Drying and Heat Transfer Behavior of Combined Low ... - JGSEE

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”E-037 (O) 21-23 November 2006, Bangkok, ThailandFig. 3 Changes in moisture content and temperature of banana slices undergoing LPSSD-FIR at different drying conditions.moisture ratio; drying medium temperature; sample temperatureFig. 4 Changes in moisture content and temperature of banana slices undergoing VACUUM-FIR at different drying conditions.moisture ratio; drying medium temperature; sample temperature4

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”E-037 (O) 21-23 November 2006, Bangkok, ThailandIn the case of VACUUM-FIR drying it was found from Fig. 4 that the temperature of the samples also suddenly dropped duringthe initial period of the process. However, the temperature of the samples after this period steadily rose to the level higher than thepre-determined medium temperature, as noted earlier in the case of LPSSD-FIR drying, without the period of constant sampletemperature. After this period the temperature of the samples also remained almost constant as in the case of LPSSD-FIR drying.To investigate the effects of radiation intensity of far-infrared radiator on the temperature evolution of the sample, the temperatureof samples dried by LPSSD-FIR and VACUUM-FIR were again compared. At the same drying conditions, it can be seen from Figs. 3and 4 that the sample temperature during the later stage of the process in the case of LPSSD-FIR drying was higher than in the caseof VACUUM-FIR drying. This is due to the fact that in the case of LPSSD-FIR drying, radiation intensity at the position of thethermocouple used for sending the signal to the PID controller (at 30 mm above the sample surface) was more attenuated due to theabsorption of superheated steam, resulting from higher far-infrared radiation absorptivity of superheated steam due to the presence ofa large proportion of water vapor [10]. Therefore, the far-infrared radiator during LPSSD-FIR drying was used more often tomaintain the desired level of drying medium temperature leading to higher surface temperature of the far-infrared radiator (data is notshown). Consequently, the radiation intensity, depending on the surface temperature of the far-infrared radiator, experienced byLPSSD-FIR sample was greater hence higher level of the sample temperature. It was also observed that in the case of VACUUM-FIRdrying at 70 o C (see Figs. 4e and 4f) the sample temperature during the later stage of the process was very close to the drying mediumtemperature (not much higher than the drying medium temperature) compared with those dried at higher temperatures. This mayprobably due to the fact that when drying at lower temperature (70 o C in this case), the radiation intensity was lower indicated bylower surface temperature of the far-infrared radiator. Since the sample temperature was not controlled by the drying mediumtemperature but by the radiation intensity, very high sample temperatures developed in the cases of drying at higher temperaturesresulting in overheating and burning of the product hence degradation of dried product quality, especially in the case of LPSSD-FIRdrying at the highest temperature tested (90 o C). It should be noted that the effects of drying pressure on the level of sampletemperature during the later state of drying mentioned earlier was not significant.3.3 Energy consumptionTable 1 compares energy consumption along with drying time of banana slices undergoing different drying methods and conditions.It can be seen from this table that the energy consumption of the vacuum pump increased with an increase in the drying time. This isbecause electrical energy consumed by the vacuum pump at all drying conditions were the same, 1.5 kWh in this study. Regardingthe electrical energy required to generate thermal energy, it was found that the electrical energy required to generate thermal energyof LPSSD-FIR drying was higher than that of VACUUM-FIR at all drying conditions. This is because the far-infrared radiator wasused more often during LPSSD-FIR drying resulting from higher far-infrared radiation absorptivity of superheated steam asmentioned earlier in the case of temperature evolution.DryingmethodLPSSD-FIRVACUUM-FIRTemp( o C)708090708090Table 1 Energy consumption at different drying methods and conditionsPressureEnergy consumption (kWh) Specific Total steamDrying timeenergy consumption Consumption(kPa) (min) Vacuum pump b Thermal Total (kWh/kg water) (kg)10 N/A a N/A N/A N/A N/A N/A7 N/A N/A N/A N/A N/A N/A10 190 4.750 0.530 5.280 193.975 82.3337 140 3.500 0.490 3.990 146.583 60.66710 100 2.500 0.420 2.920 107.274 43.3337 90 2.250 0.410 2.660 97.722 39.00010 255 6.375 0.230 6.605 242.652 –7 185 4.625 0.200 4.825 177.259 –10 145 3.625 0.270 3.895 143.093 –7 130 3.250 0.250 3.500 128.582 –10 120 3.000 0.340 3.340 122.704 –7 110 2.750 0.330 3.080 113.152 –a N/A implies that the final moisture content of 3.5% (d.b.) was not obtainable at this condition.b The electrical energy consumption required to operate the vacuum pump was 1.5 kWh.c The flow rate of steam into the drying chamber was maintained at about 26 kg/h.Considering the total energy consumption that is the sum of the electrical energy required to operate the vacuum pump and togenerate thermal energy, it was found that although, at the same drying condition, the electrical energy required to generate thermalenergy of LPSSD-FIR drying was higher than that of VACUUM-FIR drying, the total energy consumption of LPSSD-FIR drying waslower when drying was performed at the temperature of 90 o C. This is due to the fact that this level of drying temperature (90 o C) wasabove the inversion temperature, which was somewhere between 80 o and 90 o C as noted earlier, resulted in a faster drying process forLPSSD-FIR drying compared to VACUUM-FIR dying [17]. A similar trend was also observed in the case of specific energyconsumption. It is also seen from Table 1 that LPSSD-FIR drying at 90 o C and 7 kPa provided the lowest value of the specific energyconsumption. Although the dried product quality was generally considered to optimize the operating parameters of the drying process,it was beyond the scope of this paper. Therefore, LPSSD-FIR drying at 90 o C and 7 kPa was suggested, based on the energyutilization, as the optimum condition of the process in this study. In the case of drying with the application of steam into the dryingchamber (LPSSD-FIR), it was again found from Table 1 that the total steam consumption increased with an increase in the dryingtime, as expected.5

The 2 nd Joint International Conference on “Sustainable Energy and Environment (SEE 2006)”E-037 (O) 21-23 November 2006, Bangkok, Thailand4. CONCLUSIONA drying system combining the radiative and convective heat transfer process, so called a combined low-pressure superheatedsteam and far-infrared radiation drying system (LPSSD-FIR), for drying food products was developed and studied to understand thedrying and heat transfer behavior. The results showed that with the use of far-infrared radiation, the temperature of both LPSSD-FIRand VACUUM-FIR samples during the later stage of drying were higher than the pre-determined medium temperatures. It was alsofound that LPSSD-FIR drying required more drying time than VACUUM-FIR drying at almost all drying conditions except at thetemperature of 90 o C; this indicated that the inversion temperature calculated from the overall drying rates should be somewherebetween 80 o and 90 o C. Although LPSSD-FIR drying required more energy consumption than VACUUM-FIR drying at almost alldrying conditions, the lowest specific energy consumption was obtained during LPSSD-FIR drying at 90 o C and 7 kPa. This conditionwas suggested, based on the energy utilization, as the optimum condition in this study.5. ACKNOWLEDGMENTSThe authors express their sincere appreciation to the Commission on Higher Education, the Thailand Research Fund (TRF), theNational Research Council of Thailand and the International Foundation for Science (IFS), Sweden for supporting the studyfinancially.6. REFERENCES[1] Abe, T. and Afzal, T.M. (1997) Thin-layer infrared radiation drying of rough rice, Journal of Agricultural Engineering Research,67, pp. 289-297.[2] Abe, T. and Afzal, T.M. (1998) Diffusion in potato during far infrared radiation drying, Journal of Food Engineering, 37, pp.353-365.[3] Afzal, T.M., Abe, and Hikida, Y. (1999) Energy and quality aspects during combined FIR-convection drying of barley, Journalof Food Engineering, 42, pp. 117-182.[4] AOAC (1984) Official methods of analysis (14th ed.), Washington DC: Association of Official Agricultural Chemists.[5] Barbieri, S., Elustondo, M.P. and Urbicain, M.J. (2004) Retention of aroma compounds in basil dried with low pressuresuperheated steam, Journal of Food Engineering, 65, pp. 109-115.[6] Devahastin, S., Suvarnakuta, P., Soponronnarit, S. and Mujumdar, A.S. (2004) A comparative study of low-pressure superheatedsteam and vacuum drying of a heat-sensitive material, Drying Technology, 22, pp 1845-1867.[7] Dostie, M., Seguin, J.N., Maure, D., Ton-That Q.A. and Chatingy, R. (1989) Preliminary measurements on the drying of thickporous materials by combinations of intermittent infrared and continuous convection heating, in: A. S. Mujumdar, M. A. Roques(eds.), Drying ’89, Hemisphere, New York.[8] Ginzburg, A.S. (1969) Application of infrared radiation in food processing, Chemical and Process Engineering Series, LeonardHill, London.[9] Leeratanarak, N., Devahastin, S. and Chiewchan, N. (2006) Drying kinetics and quality of potato chips undergoing differentdrying techniques, Journal of Food Engineering, 77, pp. 635-643.[10] Mills, A.F. (1999) Basic heat and mass transfer (2nd ed.), Prentice-Hall, New Jersey.[11] Mongpraneet, S., Abe, T. and Tsurusaki, T. (2002) Accelerated drying of welsh onion by far infrared radiation under vacuumconditions, Journal of Food Engineering, 55, pp. 147-156.[12] Mongpraneet, S., Abe, T. and Tsurusaki, T. (2002) Far infrared-vacuum and convection drying of welsh onion, Transactions ofthe ASAE, 45, pp. 1529-2535.[13] Mujumdar, A.S. (2000) Superheated steam drying–Technology for the future, in: S. Devahastin, (eds.), Mujumdar’s PracticalGuide to Industrial Drying, Brossard, Exergex, pp. 115-138.[14] Ratti, C. and Mujumdar, A.S. (1995) Infrared Drying, in: A.S. Mujumdar, (eds.), Handbook of industrial drying: Volume 1,Marcel Dekker, New York.[15] Sandu, C. (1986) Infrared radiative drying in food engineering: A Process Analysis, Biotechnology Progress, 2, pp. 109-119.[16] Sun, D.W. and Wang, L.J. (2006) Development of a mathematical model for vacuum cooling of cooked meats, Journal of FoodEngineering, 77, pp. 379-385.[17] Suvarnakuta, P., Devahastin, S., Soponronnarit, S. and Mujumdar, A.S. (2005) Drying kinetics and inversion temperature in alow-pressure superheated steam-drying system, Industrial & Engineering Chemistry Research, 44, pp. 934-1941.6