Study of the Interaction Between Water and Gelidium sesquipedale ( Rhodophyta ) : Part II : Kinetic of the Drying Process

The study of drying of red alga (Gelidium sesquipedale, Rhodophyta), with initial sample masses varying from 1 to 10 g, was carried at different temperatures 30 – 80 °C. The experimental drying curves show a slight effect of the initial mass. Two types of curves are obtained: those at temperatures below 50 °C are fitted into straight line according to an interfacial progression process in cylindrical symmetry. Those of high-temperature drying (50 < θ < 80 °C), are perfectly parabolic and show that the kinetic regime is controlled by the three-dimensional diffusion according to the Jander equation. The two types of water molecules identified were also highlighted during the drying kinetics process. The apparent activation energy of the strongly bounded water (20%) is about 36.25 kJ·mol, and that of the wetting water is in the range of 47.68 kJ·mol.


Introduction
Seaweed has attracted considerable attention as a potential source for several applications and is the subject of several studies (Corrigan, 1995;Miyagawa et al., 1995;Usov, 1998;Bannov et al., 1998;Ait Mohamed, 2005;Wang et al., 2009;Ye et al., 2010, etc.).Moroccan coasts are rich in red algae.Gelidium sesquipedale is the primary source for the production of agar, highly recommended product in the food industry.The industrial process for preparing the agar is always preceded by the step of storing the red algae which depends on ways and technology used in the drying phase.
In a previous studies (Hnini et al., 2013), we have presented the experimental results concerning the thermodynamic equilibrium of the sorption and desorption of water in red algae.The main aim was to understand the nature of the interaction between water molecules and the Gelidium sesquipedale.The sorption and desorption isotherms were performed using the static gravimetric method at temperatures 30, 45, and 60 °C and showed a slight hysteresis.In this study, the thermodynamic aspects has allowed to understand that 20% of water molecules are strongly linked to potential adsorption sites inside the plant cells and 80% of water molecules constitutes dampening water (Hnini et al., 2013).
To better control the behavior of the Gelidium sesquipedale with water, and to determine the optimal conditions for storage of dried seaweed, we study in this work the kinetic drying of Gelidium sesquipedale by conventional heating in order to understand:  the influences of temperature and initial sample mass on drying;  the mechanism that regulates the drying process and the kinetic parameters which can be used for modeling and optimization of the drying processes.
These results will be also compared in a further work, with those achieved under the same conditions of drying by the application of microwave technology.This comparison will allow us to better understand the interaction between water and the algae during the drying process.

Drying is drying pro desorbed m
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Studied
The studi preparation   ) and used for a)  The results obtained in this study can be compared with those obtained by Bakass et al. (1997) for the adsorption and the desorption of water vapor on a superabsorbent polymer, Aouad et al. (2006) during the drying of phosphates black and Aouad et al. (2002) during the decomposition of natural phosphate and its kerogen.

Experi
Figures 6 and 7 show respectively the variation of extent of conversion α and the instantaneous speed of drying versus time at 60 °C, for samples that have different initial masses (from 1 to 10 g).The analysis of these results show that for the initial masses used, the rate of desorption is continuously decreasing and the curves converge to the level α = 1 even faster than the initial mass of the sample decreases.

Conclusion
In this work we are interested to studying the drying kinetics of the Gelidium sesquipedale under different temperatures and for different initial masses.Two main objectives were set at the beginning:  The determination of the effects of temperature and initial mass of the sample,  The knowledge of the mechanism that can regulate the drying process and the kinetic parameters that can be used for modeling and optimizing the drying process.
From the analysis of all results, we can note:  For a given initial mass of the sample, the temperature has a significant influence on drying.Its effect is manifested by increasing the speed of drying when the temperature increases. For a fixed temperature, the initial mass of the sample has a slight influence on the rate of drying.In the area of initial mass studied the drying mechanism remains the same. The drying of samples occurs by mechanisms that depend on temperature: interfacial progression at low temperatures (range I) and diffusion at high temperatures (range II).During the drying process, these two mechanisms occur with the start of the water desorption, but at high temperature, they compete with mechanical phenomena that begin over early.
The drying kinetics is influenced by temperature but also the way in which heat transfer takes place within the algae.These results encourage us to undertake a further systematic study on the drying process under microwave irradiation.This technology is now playing an important role in energy optimization of industrial processes and drying at the quality of dried products.

Figure 6 .
Figure 6.Evolution of the rate progress of the drying process at 60 °C for samples that have different initial masses Figure 9