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Experimental study of water desorption isotherms and thin-layer convective drying kinetics of bay laurel leaves

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Abstract

The aim of this work is to determine the desorption isotherms and the drying kinetics of bay laurel leaves (Laurus Nobilis L.). The desorption isotherms were performed at three temperature levels: 50, 60 and 70 °C and at water activity ranging from 0.057 to 0.88 using the statistic gravimetric method. Five sorption models were used to fit desorption experimental isotherm data. It was found that Kuhn model offers the best fitting of experimental moisture isotherms in the mentioned investigated ranges of temperature and water activity. The Net isosteric heat of water desorption was evaluated using The Clausius–Clapeyron equation and was then best correlated to equilibrium moisture content by the empirical Tsami’s equation. Thin layer convective drying curves of bay laurel leaves were obtained for temperatures of 45, 50, 60 and 70 °C, relative humidity of 5, 15, 30 and 45 % and air velocities of 1, 1.5 and 2 m/s. A non linear regression procedure of Levenberg–Marquardt was used to fit drying curves with five semi empirical mathematical models available in the literature, The R2 and χ2 were used to evaluate the goodness of fit of models to data. Based on the experimental drying curves the drying characteristic curve (DCC) has been established and fitted with a third degree polynomial function. It was found that the Midilli Kucuk model was the best semi-empirical model describing thin layer drying kinetics of bay laurel leaves. The bay laurel leaves effective moisture diffusivity and activation energy were also identified.

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Abbreviations

aw:

Water activity

Va:

Air velocity (m/s)

T:

Temperature (°K)

HR:

Air relative humidity (%)

me :

Water mass in the product (Kg)

ms :

Dry matter mass (Kg)

X:

Product moisture content at dry basis (Kg water/Kg dry matter)

X0 :

Initial moisture content (Kg water/Kg dry matter)

Xt :

Average moisture content at time t (Kg water/Kg dry matter)

Xe :

Equilibrium moisture content (Kg water/Kg dry matter)

XR:

Moisture ratio

XRi,exp :

ith experimental moisture ratio

XRi,pre :

ith predicted moisture ratio

\(\overline{\text{XR}}\) :

Average value of experimental moisture ratio

N:

Number of observations

Z:

Number of constants in the drying model

Qst,n :

Net isosteric heat of desorption (kJ/mol)

R:

Ideal gaz constant (J/mol/K)

P:

Slope of the desorption isosters

F (φ):

Dimensionless draying rate

t:

Time (s)

r:

Space coordinate (m)

Deff :

Effective moisture diffusivity (m2/s)

D0 :

Pre-exponential factor of Arrhenius equation (m2/s)

L:

Thickness of sample (m)

Ea:

Activation energy (kJ/mol)

a, b, c, k, k0, k1 :

Models constants

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Authors and Affiliations

  1. Applied Thermodynamics Unit Research, National Engineering School of Gabès, ENIG, University of Gabès, Gabès, Tunisia

    Thouraya Ghnimi & Mohamed Bagane

  2. LETTM Laboratory, Sciences Faculty of Tunis, University Tunis El Manar, Tunis, Tunisia

    Lamine Hassini

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  1. Thouraya Ghnimi
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  2. Lamine Hassini
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  3. Mohamed Bagane
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Correspondence to Thouraya Ghnimi.

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Ghnimi, T., Hassini, L. & Bagane, M. Experimental study of water desorption isotherms and thin-layer convective drying kinetics of bay laurel leaves. Heat Mass Transfer 52, 2649–2659 (2016). https://doi.org/10.1007/s00231-016-1770-0

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