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MODELING AND EXPERIMENTAL
STUDYON
DRYING
OF
APPLE SLICES IN
A
CONVECTIVE CYCLONE DRYER
E. KAVAK AKPINAR' and
Y .
BICER
Mechanical Engineering Depamnenr
Firat University
23279
Elazig. Turkey
AND
A. MIDILLI
Mechanical Engineering Department
University ofNigde
51100. Nigde Turkey
Accepted
for
Publication June
1 8 . 2 0 0 3
ABSTRACT
The main objective pursued in thispaper is to experimentally investigate the
single layer drying behavior
of
apple slices
in
a convective type cyclone dryer
and also to perform the mathematical modeling by using single layer drying
models in literature. The experimentswere conducted at drying
air
temperatures
of
60 70 and 80C n drying air velocities of and 1.5 mls. It was concluded
that apple slices with the thickness
of 12.5
mm would perfectly dry in the ranges
of
280-540 min while those with the thickness of 8 mm would dry in the ranges
of 180 320 min in these drying conditions by using convective type cyclone
dryer. Additionally the mathematical model describing the single layer drying
curves was determined by nonlinear regression analysis and the logarithmic
model was selected as the most suitable model to obtain the drying curve
equationof apple slices. Considering the parameters suchas drying time drying
rate moisture transfer velocity and drying air temperature it is suggested that
the apple slices be
dried
at the above optimum processing conditions.
I
Corresponding author. Dr. bru
Kavak Akpinar.
Mechanical Eng . Department, Firat U niversity,
23279,
Elazig, Turkey. TEL:
+90-424-237oooO/5343;
FAX:
+90424-2415526;
EMAIL:
eakpinar@tirat.edu.@
Journal
of
Food Process Engineering 26
(2003) 515-541.
All Rights Res ewed .
Copyright
2003
by
Food d r Nurrition
Press, Inc.. Trumbull. Connecticut.
515
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516
E. KAVAK AKF'INAR,
Y.
BICER and A . MIDILLI
JNTRODUCTION
Drying is defined as a process of moisture removal due to simultaneous heat
and mass transfer. Heat transfer from the surrounding environment evaporates
the surface moisture. The moisture can be either transported to the surface of
the product and then evaporated, or evaporated internally at a liquid vapor
interface and then transported as vapor to the surface (Gogiis 1994). It is also
one of the conservation methods of agricultural products, which is most often
used and is the most energy-intensive process in industry (Dincer 1996).
Moreover, drying is one of the oldest methods of food preservation and it is a
difficult food processing operation mainly because of undesirable changes in
quality of the dried product. Longer shelf-life, product diversity and substantial
volume reduction are the reasons for popularity of dried fruits and vegetables,
and this could be expanded further with improvements in product quality and
process applications. These improvements could increase the current degree of
acceptance of dehydrated foods in the market (Maskan 2001).
Cost-effective and hygienic ways
of
preserving foods is of great importance
given the prevailing insecurity in food supplies throughout the world. Drying of
vegetables and fruits all over the world is carried out by either sunlight or dryers
using solar collectors (Tiris
ef
al.
1994;
Ratti and Mujumdar
1997;
Midilli
2001a; Yaldiz and Ertekin 2001; Togrul and Pehlivan 2002; Midilli and Kucuk
2003). In Turkey, the apples are traditionally dried in the open air and exposed
to sunlight, which usually takes 8-10 days. This practice is a common method,
yet it
has
several drawbacks such as time consuming, prone to contamination
with dust, soil, sand particles and insects and being weather dependent (Oztekin
ef al.
1999). Therefore, using convective type dryers providing uniformity and
hygiene are inevitable for industrial food drying process (Uretir
1995;
Maskan
2001).
In the literature, the fruits were generally dried via tunnel type dryers.
However, there has not been found any recent information on the drying process
by using the cyclone type dryer. In the cyclone typedryer, the samples are dried
by the swirling flow of drying air instead of the axial flow of drying air. In the
system, radial entering
of
the drying air
from
the bottom part of the drying
chamber
performed
the swirling flow.
The study of drying behavior of different materials has been a subject of
interest for various investigators on both theoretical and application grounds
during the past 60 years. Many studies including drying processes have been
presented in the literature (Saravacos and Charm
1962;
Chiang and Petersen
1985; Uretir 1995; Dincer 1996; Midilli 2001a; Yaldiz
er al.
2001; Togrul and
Pehlivan
2002;
Doymaz and Pala
2002;
Midilli and Kucuk
2003).
Some
experimental studies on apple drying were reported in the literature. Uretir
(1995) conducted an experimental drying of apple samples with 0.6-1.8 mm
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APPLE SLICES DRYING STUDY
517
layer thickness in 1.7-3.0
h
at 78-94C y using a computer-controlled-tunnel-
type dryer. She modeled the drying process by using the constant and linearly
increasing temperature. Lewicki and Korczak (1996)obtained the values of
diffusion coefficient between 6.7
x
1 0 O and
2.7
x m2/s by drying the
apple samples with 1 cm cubic shaped in 0.6-2m / s at the ranges of 45-9OC.
Karathanos e?
al.
(1995)ound that the effective diffusivity varied from 4 o 21
x lo-'' m2/s for the apple in nature samples. Ramaswamy and Nsonzi
(1998)
observed the same magnitude for blueberries: to 2 X lo-'' m2/s. Many
researchers determined the diffusion coefficients between to lo-'' m2/s for
apple samples at the ranges of
30-76C
Chirife
1980).
Under these considerations, the
main
objectives of this study are to
investigate the single layer drying of apple slices in a convective type cyclone
dryer, and perform the mathematical modeling by using single layer drying
models in literature.
MATERIALS AND
METHODS
Experimental Set-up
Figure 1 shows a schematic diagram
of
the cyclone type dryer (Kavak
Akpinar 2002). It consists of a fan, resistance and heating control systems,
air-duct, drying chamber in cyclonetype, nd measurement instruments. The air
fan has a power of
0.04
kW. he airflow was adjusted through
a
variable speed
blower and manually operated an adjustable flap in the entrance. The heating
system consisted of an electric 4000W heater placed inside the duct. A rheostat,
adjusting the drying chamber temperature, was used to supply heating control.
The drying chamber was constructed from sheet iron in
600
mm diameter and
800 mm height cylinder. The inside and outside surfaces of the drying chamber
were painted with a spray dye to prevent rust in the sheet iron surface. The
drying chamber was constructed in a concentric form and 30 mm annulus was
isolated by polystyrene. Both topside and bottom side of drying chamber was
closed. Also, the covers made of the steel were isolated by polystyrene. This top
cover was used to load or unload the chamber. Drying air was tangentially
entered in the drying chamber. In this way, the samples were dried in swirl flow
in place of uniform flow. The samples were dried in two trays in distance of 150
mm. Trays were manufactured from nylon sieve. For the weight measurement
of the trays, the second tray was centrally drilled in 5 cm-diameter and its bar
was also connected to the balance. Thus, the weights of the first and second
trays were simultaneously measured. After the second tray was measured the
weight of the first tray was determined by using the bar of the first tray.
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518
E.
KAVAK
AKPINAR,
Y.
BICER and A. MIDILLI
In
temperature measurements,
J
type iron-constantan thermocouples with
the accuracy of fO.lC in
BS
4937 standard were used with a manually
controlled 20-channel automatic digital thermometer (ELIMKO 6400). An
EXTECH 444731 model humidity thenno hygrometer was used to measure
humidity levels at various locations of the system.
The velocity of air passing through the system was measured with
0-15
ds-capacity vane probe anemometer (LUTRON, AM-4201). In the velocity
measurements, the values of the velocity in the center of the drying chamber
were taken into account. The tangential
airflow
was across the layer during
drying process. M oisture loss
was
recorded at 20
min
intervals during drying
for determination of drying curves by a digital balance (BEL,
Mark
3100). The
measurement range was 0-3100
g
with an accuracy of
f O O 1
g. The effect of
airflow on the weight measurements was little. Therefore, this effect was
calibrated.
FIG. 1 . EXPERIMENTAL SET-UP
(1)
Drying chamber
(2)
1st
Tray
(3)
2nd
tray
(4)
Digital balance
(5)
Observed windows
(6)
Digital
thermometer
(7)
The balance bar (8)
Control
panel
(9)
Thermocouples (10) Digital thermometer and
channel selector (11) Rheostat (12) Resistance (13) Fan (14) Wet and dry thermometers
(15) Adjustable flap (16) Duct
Procedure
Apple slices were dried
as
single layer with the thickness of 12.5
mm
and
8 mm at temperatures of 60, 70 and 80C n the velocity of drying air of 1 and
1.5ds rying of apple slices started with an initial moisture content around
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APPLE
SLICES DRYING STUDY 519
apples
choosing
87%
(wb) and continued until no further changes in their mass were observed,
e.g. , to the final moisture content of about
13%
(wb), which was then taken
as
the equilibrium moisture content in the later com putations.
The fresh apples were used in the experiments. Before drying process, the
apples were peeled, cut into slices
of 12.5 x 12.5
X
25
mm and 8
x 8
X
18
mm (width
x
thickness
x
length) with a mechanical cutter. The trays were
loaded as single layer. Each 125 g apple slice sample was carefully and orderly
placed at
15
mm-distance between each slice on the nylon tray
so
that the
airflow could pass across the trays. The initial and final moisture con tents of
the
apple specimens were determined at
80C
by using a METTLER Infrared
Moisture Analyzer. After dryer is reached at steady state conditions for
operation temperatures, the samples are put
on
the trays and dried there.
Drying experiments were carried out at
60, 70,
and
8OC
drying air
temperatures and 1, 1.5 m/s drying air velocity. The velocities and temperatures
were measured in the center of the drying chamber. External air temperatures
changed between
21
and
23C
and relative humidity of ambient air changed
between
40% and 43%. Drying was continued until the final moisture content
of the samples reached to approximately 13% (wb). M oisture analyzer was only
used to measure the average moisture in the samples. During the experiments,
ambient temperature and relative humidity, inlet and outlet temperatures of
drying air in the duct and dryer chamber w ere recorded. In the calculations, the
dry basis values were used. F igure 2 shows the process flow diagram of drying,
pursued in this work for apple slices. The amounts of electricity energy were
measured by using standard type energy device.
cleaning . peeling
oFapples
of apples
c
+
apple
slices
preparations for
experiments
apple slices
apple s l i a s
FIG.
2. THE
FLOW
DIAGRAM
OF PE PA R E S PROCESS OF
APPLE SLICES
FOR EXPERIMENTS
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520
E.
KAVAK
AKPINAR,
Y.
BICER
and A . MIDILLI
ExperimentalUncertainty
Errors and uncertainties in the experiments
can
arise from instrument
selection, condition, calibration, environment, observation, and reading, and test
planning (Midilli
2001b).
In drying experiments of the apple slices, the
temperatures, velocity of drying air, weight losses were measured with
appropriate instruments. During the measurements of the parameters, the
uncertainties occurred were presented in Table
1.
TABLE
1.
UNCERTAINTIES
OF
TH E PARAMETERS DURING DRYING
OF
APPLE
SLICES
P.rrmau I
Unit
I Comment
Mathematical
Modeling and
Formulation
For mathematical modeling, the single layer drying equations in Table 2
were tested to select the best model for describing the drying curve equation of
apple slices during drying process by the convective cyclone type dryer. The
regression analysis was performed using Statistica computer program. The
correlation coefficient R) as primary criterion for selecting the best equation
to describe the drying curve equation (Guarte 1996). In addition to R, he
reduced -square as the mean square of the deviations between the experimental
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APPLE SLICES DRYING ST UDY
52 1
Modified
Page
m = q
(My
ModiIiedPage
MR =e q
-kt.)
and calculated values for the models was used to determ ine the goodness of the
fit. The lower the values of the reduced
x-square
the better the goodness of the
fit
(Yaldiz and Ertekin 2001). This can be calculated
as:
Whilee-tal. 1978
Ovemultsctal.
1973
The effects of some parameters related to the product or drying conditions
such
as
slice thickness, drying air temperature, relative humidity, etc., were
investigated by many researchers (Yaldiz and Ertekin 2001; Sarsavadia
er
al.
1999). Modeling the drying behavior of different agricultural products often
requires the statisticalmethodsof regression and correlation analysis. Linear and
nonlinear regression models are important tools to find the relationship between
different variables, especially, for which no established empirical relationship
exists. In this study , the relationships of the constants of the best suitable model
with the drying air velocity, temperature and sample area were also determined
by multiple regression technique using Arrhenius, exponential and power
regression models (Guarte 1996).
Two term
MR
=oUpf-k +bexp(-k,V
Hmdrrson
1974
Twn-tm e x p m t h d
MI7
=
cmrp(-
r + ( I - ) e q ( - k
a I)
S M - E l d e n
c
a1 1980
Wang
and Singh
M R = l + a f + b t '
Wang a d
in&
1978
Appoxima(iw
of
di f fusi an
Mlt = aup f - k v+ l -
4ap(-k
bt)
YaldizandErtekjn 2001
Verma et
al.
A07 =aexp(-kt)+(l-a)exp(-gr)
Verma
et
al. 1985
Moisture ratios of apple slices
MR)
uring the single layer drying
experiments were calculated by using the following equation (Midilli 2001a)
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522
E. KAVAK AKPINAR,
Y.
BICER
and
A .
MIDILLI
Wt- We
wi
-
we
R =
Drying rate of apple slices were calculated by using Eq. 3) (Kavak Akpinar
2002).
Wt+m
- wt
Drying
rate =
a3
(3)
From the drying data analysis, it was established that the air-drying of
apples consists of no constant rate period and
the
drying
mainly
took place under
the falling rate conditions. This behavior suggested strongly an internal mass
transfer type drying with moisture diffusion as the controlling phenomena.
Hence, experimental results
can
be interpreted by using Fick's diffusion model.
To
solve Eq.
(4)
the initial moisture concentration is assumed to be
uniform, and external gas phase
mass
transfer resistance is negligible, that is,
moisture movement is controlled by internal resistance, and outer surface
concentration isnot varying in time. Under these conditions, analytical solutions
of
Eq.
(4) for
an
infinite slab geometry are given in the literature (Crank
1975).
For an infinite slab,
For sufficiently long drying times, only
the
1st term of n = 1 in Eq. (5 ) can
be used with small error. The geometry of the apple samples used in experi-
ments
can be
considered as a
3-dimensional
h t e slab. The solution for the
finite slab is obtained applying Newman's rule (Treybal 1968):
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APPLE SLICES DRYING STUDY 523
Fo r each of the falling rate periods, Eq.
(6)
allows the calculations of the
diffusion coefficients from the slope of
the
straight
line
representing
In [l?t-We)/(lV-We)l vs time (Kaymak-Ertekin 2002).
RESULTS
AND
DISCUSSION
In the scope of this study, the following variations were discussed in detail.
(1) The variations of moisture ratio of the apple slices with drying time,
(2) The variations of drying rate of the app le slices with moisture content,
(3)
The variations of diffusion coefficient with the velocity and temperature of
drying air,
Additionally, single layer drying curve equation of apple slices was
determined by applying the single layer drying models in literature.
Figures
3-7
present the variations of moisture ratio with drying time at
drying air temperatures of 60,
0
and 80C and at drying air velocity of
1
and
1.5 m/s based on the layer thickness of apple slices. Moisture ratio of apple
samples was calculated using Eq. 2).
When all these figures were analyzed, the following important points were
obtained. Moreover, the results, and initial and last conditions were listed in
Table 3 for each experiment.
(1) Generally, the samples dried more slowly at
60C
by depending on drying
air velocities and the layer thickness,
(2) Considering the same velocities of drying air and the sam e sizes of the
samples, there has not appeared an important difference between drying
times in the first and second trays of the convective cyclone dryer. This
shows that the samples were homogeneously dried in the trays at constant
velocity of drying air.
3) Considering the different velocities of drying air, the samples with the
siz
of 8 x 8 x 18 mm dried faster than the others in the velocity o f drying a ir
of 1.5
d s .
This implied that the convective cyclone dryer operated more
efficiently during drying of small-sized-samples of apple slices.
(4) Because drying time was more important parameter in drying processes, the
samples should be dried at appropriate temperatures without decomposing
the organic structure of the samples. Therefore, during drying process by
using convective cyclone dryer, it can be said that apple slices at
80C
ould
be exactly dried in less time period.
Accordingly, it was emphasized that the size of the apple slices effected
particularly on drying time further than the mass
loss
of the samples.
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524
E. KAVAK AKPINAR, Y . BICER and A. MIDILLI
-~
v=1.5
,
4 8 x 8 ~ 1 8 - l s t h y , T = 8 0 C
0.9 A 8 x 8 ~ 1 8 I Qst tray, T=7OC
0 8 x 8 ~ 1 8 m, lsttray, T=6OC
0.8
1 2 . 5 ~ 1 2 . 5 ~ 2 5m, lsttray, T=8OC
5
0.7
0
1 2 . 5 ~ 1 2 . 5 ~ 2 5nm, 1st bay, T=70C
0.6
A 1 2 . 5 ~ 1 2 . 5 ~ 2 5m, lstbay, T=60C
3 0.5
L
5
0.4
0.3
0.2
0.1
0
- fR=aexp(-kt)+c
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0
Drylns t ime
(min)
FIG.
3.
VARIATION
OF
MOISTURE RATIO
WITH
DRYING TIME AT
1.5
ms-'
OF DRYING AIR
1
V=lm-
4
8 x 8 ~ 1 8 m, lsttmy, T = W
12.5x12.SxZS
nm~,
statray, T=8OC
0 1 2 . 5 ~ 1 2 . 5 ~ 2 5
sq lsttmy,
T=70C
A
1 2 . 5 ~ 1 2 . 5 ~ 2 5m dtray,
T=6M)
e-mmb-
0
5 0 1 0 0 1 5 0 m 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 5 0
Drymg time
(min)
FIG. . VARIATION OF MOISTURE RATIO WITH DRYING
TIME
AT
1
ms-'
OF
DRYING AIR
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APPLE SLICES DRYING ST UD Y 525
1
0.9
0.8
A V=lSm/q %x8x18mm,1st
tray
A V=l.Sm /q 8x8x18mm, 2ndtray
0 V=lm/q 12.5x12.5x25mm,
1st tray
0
V=lm/s 12 .5~12 .SxZSmm,Zndtray
V=lm/q 8x%x18mm,
1st
tray
0
V=Im/s,
8x8x18mm, 2nd
tray
R=a.exp(-kt)+c
$ 0.7
0.6
x
0.5
5
0.4
$ 0.3
0.2
0.1
0
0 5 0 1 0 0 1 5 0 M o 2 5 0 3 0 0 3 5 0 4 0 0 ~ 5 0 0 5 5 0
Dryingt ime(min)
FIG. 5 .
VARIATION OF MOISTURE RATIO WITH DRYING TIME AT 8OC
OF
DRYING AIR
1
0.9
0.8
2
OS7
g
0.5
&
0.6
tv
-5
0.4
0.3
0.2
0.1
0
0 V=I.Sm/q 12.5x12.5x25mm, 2nd ray
A
V=l.Srnls
x8xt8mm, 1st tray
A
V=l.Sm/s 8x8x18mm,
2ndtny
8
V=lm/s,
12.5xI2.5x25mm, 1st tray
0
V=Im/s, 12.5x12.5x25mm. 2nd
tray
V=lm/s, 8x8x18mm, 1st tray
0 V=lm s, 8x8x18m m,
2nd
tray
R=a.exp(-kt)+c
0 5 0 1 0 0 1 5 0 m 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0 5 6 0
Drying ime
(min)
FIG. 6 . VARIATION
OF
MOISTURE RATIO WITH DRYING TIM E AT
70C
OF
DRYING AIR
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526
E.
KAVAK
AKPINAR, Y. BICER and A.
MIDILLI
1
0.9
o V=l.Smlk 12.5x12.5x25mm, 2nd tr
0.8
A
V=l.SmIg 8x8x18mm, 1st
tray
A
V=l.Smis, 8x8xl8m m, 2ndtray
0
V=lm/q 12.5x12.5x25mm,
1st
tray
0 V=lm/s, 12.Sx12.5x25mm, 2ndtray
V=lm/q 8x8x18mm,
1st tray
0.6
Y
0
V=lm/q
8xSx l8m m, 2nd tray
2
.7
L MR=a.exp(-kt)+c
0.5
0.4
0.3
0.2
0.1
0
0
5 0 1 0 0 1 5 0 a o o 2 5 0 3 0 0 3 6 0 4 0 0 4 5 0 5 0 0 5 5 0
Drying ime (min)
FIG.
7.
VARIATION
OF MOISTURE RATIO
WITH
DRYING
TIME
AT
6OC OF DRYING AIR
Figures
8-12
show the variations of drying rate with moisture contentof the
samples in the first and second trays
at
drying air temperatures
of 60,
70 and
80C and at drying air velocities of 1 and 1.5 ds rom these figures, it was
noticed that,
(1) At the beginning
of
drying process, drying rate changed by depending
on
the sample
size
and the velocity
of
drying air, and then, decreased linearly
based
on
these parameters,
(2)
Drying rate went up with the increase of the temperature of drying air and
the highest values of drying rate was obtained during the experiments at
80C
of drying air,
3) At the constant temperatures of drying air, drying rate increased with the
rise of the velocity of drying air by depending on the size of the samples.
Namely, drying rate during drying of the small-sized-samples was higher
th n
that of the large-sized-samples by the rise of velocity of drying air.
However, drying rate in the first and second trays was almost equal to each
other.
(4) During the experiments of apple slices, the constant period of drying rate
did not take place and, all drying process were carried out in the falling
period of drying rate.
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APPLE SLICES DRYING STUD Y
527
I
4
v=1.sms-
A8~8~18naq
st m y ,
T=7W
12.5~12.5~25lmn,sttray, T= 8W
A
0
2
0 . q
0
1 2
3
4 5 6 7 8
W(g
watedg dry
matter)
FIG. 8 . VARIATION OF DRYING RATE
WITH
MOISTURE CONTENT AT
1.5 m -
OF
DRYING AIR
0.12
A 8 x 8 ~ 1 8 a n Ssttray, T=7W
0
8xSx18mq 1st tray, T=6OC
0.1
g 0.08
2
0.06
n
$
.-
E
g
i
a
-0
e
M
v
3 0.04
e
3
a
0.02
0
0
1 2 3 4 5 6 7 8
W(g water& dry matter)
FIG.
9.
VARIATION OF DRYING RATE WITH MOISTURE CONTENT AT 1 m. OF
DRYING AIR
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528
E. KAVAK AKPINAR,
Y.
BICER and A . MIDILLI
0 V=1.5m/s, 12.5x12.5x2Smm, 1st tray
o V = l.5 m /~ 2 .5x12.5x2Smm, Zndtray
A
V=l.5m/s, 8x8x18mm,
1st
tray
A
V=l.Sm/g 8x8x18.2ndIray
0 V=lm/g 12.5~12.5xZSmm, s ttray
o V=lm/g 12.5x12.Sx25mm, Zndtny
B V = ld g 8 x% x1 8mm,
1st tray
n
V=lm/q %x8x18mm,
nd
tray
4
~ = l m / s2.5x12. ix~5mrn, 1st tray
V=lm/% xSx18mm, 1st
tray I3
A
V=I.Sm/q 8x8x18m m, 1st tray
A
V =1 . 5 m/ 8 x 8 ~1 8
ndtray
o
V=lm/s 12.5x12.5x25mm, 2ndtr.y
V= lm /s 8x8x18mm, 2ndtray
A
A
A m
&I
L
A
A
0
0
8 0
6
8
0
1
2 3
4 5 6 7
8
W(g
watedg
dry
FIG. 0.
VARIATION
OF
DRYING RATE
WITH
MOISTURE CONTENT AT
80C
.
2
3,O.M
3.
E
00
0.02
0
0 l
2
3 4
5
6
7
8
W(g
-edg
(11y
FIG.
1 .
VARIATION OF DRYING
RATE WITH
MOISTURE CONTENT AT 70C
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APPLE SLICES DRYING STUDY
529
0.1
-
s
3 0.08
il
s
8
$?
0.06-
00
.
6
V=l.Srn/q 12.Sx1 2.S~25 mm , st t ray
o
V=i.Sm/S 12.Sx12.SxZSmm. 2ndh-aj
A V=l.Sm/s 8x8x18m m,
1st
t ray
A V=l.Sm/s, 8x8x18.2nd t ray
V=lm/g 12.5x12.5x25mrn.
1st
tray
o V=lm /q 12.Sx12.SxZSmm, 2o dt ra y
V=lm/q 8x8xl8mm, 1st tray
0 V=lm/q 8x8x 18mm , 2nd
tray
T-60
C
A
A
8
0
A
0
1
2
3 4
S
6 7 8
W(g water/gdry matta)
FIG.
12.
VARIATION OF DRYING RATE WITH MOISTU RE CO NT EN T AT 6OC
TABLE
3.
EXPERIM ENTAL DRYING CO NDITIONS AND THE RESULTS OF THE
DRYING PROCESS
Parameter
Drying medium
Auxil iary heater
Tray number
Sample weight (each
of
(ray)
Ambient
temperature
Material
sample size
Drying
air
temperature
Air
velocity
Drying
tim
Final weightof samples
Final moistureratio
Dilhrsiw d i C S
f
samks
Laboratory conditiaw
Cyclonetype drying cupboard
Electric
furnace
1.2
125
21-23
Apple
12.sx12.sx2s
BX8X18
Is
60,70,80
I
1.5
180-540
18.17-20.95
13
D . 8 4 1 ~ 1 0 ~ -
.060x109
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530
E. KAVAK AKPINAR, Y. BICER
and
A. MIDILLI
Because there was a relationship between drying air temperatures and
drying rate, the increase of drying rate resulted from the rise of temperature of
drying air during drying process of apple slices. Accordingly, it is said that the
higher temperature of drying air, the higher drying rate during drying process.
Effective moisture diffusivity was calculated by Eq. (6), using slopes
derived from the linear regression of In (MR) vs time data shown in Fig. 13-14.
It is noticed that the drying curves have a concave form when the curves of In
(MR)-time are analyzed. Researchers explained that the linear deviation fromthe
drying curves took place by the variation of the diffusion coefficient that was
assumed
as
constant in Fick Equation versus moisture content (Bruin and
Luyben 1980). Thus, it is said that the concave form of the drying curve
equation for the apple samples will be based on the variation of diffusion
coefficient with the amount of moisture.
Figures 15-18 present the effects of the velocity and temperature of drying
air on diffusion coefficients by depending on the sizes of apple slices. It was
observed from these figures that
(1) Diffusion coefficient went up with the increase of velocity and temperatures
of drying air and the sizes of the samples. In literature many researchers
detennined the diffusion Coefficients between to m2/s for apple
samples (Chirife 1980; Karathanosef
al.
1995; Lewicki and Korczak 1996).
However, in this study, it was noticed that diffusion coefficients changed
between 0.841 x lo- to 2.060 x lo m2/s.
(2) The diffusion coefficients that were estimated during drying of the samples
with the size of 12.5 x 12.5 x 25 mm were higher than those during
drying of the samples with 8 x
8
x 18 mm. This stemmed from the
moisture transfer from the sample surfaces and the structure of the samples.
As
a result, moisture diffusion can
go
up with the rise of the temperature
of drying air. Additionally, the influence of temperature of drying air was higher
than that of the velocity of drying air. Although some researchers
assumed
that
the effect of the air velocity would be neglected during the analysis of the data
from the thin layer drying, Islam and Flink (1982) explained that the resistance
of the external mass transfer was important in 2.5 d s r lower velocities than
this and should be considered in the analysis of drying data. One of the
assumption in derivation of Eg. (6) is that the resistance of drying air
to
the
moisture transport may be omitted. This requires that the diffusion coefficient
does not depend on the velocity of drying air. However, Mulet
ef
al.
(1987)
expressed that drying
air
velocity affected the diffusion coefficient and drying
rate at interval of a
certain
flow velocity, and was possible to determine the
value of threshold velocity during the constant-temperature-drying rocess of the
certain shaped material.
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APPLE SLICES DRYING STUDY
-6 -
-10 - v=1.5
115.1
8
X
8 x 8 ~ 1 8 m,
1st
tray,
T=60C
1 2 . 5 ~ 1 2 . 5 ~ 2 5
m,
1st tray, T=80C
1 2 . 5 ~ 1 2 . 5 ~ 2 5m, 1st tray, T=70C
5 3 1
0
FIG.
3.
VARIATION OF
In
(MR)
WITH
DRYlNG TIME AT
1.5
ms-'
OF DRYING AIR
-6
-
-10
-
1 2 . 5 ~ 1 2 . 5 ~ 2 5m, 1st tray, T=80
1 2 3 ~ 1 2 . 5 ~ 2 5
m,
1st
tray,
T=60
0
FIG. 14.
VARIATION OF In (MR)
WITH
DRYING
TIME
AT
1 ms l
OF DRYING
AIR
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532
2.5
z.a
h
7
1.5
E
z
g
L.0
0.5
0.0
+
st
bay, T=70
C
+ st tray, T=80
-b
nd tray, T=60
2nd tray,T=70
2nd tray, T=80C
E. KAVAK AKPINAR, Y. BICER and A . MIDILLI
FIG. 1 5. INFLUENCE OF THE DRYING
IR
VELOCITY ON THE DIFFUSION
COEFFICIENT (12.5
x
12.5
x
25
mm)
1.6
1.4
1.2
-
1
mm
8
ss
0.8
-
0.6
0.4
0.2
0
8
--t
sttray,T=60
C
+ l s t h y , T = 7 O C
+llrttray,T=SOC
+Zndtray,
T=60 C
-B-
2 d
ray,
T=70 C
2nd trav. T=80
1.1 1.2 1.3 1.4 1.5 1.6
.9 1
Dryins
air velacity
(Ins-')
FIG.
16.
INFLUENCE OF THE DRYING AIR VELOCITY ON THE DIFFUSION
COEFFICIENT
(8
x 8 x
18
mm)
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APPLE SLICES DRYING STUDY
1.6
YJ
1.4
-
1.2
.
7
1 -
E
2
m- 0.8
-
0.6
-
0.4
-
0.2
-
533
T
8x8x18mm
-D-
lsttray,
v
=
1 mls
+Zndtray, V=1.5ds
2.5
2
-
s r n 1.5
E
2 1
0.5
0
--t
sttray,v = 1.5ds
+
sttray,
v
= 1
d s
+Mtray, V=l.Sm/s
FIG.
17.
INFLUENCE OF THE DRYING AIR TEMPERATURE ON THE DIFFUSION
COEFFICIENT (12.5
X
12.5 x 25 mm)
1
50
60
70
80
90
Drylas air
ec>
FIG.
18. INFLUENCE OF THE DRYING AIR TEMPERATURE ON THE DIFFUSION
COEFFICIENT (8 x 8 x 18 mm)
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534
E.
KAVAK
AKPINAR,
Y.
BICER
and
A. MIDILLI
For mathematical modeling, the moisture content data at the different drying
air temperatures, velocities and sample
area
were converted to the moisture ratio
then fitted against the drying time. The best model describing the single layer
drying characteristics of apple slices was chosen as the one with the highest
R
and the lowest values (Guarte 1996; Yaldiz and Ertekin 2001; Yaldiz
e?
al.
2001; Togrul and Pehlivan 2002; Midilli and
Kucuk
2003). Generally R-values
were changed between 0.89508-0.96634.R and
2
alues obtained by using the
two term, the Approximation of diffusion, the Verma
e?
al. (1985) and the
logarithmic model are too close to each other. But, the R-value of the logarith-
mic model is slightly higher than the values obtained by the two term, the
Approximation of diffusion, the Venna
ff
al.
(1985). Moreover, the
x
value of
the logarithmic model is slightly lower than the others. Therefore,
the
logarithmic model was selected to represent the single layer drying behavior of
apple according to the highest value of R and the lowest value of
x
Table
4).
TABLE 4.
MODELING
OF
MOISTURE RATIO ACCORDING TO
THE
DRYING TIME
I
I
R
r
add
Ncwton (+0.012161)
Page k=O.015031 14.953641)
Modified Page
bO.012257 ~0 .9 53 63 5)
ModifiedPa5 k=O. I10276 n=0. 110276)
Hendason and pabis (a4.992968 kO.012072)
Logarithmic(Fo.981022 M.012921 c=O.021704)
Tw-term (~0.057650,,=0.002647 M.947538 k,=0.013469)
Two-lennurpOnmtial(~O.019458M.612236)
Wang a d Sing (&.006878 b=O.ooOo11)
Approximation
ofditfusion
(Fo.950561La.013275 b=O. 174445)
VCIlM d
al.
(~0.049515 -o.002322 -0.013275)
0. 588
0. 581
0.96561
0.96634
0.96633
0. 9
0.89508
0.96633
5.74xIO-
5 . 7 8 ~
O
5 . 7 8 ~0-
5 . 6 8 ~ 1 0 ~ ~
5.69s
10
1.70~10~
5.69~10.
5.69~10.
5.77x10
To account for the effect of the drying Sariables on the logarithmic model
constants a,
k
and c were regressed against those of drying air temperature,
velocity and sample area using multiple regression analysis. All possible
combinations
of
the different drying variables were tested and included in the
regression analysis. The multiple combinations of the different parameters that
gave the highest R and the lowest x values were finally included in the final
model. Based
on
the multiple regression analysis, the accepted model constants
and coefficients were presented in Table
5 .
When
he effect of the drying air
temperature, velocity and sample area
on
the constants and coefficients of the
logarithmic model drying model was examined, the resulting model gave an R
of
0.9987,
nd 2 of
2.19
x I @ for 1st tray and an R of 0.9986, and
2
f
2.30
x
l for 2nd tray.
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5/20/2018 Trabajo Grupo 01
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T
A
5
.
T
H
A
D
M
O
T
H
L
O
T
H
M
I
C
M
O
C
A
A
C
C
E
N
M
od
M
=
a
.
e
x
p
k
.
0
t
I
t
r
a
2
t
r
a
a
=
0
4
T
V
A
4
0
5
8
2
0
8
a
=
5
T
5
V
A
-
0
0
8
k
0
0
~
.
A
o
5
7
5
9
.
e
p
0
0
%
7
T
0
9
7
8
k
0
0
V
2
A
5
0
T
0
9
c
=
4
-
~
9
6
.
V
4
5
2
5
2
.
.e
p
4
7
T
0
7
c
=
-
0
7
7
3
.
A
'0
.
e
p
1
4
T
0
7
C
a
s
a
d
c
e
n
s
R
C
a
s
a
d
w
k
n
t
s
R
0
r
A
2m
uw
u
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536
E.
KAVAK
AKPINAR,
Y.
BICER
and A .
MIDILLI
Validation of the established model was evaluated by comparing the
computed moisture ratio in any particular drying conditionswith the observed
moisture ratio. The performauce of the model at the different drying air
velocities, drying air temperatures and sample
areas
was illustrated in Fig. 19.
The predicted data generally banded around the straight line, which showed the
suitability of the logarithmic model in describing drying behavior
of
apples.
CONCLUSIONS
The following results may be drawn from
the
present work in which drying
mechanisms of apple slices have been studied.
The apple slices can be effectively dried using this system in shorter time
required to dry them
to
the 13 (wb) moisture levels on the open sheets.
Samples in dimension 12.5
x
12.5 x
25
mm and 8
x
8 x 18
mm
perfectly
dried at different air temperatures and velocities in the time period 280-540
min
and 180-320min, respectively.
In
order to explain the drying behavior
of
apples, eleven single layer-drying
models were compared according to their coefficients of determination and
reduced chi-square values. According to the results, the logarithmic model could
adequately describe the single layer drying behavior of apple samples. When the
effect of the drying air temperature, velocity and sample area on the constant
and coefficients of the logarithmic model were examined, the resulting model
gave an
R
of 0.9987, nd
x
of 2.19 x
104
for the 1st tray and an
R
of 0.9986,
and x of 2.30 x
lo4
for the 2nd tray. Accordingly, it can be said that the
logarithmic drying model adequately described the drying behavior of apple
slices at a temperature range of 60-8OC and a velocity range of 1 - 1.5
m / s
of
drying air.
The moisture transfer from the apple slices occurring during the falling rate
period of driving was characterized by determining experimentally the diffusion
coefficient into the air. It was seen that the diffusion coefficients are agreeable
with literature values.
Considering the parameters such
as
drying time, drying rate, moisture
transfer, velocity and drying air temperature, it is suggested that the apple slices
could be dried at temperatures of
8OC
in drying air velocity of 1.5 ms-.
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APPLE SLICES DRYING STUDY
537
FIG.
19.
1
0 9
0 8
3 0 6
3
07
'H
o 5
:;
0 2
0 1
0
0 01 02 03 0 4 05 06
7
0 8 0 9 1
bpsrmentdvfhc3
I
0.9
0 8
0 7
3 0.6
j 0.5
.P 0.4
2 0.3
0.2
0.1
0
I
0.9
0.8
f
0.7
1 0.6
j
0 5
.p 0.4
0.3
0.2
0.1
0
1
0.9
0.8
0.7
0.6
0.5
0
4
0.3
02
0 1
0
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
0 . 9 .
1
~ m m n l d
ahw
1
- ,
0 0 1 0 2 03 0 4 0 5 0 6 0 7 0.8
0.9
1
Exprrimntd
values
. ....
o 8 O C Z n d h a y
a
7 0 C Z n d t r a y
0
600 C 2 n d l r i ys l tray /
70
C. I d
ray
J
0 1
0 2 03
0 4
0 5
0 6
0.7 0 8 0 9 1
Eipuimntal values
COMPARISON OF EXPERIMENTAL AN D PREDICTED MOISTURE RATIO
THE LOGARITHMIC MODEL
BY
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538
E.
KAVAK
AKPINAR,
Y.BICER and A. MIDILLI
NOMENCLATURE
a,b ,c,g ,n empirical constants in the drying models
A sam ple area (m')
D diffusion coefficient m2-')
k, k,,,
k, empirical coefficients in the drying models (min-')
thickness (mm)
number constants
number of observations
moisture ratio
experimental moisture ratio
predicted moisture ratio
diffusion path (m)
regression coefficient
time s)
temperature ( C)
velocity d s )
moisture content (g water g-'
dry
matter), (dry basis)
moisture content in equilibrium state
dry
basis)
moisture content at
r
=
0
(dry
basis)
moisture content at r dry basis)
moisture content at t+d? (dry basis)
average moisture content at ? (dry basis)
chi-square
ACKNOWLEDGMENT
Authors wish to thank the Firat U niversity Research Foundation
(FUNAF)
financial support, under project number 357.
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APPLE SLICES
DRYING
STUDY
54
1
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