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

Greg's Response:

The basic loop arrangement for dryer temperature control would use cascade control of outlet temperature to set inlet temperature. However, a constant outlet temperature does not mean a constant moisture content of product exiting the dryer. The product temperature in dryers closely approximates the web bulb temperature as long as there is still some moisture present. The increase in dry bulb from inlet to outlet represents the moisture load. Of course, the minimum outlet temperature and, hence, the maximum moisture load is reached at the dew point temperature. If air flow can be manipulated directly or ratioed to feed rate, the dry bulb temperature difference between the inlet and outlet of the dryer can be used for moisture control. This strategy to control moisture load helps eliminate the interaction from separate temperature loops and cancels out noise that appears on both temperature signals.

Below the critical moisture point, where inner particle moisture has been depleted and dry spots start to appear on the surface, the rate of drying is proportional to the moisture content. Moisture content can be computed from the driving force for moisture removal that is the log mean temperature between air dry bulb and product wet bulb temperatures. If the wet bulb product temperature can be measured or computed from an inlet air humidity or dew point and dry bulb temperature measurement, an inferential measurement of moisture can be directly computed. If this is not possible, an outlet temperature set point can be computed to provide inferential moisture control. However, this calculation creates positive feedback and will cause instability unless the ratio of outlet to inlet temperature is less than open-loop gain of the outlet temperature loop, and the time constant of the filter applied to the inlet temperature is set larger than the integral time (minutes/repeat) of the outlet temperature loop. The dryer temperature or inferential moisture control system can manipulate feed rate instead of heat input to maximize throughput. A high temperature override controller protects against voids or plugging.

Simulations that include operating conditions and equipment design with Digital Twin can help find and confirm the best control strategies.

For much more knowledge, see the ISA book Advanced Temperature Measurement and Control, Second Edition (use promo code ISAGM10 for a 10% discount on Greg’s ISA books).