Simple Distillation and Fractional Distillation
What is an Azerotrope?
. A mixture of liquids of a certain definite composition that distills at a constant tempera- ture without a change in composition
What is a theoretical plate?
Each successive condensation-vaporization cycle,
What is fractional distillation?
Fractional distillation differs from simple distillation in that a fractionating column is placed between the distilling flask and the condenser.
What does the copper sponge do?
It allows multiple condensation and vaporization to take place as it ascends in the column
Is equilibrium attained instantaneously?
No so It should be an adiabatic process; that is, heat should be transferred from the ascending vapor to the descending liquid with no gain of heat or net heat loss to the surroundings.
Does constancy of a boiling point mean that the liquid consists of only one compound?
No,two miscible liquids of similar chemical structure that boil at the same temperature individually will have nearly the same boiling point as a mixture. And, as noted previously, azeotropes have con- stant boiling points that can be either above or below the boiling points of the indi- vidual components.
What you dont want a lot (dense) of copper steal sponge inside the condenser?
Pressure changes take place within the column to cause non- equilibrium conditions. Also, if the column packing has a very large surface area, it will absorb (hold up) much of the material being distilled.
What does a theoretical plate do?
Produces a vapor that is richer in the more volatile fraction. As the temperature of the liquid mixture is increased, the lower-boiling fractions become enriched in the vapor.
What does simple distillation involves?
Simple distillation involves boiling a liquid in a vessel (a distilling flask) and direct- ing the resulting vapors through a condenser, in which the vapors are cooled and converted to a liquid that flows down into a collection vessel (a receiving flask).
When to use Simple Distillation?
Simple distillation is used to purify liquid mixtures by separating one liquid component either from nonvolatile substances or from another liquid that differs in boiling point by at least 75°C.
When a series of simple distillations take place within a fractionating column, it is important...
That complete equilibrium be attained between the ascending vapors and the descending liquid.
What is the column packed with?
The column is packed with material- copper sponge- that provides a large surface area for heat exchange between the ascending vapor and the descending liquid.
The number of theoretical plates is proportional to
The height of the column, so various packings are evaluated according to the height equivalent to a theoretical plate (HETP)
The smaller the Height equivalent to a theoretical plate...
The more plates the column will have and the more efficient it will be
What is the most important variable that contributes to good fractional distillation
The rate at which the distillation is carried out.
If two different liquid compounds are mixed
The vapor above the mixture will con- tain some molecules of each component
What do you need a fractionating column?
This fractionating column allows for successive condensations and distillations and produces a much better separation between liquids with boiling points closer than 75°C
Why can you use Simple distillation when the BP are too close?
Because in simple distillation, the initial condensate will have essentially the same mole ratio of liquids as the vapor just above the boiling liquid. So the closer the boiling points of the components of a liquid mixture, the more difficult they are to completely separate by simple distillation.
What is the process behind the multiple condensation and vaporizations?
Condensing of the higher-boiling vapor releases heat, which causes vaporization of the lower-boiling liquid on the packing so that the lower-boiling component moves up while the higher-boiling component moves down.
