Air drying is a continuous and efficient solid Fluidization drying method, tobacco, chemicals, pharmaceuticals, food processing and other industries generally. In-depth research on principle of pneumatic drying and its application in tobacco processing for optimizing drying process parameters and the development of new tobacco drying equipment and give full play to advantages of pneumatic drying processing technology to improve product quality of cigarettes is important.
1, principle and characteristic of pneumatic drying
1.1, air dryer drying principle
Air drying, also known as Instant drying, heating medium is (both heat and wet, such as air) process of direct contact with dry solid material. Materials in suspension in the air, heating medium to convective heat transfer heat to the material, material in the parts of water vapour, so as to obtain a certain moisture content of solid products.
In the process of drying material, material particles in the air movement into accelerated motion and isokinetic phase. During the acceleration phase, particles are drag and buoyancy is greater than gravity, with upward acceleration, therefore particles and the relative velocity of the flow is a variable with the increase of particle speed, drag decreases until the 3 vectors and zero, particles into the constant velocity phase, relative velocity between the gas and particle at this time is a constant. Particles and air of relative movement situation on particles and air Zhijian of heat transfer rate effect larger, in initial dry stage, particles just into dry tube Shi rose speed for zero, and has high speed of heat flow met, get up of speed, at two-and-of convection heat transfer coefficient is big, material particles constantly accelerated rose, into accelerated movement dry stage, solid particles in accelerated stage by get of heat accounted for whole dry stage get heat of half above. Later in the dry, when the rate of solid material close to or even reach air speeds, convective heat transfer coefficient greatly reduces drying efficiency. Changing in the drying process of gas-solid two phase relative speed, boost around grain boundary layer turbulence intensity, maximizing contact area of gas-solid two phase, increasing the contact time of phase, are effective measures to improve drying efficiency.
1.2 characteristics, drying equipment
① heat and mass transfer between gas-solid two-phase surface area, high efficiency. Because of the solid material (particles) in the highly fragmented state in the air, the contact area between the two has increased significantly, at a high velocity (20-40m/s) under the action of high relative velocity of gas-solid two-phase, volumetric heat transfer coefficient, high thermal efficiency; second, the drying time is short. Drying process takes only a few seconds, and is particularly suitable for heat-sensitive and low melting point materials dry c flow resistance of large power consumption.
Currently air drying equipment mainly straight-tube, pulse-tube, Tornado and inverted cone Jet dryers. Application of straight tube type air dryer is prevalent; pulse tube-type air dryer drying efficiency was much higher than the straight tube, using alternate methods to refine and expand the diameter, alternating particle acceleration or deceleration, causing air and the relative velocity of the particles, and large heat transfer area, thus strengthening the heat and mass transfer rate. Meanwhile, falling within the airflow on the large diameter, to extend drying time for the item. Development direction of drying equipment is the dryer unit diversity, equipment flow pipe network and material dispersion mechanization.
2, air drying technique and its application in tobacco processing equipment
2.1, air drying technology and equipment of tobacco
As early as in 1959, Anderson came up with a hot air drying method of tobacco, then design a drying tube and cylindrical drying chamber interval pulse tube cut tobacco drying system. The principle is, high water content of tobacco is carried along by hot air drying tube up into the drying chamber, did not reach the top of the drop, so the cycle, tobacco is constantly drying by hot air, until the water reaches the set value has been conveying for dry rooms. This reciprocating drying overcomes drum drying equipment's uneven water content of tobacco, and is capable of continuous operation.
20th century 60-70, the researchers designed a variety of air drying methods and equipment for tobacco, but because the technology is not perfect, makes cut tobacco dryer residence time is too long and is easily broken. 1983, Hibbits design out has more classic of high temperature air dry tobacco equipment, by feeding device, and dry tube, and separation device and for heating process gas of heater composition, tobacco was high temperature high-speed of overheating steam gas conveying through paper Hill in tube and dry tube, in dry tube in the run Shi tobacco speed always below air speed, so heat transfer biography quality rate is high, tobacco in dry tube in the of stay time not over 1s. Wu, research of pneumatic conveying of tobacco drying machine by hot air carries the features of tobacco into tangential separator, conveying, drying and separation at about the same time, tobacco in straight pipe run in a short distance, effectively solve the problem of cut tobacco broken.
Tobacco industry tobacco drying equipment mainly used in China are imported from abroad and the digestion and absorption of, in the course of using some of the equipment was improved. In 2002, by Changzhou zhisi machinery cigarette factory and Hefei jointly developed "SH9-shaped wire line high-speed expansion system", based on tube tower structure and pulse-air transport, the heat transfer coefficient greatly enhanced, full access to the airflow in the drying process and of tobacco, which effectively reduces the moisture not uniform phenomenon of cut tobacco Pellet. In addition, libiao, Dickinson-Legg produced the HXD drying into the vertical tube of air drying system with large RADIUS arc runner and the cut surface of oval horizontal dry pipeline, and improved the distribution of hot air to make drying more uniform moisture content of tobacco exports, broken tobacco reduction.
In 1967, Wright with hot-air drying of tobacco, when added to the dry air in the steam or water, padding the result of tobacco increased. Since then, researchers using a variety of methods to improve the airflow drying of cut tobacco padding value, for the purpose of saving costs and reducing cigarette tar. Jew-ell 120-340 ℃ high temperature air drying, such as stems, adding steam to the air or vapor-air mixture, along with the increase in water vapor content in the air, pipe tobacco filling value also increased significantly. Scrunecker analysis, by adding water vapor to the dry air can improve the air wet bulb temperature, avoiding tobacco is padding reduces due to contraction in the drying process. Dipling moisture content of tobacco 10%-380-1000 ℃ hot air drying, padding the result of tobacco were 30% higher than before drying. But, higher drying temperature will cause the loss of tobacco aroma. Hibbits design is the moisture content of 48.5% superheated steam drying of tobacco with 350 ℃, padding value can be 8.3cm3/g, 63% higher than prior to drying.
In 1993, w. xiershi during the detailed design of the pneumatic drying of tobacco such as air speed, air temperature, material moisture, upper and lower limits of the material temperature and gas range. To speed up the initial drying, design speed of air flow value up to 100m/s, in addition to increased drying of cut tobacco moisture content (less than 40%), added to the drying gas vapor, drying downstream section area design is 3-5 times the cross-sectional area in the upper reaches, these measures have helped to speed up the drying rate, improve the fill value of tobacco. The technology was licensed to Dickinson-Legg company manufacturing and selling air drying equipment. Werkmeister design of drying equipment, such as different from the traditional straight-tube air dryers. Tobacco is air carries heat through two successive segmental elbow pipe, fill the drying of tobacco can be reached 5.41cm3/g. IE Tatham design for airflow drying device and similar.
With the in-depth study on cut tobacco drying equipment and technology, people use air drying equipment improved cut tobacco drying effect and physical properties at the same time, is also considering how to achieve better sensory quality. For used high of air temperature dry tobacco Shi fragrance material easy volatile, caused aroma loss and smoke taste bad of problem, plant pine macro sea, in dry tube into material mouth of downstream location to high temperature air in the spray people must volume of steam gas or water, to this to control air passed to tobacco of heat, makes tobacco in fast expansion of while can retained original of aroma. In cut tobacco pneumatic drying reduces air speed at the end, you can also achieve the objective of avoiding overheating and loss of tobacco aroma. Huang Jia thrown when using superheated vapor at high temperature drying of tobacco, tobacco dry basis moisture content down to a 15%-16.5% dry pipe position will lead to lower temperatures and with a certain moisture content of circulating air can make the hot air temperature decreases, avoid excessive tobacco aroma loss, can also reduce scorched flavor.
Current domestic tobacco industry drying equipment mainly used by British Diekinson-Legg HXD gas-cut tobacco drying machine, production capacity for 4800-10000kg/h; German company HAUNI HDT superheated steam drying machine, maximum production capacity of 10000kg/h domestic SH9 developed high-speed expansion of cut tobacco drying machine and SH963 of digestion and absorption of tobacco drying equipment. HXD currently in domestic tobacco enterprise in the application more, the system main by burning furnace, and hot Exchange device, and into material system, and air expansion dry tube and cyclone separation device composition, work wind temperature control range 260-480 ℃, process air speed can reached 60M/s around, can through row tide wind temperature, and simulation load flow, and control spray water and steam gas Jet volume, process parameter to control export tobacco of containing water rate, guarantee products quality uniform stable.
2.2, air dryer and dryer drum drying methods of comparison
Tobacco drying machine and works differently than the traditional drum dryer, heat in different ways. Main heat drying drum drying of tobacco, to convective heat and air drying is made of cut tobacco moisture evaporation, tumble dry for so long, you generally need to 6-8min, while air drying takes only a few seconds.
2.3, drying effects on the quality of tobacco
2.3.1, effect on physical properties of tobacco
Due to the expansion and high humidity and air drying is to use high temperature high velocity drying setting, but after the tumble dryer dryer pipe tobacco filling high-15%-18%. Scanning electron microscope images showed, after drying, the tobacco cells swell, specific surface area and pore volume increased significantly. Padding values increase the amount of incoming tobacco physical performance and influence of process parameters. Studies have found that filling value with process flow (29x103-35x103kg/h) increased showed an upward trend after the first, tobacco padding and temperature 260-330 ° c working temperature showed a positive correlation between. Xi Niansheng,  the study results show that added to the hot steam, using high enthalpy of superheated steam can provide more heat for tobacco, make it quick heat expansion, so as to improve the fill value. When the initial moisture content of tobacco at 25%, with the steam jet increases from 0 to 1800kg/h, Ye Si padding value is increased from 4.35cm3/g to 4.56cm3/g.
Compared with the tumble dryer, another advantage of using air drying treatment of tobacco is not obvious head-tail phenomenon. This is because the device is in normal operation, tobacco stocks at any point in the air dryer tumble dryer for only about 2% of tobacco, combined with air dryer control program with simulated load to minimize failed head and tail.
Current airflow drying technology in tobacco processing, there are still some problems, mainly because of the drying time is short, procedure before the tobacco export great influence on the moisture content of materials, exports of cut tobacco moisture content control accuracy is poor. Especially when the moisture more than the population of tobacco 28%, exporting tobacco Pellet is more clear, under the same conditions compared with the tumble dryer, exports of cut tobacco moisture fluctuations.
2.3.2, influence on the sensory quality of cigarette smoke and
Tobacco using special pneumatic drying process parameters on reducing cigarette tar and nicotine have obvious effects. Compared with the tumble dryer, after drying, the tobacco padding values to increase, as the cigarette lighter, can bring down the smoke tar 0.7-1.5mg/nicotine reduces to 0.08-0.15mg/. Ma yuping of different grades of flue-cured tobacco recipes such as tobacco compared the effectiveness of HXD, discovery will feed moisture content is increased from 22% to 34%, process gas flow when the temperature is increased from 200 c to 290 c, high-grade cigarette tar down to about 5% or so, cheap cigarette tar can be reduced by 10.6%. Due to the air dryer with high temperature and strong approach to effectively remove cigarette wooden gas gas, reduce irritant effects clear and offensive, but also to a certain extent reduce cigarette smoking aroma quality and aroma amount of smoke concentration and energy is also reduced. From the current situation, with the air drying method on low quality improved significantly the level of tobacco, on the sensory quality of high-grade tobacco has some negative effects.
2.3.3, the effects of tobacco quality
The main process parameters of drying tobacco initial water content, drying air temperature, the amount of dry air in the vapor, change each parameter set will have an impact on the quality of tobacco.
Initial moisture level of cut tobacco drying of filled values and sensory quality has important implications. Studies have shown that when the initial moisture content of tobacco at lower airflow of drying tobacco padding difference compared with the tumble dryer, cigarette weight did not change significantly. But as the initial water content (22%-34%) the increase of, air dried tobacco padding significantly increases; when the initial moisture content of tobacco increases further, filling rate of increase has slowed. For high-end smoke of formula tobacco, with tobacco initial containing water rate of increased, air dry Hou cigarette of aroma volume, and delicate degree and concentration will slightly has declined, miscellaneous gas, and stimulus sex and clean degree changes is unlikely to; and for low-grade smoke formula tobacco, is with tobacco initial containing water rate increased, cigarette aroma volume slightly has reduced, delicate degree has reduced, but miscellaneous gas, and irritation and the clean degree get improved. Drying air temperature on tobacco physical characteristics, organoleptic and chemical composition has a significant impact on the quality. Drying air temperature in a certain range and fill the drying of tobacco showed a positive correlation. Studies have shown that cigarette smoking aroma quality and aroma amount greatly influenced by air temperature, air temperature too high will reduce the aroma of elegant and diffusive. According to Kim and other studies using superheated steam drying after feeding the resurgence of Burley tobacco, process temperatures up to 150 ° c up to 320, padding of tobacco will also rise, increased from 6.08cm3/g to 7.81cm3/g. And with the increase of superheat steam temperature, and total sugar in tobacco, nicotine and total amino acid content decreased, total nitrogen and extract reduced, while changing sense quality of Burley, roasted flavour, stimulus, bitter, hot and impact strength, flavor is improved. Dai Xiang, are Virginia type cigarette blend pipe tobacco drying experiment, results showed that air temperature rose gradually from the 200 c to 265 c, cigarette sensory quality were gradually reduced, total sugar in tobacco, total nitrogen and total
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