Pulp washing line control: The ultimate balancing act

Stora Enso’s Veitsiluoto mill sees much better stability across the whole line and improved washing efficiency

Until recently, the brown stock washing area has not been highly automated and its performance not been optimized in any significant way. In many pulp mills the washers are equipped with basic instrumentation and the individual control loops are supervised by a DCS system. In most cases, the coordination of these individual control loops is done manually by the operators. In any manual operation the results vary with the individual, so the stability of the whole operation suffers. The transition periods during fiber species and production rate changes can be particularly troublesome.

Many mills are now realizing that poor performance in the washing line can have a significant impact on the stability of operation and the production costs in other related processes pulping or recovery process. For instance, poor washing of the pulp can result in higher bleaching chemical consumption. Organic materials that are not washed out of the pulp are a lost source of energy for the recovery boiler. On the other hand, if the filtrate liquor is too dilute, evaporation costs will be higher. In some cases this may result in a production bottleneck for the whole pulp mill. Also, if outlet consistency is not regulated and becomes too high the washer operation may be halted.

The operation of the brown stock washing operation is therefore a delicate balancing act, with the operators trying to achieve the best removal of soluble impurities, the highest possible outlet consistency and the highest solids content in the dilute liquor sent to the evaporation plant. But operating practices vary from operator to operator and, in some mills, the wood species and production rates change on a regular basis, thereby destabilizing the washing process. Because the washing line is a sequential, countercurrent operation, the stability of the whole line can be upset. It may take only a few minutes to destabilize, but the recovery may take several hours.

To address these issues Metso Automation has introduced a whole-line optimization control called DNAwash, which controls first the individual washing units and then balances the distribution of washing liquors and manages filtrate tank levels throughout the entire line. Most importantly, this control manages the washing operation through transitions between fiber species and during the usual production rate changes. The benefits of improved washing line control and optimization include:

• Optimized washer load distribution
• Consistent operation, no human errors
• Lower, more stable washing losses
• Steam savings in evaporation plant
• More even pulp quality
• Reduced bleaching chemical use
• Alleviation of bottlenecks and higher production rates

More pulp with the same equipment

To realize many of these goals, Stora Enso Fine Paper’s Veitsiluoto mill, in Kemi, Finland invested in Metso Automation’s DNAwash, implemented in their metsoDNA control system. The pulp mill produces approximately 350 000 tonnes per year of ECF bleached pulp for the integrated paper mill. Like many pulp mills the capacity has been increased significantly from its startup in 1977, and the demand on its capacity was being stretched ever further by recently completed or in-progress rebuilds of two of the mill’s paper machines.

Pentti Tolonen, Pulp Mill Production Superintendent, emphasizes the mill’s difficult task to produce more pulp with the same equipment in the washing line: “In the coming years we must produce 385 000 tonnes per year. This was a big challenge for the washing equipment, some of which dates back to 1977.” He explains that balancing the line, even with good, experienced operators was difficult, especially when fiber species changes are made every second or third day. “They had to balance it in their heads,” he says. The softwood fiber is produced at about 1110 tonnes per day, while the hardwood fiber runs at about 1400 tonnes per day. Kari Salminen, Operating Engineer, says that operators in different shifts handled the upsets in different ways and filtrate tank stability was a problem during the transitions. To alleviate these stability problems and to establish consistent operation, the DNAwash controls were implemented during 2003.

Multiple levels of control

Washing line optimization controls are divided into three different control levels as shown in Figure 1. The first level is washing unit control, which involves rotation speed control for drum washers or wash presses or filter screen movement control for atmospheric or pressure diffusions washers. The next level is whole washing line control by applying the right amount of wash water to a achieve consistency targets. The final level is dynamic adaptation, which determines the total washing line dilution factor control.

The optimization of the entire washing line includes control of the dilution factor and material balances throughout the washing line. The dilution factor in the whole washing line is optimized by dividing the washing load between the individual washers. Finally, the usage of washing water is optimized by dynamic adaptation so that the capacity of the evaporation plant is used effectively, without overloading it.

Figure 1: Hierarchy of optimization controls for a drum displacement washer line


Diffusion model defined

DNAwash employs a washing model, adapted to a specific to a type of washer, to stabilize its operation and to achieve the highest possible washing efficiency. The efficiency of the dilution and extraction mechanism depends mainly on the consistencies to which pulp slurry is initially diluted and finally thickened. The pulp slurry should be diluted as much as possible and the pulp consistency after thickening should be as high as possible. To enable longer retention times and thus a higher degree of diffusion, the dilution and extraction process should be slowed down as much as possible to remove solute absorbed within the pulp fibers.

In brown stock a lot of absorbed solute, both organic and inorganic, still remains, so the diffusion effect become vital in washing, regardless of the mechanism. As a starting point for the optimum control of a washing model has been developed to explain how soluble impurities are removed from fibers by the diffusion process. This mass transfer model is called Washing Potential in Diffusion. In simplified form, it is expressed as a functional relationship.

Q= f (C, t, T)

Q= Washing Potential in Diffusion
C= Consistency
t= Time
T= Temperature

The washing potential defines the capability of any washer to remove solute from the fiber. This basic washing mechanism is used for controlling the washing process in several different washing processes, all of which depend on diffusion. The model is adapted for different fiber species.

More stability, cleanest pulp

The control of the washing line is customized to the individual line, its equipment, capacities, process flows and dynamics. At the Veitsiluoto mill, the process equipment consists of two parallel pressure diffusion washers followed by a 1-stage atmospheric diffusion washer. A double drum filter washer follows the knotting and screening operation. After the oxygen delignification process, added in 1993, an atmospheric pressure diffusion washer and a wash press complete the washing before the high density storage chests. An overview of the controls in the line is shown in Figure 2.

Figure 2: Overview of washing line control at Stora Enso Veitsiluoto mill

In the pressure diffusers, the screen unit is moved slightly faster than the pulp column movement. The washing model sets the rate at about 1.1 times the pulp flow rate. This rate is significantly slower than before the controls were implemented. Salminen says, “The pressure diffusers now wash better. With the slower filter basket movement more liquor is removed without plugging.” The atmospheric diffuser screens are controlled in a similar way

In the higher level controls, outlet consistency is regulated by wash liquor flow, at a level which avoids the risk of screen plugging. The outlet consistency is calculated by mass balance with a measurement of the blow line consistency, by means of a torque indication on the digester outlet device.

The level of the pressure diffuser filtrate tank is controlled by filtrate flow. The level of this filtrate tank, and others in the line, is more precisely controlled by tank level predictors which use a volumetric balance, knowing how much is entering and leaving the tank. This more precise level control ensures a consistent supply of wash water to previous stages, even during species changes. The same level predictors are used to control the level of a large filtrate tank in the screening department by regulating the wash water to the double drum washer. The regulation of the washing water to this washer, and all other washers in the line, ensures the dilution factor for the entire line is controlled and filtrate flows are in balance, in order to maintain level stability in the tanks. Further along, in the wash press, the feed consistency is controlled to match its hydraulic water displacement capacity. The rotation speed stabilizes the outlet consistency.

Managing change, rebalancing the line

When the fiber species change, hence production rate changes, consistency and tank level targets are automatically adapted to the new conditions. The dilution factor of the whole washing line is controlled in the dynamic adaptation control level. The control automatically tries to use as much washing water as possible in the washing and therefore obtain the cleanest pulp. The control also supervises the situation in the evaporation plant and ensures that weak black liquor is not too dilute. The dynamic adaptation control adjusts the flow of weak black liquor in the digester's by pass line.

Salminen describes the benefits of this by-pass control: “It follows the needs in the evaporation plant by looking at the tank levels. If there’s room then more by-pass liquor is sent to the evaporators. Then more clean wash water can be used in the wash press and we get better washing efficiency. The optimization system always sends the maximum washing water to the wash press. Here we get the best washing results. Then the rest of the line balances. ”

The operators can follow the effectiveness of the washing efficiency in each unit by referring to an information window showing the conductivity of each filtrate as the pulp progresses though the line. Low conductivities indicate better washing efficiency. Salminen says the operators really make good use of this trend display; an 8-hour history is the most popular.

Figure 2: To see the washing effectiveness the operators can follow the filtrate conductivities throughout the line. They can select current values or those from 8 or 24 hours in the past

More consistency and maintaining it

Tolonen says the whole line control has made its operation more consistent: “All the shifts are now running the best way,” he says. This consistency has made the washing operation more uniform and its main objective – producing a consistently clean pulp- has been achieved. According to their statistics Chemical Oxygen Demand (COD) to the bleach plant has been reduced by about 18% on softwood pulp and 32% on hardwood pulp. Bleach plant chemical consumption is showing a downward trend and brightness deviations are smaller. There are a number of reasons for this, including better washing and a new wash press before the bleach plant.

The mill also measures the success of the system by how consistently the operators use the controls. In this respect, the controls have been a good success. Salminen says the balancing of operations across the line is much better. One person operates the pre-delignification washer and one person operates the post-delignification washer. Their coordinated efforts are now more effective.

These performance indicators – COD reduction and control utilization – are now being used as benchmarks in a continuing Performance Agreement signed with Metso Automation. In this partnership agreement, specialists at Metso Automation regularly monitor the washing line performance, using local people and also through a remote link to their Pulp Centre in Tampere, Finland. With this high level of performance monitoring and reporting Metso Automation and Stora Enso personnel have become adept at solving problems and continually improving the process. The mill sees a good return for the washing line process and others in the mill. The Performance Agreement, in place since 1999, covers the major unit operations in the pulping and recovery lines.

By Mark Williamson, Freelance Writer, Thornhill, Ontario, Canada and Jari Kapanen, Application Specialist, Pulp Mill Automation Solutions, Metso Automation Inc.

Published with permission of PPI, January 2005, p. 34-36.