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Tight fit. The AWP takes up a very small footprint in the mill. Here it is shown sandwiched between two old presses at Skutskär (one on the left side and one behind it).
It was at the 2008 SPCI exhibition in Stockholm that ANDRITZ introduced its new AWP wash press. In addition to highlighting the technical merits of the new design, Mikael Forslund, Senior Vice President and Division Manager, revealed that the first AWP had already been sold – to Stora Enso’s Skutskär Mill in Sweden.
The AWP is now operational and we recently had an opportunity to visit with Lasse Aspelin, Fiberline Production Engineer at Skutskär, and Göran Bröttgårdh, Product Manager, and Johan Sjöberg, Process Specialist, from ANDRITZ. Aspelin gave a tour of the mill and talked about the performance of the AWP since its start-up in June of this year.
Skutskär: always improvingSkutskär was established over 100 years ago and is a sulphate pulp mill producing about 540,000 t/a of bleached pulp and fluff pulp from birch and softwoods. “We have three digesters, three bleach plants, and four drying machines (two for baled pulp and two for fluff pulp),” Aspelin says. “We are one of a few producers of fluff pulp in Europe.”
Recently, Stora Enso shut down the nearby Norrsundet pulp mill and Skutskär will acquire some machinery from it and the Kemijarvi mill. “It is tragic when a mill shuts down, but it’s good that we can reuse equipment,” says Aspelin. “We have old drum filters here that we can retire when we install the ANDRITZ DD washers from the other mills. The future double oxygen stages will improve our fiber quality and environmental impact even more.”
The evolution of the AWP“We were discussing with ANDRITZ a service package for one of our bleach plants and, half-jokingly, told them that if they wanted to build a new wash press and put it somewhere, we would take it,” Aspelin says. “They already had plans to build one, so we began to have serious discussions.”
Two very old wash presses at Skutskär were limiting the capacity on Line #1. With a new wash press, Skutskär could maxi-mize production and pump surplus pulp to Line #2 so they could fully utilize the digester. “The digester should be at maximum production,” Aspelin says.
“While ANDRITZ has a portfolio of washers, there are applications where customers want to have filtrate extraction (water lock) for example,” says Bröttgårdh. “The AWP can boost inlet consistencies of 2-8% to outlet consistencies above 30% while delivering clean pulp to the next process stage.”
Modern design tools were used to develop the AWP, as well as simulation routines to optimize the drum design, vat geometry, and filtrate extraction. “Saying that a washer is a washer is much like saying an automobile is an automobile,” Bröttgårdh explains. “They are similar in looks and function, but the differences in details can be quite substantial.”
ANDRITZ presented its design to Skutskär. A contract was signed in May 2008 and erection began in May 2009. The agreement was that ANDRITZ would use the time from start-up (June) to fall shutdown (November) to test the wash press, make modifications, and do whatever was needed to perfect the design.
A perfect fit?“We designed the AWP for retrofit and upgrade applications where the ‘footprint’ is at a minimum,” Bröttgårdh says. “The Skutskär case was tighter than most.”
“We wanted to run the old presses in parallel so that ANDRITZ could have the ability to modify the AWP if needed,” Aspelin says. “The AWP design, with its larger diameter drum, but shorter length, is perfect for the 840 admt/d production we need.”
Sjöberg describes the situation. “Our first layout had the AWP beside one of the old presses,” he says. “But then the problem was how to divide the pulp from the AWP into the two feeding lines for the old presses. This would require conveying screws and a distribution plate to separate the pulp – much too complicated. Then we took a look at whether we could fit the unit between the two old presses and it made more sense. But it is a tight fit.”
“We knew the press would work, or ANDRITZ would do whatever it took to make it work,” Aspelin says. “Our concerns were centered around the special solution for pulp discharge, and how the pulp would react in the oxygen reactor.”
No fearsBröttgårdh explains. “With an ordinary press, you take out the pulp on one side,” he says. “Here we take the pulp out on both sides and feed it through an extra long shredding screw into a pressurized reactor. If we don’t do it right, with the right consistency, we could get a backblow, which could be quite a mess. Also, if we don’t deliver the pulp evenly on both sides, we will not have even plug flow to the oxygen stage.”
“We knew that we would have to live with this special solution for almost a year,” Aspelin says. “If it hadn’t worked, we would be in trouble. The pulp flow has been well distributed and split evenly to both sides, but a mid bearing in the long shredding screw has caused some problem.”
The other unknown factor was how the pulp would react in the oxygen reactor. “With this type of oxygen stage, unlike an MC stage, you mix when the pulp is at 28% consistency, and it just lays on tables inside the reactor,” Aspelin says. “If you don’t get it mixed right in the beginning, you get uneven pulp out of the reactor. We are seeing a reduction in kappa number, so we know that our solution is working. Future plans are to have a traditional discharge screw connected to an MC oxygen stage.”
Looking good“There are advantages and disadvantages to being the first,” Aspelin says of the mill’s decision to install the first AWP unit. “You can usually get a little reduced cost, and you have the advantage of the new technology when it works right. But sometimes you have to live with what I call child diseases, the small things that go wrong during any new product development.
“In fact, I was quite surprised. The first week when we started up the AWP, we had one plugging incident. At that time, our operators had not learned how to react when they saw a disturbance coming to the press, so they just waited it out. But after that it ran for three straight weeks and it ran extremely well. I didn’t think it would go so smoothly.”
And, the parallel operation of the old presses? “The old presses are not running very often,” Aspelin says. “Only when ANDRITZ wants to do something with the AWP. This press is very forgiving and can handle upset situations or disturbances easily. Runnability of this unit has been quite nice. You just push the button and it starts up.”
ANDRITZ is completing its tests of wash losses and efficiencies. “The thing that we can see right now is that the old presses were not really wash presses – they were only presses,” Aspelin says. “The AWP actually washes the pulp and what we can see in the oxygen stage right after the AWP is better kappa reduction. Our target now is to find the right running strategy to keep the production efficiency up all the time.”
“What we know from this first installation is that the design is sound,” Bröttgårdh confirms. “The key features and geometries are correct. We verified the design with computer models – but the proof is in the real world.”
“The production increase we are getting would probably come from any wash press,” Aspelin says. “But, we also get a lot of contact with specialists from ANDRITZ. I think we are using each other’s experience well. So far, it’s all been good. The press works, the Lo-Solids® works, and we’re looking forward to installing the DD Washers. Right now, we’re on a roll!”
Lo-Solids® at Skutskär“We have been running our #2 Digester on the Lo-Solids® concept since August and it’s going really well,” says Lasse Aspelin, Fiberline Production Engineer. “We ran it in the 1990’s in the traditional Lo-Solids® counter-current mode, but stopped when we had the digesters shortened in a retrofit. This spring, we ran a two-week capacity test with Downflow Lo-Solids® and set a new production record during the trial. We made some modifications during the summer and started up for a longer trial in August.
“It’s exciting to see how much we can push the production capacity upwards and the kappa variation is decreasing. I think that ANDRITZ has made it much easier now for the operators to work with it. We can now see that production is higher and the digester is more stable.”
