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Vacuum drying is a method of drying after vacuuming in a closed container.The advantage of this method is that the temperature is low and the product is loose and easy to crush.
It reduces the adverse effects of air on products, which is of certain significance to ensure product quality.Especially suitable for materials containing heat sensitive ingredients.Common instruments arePressure reducing dryer。The drying effect depends on the vacuum degree and the thickness of the dried material.Its characteristics include:;The drying temperature is low and the drying speed is fast;The contact opportunity between materials and air is reduced, which can reduce drug pollution or oxidative deterioration;The product is spongy and easy to crush.This method is applicable to the drying of thick extracts and heat sensitive or high temperature oxidizable materials.During decompression and drying of thick extract, the amount of tray, vacuum degree and heating steam pressure should be controlled to avoid material foaming and overflow, resulting in waste and pollution.The drying equipment isVacuum drying oven。
Drying principle
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When wet materials are dried, two processes are carried out at the same time: (1) heat is transferred from hot air to wet materials, so that the moisture on the surface of the materials is immediately vaporized and diffused into the main airflow through the air film on the surface of the materials;(2) Due to the gasification of moisture on the surface of wet materials, the moisture concentration difference between the inside and the surface of materials is generated, so the moisture is diffused from the inside to the surface.Therefore, heat and mass transfer are two opposite processes simultaneously in the drying process.The important condition of drying process is that it must have the driving force of heat and mass transfer.The vapor pressure on the material surface must be greater than the vapor partial pressure in the drying medium (air). The greater the pressure difference, the faster the drying process will proceed.
influence factor
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Drying rate and its influencing factors
According to the drying principle, the main factors affecting drying are as follows:
Dry area
Since the evaporation of water mainly takes place on the surface of the dried material, the drying area of the dried material plays an important role in drying.The drying efficiency is proportional to the drying area.The thicker the materials to be dried are, the smaller the drying area is, and the slower the drying is, and vice versa.
Drying rate
Drying shall be carried out at a certain speed.During the drying process, the surface moisture is quickly evaporated and removed, and then the internal moisture diffuses to the surface and continues to evaporate.If the drying speed is too fast and the temperature is too high, the moisture on the surface of the material will evaporate too fast, and the internal moisture will not diffuse to the surface in time, causing the surface particles to bond with each other or even melt the conjunctiva, thus preventing the internal moisture diffusion and evaporation, making the drying incomplete, causing the pseudo drying phenomenon of the external dry and internal wet, and making the material deteriorate after long storage.
Drying method
Materials dried during drying can be static or dynamic.Drying materials in the oven or drying room is static, and the drying area of materials is small, so the drying efficiency is poor.Several dry materials are in the state of tumbling or suspension, such as the fluidized drying method, in which the powder particles are separated from each other, increasing the drying area, so the drying efficiency is high.Under reduced pressure, the drying temperature and drying time can be reduced, so it is suitable for drugs with low melting point, unstable heating and difficult to remove moisture.Some drugs have low melting point or cannot be heated due to thermal instability. The desiccant drying method under reduced pressure can be used in the vacuum dryer.If the melting point of ibuprofen is 74.5~77.5 ℃Phosphorus pentoxideReduce the pressure in the dryer and dry to constant weight.adrenalineIt is specified to dry for 18 hours in a phosphorus pentoxide dryer under reduced pressure.The sample that can withstand a certain temperature can be heated and dried under reduced pressure.For example, digoxin is regulated to be dried at 105 ℃ for 1 hour.When heating and drying under reduced pressure, use a constant temperature and reduced pressure drying oven.When vacuum dryer or constant temperature vacuum drying oven is used, unless otherwise specified, the pressure shall be below 2.67kPa (20mmHg).
temperature
The higher the temperature, the faster the evaporation rate, the greater the evaporation capacity, which is conducive to drying.However, the drying temperature should be properly selected according to the nature of the drying materials to prevent some components from being damaged.
humidity
If the humidity of the material itself is large and the evaporation capacity is large, the relative humidity of the drying space is also large, the drying time of the material is prolonged, and the drying efficiency is low.For this reason, the drying room and oven often use an air blower to update the air flow in the drying space, so as to avoid the relative humidity saturation in the drying room and stop evaporation during the drying process.
pressure
Pressure is inversely proportional to evaporation, so decompression is an effective means to improve evaporation conditions and accelerate drying.When vacuum drying is used to prepare dry extract, the drying temperature can be reduced to accelerate the evaporation rate, making the product loose and fragile;The effective ingredients are not easy to destroy, and the solvent can also be recovered at the same time.
Material characteristics
Different shapes of materials have different properties and moisture states, and drying efficiency is also different.Materials are roughly divided into two categories: (1) granular or crystalline solids, such as calcium sulfate, magnesium oxide, etc;(2) Amorphous solid, such as starch, enzyme, insulin, etc.The water in crystalline solid materials is often adsorbed on the external surface of the material or in the shallow opening holes as well as in the internal particle gaps of the material. These gaps are connected with the surface, and the water is easy to remove;The water in the materials of amorphous solids (including fibrous and colloidal structures) often exists in the molecular structure or is trapped in many tiny capillaries or pores. The water moves slowly from the inside of the materials to the surface, and such materials are not easy to dry.
Nature of moisture in materials
(1) According to whether the moisture in the material can be dried and removed, it can be divided into balanced moisture and free moisture. Balanced moisture means that when the material is in contact with the air in a certain state, the material will remove or absorb moisture until the water vapor pressure generated on the surface of the material is equal to the water vapor partial pressure of the air. At this time, the moisture contained in the material.Balance moisture is the limit of material drying. As long as the air state remains unchanged, the moisture in the material will always remain constant and will not change due to the extension of the contact time with the air. Therefore, balance moisture is the moisture that cannot be removed during drying.Free water refers to the water contained in the material that is greater than the equilibrium water, that is, the water that can be removed during the drying process.
(2) According to the degree of difficulty in removing water from materials, it can be divided into bound water and non bound water. The bound water is mainly physically and chemically bound, including water in the cell wall of materials, water in the soluble solid solution in materials, and water in the capillary tubes in materials. This kind of water has a strong binding force with materials, so it is difficult to remove, and the drying speed is slow.Non binding moisture is mainly mechanically bound moisture, such as moisture on the surface of materials. This kind of moisture has weak binding force with materials, is easy to remove, and has fast drying speed.
