= Fractional distillation
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<Fractional distillation> brings rising <vapor> into contact with descending <liquid> in a <fractionating column>. Packing supplies contact area; it does not act as a sieve for individual <molecules>. Repeated exchange can enrich the rising <vapor> in the more volatile component. Some condensed <liquid> returns downward as <reflux>, rather than all being withdrawn as distillate.<Distillation by Hong Kong University of Science and Technology>[Introduction, paragraphs on reflux and plates, trays or packing]{ref}
An <equilibrium stage> is an idealization of this contact. A real <fractionating column> has finite contact time and imperfect wetting and mixing; its length is not automatically a count of <equilibrium stages>. Product withdrawal and <reflux> affect the separation together. Thus the <repeated ideal-stage enrichment> example illustrates the composition advantage of repeated equilibration, but supplies neither a column design nor a specified <reflux> requirement.<Hyper-TVT by ETH Zurich/Distillation efficiency>[Hyper-TVT by ETH Zurich; §5.6 Efficiency, equilibrium versus actual plates and flow-dependent efficiency]{ref}
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