DRYING OF MOISTURE-ABSORBING GRANULES IN A BED

A. S. Bogma, V. M. Vityugin, V. A. Prokhorovich

(Presented by the Scientific-Methodological Seminar of the Faculty of Chemical Technology)

One of the ways of intensifying the production of silicon carbide is the agglomeration of quartz sands and fine-grained carbonaceous material by the granulation method [1]. However, raw granules with a moisture content of 4–5% have insufficiently high mechanical strength (40–50 g/granule), which leads to the danger of their destruction during transportation, mixing, and charging into the core furnace.

On the other hand, there is a danger of softening of moist granules when they are heated in the furnace, with the associated disruption of the uniform gas permeability of the charge bed.

For these reasons, the drying regime of granules requires special attention in developing the general technology of granulation, especially for moisture-absorbing materials.

To determine the optimal regime for drying granules under laboratory and semi-industrial conditions, studies were carried out, respectively, on laboratory and semi-industrial drying installations.

The experimental laboratory installation consisted of a metal drying chamber (diameter 100 mm, height 400 mm), in the lower part of which there is a mixing chamber and in the upper part a vacuum chamber. As the heat carrier, the products of gas combustion were used, diluted with cold air to the temperature required for the experiment. Removal of gases and evaporated moisture from the dryer was carried out by an exhaust fan. The flow rate of the heat carrier was determined by means of an orifice plate and an inclined liquid manometer.

The granules under study were poured into a metal cup with a perforated bottom and with a longitudinal opening in the side surface for installing thermometers and for visual evaluation of the state of the granule bed after drying. The cup was tightly installed in the drying chamber, along the height of which, in turn, openings for installing thermometers are provided every 10 mm, which in practice makes it possible to measure the temperature at any point along the height of the granule bed.

In each experiment, the temperature was measured in the mixing chamber, in the middle of the granule bed, and in the space above the granules. After the experiment, the granules were poured onto a metal tray, and samples for moisture content and mechanical strength were taken from the upper, middle, and lower layers.

The design of the dryer provides for the possibility of interchanging the positions of the mixing chamber and the vacuum chamber, which makes it possible to carry out the process of drying granules both with suction and with blowing of the drying agent through their layer. The dust entrained by the gases was captured by means of a cyclone.

First of all, it was necessary to determine the effect of the temperature of the drying agent on the drying rate, caking, and mechanical strength of the granules. In the first series of experiments the height of the granule layer was 100 mm; suction and blowing of the heat carrier were carried out at a constant superficial velocity of \(1\ \mathrm{m^3/m^2\,sec}\) in the temperature range from \(20^\circ\mathrm{C}\) to \(200^\circ\mathrm{C}\). The data of the experiments of the first series showed (Fig. 1) that the drying of raw granules, as was to be expected, depends to a considerable extent on the height of their layer. Moisture removed from the hotter layers in direct contact with the heat carrier entering the dryer condenses in the colder layers, increasing their moisture content [2].

Fig. 1. Dependence of the moisture content and mechanical crushing strength of granules on the temperature of the drying agent (when it is sucked through the granule layer). \(1,1'\); \(2,2'\), and \(3,3'\)—respectively for samples from the upper, middle, and lower layers of granules after 8 min of drying.

Fig. 2. Dependence of the moisture content and mechanical crushing strength of granules on the velocity of the drying agent (when it is sucked through the granule layer). \(1,1'\); \(2,2'\), and \(3,3'\)—respectively for samples from the upper, middle, and lower layers of granules after 8 min of drying.

At a drying-agent temperature below \(100^\circ\mathrm{C}\), the binder (sulfite-alcohol stillage) softens; with increasing temperature, the temperature difference between the upper and lower layers of granules increases, which imparts a sharp nonuniformity to their moisture content and mechanical strength. At a superficial velocity of the drying agent of \(1\ \mathrm{m^3/m^2\,sec}\), entrainment of fine-grained particles is practically not observed.

In the second series of experiments, the aim was to investigate the process of drying granules as a function of the velocity of the drying agent when it moved from top to bottom and from bottom to top through the granule layer. The height of the granule layer was 50 mm; the temperature of the drying agent was \(100^\circ\mathrm{C}\); the granule size was 3–5 mm at an initial moisture content of 4.5%. The superficial velocity of the drying agent was 0.5; 1.0; 1.5; 2.0; 3.0; and \(4.0\ \mathrm{m^3/m^2\,sec}\).

The results of the experiments of the second series showed (Fig. 2) that an increase in the relative velocity of the heat carrier above \(1\ \mathrm{m^3/m^2\,sec}\), with a granule-layer height of 50 mm, helps to reduce the drying time and increase their mechanical strength, but unequal drying conditions in different layers over the height ...

layers are not eliminated. This especially leads to overwetting of the lower layers, with a change in their porosity and gas permeability when the heat carrier is drawn from top to bottom through the layer of granules. At a heat-carrier velocity of \(4\ \text{m}^3/\text{m}^2\text{s}\), entrainment of fines is observed.

Investigations of the 3rd series of experiments on drying granules were carried out at a bed height of \(25\ \text{mm}\), a temperature of \(100^\circ\text{C}\) (granule size 3–5 mm, initial moisture content 4.5%), and at various velocities of the drying agent. The results of the experiments (with the heat carrier blown through the layer of granules from bottom to top) are given in Table 1.

Table 1

With a granule bed of \(25\ \text{mm}\), a drying-agent temperature of \(100^\circ\text{C}\), and a superficial velocity of its passage through the granule bed of \(2\)—\(3\ \text{m}^3/\text{m}^2\text{s}\), the uniformity of drying, as is evident from the table, becomes equalized across the layers. Granules dried under the conditions of the fourth experiment after 8 minutes of drying showed high mechanical strength (2300—3000 g/granule) and fully satisfy production requirements.

Thus, on the basis of the laboratory investigations carried out, taking into account the capacity of the drying installation and the quality of the dried granules, the following should be considered the optimum drying regime:

1. Drying-agent temperature \(100\)—\(120^\circ\text{C}\).
2. Granule bed height 20—25 mm.
3. Superficial velocity of the drying agent \(2,5 \div 3,0\ \text{m}^3/\text{m}^2\text{s}\).

Under these selected conditions, 5 tons of quartz-carbon granules were dried on an enlarged model of the drying installation. The quality of the dried granules fully confirmed the selected optimum drying regime.

Conclusions

1. It has been established that, when drying moisture-containing granules in a bed, the main hazard is the formation of a zone of overwetted state, leading to unequal drying conditions for granules in layers at different heights.

As the bed height increases, the nonuniformity of drying rises sharply and leads to the destruction of the overwetted granules, reducing the gas permeability of the bed and increasing the load on the exhauster.

This phenomenon is not eliminated by raising the temperature of the drying agent.

1. A rational drying regime for moisture-containing granules in a bed has been established.

2. Under production conditions, the selected optimum drying regime is easy to implement on mesh belt dryers.

Literature

1. V. A. Rybakov, V. V. Karlin, V. M. Vityugin, V. A. Prokhorovich. “Ways of Further Developing the Production of Silicon Carbide,” Abrasives, issue 5, 3–5, 1963.

2. A. M. Parfenov. Agglomeration of Iron Ores. Moscow, Metallurgizdat, 137, 1954.