These process variables include MVs, CVs, disturbance variables (DVs) and constraints:
- MVs consist of controller setpoints written to by the model-predictive controller (MPC). For example, they can include first-column, side-draw temperature setpoints.
- CVs include process variables to be maintained at a specific value or set point, such as column OH pressure.
- DVs are measured variables that may also affect the value of controlled variables, such as solvent concentration, feed flowrate, reflux remperatures and first-column OH pressure.
- Constraints (LV) are variables that must be kept within an operating range, such as product stream impurity level and percent of product in the impurity stream.
Models were developed for the four process variables, and Goodyear's engineers used their own knowledge to adjust the models' parameters. Actual process data were then used to validate the models.
Johnston said the modeling and tuning resulted in improved temperature control, further reduced product loss, and improved impurity levels. "The new control scheme in our second project also reduced the rate and the product concentration in the purge stream. Overhead product quality was not negatively impacted. We reduced product losses by 7,000 pounds per day. And we reduced energy consumption a moderate amount," adds Johnston. (Figure 3)
To maintain its new APC tools and system in the future, Goodyear may need to change the limit ranges of its manipulated, controlled or constraint variables if feed rate or composition change significantly. "We may also need to tweak or rebuild our models at some point, and we might need to regenerate the controller to adjust performance versus robustness, such as in ‘penalty on move' versus ‘penalty on error' situations," she concludes.
Goodyear plans to install SmartProcess Distillation module and APC models on its additional distillation columns in the C5 refinery process unit, and explore the potential for using APC in other units on non-distillation applications.
Jim Montague is Control's executive editor.