Digester
Digester is a huge pressure vessel used for chemical pulp making and dissolved fiber cooking section. As we’ve known, it is used for cooking raw pulp materials like wood, grasses, bamboos and others. These materials react with chemical liquor and we will get the pulp that is mostly composed of cellulose. Then, the pulp will be sent to the washing and bleaching system and applied to paper processing. According to different pulping processes, there are two types of the digesters in the chemical pulp production: horizontal digester and vertical digester.
Horizontal Digester
Horizontal digester is main core part for the continuous cooking system. This type of digester is designed for grasses just like bagasse, wheat grass, straw, etc.
Vertical Digester
Vertical digester is mostly used for the supper batch cooking system. The most advanced batch cooking system adopts the displacement cooking process, which requires low cost but obtain high-quality pulp.
| MODEL | ZJG1 | ZJG2 | ZJG3 | ZJG4 | ZJG5 | ZJG6 |
|---|---|---|---|---|---|---|
| Effective volume(M3) | 50 | 75 | 110 | 135 | 175 | 225 |
| Diameter(internal) | ￠3000 | ￠3250 | ￠3600 | ￠3600 | ￠4500 | ￠4500 |
| Maximum working pressure(Mpa） | 0.8 | |||||
| Maximum working temperature (℃) | 175 | |||||
Working Principle
Digester is main equipment in the chemical pulp process, but how does it work?
Before cooking, all materials must be kept moisture content below 15%, Silicon content below 3%, ash content about 2%-3%, lignin content about 22%-25%, and cellulose content about 50%-55%.
First, the materials are cut into about 25×30mm chips, and then the latter will be sent through conveyor belt into the digester full of coolant liquor and steam.
The cooking time usually depends on specific requirements of pulping process. It is decided by yield and hardness. Generally, the cooking will last for 5-6hours, and cooking temperature shall be kept at 160-175℃ during the whole period.
The heating process is closely relevant to the indirect heating area of the tubular heater. The liquor from the center flows through the circular tube and get heated and transferred from top to bottom. Next, the liquor will get heated circularly about 8-12 times every hour. Meanwhile, steam enters for auxiliary heating through the vent at the bottom.
In the cooking process, the steam will be discharged through the vent at the upper to reduce the fake pressure caused by the air in the internal digester, which ensures the cooking uniformity and improves pulp quality.
After cooking completion, the pulp out from the digester will enter the discharge washing under the pressure, and then be stored temporarily.
