What Is Vacuum Drying?
Vacuum drying, also known as analytical drying, is a method of placing the material under vacuum negative pressure to reduce the boiling point of water. The boiling point of water under an atmospheric pressure is 100 ° C. Under vacuum negative pressure, the boiling point of water can be reduced At 80 ° C, 60 ° C, and 40 ° C, evaporation begins. The vacuum drying newly developed by Jiangyin Mingzheng Machinery can evaporate water at 20 ° C, 5 ° C, -5 ° C, and -10 ° C. Dry craft.
- Chinese name
- Vacuum drying
- Aka
- Analytical drying
- solved
- Drying process of heat-sensitive materials
- Condition
- One way to put materials under negative pressure
- Vacuum drying, also known as analytical drying, is a method of placing the material under vacuum negative pressure to reduce the boiling point of water. The boiling point of water under an atmospheric pressure is 100 ° C. Under vacuum negative pressure, the boiling point of water can be reduced At 80 ° C, 60 ° C, and 40 ° C, evaporation begins. The vacuum drying newly developed by Jiangyin Mingzheng Machinery can evaporate water at 20 ° C, 5 ° C, -5 ° C, and -10 ° C. Dry craft.
Introduction to vacuum drying
- Vacuum drying, also known as analytical drying, is a drying method in which materials are placed under negative pressure and the boiling point under negative pressure is appropriately reached by heating or the temperature is reduced to make the material solidify and then pass the melting point to dry the material.
- Vacuum drying technology has been widely used in the production of health foods, especially the extraction of natural products. Because many functional ingredients in natural products are not resistant to high temperatures, vacuum drying is widely used. Vacuum drying technology is also called vacuum drying. It is a method of drying air in a closed container by depressurizing it [1] .
- Moisture in the material
Vacuum drying principle
- Vacuum drying principle :
- In various heating and drying methods under normal pressure, the color, fragrance, taste and nutritional components will be lost to a certain degree due to the heating of the material. If the vacuum drying method is used, because the material is isolated from the air under negative pressure, some materials that are prone to chemical changes such as oxidation during the drying process can better maintain the original characteristics and reduce the loss of quality. Vacuum drying is to place the dried food materials in a closed drying room. While vacuuming with a vacuum system, the material to be dried is appropriately and continuously heated, so that the moisture inside the material diffuses to the surface through pressure or concentration differences, and water molecules After obtaining sufficient kinetic energy on the surface of the material, after overcoming the attractive force between the molecules, it escapes to the low-pressure air in the vacuum chamber and is removed by the vacuum pump [2] .
- The vacuum drying methods can be divided into two types: boiling point and melting point .
- First, by boiling point drying
- During the drying process, there are two ways to evaporate the liquid moisture, e.g. shaft evaporation and boiling . Water evaporates at a faster rate than when it boils. Evaporation of water into steam can occur at any temperature. The boiling of water into steam can only be performed at a specific temperature, but when the pressure is reduced, the boiling point of water also decreases. For example, at a pressure of 19.6kPa, the boiling point of water can be reduced to 60 ° C. The vacuum dryer is a vacuum source that provides a heat source and supplies sufficient heat of moisture in the material through heat transfer methods such as heat conduction and heat radiation , so that evaporation and boiling occur simultaneously, and the vaporization speed is accelerated. At the same time, evacuating and quickly extracting vaporized steam, and forming a negative pressure state around the material, a large humidity gradient is formed between the inner and outer layers of the material and between the surface and the surrounding medium, which accelerates the vaporization rate and achieves the purpose of rapid drying. .
- The vacuum drying process is affected by factors such as heating mode, heating temperature, vacuum degree, coolant temperature, the type and initial temperature of the material, and the amount of compression force. Generally, there are three types of heat supply : heat conduction , heat radiation, and a combination of the two. the way.
- Second, by melting point drying (also known as freeze drying)
- During the drying process, the wet material (or solution) is first frozen into a solid at a lower temperature (-10-50 ° C), and then the solid moisture is directly raised to a gaseous state under a high vacuum (130-0.1Pa). The drying process is also called sublimation drying .
- Relationship between water ice gas pressure and temperature
- Wet materials may not be pre-frozen , but the materials may be frozen by themselves by using moisture vaporization and heat absorption during high vacuum. This freezing energy consumption is small, but it is easy to cause foam or splash phenomenon for liquid materials and suffer losses. At the same time, it is not easy to obtain a porous uniform dry material.
- Generally, the heat is supplied by the heating medium through the partition wall of the drying chamber. Therefore, the heat of the wet material must be supplied to ensure a certain drying rate, and the melting of ice must be avoided.
- The sublimation temperature during the drying process is generally -35--5 ° C. The extracted water can be frozen and collected on the condenser or discharged directly to the vacuum pump. If the heat required for sublimation is directly supplied by the material being dried, in this case, the temperature of the material decreases rapidly, so that the vapor pressure of ice is very low and the sublimation rate is reduced.
Vacuum drying characteristics
- The main features of vacuum drying are as follows.
- (1) Vacuum drying is suitable for heat-sensitive materials, or materials that are easily oxidized at high temperatures, or materials that are valuable or toxic, and combustible.
- (2) The range of vacuum and heating temperature used during drying is large and the versatility is good.
- (3) The drying temperature is low, there is no overheating, the moisture is easy to evaporate, and the drying time is short.
- (4) Reduce the contact opportunities between materials and air, and avoid pollution or oxidative deterioration.
- (5) The dried product can form a porous structure, which is crunchy and sponge-like, easy to pulverize, has better solubility, rehydration, and better color and taste.
- (6) The volatile liquid can be recycled, but the production capacity is small, it needs intermittent operation, and the drying speed is fast.
- (7) Equipment investment and power consumption are higher than normal pressure hot air drying [1] .
Vacuum drying method
- 1. Principle : Use a lower temperature and dry under reduced pressure to exclude water, and the reduced amount in the sample is the moisture content of the sample. This method is applicable to foods that are easily deteriorated when heated above 100 ° C and contain water that is not easily removed. The measurement results are closer to true moisture.
- 2.Operation method
- Accurately weigh 2.00 5.00g sample weigh to a constant weight weighing dish to a vacuum oven 70 , vacuum degree 93.3 98.6KPa (700 740mmHg) bake for 5 hours cool in a drying dish weigh to constant weight
- Calculation: Moisture = G / W
- G-weight loss after drying in the sample (g)
- W sample weight (g)
- The vacuum drying method is used to measure moisture. It is generally used for samples that are easily deteriorated, damaged or difficult to remove above 100 ° C. Samples such as syrup, monosodium glutamate, sugar, candy, honey, jam and dehydrated vegetables can be measured by vacuum drying method.
Vacuum drying characteristics
- The process of the vacuum drying box is to place the dry material in a closed drying chamber, and use a vacuum system to continuously heat the material to be dried while evacuating, so that the moisture inside the material is diffused to the surface by pressure difference or concentration diffusion, and water molecules are in the material. The surface obtains sufficient kinetic energy, and after the mutual attraction between molecules is overcome, it escapes to the low-pressure space of the vacuum chamber, and is thus pumped away by the vacuum pump.
- In 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 density is low, and the oxygen content is low, so it can dry materials that are easily oxidized and deteriorated, flammable and explosive dangerous goods, etc. It can play a certain role in sterilizing medicines, foods and biological products, which can reduce the chance of contamination of materials or inhibit the growth of certain bacteria.
- Because the temperature of water during vaporization is directly proportional to the vapor pressure, the ratio relationship is shown in Figure 1-1. Therefore, the moisture in the material can be vaporized at low temperature during vacuum drying, and low temperature drying can be achieved. This is beneficial for the drying of heat-sensitive materials in certain pharmaceuticals, foods and agricultural and sideline products. For example, if the sugar solution exceeds 70 , some ingredients will
Vacuum drying vacuum drying equipment
- The vacuum dryer consists of a condensing system consisting of a drying cabinet, a condenser and a condensate collector, and a vacuum pump. Place the wet material in a shallow dish and place it on the shelf of the drying cabinet. The heated steam is introduced by the steam population. It enters the mezzanine shelf. The lower outlet of the condensate white drying box flows out through the condensation pipe to the condensate collector; The system is closely connected to the vacuum pump through the pipeline and the valve to form a complete closed system to make the drying operation continuous.
- At present, the vacuum drying equipment is also constantly updated with the development of modern machinery manufacturing technology and electrical technology. Vacuum disc continuous dryers, double cone rotary vacuum dryers, vacuum harrow dryers, plate vacuum dryers, and low temperature have appeared. Various types of vacuum drying equipment, such as belt continuous vacuum dryers and continuous vacuum dryers [1] .
Influencing factors of vacuum drying
- The main factors affecting vacuum drying are: the relative density of the concentrated liquid, the vacuum drying temperature, the vacuum drying vacuum degree, and the vacuum drying time.
- The relative density of natural material extraction concentrated liquid should be controlled to be 1.30 1.35 (60 heat measurement) thick paste; vacuum drying temperature is generally not higher than 70 , often controlled at about 60 ; vacuum drying vacuum degree Generally controlled at about 0.08MPa, the vacuum degree is too low, the drying speed will be slow, and the vacuum degree is too high, the materials are prone to splashing; the vacuum drying time is generally controlled by the degree of complete drying, or controlled by the moisture content, and the drying time of different materials is different. Great.
- In addition, when viscous materials such as extracts are drying, the loading volume should not be too much, so as to prevent foaming from overflowing the tray, contaminating the dryer, wasting materials, and affecting the drying effect; the valve on the vacuum pipe should be opened slowly during operation, otherwise foaming is also prone Overflow phenomenon [1] .