Advanced Continuous Digester Solution

Honeywell’s Continuous Digester Solution delivers tight control of the cooking, resulting in high-quality pulp at a lower cost and higher yield. Continuous Digester controls provide precise control of the kappa number, improving pulp uniformity out of the digester and stabilizing unit processes in a variety of areas throughout the mill, including washing, screening, the bleach plant, chemical recovery, and the paper machines.

Continuous Digester control allows your continuous digester to be operatedcloser to the economic optimum, without compromising product quality or yield. For example, typical results achieved with the package include a 30 to 40% reduction in kappa number variation and an increase in yield of 0.5 to 2%. The flexible application package achieves excellent results in controlling digesters of all configurations: single and dual vessel, hydraulic and vapor phase, cooking pulp by conventional means or using extended delignification. 

Highlights

• Powerful control strategies—first stabilize, then
  optimize the digester operation to a more economical
  and environmentally safe level, while maintaining or
  improving pulp quality within existing specifications. 

• Model-based control techniques—utilize both
  feedforward and feedback strategies to control key
  process variables resulting in increased quality while
  reducing cost. 

• Statistical process control techniques—identify
  process trends, reduce process and product variability,
  and improve product quality. 

• Configurable modules—optimize the continuous
  digester operation while providing flexibility to meet the
  mill’s changing needs. 

Description

The primary objective of Continuous Digester control is to enable mills to safely produce pulp of the highest quality, at the maximum rate and at the lowest cost. Control strategies for the continuous digester extend beyond the digester unit; they enhance the efficiency of the entire mill. The Continuous Digester control package offers advanced control functions presented in a modular form. The control functions are designed to stabilize and optimize any or all of the three basic parameters of pulp digesting cooking time, cooking temperature, and cooking liquor concentration. Custom graphic displays, reports, and services are included to enhance operator effectiveness.

Cooking Time Functions

Chip Bin Level Control minimizes variations in the chip bin level by manipulating the mass flow rate of wood to the chip bin. A unique model-based control concept known as Horizon Predictive Control (HPC) is employed to compensate for the deadtime between the feed actuator and the bin, enabling superior performance. Control actions are either feedback from the bin level error or feedforward from chip demand changes in the digester, as indicated by the chip meter speed.

Production Rate/Chip Meter Calibration Control calculates and controls from the front end of the digester. The operator-entered production rate target and pulp yield setpoint determine the dry wood feed rate target. An on-line program periodically updates the chip meter calibration (i.e., the dry weight of chips delivered per revolution of the chip meter) by conducting a dynamic mass balance around the chip bin. The control compensates for changes in the measured chip meter calibration by adjusting the chip meter speed to maintain the calculated dry wood feed rate at target. All chip meter speed changes are gradually ramped to the new setpoint minimizing disturbance to the operation.

Digester Chip Level Control uses strain gauge inputs for hydraulic digesters and gamma gauge input for vapor phase digesters. For hydraulic digesters, a special processing routine uses statistical techniques to accurately convert the signals to a 0 to 100% analog readout. The routine includes logic that automatically determines the effective span for each strain gauge signal and relies on a comprehensive truth table to confirm the validity of the level computation before using it for display and control.  Blow flow is used as the primary controller of digester chip level. The model-based, deadtime-compensated HPC algorithm is employed to minimize level variations. Control strategy constraints limit blow flow deviation from its nominal value, which is production rate dependent and determined from operating history. This approach enables the nominal value to compensate for such things as instrument drift, variations in chip meter pocket fill, and changes in wood quality.

Dilution Factor Control stabilizes the end point of the cooking zone and assistsin chip level control by reducing  wash upflow variations. Dilution factor is typically calculated from a mass balance around the outlet of the digester and controlled using the extraction flow (hydraulic digesters) or cold blow flow (vapor phase digesters). Great care is taken in adjusting the manipulated variable to avoid plugging the  screens and disturbing the chip plug movement. The differential pressure measurement across each extraction screen is constantly monitored for indications of plugging. If plugging is detected, the dilution factor target is automatically reduced. Once the condition clears, the dilution factor target is gradually ramped back to its initial value.  Blow Consistency Control helps steady the plug movement in the digester by stabilizing the fiber discharge from the digester. The blow consistency signal used for control can be either a direct measurement from a sensor located in the blow line, or an inferred measurement derived from other blow parameters. Although direct measurement is preferred for control, the latter approach allows the control system to detect and correct for blow consistency trends.

Blow consistency is controlled by adjusting the bottom dilution flow. Under normal operating conditions, flow corrections are gradual in order to minimize disturbance to the process. If, however, the consistency signal suddenly drops off indicating a plug hesitation bottom dilution flow is rapidly decreased to reestablish chip plug movement.

Cooking Temperature Functions

Kappa Number Control employs a number of advanced statistical process control (SPC) techniques to minimize the total kappa number variation by taking corrective action only for true process shifts. Control actions are taken only when they are statistically valid and no cause for the kappa disturbance can be found. This SPC approach is combined with a modified H-factor model of the cooking process to provide comprehensive control of the kappa number via adjustment of the cooking temperature setpoints. In essence, the SPC determines when to make a temperature correction and then triggers the cooking model to determine the amount of correction to make. This feedback portion of the control function is based on periodic kappa test entries or on-line kappa measurements. In addition, the cooking model is responsible for providing feedforward temperature corrections whenever the digester is in a dynamic state, such as during rate changes and grade changes.

The mathematical techniques included in the SPC package eliminates over-control of the kappa number, a phenomenon often observed in traditional control systems. These techniques also provide insight into the cause of a variation, allowing for reduction or elimination of frequent un-assignable  causes.

Cooking Liquor Concentration Functions

Chemical-to-Wood Ratio Control maintains the initial chemical concentration to attain the desired pulp properties at the target kappa number. This control function calculates and controls the mass flow of (active or effective) cooking chemical per mass flow of dry wood charged to the digester. For digesters equipped with multiple chemical injection points, each component can be controlled at a desired percentage of the total chemical. The chip moisture and chemical concentration measurements used in this control scheme can be entered from laboratory data or as inputs from reliable on-line sensors.

Residual Chemical Control maintains the desired concentration in the cooking zone of the digester by manipulating the chemical-to-wood ratio target. The control strategy uses a continuous measurement of liquor strength, from a titration analyzer, mounted in a cooking liquor irculation line, to measure chemical concentration. Control action is based on the same SPC techniques employed in the kappa number control function, so control actions are made only when statistically valid and no assignable cause of the concentration disturbance can be determined. Once a change to the chemical-to-wood ratio target is made, no further control action takes place until the effect of the change appears at the residual measurement location. Safety limits are placed on the chemical-to-wood ratio target to prevent driving this parameter beyond mill specifications.

Liquid-to-Wood Ratio Control manipulates the flow of weak black liquor to the top of the digester to maintain a constant ratio of total liquid charge to dry wood flow. The liquid-to-wood ratio is determined from measurements of chip moisture, chemical flow(s), and weak black liquor flow, plus calculations of dry wood feed rate and condensed steam.Feedforward control is provided whenever the total chemical flow setpoint is changed.

Time, Temperature, and Concentration Functions

The Production Rate Change Control strategy coordinates several advanced controls to achieve a change in the production rate with minimum effect on kappa number variability. Control is based on properly balancing time and temperature variables to maintain the same H-factor during the change. This function can accommodate both scheduled and emergency rate changes. In emergency situations, an immediate cutback in the digester production rate can be implemented when problems upstream or downstream of the unit necessitate such action.

The Grade Change control strategy provides a smooth transition during yield or species changes. The position of the new pulp grade interface is tracked using a volumetric integration of the wood flow through the chip meter. This feature tracks the interface from the chip bin to the high-density storage chests. As the interface reaches key points in the digester, operating targets for the new pulp grade become active. Specific techniques are incorporated into the control to make the interface between the grades as sharp as possible, minimizing the amount of transition pulp produced.

Custom Graphic Displays and Reports

Improved process visibility promotes better overall control of the continuous digester. Honeywell has designed intuitive graphic displays and reports for operators, engineers, and mill managers to ensure that the mill realizes maximum benefit from its advanced controls. The Advanced Control Summary graphic provides a comprehensive summary of the control status by displaying both the controlled and manipulated variables associated with each control function, even when the control is inactive.

The Production Rate Change and Grade Change graphics provide dedicated displays for the operator during important process changes. Custom graphics are designed to make it easy for your mill personnel to understand and support the advanced controls. Help displays function as an effective operator training tool and are also useful in process troubleshooting. Tuning displays allow mill engineers to easily maintain optimum performance of the advanced controls.

The Production Summary Report is automatically generated at the end of every operating shift, day, month, and grade. It provides a detailed breakdown of raw material usage, pulp quantity and quality, process variability, and advanced control utilization for the reporting period. Each report is configurable, allowing any process measurement, calculation, or test entry to be collected, statistically analyzed, and displayed for your benefit.

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