For many cases, the boiling points of the components in the mixture will be sufficiently close that Raoult's law must be taken into consideration. Therefore, fractional distillation must be used in order to separate the components well by repeated vaporization-condensation cycles within a packed fractionating column.

    As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component.　In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description.

    More theoretical plates lead to better separations. A spinning band distillation system uses a spinning band of Teflon or metal to force the rising vapors into close contact with the descending condensate, increasing the number of theoretical plates.

    許多情況下，混合物中各組分的沸點(diǎn)十分接近，以致于必須將拉烏爾定律考慮在內(nèi)。因此，為了很好地分離各組分，必須通過在一個(gè)填充分餾柱內(nèi)不斷重復(fù)汽化－冷凝周期的方法進(jìn)行分餾。

    當(dāng)要被提純的液體受熱時(shí)，它的蒸汽上升到分餾柱。蒸汽上升的同時(shí)，冷卻并凝結(jié)在冷凝器壁和填充物的表面。此時(shí)，上升的熱蒸氣使冷凝物繼續(xù)受熱，并再一次汽化。然而，新生成的蒸汽的成分再一次取決于拉烏爾定律。每一個(gè)汽化冷凝周（稱為一個(gè)理論塔板）都會產(chǎn)生一種易揮發(fā)組分含量更高的液體。實(shí)際上，一定溫度下的每一周期并不是在分餾柱內(nèi)的同一位置進(jìn)行的，因而，理論塔板只是一個(gè)概念而不是精準(zhǔn)描述。

    塔板越多分離越好。旋轉(zhuǎn)帶蒸餾系統(tǒng)使用一條轉(zhuǎn)動的特氟綸帶或金屬帶迫使上升的蒸汽與下降的冷凝物緊密接觸，而提高理論塔板數(shù)。