Extending the scope
Paper sheet forming and physical property development are also influenced strongly by fiber refining. Online freeness analyzers are well accepted measurements which papermakers use to determine the specific refining energy required, and that is often controlled automatically. The next logical step is to extend the scope of that control to include regulation of papermaking indicators of drainage or even measured paper properties, either online or from the laboratory. Figure 3 illustrates how a linerboard machine online furnish freeness measurement affects couch vacuum, dryer pressure and also influences board strength tests: Mullen and STFI compression tests.
This type of real-time analysis points out the strengths of information processing which is available in DCS or IT systems. Time synchronized readings of many different papermaking variables can be compared and a multi-dimensional response and interaction model can be built up.
In many cases papermakers are already manually controlling fiber properties to achieve end-use sheet tests. For instance, an SC paper producer manages CD-tear and porosity, measured every reel by an automated test lab, so they are both within an optimum window. The success of this approach depends on operators' learned skills. However, operators must deal sometimes with process responses which affect paper web quality tests in conflicting ways when a change in operating point is made. Now and then, a compromise is called for whereby one variable is prioritized while another is kept within constraining limits. Control engineers have the means to do that with the software tools they have and, through fuzzy logic programming, determine the best approach.
Process automation systems can emulate the best operating practices and perform them repeatedly, so there is an argument in favor of automation as means to achieve that optimum operating point consistently, regardless of operator experience. Consistency is a virtue in papermaking which we know is a 24/7 operation.
Figure 4 - Wet strength dosage has a diminished effect on wet tensile after a certain point when charged reactive sites become saturated. Charge also influences wire pit foam levels.
The right balance point
Knowledge of wet end process chemistry allows papermakers to run at the right balance point where sheet additives are most effective and are not wasted. In one example, shown in Fig. 4, a linerboard producer determined that, after a certain optimum wet strength agent dosage level is reached, any additional dosage becomes ineffective because charged reactive sites on the fibers have become saturated. The mill staff concluded that charge measurements can be used to achieve better wet strength properties at lower chemical doses. A model that relates sheet additive amounts and costs, electrochemistry and sheet property tests would be a good candidate for multi-dimensional controls.
Waiting to be exploited
Many of the measurements required for a multi-dimensional control model already exist and are well proven. Fiber dimensions and freeness are now sampled online and those properties influence the forming and drainage process. Retention is well measured and controlled now, but the next step may be to control drainage as well. Many new drainage measurements have been announced recently. Electrochemistry is an established measurement but how well is its control potential realized? Formation is measured at the wet end and dry end and the range of dry end sheet measurements is increasing. And let's not forget about automated laboratory measurements that are made after every reel turn-up.
IT systems are another powerful ingredient since they can link all of this data that are gathered at different times with different update cycles. However, process control systems are good at recognizing time lags and making appropriate control adjustments.
The extra dimensions of a multi-dimensional model are waiting to be exploited. Some positive signs are emerging. For instance, one QCS supplier has published a technical paper showing how a multi-variable controller can manage and stabilize retention, drainage and sheet formation while achieving a profit motive. The goal was to increase ash content in the sheet without sacrificing quality and produce a return on investment. That goal was achieved.
Profit model is essential
For multi-variable control to be extended beyond the present boundaries the justification and the profit motive must be well calculated by papermakers and demonstrated by automation suppliers. Most importantly, there must be a profit model to go along with the control model.
Imagine controlling a paper machine process from the stock prep to the reel in order to meet all customer specs with minimum production cost and maximum runnability. That's the ideal goal of papermaking controls, and there is a substantial economic benefit and profit motive associated with that achievement. However, that's a rather broad and unfocused objective, so this all-encompassing approach may be too much to bite off all at once. A more focused, stepwise and results-centered approach may have the best chance of success.
Furthermore, suppliers must be able to show that this type of control can be economically sustained over the long run. Papermaking processes do change and the controls must change with them to prolong the results.
It's an opportunity, but is it a pipe dream for now? Time will tell.
Mark Williamsonis a journalist/engineer based in Thornhill, ON, Canada
(This article previously appeared on the website of Paper Advance (www.paperadvance.com)