Technical summary of drying equipment selection
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 more than 200 kinds of dryers have been used in industrial production. Among them, many new dryers have appeared. Some of them have improved the structure of ordinary dryers, some have absorbed the advantages of other dryers, and some are completely new ideas.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 more than 200 kinds of dryers have been used in industrial production. Among them, many new dryers have appeared. Some of them have improved the structure of ordinary dryers, some have absorbed the advantages of other dryers, and some are completely new ideas.
Drying is also a unit operation with considerable industrial energy consumption. According to records, 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 of production, and this figure is increasing constantly. At the same time, a large amount of carbon dioxide and other gases are produced by using fossil fuel as heat source for drying operation. The tail gas of drying equipment (some dust in these gases) has a bad impact on the atmospheric environment, which is a great challenge to the "environmental protection" which is increasingly attracting global attention.
Almost all industries are inseparable from the drying operation. Although a correct understanding of the drying and the working mechanism of the drying equipment is helpful to the successful completion of the drying process, we still need to invest human and material resources to further research and development of drying technology, so as to effectively utilize 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 wood drying technology
Drying technology has a wide range of applications. In the face of numerous industries, materials with different physical and chemical properties, product quality and other requirements, drying technology is an interdisciplinary, interdisciplinary and experimental science technology. Generally, there are three aspects of knowledge and technology in the development and application of drying technology. The first is to understand the physical and chemical properties of the materials to be dried and the use characteristics of the products; the second is to be familiar with the principles of transfer engineering, that is, the principles of mass transfer, heat transfer, fluid mechanics and aerodynamics; the third is to have the means of implementation, that is, to be able to carry out the 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 area. In practice, one of the three aspects of knowledge and technology is indispensable. So drying technology is a cross industry, interdisciplinary technology.
Although modern drying technology has a history of more than 100 years, it still belongs to the category of experimental science. At present, most drying technologies are lack of scientific theories and design methods that 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. The reasons for this situation are as follows:
One of the reasons is that some basic disciplines of drying technology (mainly subject to the category of transfer engineering) have the characteristics of experimental science. For example, the research and development of aerodynamics need to be promoted by "wind tunnel" experiments, which shows 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 multi-disciplinary technology convergence, involving a wide range of factors, 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, and the flow direction and velocity of other droplets and hot air around it. However, due to the process of mass transfer and heat transfer, these parameters change all the time, and it is impossible for the droplet size or hot air distribution to be uniform in the initial state. 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 the types of materials to be dried are various, and their physical and chemical properties are also different. Even under the same drying conditions, the rate of mass transfer and heat transfer of different materials may vary greatly. If it is not treated differently, it may result in 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. Manufacturers often avoid drying experiments due to lack of experimental equipment or incomplete models (which is a common phenomenon in China), and users often give up the requirement 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 worth 20 million yuan was idle because it could not meet the requirements. Therefore, before the construction of industrial drying devices, especially large ones, we must carry out sufficient and convincing experiments, and take the experimental results as the basis of industrial device design. This is the remarkable characteristic of drying technology application.
In addition, a 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, it is necessary to select the suitable drying technology according to the specific situation. This will have a significant impact on investment costs, operating costs, product quality, environmental protection requirements and so on. For example, in a certain enterprise, three types of drying of silica cake were selected: box drying, spray drying and swirling airflow drying. ^The results show that each of the three technologies has its own advantages. Although the production efficiency is low and the labor intensity is high, the product quality is good. The tensile strength of the product after mixing with rubber is higher. The rotary airflow drying equipment is compact, low investment and high production efficiency, but the strength index of the rubber products produced is poor among the three. 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 problem of which drying method is more advanced in the production of silica was once debated in China's drying industry. In fact, each of the three types of equipment has its own characteristics. Which model to choose depends on the user's own conditions and product requirements. There is no conclusion that the technology is more advanced. There are many similar examples, which show that there are many kinds of drying equipment and each has its own characteristics. Therefore, we should carefully compare and carefully select the technical scheme in the application, and it is also an essential step to evaluate the technical scheme through drying experiment.
2、 Development status of industrial drying equipment
Drying is an important unit operation in many production, because drying is not only a simple solid-liquid separation process, but also a later process in the production process. The product quality and dosage form 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, the thermal efficiency fluctuates in the range of 15% - 80%, and the operating cost of equipment is closely related to the design and selection of dryers, so the decision makers of enterprises have always attached great importance to it. There are many kinds of materials to be dried, and their physical and chemical properties are very different. Even the same variety of different production processes, the same variety of different product requirements, resulting in drying conditions may be different, so it 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 was backward and production equipment was primitive. It was not until 1957 that the vacuum rake dryer appeared. After 1964, the drying technology had a rapid development. Throughout the development history of China's drying technology and equipment, it has experienced the development stage from simple to complex, from low to high level in several decades. Now, there are more than ten types of drying equipment commonly used in production, and 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 select these drying equipment reasonably The premise is to understand the basic working principle, structural characteristics and applicable material range of these equipment, so as to avoid detours in the selection.
In recent years, due to the development of drying technology, more complex factors have been brought to the screening equipment. Even the design, manufacture or users of drying equipment often do not know how to choose the right equipment. As the salesmen of drying equipment are only interested in the types of dryer they sell in the market, and do not introduce other types of dryers, users have to rely on the reference materials of modern drying technology to decide the post selection of the equipment. There is no doubt that users need information about the laboratory, the scope of the experiment and the technical and economic aspects provided by the salesman. Therefore, it is necessary to be familiar with most of the drying equipment in order to select a reasonable equipment. It should be emphasized that in a specific production and operation state, there may be many more suitable dryers. However, it must be noted that in a specific working state, there is no strict rule to specify the best drying equipment. Each product has its own unique production mode. There are many factors affecting the selection of the best drying device, such as the choice of intermittent drying or continuous drying, the consumption of fossil fuels, power consumption, local environmental laws or noise pollution limits, etc. The output of the product 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 many types of drying equipment? This is mainly due to the different types and properties of drying materials and the different specific requirements of processed materials.
With the rapid development of production technology in various industries in China, domestic drying technology and equipment have also been rapid development. In the drying of granular materials, fluidization technology has been widely used and developed in recent years. The fluidization drying can improve the gas-solid contact conditions (the evaporation surface area increases), and the violent agitation of the materials greatly reduces the film resistance, which creates extremely favorable conditions for the heat transfer medium. In addition to the rapid development of airflow drying, which was used earlier in domestic drying technology, 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 drying materials to create a lot of new and efficient fluidized dryers, which are described as follows.
The straight tube air dryer is an earlier fluidized drying equipment in China. After several years of production practice, it is considered that the air flow drying is an ideal drying equipment for the drying of granular materials, especially heat sensitive materials. It is superior to oven drying in terms of production capacity and floor area, so it is widely used in pharmaceutical, plastic, food, fertilizer and other industries. However, there are still some disadvantages in air drying, such as low heat utilization rate, high equipment and low relative velocity of gas-solid phase. In recent years, new air flow equipment, such as pulse air dryer, cyclone air dryer and crushing air dryer, have been created to overcome the shortcomings of straight tube air drying. In addition to reducing the height of the air dryer, the application scope of the air dryer is expanded, 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 production and quality of the products. In addition, the multi-stage air flow drying process and the combined air flow drying process are adopted. In the application of the air dryer, the two-stage series connection mode is adopted in many projects It is more reasonable in drying some materials and improves the thermal efficiency. Straight tube air drying has been mature in production and operation. Pulse air drying and cyclone 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.
Most of the materials with strong thermal sensitivity and easy oxidation are air dried. Generally, it can dry the material with initial humidity of 10% - 25% to 1% - 0.05%, the particle size of dried material is generally 60-100 mesh, and the output is generally 100-200kg / h. At present, it is widely used in pharmaceutical, food, plastic and other industries in China. With the rapid development of China's production technology, air drying will be more widely used in the future industrial production.
Fluidization drying is another drying technology developed in recent years. It has been proved by production practice that it has many advantages and can realize large-scale production with small equipment. Because of the large heat capacity coefficient and the residence time can be adjusted arbitrarily, it is suitable for the materials containing surface water and materials that need to go through the slow drying stage, especially for the drying of bulk materials. ^The following types have been developed and industrialized recently: single-layer cylindrical type, multi-layer round tube type, vibrating fluidized bed, horizontal multi chamber fluidized bed dryer, agitated fluidized bed and heat pipe fluidized bed with internal heat pipe, etc., among which the latter is developing rapidly. At present, it has been widely used in pharmaceutical, chemical fertilizer, food, plastic, petrochemical and other industries. After several years of practice, domestic fluidized drying has developed to a more mature stage in terms of operation and equipment structure. In view of its application, the horizontal multi chamber fluidized dryer has the advantages of simple structure, convenient operation and stability, wide adaptability of materials, and can obtain products with uniform water content and less power consumption. It is an ideal equipment for fluidized drying of granular materials, which is worthy of promotion and development in the future. The inner heat pipe is the product of the combination of convective heat transfer and conduction heat transfer in fluidized bed. It has high thermal efficiency and good drying effect. It is a new model recommended in recent years.
There are three types of conical fluidized bed in China according to operation: one is dense phase overflow discharge, which is widely used in fluidized granulation in China in recent years; the other is spouted bed drying, in which the material is 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. Compared with the fluidized bed, this structure has the advantages of simple structure, small equipment, large output, high drying intensity, strong bed isotherm and no local overheating. In the past, it was only suitable for large particle materials (PVC), but in recent years it has been developed to be suitable for drying fine materials. At present, it is used in plastic, cereal, pharmaceutical and other sectors. However, due to the large power consumption, its 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 drying chamber, more experimental research work has been done on the performance of spray nozzle atomizer, and remarkable results have been achieved;
(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.
(4) at present, low temperature spray drying experiment is being carried out. It uses air which has very low moisture content and low temperature as carrier, and the air is dehydrated and dried in advance. The temperature of the product in drying process does not exceed 35 'C, so it is suitable for drying of heat sensitive materials, such as medicine, food and dehydration.