Have You Closed the Loop Around “Automatic” Grade Change Yet? - Achieving Diverse Performance Targets, 18PaperCon

The primary objective of a grade change in a paper mill is to guide the paper machine operations from the production of the current grade to a new grade. Therefore, grade changes generally entail production and quality target shifts such as weight, moisture, paper ash, color, production speed, jet-to-wire ratio, and/or other quality and process variables to achieve the specification of the new paper grade at the desired production rate. However, the trajectory from the current grade to the new grade and the desired grade change performance in this transition are generally different in different applications and machines.  This also leads to different economies. 

Many production lines may prefer the target shift to be as quickly as possible, with a minimum off-spec production and lost time due to breaks. However, for some other mills, it may be desirable that certain paper quality parameters remain within a specified range during the entire grade change to maximize saleable paper and/or to avoid production breaks.  In this mode, the controller may limit the rate of change for some variables to ensure the required machine runnability. We would like to propose and show that in an MPC-based closed-loop grade change, the different performance requirements can be met through different control settings and thereby, the desired grade change performance achieved. 

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