Expanding controls: Is it in the cards?

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Expanding controls: Is it in the cards?

July 22, 2012 - 16:00

BRUSSELS, July 23, 2012 (RISI) -Paper machine wet end multi-variable controls and applications that also integrate feedback controls from the dry end scanner are well proven and have provided some impressive benefits. In fact, they are so well established that process control suppliers commission their multi-variable model control as a mainstream product. These controls provide wet end stability that carries through the papermaking process.
Figure 1 - The release angle on a center press roll is affected by furnish charge demand. A similar relationship between charge and wet press sheet moisture is also documented.

A pipe dream or a step forward?

Until now, mainly the papermaking basics are controlled: white water and headbox consistency, electrochemistry, filler addition rates, basis weight, moisture and sheet ash content. By keeping these variables relatively constant wet sheet and dry sheet properties are stabilized, thereby reducing start-up losses, improving sheet runnability and break recovery times, and allowing some furnish cost optimization.

However, there is much more that could be done to extend today's multi-variable controls. More dimensions could be added by including the furnish preparation and sheet forming processes where many paper properties are developed. By combining the management of these processes many additional wet end and dry end measurements could be used. These include fiber properties, fines levels, freeness, on-wire drainage, sheet formation, porosity and other dry sheet measurements. Conceivably one could add wet sheet moisture and sheet release from press rolls to the control matrix, since they are affected by wet end conditions as well. Furthermore, by extending the controlled variables to include laboratory quality tests, even more dimensions could be added to a comprehensive overall strategy.

Extending current multi-variable controls to a multi-dimensional model would be a step forward for the science of papermaking. As we know, adjusting chemistry and retention by additive flows affects a host of related variables. Similarly, many sheet properties and runnability are affected by the furnish preparation and sheet forming processes. All told, it's often difficult to find the best operating point, or window as papermakers call it.

Are these more extensive multi-dimensional controls in the cards, or are they just a pipe dream for now?

Figure 2 - The sheet release point can be continously monitored by a web image analysis system. Why not calibrate this release point by pixel mapping and link this real-time measurement to an automated control in a QCS or DCS? Camera image courtesy of Metso.

Interrelationships proven

There are already documented cases that prove the interrelationships between manipulated and controlled variables beyond the effect that retention aids and electrochemistry have on fiber and filler retention. For instance, drainage is affected by freeness, cationic charge demand and chemical addition rate. Furthermore, sheet formation is influenced by a variety of forming zone conditions including fluid-dynamics, fiber properties and coagulating and flocculating chemical additions.

In addition, it is known that charge demand affects the release point or release angle on press center rolls, the stability of which affects sheet tension and runnability. Says one experienced European papermaker, "Running over 1500 m/min and with lower grammage, the operating window is very narrow and the draws are critical."

Controls which help papermakers stay in that narrow window are most welcome and produce a good ROI.

Figure 1 shows that the center roll release angle is influenced by cationic charge demand. In this case the release angle was measured by a laser sensor in the press run. But why go to that trouble and expense if a paper machine is already equipped with a web break camera system which monitors the release point continuously? See Figure 2. By mapping the pixel location of the release point an automated release point stability warning could be issued and, if the camera system were linked to the QCS or DCS, a release point control would be possible. When will see linked or integrated break image analysis and DCS/QCS systems? Is it another pipe dream?

Figure 3 - Board machine freeness level changes can affect drainage indicators, like couch vacuum and dryer pressure, and quality strength tests like Mullen and STFI compression

Mark Williamsonis a journalist/engineer based in Thornhill, ON, Canada

(This article previously appeared on the website of Paper Advance (www.paperadvance.com)