Microwave drying characteristics of wood

In this paper, the dielectric absorption power of wood microwave drying is derived from the RC model of wood microwave drying, and the drying mechanism is discussed from the perspective of spectroscopy, the temperature distribution is calculated, the existing problems in microwave drying are analyzed, and the solutions are put forward.
Keywords: 

microwave dryer
Because the microwave energy penetrates into the wood interior, after the microwave energy is absorbed by the wood, the “heat accumulation” occurs in the wood interior, forming the so-called “positive heat source” of the wood interior temperature higher than the surface layer, which is different from the ordinary drying process. This is the biggest characteristic of microwave wood drying.

So we can use microwave to dry special thick wood and special materials. However, microwave drying also has shortcomings. In order to ensure the quality of products, there are many technical problems to be further studied. The investment in drying equipment is too expensive, and the cost of electric energy consumption is too high. How to improve the economic efficiency needs further discussion.

microwave wood drying. 

Study on microwave drying characteristics

Microwave drying characteristics of wood dielectric loss per unit volume cos 65507 One is to increase the voltage U, the two is the plus frequency f.

But the residual rate method is more economical than increasing voltage. Therefore,

microwave drying equipment 

is widely used to dry wood, namely wood wave drying. The absorption mechanism of microwave in wood drying process now lets us use the spectroscopic point of view, the microscopic mechanism of wood drying. Wood is made from cellulose, hemifibre and lignin. They are polymer compounds with large molecular weight and degree of polymerization. Especially for wood cellulose molecules, the degree of polymerization can reach more than 100002.

The microwave drying of wood mainly removes the water content of the material, and the water is polyatomic molecules. Because cellulose is 0% in the cell of wood, we mainly discuss the molecular and water molecules of lignocellulose when exploring the drying mechanism of microwave wood.

Because the fiber molecule is a molecule and the water is a polyatomic molecule, the rotational inertia of the molecule is larger, and the coupling of the harmonic lines is more complex than that of the diatomic partition. But their absorption mechanism for electromagnetic wave energy is still similar to diatomic molecules.

microwave drying of wood 

Mechanism of moisture movement in wood during microwave drying

Abstract: By measuring the internal temperature, pressure and stratified moisture content of wood during microwave drying, the distribution and variation of temperature, vapor pressure and moisture in wood during microwave drying were studied. During the drying process, the temperature distribution in the wood is more uniform, but in the later stage of drying, the non-uniformity of temperature distribution in the wood tends to increase; with the drying process, the moisture content gradient in the wood decreases gradually, and the moisture content distribution becomes more uniform; in the early stage of wood drying, there is moisture accumulation on the surface of the wood; in the power of 10. Under the condition of 00 W, the internal pressure of 300 mm 100 mm 65

Key words:

  microwave drying equipment, 

water movement mechanism, vapor pressure difference, Masson Pine

In conventional drying, wood microwave drying medium transfers heat to wood, and then water is exchanged to drying medium. Because heat transfer and water transfer are slow processes, the temperature of water (including free water and water vapor) in wood cell cavity increases slowly with limited energy supply, and the evaporation of free water and water vapor occur. In addition, wood cells are semi-enclosed structures, which drain part of the water through the pits on the cell wall, and to a certain extent play a “pressure relief” role. Therefore, it is generally believed that there is no total pressure difference inside and outside the wood in the conventional chamber trunk, and the water content in the wood is in the role of water content gradient (or concentration gradient). In the process of microwave drying, energy permeates directly into the interior of wood in the form of electromagnetic wave. Under the action of microwave electromagnetic field, the sharpened water molecules in wood rotate rapidly, rub against each other, produce heat, heat and dry wood. It is precisely because the energy in the process of microwave drying is in The uniqueness of generation and transfer in wood leads to the different mechanism of moisture movement between wood and conventional drying, resulting in the high speed of wood microwave drying.

wood microwave drying 

Experimental comparison between microwave drying and hot air drying

In this study, two groups of black walnut specimens were dried by intermittent radiation, 2 minutes for a radiation cycle, microwave radiation power of 200 W (second output), the actual microwave radiation time in each radiation cycle take 2 levels (35/30, 30/25). 35/30 represents the first 40 radiation cycles of microwave drying. The actual microwave radiation time per cycle is 35 seconds, the actual microwave radiation time per cycle in the follow-up radiation cycle is 30 seconds, 30/25 indicates that the actual microwave radiation time per cycle in the first 40 radiation cycles is 30 seconds, and the actual microwave radiation time per cycle in the follow-up radiation cycle is 25 seconds, and all of them are radiated at the beginning of each radiation cycle. The other group of specimens were dried by conventional hot-air drying. The dry and wet-bulb temperatures were set at 60 C and 55 C respectively in the initial stage of drying. When the moisture content was reduced to below 15%, the wet-bulb temperature was adjusted to 45 C.

microwave drying

The end and side surfaces of the specimens were sealed by J-B Weld and aluminum foil, and the aluminum foil was removed after the resin was fully cured for 48 hours.

wood microwave drying

Experiment on microwave drying of wood

According to the sectional density distribution curves of wood specimens in the initial state before drying, during drying and under dry condition, the sectional density distribution along the thickness direction can be obtained by linear interpolation and weighted average method.

In order to eliminate the influence of micro-machining defects on wood surface and the distortion of interface measurement data, the density data of 5 scan steps (thickness about 0.5 m) in the surface layer (the first and the 21st layers) were removed when calculating the average density.

microwave drying equipment, 

The moisture content distribution in the direction of wood thickness was the most uneven before drying. With the drying process, the moisture content in each layer of wood decreased, and the decrease extent of inner layer was greater than that of surface layer. When the moisture content is below 10%, the moisture content distribution in the whole thickness direction is very uniform.

In the process of microwave drying equipment, the moisture content distribution in each layer of wood is inhomogeneous, that is, the moisture content of wood from the surface layer to the central layer does not always increase, but fluctuates. Even when the wood is dried to a very low moisture content (the average moisture content is about 5%), the moisture content in the inner part of the wood appears. Under the condition of lower than the surface layer, but the difference of moisture content is within 0.5%, and not all the inner layers are lower than the surface layer moisture content, so it can be considered that there is no integral moisture gradient of low and high moisture content inside the whole microwave drying process, that is, there is no moisture gradient field contrary to conventional drying.

wood microwave drying

Dynamic distribution of water content in wood during microwave drying and conventional drying

Accurate measurement of dynamic distribution of moisture content in wood during microwave drying is the prerequisite for realizing optimal control of drying process and in-depth study of drying mechanism.Conventional hot-air drying is widely used in practical production because of its mature technology and large loading capacity. With the advantages of integral heating and fast drying speed, it has been applied to the drying of precious and Difficult-to-dry wood to a certain extent.

microwave drying

Previous studies have shown that there exists an internal high and external low moisture gradient field opposite to the temperature gradient in the wood during conventional hot air drying. Some scholars have conducted preliminary studies on the distribution of moisture content in wood during microwave drying, but the results are not consistent. Some scholars believe that the moisture content in wood during microwave drying is different. Some experimental results show that the moisture content of wood inner layer is higher than that of surface layer in microwave drying, and there is no moisture gradient field contrary to that of conventional hot air drying. In addition to the rapid moisture migration and the rapid change of moisture content distribution in wood, it is very difficult to accurately determine the moisture content. Another important reason is that an ideal method for measuring the dynamic distribution of moisture content has not been found.

wood drying 

Application prospect of microwave technology

2. Microwave drying technology has been studied and applied early in wood processing industry in China, so the principle of microwave heating and drying technology have been grasped preliminarily. Because of the high cost of microwave drying, it is mainly used for drying precious wood. The key problem of microwave drying technology is the expensive equipment, which hinders the further application and popularization of microwave technology. The solution is to carry out technical innovation and enhance the research and development ability of PLC programmable controller, magnetron, microwave transformer, frequency converter and other core components. Although it takes some time to master the core technology of microwave equipment, the industrialization prospect of microwave drying technology is very promising. Microwave drying will be the main development direction of drying technology in wood industry.

   Microwave modification technology

   microwave drying equipment

   Wood modification is an important way to improve wood properties and achieve optimum use of secondary materials. Microwave treatment can improve and overcome the defects of dry shrinkage and wet expansion, poor dimensional stability, easy discoloration, flammability, non-corrosion resistance, non-wear resistance and so on. The basic research on wood modification in microwave technology is rather weak in China, and there is no application research. Therefore, we must intensify the research on microwave modification.

   Wood microwave drying 

   
   
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