Stage operations enthalpy balances

L = constant, and V = y , = V = constant. In this case it is possible to dispense with one equation for each stage, namely the enthalpy balance. This is called the constant molal overflow assumption (which was also embodied in (8-5)) and is valid if, over the temperature and pressure operating range of the separator ... 

EXAMPLE 11.6-2. Distillation Using Enthalpy-Concentration Method A liquid mixture of benzene-toluene is being distilled using the same conditions as in Example 11.4-1 except that a reflux ratio of 1.5 times the minimum reflux ratio is to be used. The value o R = 1.17 from Example 11.4-2 will be used. Use enthalpy balances to calculate the flow rates of the liquid and vapor at various points in the tower and plot the curved operating lines. Determine the number of theoretical stages needed. 

To this point it has been assiuned that all quantities vary from tray to tray and that consequently 3(N + 2) balances will be required to describe the operation. Here N denotes the number of stages, with an additional 3x2 equation needed to balance the flow of mass and heat about the condenser and reboiler. These have to be further supplemented by expressions relating each enthalpy to the key state variables x, y, and T. Evidently, we are dealing with a model of considerable complexity and dimensionality, which would require a numerical solution. 

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