Variable chips make uniform pulp - Part II

By Tapani Poranen, Product Manager, Fiber Line Controls, Metso, Tampere, Finland and Mark Williamson, Journalist Engineer, Thornhill, ON, Canada

BRUSSELS, Aug. 15, 2011 (RISI) - Variations in wood chip quality is one the most important disturbances that affect the kraft pulp digesting process. Using a continuous digester system to even out these disturbances, Cenibra is able to stabilize pulp quality, inrease yeilds and reduce overall costs. Read Part I here.

Advanced pulp quality control

Once the chip flow, the alkali to wood ratios throughout the digester and liquor balances are under control it is possible to control the delignification process throughout the entire cooking process. This is done by optimizing the chemical reaction rate to produce pulp with a steady Kappa number at the blow line. The chemical reaction rate is controlled by managing the temperature profile of the digester and by ensuring an optimal chemical profile through the process. Disturbances introduced by chip quality variation can therefore be accommodated so an even quality pulp is produced.

This is accomplished by using a chemistry-based process model which takes into account chemical dosage, residence times and temperatures. H-factor is calculated from the residence time and temperature history of the chips as they proceed through the various zones of the digester. Since the digester's temperature profile varies from zone to zone, the total H-factor is calculated using all available and representative temperature measurements, Fig. 2. The temperature profile and H-factor are effectively controlled by calculating temperature targets for digester top (steam phase) and cooking and homogenization circulations.

Figure 2 - The H-factor is controlled in the following zones: digester top, cooking circulation screens and homogenization circulation screens

Once H-factor stability is established over the entire cooking process, the cooking conditions are suitable for a steady chemical reaction rate. However, chip quality at many pulp mills like Cenibra's can vary significantly, thereby affecting the degree of penetration of liquor into the chips and the completeness of the delignification. An even pulp quality can be achieved if the cooking model parameters are adapted to suit chip cooking conditions by feedback measurements of pulp Kappa number from automated analyzers on both lines. Updates are automatically available every 15 minutes, thereby providing a fast feedback of delignification rates and providing tighter control. The H-factor target is regularly updated by these Kappa measurements using a patented method, Fig. 3.

Figure 3 - The online reading of Kappa is used to regularly update the Kappa control model parameters with a patented method

Both fiberline 1 and fiberline 2 are equipped with Metso's automated on-line pulp quality sampling analyzers. In line 1, a kajaaniKAPPA analyzer samples after the digester blow, before and after the 0₂ delignification stage, before a DualD(hot) stage and after the EP stage in the bleach line. In line 2, there are two sampling analyzers - a kajaaniKAPPAi and a kajaaniKappaBrite -- which measure Kappa after the digester, Kappa before and after the 0₂ delignification stage, Kappa before the Dhot stage, Kappa and brightness after two parallel EP stages, and pulp brightness after the D₁ stage and after the final P stage.

The APC system also ensures steady pulping operation and minimum deviation from target by an automated production rate change management. The timing of changes to the process and their time ramps are coordinated to make sure that off-quality pulp is minimized. Chip feed rates, liquor feed and cooking variables are ramped automatically at the right time to new target values, thereby avoiding human errors, Fig. 4.

Figure 4 - During a production rate change, the timing of changes to the pulping process and the ramp rates are coordinated to make sure that off-quality pulp is minimized

New operation practices

The successful implementation of the APC control on both lines required a high level of cooperation between Metso and Cenibra staff. The main challenge for the control engineers was to adapt the digester temperature controls when the quality of the wood chips has changed. This has required very large target changes compared to the normal running situation. Of course, these new operating methods must be learned and accepted.

The new controls and the stability they provide have had a significant impact on the process operators and how they manage the cooking process. Before APC, the operators were changing the cooking temperatures in production rate situations and using rules of thumb for adjusting temperature to get the right Kappa. Then, all changes were operator specific and mistakes were sometimes made. Now the mCooking system is fully automatic and based on absolute targets and programmed responses.

Figueirêdo explains that operators' roles have changed, "The digester operator's main concern was the Kappa number; before he had to control it closely, but not today. He has just to make a few adjustments and supervise the process. There is a mathematical pattern behind it which is making the necessary corrections. Therefore, the operator interferes very little in the process and takes action only at critical situations."

At the optimum point

After commissioning and tuning of the advanced control systems, a performance test was run to validate Metso guarantees. The goal was to achieve a Rate Performance Achievement - a so-called hit rate -- over 65%. This means that 65% of the time blow line Kappa readings were within an optimal range of operation specified by Cenibra. The performance test indicated that the hit rate for fiber line 1 was 69% resulting in improvement of 93% in terms of variability. In fiber line 2, 63% of the values were within the target control range, representing a 34% improvement in variability. Figures 5 and 6 show Kappa number variability before and after the implementation of APC's in fiberlines one and two respectively. Figure 7 shows the Kappa control target hit rate during the baseline period, during the evaluation period in 2009 and to the end of 2010. The control target compliance numbers of both fiberlines show a significant increase from 2008.

Figure 5 and 6 - Kappa number variability histograms before and after the implementation of APC's on fiberlines 1 (top) and 2 (bottom)

This significant reduction of variability makes it possible to increase Kappa target, thereby increasing efficiency by reducing specific wood consumption or increasing pulp production. Particularly in cooking plant 1, it was possible to increase Kappa number target by reducing process variability provided by mCooking.

With the reduced variability, the process can now be run closer to its optimal point. This opens up the possibility to run consistently at higher Kappa numbers with higher yield and reduced specific wood consumption. Cenibra also sees the opportunity to reduce liquor and steam consumption. These gains fully justify the project financially.

Brandão explains that with help of mCooking they were able to stabilize the cooking process. Now, the pulp quality is better after the digester and the guaranteed results were achieved 100%.

He sums up the results so far, "We had an average Kappa of 15, and our objective was to reach 17. It is extremely important because so you can much better utilize the wood. With a solid partnership between the specialists from Metso and Cenibra's team, we got very good results, which were much better than expected. Today, we can work with Kappa 16.5, and it is working beautifully," says Brandão.

Figure 7 - Kappa control target "hit rate" during the baseline period in 2008, during the evaluation period in 2009, and to the end of 2010. The control target compliance numbers of both fiberlines show a significant increase from 2008

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