Abstract:
An apparatus for puffing granular material, in particular cereals and legumes, has a heating apparatus for preheating the granular material and a puffing reactor for puffing the material. For effective, fast, and uniform heating of the material, it is proposed that: the heating apparatus have a free jet fluidized bed without a flow impact floor in which a batch of the material to be heated can be acted upon, in a preheating operation synchronized with the puffing process and proceeding batchwise, by a heat-carrying gaseous medium.

Description:
CROSSREFERENCE OF PENDING APPLICATION 
     This application is a continuation of pending international application PCT/EP99/00797 filed on Feb. 6, 1999. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a device for puffing granular material, in particular cereals and legumes, having a heating apparatus for preheating the granular material, and having a puffing reactor for puffing the material. 
     The invention further concerns a method for puffing granular material, in particular cereals and legumes, in which the material is first heated in a heating apparatus, and the heated material is then conveyed to the puffing reactor. 
     2. Related Prior Art 
     A device and a method of this kind are described in DE 195 21 243 C1. 
     “Puffing” is understood to mean a treatment method for a granular material, in particular cereals and legumes, that is steam-treated under applied pressure and, when the pressure is abruptly discontinued, is inflated into looser masses. The products are marketed as, for example, puffed wheat, puffed rice, puffed corn, puffed beans, etc. It is also possible to treat other granular material, for example tobacco, in this fashion. 
     In the 1930s, the facilities were configured in such a way that the material to be treated was introduced into a horizontal rotating cylinder. First the material in the rotating cylinder was heated with gas flames until the steam pressure had risen to a specific value. Then heating with the gas flames was continued until a pressure of approximately 12 bar existed inside the cylinder. The cylinder was then abruptly opened, so that the material shot out and inflated. 
     Since this operation, which takes several minutes, is relatively time-consuming, in a development of this technology the preheating operation and the actual puffing operation were performed separately, so that these operations could take place concurrently. 
     In the case of the document cited initially, the material to be treated is first heated in a heating chamber, in which it is preheated to a preheat temperature of approximately 100° C. From the heating chamber, the material is conveyed into a holding container, where the material rests until it is transferred into the puffing reactor. 
     A similar device is known from GB-B-2 186 180. In this, the heating chamber is configured as a rotating chamber that is heated from outside with gas flames. The material to be heated is continuously passed through the rotating chamber, then drops out of it into a funnel-shaped holding container from which it is conveyed, via a screw conveyor, to the upper charging end of the puffing reactor, where it is once again temporarily transferred into a hopper. 
     A further design is known from EP-B-0 061 229, in which the material passes through several sieve-tray-like preheating chambers and is then transferred into hoppers, out of which the preheated material is then transferred into the actual puffing reactor. 
     A device for conditioning soybean fragments is known from CH 656 775 A5. In a fluidized bed system, the soybean fragments are fluidized using air that is introduced through a diffusion floor fitted with numerous nozzles. Heatable heat exchangers are arranged in the reaction space. The fluidized bed system operates continuously, i.e. soybean fragments are continuously introduced and discharged. Residence time is approximately 4 to 8 minutes. 
     The placement of heat exchangers in the reaction space interferes with fluidization and is hygienically dubious. The physical configuration of the diffusion floor is complex, and the long residence time and continuous operation are not suitable for combining with a puffing reactor that operates with short cycle times of 30 to 90 seconds. 
     In the case of DE 195 21 243 C1 cited initially, the puffing reactor is configured so that rapid and uniform heat distribution and heat transfer to the material present in the puffing reactor is accomplished, so that very short puffing cycles, in the range of 30 to 90 seconds, can take place. 
     This makes considerable demands in terms of the speed and uniformity with which the material is preheated in the upstream heating chamber. 
     Indirect application of heat to the material in the heating chamber, for example by the fact that the chamber wall of the rotating heating chamber is heated from outside with gas flames and that heat is transferred from the heated wall to the material moving along the inner side, requires a certain amount of time and is associated with high heat losses. Uniform heating of the material is also not always guaranteed, since rotating masses of material mix together in relatively uncontrolled fashion, so it is entirely possible for outer regions, which are in direct contact with the hot heating chamber wall for longer periods, to be more strongly heated than portions of the material located at the core of the rotating mass of material. 
     Maximum uniformity in the heating of the material is, however, a prerequisite for a uniformly good puffed product, since during the short heating period in the puffing reactor there is insufficient time available to completely equalize temperature differences in the batch of material. 
     It is therefore the object of the present invention to provide a remedy for this problem, and to improve a device and a method of the kind cited initially in such a way that rapid, uniform, and efficient preheating of the material can be achieved. 
     SUMMARY OF THE INVENTION 
     According to the present invention, the object is achieved by a device by the fact that the heating apparatus has a free jet fluidized bed without a flow impact floor in which a batch of the material to be heated can be acted upon, in a preheating operation synchronized with the puffing process and proceeding batchwise, by a heat-carrying gaseous medium. 
     In the case of a method, the object is achieved by the fact that a batch of the material is fluidized in a free jet fluidized bed without a flow impact floor, in a preheating operation synchronized with the puffing process and proceeding batchwise, using at heat-carrying medium, and is thereby uniformly heated. 
     The term “free jet fluidized bed without a flow impact floor” is understood to mean a design in which a batch of the material to be heated is blown up, by a powerful jet of the heat-carrying medium, into a jet-shaped fluidized bed in which no mechanical obstacles are present, so that the jet shape can develop unrestrictedly. A floor is not present, since its cross section serves as the air delivery opening. 
     The provision of a free jet fluidized bed allows direct and intensive contact between the heat-carrying medium and the material, so that efficient heat transfer can take place very rapidly without heat losses, i.e. without heating any heat-transferring walls. 
     Because the material is fluidized in a free jet fluidized bed by the heat-carrying gaseous medium, the individual particles of material are located relatively far apart from one another, so that the gaseous heat-carrying medium can flow completely around each individual particle of material, which again contributes to efficient and in particular to rapid and uniform heating. 
     This is even further promoted by the fluidizing operation, i.e. the high relative velocity between the heat-carrying gaseous medium and the fluidized material ensures rapid and uniform heating. 
     Uniform and constant conditions are present in the free jet fluidized bed, so that an entire batch, i.e. an entire charge of a puffing reactor (for example, 20 kg), can be uniformly and rapidly heated. 
     This rapid and highly effective heat transfer makes it possible, within the short puffing cycle times of approximately 90 seconds that are attainable, to deliver the material to the free jet fluidized bed, establish the fluidized bed, transfer the heat, and deliver the heated material to the puffing reactor, so that the time required for the actual heat-transfer operation in the fluidized bed is, for example, only approximately 80 seconds. 
     Because the procedure is timed to coordinate with the puffing reactor, there is no need for the heated material to stand or wait in holding containers between the heating chamber and the puffing reactor. This offers the considerable advantage of thereby preventing uncontrolled heat-initiated reactions from taking place in the heated material. One such reaction, for example, is the so-called Maillard reaction. This exothermic reaction, which is initiated by heat, results in undesirable browning of the material. Once the Maillard reaction has been initiated, it proceeds very rapidly because of its exothermic nature, and undesirable chain reactions can occur. This is now prevented by the fact that protracted holding periods are eliminated, and the preheated batch is immediately delivered to the puffing reactor in time with its cycle. 
     In a further embodiment of the invention, the free jet fluidized bed is configured as a vertical tube widening conically upward. 
     This feature on the one hand has the advantage that the free jet fluidized bed is constructed from physically very simple means, and its length and conical shape allow simple design adaptation to different material properties and different batch sizes and moreover make possible optimum development of the jet-shaped fluidized bed. The widening creates a defined zone in which the velocity of the blown-in and expanding jet has slowed sufficiently that the material separates from the gaseous medium and remains behind in the fluidized bed. 
     In a further embodiment of the invention, the free jet fluidized bed is arranged in a circuit in which the heat-carrying medium is circulated. 
     This feature offers the considerable advantage, in terms of process engineering, that the circulation system makes possible efficient and energy-saving heat delivery and transfer. 
     In a further embodiment of the invention, a heat exchanger and a circulating fan are arranged in the circuit. 
     The advantage of this feature is that by way of the heat exchanger, the necessary heat can be delivered directly to the circuit and to the medium circulating therein, so that here again operation is very thermally efficient. The circulating fan allows flexible adaptation to the material, i.e. individual adjustments can be made to the nature, size, and quantity of the material to be fluidized, so that in each case the most efficient heat transfer is attained in the shortest possible time. 
     In a further embodiment of the invention, vent valves are arranged on the delivery side and intake side of the circulating fan. 
     The advantage of this feature is that portions of the gaseous medium can be withdrawn via a venting valve that is provided on the delivery side, in order to remove moisture from the circuit. Fresh gaseous medium can then be fed into the circuit through an aeration valve that is provided on the intake side. 
     In a further embodiment of the invention, a cyclone separator or filter separator is arranged in the circuit. 
     The advantage of this feature is that in the circuit, the material being heated can be separated from smaller dust particles or other particles, which are entrained by the medium from the free jet fluidized bed and can be removed from the circuit in the cyclone separator or filter separator. This results later in an outstanding, dust-free end product. 
     In a further embodiment of the invention, the puffing reactor can be connected to the circuit via a branch circuit, so that the heated material can be delivered directly from the circuit to the puffing reactor via the branch circuit. 
     his feature has the considerable advantage that by switching in the branch circuit, the preheated material can be delivered by the heat-carrying medium directly to the puffing reactor. Because the product then stays in contact with the heat-carrying medium even during transfer, it cannot cool off but rather is delivered to the puffing reactor at the exact desired temperature. This is also extremely simple in design terms: there is no need to provide separate collecting and transport devices for conveying the hot material from the heating chamber into the puffing reactor. 
     In a further embodiment, the branch circuit branches off up-stream from the free jet fluidized bed, and there is arranged at the inlet of the free jet fluidized bed a control valve that in one position controls delivery of the gaseous medium into the circuit, and in a second position blocks the entry of medium into the free jet fluidized bed, which is then connected to the branch circuit. 
     This feature is extremely simple in terms of control engineering, and allows a rapid switchover from closed circulation in the circuit in order to heat the material, to the transport mode in order to convey the heated material via the branch circuit to the puffing reactor. Disconnecting the free jet fluidized bed reactor causes the fluidized bed to collapse suddenly, and the material can then be collected and delivered to the puffing reactor. 
     In a further embodiment of the invention, the inlet of the free jet fluidized bed is connected via a gravity line to a line of the branch circuit running below the inlet. 
     This feature offers the considerable advantage, in terms of process and control engineering, that gravity is utilized to convey the material into the branch circuit. Specifically, when the gas-carrying medium is no longer being introduced into the free jet fluidized bed, the fluidized bed collapses and drops under its own weight toward the bottom of the free jet fluidized bed. Because the gravity line has been provided, this material can now be admitted directly into the branch circuit line, and the material is then transferred, by way of the gas-carrying medium that in the meantime has been switched over into the branch circuit, to the puffing reactor. This is very simple in terms of control engineering and design, and can be performed effectively and with little heat loss in very little time. 
     In a further embodiment of the invention, a cyclone separator, into which the heated material is conveyed, is arranged in the branch circuit directly upstream from the loading inlet of the puffing reactor. 
     This feature offers the considerable advantage that the material is transferred into the cyclone separator in very finely distributed fashion and while the heat-carrying medium is still flowing around it, and then separates from the medium therein. 
     This means that directly prior to transfer into the puffing reactor, the material is still in intensive heat exchange with the heat-carrying medium, so that the transport path to the puffing reactor can in fact be utilized for heat exchange, at least to maintain the thermal status that has been attained. 
     In the case of the devices cited initially, which operate with a relatively slow volumetric throughput but with large volumes and large quantities of heat (for example, by way of gas flames), local overheating can trigger Maillard reactions on individual particles of material. Because of the holding period between the heating chamber and introduction into the puffing reactor, there is then sufficient time available for such reactions to proceed in uncontrolled fashion. Leaving this entirely aside, considerable temperature gradients can also occur in these holding periods between the heating apparatus and the puffing reactor, since a particle that, at the beginning of the charging operation, falls to the bottom of the holding container, which is usually configured as a hopper, remains in it much longer than a particle gradually piled on top of it. 
     This is ruled out by the present invention, since such holding periods of different length for each particle are eliminated, and the material is in any case much more uniformly heated, so that even if a holding container is desirable for reasons of safety and operating continuity, uncontrolled and undesirable reactions cannot occur in it, or at least are greatly inhibited. 
     In a further embodiment of the invention, a delivery apparatus opens into the circuit in order to deliver material that has yet to be heated. 
     The advantage of this feature is that the material to be heated can be introduced into the circuit at a favorable point, e.g. in the bottom region of the free jet fluidized bed, from which it is then, immediately after activation of the circuit, fluidized into the turbulent zone of the free jet fluidized bed. 
     In a further embodiment of the invention, the puffing reactor is configured as a vertical reactor, and the free jet fluidized bed is arranged above an upper inlet end of the puffing reactor. 
     The advantage of this feature is that the preheated material can be conveyed directly from the free jet fluidized bed into the puffing reactor, for example by briefly interrupting the delivery of air through the free jet fluidized bed. 
     The result is not only that the material is quickly transferred, for example with the aid of gravity, into the puffing reactor without the risk of cooling or other changes. Another consequence is that in the working cycles in the free jet fluidized bed, heating can continue almost until the end of a time cycle, and the heated product can be conveyed into the puffing reactor immediately after the puffing reactor has been emptied and re-closed. 
     It is understood that the features mentioned above and those yet to be explained below can be used not only in the respective combinations indicated, but also in other combinations or in isolation, without leaving the context of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described and explained in more detail below with reference to a selected exemplary embodiment, in conjunction with the appended drawings, in which: 
     FIG. 1 shows, in highly schematic fashion, an overall view of a device in an operating state in which the circuit for heating the material in the free jet fluidized bed is closed; 
     FIG. 2 shows a view, corresponding to FIG. 1, in which the branch circuit is connected in order to transfer the material out of the circuit to the puffing reactor; 
     FIG. 3 shows a highly enlarged partial representation of the device in the vicinity of the bottom of the free jet fluidized bed, in the operating state of FIG. 1; and 
     FIG. 4 shows a comparable portion, corresponding to the operating state in FIG. 2, for transferring the preheated material. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     A device for puffing granular material, specifically for puffing cereals, as shown in FIGS. 1 and 2, is labeled in its entirety with the reference number  10 . 
     Device  10  has a heating apparatus  12  and, separately therefrom, a puffing reactor  14 . 
     Heating apparatus  12  has a free jet fluidized bed  16  that is configured in the form of a vertical tube widening conically upward. 
     A line  20  leads from the upper end of free jet fluidized bed  16  to a cyclone separator  22 . 
     From the upper end of cyclone separator  22 , a line  23  leads to a circulating fan  24  which is followed by a heat exchanger  26 . 
     An aeration valve  29  for the introduction of fresh air is arranged on the intake side of circulating fan  24 . A venting valve  31  for discharging moisture-laden air is arranged on the delivery side of circulating fan  24 . 
     A line  27  leads from heat exchanger  26  to inlet  17  of free jet fluidized bed  16 . A control valve  30  is arranged at inlet  17 ; a flap valve  28  is arranged in line  27  directly upstream from control valve  30 . 
     More details concerning configuration and operation will be described later in conjunction with FIG.  3 . 
     The assemblage, self-contained in terms of lines, of free jet fluidized bed  16 , cyclone separator  22 , circulating fan  24 , and heat exchanger  26  forms a circuit  32  as indicated by the arrows in FIG.  1 . 
     A branch circuit  40  branches off directly upstream from flap valve  28 . 
     Arranged in a line  42  of branch circuit  40  is a flap valve  43 , following which a gravity line  44  constitutes a connection between control valve  30  and line  42  of branch circuit  40 . Line  42  leads to a cyclone separator  46  directly above puffing reactor  14 . Cyclone separator  46  is connected via metering slide valves  50  and  51  to the inlet or delivery end of puffing reactor  14 . 
     At the outlet, puffing reactor  14  is connected via a line  52  to an expansion chamber  54 . 
     A delivery apparatus  34  serves to deliver as-yet unheated material  29  to circuit  32 ; for that purpose, a delivery line  36  opens in bottom region  38  of free jet fluidized bed  16 . A flap valve  37  allows delivery line  36  to be opened and closed. 
     Delivery line  36  can also be connected directly to the inlet via a multiple-way valve, so that the volume of one batch can be drawn in each case. Delivery line  36  can also open directly into line  27 . 
     Device  10  operates as follows: 
     Circulating fan  24  circulates gaseous medium  25 , which in the exemplary embodiment shown is hot air at approximately 160° C., in circuit  32 . Heat is applied to the air via heat exchanger  26 . Hot air  25  is conveyed via line  27  (see also, in particular, the enlarged representation of FIG. 3) to the bottom inlet  17  of free jet fluidized bed  16 , where it shoots in as a jet over the entire inlet cross section and therein fluidizes the granular material  19  that is to be heated, as indicated by the flow arrows. A free jet-shaped gas flow develops, corresponding to the conical shape of tube  18 . 
     Flap valve  43  in line  42  is closed, flap valve  28  is open, and control valve  30  is in a pivoted position as shown in FIG.  3 . In this position, air  25  is circulated in the closed circuit  32 , thereby comes into intensive heat-exchanging contact with material  19 , and heats it to the desired temperature of approximately 120° C. in a very brief period, i.e. approximately 30 to 90 seconds. 
     Very small dust particles entrained out of free jet fluidized bed  16  by air  25  are separated in cyclone separator  22  and can be removed from it from time to time. 
     Once material  19  that is being heated has been sufficiently treated, control valve  30  is pivoted and is brought into a position as shown in FIG.  4 . At the same time, flap valve  28  is pivoted into the blocking position, and flap valve  43  in line  42  is opened. 
     The result of this; is that the fluidized bed in free jet fluidized bed  16  abruptly collapses, and material  19 , responding to gravity, falls toward the bottom or inlet  17  of free jet fluidized bed  16 . This falling material  19  is conveyed via gravity line  44  into line  42 . The heated material  19  conveyed into line  42  is then transported via branch circuit  40 , by the hot medium  25  that: is still being circulated, to cyclone separator  46 , as shown in FIG.  2 . In cyclone separator  46 , material  19  is separated from conveying medium  25 . Medium  25  separated from material  19  is returned back to the intake side of recirculating fan  24  through a line  48 . 
     The pathway shown with solid lines in FIG. 2 thus constitutes a branch circuit  40  that also passes through a portion of circuit  32 , namely circulating fan  24  and heat exchanger  26 . It is thus possible to use one and the same conveying and heat-exchanging apparatuses both to heat the material in circulating mode and to transfer the heated material; this is particularly efficient. 
     It is evident from the representation in FIG. 4 that when material  19  falls past control valve  30  and is discharged through gravity line  44  into the line  42 , control valve  30  can be brought back into the position shown in FIG.  3 . By opening flap valve  37 , it is now possible to deliver a new batch of as-yet unheated material  19  from delivery apparatus  34  through line  29 ; in this process, flap valve  28  prevents material  19  from entering branch circuit  40 . 
     Once flap valve  43  has been closed and flap valve  28  opened, material  19  is then forced into free jet fluidized bed  16  and fluidized therein. 
     The operations of heating as-yet unheated material  19  in free jet fluidized bed  16 , transferring the heated material  19  to puffing reactor  14 , and refilling free jet fluidized bed  16  with material  19  that has not yet been heated, can be effected at short time intervals using mechanically simple and therefore reliable means. 
     Material  19  is conveyed in batches, via slide valves  50  and  51 , into puffing reactor  14 , where it is then acted upon by steam and pressure in order to perform the puffing operation. After puffing reactor L 4  is abruptly opened, material  19  shoots through line  52  (see FIG. 1) into expansion chamber  54 , in which it inflates, drops to the bottom, and is further processed as product. 
     In the exemplary embodiment shown, free jet fluidized bed  16  is arranged next to puffing reactor  14 , which is configured as a vertical reactor. As a result, these components are located approximately on one plane. 
     In a further embodiment, provision is made for placing free jet fluidized bed  16  directly onto puffing reactor  14 . When the free jet fluidized bed is being emptied, the preheated material then falls directly into puffing reactor  14 .