Significant improvements could be made to paper quality by controlling the addition of chemicals at the wet end

Under control at the wet end

Paper machine manufacturers have developed new designs of former and headbox to improve speed and formation, but good wet-end chemistry is still crucial in order to obtain paper with the desired final paper properties. Today, there are numerous chemical suppliers and many different kinds of retention aid chemicals with varied behavior. This diversity has made it important to try and control these chemicals, which are now more aggressive and efficient than they were 10 years ago.

Figure 1 - Control strategy

The quantities, preparation and mixing of chemicals all influence their effectiveness in the process as do the dosage points. These chemicals should be injected in a controlled and intelligent way. Online retention systems could be used to achieve this goal and can provide relevant information about how the process reacts to changes in the furnish or the paper machine.

The use of recycled paper has greatly increased the complexity of the wet end and furnish can change quite dramatically. Papermakers must vary the recycled content according to stock levels and the content of recycled fibers may vary from 0-100%. These changes affect retention and, ultimately, the quality of the paper. If more recycled fibers are added, for example, the chemical balance of the system is upset by increasing conductivity and cationic demand. Under these circumstances it is necessary to increase the amount of retention chemical (polymer) in order to maintain stable paper quality.

Having doubts

Research backed up by field experience has shown that it is possible to control and understand the various phenomena at the wet end, but many papermakers are still skeptical that wet-end control can lead directly to process improvements. A recent practical demonstration at Stora's mill in Hyltebruk, Sweden, could help dispel these doubts. Results from the study show that the wet end of a PM can be controlled by controlling white water consistency.

Although this technique is well known in the industry, especially among instrument companies and chemical suppliers, the study team (Staffan Svensson from Stora Hylte and Magnus Tomicic from MoDo Holmen paper) initially faced some opposition from the mill's production department. According to Tomicic, "They did not believe that we could do it so easily without disrupting the paper machine. They thought we would cause a sheet break on the machine by changing the chemical dosage."

Stora Hylte's PM 3 produces 42-48 g/m newsprint on a vertical Valmet SpeedFormer. The furnish comprises thermomechanical, groundwood, kraft and recycled pulps. The machine runs at 1,200 m/min and has a width of 8.4 meters. Some clay is injected as filler and the polymer is a microparticle system of bentonite and polyacrylamide.

To evaluate the improvement of the paper machine, a PSA-5000 portable retention analyzer from BTG was installed on the PM. The system was calibrated and put in closed loop control (Figure 1).

The system uses standard proportional integral (PI) control and receives batch signals from the on-line retention system. The measurement update time is approximately two minutes, which is sufficient to control the polymer dosage. Paper machine dynamics are fairly slow and it takes some time to equilibrate following a polymer change, so the update time is quick enough for process control purposes.

Batch measurements are more precise and stable than continuous measurements, because the sample is deaerated before being measured. Continuous measurement could be feasible in some applications where there are no air bubbles or flocculation to worry about, but generally speaking batch measurement is the safest way to control the wet end.

The greatest improvement was in production stability. When white water consistency was controlled to a remote set point, the stability of basis weight, moisture and thickness improved by more than 50% (Table 1).

Figure 2 shows the paper machine basis weight, moisture and thickness on a normal day before the polymer loop was closed and the improvement obtained four hours later by closing the polymer loop. Automatic control automatically adjusted the polymer dosage to compensate for broke addition and variation in recycled fibers. The dosage varied from 8-28 l/min depending on the demands of the process.

Figure 2 - BW, moisture and thickness before and after control on PM 3

Figure 3 - PM 3 and PM 4 headbox and white water standard deviation before and after control

Sensitive PM 4

After substantial improvements had been achieved on PM 3, another system was installed on PM 4, which is 8.5 m wide and based on Beloit's Bel Bai Concept III design. It is capable of speeds up to 1,500 m/min and the typical variation in basis weight is between 42-48.8 g/m.

This PM is more sensitive to retention aid changes than PM 3, especially when large amounts of clay are added to control opacity. Controlling the polymer dosage has made it possible to reduce the total consistency of the white water from 8.5 to 7.5 g/l and has significantly stabilized the white water and headbox consistencies.

Figure 3 shows the improvements in white water and headbox achieved with polymer control on PM 3 and PM 4. It proved possible to reduce the standard deviation of the wet-end consistencies by more than 60%. Since the white water is used to dilute the thick stock from the machine chest, the headbox consistency becomes more stable if the white water is controlled to a remote set point.

This change has an obvious impact on retention without compromising formation and drainage on the paper machine. Control could allow the filler content to be increased for the same basis weight, which is a significant advantage in terms of raw material.

Additional benefits

Polymer control on a paper machine offers many direct advantages, but there is more to be gained by monitoring and controlling the white water. Besides paper machine stabilization, both PM 3 and PM 4 reported at least a 30% improvement in the clear filtrate stream because the disk filter operates more efficiently with a stable white water supply. This means that the clear filtrate used to wash the wire does so more efficiently and is less likely to clog the spray nozzles. This results in better wire cleaning and more efficient use of cleaning capacity.

A more stable clear filtrate and white water could also help in the wastewater treatment plant. Most mills produce too much used water, which must be treated in the treatment plant. The control of the white water would stabilize the mill's chemical oxygen demand (COD) and as COD control is expensive, a cleaner filtrate would save on costs.