Valmet has conducted research on pressing: here are the results.
Magnus Hultcrantz and Ingvar Klerelid (Valmet R&D)
Bulk and bulk development in tissue grades has been a hot subject in recent years, as more companies have begun to offer TAD grades and paper machine suppliers have sought ways to improve bulk in conventionally produced tissue. In an attempt to better understand bulk-forming in tissue grades, Valmet-Karlstad recently conducted a series of studies on both a laboratory press simulator, as well as on the full pilot tissue machine. The objective was to learn more about how bulk is developed and how different machine configurations can lead to higher bulk. It is clear that TAD is the best process for producing tissue with high bulk and softness. At the same time, TAD involves an investment that some companies are hesitant to make. Therefore, in an attempt to get higher bulk out of a conventional machine design, some researchers have suggested that pressing the tissue sheet against the Yankee dryer with a shoe press is one way to get this bulk improvement.
However, Valmet's results seem to suggest that it may not be the actual pressing against the Yankee that is the critical factor, but rather the pre-dewatering stage with the suction press.
THE CURRENT STUDY CONSISTED OF TWO PARTS. The first preliminary research was done on a lab scale press at STFI (The Swedish Pulp and Paper Research Institute) in Stockholm. The second phase was carried out on Valmet's pilot tissue machine in Karlstad. For the trials on the pilot machine, two arrangements were used. The first being a shoe press against the Yankee, while the second arrangement had a solid press roll in this position. A suction roll was used for pre-dewatering prior to this press roll. The lab scale test was carried out in order to verify the impact of only the pressing. Thus eliminating the impact from the Yankee coating and the creping process. STFI has an advanced press simulator that has previously been used on other grades for verification of how different pulps, load, tilt, etc. effect the press result. For this trial, handsheets were made with a basis weight of 19 gsm and using a furnish of 80% SW and 20% CTMP. The press surface temperature was kept constant at 90° C to simulate the Yankee surface temperature.
Other parameters of the test were the following:
Ingoing dryness: Two levels 20% and 25%
Moisture in felt: Two levels 0.3 and 0.5 kg water/kg felt
Press impulse: Roll press 12 and 21 MPa; Shoe press 14 and 28 MPa
As expected and as shown in Figure 1, the results followed the trend that had been seen on other grades. Thus at constant increase in dryness, there was an increase in bulk of between 4% - 7% tied to a longer pressing time.
PRE-DEWATERING IS AN IMPORTANT FACTOR. When separating the results from the tests with different felt moisture it could be seen that the felt moisture, at least on the extended nip, had a large impact (see
Figure 2). This indicates that some of the bulk increase may be due to the pre-dewatering that takes place over the suction roll, rather than to the actual pressing. A possible explanation may be that the pre-dewatering of the sheet and felt leads to a more gentle dewatering during pressing, thus allowing more of the bulk to be retained. The conclusion from the lab scale tests was that an extended nip press has the potential to increase the hulk for tissue in the same way as it does for other grades. The objective of the pilot machine trial was to evaluate the impact of a shoe press on paper quality in particular on bulk and tensile strength, but also on sheet dryness after the press. The trials were conducted on the pilot machine, using a C-former twin wire configuration (Figure 3). The first part of the test was made with a SymBelt TM shoe press, while the second part was done with a simple solid press roll. The machine is a Cormer with a three-layer headbox that also has dilution profiling, a 15 foot Yankee dryer with a high capacity hood, calender and a reel with center-wind-assist on both primary and secondary carriages. For the trials with the shoe press against the Yankee, the sheet was dewatered with suction roll prior to the press nip. It had earlier been tried to run a SymBelt TM press in a single felted pre-press before the Yankee with out any dewatering of the sheet and the felt before the pre-press.
However, this was unsuccessful as sheet crushing occurred in the nip.
Typical dryness levels during the trial were the following:
After Pick-up: 15%
After Suction Roll: 25%
After Press: 40-45%
During the trial the following parameters were changed:
Product:
Towel: 22 gsm, 80% SW /20% CTMP
Bath: 17 gsm, 40% SW /60% HW
Facial: 15.5 gsm, 40% SW /60% HW
Nip load: 80 - 150 kN/m
Tilt: 1.0 - 1.3
Shoe length: 55 mm and 90 mm
In Figure 4, the dryness after press can be seen for the two different shoe dimensions. As was expected, the dryness for the shorter shoe at constant nip load was higher. At the same nip load the dryness was about 2 percent higher for the shorter shoe due to higher peak pressure. When changing the tilt from 1,0 to 1.3 the dryness increased by approximately one percent at the same nip load. This result was also due to the increased peak pressure.
TENSILE STRENGTH FALLS. The increase in bulk was very good, but the tensile strength fell as bulk increased. In the trials, tensile strength was reduced by half at 40% dryness (Figures 5 and 6). The results may have been influenced somewhat by the fact that the creping was not kept constant, which is a difficulty when performing a trial like this. The conclusion, however, is that there is a price to pay for the bulk increase, and that price is a reduction in tensile strength. Following this, a trial was performed with a solid press roll that replaced the SymBeltTM press. The press roll was the same as had been used in the pilot TAD machine. It has a cover hardness of 25 P&J. The hardness of a press roll in a conventional machine is between 30-40 P&J. For a harder roll like this, the result would be a higher dryness and strength, as well as lower bulk for the same nip load. (Note: A direct comparison of the shoe press results to a conventional single suction press roll configuration would have been preferred, but such a rebuild of the pilot machine was not possible due to time constraints.)
SOLID PRESS ROLL SHOWS EVEN HIGHER BULK. With the solid press roll installed, a bath product was evaluated which earlier had been run with the 55 mm shoe. As can be seen in Figure 7, the bulk for the solid press roll trial was even higher at the same increase in dryness. Given the hardness of 25 P&J for the solid press roll cover, the press nip should not exceed 5-5mm. Therefore this result was rather surprising, with the bulk about 20% higher for the solid press roll at the same increase in dryness. Here the impact of refining can also be seen. A decrease in refining from 26 to 21 SR increased the bulk by 14 %. The dryness level was about the same for the solid press and for the 55 mm shoe cases. The tensile strength fell approximately 20% for the same dryness, which was less than for the towel product. Reasons for this may be found in the felt moisture level. It can be assumed that the pre-dewatering of the felt makes room for the water that is pressed out of the web and this in turn makes the dewatering more gentle due to the lower hydraulic pressure compared to that of a conventional machine. One of the results from the lab scale test was that a low felt moisture gave a higher increase in the bulk. The absorbency was tested on the towel product and did not show any improvement compared to a conventionally pressed sheet. The levels were around 5g water/g fiber, which is a normal value for a towel product from a conventional machine. For a TAD sheet the absorbency is significantly higher and can be on the level of 10-12 g/g. There are two main observations that can be made concerning the runnability. Firstly, there is a risk to damage the felt due to wads getting caught on the ingoing side of the SymBeltTM press. This happened during the trials and it burned the surface of the felt. The difference from a conventional press is that the shoe is not as flexible as a rubber cover on a press roll, which would allow a wad to pass. The second observation is that the felt is harder to keep open. The suction press is the most effective felt cleaner since the water is pressed through the felt and carries contamination with it. When the suction press is used for pre-dewatering of the felt there is a risk that the conditioning of the felt will not be uniform across the machine.
CONCLUSIONS. The following conclusions have been reached based on the study:
-It is possible to increase the bulk with a SymBeltTM press at constant dryness levels when compared to a conventional press.
-A suction roll is very efficient in reducing the water content in the felt before the shoe press.
-As expected, the tensile strength drops with the lower peak pressure, as the bulk rises. This probably makes the concept better suited for bathroom tissue products where the tensile strength is less important.
-Only a very small increase in bulk can be achieved with the shoe press.
-There is no improvement in water absorbency for towel grades.
-The most important benefit of running a SymBeltTM press would be an increase in production, due to higher dryness levels.
-Crown compensation is an important advantage of the shoe press, since it allows different hydraulic pressure in the CD-direction and solves the issue of crowning. However, this could also be achieved with a crown-compensating roll such as a SymRoll TM.
-With a solid press roll instead of a SymBeltTM press the same level of bulk increase can be achieved, but at a slightly lower dryness.
-The solid press gives additionally better machine runnability due to simpler operation.
FULL SCALE TRIALS. Since the time of the above trials, it has been possible to confirm the research results of the Valmet pre-press dewatering concept (patent pending) through an installation on a production machine (pre-dewatering with the suction press roll and the solid press roll against the Yankee). The results from this full scale run indicate a bulk improvement for a 25 gsm sheet, of 11 - 15 %.