Compared with conventional drying methods, microwave heating drying has obvious advantages and good economic benefits, and has been further applied in pharmaceutical industry in recent years. Looking at the present situation of pharmaceutical microwave drying equipment, there are only microwave dryers, microwave belt drier, microwave vacuum dryers and microwave rotary vacuum dryers. Although the above microwave drying equipment should use microwave technology, there are always some imperfections. In order to solve the above problems, Changzhou Kewei Machinery Manufacturing Co., Ltd. recently developed WDZ intelligent microwave vacuum continuous dryer, which can realize continuous low-temperature drying in microwave and vacuum environment. Based on the mechanism of microwave drying, this paper expounds the principle, structure and characteristics of intelligent microwave vacuum continuous dryer, and analyzes its application.
1 microwave drying mechanism
1. 1 microwave characteristics
Microwave is a kind of electromagnetic wave with very high frequency, the frequency is 300 MHz to 300 kMHz, and the wavelength is 1 m to 1 mm. Electromagnetic waves with a wavelength greater than 1 m are radio waves, and those with a wavelength less than1mm are light waves. When microwave encounters different kinds of substances in the transmission process, it will produce reflection, transmission and absorption, and the strength of these phenomena depends on the properties of the substances themselves.
The relationship between 1.2 material and microwave field
According to the relationship between materials and microwave field, it can be divided into four categories:
(1) conductor, which can reflect microwave and can be used to store or guide microwave, that is, conductor can be used as the material of drying chamber and waveguide; (2) Insulators are also called "lossless dielectrics". Almost all insulators do not reflect or absorb microwaves, but microwaves can penetrate insulators, so these materials (such as ceramics, glass, plastics, etc.). ) can be used as a supporting device for heated materials in microwave fields; (3) Dielectric, whose characteristics are between the first two, most of which can be called "lossy media", can absorb microwave energy to varying degrees and convert it into heat; (4) Ferromagnets also absorb, reflect and penetrate microwaves, interact with the magnetic field components of microwaves and generate heat, and are often used as materials for protective or choke devices to prevent the leakage of microwave energy.
1.3 microwave heating mechanism
Microwave drying depends on microwave heating. The heating mechanism of microwave is completely different from the traditional heating methods (conduction, convection and radiation). When water-containing substances are irradiated by microwave, because water molecules are polar molecules, the arrangement of polar molecules changes from disorderly nonpolar state to orderly arrangement. When the direction of external electric field changes repeatedly, polar molecules change repeatedly and swing frequently. In the process of swinging, due to the similar friction between molecules, a lot of heat is generated, and the temperature of the material also increases. Microwave heating is to use the principle of dielectric loss [1] to convert microwave energy into heat energy needed for heating materials, and the heat absorbed by materials is proportional to the dielectric loss factor of materials. Because the dielectric loss factor of water (or other solvents) is much larger than that of other substances, water (or other solvents) molecules preferentially absorb microwave energy, and water molecules move from the inside of the substance to the surface, and continue to absorb microwave energy, and water turns into steam and is discharged, thus quickly completing the drying purpose.
Structure and characteristics of intelligent microwave vacuum continuous dryer
2. 1 structure of intelligent microwave vacuum continuous dryer
The structure of WDZ intelligent microwave vacuum continuous dryer is shown in figure 1. It consists of the following main parts: drying cylinder, microwave generation system, control system (PLC controlled dynamic display system), vacuum system, transmission system, dehumidification system, crushing system, feeding and discharging system, cleaning system, etc.
The working process of the intelligent microwave vacuum continuous dryer is as follows: (1) Start the vacuum pump to vacuum the inside of the drying cylinder 3; (2) Start the conveying system 6 and the crushing system12; (3) Start the feed pump 2 to spray the materials in the barrel 1 onto the conveyor belt through the feed mechanism 4; (4) the microwave heating system 5 is started, and at this time, the drying process starts, the microwave energy is rapidly converted into heat energy, and the moisture in the material is converted into steam, which is pumped out by the vacuum pump; (5) The continuously dried materials are crushed after being cooled by the cooling system 8, and are collected by the specially designed discharging systems 10 and 1 1.
In the process of operation, according to the process requirements and the actual drying state of the material, the temperature, conveying speed and feeding amount of the material are automatically controlled according to the program compiled in PLC, so that the material characteristics can be maintained and the system operation can achieve the best effect. The system is equipped with a liquid crystal display screen, which can display the drying state of the materials in the box and the running state of each mechanism.
After the water in the material evaporates, part of the steam will condense into condensed water on the inner wall of the box, and the condensed water will flow down the box wall and be collected and discharged from the system.
After the material is continuously dried, turn off the microwave generating system, then turn off the conveying system, finally turn off the vacuum system, turn on the ventilation respirator, and introduce clean air into the dryer to balance the negative pressure in the box.
The cleaning system 7 arranged inside the dryer can conveniently clean the inside.
3.2 Structure of Intelligent Microwave Vacuum Continuous Dryer
WDZ intelligent microwave vacuum continuous dryer integrates microwave technology, belt continuous motion technology, vacuum drying technology and automatic control technology, highlighting the following characteristics:
(1) has fast drying speed. The conventional heating method is to use hot air, gas, steam or superheated steam to heat the material, and its essence is to realize heat transfer through the temperature gradient inside the material, so the heating speed is slow. Microwave heating, on the other hand, uses the interaction between microwave field and the whole material to generate thermal effect quickly inside the material, and the heating speed is very fast, usually the heating process can be completed in a few seconds.
(2) Dry evenly. The material in the microwave field is subjected to "volume thermal effect" [2], thus avoiding the large temperature gradient in the conventional heating system, that is, the material heated by microwave is very uniform, and the surface of the material will not be overheated and crusted. At the same time, the material dried by this machine moves continuously and uniformly in a straight line, which makes the material receive microwave energy more evenly and greatly contributes to the improvement of the quality of dried products.
(3) continuous drying. This machine adopts conveyor belt with automatic deviation correction, crushing mechanism, charging gun and special material unloader, which can carry out continuous drying operation with large output. Therefore, in the structural design, the technical problems of realizing feeding and discharging in vacuum state and preventing microwave energy leakage are well solved.
(4) low-temperature drying. On the one hand, in the microwave field, the microwave is only coupled with the solvent in the material, but not with the matrix, so the water is heated, vaporized and discharged, while the carrier (matrix) of the water is mainly heated by heat conduction, so the temperature of the material is not high, which is called "selective heating" [3]; On the other hand, the machine dries materials in a vacuum environment, thus greatly reducing the evaporation temperature of the solvent. The temperature of the dried material is not high and controllable, which is very suitable for drying low melting point and heat-sensitive materials.
(5) sterilization function. Because of the "biological effect" of microwave, bacteria can be killed at low temperature. When it is applied to the drying of medicines and foods, it can achieve sterilization while drying.
(6) High raw material yield and simple operation. In the whole drying process, the material will not be lost, and its output is extremely high, which is more important for the drying of precious materials. At the same time, the whole operation process is equivalent to automatic loading and unloading, and the operation is relatively simple.
(7) Comprehensive application of high technology. The machine adopts many advanced electronic technologies, such as infrared remote control to measure and control the temperature of materials, dynamic display of materials during drying (camera and display), PLC control and so on. , improve the technical content of the whole machine.
(8) Safe and reliable. The equipment is equipped with a special structure of microwave choke device and vacuum sealing mechanism, and its microwave leakage can be strictly controlled below the national standard (less than 1 mW/ cm2 at a distance of 5 cm from the equipment after factory inspection).
(9) Easy to clean. Cleaning balls are installed in many places inside the equipment, which can clean the inside conveniently and effectively.
(10) energy saving. Because of the selectivity of microwave heating and the above characteristics, not only the heating energy required for drying is saved, but also a lot of electric energy is saved, which greatly reduces the total energy consumption of drying.
Application of Intelligent Microwave Vacuum Continuous Dryer
WDZ intelligent microwave vacuum continuous dryer not only has the advantages of fast, uniform and energy-saving microwave drying, but also has the advantages of vacuum low temperature drying. It can also make the conveyor belt continuously feed and discharge materials under the action of uniform straight line, thus improving the efficiency of material drying. Therefore, all materials and products in industrial production need microwave, vacuum and continuous drying, which can be widely used.
WDZ intelligent microwave vacuum continuous dryer is mainly used for drying powdery, granular, flaky and strip materials in pharmaceutical, chemical and food industries, especially for drying paste materials (such as extractum).
Among them, because of the special physical properties of paste materials, such as viscosity and heat sensitivity, it has always been a technical problem in the drying field. If vacuum drying equipment is used, the drying time is long and uneven because of the slow conduction heating speed in vacuum state; Another example is the microwave vacuum dryer, which can't discharge continuously. Its biggest disadvantage is that the dried materials are in a static state, and the microwave energy received by materials in different positions is very different, which seriously affects the drying quality. If you choose other types of dryer equipment, there will be defects such as large volume of heating device and slow drying speed. In a word, the intelligent microwave vacuum continuous dryer overcomes the imperfections of many drying equipment mentioned above, and has also achieved very satisfactory results for extract materials that are extremely difficult to dry.
5 conclusion
Based on the mechanism of microwave drying, this paper expounds the principle, structure and characteristics of WDZ intelligent microwave vacuum continuous dryer newly developed by Changzhou Kewei Machinery Manufacturing Co., Ltd., and draws the following conclusions:
(1) The equipment adopts microwave energy and vacuum technology, and also adopts the dynamic structure and dynamic display means of the drying process, so that the materials move in a straight line and continuously in the dryer, and the whole drying process is completed in the microwave field and vacuum environment, so that the received microwaves are basically the same all the time, the drying degree of the materials is basically the same, and the materials are dried evenly, thus realizing the continuous drying operation and greatly overcoming the shortcomings of low yield and inconvenient operation of intermittent drying.
(2) The equipment also adopts vacuum drying technology, so that the vaporization temperature is greatly reduced, and the water in the material can be quickly converted into steam, so that the material can be dried at low temperature, which is especially suitable for drying low-melting-point and heat-sensitive materials, especially for viscous, gelatinous and extremely difficult-to-dry extract materials.