It is known that the kraft cook can be separated into three stages; initial phase, bulk delignification phase and final phase. A chemical reaction is a rate-determining step for delignification under the condition in which the chip thickness is below certain value, except in the initial phase. Delignification rate in the two stages, bulk delignification phase and final phase, was studied.
Assuming that the delignification is reversible reaction, it was found that the delignification rate in the two stages, bulk delignification phase and final phase, could be expressed as a single kinetic equation. In addition, the effect of reaction temperature and concentration of cooking chemicals on the delignification rate was revealed quantitatively. The rate equation, which was established in the previous report for effective alkali consumption by wood components, was applied to the kinetic equation of delignification in order to determine the numerical solution. These numerical solutions fit to the experimental results successfully. While this method involves some assumption, it is useful to describe the delignification rate quantitatively under the condition of timevarying liquor concentration.