Advanced Batch Digester Solution 

Honeywell’s Advanced Batch Digester Solution precisely and consistently controls your batch digesters, enabling production of high-quality pulp at a lower cost and higher yield. Advanced Batch helps to reduce pulp quality variability resulting from inconsistency in charging, cooking, steaming, and venting. Precise control of the kappa number improves pulp uniformity out of the digesters and helps stabilize unit processes in a variety of areas throughout the mill, including washing, screening, the bleach plant, chemical recovery, and the paper machines.

Advanced Batch controls maintain tight control of the loading and cooking processes. This allows the batch digester house to be operated closer 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 application package achieves excellent results in controlling digester houses of all configurations cooking single or multiple grades. 

Highlights

• Improve pulp quality by regulating chip/liquor loading
  and the cooking process, leading to reduced K/kappa
  number variations. 

• Reduce chemical demand by tightly controlling the
  chemical-to-wood ratio during the charge. 

• Reduce powerhouse steam demand swings by
  maintaining on-time blows, running a consistent
  production schedule, and providing steam leveling.

• Ease operator workload by automating routine functions
  and presenting operating information in a concise and
  useful manner. 
  

Description

The Advanced Batch package offers a modular arrangement designed to stabilize and optimize your batch digester house. The control functionality is divided into three levels, Base, Advanced, and Optimization controls. The base-level regulatory and logic functions provide essential process stability. Base-level controls provide safety interlocking, pump and motor start/stop logic, and analog regulatory controls. Advanced controls include functions such as automated charge, cook control, and blow control. Optimization controls serve the supervisory role and help close the entire control loop for the batch process. The digesters are automatically scheduled and sequenced properly based upon the mill’s production demand. When a fully instrumented digester house is on optimization, the only actions the operator needs to perform are to confirm the blow and provide test entries if on-line test measurement is not available.

 
Chip and Liquor Charging

Accurate and consistent addition of chips and liquor to the digester is the key to producing uniform, high-quality pulp. Measurement of the chip flow to the digesters is used to maximize the packing efficiency in each digester. This results in better cooking liquor circulation in the digester and often increases the volume of chips charged. Timely introduction of the cooking liquor during the loading cycle prevents liquor flashing and decreases chip floating while minimizing excessive delays that may extend the load.

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Chip and Liquor Ratioing: The digester load time is minimized by dynamically ratioing the chip and liquor flow during the charge. Digester cooking efficiency is improved by providing for consistent liquor impregnation into the chips throughout the digester. 

Chemical-to-Wood Ratio Control: Chemical-to-wood ratio control charges the digester with the optimal quantity of white liquor to achieve the desired pulp properties at the target K/kappa number.

Based on the dry wood charge to each digester and the white liquor strength, the control function determines the correct volume of white liquor to charge. The source of the white liquor strength measurement can be either operator input based on lab tests or an in-line measurement from a reliable liquor-strength sensor.

Liquor-to-Wood Ratio Control: Liquor-to-wood control loads the digester with the optimum volume of total liquor needed to achieve good cooking liquor circulation without overfilling. Control of the liquor-to-wood ratio is critical because insufficient liquor volume in the digester causes inadequate circulation during the cook. More liquor in the digester than is needed for circulation results in extra liquor that has to be heated. Excessive liquor also increases the risk that liquor may carry over into the relief line; this may promote excessive foaming and may increase the frequency of screen plugging.

White Liquor Measurement: The optional white liquor strength measurement module permits the operator to select the white liquor strength input from a sensor or a manual strength test entry. When the sensor signal is selected for use, the input is checked for validity and compared with predefined high and low manual test limits. If the signal is valid and within limits, the filtered average sensor value will be used to permit on-line adjustment of the white liquor fill target to achieve the target chemical-to-wood ratio. A sensor reading above or below the high and low limits will be clamped to the respective limit. If the sensor is found to be invalid, the manual test entry will be used for white liquor strength.

Cooking Control

The objective of Advanced Batch cooking control is to deliver to each digester  the amount of heat needed to stabilize the delignification reaction. Stability of the cooking process contributes to uniformity of the final process parameter, kappa number. Employing a modified H-factor equation, the controls determine the steam flow rate required to achieve the desired "time to temperature" and "time at temperature" setpoints.

Dynamic cooking control feedback: Automatically analyzed cook time and temperature parameters are used to maintain consistent cooking conditions across the digester house. Corrections are also made to ensure that the digester schedule is maintained.

Ready Time Optimization: If extra time is available in the digester house schedule, the digesters may be requested to steam earlier than scheduled and cook at a lower temperature. Cooking at a lower temperature and pressure will typically provide steam savings, improve tear and tensile strength, and reduce wear on digester blow valves.

False Pressure Control: False pressure in a digester may severely impede digester circulation, resulting in poor pulp quality. This condition indicates severe screen plugging, which restricts the relief line’s ability to release non-condensable gases. If excessive false pressure is detected, controls initiate immediate blast of blowback steam to clear the blocked screen. If the condition persists, a more advanced routine that manipulates the digester steam, blowback, and relief valves is initiated.

Cook Pressure Differential Control: Excessive pressure / temperature differential within the digester is eliminated to promote good digester circulation. Based on a dynamic pressure differential target, relief, steam, and blowback valves are manipulated to maintain optimum digester circulation.

Relief Valve Strategy: Proper control of the digester relief valve is critical to good digester cooking. The objectives of Advanced Batch relief control include:

• evacuation of non-condensable gases and entrained air
  from the digester to control "false pressure" while
  minimizing steam waste 

• dynamic positioning of the relief valve to maximize
  turpentine recovery in a softwood mill 

• emergency pressure relief when the digester dome
  pressure rises above a predefined safe limit. 

Blowback Control: This control clears the relief screens to allow proper venting of the digester and enhances uniformity of the pulp mass through coordination with relief control Screen blowbacks can be initiated on a timed basis or as a function of the differential pressure between the digester and the relief line. Separate timers are provided for maximum flexibility.

Optimization Controls
Grade Change

Grade change control offers a mill the flexibility to meet changing market needs. However, handling grade changes in a batch digester house is a difficult and time-consuming process. Advanced Batch grade change control strategy provides a smooth transition during yield or species changes. The module is designed to handle up to nine separate grades and automatically transfers operating targets for the new pulp grade into the current grade when the digester completes the current cooking cycle.

Grade handling is done on a digester basis to minimize pulp variability between digesters.

SPC K/kappa Control

Kappa number control employs 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 assignable 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 H-factor target and the chemical-to-wood ratio target. The control function is based on periodic kappa test entries or on-line kappa  measurements.

Steam Leveling

Advanced Batch steam leveling control reduces load disturbances to the powerhouse with a minimum impact on digester production. This control coordinates the individual digester steam flow targets to reduce excessive swings of steam demand. This is accomplished by restricting the rate of change of steam to be used at any given time. The limit for steam leveling can be manually entered or can beautomatically calculated as a function of  production rate.

The steam leveling module also monitors the steam header pressure to ensure minimum impact on the header by the digester house. If the main steam header pressure falls below a configurable level, the steam will be cut off to all digesters. When the steam pressure comes back up above the cut-off limit, the steam will either return to the previous limit or remain at zero and alert the operator to engage the control.

Production Rate Scheduling - Line or House

Production rate scheduling enables the operator to easily change pulp throughput, allowing quick response to downstream demands without sacrificing pulp quality. The scheduler determines a target blow time for each digester and maintains the desired digester blowing sequence. The operator simply enters the desired production rate in one of the following forms: a digester spacing time, a total cycle time, a production rate in tons per day, or a target number of cooks per day.

House Production Rate Control: This control simplifies operation of the most complicated batch digester process. The control allows the operator to enter the desired tonnage per day or tonnage per grade, and the controls make the necessary calculations and adjustments based on house constraints. A Production Target Worksheet tool is included that allows the operator to verify that a desired production rate is possible before the target is entered into the system.

The primary objective of the House Scheduler is to optimally sequence the charging and blowing of digesters. The House Scheduler offers a number of additional benefits, including consistent steam usage, efficient digester loading, and improved blow tank level control.

Blow Tank Level Prediction: This forecasts the blow tank level based on the current blow tank level, washer production rate, dilution flow, and the blow schedule to the blow tank. This control promotes improved product quality by enabling the wash plant operator to set the washer production rate to a consistent value, increasing washing efficiency while maintaining on-time blow spacing in the digester house. 

Production Rate Control: This control is designed to calculate digester production targets based on washer rate and levels in the high-density storage tanks. The control calculates house production targets based on the operator-defined high-density storage tank level setpoint.

Advanced Batch Digester Control Benefits:

• Reduction in kappa number variation of 30 to 40% 

• Increases in yield of 0.5 to 2% 

• Increases in throughput of 2 to 5% 

• Reduction in cooking chemical usage of 3 to 6%

• Reduction in cooking chemical usage of 3 to 6%

• Reduction in bleaching chemical usage 

• Stabilization of downstream unit operations 

• Enhancement of process monitoring and management
  capabilities to increase productivity in troubleshooting,
  monitoring, and tuning of the control.  

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