Predictive soft sensors could help operators to zero in on key operating windows while quality controls move upstream.
Papermaking multivariable predictive controls (MPCs) or so-called advanced controls have matured; they are proven, robust and provide continuing results. But potential new applications are still a work in progress, and much more can be expected.
Just ask papermakers how they do their jobs and you will see how multi-dimensional and intuitive they are. In this respect, advanced controls are a good fit since they are suited to a continuous process where the change in one setting affects several others and several settings can be changed at the right times and by the right amounts to achieve the best result. That's papermaking! In fact, process operators with a depth of experience use their own multivariable control instincts. Even so, from shift to shift operators may control the process differently, with personal, favorite settings and varying results. On the other hand, the controls do the same job day in and day out, with seamless shift changes. In short, they are consistent.
Predictive controls have been around for quite a while. One can argue reasonably that feedforward basis weight control introduced in the 1970s was the first. It is a simple form, where the concept of a basis weight soft sensor was born. Soft sensors today are based on the same idea, but can be more complex multi-input calculations.
Today, MPCs are a standard part of quality control systems. They are well suited to CD controls where multiple profilers interact with each other. Wet end and dry end controls have been merged using multivariable techniques, and they have been well established for over a decade. The strength is apparent when the papermaking process is in transition - in the unsteady state during machine startups, after long web breaks when broke recycle is increasing, and during grade changes. The investment payback is made when recoveries are quick, repetitive breaks are avoided, and waste is reduced.
Drying with multiple heat transfer mechanisms and different sources of energy is another important application for the waste and cost reduction potential. This is particularly true in tissue drying where natural gas, steam and electrical energy must be balanced and used judiciously to run a machine with the lowest costs while maintaining quality and efficiency. As you can see in the control response matrix below there can be quite a few manipulated and controlled variables. This concept is applicable to drying of paper and board coating, as well.
Predictive control has found an application in paper brightness and shade control where there are long time delays and chemical reaction latencies from pulp storage to the machine reel. In this situation, quality control has moved upstream. Valmet has implemented a strategy in which soft sensors predict paper brightness at chemical addition points well before it is measured by the QCS. This saves a considerable amount of bleaching chemicals.
Today, there are established MPC applications, but there could be and really should be more to come, either being worked on now or in the future. Let's take a look into what could be developed over the course of time:
Strength prediction and stock prep management
Valmet has developed a soft sensor-based technique for dry sheet strength prediction which signals a significant shift in the focus of paper quality management into the stock preparation area. The prediction is derived from measurements of fiber furnish properties, including fibrillation, by an online pulp fiber analyzer. Presumably, some sort of modeled refining control could be the next step. This is definitely a technology to watch.
Sheet release and draw management
The only open draw in the wet end of many modern machines occurs when the sheet releases from a center press roll. At this point the delicate sheet is exposed to a variety of forces including its own weight and velocity, air pressure balance and adhesion to the center roll. To achieve the lowest stress on the sheet, hence the best runnability and speed, these forces must be in perfect balance. Models of how these forces interact have been developed.
A major WFU machine at UPM's Kymi mill in Finland is equipped with a variety of measurements which give papermakers a window into this area, plus an opportunity exert some control over it. Average moisture and CD profiles are measured after the press section by an electronically-scanned sensor beam, cationic charge demand by an online analyzer and release point by a laser sensor in the press run. Papermakers are very aware of changes in release points and draws, and make adjustments to stay within a narrow operating window. These measurements have shown how the release point varies with sheet moisture and charge, and that is where MPC could fit in. Draws influence final sheet tests like strength and porosity so they could be another part of the multivariable puzzle.
Instead of a laser sensor a web break camera system has been used in other paper machines to provide the operators with an image of the release point from a distance where it is easier to keep the camera clean. For release point applications, the pixel image map would be calibrated.
Drainage and dry line management
Stable drainage (or dry line in open-wire formers) is particularly important during grade changes since it has an impact on sheet formation, runnability, drying limitations and ply bonding. It is another unsteady state process which could benefit from soft sensors and MPC. In fact, controlling the dry line was looked at almost twenty years ago by developers at Sweden's Luleå University of Technology who implemented a fourdrinier dry line detection system based on CCD cameras. The dry line is affected by a variety of factors including machine speed, headbox total head, slice opening, and refining energy. These factors all come into play during grade changes, so it is a complex situation for operators to handle.
To me, the grade change graphs in the figure below look remarkably like an MPC matrix. Perhaps reliable online freeness measurement was not available in 1998, but it should be added to the control strategy today along with chemical addition rates. Online measurements of fiber fibrillation, fines levels and entrained air could be useful since all of them affect drainage. The ideas from 1998 could be dusted off, refined and expanded to help operators make grade changes more efficiently with less waste.
Researchers at Luleå University of Technology used a CCD camera to measure the dry line during grade changes. Source: American Control Conference Proceedings, Philadelphia, PA, June, 1998.
Multi-ply board forming
As all papermakers know, freeness is a primary indicator of drainage potential which is usually manipulated by refining intensity. At the same time, the degree of refining affects quality tests, adding another layer of complexity. With multiple furnishes and forming zones typical of a board machine it is even more daunting. Operators at the former Weyerhaeuser containerboard mill in Albany, OR developed a feel for these interactive effects by using online freeness measurements of top, bottom and filler ply furnishes to control each refiner train, stabilize how the sheet plies form, and assess the impact on quality tests (Mullen and STFI). Couch vacuum and dryer pressure were trended, as well. In addition, the machines were equipped with white water consistency and electrochemistry measurements to get a better handle on retention aid and strength additive dosing. The expected improvements from operator-initiated actions included improved dry and wet web strength, fewer breaks, higher production rated with higher freeness, refining energy savings, faster grade changes, and less cull or downgrades. The full potential of these goals has not been sustained as the mill was closed and is now demolished. Nevertheless, with these promising objectives, the optimization of multi-ply board forming sections could be a significant MPC application waiting for a developer to take the lead.
Linking pulping to papermaking
The ability to control multiple variables in an integrated fiber line and paper production line from "chipping to shipping", as the old marketing slogan goes, has been a pipe dream of control suppliers for a long time. Now, that dream could be coming closer to reality, at least in virgin-fiber brown grades where there is direct link between unbleached pulp Kappa variation, board machine runnability and final sheet quality. A recent World Pulp and Paper Magazine article revealed Green Bay Packaging's Arkansas Kraft Division has commissioned a BTG single point Kappa analyzer which the pulp mill and board mill operators use to stabilize their operations and quality. The measurement allowed closer control of Kappa number, so operators could raise the target, lower liquor/wood ratios and increase pulp yield. Downstream, the board machine operators work with a more consistent pulp and they use the frequently updated measurements to adjust refining and machine speed to avoid web breaks and quality problems. The result is a good return on investment in both sides of the mill.
Admittedly, the results are not based on automatic control; the operators have learned the ropes and do a fine job. However, the door is open to a more comprehensive strategy in integrated mills to link the pulp and the paper sides of the production chain. The strategy to get more consistent paper properties by controlling the pulping operation was implemented very successfully by the Billerud mill in Karlsborg, Sweden. They used Metso (Valmet) analyzers and controls to get more even Kappa number and therefore more consistent porosity and strength in kraft sack paper used for cement bags. However, there was no direct control linkage between the pulp and paper mill.
This linkage applies to recycled fiber (RCF) plants since the results of ink removal, brightening, fiber fractionation, fiber blending and refining are passed on to the paper machines. As a case in point, Valmet has documented results in a RCF line where brightness, ash and ERIC levels of the paper furnish have been stabilized, while paper brightness is more uniform. The ROI comes from cutting the consumption of deinking and brightening chemicals and eliminating the addition of fresh filler to the paper machine.
Connecting upstream to downstream
Transforming a variable raw material into a standard or a specialized product, meeting quality tests, and running a machine efficiently is not easy. Incoming fiber properties, the operation of stock prep and papermaking equipment, and end use tests are interactive, so the objectives and the control moves must be in balance. It's clear that paper quality control is moving upstream of the paper machine to fiber and furnish preparation, and maybe even further back. Therefore, there is a need for more soft sensors and real-time multivariable control to connect the upstream process to the downstream process. Some are in place already, but others will need further development. As a final note, let's keep the profit motive in mind since an investment payback is crucial.