By Jarkko Peuhkuri, Pulp Mill Production Manager, Stora Enso
Oulu, Finland,
Oct. 12, 2009
(RISI) -
Producing an even quality pulp at a consistently high production rate is a demanding task for pulp mill operators, considering the raw materials they have to work with are naturally variable. They have to watch the process all the time to make sure that disturbances are managed properly and pulp quality is what the papermakers expect day after day. Wood chip fiber quality and moisture levels (frozen or thawed) tend to change with the seasons, natural geographical factors and the wood source. Moreover, scheduled swings in wood species, from hardwood to softwood, makes the process management task even more complex. It takes years for operators to learn and perfect these process management skills.
If those years of experience and knowledge can be incorporated together with modern process measurements and process modeling and control techniques, pulping stability issues can be effectively managed. The stability that such an optimizing automation system provides is consistent - from shift to shift and during transition periods. That consistency can have positive effect on product quality and productivity.
Establishing pulping process stability
By implementing process optimization controls supplied by Metso, Stora Enso's bleached kraft pulp mill in Oulu, Finland has dealt conclusively and successfully with these process stability and disturbance management problems. The production capacity of the fiber line, started in 1975, has risen from the original capacity 600 tpd to over 1000 tpd. The bleached pulp is supplied to the integrated paper mill and to paper mills in Europe. Such a large step in production sometimes aggravates process stability problems. The pulp quality from the mill's Kamyr digester is particularly susceptible to instabilities in chip level and flow as it has a narrow bottom diameter (5.8 meters) and a correspondingly high chip flow rate. The residence time in the digester is less than three hours.
Swings from hardwood to softwood pulp also destabilized the pulping process. Managing the fiber species swings and avoiding production and quality disturbances was a major justification for renewing their existing digester controls. Avoiding chip flow disturbances and producing a very consistent product for the papermakers were also high on the list of objectives. The decision to implement a new system with advanced controls and diagnostic tool was justified mainly by expected improvements in papermaking fiber quality.
The existing supervisory controls system, based on a 1980s-vintage minicomputer, had reached its limits and could not be improved any more. The chip level control was a particular problem that needed a remedy. Metso's optimization control system chosen for this project was implemented in early 2005. The optimization system, called mCooking, is linked to the existing DCS system which executes the base level controls. A control simulation system was run in Metso's Oulu office before the actual commissioning. The pre-tuned demo system simulated typical grade and production rate changes.
Pulp quality control
The control package consists of a number of digester operating functions arranged in a hierarchical structure. Calculation and stabilizing control functions include production rate by chip feed control, alkali to wood charge based on recipes and residual alkali, liquor balance control and temperature controls based on H-factor target. Optimization controls include adaptive, model -based pulp quality control with Kappa feedback, species and production rate change management and disturbance management - using fuzzy logic to determine and correct for irregularities in chip column flow. Short term digester production halt planning is also included in the optimization package.
The quality optimization control is the key function which ensures even pulp Kappa number and fiber properties. The control tracks and adjusts the targets for chip alkali charge and cooking conditions during the 3-hour residence time in the digester vessel. See Figure 1.
Figure 1: A multi-zone H-factor calculation and a dynamically updated Kappa model ensure even Kappa number and pulp quality
The H-factor is the key control target which ensures repeatable cooking conditions. Because temperature changes as the chips move through the digester vessel a sectored, multi-zone H-factor calculation gives the most comprehensive picture of the combined effect of time and temperature variables which effect reaction rate. Temperature and alkali to wood ratio changes are tracked all the way through the digester. The H-factor model segments the digester into small zones or "slices" of 1 to 3 minutes of residence time. In each zone the realized H-factor is calculated using residence time and a digester temperature profile which is derived by temperature measurements at strategic locations and soft temperature sensors which are calculated from multiple temperature inputs.
The realized H-factor and alkali to wood ratio information is input to a Kappa model to estimated blow line Kappa. The estimated Kappa is compared to a real blow-line Kappa number measure by a kajaaniKAPPAi analyzer installed for the digester optimization control project. The updates to Hatton's model, which compensate for wood properties and actual cooking conditions, are a patented development. Because of precise tracking of the realized H-factor compared to H-factor target, unusual and uncontrollable temperature disturbances are noticed, thus avoiding unnecessary and destabilizing control actions. The realized H-factor can also indicate digester plugging. The automatically updated quality controls look after the digesting process so operators can spend more time looking for process upsets.
Alkali charge is another important value which ensures a consistent cooking endpoint. White liquor strength, hence alkali charge, is measured by an alkali analyzer previously installed in the causticizing plant. The kappa and alkali analyzer measurements have proven to be very reliable.
Chip level and liquor balance control
Since the digester is quite narrow, the chip column movement is susceptible to disturbances. This creates a great challenge for chip level control. A special chip level control strategy uses non-linear, adaptive gains to achieve chip level stability. Chip level in the impregnation vessel is primarily controlled by the outlet scraper device speed, and secondarily by bottom dilution flow. Digester chip level is controlled by the blow flow utilizing the same non-linear adaptive gain concept.
To ensure uniform cooking liquor strength, even chip packing density and column flow liquor to wood ratios are maintained by using a liquor balance calculation. Moisture in the chips, updated by a laboratory test, white liquor flows, black liquor flows and condensed flash steam are factors in the calculation. Flash steam, which cannot be directly measured, is calculated by an enthalpy (heat) balance. Liquor to wood ratio is controlled in the impregnation vessel and at the top of the digester.
Production rate and wood species changes
Production rate and wood species changes are accomplished with a sharp transition, with minimum disturbances to minimize off-grade quality. Chip feed rates, liquor feed and cooking variables are ramped automatically at the right time to new target values, thereby avoiding human errors. For short production halts of a few hours cooking conditions are ramped toward the planned stoppage, using the Kappa model to ensure realized H-factor is controlled right to the end point. This ensures the pulp in the digester is prime quality up to the halt. The startup phase is similarly ramped up to ensure quality pulp.
Disturbance detection and management
Increased production rates have made digesters more sensitive to disturbances. With disturbances, pulp strength properties are affected because of fluctuations in the kappa number. If the disturbance is great enough, production rates must be reduced.
Disturbances in the chip column movement are calculated, using fuzzy logic functions from inputs of pulp consistency at the digester outlet, chip and liquor levels, kappa number, pressure differences over the extraction screen, liquor level differences and temperature differences in the extraction flows of two screens. See Figure 2. The construction of the set of "fuzzy rules" is based on operator interviews and their collective knowledge.
The disturbance index ranges from 0 (undisturbed plug flow) to 100. Since the operators are less busy managing cooking zone control they can devote more time to diagnosing and correcting disturbances indicated by the disturbance index. If a chip flow disturbance is detected in its early stages, maybe a half hour before, corrective action can be taken quickly and the impact on production and pulp quality can be minimized. Therefore, a problem can be avoided before it becomes a major production or quality problem. For instance, changing pressure differences can indicate screen plugging. Chip column channeling can be indicated by changes in temperature in the extraction flows. A remedy is to reduce the washing factor.
Since the rules were made by the operators, they react right away if they see a problem. The control system displays and diagnostics also help younger operators to learn the process and control it more effectively.
Figure 2: Disturbance detection. If a chip flow disturbance is detected in its early stages corrective action can be taken quickly and its impact on production and pulp quality can be minimized.
Control project results
The Oulu mill asked Metso to guarantee that Kappa number variability must be within tight quality control limits so they could say that final pulp quality was similarly uniform. The guarantee was to keep 70% of the measured Kappa values within the control target range. The reference baseline was 20% for hardwood pulp and 44.8% for softwood pulp. After the 30-day guarantee run with the optimization controls both guarantees were achieved. For hardwood pulp 73.1% of the Kappa tests were in the target zone and 87.9% of the softwood pulp Kappa tests were in the target zone. See Figure 3.
Figure 3: The optimizing controls keep pulp kappa number within very tight limits. The histograms on upper part of the figure show the distribution of kappa sample results for hardwood and softwood pulps. The bar graphs on the bottom show what percentage of Kappa samples are within Stora Enso's target zone. The goal of 70% was reached or bettered by the optimization controls.
More stable pulp, better papermaking properties
The optimization controls ands disturbance management tools have positively affected the fiber line operation operations. The number of disturbances has been reduced significantly, which allows the Stora Enso Oulu mill to produce more stable pulp with better paper making properties. Higher production can be maintained with fewer process disturbances. The oxygen delignification stage works much better now because of stable in coming kappa, allowing chemical savings in bleaching.
The mill staff knows that learning about the pulping process and improving the process operation doesn't stop after the project phase. To further develop the system's control potential the Stora Enso Oulu mill has contracted Metso to help develop new control applications and procedures on a continuing basis. The supplier's engineers regularly discuss with mill operators and engineers how to make the controls better and implement solutions which make the process more stable.
This article is based on a technical paper presented at the Brazilian ABTCP congress in October, 2008 and Metso's Pulp and Paper Technology Days in Munich, in October, 2009
