The present invention relates to a method and to an arrangement for determining the moisture content of a finely divided material during the process of drying of said material, as defined in the preamble of the independent method Claim and in the independent apparatus Claim respectively.

In conjunction with drying waste material, for instance organic waste and then particularly food rests and similar waste material, it is known to disintegrate the material to generally one and the same particle size and to heat the material by microwave radiation whilst stirring or agitating said material.

Microwave heating has certain advantages, of which one is that heat is distributed generally uniformly throughout the material mass. However, the material is normally stirred or agitate, in order to equalise any temperature differences in the material.

It is also known to keep the container in which the material is dried at a selected pressure, for instance a sub- atmospheric pressure, so that water present in the material will boil at a corresponding temperature. Heat is delivered to prevent the material from cooling when the water evaporates. In the case of certain materials, for instance fish rests or residues, a maximum drying temperature of about +40°C is chosen, since this enables valuable oils/fats to be won from the dried product. Such oils/fats will be destroyed

at somewhat higher temperature, for instance at a temperature of about +45°C.

The material subjected to a drying process of the aforesaid kind may vary both in nature and in homogeneity or consistency. In the case of fish residues, the material concerned may include fins, backbones, heads, etc., all of which are disintegrated to generally one and the same particle size.

Alternatively, the waste may comprise other food residues, such as bread residues, vegetable residues, meat residues and paper from paper plates, serviettes and the like.

Naturally, the disintegrating process is desired to produce a uniform particle size/fragment size, although this desire cannot always be met.

Consequently, in practice, the material mass is heated unevenly in the drying process, due to the fact that the pieces of material may have varying water contents, may vary in their ability to receive microwave energy, and may vary in their ability to emit water vapour or steam. It must also be possible to determine whether or not the material has reached a desired chosen moisture content at the final stage of the drying process. However, it is difficult to determine directly the moisture content of the material to any great degree of accuracy when using conventional moisture sensors.

This is again due to the fact that the material is inhomogenous, is heated (and cooled) unevenly, and is not thoroughly mixed. It can take a relatively long time to equalise the moisture distribution throughout the material

mass, so as to enable a conventional moisture sensor to correctly determine the moisture content of said material.

The storage life of dried material whose moisture content is substantially higher than the intended moisture content can be unacceptably low. When the moisture content is too low, this results in a waste of energy and an unnecessarily long stay time in the drying apparatus. Furthermore, the risk of the material igniting has been increased unnecessarily.

An object of the present invention is to provide a technique which will enable the moisture content of the material to be determined during the drying phase, so that the drying process can be controlled in a manner that will ensure that the material will be dried to a predetermined water content with high probability in the shortest possible drying time and within a predetermined temperature range.

This object is achieved by the method set forth in the accompanying independent method Claim. The object is also achieved with an arrangement according to the accompanying independent apparatus Claim.

Further embodiments of the invention will be apparent from the dependent Claims.

The invention enables the time variation of the magnetron power in maintaining the material drying temperature within a chosen temperature range to be utilised to predict when the supply of heat to the drying process can be stopped in order for said material to have the desired moisture content after cooling.

The invention will now be described by way of example with reference to the accompanying drawings.

Fig. 1 illustrates adjustment of the power of the magnetrons for keeping the material within a chosen temperature range.

Fig. 2 illustrates a measurement of the variation in magnetron power with time.

Fig. 3 is a block diagram illustrating the most important parts of an inventive arrangement.

In drying finely divided organic waste in the form of food residues together with paper plates and paper serviettes, the material was dried conventionally by feeding the finely divided material into a drying container and there stirring said material while keeping said container at a pressure below atmospheric pressure. The container was transparent to microwave energy and the material was heated by irradiating the material with microwave radiation emitted from magnetrons. The container was provided with heat sensors for regulating the power of the magnetrons so as to maintain the temperature of the material within the chosen temperature range.

It will be seen from Fig. 1 that the power of the magnetrons was regulated by causing them either to deliver full power Po or to deliver no power at all, wherewith the temperature sensors functioned to determine the time periods tl over which the magnetrons were activated to deliver full power.

There is formed from the information according to Fig. 1 a factor f = (I tl)/t2l where z tl is the sum of the time periods tl during a predetermined time period t2 which extends up to the current time.

The factor f is thus a continuous mean value.

It will be seen from Fig. 2 that the factor f varies in time during a drying process. A limit value fo is chosen for the factor f. The limit value fo lies in the proximity of the minimum value of the factor f. We have found that a time period t3 during which f is lower than fo is indicative of the moisture content of the material. Thus, we have found that when t3 is greater than a minimum value of t3g, it is possible to continue the drying process over a predetermined time period t4 up to a final time point tx, at which the moisture content of the material corresponds to the predetermined moisture content to a surprisingly high degree.

It has been found that the inventive technique is highly effective when drying organic waste material of the aforesaid kind within a temperature range of 30-80°C, with the intention of achieving a predetermined moisture content in the range of 5-30%.

As will be seen from Fig. 3, the magnetron unit 1 is switched on and off by a thermostat 8 which senses the temperature of the material and breaks and makes the power supply to the magnetrons 1 at respective temperature limits T2 and T1.

A clock 2 senses and summates the time periods over which the magnetrons 1 are activated. A clock 3 determines a time

interval during the drying process. A calculator 4 calculates the factor f and delivers a corresponding signal to a device 5 that compares the signal f with a set signal fO and registers the time period t3 over which the signal f is smaller than fo. The signal t3 is compared in a device 6 with a preset time limit value t3g. When t3 is greater than t3g, the device 6 triggers a timer 7 which stops operation of the device after a predetermined time period t4 at time point ts (for instance by switching off the magnetron 1) (Fig. 2).

Fig. 3 also shows that the temperature sensors (8) sense the temperature of the contents of the container (9), which is equipped with a stirrer or agitator (10). Steam is evacuated to a condensation container (14) via conduit means (13) and a condenser (12), and a pump (11) is provided for maintaining a selected container pressure. The pump (11) is preferably connected via the tank (14).