BRUSSELS, Aug. 8, 2011 (RISI) -Variation in wood chip quality is one of the most important disturbances that affect the kraft pulp digesting process, and one which pulp makers struggle to deal with the world over. Uneven wood chips can contribute to irregular cooking conditions and variable pulp quality. So, to effectively deal with these chip quality issues and other process instabilities, Cenibra in Belo Oriente, Minas Gerais, Brazil, commissioned Metso mCooking advanced process control (APC) supervisory automation systems on fiberlines 1 and 2. The startup was in May 2009.
The continuous digester mCooking systems were intended to even out disturbances to the pulping processes and reduce Kappa number variability, thereby stabilizing pulp quality. With this steadiness, pulp yield could be increased, thus reducing the cost of wood per tonne of pulp. And the goals have become reality, as the pulp quality control objectives of Cenibra have been fully met by complying to control targets set by Cenibra, opening the door to reduced specific wood consumption or increased pulp production.
Higher Kappa, higher profits
Cenibra (Celulose Nipo-Brasileira) is one the largest pulp producers in Brazil, and is also a pioneer in eucalyptus pulp production since it was established in 1977. The mill is owned by Japan Brazil Paper and Pulp Resources Development. The total production from two fiber lines is 1,170 000 tonnes/yr of elemental chlorine-free (ECF) bleached eucalyptus pulp, 92% of which is exported. The original line 1 produces 470,000 air dry tonnes/yr, and line 2, started up in 1996, produces 700,000 air dry tonnes/yr.
Variable chip quality was one of the known destabilizing factors that were difficult for operators to handle manually. The terrain around Belo Oriente is quite mountainous, with significant differences in elevation, so chip quality can vary naturally. By closely controlling the process using predictive pulp delignification models and by providing the operators with up-to-date process information on the digesting process it was expected that the cooking process would adapt to changing chip quality and produce a more even Kappa number at the blow line.
Leonardo Figueirêdo, fiberline process engineer, explains the challenges and the expected results: "We could not raise the Kappa since it was a risk, especially in line 2. Our screen room is very sensitive to high Kappa numbers. Therefore, we worked with a lower Kappa number as a precaution. Hence, the project's expectations were to decrease the variability in order to raise Kappa. The higher the Kappa number, the higher the profits."
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Leonardo Figueirêdo: “The project’s expectations were to decrease the variability in order to raise Kappa. The higher the Kappa number, the higher the profits.”
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Hamilton Brandão, coordinator of cooking and bleaching, adds, "Because we are in an uneven and irregular topography, our wood is equally irregular and this makes the cooking stage a little more complicated. Consequently, the biggest challenge of the mCooking was to correct the process beforehand (while the chips are cooking) so the end result would be even."
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Hamilton Brandão: “The biggest challenge of the mCooking was to correct the process beforehand (while the chips are cooking) so the end result would be even.”
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Coordination and optimization
The APC is structured so that certain key digester functions are coordinated and optimized through special custom-made operator display screens. These functions, Fig. 1, include:
- Chip charge and digester production rate control
- Alkali charge control
- Liquor balance control
- Pulp quality control, (Kappa number)
- Production rate change management.
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Figure 1 - Ccontrol hierarchy structure of McCooking continuous digester control
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The APC calculates the required control loop target values and ramp rates (if required) for the base level controls. The transfer of loop setpoints is accomplished through an OPC link to the DCS system.
The basics
First requirement for steady fiber line operation is constant raw material feed to the process. Metso's chip feed management takes care of controlling chip transportation from chip piles to the digesters' chip bins. The speed targets of the reclaiming screws and belt conveyors are calculated using target digester production rate and chip bin level. A stabilized chip bin level enables successful steaming and further liquor impregnation to the chips, which are the basic requirements for steady cooking.
The digester production rate depends on the chip feed rate and finally on pulp flow from the digester. The production rate target is set by the chip meter speed. Actual production rate is calculated according to the chip meter speed, pulp yield, chip moisture, the filling degree and volume of the chip meter. Steady packing and movement of the chip column in the digester requires a steady chip level in the digester. The strategy of how chip feed and blow flow are controlled is decided case by case. At Cenibra, the digester chip level is controlled with blow flow.
The alkali charge controls include total chemical dosage, chemical distribution to various addition points and dosage correction based on residual alkali measurements. The white liquor dosage is controlled to meet the alkali to wood ratio target. The white liquor flow target is calculated according to the flow of dry wood and the white liquor strength as measured by a Metso kajaaniALKALi analyzer. The residual alkali in the cooking circulation is measured with a Kemotron on-line alkali sensor.
Liquor balance control is based on the liquor to wood ratio in different digester zones: the top zone, the cooking circulation zone, the homogenization circulation and the wash extraction zone. The liquor to wood ratio is controlled at the digester top by black liquor flow to the feed circulation and in the homogenization zone by black liquor flow. In the washing zone, the dilution factor rather than liquor to wood ratio is controlled by wash flows. The digester liquor level is controlled by the extraction liquor flow, which corrects for changes in the washing factor.
To be continued ... Read Part IIhere.
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