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By Ian Lang
 Through-air
drying (TAD) is recognized as the process of choice for
the production of premium tissue and towel products. One
drawback associated with the TAD process is the high cost
of drying, owing largely to the low solid content of the
web entering the dryer section, although a significant
amount of the evaporative load is due to the water carried
by the TAD fabric from the fabric conditioning showers
into the drying process.
Recent work by AstenJohnson has focused on the development
of new materials to modify the behavior of fabrics, specifically
to enhance hydrophobicity and oleophobicity to reduce
drying cost and improve contamination resistance.
This paper deals with investigations carried out on a
pilot TAD machine with new and conventional TAD fabrics.
The results confirm reduced TAD energy consumption due
to a reduction in water carry to the drying section. Results
from field trials provide evidence of reduced fiber carrying
by the fabric.
THE TAD PROCESS
 In
a through air dried (TAD) process the web is transferred
to the TAD dryer section without prior mechanical dewatering.
Web transfer from the forming fabric to the TAD fabrics
is by means of a vacuum pickup or roll. A vacuum moulding
box may be used after the pick-up to “mould”
the web to the TAD fabric, imparting a 3-D structure to
the web and creating areas of alternating high and low
density. The moulding process is generally used for TAD
towel whereas TAD tissue uses little or no moulding. The
TAD fabric carries the web over the TAD roll where it
is partially dried and then carried through a nip onto
a Yankee cylinder and dried to completion. Alternatively
the web may be dried to completion without the use of
a Yankee cylinder.
The absence of pressing prior to the drying step, in conjunction
with the moulding step, produces a web which is considerably
bulkier. The absence of densification, along with shaping
generates the unique properties of TAD towel and tissue,
namely high bulk, increased water absorbency and “feel”
or softness. The superior water absorbency (on a gram
per gram basis) of TAD tissue and towel allow the papermaker
to reduce the basis weight of the web resulting in considerable
fiber savings. From a drying perspective, the process
is energy intensive as the low incoming solids content
at the start of the drying phase results in evaporative
loads two to three times that of wet pressed tissue.
TAD fabrics are subject to contamination from fibers as
well as additives, in particular wet strength resin, requiring
the use of cleaning showers. A typical TAD fabric conditioning
system consists of flooded nip showers in conjunction
with high pressure showers. Traversing needle jet showers
may also be used. Conditioning showers are followed by
a Uhle box to remove water from the fabric. Air showers
may be used in addition to the Uhle box.
Water remaining in the TAD fabric following the conditioning
step is carried through to the web pick-up and moulding
phase where it contributes to the evaporative load in
the dryer section. Recent work by AstenJohnson has focused
on TAD fabrics, with an aim to develop an understanding
of their water carrying behavior and means to reduce their
impact on drying energy consumption.
Research in the field of materials has lead to new treatments
which impart desirable properties, such as oleophobicity
and hydrophobicity. TAD fabric designs incorporating this
treatment are supplied under the name of “ArmorTec
TM”, an example of which can be seen in Figure 1.
This work has led to fabrics with improved dewatering
performance and resistance to contamination, resulting
in reduced showering and less frequent production interruptions
for cleaning.
BACKGROUND
Early work (1) was carried out on laboratory dewatering
and drying apparatus at the Institute of Paper Science
and Technology (IPST) in Atlanta, USA and Karlstad University
in Karlstad, Sweden. Trials were carried out to simulate
the TAD fabric conditioning process to determine the potential
for water carrying by the TAD fabric into the moulding
and drying stage. In addition, the moulding phase was
simulated to determine the residual water carry by the
web as well as the TAD fabric with both conventional and
ArmorTec treated fabrics being tested.
Results of one of the conditioning tests with two different
fabric types, a novel double layer fabric designated as
“A” and conventional single layer, designated
as “B” are shown in Figure 2. Water carry
with the ArmorTec treatment was reduced by approximately
50 percent as a result of the hydrophobic nature of the
modified surface. Interestingly the treated double layer
fabric carried less water than the conventional single
layer fabric.
Trials on the Karlstad lab apparatus were done with vacuum
residence times closer to those found in industrial practice.
The results of one test simulating the moulding process
can be seen in Figure 3.
The treated fabric carried approximately 50 percent less
water than the conventional fabric. In addition the solids
content of the web shaped on the treated fabric was slightly
less, 1 percent or so, compared to the web shaped on the
conventional fabric. The reduction in sheet solids was
felt to be due to reduced rewet of the sheet from water
remaining in the fabric after vacuum moulding.
Data from mill trials is somewhat limited. Reductions
in gas consumption at one mill in the order of 5 percent
were reported. Reduced downtime for batch washing of TAD
fabrics as well as reduction in conditioning shower water
fiber contamination has also been reported. Detailed results
on gas and electrical power consumption are difficult
to obtain owing to the highly secretive nature of the
business.
Given the limitations of bench top test equipment and
the difficulty in obtaining detailed operating data from
commercial machines it was decided to carry out trials
at the Metso tissue R&D center in Karlstad, Sweden.
PILOT MACHINE TRIALS
The goals of the trials were as follows:
• Quantify TAD fabric water carry and its effect
on drying energy consumption
• Determine the effect of ArmorTec coating on TAD
energy consumption
A trial plan was set up to investigate two extremes of
wet end dewatering and moulding box vacuum. One condition
was designed to reproduce the conditions normally used
for towel production, with moderate wet end vacuum and
the maximum moulding box vacuum. This condition will be
referred to as “Moulding Box On”. The second
condition represented the other extreme in terms of dewatering,
with vacuum maximum wet end vacuum, but no moulding box
vacuum. This condition will be referred to as “Moulding
Box Off”. This arrangement is occasionally used
for the production of bath tissue.
In both trials a 20gsm sheet comparable to “normal”
TAD towel was produced. Furnish was 100 percent softwood.
Refining power was 40 kW/ton. Wet strength resin was added
at the rate of 12 kg/ton. TAD fabric conditioning system
Uhle box vacuum was -30 kPa. Two TAD fabrics were used
for the trials, one without and one with the ArmorTec
treatment. For the first trial the moulding box was on
and only one TAD was in operation. The results with an
untreated TAD fabric are shown in Table 1.
Runs 10, 20 and 30 represent normal running condition
with showers on. Run 60 was carried out with the cleaning
showers off. This represents a condition outside normal
operating parameters, but was done in order to quantify
the impact of shower water on TAD energy consumption.
With showers off, energy consumption was reduced by approximately
16 percent. The reduction was due in part to reduced cooling
of the TAD fabric as well as reduced water carry to the
dryer. Sheet solids at the TAD inlet in runs 10-30 were
1.4 percent lower than with showers off, clearly the result
of water in the TAD fabric wicking into the sheet.
Runs 40 and 50 were carried out with reduced vacuum in
the TAD fabric Uhle box, at -20 and - 10 kPa vacuum respectively.
Comparing the incoming sheet solids and total heat consumption
for these two runs with runs 10-30, no measurable difference
was seen. This suggests that TAD fabric water carry downstream
of the moulding box was not significantly affected by
the lower vacuums and that some potential for energy savings
may exist by reducing Uhle box vacuum.
Table 2 shows the result from the second trial with a
standard and an ArmorTec coated fabric. Furnish, refining
and basis weight in the second trial were comparable to
that of the first trial. Both extremes of vacuum dewatering
and moulding were used. The standard fabric was run first
and the ArmorTec coated fabric second. Both TAD hoods
were in operation for the second trial.
Comparing the conventional and ArmorTec coated fabric
runs, total energy use by TAD 1 and 2 was reduced by 4.2
percent and 5.2 percent for the case with the moulding
box on and moulding box off respectively. For both the
standard and ArmorTec coated fabrics, sheet solids into
TAD1 were higher for the set-up with the moulding box
on than with the moulding box off. Clearly a larger amount
of rewet occurred on the TAD fabric without the moulding
box, which explains why the coating had a bigger impact
on energy consumption with the machine operating in that
arrangement.
From the first trial we observed that average heat input
with showers on was 16 percent higher than with showers
off, whereas the evaporative load, as determined from
grab samples was only about 5 percent higher. This suggested
that water was being evaporated in excess of that which
was determined from the grab samples, the likely explanation
being that a part of the water carried by the fabric after
the conditioning Uhle box was being “wicked”
into the sheet after the pick-up and an unmeasured part
remaining “unwicked” the fabric.
A method of determining the “wicked” and “unwicked”
components of TAD fabric water carry was developed. The
solution employed taking the difference between actual
TAD energy consumption and the predicted theoretical amount,
based on a dry TAD fabric. This, along with an estimate
of the amount of wicked water, determined from the sheet
solids from the runs with showers on and off, allowed
a determination of the water carry. The results of the
calculations for the first trial are shown in Table 3.
The difference between the actual and theoretical energy
consumption, divided by the theoretical energy was multiplied
by the evaporated sheet water for the TAD. This yielded
an amount of water equal to the “unwicked”
water i.e., water not accounted for in the sheet.
From run 60 (dry TAD fabric), the sheet water evaporated
by TAD 1 was divided by the basis weight – to determine
the sheet water, normalized to basis weight. Multiplying
this normalized sheet water (dry fabric) by the web basis
weight for runs 10-30 gave the adjusted sheet water (with
no wicked water) for those runs. The adjusted incoming
moisture content was determined by multiplying the difference
between the measured ingoing and outgoing water content
by the adjusted sheet water, divided by the original sheet
water and adding the outgoing water content. From the
difference between the measured and adjusted water content
the amount of wicked water was obtained.
From this it was determined that the average amount of
water carried by the TAD fabric, with a moulding box on,
was equal to approximately 7.2 gsm (average of runs 10-30)
or 14.6 percent of the adjusted sheet water.
A similar technique was used to determine the effect of
the ArmorTec treatment on TAD fabric water carry. The
results from the second trial are shown in Table 4.
As before, the difference between the actual and theoretical
energy consumption, divided by the theoretical energy
was multiplied by the evaporated sheet water for the TAD
to yield the unwicked water. For the runs with the moulding
box (10, 20 and 50, 60), the wicked water was assumed
to be equal to the average result determined from the
first trial, 3.4 gsm (the average of runs 10-30 in table
1). Note that this assumed that the amount of wicked water
is independent of speed – possibly a conservative
estimate. For the runs with no moulding box (30, 40 and
70, 80) the wicked water was determined from the difference
between the sheet moisture content after the wet end and
the moulding box, it being assumed that the moisture gain
was due entirely from the sheet wicking up water from
the TAD fabric.
The results show that for the standard fabric with moulding
box, the fabric water (wicked and unwicked) was equal
to 8.6 gsm, approximately 19.4 percent of sheet water,
slightly higher than what was observed for the first trial.
For the case without moulding box, the result is more
significant, with fabric water being 14.2 gsm or 27.3
percent of the sheet water.
With the ArmorTec treatment, fabric water carry was equal
to 4.9 gsm or 11 percent of the sheet water, with the
moulding box on and 7.3 gsm or 13.4 percent of the sheet
water with no moulding box. This represents an average
reduction in TAD water carry of approximately 43 percent
and 49 percent, for the case with and without moulding
box, as compared to a standard fabric.
CONCLUSIONS
TAD fabrics require the use of continuous showers to maintain
fabric cleanliness. TAD fabric conditioning showers contribute
to drying energy consumption as the result of residual
water being carried by the fabric into the TAD, as well
as the cooling effect of the shower water on the TAD fabric.
The contribution to overall drying energy consumption
is significant.
From pilot trials it was found that, compared to normal
operation with conditioning showers on and operating with
a moulding box, TAD drying energy was reduced by 16 percent
when operated without showers. With no moulding box, energy
consumption due to TAD fabric water carry would be substantially
higher. This underlines the importance of reducing TAD
fabric water carry by vacuum or other means.
TAD fabric water carry was found to be between 14 percent
and 19 percent of sheet water when operating with a moulding
box, approximately 27 percent with no moulding box.
ArmorTec was found to be effective at reducing water carried
by the TAD fabric by between approximately 43 and 49 percent.
For the case of operation with, and without a vacuum moulding
box, ArmorTec treatment reduced drying energy consumption
by 4.2 percent and 5.2 percent respectively. TW
Ian G. Lang, P.Eng., M.Eng. is team leader - Advanced
Product Development AstenJohnson, Kanata, Ontario, Canada
| References |
| 1. |
Lang, I., New Through Air Drying Fabrics
for Reduced Energy Consumption, Proceedings of Tissue
World Americas Conference (Miami, November 2004) |
| 2. |
Graneveld, R., Nilsson, L.B., Nilsson, L.S., Stenstrom,
S., Development of A Laboratory Apparatus for Investigation
of Vacuum Dewatering of Low Basis Weight Paper Sheets,
Proceedings of Nordic Drying Conference (Copenhagen,
June 2003) |
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