Albany: increased paper machine efficiency using start-up enhancements

In the packaging grades, the necessity to lower costs shows in an extensive usage of recycled furnish with short fibers (OCC, MW, DIP, ONP etc.) and a higher rejects percentage (up to 30% for some corrugated medium manufacturers). This condition led to more synthetic contaminant building up on the machine clothing which caused poor machine efficiency and a shorter usage period of the PMC.

Problematic

The extensive usage of the recycled fibers plugs the press Fabrics since the contaminant can’t be flushed with the standard conditioning. Higher contamination creates several issues: more frequent shut downs to batch wash the press fabrics, high vacuum needs as the dewatering of most of the packaging machines is flow controlled, and extensive usage of chemicals during shut downs and while the machine is running. To tackle this issue, press fabric manufacturers started to produce high pore size press fabrics (high air and water permeability) that allow the synthetic contaminants to be flushed and the requested fabrics lifetime to be reached. High pore size press fabrics reduces the stickies and lowered the vacuum needs, but creates a new issue: reduction of machine speed and efficiency at start-up after press fabrics changes due to lower dryness after press section.

The issue is more severe on first presses in general and particularly on pick up positions where it is critical that the adhesion between the felt surface and the paper is strong enough to transfer the sheet. In the worst case, if adhesion is lost, the paper can follow the wrong path in the press section or cause runnability issues such as drop offs or stealing. The capillary action of the felt surface and the moisture reached can ensure the sheet adhesion to the felt (TappiJournal, 1993), yet simultaneously cause higher vacuum values and shorten felt life. The papermaker, in cooperation with the fabric manufacturer, often has to make compromises to reach the primary targets: sheet transfer vs. fabric lifetime, drop off vs. cleaning cycles, etc.

The Experiment

The lower dryness in the press section after the press fabrics change is due to the non-saturation of the press fabrics in all the paper grades – saturation being the filling of all pores in the press felt with water. Previously, to saturate or “Break in” a press fabric, papermakers had to wait days or even weeks to reach the expected machine speed, steam consumption, and production output levels. Till now, the standard method employed by press fabric manufacturers was to supply fabrics with larger pore sizes (high permeability press felts) that decrease in size during press fabric usage.

The Albany International Research and Development Center worked on developing a new concept that did the invers; the pore size of the press fabric was to be smaller when starting up with new clothing and increasing in size during the machine run. This new concept eliminates the need to compromise between machine speed after felt change, vacuum needs, washing cycles, and press fabrics life. The innovation is a startup enhancement to Albany International Press fabrics, called Rapid™ which is a polymer based material applied to the press fabric during manufacturing in order to temporarily reduce the mean flow pore size (MFP). Figure 1 is a visual comparison between the same felt with and without Rapid™.

On a pilot single felted roll Press with an ingoing dryness of 21%, a dewatering study compared sheet dryness with and without the Rapid™ start-up enhancement. The sheet was a mixture of a 60% hardwood / 40% softwood. A non-woven press fabric was used for the trial: Nip dewatering was studied at 30 kN/m at 1200 m/min using -20 kPa uhle box vacuum; Sheet dry content was studied at 30 kN/m, 1000 m/min, -20 kPa uhle box vacuum. Figure 2 shows the nip dewatering percent in relation to total dewatering for a Non Woven felt with and without the start-up treatment. From day one, it was clear that the start-up enhancement facilitates a higher degree of nip dewatering which implies a faster saturation of the felt.

Figure 3 shows the sheet dryness. The press fabric utilizing the start-up treatment was found to produce approx. 3% higher sheet dryness when compared to the press fabric without the start-up treatment. Sheet dryness was 40.7% using the start-up treatment and 37.6% without the start-up treatment.

Results analysis

The higher nip dewatering and higher dryness with the Rapid™ enhancement is explained by the smaller mean flow pore size (MFP) in the press felts at start-up. Smaller MFP allows a faster saturation of the felt, thus higher nip dewatering (Gullbrand, 2004). Higher nip dewatering in addition to the same amount of suction box dewatering (same felt weight and openness needed for suction box dewatering) increased the solid content in the grab test which and led to a higher dryness.

Market Launch and results

Rapid™ is a water based polyurethane with a low concentration; it is dissolved while the paper machine is running. The nip dewatering and the conditioning application (high pressure showers, suction boxes, and nip dewatering), removes the Rapid™ material from the felt bringing the pore size to the target size for the loading and life conditions (Table 1).

On a paper machine the initial smaller pore size (MFP) lowers water permeability and increases the saturation of the felt at beginning of life. After a certain number of nip cycles (depending on the machine speed, dewatering capacity, and conditioning) the Rapid™ enhancement will be washed out and water permeability/moisture will return to the original specifications of the press fabric. The temporary reduction in pore size at the beginning of press fabric life results in a faster saturation of the fabrics and a higher dewatering capacity during the startup period. Higher dryness after a fabric change using Rapid™ results in a substantial increase of machine speed (equating to higher paper machine efficiency). The filed results are all unanimous; the Rapid™ start-up enhancement brings benefits to users.

Success Case 1

A Rapid™ trial on a Linerboard papermachine in Germany (70 to 140 gsm) has been running at over 1600 mpm on the PU Position (Tandem Nipcoflex Press Section Layout). The trial goal was to reduce the saturation time of the pick up fabric in order to reach a higher amount of nip dewatering and dryness. Previously, the machine always reached higher dewatering figures at the end of press fabric lifetime. The idea was to simulate the press fabrics properties on the run, to reach the expected dryness immediately after the press fabrics changes and start-up. The water permeability using a Felt Perm measurement device was the best measuring method to assess the trial results. Figure 4 shows the water permeability measured on the Pick Up position. The water permeability on the pick up felt treated with Rapid™ started on the low side which helped the customer to reach the desired speed within 3 hours (compared to 2 days with the non-treated felts). The Rapid™ enhancement then washed out and the water permeability increased to reach the normal level.

Success Case 2

A significantly higher dryness was achieved with a trial pick up press fabric on a Trinip linerboard machine in Italy. Table 2 shows the recorded speed results where Rapid™ has been applied to the pick up position (5 m reel width, 70 gsm linerboard). The machine had an increase of over 350 mpm the first day while using the Rapid™ enhancement. The machine speed while trying Rapid™ was always higher which led to an increase in paper production efficiency.

Success Case 3

Reducing the pore size early in life to saturate the press fabric has been proven to increase dewatering in the press section, lower steam consumption, and allow a higher machine speed. One of the earliest trials was in Germany on a Kraftliner producer running at over 1200 mpm which had ecoflows to monitor the nip dewatering, suction box dewatering, and total dewatering. Rapid™ was tried on the same machine set up with a standard laminate 2+1 pickup fabric. The results showed an increase in nip dewatering while retaining the same amount of suction box dewatering. The total dewatering on the pick up position was increased by 15% increasing the total dewatering in the press section by 2%. This dryness increase automatically tled o a lower steam consumption and higher machine speed (higher machine efficiency). The same trial was repeated on a liner and medium produced in Australia running at over 1300 mpm on a pick up position and the 2nd top position. The Rapid™ application improved dewatering by 10l/m.

Rapid application in Packaging grades

The Rapid™ application has been used for packaging grades as well as for high speed publication machines where the lifetime is short due to the high amount of nip cycles reached in a short period. The polyurethane based technology allows papermakers to reach a very high production right after changing the press fabrics and eliminates the need to make a compromise between felt lifetime and high machine speed at start-up. The main benefits of the technology are: Higher dryness after press section while starting up new fabrics, this results in an increase in machine speed and efficiency; Reducing fabric washing frequency which increase paper machine overall efficiency; Increased press fabric lifetime.

So far, Rapid™ has been applied on all container and carton board grades. It has been applied on all press section configurations, different positions, and different speeds. The lowest machine speed applied was 450 mpm on a high quality White Top Kraftliner machine (250 to 500 gsm). The highest speed was on a light producer of corrugating medium at 1600 mpm (50 to 90 gsm). The best effect is certainly on earlier presses where the needs are a high void volume and high permeability press fabrics and on nip dewatering positions. The Rapid™ enhancement has been used mainly for achieving higher dryness in the press section, but there are other applications as well such as reducing the initial shedding, blowing, or stealing. Incremental development in this technology is still ongoing in order to reach the entire potential in all segments where the technology can be applied.