.

Warm air is likewise supplied to the annular chamber 43 through a further hot-air duct 42 and contributes to the further optimum drying of the solid substance. The warm air temperatures are preferably graduated so that warm air at 300 C for example (pipe 10), 200 C (pipe 11) and 100 C (pipe 12) is blown in through the pipes and warm air at below 100 C is blown in through the duct 37 while the temperature of the warm air conveyed in the duct 42 may be somewhat lower still.

The method described is distinguished from the known systems above all in that the dwell time of the drying substance is relatively great and moreover can be controlled by deliberate selection of the air velocity (pipe 11, Fig. 2) and angle of flow (nozzle 7, Fig. 2) and that adhesion to the chamber wall is avoided with certainty.

Whereas atomizing driers could hitherto not be combined with fluidized beds in a single unit because of their large dimensions, the device described, because of its compact construction enables this to be disposed directly above a fluidized bed. As a result there is not only a saving in space but at the same time a considerable saving in heat requirements because the heat losses are drastically reduced.

Other combinations, for example the provision of multi-stage fluidized beds with chicanes for the compulsory delaying of the fluidized material can be realized by one skilled in the art within the scope of the idea of the invention.

Claims (17)

1. A method of drying a pumpable substance containing a liquid comprising the steps of spraying the substance into a chamber, preheating a flowing gas, and so directing the flow of gas into the chamber that a rotating cushion of gas is produced at the inner wall of the chamber over at least a part of the height of the chamber to prevent the substance from contacting the inner wall.

2. A method as claimed in claim 1, wherein the gas comprises air.

3. A method as claimed in claim 2, wherein the preheated air is blown tangentially into an annular chamber surrounding the chamber, and therefrom enters the chamber through a gas-permeable boundary wall thereof in a radial and/or tangential manner at such a speed that the gas cushion is formed.

4. A method as claimed in claim 3, wherein the preheated air is introduced into the annular chamber and therefrom into the chamber in two or more streams, each at a different height.

5. A method as claimed in claim 4, wherein the temperature of each stream of air is different and the streams are so arranged that the hottest air is blown into the upper most portion of the chamber and the coolest air into the lowest portion of the chamber.

6. A method as claimed in either claim 4 or claim 5, wherein each stream of air is blown into the annular chamber in the same tangential direction.

7. A method as claimed in either claim 4 or claim 5, wherein each stream of air is blown into the annular chamber in an oppo site direction from the or an adjacent stream.

8. An apparatus for carrying out a method as claimed in any one of the preceding claims, the apparatus comprising a housing provided with at least one connection for the supply of a preheated gas, at least one spraying device for the substance, a chamber disposed within the housing and bounded by a substantially cylindrical wall which comprises, at least over a part of its surface, a plurality of gas pas sages, the or each connection leading into the gap between the housing and the cylindrical wall.

9. An apparatus as claimed in claim 8, wherein the housing is also cylindrical and at least some of the gas passages disposed in the wall and the or each connection are inclined with respect to the surface of wall.

10. An apparatus as claimed in claim 9, wherein the or each connection and the at least some of the gas passages are disposed at an angle substantially tangential to the surface of the wall.

11. An apparatus as claimed in any one of claims 8 to 10, wherein the spraying device comprises a spray nozzle disposed on the axis of rotation of the chamber and provided with an upwardly directed mouth, and the annular chamber defined between the cylindrical wall and the cylindrical housing is divided by an nular plates into a plurality of annular cham bers arranged one above an other, a connec tion for the supply of preheated gas leading into each annular chamber, the gas being preheated to a temperature progressively de creasing from the top to the bottom annular chamber.

12. An apparatus as claimed in claim 11, wherein the connections of two adjacent an nular chambers are so offset in relation to one another that the rotating gas flows in the respective annular chambers are in opposite directions.

13. An apparatus as claimed in any one of claims 8 to 13, wherein the wall comprises perforated sheet metal, a sintered metal or a sintered plastics plate.

14. An apparatus as claimed in any one of claims 8 to 13, further comprising a dust arrester installation wherein the dried substance is drawn off downwardly and the gas saturated with moisture is drawn off upwardly through filter elements of the dust-arrester installation.

15. An apparatus as claimed in claim 14, further comprising a fluidised bed into which the dried substance is passed to be finishdried.

16. A method of drying a pumpable substance containing a liquid substantially as hereinbefore described with reference to the accompanying drawings.

17. An apparatus for drying a pumpable substance containing a liquid substantially as hereinbefore described with reference to the accompanying drawings.