The effect of acoustic and solar energy on drying ...

Energy Conversion and Management 67 (2013) 351–356

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The effect of acoustic and solar energy on drying process of pistachios Ahmad Kouchakzadeh ⇑ Department of Agricultural Machinery Engineering, Ilam University, Pazoohesh Ave., Ilam, Iran

a r t i c l e

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Article history: Received 6 November 2012 Received in revised form 9 December 2012 Accepted 11 December 2012 Available online 21 January 2013 Keywords: Solar-ultrasonic drying Thin layer Pistachios

a b s t r a c t A new approach of ultrasound-assisted sun drying was tested in this study using a flat bed as product support and two extensional piezoelectric Bolt-clamped 20 kHz transducer elements. The mono layer of moist unshelled pistachios was dried under open sun by applying 500 and 1000 W power ultrasound. The results showed that the Page model was found to be the most suitable for describing drying curve of pistachios. But, the Logarithmic model was described to satisfactorily drying curve of pistachios for open sun method when the ultrasound power was turned off. By applying the ultrasound about 17 W/kg moist pistachios in thin layer, the drying period could be reduced to 4 h. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Pistachio (pistacia vera l) is the most important agricultural crop cultivated in Iran’s tropic. Pistachio export earnings are the largest among non-petroleum Iranian export industries. Pistachio nuts have outer skin with the name of hull, which encase each nut. When the hull ripens the inside shell splits, it shows the nut is ready to be harvested. Harvest usually starts in early September and continues for 4–6 weeks. Iranian pistachios are mechanically shaken from the tree (in under a minute) or by hand at a low rate of speed and fall directly onto a catching frame. At the processing plant workers use machines to remove the hull and dry the nut in 12–24 h after harvest [1]. The pistachios moisture at harvesting time is about 40–50% (dry basis (d.b.)) according to date and climatic location. But, for storage and consumption pistachios need to dry 5–7%. So pistachios dryers are needed where pistachios in bulk expose hot air at temperatures 50–93 °C for 3–8 h. Two different commercial dryers such as flat plate continuous dryer (Fig. 1) and vertical cylindrical dryer (Fig. 2) were used in Iran to dry pistachio nuts. These dryers consume about ten million liters of mostly diesel fuel and natural gas firing each year [2]. The pistachio drying process may be conducted by using solar energy. Solar drying is a well-known food preservation technique to reduce the moisture contents of agricultural products. Pistachio products are dried either on paved ground under the sun or with a drying system. The natural sun drying method has some inherent disadvantages. Rate of drying pistachios in free air is slowly and needs 2 or 3 days period that can produce conditions in with Aflatoxin (fungal metabolites showing toxin) growth. The pistachio ⇑ Tel.: +98 9124532128; fax: +98 8412227015. E-mail address: [email protected] 0196-8904/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.enconman.2012.12.003

drying process is influenced by several factors of the crop, to the ambient conditions and to the harvest treatments [3]. Despite many disadvantages, solar drying of pistachios is experienced in many places in Iran. Solar energy is an important alternative source of energy and preferred to other energy sources because it is plentiful and free. Among emergent new technologies, ultrasonic dehydration is promising because the effects of power ultrasound are more important at low temperature [4]. When a high-intensity ultrasonic wave is directed coupled to the foodstuff, it moves through the solid medium and makes a rapid series of alternative compressions and expansions, in a similar way to a sponge when it is squeezed and released repeatedly (sponge effect). The forces involved by this mechanical mechanism can be higher than surface tension that maintains the moisture inside the capillaries of the material creating microscopic channels that may remove the moisture easier [5]. In the ultrasonic drying method, the air ventilation is helped to carry away the water vapor, driven from the interior of the food to its surface. High intensity ultrasound has been considered to enhance mass transfer for different products and processes such as drying apple and red bell pepper [6], orange peel during hot air drying [7] and in many food and vegetables applications, such as sterilizing, extracting, degassing, filtrating, drying and enhancing oxidation [8]. The combination of solar and acoustic energy for drying of foodstuffs has not yet been tried. The aim of the present work is to study the effect of power ultrasound wave in open sun heating with natural convection method on pistachios drying kinetics. 2. Method and material A pistachio processing plant in Qom province central Iran was selected for this study. In this region, pistachio-harvesting season

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Fig. 1. Continues flat bet pistachios dryer.

starts in late August and last through late September. The pistachio nuts were daily harvested to match the capacity of the post-harvest processing units. After dehulling, washing and separation the nuts were sent to drying. For drying the mono layer of moist unshelled pistachios were scattered in the horizontal square 2 2 m stainless steel flat bed under open sun. As illustrated in Fig. 3, the bed was equipped with load sensor (SM601, Sewha, Inc., Korea) and two bolt clamped 20 kHz ultrasound transducer. The load sensor was situated at the middle of the plate that contacts the ground surface with the basis and the ultrasound transducers were placed at opposite the corners. The driving transducer consists of an extensional piezoelectric Bolt-clamped Langevin type transducer element (SMBLTD50F20 HA) with aluminum body and dimensions: 109 50 40 mm, resonant frequency 20 ± 1 kHz and maximum output power 1000 W made by Steiner & Martins, Inc., USA. Bolt-clamped Langevin type transducers (BLT) are common vibration sources in high-power ultrasonic applications such as ultrasonic plastic welding, bio diesel mixer transducer, solid separation transducer and high-torque traveling wave ultrasonic motor. An ultrasound

generator, MSG.X00.IX.YF with maximum-pulsed power 3000 W and carrier frequency range 17.5–28.5 kHz made by Mastersonic, Inc., Switzerland was applied during experiments. The LED display shows the ultrasonic generator power level as a percentage of its maximum power. To measure the solar radiation flux that is incident on a plane surface in W/m2 from a 180° field of view, a pyranometer (LP02LI19 hukseflux, Netherlands) was used. The relative humidity and temperature of the surrounding air were checked with the aid of a Hand-held thermo-hygrometer (LUTRON, HT-3015, Taiwan) and wind speed was measured continuously by anemometer (LUTRON, AM-4201, Taiwan) at 1 and 2 m above the surface of the drying bed. The initial moisture content of samples were determined by oven drying at temperature of 130 °C for 6 h according to a standard method ASABE [9]. About 150 g of pistachios was placed in an oven, its final weight was taken, and the difference in weight was taken as water loss and expressed as grams water for each gram dry matter. Then mono layer of pistachios was placed on drying chamber of device dryer and variation of weight of pistachio with the resolution of 10 g recorded continually. The signals from load cell are recorded by a high speed data acquisition system (TML DC-204R) that is a compact recorder equipped with signal conditioners and sensor input conversion cable (CR-6180) that used for connecting transducers. 2.1. The tests About 30 kg of moist unshelled pistachios with the initial moisture content of 58.6% (db) were scattered in mono layer in device dryer under open sun, then the variation of weight of pistachio recorded and moisture content were determined for any time. Drying experiments were first carried out without ultrasound and then with 500 and 1000 W, which were 50% and 100% of maximum-pulsed power of ultrasound generator. Experiments were

Fig. 2. Vertical cylindrical pistachios dryer.

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Fig. 3. Schematic solar flat bed pistachios dryer test apparatus.

Fig. 4. Hourly solar radiation intensity and ambient air temperature (averages of 9 days).

Table 1 Some parameters during drying of pistachios.

Average ambient air temperature (°C) Average wind speed (m/s) Average relative humidity of air (RH) Initial mass of pistachios (kg) Final mass of pistachios (kg) Mass of evaporated water (kg) Time interval (s) Acoustic energy (W) Acoustic energy per unit mass of wet pistachios (W/kg) Global solar energy (W/m2) Solar energy in time interval (W/m2) Solar energy on drying bed in time interval (W) Total energy (W) Drying efficiency (%)

Day 1

Day 2

Day 3

Day 4

Day 5

Day 6

Day 7

Day 8

Day 9

26.5 1.29 30 30 25.21 4.79 14,400 0 0 8004 2668 10,672 10,672 8.6

25.7 1.39 30 30 22.53 7.47 14,400 500 16.7 7566 2522 10,088 10,588 13.5

26.2 1.18 29 30 12.81 17.19 14,400 1000 33.3 8128 2709 10,837 11,837 27.8

25.3 1.90 24 30 25.54 4.46 14,400 0 0 7907 2636 10,543 10,543 8.1

27.1 1.34 24 30 21.28 8.72 14,400 500 16.7 8029 2676 10,705 11,205 14.9

25.0 2.01 31 30 12.86 17.14 14,400 1000 33.3 7982 2660 10,640 11,640 28.2

23.3 1.75 28 30 24.97 5.03 14,400 0 0 8210 2737 10,947 10,947 8.8

25.9 2.06 25 30 21.02 8.98 14,400 500 16.7 8037 2679 10,716 11,016 15.6

23.0 1.85 26 30 12.74 17.26 14,400 1000 33.3 8131 2710 10,841 11,841 27.9

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Table 2 Mathematical models applied to drying curves.

1 2 3 4 5 6

Models

Name

MR = exp(kt) MR = exp(ktn) MR = a exp(kt) MR = a exp(kt) + c MR = a exp(k0t) + b exp(k1t) MR = 1 + at + bt2

Newton Page Henderson Pabis Logarithmic Two term Wang and Sing

performed in triplicate in 9 days. Drying without ultrasound was down in day 1 and repeated in day 4 and day 7, drying with 500 W ultrasound was performed in day 2 and down again in day 5 and day 8, separately for 1000 W in day 3, day 6 and day 9. Al tests were started at 10 am and continued until the weight of pistachios stayed constant (approximately during 2–4 h). Except drying without acoustic power that took longer time and more than 1 day was required to for completely drying, therefore because of comparison the end of tests were adjusted to 240 min.

The averages of hourly solar radiation intensity and ambient air temperature for 9 days experiments are shown in Fig. 4. Some local weather data during experiments were presented in Table 1. 2.2. Mathematical modeling The moisture ratio (MR) was calculated by dividing present moisture by its initial moisture ((M/M0) [10]. Drying curves were fitted with six moisture ratio models that were tried by several researchers such as Midilli and Kucuk [11], Kashaninejad et al. [19], Kouchakzadeh and Shafeei [12] and Kouchakzadeh and Haghighi [1] for drying kinetic of pistachios. These models were used to describe the thin layer drying of biological materials that are Newton, Page, Henderson Pabis, Logarithmic, two term and Wang and Sing models that are represented in Table 2. The acceptability of models was determined by the coefficient of determination R2, and the reduced value of mean square of deviation v2. The reduced chi-square can be calculated as [13]:

v2 ¼

Pn

MRpre;i Þ Nn

i¼1 ðMRexp;i

Fig. 5. Plot of moisture contents of pistachios vs. time.

Fig. 6. Plot of pistachios moisture ratio vs. time.

ð1Þ

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A. Kouchakzadeh / Energy Conversion and Management 67 (2013) 351–356 Table 3 Modeling of moisture ratio according to drying time in various ultrasound powers. Ultrasound power (W) Newton model

0 k R2

v

2

Page model

k n R2

Henderson Pabis model

a k R2

v2

v2 Logarithmic model

a k c R2

v2 Two term model

a k0 b k1 R2

Wang and Sing model

a b R2

v2

v2

500

1000

0.0894 0.9430 6.880 104

0.1213 0.9423 1.043 104

0.0691 0.9210 1.114 103

0.0587 0.8486 0.9790 2.740 104

0.0164 2.0260 0.9885 7.788 105

0.0383 1.4539 0.9890 1.639 105

0.9167 0.0734 0.9570 5.750 104

0.5949 0.1805 0.9132 3.892 104

0.9049 0.0510 0.9620 5.935 104

1.1422 0.1160 0.0126 0.9885 7.788 105

0.8503 0.9199 0.9118 0.9790 1.639 105

0.2825 0.3019 0.7238 0.9679 1.209 105

0.1090 0.1919 0.4130 0.1950 0.9667 4.590 105

0. 0698 0.8080 0.1987 0. 6296 0. 9671 6.569 104

0.8369 0.0368 0.0997 0.2203 0.9710 5.689 104

0.6460 0.0654 0.9860 5.432 105

0.3976 0.0086 0.9770 3.128 104

0.0927 0.0106 0.9780 4.835 104

Fig. 7. Effect of acoustic power on drying efficiency.

where MRexp,i is the experimental moisture ratio, MRpre,i is predicted moisture ratio, N is number of observation, and n is number of constants. Non-linear regression analyses were down by using statistical computer program. 2.3. Drying efficiency The drying efficiency was calculated as the ratio of heat energy used for evaporating water from the pistachios to the heat supplied by the device dryer. The cumulative drying efficiency values were calculated as the averaged energy consumption for water evaporation divided by the supplied power and time [14]:

mw kw nd ¼ 100 P Dton

ð2Þ

where nd is the drying efficiency in percentage; mw is the mass of evaporated water in kg; kw is latent heat of vaporization in J/kg; P is the average power in Watt and Dton is the time interval in s. Latent heat of vaporization at the evaporating temperature (100 °C) was taken as 2257 kJ/kg [15]. By multiplying the average of global solar radiation in 9 days experiments by surface area of dryer (2 2 m2) plus acoustic power, the value of P was determined.

3. Results and discussion The moisture of pistachios as a function of drying time is presented in Fig. 5. The experiential data were presented as a plot of log(M/M0) vs. time (min) as shown in Fig. 6. From slope of the curves, drying rate under open sun has a steady slope but after

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applying ultrasound the drying rates have more than one falling rate period. Kouchakzadeh and Shafeei show that when pistachios were exposed in microwave field, the drying rates have two stages [12], but each stage period is 15–20 times shorter than similar stages in combined acoustic-solar drying, this were different from that reported for rice, grain sorghum, apricot, potato drying and onion by Tsamo et al. [16]. This may be due to capillary property and cell structure of the pistachio nut as indicated by the rate of drying, which was not constant [17]. During the first period, the surface of product behaves as a surface of free water. The rate of moisture content removal from the surface is dependent on condition of places that drying is occurred, but in second stage the moisture migration from the inter layers of products to surface, this stage is dependent on the rate of diffusion of moisture from n the product to the surface and also moisture removal from the surface. Both the external factors and internal mechanism controlling the drying process in two main rate regimes are important in determining the drying rate of products [18]. According to highest R2 and lowest v2, as shown in Table 3 the best fitted model for solar drying (no acoustic power) during time interval is Logarithmic model and the best fitted models for combined solar and acoustic are Page model. The Logarithmic model was selected to represent under open sun drying behavior of pistachios. Similar finding was reported by Midilli and Kucuk for prediction of behavior of thin layer drying of pistachio by using solar energy [11]. But with applying power ultrasound in solar drying the model converted to page model. Kouchakzadeh and Shafeei showed that the Page model was adapted for microwave–convective drying of pistachios [12], Kashaninejad et al. showed that the Page model was most suitable for describing the drying behavior of the pistachio nuts by convective heating [19]. As shown in Table 1 the drying efficiencies raised when acoustic powers were applied to dryer. The averages of efficiencies in similar acoustic powers are illustrated in Fig. 7. 4. Conclusion In this study, accelerated drying of pistachios under open sun by applying power ultrasound was investigated. No constant falling rate period of drying was observed. The results showed that the Page model was found to be the most suitable for describing drying curve of pistachios. But, the Logarithmic model was described to satisfactorily drying curve of pistachios for open sun method when the ultrasound power is turned off. According to data presented in Table 1 the average drying efficiencies increase from 8.5% without ultrasound power to 14.7% and

28% with 500 W and 1000 W, respectively. Due this period, the moisture content of pistachios were reduced by 37.4%, 4.3% and 0.5% in 0, 500 and 1000 W ultrasound power, respectively. One of the inherent disadvantages of natural sun drying method of pistachios has slow rates that may produce conditions in with Aflatoxin growth. We estimate by applying the 20 kHz ultrasound waves about 17 Watt per kilograms moist pistachios in thin layer, the drying period could be reduced to 4 h. References [1] Kouchakzadeh A, Haghighi K. Modeling of vacuum-infrared drying of pistachios. Agric Eng Int: CIGR J 2011;13:1–6. [2] Kouchakzadeh A, Tavakoli T. The effect of moisture and temperature on thermophysical properties of Iranian pistachios. World Appl Sci J 2009;7:1552–8. [3] Kouchakzadeh A, Tavakoli T. New practical method for evaluation of a conventional flat plate continuous pistachio dryer. Energy Convers Manage 2011;52:2735–40. [4] García-Pérez JV, Cárcel JA, Benedito J, Mulet A. Power ultrasound mass transfer enhancement in food drying. Food Bioprod Process 2007;85:247–54. [5] Yao Y, Zhang W, He B. Investigation on the kinetic models for the regeneration of silica gel by hot air combined with power ultrasonic. Energy Convers Manage 2011;52:3319–26. [6] Schössler K, Jäger H, Knorr D. Effect of continuous and intermittent ultrasound on drying time and effective diffusivity during convective drying of apple and red bell pepper. J Food Eng 2012;108:103–10. [7] Ortuño C, Pérez-Munuera I, Puig A, Riera E, Garcia-Perez JV. Influence of power ultrasound application on mass transport and microstructure of orange peel during hot air drying. Phys Procedia 2010;3:153–9. [8] Leadley C, Williams A. Power ultrasound—current and potential applications for food processing, review no. 32, Campden and Chorleywood Food Research Association; 2002. [9] ASABE. Standard for measurement of moisture in grin and seed. Agricultural Engineers Yearbook of 2005. p. 564–5. [10] Tripathy PP, Kumar S. Mathematical modelling of drying of bay leaves. Energy Convers Manage 2005;49:2941–8. [11] Midilli A, Kucuk H. Mathematical modelling of thin layer drying of pistachio by using solar energy. Energy Convers Manage 2003;44:1111–22. [12] Kouchakzadeh A, Shafeei S. Modeling of microwave-convective drying of pistachios. Energy Convers Manage 2010;51:2012–5. [13] Mohamed LA, Ethmane Kane CS, Kouhila M, Jamali A, Mahrouz M, Kechaou N. Thin layer modelling of Gelidium sesquipedale solar drying process. Energy Convers Manage 2008;49:940–6. [14] Mousa N, Farid M. Microwave vacuum drying of banana slices. Drying Technol 2002:2055–66. [15] Kavak Akpinar E. Drying of mint leaves in a solar dryer and under open sun: modelling, performance analyses. Energy Convers Manage 2010;51:2407–18. [16] Tsamo CVP, Bilame AF, Ndjouenkeu R, Nono YJ. Study of material transfer during osmotic dehydration of onion slices and tomato fruits. LWT Food Sci Technol 2005;38:495–500. [17] Kouchakzadeh A. Moisture diffusivity of five major of Iranian Pistachios. Am J Food Technol 2011;6:253–9. [18] Ekechukwu OV. Review of solar energy drying systems I: an overview of drying principles and theory. Energy Convers Manage 1999;40:593–613. [19] Kashaninejad M, Mortazavi A, Safekordi A, Tabil LG. Thin-layer drying characteristics and modeling of pistachio nuts. J Food Eng 2007;78:98–108.