Journal of Food Engineering 64 (2004) 129–133

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Research note

Drying performance of a batch type vibration aided infrared dryer

Ipsita Das, S.K. Das *, Satish Bal

Post Harvest Technology Centre, Department of Agricultural and Food Engineering, Indian Institute of Technology, Kharagpur 721 302, India

Received 1 August 2003; accepted 13 September 2003

Abstract

A batch type vibration aided infrared dryer was developed and studied for drying characteristics of three varieties of high

moisture paddy using 0.8 kg each (variety: ADT-37 of slenderness ratio ðlÞ ¼ 2:70, Shankar of l ¼ 4:48 and Basmati of l ¼ 5:77)under the optimum range [Simultaneous parboiling and drying of high moisture paddy using infrared, PhD thesis, Indian Institute

of Technology, Kharagpur, India, unpublished] of radiation intensity (3100 and 4290 W/m2) and grain bed depth (12 and 16 mm).

The drying rate was found to be dependent on levels of radiation intensity. Two distinct drying rates periods were observed i.e., an

initial heating up period and falling rate period. The drying took place almost entirely in the falling rate period. For any particular

radiation intensity level, the drying rate among the bed depths was found to be insignificant (p < 0:01). The Page model adequately

fitted the experimental drying data.

� 2003 Elsevier Ltd. All rights reserved.

Keywords: Infrared drying; Drying kinetics; Paddy drying

1. Introduction

Infrared (IR) heating involves the exposure of a

material to electromagnetic radiation in the wavelength

range of 0.8–1000 lm (Mohsenin, 1981). IR radiation

drying is fundamentally different from convection dry-

ing where the material is dried directly by absorption of

IR energy rather than transfer of heat from the air (Bal,Wratten, Chesnen, & Faulkner, 1970). IR radiation

drying has significant advantages over conventional

drying. The advantages are versatility, simplicity of the

equipment, easy accommodation of IR heating with

convective, conductive and microwave heating, fast

transient response and significant energy saving (Sandu,

1986). Earlier attempts to apply IR to drying of agri-

cultural materials have been reported in the literature(Ginzburg, 1961). Many researchers reported that,

though IR heating provides a rapid means of heating

and drying, it is attractive for only surface heating ap-

plication. As the IR energy is absorbed on the surface, it

allows only a shallow layer to be dried (Jenkins & Forth,

1965). With increased bed depths, a wide variation in

*Corresponding author. Tel.: +91-3222-283112; fax: +91-3222-

755303.

E-mail addresses: ipsit@agfe.iitkgp.ernet.in (I. Das), skd@agfe.

iitkgp.ernet.in (S.K. Das), sbal@agfe.iitkgp.ernet.in (S. Bal).

0260-8774/$ - see front matter � 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2003.09.020

moisture distributions was obtained. The layer, close to

the IR source, dried more rapidly compared to the one

that was deep inside. Hence, to dry grain in a deepbed,

external agitation like vibration would be helpful to turn

the grains in the bed so that each grain can receive the

radiation uniformly (Nindo, Kudo, & Bekki, 1995).

Very few published literature is available on IR drying

of rough rice/parboiled rice (Abe & Afzal, 1997; Sch-roeder & Rosberg, 1961). They have indicated favour-

able results for drying of stationary and single kernel

layers of rough rice using IR energy.

Considering the surface heating by the IR radiation,

drying of high moisture paddy was carried out using

vibratory bed for faster and uniform drying of grains.

Drying experiments have already been conducted (Das,

2003) at five different levels of radiation intensities(1509, 2520, 3510, 4520, and 5514 W/m2) and four dif-

ferent levels of grain bed depths (single kernel thickness

of 3 mm, 6, 12 and 25 mm). Optimization of the process

variables was done using response surface methodology

(RSM) on the basis of specific energy consumption and

quality attributes of the product like, maximum percent

head yield and percent-gelatinized kernel with minimum

colour (yellowness index). The optimum radiation in-tensity and grain bed depth was found to lie between

3100 and 4290 W/m2 and 12 and 16 mm, respectively

(Das, 2003). The present study discusses a laboratory

130 I. Das et al. / Journal of Food Engineering 64 (2004) 129–133

scale batch type vibratory IR dryer and drying charac-teristic of high moisture paddy under optimum drying

conditions.

2. Materials and methods

The experimental set-up for IR drying of the paddy is

shown Fig. 1. The dryer consisted of mainly two basic

units, i.e., a heating unit and a vibrating unit.

Fig. 1. View of the vibration-aided IR dryer (1––Variac; 2––Wattmeter; 3––I

springs; 7––Drying tray; 8––Motor; 9––Base plate).

2.1. Heating unit

This unit had two IR lamps of 250 W (Philips, India)

connected in series, with provision for varying the

electrical input to the lamp and thereby radiation in-

tensity. This unit was operated from a 220 V AC power

supply. The output radiation intensity was varied by

regulating the electrical power input to the lamp and by

varying the distance between the lamp and the receivingsurface. The electrical input to the lamps was varied

using a variable transformer. The output radiation in-

R lamps; 4––Main frame; 5––Motor speed regulating unit; 6––Helical

I. Das et al. / Journal of Food Engineering 64 (2004) 129–133 131

tensity of the IR lamps was measured with a pyrano-meter (National Instruments Limited, India) in con-

junction with a milli-voltmeter, while a wattmeter

(Automatic Electrical Limited, India) measured the

electrical power input to the lamp. The whole set-up was

mounted over the vibrating unit. The heating unit had

the provision for the uniform distribution of IR radia-

tion throughout the entire length of the drying section.

2.2. Vibrating unit

The vibrating unit had the provision for vibrating the

drying tray on which the paddy sample was placed.

Vibration was transmitted to the system through a link

attached to a variable speed motor. The link has beenattached to the axis of the motor with an eccentricity of

10 mm equal to the vibration amplitude required during

experiments. The variable speed motor (368 W, DC) was

fixed at the base of the supporting structure. A speed

regulator unit was used to change the speed of the motor

and consequently the frequency of vibration. The whole

structure was made of rigid 25 mm mild steel angle iron.

The square shaped vibrating platform was made up ofmild steel sheet. It was supported by four helical springs.

The vibrating bed was so arranged that four helical

springs pushed it down from the top and the link pushed

it up from the bottom. The above arrangement made the

platform vibrate in a vertical plane. The drying tray was

detachable to facilitate its weighing at different intervals

of time.

2.3. Experimental procedure

Three Indian varieties of paddy having different

slenderness ratios (length to breadth ratio (l)) were

chosen, e.g., ADT-37 (l ¼ 2:70), Shankar (l ¼ 4:48) andBasmati (l ¼ 5:77), designated as short bold, mediumslender and long slender, respectively. The samples were

first cleaned to remove impurities and over-and under-

size grains using a laboratory-cleaning device (Burrows

Equipment Company, Illinois). The cleaned sample was

then soaked in hot water for 24 h. Temperature of water

was initially raised to 95–97 �C (near boiling point) and

cleaned paddy was dumped into it, bringing the mixture

temperature nearly to 70 �C. The mixture was kept for24 h without any further heating. A standard hot air

oven method (ASAE, 1996) was used to determine the

moisture content of grain in triplicate. The moisture

content of the samples after soaking (around 42± 2%

db) was considered as the initial moisture content of the

grain. The drying process was conducted using four

combinations of radiation intensity (3100 and 4290 W/

m2) and grain bed depths (12 and 16 mm) with all thethree varieties of paddy separately.

Prior to drying, experiments were carried out to ensure

proper mixing of grain in the sample-holding tray

(300 · 300 mm). In order to avoid accumulation of grainon one side under the influence of vibration, the tray was

divided into 25 small compartments. A layer of coloured

grain was put on the grain beds in each compartment as a

tracer. Then the vibration (frequency of 22Hz, amplitude

of 10 mm) was applied. A mixing index was observed

from the analysis of the sample following the methodol-

ogy described elsewhere (Das, Das, & Bal, 2003).

Before placing the loaded sample tray, the wholedrying set-up was kept at the operating conditions for 5

min prior to start the actual experiment so as to obtain a

steady state condition. Sample weights were taken at

regular intervals throughout the total drying period using

an electronic balance (Mettler, PB-3001, Switzerland)

with an accuracy of 0.001 g. The weight loss was recorded

with time by removing the tray each time at an interval of

1 min with an interruption of drying for 20 s. The dryingwas continued until the weight of the sample reduced to a

level corresponding to a moisture content of about 14–

16% (db). Each set of experiments was replicated thrice.

Moisture loss, drying rate and moisture ratio were cal-

culated on the basis of weight loss of the sample.

2.4. Modeling of drying kinetics

Methods of describing the drying process with thin

layer drying models are widely reported in the literaturefor the purpose of simulation and scale up of the process

(Sharma & Prasad, 2001). The empirical Page’s equation

(Eq. (1)) has been used to describe the drying kinetics of

high moisture paddy (Hasan & Hobani, 2000).

MR ¼ exp½ð�ktnÞ� ð1Þwhere MR is the moisture ratio, k and n are the model

constants, and t is drying time. The coefficient of de-

termination (COD) and the reduced chi-square (v2) be-tween the experimental and calculated values was usedto evaluate the goodness of fit of the model. The higher

the value of COD and the lower the values of chi square

(v2), the better the model fit. By definition (Hasan &

Hobani, 2000)

v2 ¼PN

i¼1ðMrexp �MrcalÞ2

N � cð2Þ

where Mrexp is the experimental moisture ratio at ithobservation, Mrcal is the calculated moisture ratio at that

observation. N is the number of observation and c is thenumber of constants in a model.

3. Results and discussion

The initial moisture content of the sample was notsame for all the drying treatments. In order to normalize

the drying curves, the data on moisture content was

transformed to dimensionless moisture ratio defined by

Variety: ADT 37Bed depth

0.2

0.6

1.0

1.4

1.8

0 5 10 15 20 25 30 35

Drying time, min

Dru

ing

rate

, kg/

kg.m

in12 mm

16 mm

Fig. 4. Drying rate against time at optimum bed depths for a radiation

intensity of 4290 W/m2.

Variety: ADT 37Radiation intensity, W/m2

0.35

0.45

0.55

0.65

0.75

0.85

0.95

0 5 10 15 20 25 30 35Drying time, min

Moi

stur

e ra

tio 4290

3100

Fig. 2. Variation of moisture ratio with drying time at optimal radia-

tion intensity for a grain bed depth of 12 mm.

Variety: ADT 37 Radiation intensity, W/m2

0.4

0.8

1.2

1.6

2.0

0 5 10 15 20 25 30 35Drying time, min

Dry

ing

rate

, kg/

kg-m

in

4290

3101

Fig. 3. Drying rate with time for grain bed depth of 12 mm under

optimum radiation intensity levels.

132 I. Das et al. / Journal of Food Engineering 64 (2004) 129–133

MR using Eq. (3). The equilibrium moisture content, Me,

has been numerically set to zero, since prolonged ex-posure of grain to IR radiation eventually caused the

burning of the material and this happens only at nearly

zero moisture content (Fasina, Tyler, & Pickaw, 1998).

Thus the moisture ratio (MR) becomes

MR ¼ M �Me

M0 �Me

¼ MM0

ð3Þ

Fig. 2 shows the change in moisture ratio of the

sample (Variety: ADT 37) with drying time at two dif-

ferent radiation intensity levels for a fixed grain bed

depth of 12 mm. The curves indicated exponentialdecay. For all the drying treatments, the drying time was

found to reduce with increase in radiation intensity. A

similar result has already been reported in earlier studies

while drying with IR energy (Abe & Afzal, 1997; Afzal

& Abe, 1999). As expected at higher radiation intensity,

the higher mass transfer driving force resulted in faster

drying rate and consequently lesser drying time.

Fig. 3 gives the drying rate of the same paddy atdifferent radiation intensity levels. Two distinct drying

periods were observed, namely, an initial heating up

Table 1

Page equation’s parameter for drying of high moisture paddy using IR vibr

Variety Radiation intensity

(W/m2)

Grain bed

depth (mm)

k

Shankar 4290 12 0.028

3100 12 0.025

4290 16 0.030

3100 16 0.029

Basmati 4290 12 0.028

3100 12 0.026

4290 16 0.029

3100 16 0.028

ADT 37 4290 12 0.028

3100 12 0.025

4290 16 0.031

3100 16 0.026

period during which drying rate increased attained a

peak, followed by a falling rate period. The drying rate

became a maximum at about 9 min. No constant rate

period was observed. The moisture content of the sam-

ple is responsible for absorption of radiation. As the

drying proceeded, the loss of moisture in the product

atory dryer

n COD v2 (·104)

1.082 0.992 3.54

1.036 0.993 3.28

1.008 0.991 3.80

0.963 0.991 3.54

1.069 0.991 4.07

1.024 0.990 4.21

1.006 0.989 4.82

0.980 0.990 5.22

1.088 0.992 3.66

1.035 0.991 4.34

1.079 0.992 3.77

0.993 0.987 5.90

I. Das et al. / Journal of Food Engineering 64 (2004) 129–133 133

decreased the absorption of radiation by the sample,and decreased the drying rate. A similar trend in drying

rate was reported by Hashimoto and Kameoka (1999),

while studying the IR radiation drying characteristics of

wet porous non-food materials.

Fig. 4 shows the effects of grain bed depth on drying

rate as a function of drying time for short and bold

paddy (Variety: ADT 37), corresponding to a radiation

intensity of 4290 W/m2. The rate of moisture removalamong the different bed depths was found to be insig-

nificant (p < 0:01) at any particular intensity level. A

similar trend was observed for other radiation intensity

level, i.e., 3100 W/m2.

The Page model expressed by Eq. (1) was applied to

describe the IR radiation drying of high moisture paddy.

Estimated model constants and the corresponding COD

and v2 are presented in Table 1. The analysis yieldedhigh values of COD, ranged from 0.987 to 0.993, a low

value of v2 ranged from 3.3 · 10�04 to 5.9 · 10�04 indi-

cating a good fit of the model to the experimental data.

4. Conclusions

The drying characteristics of high moisture paddy

under optimized radiation intensity levels (3100 and

4290 W/m2) and grain bed depths (12 and 16 mm) have

been determined. Drying rate was found to be directly

dependent on radiation intensity level. Irrespective ofthe bed depth and variety, the drying time was found to

reduce with increase in radiation intensity. Two distinct

drying periods were observed, namely, an initial heating

up period followed by a falling rate period. The drying

rate among the different bed depths was found to be

statistically insignificant (p < 0:01) for any particular

radiation intensity level. The Page model adequately

described the experimental data on drying of highmoisture paddy in a vibratory IR dryer.

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