METHOD AND APPARATUS FOR FLUID-BED DRYING

Abstract: The invention relates to a method and an apparatus for fluid-bed drying of heat sensitive material. The heat sensitive material is led to the bottom of the drying apparatus and caught in an upward stream of fluidization and drying medium. The residence time is adjusted by orifices in an annular protruding wall.

Description: Method and apparatus for fluid-bed drying The present invention relates to a method and apparatus for fluid-bed drying, particularly for simultaneous drying and disintegrating a heat sensitive material. Fluid bed treatment of various materials has been known for decades and has become a widely used technology in industry and manufacturing, due to the many advances of the technology. EP 0 141 403 discloses an apparatus for heat treatment of a material in a fluid-bed. The material to be treated in the fluid-bed is led to the fluidised bed through an inlet in the side wall of the apparatus above a rotating fluidised layer. The drying air is supplied via an annular distributor surrounding the lower part of the cylindrical wall of the chamber and a circular slit between the conical bottom and the cylindrical wall. In this apparatus larger and smaller lumps of material fall by gravity downwards in the chamber through the fluidized layer and towards the bottom and the slit. Consequently the largest particles of the paste material will fall deepest down in this space and be exposed to a violent impact from the stirrer arm and to a simultaneous, intensive drying of their surfaces from the drying medium, which has its highest temperature in this area. By this combined effect the larger particles will be disintegrated and dried. Smaller and lighter particles will be carried upwards under a simultaneous final drying and eventually leave the drying chamber together with the drying medium through the outlet duct at the top. The drying medium is introduced tangentially through the supply duct into the annular distributor and the circular slit, in order to obtain a preferred rotation of particles and drying medium in the drying chamber. A large variety of cakes and pastelike materials can be successfully dried in the apparatus described in EP 0 141 403. Some materials, however, in particular in their moist form, are of a very sticky nature and tend to deposit on the interior surface of the drying chamber walls. This is particularly inconvenient for heat sensitive materials because of the risk of discolouring and decomposition. Furthermore, the apparatus described in EP 0 141 403 has the disadvantage of requiring a very rigid structure and a very high energy consumption for the stirrer arm at high capacities because of a required wide diameter of the dryer. German patent no. 3612049 and additionally application no. 3633125 discloses an apparatus for drying of hen droppings. The hen droppings are led to the drying chamber via the bottom of the apparatus and introduced to the drying chamber by use of compressed air or a fan which creates an upward stream of air. In the chamber the hen droppings are dried by use of warm air from the chicken run. In general the disclosed apparatus is very ineffective as a drying apparatus, as the temperature is low and the compressed air does not create a fluidised layer . Consequently there has been a need for an apparatus for continuous fluid-bed drying and disintegration of a material in the form of a paste or a filter cake, which is insensitive to a sticky nature of the material. Furthermore there has been a need for an apparatus for continuous fluid-bed drying which allows for large constructions with high capacity without the need for rigid, wide diameter stirrer arms. It is an object of the present invention to provide a method and an apparatus which are particularly useful for drying of heat sensitive materials. This object and other objects are obtained by the invention as defined in the claims. The present invention provides a method and apparatus by which heat sensitive material can be dried without the risk of discolouration and decomposition which may appear when drying heat sensitive material. The invention provides a method and apparatus in which the material to be dried is almost instantly after entering the drying chamber caught in an upwards stream of fluidization and drying medium thereby avoiding contact with the hot walls of the apparatus. As mentioned previously one aspect of the invention relates to a method for fluid-bed drying of wet material in a drying chamber comprising an upper part and a lower part, the upper part having an outlet for dry material and the lower part comprising a substantially horizontal bottom part with a least one inlet for wet material and at least one inlet for a stream of fluidization and drying medium, said method comprising the steps of: i) providing the wet material to be dried into the drying chamber vertically through an inlet in the horizontal bottom part of said drying chamber, ii) exposing the wet material to an upward stream of fluidization and drying medium (definers) introduced into said drying chamber, said stream of fluidization and drying medium preferably being introduced through openings partly or totally surrounding said inlet for the wet material, iii) drying the wet material in the upward stream of fluidization and drying medium to obtain dry material, iv) removing the dry material from the upper part of the drying chamber through the outlet. The fluidization and drying medium is normally a gas, preferably atmospheric air which is heated to a temperature between 100-500 °C. The horizontal bottom part is constructed to be substantially horizontal and comprises one or more inlets for the fluidization and drying medium placed around one or more inlets for the wet material to be dried in such a away that the fluidization and drying medium constitutes a substantially circular or cylindrical upward stream of fluidization and drying medium, and the wet material to be dried enters substantially into the centre of the circular or cylindrical upward stream thereby being caught in the circular or cylindrical upward stream of fluidization and drying medium. In a preferred embodiment of the method according to the invention the wet material to be dried is in the form of a paste or filter cake. Preferably the wet material to be dried is in the form of a mixture of liquid and solid particles wherein the mixture contains 10-90 % by weight of solid material. In a preferred embodiment of the method according to the invention the wet material is stirred and/or disintegrated by stirring and/or disintegrating means in the drying chamber. Preferably the stirring or disintegrating means are blades, paddles, knifes or similar devices fastened to a vertical rotating shaft protruding from the centre of the horizontal bottom. Preferably the residence time for exposing the wet material to be dried to the stream of fluidization and drying medium is adjusted by adjusting the size of orifices. The residence time is the average time required for a given material to be exposed to the fluidization and drying medium to obtain a satisfactory dryness. The skilled person is able to determine the residence time for a material as a matter of routine. In a preferred embodiment of the method according to the invention the adjustable orifices are placed in the upper part of the drying chamber. Preferably the orifices are adjustable in an annular wall protruding horizontally into at least a part of the upper drying chamber. In a preferred embodiment of the method according to the invention the wet material to be dried is provided to the substantially horizontal bottom part of the drying chamber vertically upwards, preferably by means of paddles or transporting screws or belts, etc. The invention also comprises use of the method for drying wet material, and preferably for drying heat sensitive material. In one aspect the invention relates to an apparatus for fluid-bed drying of wet material with a drying chamber comprising an upper part and a lower part, the upper part having an outlet for dry material and the lower part comprising a substantially horizontal bottom part comprising means for providing an upward stream of fluidization and drying medium in the drying chamber and an inlet for the wet material to be dried. In a preferred embodiment of the apparatus for fluid-bed drying according to the invention the means for providing an upward stream of fluidization and drying medium is a slit in the substantially horizontal bottom part of the drying chamber, preferably the slit is annular-shaped and preferably the annular slit partly or totally surrounds the inlet for the wet material to be dried. Hereby it is possible to catch the wet material to be dried in the upward stream of fluidization and drying medium and avoid contact with the walls of the drying chamber. It is preferred that the apparatus for fluid-bed drying according to the invention has a drying chamber with an upper cylindrical part and a lower conical part tapering downwards to the substantially horizontal bottom part. Such a construction establishes the optimal conditions for the material flow in the drying chamber. Preferably the inlet for the wet material to be dried is placed substantially in the centre of the substantially horizontal bottom part of the drying chamber to provide the best distribution properties of the wet material to be dried in the upward stream of fluidization and drying medium. In a preferred embodiment of the apparatus for fluid-bed drying according to the invention the drying chamber is supplied with means for stirring and/or disintegrating said stirring and/or disintegrating means preferably being a coaxially placed rotating shaft with wings, paddles, knifes or rods. As the wet material to be dried is often cake or paste-like, there is a need for disintegrating the material to obtain a better drying. Preferably the wet material to be dried is provided upwards to the inlet in the horizontal bottom by means of paddles or transporting screws, belts or pumps. In a preferred embodiment of the apparatus for fluid-bed drying according to the invention the upper part of the drying chamber comprises means for adjusting the upwards stream of fluidization and drying medium, and these means are preferably a substantially horizontal projecting edge in form of an annular-shaped plate fastened to the wall of the upper chamber and with adjustable orifices. The invention also comprises use of the apparatus for drying wet material, and preferably for drying heat- sensitive material. The invention will hereafter be described in further details with reference to an example and drawings wherein Figure 1 shows a schematically vertical axial section of the apparatus according to the invention. Figure 2 shows the adjustable orifice to determine the residence time for the material in the drying chamber. The apparatus comprises a drying chamber 1 with a cylindrical part 2, a top part 3 and a conical part 4. The cylindrical part has an orifice 6 and an exhaust aperture 5 for the dried product, carried by the drying medium. The drying medium is introduced tangentially through the aperture 7 to the annular distributor 8 and directed into the drying chamber through an annular slit 11, delimited by the conical guide ring 9 and the outer member 10 of the central member 16. A rotating driving member 12 with a shaft 13 is placed coaxially in the chamber 1 and in the member 16 and is equipped with paddles 14 in the part enclosed by the member 16 and with wings 15 in the part enclosed by the conical part 4 of the chamber 1. The member 16 is provided with a supply member for paste material in the form of a member 17 with a conveyor screw 18. The member 16 and its outer part 10 forms a water jacket containing a medium for control of the temperature of the member 16. Circulation of the medium can be provided for by means of an inlet pipe 19 and an outlet pipe, not shown. The apparatus operates as follows. The material in the form of a cake or paste, which is to be dried and disintegrated, is supplied to the member 16 by means of the screw 18. By rotation of the shaft 13 with the paddles 14 the paste is conveyed upwards and - by act of centrifugal forces - introduced radially outwards into the drying chamber. The diameter of the member 16, the number and design of the paddles 14 and the revolution of the shaft 13 are chosen to allow for a vertical transport mass rate of the paste several times that of the supply screw 18. The reason for this is that whereas the operation of the screw 18 is a part of the overall control of the drying process, the paddles 14 shall supply the paste material to the central drying zone of the chamber in the form of a dispersed cloud of wet material. The wet cloud is met by the hot drying medium emanating form the slit 11 and a violent fluidised bed is established in the lower part of the chamber. Immediate initial evaporation from the dispersed wet material takes place. This is supported by the impact from the wings 15, and possible remaining lumps of paste will dry and disintegrate. A part of the material in the fluidised bed will leave the bed in the form of small lumps and be carried by the rotating drying medium along the conical walls 4 to the upper parts of the chamber. These lumps will however be dry on the surface and consequently not present any risk of adhering to the chamber walls. Lumps, which have a still wet interior, will move upwards along the conical walls 4 until the flow pattern in the chamber will direct them back into the bed in the bottom. For most products it is essential to control the residence time of the product in the drying chamber in order to ensure sufficient drying. The diameter of the central aperture in the orifice 6 has a known effect in terms of the product residence time in the chamber. Due to the rotation of the drying medium the material in the chamber moves upwards in the form of a rotating fluidized bed, the inner limitation of which is the aperture diameter of the orifice 6. When dry material from the process in the bottom of the chamber is added to the rotating particle mass, the particles from the surface will escape over the edge of the aperture and a constant particle mass will be maintained in the chamber for a given diameter of the aperture. Consequently a given diameter of the aperture in the orifice together with a given supply of material to the process will define a thereby given residence time for the product mass in the chamber. The present invention incorporates means for control of the residence time during operation by introducing a segment cut out of the orifice ring 6 and equipped with a slide gate 6a which can be adjusted between an open and a closed position by means of an external activator, see figure 2. The activator can be manual or remote operated and in such a way that the gate can be adjusted to any position between open and closed. In the open position the gate edge towards the centre of the chamber is in its maximum distance from the centre line of the chamber. In its closed position the gate edge towards the centre of the chamber is in line with the rim of the circular opening in the orifice 6, By nature of dynamics the mass of the rotating fluidized material will be defined by the position of the controlling edge of the gate. Consequently the residence time of the particle mass in the chamber can be controlled by adjustment of the gate position. Various modifications of the apparatus are possible within the scope of the invention: Various forms of the paddles 14 or even a full screw may be provided as required for pasty products of different natur . As the drying medium is supplied to the annular distributor 8 through the tangentially directed supply pipe 7, guide blades may be provided in the distributor 8 to change direction of the medium flow through the slit 11. Also one can imagine other executions of the slide gate 6a within the nature of the invention. As an example more gates than one can be applied and the geometry of the individual gate can be varied as well. Though the invention is particularly advantageous for the drying of heat sensitive materials and for high capacity fluid-bed dryers, the apparatus might also be advantageous for the drying of materials, which can be successfully dried on known apparatuses and at small/medium capacity Example The invention and the advantages by using it are elucidated by the result from the following experiment, performed in an apparatus as depicted in the drawing. The diameter of the cylindrical part of the apparatus was 1.6 m, and the height from air inlet (9) to the top (3) was 4. m. The opening in the orifice 6 was 1 meter . 4161 kg/h wet, pastelike whey concentrate with a total solid content of 85% was fed vertically upwards into the drying chamber and disintegrated by the rotating wings. The drying and fluidizing air in an amount of approx . 20,000 kg/h was introduced into the drying chamber through the annular slit (11) at a velocity of 55 m/s and a temperature of 140 °C. Partly dried material formed a rotating fluidised bed in the chamber, limited by the orifice 6 and the slit 6a, with 50 % opening. Outlet drying air (5) temperature was 70 °C. Dried powder was separated from drying air in a cyclone (not shown) . The experiment was closed by the operator after 4 hours. The chamber was free of severe deposits. Flowability of a powder was measured by pouring approx. 10 grams of powder in a wide tipped funnel, resting on a table. When the funnel is removed carefully by hand, the powder will be left as a more or less cone-shaped pile. Flowability is expressed as the angle of repose, measured by means of a handheld goniometer. The normal range for spray dried powders is from 30 degrees or less ("good") to 45 degrees or more ("poor") . (Reference is made to Cheremisinoff, N., Hydrodynamics of Gas-Solids Fluidization, (1984) , p. 6.) Residual moisture was measured by weight-loss after 6 hours in oven at 87 °C. Measurement of particle size was made by laser diffraction: The powders were distributed on a small vibrating powder-feeder, suspended in air and blown/sucked through the laser beam of a Malvern Particle Sizer Series 2600. Particle size measurements are expressed as Rosin- Rammler-Bennett (RRB) mean diameter d' and as RRB inclination parameter, n (See e.g. DIN Standard 66145 or many textbooks, including Perry's Chemical Engineers' Handbook, 6th edition (1984) p. 8-5). Caking properties are estimated by examining the crust formed on a spoonful of powder, subjected to 60% and 80% relative humidity at 20 °C for 24 hours. The obtained powder was characterised by a good flowability, residual moisture 2.0 %, no visible discolouring, RRB-particle size d'/n = 132 mum/2.9 and it was non-caking (NB) .

Claims: P a t e n t C l a i m s : 1. A method for fluid-bed drying of wet material in a drying chamber comprising an upper part and a lower part, the upper part having an outlet for dry material and the lower part comprising a substantially horizontal bottom part with a least one inlet for wet material and at least one inlet for a stream of fluidization and drying medium, said method comprising the steps of: i) providing the wet material to be dried into the drying chamber vertically through an inlet in the horizontal bottom part (definers) of said drying chamber, ii) exposing the wet material to an upward stream of fluidization and drying medium (definers) introduced into said drying chamber, said stream of fluidization and drying medium preferably being introduced through openings partly or totally surrounding (definers) said inlet for the wet material, iii) drying the wet material in the upward stream of fluidization and drying medium to obtain dry material, iv) removing the dry material from the upper part of the drying chamber through the outlet. 2. A method according to claim 1, characterized in that the wet material is in the form of paste or filter cake. 3. A method according to any one of claims 1 and 2, characterized in that the wet material is heat sensible. 4. A method according to any one of claims 1-3, characterized in that the wet material is stirred and/or disintegrated by stirring and/or disintegrating means in the drying chamber. 5. A method according to any one of claims 1-4, characterized in that the residence time (definers) for exposing the wet material to be dried to the stream of fluidization and drying medium is adjusted by adjusting the size of orifices. 6. A method according to claim 5, characterized in that said orifices are placed in the upper part of the drying chamber. 7. A method according to any one of claims 1-6, characterized in that the wet material to be dried is provided to the substantially horizontal bottom part of the drying chamber vertically upwards, preferably by means of paddles or transporting screws. 8. Use of a method according to claims 1-7 for drying wet material, preferably heat sensitive material. 9. An apparatus for fluid-bed drying of wet material with a drying chamber comprising an upper part and a lower part, the upper part having an outlet for dry material and the lower part comprising a substantially horizontal bottom part comprising means for providing an upward stream of fluidization and drying medium in the drying chamber and an inlet for the wet material to be dried. 10. An apparatus for fluid-bed drying according to claim 9, characterised in that said means for providing an upward stream of fluidization and drying medium is a slit in the substantially horizontal bottom part of said drying chamber, preferably said slit being annular and preferably said annular slit partly or totally surrounds the inlet for the wet material to be dried. 11. An apparatus for fluid-bed drying according to any one of claims 9 and 10, characterized in that said drying chamber has an upper cylindrical part and a lower conical part tapering downwards to the substantially horizontal bottom part. 12. An apparatus for fluid-bed drying according to any one of claims 9-11, characterized in that said inlet for the wet material to be dried is placed substantially in the centre of the substantially horizontal bottom part of said drying chamber 13. An apparatus for fluid-bed drying according to any one of claims 9-12, characterized in that said drying chamber is supplied with means for stirring and/or disintegrating said stirring and/or disintegrating means preferably being a coaxially placed rotating shaft with wings. 14. An apparatus for fluid-bed drying according to any one of claims 9-13, characterized in that the wet material to be dried is provided upwards to the inlet in the horizontal bottom by means of paddles or transporting screws. 15. An apparatus for fluid-bed drying according to any one of claims 9-14, characterized ' in that said upper part of the drying chamber comprises means for adjusting the upwards stream of fluidization and drying medium, said means preferably being a substantially horizontal projecting edge with adjustable orifices. 16. Use of an apparatus according to any one of claims 9- 15 for drying wet material, preferably heat-sensitive material .

Inventors: KRAGH OLE TEGLHUS, KRAGH, OLE, TEGLHUS
Applicants: APV NORDIC ANHYDRO, KRAGH OLE TEGLHUS, APV NORDIC, ANHYDRO, KRAGH, OLE, TEGLHUS
Application year: 2001
Main classification: F26B3
IPC codes: F26B3, F26B17