Energy-saving and alternative technologies for thermal vacuum drying of disperse materials :: SPE RESST

Energy-saving and alternative technologies for thermal vacuum drying of disperse materials

Drying of a moist material is the process including the moisture transfer from the internal layers of a material to its surface and the moisture removal from the surface into the ambient medium. Thermal and physical processes of material mass-and-energy transfer are the most essential physical fundamentals of disperse material drying technologies. To increase the reliability and the efficiency of drying units, studied was the thermal power of the processes used in the drying units for production of high-quality dried materials.

Investigations were carried out by the examples, currently important for the national economy of Ukraine, including the vacuum drying of grain, wood, large-size ceramics and zirconium hydroxide Zr(OH)₄ for the purpose of obtaining from the latter zirconium dioxide ZrO₂. The investigations have demonstrated that for every kind of a product it is necessary to develop specific drying conditions and equipment (in particular, heating elements of drying units) providing a high-quality final product.

Drying of grain

For drying of grain two modes were used. Fodder grain is dried at a temperature to 70°C, and seed grain – at a temperature not higher then 50°C, the duration of drying being increased.

Experiments have shown that the relation between the moisture diffusion coefficient of the wheat grain, its moisture content and temperature can be written in the following form

,
where T is the temperature in the vacuum chamber; u is the relative moisture (mass fraction of Н₂O). For several materials – grain, ceramics, Zr(OH)₄ – the coefficients A, B, C, k were determined.

A maximum allowable temperature of grain heating is determined by the thermal stability of its protein content and depends on the initial properties of gluten. A grain is quickly heated to the maximum limiting temperature but slowly gives away the moisture. For grain drying it is necessary to apply an oscillating drying mode when after heating the grain is subjected to the intermediate cooling, and then the drying cycle is repeated. This drying mode holds nutritive materials in the grain and its growth power remains (Figure 1). As the drying temperature exceeds the limiting one (see Figure 1(b)), grain cells lose their viability and the grain may be utilized only as a fodder grain.


а)	б)

Figure 1 – Microstructure of the grain dried at T = 50°С (а) and T = 70°С (b).

Desiccation of wood

The process of desiccation of wood (as well as, the drying of large-size refractory products) requires another drying mode differing from the grain heating mode.

Under conditions of intensive heating the above-mentioned materials are inclined to cracking and therefore they should be first heated to 30°C and then held at this temperature during 4...5 hours in order to avoid an intense evaporation from the material surface and to exclude a high pressure gradient occurrence within the body. By heating the wood and refractory products the temperature should be slowly increased at a certain interval. The maximum temperature at a final stage of drying reaches 80°С for wood and 50°С for refractory products.

In Figure 2 presented is the oak fragment before (a) and after (b) drying.


а)	b)

Figure 2 – Structure of the oak before (a) and after (b) drying in the vacuum chamber.