1.1 Influencing factors from the perspective of basic vacuum drying mechanics
During the vacuum drying process, the following factors restrict the drying speed:
1.1.1 The condition of the material to be dried (such as material shape, size, stacking method), the moisture content, density, viscosity and other properties of the material itself. In addition, if the method of increasing the initial temperature of the material, pre-treatment by vacuum filtration, and reducing the moisture content of the material can all increase the vacuum drying speed.
1.1.2 The higher the degree of vacuum, the more favorable it is for the vaporization of water at a lower temperature, but if the vacuum is too high, the heat conduction will be less beneficial, which will affect the heating effect on the material.
In order to improve the drying speed of materials, the degree of vacuum should be considered comprehensively according to the characteristics of materials. Usually, the vacuum degree should be less than 1×104Pa.
1.2 Issues Considered in the Selection of Double Cone Rotary Vacuum Dryer
1.2.1 Characteristics of materials
(1) Viscosity of the material: The viscosity of the material changes with the humidity. The viscosity can make the material agglomerate and stick to the wall of the cylinder. After drying, it is easy to stick to the dust collector and the pipe.
In order to solve this problem, the suspension type double cone rotary vacuum dryer newly developed by Changzhou Fanqun Drying Equipment Co., Ltd. adopts a pneumatic vibration and crushing device.
(2) The allowable temperature of the material: The allowable temperature is the highest temperature that the material can withstand. Exceeding this temperature will lead to changes in the potency of the material, decomposition, damage or discoloration. Heat-sensitive medicines can generally be used at ≤60°C.
(3) Bulk density of materials: The bulk density of materials refers to the mass of materials per unit volume. Because the material is a mixture of dry material and moisture, and the dry material is often granular or powdery, the gap between the dry material particles is also different due to the difference in moisture in the material. Therefore, the bulk density of the original wet material and the material in the drying process is also very different, which must be paid special attention to when preparing raw materials.
(4) Material slip angle: When stacking granular or powdery materials, when the material is inclined and the angle between the bottom surface increases to a certain angle, the side material will slide down. The angle between the slope where the material slides and the bottom is called the slip angle of the material. The slip angle is related to material composition, moisture content, particle size and viscosity.
Therefore, when designing and selecting the double-cone rotary vacuum dryer, the angle of the double-cone cylinder should be selected according to the slip angle of the material.
1.2.2 Drying temperature
Depending on the nature of the material, an appropriate temperature can be selected, and the variable temperature drying method is often preferred, that is, the temperature is low at the initial stage of drying, and the temperature is gradually increased to increase the drying speed.
1.2.3 Working vacuum degree
Usually choose 1×103-1×104Pa vacuum degree, high vacuum degree, low vaporization temperature of moisture in the material, fast drying speed.
However, if the vacuum is too high, the cost of the air extraction system will increase, and the price of the dried material will increase, which is economically beneficial.
1.2.4 Filling rate
The ratio of the actual charging volume to the drying cylinder volume is the filling rate. The filling rate of the double cone rotary vacuum dryer is usually between 30% and 50%, which is also related to the bulk density of the material.
1.3 Discussion on key issues of double cone rotary vacuum dryer
1.3.1 Rotation speed of double cone rotary vacuum dryer
The faster the rotation speed of the cylinder of the double cone rotary vacuum dryer, the higher the drying speed. However, in the later stage of drying, as the moisture content of the material decreases, the drying speed also decreases. At this time, increasing the rotating speed is not beneficial to the improvement of the drying speed. In addition, in the early stage of drying, the faster rotation speed will easily cause the moisture to vaporize too fast and cause the material to stick into agglomerates.
Therefore, a lower speed should be used in the initial stage of drying, and the speed should be increased when the surface of the material is dry and no agglomeration, so as to speed up the drying speed and shorten the drying time.