Reflux, enthalpy manipulation

The prior discussion assumes that the feed rate, feed composition, and heat content (enthalpy) are fixed. My purpose in presenting this review of the phase rule is to encourage the routine manipulation of tower operating pressures, in the same sense, and with the same objectives, as adjusting reflux rates. Operators who arbitrarily runs a column, at a fixed tower pressure, discards one-third of the flexibility available to them, to operate the column in the most efficient fashion. And this is true, regardless of whether the objective is to save energy or improve the product split. 

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. 

Consider Figure 19.2 where top-product flow is set by flow control, reflux flow is set by condensate receiver level control, boilup is fixed by flow control of steam or other heating medium, and bottom-product flow is determined by column-base level control. As shown by the dotted line, we wish eventually to control column top composition by manipulating distillate flow. Let us assume that feed rate, feed composition, feed enthalpy, and boilup are fixed and that we wish to find the changes (i.e., gains ) of top and bottom compositions in response to a change in D, the top-product rate. 

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See also in sourсe #XX -- [ , ]

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