hmmmmm, chemical engineering
Distillation is a liquid-liquid separation technique based on
differences in boiling points. Separation of a mixture
of two components is called binary distillation, three components
ternary distillation, and so on.
Several other fluid separation techniques exist, such as
absorption, adsorption, and liquid-liquid extraction. However,
distillation is by far the most common separation process. The
importance of distillation as a unit operation in the chemical
process industry (CPI) is enormous. For instance, consider the
petrochemical industry: crude oil is catalytically
cracked into lower molecular weight organic chemicals that in turn are
distilled to form pure reactants for all sorts of products
(gasoline, solvents, plastics. Industrial gases are generally obtained
by cryogenic distillation; a low temperature distillation technique in
which the process gas is cooled down below the boiling point of its
components. Air is separated in this way to form oxygen, nitrogen,
and several inert gases (many billion pounds per year
production).
A typical schematic for a (continuous) distillation column is given in
the figure. The feed enters the column at the feed tray on a chosen
height in the column. The liquid trickles down past several separation
stages, where it comes into contact with its vapor. The separation
stages consist of contacting plates or bubble caps. Contact between liquid and vapor is essential, since
vapor-liquid equilibrium (VLE) is the fundamental principle for
separation. The liquid stream flows down through the bottom section of
the column (called the stripping section). The product with the lowest
boiling point is collected as bottoms, but part of the liquid
is vaporized in a condenser, and re-enters the column. The condenser
supplies the energy input to the system, required for separation.
The vapor follows its way through the top section, called the
Rectifying section, and enters a reflux drum. In the drum, the vapor
is cooled down to liquid. Part of the product is returned to the column
(called the reflux), and the remainder of the (high boiling point)
product leaves the column as distillate.
__________
__|__ _|_
| |___|~~~| Reflux drum
| | |___|
Rectifying |_____| |__________
Section |_____| Distillate
|_____|
|_____|
______|_____|
Feed |_____|
|_____|
|_____|
Stripping |_____|
Section | |_______
| | _|_
|_____| |~~~| Condenser
| |___|
|_________|__________
Bottoms
Batch distillation, such as it is often applied used for small
scale operations, sensitive or expensive products, or noncontinuous
feeds generally works on the same principle. In this case there is no
feed stream, but a large heated bottom flask. Separation is usually
carried out with a high surface area column packing, and distillate is
collected in a similar way to continuous distillation. The bottoms
product remains inside the bottom flask until bottoms or distillate meet
the required product purity.
Distillation is based on the chemical equilibrium stage concept.
The liquid to be separated is in contact with its corresponding vapor at
any location inside the distillation column. At each point, the vapor
stream and liquid stream are in equilibrium (VLE), i.e. the
pressure, temperature and chemical potential of the liquid
and vapor phase are equal. The equilibrium conditions are governed by
the Gibbs Phase Rule.
Consider a mixture of ethanol (EtOH, alcohol) and water (w) at an
equilibrium stage inside the distillation column. A vapor stream (V)
flows upward, contacting a downward flowing liquid stream (L). The
concentrations of the liquid phase are given by xw and
xEtOH. The concentrations of the vapor phase are given by
yw and yEtOH. At the given operating conditions
(e.g. T= 85.3 C, P= 1 atm.), the mole fractions
(concentrations) of the liquid phase are: xEtOH= 0.1238
(12.38%), xw= 1 - 0.1238 = 0.8762 (87.62%). The corresponding
vapor phase compositions in equilibrium with the liquid phase
are: yEtOH = 0.4704, yw = 0.5296 (Note that the mole fractions in each phase add up to 1, or 100%).
yEtOH=0.4704
L yW=0.5296
↓ ↑
___________________ Equilibrium Stage
↓ ↑
xEtOH=0.1238 V
xW=0.8762
As we can see, at
this equilibrium stage, the ethanol concentration in the vapor phase is
significantly higher than in the liquid phase. We have enriched
the vapor stream with ethanol. The vapor stream flows up and
interacts with more liquid at a higher equilibrium stage. The vapor
stream leaving this stage will have an even higher ethanol
concentration. The purity of the product (bottoms and distillate) can be
improved by adding sufficient equilibrium stages for separation.
It is important to note that it is impossible to obtain a 100% purity
of products. As the purity of the product goes up, the efficiency of the separation
becomes less, em>e.g. it is more difficult to increase a
concentration from 90% to 95% than it is from 50% to 55%. For 100%
product purity, an infinite number of equilibrium stages is required.
Another complication is the interaction of the liquid components called
an azeotrope. For instance, an azeotrope is the reason why a binary
mixture of ethanol and water normally can't be separated beyond 89%
alcohol content.