<Li> are more viscous </Li> <Li> are more difficult to ignite and to burn </Li> <P> Large - scale industrial towers use reflux to achieve a more complete separation of products . Reflux refers to the portion of the condensed overhead liquid product from a distillation or fractionation tower that is returned to the upper part of the tower as shown in the schematic diagram of a typical, large - scale industrial distillation tower . Inside the tower, the reflux liquid flowing downwards provides the cooling needed to condense the vapors flowing upwards, thereby increasing the effectiveness of the distillation tower . The more reflux is provided for a given number of theoretical plates, the better the tower's separation of lower boiling materials from higher boiling materials . Alternatively, the more reflux provided for a given desired separation, the fewer theoretical plates are required . </P> <P> Fractional distillation is also used in air separation, producing liquid oxygen, liquid nitrogen, and highly concentrated argon . Distillation of chlorosilanes also enable the production of high - purity silicon for use as a semiconductor . </P>

Is crude petroleum a solution or mechanical mixture