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COATING DRYING MEASUREMENT & CONTROL REDUCES VARIATIONS AND COSTS
Ross MacHattie, Antti Paavola and David Watson
Honeywell Process Solutions, 500 Brooksbank Avenue, North Vancouver
Presented at the 59th APPITA Annual Conference, Auckland, New Zealand
Producing uniform and consistent coating quality in coated paper and board products at the lowest cost is the ultimate goal for all coating machines. It is not an easy goal to accomplish, since the coating process is affected by a large number of interrelated variables, ranging from the recipe, rheology and the applied coating solids, to base sheet porosity and the unique characteristics of the coater itself. One key element for final coating uniformity and consistency is control of the coating consolidation process. Traditionally, there has been no way of measuring this coating consolidation, so operators have typically overdried the coating to prevent "painting" the machine, and have used the coating drying process to control the sheet bulk moisture, which is rarely the same as the moisture content of the coating.
We will examine the development of a measurement and control system, GelView, which uses remote distributed sensing to give direct measurements of coating consolidation, enabling true control of the coating drying process. This ability to control the rate of evaporation requires continuous control of both air dryers and infrared (IR) dryers, providing papermakers with a valuable tool to significantly enhance their product quality while minimizing costs. We will discuss the system development from laboratory testing and pilot coater testing, right through to results from extended testing on a full production machine. The sensitivity of the measurement to normal operating conditions and process upsets, and the response of the control system are presented.
The key factors in a coating process are applying an even layer of coating on a substrate and removing fluid mass from the coating in a controlled fashion to deliver high quality product at maximum production rates.
The tools generally available for fluid removal are IR, air, and cylinder dryers. IR and air dryers are non-contacting drying elements and must provide enough drying before contact with the cylinder surfaces to prevent sticking or picking. IR dryers are primarily energy transfer devices, but they also provide some degree of mass transfer depending on their configuration. Air dryers are primarily mass transfer devices, but by default also provide energy transfer to balance latent heat of evaporation. Mass transfer also occurs in open draws, depending on the degree of energy that has been transferred into the coating. Cylinder dryers are used primarily for finish properties and end point moisture content control.
Current measurement and supervisory control systems exist to effectively control the final moisture through the cylinder dryers. Lowlevel temperature and plenum pressure controllers traditionally control air dryers in an effort to provide constant heating at a constant jet velocity. These simple regulatory controls, while very effective in keeping the equipment conditions constant do not, in practice, keep the evaporation rate constant. A void exists for the supervisory control of the evaporation rate through the preceding open draws, IR and air dryers.
To fill this void, a simple, cost-effective and robust measurement is needed. In 1982 it was described how the reflective properties of coating change as the water leaves the coating1. As Watanabe and Lepoutre described, the gloss of a drying surface changes abruptly at the point when a continuous film of liquid is no longer present, and drops off rapidly until the point that air begins entering the coating. Conversely, the scattering of light increases during this same process, as indicated in Figure 1.
By using a ratio of diffuse to specular reflection we created a robust measurement of these reflectivity changes, suitable for the hostile online environment. The result was a system that delivered light to a freshly coated surface and detected both the specular and diffuse reflections from that surface. We then scaled the ratio of the two signals as described in Equation 1 and termed the new unit a "gel". One of the sensors is shown in Figure 2.
Typical laboratory results from a sensor are shown in Figure 3. In this example, the coating was applied to a non-absorbent substrate and allowed to dry by natural convection at ambient conditions. At the same time, the weight of the sample was recorded and plotted in terms of percent moisture. From this, two things are observed. First, the moisture measurement is not sensitive to the locations of either the first critical concentration (FCC) or the second critical concentration (SCC), and second, the sensitivity of the moisture measurement between these critical concentrations is much less than that of the "gels" measurement. This is significant because it has been reported that quality problems, such as mottle, occur in this critical solids range (CSR)2,3.
Laboratory tests have shown that the measurements are very repeatable for any given formulation; however the overall reflective properties will vary between formulations. The significance of the absolute level of gels observed has not yet been determined. Instead, the useful information comes from the change in gels. Figure 4 shows the drying curves for the six different formulations listed in Table 1.
Having shown that the measurement in the lab identified the FCC and SCC, and was very linear between those two points for all coatings that were looked at, the next challenge was to show that the same relationship between gels and water weight held true for a pilot coater environment. Of course on the pilot coater, many sensors are needed along the path of the coater to sample the drying in progress, since the paper is in motion. Time in the laboratory is like distance on the coater when machine speed is taken into account.
PILOT COATER ENVIRONMENT
The ability to relate the sensor measurements to the operating conditions of the dryers is essential in determining the feasibility of supervisory control. For simple feedback control, it is desirable that control variables (e.g. SCC position) have a linear relationship to the process variables being manipulated. In the case of dryers, water removal is being manipulating by changing one or more of the state variables; which determine the overall mass and energy transfer rates at the surface of the coating.
In the pilot coater studies it was not possible to compare the measurement to a sample weight; since it was not possible to take samples at the points of measurement, indicated by the red dots in Figure 5. To get around this problem the experimental trials were constructed such that the CSR was always within one dryer element, air floatation 1.
Under these conditions data was always collected in the linear region with respect to delta gels (i.e. the ratio Δ gels/Δ water weight = constant). If there were no differences in the behaviour of the sensors under pilot coater conditions; then it would be possible to analyze the Δ gels data against the theoretical mass transfer as calculated in equation 2, using the defining equation for mass transfer from a surface by forced convection4,5,6. A schematic describing the mass transfer is shown in Figure 6.
To prove this relationship, three trials were run on the pilot coater; each with 21 different operating conditions. The results of these 63 runs have been previously published7. Using measurements of the operating conditions around the first air dryer in the defining equation for mass transfer, the theoretical mass transfer for the first air dryer was equated to the measured Δ gels. This relationship is shown in Figure 7.
In 1998, results from pilot coater studies described how the drying of paper coatings, can significantly affect the back-trap mottle of coated paper8 . In these studies a direct measurement of coating consolidation was not available and some of the analysis had to be based upon mathematical models to calculate critical solids and net evaporation rates. Kim et al. described, to control back-trap mottle, the drying strategy must strike a balance between early-harsh and late-mild drying while restricting net evaporation rates over the critical solids range.
Having proven the relationship between the sensor measurement and evaporation rate, limited trials were started to support these findings. Six trials were conducted using similar formulations and base sheets. The gel curves and print results are summarized in Figure 8. Even within this limited data set, a relationship between the gel curves and the level of back -trap mottle can be seen. Also, as Kim et al. implied8, there is an obvious threshold in drying conditions outside which back-trap mottle does not occur.
Since pilot coater runs are typically fifteen minutes or less in duration, the implementation of supervisory controls was not feasible. Therefore a move to a production machine was required to implement and test the controls for extended periods of time.
The beta test site for this new measurement and control system was a multi-layer board machine with four on-machine coaters. Results from this site have previously been published9. Proper machine direction positioning of the sensors was critical to the implementation of closed loop control. For optimal control it is essential to locate measurements within the CSR so that accurate measurements can be taken of both critical concentration positions and the "gel-rate" within the critical drying phase. The positioning of the sensors (designated by letters) is shown in Figure 9.
On this machine, the supervisory control system regulated the three coat weights (both MD and CD) and the base sheet weight. The final moisture regulated the base sheet moisture and the coater dryers had fixed operating levels.
The new sensors showed that under these conditions, the coating dryness varied over the life of the coater blade. This is shown in Figure 10, as a logarithmic decline in gels after each coater blade change.
Specifically, the blade run from 00:00hr to 08:00hr contrasts the regulation of coat weight (Coater 4), final moisture (Coater 4 Moisture) and base sheet moisture; to the gradual decline of the coating gels measurement in Figure 11. At roughly 01:30hr the coat head on coater 4 is retracted to replace the coater blade during which the supervisory controls are suspended.
Once the blade is replaced the supervisory controls are reactivated and maintain their original targets.
In Figure 11 the final moisture and base sheet moisture track each other, which indicate that base sheet moisture is the dominant source of final moisture variation. This can be contrasted to the "gels" measurement, which is directly related to the moisture content of the coating only. Both the "gels" measurements for sensors B and F decline, which indicates that the final moisture content of the coating is varying over the life of the blade; as the blade wears, the coating is easier to dry.
The coating consolidation supervisory control system uses measurements in the CSR to calculate the position of a nominal gels value; the second critical concentration position (SCCP). The SCCP supervisory controller manipulates the power level of the electric infrared dryer to maintain a target SCCP. A schematic of the control system is shown in Figure 12.
The SCCP supervisory controller response to set-point changes and load changes is shown in Figures 13 and 14. In both cases the supervisory controller is capable of maintaining the Second Critical Solids Position to within 0.1 meters of target.
In Figure 15, we can see the OFF control variation, which is typically 1.5 meters. This variation in SCCP means that coated product is produced over a wide range of coating dryness.
For this machine, the product produced at either extreme is within quality limits. This allows the flexibility to move the SCCP to its maximum position via the SCCP supervisory control by either increasing production or decreasing IR energy.
The on-control histogram of SCCP variation for this grade is also shown in Figure 15. Not only has the mill reduced the level of variation, but has also realized the economic benefits associated with moving the SCCP as far down the machine as possible.
Future benefits of stabilizing the process and maintaining an SCC position will come from a better understanding of the coating drying process and how it affects the quality of the printed surface.
It has been demonstrated through production machine trials that direct measurement of the reflectivity changes in the critical solids region provides robust control of the coating consolidation process. By manipulating either or both the IR dryers and air dryers to compensate for changes elsewhere in the process, variation of the second critical solids location has been reduced. Since the measurement is directly related to the change in water weight of the coating only, it is possible to manipulate conditions on the machine that will compensate for changes that, ideally could be more tightly controlled. These changes could include base sheet porosity, exact coating formulation, percent solids at application, coating color temperature, web temperature, air pressure, relative humidity and others.
The authors would like to acknowledge the Stora Enso, Fors mill and The Dow Chemical Company, Emulsion Polymers Business, Pilot Coater for their collaboration and use of their facilities.
1 Watanabe, J. and Lepoutre, P., "An investigation of the development of mechanism for the consolidation of the structure of clay-latex coatings", TAPPI Press Coating Conf. Proceedings pp181-186 (1982)
2 ASCHAN, P., "Solving problems of print mottle on coated board", TAPPI Press Coating Conference Proceedings pp73-78 (1986).
3 Norrdahl, P.C., "Effect of drying conditions on paper quality on wood-containing light weight coated paper", TAPPI Press Coating Conference Proceedings pp417-436 (1991)
4 Welty, J.R., Wicks, C.E. and Wilson R.E., Fundamentals of Momentum, Heat & Mass Transfer, Chapter 19, NY, John Wiley & Sons.
5 Parsons, R.(editor), "Fundamentals:2002 Ashrae Handbook: Inch-Pound Edition", American Society of Heating, Refrigeration and Air Conditioning Engineers (2002)
6 Perry R.H., Chilton C.H., Chemical Engineers' Handbook, 5th Ed, Solids Drying Fundamentals, Chapter 20, McGraw Hill
7 Alexander, C., MacHattie, R., Watson, D., "Dynamic Control of Coating Consolidation", PAPTAC Proceedings, EXFOR (2003)
8 Linda H. Kim, Mark J. Pollock, Edward L.Wittbrodt, John A Roper, Iii, David A. Smith, John W. Stolarz, Michael J. Rolf, Terrell J. Green, And Barbara J. Langolf, "Reduction of back-trap mottle through optimization of the drying process for paper coatings, part 1", TAPPI Journal 81(8) pp153- 164 (1998)
9 Skrgrd, O., Watson, D., MacHattie, R., Alexancer, C., "Production Results From Control Of The Coating Consolidation Process On Coated Carton Board" PAPTAC Proceedings ppC-1707-1710, (2004)