The relationship between press dewatering and papermaking performance of paper machine

1. Wet end dehydration mechanism

A constant function of the forming section, press section and dryer section of a paper machine is dehydration. In the wet part, mechanical methods are used for dehydration, while in the drying part, evaporation is used for dehydration. The relative cost of removing 1 ton of paper is: forming part: pressing part: drying part = 10:12:78; therefore, papermakers must strive to Improve the dewatering efficiency of the wet section, especially the efficiency of the press section, to reduce the evaporation load of the dryer section, and the transverse dewatering should be uniform.

The dehydration of the paper sheet is the process of solidification, forming a network in the wire part, and only using the vacuum effect to make the paper sheet dryness of the couch roll up to 18~23%, in this moisture range, most of the free water has been removed, Further dewatering is accomplished by pressing the sheet structure to compress the internal water pressure to increase the outward dewatering.

The void volume of the sheet is reduced after the moisture is squeezed out of the web during the pressing process, and a certain degree of fiber compression occurs where the fibers are in contact with the fibers, and the applied pressure helps to evenly distribute the moisture throughout the sheet.

Press dehydration is divided into four stages: the pressure of pressing is the sum of water pressure and mechanical pressure, and the wet pressing nip is divided into four stages according to the interaction of its water pressure and mechanical pressure.

In the first stage, the paper sheets and felts discharge air until the paper sheets reach saturated moisture and there is no more residual air. At this stage, the water pressure in the paper sheets is not large, and the dryness does not change.

In the second stage, the paper sheet is saturated with water, and the water pressure in the paper sheet rises, so that the water is transferred from the paper sheet to the felt. When the felt is also saturated, the water is discharged from the felt. The second stage has been to the middle of the nip. pressure is at its highest.

In the third stage, the nip seam begins to expand, and further dewatering is performed until the water pressure in the sheet drops to zero, and the dryness of the sheet is maximum at this time.

In the fourth stage, the paper sheet and felt begin to expand, and the paper sheet becomes unsaturated. At this time, the paper returns to the paper sheet by means of capillary action, vacuum inside the paper sheet or the mechanism of void rewetting. The press exit sheet should be separated from the felt as early as possible to reduce the rewet effect.

Influencing factors of press dehydration:

① Pressure applied to unit area;

②The duration of the applied pressure;

③ The number of pressing devices;

④The structure of the press roll;

⑤Characteristics and state of press felt;

⑥ Viscosity (or temperature) of water;

⑦ Paper properties, including fine fiber content, additive amount and air permeability of wet paper.

The main factors that promote dehydration are the water pressure difference through the paper, the residence time of the nip and the uniformity of the pressure. The product of the pressure in the nip and the residence time is called the pressing impulse; the main resistance is the flow resistance of the paper itself. When the water temperature increases by 1 degree, the flow performance increases by about 2.5%. During the pressing process, the temperature of the paper sheet is increased from 20 degrees to 30 degrees, and the dryness of the pressed paper sheet increases by 2~3%.

Press dewatering is divided into two categories: pressure control type and flow control type.

The pressure-controlled type is mainly used on paper sheets with low basis weight and high freeness. Because the paper is very thin, the structure of the paper sheet does not significantly restrict the flow of water. Determined by the possibility of rewetting from the felt to the sheet after the midpoint.

The flow control type is mainly used on high basis weight sheets, which are quite thick, and the structure of the sheet restricts the flow of water from the sheet. Sheet density, temperature and felt porosity have a significant effect on water resistance, and residence time in the nip becomes a limiting factor. Pressure-controlled dewatering and flow-controlled dewatering, respectively, point to the direction of efforts to improve press efficiency.

Crush is the direct result of water pressure caused by the lateral movement of water flow in the sheet. The method of slow pressurization can be avoided, and the structure of the press roll and the selection of the felt can be improved, and the dewatering speed can be accelerated, thereby avoiding the occurrence of crushing. Press dehydration also has a critical point problem. Exceeding the critical point, pressing dehydration is uneconomical, or it will over-compress the paper sheet, and the water left in the paper sheet must be removed by evaporation. Depending on the type of pulp, pressing form and operating conditions, the critical point is 50-57%.

Second, the paper machine performance

The papermaking performance of the paper machine refers to the convenient operation and stable performance of the paper machine during operation, which is mainly reflected in the speed and stability of the transfer of paper sheets, and the main purpose is to reduce paper breakage. The transfer of the paper web requires force, such as speed differential traction, vacuum suction and foil. The technologies used to improve the running performance are: vacuum transfer threading, paper transfer vacuum box, paper stabilization bellows, paper transfer bellows, single hanging dryer, vacuum cylinder, pocket ventilation, automatic tension control, curved rollers.

Tapping at the couch roll: The paper machine variables that affect the peeling of the wet sheet from the couch roll are the peel angle, the machine speed, and the relative adhesion of the formed sheet and the forming wire.

The minimum tension peeling angle is usually between 60 and 120 degrees; the adhesion to the forming wire depends on the initial forming conditions, such as the concentration of the mesh, the degree of pressure forming, and the cleanliness of the mesh. There are two types of threading at the couch roll: open type and closed type. The closed type threading is divided into three types: ordinary roller threading, vacuum roller threading and vacuum threading and pressing.

1) Open threading is limited by the strength of the wet paper web, so the operating speed is low. When the vehicle speed exceeds 7.6m/s, the tension of the paper sheet quickly becomes the dominant factor. When the vehicle speed reaches 10.2m/s, open threading 60% of the wet web tension in the paper is due to the effect of vehicle speed.

2) In the closed threading of ordinary rollers, the paper sheets and felts at the threading part must be quite wet before they can be drawn, which is easy to cause embossing, so it is only suitable for lightly pressed low-weight paper.

3) Vacuum Tipping A vacuum roller is used inside the pickup felt to apply the suction force required to make the wet paper sheet detach from the mesh, and at the same time, the pressure exerted by the pickup roller on the sheet is further dewatered. Since the wire speed is the same as the felt speed, the vacuum threading completes the transfer of the sheet without any change in the traction force.

The papermaking performance of the drying section: the instability of the paper in the drying section is due to the flow of air, and the flow of air is not only related to ventilation, but also related to the moving surfaces such as the dryer wire, drying cylinder and guide roll. Disturbing the paper sheet, with the increase of the speed of the vehicle, the problem further deteriorates. In order to stabilize the paper web, a large speed difference is used to tighten the paper web, which increases the frequency of paper breaks and loses a lot of paper performance. Unconventional arrangements are used for this on high-speed paper machines.

Sheet transfer blow box from press section to dryer: the turning roll of the first set of dryer fabrics is as close as possible to the last press roll, and the sheet reaches the bottom of the dryer fabric only after a short distance without support section. The box is arranged on the bottom surface of the dryer fabric, and the nozzles emit a pressurized jet to control the air in the interface layer carried by the dryer fabric to form a negative pressure suction force, and the adsorbed paper web is attached to the dryer fabric and enters the drying section.

Drying section steady paper blow box:

The paper coming down from the upper cylinder tends to follow the drying cylinder, which causes the formation of air pockets in the paper sheet and causes wrinkles; the dryer wire carries a large amount of air from the interface to form a positive pressure in the inlet area of the lower drying cylinder, and the quilt is forcibly pressed into the collecting area of the mesh cylinder , a lot of air is pressed into the drying wire, and is intercepted and gathered between the drying wire and the paper sheet along the lower drying cylinder, so that the paper sheet floats away from the drying wire and loses its stability;

When the sheet enters the upper cylinder, the trapped air is forced out, which may also cause wrinkling. Use a blow box to blow air against the running direction of the dryer to remove the interface gas, and the blowing generates a negative pressure to make the paper stick to the dryer without trapping air.

There are two types of blower boxes, one is a unilateral blower box and the other is a double side blower box. The unilateral blowing box blows on the upper and lower drying cylinders, the upper and lower drying cylinders, and the sides of the mesh on both sides, forming a unilateral vacuum area in the upper and lower drying cylinders. The double blower does not blow the drying surface.

specific measure:

The specific measures to ensure the running performance of the paper machine in production include:

1) For vacuum transfer paper, it is necessary to adjust the width of the suction area suitable for the width of the paper comrade to avoid edge stripping and edge drop;

2) In the paper transfer vacuum box, the moisture in the press felt should be removed evenly across the entire width, and the paper web should be drawn down to the felt to achieve stable separation and then lead to the next process. Pay attention to the necessary degree of vacuum during use and not to be partially blocked;

3) In the paper-stabilizing vacuum box, it is necessary to adjust the air volume to ensure that a suitable negative pressure is generated to stabilize the paper web and avoid folding and flickering of the paper web;

4) In the paper transfer bellows, the air volume should be adjusted to ensure that a suitable negative pressure is generated to stably transfer the paper web;

5) For a single-hanging drying net, the sticky material should be scraped off to maintain proper air permeability so that it can be combined with the vacuum cylinder and the paper-stabilizing bellows, so as to realize the drying of the wet paper web at the beginning of the drying and no open running support; the vacuum cylinder: pay attention to the hole The adjustment of the dredging and vacuum suction doors makes the paper web run stably against the dryer fabric;

6) Ventilation in the bag area, adjust the ventilation and air supply volume to balance the air pressure in the dryer bag area to avoid excessive jitter and paper breakage due to the unbalanced and unstable air pressure on both sides of the paper web;

7) For automatic tension control, the appropriate setting value should be set according to the different quantitative and position, and the automatic adjustment should be performed according to the setting value to ensure the smooth running of the paper web;

8) For the curved roller, the angle of the curved roller should be adjusted according to the different horizontal tightness of the paper web to stretch the paper web and keep the tension uniform across the entire width to avoid folding.