The step-by-step procedure implied by the equations written in the previous section is laborious to solve, often involving a trial and error solution.

The solution becomes far simpler if we follow a suggestion of Lewis of assuming constant molal overflow (CMO).

**Constant Molal Overflow (CMO)** is the assumption that all
liquid (L) and vapor (V) flows (on a molar basis) in the rectifying
section are constant and that all liquid (L') and vapor (V')
flows (on a molar basis) in the stripping section are constant (but
*not* the same as those in the rectifying section).

Symbolically this means we take the following relations to be true:

L_{1}=L_{2}=...=L_{j}=L

V_{1}=V_{2}=...=V_{j}=V

L'_{1}=L'_{2}=...=L'_{f}=L'

V'_{1}=V'_{2}=...=V'_{f}=V'

The conditions under which these assumptions hold true are as follows:

- The column (except the reboiler and condenser) is adiabatic
- Enthalpic contributions due to temperature changes are negligible relative to those due to phase change
- The heat of vaporization per mole is not concentration dependent (i.e., the two components have similar heats of vaporization)

It is possible that constant **mass** overflow is valid
(instead of molal), if the third condition holds true on a mass
basis, rather than a molar basis (which is sometimes true for
hydrocarbon mixtures).

Explain what constant molal overflow (CMO) is, and determine if it is valid in a given situation