Study on Microwave Drying of Materials with Low Water Content

In terms of microwave drying kinetics, the driving force of moisture evaporation inside the material is directed from the material to the outside, that is, the internal vapor pressure is greater than the material outer layer. This dynamic performance is especially suitable for drying materials with low moisture content. The drying time of low moisture content materials by conventional heat medium heating is very slow, which is mainly due to the heat transfer mechanism of conventional heat medium heating. It can not overcome the contradiction between heat transfer intensity and heat and mass transfer direction. If we want to improve the heat transfer intensity, we can't solve the influence of excessive heat accumulation on the material quality;

that is, we want to speed up the heat transfer rate to the material interior, but we can't solve the restriction of quality (water vapor) and its reverse contradiction and so on. Only by changing the heating mode and choosing a new heating mechanism, we can improve the drying efficiency of materials with low moisture content. 。 The drying curve shown below shows that it is very difficult to remove about 3% of the moisture from the small square potatoes by conventional heating method. The yellow line represents the hot air drying curve at 71 C, and the blue line represents the microwave heating drying curve. Obviously, microwave is especially suitable for the later drying treatment of general drying and dehydration. The unique advantages of microwave drying are as follows:

The drying layer first forms in the inner layer of the material and then expands from the inner layer to the outer layer. The reason is that microwave energy penetrates into the material and is absorbed, and its energy instantaneously turns into heat energy, which makes the whole material warm up (including the inner material and its moisture temperature). At this time, the water vapor pressure in the inner layer rises sharply, which drives the water vapor to be discharged to the surface of the material. The drying layer first appears in the inner layer of the material and gradually expands outward. The drying efficiency of microwave drying is much higher than that of conventional drying method in the drying process of materials with low moisture content (less than 5%).

3) Energy saving of microwave drying can be used to judge whether two kinds of heating equipment are energy saving or not. It is necessary to compare the total energy required by the two heating equipment when they meet the same processing requirements. The total energy consumption in the heating process of

material is as follows:

total energy dissipation P≈ P1+P2+P3

formula: P1-energy required for material heating

P2-energy dissipated in the surrounding environment of material, including energy required for heating equipment components heating

P3-energy needed for water evaporation into water gas in material.

P1 and P3 are the energy consumption necessary for drying and dehydration of materials, which is called effective energy consumption.