Column Variables and Their Pairing

Controlled variables include product compositions (x,y), column temperatures, column pressure, and the levels in the tower and accumulator. Manipulated variables include reflux flow (L), coolant flow (QT), heating medium flow (Qb or V), and product flows (D,B) and the ratios L/D or V/B. Load and disturbance variables include feed flow rate (F), feed composition (2), steam header pressure, feed enthalpy, environmental conditions (e.g., rain, barometric pressure, and ambient temperature), and coolant temperature. These five single loops can theoretically be configured in 120 different combinations, and selecting the right one is a prerequisite to stability and efficiency. 

Column pressure almost always is controlled by heat removal (QT). This loop closes the heat balance around the column, while the levels are controlled to close its material balance. Therefore, the key task is the assignment of the manipulated variables to the composition controllers. No matter how we make that selection, these two loops will interact (a change in one will upset the other). This is because whenever they change the openings of their control valves, the material and heat balance of the column will also change. 

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