# Constant Molal Overflow (CMO)

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).

##### DEFINITION:

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:

L1=L2=...=Lj=L

V1=V2=...=Vj=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)
##### NOTE

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).

##### OUTCOMES:

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