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Dec
28

Biggest Opportunities for Process Control Improvement – The Operator (Online Metrics)

Who is living with the process every minute? Who changes the feed rates or charges? Who changes the modes and set points of the control system? Who starts or stops batches or unit operations? In most plants, it is the operator, yet the displays and education of the operator haven’t changed much in the last 20 years. We still have faceplates, trend charts, and digital values of process variables, and changing or flashing colors or shades. We still have minimal operator training based more on tiebacks and interface familiarization than on first principles and process understanding.

If the operator knew the yield and cost per pound of product for the last eight hours of each shift, the operator could be more recognizant and probably more competitive. This could be achieved by flows that are synchronized, shift totalized, and ratioed with dollar amounts assigned for each flow. Consider a reactor and an 8 hour shift. Here the total flow of each reactant and utility for the last 8 hours would be ratioed to the total product flow for the last 8 hours for each shift. Each flow total would be multiplied by the cost of the stream ($/lb) to provide cost to product ratios for the last eight hours. The reactant and utility flows could be delayed to match them up time wise with the product flow. The use of totals for the last 8 hours reduces the accuracy requirement of this synchronization besides decreasing noise. The use of ratios decreases the effect of production rate on metrics. Also, changes in ratios offer keys to tracking down disturbances and changes in concentrations of feeds (e.g. raw materials, intermediate, or recycle streams). Both totals and ratios for each shift could be indicated. Shift metrics could be treated similar to batch metrics where each shift is like a different piece of equipment running the same batch process. The shift metrics could be plotted similar to batch metrics.

For waste pH systems, it would simply be the total reagent flow ratioed to the total effluent flow ratio for the last eight hours. I developed a real time virtual plant in DeltaV using this concept a couple of years ago to show the value of adaptive controller tuning for pH control. If you want a copy, contact your rep.

The concept could be expanded to use totals to cover the last week or month or the last “n” number of batches for each shift and all shifts.

If the operator could plot these ratios versus changes in operating points, what insight could be gained on process nonlinearity and for process optimization? What if the operator had XY plots, worm plots, and 3-D plots built into the operator graphics for all historized variables like what engineers generate in Excel and statistical packages?

When comparing the performance of similar plants in the USA and Belgium, it was found that the Belgium plants had consistently better yields. The Belgium operators lead the design of experiments and guided the process improvements. Could better online performance metrics and process training be the key for operators to perform roles of the increasingly scarce process and process control engineers?