Drying is an important and indispensable part of the production process in many industries. The selection of drying equipment is reasonable and the quality of production directly affects product quality, production efficiency, production cost, energy consumption, labor intensity and other indicators. And a variety of drying equipment, the same material has a variety of drying methods, can use a variety of types of dryers, drying equipment, the same drying equipment can dry a variety of materials, therefore, the rational selection and proper use of drying equipment It is very important. In order to facilitate the user to choose an ideal drying device, a brief description of some related issues is given here.
Material and moisture combination
According to the ease of removal of moisture in the material, it is divided into the following two types:
(1), non-binding moisture:
The non-bonded moisture includes moisture absorbed by the material when the wetted water, pore water, and the like which are present on the surface of the material are in direct contact with the moisture. Because of the small bonding strength with the material, it is easy to remove.
(2), combined with moisture:
It includes the moisture contained in the material cells or fiber tubes and capillaries. This moisture can be subdivided into chemically bound water, physicochemically combined water and mechanically bound water. Among them, the chemically bound water mainly includes crystallization water, and the bonding strength is large, so it is difficult to remove. The process of removing the crystallization water does not belong to the drying process; the physical and chemical combination water includes adsorption, permeation and structural moisture, and the combination of adsorbed water and material is the strongest. The moisture can be adsorbed by the outer surface of the material or adsorbed on the inner surface of the material. When the moisture is combined, the heat is released. When it is removed, the heat is absorbed. The combination of the water and the material is due to the inside and outside of the material wall. The osmotic pressure is caused by the difference in the concentration of the dissolved matter, the bonding strength is relatively weak, the structural moisture is present inside the material structure, and the water is combined during the formation of the colloid, and the dissociation of such moisture can be caused by evaporation, external pressure or tissue. Destruction; mechanically combined moisture includes capillary water, etc. Capillary moisture is present in the wet material in which the fibers or fine particles are agglomerated, and its bonding strength with the material is weak.
Materials containing moisture are called absorbent materials such as wood, grain, leather, fiber and fabric, paper, synthetic resin pellets, and the like. Materials containing only non-bound water are called non-absorbent materials, such as foundry sand, various crystal particles, and the like. In terms of ease of drying, non-absorbent materials are much easier to dry than absorbent materials. The crystallization water of the material is chemically bound water, and the drying process generally cannot remove the crystal water. The binding energy of water of different structures is about 100-3000 J/mol. The different combinations of materials and moisture make the energy consumption of water removal different, which means that the heat energy required for drying is different.
According to the material under certain drying conditions, whether the water can be separated by drying method can be divided into balanced moisture and free moisture. In life, it is often encountered that some materials "return to the tide" in the humidity of the air, and these materials return to the "dry" state in the dry air. Regardless of the "return" or "drying" process, after a certain limit, the water content in the material will tend to a certain value, which is called the equilibrium moisture in this air state. The part of the material that contains more than the equilibrium moisture can be removed from the wet material during the drying process, which is called free moisture.
Drying process of wet materials
1. Drying process of wet materials
The drying conditions are the flow rate, humidity and temperature of the drying medium (usually hot air). When hot air flows stably from the surface of the wet material, because the temperature of the air is high and the temperature of the material is low, there is a heat transfer driving force between the air and the material, and the air transfers the heat to the material in a convection manner, and the material is accepted. This heat is used to vaporize the water and is constantly carried away by the air stream, while the moisture content of the material is continuously decreasing. The drying process ends when the moisture content of the material drops to equilibrium moisture.
During the drying process of the material, there are two mutual processes of heat transfer and mass transfer. The so-called heat transfer is the transfer of heat to the material by the hot air, which is used to vaporize the water and heat the material. The mass transfer is the evaporation of the water in the material. Move to hot air to gradually reduce the moisture of the material and get dry.
2. Characteristics of the drying process:
In the drying process, since the material always has a certain geometric size, even a very fine powder can be regarded as a certain size of particles from the microscopic. In fact, the above heat and mass transfer process is in the hot air flow and material. The mechanism between the particles and the inside of the material particles is different. In the theory of drying, the heat and mass transfer process is divided into the heat and mass transfer process of the hot gas flow and the surface of the material and the heat and mass transfer process inside the material. Because the two processes affect the drying process of the materials, the two play different leading and restraining effects in different drying stages, which leads to the faster and stable of the previous stage when the general wet materials are dry. The speed is carried out, while the latter stage is carried out at a slower and slower speed, so we divide the drying process into a constant-speed drying stage and a slow-down drying stage.
(1) Constant velocity drying stage
In the constant-speed drying section, the velocity of the internal moisture of the material diffuses to the surface, so that the surface of the material is sufficiently wetted, that is, the moisture content of the surface is greater than the maximum moisture absorption capacity of the drying medium, so the drying speed depends on the surface gasification speed. In other words, the constant velocity section is the stage of gasification control. Since the drying conditions (airflow temperature, humidity, speed) remain basically the same, the drying and dehydration speeds are also basically the same, so it is called the constant-speed drying stage. The heat and mass transfer process between the hot gas flow and the material surface plays a role in this stage. Leading role. Therefore, increasing the airflow speed and temperature and reducing the air humidity are all beneficial to increase the drying speed in the constant speed phase. Almost all of the heat absorbed by the material in the constant velocity stage is used to evaporate the water, and the material is rarely heated, so the thermal efficiency is high. It can be said that the dehydration in the constant velocity section is relatively easy, and the moisture removed is purely unbound moisture.
(2) Slow down drying stage
As the moisture content of the material decreases continuously, the migration speed of the internal moisture of the material is less than the gasification speed of the surface of the material. The drying process is restricted by the heat and mass transfer inside the material, and the drying speed is slower and slower. The drying stage has the following characteristics:
The drying rate of the deceleration section is related to the moisture content of the material, and the lower the moisture content, the smaller the drying rate. This is the first feature that is different from the constant velocity segment;
The drying rate of the deceleration section is closely related to the thickness or diameter of the material. The thicker the thickness, the smaller the drying rate. This is the second feature;
After the start of the deceleration phase, as the drying rate is gradually reduced, the heat transferred from the air to the material, in addition to being used as vaporized moisture, will increase the temperature of the material until it finally approaches the temperature of the air. This is the third feature;
The moisture in the deceleration section is vaporized inside the material and then diffused to the surface in the form of steam, so the drying rate in the deceleration phase is completely dependent on the diffusion rate of moisture and steam inside the material. Therefore, the slowdown section is also referred to as the internal diffusion control phase. This is the fourth feature.
In the slowdown phase, the key to increasing the drying speed is no longer to improve the conditions of the drying medium, but to raise the problem of the moisture diffusion rate inside the material. Increasing the temperature of the material and reducing the thickness of the material are all effective methods. This is the fifth feature.
Relative to the constant-speed drying stage, the drying and dewatering of the deceleration section is much more difficult and the energy consumption is much higher. Therefore, in order to improve the drying speed, reduce the energy consumption, and ensure the quality of the product, in the case of the production process, the method should be adopted to reduce the geometrical size of the material by means of breaking, crushing and cutting, so as to facilitate the drying process.
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