Efficiency of chip presteaming – result of heating and air escape processes.

Efficient removal of air from the wood chips prior to impregnation with cooking liquor is important for achieving pulping uniformity and improving the overall digester performance. Because of its effectiveness, relative simplicity and other advantages, steaming of wood chips became a primary process in pulp industry to fulfil this task. In the present article the process of chip presteaming is examined from viewpoints of chip heating and air escape. A three-dimensional model was proposed to estimate the efficiency of heating the single chip by saturated steam. Model simulations and experimental results indicated that the time required to heat the single pine chip with steam was very short, only a few minutes. Under industrial conditions, however, longer heating time may be expected due to the heat transfer limitations. The process of air removal does not proceed as fast as the chip heating process. Based on the experimental data it was shown that the process of air escape fell to the negative exponential dependency. To achieve the air removal degree of 90 % for heartwood pine chips, half an hour of steam
ing with saturated steam at 105 °C was required, for sapwood pine chips only 5 minutes. The steaming temperature was found to influence the efficiency of air removal, probably by affecting the structure of wood chips. To achieve a high degree of air removal would require very long steaming times, especially with heartwood chips. Complete removal of air, however, maybe difficult to achieve, even by applying optimal steaming conditions and long steaming times.