Étude du séchage par contact de milieux granulaires agités. Application à l'opération de cuisson-séchage de la pulpe de manioc

Gévaudan Alain. 1989. Étude du séchage par contact de milieux granulaires agités. Application à l'opération de cuisson-séchage de la pulpe de manioc. Lyon : INSA de Lyon, 228 p. Thèse Dr 3ème Cycle : Transferts thermiques : INSA de Lyon

Encadrement : Dany, Griffon

Résumé : Après avoir mesuré les propriétés thermodynamiques (isothermes de désorption) physiques (masse volumique granulométrie) et thermiques (conductivité,chaleur massique) de la pulpe de manioc en fonction de la température et de l'humidité, nous avons déterminé expérimentalement les cinétiques de séchage de ce produit ainsi que celles d'un produit modèle (billes d'alumine) en fonction des variables opératoires : température de la plaque, humidité initiale et vitesse de rotation de l'agitateur. Par ailleurs nous avons étudié l'évolution du taux de gélatinisation de l'amidon en fonction de ces mêmes variables opératoires. Les transferts couplés de masse et de chaleur (cinétiques de séchage,température moyenne de la couche) ont été interprétés à l'aide du modèle de la pénétration de la chaleur à travers la couche. Ces résultats serviront à optimiser un procédé industriel de fabrication du gari.

Résumé (autre langue) : The work presented here is a contribution to the study of the physico-chemical phenomena encountered in the cooking-drying operation of cassava mash, which is the last stage of gari processing. After presenting briefly cassava root as raw material and then gari and its process, we describe shortly these phenomena, hat appear to be cassava starch gelatinization and contact drying of the mash in agitated bed (Chapter 1). Chapter 2 is devoted to an experimental study of cassava mash gelatinization using Differential Scanning Calorimetry (D.S.C.). This method allowed us to determine the evolution curves of the degree of gelatinization of cassava starch at different humidities and for temperature ranging from 60 to 130 °C, as well as the transition enthalpy of this endothermic reaction, which is about 15,8 kJ/kg (d.m.). A kinetic study pointed out that cassava starch gelatinization happens very quickly provided that the required hydrothermal conditions are met.The apparent activation energy as well as an empirical reaction order were calculated under the assumption that the phenomenon is governed by an ARRHENIUS type law. As the analysis of the contact drying requires the knowledge of the main intrinsic and thermodynamical properties, we determined experimentally the desorption data of cassava the mash in a temperature range from 40 to 90 °C. The desorption isotherms were correlated using the well-known G.A.B. model, which is well adapted to describe these curves. The comparison of the desorption data gained to those obtained with gelatinized products from cassava showed that gelatinization has only a slight influence on the desorption phenomenon. Finally, the desorption enthalpy was calculated using the isosteric curves (Ln (aw) = f (1/f) plots) and it was stated that the desorption heat can reasonably be neglected in the drying energetic balance. In chapter 4 are gathered some intrinsic properties of cassava mash; they are : physical properties : particle diameter, density, particle and bed porosity, thermophysical properties : specific heat as a fonction of temperature and effective thermal conductivity of cassava mash at two different humidities (0 and 52 % (w.b.)). The experimental values of the thermal conductivity were compared to those predicted by various theoretical models. The best results were given by ZEHNER and BAUER's 15 model improved by TSOTSAS and SCHLÜNDER in order to take into account vapor diffusion due to temperature gradient ("heat pipe effect"). This new model was therefore adopted to predict the effective thermal conductivity of the cassava mash in modelling the contact drying phenomenon. The fundamentals of the contact drying in agitated granular beds, based on heat penetration theory, are presented in chapter 5. A mathematical model for the contact drying in the presence of inert gas first proposed by TSOTSAS and SCHLÜNDER was adapted to describe the drying of hygroscopie products. The validity of this model was checked by comparing with the experimental results obtained through drying experiments of an highly hygroscopie product (activated alumina particles). To this purpose, an experimental pilot was designed to measure both the drying rates and the mean temperature of the bed. The drying runs were carried out at different plate temperature (from 70 to 130 °C) and for agitation speeds varying from O to 42 r.p.m. They revealed that the plate temperature has a strong influence on the drying rate, while on the contrary the effect of rotation speed is not significant. The evolution of the drying kinetics and the bed temperature curves can easily be interpreted within the scope of heat penetration theory; moreover, the mathematical model provides results very close to those obtained experimentally. The contact drying and the starch gelatinization phenomena involved in the cooking-drying operation of cassava mash were analyzed experimentally. The drying runs showed in general a significant influence of the main parameters (initial moisture content, plate temperature, rotation speed of the agitator) on the drying rate, while the final degree of gelatinization seems to be only affected by the plate temperature. During the operation the mash undergoes drastic modifications of its physical and rheological properties, as it passes from a paste-like product to a free-flowing granular one, the major changes occurring at a moisture content close to 0,6 kg/kg (d.b.). This modification of behaviour has a strong effect on the mixing conditions and consequently on the drying rate and the bed temperature. Drying rate curves as well as bed temperature evolution curves were predicted using the mathematical model depicted in chapter 5, which was adapted by introducing a supplementary heat transfer resistance; this resistance takes into account the conduction phenomenon in the stationnary layer of dry particles, located on the surface of the plate during the drying process. The predictions of this two parameters model are in good agreement with the experimental results in the moisture range where the product is free-flowing granular one.

Classification Agris : Q02 - Traitement et conservation des produits alimentaires
Q04 - Composition des produits alimentaires

Auteurs et affiliations

• Gévaudan Alain

Source : Cirad-Agritrop (https://agritrop.cirad.fr/585011/)

Métriques

[ Page générée et mise en cache le 2022-04-28 ]