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The practice of drying is one of the oldest techniques used in various fields of industry, while many studies have dealt to the exploration of different designs and types of dryers. Low temperature drying of thin and fixed film products is important in the food industry. Moist agricultural products require short drying with ventilation (Mercier et al. 2014, Paul et al. 2008). While other wetter products can be dried for long periods with conditions close to the ambient environment, they are very economical in terms of energy (Jekayinfa, & Bamgboye, 2006, Nawshad & Michael, 2013, Ehsan & Nikbakhsh, 2018, Isdore, Agnes, & Benedict, 2018, Luciandra et al. 2018).
Furthermore, the drying of agro-food products is done in sophisticated dryers, ensuring a rational implementation of the operation (Amer, Hossain & Gottschalk, 2010), from the most used industrial dryers are convective tunnel dryers.
Most of these dryers operate in forced convective mode where the main thermal energy generator can be conventional (gas or fuel burner), solar or combined. These dryers are equipped with fans for air circulation and a regulation system to achieve the desired performance (Bala et al. 2003, Salah & Ali, 1994, Hossain & Bala, 2007). Food dehydration is an energy intensive process in which energy cost constitutes a major portion of drying cost. Optimal operation of dryer is one of the feasible methods for energy saving. Tunnel dryer is widely used for dehydrating fruits and vegetables specially in Mediterranean countries. The operation of hybrid solar dryers is ensured by two energy sources, solar and conventional, which allows them a continuous operating mode whatever the weather conditions (Amer, Hossain & Gottschalk, 2010, Vidaña, Lagunas & Ramírez, 2013, Mohajer et al. 2013, Hatem, Salah & Abdelkader, 2014).
Before any sizing, identification or simulation application, the modelling of the drying processes seems essential. In the literature (Dinghua, Meibao & Haili, 2013, Salah, Elaid, & Hatem, 2012, Khatchatourian, Vielmo, & Bortolaia, 2013, Naghavi, Moheb & Ziaei-rad, 2010) several models are presented which models the different drying techniques and phenomena. The complexity of the drying phenomenon requires the choice of a well-determined modelling technique. In this work we will use the models developed in previous work (Hatem, Salah & Abdelkader, 2014) based on the bond graph methodology. This modelling tool determine the dynamic mathematical model, most often based on ODE equations (Karnopp, Margolis & Rosenberg, 1990, Hatem, Salah, and Abdelkader, 2014, Pichardo &. Delgado, 2003).
To obtain the desired product quality, it would be interesting to optimize industrial drying processes. The area of control of drying process has not been sufficiently studied due to the fact that the dynamic drying models developed tend to be complex because they include systems of coupled and nonlinear equations. The drying rate and the phenomena of heat and mass transfer are well described by these equations.
Likewise, the measurement of the moisture content of agricultural products during drying in industrial or semi-industrial dryers is often difficult for several reasons of instrumentation and location of appropriate sensors (Fionn et al.,2014).
To remedy the problems encountered in the dryers to optimize and improve the performance of the drying processes, different control techniques developed can be applied.