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
Control variables for distillation column
Control variables for distillation column considered as disturbances. Heat input qR and external reflux L0 can be adjusted to achieve the desired control objectives. These are called manipulated variables. Distillate and bottom product rates can also be manipulated, so there are four variables that can be adjusted: L0, qR, D, and B. They are not independent, however. If, for example, D is controlled, then B is dependent.
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