By Ingvar Klerelid and Ola Thomasson
Metso
Paper's new Advantage NTT machine can operate in two different
modes, one producing high quality textured products with high
bulk, the other a conventional mode making products similar to
a conventional dry crepe machine. The swing capability is important
for tissue producers building a market for high bulk products.
In the NTT textured mode it is possible to produce tissue products
with 50-80% more bulk compared to conventional dry crepe machines
(DCT®). Fibre savings of 10-30% can be achieved in the finished
product. The sheet transferred to the yankee has a dryness of
43-45%, which makes the NTT process lower in energy demand than
DCT machines. In the NTT conventional mode, the tissue products
are same or better compared to DCT. The dryness is significantly
higher than DCT. With the NTT process it is possible to reach
dryness level of 45-47%, which results in a drying energy saving
of more than 20%.
The NTT machine is easy to operate and has high machine efficiency.
This has been demonstrated on the pilot machine. The operating
cost including fibre, energy, clothing, chemicals, labour etc
is on the same level as for DCT. When fibre savings are included,
the NTT is by far the most attractive concept.
THE MACHINE IN DETAIL
Metso Paper in Karlstad,
Sweden, has developed the Advantage NTT technology in close cooperation
with Albany International during the past three years. This cooperative
effort has been the key to achieving the aggressive targets for
the new tissue process and the machine is now available for the
tissue industry.
The machine has a forming section, a pre-press for dewatering
and structuring the sheet and a transfer press to the yankee dryer.
The drying is carried out on a yankee dryer using a high performance
hood. In the dry end there is a new type of reel. Figure 1 shows
the machine layout for Advantage NTT™ 200.
The forming section is basically a crescent former, as the sheet
is formed in a gap between a forming fabric and a press fabric.
The position of the headbox on the NTT differs from DCT. On the
NTT, the headbox is below the forming roll like on a C-wrap type
twin-wire former. The save-all has the latest design for water
handling and guiding the water from the headbox into a flume on
the drive side of the machine. The geometry in the forming section
with respect to jet setting and wire/felt separation is the same
as on the DCT machine.
The OptiFlo II TIS with layering in two or three layers is the
recommended headbox for NTT. It is a headbox of latest design
at Metso and, compared to headboxes from earlier generations,
it improves the formation. A dilution system can be added to the
headbox to improve basis weight profile, but it is not critical
for the operation of the NTT process.
The web is carried on the press fabric from the forming section
to the pre-press. Prior to the pre-press nip there is a suction
turning roll to dewater the felt and the paper web. The dewatering
of the felt and the web is very important for the performance
of the pre-press.
Pressing, from energy point of view, is the most efficient way
to remove water from the web and this is the reason it is used
in NTT. In the pre-press nip the dryness of the paper web is increased
up to 45% for the textured mode and up to 48% for the conventional
mode. In the nip the web is also transferred to the belt and the
belt then carries the web to the yankee dryer. Rewetting of the
web is minimal during the transportation to the yankee dryer and
because of this it is possible to achieve and maintain a high
dryness when web is transferred to the yankee dryer.
Albany has developed special belts for the NTT concept. For
production of textured products, the belt brand is NTTBelt T.
These belts can be supplied with different embossing patterns
depending on type of tissue product to be made. For production
of conventional tissue products, a plain belt is used and the
brand name is NTTBelt P. The belt has good wear resistance both
for textured and conventional modes and the estimated life time
is at least the same as for the felt. The designs of the forming
fabric and the press fabric designs are basically the same as
used in DCT and also the expected lifetime.
The machine components used in the Advantage NTT machine are
all proven in the field at running conditions very similar to
NTT.
The dry end design and equipment is critical for the performance
of the textured mode of operation. Conventional dry crepe machines
can run up above 2000 m/min using a standard reel. This is not
the case for TAD machines where more sophisticated reels are used.
The most difficult product to run at high machine speed is toilet
tissue because of low tensile strength and high bulk. Metso has
developed a new reel concept which improves the winding and the
turn-up efficiency. The name of the new reel is Advantage SoftReel
Belt and is an important part of the NTT process to achieve high
machine speed for the textured products. Experience from pilot
trials show that the NTT process is simple and easy to operate
at high machine speed. The forming principle is the same as on
crescent formers, which means it is also possible to achieve a
very uniform formation also at very low basis weight. The transfer
of the web from the press fabric to the belt in the pre-press
nip has on the pilot machine not been an issue. However, the surface
properties of the belt are important for achieving sufficient
adhesion of the web during the transportation to the yankee dryer.
The coating applied to the yankee controls the transfer of the
web and the creping process. In the textured mode typical TAD-type
coating chemicals are used. The add-on rate is lower than with
TAD and in general it has been easier to achieve a stable coating
compared to TAD when producing toilet tissue.
The focus in the development work has mainly been on toilet
and facial tissues. Excellent products have been made with 50-80%
higher bulk or caliper compared with conventional dry crepe machines
and close to TAD if compared at the same softness.
Figure 3 shows sheet bulk vs tensile strength index for toilet
with a basis weight of 18 g/m2. The products were made in the
NTT textured mode with furnish consisting of 40% SW and 60% Eucalyptus.
The structuring of the web is carried out during the dewatering
process in the pre-press and this structure is maintained and
transferred to the yankee dryer. The creping process will then
magnifies the web structure transferred to the yankee dryer and
the result is the textured product on the reel. By carefully selecting
the belt surface embossing pattern it is possible to achieve different
coarseness in the textured sheet. The design of the belt surface
is also very important for the dryness out of the prepress.
More MD orientation in the surface of the belt improves the
creping process and increases sheet softness. The surface area
of the belt is important for the sheet transfer to the yankee.
It also affects the yankee share of the drying and the creping
process, which affects sheet softness.
A fine pattern gives a sheet with lower caliper, but at the
same time high softness. This is similar to the experience from
TAD machines. The fine pattern is suitable for both facial and
toilet tissues. A coarser pattern is more suitable for toilet
tissues and towel. The coarser pattern will also give higher sheet
caliper. The surface area of the belt can be selected, and with
higher surface area a softer product can be produced. TAD products
normally need to be calendered to achieve an acceptable softness.
The pilot NTT trials have shown that calendering is not needed
as much as for TAD.
Figure 4 shows a product comparison for 22 g/m2 toilet tissue
between different technologies. The bulk improvement compared
for NTT compared to DCT was more than 60%.The softness of the
NTT and TAD products was about the same.
Figure 5 shows a cross section of the NTT textured product.
It clearly shows the three-dimensional texture in the product.
Although towel has not been the main focus for the project,
it is of course possible to produce towel and achieve higher bulk
compared to DCT. However, it is not yet demonstrated to achieve
same quality as on a TAD machine. The base sheet for TAD has a
bulk level of 18 cc/g compared to about 7 cc/g on a conventional
dry crepe. The water absorbency is twice as high for the TAD product,
16-18 g/g measured with the basket method. The preliminary trials
with towel indicate that NTT falls in between with a bulk of 11-
12 cc/g and a water absorbency of 10 - 12 g/g.
NTT gives a combination of high bulk and high dryness. Figure
6 shows the relation between bulk and dryness, when drying is
started for TAD, conventional dry crepe and NTT. The bulk of NTT
toilet tissue is slightly less than TAD but a significant higher
dryness. The data points for TAD toilet is with relatively fine
TAD fabric, which gives high softness without calendaring. If
a coarser TAD fabric is used it is necessary to calender to achieve
sufficient softness. When considering operating cost, the high
dryness after the pre-press is not the only advantage with NTT
looking at operating cost. For instance is the vacuum requirement
for NTT machine on the same level as for a DCT machine. In comparison
to TAD, the vacuum consumption is less than 25% of TAD.
Pilot machine operation revealed that the requirement for refining
energy for NTT is lower than for DCT. This makes it possible to
run with higher amount of HW. Less refining and more HW additionally
reduce operating cost. The reason to high tensile strength development
is the relatively high load in the surface area of the belt. This
area has also the best adhesion to the Yankee dryer. Compared
with dry crepe technology, NTT has lower energy cost both in textured
and conventional running modes. The difference in textured mode
is small, but in conventional mode the energy cost is about 20%
lower than conventional dry crepe. Figure 7 shows an energy cost
comparison for three different machine concepts, namely DCT (dry
crepe), TAD and NTT in textured mode. The energy consumption is
for a whole tissue line running virgin fiber, which means stock
preparation, vacuum, refining, machine drives and all drying energy.
The machine room ventilation is not included. The numbers are
at 100% machine efficiency. If the machine operates at 90% machine
efficiency the numbers for the energy consumption will be 10%
higher.
The energy consumption in the table is at different machine
speeds for the three
different processes.
The machine speeds used in the comparison are for DCT 2000 m/min,
for NTT 1800 m/min and TAD 1300 m/min. These are realistic machine
speeds for toilet tissue production and on the high end of each
technology. NTT in textured mode using central Europe energy prices
is €119/ton
(gas €0.32/kWh and electricity €0.07/kWh). The energy
cost for TAD is about €248/ton. The difference is €129/ton
(Figure 8). This
difference in energy
cost corresponds to fibre saving of 23% if the pulp price is €550/ton.
The above numbers are for the instantaneous
consumption which
is the same as at 100% machine efficiency. Dry crepe machines
have normally higher machine efficiency which makes the difference
in energy cost even bigger.
The high bulk of the textured mode creates possibilities of
reducing the basis weight or reducing the weight of the finished
product. In Europe, it is common to produce threeply products
with a total basis weight of about 50 g/m2. With NTT it is possible
to make a two-ply product with total basis weight of 45 g/m2 and
at the same time maintain the roll firmness (hardness). In such
a product the sheet count can remain the same or be reduced. Another
possibility is to maintain the basis weight in the three-ply product.
The sheet count or length of such a product has to be reduced
at the same roll firmness. The fiber saving in finished product
can be at least in the range 10-20%, may be up to 25%. This estimate
is based on finished products made with base paper from the pilot
machine.
The combination of low energy cost and possible fibre savings
in finished products
makes the NTT concept
very attractive for the tissue maker. It is however important
to include all operating costs. Figures 9 and 10 compare product
cost for a unit finished product including variable cost (pulp,
energy, clothing and chemicals) and fixed cost (labor, overhead,
maintenance, consumables) at two different levels of fiber saving
for the NTT and TAD products. The capital cost is not included.
The cost reference is the finished product made on DCT machines
and it is set at 100%. TW
