Ind. Eng. Chem. Res. 2006, 45, 7733-7736
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Drying Characteristics of Cotton Lint in a Recirculating Fixed-Bed Dryer with a Condenser Kyu Sun Lee and Dong Hyun Lee* Department of Chemical Engineering, Sungkyunkwan UniVersity, Suwon 440-746, Korea
Sang Done Kim Department of Chemical and Biomolecular Engineering and Energy and EnVironment Research Center, KAIST, Daejeon, 305-701 Korea
The effects of gas velocity (0.094-0.189 m/s), gas temperature (60-120 °C), cooling water flow rate (4.8 × 10-3-9.0 × 10-3 kg/s), and the cooling method (direct or indirect) on the drying characteristics (drying rate and specific energy consumption) of cotton lint in a 0.15-m-inner diameter (ID) × 0.9-m-high recirculating fixed-bed dryer have been determined. The drying rate increases, but the amount of specific energy consumption decreases, as the air temperature increases. With the direct cooling method, the drying rate increases as the cooling water flow rate increases. Further research is required to investigate the drying rate and the specific energy consumption as a function of the direct and indirect cooling methods. 1. Introduction The fixed-bed drying process has low capital and maintenance costs for drying moist particles. Drying processes in the form of deep-bed and thin-layer drying are widely used in agricultural and chemical processes; materials such as grains, coffee, tobacco, wood chips, coal, and many chemical products are usually dried this way.1-4 An improved model was developed by Abernathy et al.5 to predict the equilibrium moisture content of cotton lint using the experimental data of Griffin6 and Urquhart and Willians7 at temperatures up to 90 °C. They also evaluated several models, including the Henderson and Perry model,8 and the BET model of Brunauer, Emmett, and Teller,9 as modified by Dent,10 and reported that the BET model is the best to predict the equilibrium moisture content. Barker11 determined equilibrium moisture contents of cotton plants parts (leaves, sticks, and burrs, which are all debris) at 5-80 °C and a relative humidity of 0%-98% in a moving air stream. He observed a linear increase in the equilibrium moisture content between the relative humidities of 20% and 70%, followed by an exponential increase as the relative humidity approached 100%. Barker and Laird12 determined the effect of temperature onthe moisture absorption and desorption rates for cotton lint under controlled temperature (50-90 °C) and relative humidity (0.28-99.9%) conditions. Their results indicated that both the temperature and air conditions (dry or humid) significantly affect the “diffusivity” parameter in the nonlinear equations. Also, higher moisturetransfer rates occur at higher temperatures and the drying rate is significantly higher than the humidification rate for cotton lint at the same temperature. Previous studies have determined the effects of temperature, gas velocity, and the type of dryers on the drying characteristics. Normally, a drying agent that is flowing past or through a body removes evaporated water and transports it from the dryer. However, previous investigators have not reported the drying characteristics in a recirculating fixed-bed dryer with a condenser. * To whom all correspondence should be addressed. Tel: +82-31290-7340. Fax: +82-31-290-7272. E-mail address:
[email protected].
Therefore, the objective of this study is to determine the effects of gas velocity, gas temperature, basket size, the mass flow rate of cooling water, and the cooling method (direct or indirect) on the drying characteristics of cotton lint in a recirculating fixed-bed dryer with a condenser. 2. Experimental Section Drying experiments of cotton lint were conducted in a 0.15m-inner diameter (ID) × 0.9-m-high stainless steel column, as shown in Figure 1. The outer wall of the column was insulated with 20-mm-thick glass wool. A distributor was located between the main column and a windbox (0.15 m ID × 0.30 m high) into which air was fed through a blower and a calibrated flowmeter (FLT, Korea Flow Cell Co.) in the range of 0.0940.187 m/s. Two electric heaters (each with a heating power of 1.5 kW) were inserted into the windbox with ceramic packing materials. Wetted cotton lint was loaded in a basket (0.15 m high) that had a perforated plate at the bottom with 100 evenly spaced holes (2.0 mm ID). Four temperature sensors were installed along the basket height, to determine temperature (see Figure 3, presented later in this work). Temperatures at various points in the dryer were measured by residence time distribution (RTD) sensors (pt-100Ω), as shown in Figure 1. Relative humidities and temperatures of the inlet and outlet air were measured by a hygrometer (Vaisala, Inc., Model No. HMT237), from which the drying rates were deduced.13-15 The temperature, relative humidity, and electric power consumption (using a Model No. WT230 digital power meter, Yokogawa Co.) were monitored with a personal computer at a sampling frequency of 0.2 Hz during the drying period. As can be seen in Figure 1, the hot and humid air was condensed by the direct or indirect cooling method and the condensed water was separated by a cyclone. The direct cooling method is the heat exchanging process, which involves hot and humid air in the main column passing through a condenser and coming into direct contact with cooling water. The hot and humid air then travels a zig-zagtype path marked on a plate and the humid air is condensed by concurrent contact with the cooling water. The cool and dry air was recirculated into the blower. The moisture balance was determined from the moisture content of cotton lint before and after drying.
10.1021/ie0607447 CCC: $33.50 © 2006 American Chemical Society Published on Web 09/26/2006
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Figure 1. Schematic diagram of the recirculating fixed-bed dryer. (Legend: DP, differential pressure transducer; H, hygrometer; T, thermocouple; and V, flow meter.) Table 1. Experimental Conditions of the Drying Test variable
range
heater temperature superficial gas velocity cooling water mass flow rate condensing methods
60-120 °C 0.094-0.189 m/s 4.8 × 10-3 and 9.0 × 10-3 kg/s direct and indirect
The ranges of experimental variables are listed in Table 1.
Figure 2. Variation in temperature, relative to the elapsed time.
3. Results and Discussion The variation of temperature for a given experiment, relative to the elapsed time, is shown in Figure 2. In this case, the air temperature from the electric heater was maintained at 82 °C and the gas velocity (Ug) was 0.157 m/s. The outlet humid air from the main column was cooled via the indirect method with a flow rate of mCW ) 4.8 × 10-3 kg/s. As can be seen in Figure 2, the end point of the drying process is determined to be 13 000 s, because the outlet gas temperature was maintained constant afterward. The outlet gas temperature from the drying column initially decreases, because the moisture in the wetted cotton lint was initially vaporized by the hot inlet gas, but gradually increases over 3000 s, because of the microdiffusion of moisture into the lint pores. The average temperature of the inlet cooling water was 18 °C, and that of the outlet cooling water was 22 °C with mCW ) 4.8 × 10-3 kg/s. The heat, which was ∼278.4 kJ, was transferred to the cooling water for total drying time of 4 h. For another experiment, the temperature variation of cotton lint, relative to the elapsed time in the fixed-bed dryer, is shown in Figure 3. The air temperature from the electric heater was maintained at 61 °C and the gas velocity was Ug ) 0.189 m/s. In the receding front model,16 experimental observations indicate that, during the drying of some porous cotton lint, a distinct evaporating zone may be observed. It divides the cotton lint into dry and wet parts, and as the drying process in this zone proceeds, as shown in Figure 3, the ratio of dry to wet parts of the cotton lint increases. The exhaust gas temperature from the dryer reached a steady state after 13 000 s; complete drying occurs concurrently at this time, and the temperature of the cotton lint was 56 °C within the basket. The temperature drop of 4 °C in the cotton lint may be due to heat loss through the wall and the energy used to heat the column materials themselves (steel wall and other parts) and the cotton lint that is to be dried. The temperature and relative humidity of the hot gases at the entrance and exit of the dryer are shown in Figures 4a and
Figure 3. Temperature variation of the cotton linte, relative to the elapsed time in the fixed-bed dryer.
4b, respectively. From the relative humidity and temperature data, relative to time, the absolute humidity can be calculated. In Figure 4a, the relative humidity in the exhaust gases begins at 60% and ends the drying process at 20%, whereas the relative humidity of inlet gas was in the range of 70%-80%. This phenomenon is caused by the relative humidity of the gases reaching 100% at the condenser; however, this gas was heated by the heater through the blower and there was a slight increase in temperature, which is why 100% relative humidity was not reached. The absolute humidity, which is calculated from the relative humidity and temperature, relative to time, is shown in Figure 4c. The difference between the absolute humidity of the incoming and exhaust gases is related to the evaporated volume of water. If the absolute humidity is integrated with respect to the total drying time, the volume of water that evaporated during the drying process can be calculated. The recorded evaporated water mass (calculated value) in this experiment was 205.1 g
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Figure 4. (a) Temperature, (b) relative humidity, and (c) absolute humidity of the hot gases at the entrance and exits of the dryer.
Figure 5. (a) Drying rate and (b) the amount of specific energy consumption, relative to variations in the moisture content.
and the evaporated actual mass (measured value) of water was 199.7 g, which indicates that the experimental error was 2.7%. The drying rate, as a function of moisture content, is shown in Figure 5a at a hot gas velocity of 0.126 m/s and a temperature in the range of 60-120 °C. When the moisture content was at 1.0 kg H2O/kg solid (dry basis), the drying process began until it reached the falling drying rate, where the drying rate slowly decreased. Heat can supply more energy with increasing gas temperature, thereby increasing the drying rate. Figure 5b shows the amount of specific energy consumption with variations in the moisture content. The amount of specific energy consumption refers to the amount of energy required to evaporate 1 kg of water; therefore, the theoretical specific energy consumption required for the evaporation of water amounts to 2.42 MJ/kg of water. However, in the actual experiment, the
Figure 6. Effect of (a) moisture content on drying rate and (b) the amount of the specific energy consumption, relative to variations in the moisture content, as a function of the direct and indirect cooling methods.
energy supplied to the dryer was not only used to evaporate the water but also to replace the heat loss. The difference in the utilized heat and the total energy consumption was not only caused by heat loss through the wall, butwas also due to the energy that is required for heating the total mass of the main column and the cotton lint from the starting temperature to the end temperature. The amount of specific energy consumption decreases as the gas temperature increases. The significant increase in the amount of water evaporating is due to the increase in temperature being larger than the increase in energy required for the heater to increase the temperature of the gas. The effect of moisture content on drying rate, as a function of the cooling method and mass flow rate of cooling water, is shown in Figure 6a at a gas temperature of 100 °C, a gas velocity of 0.157 m/s, and a basket height of 0.15 m with alternation of the cooling method from indirect to direct. With the indirect cooling, the drying rate does not change appreciably at the cooling water flow rate from 4.8 × 10-3 kg/s to 9.0 × 10-3 kg/s; however, with the direct cooling method, the drying rate increases as the cooling water flow rate increases, because it is convenient to reduce humidity efficiently by condensing water in the humid and hot air. Therefore, it is possible to dehydrate at lower temperatures, compared to water condensation via indirect cooling. Figure 6b shows the amount of the specific energy consumption, relative to variation of moisture content, as a function of direct and indirect cooling methods. The amount of the specific energy consumption is lowest with the direct cooling method at a mass flow rate of cooling water of 9.0 × 10-3 kg/s. However, further work is needed to investigate the specific energy consumption, as a function of direct and indirect cooling method. 4. Conclusion The effect of the gas temperature, cooling water flow rate, and the cooling methods (direct/indirect) on the drying characteristics of cotton lint in a 0.15-m-inner diameter (ID) × 0.9m-high recirculating fixed-bed dryer with the condenser have been examined. In the recirculating fixed-bed dryer, the drying
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rate increases, but the amount of specific energy consumption decreases, as the air temperature increases. With the direct cooling method, the drying rate increases as the cooling water flow rate increases. Further research is required to investigate the drying rate and the specific energy consumption, as a function of the direct and indirect cooling methods. Nomenclature DP ) differential pressure transducer in Figure 1 H ) hygrometer in Figure 1 Hbasket ) basket height (cm) mCW ) cooling water mass flow rate (kg/s) Q ) volumetric flow rate of air (m3/h) T ) thermocouple in Figure 1 T ) temperature (°C) Theater ) heater temperature (°C) t ) time (s) td ) drying time (s) Ug ) superficial gas velocity (m/s) V ) flowmeter in Figure 1 φ ) relative gas humidity (%) Literature Cited (1) Sabbah, M. A.; Meyer, G. E.; Keener, H. M.; Roller, W. L. Simulation Studies of Reversed-Direction Air-Flow Drying Method for Soybean Seed in a Fixed Bed. Trans. ASAE 1979, 22, 1162. (2) Sheikholeslami, R.; Watkinson, A. P. Drying of Wood Residues in a Fixed Bed, ConVectiVe Heat and Mass Transfer in Porous Media; Kakac, S., Kilkis, B., Kulacki, F. A., Arinc, F., Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1991. (3) Berbert, P. A.; Queiroz, D. M.; Silva, J. S.; Filho, J. B. P. Simulation of Coffee Drying in a Fixed Bed with Periodic Airflow Reversal. J. Agric. Eng. Res. 1995, 60, 167.
(4) Wang, Z. H.; Chen, G. Heat and Mass Transfer in Fixed-Bed Drying. Chem. Eng. Sci. 1999, 54, 4233. (5) Abernathy, G. H.; Hughs, S. E.; Gillum, M. N. Improvements of Equilibrium Moisture Content Models for Cotton. Am. Soc. Agric. Eng. 1994, 37 (2), 561. (6) Griffin, A. C. The Equilibrium Moisture Content of Newly Harvested Cotton Fibers. Trans. ASAE 1974, 17 (2), 327. (7) Urquhart, A. R.; Williams, A. M. The Moisture Relations of Cotton Taking Up of Water by Raw and Soda-Boiled Cotton at 20 °C. J. Textile Inst. 1924, 15, T138. (8) Henderson, S. M.; Perry, R. L. Agricultural Process Engineering, 3rd Edition; AVI Publishing Co.: Westport, CT, 1976. (9) Brunauer, S.; Emmett, P. H.; Teller, E. Adsorption of Gases in Multimolecular Layer. J. Am. Chem. Soc. 1938, 60 (2), 309. (10) Dent, R. W. A Multilayer Theory for Gas Sorption. Textile Res. J. 1977, 47 (2), 145. (11) Barker, G. L. Equilibrium Moisture Content of Cotton Plant Components. J. Agric. Eng. Res. 1996, 63, 353. (12) Barker, G. L.; Laird, J. W. Drying and Humidification Rates for Cotton Lint. Trans. ASAE 1993, 36 (6), 1555. (13) Lee, D.-H.; Kim, S.-D. Drying Characteristics of Starch in a Inert Medium Fluidized Bed. Chem. Eng. Technol. 1993, 16, 263. (14) Park, Y.-S.; Shin, H. N.; Lee, D. H.; Kim, D. J.; Kim, J.-H.; Lee, Y. K.; Sim, S. J.; Choi, K.-B. Drying Characteristics of Particles Using Thermogravimetric Analyzer. Korean J. Chem. Eng. 2003, 20 (6), 1170. (15) Stromillo, C.; Kudra, T. Drying: Principles, Applications and Design; Gordon and Breach Science Publishers: New York, 1986. (16) Pakowski, Z.; Mujumdar, A. S. Basic Process Calculations in Drying, Handbook of Industrial Drying; Mujumdar, A. S., Ed.; Marcel Dekker: New York, 1995.
ReceiVed for reView June 12, 2006 ReVised manuscript receiVed August 19, 2006 Accepted September 18, 2006 IE0607447
