Continuous Digester Optimization

Digester
CONTINUOUS DIGESTER OPTIMIZATION

Objectives

Stabilize the process
Provide consistent operation from shift to shift
Minimize pulp quality variation – K/Kappa
Enable operations to control kappa shifts
Reduce wood, chemicals, and energy consumption
Provide for seamless changes in production rate
Change pulp or species grade with optimal pulp quality
Increase yield by alleviating bottlenecks

Continuous digesters (both hydraulic and vapor phase versions) are highly interactive processes. Virtually all of the major control loops are coupled to some extent. However, many of these loops are often treated as Single-Input Single-Output (SISO) loops with little regard for the consequences of interactions. This makes it difficult to obtain both stability and "tight" control of Kappa number. Overall system stability should be a major control objective of supervisory control if process variability is to be minimized. Maintaining a stable fiber mass balance around the Digester Vessel, Impregnation Vessel and Chip Bin is very important if minimum variance control of Kappa number is to be achieved.

Keeping the major regulatory controls in a controllable range is important. This is accomplished in some loops by the use of mid-ranging control. However, quite often this is the source of process variability as it is difficult to design and tune these controls to function smoothly over the entire range of operating conditions. To overcome this difficulty, AAI employs Fuzzy Logic mid-ranging algorithms which have proven to be robust in these situations.

When process upsets occur, the control system should respond with coordinated control actions to return the system to a stable mode of operation. The strategies for control presented herein are intended to accomplish exactly this.

Fuzzy Logic is presented as a tool to be utilized in parts of the Digester control implementation. It is a technology that allows implementation of rule based control which is easy to configure, modify, and tune. AAI has developed Fuzzy Logic software with a graphic interface that allows a user to develop Fuzzy Logic control without writing software. Typically, all of the control functionality resides in a Distributed Control System (DCS).

Chip Bin Level Control – Stable chip bin level control facilitates proper pre-steaming in the chip bin which ensures the best absorption of white liquor in the Impregnation Vessel (IV) and also improves chip column movement in the digester vessel.

Dry Wood Feed Rate – Dynamically controls feed rate based upon correcting ‘fill factor’, thus contributing to better Kappa control by reducing variations in chemical to wood ratio.

IV Chip Level Control - The primary manipulated variable for Impregnation Vessel Chip Level Control is sluice flow with chip meter speed serving as the secondary manipulated variable. The IV chip level control is tuned to respond significantly faster than the digester.

IV Outlet Device Control - The Impregnation Vessel Outlet Device nominal speed target is set by the production rate target and can be biased by the operator. A mid-ranging control scheme incrementally adjusts the outlet device speed target around the nominal value.

Digester Chip Level Control - The primary manipulated variable for Digester Chip Level Control is blow flow. The secondary manipulated variables are Chip meter speed and Sluice Flow.

Digester Liquor Level Control - Digester Liquor Level control is accomplished by modulating the total extraction flow setpoint. A rate limiter is used to prevent sudden flow changes which could plug the extraction screens.

Dilution Factor Control - Dilution Factor is controlled by adjusting the total dilution flow setpoint. This loop is tuned for a slow response (closed loop time constant of about 15 minutes). The dilution factor is temporarily reduced when the plug is hesitating or the extraction flow exceeds a maximum desired flow rate

Blow Consistency Control - The OD speed is manipulated to keep the estimated Blow Line Consistency near a desired value. The estimate of Blow Line Consistency is based on measured Blow Line Consistency, outlet device DP, outlet device load (amps) and a calculated Blow Line Consistency.

Digester Bottom Dilution - Mid-ranging control adjusts total dilution ratio slightly to bring the outlet device speed back into the desired operating range.

Production Rate Control – Production Rate Control establishes the nominal targets for blow flow, chip meter speed, sluice flow and IV outlet device speed. The desired production rate, nominal blow flow consistency, and expected yield are used to calculate the nominal blow flow target. The incremental correction from the level control module is maintained. The nominal chip meter speed, sluice flow and IV outlet device speed are also calculated.

Chemical to Wood Control - The Chemical to Wood control module adjusts the main white liquor flow control setpoint to maintain a desired chemical to wood ratio (% EA). This ratio is initially set by either the operator or by production rate control, and is trimmed by BC Availability Control.

BC Availability Control - Bottom Circulation (BC) Availability Control has two objectives. The first objective is to hold the Effective Alkali (EA) in the BC line at a desired value. This is done by modulating the white liquor flow addition to the BC line.

Liquor Management - White liquor addition flows are adjusted to each line until the EA, as measured by an alkali meter, matches the target.

Kappa Number Control - Isothermal Cooking extends cooking or delignification to the bottom of the digester while maintaining a nearly constant temperature profile (isothermal) down the digester. The temperatures, EA, and residence time in the digester are the three factors which control the delignification process in the digester.

Enhanced User Interface - With the Advanced Control System, a set of User Interface displays is included to simplify operation of the Digester Process. Through these displays functionality such as ramped load changing is provided. In addition, a single display screen gives operators an overview of the entire process. The overview screen emulates the traditional operator panel board. Visual indication of process status is displayed using "analog meters" rather than engineering units.

Stability in the Digester Process is critical to production of quality pulp with optimum process operating cost. Methods to achieve process stability include putting all process parameters on automated computer control and implementing applicable advanced control techniques. This is accomplished with implementation of the Digester Advanced Control System.

Performance of the control system may be measured by comparing certain process data after start-up to that collected during a benchmark period prior to the project. Targets for standard deviation of key process parameters might include:

Through the implementation of AAI’s Digester System, a mill can realize significant benefits. An increase in quality pulp yield with reduction in wood and chemical input can produce additional tonnage per year of pulp, resulting in additional bottom line profits, while at the same time reducing operating costs. A 1,000 to 2,000 Ton/ADT increase per year of pulp, at today’s market prices will result in $450,000 – $900,000 additional profit in a given year.