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