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This article was downloaded by: [Iran University of Science &]On: 19 November 2012, At: 05:02Publisher: Taylor & FrancisInforma Ltd Registered in England and Wales Registered Number: 1072954 Registered office: MortimerHouse, 37-41 Mortimer Street, London W1T 3JH, UK
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INDUSTRIAL-SCALE PROTOTYPE OF CONTINUOUS
SPOUTED BED PADDY DRYERThanid Madhiyanon
a, Somchart Soponronnarit
a& Warunee Tia
b
aSchool of Energy and Materials, King Mongkut's University of Technology, Thonburi,
Suksawat 48 Road, Bangkok, 10140, ThailandbSchool of Energy and Materials, King Mongkut's University of Technology, Thonburi,
Suksawat 48 Road, Bangkok, 10140, Thailand
Version of record first published: 06 Feb 2007.
To cite this article:Thanid Madhiyanon, Somchart Soponronnarit & Warunee Tia (2001): INDUSTRIAL-SCALE PROTOTYPECONTINUOUS SPOUTED BED PADDY DRYER, Drying Technology: An International Journal, 19:1, 207-216
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DRYING TECHNOLOGY, 19(1), 207216 (2001)
TECHNICAL NOTE
INDUSTRIAL-SCALE PROTOTYPE
OF CONTINUOUS SPOUTED
BED PADDY DRYER
Thanid Madhiyanon, Somchart Soponronnarit,
and Warunee Tia
School of Energy and Materials, King Mongkuts
University of Technology, Thonburi, Suksawat 48 Road,
Bangkok 10140, Thailand
ABSTRACT
An industrial-scale prototype of spouted bed dryer with a
capacity of around 3500 kg/h was constructed and tested. The pro-
totype is shown to have a desirable feature of a spouted bed as well
as the capability of continuous drying and offering consistent re-
sults 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 130160C were applied to
dry paddy from various initial moisture contents to the range of
1425%, dry basis without significant change in quality. Thermal
energy consumption, in the range of 3.13.8 MJ/kg water, is com-
parable with other commercial dryers.
Key Words: Continuous spouting; Drying; Grain.
Corresponding author. E-mail: [email protected]
207
Copyright C2001 by Marcel Dekker, Inc. www.dekker.com
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208 MADHIYANON, SOPONRONNARIT, AND TIA
INTRODUCTION
The combination of two distinct hydrodynamic features, viz., the pneumatic
transport of particles in the spout which allows intensive heating and moisture
evaporation and a falling bed in the downcomer in which tempering of particles
takes place are the main features of the spouted bed. To overcome some of the
limitations of the conventional cylindricalconical spouted bed, Mujumdar (1)
proposed the two-dimensional spouted bed in which scaling up can be easily
performed. Kalwar et al. (2), Kalwar and Raghavan (3,4) studied drying of grains
in two-dimensional spouted beds with draft plates, using soybeans, wheat, corn,
and shelled corn as test materials. It was found that thin-layer drying behavior as
that predicted by Pages equation is in very good agreementwith experimental data.The circulation of particles strongly depends on the entrance height, spout width
and slant angle. It is also shown that the drying rate is signi ficantly influenced by
grain circulation rate. In another experiment conducted by Wetchacama et al. (5), a
linear equation was found to be suitable to describe the drying rate of paddy which
depends on hold-up and drying temperature. The milling quality of paddy in terms
of head rice yield as well as the drying characteristics of paddy was investigated by
Nguyen et al. (6,7). A triangular spouted bed was proposed in their experiments.
The result of head rice yield is satisfactory as long as the moisture content is kept
above 17.6% dry basis regardless of the high inlet air temperature up to 160C.
Although extensive research has already been performed on the spouted
bed grain drying, the focus was only on batch drying operation. For drying on a
commercial scale, continuousoperation is always preferable. It is thus the objective
of this project to design and test an industrial-scale prototype of a continuous
spouted bed dryer for use in a rice mill. In addition to the fluidized bed dryer, which
has been commercialized, the spouted bed dryer is an alternative for continuous
drying of grain with high initial moisture content. Moreover, the expectation is that
grain quality is preserved, though it is continuously dried to a moisture content
safe for storage.
MATERIALS AND METHODS
Drying studies were conducted in an industrial-scale, prototype spouted bed
dryer with a capacity of 3500 kg/h as shown in Figures 1 and 2, using paddy as a
test material. The dryer consists of a vertical rectangular chamber, 0.6 m in width,1.45 m in height, and 2.1 m in length. The front wall and both of the sidewalls
just above the slanting base of the drying chamber arefitted with glass windows
to permit visualization of the grain flow pattern. The slanting base is inclined at
60. The air entrance and spout widths are 0.04 and 0.06 m, respectively. The
draft plates with 0.62 and 0.82 m in height were centrally installed in the first and
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INDUSTRIAL-SCALE CONTINUOUS SPOUTED BED DRYER 209
Figure 1. Schematic diagram of the overall experimental set-up.
Figure 2. Dimensions of the continuous spouted bed dryer.
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210 MADHIYANON, SOPONRONNARIT, AND TIA
second phase of experiments, respectively. The entrance heights of 0.10, 0.125,
and 0.15 m were varied from one experiment to the other to match the feed rate of
paddy. Heat derived from burning diesel fuel was supplied to heat the air before
it was drawn through the blower. The inlet air temperature was controlled by a
P.I.D. temperature controller to be within 2C of the set values. The heated air
was then forced through the ductwork connected to the drying chamber. The hot
and humid air leaving the drying chamber was discharged into a cyclone with
some portion of it exhausted to the ambient. The rest (approximately 60 70% of
total circulation air) wasrecirculated and mixed with ambient airat the combustion
chamber. Paddy was continuously fed into the drying chamber. The paddy traveled
upward through the draft channel before raining back onto the downcomers and
moved vertically downward. The paddy movement alternated between spout anddowncomer until paddy came outof theoutlet port at thebottom of dryingchamber.
Air and paddy temperatures were measured using k-type thermocouples connected
to a data logger with an accuracy of1C. The pressure drop across the bed was
measured using a U-tube manometer. A hot air anemometer with an accuracy
of4% was used to measure the air velocity. Paddy samples were taken from
the outlet port of the drying chamber for measuring moisture content, head rice
yield, and whiteness at 10-min intervals for a period of 80 min. Moisture contents
were determined using a hot air oven set at 103C for 72 h. Head rice yield
was determined according to the method of the Rice Research Institute, and the
whiteness was measured using the Kett meter.
RESULTS AND DISCUSSION
Paddy Motion in the Spout and Bed Shape
It was observed that the flow of paddy was relatively uniform through the
entirelength of thedryer dueto theproper functioning of ductwork connectedto the
inlet of the drying chamber. In the first phase of experiments, the spouting did not
occur continuously. This could be attributed to the improper blower performance,
which resulted in a low pressure drop across the bed and low inlet air velocity at
the entrance of the drying chamber. To solve this problem, an existing blower was
replaced with a high-pressure blower during the second phase of the experiments.
Figure 3 presents the variation of the bed height along the length of dryer at afeed rate of 3550 kg/h and hold-up of approximately 310 kg. The shape of both
downcomer sides is not quite symmetrical. This is because the draft plates may
not be positioned exactly at the center of the drying chamber. The effect of the bed
height difference between each side on the moisture distribution in the bed cannot
be identified since the moisture distribution was not measured in this study.
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INDUSTRIAL-SCALE CONTINUOUS SPOUTED BED DRYER 211
Figure 3. Variation of bed height with length of dryer: feed rate, 3550 kg/h; hold-up,
310 kg.
Drying Efficiency and Milling Quality
Fifteen experiments were carried out in phases 1 and 2. The results of the
first and second phase experiments are summarized in Tables 1 and 2, respectively.
To study dryer performance in terms of moisture content reduction, milling quality
and energy consumption, feed rate of paddy, and the inlet air temperature were
varied from one experiment to the other.
Moisture Content
Cycle time strongly affects the moisture distribution inside the paddy kernel
in the downcomer as well as the number of turns of paddy flowing through the
draft channel. Partial tempering occurs in the downcomer while intense heating
and moisture removal take place in the draft channel. However, it is impractical
to record cycle time in these experiments because of the continuous operation.
Instead, the mean residence time in the dryer (tm) is defined by the following
equation:
tm =hold-up
feed rate(1)
The moisture reduction result of test No. 1 (Table 1) was not satisfactory.This is probably due to nonmatching between the airflow rate and the high feed
rate, which led to residence times that were too short. Therefore, feed rates in
all subsequent experiments were limited to 1000 kg/h, which consequently led
to good results in terms of moisture reduction. The results in Table 1 show that
paddy was dried from a moisture range of 20.030.3% to 14.421.5%, dry basis
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INDUSTRIAL-SCALE CONTINUOUS SPOUTED BED DRYER 213
Table 2. Summary of the Second Phase Experimental Results
Test No.
Description 8a 9b 10b 11c 12c 13c 14c 15d
Feed rate (kg/h) 2470 2440 1990 2370 2400 3160 3280 3550
Hold-up (kg) 60 360 340 240 210 335 290 310
Residence time (min) 1.5 8.9 10.2 6.0 5.3 6.4 5.3 5.2
Average inlet air temp. (C) 154 149 141 152 159 154 156 160
Exit grain temp. (C) 68 71 66 71 68 67 67 68
Average moisture content
BFD (% dry basis) 26.1 21.7 28.5 22.1 25.0 29.4 28.2 22.8
AFD (% dry basis) 22.7 17.5 23.6 17.1 19.7 25.0 23.3 19.0
Head rice yield
BFD (%) 56.1 45.9 53.3 49.0 49.6 50.1 54.3 41.2
AFD (%) 58.9 45.7 54.2 47.4 48.4 52.4 55.0 40.8
Whiteness
BFD 41.9 45.7 44.2 47.6 47.6 47.8 46.4 46.7
AFD 41.6 45.4 45.0 46.8 46.7 47.5 46.3 45.9
Drying rate (kg water/h) 68 87 79 101 106 111 130 101
Energy consumption
(MJ/kg water evaporated)
Heat 6.5 7.0 N/A 4.4 5.2 3.5 3.8 3.1
Electricity 0.60 0.60 0.60 0.50 0.50 0.46 0.40 0.46
AFD, after drying; BFD, before drying; N/A, not available; temp., temperature.a Draft channel blade angle 85; entrance height, 10 cm.b
Draft channel blade angle 89
; entrance height, 10 cm.cDraft channel blade angle 89; entrance height, 12.5 cm.dDraft channel blade angle 89; entrance height, 15 cm.
Figure 4. Comparisonof moisture contentbetween before andafter drying:inlet airtemp.,
154C; feed rate, 3160 kg/h; hold-up, 335 kg; tm, 6.4 min.
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214 MADHIYANON, SOPONRONNARIT, AND TIA
Milling Quality
The milling quality, in terms of head rice yield and whiteness, is very sen-
sitive to the drying method and is usually employed in appraising the success
or failure of a grain-drying system. As seen in Tables 1 and 2, it is clear that the
present drying process does not have much effect on milling quality. No significant
quality change was observed when paddy was dried down to 1425% dry basis,
regardless of the high inlet air temperature of up to 130160C. For the head rice
yield, this was due to the fact that the moisture gradient inside the grain kernel
was somewhat redistributed and equalized while passing through the downcomer,
which resulted in relaxation of the stress developed within the grain kernel and
thus gave a satisfactory result in the head rice yield. The high inlet air temperaturemakes no significant change in the whiteness, since grain is exposed to the hot air
in the spouting region within a very short time. In addition, the alternative method
for preservation of milling quality is to apply a zoning concept, that is, using a
high temperature in the first zone for fast drying and a lower temperature in the
subsequent zone of the dryer for gentle drying. This is analogous to employing
the different temperature level concept as described by Nguyen et al. (6,7). The
head rice yield and whiteness results of test No. 13 are presented in Figure 5.
A similar trend of consistency was observed in all experiments.
Energy Consumption
Thermal energy consumption in the first phase of experiments (Table 1)
was relatively high, that is, in the range of 5.67.7 MJ/kg water removed. This
Figure 5. Comparison of head rice yield and whiteness between drying with ambient air
and drying with spouted bed dryer (inlet air temp., 154 C; feed rate, 3160 kg/h; hold-up,
335 kg;tm, 6.4 min).
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INDUSTRIAL-SCALE CONTINUOUS SPOUTED BED DRYER 215
be attributed to noncontinuation of the spouting, which would result in poor paddy
circulation. However, in the second phase of the experiments, thermal energy
consumption was reduced to the range of 3.13.8 MJ/kg water according to a
feed rate of 31003500 kg/h (test Nos. 1315 in Table 2). The decrease of thermal
energy consumption in thesecond phase, especially fortest Nos. 1315 is probably
attributed to a much higher circulation rate than that achieved in thefirst phase of
the study.
CONCLUSIONS
An industrial-scale continuous spouted bed dryer with a capacity of 3500
kg/h was developed and tested. No serious loss in quality was observed during
the experiments while the high drying air temperature was used. Final moisture
content and milling quality results appear to be consistent throughout the testing
periods. However, with the limitation of the existing drying chamber length, a
high percentage of moisture reduction corresponding to the high paddy feed rate
could not be achieved. The energy consumption is comparable to those of other
commercial dryers.
ACKNOWLEDGMENTS
The authors would like to thank the Thailand Research Fund for financialsupport and Rice Engineering Supply Co. Ltd. for supplying facilities for the con-
struction of the prototype dryer. The Kungleechan Rice Mill is also acknowledged
for its cooperation during this study.
REFERENCES
1. Mujumdar, A.S. In Spouted Bed Technology: A Brief Review, Drying84;
Hemisphere: Washington, 1984; 151157.
2. Kalwar, M.I.; Kudra, T.; Raghavan, G.S.V.; Mujumdar, A.S. Drying of Grains
in a Drafted Two-Dimensional Spouted Bed. J. Food Proc. Eng. 1991, 13,
321332.3. Kalwar, M.I.; Raghavan, G.S.V. Batch Drying of Shelled Corn in Two-
Dimensional Spouted Beds with Draft Plates. Drying Technol. 1993,11 (2),
339354.
4. Kalwar, M.I.; Raghavan, G.S.V. Circulation of Particles in Two-Dimensional
Spouted Beds with Draft Plates. Powder Tech. 1993,77, 233242.
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216 MADHIYANON, SOPONRONNARIT, AND TIA
5. Wetchacama, S.; Soponronnarit, S.; Swasdisevi, T.; Panich-ing-orn, J.;
Suthicharoenpanich, S. In Drying of High Moisture Paddy by Two-
Dimensional Spouted Bed Technique, Proceedings of the First Asian-
Australian Drying Conference (ADC99), Bali, Indonesia, Oct. 2427, 1999;
300307.
6. Nguyen, L.H.; Driscoll, R.H.; Srzednicki, G.S. In Drying Characteristics of
Paddy in a Triangular Spouted-Bed, Proceedings of the 11th International
Drying Symposium (IDS98), Halkidiki, Greece, Aug. 1922, 1998; 1397
1404.
7. Nguyen, L.H.; Driscoll, R.H.; Srzednicki, G.S. InFlowing Performance and
Drying Characteristics of Paddy in a Triangular Spouted-Bed, 7th Interna-
tional Working Conference on Stored-Product Protection, Beijing, China, Oct.1420, 1998.
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