There are many types of vacuum drying equipment, which have a wide range of applications and are developing rapidly. This article only elaborates on the domestic and international development trends of several types of vacuum drying equipment, with the aim of exchanging information, identifying the problems that need to be solved in the development of vacuum drying equipment, and improving the level of vacuum drying equipment in our country.
Key words: Vacuum drying Drying equipment; Freeze-drying
Vacuum drying has many advantages: when drying under low pressure, the oxygen content is low, which can prevent the oxidation and deterioration of the material being dried, and it can dry flammable and explosive dangerous goods. It can vaporize the moisture in the material at low temperatures, making it easy to dry heat-sensitive materials. It can recover the valuable and useful components in the dried materials. It can prevent the emission of toxic and harmful substances in the dried materials and can become an environmentally friendly type of "green" drying. Therefore, the application of vacuum drying equipment is becoming increasingly widespread.
The main drawback of vacuum drying is that it requires a vacuum system capable of extracting steam, which leads to high equipment investment and operating costs. The production efficiency of the equipment is low and the output is small. To overcome these shortcomings, many scientific and technological workers have made a lot of efforts. At the same time, due to the many advantages of vacuum drying, some products have to use vacuum drying equipment. Therefore, the development of vacuum drying equipment will have a very promising future.
1. The development of continuous vacuum drying equipment at home and abroad is unbalanced
To increase the output of equipment and ensure product quality, various continuous vacuum drying devices were developed abroad over a decade ago. However, in China, due to the limitations of technological levels and people's cognitive concepts, its development has been relatively slow.
1) Belt continuous vacuum drying equipment
The WL-VAQ type belt continuous vacuum drying equipment produced by Nisaka Manufacturing Co., Ltd. of Japan is suitable for drying liquid materials, slurries, pastes, high-concentration and high-viscosity materials. The BV-100.5 vacuum belt continuous drying device produced by Okawara Co., Ltd. of Japan adopts steam and conduction heating. The temperature of each section is adjustable, and the tension and speed of the conveyor belt are also adjustable. The Buch-Gade company of Switzerland has developed a series of belt continuous vacuum drying equipment with automatic cleaning devices.
Since 1995, we have been engaged in the design, manufacture, installation and service of belt continuous vacuum drying equipment, and our technology is relatively mature.
Domestic belt continuous vacuum drying equipment is not common. In 2004, the Guangdong Academy of Agricultural Sciences successfully developed a small-scale experimental device for the drying of banana powder, with very good results.
2) Continuous vacuum drying equipment for grains
The amount of grain to be dried is very large, so continuous drying equipment must be used. In the past, many countries around the world developed a large number of grain drying equipment. Vacuum drying of grains was mostly used for seed drying because the cost of vacuum drying was too high. In fact, this is a misunderstanding. According to Senior Engineer He Xiang from Zhengzhou Grain Research and Design Institute, the continuous corn vacuum drying equipment they developed has a production capacity of 60 tons per day. The fixed investment is slightly higher than that of hot air drying, and the operating cost is the same as that of hot air drying. If the quality of the dried product is taken into account, such as the breakage rate and the rate of bulging during the drying process, the total cost of vacuum drying at low temperatures is not higher than that of hot air drying.
3) Continuous vacuum freeze-drying equipment
Food raw materials are abundant, and the output of products that need to be freeze-dried is very large. Therefore, continuous food freeze-drying equipment emerged relatively early. In 1985, the Danish company ATLAS produced the CONRAD - 800 continuous freeze-drying equipment for freeze-dried coffee production, with a daily production capacity of 13 tons. Figure 2 is a schematic diagram of this equipment. Figure 3 shows a continuous freeze dryer produced in Germany.
The first domestic continuous vacuum freeze-drying equipment was successfully developed by the Shenyang Refrigeration Technology Research Institute in 2000. The vacuum chamber adopts a rectangular structure. Isolation plates are set between the feeding and discharging bins and the drying bin. Both the feeding and discharging bins are equipped with automatic weighing systems, which can determine the drying rate, water output and final drying degree of freeze-dried food. Two external water traps work alternately to achieve continuous water capture and ice melting.
There is a significant gap in the sales volume of continuous freeze-drying equipment at home and abroad. From 1985 to 1990, the Danish ATLAS Company sold 18 continuous freeze-drying machines in just five years. Among them, one was purchased from Taiwan, China. However, no such equipment has been introduced to any province or city on the Chinese mainland to date. Only one domestically produced machine has yet to be sold.
2 The varieties of vacuum drying equipment both at home and abroad are increasing
For different purposes, various new types of vacuum drying equipment have been developed.
1) Microwave vacuum drying equipment is energy-saving and increases the drying rate.
2) The combination of spray and freeze-drying in spray freeze-drying equipment is determined by the nature of the material and the output of the product. The material must be a liquid slurry. After being sprayed and frozen, it forms micro-nano scale particles, which increases the drying rate and enhances the output of the product.
3) Vacuum vapor phase drying equipment is a device specifically designed for drying large oil-immersed transformers and other similar electrical equipment. Switzerland developed this product in the 1970s. China introduced this kind of equipment in the 1980s. After the 1990s, the introduction and digestion were basically completed, and now it can be produced in complete sets.
4) Small-scale laboratory freeze-drying equipment: To meet the needs of schools and research institutions, several types of small-scale laboratory freeze-drying equipment with different functions have been developed successively at home and abroad. Figures 4 to 7 show several small-scale laboratory freeze dryers produced by the German company Martinchrist. They are small in size, light in weight and have multiple functions. It can basically meet the freeze-drying experiments of various materials. Several domestic manufacturers, such as the Fourth Factory of Haimen Light Industry Machinery, have begun to digest, absorb and introduce designs, and have developed new products, but their performance is still not stable enough.
Single-chamber system
Under aseptic conditions, both pre-freezing and drying are carried out in the condensation chamber.
Dual-chamber system
Pre-freezing is carried out in a low-temperature refrigerator or a rotary freezer, and drying is conducted in a drying chamber above the condensation chamber.
5) Freeze-drying separation type freeze dryer for pharmaceutical use
Food freeze dryers all operate the freezing and drying processes separately to achieve the goals of energy conservation, time saving and increased output. In traditional medicine, freeze dryers complete both freezing and drying in one go within the machine. Some companies in Western Europe and the United States, in order to achieve the same purpose as freeze-drying food, pre-freeze medicines and then send them into dryers for drying. Figure 8 shows a photo of the continuous drug pre-freezing equipment.
3 The functions of vacuum drying equipment are increasing
With the advancement of electronic and computer technology, the control systems and display functions of vacuum drying equipment have generally increased. The corn vacuum drying equipment developed by Zhengzhou Grain Science Research Institute has the functions of automatic control and simulation screen display. Vacuum freeze dryers produced by manufacturers such as Shanghai Dongfulong and Beijing Suyuan have multiple functions including determining the end point of freeze-drying, designing freeze-drying curves, checking the operation status, storing multiple freeze-drying curves, and printing production information.
Vacuum drying equipment from abroad has more functions. Most of them are equipped with mechanical arms and can take samples for testing during the drying process. Some rotary vacuum drying equipment has the functions of crushing and granulation. Pharmaceutical freeze-drying equipment can also be used for granulation.
4 Several views on the Development of vacuum drying equipment
Innovation is the fundamental way out for the development of vacuum drying equipment
At present, the development of vacuum drying equipment is relatively slow. One important reason for this is the mutual imitation, remaining at the same technical level and lacking innovation. The ability to imitate is strong, the speed is fast, the concept of innovation is poor, and the investment is low. They focused their competitive efforts on seizing the market, building connections and engaging in price wars. A better approach is to put in some effort in terms of quality and after-sales service. However, none of these are good ways to develop vacuum drying equipment. It is necessary to invest human, material and financial resources to innovate.
Energy conservation is the key to the development of vacuum drying equipment
Energy shortage has drawn worldwide attention. The traditional view holds that vacuum drying equipment has high energy consumption because it requires a vacuum pump. However, I have never witnessed a true comparison of energy consumption for the same material when it is dried to the same moisture content. In fact, vacuum drying is a low-temperature drying process carried out in a closed space, resulting in relatively less energy waste. Despite this, energy conservation of vacuum drying equipment remains the key to its development. Compared with other types of drying equipment, the energy-saving focus of vacuum drying equipment should be
It should be placed on the rational design, selection and use of vacuum systems.
There are not many vacuum systems capable of extracting steam. Currently, there are mainly two categories. One is pumps that can directly extract steam, including water jet pumps, steam jet pumps, water ring pumps and wet Roots pumps. Another type is to capture water by condensation. Both of these methods consume relatively high energy. The direction of energy conservation for the first type should be to enhance the pumping efficiency of the pump. The latter type of energy-saving direction is to design the structure and thermal efficiency of gas-solid phase change heat exchangers.
3) Continuous vacuum drying equipment is an important method to increase product output and save energy
Compared with periodic vacuum drying equipment, continuous vacuum drying equipment requires less auxiliary time, saves time and increases output. At the same time, it does not have to be like the periodic vacuum drying equipment, where the temperature of the equipment changes once after each production cycle, resulting in some energy being wasted on the repeated heating of the equipment components. Therefore, energy was saved.
4) The combination of vacuum drying equipment with other drying methods is a production approach that should be advocated
The combination of vacuum drying and other drying methods can enhance production efficiency and reduce product costs. For instance, for products such as vegetables, aquatic products and fruits, vacuum drying can ensure product quality. If, before vacuum drying, they are washed and blanched, then subjected to centrifugal dehydration, followed by cold air drying to remove surface water, and then sent into vacuum drying equipment, it can significantly improve production efficiency and reduce production costs at the same time. The combination of spray and freeze-drying is also one of the combination methods. This kind of similar
There are many combination methods that can be found through experiments, but of course, it also requires a certain amount of time, energy and financial resources.
5 Closing remarks
Vacuum drying equipment is constantly being innovated There are still quite a few existing problems. If industry, academia and research can cooperate closely, the results will be more remarkable.
The materials I have consulted are limited and my insights may not be accurate. Corrections are welcome.