Features Equipment & Systems
Achieving Efficient pulp washing

While the capacity to produce pulp has increased significantly for many Canadian mills, one of the greatest challenges remains the need for modernization of the pulp washing process. Now being done by equipment that is well below potential washing efficiency, through age and mechanical shortcomings, the optimization of washer performance affects both the financial and environmental success of a pulp mill.

The importance of this process was stressed by Dr. Robert Pelton of McMaster University. “One of Canada’s biggest exports is kraft pulp, a dried pulp that can be ‘re-slushed’ to make paper,” he explained. “Part of the process of making the pulp is washing it. The better you can wash it, the less you have to bleach it, and the less bleach you use, the less effluent there is.”

While most of the residual chemicals from kraft pulping and oxygen delignification have been recovered through the chemical recovery cycle, proper pulp washing constitutes an important part of the operation, separating the cooking chemicals from the pulp. Lignin breakdown by-products and chemicals are removed as a prerequisite to pulp drying, bleaching, and papermaking. In addition to the dissolved inorganic material, the process can economically remove the maximum amount of lignin to be burned in the recovery boiler, providing electricity and steam for the mill.

What’s the problem?

The importance of maintaining pulp consistency and desired brightness in the production of fine papers has lead to many of the washers in the older mills being operated above design capacity.

Other problems leading to financial losses for the mill can include insufficient washing, scaling, higher evaporation costs and increased costs relating to energy and bleaching chemical consumption. Organic materials that are not washed out of the pulp are a lost source of energy for the recovery boiler.

Environmental concerns

With growing environmental concerns surrounding pulp and paper production, pulp washing is being closely scrutinized as a critical step. In order to make finer papers, the lignin has to be either washed or bleached out and manufacturers have been striving to reduce chemical consumption and energy usage, while also minimizing both the amount of water used and the organic material, BODs and CODs, in the effluent.

One of the primary uses of water in a pulp and paper mill is the pulp washing but efficient washing will supply the bleach plant with clean pulp, while minimizing the amount of wash water used. There is also a pronounced effect of temperature on washing and reducing water usage can significantly reduce energy usage.

Proper technology can not only improve the efficiency of pulp washing, but also reduce the need to vent vapours to the environment.

Maintaining a high standard in the brownstock washing operation can make a significant difference for a kraft pulp mill. A successful balance will consist of the highest quality of pulp, the best removal of soluble impurities and the greatest amount of solids content in the dilute liquor sent to the evaporation plant.

Efficiency in these areas is directly reflected in savings to the company.

How it works

Most mills operate on a multi-stage, counter-current washing process, allowing the mill to minimize the use of fresh water while increasing the concentration of black liquor going to the evaporators.

Basically, this consists of a sequence in which the cleanest wash water is applied to the last stage of the process which has the cleanest pulp. The filtrate from this last stage is then used as the wash water for the preceding stage.

Originally, many mills employed a standard method of washing which consisted of a series of rotary vacuum washers. Several different methods exist now, of which the four most popular are vacuum drum, pressure, diffusion, or Chemi. Mills will sometimes combine different types of washers to achieve higher washing standards.

Vacuum drum washing

By far the most abundant washers in the industry, vacuum drum washers usually consist of three to six counter-flow stages. Each stage consists of a rotating screen drum which has a partial vacuum applied to the interior. The drum sits in a tank where pulp is diluted with recirculated filtrate. The vacuum draws a pulp mat against the surface and wash water through the mat. The drum rotation advances the washed pulp mat to the next stage. Wash water discharged from this wash stage is sent to the previous washing stage.

“Many of these are older and run far beyond their original design capacity, giving them an undeserved reputation of being poor washers,” said Mike Towers, senior research engineer, Process Development, at Paprican. “However, they are very easy to operate.”

“The main negative with vacuum drum washers is the space and building requirements,” he explains. “They require a large tank to deaerate the filtrate. The washer must also be located about 30 to 35 feet above the filtrate tank to get adequate vacuum in the drop leg. Another issue with vacuum washers is that they are not the best choice as the first stage out of the digester, because it is generally too hot and this can cause flashing of the filtrate in the drop leg of the washer and cause it to shut down.” He concludes, “Many mills have installed new vacuum drum washers and/or replaced existing drums with newer designs and greatly improved washing efficiency.”

Low operating vacuum with low differential pressure has also been known to cause problems such as low discharge consistency.

For their latest technology in pulp washing, Andritz points to their vacuum washer system which incorporates many features from the many machines it has designed and manufactured over the years. Because the patented Maxton Deck has less pressure drop than the conventional deck, the discharge consistency is higher and the washing efficiency is greater. Improved washing efficiency translates to lower operating costs.

In a recent case where the Andritz team replaced an 11.5’x14′ D100 washer, the discharge consistency rose more than two percent. While the replacement was made for maintenance reasons, this increase in discharge consistency was a very nice “unplanned” benefit, because it significantly reduced the caustic required in the following stage. This improvement can be expected from either bleached or brownstock drums.

GL&V has also brought together a comprehensive line of washer drum technologies. Their selection ranges from the reliable Dorr-Olivier and Canron washers, through to the advanced composites for severe corrosion environments of the GL&V/LaValley FRP composite washer drum. They are reputed to handle high freeness pulp with a high discharge consistency, while being maintenance friendly and efficient.

Pressure washing

Pressure washing is similar to vacuum drum, but differs by spraying water under pressure through the pulp mat as the drum rotates.

According to Towers, the compaction baffle (CB) filter is a popular washer based on this concept. “The CB filter is an extremely efficient washer and probably the most popular choice for new washing equipment. Since it is pressurized, high temperatures are not a problem and the filtrate tank does not need to be located below the washer, as is the case with the vacuum washers. The only real negative with these is that they are more complex to run, resulting in a fairly steep learning curve for the operators.”

Diffusion Washing

Diffusion washing is a counterflow process that takes place in one or more stages. Pulp flow is upward and is carried on a perforated plate; water flow is downward through a series of baffles. This method offers a high degree of cleaning with low water use.

Towers explains the technology. “Pressure diffusers are now the preferred washer technology out of a continuous digester.They can be considered almost as an extension of the digest
er wash zone, and as many mills are cooking farther down the digester, they can recoup some lost washing capacity. Higher temperatures are generally not a problem. Washing efficiency is greatly improved at higher temperatures due to the greater diffusion rate of material from inside the fibres and the greater solubility of soaps. Atmospheric diffusers are not as popular these days, mostly because of the high efficiency of the pressure diffusers. They are generally not a good option as the last washer of a line because of their relatively low discharge consistency.”

The pressure diffuser from Andritz is a high efficiency, diffusion-type washer that is fully enclosed and operates under pressure, pushing the pulp through the machine without the use of pumps, reducing energy and maintenance expenses. Also with the pressurized wash, higher blow temperatures from the digester can be used. Since the pressure diffuser is totally enclosed, there are no gas emissions or pulp spills. The vertical pressure diffuser requires very little space for installation.

The atmospheric diffuser from Andritz is a compact washing system that maximizes washing efficiency while minimizing energy consumption. It eliminates the need for intermediate dilution between washing stages and can use the pressure from a continuous digester to transport pulp through the diffuser. This eliminates the pumps required by other washing systems.

Chemi or belt washing

Belt washing is a counter flow process where pulp enters the washer area on a wire belt and washing takes place under a series of showers. Clean water enters on the opposite end from the pulp and is sprayed on the pulp to displace the liquor in the pulp mat. The used wash water is then collected and reapplied to the dirtier pulp by the next washing head countercurrent to the direction that the pulp moves. This process is continued through several stages until the wash water is saturated with liquor after washing pulp entering this area. The wash water is then sent to the recovery process. Reduced dilution of the liquor recovered from washing will result in reduced energy usage in the recovery process.

While perhaps the simplest washing system design, the Chemi or belt washer tends to be sensitive to feed consistency, production rate and dilution factor. Despite the reduced water usage, there are not many installed in Canada.

Wash presses

“These are typically easy to operate and very efficient,” commented Towers. “They are very good as a last stage of washing because of their high discharge consistency. As a result, they create a good water barrier between mill areas (ie. between brownstock and the bleach plant) and fresh shower use is less than that for a washer with medium consistency discharge.”

Towers explains that, for efficient washing, the wash presses rely on high discharge consistency, and this means they are not as good in areas where the liquor has a higher viscosity, such as at the start of brownstock washing. They are mostly used after oxygen delignification stages or even in bleach plants.

Metso Paper introduced the press wash technology and is the world’s leading supplier. This new generation of presses, TwinRoll, is based on only two roll diameters. With different roll lengths, production rates of over 3,000 ATD/d can be accomplished on single machine. Recent extensive modularization has resulted in more efficient production, in addition to improved wear and spare part economy.

Drum displacer washers

According to Andritz, their drum displacer anticipates the new demands being placed on washing performance: improved efficiency, environmental compliance and reduced costs for installation, maintenance and operation.

The washer family includes drum displacers with low consistency feed and medium consistency feed, with between one and four washing stages. The washing efficiency is very high, in part due to the drum displacer’s fractioned counter-current washing.

These are fairly complex units but are suited to mills where space is a critical factor. There is generally no diffusion of liquor from within the fibres between washing stages.

Conclusion

Towers points out that the overall choice of equipment is sometimes dictated by circumstances such as inlet consistency. For example, vacuum drum washers require a low vat consistency (~1.5%) to form a good uniform mat on the washer. Pressure drum washers and wash presses require an inlet consistency of about 4%, while diffusers require an inlet consistency of about 9-10%. Therefore, for thickening after low consistency screening, vacuum drums are still the choice. However, if space and lower hydraulic recirculation (ie. inlet dilution) are important, more modern washers may be required.

“Efficiency-wise, modern vacuum drum washers can compete, but they need to be properly designed for the production load,” Towers explains. Modernizing an existing drum washer with anti-rewet drums, high efficiency end-valves, or possibly resizing the drop-leg can only get you part of the way there if the washer is severely overloaded.

Focused engineering in this area can help a mill achieve uniform sheet formation and maximum chemical recovery during pulp production.

Acknowledgements

PPC would like to thank Mike Towers of Paprican, Dr. Robert Pelton of McMaster University and Grace Todd of Andritz, Tuomo Nykanen of Metso, and GL&V for their contributions to this article. If any of the information submitted by the contributors was inadvertently mis-communicated through the consolidation of that information within the wider context of the article itself, the responsibility for that error rests with the author.