Preliminary discussion on the development of low temperature vacuum continuous drying technology and its tower-shaped equipment
Abstract: Reducing the loss of heat sensitive materials in materials during drying process, reducing the energy consumption of drying processing and equipment operating costs, and lower equipment manufacturing costs are the research and development direction of drying technology and equipment. The low-temperature vacuum continuous drying tower equipment successfully used the vacuum drying technology, the high-air-tight rotary valve to enter and discharge the material, the high heat exchange area and the volume ratio of the dry silo, and the material in the silo is formed by the gravity between the heat exchange tubes. The top-down mixing flow in the reasonable open cavity is heated, thereby achieving high-quality, high-yield, low-energy, low-running drying processes. 2, heating temperature: in order to reduce the drying time in order to reduce the drying cost, the heat transfer medium temperature should be properly increased, because the temperature inside the material particles in the vacuum drying chamber as long as there is moisture for rapid boiling, vaporization and gas pressure in the chamber The association is constant. It is recommended that the upper temperature limit is 110-160 °C, and the throttle valve should be installed in the heat supply pipeline to make the temperature of the heating section in the warehouse form a continuous temperature gradient from top to bottom. The temperature difference is 20 ~ 30 °C to avoid the difference of material moisture is too large, the temperature rise of individual materials exceeds the upper limit, so the moisture difference of the materials should be controlled not to be too large, should be ≤ 3%. This item is expressed in a popular saying. When cooking with high-temperature flame, as long as there is water in the pot, the food in the pot will not exceed 100 °C. In Tibet, 7-8 km above sea level, the boiling temperature is 70-80 °C, and you can't eat cooked rice without using a pressure cooker. When calculating the vacuum pumping speed when selecting the specification, calculate the leakage pressure of the working pressure of the vacuum drying chamber, the amount of gas entering and leaving the warehouse, the amount of gas in and out of the material, and the amount of air leakage connected to each valve in the inlet and outlet. The vacuum power required to remove the total amount of gas outside the chamber should also be calculated. And consider the reasonable spare factor, choose the appropriate specifications. It is necessary to pay attention to the process level and sealing material types of the various parts of the drying equipment manufactured by the manufacturer. Selecting the components and assembly process level at the time of installation determines the model specification of the vacuum pump and the size of the selected module directly affects the motor capacity of the vacuum pump and the amount of power consumption. a Product quality is better than hot air drying after vacuum drying. Process flow diagram bibliography:
Key words: low temperature vacuum, continuous drying, tower equipment, high air tightness rotary valve
Introduction: During the period from 2002 to 2005, the author was fortunate to participate in the research on low-temperature vacuum continuous drying technology of northeast corn and Sichuan Tianfu peanuts, as well as the development, installation, commissioning equipment and drying test of tower drying equipment. Now I will discuss some of the experiences in the research and development of low-temperature vacuum continuous drying technology (hereinafter referred to as vacuum drying) and its tower-shaped drying equipment.
China is a large agricultural country. The soil conditions vary widely across the country. The agricultural and forestry fruit and vegetable products are very rich and the output is huge. With the growth of national strength, the people's living standards are increasing, high-yield, high-quality, low-cost, fast and timely machinery. Equipment for drying agricultural and forestry fruit and vegetable products to meet the urgency of industrial raw material processing and high-quality, people's food needs, is placed in front of the dry workers. At present, most of the equipment used for drying agricultural and forestry fruit and vegetable products, cereals, forest fruits, tea, Chinese herbal medicines and vegetables is atmospheric pressure hot air tower-shaped drying equipment. Due to decades of research and production, its equipment, its technology, The process industry is very mature. This type of equipment has the outstanding features of relatively simple structure, reliable manufacturing process, reliable drying process parameters and low cost of drying and dewatering. At present, it is widely used in various parts of the country, and various models are estimated to be used in more than 10,000 units.
Opportunities and Challenges:
Due to the technical characteristics of the atmospheric hot air drying technology; the dried material is heated by atmospheric hot air, and the material and internal moisture are dehydrated, relying on the difference between the unsaturated vapor pressure and the saturated steam pressure in the hot air and temperature conditions. The inside of the particles is evaporated and discharged with the hot air out of the drying chamber. Then the higher the heating temperature, the less water vapor in the air, and the higher the drying efficiency. If the high temperature hot air dries the grain materials, such as rapid drying, the grain bursts, the seed coat cracks or even ruptures, reducing the quality. Even if the temperature is lowered, multiple stages of slowing down must be used to reduce the breakage. When the high temperature hot air is used to dry the grain and other materials, the material is easily oxidized, acidified, deteriorated, and loses edible value under high temperature and rich oxygen conditions, and the starch is easily denatured. Protein denaturation at high temperature ≥83°C, starch denaturation at ≥64°C, inactivation of active enzyme at ≥55°C, decomposition of vitamin С at ≥40°C, and seeds are more sensitive to temperature. When the rice seeds are ≥35 °C, the physiological activities such as cell tissue will decrease with the increase of temperature until the germination ability is lost. When the material having a heat sensitive substance is dried at a low temperature, there are strict conditions for the hot air temperature and the air humidity. For example, in the high temperature hot and humid weather environment in the south, the air is already in a state of high temperature water vapor saturation, it is difficult to use ambient air to heat the low temperature hot air drying; in the north, low temperature air is used as the heat medium, but because the low temperature air is hot and low, the air in the air is low. The steam partial pressure and the force difference are small, and the evaporation amount is greatly reduced when compared with the high-temperature air, so that the supply power consumption is increased, the drying efficiency is low, the time is long, and the drying cost is greatly improved. It should also be pointed out that the hot air at high temperature after drying the material by hot air must be discharged outside the warehouse. The hot air discharged from the air outside the chamber is not only vaporized latent heat, but also the heat from the outlet air accounts for 30-60% of the heat before entering the warehouse. The lower the air temperature in the hot air furnace, the more the air humidity Large, the material allows the lower the hot air temperature, the lower the utilization rate of dry heat energy, the low temperature hot air drying heat sensitive material, the comprehensive utilization rate of heat energy is only about 20%. According to the data, the energy consumption of drying equipment accounts for about 12% of the total energy consumption of the national economy, while the drying of agricultural and forestry raw materials and the drying of finished products occupy an important position in industrial and agricultural production, and are also bulk products. Its energy consumption occupies a certain proportion in the national economic production, and it is one of the “big households†of energy consumption. At present, the world's energy shortage is scarce. China is at an extremely low level in the petrochemicals, especially oil and gas resources. The country has therefore set the direction for building a scientific and technological development path of economical economy. Coupled with the demand for high quality of life, we have provided us with a good opportunity to develop new equipment that is efficient, energy efficient, low cost and high quality dry materials. The vacuum drying technology equipment provides good technical support, but the original vacuum drying equipment cannot meet the low price, low cost operation and large output demand due to the technical and raw material supporting equipment; Low-priced, high-volume, low-cost dry, easy-to-operate low-temperature vacuum continuous drying technology equipment presents us with serious challenges to overcome insurmountable difficulties. Due to the country's reform and opening up, the emergence of new technologies and new materials has provided us with the possibility of solving one technical problem. After four years of research and development, the possibility of production of low-temperature vacuum continuous drying technology equipment has been initially realized. Today, 20 tons of Haotian, 60 tons of Haotian, 300 tons of Haotian, vacuum drying and processing of Northeast Maize have been developed. Dry processing moisture 36% dry to 4% Sichuan Tianfu peanut prototype. After the test, the equipment after drying has excellent quality and remarkable energy saving effect. The technology has passed expert review and is now being criticized for correcting the relevant issues.
Low-temperature vacuum continuous drying technology characteristics and equipment characteristics:
There are many classification methods for drying technology equipment. For example, according to the pressure conditions around the dry material, it can be divided into two categories: atmospheric drying and vacuum drying. Hot air drying is normal pressure drying. When hot air is used to dry the material, the hot air must be pressurized to the drying chamber with appropriate pressure, and the heat is transferred to the material and discharged to the atmosphere by the residual pressure. The pressure around the material in the drying chamber is higher than the atmospheric pressure in the environment outside the warehouse by hundreds to thousands of Pa, which increases the difficulty of evaporation of water in the material. Vacuum drying is the condition that the pressure of the vacuum drying internal environment is usually between 5000 and 15000 Pa under low pressure and anoxic conditions. The ambient pressure and oxygen partial pressure of the material are only 5.5 to 14.8% of hot air drying. The drying temperature of the material is in the low temperature range between 35 and 54 °C. The actual temperature of the material in the warehouse is only saturated with the free water in the material and the mechanical free water in the large capillary (water is more than 8%) for boiling vaporization. The steam pressure is associated with one-to-one correspondence with the vacuum pressure. Vacuum drying can conveniently realize that the heating medium is not in contact with the material, and the heating medium realizes closed cycle heating without external heat energy. The above basic technical features are aimed at the low temperature in the northeast, large corn yield, high water content, high energy consumption, and must be dried by large mechanical equipment. On the basis of small-scale experiments, a new technology research and development project with a low temperature of 43 ° C (corn seed grade drying temperature) and a vacuum of 8600 Pa continuous drying was proposed. The authors participated in a 20m2 small freeze dryer, vacuum 6600 Pa material temperature 1 ° C, dry corn seed 20 T / D, vacuum 5300 Pa temperature 35 ° C, dry northeast corn 60 T / D, vacuum 8000 ~ 10000 Pa Temperature 43 ~ 47 ° C, dry northeast corn 300 T / D, vacuum 8600 Pa, temperature 43 ° C, dry northeast corn, vacuum 15000 Pa, drying temperature 54 ° C, dry Sichuan Tianfu peanuts and other equipment research and development, installation, commissioning Dry test work, experience and experience, and ask the following questions for discussion.
1. The vacuum drying temperature of the equipment during vacuum drying is not as low as possible, and should not be higher than the upper limit of the allowable heating temperature of the material, whichever is lower. The recommendations are as shown in the table: 
3. Vacuum drying chamber and drying section structure: feeding and discharging sections, the discharging mechanism can follow the original structure of the hot air drying tower, and the discharging mechanism should be vacuum sealed. Due to the structure of other ordinary vacuum drying equipment, the heating area per unit volume is small, and the vacuum tube of the heating tube is difficult to seal under the rotating motion state, so the equipment cost is high. According to the "solid particle material flow theory", we designed a similar column-tube heat exchanger arrangement structure, the material S-shaped flow formed by the self-weight in the tube arrangement opening, the oblique opening suitable size, the top-down uniform mixing flow process The neutralization heating tube contacts the heat. The large-volume dry corn and peanut test proves that the “solid particle material flow theory†provides accurate data, ie D≥4d. The S-shaped mixing flow between the above-mentioned materials in the dry shaped heating tube is smooth, the heating is uniform, and the moisture is dry after drying. The heating tube assembly with special shape structure can heat the area of ​​heat transfer per cubic meter in the drying chamber to 15~30m2. The outer wall of the heating tube assembly is coated with a high emissivity far-infrared wear-resistant coating, and the spectrum and material absorption spectrum tend to Nearly consistent, the long-wave far-infrared penetrating ability is strong. After testing, it can penetrate 7~12mm corn and peanut granules. The above technical measures can supply heat to the material quickly and efficiently, and the heat exchange component can greatly reduce the amount of metal.
4, vacuum drying warehouse wall, heating capacity cavity structure: vacuum drying warehouse is a pressurized container when working, cylindrical warehouse wall, spherical shape warehouse top, bottom is a reasonable structural shape. However, it is difficult to realize the process of sealing and connecting the heating pipe on the wall surface of the silo, so that the arrangement of the heating pipe assembly is arranged and the connection is made by a reasonable process, and the wall of the square and rectangular section is adopted. The structure is reasonable. When calculating the strength of the pressurized container for the square structure, the square structure strength should be calculated according to the working pressure ≥0.1Mpa, and the reinforced or mesh-like structure of suitable size should be used to increase the structural rigidity, and the material in the warehouse can be ensured smoothly. The mixed flow can be strengthened by using the support member and the tie rod under the premise that the heat medium in the heat medium cavity is not blocked, and the amount of metal for reinforcing the rigid structure can be greatly reduced, and the manufacturing cost of the device is reduced.
5. Drying of the material in the vacuum drying chamber should be carried out by thin layer drying: in other types of vacuum drying equipment, the materials are not filled with the contents of the drying chamber, and the accumulation layer is thin. When the accumulation layer is thick, the material is continuously turned and stirred in the warehouse, and the resistance of the water vapor and the like in the drying chamber to the outside of the warehouse is small. The vacuum drying section of the equipment is filled with materials from several meters to ten meters. The gas is removed from the material and the resistance is large. It is bound to consume large vacuum power. The thickness of the dry layer of the material is slightly thinner than the thickness of the hot air drying material layer. The design has a good trial effect, and the vacuum gauge shows that the pressure difference between the inside and outside of the chamber is small.
6. Vacuum drying equipment is equipped with high-air-tight rotary valve: vacuum drying equipment can only achieve high-yield, low-cost vacuum drying operation with high-air-tight rotary valve for continuous feeding and discharging. The bottleneck of cost and high cost of use can be solved well. Various types of rotary valves that can be used at home and abroad have complex structure, high manufacturing process requirements, low air tightness and high price, and are difficult to use on vacuum drying equipment. On the Sichuan Tianfu peanut drying equipment, we used a rotary valve with a simple structure, a low manufacturing process and a low manufacturing cost, and a vacuum drying requirement. After being used well, it meets the requirements for the allowable air leakage of the equipment. The leaked gas with a measured pressure difference of 0.1 MPa is about 50 liters. Using the rotary valve to enter and discharge materials can reduce the height of the equipment by 5 to 8 meters, which can reduce the cost by 50,000 to 80,000 yuan. The above-mentioned 3, 4, 5, and 6 items are named as low-temperature vacuum continuous drying tower-shaped equipment.
7. Vacuum drying material for heat source and heating equipment: When vacuum drying, the material is not in contact with the heat medium, so it is possible to select the heat source of the user's location to obtain heat. For example, power plant waste water steam, large boilers, cement plants, ceramic plants, and other high-temperature exhaust gas in kiln, centralized heat source, heat transfer oil, etc. When the user must purchase heating equipment, it is recommended to use a micro-pressure steam boiler of ≤0.1Mpa, using the heating pressure ≤0.1Mpa. First, when manufacturing vacuum drying equipment and installation equipment, and laying heating pipes, it is not subject to the mandatory restrictions on the manufacture and installation of pressure vessels, and the strict management of local departments. The second is to achieve unpowered heating and reduce the amount of electricity used.
8. Vacuum drying equipment should use vacuum pump correctly: There is a large amount of vacuum in the process of vacuum drying materials. The large volume of water vapor should be removed from the warehouse by the vacuum power generated by the vacuum pump. The following table is recommended for the type selection: 
9. Recycling of condensate in the warehouse when vacuum drying: When vacuum drying materials, there is a large amount of condensed discharge of water vapor from the warehouse, which is estimated to dry 300 tons of scorpion and 10% of corn precipitation, and can be recycled by non-contact condenser. Condensed soft water contains more than 30 tons of soft odor oil. The value of water per ton is 50-100 yuan, and the water in barrels is 1-2 yuan, which can increase the income by 1,500-3,000 yuan per day. The income of 100 working days in the northeast region is 15000-30,000 yuan. Recycling equipment may be considered in areas where drinking water is lacking, and if it is not recycled, it can be directly discharged into the water tank of boiler equipment.
10. Improve the comprehensive utilization rate of heat energy: further improve energy efficiency. Compared with hot air drying, vacuum drying has a significant energy saving advantage due to the fact that there is no external heat energy and the heat source is minimally affected by the climate. When you have a self-contained steam boiler in a cold area, you can greatly improve energy efficiency. First, the use of steam working pressure is lower, the outlet steam temperature ≤ 119 ° C boiler exhaust flue gas temperature can be reduced, can improve the boiler thermal efficiency of 1 ~ 3%. The second is to make full use of the exhaust gas of the boiler with a high temperature of about 200 °C, install a high-efficiency heat exchanger to heat the material before entering the warehouse, and raise the temperature to the vacuum drying temperature. This is because the moisture in the corn and other materials in the cold season in the northeast is in the form of ice. Because the corn in the northeast is dry in the low temperature season, for example, the average temperature in January: Heilongjiang -20 ° C; Jilin - 16 ° C; Liaoning - 10 ° C Inner Mongolia -16 ° C, the average moisture of corn varies from 22 to 32%, but the moisture in corn is in the form of ice. When entering the drying chamber, the heating process requires a large amount of heat energy. The temperature is raised from -10 °C to 40 °C, the temperature difference is 50 °C, and the energy consumption is calculated by using 25% of water at 1000 Kg as follows: temperature rise 50 °C 25% moisture, ie 250Kg ice dissolved water, 335KJ∕Kg×250=83750KJ, 1000Kg corn temperature rise 50°C, 2.2 KJ/Kg×1000Kg×50°C=110000KJ, 120Kg dehydrated after drying to 14% water, energy consumption 2968KJ/Kg×120Kg=356160KJ total heating energy consumption 83750KJ+110000KJ=193750KJ; dry 1000Kg corn energy consumption 193750KJ+356160KJ=549910KJ; The proportion of total energy consumption: 193,750 ÷ 549,910 = 35.2%, so the design of the use of hot air stove waste heat and out of the air to preheat the corn before drying can save more than 35%. In addition, the design can also send the water vapor condensation drying layer in the humid air to the hot air furnace heat exchanger, and can save energy by 20-30%. The total energy saving (coal) can reach 50%. Calculated by a 300-ton/day drying tower, the average daily savings can be 7-8 tons × 400 yuan ≈ 3,000 yuan, calculated on the working day 100 days, the savings per dry season can reach more than 300,000 yuan.
Preliminary conclusion
1. Applicable to the range of vacuum drying materials, suitable for large dry batches, 5~1000T/D when dewatering 10% of the vacuum drying capacity of a single set. Any moisture content of granular material with an average size of ≤30mm. For example, all kinds of cereals, their seeds, fruit and fruit products, jujube, ginkgo, camellia seed, cocoa beans, coffee beans, all kinds of fruits, hawthorn, strawberries, apples, all kinds of vegetables, mountains, vegetables, Chinese herbal medicine, Slices, granules, chemical pellet products, peanuts, melon seeds and other particles. Pellet feed, puffed feed, granules and post-expansion additives are dried again. Rice granule foods are added after nutrient fortification. It is especially suitable for low temperature requirements of various types of seeds. After drying, the water content is ≤ 8%, the storage time is long, and the germination rate is high.
2, advantages;
b Energy saving effect is significantly 30 to 60% less than hot air
c Green, safe, no flammable and explosive dangers d High moisture material has a short drying time
3, using the necessary technical means to produce the market price and hot air drying compared to the price is relatively high or slightly higher than 5 to 15% of the low drying cost, large-volume, high-efficiency low-temperature vacuum continuous drying tower-shaped equipment has become a reality.
4. A major technological breakthrough in the field of drying technology equipment.
Postscript: Due to the limited personal level of the author, we are engaged in the scientific research of vacuum drying equipment, research and development test, the time is short, and the vacuum drying technology is unknown. Some of them are known but not much solved, and the understanding of vacuum drying technology theory is obvious. There are many delays in the above discussion. Please ask experts, scholars, and industry colleagues to criticize and correct.
In the above research and experiment, I got the care and support of many friends, and I got the specific guidance from the comrades of Jiliang Group Gonglingling Grain Depot, Henan Jidi Company, Zhengzhou Sanrong Vacuum Company and Sichuan Deyang Jinfuwang Food Company. And thanks. 
Pan Yongkang Wang Xizhong "Modern Drying Technology" Chemical Industry Press Xu Chenghai Zhang Shiwei Guan Kuizhi Editor-in-Chief "Vacuum Drying" Chemical Industry Press