Distillation

Distillation

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by Yanty
 an introduction
• Distillation is defined as: a process
in which a liquid or vapour mixture
of two or more substances is
separated into its component
fractions of desired purity, by the
application and removal of heat.

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 • Distillation is based on
the fact that the vapour
of a boiling mixture will
be richer in the
components that have
lower boiling points.
• Therefore, when this
vapour is cooled and
condensed, the
condensate will contain
more volatile
components. At the
same time, the original
mixture will contain
more of the less volatile
material.
• Distillation columns are
designed to achieve this
separation efficiently
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 • distillation is the
• Although many most common
people have a separation
fair idea what technique
“distillation” • it consumes
means, the enormous amounts
important of energy, both in
aspects that terms of cooling
seem to be and heating
missed from the requirements
manufacturing • it can contribute to
point of view are more than 50% of
that : plant operating
costs

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the terminology used in distillation practice
and to give a very basic introduction to:
 types of columns
 basic distillation equipment and operation
 column internals
 reboilers
 distillation principles vapour liquid equilibria
 distillation column design and

the factors that affect distillation column o
peration
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 types of columns
• There are many types of distillation
columns, each designed to perform
specific types of separations, and each
design differs in terms of complexity.
• One way of classifying distillation
column type is to look at how they are
operated. Thus we have:
o batch and
o continuous columns.

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• Batch Columns • Continuous Columns
In batch operation, the In contrast, continuous
feed to the column is columns process a
introduced batch-wise. continuous feed stream.
That is, the column is No interruptions occur
charged with a 'batch' unless there is a problem
and then the distillation with the column or
process is carried out. surrounding process units.
When the desired task They are capable of
is achieved, a next handling high throughputs
batch of feed is and are the most common
introduced. of the two types. We shall
concentrate only on this
class of columns.

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 Simple Distillation
• The process of vaporizing a liquid mixture in
one vessel and condensing the vapors into
another vessel is called distillation.
• The liquid being distilled is heated in a flask,
which is sometimes called a distillation flask
or distillation pot.
• The vapors are condensed on a cool surface,
usually a water-cooled condenser.
• The resulting liquid is called the distillate and
is collected in a receiving flask.

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• A vaporization-condensation cycle is when a
substance vaporizes and condenses one time.
• A simple distillation involves only one vaporization-
condensation cycle.
• Purification occurs as the volatile liquid vaporizes
and leaves behind the nonvolatile (or less volatile)
impurity.
• Purification by simple distillation is appropriate for
liquids that contain either nonvolatile impurities, or
very high boiling point impurities.
• When a liquid contains an impurity with a boiling
point close to its own (~25°C), then fractional
distillation, which involves multiple vaporization-
condensation cycles, would be required to separate
the two substances.

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 Types of Continuous
Columns
• Continuous columns can be further
classified according to:
– the nature of the feed that they are
processing
• binary column - feed contains only two
components
• multi-component column - feed contains more
than two components
– the number of product streams they
have
• multi-product column - column has more than
two product streams
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– where the extra feed exits when it is
used to help with the separation,
• extractive distillation - where the extra
feed appears in the bottom product stream
• azeotropic distillation - where the extra
feed appears at the top product stream
– the type of column internals
• tray column - where trays of various designs
are used to hold up the liquid to provide
better contact between vapour and liquid,
hence better separation
• packed column - where instead of trays,
'packings' are used to enhance contact
between vapour and liquid

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 BASIC DISTILLATION
EQUIPMENT AND
OPERATION
• Main Components of Distillation
Columns
– Distillation columns are made up of several
components, each of which is used either to
tranfer heat energy or enhance materail
transfer. A typical distillation contains
several major components:
• a vertical shell where the separation of liquid
components is carried out
• column internals such as trays/plates and/or
packings which are used to enhance component
separations
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 • a reboiler to provide the necessary vaporisation
for the distillation process
• a condenser to cool and condense the vapour
leaving the top of the column
• a reflux drum to hold the condensed vapour from
the top of the column so that liquid (reflux) can
be recycled back to the column

The vertical shell houses the column

internals and together with the condenser
and reboiler, constitute a distillation column.

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A schematic of a typical distillation unit with a
single feed and two product streams is shown
below:

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 Basic Operation and
Terminology
• The liquid mixture • The feed tray
divides the column
that is to be
into a top (enriching
processed is known
or rectification)
as the feed and this
section
is introduced usually
• The feed flows down
somewhere near the
the column where it
middle of the
is collected at the
column to a tray
bottom in the
known as the feed reboiler
tray.

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 • Heat is supplied to the
reboiler to generate vapour.
• The source of heat input can
be any suitable fluid,
although in most chemical
plants this is normally steam.
• In refineries, the heating
source may be the output
streams of other columns.
• The vapour raised in the
reboiler is re-introduced
into the unit at the bottom
of the column.
• The liquid removed from the
reboiler is known as the
bottoms product or simply,
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• The vapour moves up the
column, and as it exits the
top of the unit, it is
cooled by a condenser.
• The condensed liquid is
stored in a holding vessel
known as the reflux
drum.
• Some of this liquid is
recycled back to the top
of the column and this is • Thus, there are internal
called the reflux. flows of vapour and liquid
• The condensed liquid that within the column as well
is removed from the as external flows of
system is known as the feeds and product
distillate or top product. streams, into and out of
the column
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 COLUMN INTERNALS
• Trays and Plates
– The terms "trays" and "plates" are used
interchangeably.
– There are many types of tray designs, but
the most common ones are :

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 Bubble cap trays
• A bubble cap tray has riser
or chimney fitted over each
hole, and a cap that covers
the riser.
• The cap is mounted so that
there is a space between
riser and cap to allow the
passage of vapour.
• Vapour rises through the
chimney and is directed
downward by the cap, finally
discharging through slots in
the cap, and finally bubbling
through the liquid on the
tray.
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 Valve trays
• In valve trays, perforations
are covered by liftable caps.
• Vapour flows lifts the caps,
thus self creating a flow area
for the passage of vapour.
• The lifting cap directs the
vapour to flow horizontally
into the liquid, thus providing
better mixing than is possible
in sieve trays.

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 Sieve trays
• Sieve trays are
simply metal plates
with holes in them.
Vapour passes
straight upward
through the liquid on
the plate. The
arrangement, number
and size of the holes
are design
parameters.
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• Because of their efficiency, wide
operating range, ease of maintenance
and cost factors, sieve and valve
trays have replaced the once highly
thought of bubble cap trays in many
applications.

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Liquid and Vapour Flows in
a Tray Column
• The next few
figures show the
direction of vapour
and liquid flow
across a tray, and
across a column.

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 • Each tray has 2
conduits, one on each
side, called
‘downcomers’. Liquid falls
through the downcomers
by gravity from one tray
to the one below it. The
flow across each plate is
shown in the above
diagram on the right.

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• A weir on the tray
ensures that there
is always some
liquid (holdup) on
the tray and is
designed such that
the the holdup is at
a suitable height,
e.g. such that the
bubble caps are
covered by liquid.

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 • The picture on the left
is a photograph of a
section of a pilot scale
column equiped with
bubble capped trays.
The tops of the 4
bubble caps on the tray
can just be seen. The
down- comer in this case
is a pipe, and is shown on
the right. The frothing
of the liquid on the
active tray area is due
to both passage of
vapour from the tray
below as well as boiling.
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• As the hotter vapour passes through the liquid on
the tray above, it transfers heat to the liquid. In
doing so, some of the vapour condenses adding to the
liquid on the tray.
• The condensate, however, is richer in the less
volatile components than is in the vapour.
Additionally, because of the heat input from the
vapour, the liquid on the tray boils, generating more
vapour.
• This vapour, which moves up to the next tray in the
column, is richer in the more volatile components.
• This continuous contacting between vapour and liquid
occurs on each tray in the column and brings about
the separation between low boiling point components
and those with higher boiling points.

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 Tray Designs
• A tray essentially acts as a mini-column, each
accomplishing a fraction of the separation task.
• From this we can deduce that the more trays there
are, the better the degree of separation and that
overall separation efficiency will depend significantly
on the design of the tray.
• Trays are designed to maximise vapour-liquid contact
by considering the liquid distribution and vapour
distribution on the tray. This is because better
vapour-liquid contact means better separation at each
tray, translating to better column performance.
• Less trays will be required to achieve the same
degree of separation.
• Attendant benefits include less energy usage and
lower construction costs.

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• Liquid distributors - Gravity (left),

• Spray (right)

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 Packings
• There is a clear trend to improve
separations by supplementing the use
of trays by additions of packings.
• Packings are passive devices that are
designed to increase the interfacial
area for vapour-liquid contact.
• The following pictures show 3
different types of packings.

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• These strangely shaped pieces are
supposed to impart good vapour-liquid
contact when a particular type is placed
together in numbers, without causing
excessive pressure-drop across a packed
section.
• This is important because a high pressure
drop would mean that more energy is
required to drive the vapour up the
distillation column.

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• Structured packing (photo courtesy of Paul Phillips)

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 Packings versus Trays
• A tray column that is facing throughput problems may
be de-bottlenecked by replacing a section of trays
with packings. This is because:
 packings provide extra inter-facial area for liquid-vapour
contact
 efficiency of separation is increased for the same column
height
 packed columns are shorter than trayed columns

• Packed columns are called continuous-contact

columns while trayed columns are called staged-
contact columns because of the manner in which
vapour and liquid are contacted.

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 COLUMN REBOILERS
• There are a number of designs of reboilers. It
is beyond the scope of this set of
introductory notes to delve into their design
principles.
• However, they can be regarded as heat-
exchangers that are required to transfer
enough energy to bring the liquid at the
bottom of the column to boiling boint.
• The following are examples of typical reboiler
types.

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• Tube bundle
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 • Tubesheet

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• A novel development in reboiler design
is the self-cleaning shell-and-tube heat
exchangers by Klarex Technology for
applications where heat exchange
surfaces are prone to fouling by the
process fluid.
• Particles are introduced into the
process stream and these produce a
scouring action on the heat exchange
surfaces. An example is shown in the
diagram on the left

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 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.

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 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:

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• 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

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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.
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• The boiling point of A is • The dew-point is the
that at which the mole temperature at which
fraction of A is 1. the saturated vapour
• The boiling point of B is starts to condense.
that at which the mole • The bubble-point is the
fraction of A is 0. temperature at which
• In this example, A is the liquid starts to boil
the more volatile
component and
therefore has a lower • The region above the dew-
boiling point than B. point curve shows the
• The upper curve in the equilibrium composition of
diagram is called the the superheated vapour
dew-point curve while while the region below the
the lower one is called bubble-point curve shows
the bubble-point curve. the equilibrium
composition of the
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• 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.

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 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

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 • yi = mole fraction
of component ‘i’ in
the vapour
• xi = mole fraction
of component ‘i’ in
the liquid

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• 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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