CALENDERING: Online Papermaking
August 1, 2001 By Pulp & Paper Canada
As today’s papermakers are striving to maximize the returns from their capital investments, there are a number of interrelated driving forces favouring on-line coating and surface finishing solutions….
As today’s papermakers are striving to maximize the returns from their capital investments, there are a number of interrelated driving forces favouring on-line coating and surface finishing solutions. First, the relative market shares of high quality, calendered printing papers are increasing, creating new prospects for upgrading newsprint machines to produce new higher value products — SC grades, coated grades and improved news.
Secondly, the ROI potential of rebuild projects is, in most cases, more attractive with on-line solutions. Moreover, many older mills simply don’t have the physical space for additional off-line equipment. The capital costs of new off-line finishing equipment plus the costs of new buildings have been a major limiting factor for the viability of quality upgrade projects in existing mills. This situation has dramatically changed in the past few years. With the introduction of both on-line coating and finishing, the investment cost has been reduced by 20-40%. At the same time, the production costs have been reduced due to savings in personnel, energy consumption and finishing broke.
These quality and capital cost advantages have been made possible by new film coating and calendering equipment. Metso Paper’s OptiSizer film coater, for example, coats both sides of the web simultaneously at line speeds up to 2000 m/min. Film coating has been used successfully on both wood-free and wood-containing sheets, including those with recycled fibre. The film coater tolerates holes better and produces fewer breaks than blade coating.
There are several important technological innovations that opened the door to on-line, multi-nip calendering. These developments include the use of polymer rolls, new, more flexible nip-loading methods and the use of significantly higher calendering temperatures.
Polymer rolls have demonstrated a service period 10 times that of fibre rolls and, unlike fibre rolls, they do not lose their elasticity with persistently high loads. Those qualities make them suitable for on-line operations. The OptiLoad loading method made it possible to build a calender free from the conventional gravity restrictions of a supercalender. By using the same loading level in all nips, the calendering capacity needed for the higher on-line speeds was made possible. With the flexibility in nip loading and temperature control of thermo rolls papermakers can influence the surface finish of each side of the sheet to reduce two-sidedness.
But, let’s consider the other machine design criteria necessary for good on-line quality and efficiency.
The process study
In an off-line process, a parent reel has usually been stored for some time. Because of the inherent relaxation time, the moisture, temperature and mechanical stress gradients are essentially at equilibrium. In contrast, the on-line process is not in a state of equilibrium chemically, thermally or mechanically. For example the moisture may be non-uniformly distributed through the thickness of the sheet and some of it may exist as free water rather than being bound to the fibres. Moreover, temperature gradients may exist in all directions in the sheet. On-line processes are much more dynamic so the moisture and temperature-related properties of the paper are more critical. Their on-line control is very important.
There are fundamental differences between the off- and on-line calendering processes, so the success of the investment may depend on the correct anticipation of these differences. The ideal process design is therefore a continuous chain of compatible, sequential unit processes.
Rebuilds are the most demanding because old and new technology must be combined. Old components may not fulfill the increased requirements of an upgraded line. A process study to troubleshoot the performance of the existing stock approach process, forming, pressing or drying sections is highly recommended to ensure successful start-ups.
Forming and pressing
The forming process is the key to developing the basic structural properties of the paper web. For better drainage and more even filler distribution, a conventional top wire former may be rebuilt with a loadable blade type of former such as the Metso Paper MB former. New headbox additive layering techniques and filler property developments have given new possibilities to build a paper structure that results in a better printing surface.
The right press section concept can produce uniform, two-sided drainage and densification in both in the CD and MD. The press section contributes to both density (absorption) and roughness properties on both sides. The press concept, number of nips, linear load levels, felt properties all influence paper surface properties prior to calendering.
With low draws in the press section, lower porosities are possible because the fibre structure is not broken by excessive stress. Sheet support by press felts and dryer fabrics and blow boxes have helped to lower the draws through the press and dryer sections, improving quality as well as runnability.
and MD variations
The stability of fibre and filler consistency is essential for the well-being of any SC or fine paper mill. The fines and fillers that are returned to the paper machine from disc filters or recovery processes may create changes and variations in consistencies if not properly controlled or fed back to the stock system. The mill broke system is another source of strong variations in fibre or filler consistencies. Coated broke also introduces colloidal substances that can cause upsets in wet end chemistry and retention. DIP can cause similar upsets.
Good wet end stability is established by measuring and controlling both fillers and fibre consistencies in the short circulation system and controlling retention on the former. In modern control systems, this is done continuously by multi-variable models that integrate short circulation controls with feedback controls from the dry end ash and basis weight sensors. New furnish charge measurement sensors are now being used to stabilize retention in conjunction with polymer addition controls.
Low variability in the stock and white water consistencies also has a positive influence on CD grammage control using dilution controlled headboxes, since the white water is used to regulate the dilution in each CD zone. Recent start-ups show that dilution controlled headboxes produce excellent CD-profile for dry weight and thus form a good basis for other important profiles further downstream.
The profile controls challenges get even bigger when a calender is built on-line with a coater. The moisture content must be sufficiently high to compensate the moisture loss in the calender. At some critical moisture level, however, the risk of coating sticking to the calender becomes too big. This means that the moisture profile must stay within very tight limits.
On SC and coated paper machines, the moisture controls and interrelated gloss control strategies are different, with infrared profiling after coating or remoisturization of a dry SC sheet plus steam application on the calender. Because of the dynamic nature of an on-line process, these controls must be responsive and coordinated to achieve the desired sheet qualities. For SC sheets, recent advances in a remoisturizing technique called “moisture gradient calendering” provides significantly better gloss and air permeability to density ratios.
New scanning sensor technology allows the simultaneous measurement of the film coating profiles on both sides of the sheet. This means that two-sided coating applications can now be independently controlled for the best two-sided quality. A moisture measurement in the same sensor allows simultaneous moisture control.
The sequential forming, pressing, drying and remoisturizing in an on-line process can influence the dimensional stability of the sheet on the papermaking line and eventually in the customer’s mills. The measurement of the on-line tension, a new profiler called IQTension, provides a valuable dia
gnostic measurement that can be used to optimize the process for the best runnability and pressroom performance.
The present understanding of the complex nature of on-line papermaking is based on intensive research and a large amount of pilot studies as well as field measurements and observations. Many successful start-ups confirm that these investments are starting to pay back as the processes of the future are being created for various paper grades and machine concepts. On-line papermaking which integrates unit processes is a trend that will undoubtedly stay in the headlines for years to come.P&PC
Antti Heikkinen is the process development manager and Olavi Kokkonen is the senior paper technology manager at Metso Paper in Jyvskyl, Finland. Mark Williamson is a consultant based in Thornhill, Ontario.
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