Module 3. Food dehydration

Lesson 31


31.1 Introduction

The design of industrial dryers is based largely on empirical knowledge, while modeling and simulation can generalize and improve the design procedure. The diversity of drying processes and dried products has resulted in a multitude of drying equipment, specific for each class of products. Thus, selection of a particular dryer becomes as important as the engineering design of the equipment. In recent approach to the design of convective dryers, the thermo physical, transport, and equilibrium (isotherms) properties of the material are very important in specifying the proper dryer and drying conditions. Mechanical properties of solids and solid particles are important in handling and processing of the various materials. Drying rates (kinetics of moisture removal) are useful for preliminary estimation of the drying time. The estimated time is taken as the approximate mean residence time in the dryer, an important parameter in dryer design. The specifications of an industrial dryer should be listed in an equipment specification form, and should include the properties of the wet and dried material, the temperature sensitivity and water activity of the product, the capacity and evaporation duty (kg/h), the energy supply and cost, the environmental impact, and the relationship to the other plant operations. There are about 40 classes and over 100 subclasses of dryers, which are classified by different methods, based on type of operation (batch, continuous), type of feed (liquid, suspension, paste, granules, fibrous solids, porous solids, dense solids, and sheets), heating method (convection, contact, radiation, dielectric), product sensitivity (vacuum, low temperature). The size of dryers can be small (up to 50 kg/h), medium (50-1000 kg/h), or large (above 1000 kg/h). The capacity of the dryers can be expressed also as kilograms of water evaporated per unit surface and unit time, which is very high in rotary dryers (about 50 kg/m2h) and low in tray dryers (about 1 kg/m2h). The cost of drying is an important factor in dryer design, especially for large volume products of relatively low value. Energy (fuel) is the major cost in drying operations (62%), followed by capital and labor costs.

The major energy use is for the evaporation of water (moisture), which varies considerably for the different dryers, e.g., from 3 MJ/kg water (spray dryers) to 6 MJ/kg (tray dryers). The energy efficiency of the dryers (ratio of the heat of evaporation to the heat input to the dryer) depends strongly on the type of dryer. It is higher in contact than in convective drying, e.g., (40-80%) versus (20-40%). Rotary dryers are more efficient than tray, fluid bed, and spray dryers. Four categories of industrial dryer problems have been identified by Kemp and Gardiner: a) Under performing (corrected by performing material and energy balances, by drying kinetics, and by applying moisture equilibria); b) materials handling; c) product quality; and d) mechanical breakdown.

Last modified: Thursday, 27 September 2012, 7:16 AM