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Cascade Control on Distillation Units

Cascade control, where it is used, and why on distillation units

 

 

Subject: Cascade Control
Date: Thu, 08 Mar 2001 13:50:01 "GMT"

Dear Sir,

Could someone answer a question I've always had about column control? Some of our columns have direct level control from the bottom of the column to the bottoms flow rate. Others have a level that resets a flow controller on the bottom product rate. I don't see much performance difference between these. Can you explain why one or the other is better.

M., Asian Chemical Plant

Subject: Cascade Control
Date: 03/11/2001 2:20 PM

M.,

Cascade control is often used with levels on distillation columns and in reflux drums (condensate receivers). Figure 1 shows bottoms level control with the bottoms level directly controlling the bottoms product rate. Figure 2 shows bottoms level control with the bottoms level resetting a flow controller on the bottoms rate. Connected loops with one loop resetting the set point of a second loop are usually referred to as cascaded loops or cascade control. The outer loop is the primary loop or master loop. The inner loop is the secondary loop or slave loop (Figure 3).

Figure 1
Direct bottoms level control on a fractionator

Figure 2
Cascade bottoms level control on a fractionator

Figure 3
Cascade loop terminology

Cascade control has the potential advantages of:

  1. Catching an upset quicker. If the secondary (inner) loop has a response that is five times faster (or more) than the master (outer) loop then upsets measured by the inner loop can be caught and corrected before they affect the primary loop.
  2. Improved control linearization.

Actual performance of cascade control systems is often barely better than the non-cascade direct control. Usually this is a sign that the overall control scheme is ineffective or that the loops have been tuned incorrectly. Cascade control is typically used with a column level, temperature, composition, or pressure as the master loop to a flow loop. Cascade control is rarely used for liquid pressure systems.

If the standard column level, temperature, composition, or pressure control loops do not show an improvement with cascade control you should review your overall control scheme, instrument performance, and system tuning. Many cascade control systems work best with some feed-forward elements added to the system.

Andrew Sloley
DGI

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This page updated 15 March 2001.
© 2001 Andrew W. Sloley. All rights reserved.