(195c) Main Fractionator Control - Regulatory Control

This paper reviews the fundamentals of regulatory control on main fractionators. Product composition control is the primary regulatory target. Most operating units use some form of inferred composition control based on measuring column temperatures. Temperature measurement has the following benefits compared to direct composition measurement:

• Inexpensive measuring devices.
• Rapid response.
• Good accuracy and high precision.
• Inherent link between temperature measurement and ASTM distillation specifications.

However, many units have not moved the temperature control into closed-loop operating mode. Why is this? This paper reviews the following elements of using inferred temperature monitoring in primary fractionators:

• Sensitivity to pressure changes
• Temperature measurement location
• Sensitivity to feed composition changes
  • Process gain versus specification changes