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- 1. International Journal of Advanced Research in Engineering and Technology (IJARET), ISSN 0976 –
INTERNATIONAL JOURNAL OF ADVANCED RESEARCH IN
6480(Print), ISSN 0976 – 6499(Online) Volume 4, Issue 6, September – October (2013), © IAEME
ENGINEERING AND TECHNOLOGY (IJARET)
ISSN 0976 - 6480 (Print)
ISSN 0976 - 6499 (Online)
Volume 4, Issue 6, September – October 2013, pp. 256-268
© IAEME: www.iaeme.com/ijaret.asp
Journal Impact Factor (2013): 5.8376 (Calculated by GISI)
www.jifactor.com
IJARET
©IAEME
OPERATING PARAMETERS EFFECTS ON DRYING KINETICS AND
SALTED SUNFLOWER SEED QUALITY UTILIZING A FLUIDIZED BED
Phairoach Chunkaew1, Aree Achariyaviriya1, Siva Achariyaviriya1,
James C. Moran1, Sujinda Sriwattana2
1
Department of Mechanical Engineering, Faculty of Engineering, Chiang Mai University,
239 Huay Keaw Road, Muang District, Chiang Mai 50200, Thailand
2
Department of Product Development Technology, Faculty of Agro-Industry, Chiang Mai University
ABSTRACT
Drying salted sunflower seed using a fluidized bed was studied to investigate the effects of
various operating parameters. These parameters were the drying air temperature, the static bed height
and the superficial air velocity. Their effect on sunflower seed quality was investigated. The quality
was determined from the kernel color, rupture force and consumer acceptance test scores.
Additionally, the drying kinetics was modeled using an exponential relationship. The drying constant
was determined using the least squares method and correlated with the operating parameters. The
results show that an increasing drying temperature causes a decrease in the rupture force of dry seed.
It also results in decreasing the redness and yellowness. However, the drying temperature has no
effect on lightness. Besides, the static bed height and the superficial air velocity had no effect on the
quality. Sunflower seeds dried at 170 °C received the highest approval rating from consumer
feedback.
Keywords: Operating parameters, Fluidized bed, Quality, Consumer Acceptance Test
1. INTRODUCTION
Sunflower seeds (Helianthus annuus L.) are normally classified by the pattern on their husks.
Black oil sunflower seeds have black husks, which are usually pressed to extract their oil. Confection
sunflower seeds have striped husks (black and white), used for food [1]. The physical properties of
sunflower seeds were studied by several researchers
(Perez et al., 2007[2] ; Figueiredo et al.,
2011[3]; Sharma et al., 2009 [4]; Bax et al., 2004[5] etc.). Dried salted sunflower seeds are used as
snack products. They are a nutritious snack with high nutrition. The nutrient content of sunflower
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seeds consists of 47.5 g fat, 18.6 g of carbohydrate, 19.8 g of protein and 6 g of fiber per 100 g. The
vitamin content consists of E 37.77 mg, 9.1 mg of niacin equivalent, 3 mg of sodium, 710 mg of
potassium, 110 mg of calcium, 640 mg of phosphorus, 4.9 mg of iron, 390 mg of magnesium and 5.1
mg of zinc [6]. In 2012, Chiang Rai businesses on the Thai border imported 39.56 million Thai baht
of sunflower and watermelon seed [7]. In the production process, sunflower seeds are pretreated in
salted aqueous solution with a ratio of 1/4 to 1/2 cup of salt per two quarts of water for about 17
hours. They are then roasted at 150oC using a shallow pan or a conventional oven for about 30-40
minutes [8].
Fluidized beds are widely used for drying particulate or granular solids in agricultural and
food industries. They promote excellent mixing and have high effective heat and mass transfer
coefficients. The hydrodynamics of the bed result in coupling between the heat and mass transfer
coefficients. This research will use a fluidized bed dryer for the salted sunflower seed drying process
in an attempt to reduce the drying time and energy consumption. The air flow rate was selected
according to the drying kinetics, the lower this value the lower the required operational energy.
However, to reach a fluidized state the minimum fluidization velocity (Umf) is required. This is one
of the significant parameters in the analysis and design of fluidized beds. Several researchers
(Mawatari et al., 2003[9]; Harish Kumar and Murthy, 2010[10]) determined Umf by the pressure drop
method which is an accurate and widely used method. The pressure drop across the air distributor is
a function of its open area and the air velocity[11]. For drying kinetics, most results are presented
showing the moisture content reduction, drying time, drying rate and energy consumption [12-13].
Drying condition affect the physical properties of food and agricultural products. Research on
dried product quality has focused on color, texture (hardness, crispness and toughness) and
microstructure. Presently, there is no publication of these qualities for sunflower seed dried by the
fluidized bed technique.
In this study, a fluidized bed is proposed as a novel drying method for salted sunflower seed.
The effects of various operating parameters on the specific drying rate and product quality are
investigated and discussed. The drying temperature, static bed height to hydraulic diameter ratio
(H/Dh) and superficial air velocity ratio (Uo/Umf) are the investigated parameters. The product quality
was assessed in terms of the kernel color, rupture force and consumer satisfaction scores.
Additionally, a model for the drying kinetics was developed. Comparison between drying curves
verified the drying kinetics model. Furthermore, the quality of dried product was investigated and a
product evaluation on consumer preference levels was performed.
Nomenclature
DT
Dh
H
H*
k,
md
Min
MR
Mt
RMSE
R2
SDR
t
T
Umf
Uo
L*, a*, b*
total drying time, min
hydraulic diameter, cm
static bed height, cm
fluidization height, cm
drying constant
dry bone mass of product, g
initial moisture content of sample, %dry basis
moisture ratio
moisture content of sample at time t, %dry basis
root mean square error
coefficient of determination
specific drying rate, kgwater/ min kgp
drying time, min
temperature, oC
minimum fluidization velocity, m/s
superficial air velocity, m/s
color parameters
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2. MATERIALS AND METHODS
2.1 Experimental apparatus
A laboratory fluidized bed dryer with an open loop air system is shown in Fig. 1. It consists
of: a blower, an electrical heating chamber, a distributor, a fluidized bed riser and an anemometer
(VELOCICALC PLUS model 8385-M-GB). The anemometer was installed at the top of chamber for
measuring the superficial air velocity. The blower was driven by a 0.75 kW motor with inverter.
Eleven heaters, totaling 11 kW with a PID temperature controller were used. The distributor had a
4.5 mm diameter with a 33.3% open area [11]. The riser had a uniform rectangular, 15cm x 15 cm,
cross section and a height of 150 cm. The hydraulic diameter (Dh) was 15cm.
In the high drying temperature and high air velocity (150oC to 170oC; 1.5Umf to 2.5 Umf )
cases the ambient air was preheated by a 5 kW heater to between 45 - 60oC before flowing to the
blower.
Fig.1. The experimental apparatus of laboratory fluidized bed dryer
2.2 Material
The Chinese sunflower seed 5009 was purchased from the Mai Sai district market, Chiang
Rai Province, Thailand. Their moisture content was determined by experiment, samples were dried
in a hot air oven at 130oC for 3 hours to obtain the dry bone mass. The moisture content was 9.81 ±
0.14 on a % dry basis. The shape of seed approached an ellipsoid. The actual dimensions were 18.2
± 1.3 mm in length, 10.1 ± 1.3 mm in width and 5.6 ± 1.1 mm in thickness. The volume-equivalent
diameter was 8.18 ± 0.08 mm. Prior to drying, the sunflower seeds were pretreated by immersion in a
salt solution for 17 hours. The ratio of sunflower seed to water and salt was 48:60:5 by volume. After
draining they were left on a sieve for 1 hour. The initial moisture content (Min) of the samples was
90.65 ± 5.37 % dry basis. The samples were kept in a closed plastic bag and stored at 4oC.
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2.3 Experimental method
The minimum fluidization velocity of the sample was determined by the pressure drop
method using the apparatus shown in Fig. 1.The static bed height inside the square bed is one of the
important parameters which affects Umf [11]. Experiments were performed at ambient air
temperature and various static bed height. At static bed height of 0.3Dh, 0.7Dh, Dh and 1.3Dh, the
corresponding minimum fluidization velocities were 2, 2.1, 2.4 and 2.8 m/s, respectively.
2.3.1 Drying experiment
The drying rate is dependent on the fluidized bed operating conditions. Experiments were
performed under three different conditions. The first one kept a constant air velocity of 1.5Umf and a
static bed height of 0.3Dh whilst testing at 4 different temperatures (110, 130, 150, 170oC). The
second condition kept a constant drying air temperature of 150oC and constant static bed height of
0.3Dh whilst testing at 5 different air velocities, Uo (0.5Umf, Umf, 1.5Umf, 2Umf and 2.5Umf). The
third condition kept a constant drying air temperature of 150oC and an air velocity of 1.5Umf whilst
testing at 4 different static bed heights (0.3Dh, 0.7Dh, Dh, 1.3Dh). In each experiment, the fluidized
bed dryer maintained the chamber temperature at the desired drying condition 40 min before putting
the sample in the drying chamber. After drying for a definitive period such as 2, 4, 6, ... min, the
samples were withdrawn from the dryer and their weight was measured by a digital balance (±0.1 g).
After weighing, a new sample was placed into the drying chamber and the process repeated. For each
test the fluidization height (H*) of the product was recorded. Samples were dried until they reached a
moisture content lower than 5.2 %dry basis [14]. All experiments were carried out in triplicate.
2.3.2 Quality measurement
The quality of dried sunflower seed was investigated in terms of hardness (rupture force),
kernel color and a consumer survey. Each shall be explained in more detail below.
1) The rupture force of dried seed was determined using a universal testing machine
(HOUNS model). This was used for rupture force compression testing of dried seed at their final
moisture content. A compression load cell of 10 kN was used to determine the maximum rupture
force on the horizontal and vertical orientation of seeds. After each drying
condition, thirty seeds
were tested and the rupture force data was reported as an average value.
2) The color of dried kernel samples were measured by a colorimeter (Hunter Associates
Laboratory, Inc., model MiniScan XE Plus, VA, USA) based on standards from CIE (Commission
International de I’Eclairage). Three parameters were measured, namely L* (lightness), a*
(redness/greenness) and b* (yellowness/blueness). At least three different measurements were taken
for each sample at different positions. For each drying condition, twenty dried seeds were tested and
the data was reported as an average value.
3) A consumer product evaluation was conducted at the laboratory of sensory evaluation and
consumer testing, in Chiang Mai University, Thailand. A hundred consumers were randomly
recruited from students and staff at Chiang Mai University. The criteria for recruitment of the
participants were that they like sunflower seed, had no food allergies and were available and willing
to participate. Consumers were briefly instructed on the testing procedure, especially the use of the
consumer ballot.
Four samples, each coded with a 3-digit random number, were served. Each sample consisted
of 15 g of the sunflower seed in a 2 oz plastic cup, covered with a lid. The order of serving to each
panelist was randomized to minimize bias [15]. Samples were evaluated under controlled laboratory
conditions in partitioned booths. Panelists rated each sample using a 9-point hedonic scale [16],
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wherein 1 = dislike extremely, 5 = neither dislike nor like, and 9 = like extremely. Panelists rated
their overall preference, the seed color, flavor and crispiness using a computer ballot. Distilled water
was supplied to panelists to cleanse their palate and minimize any residual effect between samples. A
small gift was provided to panelists for participating in the study.
2.4 Analysis
2.4.1 Drying and Modeling Analysis
The experimental data was used to obtain the specific drying rate (SDR). It is defined as
follows:
SDR =
evaporative water from product
DT × md
(1)
where DT is total drying time and md is the dry bone mass of product.
To model the drying behavior of samples an appropriate empirical equation (exponential
model) was fitted to the experimental moisture removal data. For air drying at high temperatures, the
moisture equilibrium (Meq) of the samples can be taken to approach zero [17-19]. So, empirically the
drying rate can be expressed as:
MR =
Mt
= exp(− kt )
M in
(2)
where Mt is the moisture content of the sample at drying time t and Min is the initial moisture
content (gwater/gdry mass). MR is the moisture ratio (dimensionless term).
The drying constant (k) was determined by fitting experimental data to the drying kinetics
equation, Eq. (2), using the least squares method. The dependence of k on the drying air temperature,
H/Dh and Uo/Umf was evaluated. The moisture ratio is a function of the drying time and can be
estimated using Eq. (2) with Eq. (3), which is expressed as follows:
MR = exp [ ( − k1k2 k3 )t ]
where k ,
respectively.
1
k2 and
k3 are
(3)
drying constants for the drying air temperature, H/Dh and Uo/Umf,
2.4.2 Statistical analysis
The SPSS for windows, version 17.0 [20] was used for variance analysis (ANOVA). Duncan’s
test was used to analyze the drying constant, rupture force and kernel color. Tukey’s studentized
range test was used to analyze the consumer acceptance data at a significance level of 0.05. A
coefficient of determination (R2) and a root mean square error (RMSE) were evaluated to determine
the fit of the exponential model. These parameters can be calculated as shown below:
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N
∑ (M R
R2 =
p re , i
− M R ex ) 2
i =1
N
∑ (M R
ex , i
− M R ex ) 2
(4)
i =1
1
RSME =
N
∑ (MRpre,i − MRex,i )
i =1
N
0.5
2
(5)
where MRex is the average experimental moisture ratio, MRex ,i , and MRpre,i are the experimental
and predicted moisture ratios, respectively.
3. RESULTS AND DISCUSSION
3.1 Drying behavior and drying kinetics model
All experimental results showed that the drying curve was in the falling drying rate period.
The effects of drying under different experimental conditions: drying air temperature, static bed
height to hydraulic diameter ratio and superficial air velocity ratio are shown in Table 1 along with
the calculated average specific drying rates and drying constants.
Drying with an air temperature range of 110-170oC at a Uo/Umf of 1.5 and an H/Dh of 0.3, the
drying time decreases with increasing air temperature. Moreover, higher air temperatures led to
higher values of the drying constant and hence higher specific drying rates. The temperature of the
seed increases with increasing air temperature, which results in an increase in the water vapor
pressure inside the seed and thus to higher mass transfer coefficients.
Increasing the superficial air velocity from 0.5 Umf to 2.5 Umf , at 150oC and an H/Dh of 0.3,
results in an initial increase and then a reduction in the specific drying rates and drying constants.
The peak specific drying rate was at a Uo of 1.5Umf. The drying time was a constant 10 minutes
except at Uo/Umf of 0.5 where the drying time was 20 minutes. These results are similar to previous
research on drying hazelnuts [21] and drying soybean meal [22]. At high velocities the reduction in
the drying rate results from heat loss to the walls of the fluidized bed as shown in Fig. 2(a). The
fluidization height (H*) increases with superficial air velocity, leading to lower air temperatures at
the top of the bed height.
Varying the static bed height to hydraulic diameter ratio, at 150oC and a Uo/Umf of 1.5, leads
to higher drying time and lower average specific drying rates and drying constants value. A possible
reason for this is the increase in fluidization height with static bed height. The temperature of
sunflower seed at the upper level in the bed has a temperature lower than 150oC. The results of
varying H/Dh whilst keeping other parameters constant are shown in Fig.2 (b). The results are in
agreement with previous research on olive pomace [23] and granular materials [24].
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e)
(a) T=150 oC, H/Dh=0.3, Umf=2 m/s
(b) Uo/Umf=1.5, T=150 oC
Fig. 2. Comparison average fluidization height and moisture content
of various H/Dh and Uo/Umf.
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Table1. The calculation values of SDR and drying kinetics under
different experimental conditions
Tempe Superfici
Static
rature
al air
bed
velocity height to
(oC)
ratio hydraulic
diameter
ratio
Total
Initial
Final moisture
moisture
content (%dry
drying
basis)
time (min)
content of
sample
(%dry basis)
(Specific drying
rate, (kgwater/ min
kgp)
Drying constant
(min-1)
110
0.3
95.99±1.13
4.58±0.15a
30
0.0305±0.0004a
0.211±0.020a
130
1.5
0.3
95.99±1.13
4.43±0.10a
24
0.0382±0.00049a
0.344±0.001bf
150
1.5
0.3
94.00±0.00
3.08±0.75a
10
0.091±0.00073f
0.434±0.021cd
170
1.5
0.3
87.29±7.47
4.88±0.09a
7
0.125±0.02302g
0.535±0.064e
150
1.5
0.3
94.00±0.00
3.08±0.75a
10
0.091±0.00073f
0.434±0.021cd
150
1.5
0.7
92.13±0.00
4.51±0.12a
12
0.073±0.00010cd
0.235±0.008a
150
1.5
1.0
95.89±0.00
3.46±1.60a
13
0.071±0.00130c
0.234±0.010a
150
1.5
1.3
87.33±1.89
4.32±1.73a
15
0.054±0.00083b
0.226±0.041a
150
0.5
0.3
83.73±0.00
2.98±2.05a
20
0.0403±0.0010a
0.220±0.005a
150
1.0
0.3
81.00±4.24
3.02±2.17a
10
0.078±0.0064cde
0.387±0.0045df
150
1.5
0.3
94.00±0.00
3.08±0.75a
10
0.091±0.00073f
0.434±0.021cd
150
2.0
0.3
94.22±0.00
4.22±0.25a
10
0.085±0.00034ef
0.369±0.008b
150
2.5
0.3
88.66±2.22
3.62±1.5a
10
0.071±0.0010def
0.350±0.0108bf
170
a-g
1.5
1.2
0.7
84.00±0.00
3.83±0.14a
12
0.0668±0.0001c
0.326±0.0057b
Means in the same column with different superscripts are significantly difference (p<0.05).
It is apparent that the drying constant depends on the air temperature, Uo/Umf and H/Dh. A
regression equation for estimating the drying constant as a function of these operating parameters can
be expressed as
3
2
H
H
H
k =[0.062530T −0.467131] −0.430620 +1.260717 −1.20132 +0.488506
Dh
Dh
Dh
3
2
Uo
Uo
0.270367 −1.37748 + 2.097390 Uo −0.534744
Umf
Umf
Umf
263
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e)
Fig. 3 shows the results for k as predicted by Eq. (6). The predicted k values are very close to
their experimental values. Curve fitting the regression equation produces optimal correlation
t
coefficients (R2) of 0.80, 0.816, and 0.60 as shown in Fig. 3(a), 3(b) and 3(c), respectively.
,
(c),
(a) Uo/Umf=1.5, H/Dh=0.3
(b) Uo/Umf=1.5, T=150 oC
(c) T=150 oC, H/Dh=0.3
Fig. 3. Comparison of prediction k values to experimental results.
omparison
Fig. 4 shows the model for the moisture ratio as calculated by Eq. (3) and Eq. (6) and compares
it with the experimental results.
From Fig. 4(a), the moisture ratio and drying time decrease rapidly with increasing air
temperature. The model is in close agreement with the experimental values (R2 = 0.839 to 0.981 and
RMSE = 0.0295 to 0.054).
From Fig. 4(b), the moisture ratio increases with increasing static bed height to hydraulic
diameter ratio. The fluidization height increases with increasing H/Dh as expected. The model is in
eight
.
close agreement with the experimental values (R2 = 0.963 to 0.999 and RMSE = 0.016 to 0.047).
(
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e)
(a) Uo/Umf=1.5, H/Dh=0.3
(b) Uo/Umf =1.5, T=150 oC
(c) T=150 oC, H/Dh=0.3
Fig. 4. Comparison of prediction moisture contents to experimental results
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Fig. 4(c) shows that increasing the gas velocity initially causes the specific drying rate to
increase but then further velocity increases cause a reduction in the specific drying rate. This results
in a peak specific drying rate at a velocity of Uo of 1.5Umf. The model is in close agreement with the
experimental values (R2 = 0.938 to 0.983 and RMSE = 0.020 to 0.040).
3.2 Effect of operating parameters on quality
Table 2 presents the results of the colors of dried kernels, average rupture force and
the
sensory acceptability of dried salt sunflower seed using statistical analysis software ANOVA at a
significance level of 5% (p < 0.05). It was found that the redness (a*) and yellowness (b*) of the
kernel color decreased with increasing temperature. But the lightness (L*) for various temperatures
remained constant even when the thin outer film of the kernel was cracked. The lightness decreased
with increasing static bed height to hydraulic diameter ratio because at a constant temperature
(150˚C) used the thin outer film did not crack.
The average rupture force, of horizontal orientation (Rh), decreases with increasing air
temperature. This rupture force for dried seed (p < 0.05) was not affected by the static bed height to
hydraulic diameter ratio. Nor was it affected by the superficial air velocity ratio. Gupta and Das
(2000) [25] found that sunflower seed at a moisture content of 4.21 %dry basis had rupture forces of
50.2 ± 1.0 N and 59.12 ± 1.20 N for horizontal and vertical orientations, respectively. These
experimental results, presented here, of the vertical orientation rupture force show lower values than
the results of Gupta and Das [25].
The consumer product evaluation preferences of sunflower seed are shown in Table 2. There
were 4 processing conditions used, the overall taste, color, flavor and crispiness. Significant
differences (p < 0.05) were observed for the preferences of these attributes. All conditions had mean
ratings of at least 6.0 (like slightly) for overall taste and crispness, except sunflower seed dried at 110
°C. It was clear that sample dried at 110 °C did not perform well with any respect. In this study,
increasing the drying temperature tentatively increased the score of the seed. Sunflower seed dried at
170 ° received the highest preference scores in all attributes.
Table 2. The Rupture Force, Kernal Color and Consumer Acceptance Test Scores of Dried
Sunflower Seed
a-d
Means in the same Column with Different Superscripts are Significantly Difference (p<0.05)
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4. CONCLUSIONS
Salted sunflower seeds were dried by a laboratory fluidized bed dryer. The operating
parameters were the drying air temperature, the static bed height and the superficial air velocity. The
samples had an initial moisture content of 90.65 ± 5.37 % dry basis and were dried until the moisture
content was 3.73±0.73 % dry basis. All experiments showed that the drying curve was in the falling
drying rate period. A model for the drying kinetics was developed and the drying constant of the
empirical model was determined. It is a function of the drying air temperature, the static bed height
to hydraulic diameter ratio and the superficial air velocity ratio. Comparison between the predicted
results and the experimental data gave good agreement. The quality of dried salted sunflower seed
was determined by the kernel color (L*, a* and b*), the rupture force and a consumer survey. It was
found that the rupture force, the redness and yellowness decreased with increasing temperature.
However, the drying temperature had no effect on the lightness. Moreover, the static bed height and
the superficial air velocity had no effect on the quality. From the consumer survey, it was found that
the sample dried at 170 °C was the most popular. Lastly, the appropriate drying conditions for drying
salted sunflower seed using a fluidized bed should be the following: An air temperature of 170oC,
static bed height of 0.3Dh and a superficial air velocity of 1.5Umf. This drying condition gave the
highest drying rate, shortest drying time and had high product quality.
ACKNOWLEDGEMENTS
The authors express their sincere appreciation to the Faculty of Engineering, Rajamangala
University of Technology Lanna Tak (Thailand) and the Graduate School and Faculty of
Engineering, Chiang Mai University (Thailand) for financially supporting this study.
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