FOCUS:

Mill experience shows use of compact systems provides faster grade changes, as well as easier and cleaner operation.

By PAUL OLOF MEINANDER, JOHN CUTTS, MARIKA KONTKANEN, and RISTO NYKÄNEN

Compact Wet End Designs Deliver More Agile Papermaking Operations

PAUL OLOF MEINANDER
is president, POM Technology Oy Ab, Helsinki, Finland;

JOHN CUTTS
is president, POM Technology Americas Inc, Birmingham, Ala.;

MARIKA KONTKANEN
is application engineer and RISTO NYKÄNEN is process engineer, POM Technology Oy Ab, Helsinki, Finland.

SINCE the concept of compact wet end systems for paper machines was introduced at the TAPPI Papermakers' Conference in 1993,1 the idea has been further developed and now implemented into practice.2 While some problems were initially anticipated, the concept has—in addition to meeting the original objective of offering agility—proven to offer a number of unanticipated advantages.

With seven systems delivered and four in operation, it has been concluded that a compact, hydraulically-closed system is more stable, rather than more unstable, compared to a conventional system with free surfaces in tanks, towers, or vacuum deaerators. There is also confirmation that centrifugal degassing of the white water is sufficient to provide the gaslessness required for quality papermaking.

The hydraulic compact system stays cleaner than a system with large volumes, open surfaces, and slow flow. It also offers a simplification which saves energy and investment. The general trend among paper machinery suppliers throughout the world confirms this concept, and major equipment suppliers now offer solutions for compact wet end systems.

THE COMPACT WET END. Papermaking agility demands a fast response, including fast and exact adjustment in process parameters or stock formulation after any change. Therefore, it is obvious that changes should be done as close to the paper machine as possible. It is also obvious that for fast and exact adjustment, the volume of circulation material should be as small as possible. One example of a compact wet end system—referred to as the POM Concept and shown in Figure 1—shows how these criteria can be met.

Stock composition: Stock is composed in a compact stock processor.3 To have a fast and accurate proportioning of the components, the metering is preferably done on a bone dry basis, i.e., controlling the product consistency times volumetric flow of each component. With modern, accurate consistency gauges, this is feasible. Consistency control by dilution before metering is slow and would introduce one additional insecurity.

An advantage of bone dry control is that dilution for consistency control may be eliminated and consistencies increased. This contributes to a better materials and water balance in the system. If less water is introduced with the thick stock, then the amount of excess water discharged will decrease.

Cleaner system: Cleaner systems are often over-sized, especially in connection with vacuum-deaeration, where a part of the accepts are recycled over an overflow weir. In a common double dilution system, the cleaners are usually sized smaller than the smallest headbox flow, which may lead to high consistencies and poor cleaning.

A proposal to solve this problem is a flexible cascade4 (a patented and trademarked concept), which may be sized for optimum cleaning consistency in most process conditions (Figure 2). Even if there are many variations, in a typical case, the first stage cleaners would be sized for an accept flow corresponding to the minimum predicted headbox flow or for an optimum cleaning consistency.

If the headbox flow (or, actually, the machine screen inlet flow) is larger, the headbox will draw accepts from the second cleaner stage, reducing the consistency in the first stage. If the required flow is larger than the combined first and second stage accepts, the balance will be drawn from the white water distribution. Therefore, the cleaners flexibly change working mode from a full cascade with recycling to combining first and second stage rejects in a so-called "open cascade," and then further to a double dilution open cascade.

White water distribution: This is made possible by the compact hydraulic white water distribution system,5 which is the core of the compact wet end process shown in Figure 1. The drained white water is fed to centrifugal degassing pumps which remove all entrained gas from the water.

By applying suction to the pumps, even a part of dissolved gasses are released and removed. Thus, the gas is removed before the water is pumped into the hydraulically-closed process.

This process has a couple of positive consequences. First, there is no entrained gas left to be dissolved at a higher pressure, and, consequently, there will be less dissolved air in the process. Second, there is no compressible gas present in the system to destabilize pumping and amplify pulsations. Also, biological activity seems to decrease, and there is less tendency for foaming.

The white water is then fed over the hydraulic distribution system to the respective dilution points in a manner considering the different solids contents of the different white water fractions. The water with the highest solids content is taken closest to the headbox, and only the leanest fraction goes to the overflow.

In the closed system, water may be conducted so that no or little mixing occurs, and hence the overflow has the lowest possible solids. Combining the low solids with a low overflow volume, given by increased thick-stock consistency, yields a significant reduction in the amount of solids lost to the long circulation.

Degassing pump: A degassing pump6 combines the features of a degassing centrifuge and a centrifugal pump and is designed for automatically adjusting to the prevailing process conditions (Figure 3). The inlet is controlled by a level control of the inlet vessel. The entering water is fed through a showel-wheel, accelerating the water in a spiral movement along a degassing drum. Due to the centrifugal forces, the water is pressed against the drum, and the air is separated and moves toward an open air column in the center of the drum. From there it exits through an air outlet pipe.

The drum is designed as a function of flow capacity and centrifugal force so that at the outlet end all gas bubbles exceeding a certain diameter will have time to exit. At the end of the drum, the water is pressed into the white water distribution system through an impeller and a pumping spiral. Whereas the greatest portion of the pumping energy is introduced to the water by the inlet showels, the number of vanes in the impeller is high (approximately 30 to 40), which, at 600 rpm, gives a very high vane frequency (300 Hz to 400 Hz). Therefore, the degassing pump does not cause disturbing pulsations in the system.

The degassing pump removes all air bubbles up to a critical size.7 When sizing it for a critical bubble diameter of 40 µm, practically all entrained gas will be removed. By applying vacuum to the gas column in the center of the degassing pump, the bubbles may expand, which further improves the degassing.8 Applying vacuum also causes a part of the dissolved gasses to be removed.

Depending on the capacity required, the degassing pumps may be built vertically or horizontally, with the vertical type particularly well suited for major paper machines with high formers and basements.

Suction sealing: In many paper mills, the sealing pit is the dirtiest and most mismanaged part of the system. A solution is either a closed sealing system9 or individual water locks with visible overflows to a channel leading to a degassing pump. In some cases a normal centrifugal pump may be used.

The closed system is equipped with a buffer vessel, the level of which is controlled by the degassing pump. Air collecting at the top of the sealing collector rises into the buffer through the first pipe. Less air-containing and heavier water sinks through the second pipe, granting a circulation of water in the vessel.

The late Lars Mikander, an experienced papermaker, used to demand that all flows of a system be visible for control. This requirement is met in an alternative sealing solution where every suction pipe has its own water lock, overflowing into a common channel. This solution has proven to be a good instrument for allowing the machine tenders to adjust their vacuums.

RESULTS OF THE TECHNOLOGY. In retrospect, it is easy to understand why the compact process yields good results. A decade ago, when the ideas were taking shape, there were doubts regarding controllability, stability, as well as other various issues. Even now not every aspect of the concept can be explained scientifically, but the results have been overwhelmingly positive. Some of the results and their practical impacts are discussed below.

Deaeration efficiency: Centrifugal deaeration has proven to be excellent. Entrained air has been eliminated up to 99.9%, and drainage improvements up to 20.0% have been reached due to reduced air content in the headbox.

Initially, it was feared that centrifugal degassing—removing only the entrained gasses—would not be sufficient. Especially removing the gas from the white water only could have left a problem with the gas introduced with non-degassed thick stock. When starting up the first installation at MD Albbruck's No. 7 paper machine, where centrifugal white water degassing was substituted for vacuum deaeration, the content of entrained gasses in the headbox approach was actually as low or lower than with vacuum deaeration at less than 0.02% measured by compression. In other cases, when applying vacuum in the degassing pumps, the levels of dissolved gas decreased.

The solubility of gas is pressure dependent, and the dissolved gas content can also be expressed as the partial pressure of the dissolved gasses. With centrifugal degassing, the entrained gasses are removed at atmospheric pressure or even under a modest vacuum, which leaves little dissolved gasses in the degassed water. Thus, there is still space for dissolving gas carried by the thick stock.

The amount of gas acceptable in papermaking certainly depends on the particular mill case. However, the technology discussed in this article has been shown to reach levels of gaslessness well in line with most stringent demands of quality papermaking.

Paper quality: As is well known, since the introduction of vacuum deaerators, airlessness improves paper quality. Centrifugal deaeration has shown the same effects, improving dewatering capacity up to 20% and eliminating problems with free spots and pinholes usually caused by foam and entrained air. In one case, this contributed to a more than 12% improvement in formation.

Centrifugal degassing before the first pressure increase in the cycle decreases the risk of collapsing air bubbles forming dark or sticky spots. The mechanism is that the air bubbles collect hydrophobic material on their surface, which is compressed to an agglomerate when the bubble is compressed and ultimately collapses.

Process volume and response times: In all cases where the compact wet end system was installed, considerable volume reductions were reached, reducing the short circulation volume by more than half. The thick stock volume of a mechanical coating base machine was reduced by more than 95%; the thin stock and white water system by more than 80%. The process response time was shortened in the same proportions.

Examples of improvements compared with the original wet end design comprise 65% reduction in response time at color bump tests when the system volume was reduced to one-third of the original. In one case, an 85% reduction in color changing time was reached, and color changes are now done without cleaning or bleaching the wet end system, which was necessary before.

Process stability: Contrary to doubts, the wet end stability has remained as good or better than before. In one case, wet end breaks were reduced by 80% after installing a compact wet end system. Other factors may have played a role, as well.

Generally, no disturbing pulsations have been observed, and machine direction basis-weight profiles are as good as or better than previous profiles. This can be understood when looking at the open surfaces in conventional systems. More often than not, they show swinging or large scale turbulence, which is not possible in a closed piping system with defined flow patterns.

Also, the influence of air in the process pumps is decisively reduced when the white water is degassed before pumping. In addition, the system is less prone to swinging when the compressible gasses have been removed from the system.

Finally, it has been determined that a heavy white water system cannot stabilize the core process. In the process, the white water is always lagging behind the core process and can, at most, delay reaching a new equilibrium after a disturbance has already occurred.

All this makes the compact process easy to handle. A machine tender at one North American installation commented that he was happy about the easiness with which the process responds. "I adjust what I want, and the machine just follows," he said.

System cleanliness: At Albbruck, the interval between boilouts was extended from six to twelve weeks, and a mill producing colored paper reports that it does not need to dump the water between colors and does not bleach the system anymore.

One of the reasons for the improved cleanliness may be due to less formation of agglomerates, as explained above. A main reason, however, would be that in a system without tanks or stagnant waters, there is no place where precipitation of material would happen, and continuous and controlled flow keeps the system clean. There are also no or few free surfaces where foam would build up and cause dirt, and the reduced air content may reduce biological slime buildup.

Energy consumption: Degassing pumps consume more energy than ordinary ones. The energy introduced into the white water will, however, be retained in the system, which minimizes that disadvantage. Further, a compact wet end process eliminates many tanks, pumps, level controls, mixers, and other pieces of equipment. Also, the dimensions of the cleaner system and some other equipment may be reduced.

Therefore, the power consumption of the compact system clearly will be lower than that of an ordinary one. If the savings due to less waste and time losses are considered, the energy consumption per net production is significantly decreased.

When a vacuum tank deaeration is avoided or eliminated, already the power consumption for the impact feeding and vacuum pump will compensate for the additional power consumption of the degassing pumps. At Albbruck, the wet end-process energy consumption was reduced by 25%—from 700 kW to 530 kW—due to elimination of pumps and tanks when the vacuum deaeration was also eliminated.

Materials balance: In a compact hydraulic system, white water fractions are kept separate until the point where they are used. Selecting the white water with the very lowest solids content for overflowing to the long circulation and also reducing the overflow volume with higher thick stock consistencies can yield great savings in fiber losses.

In one case, the water discharged was so clear that it did not make sense to pass it over a fiber recovery screen. This will not be the case everywhere, but, in most cases, the capacity of a fiber recovery plant could be smaller than usual.

Additives consumption: One would expect that a lot of additives might be saved by using a more compact and deaerated system. Some savings are reported, but, generally, they are less significant. Boil-out chemicals consumption is, of course, reduced.

Slimicides and defoamers can similarly be saved, whereas some deaeration chemicals still seem to be necessary. Albbruck reports a 50% reduction of slimicide consumption together with changing to a 50% less toxic slimicide. Shorter dwell time in the short circulation should have an influence on hydrolysis of sizing agents and some other chemicals, but this has not been studied.

All in all, chemical consumption has been marginally reduced, mainly due to better system cleanliness and smaller systems.

Paper machine efficiency: Together, the effects of the compact system have yielded an improved overall paper machine efficiency, which, in a case involving many color changes, was reported to be 9.5%. Some other general efficiency improvements in trials have included:

REFERENCES
1. Paul Olof Meinander, "An Approach to Just On Time Papermaking," Proceedings of the 1993 TAPPI Papermakers' Conference.
2. Paul Olof Meinander and Lars-Hugo Olsson, "Compact, Airless Wet End System" Proceedings from TAPPI å99, TAPPI Press, pp. 1213-1218.
3. Paul Olof Meinander, "Apparatus and Process for Feeding Stock to a Papermachine," PCT Patent application, PCT/FI96/00052 (Jan 25, 1996).
4. Paul Olof Meinander, Risto Nykänen, and Juha Lahti, "Arrangemang för Rening av Pappersmassa," Finnish patent application, FI-P-20000940/19.4.2000.
5. Paul Olof Meinander, "Process and Apparatus for Circulating Backwater in a Papermachine," U.S. Patent 5,567,278 (Oct 22, 1996), FI Patent 89728 (July 30, 1993, Prior: May 19, 1993, FI).
6. Paul Olof Meinander, "Apparatus and Process for Pumping and Separating a Gas and a Liquid," U.S. Patent 5,861,052; European Patent 0735913, (Prior. Dec 23, 1993, FI).
7. Topi Helle, "The Behaviour of Air in the Fibre Suspension," Helsinki University of Technology, Forest Products Technology Department, Laboratory for Paper Technology, Otaniemi 1997.
8. Topi-Matti Helle, Paul Olof Meinander, and Hannu Paulapuro, "Removal of Entrained Air From White Water by Application of Centrifugal Force," Paperi ja Puu—Paper and Timber, Vol. 80, No. 5, pp. 379-382.
9. Paul Olof Meinander, Förfarande och Arrangemang vid Sugelement, Finnish patent application, FI-P-20000938/19.4.2000.

This article is based on a presentation at the 2001 TAPPI Papermakers' Conference, Philadelphia, Pa.