Controlled Variables and Manipulated Streams

A column control system has three main objectives  

Single-loop variables These include pressure and levels. They are controlled in order to achieve the first objective, i.e., setting stable conditions for column operation. The set points at which these are controlled are established by stability considerations alone, regardless of product specifications. 

Controlling pressures and levels regulates material accumulation in the column. Keeping the levels constant prevents liquid accumulation, while keeping the pressure constant prevents vapor accumulation. Unless accumulation (positive or negative) is prevented, a continuous system will not operate at steady state and will not be stable. 

Unit objective variables These include top and bottom compositions. They are regulated to achieve the second objective, i.e., meet- 

Hgure 16.1 Controlled variables and manipulated streams in a typical colunm. 

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