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Separation Process Engineering: Includes Mass Transfer Analysis, Third Edition by Phillip C. Wankat

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4.10 Limiting Operating Conditions

It is always useful to look at limiting conditions. For distillation, two limiting conditions are total reflux and minimum reflux. In total reflux, all of the overhead vapor is returned to the column as reflux, and all of the underflow liquid is returned as boilup. Thus distillate and bottoms flow rates are zero. At steady state the feed rate must also be zero. Total reflux is used for starting up columns, for keeping a column operating when another part of the plant is shut down, and for testing column efficiency.

The analysis of total reflux is simple. Since all of the vapor is refluxed, L = V and L/V = 1.0. Also, = and / = 1.0. Thus both operating lines become the y = x line. This is illustrated in Figure ...

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