Experimental and Theoretical Investigation of Moisture Dynamics in Intermittent Drying of Rough Rice

Document Type : Original Paper


School of Chemical Engineering, College of Engineering, University of Tehran, Iran


Intermittent drying is a satisfactory technique for drying of heat sensitive grains especially rough rice. This method consists of two separate stages applied intermittently: drying and tempering. In this work, intermittent drying of rough rice is investigated both theoretically and experimentally. In order to characterize this process; a multi-scale model consisting of macroscopic and microscopic submodels has been developed. Macroscopic model calculates the moisture changes of the rough rice bed assuming a lumped model for grains, while the microscopic submodel determines the rate of moisture gradient removal during the resting period. In latter submodel, moisture is assumed to diffuse through the grains governing by Fick’s law. Further, a set of experiments were designed and carried out in a lab-scale fluidized bed dryer to estimate the model parameters as well as to evaluate the effects of different parameters such as temperature, air velocity, and tempering time on the drying rate. The model estimates show good agreement with experimental data. Model's results reveal that thermal equilibrium is rapidly obtained within the first two minutes of grain-hot air exposure. Air velocity shows to have no significant effect on drying rate when fluidized condition is prevailed. In addition, a significant kink occurs in drying rate diagrams which implies that drying rate is improved as a result of moisture gradient removal by applying tempering period.


[1] Brooker, D.B., Bakker_Arkema, F.W. and Hall, C.W. (1992).“Drying and storage of grains and oil seed.” Van NostrandReinold, New York.
[2] Nishiyama, Y., Cao, W. and Li, B.M. (2006).“Grain intermittent drying characteristics analyzed by a simplified model.”J. Food Eng., Vol. 76, No. 3, pp. 272-279.
[3] Golmohammadi, M., Rajabi-Hamane, M. and Hashemi, S.J. (2012). “Optimization of drying–tempering periods in a paddy rice dryer.”Dry. Technol., Vol.30, No. 1, pp. 106-113.
[4] Schluterman, D.A. and Siebenmorgen, T.J. (2007).“Relating rough rice moisture content reduction and tempering duration to head rice yield reduction.”Transactions of the ASAE, Vol. 50, No. 1, pp.137-142.
[5] Golmohammadi, M. (2010). “Energy optimization of a fluidized bed paddy rice dryer.” M.Sc. Thesis, School of Chemical Engineering, College of Engineering, University of Tehran, Tehran, Iran.
[6] Steffe, J.F., Singh, R.P. and Bakshi, A.S. (1979). “Influence of tempering time and cooling on rice milling yields and moisture removal.”Transactions of the ASAE, Vol. 22, No. 5, pp.1214-1218.
[7] Li, Y.B., Cao, C.W., Yu, Q.L. and Zhong, Q.X. (1999).“Study on rough rice fissuring during intermittent drying.” Dry. Technol., Vol. 17, No. 9, pp. 1779-1793.
[8] Cnossen, A.G., Siebenmorgen, T.J. and Yang, W. (2002).“The glass transition temperature concept in rice drying and tempering: effect on drying rate.”Transactions of the ASAE, Vol. 45, No. 3, pp. 759-766.
[9] Wu, B., Wang, W. and Jia, C. (2004).“A three dimensional numerical simulation of transient heat and mass transfer inside a single rice kernel during the drying process.” Biosystems Eng., Vol. 87, No. 2, pp.191–200.
[10] Dong, R., Lu, Z., Liu, Z., Nishiyama, Y. and Cao, W. (2009).“Moisture distribution in a rice kernel during tempering drying.” J. Food Eng., Vol. 91, No. 1, pp. 126–132.
[11] Cihan, A., Kahveci, K., Hacihafizoğlu, O. and De Lima, A.G.B. (2008).“A diffusion based model for intermittent drying of rough rice.”Heat and Mass Transfer, Vol. 44, No. 8, pp. 905–911.
[12] Palancz, B. (1983).“A mathematical model for continuous fluidized bed drying.” Chem. Eng. Sci., Vol. 38, No. 7, pp. 1045-1059.
[13] Chung, L.L. and Mujumdar, A.S. (2006). Handbook of Industrial Drying. 3rd ED. Chapter 8, Marcel Dekker, New York.
[14] Soponronnarit, S., Wetchacama, S., Trutassanawin, S. and Jariyatontivait, W. (2000).“Development of a commercial scale vibro-fluidized bed paddy dryer.” Kasetsart J (Natural Science), Vol. 34, No. 3, pp. 423-430.
[15] Izadifar, M., Baik, O. and Simonson, C.J. (2006).“Modeling of the packed bed drying of paddy rice using the local volume averaging (LVA) approach.” Food Res. Int., Vol. 39, No. 6, pp. 712–720.
16- Iguaz, A., Arroqui, C., Esnoz, A. and Virseda, P. (2004).“Modeling and simulation of
heat transfer in stored rough rice with aeration.”Biosystems Eng., Vol. 89, No. 1, pp. 69-77.
[17] Chung, D.S. and Pfost, H.B. (1967). “Adsorption and desorption of water vapor by cereal grains and their products, part II: development of the general isotherm equation.” Transactions of the ASAE, Vol. 10, No. 4, PP.549–557.
[18] Iguaz, A., San Martin, M.B., Arroqui, C., Fernandez, T., Mate, J.I. and Virseda, P. (2003). “Thermo physical properties of medium grain rough rice (LIDO cultivar) at medium and low temperatures. ”Eur. Food Res. Technol., Vol. 217, No. 3, pp. 224-229.
[19] Chang, C.S., Converse, H. and Steele, J.L. (1993).“Modeling of temperature of grain during storage with aeration.”Transactions of the ASAE, Vol. 36, No. 2, pp. 509-519.
[20] Pinazo, J.M. (1995). Handbook of Air-Conditioning, Vol I: Psychrometric Transformations. NiversidadPolitecnica de Valencia, Spain.
[21] Steffe, J.F. and Singh, R.P. (1982). “Diffusion coefficients for predicting rice drying behaviour.” J. Agr. Eng. Res., Vol. 27, No. 6, pp.489-493.
[22] ASAE. (1996). ASAE Standards: Moisture Measurement–Unground Grains and Seeds.
American Society of Agricultural Engineers, St. Joseph, MI.
23- Hatamipour, M.S. and Mowla, D. (2007). “Mathematical modeling of the fluidized bed drying of cubic materials.” Chem. Eng. Technol., Vol.30, No. 11, pp. 1584-1589.
[24] Garnavi, L., Kasiri, N. and Hashemabadi, S.H. (2006).“Mathematical modeling of a continuous fluidized bed dryer.” Int. Commun. Heat Mass Transfer, Vol. 33, No. 5, pp. 666-675.