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Variations in internal vapor and liquid flows inside a distillation
column depend on the enthalpies of the mixtures. Equimolal overflow
assumes the rates are independent of the energy
balance. To remove the assumption we must consider the enthalpy balances
The overall enthalpy balance on a distillation column with NT trays is:
where the Q terms are the condenser and reboiler loads. This equation reduces
at steady-state to:
For a given feed and products, only one of QC and QR may
be independently set. Normally, QC is chosen to get the desired
operating pressure, product and reflux rates, and QR found by
balance. In operation, however, the reboiler duty is usually varied to control
an inventory or product composition.