A conventional distillation column with a single feed stream and two product streams is shown in Fig. 13.2. The feed conditions are disturbance variables. Determine the control degrees of freedom NFC and identify potential MVs and CVs.

Question

A conventional distillation column with a single feed stream and two product streams is shown in Figure below. The feed conditions are disturbance variables. Determine the control degrees of freedom [latex]N_{FC}[/latex] and identify potential MVs and CVs.

Accepted Answer

For a typical distillation column, five input variables can be manipulated: product flow rates, B and D, reflux flow rate R, coolant flow rate [latex]q_{c}[/latex], and heating medium flow rate [latex]q_{h} [/latex]. Thus, according to the General Rule, [latex]N_{FC} [/latex] = 5. This result can also be obtained from Equations
[latex]N_{F}=N_{V}-N_{E}[/latex]
and
[latex]N_{F}=N_{FC}+N_{D}[/latex],
but considerable effort is required to develop the required dynamic model. Although five output variables could be selected as [latex]CVs, x_{D}, x_{B}, h_{B}, h_{D}, \space and \space P[/latex], for many distillation control problems, it is not necessary to control all five. Also, if it not feasible to measure the product compositions on-line, tray temperatures near the top and bottom of the column are often controlled instead, as discussed in the next section.

Question 13.2: A conventional distillation column with a single feed stream...

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