The reaction process of cooking, the dissolution characteristics and change laws of lignin, carbohydrates and other components in the raw materials at each stage of the cooking process.
The cooking reaction process mainly includes: delignification reaction process and carbohydrate reaction process.
Understanding and mastering the cooking reaction process is of great significance for determining a reasonable cooking process, quickly and effectively removing lignin, reducing fiber damage and controlling hemifiber degradation.
What is the delignification reaction process?
During the cooking process, the change of lignin removal with temperature and time is called the delignification reaction process.
It reflects the speed and amount of delignification during the cooking process.
Due to differences in chemical composition and tissue structure of different plant fiber raw materials, the delignification process will also be quite different during the alkaline cooking process. Especially the difference between wood raw materials and grass raw materials is very significant.
During kraft cooking of wood raw materials, delignification can be divided into three stages:
Initial delignification stage:
It usually occurs from the beginning of temperature rise to 140 degrees Celsius. This stage is the liquid infiltration stage. At this stage, the dissolution of lignin is relatively small, and its dissolution amount generally accounts for 20%-25% of the total lignin of the raw material. At this stage, lignin does not degrade, and only those easily soluble lignins with a small enough molecular weight will be extracted from the S2 layer of the cell wall.
Large-volume delignification stage
When the cooking temperature reaches above 140 degrees Celsius, the delignification rate increases rapidly. In this large-volume delignification stage, 70%-80% of the lignin is dissolved. When the delignification stage reaches about 90%, the large-volume delignification stage ends.
Residual lignin removal stage
Enter the residual lignin removal stage.
Relatively speaking, the delignification rate is relatively small and the removal amount is relatively small.
For grass raw materials, especially straw, wheat straw, bagasse, etc., the delignification reaction process is quite different from that of wood, and is divided into the following three stages:
Massive delignification stage
This stage is before the temperature rises to about 100 degrees. At this stage, about 60% of the lignin is removed.
Supplementary delignification stage
This stage is the stage from about 100 degrees to the highest temperature. After this stage, the total lignin removal reaches more than 90%, so that the raw materials can be dispersed into pulp.
Residual lignin removal stage
In the insulation stage at the highest temperature, the lignin removal in this stage is generally less than 5%,so in the steaming process of grass raw materials, only a short insulation time is required, and the insulation time can even be eliminated.
