Application of air drying technology in tobacco processing

1 As early as 1959, Anderson put forward the method of drying tobacco with hot air, and then designed a set of pulse tube tobacco drying system composed of drying tube and cylindrical drying chamber The principle is that the cut tobacco with high moisture content is carried by hot air to rise along the drying pipe and enter the drying chamber, falling before reaching the top In this way, the cut tobacco is dried by hot air continuously until the moisture content reaches the set value and is transported out of the drying chamber The reciprocating drying method overcomes the problem of uneven moisture content of cut tobacco in the former drum drying equipment, and can work continuously In the 1960s-1970s, researchers also designed a variety of air drying methods and equipment for tobacco, but due to the imperfect technology, the tobacco stay in the dryer for a long time, and it is easy to be broken In 1983, hibbits designed a more classic high-temperature air flow drying equipment for tobacco, which consists of a feeding device, a drying tube, a separator and a heater for heating process gas The tobacco is transported by high-temperature and high-speed superheated steam through a venturi tube and a drying tube When the tobacco is running in the drying tube, the speed of the tobacco is always lower than the air flow speed, so the heat and mass transfer rate is very high, and the tobacco is dry The retention time in the drying tube shall not exceed 1s The characteristics of the air conveying tobacco dryer developed by Wu et al Are that the tobacco is carried into the tangential separator by the hot air flow, and the conveying, drying and separation are almost carried out at the same time The running distance of the tobacco in the straight pipe is very short, which effectively solves the problem of tobacco fragmentation The air drying equipment used in China's tobacco industry is mainly imported from abroad and digested Some improvements have been made in the use of the equipment In 2002, the "sh9 type on-line high-speed expansion system for tobacco leaves" was jointly developed by Changzhou zhisi Machinery Manufacturing Co., Ltd and Hefei Cigarette Factory The heat transfer coefficient was greatly improved by adopting the tube tower structure and pulse air flow transportation During the drying process, the air flow fully contacted the tobacco, effectively reducing the tobacco agglomeration caused by the uneven moisture content In addition, Libiao et al Transformed the vertical drying pipe of HXD air flow drying system produced by Dickinson Legg company into a horizontal drying pipe with a large radius arc-shaped flow channel and an elliptical cross section, and improved the proportion of hot air distribution, so that the moisture content of the dried tobacco at the outlet is more uniform, and the fragmentation of the tobacco is reduced In 1967, Wright added steam or spray water to the dry air when drying tobacco with hot air flow As a result, the filling value of tobacco increased significantly Since then, researchers have also used a variety of methods to improve the filling value of cut tobacco after air drying, in order to achieve the purpose of saving costs and reducing cigarette tar Jew ell et al Used high temperature air flow of 120-340 ℃ to dry the stem and added steam or mixture of steam and air into the air flow With the increase of water vapor content in the air, the filling value of tobacco increased significantly Scrunecker and other analysts believe that by adding water vapor to the dry air, the wet bulb temperature of the air flow can be increased, and the filling value of the cut tobacco can not be reduced due to shrinkage during the drying process Dipling dried the cut tobacco with moisture content of 10% - 60% in 380-1000 ℃ hot air flow The filling value of cut tobacco increased by 30% compared with that before drying However, it was found that too high drying air temperature would cause the loss of tobacco flavor substances Hibbits is designed to dry the cut tobacco with moisture content of 48.5% with superheated steam at 350 ℃, and the filling value can reach 8.3cm3/g, which is 63% higher than that before drying In 1993, w searsh et al Designed the air velocity, air temperature, moisture content of material, upper and lower limits of material temperature and the range of air material ratio in the process of air drying tobacco In order to speed up the initial drying speed, the design value of air flow speed is as high as 100M / s In addition, increase the moisture content (not more than 40%) before the tobacco drying, add water vapor to the drying gas, and design the downstream cross-sectional area of the drying section as 3-5 times of the upstream cross-sectional area These measures are helpful to speed up the drying speed and improve the filling value of tobacco The technology is licensed to Dickinson - Legg to manufacture and sell air drying equipment The air dryer designed by werkmeister is different from the traditional straight tube air dryer The cut tobacco is carried by hot air through two continuous bow elbow tubes, and the filling value of dried cut tobacco can reach 5.41cm3/g The air drying device designed by IE tessam is similar to this one With the development of air drying equipment and technology, people are considering how to obtain better sensory quality while using air drying equipment to improve the drying effect and physical properties of tobacco In view of the problem of volatile aroma substances when the tobacco is dried at a higher air temperature, resulting in the loss of aroma and the deterioration of smoke taste, tsumatsu Honghai and others spray a certain amount of steam or water into the high-temperature air flow at the downstream of the drying pipe feed port, so as to control the heat transferred by the air flow to the tobacco, so that the tobacco can retain the original aroma while expanding rapidly At the end of air drying, reducing the air velocity can also avoid the loss of aroma due to overheating of tobacco When using superheated steam to dry tobacco at high temperature, Huang Jiadi proposed that the drying pipe with moisture content of 15.0% - 16.5% on the dry basis of tobacco should conduct a circulating air flow with low temperature and certain moisture content, which can rapidly reduce the temperature of hot air flow, avoid excessive loss of tobacco aroma, and also reduce dry and burnt flavor 　　 At present, the air dryer used in domestic tobacco industry mainly includes HXD high temperature air dryer of diekinson Legg company in the UK, with a production capacity of 4800-10000kg / h; HDT superheated steam dryer of haui company in Germany, with a large production capacity of 10000kg / h; sh9 high-speed expansion dryer and sh963 air dryer for digestion and absorption developed in China Prepare At present, HXD is widely used in domestic tobacco enterprises The system is mainly composed of combustion furnace, heat exchanger, feeding system, air expansion drying pipe and cyclone separator The control range of working air temperature is 260-480 ℃, and the process air flow speed can reach about 60m / s It can control the outlet through process parameters such as moisture exhaust air temperature, simulated load flow, control of water spray and steam spray The moisture content of cut tobacco ensures the product quality to be even and stable 2 Comparison of drying methods between air dryer and drum dryer the working principle of air dryer is different from that of traditional drum dryer Drum drying mainly uses heat conduction to dry tobacco, while air drying uses convection heat transfer to evaporate moisture in tobacco Therefore, drum drying takes a long time, generally 6-8 minutes, while air drying only takes a few seconds 3 The influence of air drying on the quality of cut tobacco 3.1 The influence on the physical properties of cut tobacco Because air drying adopts high humidity expansion and high temperature and high speed hot air drying, the filling value of cut tobacco after drying is 15% - 18% higher than that of drum dryer The results of SEM showed that the specific surface area and pore volume of tobacco increased significantly after air drying The increase of filling value is affected by the physical properties and technological parameters of the incoming tobacco It was found that the filling value first decreased and then increased with the increase of process gas flow rate (29 × 103-35 × 103kg / h), and there was a positive correlation between the filling value of cut tobacco and the working air temperature between 260-330 ℃ According to the research results of Xi niansheng et al [22], adding steam into the hot air stream can provide more heat for the cut tobacco by using the high enthalpy of the superheated steam, so that it can rapidly heat up and expand, thus increasing its filling value When the initial moisture content of tobacco is 25%, the filling value of tobacco leaves increases from 4.35cm3/g to 4.56cm3/g with the increase of steam injection from 0 to 1800kg / h Compared with the drum dryer, the other advantage of air drying is that there is no obvious dry end phenomenon This is because when the equipment is in normal operation, the amount of tobacco in the air dryer at any time is only about 2% of the amount of tobacco in the drum dryer In addition, the control program of the air dryer has a simulated load, which can reduce the generation of unqualified material head and material tail to a large extent At present, there are still some problems in the tobacco processing of air drying technology, mainly due to the short drying time, the influence of moisture content of tobacco materials in the previous process is greater, resulting in the poor control accuracy of moisture content of tobacco exports Especially when the moisture content of the population is more than 28%, the agglomeration phenomenon of the exported tobacco is obvious Under the same conditions, compared with the drum dryer, the moisture content of the exported tobacco fluctuates greatly 3.2 The effect on cigarette smoke and sensory quality It has obvious effect on reducing cigarette tar and nicotine by using specific air drying process parameters Compared with drum dryer, the filling value of cut tobacco is increased after air flow drying As the weight of a single cigarette is reduced, the tar content and nicotine content of the smoke can be reduced by 0.7-1.5mg/cigarette and 0.08-0.15mg/cigarette respectively Ma Yuping et al Made a comparative study on the effects of different grades of flue-cured tobacco formula cut tobacco treated by HXD It was found that when the moisture content of the feed was increased from 22% to 34%, and the air flow temperature of the process was increased from 200 ℃ to 290 ℃, the tar content of medium and low grade cigarettes decreased about 5% and 10.6% respectively Due to the high temperature and strong treatment of the air dryer, it can effectively remove the wood gas and green gas of the cigarette and reduce the irritant effect, but to a certain extent, it can also reduce the aroma quality and quantity of the cigarette, so that the smoke concentration and strength are also reduced From the current use, the air drying method can improve the sensory quality of low-grade tobacco obviously, but has a certain negative impact on the sensory quality of tobacco 3.3 The main process parameters affecting the processing quality of cut tobacco are the initial moisture content of cut tobacco, the temperature of drying air flow, the amount of steam added into the drying air flow, etc changing the setting of each process parameter will affect the processing quality of cut tobacco The initial moisture content of cut tobacco has an important influence on the filling value and sensory quality after drying The results show that when the initial moisture content of the cut tobacco is low, the filling value of the cut tobacco treated by air drying is not different from that of the drum dryer, and the weight of a single cigarette has no significant change However, with the increase of the initial moisture content (22% - 34%), the filling value of cut tobacco after air drying increased significantly; when the initial moisture content of cut tobacco further increased, the increase rate of filling value would slow down For the formula cut tobacco, with the increase of the initial moisture content of cut tobacco, the aroma amount, fineness and concentration of the cigarette will slightly decrease after air drying, and the change of impurity, irritation and cleanliness will not be significant; for the low-grade formula cut tobacco, with the increase of the initial moisture content of cut tobacco, the aroma amount of the cigarette will slightly decrease, and the fineness degree will be reduced, but the impurity, irritation and cleanliness degree will be slightly reduced Improved The drying air temperature has a significant effect on the physical properties, sensory quality and chemical composition of cut tobacco There was a positive correlation between the temperature of drying air flow and the filling value of dried tobacco in a certain range Some studies have shown that the aroma quality and quantity of cigarettes are greatly affected by the hot air temperature Too high hot air temperature will reduce the elegance of aroma