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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:

[email protected].@

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


2/27

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


3/27

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.


4/27

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


5/27

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


6/27

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


7/27

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)


8/27

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):


9/27

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.


10/27

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


11/27

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


12/27

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.


13/27

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


14/27

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


15/27


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


16/27

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.


17/27


-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


18/27

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)


19/27


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)


20/27

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.


21/27

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


22/27

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-.


23/27


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


24/27

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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