Bleaching Tower

Bleaching tower is required for various bleaching sections and related to the pulp concentration of each section. For bleaching or whole bleaching line, bleaching can be carried out at three different pulp concentrations of low, medium and high. From current engineering examples, it is generally considered that the pulp concentration is 3.5-6% for low concentration bleaching, 7-15% for medium concentration bleaching, and 25-30% for high concentration bleaching. The waste liquor produced by traditional low concentration chlorine bleaching has always been a serious source of pollution for pulp and paper mills. Therefore, the bleaching under low-concentration conditions is eliminated in modern times, medium or high concentration bleaching is trendy in order to achieve less chlorine or chlorine-free bleaching. They are called ECF or TCF bleaching in the industry.

If the pulp is bleached with oxygen as its bleaching agent under medium concentration conditions, the medium concentration oxygen bleaching tower is a pressurized upflow bleaching tower. In addition to tower body, the tower is also equipped with pulp disperser at the bottom and discharger and spray pipe at the top. The medium concentration oxygen bleaching is carried out under a certain pressure. The process generally requires the top pressure of the tower to be above 0.4MPa, so the tower belongs to pressure vessel. In the design of the oxygen bleaching tower, the strength design calculation is also carried out to ensure the reliability and safety of the tower during operation.

In view of the degree of environmental pollution caused by traditional three-stage or multi-stage bleaching, in order to reduce the content of organic halides in the pulp bleaching waste liquor, the modern pulp and paper mill has generally applied the medium concentration chlorine dioxide bleaching technology. Especially when CLO2 is used for supplementary bleaching, the whiteness, strength and whiteness stability are good, and it is widely used in multi-stage bleaching.

Medium concentration chlorine dioxide bleaching tower is commonly divided into two types: one type is upflow-downflow type, that is, the combination of the upflow pre-reaction tower and the downflow tower; the other type is the downflow tower. The upflow-downflow bleaching tower is also known as a pre-reaction chamber downflow tower. Generally, the diameter of the upflow tower is much smaller than that of the downflow tower, so it is generally said that the upflow tower is the upflow pipe. The pulp mixed with CLO2 enters and goes up from the center of the bottom of the upflow tower, then starts the bleaching reaction. As reaching the top, it turns into the top of the downflow tower, and is put in from the center of the tower, and continues to react in the downflow tower until the bottom of the downflow tower. The bleached pulp is diluted with dilution water and pumped to the washer by a low concentration pump. Therefore, the upflow-downflow bleaching tower essentially consists of two parts: a upflow tower and a downflow tower.

Medium concentration chlorine dioxide bleaching belongs to chlorine-free bleaching. Compared with low concentration chlorine bleaching, the AOX content and wastewater volume of bleaching waste liquor are greatly reduced. And CLO2 has a strong dissolution effect on lignin in pulp. It rarely damages the strength of carbohydrates and pulp, and has little degradation to pulp. Therefore, medium concentration chlorine dioxide bleaching has been widely used in pulp bleaching.

In engineering practices, hydrogen peroxide is used as a bleaching agent in the bleaching of medium or high concentration pulp only after the development of key equipments such as medium concentration pulp pump, medium concentration high shearing mixer and high concentration mixer can the conditions be realized. Medium concentration hydrogen peroxide bleaching uses the upflow bleaching tower just like medium concentration oxygen bleaching. However, since medium concentration oxygen bleaching belongs to pressure bleaching, medium concentration hydrogen peroxide bleaching is atmospheric pressure bleaching, so the internal structure of the tower will not be exactly the same, especially for the top of the tower.

The ozone bleaching is also a bleaching method without (less) pollution, belonging to TCF bleaching. On the other hand, if there is no ozone, bleaching the kraft pulp with only oxygen delignification and hydrogen peroxide can’t reach high whiteness. However, the investment and operation costs of ozone generator and ozone bleaching units are higher.

In the case of the bleaching process, it is difficult to achieve the whiteness requirement of the pulp only by applying a certain bleaching section. The pulp can only achieve the whiteness and strength required by the bleaching process of multi-stage bleaching system combined with different number of sections.

The corrosion protection of the bleaching tower is a very important issue in the pulp bleaching line, and the corresponding equipment materials must be used depending on the corrosion degree of the equipment used by the bleaching agent.

Bleaching tower is often used in corrosive media, so the choice of materials is important for bleaching equipment. The relationship between the working conditions of the equipment and the corrosion resistance of the material should be considered, especially in the following three aspects:

1. Temperature: the corrosion rate usually increases with increasing temperature.
2. Pressure: the higher the pressure is, the higher the corrosion resistance requirement of the material is. In pressure equipments, crystal orientation corrosion, stress corrosion, crevice corrosion, corrosion fatigue, etc. may occur.
3. Medium corrosion: the higher the corrosion degree of the medium is, the higher the material requirements are.