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Virtual pulp - bringing reality to predicting quality

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Virtual pulp - bringing reality to predicting quality

March 13, 2011 - 16:00

BRUSSELS, March 14, 2011 (RISI) -Better the devil you know" is often the mantra in paper mills when it comes to changing pulps, and with good reason: testing a new furnish is time consuming and costly, and if you can't predict exactly what the new mix will deliver, it's also risky.

In the past, customers would give Södra a pulp sample to refine and benchmark. It took several weeks to deliver just one result. Yet the need to evolve is constantly pressing, and now more so than ever as papermakers seek optimum results from their furnish.

In response, Södra has developed the virtual refiner, a tool to mix different pulps, put together a recipe and mathematically calculate what is needed to create an optimal furnish for a customer's requirements. Testing and pilot trial costs are all but eliminated, results are practically immediate, and for the first time, a technical manager can present the case for a furnish change to the mill technicians without the need for weeks of expensive testing and waiting.

Figure 1 - Introducing fiber coarseness in tensile index model

Södra Cell UK's managing director, Nigel Holland, explains how the virtual refiner will enable faster decision making: "As birch becomes ever more scarce, for example, customers are increasingly looking at what alternatives there might be. The refiner makes it a lot easier to take the first step. You can easily compare a eucalyptus with a birch in the model; for example, mix them with a softwood, and see how differently they react. In the old days, a pulp salesman would have to persuade the mill to trial a new pulp, but now they won't have to. The customer can see via the model the hard evidence of how a certain furnish will perform."

Minimizing softwood pulp in a customer's furnish might be seen as counter-productive for a softwood producer. "They'll do it with or without us," reflects Kilian Kleinheinz, Södra's technical product manager for pulps sold to speciality pulp producers, "so we may as well be involved. Refining optimisation is always a trade-off and very different fiber properties are required for, say, tear and tensile, but now at least we can see where we have to make the least compromises, the model makes it very easy."

The virtual refiner is a flexible tool to calculate different refining properties from different blends to optimise cost and quality for papermakers. It's the result of combining a planning model, refining curve and Södra's extensive database of pulps. It can measure some 16 properties and produces various flexible beating curves so, for example, a papermaker can view exactly what will happen if a highly beaten Södra softwood is combined with an unbeaten hardwood.

Södra's customers can simply go the PulpServicesOnLine (PSOL) website and tick the virtual refiner. The model requires some input numbers before it can compare up to three different pulp mixes. Users need to think about what range of refining is of interest, what properties they need (everything from strength-related properties such as tensile and tear to air permeance and water retention values are included), as well as the proportion of softwood and hardwood they intend to use. "Maybe you have a Brazilian eucalyptus of a certain species," Kleinheinz explains. "We can take it, analyse it, compare it to another grade and see how it behaves when mixed with a variety of options. What would be the effect on refining if we did x, y or z. If you know what you want, you can put in the tensile target and then calculate possible savings in energy."

Figure 2 - Tensile index variations 2000 PFI

Since the results are based on lab trials, the diagrams give trends in changes but not absolute values, so the absolute values might not correspond to what the papermaker will actually receive. However, for customers wishing to delve deeper than the on-line tool, Södra has a highly sophisticated version of the system with a comprehensive database of pulps which can build a furnish with up to four different components (competitors' pulps are referred to by a code to preserve confidentiality). The model offers separate refining as well as co-refining.

"We really expect this tool to make a difference when it comes to achieving optimum refining," Kilian comments. "It should now be much easier for our customers to make the switch in their furnish to achieve a specific result. It brings the mill a step closer to optimum refining at minimum cost."

Figure 3 - Over a period of time, the system will deliver 10,000 observations online compared with 12 with traditional testing methods

The end of handsheets?

A lack of consistency in paper properties has long been a major issue for the industry; a paper mill needs to know that the fiber it purchases is as consistent as possible if it is to deliver a paper of the highest consistent standard.

For the last decade, Södra has been using on-line fiber analysis in the form of an L&W Fibermaster system to measure the traditional properties of its pulp. In 2005, the company developed an internal model to predict some paper properties using fiber length, shape factor and single fiber strength to predict unrefined tensile index and tensile index and tear index at 2000 PFI. Then in 2007, those predicted values became available to customers via PSOL, making it possible for customers to access quality data for their pulp. At this point, the on-line service was seen as an add-on to run in parallel to more traditional testing. It's based on empirical, linear regressions and describes the pulp's quality in general rather than detail, but still requires traditional manual lab testing. Back then, important properties, such as fiber coarseness and kink index could not be measured on-line, while only a few hand-sheet properties were covered at single beating points.

Recognizing that the correlation between fiber properties and traditional hand-sheet properties such as tensile index needed further investigation, Södra began working on the prediction of fiber and energy qualities more closely, developing a second-generation prediction model under Dag Molteberg at Södra Innovation. In the process, Södra identified the need for a new analyser to measure different fiber properties and a new level of quality control, including high-precision, on-line measurements also for coarseness and kink. Södra is currently installing the new L&W Pulp Tester in all mills. The new model is still empirical but is based on a different modelling method called random coefficient mixed models. It will predict all the properties currently covered by standard hand-sheet testing, in addition to providing a whole beating curve, in the same manner as the virtual refiner. By introducing coarseness in a tensile index model, the degree of explanation increases while the prediction error decreases.

Just as with the virtual refiner, the new model, PulpViz, will remove a whole level of manual testing, saving time and money. Traditional hand-sheet property testing can take over two weeks, during which time a mill lab typically collects five samples which are then sent to a PFI refiner for a further week of testing and refining. The data then have to be sent to the customer, several weeks later.

PulpViz, like the virtual refiner, enables immediate testing and prediction with a high level of accuracy. "We understand the correlation between fiber and paper properties," says Henric Brage, manager process development at Södra Innovation, the company's R&D division. He describes the new model as a tool for working with pulp consistency and a visualisation tool for dialogue with customers. "We sell fiber and we want to talk fiber properties with our customers, not PFI paper properties.

Figure 4 - Mathematical calculation of refining: separate refining (top) and mixed refining (bottom)

"The new language for pulp is fiber properties. Now we will be able to predict the entire beating curve with a new level of detail and explain any variance for our customers. We will provide them with a model explaining why different pulps have specific beating curves, explain what happens to flexibility, thickness, coarseness etc, and we will be able to make quick decisions if we see any variance occurring within our pulp. Customers don't need to rely on predictions because we know our fiber properties. We do a fiber quality control check every six minutes. Once the system is fully up and running, a customer will simply log on to PSOL, enter the pulp he's using and the date it was produced, e.g. the delivery number, invoice number or, if known, unit number, and then access its exact properties."

That means that a paper mill could actually view the specific properties of a pulp about to arrive at the mill before it even gets to stock preparation, eliminating the need for the paper mill to conduct any kind of quality control on the pulp. For example, over a period of time the system will deliver around 10,000 observations online compared with around 12 observations with traditional testing methods.

Figure 5 - Mathematical calculation of furnish

At present, the new model, which will be rolled out to all of Södra's pulp mills, is based on old data. The full quality control system will not be readily available for some months yet, until late 2011 or early 2012, but, promises Brage, it will be worth the wait. Comparison with the old model shows an improvement of RMSE from 3.7 Nm/g to 2.5 Nm/g
for the new model for tensile index at 2000 PFI.

PulpViz will mean that customers can see what happens to the beating curve when fiber properties are changed. The user will be able to change fiber properties one at a time (the properties are selected so that they match Södra's portfolio of pulps), and see the result for two hand sheet properties at the same time. The two hand sheet properties are then plotted against each other in a third graph. Three different sets of fiber properties can also be shown simultaneously.

Brage concludes, "We hope the new system will enable our customers to really make the most of our watch this space".