The main purpose of cooking is to remove lignin and other impurities from plant fiber raw materials according to different types of pulp, and to keep cellulose and hemicellulose as far as possible, so that the fibers can be separated into pulp, usually by chemical methods. Alkali can also be used.
According to the equipment and operation technology, the cooking process can be divided into super batch cooking and continuous cooking (independent of cooking method). Batch cooking is an efficient pulping process based on traditional cooking technology, which can save about 60% of cooking gas. In addition, the displacement cooking method is commonly used at present, which utilizes the replacement circulation of black liquor in intermittent cooking digester and the principle of diffusion cleaning. The spent cooking liquid is replaced by washing water, the cooking liquid and heat are replaced, and the cold spray is completed. Displacement cooking can make full use of black liquor over and over again, and can be steamed completely from the wood removal stage. Lignin and dissolved hemicellulose are extracted from the cooking system. The cooking liquid can be re-added during the dewoving phase. In this way, the heat and chemicals can be recycled and reused. During the whole cooking process, the cooking condition is kept in balance, and the pulp quality and production efficiency can be improved.
Continuous cooking process includes the whole continuous process from feed to discharge, mainly used in alkaline pulping, nitrite cooking, prehydrolytic sulfate and sulfite cooking. Today, non-wood fibers, such as bagasse, reeds, wheat straw, straw and so on, are developed for pulping of various raw materials, including wood chips and loose and porous raw materials, such as bagasse, reed, wheat straw, etc.
The common equipment used in the cooking process is pulp digester, continuous tubular digester, displacement digester system, blow tank and so on.
According to different pulp cooking processes, there are various types of pulp digester applied in the paper pulp making. Rotary digester and vertical digester are two main types of batching cooking digester. The continuous digester includes continuous tubular digester, M & D digester and vertical digester.