01 February 2005

ISA Certified Automation Professional (CAP) program

This question is from the CAP study guide, Domain II.

Definition: Identify customer requirements and complete high-level analysis of the best way to meet those requirements.

CAP question

To MINIMIZE the effect of feed upsets to the top of a distillation column, the flow feed forward signal should be connected to a tray temperature controller output that then directly manipulates a(n):

A. Overhead pressure controller.

B. Linear reflux valve.

C. Reflux controller.

D. Reboiler stream ratio controller.

CAP answer

The correct answer is C, the reflux controller.

A distillation tower has feed upsets that can produce a feedforward signal. A reflux flow controller remote setpoint becomes the feed multiplied by a reflux-to-feed ratio that the temperature controller output will correct. This is the most linear and accurate way of maintaining the material balance as long as the flow measurements are above their low range ability limit.

Changes in the pressure drop of a linear reflux valve and the valve installed characteristics and deadband will introduce errors into the feed-forward. Manipulating a pressure controller setpoint will not set up a flow ratio to maintain the material balance. A ratio controller uses a ratio of flows as its controlled variable. This creates a nonlinear loop. Also, the remote set point would be incorrect, and steam would have a slower, and generally smaller, effect than reflux flow on the top.

Reference: Blevins, et al. Advanced Control Unleashed: Plant Performance Management for Optimum Benefit, ISA Press, 2003. 

Nicholas Sheble (nsheble@isa.org) edits the Certification department. For information about the CAP program, go to www.isa.org/CAP.