Like other industrial technologies, drying technology has also made significant progress in its application process. Currently, there are over 400 types of drying machines developed, and about 200 types of drying machines have been applied in industrial production. Among them, many new types of drying machines have emerged, some of which are structural improvements to ordinary drying machines, some have borrowed and absorbed the advantages of other drying machines, and some are completely new ideas.
Drying is a unit operation that consumes a considerable amount of energy in industry. According to records, 14% of the energy consumed by developed industries is used for drying, and in some industries, drying energy even accounts for 35% of the total production energy consumption, and this number is constantly increasing. Meanwhile, using mineral fuels as heat sources for drying operations generates a large amount of gases such as carbon dioxide. The exhaust gas from drying equipment (which carries some dust) has adverse effects on the atmospheric environment, posing a great challenge to the increasingly global concern of "environmental protection".
Almost all industries cannot do without drying operations. Although a correct understanding of the working mechanism of drying and drying equipment is helpful for successfully completing the drying process, we still need to continuously invest manpower and material resources in further research and development of drying technology, so that it can effectively utilize energy, reduce adverse effects on the environment, and make process operation and control easier to achieve while producing high-quality products.
1、 Characteristics of Drying Technology
Drying technology has a wide range of application fields. Faced with numerous industries, materials with different physical and chemical properties, product quality, and other diverse requirements, drying technology is a cross industry, interdisciplinary, and experimental scientific technology. Generally, knowledge and skills in three aspects are required for the development and application of drying technology. It is necessary to understand the physical and chemical properties of the dried material and the usage characteristics of the product; The second is to be familiar with the principles of transfer engineering, including mass transfer, heat transfer, fluid mechanics, and aerodynamics, which are the principles of energy transfer; The third requirement is to have means of implementation, which includes engineering design for drying processes, main equipment, electrical instrument control, and other aspects. Obviously, these three aspects of knowledge and technology do not belong to the same disciplinary field. In practice, knowledge and skills in these three aspects are indispensable. So drying technology is a cross industry and interdisciplinary technology.
Although modern drying technology has a development history of over a hundred years, it still belongs to the category of experimental science. Most drying technologies currently lack scientific theories and design methods that can guide practice. In practical applications, relying on experience and small-scale experimental data to guide industrial design is still the main way, and the reasons for this situation are as follows:
One of the reasons is that the basic disciplines on which drying technology relies (mainly disciplines belonging to the field of transmission engineering) themselves have the characteristics of experimental science. For example, the development of aerodynamics research still relies on "wind tunnel" experiments, which indicates that it has not yet departed from the scope of experimental science, and the level of development of these basic disciplines directly affects and determines the level of development of drying technology.
The second reason is that many drying processes involve the intersection of multiple disciplines and technologies, involving a wide range of aspects, multiple changing factors, and complex mechanisms. For example, in the field of spray drying technology, the trajectory of atomized droplets in the drying tower is the key to engineering design. The trajectory of a droplet is related to its own volume, mass, initial velocity and direction, as well as the flow direction and velocity of other droplets and hot air around it. But these parameters are constantly changing due to the process of mass and heat transfer, and in the initial state, neither the size of droplets nor the distribution of hot air can be uniform. Obviously, relying solely on theoretical calculations for engineering design of such a complex and variable process is unreliable.
The third reason is that there are various types of materials to be dried, and their physical and chemical properties are also different. Even under the same drying conditions, different materials may have significant differences in their mass and heat transfer rates. If not treated differently, it may lead to unsatisfactory consequences. For example, in the drying of certain Chinese herbs, although they belong to the same medicinal herb, the drying conditions need to be changed simply because of differences in the origin or harvest period of the herbs, 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. However, these characteristics of dry search techniques are often intentionally or unintentionally overlooked by people. Manufacturers often avoid conducting necessary drying experiments due to a lack of experimental equipment or incomplete models (which is a common phenomenon in China), while users often give up the requirement to conduct necessary experiments due to a lack of understanding of the characteristics of drying technology. The outcome is that the device has poor performance and even causes the design of the solution to fail. In our country, such cases are not uncommon. There was a lesson learned from a set of industrial drying equipment worth 20 million yuan being idle due to failure to meet usage requirements. Therefore, before constructing industrial drying equipment, especially larger ones, it is necessary to conduct sufficient and convincing experiments, and use the experimental results as the basis for industrial equipment design. This is a significant characteristic of drying applications.
In addition, the wide variety and diverse uses of drying equipment are also a characteristic of drying technology. Each technology has its own suitable application areas. In engineering practice, the appropriate type of drying technology should be selected based on specific circumstances. This will have a significant impact on investment costs, operating costs, product quality, environmental requirements, and other aspects. For example, an enterprise once used box drying, spray drying and rotary airflow rapid drying for silica filter cake drying. The final result proves that each of these three technologies has its own strengths. Although the production efficiency of white carbon black by box drying is low and the labor intensity of personnel is high, the product quality is good. The tensile strength value generated after mixing with rubber is relatively high. The rotating airflow rapid drying equipment is compact, requires less investment, and has high production efficiency, but the strength indicators of the rubber products generated are inferior among the three. White carbon black is produced by spray drying, and the product indicators are in the middle of the three, but it has the characteristics of good product mobility, low dust pollution, and is welcomed by users and operators. In the 1990s, there was a debate in the drying industry in China about which drying method was more advanced for the production of white carbon black. In fact, each of the three devices has its own characteristics, and the choice of which model to use depends on the user's own conditions and product requirements. There is no conclusion that any technology is more advanced. There are many similar examples, all of which demonstrate the diverse types of drying equipment and their respective purposes. So in application, it is necessary to carefully compare and carefully choose technical solutions, and evaluating technical solutions through drying experiments is also an essential step.
2、 The Development Status of Industrial Drying Equipment
Drying is a crucial unit operation in many production processes, as it is not only a simple solid-liquid sharing process, but also often a crucial step in the production process. The quality and formulation of products largely depend on the comprehensive application of drying technology and equipment. From an economic perspective, dryers are expensive and require significant engineering investment. On the other hand, drying is a high energy consuming process, with thermal efficiency fluctuating within a large range of 15% -18%. The operating cost of equipment is closely related to the design of the dryer, so decision-makers in enterprises have always attached great importance to this. There are many types of materials to be dried, and their physical and chemical properties vary greatly. Even different production processes and product requirements for the same variety may lead to differences in drying conditions, which determines the complexity of drying engineering. It can be seen that the drying process has higher technicality compared to other unit operations.
Before the liberation, China's drying equipment was basically blank, with only drying rooms, ovens, and drum dryers, outdated drying technology, and primitive production equipment. The vacuum rake dryer did not appear until 1957, and drying technology developed rapidly after 1964. Throughout the development history of drying technology and equipment in China, it has gone through stages of development from simple to complex, and from intermediate to low-level over the past few decades. Currently, there are more than ten categories and thirty series of drying equipment commonly used in production, plus about fifty types of combined drying equipment. In addition, specialized drying equipment is even more difficult to count, and it is not an easy task to choose these drying equipment reasonably. The premise of selection is to understand the basic working principles, structural characteristics, and applicable material range of these equipment, so as to avoid detours when selecting.
In recent years, the development of drying technology has brought more complex factors to screening equipment. Even the design, manufacturing, or users of drying equipment often struggle to figure out how to choose the appropriate equipment. Due to the fact that salespeople of drying equipment are only interested in the types of dryers they promote in the market and do not introduce other types, users have to rely on relevant modern drying technology reference materials to decide on equipment selection. Undoubtedly, users need laboratory, experimental scope, and technical and economic information provided by salespeople. Therefore, it is necessary to be familiar with most drying equipment in order to select a reasonable one. It should be emphasized that there may be many suitable drying machines in specific production operation states, but it must also be known that there is no strict rule that specifies the optimal drying equipment in a specific working state, and each product has its own unique production method. There are many factors that affect the selection of drying equipment, such as whether to choose intermittent or continuous drying, the consumption of mineral fuels, electricity consumption, local environmental laws or noise pollution restrictions, etc. The product output is a major factor in the selection of drying machines.
3、 Overview of Drying Equipment Usage
As mentioned earlier, drying equipment is widely used in many industrial productions. Over the years, there have been many types of models used in industrial production, such as airflow 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 dry materials, as well as the various specific requirements of the processed materials.
With the rapid development of production technology in various industries in China, domestic drying technology and equipment have also experienced rapid growth. In recent years, fluidized bed technology has been widely applied and developed in the drying of granular materials. Liquid state drying greatly improves the gas-solid phase contact conditions (increasing the evaporation surface area), and the vigorous agitation of materials greatly reduces the gas film resistance, creating extremely favorable conditions for the heat transfer medium. In addition to the early use of airflow drying in domestic drying technology, fluidized drying equipment has developed rapidly in recent years. Mainly manifested in the use of fluidized bed technology combined with various characteristics and requirements of the dried materials to create many new and efficient fluidized bed dryers, which are described as follows.
The straight tube airflow dryer is an early fluidized drying equipment used in China. After years of production practice, it is believed that airflow drying is still an ideal drying equipment for the drying of granular materials, especially thermosensitive materials. It is superior to oven drying in terms of production volume, footprint, and other aspects, so it is currently more widely used in industries such as pharmaceuticals, plastics, food, and fertilizers. However, airflow drying still has disadvantages such as low heat utilization efficiency, high equipment, and low relative velocity between gas-solid phases. In recent years, new types of airflow equipment such as pulse airflow dryers, cyclone airflow dryers, and pulverization airflow dryers have been developed to overcome the shortcomings of straight pipe airflow drying. In addition to reducing height, crushing airflow also expands the range of use of airflow dryers, allowing easily oxidizable materials to use air as the drying medium. This not only reduces drying power consumption, but also improves product yield and quality. In addition, multi-stage airflow drying processes and combined airflow drying processes are used. In the application of airflow dryers, many projects adopt a two-stage series connection method, which is more reasonable for drying some materials and improves thermal efficiency. Straight tube airflow drying has been mature in production operations, and pulse airflow and cyclone airflow drying have been industrialized for many years. The operation is relatively mature, but there is still a lack of theoretical design. Further perfection is needed in future practical development.
Most materials with strong heat sensitivity and easy oxidation are dried using airflow. Generally, materials with initial moisture of 10% -15% can be dried to 1% -0.5%. The particle size of the dried material is generally between 60-100 mesh, and the yield is generally between 100-200% kg/h. At present, it is widely used in industries such as pharmaceuticals, food, and plastics in China. With the rapid development of production technology in our country, airflow drying will definitely be more widely used in industrial production in the future.
Fluidized drying is another drying technology developed in recent years. Through production practice, it has been proven that it has many advantages and can achieve large-scale production with small equipment. Due to its large heat capacity coefficient and adjustable residence time, it is suitable for materials containing surface water and those that need to go through a reduced speed drying stage, especially for drying granular materials. There are several types of fluidized bed dryers that have recently developed and industrialized, including single-layer cylindrical, multi-layer circular tube, vibrating fluidized bed, and horizontal multi chamber fluidized bed dryers. Due to their simple structure, easy operation, stability, and wide material adaptability, they can obtain products with uniform water content and low power consumption. They are ideal equipment for fluidized bed drying of granular materials and are worth promoting and developing in the future. The internal heat pipe is a product of the combination of convective and conductive heat transfer in fluidized beds, with high thermal efficiency and good drying effect. It is a highly recommended new model in recent years.
There are three types of conical fluidized beds in China according to operation: one is concentrated phase overflow discharge, which has been widely used in fluidized granulation in China in recent years; Another type is fluidized bed drying, which involves discharging the material from the top of the bed and collecting the product in a cyclone separator or intermittently discharging the material from the bottom of the bed. This structure is simpler than fluidized bed structure, with smaller equipment, higher output, higher drying intensity, stronger bed isothermal properties, and no local overheating. In the past, it was only suitable for large particle materials (polyvinyl chloride), but in recent years it has developed to be applicable for drying fine particle materials. Currently used in sectors such as plastics, grains, pharmaceuticals, etc. However, due to high power consumption, its use is subject to certain limitations.
In addition to the new development of spray drying, which is widely used, the conical fluidized bed has been successfully used for spray granulation production in recent years and has gradually been developing and improving. Spray fluidized granulation dryer was first used in chemical fertilizer, and has been used in medicine, food and other industries at present. 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 made progress in the following aspects:
(1) In addition to developing towards large-scale drying rooms, relevant units have also conducted extensive experimental research on the performance of nozzle atomizers, and significant results have been achieved;
(2) In addition to the more extensive use of spray drying for thermosensitive solutions, the slurry has also been successfully dried by spray in recent years;
(3) Spray drying is combined with other drying technologies to achieve the purpose of simultaneous drying or drying granulation, which is also the embodiment of the further development of drying technology in China;
(4) At present, the experiment of low-temperature spray drying is under way. It uses air with extremely low moisture content and low temperature as a carrier. The air is pre dehydrated and dried. During the drying process, the product temperature does not exceed 35 ℃, so it is suitable for drying heat sensitive materials, such as medicine, food, etc.