Benefits, Structure and Operation

Measurements and ApplicationsThe purpose of charge management is, first and foremost, to maintain a stable charge in the short circulation and to keep it at the desired level. One key idea of measuring cationic demand is to find that average pulp charge, which is the most suitable for the used retention system – may it be mosaic, bridging or microparticle mechanism. Cationic demand can be measured from many places in the process. One of the most important places is the short circulation, as this measurement combines the effects of the different pulp types, broke, water circulations, and the various process chemicals. It is very important to know the final charge level of the short circulation after all chemical additions, because it has a direct effect on the paper machine runnability and final product quality.

Wet end chemistry plays a crucial role in water removal, retention and formation – thus affect-ing, more or less directly, numerous paper quality properties and PM runnability. A central factor in the interaction between fibers, fines, fillers, and trash (dissolved and colloidal material) is the charge of each component. The control and stabilization of charge is one cornerstone of modern, efficient process manage-ment.

Charge MeasurementCharge is defined as the ability of the papermaking process to adsorb charged polymer. Charge indicates the total number of charged groups in the pulp. An anionic process has a negative charge, and thus its cationic demand can be measured. The measurement is carried out by titrating the process water with a suitable polymer – using either direct or back-titration – to determine the amount required to reach the neutral point.

Automatic Charge Control

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Wet end charge level is important, because• it influences the retention aid and its effect on process – a very important factor in retention control! – and is thus essential for runnability and paper quality• it plays a crucial role in sizing and dyeing: the efficiency of many chemicals is dependent on charge• wet end charge has a direct impact on whether or not contaminants build up in the process• wet end charge level correlates with center roll release angle – has an effect on runnability• overcationizing the wet end means that runnability and retention are totally lost!White water is good place to measure charge, because the material gone through the wire carries the infor-mation essential for flocculation. Every sudden bigger change in white water charge will disturb also HB consistency and drainage. For automatic control of white water cationic demand the fixing agent addition point can be in blend chest. Another obvious measurement point is the primary source for charge fluctuations on the paper machine, usually coated broke or groundwood line. Controlling only the charge of individual thick stock components is not enough: the quantity of stocks fed to the blend chest changes, too, and this will cause charge variations after the blend chest. Charge measurement after blend chest combines all the information of furnish component quality and quantity changes that affect charge. While many chemicals and fillers are added after the blend chest, white water measurement is needed to ensure proper level of charge in short circulation.

Requirements for charge managementCharge management requires a reliable on-line analyzer: the Valmet WEM. Laboratory analyses are significant only for research purposes, for example when the analysis results are compared to other simultaneous measurements or analyses. For chemical dosage control, laboratory analyses are hopelessly slow and cause unacceptably long delays. Measurements that are used for control purposes must meet extremely hard requirements. The most vital of these are:• Reliable – must remain clean without tedious manual cleaning operations• Repeatable – gives a stable signal, detects changes in the processMany different chemicals are applied to control charge in the process, such as fixing aids PEI, PolyDADMAC and alum. Usually they are injected in several points in the process, prior to retention chemical addition.

Constructing a charge control system

The ideal situation is a slightly anionic system with a stable charge. Especially charge reversal – variation between anionic and cationic – should be avoided.Main steps in the construction of automatic control are:1. Installing the analyzer2. Studying the process, locating the main source of charge variations3. Step tests with chemicals4. Programming the control system5. Start-up of control system & training

Step testsExtremely valuable information is obtained from step tests with the chemical to be used for control. They help to ensure that the chemical dosage equipment operates with sufficient speed and accuracy, and also that the chemical has a clear enough effect on charge. Fig. 1 shows an example of a step test with fixing aid.

Sources of charge disturbancesFig. 2 shows an example from a mill where the amount of groundwood in the process was observed to be the single most important source of charge variations. When the groundwood content increases, so does the charge of white water – the connection between groundwood and cationic demand is very clear.

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Fig. 1. Step test with fixing aid.

Fig. 2. Effect of groundwood dosage on the charge level of short circulation.

Another noteworthy factor is that fixing aid was also added to the groundwood. This chemical was added using a certain percentage per amount of ground-wood. As Fig. 2 shows, this linear dosage proved too small for larger groundwood amounts, as on these occasions the process became highly anionic.

Case: White water charge control, coated fine paper machine• Measurement points: white water, coated broke• Source of charge disturbances: quality and quantity of coated broke• Chemicals: 1 chemical (fixing aid)• Injection points: 2 locations (broke, blend chest) The fixing aid flow to the broke was controlled by the coated broke measurement, using a certain ratio between chemical and coated broke flow. The PM operator may change the ratio by changing the dosage factor from the operator interface. This option is avail-able in order to prepare for situations when the coated broke chest contains e.g. machine broke. Fixing aid to the blend chest is also controlled by the white water charge measurement. A flow diagram of the charge control system is shown on the front page.Figure 3 shows clearly the big difference between manual and control run: automatic control is put on at 42 hour mark, and after this the variation in white water cationic demand decreases considerably. In figure 4 we can see how the control operates in the long run and how it handles the situation when the dosage of coated broke – the main disturbance source – changes. The graph shows that the control is able to keep the white water cationic demand stable all the time, even during the disturbance, by changing the flow of fixing agent to the blend chest. We can also see that the dosage changes the control makes to counteract the situation are nice and smooth. The cationic demand of coated broke has also been controlled separately, and these results are seen in figure 5. At first there is no control, and cationic demand varies from -200 to -500 μekv/l or even more. Automatic control was then put on around the 140 hour mark. The setpoint was at first -250 μekv/l, after some time it was changed first to -200 μekv/l and finally to -150 μekv/l. As the graph shows, the control is able to keep coated broke cationic demand very stable at the given setpoint.

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Fig. 3. Stabilized charge: automatic control has reduced the variation of white water charge level.

Fig. 4. Source of disturbance eliminated: white water charge control has eliminated the primary source of disturbances (coated broke).

Fig. 5. Charge of coated broke with manual and automatic control.

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For more information, contact your local Valmet office. www.valmet.com Specifications in this document are subject to change without notice. Product names in this publication are all trademarks of Valmet Corporation.

Figure 6 shows how controlling the cationic demand of coated broke enhances the results achieved with white water cationic demand control. In the graph, white water cationic demand control is on all the time and coated broke control is switched on around the 40 hour mark; after this point the cationic demand of white water is visibly stabilized. The control aims to stabilize cationic demand in the broke line and short circulation. As the results illustrated by these graphs show, the control improves runnability and helps prevent problems during machine start-ups.

Benefits of Charge Control

• Better runnabilityThe control helps to prevent sudden, large changes in cationic demand and consistency. Swings in charge and consistency result in variations of paper ash, moisture, and strength – these make the web more prone to break. When the charge is kept in the anionic side, problems such as web sticking to stone rolls during machine start-ups are avoided. Broke dosage to the PM must be changed stepwise, depending on the tower level. Charge control means that changes in the quality and amount of coated broke will no more upset the bal-ance of the the short circulation and cause long-term variations in the wet end chemistry. Controlled charge and consistency on the PM also mean longer cleaning intervals. When felt clogging is prevented, and this in turn helps to avoid too frequent felt changes and unplanned shutdowns.

• Chemical savingsAutomatic control only uses the amount of chemical needed to reach the target – not more.

• Optimized use of other chemicals A stable charge in the process prepares opti-mum conditions for the retention chemicals, ensuring that its effect remains good and predictable. When charge control is combined with Valmet RM3 white water consistency control, fewer changes in retention chemical flow are needed. Starch is no more consumed to neutralize anionic trash, and thus it gives the required strength without harmful overdosages.Microbe activity and subsequent sliming can better be prevented when the amount of starch – their main nutrient – in the process is optimized. The dosage of anti-sliming agents and other chemicals can also be kept to the minimum.

• Makes the task of machine operators easierAutomatic control keeps an alert eye on the chemical dosages – flexibly, without fatigue, 24 hours a day, 7 days a week.

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Fig. 6. Effect of coated broke control on white water charge level.