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How the paper machine works

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Update time : 2020-09-15 09:57:04
  In the process of papermaking, mechanical stirring, white water reuse, surfactant or other wet end chemicals, and the existence of fine fibers may lead air into the pulp system.
  Generally speaking, there are three forms of air in pulp
    1) Air dissolved in liquid is formed in normal pressure liquid;
    2) Air entrained by agitation;
    3) the air brought by bubbles will change with the temperature.
  Dissolved air does not cause trouble to the pulp system, and their concentration is very low. The size of small bubbles is very small, which generally does not affect dehydration. When the viscosity is suitable, the air dissolved after stirring can form large bubbles and become the air entrained by mist. The entrained air contains more small bubbles, which have affinity for pulp and fine fibers. They are larger than the amount of dissolved air, will affect the dehydration of the mesh, the air entrained is stable, therefore, it is not easy to be discharged from the system.
  At the same time, the entrained bubbles will reduce the efficiency of the equipment and affect the operation performance of the paper machine. Even a very small air bubble will interfere with the normal operation of some measuring instruments. In serious cases, the measuring instruments will stop working, thus reducing the paper quality.
  The air in the foam slows down the filtration rate and brings about cavitation and so on, but the air form is most easily destroyed. No matter what form of air in pulp exists, it will cause a variety of problems. It will not only reduce the efficiency of chemicals, block the dehydration of the web, reduce the speed of the paper machine and the paper characteristics (such as tensile strength and wet paper strength), but also lead to the decline of evenness, the occurrence of pinholes, paper holes and other paper diseases, affecting the air permeability, hindering the normal operation of the paper machine and reducing the output.
  Many paper mills take the closed circulation system as the goal of further development, but the closed cycle will aggravate the adverse effects of air in pulp. It is reported that 0.4% air content can prolong the filtration time by 200% and reduce the wet paper strength by 30%. Therefore, it is very important to reduce the air content in pulp on modern high-speed paper machines. It is found that the air content in headbox is less than 0.5%, which not only has good paper evenness, but also is conducive to the dehydration of the web.
  In order to remove the air from the pulp, mechanical equipment (such as degassing tank or degasser) or adding chemicals to the pulp are usually used for degassing. Mechanical degassing is to heat the pulp suspension to near boiling point in vacuum, and then spray and extract air. In vacuum, the air bubble expands and breaks and is removed from the system by pumping. The efficiency of this method is very high, but the pulp with air completely removed will still have the tendency of air suction, so it can only ensure that the pulp does not contain air in a short period of time.
  The degasser can effectively remove the above three forms of air. The degassing operation is to introduce vacuum into white water to reduce the boiling point of liquid, which can remove air from the pulp to improve the dehydration effect. The degasser can be used without or less defoamer.
  Degassing with degasser can not only reduce the consumption of chemicals and the number of segments in screening operation, but also reduce the deposition and defects in the web and improve the performance of the paper machine. However, when the vacuum degree of degassing system is not suitable, the stirring in the container will lead to more air entrained in the slurry. Therefore, in the actual production operation, the vacuum degree of the degassing system should be paid great attention to. In addition, improper operation of degasser will aggravate the accumulation of pollutants and bring negative effects on sizing.
  Due to the negative pressure, the liquid without air will absorb the surrounding air quickly to reach balance when conditions permit, so there is still air in the mesh slot / mesh of the degasser system used alone. The paper mill can use degasser or defoamer alone or both; if degasser and defoamer are added to pulp at the same time, the synergistic effect between them will make the whole pulp system achieve better effect than using degassing equipment alone.
  It is found that chemical defoamers or antifoaming agents will maintain their effectiveness in the whole system and destroy the entrained air hidden in the whole system if they are properly used or shared with the degasser. Moreover, some special defoamers can not only reduce the interference of entrained air, but also improve the filtration by reducing the surface tension.
  The production practice shows that after adding silicone defoamer in front of the white water tank, the air content of the pulp can be reduced from 4.60% without any defoaming and degassing agent to 3.99% and 13%; the air content of the pulp can be reduced to 1.61% and reduced by 65% with the addition of silicone defoamer and degasser at the same time; the air content of the pulp can be reduced from 4.63% to 1.73% when the dosage of the defoamer is 0.3kg/t %The air content of 0.17kg/t paper can be reduced by 26.3%.
  The addition of degassing agent can effectively reduce the air content of the pulp, improve the water filtration of the screen and reduce the number of pinholes in the paper. However, this method can not completely remove the air in the pulp, and only relying on the degassing agent to reduce the air content in the pulp to a very low level will greatly increase the production cost. Therefore, defoamer and degassing agent should be used together with mechanical equipment to achieve better degassing effect.