Question

Which method is better for purifying a mixture of two liquid that boil at nearly the same temp: simple distillation or fractional distillation? Explain your answer. (Explain the term "theoretical plates" and how they apply in this situation.

Answer 1

The choice of whether to use fractional distillation or simple distillation depends on the two liquids being separated. Typically, using simple distillation is preferrable because the apparatus is, well, simpler, and a simple distillation typically goes faster than a fractional distillation (and requires less energy). On the other hand, fractional distillation gives better separation between the liquids. The choice of whether to use simple or fractional distillation, then, depends usually on the difference in boiling temperatures between the two liquids. If there is a large difference in the boiling points (>70^oC)between the two liquids then simple distillation is probably the best option. On the other hand, if there is only a small temperature difference between the two liquids a fractional distillation is the preferrable option.So,here we use fractional distillation.

A theoretical plate in many separation processes is a hypothetical zone or stage in which two phases, such as the liquid and vapor phases of a substance, establish an equilibrium with each other. Such equilibrium stages may also be referred to as an equilibrium stage, ideal stage, or a theoretical tray.

If a simple distillation column is called a “plate”, then the efficiency of a fractional distillation can be represented by the number of theoretical plates-it’s equivalent to- the more plates, the better the distillation. But distillation setups can vary in size (or height). Which do you think is more efficient- a distillation setup that fits on your lab bench that has an efficiency of 10 theoretical plates, or another the size of Mendel Hall that’s also 10 theoretical plates? The smaller one is of course. This is expressed in the height equivalent to one theoretical plate (HETP). The lower the HETP the more efficient the distillation. HETP is measured in distance per plate (example: 1cm/plate).