Considering that many U.S. paper mills operate with low margins (or even at a net income loss), the need to offset increasing energy costs has never been more important.
Attention to the wet press section can lead to significant cost reduction related to web moisture content and profile control. An old paper maker once remarked to me that the wet press is the "heart" of the paper machine, and I believe that statement remains true today.
This column will explore a few wet pressing fundamentals with the hope readers will find renewed interest in improving wet press operation. The focus will be primarily in the sectors of Printing/Writing and Newsprint grades, for reasons subsequently discussed.
It is obvious that much of the U.S. paper industry continues to experience capacity rationalization and consolidation due to decreased demand for many grades. For example, the demand for Printing/Writing and Newsprint paper has declined due to the rapid rise in electronic media. And select environmental groups have even stated in their agendas the goal of eliminating these types of papers. Bear in mind these grades combined have accounted for at least 30% of U.S. production in the past. I suppose some "comfort" can be found in knowing that global production of pulp, paper and publishing sectors continues to increase, but this is dominated by the Asian-Pacific region. But in the U.S. Printing/Writing and Newsprint sectors, a survival mode exists today.
There have been exhaustive studies on different technologies to increase web dryness from the press section in the past, and promising newer technologies, such as displacement pressing and the development of fillers designed to behave like fibers may offer significant benefits in the future. The technologies have the promise of reducing energy consumption required in conventional drying, since less water needs to be removed in the dryer section. Unfortunately, the time frame for complete commercial utilization may prove to be too long for many mills in the current market situation. This leads to the need for renewed focus on pressing efficiency improvements using our existing knowledge of methods already known to be effective.
Reducing energy costs associated with the drying of paper and adapting to operating with increased recycle content in the furnish are two of the major areas providing opportunities for significant cost reduction. It is well known that basically each 1% increase in sheet dryness from the wet press section can be used to reduce energy consumption in the dryer section by about 4.5%. This stands out as a critical area of focus since about 36% of the gross energy consumption occurs in the paper mill, and the majority of that is directly related to paper drying. Recycling is still experiencing strong growth, with levels inevitably approaching the 65% level used in European mills, in my opinion. Increased knowledge of recycle fiber properties, in particular the effect on water removal, could lead to new methods for wet press optimization.
Let's examine a few wet press related considerations with the goal of identifying water removal opportunities. These are focused on the above paper grades, which are considered to be pressure controlled in the press nip.
• Maximize the press impulse within the constraints of finished paper properties. Press impulse is simply the linear loading divided by the machine speed, with units often expressed in psi.sec or kPa.sec. Depending on the grade, increasing the press impulse in the second and third presses (roll press) by only 12% can result in a press section exit dryness increase of about 0.5-0.75%. This improvement can be taken in reduced drying energy cost in today's market, since production increases may be limited due to demand. Making a complete press analysis is the starting point for achieving improvements.
• Increase web temperature to the maximum level compatible with the grade furnish and safe operation. Numerous studies have confirmed that water removal due to elevated temperatures increase linearly with increased heat transfer to the web. As a guideline, a rise in temperature of 9 degrees Celsius can increase dryness over 1%, due to the lower water viscosity effect. The starting point for this possible improvement is a temperature survey of the press section.
• Study and determine the flow resistance characteristics of the specific furnish. In this regard, particular attention should be made to water retention value, compressibility, and permeability. From these data, it would be possible to develop mathematical models for prediction of press water removal on a proactive basis, and to make decisions concerning optimum press operating conditions as the stock furnish is varied. Remember, it is not enough to assume that all recycle behaves the same. Knowing beforehand what effect variations in recycle content make on wet pressing would be very useful for improving machine operating and water removal efficiency.
• Be keenly aware of press rewet. The expansion in the web when mid-nip pressure is released is generally higher than that of the wet felt. In addition, felt water penetrates into the web during the exit pressure zone as a result of capillary action. This is due to the capillary "tubes" in the web being smaller than those in the wet felt, causing water absorption. On lighter basis weight sheets, the effect of rewet can be in the range of 10-20% loss in dryness from the press. Since press geometry is generally fixed, minimizing the capillary rewet and/or flow-back effect is currently heavily dependent on the wet felt structure. New technology felts have been developed to not only improve the pressure uniformity in the nip, but to also provide internal "barriers" to restrict the flow-back effect.
In spite of the tremendous challenges facing U.S. printing/writing and newsprint production today, I remain optimistic that mill operating and technical personnel, with assistance from suppliers, will continue to find ways to supply the U.S. market on a cost effective basis. I also feel a stronger marketing effort, refuting environmental concerns and emphasizing the advantages of using printed media, is needed.
Robert Moore is a retired chemical engineer, and is an experienced technical and fictional writer. His past work experience spanned the chemical, paper and equipment manufacturing industries, including holding management positions at Voith Paper, Scapa plc, and The Mead Paper Corporation.