PAPER CHEMISTRY LABORATORY, INC. .....the acknowledged leader in papermaking chemistry Instrumentation
PAPER CHEMISTRY LABORATORY, INC.
.....the acknowledged leader in papermaking chemistry Instrumentation
The Use of On Line Wet End Sensors
Papermaking is a continuous process with a variable feed stock. It is variable for a number of reasons, beginning with the fact that it is based on a naturally occurring raw material, notably wood pulp, the properties of which vary by species, by climate, and by elapsed time between harvesting and pulping. Wood pulp increasingly is re-cycled, which can include de-inking and a resultant chemical carryover. Papermaking must also cope successfully with a variable amount of broke. This problem is exacerbated when dealing with coated broke which is highly anionic due to the presence of latex and fillers supplied as anionic slurries.
To make paper or board of consistent quality from a raw material with a wide and changing spectrum of qualities is difficult at best, but can be accomplished with effective use of on line sensors. Two papermaking process chemistry parameters are spoken of so frequently that they have come to characterize the process of papermaking: "retention" and "drainage". Therefore, on line sensors for measurement of retention and drainage occupy the first two places in prioritizing a list of wet end sensors.
Others need to be included, and we will select three more, beginning with pH. First, we need to distinguish between the acid process, in which alum (i.e. Al+++) is used as an inexpensive charge-neutralizing ingredient, which is highly pH dependent; and the alkaline process, employing calcium carbonate, which buffers to maintain a stable neutral to slightly alkaline pH value, usually between 7.0 and 8.0. PH should be measured on line in acid papermaking, but it is not necessary in alkaline papermaking because as little as 2% calcium carbonate buffers the pH to a fairly stable value.
The electrostatic charge on the surface of the papermaking stock needs to be measured. The most viable on line means is called the "streaming potential", and involves forming a pad between two electrodes and against a screen, drawing white water through the pad, while measuring the difference in charge of the two electrodes. The polarity and magnitude of the difference in charge are combined with other parameters and expressed as the "zeta potential", expressed in millivolts (mV).
In a typical uncontrolled alkaline fine paper system, for example, the headbox zeta potential might fall someplace in the range between about –10mV and –20mV. It has been recognized for many years that the process works better when the repulsive effect of this relatively high negative charge is reduced so the particles no longer repel one another. This requires reducing the zeta potential to someplace in the vicinity of zero by adding what we have come to call a "cationic scavenger" to neutralize the "anionic trash".
Internal sizing, for example, is particularly sensitive to zeta potential control. In our paper, "Process Chemistry Optimization, Revision 1" we show that in the controlled process internal sizing is an order of magnitude superior to an uncontrolled process. We believe this may well represent the first demonstration of the exceptionally powerful influence of van der Waals force in papermaking science. Van der Waals force is known to exert an exponentially increased beneficial influence when small particles are brought closely together.
Finally, it is instructive to measure the white water conductance. Increasing closure raises the conductance and commensurately reduces the effectiveness of chemical additives. As a rough rule of thumb, an open system has a conductance no greater than 600-800 micro-Siemens per centimeter (uS/cm). When the conductance rises to 1200-1400uS/cm, chemical effectiveness is significantly impaired. In the range of 2000-2500uS/cm, conventional functional additives have so little effect that a quite different strategy must be adopted, with more limited objectives.
It is of critical importance in the manufacture of fine paper that the cost effectiveness of chemical additives be maintained at the highest level. In the author’s experience, this requirement is tacitly recognized around the world, and the great majority of fine paper white water systems do not exceed 800uS/cm.
Conclusions
How do we best use on line wet end sensors to cope with the inherently variable papermaking process? Closed loop control of cationic scavenger feed rate is an important option. Since this entails a leap of confidence for the papermaking team, we reduce Phase One to bare essentials. Program the Distributed Control System (DCS) to operate the headbox at the optimum zeta potential by bumping the feed rate of the cationic scavenger so as to continuously maximize drainage. Closing the loop is key to effectively coping with feedstock variability. When the grade is next run, start up at the final zeta potential of the previous run.
During a recent Zeta Data installation on a world class alkaline fine paper machine, the Technical Director calculated the value of a 1% filler for fiber savings at $300-400,000/year. Our lab results in the paper previously cited show that a controlled process typically has a 5% higher ash level and much greater Scott Bond. Therefore savings in the range of $1,500,000-2,000,000 appears to be a reasonable initial target for a large fine paper machine.
Phase One is merely the beginning. In Phase Two we work on balancing the process by making additional synergistic cost effective process modifications to further increase retention with minimum adverse effect on formation. We also plan to further improve process and physical property parameters. As soon as the team sees the remarkable benefits in cost, quality and productivity gained from effectively dealing with feedstock variability, the rest of the program will rapidly fall into place.
Phase Three consists in exploring the limits of the available cost/performance envelope for each grade. The primary objective is to further reduce costs by increasing the filler for fiber substitution to the maximum (hopefully by another 5-10%) while simultaneously maintaining both the process parameters and physical properties at the most cost-effective levels.
John G. Penniman
August, 1998
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