DISTILLATION PRINCIPLES
>Separation of
components from a liquid mixture via distillation depends on the differences in
boiling points of the individual components. Also, depending on the
concentrations of the components present, the liquid mixture will have different
boiling point characteristics. Therefore, distillation processes depends on the
vapour pressure characteristics of liquid mixtures.
Vapour Pressure and Boiling
>The vapour pressure
of a liquid at a particular temperature is the equilibrium
pressure exerted by molecules leaving and entering the liquid surface.
Here are some important points regarding vapour pressure:
>
>
energy input raises vapour pressure
>
>vapour pressure is
related to boiling
>
>a liquid is said to
�boil� when its vapour pressure equals the surrounding pressure
>
>the ease with which
a liquid boils depends on its volatility
>
>liquids with high
vapour pressures (volatile liquids) will boil at lower temperatures
>
>the vapour pressure
and hence the boiling point of a liquid mixture depends on the relative amounts
of the components in the mixture
>
>distillation occurs
because of the differences in the volatility of the components in the liquid
mixture
The Boiling Point Diagram
>The
boiling point diagram shows how the equilibrium
compositions of the components in a liquid mixture vary with temperature at a
fixed pressure. Consider an example of a liquid mixture containing 2 components
(A and B) - a binary mixture. This has the
following boiling point diagram.
>
 The
boiling point of A is that at which the mole fraction of A is 1.
The boiling point of B is that at which the mole fraction of A
is 0. In this example, A is the more volatile component and
therefore has a lower boiling point than B. The upper curve in
the diagram is called the dew-point curve
while the lower one is called the
bubble-point curve.
>The dew-point
is the temperature at which the saturated vapour starts to condense.
>The
bubble-point is the temperature at which the liquid starts to boil.
>The region above
the dew-point curve shows the equilibrium composition of the
superheated >
vapour while the region below the bubble-point curve shows the equilibrium
composition of the
subcooled>
liquid.
>For example, when
a subcooled liquid with mole fraction of A=0.4 (point A) is heated, its
concentration remains constant until it reaches the bubble-point (point B), when
it starts to boil. The vapours evolved during the boiling has the equilibrium
composition given by point C, approximately 0.8 mole fraction A. This is
approximately 50% richer in A than the original liquid.
>This
difference between liquid and vapour compositions is the basis for distillation
operations.
Relative Volatility
>
Relative volatility is a measure of the differences
in volatility between 2 components, and hence their boiling points. It indicates
how easy or difficult a particular separation will be. The relative volatility
of component �i� with respect to component �j� is defined as
>
>yi
= mole fraction of component �i� in the vapour
>xi
= mole fraction of component �i� in the liquid
>Thus if the
relative volatility between 2 components is very close to one, it is an
indication that they have very similar vapour pressure characteristics. This
means that they have very similar boiling points and therefore, it will be
difficult to separate the two components via distillation.
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