(22g) Use of External Targets to Improve Dmcplus Controller Performance and Long-Term Benefits

Authors: 
Mechielsen, S., Dow Chemical Company
Celendano, M., Dow Chemical Company
Amos, D., Aspen Technology
Basteris, J., Control Consulting, Inc.


Important control variables in DMCplus ethylene plant controller designs are frequently provided with External Targets (ETs) to support RTO applications. This paper reviews how ET use was augmented in a number of ethylene plant controllers to improve operator understanding and agility in using the controllers and increase on-line time at optimal control conditions. Practices included use of ETs to robustly manage analyzer variables and simplify allocation and back-up strategies. ET ranking was used to pivot complex controller strategies for handling different operating modes in a simple to interpret manner. The resulting control variable interface was intuitively understood by operators and provided a convenient ?reach-in? capability to influence controller action without resort to changing control or manipulated variable limits. The controllers using a rich ET structure were generally observed to have maximum on-line times while always doing the right thing for stable optimal operation. This increased controller benefits in the long-term and facilitated capture of additional benefits associated with composite applications and RTO applications implemented following the introduction of the controller. Examples of how ETs were used will be provided for both ethylene furnace and back-end controllers.