Thanid Madhiyanon

A MATHEMATICAL MODEL FOR CONTINUOUS DRYING OF GRAINS IN A SPOUTED BED DRYER

ABSTRACT A mathematical model for a continuous spouted bed dryer has been presented to predict moisture content, air and grain temperatures as well as energy consumption. To better understand the interactive influence of processes in each region of the spouted bed, solution schemes for the spout and downcomer were treated separately. The behavior of dryer was investigated experimentally and found that the dryer behaved differently from an ideal plug flow. The drying rate as simulated by the model is almost constant during grain movement in the dryer. Absence of airflow in the downcomer leads to a tempering process that takes place in the downcomer while intense heat and mass transfer occurs mainly in the spout due to the high airflow rate there. Furthermore, by considering the predicted grain temperature history as one of the indicators of product quality, one can, in principle, design appropriate successive processes in a continuous spouted bed dryer to minimize product damage.

High Temperature Spouted Bed Paddy Drying with Varied Downcomer Air Flows and Moisture Contents: Effects on Drying Kinetics, Critical Moisture Content, and Milling Quality

Abstract This research is an experimental study on paddy drying conducted in a two-dimensional spouted bed batch dryer to investigate the effects of downcomer airflow, drying temperature, and initial moisture content on drying kinetics, milling quality, and thermal energy consumption. The system of spouted bed drying and a comparison of spouted bed drying in the present study, and fluidized bed drying in the related literature, are also discussed. Downcomer air flows of 0, 20, and 30% ([defined as mass flow rate of downcomer air/total mass flow rate of air] × 100%), inlet air temperatures of 110, 130, and 150°C, and initial moisture contents of 18–35% d.b. were used. It was found that moisture transfer did not only occur in the spout region but also took place in the downcomer region. The moisture content and temperature of the paddy dropped as grain moved downward in the downcomer, which resulted from the presence of an evaporative cooling phenomenon. The characterization of drying curves regardless of any drying condition could be described by nearly linear relationships between moisture content and time. Although high downcomer air flow and drying temperature could enhance effective moisture reduction, they caused an adverse decrease in critical moisture content and HRY (head rice yield). Critical moisture content and HRY could also increase with an increase in initial moisture content. The difference in moisture content between the initial and critical moisture contents varied between 4.5 and 8.0%d.b., depending upon drying conditions. No significant effect on color was evident. From the point of view of HRY, the correct management of a two-stage spouted bed drying system could be a suitable and attractive alternative for rice mills. Finally, a comparison between the spouted bed drying and fluidized bed drying showed that the spouted bed had advantages over the fluidized bed in terms of product quality. The specific drying rates (kg water evaporated h−1 m−3) of both techniques were comparable. With respect to energy consumption, spouted bed drying was not as efficient as fluidized bed drying for intermediate initial moisture content, but a contrary result was obtained for low initial moisture content.

INDUSTRIAL-SCALE PROTOTYPE OF CONTINUOUS SPOUTED BED PADDY DRYER

An industrial-scale prototype of spouted bed dryer with a capacity of around 3500 kg/h was constructed and tested. The prototype is shown to have a desirable feature of a spouted bed as well as the capability of continuous drying and offering consistent results throughout the testing period. Experimental results show that the prototype performs well in reducing the moisture content of the paddy and yields high product quality in terms of the milling quality. The high temperatures up to 130–160°C were applied to dry paddy from various initial moisture contents to the range of 14–25%, dry basis without significant change in quality. Thermal energy consumption, in the range of 3.1–3.8 MJ/kg water, is comparable with other commercial dryers.

A TWO-REGION MATHEMATICAL MODEL FOR BATCH DRYING OF GRAINS IN A TWO-DIMENSIONAL SPOUTED BED

A two-region non-equilibrium model of a two-dimensional spouted bed dryer has been developed. The model, based on heat and mass transfer interactions between the spout and downcomer regions, predicts the air and grain temperatures and moisture content throughout each elementary thin layer of grain in the spouted bed. The model was validated with the experimental results on batch drying of corn. Drying and heating processes are shown to occur in both the spout and downcomer regions. Overall, the agreement between experimental and simulated results is satisfactory.

Different Strategies of Far-Infrared Radiation Application in Paddy Drying

The applied strategy of far-infrared radiation in paddy drying is divided into two main processes with some overlapping, namely, series and combined drying processes. The first comprises fluidized-bed drying, far-infrared radiation, tempering and ambient air ventilation as well as transport of paddy, and the latter is the combination of far-infrared radiation and hot-air convection in fluidization technique. The changes of average moisture content, grain temperature and milling quality in each paddy drying process are presented together with mathematical models. The experiments showed that series drying process was more effective to apply in the aspects of paddy drying and milling qualities than combined drying process. Due to less effectiveness in combined drying process, heat and mass transfer models, only series drying process were developed. The simulation results indicated that the models were capable of satisfactorily predicting the simultaneous average moisture contents and grain temperatures of a paddy grain as compared with the experimental results.