Foam control in paper machine sizing system(paper defoamer)

Foam control in paper machine sizing system(paper defoamer)

Combined with the production practice of Nan Paper #5 paper machine, conclude the formation, type, hazard and control measures of foam in the slurry. Edited by Mr Hu. Foam is generated when air is mixed into the slurry. The presence of foam can have a number of adverse effects on the papermaking process, especially on high speed paper machines where foam control is more important.

1.The cause and harm of foam formation

Studies have confirmed that foam is a colloidal system formed by the dispersion of gases in a liquid. In papermaking systems, the gas is typically air or CO2, and the liquid medium is typically water.
In the papermaking process, many operations cause the process water and white water to mix into the air to form a foam. For example, the slurry falls down in a waterfall, the air of the slurry pump leaks, the agitator of the slurry pool agitates too strongly or the agitator exposes the liquid surface, the pressure of the slag discharge port of the cone cleaner is too low, and the design of the white water or slurry pipeline is unreasonable. The recycling of white water, the recovery of broke paper, etc.; the fermentation of the slurry and the acidification decomposition of the carbonate will release CO2 and also form a foam.
The bubbles in the slurry have an important influence on the operation of the paper machine and the quality of the paper. High bubble content not only affects the dewatering of the net, causing paper disease, but also cavitation, which causes the performance of the slurry pump to decrease, increasing energy consumption; making the entire sizing system unstable, even causing the sizing pump to trip and stop; The pressure in the slurry tube and other pressure devices can also fluctuate.
The bubbles in the slurry have little effect on the lower speed paper machine, but have a significant impact on the high speed paper machine. Therefore, high-speed paper machines must be equipped with a complete defoaming system.

2. Types of foam

The most common foams in papermaking systems can be divided into three types: surface foams, carrying foams and static foams.
Surface foam is a familiar polygonal foam aggregate, which is commonly found in the pool, in the white water tank outside the machine, on the tray, and in the paper machine network. When the surface foam is severe, it will affect the system performance and paper quality.
The carrier foam is a fine spherical foam that is adsorbed on the surface of the fiber or dispersed in the slurry system. In general, the boundary tension of the fiber that adsorbs the foam is insufficient to overcome the surface tension of the bubble wall and the fiber surface, so that the bubble can be firmly adsorbed on the fiber. At the same time, since the diameter of the bubble of this type is very small, the boundary tension of the liquid on the surface of the bubble is weak, so that the rate of rising to the liquid surface is very slow.
The carrier foam dispersed in the slurry has a tendency to combine with each other to form large bubbles. If there is no impurity in the slurry, the foam will float in combination with each other to form a surface foam. However, the impure material present in the slurry increases the surface tension of the wall carrying the foam, making it more elastic and preventing the bond between the foams. Therefore, in the entire papermaking water and slurry system, the carried foam is still present in a highly dispersed state, and the fibers are tightly adhered by the carrying foam.
In papermaking systems, there are two main types of substances that stabilize the foam:
(1) Surfactants dissolved in a liquid, such as fatty acids, resin acids, lignin compounds, bleaching chemicals, and certain enhancers and dyes;
(2) Fine solid particles adsorbed on the gas liquid interface, which can form a stable film between the bubbles, such as a filler, a retention aid, a fine fiber, an alum, an ink dot, and the like.
The above substances in the slurry provide a good external environment for the stable presence of bubbles. Because pure liquid does not foam even if it is strongly mixed with gas. A stable bubble is formed only when a third type of impure substance is present.
Static foam is an "aged" surface foam that is formed by the presence of large amounts of fine fibers and particles in the slurry. This type of foam generally does not cause the paper machine to malfunction, but it affects the cleaning of the paper machine and causes the slurry.
Among the above three types of foams, carrying foam has the greatest influence on the sizing system and paper quality, and is the most difficult to eliminate; secondly, surface foam, and again static foam. In general, when the surface of the system has a high foam content, the amount of foam carried is also high, but less surface foam does not indicate that the amount of foam carried is also low.

3. Physical measures for degassing

In order to reduce the air content in the slurry, some necessary measures can be taken during the preparation of the slurry to prevent air from being mixed into the slurry. For example, extend the outlet of the slurry pipe and white water pipe below the liquid level; try to avoid small angle bends or joints to reduce the pulse and reduce the probability of air mixing; the pressure at the outlet of the slag remover should not be too low, or choose the row The slag port is a parallel closed type slag remover; the agitator of the slurry pool cannot expose the liquid surface, and the stirring speed should be moderate, not too fast [3]. However, to reduce the air content in the slurry to a low level, it is not enough to rely solely on the above measures. Paper machine systems, especially high-speed paper machines, should be equipped with a complete degassing device.

3.1 Paper machine degassing system

For modern high-speed paper machines, it is very important to set up a complete degassing system in the process. The system mainly includes the deaerator and related vacuum system. The degassing system of Nan Paper #5 paper machine includes:
The slurry is pumped to the deaerator through a slurry pump, and the bubble is released from the slurry by the principle of diffusion, acceleration and re-diffusion under the vacuum of 80-90 kPa. The released air bubbles are destroyed by the defoaming sealing water and enter the degassing sealing groove. In general, the air content in the slurry can be reduced from 2.5% to less than 1.5% after passing through the deaerator.

3.2 Auxiliary degassing measures

Although the deaerator can effectively remove the air in the slurry, if there is too much air in the slurry and white water, it is difficult to maintain the stability of the system. Therefore, some auxiliary measures must be taken to reduce the slurry entering the deaerator. Air content.
Auxiliary degassing measures include installing an exhaust pipe on a centrifugal pump (such as a slurry pump, a sizing pump, etc.) and a slag remover to remove air bubbles enriched in the centrifugal pump and the slag remover. The pumping of the outer white pool and the paper machine net section can effectively eliminate the surface foam of the paper machine system.

4. Defoaming agent defoaming

Practice has shown that it is impossible to completely eliminate foam by relying entirely on mechanical methods and good slurry conditions. In modern paper mills, defoamers are an essential wet end chemical. Typical papermaking defoamers generally include the following ingredients:
----Organic or inorganic hydrophobic microparticle active ingredients, such as amino compounds, silica ----Surface active ingredients such as fatty alcohols, lipids  Emulsifying and dispersing agents  Oil or water as a carrier
According to the use of the carrier of the antifoaming agent, it can be divided into a water-based defoaming agent and an oil-based defoaming agent. The active ingredient of the oil-based defoamer is emulsified or dispersed in a hydrocarbon carrier. This carrier technology is very suitable for industrial production, but its price is high; the emergence of water-based defoamers not only solves the cost problem, but also the performance of the defoamer. This is because hydrocarbons in oil-based defoamers are capable of dissolving similar resinate-like substances, preventing them from binding to fibers, and thus have a certain negative effect on the effects of certain retention aids and sizing agents [4].
Whether it is a water-based or oil-based defoamer, it basically works according to the following mechanism:
--- Intrusion of particles or hydrophobic particles in the wall of the bubble, these particles smashed the arrangement of the molecules of the bubble wall, creating local weaknesses. These ruptured bubble walls allow the carrier foam to bond to each other, causing the collapse of the bubbles to disappear.  preferential surface replacement, or replacement of undesired foam stabilizing impurities with non-foam stabilized surface active ingredients. Unstable bubble walls make bubbles more susceptible to cracking. ----Film formation is also effective for controlling the surface foam. Low surface tension insoluble agents can rapidly spread into a film on the surface of water. This rapid spread of film and bubble wall surface tension reduces the surface foam, but this does not enhance bubble bonding and does not release the carrier foam. Since a film forming agent such as kerosene does not prevent the formation of a new foam, it is more specifically a defoaming agent than an antifoaming agent.
The addition point of the defoamer directly affects the effect of defoaming. Nan Paper & ' Paper Machine, the initial addition point is the entrance of the white water channel to the white water pool outside the machine. There is only one addition point, the defoaming effect is not very good; the point will be changed to the white water channel of the internal network and the external network. The inlet is added at the same time, and the amount added is proportional to the flow rate of the white water, ensuring that the defoaming agent is thoroughly mixed with the white water. After the improvement, the bubble content in the white water decreased from 1.7% to 1.4%, the sizing system was more stable than before, the paper formation was improved, and the amount of defoaming agent was also reduced from 0.06% to 0.04%, which reduced the cost.

5.Impact of wet machine chemistry on system foam

The amount of bubbles in the slurry and the degree of damage to the system depend in some way on the wet chemical environment of the paper machine. The content of the water-soluble high molecular substances and the rubber in the slurry, the pH value, the type and amount of the chemicals selected for the paper machine, etc., all have an effect on the air content of the slurry system.

5.1 Effect of slurry and water properties on foam

When the content of water-soluble polymer materials and rubber in the slurry is high, the foam content of the system will change with the fluctuation of the pH value of the water and the slurry, and the rotation speed of the slurry pump will also fluctuate, resulting in the sizing system. Unstable. Because Pinus massoniana GP and TMP contain a large amount of resin, the content is as high as 0.5% to 0.6%, which causes resin barriers such as battering and clogging on the paper machine. In order to overcome the difficulties caused by the resin, lipase is added to the TMP and GP workshops to break it down into glycerol and free fatty acids. When the pH is high, the fatty acids form corresponding saponates, which are a strong surfactant. Once air is mixed into the slurry, bubbles are easily generated.
Therefore, controlling and stabilizing the pH value of the system is of great significance for reducing the foam content of the system and stabilizing the sizing system. For Pinus massoniana GP and TMP systems, the pH is stable at 6.5 to 6.8.

5.2 Configuration of wet machine chemicals in paper machine

The chemicals selected for the wet end of the paper machine, such as fungicides, alum, dyes, fillers, etc., also have an important effect on the air content of the system. The addition of the bactericide can inhibit the slurry and reduce the CO2 gas generated by the rot of the slurry; the alum can adjust the pH of the slurry, which helps to alleviate the resin barrier, but the excess alum also increases the foam content of the system.
When talcum powder is used as the filler, the pH of the system should not be too low. This is because the general talcum powder contains 0.2% to 1.0% of CaCO3 component, and CaCO3 decomposes very slowly in the environment of pH ≧7. When PH=6.0 or so, the decomposition speed will increase a lot, releasing a large amount of CO2 and generating foam.
The particle size of the filler also affects its rate of decomposition. The finer the particle size, the larger the specific surface area, the greater the surface activity and the faster the decomposition rate; the larger the specific surface area, the easier it is to adsorb air and produce more foam.
Dyes are also a blowing agent that tends to cause foaming in the system. However, due to the small amount of addition, the impact on the whole system is not obvious.

6 Conclusion

The amount of foam in the slurry directly threatens the normal operation of the paper machine and is particularly sensitive to high speed paper machines. Controlling foam content and stabilizing the wetted chemical environment are important for stable production. For operators, the foam content and source of the system should be actively observed and analyzed, and necessary measures should be taken to solve them. Practice has shown that physical and chemical foam control can effectively control and stabilize the foam content of the system.
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