Combination microwave drying technology Research
Abstract: This paper describes the domestic and foreign research status of microwave vacuum drying , the focus of a variety of microwave drying, and analyzed the microwave vacuum drying technology on several issues .
1 Preamble
Microwave is the frequency of 300MHz ~ 300GHz, lmm ~ lm wavelength of the electromagnetic wave . Its drying principle is : the microwave generator to the microwave radiation on the material to be dried , when the microwave radiation within the material when the person , so that the material of water and other polar molecules at microwave frequencies for synchronous rotation and swing ; water and other polar molecules result of the rotation speed , the frictional heat is generated within the material instantaneously , causing the surface within the material and the temperature was raised , a large number of water molecules evaporate from the material to escape , so as to achieve the purpose of drying .
Microwave vacuum drying with microwave drying technology was developed a new combination drying technology . It not only has a fast-drying, when asked short , the material temperature is low, the color, flavor and nutrients retention and good, and easy to control parameters , can dry many different types of materials. At present , although some units being studied , but also need to improve its technical performance , and technological aspects of the mechanism are also many issues that need to deepen and research.
(2) research status
Back in the 1980s , the United States , Canada, Britain and Germany began to study the microwave vacuum drying technology , mainly in the United States, University of Wisconsin , University of California , Canada, Britis C0lumbia University , Germany University of Karlsruhe , British Queen University, Greece the National University of Technology, Institute of Albi in France and so on. Content of the study involving microwave vacuum dryer manager, heat and mass transfer analog microwave vacuum drying , microwave vacuum drying process as well as energy consumption and a variety of different types of materials ( banana, carrot slices, pectin , potatoes , berries , etc. ) of the microwave vacuum drying operation and so on.
Current domestic research units Jiangnan University College of Food , Northeastern University, Dalian Fisheries University, China Agricultural University , Zhejiang University, Shanghai University of Engineering , South China University of Technology, South China Agricultural University , Tianjin University of Light Industry , Shanghai Light Industry Machinery Corporation Chen Chan , Sichuan University, College of food Science and food Engineering , etc.
Food Science and Technology , Jiangnan University conducted a cabbage and hot air combined microwave vacuum drying test . The results showed that : Microwave Vacuum Drying shorten the drying time to ask 48% , improved preservation of nutrients and chlorophyll , improving the drying quality .
Dalian Fisheries University Zhangguo Chen had scallops microwave - vacuum - combined drying , experimental study of microwave power , vacuum, microwave hoist ratio, pre- salt concentration and the size of the dried scallop effect , establish a scallop microwave vacuum drying kinetic model .
3 combination microwave drying technology
Drying is a combination has broad prospects for development of drying technology , it can play a variety of drying technology strengths, overcome their shortcomings, borrow , complementarity , to achieve high efficiency , low energy consumption , excellent quality drying purposes. Because microwave drying is a completely different from other drying drying technology, so it is also combined with other drying up as a drying technology , but also the most current international research on a dry technique . Here are some of the more common combinations.
3.1 Combination microwave hot air drying ( also known as microwave convection drying )
Combined with the microwave drying method , the microwave drying of hot mix most studied . Because hot air drying time is long , the quality is poor, it is not suitable for drying heat-sensitive materials ; combination microwave drying using hot air can overcome these drawbacks. In addition , the cost of microwave drying and hot air drying or high compared , microwave drying alone is not economical . Hot air drying of the material is dried from the surface inwards , temperature gradient and the transfer of water in opposite directions, while the microwave drying is heated from the inside , the temperature gradient and the same direction of the transfer of water , the combination can be achieved not only shorten the drying time and reduce costs . Microwave and hot air drying can have a combination of three ways .
3.1.1 In the critical moisture content by adding microwave
When the drying section from the constant deceleration segment into ( ie, material moisture content reaches a critical moisture ) when microwave energy into the dryer , so that the material internally generated heat and vapor pressure , so that the moisture diffusion to the surface of the material and excluded , then use microwave be very significantly improve the drying speed.
3.1.2 Add microwave terminal of the dryer
A single drying system near the end of the least efficient drying to remove a few percent of the water often takes a long time , the use of microwave drying time can be significantly reduced .
3.1.3 In the initial warm-up phase to join the microwave
Before drying, the moisture content of materials is high, the material can be heated by microwave to the evaporator temperature , using an ordinary hot air drying , to remove surface moisture , the drying time can be shortened.
3.2 Combination microwave vacuum drying
Microwave , while having fast heating, drying time is short , good selectivity and high energy efficiency and ease of control , etc., but simply using microwave food drying , prone to the phenomenon due to overheating caused by burns and food edges coking, crusts and hardening and other phenomena ; these phenomena is mostly due to high temperature and drying too fast cause. Vacuum evaporation of water can reduce the temperature of the material at a lower temperature rapid evaporation , but also to avoid oxidation , thereby improving the drying quality . In the pharmaceutical , food and chemical industries have a lot of sensitive materials require low-temperature fast drying , so the microwave technology combined with vacuum technology has become a very promising and practical value of new technologies. From domestic and foreign research status of microwave drying of view, combination microwave vacuum drying is currently developing a combination of rapid drying technology .
3.2.1 Pulse intermittent microwave vacuum drying
Notwithstanding the many advantages of microwave drying , but often local overheating occurs , surface hardening , color errors and the phenomenon of uneven heating ; Furthermore , the energy efficiency is not high is a drawback. One of the reasons for these phenomena is the heat and mass transfer controlled, one solution is to use pulsed microwave energy input , ie short microwave heating and longer interruption. Tests to prove: When the critical moisture after drying , microwave energy is applied continuously and not accelerate the evaporation of water ; intermittent drying method can not only save energy, improve drying efficiency , but also can improve the quality of dried materials . Intermittent pulses of microwave vacuum drying technology is Edh0lm proposed in 1933 . Using this technology is characterized by the material in the water and the temperature in the intermittent phase equilibrium redistribution can reduce the moisture gradient , which will help improve the next stage of the drying rate .
Results also showed that , pulsed microwave drying , the microwave on the longer the OFF time shorter the higher the temperature the material . Therefore, by adjusting the degree of vacuum pulse ratio or the temperature of the material can change .
3.2.2 Variable power microwave vacuum drying
Canadian Food Engineering Christene H. Etc. radish slice variable power microwave vacuum drying, microwave frequency of 2450MHz, adjustable microwave power 4kW , the vacuum of 13.3kPa, carrot pieces of the final moisture content of 10% , a microwave cavity is cylindrical , diameter 350mm, length 500mm, using a dry process as follows: after the initial start of drying 19min microwave power 3kW, intermediate 4min to 1kW, the final 10min of 0.5kW. Studied during the test color, rehydration , density and carotene, vitamin content and other quality indicators. The results showed that: If taken together, microwave vacuum drying performance even better vacuum freeze-drying . Developed at the University of California, microwave vacuum drying equipment is a resonator length of 12.2m of the stainless steel cylinder , the middle conveyor belt, in the longitudinal direction of the drying zone is divided into three , the first drying zone of the microwave power is larger, the degree of vacuum of 1.33 ~ 3.99kPa, second, third drying zone decreasing microwave power . Description Variable power microwave vacuum drying is a research direction.
3.2.3 Combination microwave hot air and vacuum drying technology
Maskan combination microwave and hot air dried kiwi, dried kiwi found shrinkage ( 76% ) less than a simple microwave drying ( 85% ) , and the color is also greatly improved. Szab0 use hot air + microwave + combinations for mushroom drying test and found to improve the quality of dried mushrooms . Dalian Fisheries University study shows , hot air drying shrinkage scallops with smaller Durance hot air combined with the use of microwave vacuum drying tomatoes, tomatoes found rehydration rate improved. Thus, the microwave vacuum drying and hot air drying with a complementary. In recent years, the high moisture content and drying heat sensitive materials , a combination of microwave vacuum drying and hot air gradually been applied.
4 A few issues worth exploring
4.1 on the size and shape of the material
Microwave drying wide variety of materials , components and states also vary , according to the shape of a liquid , paste, paste, granules , flakes , powder ; divided by type of vegetables , fruits, grains , medicines, aquatic products and agricultural products ; About the size can be as small rapeseed , large ginseng, mushrooms. Microwave drying studies show that the size of the material , shape, number , moisture, and the position of a microwave resonator has a certain influence on the drying effect . Dr0uzas pectin using microwave drying test, five trays with five different positions on the furnace and found a significant difference between the drying rate . Therefore, the material should be based on microwave drying characteristics ( thermal physical properties of dielectric properties , water content and shape and size ) selected drying process and parameters, the following principles:
① microwave power should match the amount of dry material .
② the size of the material to be dried and the moisture content should be as uniform as possible .
③ considering the penetration depth of the microwave , the bulk material is best processed into small granules or flakes .
④ If the powder material should be stacked together as a whole .
⑤ pellets materials used microwave power (w / g) can be appropriately reduced.
⑥ suitable for sensitive materials can increase or decrease the degree of vacuum microwave power .
4.2 With regard to the degree of vacuum
Seen from the steam characteristics table , the higher the degree of vacuum , the lower the boiling point of water , more water to evaporate . However, the microwave vacuum drying , the higher the degree of vacuum is not possible , the degree of vacuum increased energy consumption increase, plus drying costs , and will produce breakdown discharge phenomenon. When the microwave frequency of 2450MHz , the vacuum degree of 2 ~ 7kPa enough, the corresponding water vaporization temperature is 20 ℃ and 40 ℃. For heat sensitive materials require material temperature is low, so the vacuum will be higher. Prof. Pere France under vacuum conditions at different microwave vacuum drying tests , tests showed that the degree of vacuum under the same conditions to 7kPa from 1kPa , each unit using a vacuum a great difference values ​​, indicating the microwave vacuum drying a reasonable choice in vacuum requires further study.
5 Precautions
Microwave vacuum drying , there are some issues that need attention :
① microwaves can be metal reflector , dry material and test sensors can not be mixed metals.
② upon the size and shape of the dry material should be basically similar.
③ microwave drying equipment non -load operation .
④ microwave can penetrate glass and polymer without loss of energy .
⑤ microwave materials should be scattered arrangement rather than accumulation.
⑥ drying process material is best to exercise .