Assessment and Future Directions of Nonlinear Model Predictive ...

396 K. Naidoo et al.7 CommissioningCommissioning is usually phased over several visits to a site. The aim of the firstvisit is to put the inferential quality models online so they can be observed overseveral operating conditions to ensure accuracy and models can be improved ifnecessary.Follow up visits will involve commissioning of the controllers. Some practicalissues:1. Slave (gas composition) controllers are commissioned prior to master (quality)controllers.2. Controllers are commissioned incrementally adding one or two handles at atime - the software must support this incremental workflow.3. Controllers are initially commissioned for in-grade control. Only when thisis satisfactory are transitions commissioned.4. Windows of opportunity for a given grade may be very short - sometimesa window of only a day or two within a period of several weeks. This hasseveral implications:• Commissioning must be well planned and well prepared to make the mostof these opportunities.• Models must allow for fine tuning online so as to make the most of theshort time window5. Valve properties may require some practical approaches• Accumulate several moves before implementing them• When writing valve outputs directly, it is better to explicitly separateout valve non-linearities from the model structure6. Identify disturbances at the source and address them early - robust controlis still necessary, but not sufficient• Examples of disturbances in a UNIPOL PP process:– Cooling water inlet temperature variance.– Catalyst batch changes– Day/night feed quality changes– Bed instability (monitor bed levels, bed weight)– Powder discharge system problems• For example, one might check on the feasibility of including cooling waterinlet temperature control to minimize ambient effects.8 Operating the SolutionBoth operators and plant engineers have their own interface to the online solution.The operator interacts with the system through a simplified interface whichallows monitoring of key quality and composition variables, allows changing ofoperating limits, in-grade set-point changes, validation of lab results, triggeringof transition sequences, and the ability to turn the controller off and change tomanual control.