Chemicals: Yankee coating
Last year we saw a huge global surge
in the cost of petroleum-based energy.
The recession has lowered the cost for
now, but as economies recover energy prices
will certainly increase again. To save energy,
we should focus on evaporation load, coating
nature, coating thickness, sheet intimacy
development promoting heat conduction, and
optimization of hood and Yankee drying.
For every ton of dry crepe tissue produced,
1.3-1.5 tons of water must be evaporated either
with Yankee steam or heat from the hoods.
Below 28% moisture content, the bound water
in hydrogen bonds requires more energy than
the latent heat of vaporization.
The mechanisms in the Yankee area are
about attaching the sheet to the Yankee,
increasing the bond strength between the
Yankee and the sheet, flash drying the sheet,
and then presenting the sheet to the creping
blade in a manner that will result in desirable
properties.
At the pressure roll the sheet contacts the
Yankee surface. The coating should grab the
sheet. The pressure and vacuum of the pressure
roll remove water and force the sheet against
the Yankee surface and into the hopefully
sticky adhesive. Attachment bond must be
greater than the vacuum. A new layer of
adhesive is applied before the pressure roll
nip.
Figure 1 shows the set zone, the
polymerization / drying zone, and the creping
zone of the Yankee. First, in the set zone the
coating shower sprays a mixture of water,
adhesive, releases/modifier, extenders, process
aids such as phosphates onto the Yankee
surface. Once in contact with the Yankee,
factors such as dwell time, temperature,
amount of water sprayed through the shower,
amount of release, and nature of the adhesives
and releases/modifiers used, interact to bring
the applied coating to a point of maximum
tack.
The timing for reaching maximum tack
(Figure 2) is critical. Some adjustments can
conserve energy while others increase energy
consumption. This point of maximum tack,
or set point, should happen at the pressure
roll nip where the sheet first contacts the
coating. If it does not, then one of two
situations will occur. Either the coating sets
too soon, -‘glasses over’ - and does not adhere
well to the fibre or, if it sets too late, the
coating washes into the sheet and felt and
again the sheet does not adhere well. A sheet
following the Yankee does not necessarily
have good attachment. See “Set characteristics
of Yankee dryer coatings” shown in the
references. Good creping requires controls to
insure the coating sets properly in the nip.
Sheet attachment is important to dryer
energy consumption. Attachment does more
than make the sheet follow the Yankee; it
increases the contact area between the sheet
and the coating. Figure 3 represents levels of
attachment. The point is maximum contact
area promotes heat transfer; more heat passes
from the Yankee into the sheet! Increasing
sheet contact or intimacy reduces drying load.
In fact, one indicator of good attachment is
reduced heat consumption.
More pressure at the pressure roll nip
improves intimacy but also reduces bulk and
softness. Increasing pressure roll loading is
seldom the best solution for intimacy
improvement, and is a poor tool for coating
set adjustment.
Other advantages of intimate contact are
improved micro creping, increased crepe
count, and increased stretch. With increased
stretch and finer crepe one can either increase
reel speed or improve sheet softness.
Also, dryer coating thickness can impact
energy consumption. Most coatings are poor
transmitters of heat; thus, a thicker coating requires more energy to reach a specified
moisture content.
The drying scheme (hoods versus Yankee)
impacts coating. Yankee preference tends to
harden coatings faster and requires adjustments
to keep the set point at the nip. Hood
preference reduces heat to the coating resulting
in softer, slower setting coating.
Different mills have different relationships
between the cost of energy for heating hoods
and heating dryers. If you want to save money,
know the drying costs for your machines and
tune your system for drying with the lowest
total energy cost. Remember that changing
your drying balance will change the set of
your coating.
Each machine will have a specific distance
from the coating shower to the pressure roll
nip, but this distance varies widely from
machine to machine with observed distances
of 0.3-3 m. Distance and machine speed
together determine dwell time. Dwell time is
the time, usually expressed in milliseconds,
for a spot at the line of coating shower contact
to rotate into the pressure roll nip. Normal
dwell time would be 35-70 ms and 20-120
ms would cover the full range seen. When
machine speed changes, dwell time changes.
Along with dwell time other factors that
determine coating set are:
- Amount of release used (release retards set)
- Amount of water sprayed with the coating through the shower. (pressure, nozzle size, coverage)
- Temperature of the coating mixture (a fine tuning factor with some severe problems when the mixture temperature gets too hot)
- pH of the coating mixture (only with crosslinking coatings). Higher pH accelerates set while lower pH retards the set). Stock pH can also have an impact.
- Temperature of the Yankee dryer surface after the creping doctors and before the shower contact (what the coating will see)
- Type of coating chemistry being used (heat demand of the coating package)
Changes to most of these control
parameters have secondary and even thirdlevel
effects. Some are desirable and some
are not. The creping expert considers all effect
levels when adjusting. For example, adding
more release retards set time but also lowers
coating bond strength, and additional release
can cause the sheet to fly off the Yankee. On
a third level some oil releases may hurt
absorbency. Release variation is a complex
control for controlling coating set rate. It is
probably near energy neutral though. Here
we discuss only a few factors with strong
energy implications.
The amount of water applied with the
coating is a common control. Generally, it is
better to use the minimum amount of water
to effectively deliver the coating package, but
water is an effective set control. It has little
negative effect as the sheet progresses around
the Yankee and through creping, except it
must be evaporated. A coating shower usually
contributes 4-11% of the drying energy
demand, but loads of 20-30% have been seen.
Given 7% as normal, and an actual load of
20%, then 13% of your drying load is being
used to evaporate shower. Calculate how much
money that 13% is for your machine and that
is the cost of adjusting the set point using
water. Shocking! For example, a 90 ton/day
dry crepe tissue machine with a 40% postpressure-
roll consistency dries the sheet to
5%. Some 59.375 kg of dry fibre pass over
the Yankee each minute as do 89.1 kilograms
of water.
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Figure 1
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Figure 2
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Figure 3
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Figure 4
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*1 The cost of the standard program is not available and it was not considered on the ROI calculation. *2 The cost of this SELECT program was $9.50/ton of paper, with a production rate of 90 tons/day.
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Energy Consumption
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To get to 5% moisture content we
must remove 85.9 kilograms of water/minute
through evaporation. With a shower flow of
7 l/min the shower energy is 7/7+85.9*100%
=7.5%. Double the amount of water and the
percentage rises to 14%. Calculate 1% of your
per day drying cost and you can see that the
impact of using 1% more water to control
coating set is significant.
A wrong coating package requires water
to make it work. Using too much water is
often not by design, but more evolved as
operators work to find a ‘sweet spot’ for
coating performance. In the past it has been
common practice to adjust for a long dwell
time and inexpensive quick set coating by
using more water. A lower adhesive can result
in much higher drying expense than the coating
cost savings.
Short and long dwell time machines are
challenges. Short dwell time machines are
harder to handle than long dwell time
machines. Dwell time and heat transfer is too
short for conventional coating set. Shoe presses
present very long dwell times and extra water
for control is common on these machines.
Case hardening a heavy sheet with lots of
fines, fillers, ash, and resins can boost energy
consumption. Case hardening is rapid drying
of the exterior (hood side) of the sheet, causing
capillary closure. Water is removed from a conventional tissue sheet by the water
migrating to the surface through capillaries
where it is vaporized. Close the capillaries
needed and energy consumption increases.
Hood heat and humidity control, especially
on the wet side hood, will prevent case
hardening.
Yankee dryer coatings are one of the fastest
changing areas of tissue manufacturing
technology. Fifty years ago adhesives and
releases such as animal glue, starch, and then
wet strength resin were used with little
understanding of the mechanisms. Today’s
engineered multi-component adhesives,
releases and extenders produce much better
results. Factors such as differing dwell times,
Yankee and hood temperatures,
polymerization time and grade requirements
demand these specialized chemistries. In the
last 50 years compounded multi-component
products with controlled heat demand, ability
to rewet and redevelop tack, improved wet
and dry film forming, softer cured films, and
high temperature tolerance have been
introduced. This variety allows coating design
to meet specific product and machine needs
including energy optimization.
On one machine with a very short dwell
time (>30 milliseconds), a relatively cold
Yankee used a conventional cross-linking
adhesive. It did not set properly at the pressure
roll nip, filling felts and giving poor sheet
quality. Development of an ultra-low-heatdemand
rewettable adhesive greatly improved
this situation.
Another machine with a 21-ft diameter
Yankee was part of an integrated kraft pulp
mill with inexpensive recovery boiler steam.
The energy saving option was to use maximum
Yankee drying. At high Yankee temperature
the coating over-hardened at the doctor blade.
The Yankee had to be sanded about once per
shift. An ultra high temperature tolerant
adhesive was developed. The machine now
runs with a Yankee surface temperature of
about 120-125° C, allowing the mill to achieve
its energy cost objectives.
Shoe presses present a special challenge
for a coating. The coating shower must not
be above the felt and vacuum roll. One
machine dwell distance was almost 3 m.
Before the shoe press installation a low heat
demand coating was being used on a normally
hot Yankee. With the shoe press, much more
shower water was needed to control set. Even
then the crepe was coarse. A very robust
adhesive needing the long dwell time and
Yankee temperature was developed and solved
this situation.
Finally, we offer an example from a mill
making high quality dry creped toilet tissue.
Its standard coating was a conventional
polyamide/EPI base adhesive, PVOH
extender, and release. Coating work was
initiated to improve softness. This machine
was also drying limited. A secondary objective
was improved drying efficiency and machine
productivity. A Buckman SELECT dual
adhesive coating program was designed
specifically for this machine’s conditions
focusing on improving the sheet to coating
intimacy with less insulating coating thickness.
The evaluation produced the anticipated
softness improvement and with improved and
more consistent crepe quality. Machine speed
was increased. Crepe percentage was reduced,
increasing reel speed and tonnage per day.
Energy consumption was reduced due to better
heat transfer and more uniform sheet adhesion.
The softness and stretch improved because of
improved sheet intimacy and more microcrepe.
The financial return (ROI) for using this
SELECT Program is based on softness
improvement, increased drying capability and
production rate, and reduced energy
consumption – steam and gas.
Theory and practice prove that
considerable energy can be saved by properly
managing a Yankee dryer coating program.
Definite objectives of energy conservation,
proper Yankee dryer coating application and
control, and newly designed chemistries are
fundamental to dryer area energy conservation.
References
“Set Characteristics of Yankee Dryer Coatings” Presentation at Tissue World-Nice Yankee Dryer Workshop, March 2007, and paper in the proceedings; John Stitt – Buckman International Inc. “Every Yankee has its Coating” Presentation at Tissue World-Nice March 2005, and paper in the proceedings; John Stitt – Buckman International, Inc.
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John Stitt is Market Manager - Creped Technology with Buckman International, USA. |
