Ask Greg McMillan - What role do you see dynamic simulation playing in the future of best distillation column temperature control tray?

Greg's Response:

Process simulations or plant tests can be run to determine the tray with the largest and most symmetrical (linear) response to a change in the split that is the distillate to feed ratio (D/F) or reflux to feed ratio (R/F). This is also generally the most binary tray and the tray with the fastest dynamic response for a change in the D/F ratio. It is also the tray that usually minimizes the maximum excursion of all other trays. While other locations (e.g., closer to the end of the column where the flow is manipulated) may provide a smaller dead time, the importance of sufficient process gain (process sensitivity) is paramount. It does you no good to have a faster loop if the temperature is no longer a good inference of composition. The composition error for a given temperature measurement error can be so large, due to small temperature changes that control of a single tray temperature is meaningless. In some cases, a delta temperature difference between two different trays can be used as the controlled variable to improve the inference of composition.

Changes in column pressure will cause changes in column temperature even though the composition is constant. Pressure composition can be used, but it assumes a two-component (binary) mixture. The use of the tray that shows the largest temperature change for a composition change is often the most binary tray and usually exhibits the least pressure effect.

Since the use of the best tray location is critical and can change with operating conditions, it is 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, plus 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.

For much more knowledge, see the ISA book Advanced Temperature Measurement and Control, Second Edition (LIST PRICE REDUCED by 50%. Use promo code ISAGM10 for an additional 10% discount on Greg’s ISA books).