Drying is the operation of removing moisture or other volatile components from materials. It involves a wide range of dry and wide variety of drying equipment. If it is divided according to the pressure of the environment in which the material is placed in the process, it can be divided into atmospheric pressure drying and vacuum drying. The former is used more, relatively simple, and has been applied since ancient times; the latter is developed late and relatively complicated. However, vacuum drying has many advantages of its own and has been widely used in recent years, especially in the pharmaceutical, food and chemical industries. Pellets are a dosage form of the pharmaceutical industry. The drying method can be dried normally at normal pressure or vacuum dried with double cones. However, the drying effect of the two is different, and the quality of the pills dried by vacuum is obviously better.
1 advantages of vacuum drying
Vacuum drying has many advantages: (1) In the vacuum environment, the boiling point of water is reduced, and low-temperature drying can be achieved, which provides favorable conditions for the drying of heat-sensitive materials, such as some vitamins and the like, which will decompose when exposed to high temperatures. (2) During the vacuum drying process, the pressure in the drying chamber is always lower than the atmospheric pressure, the number of gas molecules is small, the partial pressure of oxygen is reduced, and the oxidative deterioration of the material during the drying process is avoided, thereby ensuring the dry material. Quality, in the state of hypoxia can kill some bacteria, inhibit the growth of some bacteria; (3) vacuum drying can eliminate the surface hardening phenomenon easily generated under normal pressure; (4) vacuum drying process can recover some toxic and harmful gases To prevent pollution to the environment.
The double cone rotary vacuum drying machine is applied to the dry pellet operation, the container is easy to clean, the dried pellets are clean and hygienic, the surface of the particles is firm, the sphericity is high, the size is uniform, the content is uniform, the water is qualified, the color is bright and consistent, and the fine powder is fine. Less, the quality of the drug is stable and the preservation time is long.
2 working principle of vacuum drying
Vacuum refers to the state of gas in which the density of gas molecules is lower than the density of gas molecules at atmospheric pressure in a given container. The degree of thinness of gas under vacuum is called vacuum. Vacuum drying is a process of indirectly heating a material under a condition of lower than normal pressure, and vaporizing the moisture in the material at a corresponding temperature to reduce the moisture content in the material.
The basic principle of vacuum drying: the saturated vapor pressure of water is closely related to temperature. Under vacuum, the boiling point of water will decrease, so the moisture in the material can be vaporized at low temperature during vacuum drying, and low temperature drying can be achieved. As we all know, under normal pressure, if the temperature reaches 100 ° C, the water begins to boil and vaporize. Why is it not easy to cook when cooking rice or other foods in the plateau? This is because the atmospheric pressure in the plateau is low and the boiling point of water is lowered. If the pressure of the container has been decreasing, when it drops to an absolute pressure of 613.28 Pa, the boiling point of the water becomes 0 °C. By maintaining a different degree of vacuum, the drying temperature of the material can be changed. The vacuum sensitivity of the vacuum drying is usually determined by the thermal sensitivity of the material. Since the vacuum pump tends to reduce the gas flow rate to 0 when the degree of vacuum tends to zero, it is not conducive to the flow of gas and the discharge of moisture. Therefore, the reading of the vacuum gauge is generally controlled below 0.095 MPa in production.
Vacuum drying is the free water in the dry material. The free water can be divided into gap water, surface water, capillary water and wrapped water. In the drying process, first, the heat source transfers heat energy to the material to heat it up and vaporizes the moisture; second, the vaporized moisture vapor migrates to the drying chamber space, so that the moisture vapor is separated from the surface of the material and discharged into the drying chamber; The moisture inside the material migrates to the surface of the material in a liquid state to facilitate vaporization on the surface. Vacuum drying is the three processes that are accompanied and coordinated. The coordination is good or bad, which directly affects the vacuum drying speed and the quality of the materials.
When the vacuum is started and heated, the drying speed is slow. When the material is heated above the vaporization temperature of the water, the drying speed will suddenly increase, and the material will be heated to the vaporization temperature under the corresponding pressure within the allowable temperature range and vaporized in a large amount. The temperature remains unchanged, the added heat is used for the latent heat of vaporization and various heat losses, and the vacuum system continuously discharges the vaporized vapor to maintain the pressure difference between the evaporation surface and the space, so that the drying continues.
The moisture content of the material is reduced to a certain extent, and the water evaporated from the material is reduced. When the heating amount is constant, the temperature of the material begins to rise, the pressure difference between the evaporation surface and the space decreases, and the drying shifts into the deceleration phase. The drying speed gradually decreases until it reaches zero.
3 factors affecting the drying speed
In the vacuum drying process, people always want to speed up the drying and shorten the drying time. However, there are many factors that affect the drying speed, such as the condition of the material to be dried, the moisture content, the density, and the viscosity. If the initial temperature of the material is increased, the moisture content of the material is lowered, etc., it is beneficial to increase the vacuum drying speed; if the temperature rises too fast, the water is too late to be discharged, and the pellets are easily agglomerated and adhered to the wall of the pot. Therefore, the working vacuum of the drying chamber is high, which is favorable for vaporization of water at a lower temperature, but the degree of vacuum is high, the fluidity of the air is deteriorated, the moisture is not easily discharged to the outside of the container, and the requirements for the vacuum device are improved. . Therefore, we should consider the influence of vacuum according to the characteristics of the material.
In addition, the structure of the vacuum drying equipment, the heating method, the experience of the operator, etc. all affect the vacuum drying speed.
4 double cone rotary vacuum drying system
The double cone rotary vacuum drying system mainly consists of a double cone rotary vacuum dryer, a condenser, a dust collector, a vacuum pumping system, a heating system, a cooling system, and an electric control system.
When the pellets are dried, the temperature cannot be too high, so it is more suitable to heat the heating system with water. The cooling system and the water heating system can share one set of water pipes to reduce the investment. If the moisture content in the pellets is water, the condenser can be omitted in the equipment; if there is not much dust, the dust collector can be simply replaced with a buffer tank.
For the double cone rotary vacuum dryer body, the cone jacket is wrapped with an insulation layer to reduce the temperature of the room. The heated water in the jacket first transfers heat to the inner wall of the cylinder, and the inner wall transfers heat to the pellets in contact with the cylinder wall, and the generated wet steam is pumped away by the vacuum suction pipe. While the cone is rotating, the pellets are continuously tumbling in the cylinder to obtain sufficient mixing, and the material to be discharged is uniform in water and color.
5 double cone rotary vacuum dryer in the application of pellet drying
Take a pellet drying operation as an example. The parameters of the pellet: dry material density 0.70g / cm3, initial moisture content 15%, final moisture content 2% ~ 3%; double cone rotary vacuum dryer dryer parameters: volume 500L, speed 4 ~ 6r / min; Heating temperature: water heating, below 100 °C.
The pellets are loaded into the double-cone pot, and the loading amount is required to be controlled below 50% (or the level of the horizontal tube is not higher than the suction tube). Cover the lid and start to rotate the double-cone body. Adjust the rotation speed to 4~6r/min. The pellets flow in the double-cone pot and get sufficient mixing.
However, the pellet drying cannot be heated by steam like other materials. There are two reasons: one is that the temperature of the steam is too high, which will affect the performance of the material; the other is that the material is directly heated at a high temperature, and the moisture in the pellet is fast. When it migrates to the surface, the surface moisture is too late to vaporize, causing the pellets to stick to each other, not to flow, and easy to form agglomerates. The entire wall of the pot will also stick a thick layer of powder and cannot transfer heat. Therefore, the use of water heating is better. According to the characteristics of the material, choose an initial water temperature, then slowly warm up, and vacuum the inside of the double-cone pot, the water generated in the pot should be discharged in time, otherwise it will agglomerate.
The drying of the pellets can be divided into four stages: the first stage is the initial stage, and the material is gradually heated. Since the vaporization temperature has not been reached, the surface of the pellets has little vaporization and the drying is slow; when the temperature reaches the vaporization temperature, Known as the second stage, the water is vaporized in a large amount, the drying speed is fast, the temperature rise becomes inconspicuous, and the degree of vacuum is lowered.
Strictly speaking, the vaporization temperature at this stage cannot be a point, it should be said to be a small range. Because the material is always in a state of heating, vaporization will take away a lot of heat, both of which will cause temperature fluctuations, while vaporization will cause a change in the degree of vacuum, which in turn causes a change in the vaporization temperature, which is a dynamic the process of. If the reading of the vacuum gauge fluctuates around 0.093 MPa, the vaporization temperature will fluctuate around 40 °C; in the third stage, as the moisture content of the pellets becomes less, the moisture that migrates to the surface becomes less, the temperature rises significantly, and the degree of vacuum Also, the drying speed is slower and slower; in the fourth stage, when the moisture content of the pellets reaches the set requirements, the material is cooled. Turn off the heating system, pass the tap water to the jacket, and cool and cool the pellets to prepare the discharge.
6 conclusion
In this paper, the advantages and working principles of vacuum drying are discussed. The factors affecting the drying speed are discussed. Combined with the practical application of the company, the application of double cone vacuum drying equipment in the field of pellet drying is expounded.
Strictly speaking, the vaporization temperature at this stage cannot be a point, it should be said to be a small range. Because the material is always in a state of heating, vaporization will take away a lot of heat, both of which will cause temperature fluctuations, while vaporization will cause a change in the degree of vacuum, which in turn causes a change in the vaporization temperature, which is a dynamic the process of. If the reading of the vacuum gauge fluctuates around 0.093 MPa, the vaporization temperature will fluctuate around 40 °C; in the third stage, as the moisture content of the pellets becomes less, the moisture that migrates to the surface becomes less, the temperature rises significantly, and the degree of vacuum Also, the drying speed is slower and slower; in the fourth stage, when the moisture content of the pellets reaches the set requirements, the material is cooled. Turn off the heating system, pass the tap water to the jacket, and cool and cool the pellets to prepare the discharge.
6 Conclusion In this paper, the advantages and working principles of vacuum drying are discussed. The factors affecting the drying speed are discussed. Combined with the practical application of the company, the application of the double cone vacuum drying equipment in the field of pellet drying is expounded.
Yunnan NBC Agricultural Development Co., Ltd deals in (heat pump air drying) Yunnan Plateau agricultural products. Including Flowers And Leaves, Fruits, rhizomes, poultry meat, aquatic products and other drying products. All products are free of additives, agricultural residues and phosphorus and sulfur. It has obtained the national SC food safety production certification, and at the same time, it is produced according to the international standards, reaching the export level, and can be OEM.
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