Continuous Digester Optimization
- 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).
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:
- Chip Bin Level IV chip level
- Digester chip level
- Digester liquor level
- BC concentration
- Dilution factor
- Kappa number
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.