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Performanceanalysisofdryingofgreenoliveinatraydryer

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NeslihanColak

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ArifHepbasli

YasarUniversity

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Journal of Food Engineering 80 (2007) 1188–1193

Performance analysis of drying of green olive in a tray dryer

Neslihan Colak a,*,1, Arif Hepbasli b

a Solar Energy Institute, Ege University, 35100 Bornova, Izmir, Turkeyb Mechanical Engineering Department, Faculty of Engineering, Ege University, 35100 Bornova, Izmir, Turkey

Received 26 January 2006; received in revised form 27 August 2006; accepted 14 September 2006Available online 7 November 2006

Abstract

This paper deals with the performance evaluation of a single layer drying process of green olives in a tray dryer using exergy analysismethod. Green olive was used as the test material being dried. Drying process was realized at four different drying air temperatures (40,50, 60 and 70 �C) and a constant relative humidity of 15%. The effects of temperatures and mass flow rates were investigated. Maximumexergy efficiency of the drying chamber was obtained at a temperature of 70 �C and a drying air mass flow rate of 0.015 kg/s with0.0004 kg/s of olive. The exergy efficiency values were found to be in the range of 68.65%–91.79% from 40 �C to 70 �C with dryingair mass flow rates of 0.01 kg/s–0.015 kg/s.� 2006 Elsevier Ltd. All rights reserved.

Keywords: Drying; Exergy analysis; Green olive

1. Introduction

Most of the olive production is destined for olive oil;however, a considerable part of its processed to differenttypes of olives for direct human consumption. Amongthese products, fermented and black table olives are veryimportant products for South Europe, including Turkey.According to FAO (2003), 6,000,000 tons of table olivesare produced worldwide. Spain, Greece, Italy, Tunisiaand Turkey together, which are important olive oil produc-ers in the Mediterranean basin, have 97% of worldwideolive oil production (Lopez-Villalta, 1998). Amount ofolive production in Turkey is estimated to be1,800,000 tons (DIE, 2002).

Exergy analyses can reveal whether or not and by howmuch it is possible to design more efficient thermal systemsby reducing the sources of existing inefficiencies (Dincer &

0260-8774/$ - see front matter � 2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.jfoodeng.2006.09.017

* Corresponding author. Tel.: +90 232 388 4000x1918 17; fax: +90 232388 8562.

E-mail addresses: neslihan.colak@ege.edu.tr (N. Colak), arif.hepba-sli@ege.edu.tr (A. Hepbasli).

1 On leave: Food Engineering Department, Faculty of Engineering,Pamukkale University, 20020 Camlik, Denizli, Turkey.

Sahin, 2004). Exergy analysis has been applied successfullyto various areas of engineering applications (Szargut, Mor-ris, & Stewart, 1988).

During the past few decades, thermodynamic analyses,particularly exergy analyses, have appeared to be an essen-tial tool for system design, analyses and optimization ofthermal systems (Dincer & Sahin, 2004). For evaluatingthe performance of food systems, energy analysis methodhas been widely used, while the studies on exergy analysisare relatively few in numbers. The studies conducted onexergy analyses of food systems may be reviewed in twogroups as follows: (i) Exergy analyses of various food pro-cesses in general (i.e., Balkan, Colak, & Hepbasli, 2005;Bayrak, Midilli, & Nurveren, 2003; Tekin & Bayramoglu,1998; Topic, 1995) and (ii) exergy analyses of food drying(i.e., Akpinar, 2004, Akpinar, Midilli, & Bicer, 2005,2006; Dincer & Sahin, 2004; Midilli & Kucuk, 2003; Syah-rul, Hamdullahpur, & Dincer, 2002).

Topic (1995) presented a mathematical model for exergyanalyses of an industrial system for a high temperature for-age drying. Tekin and Bayramoglu (1998) studied aboutexergy loss minimization analyses of a sugar productionprocess from a sugar beet. Bayrak et al. (2003) performedenergy and exergy analyses of sugar production stages.

Nomenclature

C specific heat, kJ kg�1 K�1

_E rate of net energy transfer, kJ s�1

_Ex exergy rate, kJ s�1

h specific enthalpy, kJ kg�1

_I rate of irreversibility (exergy destroyed), kJ s�1

I _P rate of improvement potential, kJ s�1

_m mass flow rate, kg s�1

P pressure, kPa_Q heat transfer rate, kJ s�1

R gas constant, J kg�1 K�1

_S rate of entropy, kJ s�1 K�1

s specific entropy, kJ kg�1 K�1

T temperature, �C or Kt time, s_W work rate, kJ s�1

X weight fraction of dry matter, dimensionless

Greek symbolsg exergy efficiency, dimensionlessw flow exergy, kJ kg�1 sw specific humidity, g g�1

Subscripts

a airb boundaryc carbohydrateda drying airdest destroyed, destructione energyevap evaporationex exergetic, exergyf fatfi fibergen generationin inletk locationL lossm materialout outletp proteinv vaporw water0 restricted dead state

N. Colak, A. Hepbasli / Journal of Food Engineering 80 (2007) 1188–1193 1189

Balkan et al. (2005) conducted a study on the performanceevaluation of a triple effect evaporator with forward feedusing exergy analysis method.

Syahrul et al. (2002) carried out a thermodynamic anal-ysis of the fluidized bed drying process of moist particles tooptimize the input and output conditions using energy andexergy models. Midilli and Kucuk (2003) performed theenergy and exergy analyses of the drying process of shelledand unshelled pistachios using a solar drying cabinet. Din-cer and Sahin (2004) developed a new model for thermody-namic analyses, in terms of exergy, of a drying process.Akpinar (2004) studied on energy and exergy analyses ofdrying of red pepper slices in a convective type dryer. Akp-inar et al. (2005, 2006) performed energy and exergy anal-yses of potato and pumpkin drying processes via cyclonetype dryer.

This study performs an exergy analysis of thin layer dry-ing of green olive in a tray dryer. The dried green olive is anew product, which is proposed as snack food.

2. Materials and procedure

2.1. Experimental set-up

Drying experiments were performed in a laboratoryscale dryer constructed in the Department of AgriculturalMachinery, Faculty of Agriculture, Ege University, Izmir,Turkey (Gunhan, Demir, Hancioglu, & Hepbasli, 2005;Ongen, Sargın, Tetik, & Kose, 2005; Yagcioglu, Demir,& Gunhan, 2001). The dryer consists of mainly three sub-systems, namely (a) air supply unit, (b) drying unit with

heater and humidifier, and (c) data acquisition and elec-tronic control unit.

Temperature control, data acquisition and storage aswell as the general supervision of the unit, start-up andshut down electric heaters, injecting hot water into the airstream and circulating cold water through the coolingtower are done by the GENIE data acquisition software.

2.2. Experimental procedure

Measurements were performed to determine exergy effi-ciency of the system. Before starting experiments, the sys-tem was run for at least one hour to obtain steady-stateconditions.

Olive samples (Domat variety) were obtained locally.They were calibrated (140–180 particles/kg) and storedovernight at T = (10 ± 2) �C before processing. The proce-dure for preparing the product studied consists of treatingthe fruits with 2% NaOH solution, which hydrolyses thebitter glycoside oleuropein and increases the permeabilityof the olive skin, followed by water washes to remove theexcess alkali. Subsequently, a 7% (w/v) NaCl solution isadded to the fruits, in which they undergo spontaneous lac-tic acid fermentation (Fernandez-Diez et al., 1985). Whenthe fermentation was completed acidity and brine concen-tration were kept constant during storage. After the fer-mentation process, stones were removed. The amount ofolive to be dried is 4.48 kg.

After the dryer reached steady-state conditions, theolives were put on the tray of dryer and left to dry. Dryingexperiments were carried out at four temperatures (40, 50,

Table 1Total uncertainties of the measured parameters and experimental results

Description Unit Total uncertainty(%)

Temperature of drying air �C 1.59Temperature of product �C 1.59Boundary temperature of drying

chamber�C 1.59

Mass flow rate of air kg s�1 3.00Mass flow rate of product kg s�1 1.00Relative humidity of drying air % 0.10Water content of product % 1.00Enthalpy of drying air kJ kg�1 0.10Entropy of drying air kJ kg�1 K�1 0.10Specific heat of product kJ kg�1 K�1 0.10Entropy of product kJ kg�1 K�1 0.10

1190 N. Colak, A. Hepbasli / Journal of Food Engineering 80 (2007) 1188–1193

60 and 70 �C) in order to evaluate the effect of air temper-ature on the drying process. Drying air velocity was keptconstant at 1 m/s and relative humidity was maintainedat 15%. Initial temperature of olive is 18 �C and inlet watercontent is 76.29% (wet basis). Drying was continued untilthe mass of the samples reached a constant value. Duringthe experiments, ambient temperature and relative humid-ity, inlet and outlet temperatures of drying air in the dryerchamber were recorded.

Moisture content of olive fruit was determined byAOAC method at 70 �C and 400 mmHg in a vacuum oven.Oil content was analyzed by using Abencor system and cal-culated as mass fraction of oil in %. Protein (Nx6.25) wasdetermined as total nitrogen according to the Kjeldahlmethod with the addition of Kjeltabs ST as catalyst. Thesalt content of samples was analyzed by using the Mohrmethod (AOAC, 1975) (Ongen et al., 2005).

3. Analysis

3.1. Uncertainty analysis

Uncertainty analysis is needed to prove the accuracy ofthe experiments. An uncertainty analysis was performedusing the method described by Holman (1994). In the pres-ent study, the temperatures, pressures and flow rates weremeasured with appropriate instruments clarified before.The total uncertainties of these parameters calculated aregiven in Table 1.

Table 2The composition of olives used (Ongen et al., 2005)

Components Mass fraction (%)

Water 76.29Oil 14.67Protein 1.13Carbohydrate 3.32Fibre 4.09Ash 0.50

3.2. Exergy balance and exergy improvement potential

equations

The general exergy balance can be expressed in the rateform asX

_Exin �X

_Exout ¼X

_Exdest orX

1� T 0

T k

� �_Qk � _W þ

X_minw

�X

_moutw ¼ _Exdest ð1Þw ¼ ðh� h0Þ � T 0ðs� s0Þ ð2Þ

The exergy destroyed or the irreversibility may beexpressed as follows:

_I ¼ _Exdest ¼ T 0_Sgen ð3Þ

where _Sgen is the rate of entropy.Van Gool (1997) has also proposed that maximum

improvement in the exergy efficiency for a process or sys-tem is obviously achieved when the exergy loss or irrevers-ibility ð _Exin � _ExoutÞ is minimized. Consequently, hesuggested that it is useful to employ the concept of an exer-getic ‘’improvement potential’’when analyzing different pro-cesses or sectors of the economy. This improvementpotential in the rate form, denoted I _P, is given by (Ham-mond & Stapleton, 2001)

I _P ¼ ð1� gÞð _Exin � _ExoutÞ ð4Þ

3.3. Determination of thermal properties of olive

3.3.1. Determination of specific heats

In this study, the specific heat for foods was determinedusing the relations proposed by Choi and Okos (1986).

C ¼X

CiX i ð5Þ

with the specific heat of pure components given as (Rah-man, 1995).

C ¼ CwX w þ CpX p þ CfX f þ CcX c þ CfiX fi þ CashX ash ð6ÞThe composition of olive which used in the calculation ofspecific heat is presented in Table 2.

3.3.2. Determination of entropies

The specific entropy of olive at the inlet temperature(Tm1) is calculated as (Syahrul et al., 2002)

sm1 � sm0 ¼ Cm1 lnðT m1=T m0Þ ð7Þwhere Tm0 is the reference temperature, which is taken tobe 15 �C in this study.

3.4. Performing exergy analysis

Total exergy inflow, outflow and losses of the tray andthe drying chamber were estimated based on the exergyanalysis, which determines the exergy values at steady-statepoints and the reason of exergy variation for the process.

N. Colak, A. Hepbasli / Journal of Food Engineering 80 (2007) 1188–1193 1191

To evaluate the entropy of moist air, the contribution ofeach component in the mixture is determined at the mix-ture temperature and the partial pressure of the component(Syahrul et al., 2002):

sda ¼ sa � Ra lnP a

P 0

þ w sv � Rv lnP v

P 0

� �ð8Þ

Exergy balance equation for the tray dryer is,

_Exm2 � _Exm1 ¼ _Exda1 � _Exda2 þ _Exevap � _Exloss � _Exdest ð9ÞThe specific exergies at the inlet (wm1 ) of the material

and with a stream of drying air entering the dryer (wda1)are calculated as follows, respectively (Syahrul et al., 2002)

wm1 ¼ ðhm1 � hm0Þ � T 0ðsm1 � sm0Þ ð10Þwda1 ¼ ðh1 � h0Þ � T 0ðs1 � s0Þ ð11Þ

The heat transfer rate due to phase change ð _QevapÞ, therate of exergy transfer due to evaporation of the dryerðE _xevapÞ, the heat transfer rate to the environment ð _QlossÞ,and the rate of exergy loss to the surrounding (E _xlossÞ aredetermined as follows, respectively (Syahrul et al., 2002):

_Qevap ¼ _mw � hfg ð12Þ

_Exevap ¼ 1� T 0

T m2

� �_Qevap ð13Þ

_Qloss ¼ _Qevap � _m1ðhm2 � hm1Þ þ _mdaðhda1 � hda2Þ ð14Þ

_Exloss ¼ 1� T 0

T b

� �_Qloss ð15Þ

3.5. Exergy efficiencies of tray drying

The exergy efficiency of the dryer can be defined as theratio of the product exergy to exergy inflow for the cham-ber. Thus, the general form of exergy efficiency is written as(Akpinar, 2004):

gex ¼ 1�_Exloss

_Exin

ð16Þ

Fig. 1. Variation of exergy efficiencies at different mass flow rates ofdrying air (Mass flow rate of olives: 0.00015 kg s�1).

Fig. 2. Variation of exergy efficiencies at different mass flow rates of olive(Mass flow rate of air: 0.01 kg s�1).

4. Results and discussion

In this section, the effects of the drying air temperature,the mass flow rate of drying air and olives on the system per-formance are discussed. The polynomial relations for thevariations of specific heat and enthalpy of olives with tem-perature and composition were obtained with the help ofa regression program using the numerical values of specificheats and enthalpies at some temperatures from Mannape-ruma and Singh (1989). The entropy of olives at the inletwas calculated from Eq. (7), while that at the outlet wasmade in a similar manner. Exergy analyses of the tray dryerwere determined for 15% relative humidity, 40, 50, 60 and70 �C drying air temperatures and 1 m s�1 air velocity. Inaddition to these, a parametric study was undertaken to cal-culate exergy efficiencies for different drying air velocitiesand mass flow rates of olives.

The effect of hot air drying on the quality characteristicsof green olives was investigated by Ongen et al. (2005). Inthis study, it was reported that samples dried at 70 �C hadsignificantly different color as compared to other tempera-ture applications because of increasing in the degree ofbrowning. Ongen et al. (2005) also studied on the qualitycharacteristics of the oil after drying process. Based onthe composition, color, oil and sensory analyses, dryingof green olives at 50 �C gave the acceptable final product.

The exergy analyses of a single layer drying process ofgreen olives were performed by using data obtained fromthe experiments. The results obtained from these calcula-tions are presented in Figs. 1–4 and Table 3, while theyare discussed as follows:

Fig. 1 presents the variation of exergy efficiency as afunction of mass flow rate of drying air at temperaturesbetween 40 and 70 �C. From this figure, increasing massflow rate augments the exergy efficiency. Maximum exergyefficiency of 91.79% is obtained at a drying air of 70 �Cwith a mass flow rate of 0.015 kg/s. Minimum value ofexergy efficiency is 68.65%, while drying air temperatureis 40 �C and mass flow rate is 0.01 kg/s. By comparison,Akpinar (2004) reported the exergy efficiency valuesbetween 71 and 96.68% and 69.81and 97.12% at dryingtemperatures of 55 and 60 �C for drying of red pepper slicesin a convective type dryer, respectively. The exergy effi-ciency values between 30.81 and 100 and 46.97 and 100were also obtained by Akpinar et al. (2006) at drying tem-peratures of 60 and 70 �C for drying of pumpkin slices in acylone type dryer, respectively. It can be seen from Fig. 2,increasing the weight of olive influences the exergy effi-

0.05

0.10

0.15

0.20

40 50 60 70Temperature of drying air (oC)

Impr

ovem

ent p

oten

tial

rat

es (k

J s-1

)

Fig. 4. Variation of improvement potential rates at different temperaturesof drying air (Mass flow rate of drying air is 0.01 kg s�1).

Table 3Total uncertainties of the calculated parameters

Description Nominal value Unit Total uncertainty (%)

_Exloss 0.1795 kJ s�1 ± 1.59_ExD 0.1041 kJ s�1 ± 1.83_Exevap 0.3646 kJ s�1 ± 0.09ð _ExPÞin 0.0001 kJ s�1 ± 0.20ð _ExPÞout 0.0036 kJ s�1 ± 0.20ð _ExdaÞin 1.2611 kJ s�1 ± 0.60ð _ExdaÞout 1.3386 kJ s�1 ± 0.60gex 0.8577 ± 7.31I _P 0.1032 kJ s�1 ± 1.99

Fig. 3. Variation of exergy loss and efficiency at different boundarytemperatures of dryer (Drying air temperature is 70 �C and mass flow rateof air is 0.01 kg s�1).

1192 N. Colak, A. Hepbasli / Journal of Food Engineering 80 (2007) 1188–1193

ciency. Exergy used for drying the product increases withthe increase in the product mass. For this reason, exergyefficiency rises. Exergy efficiency would be increased if theamount of the product dried in the drying cabinet wereincreased. However, this increase in the efficiency wouldbe relatively lower than the efficiency obtained from theincrease in the drying air mass flow rate. Fig. 3 exhibitsthe variation of exergy losses and exergy efficiency of thedryer at different boundary temperatures of the dryer at70 �C drying air temperature. Exergy loss of the dryingchamber at different drying air temperatures is approxi-mately constant. If the boundary temperature of the dryingchamber increases, exergy loss increases. Nevertheless, thisaugmentation causes the falling of exergy efficiency. Fig. 4illustrates the variation of improvement potential at differ-ent drying air temperatures. The values of improvementpotential change from 0.142 to 0.103 kJ s�1, while the tem-

perature of drying air increases from 40 to 70 �C. Totaluncertainties associated with the calculated values are listedin Table 3.

5. Conclusions

This paper has presented an exergy analysis of dryingprocess of green olive. The experimental data obtainedfrom the measurements were utilized to conduct a systemperformance evaluation of energy and exergy efficienciesand its exergetic improvement potential. Exergy destruc-tions (representing the losses) in the system were alsoquantified.

The following main conclusions may be drawn from themain results of the present study:

(a) Minimum and maximum exergy efficiency values areobtained to be 68.65% and 91.79% at 40 �C and 70 �Cwith mass flow rates of 0.01 kg s�1 and 0.015 kg s�1,respectively.

(b) It is proposed that the boundary temperature of thedrying chamber should be decreased in order toobtain a lower exergy loss and higher exergyefficiency.

(c) Based on the quality analyses, the green olive dried ata temperature of 50 �C was found to be in an accept-able property. In terms of exergy efficiency, the dry-ing process realized at a temperature of 70 �C wasobtained to be the best. When the quality propertieswere investigated, it was seen that the drying resultsat 50 and 60 �C were not very much different fromeach one. This temperature difference of 10 �Cresulted in an increase of 13.66% in the exergyefficiency.

(d) The analysis should provide a designer with a better,quantitative grasp of the inefficiencies and their rela-tive magnitudes. Furthermore, the results can focusan engineer’s attention on components where thegreatest potential is destroyed and quantify the extentto which modification of one component affects,favorably or unfavorably, the performance of othercomponents of the system.

Acknowledgements

The authors thank Prof. Dr. Abdulkadir Yagcioglu andhis team from Department of Agricultural Machinery,Faculty of Agriculture, Ege University in Izmir, Turkey.In addition, the valuable comments of the reviewers aregratefully acknowledged.

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