Global Veterinaria 5 (3): 158-163, 2010 ISSN 1992-6197 © IDOSI Publications, 2010

Accelerated Drying of Alfalfa (Medicago sativa L.) by Microwave Dryer A. Farhang, 2A. Hosinpour, 3H. Darvishi, 4 M.H. Khoshtaghaza and 4T. Tavakolli Hashtjin 1

Department of Veterinary Medicine, Tehran University, Iran Department of Farm Machinery Mechanical Engineering, Ilam University, Ilam, Iran 3 Department of Mechanical Engineering, Islamic Azad University, Islamshahar Branch, Tehran, Tehran 4 Department of Farm Machinery Mechanical Engineering, Tarbiat Modares University, Iran 1

2

Abstract: In this paper, a laboratory microwave oven was used to dry the alfalfa, applying microwave power in the five levels of 180, 360, 540, 720 and 900W. The drying rate curve of alfalfa contained no constant rate period, but showed a falling rate period. Drying processes were completed between 4 and 9 min depending on the microwave power level. The fitting of experimental data with three models (Lewis, Henderson - Pabis and Wang - Singh) showed that drying curves were best described by the Wang - Singh model. Effective moisture diffusivity was estimated using the analytical solution of Fick's law. The effective moisture diffusivity was found to be in the range of 2.01×10 6m2/s to 4.344×10 6m2/s for alfalfa. In conclusion, showed that higher microwave power cause shorter drying time. Key words: Microwave drying

Effective diffusivity

Alfalfa

INTRODUCTION

Rotary dryers used to dry agricultural products like alfalfa. The hot air is introduced in the dryer where it contacts the products to be dried [2]. Major disadvantages of hot air drying of products are low energy efficiency and lengthy drying time during the falling rate period. Higher temperature and longer drying time in conventional drying may cause serious damage to the quality parameters of the product such as colour, nutritional value and taste [21]. Microwave drying is caused by water vapour pressure differences between interior and surface regions, which provide a driving force for moisture transfer. Microwave drying results in a high thermal efficiency, uniform heating, shorter drying time and improved product quality compared to conventional hot air drying [22-25]. It has also been suggested that microwave energy should be applied in the falling rate period for drying [25, 26]. Because of the concentrated energy of a microwave system, only 20-35% of the floor space is required, as compared to conventional heating and drying equipment [23, 25]. In microwave drying, operational cost is lower because energy is not consumed in heating the walls of the apparatus or the environment [27, 28].

Alfalfa (Medicago sativa. L) is one of the most important forage that is often called “Queen of forages” because it provides high levels of energy, protein and nutrients for livestock. Fresh alfalfa must be dried to moisture contents of 8~10% for safely stored and baled [1, 2]. Typically, alfalfa is dried in the field. The average time to reach a safe moisture level in field is form two to six days. But, in most climatic zones, weather variations result in variable moisture content at baling. Furthermore, the long drying time led to significant nutrient loss of the dried alfalfa. Drying at a faster rate reduces the risk of weather and rain damage, increases harvest yields and leaf retention, reduces damage to regrowth and extends the hay cutting season [3]. For these reasons, several researchers have studied the drying to better control the final moisture and quality of alfalfa [1, 4-20]. Most of the above studies examined on convective drying kinetics of alfalfa. But, limited study concerning microwave drying kinetics of alfalfa has been performed up to now [18]. Corresponding Author:

Drying time

H. Darvishi, Department of Mechanical Engineering, Islamic Azad University, Islamshahar Branch, Tehran, Tehran, Iran, Tel: +98 (21) 44194911-4, E-mail: [email protected].

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Global Veterinaria, 5 (3): 158-163, 2010

The aim of this study was to (i) describe the influence of microwave output power on drying kinetics and (ii) compare the measured findings obtained during the drying of alfalfa with the predicted values obtained with three semi-empirical equations for the purpose of simulation and scaling up of the process.

interfaced to a computer by a RS-232 cable and the weight loss and temperature of the layer alfalfa were recorded online every 15s throughout drying using software for the balance and multi-meter. Three replications of each experiment were performed according to a preset microwave output power and time schedule and the data given are an average of these results.

MATERIAL AND METHOD

Mathematical Modeling: The moisture ratio (MR) was calculated using the following equation:

Materials: The alfalfa was harvested from farm of alfalfa in Ilam, Iran, in September 2009. Alfalfa had an initial moisture content of 81.51% wet basis, which was determined by drying in a convective oven (Memmert, DO6836, Germany) at 103±1°C for 24h [1].

MR =

Mt − Me M0 − Me

(1)

Whereas, MR = Moisture ratio (dimensionless); Mt = Moisture content at t (kg water/ kg dry mater); Me = Equilibrium moisture content (kg water/ kg dry mater); M0 = Initial moisture content (kg water/ kg dry mater). The values of Me are relatively small compared with M t or M0 for long drying time. Thus, the moisture ratio can be simplified to Mt/M0 [1, 22]. Numerous mathematical models have been proposed to describe the drying characteristics of agricultural products. Drying curves were simulated using three empirical models of reduced moisture content: Lewis [29], Henderson - Pabis [30] and Wang - Singh [31] models.

Drying Equipment and Drying Method: Fig.1 shows the microwave drying system. The drying apparatus used consisted of a laboratory microwave oven (CH-3071W, LG Electronics Ins) with technical features of 230V, 50 Hz with a frequency of 2450 MHz. Drying trial was carried out at five different microwave generation power being 180, 360, 540, 720 and 900W. In the measurements of temperatures, K type iron-constant thermocouples were used with multimeter (ET-2076, Minipa, China). Samples were suspended beneath a digital balance (GF-600, A & D, Japan) into the microwave oven by using a mesh basket. The digital balance and multi-meter were

Fig. 1: Lab-scale microwave dryer with measurement instrument, (1) microwave oven, (2) digital balance, (3-4) RS-232 cable, (5-6) multimeter, (7-8) thermocouple, (9) computer 159

Global Veterinaria, 5 (3): 158-163, 2010

The Lewis model:

Mt MR = = exp ( −kt ) M0 The Henderson - Pabis model Mt MR = = a exp ( −kt ) M0 and the Wang - Singh model: M MR = t =1 + bt + at 2 M0 where, a, k and b = Drying constants in models.

DR =

(8) Where, DR = Drying rate (kg water/ kg dry mater min).

(3)

RESULTS AND DISCUSSION Drying Curves: Variation of moisture content versus drying time for alfalfa is shown in Fig. 2. As it can be observed in this figure, with increasing drying power microwave in the tested range, the amount of moisture removed from alfalfa increased and the time to achieve final moisture content in finished products was reduced. The moisture ratio versus drying time for the alfalfa at the selected powers is shown in Fig. 3. Difference between moisture ratios increased gradually as from starting of drying. The total drying times to reach the final moisture content for the fresh alfalfa sample were 9, 7.5, 6, 5.5 and 4 min at 180, 360, 540, 720 and 900 W, respectively. The drying time until the moisture ratio was up to 0.5 was 4.25, 3.85, 3, 2.6 and 1.9 min for the alfalfa at the output powers of the tested range, respectively.

(4)

Effective Moisture Diffusivity: Fick’s second law of the unsteady-state diffusion as in equation: (5)

∂M = Deff ∇ 2 M ∂t

The solution of Fick's second law in thin layer, with the assumptions of mass transfer being by diffusion and constant diffusion coefficient were as follows: = MR

8 2

∞



1

∑ (2n + 1)2 exp  −(2n + 1)2

2

n =1

Deff t   H2 

M t − M t + dt t

(2)

(6)

Modeling Drying Data: The constants of models for microwave drying of alfalfa in microwave powers are presented in table 1. The best model to describe the drying behaviour of alfalfa was selected on the basis of high R2 value. It is observed from Table 1 that the high values of coefficient of determination (R2) are indicative of good fitness of the empirical relationship to represent the variation in moisture ratio with time of alfalfa. Thus, the Wang - Singh model may be assumed to represent the thin layer drying of alfalfa hay in a microwave dryer. The validation of the Wang - Singh model at different microwave powers is shown in fig. 4. As can be seen, the dots in fig. 4 are closely banding around at a 45° straight line - a very good agreement between calculated and experimental data, which indicates that the Wang and Singh model could adequately describe the drying behavior of alfalfa.

Whereas, Deff = Effective diffusivity (m2/s); t = Drying time (s); H = Thickness of layer (m). When the mass transfer Fourier number is greater than 0.2, equation (6) can be simplified to equation in the form:  H2  8 Mt  ln( t= 2 ) (7) 2 M  Deff  0   The effective moisture diffusivity can be determined from the slope of the normalized plot of ln(MR) versus drying time. Drying Rate: Drying rate, DR, is expressed as the amount of the evaporated moisture over time. The drying rate of the alfalfa during drying process can be determined using the following equation: Table 1: Values of model constants and coefficient of determination

P(W)

MR = exp(-kt)

MR = aexp(-kt)

MR = 1+ bt + at2

---------------------------------

-----------------------------------------------------

----------------------------------------------------------

k

A

A

R2

k

R2

b

R2

180

0.195

0.911

1.142

0.225

0.940

0.0003

-0.1176

0.994

360

0.216

0.905

1.108

0.243

0.924

-0.0017

-0.1228

0.999

540

0.290

0.917

1.109

0.325

0.936

0.0006

-0.174

0.998

720

0.319

0.921

1.110

0.357

0.940

0.0019

-0.1949

0.997

900

0.461

0.914

1.103

0.509

0.930

0.0051

-0.2792

0.993

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Global Veterinaria, 5 (3): 158-163, 2010

5

180W 360W 540W 740W 900W

4 3

4.5

3.5

2.5

2 1 0

Deff =6E-06P 2 - 0.0028 P + 2.5678 R2 = 0.9482

1.5

0

1

2

90

180

270

630

720

810

3

1.6

180W 360W 540W 740W 900W

1.4 1.2

1

180W 360W 540W 740W 900W

0.8 0.6

900

Fig. 5: Effective diffusion coefficient variation with microwave power

4 5 6 7 8 9 Drying time(min) Fig. 2: Variation of moisture content with drying time for alfalfa under different drying powers

1 0.8 0.6 0.4

0.4

0.2

0.2

0 0

1

2

0 0

1

2

3

4 5 6 Drying time(min)

7

8

3

4

5

6

7

8

9

Drying time(min)

9

Fig. 6: Variation of drying rate with drying time for alfalfa under different drying powers

Fig. 3: Variation of moisture ratio with drying time for alfalfa under different drying powers

1.6 180W 360W 540W 720W 900W

1.4

1

1.2

0.8

1 0.8

0.6

180W 360W 540W 720W 900W

0.4 0.2 0

360 450 540 Microwave power(W)

0

0.2

0.4 0.6 0.8 Experimental moisture ratio

0.6 0.4 0.2 0 0

1

1

2

3

4

5

Moisture content(kg water/kg dry mater)

Fig. 7: Variation of drying rate with moisture content for alfalfa under different drying powers

Fig. 4: Experimental and predicted moisture ratio values at different microwave powers for the Wang and Singh model

within the general range of 10-9 to 10-11 m2/s for food materials. As expected, the values of Deff increased with the increase of output power. This result is similar to the results of drying apple pomace [22], mint leaves [32] and tomato pomace [33]. A second polygonal equation was fitted between effective diffusivity and microwave power as follows:

Effective Moisture Diffusivity: Variation of effective moisture diffusivity versus microwave power for the alfalfa is shown in fig. 5. The effective diffusivities of alfalfa of microwave drying at 180 to 900 W ranged from 2.01×10 6 to 4.34×10 6 m2/s. While, the values of Deff are 161

Global Veterinaria, 5 (3): 158-163, 2010

= Deff

R2 ( 2.5678 − 0.0028P + 6×10−7 P2 ) ×10−5 =

where, P = Microwave power (W).

0.9482

4. (9)

Drying Rate: The variation of drying rate with drying time and moisture content are shown in figs. 6 and 7. Drying rate decreases continuously with time and decreasing moisture content. In these curves, increasing the microwave power increases the drying rate and there was not constant-rate period but it seen to occur the falling-rate period. This shows that diffusion is dominant physical mechanism governing moisture movement in the alfalfa. These results are in good agreement as compared to the earlier studies of various vegetables [32, 34-39]. The high drying rate at high microwave power could be due to more heating energy which speeds up the movement of water molecules and results in higher moisture diffusivity. This result is in agreement with the earlier observations of Ozbek and Dadali [32], Ozkan et al. [37] and Wang et al. [22]. It was concluded that drying kinetics of alfalfa in a microwave dryer was studied at microwave powers of 180, 360, 540, 720 and 900W. Drying of alfalfa occurred in falling rate period. The results showed that alfalfa drying kinetics were best fitted by Wang - Singh model. The drying time decreased with increase in microwave power output. The effective diffusivity varied from 2.01×10 6 to 4.34×10 6 m2/s and increases as microwave power increases. Microwave drying can replace conventional drying because it greatly reduces the drying time while producing the same product quality as that of conventional drying. Experimental work has shown that the use of microwaves in drying improves colour and rehy dration capacity and reduces product shrinkage [25, 40, 41].

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8.

9.

10.

11.

12.

13.

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