On the Study of the Drying Process of Bulk Materials

I. P. Chashchin, V. A. Lotov, N. T. Shalavin

(Presented by the scientific seminar of the Department of General Chemical Technology)

It is known that, in the process of drying granulated disperse materials, the change in their moisture content and strength depends on the method of drying [1]. At present, in chemical technology, such drying methods as convective, contact, radiation, and drying in a fluidized bed have become widespread.

Convective and contact methods of drying have been studied sufficiently well by A. V. Lykov [2], whereas the radiation method, which is the most effective, still remains insufficiently studied.

In this connection, we carried out studies of the drying process of a granulated bulk material by the radiation method. To determine the optimal drying regime for granules consisting of a definite ratio of finely ground bauxite, technical alumina, and white electrocorundum sludges, a lamp radiation dryer from the “Vlagomer-20” instrument was used. The power of the thermal radiation, and consequently the temperature, were regulated by a laboratory autotransformer.

Fig. 1. Characteristics of drying regimes for different drying methods: 1 — convective method; 2 — radiation method; 3 — radiation-convective method

The granules had a cylindrical shape, 4–6 mm long and 2.5 mm in diameter. Drying was carried out in a monolayer on a special tray, which had heat-protective insulation, at temperatures of 100–250°C with an interval of 20°C to a final moisture content of the granules of less than 0.5%. Under the same conditions, the radiation-convective drying method was studied with air blowing at a rate of 0.5 m/sec.

As a result of the investigations carried out, dependences were established for the change in the moisture content of the granules over time at various temperatures.

drying. For convenience in selecting the optimal drying regime, the experimental data are presented in the coordinates drying time—temperature (Fig. 1). It is evident from the figure that, in comparison with the convective method of drying (curve 1), the radiation method requires much less time for drying a granulated material having an initial moisture content of 20–22%. Curves 2 and 3 characterize the drying regime for the radiation method and the convective-radiation method (i.e., with air blown over the layer of material), respectively.

It is evident from these curves that the use of air blowing shortens the drying time, other conditions being equal, and that increasing the temperature above the layer of the material being dried reduces the drying time. However, increasing the temperature gives an effect only up to a certain limit (200–220°C). A further increase in temperature leads to an increase in the drying time. This phenomenon can be explained by using Lykov’s law of heat-and-moisture transfer [3], which is written as

\[ g_m = a_m \gamma_0 \frac{\partial u}{\partial x} + \delta a_m \gamma_0 \frac{\partial t}{\partial x} + a_p \gamma_0 \frac{\partial P}{\partial x}, \tag{1} \]

where

\[ \begin{aligned} g_m &\text{— amount of evaporated liquid;}\\ a_m &\text{— coefficient of potential conductivity, } m^2/\text{h;}\\ \gamma_0 &\text{— specific weight of the absolutely dry material;}\\ \delta &\text{— thermogradient coefficient, } 1/\text{deg;}\\ \frac{\partial u}{\partial x},\quad \frac{\partial t}{\partial x},\quad \frac{\partial P}{\partial x} &\text{— partial derivatives proportional to the gradients} \end{aligned} \]

of moisture content, temperature, and pressure.

The third term of the equation characterizes the driving force of the moisture-transfer process at temperatures below 100°C and a pressure of 1 ata. At a temperature above 100°C it is equal to zero, and the driving force of the moisture-transfer process under these conditions is the second term of the equation.

However, in capillary-porous bodies, a typical representative of which is the material under study, at low residual moisture contents the thermogradient coefficient becomes negative, since in this case moisture, owing to the relative thermal diffusion of vapor and air, moves in the opposite direction—from cold zones to hot ones; this explains the reason for the increase in drying time at temperatures above 200°C.

Thus, on the basis of the investigations carried out, it may be concluded that, for drying a granulated material of the above-mentioned composition, it is advisable to use the radiation-convective method of drying in a monolayer under the following regime: drying temperature—180°C, drying time—3–5 min.

REFERENCES

1. A. S. Bogma; V. M. Vityugin, V. A. Lotov. Proceedings of the First Scientific Conference of the Tomsk Branch of the All-Union Chemical Society named after D. I. Mendeleev. Tomsk State University Press, 1969.

2. A. V. Lykov. Heat and Mass Transfer in Drying Processes. Gosenergoizdat, Moscow, 1956.

3. A. V. Lykov. Theory of Drying. Energiya Press, Moscow, 1968.