Abstract:
The invention relates to a method of drying a material, that includes the steps of: providing a gaseous atmosphere with superheated steam in a housing, transporting a material into the housing, and drying the material in the gaseous atmosphere. Thereafter, transporting the dried material out of the housing and extracting from the gaseous atmosphere volatile substances which escape from the material into the gaseous atmosphere, especially flavors. The invention is also direct to an apparatus for drying the extruded material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a national phase filing of PCT/EP2008/006428 filed on Aug. 5, 2009, claiming priority from DE 10 2007 037 605.9 filed on Aug. 7, 2007. 
     
    
     TECHNICAL FIELD 
       [0002]    The invention relates to a method and an apparatus for drying a material 
       BACKGROUND OF THE INVENTION 
       [0003]    Numerous materials first have to be dried before further processing or packing. Furthermore, especially if the material is a food product, the drying process must as far as possible be performed in such a way that any contact between the material to be dried and germs or other micro-organisms can be ruled out. With conventional drying methods, in which there is contact between the material and dry air, it is, however, difficult to achieve general sterility. Furthermore, the drying inevitably involves a certain loss of volatile components, which escape from the material because of the elevated drying temperature. 
         [0004]    The problem of the invention consists in providing a method and an apparatus which make it possible to dry a material efficiently without this entailing any contamination with undesirable germs and without any excessive loss of volatile components occurring. 
       BRIEF SUMMARY OF THE INVENTION 
       [0005]    From the point of view of process engineering, this problem is solved by a method of drying an extruded material, comprising the steps of: providing a gaseous atmosphere with superheated steam in a housing, transporting a material into the housing, drying the material in the gaseous atmosphere, transporting the dried material out of the housing, and extracting from the gaseous atmosphere volatile substances which escape from the material into the gaseous atmosphere, especially flavors. 
         [0006]    It can be provided for the material to be dried to a water content of less than 50% by weight, 40% by weight, 30% by weight or 20% by weight. 
         [0007]    It can also be provided for the material to be dried to an AW value of less than 0.9, 0.8, 0.6, 0.5 or 0.4. AW (Activity of Water) is defined as the quotient of water vapor pressure over the material (p) to the water vapor pressure over pure water (p 0 ) at a defined temperature: AW=p/p 0 . 
         [0008]    It can be provided for the gaseous atmosphere, at least in a horizontal partial layer, to be at a temperature of more than 100° C., 120° C., 140° C., 160° C. or 180° C. 
         [0009]    The gaseous atmosphere may be at ambient pressure or at an elevated or reduced pressure. 
         [0010]    It is preferably provided that the gaseous atmosphere is a mixture of a first component, consisting of air and/or another gas, such as CO 2 , nitrogen or another inert gas, and water vapor as a second component, and, at least in a horizontal partial layer, has a steam content of at least 50% by weight, 60% by weight, 70% by weight, 80% by weight, 90% by weight, 95% by weight, 98% by weight or 99% by weight. The gaseous atmosphere may also consist of pure water vapor. 
         [0011]    In a convenient variant, it is provided that the substances extracted are added to the material again either proportionately or in a predetermined quantity. The substances can either be added directly or mixed in or supplied in the form of a carrier material, e.g. a coating or filling, to which the extracted substances are first of all added in the form of either liquid or paste. 
         [0012]    The material can be packed after being moved out of the housing. In this context, it can be provided that substances extracted from the gaseous atmosphere are introduced into a packing container during the packing process. 
         [0013]    From the apparatus point of view, the problem of the invention is solved by an apparatus for drying an extruded material, comprising a housing for holding a gaseous atmosphere with superheated steam, the housing having an inlet port and an outlet port, a means for generating a gaseous atmosphere with superheated steam inside the housing, a first transport means for transporting material through the inlet port into the housing, a second transport means for transporting the material through the outlet port out of the housing, and an extraction unit for extracting volatile substances from the gaseous atmosphere. 
         [0014]    It is appropriate for the inlet port to be disposed at a free end of an inlet duct. 
         [0015]    The outlet port can be disposed at a free end of an outlet duct. 
         [0016]    The inlet and/or the outlet duct can extend downwards from the housing. 
         [0017]    It can be provided that a ventilation line communicates with the housing and has an exit aperture at a height below the housing and above the inlet and outlet ports and leading to the outside. 
         [0018]    Alternatively or in addition, it is possible for an extraction line to communicate with the housing and to be conducted via a fan to a condenser. 
         [0019]    The first and/or second transport means may have a perforated conveyor belt. 
         [0020]    A conduit subjected to elevated or reduced pressure can be disposed along a part-section of the first transport means, communicating with the housing, especially with the lower portion thereof. 
         [0021]    A flow guide or sealing means may be disposed between the conduit and the partial section of the first transport means in order to ensure that there is an intensive flow round or through the transport means through the gaseous atmosphere. 
         [0022]    It is also contemplated that there may be a packing station associated with the apparatus for packing the dried material. 
         [0023]    It is convenient for the second transport means to extend as far as the packing station. 
         [0024]    It is preferably provided that the extraction unit takes the form of a spinning cone column for extracting volatile substances from condensate from the gaseous atmosphere. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    Further advantages and features of the invention will become clear from the following description of a preferred embodiment, reference being made to a drawing in which: 
           [0026]      FIG. 1  shows a schematic side view of an apparatus in accordance with the invention, with which the method of the invention can be carried out, 
           [0027]      FIG. 2  shows a variant of the apparatus according to  FIG. 1 , and 
           [0028]      FIG. 3  shows density values of steam and air at different temperatures. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    The apparatus consists first of a housing  2  with a bottom wall  4 , a right-hand side wall  6   a , a left-hand side wall  6   b , a rear wall  8 , a corresponding front wall, not shown, and an upper wall  10 . The bottom wall  4  is arranged substantially horizontally. 
         [0030]    The front wall, not shown, is substantially parallel to the rear wall  8  and abuts the lateral edges of the upper wall, the side walls and the bottom wall in such a manner as to create a seal, so that, apart from the apertures, which will be explained below, the housing  2  surrounds an interior space which is enclosed on all sides. 
         [0031]    A transport means  30 , which in this example forms the first and second transport means, is formed by an endless supporting means (chain, cable, pair of chains or the like), not shown, with bowls or trays  34  held on it. The transport means  30  extends through an inlet port  40  into or beneath the bottom wall  4  upwards in the direction of the upper wall  10 , runs horizontally along the upper wall, spaced apart from the latter vertically, and then back down in the direction of the bottom wall  4  and through an outlet port  42  out of the housing. Each tray  34  has a bottom wall  34   a  and a trough-like peripheral side wall  34   b  and is intended to receive a particular quantity of the material to be dried. The trays  34  are preferably gas-permeable, in order to permit the gaseous atmosphere present in the housing  2  to pass through to the material unhindered, and may, for example, have a perforated bottom and/or side wall or consist entirely or partially of a porous material, rib mesh or the like, the size of the openings provided in the walls or the porosity depending on the material to be dried. 
         [0032]    Instead of the trays, a conveyor belt can be provided as the transport means, on which the material to be dried is located, e.g. a flat conveyor belt or one which is concave like a trough, possibly perforated or porous. 
         [0033]    The trays  34  are each conveniently hung on the carrier on a horizontal pivoting axis  36  running higher than their centre of gravity, allowing them to swing freely in pendulum fashion, so that they maintain their orientation with their bottom walls facing downwards despite the changes in direction of the supporting means. 
         [0034]    The course followed by the transport means  30  before it enters the inlet port  40  and after it leaves the outlet port  42  is dependent on the specific circumstances of the particular application. Outside the housing and before the transport means enters the inlet port, a filling means may be provided, for example, where the trays  34  are filled with the material. The transport means  30  is preferably guided in an endless loop, constantly circulating between the filling means, inlet and outlet port, though as an alternative, it can also be designed for batch operation, moving to and fro, either endlessly or as a section of specific length, with a separate beginning and end. 
         [0035]    In the embodiment illustrated, a discharge means  52  is provided, at which the dried material is emptied from each individual tray  34  via collector trays  53  and a chute  55 , if necessary with a lock  55   a  to cut off the pressure, onto a conveyor  54 , which transports the material to a packing station  60 . It can, for example, be provided that trays  34  are swivelled by 180° about their pivoting axis  36  at the discharge means  52 , so that the material contained in them falls onto the collector trays  53 . 
         [0036]    A packing conveyor  62  transports packing containers  64  such as screw-topped jars, tins, pouch packs or the like from a charging apparatus  66 , where the packing containers are delivered to the packing conveyor, first to a filling station  68  of the packing station, where the dried material is filled into a respective packing container. 
         [0037]    At an adding station  70 , extracted volatile substances, such as flavors, can be added to the packing container. The substances can be added either in the form in which they have been extracted, i.e. in pure form or dissolved in a solvent such as water. Alternatively, there is the possibility of first mixing the extracted substances, before they are added to the packing container, with a carrier material, which will preferably be a material that is additionally added to the dried material in the packing container anyway, such as a coated layer, coating, filling, cream, gravy or the like. The extracted substances may, for example, be added proportionately, i.e. in the same amount as they were released from the amount of material contained in a packing container during the previous drying process. In this way, the original content of volatile substances in the material is restored to a very great extent. Alternatively, it is possible to add a larger or smaller dose of volatile substances or to put them to some other use. 
         [0038]    At a sealing station  72 , the individual filled packing containers are sealed, i.e. with a screw-on lid or can lid, for example, or, in the case of a pouch pack by sealing. 
         [0039]    An extraction unit  80  for recovering volatile substances, such as flavors, preferably takes the form of a spinning cone column or spinning table column. This is a vertical cylinder  81 , in which an inert separating gas such as steam at normal or reduced pressure separates a vaporous stream of volatile components from a fluid input or a slurry. From top to bottom, there is an alternating arrangement of fixed conical sheets of metal  83  attached to the interior wall of the cylinder, and conical sheets of metal  87  attached to a rotating shaft  85 . 
         [0040]    The fluid input to be extracted, in this case the condensate forming in the housing  2 , is introduced into a product input  82  at the top of the column via an extraction line  84 . Under the force of gravity, the liquid flows downwards and inwards on the upper surface of the first fixed cone  83  and, on the inside, reaches the first rotating cone  87 , on which the liquid is distributed into a thin, turbulent film because of the centrifugal forces acting on it, and then flows upwards and outwards and passes from the outer edge of the rotating cone onto the next-lower stationary cone  83  until, having passed through all the cones, it reaches an outlet  86  at the foot of the column, where there is only a small content of volatile substances left in it. 
         [0041]    The inert separating gas, steam  192  in this case, which is fed in counterflow, flows through the column from bottom to top and absorbs volatile components from the condensate. When the steam enriched with volatile substances reaches the head of the column, it is condensed in a condenser  88 , so that the volatile substances are available in a concentrated form, dissolved in water, and can be delivered in the manner described to the dried material and/or a packing container or used in some other way. 
         [0042]    In order to generate the desired gaseous atmosphere with superheated steam or an atmosphere of pure superheated steam with little or no air content at all inside the housing  2 , such as is described in U.S. Pat. No. 5,711,086, there is a heating means  100  inside a conduit  102 , which leads, on the one hand, into the upper wall  10  in an opening  101  located above the transport means  30  and, on the other hand, into an opening  104  in the rear wall  8  at a certain height above the bottom wall  4 . A flow generator, in this case fan  106 , provides for a current, such that it extracts via the opening  101  and blows out into the opening  104 . 
         [0043]    Attached to the upper wall  8  beneath the opening  101  are baffle plates  130 , directed towards one other, as flow guide or sealing means which ensure that the greatest possible portion of the steam atmosphere extracted from the housing  2  through the opening  101  flows through the perforated tray or trays  34 , which are located just beneath the opening  101 . This ensures that the product located in the trays comes into intimate contact with the superheated steam. 
         [0044]    Two further baffle plates  132 ,  134  are arranged substantially parallel to the left-hand side wall  6   b  and overlapping with a slight space between them. The task of these baffle plates is, as far as possible, to prevent excessive amounts of ambient air being introduced into the housing  2  by the moving transport means  30 . A gap between the baffle plates in the vicinity of the bottom wall  4  allows air or an air-rich steam atmosphere of relatively high density to escape from the housing to the outside. 
         [0045]    Alternatively or in addition, a nozzle-like flow guide for the steam can be provided by means of a constriction, in order to cause a local increase in the flow speed and thus to achieve an intensified application and improved contact of the steam with the material to be dried. 
         [0046]    In a lower region of the housing, preferably in the region of a deepest point of the housing directly above the bottom wall  4 , an extraction line  120  leads into an extraction port  122  in the rear wall  8  and to a condenser  124 , from which condensate is conducted into a container  126 . Inside the extraction line  50 , there is a controlled fan  128 , which is controlled by a control unit on the basis of temperature and moisture or steam content information. For this purpose, at least one temperature sensor and at least one steam content sensor are disposed inside the housing to determine the condition of the steam (relative humidity and/or degree of saturation, or steam content), preferably each in the vicinity of the upper and lower walls and/or in the vicinity of the extraction port  122 . If there is any superheated steam present, it is possible in this way to determine the degree of superheating or the temperature difference relative to the saturation state. By means of an enhanced extraction of relatively moist steam, while at the same time supplying heat, the composition of the gaseous atmosphere inside the housing can be shifted in the direction of a higher content of superheated steam at a higher temperature. The extraction port can be disposed on the same level as the bottom wall, in it or above it. 
         [0047]    The gas/steam atmosphere conducted in a closed circuit via the conduit  102  and the housing  2  can be heated to a desired temperature in this way. The material entering the housing  2  causes an input of water or steam in the housing, so that, if sufficient energy or heat is delivered into the housing, the steam content inside the housing increases. By means of an appropriate open or closed-loop control of the steam extracted via the extraction line  120  and condensed in the condenser  124 , the steam content in the housing  2  can be adjusted. If there is no extraction, surplus steam is released to the outside through the inlet and/or outlet port  40 ,  42 . 
         [0048]      FIG. 1  shows an additional possibility of removing steam from the housing, the saturated steam and thus the surplus humidity being removed from the housing, instead of by extraction via the extraction port  122 , alternatively via a ventilation line  140 , or both. The ventilation line  140  is connected to three-way valve  142 , which is also connected, via a connection line  144  to the extraction line  120  and to a further connection line  146  with an opening  148  in the bottom wall  4 . The ventilation line  140  has a ventilation aperture  141  leading to the outside 
         [0049]    The three-way valve  142  can be placed in a first position, in which the lines  144  and  146  are connected to one another, while the ventilation line  140  is closed off, so that extraction is effected via the openings  122  and  148 . If so desired, a check valve can be provided in the line  144  in order to ensure that, in the first position of the three-way valve, extraction occurs exclusively via the opening  122 . 
         [0050]    The three-way valve  142  can be placed in a second position, in which the vent line  140  communicates with the conduit  146 , while the conduit  144  is sealed off and the fan  128  is switched off, so that the steam atmosphere within the housing communicates with the environment via the port  148  and the conduits  146  and  140 . 
         [0051]    The apparatus illustrated in  FIG. 1  includes guide ducts, or inlet and outlet ducts  150 ,  152 , which, in the region of the openings in the bottom wall, namely the inlet port  40  and the outlet port  42 , extend the housing downwards and terminate openly at substantially the same height. These guide ducts enclose the transport means  30  at the inlet and outlet sides and, together with the ventilation line  140 , which terminates on a higher level, ensure that surplus steam is released from the interior of the housing  2  via the ventilation line  140 , in the second position of the three-way valve  142 . The steam located inside the housing has a tendency to flow downwards through the guide ducts, but encounters relatively cold ambient air in the process, so that a substantially horizontal boundary layer forms in all the guide ducts at the level of the ventilation aperture  141 . The height h 0  at which the vent aperture  141  is located above the height of the free end of the guide channels may, for example, 10%, 20%, 30% or 50% of the height H of the housing, H referring to the vertical distance between the highest and lowest point of the interior of the housing. In addition, the height h 0  is preferably between about 30% and 70%, e.g. 50%, of the vertical extent hb of the guide ducts, beginning at the lower wall  4  or the lowest point of the housing. 
         [0052]    In all embodiments, a height hs of the extraction port  122  above the lower wall  4  of the housing or the lowest point of the housing may be virtually zero in effect, or it may be about 5%, 10%, 15%, 20% or 30% of H. A height ht at which the actual drying process mainly takes place and at which or above which the horizontal sub-layer is preferably located, in which the gaseous atmosphere is of the desired high temperature and exhibits low oxygen values, may be about 50%, 60%, 70%, 80%, 90% or 95% of the height H of the housing, measured in each case from the lower wall of the housing  4  or the lowest point of the housing. 
         [0053]    In order to accelerate or intensify the generation of the desired gaseous atmosphere, a steam feed line, not shown, can be used to introduce superheated steam directly into the housing  2 . Alternatively, it can be provided that a steam or water feed line, such as a water atomizer, leads into the conduit  102  upstream of the heating means  100 , so that by heating the steam or evaporating the water, a superheated steam atmosphere can be introduced in the region of the opening  104 . 
         [0054]    Because of the different densities of steam at different temperatures and because of the influence of any air that might be mixed in, as is shown in  FIG. 3 , it is possible and intended for vertical layers to form inside the housing, with superheated steam collecting at the top and moister steam and/or a steam-air mixture collecting at the bottom. For this reason, the transport means  30  runs partially in an upper region of the housing at height ht in the vicinity of the upper wall  10 , i.e. inside a zone of superheated steam, after which the material is then conveyed towards the outlet port  42  located on a lower level. The low position of the outlet port  42  makes it difficult for superheated steam to escape directly from the housing, which would be an undesirable loss. 
         [0055]      FIG. 2  illustrates a variant of the invention in which—unlike the embodiment illustrated in FIG.  1 —the aim is not to have a distinct horizontal succession of layers, namely a steam atmosphere that, moving from bottom to top, becomes increasingly hotter, poorer in air and oxygen and increasingly contains only superheated steam, but rather a steam atmosphere which is mixed as thoroughly as possible and homogenized within the entire housing. This is achieved in that the interior of the housing is evenly mixed with the aid of at least one circulation fan  160  ( FIG. 2  shows two of them), so that virtually no stratification or uneven mixing can become established in the vertical direction. 
         [0056]    In addition, the contact between the material to be dried and the steam atmosphere is improved with a forced circulation system consisting of a cyclone  162 , a fan  164 , a heat exchanger  166 , fans  168   a, b, c  and, connected to them, steam guide boxes  170   a, b, c . Depending on what is more appropriate, the cyclone  162 , fan  164 , heat exchanger  166  and fans  168   a, b, c  may be disposed inside or outside the housing  2 . Depending on the flow conditions, either the fan  164  or the fans  168   a, b, c  may be dispensed with. The fan  164  sucks in the steam atmosphere across the cyclone  162  upstream, in which particles originating from the material to be dried are deposited. The cyclone for its part sucks in the steam atmosphere at any suitable point or area within the housing. Downstream of the fan  164 , the steam atmosphere flows through the heat exchanger  166 , having optionally been enriched with steam beforehand by means of a steam generator  165 . In the heat exchanger  166 , heat may be supplied or removed as required, whereupon the steam atmosphere then enters the steam guide boxes  170   a, b, c  via the fans  168   a, b, c . The steam guide boxes guide the steam atmosphere through a preferably perforated conveyor belt of the transport means  30 , so that the material on it is brought into intimate contact with the steam atmosphere. 
       LIST OF REFERENCE NUMERALS 
       [0000]    
       
           2  Housing 
           4  Bottom wall 
           6   a, b  Right-hand, left-hand side wall 
           8  Rear wall 
           10  Upper wall 
           30  Transport means 
           34  Tray 
           34   a  Bottom wall 
           34   b  Side wall 
           36  Pivoting axis 
           40  Inlet port 
           42  Outlet port 
           52  Discharge means 
           53  Collector tray 
           54  Conveyor 
           55  Chute 
           55   a  Lock 
           60  Packing station 
           62  Packing conveyor 
           64  Packing container 
           66  Charging apparatus 
           68  Filling station 
           70  Adding station 
           72  Sealing station 
           80  Extraction unit 
           81  Cylinder 
           82  Product input 
           83  Fixed cone 
           84  Extraction line 
           85  Rotating shaft 
           86  Outlet 
           87  Rotating cone 
           88  Condenser 
           100  Heating means 
           101  Opening (in  10 ) 
           102  Conduit 
           104  Opening (in  8 ) 
           106  Fan 
           120  Extraction line 
           122  Extraction port 
           124  Condenser 
           126  Container 
           128  Fan 
           130  Baffle plate 
           132 ,  134  Baffle plate 
           140  Ventilation line 
           141  Ventilation aperture 
           142  Three-way valve 
           144 ,  146  Connection line 
           148  Opening 
           150  Inlet duct 
           152  Outlet duct 
           160  Circulation fan 
           162  Centrifugal cyclone separator 
           164  Fan 
           165  Steam generator 
           166  Heat exchanger 
           168   a, b, c  Fan 
           170   a, b, c  Steam guide box