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BTG helps Swiss mill boost process stability and additive performance

Installation of a new BTG online charge control system on M-real Biberist’s PM 9 has enhanced retention by up to 5 % and paved the way to more accurate online control of retention aids. Additive dosages have already been cut by 10 - 25 %, with promising prospects of further savings in the future.

Located about 25km north of the Swiss capital, Bern, M-real Biberist produces wood-free coated fine papers for the graphic arts industry and offset printing applications, and wood-free uncoated paper for office, preprint and offset printing.

A member of Finland’s M-real Group, the plant’s PM 9 machine produces base papers with a basis weight range of 60 – 210gsm, but until recently has been suffering from chronic performance problems. The cause: unstable performance of applied additives, due to high levels of anionic trash entering the circuit, together with coated and uncoated broke.

A BTG wet end survey undertaken to identify ways of rectifying the problem revealed considerable scope for optimization of additive performance, particularly in the area of retention aids. That prompted a project aimed at increasing the efficiency of chemical additives, to reduce costs while improving output quality. Project partners included BTG Mütek GmbH, BASF, and post-graduate student Stefan Franke, who developed the project as part of his Diploma thesis at Munich University of Applied Sciences (FH München).

Project preparation

Before commencing the project on-site, the efficiency of the relevant additives and their effects on stock suspension and charge level were investigated in the laboratory using a Mütek^™ PCD-03 Particle Charge Detector and a Mütek^™ SZP-06 System Zeta Potential.

Since lab-scale measurements only cover random samples, and fail to reflect the changes in actual production conditions over time, online measurements were taken for both charge and retention levels for subsequent setup of closed loop systems.

Online charge measurement

FIGURE 1
Charge evolution on PM 9 without charge control and PAC in the HC stock

To begin, a Mütek^™ PCT-20 Charge Analyzer and a Mütek^™ TSS-70 Filtration Sampler were used to identify the charge level at the outflow of the mixing chest. Measurements covered all components of the fibrous suspension before the introduction of additives (starch, AKD, retention aid), and the process was also monitored by measuring white water charge.

Despite the addition of constant dosages of polyaluminium chloride (PAC) to the white water to control deleterious matter, charge level underwent marked fluctuation.

By reacting with the deleterious material, the additives were actually acting as fixatives in non-quantifiable amounts. This in turn destabilized chemical performance, prompting M-real Biberist staff to routinely overdose additives to ensure satisfactory product quality under worst case conditions.

Aside from the cost overhead, this practice also entailed the risk of unwanted side effects in the form of reactions with anionic trash, and resulting negative impacts on machine runnability, such as deposit formation and deteriorating dewatering and retention levels.

Online fixative control

FIGURE 3
Charge evolution on PM 9 with optimized controller

FIGURE 2
Charge evolution during the first charge control trial on PM 9

While the constant addition of fixing agent successfully reduced the level of deleterious matter, it failed to compensate for charge variation. Manual dosage adjustment proved ineffective, because of the delayed response to charge fluctuations.

Against this background, a new closed loop charge control system was implemented to control the dosing of PAC. A conventional PI controller was applied to set the control value for the metering pump.

FIGURE 4
Grade-specific fluctuations of PAC consumption for charge control

After activation of the control loop, the charge level measured in the mixing chest remained stable at the setpoint. A failure of the PAC pump subsequently caused the system to revert to its original state of constant charge fluctuation, but when the metering pump was replaced and the controller optimized, the charge level again re-stabilized in line with targets. Retention values, too, remained even and constant, except during grade changes. Stabilization of the system was also reflected in a PAC dosage trend with no short-term variation.

Variation in the charge level was primarily evident from the changing dosages of PAC. The principal reasons for grade-specific variation in PAC consumption levels were fluctuating basis weights or fibre compositions.

The impact of fixative control

FIGURE 5
Effects of charge variation on retention levels

The principal factors influencing retention include changes in basis weight, machine speed, stock composition and charge level. As interfering substances were fixed and charge control activated, retention was found to improve by 2-5%, depending on the paper grade – a positive trend that also reduced the frequency of undesirable reactions between retention aids and deleterious matter.

The failure of the PAC metering pump saw cationic demand start to rise in the mixing chest and, after a corresponding time interval, in the white water as well. Similarly, over time, overall retention and ash retention decreased by approximately 5 %. After charge level had re-stabilized, retention attained its original level.

Retention measurement

As with charge measurement, laboratory studies into retention failed to reflect short-term changes. The total consistency and the fines/filler content of the stock suspension were determined via online retention measurement using a Mütek^™ RET-20 Retention Inline and two sensors – one installed in the headbox, and one in the white water. The results enabled accurate calculation of total retention and ash retention.

The Mütek^™ RET-20’s ability to provide reliable and precise real-time results made it an excellent process monitoring tool, allowing for trials or process changes to be evaluated very quickly.

Online retention aid control

FIGURE 6
White water consistency with retention control

Automatic dosage control of retention aids was based on the headbox and white water consistencies measured with the Mütek^™ RET-20 Retention Inline. The white water consistency was taken as a controlled variable, because it directly impacts the process and quickly and directly reflects retention variation.

By ensuring appropriate retention aid dosages, retention control quickly reached and maintained the setpoint for the white water consistency. In spite of continual changes in the metering pump flow, the white water consistency kept constant at a setpoint of 0.23 %.

Benefits of retention control in the Process Control System (PCS)

Taking into account the frequency of grade and basis weight changes, a transition programme had already been provided in the PCS which ensured the smooth transition of control values for the metering pumps.

Integrating retention control into the transition programme and the PCS as a whole provided the option of comparing newly-acquired data with other data visualized in the PCS, greatly facilitating rapid identification of potential trouble sources and process disturbances.

Another win for BTG Mütek

The highly successful results obtained on PM 9 has prompted M-real Biberist to go ahead with the installation of a new online measuring system for charge and retention on its PM 8 line. Closed loop control will be introduced as soon as possible, to enable closer observation of the process and optimization of the machine.