x, mole fraction benzene in liquid
Figure 2.10 (Continued)
is (0.47, 0.69) in Fig. 2.10a. These points change when the component balance lines do.
A column is said to be "pinching" when the component balance line is too close to the equilibrium curve. Physically, this represents a situation where several stages are doing very little separation and are practically wasted.
Pinching in a column design does not only waste stages, thereby leading to an oversized column, but is also risky. Even minor inaccuracies in relative volatilities and enthalpies may bring the component balance line and the equilibrium curve closer, or even to cross, earlier than anticipated. This exponentially escalates the stage requirement. If the additional stages required are unavailable, the column will not accomplish the desired separation. It is therefore imperative to design a column away from the pinched region.
In an existing column, mechanical reasons often make it difficult to change feed location. A pinch can then be remedied by increasing reflux and reboil. This draws the component balance line and the equilibrium curve further apart, enlarging the steps, and thereby per
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