Distillation, Part 5: Multiple Products

Heat balance is often controlled by throttling the flows of pump around reflux streams (right side of Figure 1). The goal is to maximize the amount of heat transferred to the feed.

Multivariable Control

Traditional PID controls usually implement single-input-single-output (SISO) algorithms, while advanced controls work with multiple inputs and outputs (MIMO). The modeling of a process can be of the “white box” or “black box” type. White box modeling is used for well-understood processes, such as distillation, where the knowledge of mass, energy and momentum balances allows the development of accurate dynamic models. These internal model control (IMC) systems are useful in optimizing the process and in anticipating future events.

“Black box” or model-free controls (MFC) include the ANNs, fuzzy logic and statistical process control strategies. These algorithms are trained on the data obtained from the past operation of the controlled process. Their limitations include the relatively long learning period they require and the fact that their knowledge is based on the past performance of the process. Therefore, they’re not well-suited to anticipating future events, and if operating conditions change, they require retraining. 

Feed Composition Compensation

Feed-forward control configurations are required if fast changes in feed composition occur, because such sudden changes can’t be handled by feedback control alone. If z is the concentration of the key component in the feed, the distillate flow is calculated as:

Therefore, when z is measured, the equation for distillate can be simplified to:

where m is the output of a feedback trim controller (ARC-1 in Figure 2), which maintains the overhead composition.

The feed composition analyzer measurement is sent through an auto manual station (HIC), so that during periods of analyzer failure or maintenance, the signal can be substituted by a manually generated one. The feed-flow measurement (F) is dynamically compensated (FY) to reflect the dead time and time constants of the process. The feed-forward product of the feed composition (z) and the feed flow (F) is divided by the feedback overhead composition controller (ARC-1) output to arrive at the set point of the distillate flow controller (FRC).

The reboiler heat input also is feed-forward compensated, because the ratio controller (RIC) maintains the feed to boil-up ratio (F/V) on the basis of the dynamically compensated (FY) feed flow. The material balance of the column is closed by the bottoms level controller (LIC), which is modulating the bottoms flow.

Béla Liptákis a regular contributor to Control. He can be reached at liptakbela@aol.com. He is editor of the Instrument Engineers’ Handbook, former chief engineer of C&R (formerly John Brown) and former adjunct professor of Yale University.