Focus on Wet Degassing
Wet end with active degassing
LC Paper Mill in Besalú, Spain, has installed a complete POM compact wet end system in a new Crescent former tissue machine. It offers fast grade changes in coloured grades. Degassing has been tested as an afterthought
By Dr Topi Helle, Juha Iso-Herttua, Joan Vila & Carles Jord
LC Paper’s new machine is 2.7 m wide and can make 35 000 tons/yr of coloured grades of tissue. It is fitted with a compact POM system, which offers small system volume for fast grade changes. The company decided to run mill trials to find out the effects of compact wet end with effective degassing against conventional wet end. Normally tissue machines are not equipped with degassing. The hypothesis of this work was that degassing would bring enough positive results to justify the additional investment cost of the degassing, even for a tissue machine producing standard grades. Studies were performed using different levels of air in the headbox furnish.
The entrained air in papermaking normally consists of several gases and water vapour. Gas is also developed from microbiological and chemical reactions, such as breakdown of calcium carbonate filler into carbon dioxide at acid pH. The different gases dissolve in fibre-water suspension at concentrations related to their solubility. In this study air refers to any gas which is present in the papermaking stock.
In fibre suspensions air exists in three forms: free air, bound air and dissolved air. Free air consists of large air bubbles which tend to rise to the surface as foam. Free air can also become bound to fibres through mechanical action such as pumping. Bound air consists of small air bubbles which are tightly attached to fibres and attract fibres to form fibre bundles. Free and bound air are together called entrained air. Dissolved air has no known effect on the papermaking characteristics, as long as the air remains dissolved. It can convert to free and bound air if the temperature and pressure of the fibre suspension change.
Air has many detrimental effects on paper properties and papermaking processes. The most common effect of air is surface foam, which causes foam spots in the sheet. Furthermore, wet web strength and dry strength are lower than for sheets formed from deaerated stock. Air can also reduce the drainage rate on the wire and lead to poorer formation. In addition, air in the fibre suspension causes energy losses at the pumps and pressure pulses in the paper machine approach flow system.
Deaeration is used to avoid the adverse effects of air and other gases in the paper machine. Air can be removed either chemically or mechanically. Good process design, including pipeline design and stock composition, and the right temperature, pH and pressure will prevent aeration of the fibre suspension. Although this reduces the need for deaeration, some mechanical deaeration is still required. Some free air is removed in the wire pit, but for complete air removal a vacuum deaerator or POMp degasser is used. Normally tissue machines only have a wire pit or flume for the degassing. The main reason for this is that the cost of a vacuum deaerator for a tissue machine would be very high due to the low consistency in the headbox, which makes the slice flow very high. The vacuum deaerator would be very big in size compared to the production capacity of the tissue machine.
COMPACT WET END
POM compact wet end system is a simple and compact solution for an effective wet end process. Material flows are controlled directly and system is hydraulically closed with minimal open surfaces. In addition air is removed right after the wire sector, which makes the whole short circulation airless (Figure 1).
The Compact wet end is easy to operate and fast to control during grade changes and start-ups. It has fewer sources of potential disturbance than the complex and large volume conventional system. Over to 70 installations running in all kinds of paper and board machines have proved the system to give savings in broke, fresh water, chemicals and energy. When the paper machine approach flow system is designed as compact as possible the entrained air has to be removed from the process immediately at is origin, ie where the white water exits the former.
In the POM System the degassing is accomplished by the POMp degasser, which uses centrifugal force for the degassing (Figure 2). Entrained air is removed with the help of high centrifugal force by pressing the water against a rotating drum, which releases the gas bubbles from the solids and water.The degassed white water is then fed directly to a hydraulic, pressurised distribution system without any open tanks. Application of vacuum allows the dissolved air to be released and removed. The POMp itself is designed for stability and low pulsation.
The overflow water from the POMp degasser and also the suction waters are led to a Cyclopipe. In the Cyclopipe the white water flows tangentially and large air bubbles are removed in order to pump the water to the POM header. The header together with overflow tank forms a pressurized stock distribution system from which white water can be selected according to its material content, ie the richest water is used closest to the headbox and only the cleanest is led to the long circulation and in the fibre recovery.
The POMix compact stock processor is a small volume, high intensity mixing vessel, in one or several stages. In this tissue installation there are two compact stock processors, both with pre-mixing tank for colouring. One stock processor is for the short fibre line and the other one is for the long fibre line. POMix allows multiple stock furnishes and wet end additives to be mixed and blended in a compact unit. From the POMix the stock is fed to the machine by controlling the bone dry stock flow. The POMix is designed for quick and accurate grade changes, and substitutes both the conventional machine and mixing chests (Figure 3).
A three-day mill trial was run on LC Paper’s PM3. In the trial the air content was changed to see the effect of increased air content to paper making, while all the control parameters were kept as constant as possible. The machine ran the same grade (quality and basis weight) during the trial (Table 1).
Air content was measured with an expansion method. The air content was changed by changing the POMp degasser’s rotation speed in order to compare the situation without effective degassing.
RESULTS AND DISCUSSIONS
The headbox entrained air content was increased from 0.03% to about 1.0% during Day 2 (Figure 4). On Day 3 the headbox air content was kept at about 0.4%. At the white water tray before the degasser the entrained air content was about 9.0% and dissolved air content about 2.0 %. In this trial the degasser did not effectively remove dissolved air.
However the POMp degasser could remove effectively dissolved air, if a higher vacuum were used inside the degasser.
Consistency was measured optically online with Lange SC1000, which is sensitive to air bubbles. However the pressure was above 2 bar at the measurement location, which should dissolve the air bubbles. Dissolved air does not affect optical consistency measurement. First-pass retention fell from about 89% to about 78% with the increasing air content.Wire water consistency fell from 434 mg/l to 226 mg/l, with effective degassing. The white water consistency after fibre recovery was 70 mg/l with high air content and only 25 mg/l with effective degassing.
In previous studies with a fourdriner pilot paper machine, the retention has also been decreased with increasing air content. In those studies the paper machine speed was increased with air removal, which is not the case in these tissue machine trials. Also the pilot paper machine trials with a gap former with newsprint furnish has shown no obvious effect on the paper machine speed. Probably the turbulence in a crescent former or a gap former is so high that the small air bubbles do not much affect water removal at the wire. Paper machine speed is clearly decreased with increasing air content in a fourdriner former, which is probably because air bubbles block the wire and increase turbulence, which impairs the water removal at the former. According to the mill production people the paper machine had runnability problems due to the high air content (entrained air 1.0%) at the end of Day 2. Bad runnability with several breaks was believed to be caused by low firstpass retention because of high air content. Air also affected the system stability. Pressure variation was clearly increased in the headbox. The negative effect of air on wet-end stability is well known.
During the mill trial the paper produced was within the normal quality limits and was sold to customers. From each production reel the standard paper analyses were made, such as basis weight, tensile strength, thickness, moisture, brightness and whiteness. The tensile strength and formation variations were slightly increased. Formation variation, which was indicated statistically with t-test, is shown in Figure 5. Paper formation was very good with degassing, when visually analyzed. No other clear differences in paper properties were found.
Because of some improvement in stability with degassing, such as pressure variation, some more improvements in paper properties would be expected. Headbox consistency is very low in tissue (0.2%) and the paper is very porous. Probably because of that we cannot find more clear improvements in paper properties.
Water consumption of the tissue machine was 2.5 m3/ton, which is well below the estimated water use of 4.0 m3/ton of a conventional wet end. 1.0 m3/ton was saved with vacuum blowers, which do not use sealing water. 0.5 m3/t is believed to be saved from the use of fresh water showers by the POM system cleanliness and improved retention due to the compact and airless wet end. In addition there is potential for even higher fresh water savings due to the fast grade changes and paper machine start-ups which will reduce the broke amount.
It appears that POM compact wet end with effective degassing has several benefits in tissue making. The following conclusions can be drawn from the research results obtained from the mill trials, in which the entrained air content was increased:
• The approach flow system is over three times faster than a conventional system. The reason is the small system volume of the compact wet end.
• The Compact wet end is more stable than a conventional wet end. The headbox pressure shows less variation. Air bubbles appear to disturb pumping.
• Paper formation and tensile strength had less variation.
However there were no other clear changes in the paper properties. This was probably due to the low headbox consistency and high paper porosity.
• First-pass retention was improved by over 10%. This is probably due to the improved drainage with effective degassing. Air bubbles can block the wire an increase turbulence.
• Water consumption is estimated to be reduced by over 10% with the POM system. This is probably because of the cleanliness and improved retention of the Compact and airless wet end. In addition there is potential for even higher fresh water savings due to fast grade changes and paper machine startups. According to these results it seems that the Compact wet end and effective degassing have several benefits in tissue paper making, eg fast grade changes, improved stability and enhanced retention and cleanliness with reduced water use. These benefits are obvious for tissue machines with several grade changes; however they should be interesting also in tissue machines making standard tissue.
The authors wish to thank the paper machine personnel at LC Paper for the mill trial. We would also like to thank Professor Hannu Paulapuro and Dr Eero Hiltunen from Helsinki University of Technology for valuable discussions during the research.
Topi Helle is sales director, process and equipment solutions with Aikawa Fiber Technologies (AFT) Oy, Finland.. Juha Iso Herttua is project manager with the same company. Joan Vila is director general of LC Paper based in Gerona, Spain. Carlos Jord is JC’s production manager.