Vacuum drying has many advantages: low oxygen content when dry at low pressure, can prevent oxidative deterioration of dried materials, dry flammable and explosive dangerous goods; can vaporize moisture in materials at low temperature, easy to dry heat sensitive materials; Recycling valuable and useful ingredients in the dried material; preventing the emission of toxic and harmful substances in the dried material, and becoming an environmentally friendly type of "green" drying. Therefore, the application of vacuum drying equipment is increasingly widespread.
The main disadvantage of vacuum drying is that it requires a vacuum system capable of pumping steam, which makes the equipment investment cost high and the operation cost high; the equipment production efficiency is low and the output is small. In order to overcome these shortcomings, many scientific and technological workers have made many 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 be promising.
1. Unbalanced development of continuous vacuum drying equipment at home and abroad
In order to improve equipment production and ensure product quality, a variety of continuous vacuum drying equipment have been developed abroad more than ten years ago. However, due to the limitations of the technical level and people's perceptions, the development is relatively slow.
1) Belt continuous vacuum drying equipment
KK-VAQ belt continuous vacuum drying equipment manufactured by Japan Hiroshima Co., Ltd. is suitable for liquid materials, mud-like, paste-like concentration and high viscosity boring drying. The BV-100.5 vacuum belt continuous drying device produced by Okawa, Japan, uses steam and conduction heating. The temperature of each section is adjustable, and the belt tension and speed are adjustable. The Swiss company Buch-Gade has developed a range of belt-type continuous vacuum drying units with automatic cleaning.
Since 1995, he has been engaged in the design, manufacture, installation and service of belt continuous vacuum drying equipment, and the technology is relatively mature.
Domestic belt continuous vacuum drying equipment is rare. In 2004, Guangdong Agricultural Science Research Institute successfully developed a small test equipment for the drying of banana powder with good effect.
2) Grain continuous vacuum drying equipment
The amount of grain dried is large and continuous drying equipment must be used. In the past, many countries have developed a number of grain drying equipment, and vacuum dried grains are often used in seed drying because the vacuum drying cost is too high. In fact, this is a misunderstanding. According to the senior engineer of Zhengzhou Food Science and Design Institute He Xiang, the continuous corn vacuum drying equipment developed by them has a production capacity of 60t/d. The fixed investment is slightly higher than hot air drying, running cost and hot air. Dry and flat. If considering the quality of the dried product, the breakage rate of the drying process, the waisting rate, etc., the total cost of vacuum drying at low temperatures is not higher than that of hot air drying.
3) Continuous vacuum freeze drying equipment
The food is rich in raw materials and the output of the products that need to be lyophilized is large. Therefore, continuous food freeze-drying equipment appeared earlier. In 1985, the Danish company ATLAS produced the CONRAD–800 continuous freeze-drying equipment for lyophilized coffee production with a production capacity of 13t per day. Figure 2 is a schematic diagram of the equipment. Figure 3 is a continuous lyophilizer produced in Germany.
The first continuous vacuum freeze-drying equipment made in China was successfully developed by Shenyang Refrigeration Technology Research Institute in 2000. The vacuum chamber adopts a rectangular structure, and a separator is arranged between the inlet and outlet bins and the drying bin, and an automatic weighing system is provided in the inlet and outlet bins, which can determine the drying rate, the water output and the final drying degree of the freeze-dried food. Two external water traps work alternately to achieve continuous water trapping and ice melting.
The sales volume of continuous freeze-drying equipment at home and abroad is very different. ATLAS of Denmark sold 18 continuous freeze-drying equipments from 1985 to 1990. Among them, one Taiwan was purchased in Taiwan, but the mainland provinces and cities have not introduced such equipment, and only one domestic equipment has not been sold yet. Go out.
2. The variety of vacuum drying equipment at home and abroad is increasing
Various new types of vacuum drying equipment have been developed for different purposes.
1) Microwave vacuum drying equipment saves energy and increases drying rate.
2) Spray freeze-drying equipment The combination of spray and freeze-drying is determined by the nature of the material and the yield of the product. The material must be a liquid slurry. After spray freezing, it forms micro-nano-sized particles, which increases the drying rate and improves the product. Yield.
3) Vacuum vapor phase drying equipment is a device specially 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, it has basically completed the introduction and digestion, and now it can be produced in sets.
4) Small-scale experimental freeze-drying equipment In order to meet the needs of schools and scientific research units, several small-scale experimental freeze-drying equipments with different functions have been developed at home and abroad. Figures 4 to 7 are several small experimental freeze dryers produced by Martin Christ of Germany. They are small in size, light in weight and versatile. It basically satisfies the freeze-drying experiment of various materials. Several domestic manufacturers such as Haimen Light Industry Machinery No. 4 have begun to digest and absorb and introduce designs, and have developed new products, but the performance is not stable enough.
Single cavity system
Under aseptic conditions, both pre-freezing and drying are carried out in a condensation chamber.
Double chamber system
The pre-freezing is carried out in a low-temperature refrigerator or a jellyr, and drying is carried out in a drying chamber above the condensation chamber.
5) Freeze-dried and separated pharmaceutical lyophilizer
Food lyophilizers are separately cooled and dried to save energy, save time and increase production. Traditional medical freeze dryers are all frozen and dried in the machine at one time. Some companies in Western Europe and the United States pre-freeze the drug for the same purpose as food lyophilization, and then send it to a dryer for drying. Figure 8 shows a photograph of a continuous drug pre-freezing device.
3. The function of vacuum drying equipment is increasing
With advances in electronics and computer technology, control systems and display functions for vacuum drying equipment have generally increased. The corn vacuum drying equipment developed by Zhengzhou Grain Science Research Institute has automatic control and analog screen display functions; the vacuum freeze dryer produced by Shanghai Dongfulong, Beijing Suwon and other manufacturers has the freeze-drying end point judgment, design freeze-drying curve, and check the operation status. , store multiple freeze-drying curves, print production information and other functions.
Foreign vacuum drying equipment has more functions, most of which have robots, which can be sampled and tested during the drying process; some rotary type drawing machines have pulverizing and granulating functions; pharmaceutical lyophilizing equipment can also granulate .
4. Views on the development of vacuum drying equipment
1) Innovation is the fundamental way to develop vacuum drying equipment
At present, the development of vacuum drying equipment is slow. One important reason is that they are copied to each other, and they lack innovation at the same technical level. The ability to copy is strong, the speed is fast, the concept of innovation is poor, and the investment is small. Put the energy of competition in the market, pull the relationship, price war. A better way is to do some work on quality and after-sales service. However, these are not good methods for developing vacuum drying equipment. They must invest human, material and financial resources to innovate.
2) Energy saving is the key to developing vacuum drying equipment
Energy shortages have received worldwide attention, and traditional wisdom is that vacuum drying equipment consumes a lot of energy because it requires a vacuum pump. However, there has not been a real energy consumption comparison of the same material when it is dried to the same moisture content. In fact, vacuum drying is low temperature drying in an enclosed space, and energy waste is relatively small. Despite this, the energy savings of vacuum drying equipment are still the key to development. Compared with other types of drying equipment, the energy saving focus of vacuum drying equipment should be
This should be placed in a rational design, selection and use of vacuum systems.
There are not many vacuum systems that can pump steam. At present, there are two main types, one is a pump that can directly pump steam, mainly water jet pump, water jet pump, water ring pump and wet Roots pump; It is condensed to catch water. The energy consumption of these two methods is relatively high. The direction of the former type of energy conservation should be to improve the pumping efficiency of the pump; the latter type of energy saving direction is to design the structure and thermal efficiency of the gas-solid phase change heat exchanger.
3) Continuous vacuum drying equipment is an important method to increase product output and save energy.
Continuous vacuum drying equipment has less auxiliary time than cycle vacuum drying equipment, saving time and increasing production. At the same time, he does not have to change the temperature of the equipment once every one cycle, like the periodic vacuum drying equipment, so that part of the energy is wasted on the repeated heating of the equipment components. Therefore, energy is saved.
4) The combination of vacuum drying equipment and other drying methods is the production method that should be promoted.
Vacuum drying combined with other drying methods can increase production efficiency and reduce product costs. For example, vegetables, aquatic products and fruits, such as vacuum drying, can guarantee the quality of the product. If it is washed before vacuum drying, after being washed and blanched, it can be dewatered by centrifugation, then dried by cold air to remove surface water, and then sent to vacuum drying equipment. Significantly increase production efficiency while reducing production costs. Spraying in combination with freeze drying is also one of the combinations. This kind of similar
The combination method can be found through experimentation, and of course it takes a certain amount of time, energy and financial resources.
5. Conclusion
Vacuum drying equipment is constantly innovating. There are still many problems. If production, learning, and research can work closely together, the results will be more significant.