Four major causes of microwave vacuum drying

At present, the application of microwave vacuum drying is becoming increasingly widespread. The following discusses several important factors that affect the effectiveness of microwave vacuum drying:

1. Product type and size: The type and state of the product vary greatly, and the process of microwave vacuum drying is not fixed. In fact, during the microwave vacuum drying process, porous materials gradually form internally, and the internal thermal conductivity begins to weaken, meaning that the material gradually becomes a poor thermal conductor. With the development of microwave vacuum drying technology, its internal temperature will be higher than the external temperature. The larger the material volume, the greater the internal and external temperature gradient. Internal heat conduction cannot balance the temperature difference generated by microwaves, resulting in temperature gradients reaching unacceptable levels. Therefore, the material should be processed into smaller particles or flakes in advance to improve the drying effect. Powder products have their own characteristics during microwave drying. When they pile up together, they should not be seen as many small particles, but as a whole. Special attention should be paid to the temperature difference inside and outside the material layer. In general, when the material appears in a larger form and approaches the deceleration drying period, it is necessary to reduce the microwave power to effectively reduce the temperature difference between the inside and outside, but vice versa, it is necessary to extend the drying time.

2. Equipment vacuum degree: The lower the pressure, the lower the boiling point temperature of water, and the faster the diffusion rate of water in the material. The vacuum degree of a microwave vacuum resonant cavity is mainly limited by the strength of the breakdown electric field, because in a vacuum state, gas molecules are easily ionized by the electric field, and the breakdown electric field strength of air and water vapor decreases with the decrease of pressure; The lower the frequency of electromagnetic waves, the smaller the breakdown electric field strength of the gas. Gas breakdown is most likely to occur at the coupling port of microwave energy and the location of field strength concentration. Breakdown discharge not only consumes microwave energy, but also damages the device, resulting in significant microwave reflection and shortening the service life of the magnetron. If breakdown discharge occurs on the surface of food, the food will be charred. Generally speaking, a field strength of 20kV/M can penetrate food (with different dielectric constants). Therefore, choosing the appropriate vacuum degree is very important. The higher the vacuum, the better. The higher the vacuum, the greater the energy consumption, and the greater the possibility of breakdown discharge.

3. Equipment power: Microwave can selectively heat materials. Water is a substance with strong molecular polarity and is easily heated by microwaves. Therefore, the higher the water content, the easier it is to absorb microwaves and the faster the heating. When the water content decreases, the ability to absorb microwaves correspondingly decreases. Generally, in the early stage of drying, the moisture content in the material is high. The input microwave power has a higher impact on the drying effect, as shown in Figure 1. Continuous microwave heating can be used. At this time, most of the microwave can be absorbed by water, and the water quickly migrates and evaporates; During constant and slow drying periods, as moisture decreases, less microwave energy is required, and pulse gap microwave heating can be used

4. Equipment working time: The selection of microwave vacuum drying time is very important and is also influenced by many factors. The effect of microwave time on the vacuum microwave drying of pineapple powder was studied. Under a vacuum degree of 98.2~99.2 kPa, due to insufficient absorption of a large amount of microwave energy by water molecules inside the material and insufficient heat source, the moisture content of the material does not change significantly during the initial drying stage. As the drying continues, the polar molecular vibrations inside the material increases, and more energy is converted into heat, thus promoting the movement of water molecules, and the water content of the material has changed greatly. In the later stage of microwave vacuum drying, porous materials gradually form, the internal thermal conductivity begins to weaken, and the water content tends to stabilize. In addition, the drying time is also affected by the moisture content requirements of the finished product. The finished product is generally dry, and the moisture content can be controlled between 3% and 5%. If the requirement is as low as 1% or less, the drying time should be extended accordingly.