What's cooking?

Ozone innovations, space efficient washing and super-fast drying are among pulping and bleaching's latest tricks

by Jonathan Roberts

It must be hard to focus on investing in the pulp production process when every tonne produced adds to your losses. This is the reality for many producers across the globe with the dire state of pulp prices. But cycles come and go and whatever the rights and wrongs of investing in pulp capacity at the moment, one can only praise any initiative to make pulp production more efficient.

Fortunately for the market, there are relatively few capacity-increasing projects underway at the moment, the present climate calls for a focus on what technology can do to make a mill better, rather than, necessarily, bigger.

Papier Masson has certainly made the most of its assets having set a new thermomechanical pulp (TMP) production record at its Masson Angers mill in Quebec, Canada. During a 72 hour run, 2,277 bone dry tonnes of newsprint grade TMP were produced on a single line of two-stage Metso RGP 82 CD refiners. Plant availability was more than 95.5% during the run, with only three short production interruptions. The peak operating rate during the period of 760 tonnes/day makes Papier Masson the largest single line TMP production unit in the world. The line started up in the middle of last year.

The claimant for the title of largest single line for kraft pulp is Metsä-Botnia Joutseno of Finland, which started up its new fiber line on May 15 this year. The 2,140 tonne/day installation takes the mill from its present capacity of 410,000 tonnes/yr to 600,000 tonnes/yr. Environmental load is also reduced.

Kvaerner Pulping supplied a continuous digester plant and washing facilities, including an impregnation vessel and digester vessel which are the largest of their kind in the world. The digester has a diameter of more than 10 m and height of 64 m. Cooking is based on Kvaerner's two vessel steam/liquor phase Compact Cooking Process, which is said to be particularly well suited to the production of the reinforcement pulp which is Joutseno's specialty. The Kvaerner order also includes a Compact Feed system and brown stock washing in a two-stage atmospheric diffuser.

Ahlstrom Machinery supplied equipment for oxygen delignification, screening and bleaching. Ahlstrom's delivery included MC (medium consistency) technology, MODUScreens and DrumDisplacer washers designed for high unit capacity.

Cooking class

A landmark project for the newly-formed Andritz-Ahlstrom is Willamette's $600,000 capacity doubling project at Hawesville, in the US. The centerpiece technology is a Lo-Solids continuous cooking system and the first Drum Displacer (DD) Washer bleaching system to be installed in North America. Willamette has experience with Lo-Solids cooking at its Johnsonburg Pennsylvania mill.

Willamette's pulp production manager Barry Zuercher, comments, "Lo-Solids cooking helps us increase pulp strength and viscosity more than we ever could with batch. We immediately noticed improved washing in the digester and were using less bleaching chemicals compared with extended delignification cooking methods."

The enthusiastic theme continues with Maika Carman, digester cook, who says, "Operating the batch digester was like tending six little children. Each child had to be treated individually - one steamed up hotter in the bottom; one would take longer to cook. The Andritz-Ahlstrom equipment is so forgiving. The pulp is much more consistent and cleaner. If there are upsets, we can handle them and still send quality pulp to the bleach plant."

Bleaching bright spots

"Reduced capital outlay". That's music to the ears of those with the task of pleasing and pacifying pulp mill stakeholders. With the plug screw feeder, the refiner fluffer and one washing stage eliminated, Metso Chemical Pulping's new ozone bleaching technology must be worth a tune or two.

Ozone bleaching became commercial in the early 1990s but development until now has been rather slow. Lately it has taken off, relatively speaking. This is thanks to high consistency techniques that keep the cost of ozone bleaching down and the realization that with ozone applied in the first stage, pulp strength can be preserved effectively.

Metso's ZeTrac system removes the washing stage directly after the ozone stage, so washing takes place after extraction. The filtrate is then fully recoverable to the brown stock or post oxygen washing system. With a strong ozone stage at the beginning of the bleaching sequence, bleaching costs and effluent volumes and load can be reduced significantly.

Since 1995, the Burgo Ardennes mill in Belgium has been converting its original C/D(EO)DED bleaching system phase by phase to a modern D(Z(EOP))(DD) sequence. The original capacity of 240,000 tonnes/yr has been increased to 360,000 tonnes/yr. The bleaching sequence can be run with or without the Do stage. At the moment there is no recirculation of the (Z(EOP)) stage effluent, but a tight closure will be possible in the near future applying the sequence without the Do stage.

Click for full image Votorantim Celulose e Papel will produce more than 2,100 tonnes/day of bleached eucalyptus pulp at its Jacarei mill in Brazil, using a new ozone process

The Nichinan mill, Oji Paper, starts up its new ZeTrac bleaching sequence imminently. Nichinan's bleaching capacity will be 750 tonnes/day of mixed hardwood pulp at a brightness of 86% ISO. The main change is the rebuild of the bleach plant to ZEPD.

With the same technology, the new fiber line at Votorantim Celulose e Papel's (VCP) Jacarei mill in Brazil will produce more than 2,100 tonnes/day of bleached eucalyptus pulp. The bleaching sequence is O(Ze)DP. Pulp is bleached in an existing line with ozone at medium consistency but in the new line, the ozone stage will be run at high consistency with the ZeTrac. The ZeTrac process and the use of Twin Roll presses for washing give very low effluent volumes and load, while bleaching cost is lower compared with a similar sequence with MC ozone.

Wash press

Kvaerner Pulping's new washing technology is designed to require minimal space and provide significant environmental benefits. The Compact Press is the result of three years' development work in close cooperation with Billerud's Gruvön mill in Sweden, where it has helped reduce water flow to the aerated lagoon by 4 m3/min. According to Kvaerner, the goal in the development work was to develop the best washing equipment on the market, while achieving high runnability and minimum maintenance requirements.

The Compact Press covers capacities between 500 and 3,000 tonnes/day and is designed for a wide range of inlet consistencies. It provides high loading in terms of tonnes/m², thanks to the compact design, which also makes the unit an obvious choice for retrofits.

The Compact Press combines new technology with well-proven components. There are two specially designed distribution screws located at the top of the machine, one on each side, which are designed to make possible the uniform distribution of pulp length wise at the same basic weight, independent of feeding consistency. With a uniform pulp web, the necessary conditions are created for efficient washing. Other main features include two opening flaps and one opening vat, one shredding screw and a rigid frame. Inside the unit are two drums, one with a diameter suitable for higher production and the other with a smaller diameter for lower production. The flap/vat configuration is designed to reduce the machine's sensitivity to consistency variations. Three quarters of the drum is used for dewatering and washing and this is the main reason why the press is so compact. All bearings and such like are located outside wet areas.

In the washing area, special zones are used to distribute the washwater in order to provide efficient, high standard displacement washing. The full-size unit has been operating at Gruvön since May 2000. It was sold to the mill a year before. Environmental concerns motivated the mill to replace a drum filter with the Compact Press but the space consideration was also persuasive.

The unit was commissioned for a production of 1,200 tonnes/day with an inlet consistency of around 5%. Feeding with an MC pump raises the capacity considerably.

The Compact Press was installed as final wash stage before the bleach plant. Do filtrate is used for dilution after the press to minimize bleach plant effluent. Condensate and warm water are separated together for use as wash liquor and the DF (dilution factor) runs counter current in the fiber line from the Compact Press.

After a good startup, the results look promising and the mill's ambitions for the project have been fulfilled.

With softwood, an outgoing consistency above 32% is possible and operational experience has been characterized by high availability. "Our operators are very content," says production superintendent, Christina Larsson. "It doesn't leak and is very clean," she adds.

The Compact Press has proved itself able to handle big variations in inlet consistency without sacrificing runnability. Washing results have been very good with low COD (chemical oxygen demand) carry over to the bleach plant. The system has also been operation with MC-consistency feed and this has worked well, as expected.

Since Gruvön is an integrated mill, the press has to cope with big production variations. But it has worked from nominal capacity down to 35% without any perceived problems.

There are several Compact Presses on order. One is for Brazilian producer, Ripasa, which has chosen the Compact Press for its new bleach plant. The mill has also specified a new evaporation plant and an upgrade of the recausticizing facility to handle 750 tonnes/day of bleached pulp.

Quality control

The term "off-grade" is handy for pulp producers that want to shift pulp at a bargain price in a weak market without losing face. But in an ideal world a pulp mill maintains constant control of its quality and instruments. Södra Cell Folla in Norway was in the market for an analysis tool, which would help it identify the cause of an unexplained drop in peroxide dose. The problem was with the 25% of the mill's chemi-thermomechanical pulp (CTMP) production which was bleached in the 75-80% ISO brightness range.

Click for full image Södra's Folla mill is feeling the benefits of its new RPA Residual Peroxide Analyzer

Folla's production manager, Odd Morten Aalberg, echoes the sentiments of many pulp producers when he says, "Producing uniform product consistently is essential if we are to operate profitably and fulfill our ambition of providing our customers with tailor-made pulp qualities."

BTG offered Folla an RPA-5000 Residual Peroxide Analyzer for trial to check the filtrate from the post HC (high consistency) bleaching tower wash stage. High peroxide residuals were the reason for brightness targets not being met. Aalberg explains the results, "The RPA controls hydrogen peroxide dosages constantly and this provides a uniform chemical environment and minimum brightness variation in the higher brightness ranges."

The instrument operates on a five minute measuring cycle and feeds information continuously to the mill's distributed control system (DCS). Optimized peroxide doses are the sum of the residual peroxide in the filtrate and the fresh peroxide which may need to be added in the bleaching process to achieve the required brightness.

The RPA-5000 has reduced hydrogen peroxide consumption by more than 5% and the mill produces up to 10 tonnes less off-spec pulp each day.

Another mill striving to know more about its process and find better ways of controlling them is Ence's Huelva mill in Spain. Huelva features one of the world's first three-tier multi-site implementations of SampleManager LIMS.

SampleManager was installed in a move to ensure improved automation of quality control procedures in the manufacture of elementally chlorine-free (ECF) eucalyptus pulp.

To help the mill maintain and exceed standards, as well as increase production capacity, Ence undertook an investment program which included an Environmental Management System (SIGMA). On top of that, the mill installed SampleManager LIMS in its quality control and analytical laboratories to help automate improved laboratory procedures and ensure data integrity.

Ence's pulp mills had experienced problems with their in-house laboratory systems for a number of years. They were proving both expensive to support and incapable of handling the data volumes required. Systems had become increasingly difficult to modify and configure as new improved procedures were introduced. Additionally, Ence required new statistical control capabilities and integration with laboratory instruments.

The initial stage of the project involved Thermo LabSystems Pathfinder analysts undertaking an extended period of requirements gathering and analysis via interviews with users, laboratory management and IT support personnel. This resulted in a weighty document which comprehensively examined all technical and human factors involved in a LIMS replacement project. It became clear that significant customization was required to satisfy Ence's requirements, in particular the development of a real-time batch production subsystem. During these early stages, Thermo LabSystems demonstrated both the capabilities of SampleManager and a clear understanding of Ence's additional requirements. Ence's Pontevedra and Navia mills in Spain go live with replica SampleManager implementations this year.

Pulp dewatering and drying

It is getting hard to find an area of pulp and papermaking which has not been infiltrated by the shoe press. With tissue and calendering now under its spell, the shoe press has moved in on pulp dewatering. Andritz has borrowed Voith Paper's NipcoFlex shoe press technology for the purpose.

The dwell time in the nip for a shoe press is up to 10 times longer than in a conventional roll press so the potential is clear. Metsä-Botnia Äänekoski in Finland has had a NipcoFlex shoe press fitted as part of a rebuild of its fourdrinier-type pulp dryer. Trials with birch and pine showed that dryness points of 4-5% higher were possible with the new technology at the same time as increasing the rate of production by 20%. The other advantage with the shoe press is that dewatering is carried out more gently, which helps eliminate sheet damage.

NipcoFlex shoe presses are also in operation at Mercer's Rosenthal mill in Germany and APRIL's Riau Andalan mill in Indonesia. Aracruz is set to start up another one in Brazil next year. The Aracruz order is part of a project to increase market eucalyptus pulp capacity by 700,000 tonnes/yr. The wet end of the pulp dryer, to be built by Andritz, will feature the largest ever twin-wire former built for a sheet drying line. The working width will be 8 m.

The design speed of Stora Enso Kaukopää's new pulp drying line is 300 m/min

Meanwhile, Metso has developed what it claims to be a totally new concept in pulp drying. DryWay uses twin-wire former technology and an airborne dryer with an advanced nozzle, fan module and steam coil design which results in higher production rates and energy savings. Stora Enso Kaukopää in Finland took delivery of the technology early this year (see separate article). The line features new solutions from headbox to baling and its design speed is 300 m/min. The cutter layboy achieved a world record speed in the tune-up period. Bearing in mind most pulp machines struggle to exceed 180 m/min, this is quite a significant advance.

What's next

Pulping will always attract environmental scrutiny as does any major industrial process. One of the great challenges for developers of pulping technology has been how to cook and bleach non-wood fibers in a way that does not create excessive environmental load. Natural Pulping of Germany has been tantalizing the industry with talk of new environmentally-friendly pulping processes. One can assume fairly confidently that a production rate of 1,000 tonnes/day is not around the corner but Natural Pulping has just courted, successfully, an industrial partner, which could bring its ideas much closer to fruition. The prospects, particularly for fiber-poor regions, are intriguing.