(6d) The Forgotten Science of Distillation Control

Distillation is the heart of all refining and petrochemical plants, as it determines the ability of a plant to

generate on‐specification product(s). In spite of the vast body of fundamental research and practical

design experience within the industry, the design and tuning of the regulatory controls, which plays a

pivotal role in achieving on‐specification production, became and remains largely a lost and dying art.

This is evidenced by the author’s experience across multiple sites and companies wherein similar, if not

identical, controls design and control‐ability issues were discovered and rectified on and around

distillation units, as well as by the precipitous drop in attendance of control‐related sessions at industrial

and academic conferences.

Historically, there was a great amount of attention given to the design and performance of the

regulatory and advanced regulatory controls (ARC) implemented on distillation units. As more advanced

technologies came into common use, less attention was given to the underlying regulatory control

structures, in spite of the fact that it provides the foundation upon which the new technologies rely. IN

parallel to the shift in technology usage, the past decades have seen fundamental changes in the modus

operandi of refineries: the design and operations resemble that of chemical plants. This was primarily

the result of end‐user quality programs that propagated more stringent product specifications

“upstream” in the supply chain. Despite the changes to the operational criteria these tighter

specifications created, refinery designers have effectively maintained the historical “status quo”,

especially regarding controls design.

With these technological and historical aspects in mind, this paper presents fundamental and practical

guidelines for designing robust and reliable control structures on these vital units. The authors draw

from 30 years of experience in process and controls design, and tuning such controls, on multiple units

across a wide array of refining, petrochemical and LPG/LNG plants. In addition to the guidelines, a

sampling of real‐world situations and the solutions for same will be presented to illustrate how the

principles are practically implemented. Topics included in the presentation will include:

• Only Two “Handles” for Every Distillation Tower
• Accumulator Level Control Myths
• Relative Gain Analysis Made Easy
• “Closing the Loop” in Controls Design
• When Tray Temperatures act like Levels
• Flooded Condenser Design: Keep it Simple!
• APC: Love it, Hate it, Need it?