Chemicals: Optimizing Yankees

Started up in October 2008, SCA’s new US deink plant and tissue machine in Barton, Alabama, are well on their way to full production

R&D into Yankee coating chemistries has resulted in improved coating performance. But often problems are rooted elsewhere, writes Andrew McNab from Ashland Hercules Water Technologies

Five key areas of the paper machine that affect Yankee coating performance are: the spray bar; coating equipment; creping blade; Yankee surface temperatures; press and felt. By optimizing these areas, papermakers can achieve better Yankee coating performance at little or no cost.

Spray Bar Set-Up

The spray bar can offer a uniform, optimized coverage of coating and release agents to the Yankee surface, depending on pressure, type and number of nozzles and the position of the spray bar. In most cases, it is good practice to have triple coverage (double overlap) spray pattern across the whole Yankee width, while minimizing over-spray at the edges. Calculations are available and can be devised to help predict and design the correct spray bar and pattern for a given application. Incorrect spray application can lead to: streaky or even no coating on the Yankee surface; excessive add-on and sprayed water inhibiting the cross linking or tackiness of the coating; poor overlap and uneven coating profile; loss of chemical and overspray hazards. These can lead to poor quality product and poor moisture profiles in the reel with the potential to damage the Yankee surface if left unchecked, so it is important to ensure that the correct type of spray bar is fitted and is maintained to offer the correct flow rates and spray pattern at all times.

Spray Bar Design - Double Overlap (triple cover)

In some cases a separate edge spray setup should be included on the edges of the Yankee, particularly if edge build-up occurs and causes blade movement and chatter. Specific chemistries have been developed to help lubricate and clean the edges of deposits. Careful set-up will ensure the system is offering the correct level of protection and not washing off excess coating. The distance of the spray bar from the suction press should allow enough dwell time between addition point to the Yankee surface and the nip on the suction press and pick up of the sheet. Incorrect positioning will affect the coating performance, inhibiting the ability of the polymer chemistry to react in time and not offer the correct tack for pick up of the sheet due to incorrect heat of evaporation. The typical dwell time is 25-75 msecs which is the measured time from application point of the coating spray to the Yankee to the nip point at the suction press. The coating chemistries developed have to be able to react and form to a ready state and correct rheology during this dwell time in order to pick the sheet up and adhere it evenly to the Yankee surface. Figure 2 shows the typical phases for the coating formation of a thermo-setting resin. The initial stage is dehydration so it is important that the spray water add-on is within the correct range to allow the right rate of evaporation.

Coating Information

Coating Equipment

Correct dosing and make down equipment are essential as are the dose point and application. Without proper investment and application the chemical cannot perform as required and results will be variable at best.

The basic set-up must have: chemical dosing pumps with flow control and, if appropriate, high viscosity heads; mixing tank (static mixer rather than air) not too large and set at 50-55º C; a recirculation facility to allow good flow control and also help mixing; correct size and pressure rated dosing pump; filters sized to ensure that any particulates are captured and do not block the nozzles; correct size spray bar and position.

The incorporation of flow meters, lowflow sensors, alarms and dry-run protection should also be considered. If possible, remote control and read-out on the DCS or separate system would be an advantage.

The dosing equipment should ensure: correct type and sizing of pumps; good mixing in the mixing tank, with temperature at 50- 55º C; filters sized to ensure particulate capture and prevent nozzle blockages; protection and alarms to ensure continuous operation; remote operation and conversions to add on rates as mg/m²

Crepe Blade Set-Up

The physical attributes of the tissue are largely determined during creping. Often the blade is not optimized or changed to suit product expectations. Sometimes the blade geometry and set-up remain unchanged regardless of the product. This lack of optimization reduces crepe performance and product quality.

Blade type and material strongly influence the crepe performance. In general steel blades are used for kitchen towel and low-grade bathroom tissue products. For premium soft bathroom tissue and facial tissue a ceramic blade is preferred as it offers superior performance and longevity, though cost is higher.

By correct geometry and set-up of the crepe blade it is possible to control softness, tensile strength, stretch, bulk of the tissue and absorbency. Keys to crepe performance are: crepe holder angle; crepe blade fit to the Yankee surface, stick-out ( free height above blade holder) and bevel angle

The success of the creping process relies on coating performance and sheet properties including moisture levels. The geometry of the crepe blade is a key aspect and one the papermaker can influence most easily and cheaply.

Results from the use of portable vibration monitoring equipment on the crepe doctor are open to misinterpretation but studies by the Ashland Hercules Tissue Applications Group have found good correlation between the vibration frequency and blade chatter induced by hard coating. Vibration analysis can be a useful tool.

Yankee Considerations

Generally 40% of the drying is performed by the Yankee (steam fed) and 60% by the hood (gas). The process of drying involves conducted heat from the Yankee and diffused heat from the hoods. The Yankee should be regularly inspected to ensure that the internals are intact and the soda straws are deposit free and in good working order. Any defects need rectifying as internal problems will lead to external issues with hot or cold spots on the Yankee surface. This will affect the performance of the coating and crepe ability and thus product quality. In the worse cases it is possible to cause loss of coating and surface damage. Points to ensure with the Yankee include: good surface condition; condensate headers and soda straws are not blocked or leaking; condensate soda straws are correctly profiled to give an even condensate depth; thermocompressor and steam supply are even; and boiler steam and condensate are adequately treated to avoid corrosion and scale.

Yankee Cylinder

Mill engineering staff and/or the Yankee manufacturers normally help control and monitor the Yankee internals and recommend any changes required to keep an even temperature profile across the Yankee surface. Variations in surface temperatures can cause variation in moisture profile and quality issues. In extreme cases the coating itself can be affected as it relies on a set temperature range to function as required. Relatively cheap infrared or thermal image cameras are available and can be put to good use in helping to measure and monitor surface temperatures.

Press And Felt

Among the greatest influences on the sheet’s ability to adhere to the Yankee correctly and uniformly are the suction press fit to Yankee and the felt condition and type. Failure to produce a sheet with even moisture profile and within expected range of solids can cause uneven adhesion to the Yankee, thus poor moisture profiles and uneven creping. The correct set-up and fit of the suction press are vital to ensure even pressure across the profile.

As the Yankee and press run at higher speed and heat up, the Yankee deforms and the process becomes complex. It is important to calculate the press-fit profile to account for the degree of deformation and that the correct crown is used.

Aims with the crowning are to: achieve uniform dryness and paper properties; minimize roll wear and paper trim; improve runnability leading to higher speed and production.

The type of the felt and its condition also influence the sheet, particularly the moisture profile. As with the suction press, it is important that the felt is chosen to offer the correct degree of dewatering and drying for the sheet quality required. Over time, the felt wears due to thermal and frictional forces which compress and stretch it, reducing its performance. It is also subject to filling with fines and fillers, particularly if recycled furnish is used.

Wet-strength resins can also have a detrimental effect on felt performance; felts need conditioning and cleaning regularly. Poor felt performance can markedly affect Yankee coating performance. It can even cause wash-off of the coating and hence poor drying and creping of the sheet. There can be so much loss of coating that the crepe or cleaning blades contact the Yankee surface and potentially cause chatter marks and damage the surface.

It is important to realize that deviation from centerlines, often with the goal of shortterm cost savings, can in fact seriously compromise machine performance and tissue quality.

Chemical suppliers can also recommend a range of continuous conditioning and cleaning programs for fabrics. For example, neutral continuous conditioning products can be very effective in increasing run time and dewatering of the felt on machines using wet strength resin; this in turn can help significantly with the operability of the Yankee creping program.

In summary, close attention to all details and following the recommended centerlines from machine vendors and consumable suppliers will ensure that the chemical and mechanical interaction on the tissue machine deliver the optimum machine operating performance and tissue quality.

Andrew McNab is Applications Team Manager – Strength and Tissue with Ashland Hercules Water Technologies. The article is based on his presentation atTissue World Nice 2009. To request a copy of the fullpaper please email amcnab@ashland.com. The author would like to thank Metso Tissue for its permission to use the illustration above.