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ABSTRACT
Today, the marketplace trends demand ever larger single pulping lines
to reach the best overall economy. The latest mega-mill plans call for design capacities as high as 2500 to 3000 tons per day. The bottleneck has been in pulp drying, where such machine capacities have not been
reached with a single drying line. Metso Paper's development work in the field has focused on a single pulp drying line that would capacity-wise match with a corresponding new greenfield mega-mill fiber line.
It is widely recognized that the typical bottlenecks encountered while
increasing pulp drying capacity lie in the former and cutter areas. The web speed in present pulp drying machines is approximately 150–180 m/min and production is about 280 ADT/m. Metso Paper has developed a totally
new pulp drying technology concept called DryWay™. With its new former and cutter design and other process improvements, production speed and capacity has increased dramatically. The DryWay concept features web
speeds of 200 to 300 m/min and production rates of 300 to 500 ADT/meter. In addition to savings in investment and operational costs, the new capacity range enables reaching of up to 30 % savings in drying machine
building volume compared with conventional systems.
Reliable and optimum operation of the pulp drying process at
high-speed levels also requires sophisticated automation and control systems. The new DryWay concept automation system covers process, machine, drive and quality controls, as well as condition monitoring and baling
management. Recently this technology was implemented and started up at the Stora Enso Kaukopää mill in Finland. This project is the culmination of all the combined knowledge in the field, featuring totally new
solutions in many process areas from the headbox up to pulp baling. The presentation describes the experience obtained in mill scale using the DryWay technology.
INTRODUCTION
In the early 1990's, the marketplace trend was towards large
single-line pulping lines in an attempt to reach the best overall economy.
The latest mega-mill plans call for design capacities as high as 2500 to 3000 tons per day. The bottleneck has been in pulp drying, where such machine capacities have not been reached with a single drying line. In 1994, Metso Paper began a development project with the goal to make a cost effective, single, pulp drying line that would match the capacity of a corresponding new Greenfield mega-mill fiber line in the 2500 to 3000 ADMT/D range.
This paper describes the development project process and results.
CURRENT TECHNOLOGY REVIEW
With the goal established, it was necessary to determine how large a
gap existed between current technology and the goal.
Thus, prior to creating a new concept, a study of the top pulp drying machines in the world was made. As expected, the most production was made on the widest machine. However, not all machines are at the same capacity loading in terms of ADMT/ meter width of trim. In fact, the top eight machines' trim ranged from 6.4 meters to 8.1 meters, with capacity loading from 200 tonnes/trim meter to 272 tonnes/trim meter (Fig 1). Speeds ranged from 150 meters/minute to 180 meters/minute. The conclusion reached from this study was that existing technology is limited to around 280 tonnes/trim meter and around 180 meters/minute. Thus, using conventional technology to make 3000 tpd would require a drying machine 10.7 meters wide. While paper machines have been built this wide, to date no pulp dryers have been built this wide.
Figure 1
ECONOMIC REVIEW
Next a study of the economics of a wider machine was made. Because of Metso Paper's large paper machine experience, it was quickly concluded that the structural requirements of
a wider pulp machine would require heavier members and stronger foundations and therefore be expensive on a Total Installed Cost basis.
Looking at the cost of the machine hall identified some large cost numbers also. Visualize
the machine hall width in multiples of trim widths. First, of course, there is the width of the machine made up of the trim plus some space for the bearings and frames. Define this as 1+
trim widths. On the backside of the machine, there is another 1+ trim widths filled with the cantilever beams, drives, tanks, pumps, vacuum pumps, hydraulics, and other related
equipment. On the tending side, space is required for roll changes, wire changes, and room to walk around when these activities are going on. Define this area as 1+ trim width also.
Summing the widths results in a machine hall width of 4.0 to 5.0 times the trim width. For the 10.7 meter trim machine previously discussed, the machine hall would be 40 – 50 meters
wide. This would be very costly to construct.
BRAINSTORMING IDEAS
Other questions were raised and studied but will not be addressed in this paper. Some of
them were:
- What is the most cost-effective solids level going to the dryer?
- What unit operations are limiting the speed of a drying machine?
- What is the practical maximum web length in a dryer before the web stretch requires
an intermediate draw control?
- How fast can a baling line run?
- How much automation should be modularized and embedded into the unit operations?
And how much should be in the DCS?
DEVELOPMENT PARAMETERS DEFINED
The various studies caused the goal of a single drying line for 2500 – 3000 ADMT/D to be
defined into several development objectives that had to be solved in order to meet the goal.
1. Increase web speed from 150-180 m/min to 300+ m/min
2. Increase machine loading capacity significantly over 300 ADMT/trim meter
3. New energy-efficient wet end design
4. Energy-efficient high speed dryer
5. New high speed cutter design
6. Baling equipment designed for 300 bales/hr
7. Intelligent process automation
To accomplish the objectives and to develop and prove the new ideas, three new pilot
machines where built; one for forming, one for drying, and one for cutting. Existing press section pilot machines were used. Mill-scale baling equipment prototypes were also built.
WET END CONFIGURATION
Several forming concepts were quickly narrowed to one which concentrated on removing
water from both sides of the sheet, dewatering in a sealed environment to avoid introducing air into the web or white water, and using gravity, not vacuum pumps, to aid dewatering.
The final configuration chosen is called the DryWay™ Former (Fig 2). Pilot machine results
are shown in Figure 3 and can achieve 50% higher capacity loading than conventional technology.
Figure 2
Figure 3
The former consists of a headbox and a series of specially designed boxes with perforated covers positioned on the top and bottom of the sheet. Two wires carry the web past the dewatering boxes with the bottom wire carrying the web through the first press. The
former is sealed on all sides to prevent air from entering the pulp or the whitewater. The seal is comprised of the headbox, the top and bottom dewatering chambers, the side plates,
and the web leaving the forming zone. The headbox accepts feed consistencies between 0.6% - 1.4%. Dry solids after the former are 23 – 25% OD. Solids going into the airborne
dryer are 50-54% OD. While these solids levels are similar to conventional technology, it must be remembered that this machine is operating at much higher speeds and capacities, 300 m/min and 300+ tonnes/trim meter.
PRESS DEVELOPMENT
The press section has only two nips. The first nip is a long nip shoe with a grooved
polyurethane counter roll called the DryWay™ PrePress. The maximum linear load is 800 kN/m. The second press, a SymBelt Shoe Press, is a long nip shoe press with a rubber
covered Sym ZL counter roll and a maximum linear load of 1500 kN/m.
DRYER DEVELOPMENT
The airborne dryer design parameters were:
- High evaporation effect to decrease the size of the dryer
- Straight web run to avoid contact with the dryer
- Non-contact drying to avoid contact with the nozzles
- Automatic tail threading system to increase threading speed
Extensive pilot machine work was done to maximize evaporation efficiency. During the trials
, efficiency gains where observed with specific nozzle patterns, specific nozzle shapes and a specific blowbox configuration. The final configuration chosen optimized the three
parameters resulting in a 6% gain in overall evaporation efficiency. There is excellent floatation and web support, even at the wet end. The net effect is a shorter drying length.
Higher web speeds raises the requirements on the design and construction of the turning
rolls, as well as the other rolls. The design team drew on their experiences with high speed paper machines to design rolls that will ensure even and vibration free web runs in the roll towers.
Even though the drying length is shorter, there is still over a kilometer of web in the dryer.
Web tension puts considerable forces on the dryer turning roll frames. So special attention must be given to torsional stiffness of the frames to ensure straight and even web runs. All
of this practical application of science and experience ensures a straight and true running web that requires minimal web guiding.
In keeping with the goal of cost effective solutions, an all-steel steam coil design with a
specific configuration of elliptical tubes and metallic contact between tubes and fins was chosen to ensure the most effective use of low pressure steam.
CUTTER DEVELOPMENT
How does one cut a web going 18 km/hr into rectangles, and reduce the speed to zero while
piling the rectangles into bales? And do so for sheets and wrapper? This was the challenge faced by the research team for the cutter/layboy.
It is common knowledge that around 180 m/min, individual pulp sheets tend to float, skew,
and begin to interweave. Also, a web speed of 180 m/min is approaching the upper limit of single overlap cutting. The cutter development project focussed on controlling the individual
sheets at speeds above 300 m/min. After many trials and many hours of reviewing high-speed videos, the high-speed cutter concept came together. The cutter is designed to run
sheets and wrapper in a double overlap mode at all speeds up to 300+ m/min.
INTELLIGENT INSTRUMENTATION
As speeds increase and quality parameters increase, the need for intelligent instrumentation
has increased. Not only must each unit operation be controlled, but also all of the units must work together to optimize the machine. And as technology advances, more
sophisticated control systems are being used to run today's machines. These systems are able to produce the quality desired, at higher speeds, freeing up operators for other duties.
But where should one put the controls? One can generate strong opinions when it comes to
deciding whether to put the control programming for a unit operation into a local PLC or to put it in the DCS. There are valid reasons on both sides of the discussion. Metso has
chosen to develop "Intelligent Machines." Unit operations are automated and embedded into each section of the machine. For example, the headbox control functions are within the
headbox unit. Dryer controls are within the dryer unit. Overall machine operating controls are built into the DCS level, as well as are the links to the performance and quality systems.
The net result is proper control of each section of the machine and overall optimization of
the entire machine. Performance and quality parameters are at the operators' fingertips, and operators have the time to optimize the process.
BALING DEVELOPMENT
Bales of pulp are the finished products for market pulp producers. The requirements for the
finished bale are:
- A neat and tight package
- Uniform packaging
- Accurate, consistent wire placement
For years the existing baling technology of brute force was adequate. Today, it is too slow
to match the large single line pulp dryers. Higher capacity lines are needed.
The high speed baling development project is a paper unto itself. In summary, the
development of a baling system to run at twice the speed of existing technology was achieved by abandoning hydraulic and pneumatic operations and replacing them with
servomotor technology. This permitted faster cycle times and greater precision in bale placement and wire placement.
Dedicated control modules on each unit operation and Fieldbus technology permits "plug and
play" operations with a lower installed cost over conventional solutions.
MILL SCALE
After six years in development and 16 months in construction, the result is called DryWay™.
Starting up in April 2001 at the Stora Enso mill in Kaukopää, Finland, the machine is designed for 1000 ADMT/d on 3.2 meters trim. This project is the culmination of all the combined
knowledge in the field, featuring totally new solutions in many process areas from the headbox up to pulp baling. As designed, the machine loading is 313 ADMT/meter width of
trim. Shortly after startup Stora Enso set the pulp drying world records for web speed and for capacity.
Typical Operating Parameters:
- § A basis weight target of 700 - 800 gsm, with a 2-Sigma of 0.6%
- A feed consistency of 1.2 - 1.8%
- Solids after the press section are 52 - 54% on Nordic HW
- Web solids after the dryer are 90%
- Web speed is about 220 m/min
Machine Physical Dimensions:
- The wet end (Headbox and former) length is approximately 10 meters
- The Press Section length is 10 meters
- The Dryer is 33.5 meters long, with 32 drying decks and one cooling deck
- Cutter-Layboy is approximately 10 meters long
Operator acceptance of the technology is very high. As with most World Record Holders,
they are proud of their achievements, as they should be. They even thread the machine at 180 meters per minute because they say it threads easier at the higher speeds.
SUMMARY
The goal of a single drying line for 2500 – 3000 ADMT/d has been achieved. And each of the specific development objectives were met or exceeded.
1. Web speed has increase to 300+ m/min.
2. Machine loading capacity range is 300 – 500 ADMT/trim meter, even higher based upon
pilot machine runs.
3. An energy-efficient wet end and an energy-efficient high-speed dryer are operating.
4. Several high-speed cutters are operating.
5. Many Baling systems are operating near 300 bales/hr.
6. Intelligent process automation is operating on several machines.
With the commercialization of the development project a success, it would seem to be the
end of the development project. But actually this is just the beginning! The project goes on with the goal to further improve the overall economy of the system, starting with new ways
of stock screening and feeding.
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