Dewatering (Too Slow, Too Fast)

The most common problems related to dewatering involve slow or variable drainage and response to vacuum. Slow dewatering may involve "slow initial drainage" on the forming table, low solids after vacuum application, low solids after wet-pressing, or just a consequent lower operating speed of the paper machine. If you have that kind of problem, then you should consider yourself fortunate; there are ways to accelerate dewatering, as will be described. Variable dewatering can be much more of a headache. Until the root causes of variability are identified and overcome, the results can include web breaks, roll-winding difficulties, and nonuniform product performance. And if you are really lucky, your problem is that dewatering is too fast. If that's your problem, then you probably can select among various options such as increased refining (for more strength), increased levels of hardwood or mechanical pulps (which often can improve formation uniformity), lower hydrofoil angles or fewer hydrofoils, lower vacuum levels (to reduce pumping energy and drive energy), or reduced wet-press nip pressure (which also is expected to increase the life expectancy of the felts). Choose carefully, since it's likely that some of these options will be contrary to what you need for your product goals.

SLOW INITIAL DRAINAGE

Drainage rates are highly dependent on the design and set-up of forming equipment, but here let's concentrate on issues related to the furnish and chemical additives. Some of the most likely causes of slow or decreased initial dewatering rates include high or increased fiber fines levels (often due to refining), fibrillation of the fibers, high basis weight, entrained air, and (less commonly) very high levels of water-soluble polymers. This page, below, will address all of these possibilities. Please go to the following link if you want to learn about applications of drainage chemical programs.

A fiber length analysis or "classification" of fiber length fractions can help settle the question of whether the fines content has changed. Another clue (but not proof) can consist of a higher than usual solids content in the white water. High white water solids, though, can have two basic causes. Sometimes it does mean that the furnish is over-refined or more precisely that the fibers were not strong enough to "take" the amount or intensity of energy applied during refining. Variations in pulp quality, yield, or bleaching can make the fibers more or less susceptible to cutting, even if the refining energy per ton of fiber is kept constant. Alternatively, increased fines might be due to an unexpectedly high content of mechanical fiber, hardwood, or certain kinds of recycled fibers. But be careful; a high fines content may just mean that your retention aid system is not working well (see more, a little later).

So let's start by assuming that your main problem is due to "high fines content." Before you attempt to solve your problem by dumping all of the fines to wastewater treatment, consider the following points: First, it's often found that some moderate level of fines in the product, say 10 to 40%, depending on the furnish type, yields a maximum in paper tensile strength; this has been attributed to a better sheet structure. Second, due to their higher surface area per unit mass, fines are likely to be carrying a disproportionate amount of chemical additives such as sizing agents; dumping such fines will waste chemicals. Third, different kinds of fines can have very different affects. Refining tends to produce ribbon-like fragments that come from delamination of the fiber wall; these ribbons have a very high specific surface area. They are great for inter-fiber bonding, but they can be terrible for drainage. This kind of fines can be decreased by cutting back on the amount of refining or by using a finer plate to decrease the energy imparted per impact (i.e., intensity of refining). Most deinking processes tend to remove a lot of fines, along with the toner, other ink, and fillers. Recycled mixed office waste fibers need to be handled gently, using low intensity refining; they tend to be somewhat brittle due to the processes that occur during drying, and any new fines created during refining will add to the amount already present.

Let's assume, for the moment, that the papermaking process can be modeled as "simple filtration." This is never completely true, but it helps explain why fines are so harmful to drainage, especially at high basis weights. One can envision an initial layer of fibers forming adjacent to the forming screen and the rest of the dilute furnish being filtered by this initial layer, which then becomes thicker. Any fines in the furnish will tend to be trapped at "choke points" in the structure of the fiber mat. As a consequence, the resistance to drainage increases with increasing fines content, even beyond what would be expected due to the higher surface area per unit mass. One way to overcome this kind of effect is to change the conditions of the forming process so that the "filtration model" no longer applies. Instead, one aims to establish conditions that are more consistent with a "thickening" mechanism of dewatering. For example, a higher level of "action" from increased hydrofoil angles or optimized spacing can continually redisperse the fiber mat. Conditions consistent with thickening generally hurt the retention of fines.

A moderate treatment with an effective retention aid system can help to overcome some of the worst adverse effects of fines on drainage. This can be understood in terms of the filtration model described in the previous paragraph. One of the likely effects of retention aid treatment is attachment of fine materials onto fibers. That keeps the fines from being able to follow drainage channels to "choke points." In general, retention aid treatments tend to make the solids adhere together in a more porous structure, a factor that can help drainage. But be careful; increased retention aid addition usually makes the paper less uniform, i.e. poor formation. The results can be deceiving in terms of dewatering. A floccy sheet usually drains quickly in the initial hydrofoil section of a Fourdrinier paper machine, but it is likely to respond poorly to vacuum in later parts of the forming section. Vacuum will tend to pull air through the thin parts of the web, leaving the fiber flocs wet. You might want to click over to the subject of formation problems if this is a major issue at the facility with which you are dealing.

If you think that maybe you are over-refining, it makes sense to consider backing off on the refining energy and instead relying more on the use of cationic starch or other chemical strategies to achieve dry strength. It is unlikely that dry strength additives ever can take the place of refining, but there often is room to achieve a better balance in terms of (a) less damage to the fibers, (b) better drainage, and (c) less reduction in caliper at a given basis weight.

Entrained air in stock can slow drainage considerably, though there is a lack of good published information obtained from commercial-scale paper machines. The definition of entrained air is that it consists of small bubbles that are carried along by fibers and other solids in the flowing furnish. Small air bubbles can have an effect similar to fiber fines, as mentioned above. That is, they can block drainage channels in the paper web. Entrained air consists partly of tiny bubbles formed by mechanical agitation in such unit operations as hydrocyclone cleaners, and pumps, especially if there are leaky seals. But another part consists of dissolved air that gets released as bubbles when the pressure is suddenly released at the headbox slice. Typical levels of entrained air in various papermaking furnishes are in the range from zero to about eight percent. That's a huge amount, especially if you consider that fibers usually constitute less than 1% of the volume in headbox stock. Furthermore, a typical soda drink is likely to have only about 4% air content by volume after the pressure is released.

The short-term solution to entrained air may be to optimize the defoamer system. Higher dosages are sometimes undesirable due to potential deposit problems and difficulties with hydrophobic sizing agents. Defoamers generally should be added before the hydrocyclones, since that operation has the potential to entrain air. Defoamer in the system also will tend to make the bubbles coalesce so that they are too big to be drawn down with the current in the white water silo to be circulated back to the fan pump.

It has been shown that more stable drainage can be achieved if the entrained air content is automatically controlled. This is done by on-line monitoring of the air content and control of the defoamer flow. One of the attractions of this approach is that it is often possible to run with a rather high set-point of air content. The average dosage of the defoamer can be reduced, relative to manual operation, while still making a quality product at an acceptable speed.

For completeness it also should be noted that air can be reduced to low levels by means of a deaerating system, commonly known as a Deculator, after one of the early inventors. The traditional deaerating system consists of a vacuum chamber at the outlet of a set of primary hydrocyclone cleaners. Recently, a new device known as a "pomp" has been developed to remove air from white water that is being circulated back to a fan pump. The advantage of this approach is that it avoids the flow instabilities caused by variable air content in the fan pump system.

In principle, a very high level of high-mass, soluble polymers ought to increase the viscosity of water enough to impede dewatering. In practice this is seldom a big concern. But let's consider, for a moment, the kind of polymer solution that is often used as a forming medium for wet-laid nonwoven fabrics. It is common in such applications to use a very high dosage of high-mass anionic acrylamide copolymer. A papermaker would call such a material a "retention aid." But in the wet-lay application the dosage is so high, relative to the surface area of the synthetic fibers, that most of the polymer remains in solution. The adverse impact on drainage can be very obvious, depending on the dosages. There is some evidence that anionic retention aids can have a similar effect during certain papermaking operations, though it is difficult to isolate the effect from the impact of the same polymers on formation uniformity - something else that can affect dewatering both positively and negatively. Papermakers who use excessive levels of starch or wet-strength resins will often experience poor dewatering, but that is more likely due to increased stabilization of foam, rather than an increase in solution viscosity.

Drainage - Too Fast

Experienced papermakers usually count "too fast dewatering" as a blessing, since excess dewatering capacity often can be traded for something else that is desirable. For instance, in some cases it is practical to pump more white water to the headbox, lowering the forming consistency, and improving formation uniformity. In other cases excess drainage capacity makes it possible to refine more, achieving a higher tensile strength or internal bond strength of the paper. In other cases excess drainage capacity makes it possible to use a larger proportion of lower-grade fiber, which may have a lower freeness or higher fines content.

If the dewatering rate still is too high, some other strategies to consider include (a) speeding up the paper machine, if possible, (b) decreasing the intensity of dewatering elements such as hydrofoils and vacuum boxes, (c) or omit or reduce the amounts of drainage-promoting chemicals (see earlier discussion).

References

Allen, L. H., and Yaraskavitch, I. M., "Effects of Retention and Drainage Aids on Paper Machine Drainage: A Review," Tappi J. 74 (7): 79 (1991).

Britt, K. W., and Unbehend, J. E., "Water Removal During Sheet Formation," Tappi 63 (4): 67 (1980).

Farinato, R. S., Huang, S. Y., and Hawkins, P., "Polyelectrolyte-Assisted Dewatering," in R. S. Farinato and P. L. Dubin, Eds., Colloid-Polymer Interactions, Wiley-Interscience, New York, 1999, Ch. 1, p. 3.

Raisanen, K. O., Paulapuro, N., and Karrila, S. J., "The Effects of Retention Aids, Drainage Conditions, and Pretreatment of Slurry on High-Vacuum Dewatering: A Laboratory Study," Tappi J. 78 (4): 140 (1995).

Sampson, W. W., "The Interdependence of Sheet Structure and Drainage," Paper Technol. Ind. 38 (8): 45 (1997).

Wegner, T. H., "Effect of Pulping Liquor on Drainage Aid Performance with Recycled Fiber," Tappi J. 70 (1): 100 (1987).

Wildfong, V. J., Genco, J. M., Shands, J. A., and Bousfield, D. W., "Filtration Mechanics of Sheet Forming. Part 1. Apparatus for Determination of Constant-Pressure Filtration Resistance," J. Pulp Paper Sci. 26 (7): 50 (2000).

PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University (m_hubbe@ncsu.edu). Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.


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