Like other industrial technologies, drying technology has made great progress in the application process. At present, more than 400 kinds of dryers have been developed, and about 200 kinds of dryers have been used in industrial production. Among them, many new dryers have emerged. Some of them are structural improvements to ordinary dryers, some absorb the advantages of other dryers, and some are completely new ideas.
　　Drying is also a unit operation with considerable industrial energy consumption. According to the data, 14% of the industrial energy consumption in developed countries is used for drying, and the drying energy consumption in some industries even accounts for 35% of the total energy consumption in production, and this number is constantly increasing. At the same time, the use of fossil fuel as a heat source for drying operation produces a lot of carbon dioxide and other gases. The tail gas of drying equipment (which contains some dust) has a bad impact on the atmospheric environment, which is a great challenge to the "environmental protection" that has attracted increasing global attention.
　　Almost all industries are inseparable from drying operation. Although a correct understanding of drying and the working mechanism of drying equipment is conducive to the successful completion of the drying process, we still need to continuously invest human and material resources in further research and development of drying technology, so as to make efficient use of energy and reduce the adverse impact on the environment while producing high-quality products, And it is easier to realize process operation and control.
　　1、 Characteristics of drying technology wood
　　Drying technology has a wide range of applications, in the face of many industries, physical and chemical properties of different materials, product quality and other aspects of diverse requirements, drying technology is a cross industry, interdisciplinary, experimental science technology. Generally, in the development and application of drying technology, we need to have three aspects of knowledge and technology. The first is to understand the physical and chemical properties of the dried materials and the use characteristics of the products; The second is to be familiar with the principles of energy transfer engineering, such as mass transfer, heat transfer, hydrodynamics and aerodynamics; Third, there should be means of implementation, that is, engineering design of drying process, main equipment, electrical instrument control, etc. Obviously, these three aspects of knowledge and technology do not belong to one discipline. And in practice, these three aspects of knowledge and technology are indispensable. Therefore, drying technology is an interdisciplinary technology.
　　Although modern drying technology has a history of more than 100 years, it still belongs to the category of experimental science. Most of the drying technology is lack of scientific theory and design method which can accurately guide the practice. In practical application, relying on experience and small-scale experimental data to guide industrial design is still the main way
　　One of the reasons is that some basic subjects of drying technology (mainly those belonging to the field of transfer engineering) have the characteristics of experimental science. For example, the research and development of aerodynamics depends on the "wind tunnel" experiment, which means that it has not been separated from the scope of experimental science, and the development level of these basic disciplines directly affects and determines the development level of drying technology.
　　The second reason is that many drying processes are the process of intersection of various disciplines and technologies, involving a wide range, many changing factors and complex mechanism. For example, in the field of spray drying technology, the running track of droplets in the drying tower is the key to engineering design. The trajectory of a droplet is related to its volume, mass, initial velocity and direction, as well as the flow direction and velocity of other droplets and hot air. But these parameters change all the time due to the process of mass transfer and heat transfer, and in the initial state, no matter the size of droplets or the distribution of hot air can not be uniform. Obviously, for such a complex and changeable process, it is not reliable to carry out engineering design only by theoretical calculation.
　　The third reason is that there are many kinds of dried materials, and their physical and chemical properties are also different. Even under the same drying conditions, the mass transfer and heat transfer rates of different materials may be quite different. If we don't treat them differently, it may lead to unsatisfactory results. For example, although the drying of some Chinese herbal medicines belongs to the same kind of medicinal materials, the drying conditions must be changed only because of the differences in the origin or harvest time of the medicinal materials, otherwise the product quality will be affected.
　　The above three reasons determine that the development and application of drying technology should be based on experiments. But these characteristics of dry search are often ignored intentionally or unintentionally. Due to the lack of experimental devices or incomplete models (which is a common phenomenon in our country), manufacturers often avoid the drying experiments, and users often give up the requirements of necessary experiments because they do not understand the characteristics of drying technology. The result is that the effect of the device is not good, even the scheme design fails. In China, such cases are common. There was a lesson that a set of industrial drying device with a value of 20 million yuan was idle because it could not meet the requirements. Therefore, before the construction of industrial drying device, especially the larger device, it is necessary to carry out sufficient and convincing experiments, and take the experimental results as the basis of industrial device design. This is a remarkable feature of the application of drying technology.
　　In addition, a wide variety of drying equipment, each with a purpose is also a feature of drying technology. Each technology has its own suitable application field. In engineering practice, the suitable drying technology should be selected according to the specific situation. This will have a significant impact on investment costs, operating costs, product quality and environmental protection requirements. For example, a certain enterprise has chosen three types of drying of silica cake, including box drying, spray drying and swirling airflow fast drying. The final results show that these three technologies have their own advantages. Although the production efficiency is low and the labor intensity is high, the product quality is good. The tensile strength of products mixed with rubber is higher. The rotary air flow rapid drying equipment is compact, less investment and high production efficiency, but the strength index of the rubber products is poor among them. Spray drying produces silica, and all the indexes of the products are in the middle of the three. However, they have the characteristics of good fluidity and low dust pollution, which are welcomed by users and operators. In the 1990s, the issue of which drying method is more advanced in the production of silica has caused controversy in the drying industry in China. In fact, each of the three types of equipment has its own characteristics. Which type of equipment to choose depends on the user's own conditions and product requirements. There is no more advanced conclusion. There are many similar examples, which show that there are many kinds of drying equipment with different uses. Therefore, it is necessary to carefully compare and select the technical scheme in the application, and it is also an essential step to check the technical scheme through the drying experiment.
　　2、 Development status of industrial drying device
　　Drying is an important unit operation in many production processes, because drying is not only a simple solid-liquid separation process, but also a post production process. The quality and dosage form of the product largely depend on the comprehensive application of drying technology and equipment. From the economic point of view, the dryer is expensive and the project investment is large. On the other hand, drying is a process of high energy consumption, and the thermal efficiency fluctuates in the range of 15% to 80%. The operating cost of the equipment is closely related to the design and selection of the dryer, so the decision makers of enterprises have always paid more attention to it. There are many kinds of materials to be dried, and their physical and chemical properties are very different. Even different production processes of the same variety and different product requirements of the same variety may lead to different drying conditions, which determines the complexity of drying engineering. Therefore, the drying process is more technical than other unit operations.

Before liberation, drying equipment in China was basically blank, only drying room, oven and drum dryer, drying technology is backward, production equipment is primitive. It was not until 1957 that the vacuum rake dryer appeared. After 1964, the drying technology developed rapidly. Throughout the development history of drying technology and equipment in China, it has gone through the development stage from simple to complex, from low level to high level. Now there are more than ten types and more than 30 series of drying equipment commonly used in production. In addition, there are more than 50 kinds of combined drying equipment. In addition, it is more difficult to count the special drying equipment. It is not easy to choose these drying equipment reasonably, The premise of type selection is to understand the basic working principle, structural characteristics and applicable material range of these equipment, so as to avoid detours in type selection.
　　In recent years, the development of drying technology has brought more complex factors to the screening equipment. Even the design, manufacture or users of drying equipment often do not know how to choose the right equipment. Since the salesmen of drying equipment are only interested in the types of dryers they sell in the market, but they don't introduce other types of dryers, the users have to rely on the relevant reference materials of modern drying technology to decide the later selection of the equipment. There is no doubt that users need the laboratory, experimental scope and technical and economic information provided by the salesmen. Therefore, we must be familiar with most of the drying equipment, it is possible to select a reasonable equipment. It should be emphasized that there are probably many more suitable dryers in the specific production and operation state, but it must also be known that there is no strict rule for the best drying equipment in the specific working state, and each product has its own unique production mode. There are many factors that affect the selection of drying device, such as intermittent drying or continuous drying, consumption of fossil fuel, power consumption, local environmental law or noise pollution limit, etc. Product output is a major factor in the selection of dryer.
　　3、 Application of drying equipment
　　As mentioned above, drying equipment is widely used in many industrial production. Over the years, many models have been used in industrial production, such as air dryer, fluidized bed dryer, spray dryer, drum dryer, rake dryer, freeze dryer, infrared drying and combined drying. Why are there so many types of drying equipment? This is mainly due to the different types and properties of drying materials and the different specific requirements of the materials to be processed.
　　With the rapid development of production technology in various industries in China, domestic drying technology and equipment have also been developed rapidly. In the drying of granular materials, fluidization technology has been widely used and developed in recent years. Fluidization drying fully improves the gas-solid contact conditions (the evaporation surface area increases), the violent agitation of the material greatly reduces the gas film resistance, and creates extremely favorable conditions for the heat transfer medium. In addition to the rapid development of air flow drying, which was used earlier in drying technology in China, fluidized drying equipment has developed rapidly in recent years. It is mainly manifested in the use of fluidization technology combined with the characteristics and requirements of various materials to be dried to create many new and efficient fluidization dryers, which are described as follows.
　　Straight tube air flow dryer is an earlier fluidization drying equipment used in China. After several years of production practice, it is considered that air flow drying is an ideal drying equipment for granular materials, especially for heat sensitive materials. It is superior to oven drying in terms of production and floor area, so it is widely used in pharmaceutical, plastic, food, fertilizer and other industries. However, air flow drying has some disadvantages, such as low heat utilization, high equipment and low relative velocity of gas-solid phase. In recent years, new air flow equipment such as pulse air flow dryer, cyclone air flow dryer and crushing air flow dryer have been developed to overcome the shortcomings of straight pipe air flow drying. In addition to reducing the height of crushing airflow, it also expands the application scope of airflow dryer, so that the easily oxidized materials can use air as the drying medium, which not only reduces the drying power consumption, but also improves the output and quality of products. In addition, multi-stage airflow drying process and combined airflow drying process are adopted. In the application of airflow dryer, many projects adopt two-stage series mode, The drying of some materials is more reasonable and the thermal efficiency is improved. The straight pipe air flow drying has been very mature in production operation. Pulse air flow and cyclone air flow drying have been industrialized for many years, and the operation is mature, but the theoretical design is still lacking. It needs to be further improved in the future practice and development.
　　Most of the materials with strong heat sensitivity and easy oxidation are air dried. Generally, the material with initial humidity of 10% - 25% can be dried to 1% - 0.05%, the particle size of the dried material is generally 60-100 mesh, and the yield is generally 100-200 kg / h. At present, it is widely used in pharmaceutical, food, plastic and other industries in China. With the rapid development of production technology in our country, air flow drying will be widely used in industrial production in the future.
　　Fluidization drying is another drying technology developed in recent years. The production practice has proved that it has many advantages, and can realize large-scale production with small equipment. Due to the large heat capacity coefficient and the arbitrary adjustment of residence time, it is suitable for materials with surface water and need to go through the slow-down drying stage, especially for the drying of bulk materials^ The following types have been developed and industrialized in recent years: single-layer cylinder type, multi-layer round tube type, vibrating fluidized bed, horizontal multi chamber fluidized bed dryer, agitated fluidized bed and fluidized bed with internal heat pipe, etc. At present, it has been widely used in pharmaceutical, fertilizer, food, plastics, petrochemical and other industries. After several years of practice, domestic fluidized drying has developed to a mature stage in terms of operation and equipment structure. From the point of view of application, horizontal multi chamber fluidizing dryer is an ideal equipment for fluidizing and drying granular materials because of its simple structure, convenient operation, stable and wide material adaptability, which can obtain products with even water content and less power consumption. It is worthy of promotion and development in the future. The inner heat pipe is the combination of convection and conduction heat transfer of fluidized bed, which has high thermal efficiency and good drying effect. It is a new type of machine recommended in recent years.
　　There are three types of conical fluidized bed in China: one is dense phase overflow discharge, which is widely used in fluidized granulation in recent years; The other is spouted bed drying, in which the materials are discharged from the top of the bed, and the products are collected in the cyclone separator or discharged from the bottom of the bed intermittently. This kind of structure is simpler than that of fluidized bed, with small equipment, large output, high drying intensity, strong bed isothermal property and no local overheating. In the past, it was only suitable for large particle materials (PVC). In recent years, it has been developed to be applied to the drying of fine particle materials. At present, it is used in plastic, grain, pharmaceutical and other departments. However, due to the large power consumption, the use is limited.
　　New developments have been made in the drying of solution or slurry materials. In addition to the new developments in spray drying, the cone-shaped fluidized bed has been successfully applied in spray granulation production and has been gradually developing and improving. Spray fluidized granulation dryer is first used in chemical fertilizers, and has been used in medicine, food and other industries. Spray drying has been used in China for more than 20 years, and has been mature in design and operation. In recent years, spray drying has the following progress:
　　(1) In addition to the large-scale development of the drying chamber, the relevant units have also done more experimental research on the performance of the nozzle atomizer, and achieved remarkable results;
　　(2) in addition to spray drying which is more widely used in heat sensitive solutions, spray drying has been used successfully in recent years.
　　(3) spray drying equipment combined with other drying technologies to achieve the purpose of drying or drying granulation at the same time. This is also the embodiment of further development of drying technology level in China.