Nomenclature and conventions for typical distillation column rates D and B mois/min and compositions xD and xB mol fraction light component.

Heat is transferred into the process in the "reboiler" (typically a tube-and-shell heat exchanger) to vaporize some of the liquid from the base of the column. The heat-transfer rate is qR energy units/time (e.g., Btu/hr, pcu/hr, joules/minute, etc.).

The vapor coming from the top of the column is liquified in another tube-and-shell heat exchanger called a condenser. Heat is transferred out of the condenser at a rate qc, pcu/hr.

Liquid from the condenser drops into the reflux drum. Distillate product is removed from this drum. In addition, some liquid, called "reflux" (Lc, mols/min), is fed back to the top tray of the column. This liquid reflux and the vapor boilup in the base of the column are necessary to achieve the separation or "fractionation" of chemical components. The energy required to make the separation is approximately the heat added to the reboiler.

Overall View from a Control Perspective

One can stand back and look at a distillation column with its associated reboiler, condenser, and reflux drum as a "black box" process. Feed, heat, and reflux are inputs into this box (see Figure 2.2). Outputs from the box are the two product streams D and B with compositions xD and xB. The usual situation with a distillation column is that the feed rate and feed composition must be

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