Types of Distillation & column internals

Distillation
Index
1) What is distillation?
2) Comparison of Distillation with other Mass
transfer operation
3) Reflux ratio, Relative volatility
4) Problems in distillation
5)Types of Distillation
6) Packed tower and tray tower
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1) What is distillation?
• Distillation is a method of separating the components
of a solution which depends upon distribution of the
substances between a gas and liquid phase, applied
to cases where all components are present in both
phases.
• Distillation is a method of separating mixtures based
on differences in volatilities (volatility is the
tendency of a substance to vaporize. Volatility is
directly related to a substance's vapor
pressure.) of components in a boiling liquid mixture.
Distillation is aunit operation, or a physical separation
process, and not a chemical reaction.

Distillation column
Reboiler
Residue
Condenser
Distillate
Reflux
Feed

• In Distillation vapor rising in the section above
the feed (called the absorption, enriching, or
rectifying section) is washed with liquid to
remove or absorb the less volatile component.
• The liquid returned to the top of the tower is
called reflux and the material permanently
removed is the distillate.
• The section below the feed (stripping or
exhausting section) the liquid is stripped of
volatile component by vapor produced in
reboiler.

Difference between Distillation and
other MTO
Distillation
> foreign substance is not required.
> So there is no choice required for
selection of new substance.
> Give pure product, Further purification is
not required.

Other MTO
> foreign substance required.
> So there is choice required for selection
of new substance.
> Not give pure product, Further
purification is required.

Important parameters for
Distillation
• Relative volatility
• Reflux
• Reflux ratio

Relative volatility
• It is the ratio of concentration ratio of one
component to other in one phase to other phase.
• Relative volatility is usually denoted as α.
• It indicates the ease or difficulty of using
distillation to separate the
more volatile components from the less volatile
components in a mixture
• For separation,it should be greater than 1.

Reflux Ratio
• Reflux = Liquid return to column/ Distillate
• Minimum Reflux ratio ,Maximum Reflux
ratio , Optimum reflux ratio
• Increasing reflux ratio
– Improves separation
– Increases cycle time
– Increases energy consumption

Types of Distillation
• 1) Single stage operation (Flash
Vaporization )
• 2) Batch Distillation
• 3) Azeotropic Distillation
• 4) Extractive Distillation

Single stage operation- Flash
vaporization
• Here the liquid mixture is partially vaporized, the
vapor allowed to come to equilibrium with the
residual liquid and the resulting liquid and vapor
phase separated from the apparatus.
vapor
liquid
separator
Feed

Batch Distillation
• The oldest operation
used for separation of
liquid mixtures.
• Feed is fed from bottom,
where includes Re-
boiler, to be processed.
• Numbers of accumulator
tanks are connected to
collect the main and the
intermediate distillate
fractions.

Azeotropic Distillation
• What is Azeotrope ?
• It is constant Boiling mixture.
• Azeotropic distillation usually refers to the
specific technique of adding another component
to generate a new, lower-boiling azeotrope, such
as the example below with the addition of
benzene to water and ethanol.

Extractive Distillation
• The method of extractive distillation uses a
separation solvent, which is generally nonvolatile,
has a high boiling point and is miscible with the
mixture, but doesn't form an azeotropic mixture.
• Third component change the relative volatility of the
mixture.
• A classic example to be cited here is the separation
of an azeotropic mixture
of benzene and cyclohexane, where aniline is one
suitable solvent.

Problems in tray towers
• For high tray efficiency we require deep pools of liquid
and relatively high gas velocities. These conditions
however lead to a number of difficulties. One is the
mechanical entrainment in which small droplets of liquid
will be carried by the gas to the tray above.
• Great depth of liquid and high gas velocities both result
in high pressure drop for the gas in flowing through the
tray.
• In absorber and humidifiers high pressure drop result in
high fan power to blow the gas to the tower which result
in high operating cost.
• In distillation high pressure at the bottom of the tower
results in high boiling temperatures, which in turn may
lead to heating difficulties and possibly damaged to heat
sensitive compounds.

• Flooding
• Entrainment
• Priming
• Coning
• Weeping
• Dumping

Flooding
• As the pressure difference is increased due to
increased rate of flow of either gas or liquid the
level in the downspout will rise further to permit
the liquid to enter the lower tray. And finally the
liquid level may reach that on the tray above.
Further increased in any rate, liquid will fill the
entire space between the trays. The tower is
then flooded.
• Liquid may be forced out of the exit pipe of the
tower.

Priming
• For liquid gas combinations which tend to
foam excessively, high gas velocities lead
to condition of priming.
• Here the foam persists throughout the
space between trays and a great deal of
liquid is carried by the gas from one tray to
the tray above.

Coning
• If liquid rate are too low, the gas rising
through the openings of the tray may push
the liquid away and the contact of the gas
and the liquid is poor. That is called
coning.

Weeping
• If the gas rate is too low, much of the liquid
may rain down through the openings of the
tray. Its called weeping.
Dumping
And at very low gas rates, none of the
liquid reaches the downspouts. Its called
dumping.

Packed Column
In industry, a packed column is a type of
packed bed used to perform separation
processes, such as absorption, stripping,
and distillation. A packed column is
a pressure vessel that has a packed
section

Tray column

Tray Column v/s Packed
Column
• Tray tower
• Tray is used
• Cleaning is Easy. Dirty service (plate columns are
easier to clean)
• Pressure drop is high
• Side stream withdrawal is possible.
• Presence of thermal or mechanical stress due to large
temperature changes which might lead to cracked
packings

Packed Column
• Packing used in packed column
• Side stream withdrawal is not possible.
• Cleaning is Difficult.
• More choices in materials of construction for packings
especially in corrosive service (e.g. plastic, ceramic,
metal alloys)
• lower pressure drop (important in vacuum distillation)
• less liquid entrainment · low liquid hold-up, especially
suitable for thermally sensitive material

• Thank You

Types of Distillation & column internals

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