The Net Flow A

A mass balance around the condenser above the rectifying CS shows that 

FIGURE 3.9 (a) A conventional rectifying section, and (b) a conventional stripping section. 

Unsurprisingly, the net flow in the stripping CS, As, is always negative, indicating overall net movement of material down the CS. Stripping CSs are thus identified as CSs where the bulk material flows downward. 

Notice that in both the rectifying and stripping CSs, the magnitude of the net flow is equivalent to the product flowrate. This is a property of CSs that are terminated by condensers or reboilers. The sign of the net flow thus gives an indication in which direction the bulk of the material is flowing in a CS, either upward with the vapor or downward with the liquid stream. 

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Equation 3.11 shows that since the distillate flowrate, D, is always positive, the net flow, A, is too. Rectifying sections are thus, by definition, characterized by a positive (upward) net flow, as indicated in Figure 3.9a. 

The net flow (A) and net composition (X ) are constant throughout a MCS. This can be easily deduced by overall and component mass balances (refer to Figure 9.2) ... 

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