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.

digesting equipment

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.

AGICO adopts new technologies to carry out production. We can organize production process according to the requirements of market and customer. We can manufacture various sizes of digesters, including 50m3, 75m3, 110m3, 135m3, 175m3, 225m3, 250m3 and 400m3. Our digesters are mainly used in the digesting workshop. The maximum diameter of our digester can reach 5.8m, and the maximum capacity can reach 400m3.
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.

Pulp is a lignocellulosic fibrous material prepared by chemically or mechanically separating cellulose fibres from wood, fiber crops, waste paper, or rags. Many kinds of paper are made from wood with nothing else mixed into them. This includes newspapers, magazines and even toilet paper. Pulp is one of the most abundant raw materials.

A pulp mill is a manufacturing facility that converts wood chips or other plant fibre source into a thick fibre board which can be shipped to a paper mill for further processing. Pulp can be manufactured using mechanical, semi-chemical or fully chemical methods (kraft and sulfite processes). The finished product may be either bleached or non-bleached, depending on the customer requirements.

A paper machine (or paper-making machine) is an industrial machine which is used in the pulp and paper industry to create paper in large quantities at high speed. Modern paper-making machines are based on the principles of the Fourdrinier Machine, which uses a moving woven mesh to create a continuous paper web by filtering out the fibres held in a paper stock and producing a continuously moving wet mat of fibre. This is dried in the machine to produce a strong paper web.

The pulp produced up to this point in the process can be bleached to produce a white paper product. The chemicals used to bleach pulp have been a source of environmental concern, and recently the pulp industry has been using alternatives to chlorine, such as chlorine dioxide, oxygen, ozone and hydrogen peroxide.

Chemical pulp is produced by combining wood chips and chemicals in large vessels called digesters. There, heat and chemicals break down lignin, which binds cellulose fibres together, without seriously degrading the cellulose fibres. Chemical pulp is used for materials that need to be stronger or combined with mechanical pulps to give a product different characteristics. The kraft process is the dominant chemical pulping method, with the sulfite process second. Historically soda pulping was the first successful chemical pulping method.