Not sure how to answer clearly but here goes The concept of there being a need for a minimum difference in boiling points is erroneous and probably comes from rules of thumb for separating two pure phases in a single distillation using a simple apparatus That is if you can boil off 100 of phase A without boiling off condensing phase B Think back to WWII in Norway when heavy water was separated from normal water by multiple distillation The boiling point difference is only 1 4 degrees yet heavy water was successfully produced