ON THE MECHANISM OF MOISTURE ABSORPTION DURING WET QUENCHING OF COKE

V. M. Vityugin

(Presented by the scientific seminar of the Faculty of Chemical Technology)

It is known that a high and nonuniform moisture content in coke has a harmful effect on the blast-furnace process. Moistening of coke occurs mainly during wet quenching, when incandescent coke comes into contact with water. Elucidating the mechanism of moisture absorption by coke during wet quenching is the first stage in obtaining blast-furnace coke that is not overmoistened and is uniform in moisture content.

Studies [1] have established that incandescent coke absorbs more moisture than cold coke for the same time of contact with water. During wet quenching coke is in contact with water for approximately one minute. According to the data of P. M. Chebotarev, incandescent coke absorbs approximately three times more moisture during this time than cold coke (12.1–21.55%; 4.2–6.05%). Cold coke, even when completely immersed in water for one minute, absorbs comparatively little moisture. It follows from this that the increased moisture content in coke after wet quenching is due to phenomena occurring in the quenching process itself.

Many researchers believe that coke absorbs a large amount of moisture during quenching mainly as a result of the reduction in the volume of gases during cooling, which causes water to be drawn in.

If the degree of reduction in the volume of gases in the pores of coke during quenching is taken as the principal factor in moisture absorption, then one must conclude that the higher the temperature of the coke before quenching, the more moisture the coke will absorb, and vice versa. On the other hand, acceptance of the stated proposition leads to the conclusion that during wet quenching of incandescent coke it is impossible to change its moisture content in one direction or another. The American researchers Hils and Holmes [2] arrived at such fundamentally incorrect conclusions. Thus, for example, by calculation they obtained a moisture content of 25% for coke with an initial temperature of 500°C; for coke with an initial temperature of 1000°C, a moisture content of 30%; and for cold coke, 10%.

To determine the principal factor in the process of moisture absorption during wet quenching of coke, a series of experiments was carried out. Pieces of coke at a specified temperature were immersed in water for one or another interval of time, and then dried in a drying oven at a tem—

Table 1

temperature of 105°C. In view of the fact that, under industrial quenching conditions, coke comes into contact with hot water, both cold quenching water and water heated to 70°C were used. The time of immersion of the coke in water was reduced by half in comparison with the quenching time under industrial conditions, since in production the coke is sprayed with water and is not completely immersed in water. After being removed from the water, the coke was kept in the open air for 30 minutes and then weighed. The results of some experiments are presented in Table 1.

Table 2

As can be seen from the table, the dependence of moisture absorption on the initial temperature of the coke during quenching with water is more complex than Hils and Holmes suppose. With an increase in temperature to red heat, the moisture content of quenched coke first increases and then decreases. Such a dependence of coke moisture absorption during quenching on its initial temperature cannot be explained by a reduction in the volume of gases in the pores.

In our view, moisture absorption occurs as a result of the creation of a vacuum due to the condensation of water vapor penetrating into the pores of the coke during its quenching with water. Cold coke slowly and in small quantity absorbs moisture. If, however, the coke is heated to a temperature at which, during the quenching process, the moisture is at least partially converted into vapor that condenses toward the end of cooling, then the coke will absorb much more water. At a sufficiently high temperature, which lies between 200°C and 300°C, all the open pores of the piece are first filled with vapor during quenching, and then, after condensation of the vapor, with water. The moisture content of the coke in this case will correspond to its open porosity. Consequently, if two identical pieces of coke are immersed in water, the first at a temperature of 1000°C and the second at 500°C, and they are kept there until completely cooled, they will absorb the same amount of moisture, approximately equal to the percentage of open porosity.

Because of the low thermal conductivity of coke, it cools, even in direct contact with water, over a more or less prolonged time; and if it is removed from the water while still hot, part of the absorbed water will evaporate. Therefore, under industrial conditions, with a short quenching time, the moisture content of coke is comparatively low. The higher the initial temperature of the coke and, consequently, the greater its store of physical heat, the later and in a smaller number of pores condensation of water vapor will occur, and the less moisture the coke will absorb. The data of Table 2 confirm the proposition stated above.

Conclusions

1. It has been shown that the absorption of moisture by coke during wet quenching depends not on the degree of reduction in the volume of gases, but on the condensation of water vapor in the pores of the coke.

2. By regulating the quenching time and the amount of quenching water, coke of the required moisture content can be obtained during the cooling process, regardless of its initial temperature.

3. To obtain coke with the lowest moisture content during wet quenching, it is necessary to avoid condensation of water vapor in the pores of the coke.

References

1. P. M. Chebotarev. On the moisture-absorbing capacity of coke, Coke and Chemistry, 45, No. 9, 1934.

2. D. C. Hills and C. R. Holmes. Industrial and Engineering, vol. 43, 1635, No. 7, 1951.