Update: Here are parts 3 and 4 from this excellent series on distillation column control:
- Best practices in column control distilled, part 3
- Best practices in column control distilled, part 4
If distillation is part of your production process, there is a great two four-part ControlGlobal.com series on distillation column control. The two articles are:
- Best practices in column control distilled, part 1
- Best practices in column control distilled, part 2
Emerson’s Greg McMillan and CMiD Solutions’ Mark Darby discuss the best practices in composition control strategies. Right from the opening of the part one article, you can get a good sense of the technical depth and experience shared by both Greg and Mark. Greg opens:
Distillation column control is key to achieving consistent product purity that meets specification limits without consuming excessive energy. Composition control via either analyzer or inferred prediction is critical. A proportional-integral-derivative (PID) column temperature is typically used if a temperature can be found that correlates well with the product quality or property. Often, the best tray shows large and symmetrical changes in temperature for changes in the manipulated column flow controller setpoint. The reflux or distillate flow is often manipulated to more directly control the material balance split. The manipulation of reboiler or bottoms flow generally has a smaller effect on temperature changes and hence inferred composition except at very large or very small distillate-to-feed ratios seen in high purity columns.
Since the use of the best tray location is critical and can change with operating conditions, it’s extremely important that the column have temperature connections at many trays. Sensors should be installed at the best tray and the ones immediately above and below it, as well as at enough other trays to get a good column temperature profile for diagnostics and validation of process models. The sensor should extend into the liquid froth above the tray to maximize the heat transfer coefficient, but should not interfere with column traffic. Intrusion of the thermowell into a downcomer area might cause local flooding. In packed columns, sensor location is more critical due to the possibility that channeling will cause liquid to bypass the sensor. If temperature control doesn’t provide the desired end result of column composition control, then a column composition controller can be used to manipulate the setpoint of the column flow controller. An inferential measurement of column composition may be used if an online analyzer isn’t available, or to fill in the blanks between analyzer updates that may have delays due to sample transportation and long analyzer cycle times.
There are many excellent references throughout the article including Mark’s Signal filtering: Why and How and Model Predictive Control – Past, Present, and Future, Part 1 (of 3). Greg referenced a one-hour, twenty-minute recorded webinar, Feedforward and Ratio Control. Also, Greg’s Control Talk Blog is a treasure trove of shared expertise.
This two-part series is one you’ll want to spend some time with, to absorb these shared best practices.