Tobacco Online According to Tobacco Science, airflow drying is a continuous high-efficiency solid fluidization drying method, which is widely used in tobacco, chemical, pharmaceutical, and food processing industries. In-depth study of the principle of airflow drying and its application in tobacco processing is of great significance for optimizing the drying process parameters, developing new tobacco drying equipment and giving full play to the advantages of airflow drying processing technology to improve the quality of cigarette products. 1. Principle and characteristics of airflow drying 1.1 drying principle Airflow drying, also known as instantaneous drying, is the process of bringing the heating medium (both a heat carrier and a moisture carrier, such as air) into direct contact with the solid material to be dried. The material is suspended in the gas stream, and the heating medium transfers heat to the material in a convective heat transfer mode to vaporize part of the moisture in the material, thereby obtaining a solid product having a certain moisture content. 1.2 Characteristics of drying equipment 1 The gas-solid two-phase heat transfer mass transfer surface area is large, and the drying efficiency is high. Since the solid material (mostly particles) is highly dispersed in the gas stream, the contact area between the two phases is greatly increased, and the relative velocity of the gas-solid two phases is higher at a higher gas flow rate (20-40 m/s). The volumetric heat transfer coefficient is large and the thermal efficiency is high; 2 the drying time is short. The airflow drying process takes only a few seconds, and is especially suitable for drying heat sensitive and low melting materials; 3, the flow resistance is large, and the power consumption is large. At present, airflow drying equipment mainly has straight tube type, pulse tube type, cyclone type and inverted cone type air flow dryer. The application of the straight tube airflow dryer is more common; the pulse tube airflow dryer has a much higher drying efficiency than the straight tube type, and it adopts the method of alternately reducing and expanding the diameter of the tube, so that the particle motion alternately accelerates or decelerates, resulting in air and The relative velocity of the particles and the heat transfer area are large, thereby enhancing the heat and mass transfer rate. At the same time, the velocity of the airflow in the large diameter decreases, which is beneficial to prolong the drying time of the material. The development direction of airflow drying equipment is the diversification of dryer monomers, equipment process pipe network and material dispersion mechanization. 2. Application of airflow drying technology and equipment in tobacco processing 2.1 Research on tobacco airflow drying technology and equipment As early as 1959, Anderson proposed a method of drying tobacco with hot air, and then designed a pulse tube type tobacco drying system consisting of a drying tube and a cylindrical drying chamber. The principle is that the tobacco with high water content is carried by the hot air and rises along the drying pipe into the drying chamber, and falls to the top, so that the tobacco is continuously dried by the hot air until the water content reaches the set value. Dry room. This reciprocating drying method overcomes the problem of uneven moisture content of the tobacco in the conventional drum type drying apparatus, and can be continuously operated. In the 1960s and 1970s, researchers also designed a variety of airflow drying methods and equipment for tobacco, but due to the insufficiency of technology, the tobacco stayed in the dryer for too long and was easy to break. In 1983, Hibbits designed a more classic high-temperature airflow drying tobacco device consisting of a feeding device, a drying tube, a separator, and a heater for heating the process gas. The shredded tobacco is transported through the venturi by high-temperature, high-speed superheated steam. In the drying pipe, the speed of the tobacco is always lower than the gas flow rate when running in the drying pipe, so the heat and mass transfer rate is high, and the residence time of the tobacco in the drying pipe does not exceed 1 s. The airflow conveying tobacco dryer developed by Wu et al. is characterized in that the tobacco is carried into the tangential separator by the hot air flow, and the conveying, drying and separation are carried out almost simultaneously, and the distance of the tobacco running in the straight pipe is short, effectively solving the shredded tobacco. problem. The tobacco airflow drying equipment used in China's tobacco industry is mainly imported and digested from abroad, and some improvements have been made to the equipment during use. In 2002, the "SH9 type wire wire online high-speed expansion system" jointly developed by Changzhou Zhisi Machinery Manufacturing Co., Ltd. and Hefei Cigarette Factory, using tube tower structure and pulsed airflow transmission, greatly improved the heat transfer coefficient during the drying process. The medium gas flow is in full contact with the tobacco, which effectively reduces the tobacco agglomeration caused by the uneven water content. In addition, Li Wei and others converted the vertical drying tube of the HXD airflow drying system produced by Dickinson-Legg into a circular arc-shaped flow passage with a large radius and a horizontal drying duct with an elliptical cross section, and improved the proportion of hot air distribution. The moisture content of the dried export cut tobacco is more uniform, and the shredded tobacco is reduced. In 1967, when Wright used hot air to dry the tobacco, steam or spray water was added to the dry air. As a result, the filler filling value was significantly improved. Since then, researchers have also used a variety of methods to increase the filling value of the tobacco stream after drying, in order to achieve cost savings and reduce the risk of cigarette tar. Jew-ell et al. used a high temperature airflow of 120-340 ° C to dry the stems, adding steam or a mixture of steam and air to the gas stream. As the water vapor content in the air increased, the filler filling value also increased significantly. Scrunecker et al. believe that by adding water vapor to the dry air, the wet bulb temperature of the gas stream can be increased, and the filler value can be prevented from decreasing due to shrinkage during the drying process. Dipling The tobacco with a moisture content of 10% to 60% is dried with a hot gas stream of 380-1000 ° C. As a result, the filler filling value is increased by 30% compared with that before drying. However, it has been found that excessively high dry air temperatures can cause loss of tobacco aroma. Hibbits is designed to dry tobacco with a moisture content of 48.5% with superheated steam at 350 ° C. The filling value can reach 8.3 cm 3 /g, which is 63% higher than before drying. In 1993, W. Silsh et al. elaborated the airflow velocity, airflow temperature, material moisture content, upper and lower limits of material temperature, and gas-to-battery ratio range during airflow drying. In order to speed up the initial drying speed, the design value of the airflow speed is up to 100m/s. In addition, the moisture content before the drying of the tobacco is increased (not more than 40%), water vapor is added to the drying gas, and the downstream cross-sectional area of ​​the drying section is designed as the upstream. These measures are 3 to 5 times the cross-sectional area, which helps to speed up the drying rate and increase the filling value of cut tobacco. The technology is licensed to Dickinson-Legg to manufacture and sell airflow drying equipment. The airflow drying equipment designed by Werkmeister et al. is different from the traditional straight tube airflow dryer. The tobacco is carried by hot air through two continuous arcuate elbow tubes, and the filling value of the shredded tobacco can reach 5.41 cm3/g. The airflow drying device designed by IE Tassem is similar. With the deepening of tobacco airflow drying equipment and technology research, people are also considering how to obtain better sensory quality while using airflow drying equipment to improve the drying effect and physical properties of tobacco. Aiming at the problem that the aroma is volatile when the tobacco is dried at a higher airflow temperature, causing aroma loss and deterioration of the smoke taste, Uematsu Honghai and the like spray a certain amount of steam or water into the high-temperature airflow at a position downstream of the feed port of the drying pipe. This controls the heat transfer from the airflow to the shredded tobacco, allowing the shredded tobacco to retain its original aroma while rapidly expanding. By reducing the airflow rate at the end of the drying of the tobacco stream, it is also possible to avoid the loss of aroma due to overheating of the tobacco. When Huang Jia throws out the use of superheated steam to dry the shredded tobacco, the hot water temperature is fast when the dry water content of the shredded tobacco base is reduced to 15.0% - 16.5%, and the temperature of the drying pipe is low and has a certain moisture content. Reduce, avoid excessive loss of tobacco aroma, and also reduce the smell of dryness. At present, the airflow drying equipment used in the domestic tobacco industry mainly includes the HXD high-temperature airflow type tobacco dryer of Diekinson-Legg, UK, with a production capacity of 4800-10000kg/h; the HDT superheated steam dryer of Germany HAUNI Company has a maximum production capacity of 10000kg/ h; SH9 type tobacco silk high-speed expansion dryer developed by the country and the SH963 type tobacco tobacco air drying equipment for digestion and absorption. HXD is currently used in domestic tobacco companies. The system is mainly composed of combustion furnace, heat exchanger, feeding system, airflow expansion drying pipe and cyclone separator. The working air temperature control range is 260-480 °C, and the process airflow speed can be Up to 60m/s, the moisture content of the cut tobacco can be controlled by the process parameters such as tidal air temperature, simulated load flow, controlled water spray volume and steam injection amount to ensure uniform and stable product quality. 2.2 Comparison with drum drying method The tobacco airflow drying device works differently from the conventional drum dryer, and the heat transfer mode is different. The drum type drying mainly heats the shredded tobacco by heat conduction, and the airflow drying evaporates the water in the shredded tobacco by convection heat transfer. Therefore, the drum drying time is long, generally takes 6-8 minutes, and the airflow drying takes only a few seconds. 2.3 Influence of airflow drying on the quality of cut tobacco 2.3.1 Effect on the physical properties of tobacco Since the airflow drying is formed by high-humidity expansion and high-temperature high-speed hot air drying, it can be 15%-18% higher than the dried tobacco filling value of the drum type dryer. Scanning electron micrographs showed that the tobacco expanded and the specific surface area and pore volume increased significantly. The increase in fill value is affected by the physical properties and process parameters of the incoming tobacco. It is found that the filling value decreases first and then increases with the increase of process gas flow rate (29×103—35×103kg/h), and the filling temperature value is positively correlated with the working wind temperature between 260-330°C. . Xi Niansheng et al. [22] showed that adding steam to the hot gas stream and using the high heat of superheated steam can provide more heat to the tobacco, so that it can rapidly heat up and expand, thereby increasing its filling value. When the initial moisture content of the cut tobacco is 25%, as the steam injection amount is increased from 0 to 1800 kg/h, the filling value of the leaf yarn is increased from 4.35 cm 3 /g to 4.56 cm 3 /g. Another advantage of using airflow drying to treat cut tobacco is that it does not produce significant dry head dry tails compared to drum dryers. This is because when the equipment is in normal operation, the amount of shredded tobacco at any time in the airflow dryer is only about 2% of the amount of shredded tobacco in the drum dryer. In addition, the simulation program of the airflow dryer has a simulated load, which can maximize Reduce the occurrence of unacceptable material and tail. At present, airflow drying technology still has some problems in tobacco processing, mainly because the drying time is short, the tobacco is greatly affected by the moisture content of the export material of the previous process, and the control precision of the moisture content of the exported tobacco is poor. Especially when the moisture content of the tobacco cuts exceeds 28%, the agglomeration of the cut tobacco is more obvious. Under the same conditions, the moisture content of the export cut tobacco fluctuates greatly compared with the drum type dryer. 2.3.2 Effect on cigarette smoke and sensory quality The treatment of cut tobacco with specific airflow drying process parameters has a significant effect on reducing the amount of cigarette tar and the amount of nicotine. Compared with the drum type drying machine, the filling value of the tobacco after drying by airflow increases, and the weight of the single cigarette can be reduced by 0.7-1.5 mg/piece, and the amount of nicotine is decreased by 0.08-0.15 mg/support. . Ma Yuping conducted a comparative study on the effects of HXD treatment on different grades of flue-cured tobacco. It was found that the feed water content increased from 22% to 34%, and the process gas temperature increased from 200 °C to 290 °C. The amount of tar is about 5%, and the amount of tar in low-end cigarettes can be reduced by 10.6%. Because the airflow dryer adopts high temperature and strong treatment, it is effective for effectively removing the wood gas and cyan gas from cigarettes and reducing the irritating effect, but to some extent, it also reduces the aroma and aroma of the cigarette, so that the concentration and strength of the smoke are also improved. reduce. From the current use situation, the air-drying method has improved the sensory quality of low-grade cut tobacco, but has a certain negative impact on the sensory quality of high-grade cut tobacco. 2.3.3 Influence on the quality of tobacco processing The main process parameters of airflow drying include the initial moisture content of the tobacco, the temperature of the dry gas stream, and the amount of steam added in the dry gas stream. Changing the setting of each process parameter will have an impact on the processing quality of the tobacco. The initial moisture content of cut tobacco has an important influence on the filling value and sensory quality after drying. Studies have shown that when the initial moisture content of cut tobacco is low, the fill value of cut tobacco treated by airflow drying method is not much different from that of drum dryer, and the weight of single cigarette has no significant change. However, as the initial moisture content (22%-34%) increases, the filling value of the tobacco after airflow drying increases significantly; when the initial moisture content of the tobacco further increases, the rate of increase of the filling value will slow down. For the formula of high-grade tobacco, with the increase of the initial moisture content of the tobacco, the aroma, fineness and concentration of the cigarette will decrease slightly after the airflow is dried, and the noise, irritancy and cleanness will not change much; Cut tobacco, with the initial moisture content of the tobacco, the cigarette aroma is slightly reduced, the degree of fineness is reduced, but the noise, irritation and cleanliness are improved. The temperature of the dry gas stream has a significant effect on the physical properties, sensory quality and chemical composition of the tobacco. The temperature of the dry gas stream is positively correlated with the filling value of the dried tobacco after a certain range. Studies have shown that the aroma and aroma of cigarettes are greatly affected by the temperature of the hot air. If the temperature of the hot air is too high, the elegance and transparency of the aroma will be reduced. According to research by Kim et al., the ribbed tobacco after the reheating of the feed was superheated with steam, and the treatment temperature was raised from 150 ° C to 320 ° C, and the fill value of the cut tobacco was also increased from 6.08 cm 3 /g to 7.81 cm 3 /g. And with the increase of superheated steam temperature, the total sugar, nicotine and total amino acid content in cut tobacco decreased significantly, the content of total nitrogen and ether extract also decreased, and the sensory quality of burley tobacco also changed, baking The flavor is enhanced, the irritating, bitter, burning and impact strength is reduced, and the aftertaste is improved. Dai Xiang et al. conducted a test on the drying process of flue-cured cigarettes. The results showed that the temperature of the airflow gradually increased from 200 °C to 265 °C, and the sensory quality of the cigarettes gradually decreased. The total sugar, total nitrogen and total plant alkali content in the tobacco. It will fall. When the tobacco is dried by the hot air stream, the steam is injected into the air before the heat exchanger, which can improve the heat transfer coefficient and the heat enthalpy, and is advantageous for accelerating the drying rate and promoting the expansion of the tobacco. Studies by Xi Niansheng et al. show that the effect of the amount of steam applied in hot air on the intrinsic quality of cigarettes is mainly reflected in the characteristics of aroma and taste. The lower steam application has an effect on the intrinsic quality of the high-grade cigarette formula. For the formula tobacco of low-grade cigarettes, a higher amount of steam should be applied, otherwise the aroma, aroma and cleanness of the cigarette will decrease, and the dryness will increase. The deepening of the research on airflow drying technology is reflected in the research on the relationship between the technical parameters of airflow drying. Zhou Jun and other experiments have proved that under the premise of ensuring the moisture content of the cut tobacco, the other conditions remain unchanged. The process air temperature of HXD is proportional to the moisture content of the tobacco and the flow of tobacco, which is inversely proportional to the amount of water added. In the production process, a reasonable and stable incoming tobacco flow rate and water content should be set first, then the correct hot air temperature and hot air flow rate should be set, and the hot air temperature and the moisture content of the cut tobacco can be adjusted by controlling the water spray amount and the temperature of the cyclone separator. Zhang Dabo et al. conducted experiments by changing the operating parameters of HXD, and carried out statistical analysis on the collected data. The results showed that the feed water content, material flow rate, process gas flow rate and vapor deposition amount were independent variables, and the initial gas flow temperature was the dependent variable. The regression equation can be used to calculate the combination of parameters to ensure the normal operation of HXD, so that the moisture content of the dried tobacco outlet is uniform and stable in a short time. Zhengzhou Tobacco Research Institute also conducted a comprehensive experimental study on the main technical parameters of airflow drying such as initial moisture content, material flow rate, process gas flow rate, steam application amount, equipment operating conditions, product processing quality and intrinsic quality. The influence of many single factor parameters on the quality of cigarette comprehensive processing in HXD working process. 3. Research progress on tobacco airflow drying technology 3.1 Influence of airflow drying on chemical constituents of tobacco With the deepening of research and application of airflow drying technology in China's tobacco processing, researchers are also exploring the effect of airflow drying on the chemical composition of tobacco. In 2004, Yu Ruiguo and other soots in different producing areas were tested. It was found that the chemical composition of the shredded tobacco after the airflow drying treatment was greater than that of the drum dried tobacco. Wang Lei et al. analyzed the content of free amino acids in cut tobacco before and after airflow drying by liquid chromatography. The results showed that the amino acid content of cut tobacco after drying by airflow was greatly reduced compared with that before drying, and the degree of reduction was more than that after drying with a drum dryer. cut tobacco. Liao Xudong and other studies have shown that after the tobacco is dried by HXD airflow, the moisture of the flue gas is reduced. 3.2 Mathematical Simulation of Tobacco Airflow Drying Process In recent years, more and more work has been carried out on the mathematical simulation and numerical optimization of the material airflow drying process. The analysis of the airflow drying process is based on the gas-solid two-phase flow theory. The study of the movement of the particles in the drying tube and the heat and mass transfer between the particles and the gas flow are the basis for the process simulation. According to the force of the particles moving in the drying tube, the basic equations of the accelerated motion section and the constant velocity motion section are listed, and the motion state of the particles can be described. For the study of heat and mass transfer in the airflow drying process, the convective heat and mass transfer coefficient is calculated according to the factors affecting the convective heat transfer coefficient such as fluid properties, flow state, particle properties, etc., and then the convection is calculated according to the semi-empirical and semi-theoretical equations. The heat rate and mass transfer conditions are usually discussed separately for constant-speed drying and slow-down drying. Pelegrina et al. used a one-dimensional model to simulate the airflow drying process of potato granules, and plotted the variation of particle velocity, temperature and water content along the drying tube. Skuratovsky et al. used a two-dimensional model to simulate the straight pipe airflow drying process, and obtained the velocity, temperature and particle moisture content along the drying pipe along the radial direction of the drying pipe (7/12-11/12 of the pipe diameter). Curve and simulate the change in particle diameter during drying. As can be seen from these graphs, the dry state of the material along the radial direction of the drying tube is not exactly the same, and this difference is maximized at the end of drying. In the numerical simulation study of the airflow drying process of tobacco, Fukuchi et al. regard the cut tobacco as an equal volume sphere, and define the shape of the cut tobacco as the surface area of ​​the cut tobacco/the surface area of ​​the sphere, and the size of the cut tobacco is represented by the diameter of the sphere. Through the simulation of the movement of tobacco, the mass, momentum and energy conservation equations of the continuous phase (hot air) and the force balance equation of the dispersed phase (cutting tobacco) are established. The motion characteristics of the tobacco can be obtained through iterative calculation. The experimental results show that the predicted values ​​of the model agree well with the experimental values. Pakowski et al. used the one-dimensional mathematical model proposed by Meunier to simulate the drying process of superheated steam stream, and obtained the distribution curve of temperature, moisture and velocity of tobacco and superheated steam along the drying tube, and experimented on the drying process controlled by different process parameters. The actual measured value of the moisture content and temperature of the obtained tobacco shreds is in agreement with the model prediction results. The establishment of the model is of great significance for simulating the industrial production process. 4, the conclusion Although the development of airflow drying technology in China's tobacco industry is getting faster and faster, due to the short application time, the research on the principle of airflow drying process and the influence of processing parameters on the quality of cigarette products needs to be further studied. In the future, computer simulation will be used to simulate the airflow drying process, and the fluid movement in the drying process and the temperature and humidity distribution of the tobacco and airflow in the drying tube along the axial and radial directions will be improved. Important research directions in drying technology and equipment.
During the process of air drying the material, the movement of the material particles in the air flow is divided into an accelerated motion phase and a constant velocity motion phase. In the accelerated motion phase, the sum of the drag and buoyancy of the particles is greater than the gravity, with an upward acceleration, so the relative velocity of the particles and the airflow is a variable; as the velocity of the particles increases, the drag gradually decreases until 3 The vector sum of the forces is zero, and the particles enter the isokinetic motion phase, where the relative velocity between the gas flow and the particles is constant. The relative motion of the particles and the gas flow has a great influence on the heat transfer rate between the particles and the gas flow. In the initial drying stage, the ascending velocity of the particles just enters the drying pipe is zero, and meets the hot gas flow with higher velocity to obtain an upward direction. Speed, at this time, the convective heat transfer coefficient between the two phases is very large, the material particles are continuously accelerated and enter the accelerated motion drying stage, and the heat obtained by the solid particles in the acceleration phase accounts for more than half of the heat obtained in the entire drying stage. In the later stage of drying, when the rising speed of the solid material approaches or even reaches the gas flow rate, the convective heat transfer coefficient is greatly reduced, and the drying efficiency is lowered. In the drying process, the relative velocity of the gas-solid two phases is continuously changed, the turbulence intensity at the boundary layer around the particles is increased, the contact area of ​​the gas-solid two phases is increased as much as possible, and the contact time of the two phases is increased, which is an effective measure for improving the drying efficiency.