A METHOD AND A PLANT FOR HYGIENIZING AND DRYING ORGANIC WASTE

Abstract: A method and plant for sanitising and drying water-containing organic material, in which finely divided material is poured into a treatment chamber that is transparent to microwave energy and heat is delivered from magnetrons (19) disposed outside the chamber (1). The material is kept at a prescribed temperature over a prescribed time period during the sanitising phase. If the temperature is required to be higher than 100 DEG C, the chamber (1) is set to a pressure that corresponds to a water boiling point that is in accord with at least the required sanitation temperature. The pressure in the chamber is lowered in the following drying phase, so as to set the water boiling point to a temperature that lies substantially beneath 100 DEG C, such as to ensure that material heated during the drying phase will be cooled by the water vapour produced and therewith minimise the risk of ignition.

Description: Λ method and a plant for hygienizing and drying organic waste The present invention relates to a method and to plant for sanitising and drying organic material in accordance with the preamble of the accompany independent method and independent apparatus Claims respectively. It is known to dry organic material, particularly organic waste such as fish waste, food waste, butcher's waste and similar waste, by finely dividing the material and then expelling water vapour or steam from the material in a treatment chamber. The water vapour condenses in a water container that is separate from the material treatment chamber. Water is expelled from the material by applying heat. This heat is applied, either totally or partially, by microwave radiation, as heat will then be taken-up uniformly by the waste. Once the waste material has been dried to a predetermined moisture content, it is often possible to store the material sanitarily over a given period of time without requiring special storage conditions to this end. This considerably facilitates handling of organic waste deriving from restaurants, delicatessen stores and such establishments. In some cases when the organic material to be dried includes a valuable heat-sensitive constituent, such as fish residues for instance that contain valuable fats and oils which are destroyed at temperature above about +40°C, it is known to dry the material at a relatively low temperature below the destruction temperature of said valuable constituent, therewith enabling such constituents to be won from the dried material. However, when the material to be dried is an organic waste material with which there is the risk of dangerous quantities of harmful micro-organisms developing in the dried material within a relatively short space of time, it is also necessary to sanitise the material. Because it will not normally suffice to dry the material to a given degree of dryness, the material must also be subjected to a prescribed sanitising process. The regulations concerning such sanitation can vary in relation to the properties of the material being treated, its end use, local hygiene regulations, and like criteria. One example of such prescribed sanitation that has low temperature requirements involves maintaining the material at a temperature of at least +70°C over a time period of at least one hour. However, the requirement may often involve a much higher temperature. Consequently, when an organic material to be dried must also be sanitised there is no reason to maintain a low pressure, and therewith a low temperature, in the drying chamber. On the contrary, it is necessary to maintain the pressure at such a high level that boiling water in the material will have a temperature that exceeds the requisite sanitation temperature . The invention can be applied to enable organic waste material, such as food residues or rests, meat rests and fish rests, to be stored over a longer period of time without need of refrigeration. This facilitates transportation of the dried material and enables the use or dumping of said material to be delayed. Such dried and sanitised material can be used as soil improving agents or similar agents. Other materials that do not pass directly to a recipient, however, can be used to generate biogas in digester gas plants. In this latter case, however, such waste material, including sewage sludge and the like, cannot be transported easily to and stored in a biogas plant. However, handling of such material in conjunction with such reuse is greatly facilitated when the material is dried and made hygienic. Accordingly, the object of the present invention is to provide an effective technique for both sanitising and drying organic material, particularly organic waste material, in which the material is preferably at least partially heated with the use of microwaves, both during the drying and the sanitation process. This object includes the provision of a process in which the process steps are carried out in a manner such as to minimise energy and time consumption in the process and to enhance process safety. This object is achieved by the invention defined in the accompanying independent Claim 1. Further embodiments of the invention will be apparent from the accompanying dependent Claims. In accordance with the invention, it is proposed that the material is first sanitised by maintaining said material at or above a predetermined relatively high temperature over a predetermined period of time in accordance with sanitary regulations. During sanitation of the material, water vapour extracted from the material may conveniently be removed from a chamber in which the material is contained during sanitation, wherewith the water vapour is normally condensed by cooling and collected as condensate in a condensate container. The material is dried after being sanitised. The material is heated by microwaves at least to a substantial extent during both sanitation and drying of the material. The sanitised material is dried preferably at a sub-atmospheric chamber pressure, at least in the final stage of the drying process. Since the material has been kept at a high temperature over a long period of time, the properties of the material cannot be impaired by continuing the drying process at a temperature in the region of the sanitising temperature, and consequently there is, in principle, no reasons to effect the drying process at a temperature below the sanitising temperature. However, we have still found it suitable to lower the pressure in the material treatment chamber during the drying process, and therewith also lower the chamber temperature. This is because the inflammability of the material increases with diminishing moisture contents . Another reason is because there is a greater risk of the dried material being ignited by a hot particle. A hot particle can be created by, for instance, a flake of aluminium foil that is heated to a very high temperature by microwave energy. The moisture content of the material will be so low at the end of a drying operation as to prevent said foil flake from being cooled sufficiently by the surrounding material and the flake will thus constitute a high ignition risk. By drying the material at a low pressure, the moisture in the material will convert to vapour at a corresponding low temperature therewith increasing the ability of the material to cool a hot particle. The process may conveniently include the step of rapidly lowering the chamber pressure during the process, so as to facilitate the departure of moisture from the material. When sanitising moisture-containing material, it may be desirable to maintain the material at a temperature higher than 100°C. This can be readily achieved, by maintaining the chamber at a corresponding elevated pressure at which water will boil at the desired temperature level. When the material is maintained at an elevated pressure, the pressure within the cells of the organic material will also increase. When the pressure is lowered to atmospheric pressure, or to a sub- atmospheric drying pressure, there is an increased probability that the cell walls will burst, thereby facilitating the drying pressure by virtue of the fact that water is able to depart from the interior of the cells more easily. This bursting effect can be enhanced by rapidly lowering the pressure. Heat can be more favourably spread throughout the material in the sanitising chamber/drying chamber by using an agitator or stirring device that moves within the chamber. This agitator or stirring device may be adapted to deliver heat to the material as it agitates said material, through the medium of heat exchange in a known manner. The agitator or stirring device may be comprised of material that will be heated by the microwaves and then transfer heat to the organic material upon contact therewith. The drying chamber may suitably be comprised of a material that is transparent to microwaves, such as polyethylene, and magnetrons that include waveguides may be placed outside the drying/sanitising chamber. The invention will now be described in more detail by way of example and with reference to the accompanying drawing, in which Fig. 1 is a schematic illustration of plant for carrying the inventive method; and Fig. 2 illustrates part of one variant of the plant. The illustrated plant includes a microwave transparent container 1 , which has a bottom opening 4 and which can be closed by means of a lid or cover 5. An agitator or stirring device 6 is mounted in the container 1. A number of magnetrons that include waveguides are disposed around the container 1. The illustrated plant includes an infeed container 8 which has a bottom opening 9 and a material disintegrating device 10 accommodated in the lower part of said container. The container 8 can be moved between a rest position in which it is distanced from the filling opening of the container 1 to a working position in which the outlet opening of the infeed container 8 is located above the opening 2 , with the lid 3 opened, wherewith the opening 9 of the infeed container is located in the proximity of the opening 2. Material fed into the container 8, for instance through its upper open end, is disintegrated by the disintegrating device 10 and fed into the container 1 with openings 9, 10 in register with one another. It will be seen that the upper part of the container 1 is connected by a conduit 11 to a condenser 12 which, in turn, is connected to a condensation tank by means of a conduit 13. The closed container 1, the conduit 11, the condenser 12, the conduit 13 and the condensation tank 14 together form a closed system. It will also be seen that the upper part of the condensation tank 14 is connected to a pump 15, which is used to generate an overpressure or a subpressure in the system. The condenser may be cooled with ambient air with the aid of a blower 20 or some similar device. The condensation water tank may include a bleed pipe and associated shut-off valve connected thereto. A space into which and from which an upwardly open vessel 30 can be moved is provided beneath the container bottom 5. Material treated in the container 1 can be emptied into the vessel 30, by opening the bottom cover 5. The cover 5 may conveniently be displaceable in its own plane, so as to expose the bottom opening 4. Waste prepared food products, or delicatessen, can be sanitised and dried in the following manner. Waste delicatessen is placed in the infeed container 8 and the container then moved until its bottom opening 9 is located immediately above the exposed filling opening 2 of the container 1. The waste disintegrating device is then activated and the disintegrated waste falls down into the container 1. The infeed container 8 is then moved to one side and the cover 3 closed. The agitating or stirring device is then activated, as are also the magnetrons 19. The pump 15 is activated so as to establish in the container 1 a pressure that exceeds atmospheric pressure, wherewith the boiling point of the water rises to a prescribed sanitation temperature above 103°C. Sanitising heat can then be maintained over the recommended time period. Normally, higher temperatures are required for shorter processing times. Alternatively, or additionally, the material may be maintained at a prescribed temperature, for instance a temperature of 70°C or higher, over a prescribed period of time, for instance over an hour. The chosen temperature limit is facilitated and ensured during this time period, by being able to supply heat to a relatively large extent at the same time as the temperature becomes generally self-regulating and constant in correspondence with the set pump pressure, at least for as long as the material still contains substantial amounts of water. When sanitisation of the material is complete, the pump 15 is switched to a mode in which a sub-atmospheric pressure is established in the system 1, 11, 12, 13, 14. The temperature is preferably lowered relatively quickly, so as to cause the cell walls of the material sanitised in the container 1 to burst to the greatest possible extent. The material is then dried at a temperature corresponding to the boiling point of the water at the set pressure, for instance 50°C. By setting a water boiling point of about 50°C in the container 1, the material can be kept at this temperature for as long as a significant amount of water remains in the material. Because the material has a temperature in the order of 50°C, it is still able to cool down hot particles, for instance hot flakes of aluminium foil. These particles would otherwise be heated by the microwaves to such an extent as to be capable of igniting the material when it has a relatively low moisture content. The drying process is stopped when the moisture content of the dried material has reached a predetermined threshold level, which can be detected conventionally. In practice, at least the final stage of the drying process is continued at the lowest possible pressure, for instance at a pressure that corresponds to a water boiling point of 50°C. The energy supply (the microwave effect) is switched-off when the material is judged to have reached the predetermined moisture threshold, although stirring or agitation of the material is continued for a further two minutes . Tests have shown that effective cooling of dangerously hot areas in the material (e.g. capsules, aluminium flakes and the like) can be achieved in this way with a surprisingly high degree of safety, so as to effectively minimise the risk of ignition. When all or part of the material being dried has relatively high sugar contents, in addition to water, there is a danger that there will be released a sugar-water solution that becomes sticky, gooey and tough and has a tendency to adhere strongly to the agitator/stirring device and the container walls. This can occur in particular when the material contains relatively high proportions of fruit and/or vegetables. In such cases, it can be beneficial to supply the drying material with a liquid absorbent, such as a fine fraction of paper fibres. Such fibres are in themselves essential sterile and are safe to use as animal fodder additives, therewith enabling dried material that includes such additives to be used as animal fodder if so desired. Alternatively, the material can be dumped. An addition of the aforesaid absorbent substance will also prevent sticky sugar residues from fastening to the various surfaces of the treatment vessel. The aforesaid absorbent substance may be added conveniently at the beginning of the drying process. The plant may conveniently be housed in an enclosure that is kept at a slight subpressure relative to atmospheric pressure, wherewith the enclosure atmosphere is driven or recycled through an aroma filter, for instance through a filter comprised of active carbon or some similar substance, so as to minimise the risk of contaminating the surroundings with less attractive odours. In one tested embodiment of the inventive plant, the stirring device/disintegrator in the vessel 1 was comprised of stainless steel (for instance austenitic or ferritic steel), this applying particularly to the wings of the stirring device. It has been found that the stirring device is not heated to the same high temperature that was feared during operation with microwaves, and that the wings effectively transfer sensible heat to the material. Because the material is in a dry, powdery state at the end of the drying process, we have found it possible, and convenient, to empty material from the container from above via the filling opening of the container 1 with the aid of a subpressure via a manually manageable nozzle 71 which communicates with a particle separator 8 preferably through the medium of a flexible hose 72, said particle separator 8 being in communication with a suction fan 79, as illustrated in Fig. 2. The separator 8 is able to collect the particles in a bag or sack 77 which can be removed when full and replaced with an empty bag. The separator 8 may include to this end a removable lid 74 and a perforated bag holder 75, 76 which functions to hold the bag 77 spaced from the separator vessel 73. An evacuation line 78 connects the air outlet chamber of the separator 8 to the suction fan 79. Alternatively, the separator 8 may include a cyclone separator of known design. The shaft 62 of the stirrer 6 may extend into the container 1 via its bottom and the lower end of said shaft 62 may be coupled to a drive motor 61, either directly or indirectly. One advantage afforded by the Fig. 2 embodiment is that sealing problems that are likely to occur at the bottom opening of the container 1 of the Fig. 1 embodiment are avoided, at the same time as the filling opening of said vessel need not accommodate stirrer drive means.

Claims: CLAIMS 1. A method of sanitising and drying water-containing organic material, wherein the material is introduced into a treatment chamber and heated to a sanitation prescribed temperature and maintained at this elevated temperature over a sanitation prescribed time period, and wherein the material is dried to a prescribed moisture content, characterised by first sanitising said material and then drying said material by microwave heating whilst maintaining said chamber at a reduced pressure, wherein water vapour or steam generated during the drying process is led away from the treatment chamber. 2. A method according to Claim 1, characterised by maintaining the treatment chamber (1) at a pressure higher than atmospheric pressure during the sanitising process. 3. A method according to Claim 1 or 2, characterised by rapidly lowering the overpressure in the treatment chamber to a drying pressure. 4. A method according to any one of Claims 1-3, characterised by introducing the material in a disintegrated state into the container (1) and stirring or agitating said material during the sanitising and/or drying process, and by causing heat to be transferred from the stirrer/agitator to said material. 5. A method according to any one of Claims 1-4, characterised by providing the stirrer/agitator with heat emitting devices that are heated by microwave energy delivered by magnetrons (19) disposed outside the container (1). 6. A method according to any one of Claims 1-5, characterised by bringing the treatment chamber to a pressure at which water will boil at the desired sanitising temperature and/or drying temperature; and delivering thermal energy to the material such as to prevent cooling of the material beneath the current boiling point of said water. 7. Plant for sanitising and drying water-containing organic material, comprising a treatment chamber (1) for receiving said material, wherein said chamber (1) has walls that are microwave transparent, wherein magnetrons (19) are disposed outside the container (1), and wherein the treatment chamber is provided with conduit means for withdrawing water vapour or steam given off by the material, characterised by a gas pump (15) which is connected to the container (1) for establishing therein a pressure below atmospheric pressure in a material drying operation following the sanitising operation. 8. Plant according to Claim 7, characterised in that the pump (15) is adapted to establish a pressure higher than atmospheric pressure in the chamber (1) during the sanitising phase. 9. Plant according to Claim 7 or 8, characterised in that the pump (15) is adapted to establish a rapid drop in the pressure of the container (1) after the sanitising phase. 10. Plant according to any one of Claims 7-9, characterised in that the pump (15) is adapted to set the treatment chamber (1) to a pressure at which the water boiling point corresponds to the desired sanitising temperature and drying temperature of the material respectively; and in that the material is dried at a temperature below 60°C.

Inventors: BLOM KNUT, MICKOS KAJ, BLOM, KNUT, MICKOS, KAJ
Applicants: BLOM KNUT, MICKOS KAJ, MILPROC MILJOEPROCESSER AB, BLOM, KNUT, MICKOS, KAJ
Application year: 2000
Main classification: F26B3
IPC codes: A61L2, F26B5, F26B7, F26B11, H05B6