Air Drying Kinetics of Biological Particles
Drying kinetics of several biological products (carrots, potatoes, corn, plums, coconut, parsley, cassava, ...) have been measured in warm air of various characteristics (temperature was also determined in certain cases).
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1. INTRODUCTION Reasons for measuring drying kinetics may be - fundamental : give experimental support to modelization of transfer phenomena during drying - practical : provide a starting point to calculation of industrial drying processes, specially by simulation (e.g. Daudin and Bimbenet -4-).
ABSTRACT Drying kinetics of several biological products (carrots, potatoes, corn, plums, coconut, parsley, cassava, ••• )have been measured in warm air of various characteristics (temperature was also determined in certain cases).
This paper considers the case of biological particles dryed in warm air.
The obtained curves (drying rate vs product water content), as we 11 as the evo 1uti on of the product temperature, showed no constant-rate period in most cases. Certain products exhibited a very marked initial temperature warming-up period, associated with an increasing drying rate. The classical interpretation of the falling-rate period in two distinct parts was not confirmed for most products.
2. METHODS The basic idea is to perform a drying test in "pure conditions", i.e. all the product being submitted to the same conditions. Many types of equipment and pN>cedure are used. Our own methods are described in the above-mentioned publication (4). They consist in drying a thin layer of product lying on a tray, weighing it at regular intervals during interruptions of the air flow. Automatic treatment of the data furnishes curves of the type shown further. The temperature of the product may be measured by means of an insulated thermocouple inserted in the center of a piece of product located in the warm air flow.
Discussion of the factors of these kinetics - air temperature always has a strong influence which may often be interpreted by an Arrhenius law - air humidity has very different influence according to the type of product : negligible in certain cases, important in others, but always more marked during the beginning of the drying -air velocity influence may be described in comparable terms - when possible, the influence of product size was measured and appeared to be strong -but most of all, the nature of the product was the main factor : one can speak of a "personality" of each product as to its drying behavior.
3. SHAPE OF DRYING RATE CURVES 3.1. Do we find a warming-up period ? Certain curves given further (e.g. fig. 8, 10, 12) exhibit a very marked warming-up period. It must however be observed that this period generally concerns a short part of the drying process. In other cases, no such period is clearly found.
In certain cases only the shape of the drying curve could be interpreted by a ~iffusion law with a constant diffusivity.
The existence and the importance of this period is related to : - product size (the bigger the product, the longer that period). Comparison of fig. 1 and 2 is clear at that respect
It is therefore concluded that no theory can presently predict drying kinetics of biological products. Experimentation remains essential in this field.
R. Toei et al. (eds.), Drying ’85 © Springer-Verla
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