DISTILLATION
Syllabus
Raoult’s law and Henry’s law
Thoery of distillation of binary mixtures
Clasification and study of distillation equipment used for
simple, vacuum, steam, reflux distillation, separation of azeotropes.
Questions
1.
What are the merits of boiling point diagram of a
binary mixture? Write down the procedure of calculating equilibrium data of a
binary mixture from its relative volatility value. (1999) [4+4]
2.
In a distillation column, the feed given per hour is
1000 kg. The feed contains 98 mole% A and 2 mole% B. whereas the waste contains
4 mole % A and 96 mole% B. Determine the rates of distillate waste. The
molecular weights of A and B are 78 and 92 respectively. (1999) [8]
3.
Sketch and explain working of a bubble cap tray distillation
column for distillation column for distillation of binary mixture. (1999) [6]
4.
At 1100C and 1 atm pressure the vapor
pressures of n-heptane and 2-octane are 1500 and 484 mm Hg respectively. If
their mixture is an ideal one calculate mole fraction of n-heptane in liquid
and vapour-phases at equilibrium of ideal and non-ideal binary systems. (1998) [6]
5.
Explain the constant temperature and constant pressure
vapour-liquid equilibrium of ideal and non-ideal binary systems. (1998) [6]
6.
Explain the method of azeotropic distillation in
industrial scale. (1997) [6]
7.
What is relative volatility? How do you determine the
equilibrium data of a binary mixture if the relative volatility value is known?
(1995) [2+6]
8.
In a continuous distillation column a binary mixture (A
=40%, B=60%) is fed at the rate of 2000kg/hr. The overhead product contains 98%
A and 2% B. Determine the amounts of overhead and bottom products collected per
hour. (1995) [8]
9.
In case of mixtures, for all possible concentrations it
is stated that at any constant temperature, the partial pressure of one
component of a mixture is equal to the mole fraction of that component
multiplied by its vapor pressure on the pure state at that temperature.
Illustrate with suitable example. (1995) [8]
10.
Write down the theory of molecular distillation. (1995) [8]
11.
What is the principle of steam distillation? How is
steam distillation carried out? (1994) [6]
DEFINITION
Distillation
may be defined as the separation of the constituents of a mixture including a
liquid by partial vaporization of the mixture and separate and collect the
vapor.
Such separation may include
(i)
one liquid from non-volatile impurities.
(ii)
one liquid from one or more other liquids, with which
it may be miscible, partially-miscible or immiscible
N.B.
In practice it is difficult
to distinguish between evaporation, distillation and drying.
Based on the intention:
(i) when condensation vapor is required the operation is
called distillation
(ii) when the concentrated liquid residue is required the
operation is called evaporation.
(iii) when the dried solid residue is required as product
the process is called drying
BOILING POINT DIAGRAM OF A BINARY MIXTURE
 |
The figure represents the boiling
point and equilibrium-composition relationship, at constant pressure.
Two liquids A (b.p. tA)
and B (b.p. tB) are taken in a chamber of constant pressure. Now at
any temperature the vapor composition and liquid composition will give two
lines when plotted vs. temperature.
In
boiling point diagram, temperatures are plotted as ordinates and compositions
as abcissas.
·
The diagram consists of two curves, the ends of
which coincide with the b.p. of two components (tA and tB).
·
The upper-curve describes vapor composition and
lower-curve liquid composition.
·
At any temperature t the horizontal line cuts
the vapor composition curve at ‘e’ which corresponds to vapor composition of y
(mole%A) and cuts the liquid composition curve at ‘d’ which corresponds to
liquid composition of x (mole% of A). So any two points on the same horizontal
line (such as d and e) represent compositions of liquid and vapor in
equilibrium at temperature ‘t’.
·
For all points above the top line (such as point
‘a’) the mixture is entirely vapor.
·
For all points below the bottom line (such as
point ‘b’) the mixture is completely liquefied.
·
For all points between
the two curves (such as point ‘c’) the system consists partly of liquid and
partly of vapor.
For all points between
the two curves (such as point ‘c’) the system consists partly of liquid and
partly of vapor.
RAOULT’S LAW
Raoult’s
law states that, any particular temperature, the partial pressure of one
component of a binary mixture is equal to the mole fraction of that component
multiplied by its vapor pressure in the pure state at this temperature.
e.g. to illustrate Raoult’s law,
let us consider the case of benzene and toluene mixture.
At a temperature of 1000C
toluene has a vapor pressure of 556 mm. Consequently, if partial pressure is
plotted against composition, the partial pressures of toluene at various
compositions will fall along a straight line from 556 mm for pure toluene to
zero for pure benzene. At this same temperature benzene has vapor pressure of
1350 mm, and its vapor pressure will change linearly from zero for 0% benzene
to 1350 mm for pure benzene.
The
total pressure for any composition will be the sum of the two partial pressures
at that composition.
If
the partial pressures are straight lines i.e. Raoult’s law holds then the total
pressure will be a straight line between 556 m for pure toluene and 1350 mm for
pure benzene.
Derivation
Two liquids A and B are at constant temperature.
Liquid A is more
volatile than B.
If Raoult’s law holds for this binary mixture then
from Raoult’s law pA
= PA x (1)
and
pB = PB
(1 –
x)
where pA
and pB = partial pressure of
A and B respectively
PA
and PB = vapor pressure of pure A and B
x = mole fraction of A in the solution
If P represents the total pressure, then
P = pA + pB (2)
= PA
x +
PB (1 – x)
From Dalton’s law
where y = mole fraction of A in vapor phase
[from (1) and (2)]
Example
Vapor
pressure,
|
mm Hg
|
|
Temp
0F
|
Benzene
PA
|
Toluene
PB
|
176.2
180
185
190
195
200
205
210
215
220
225
230
231.1
|
760
811
882
957
1037
1123
1214
1310
1412
1520
1625
1756
–
|
314
345
378
414
452
494
538
585
635
689
747
760
|
The
vapor pressures of benzene and toluene are as given in the table. Assuming that
mixtures of benzene and toluene obey Raoult’s law, calculate and plot the
boiling-point diagram for this pair of liquids at 760mm total pressure.
Solution:
Let us take one temperature
1800F
So at 1800F, PA = 811
Hg
PB = 314
mm Hg
We have to calculate the mole
fraction of benzene in liquid (x) and in vapor (y).
From the eqn.:
P = PAx + PB
(1 – x)
or, 760 = 811 x
+ 314 (1 –
x)
or, x = 0.897
From eqn. 
Similarly for all temperature
values corresponding x and y values may be calculated:
Temp. 0F
|
x
|
y
|
185
190
195
200
205
210
215
220
225
230
|
0.773
0.659
0.555
0.459
0.370
0.288
0.211
0.141
0.075
0.013
|
0.897
0.831
0.757
0.678
0.591
0.496
0.393
0.281
0.161
0.031
|
RELATIVE VOLATILITY
For a more volatile
phase in equilibrium with a liquid phase, the relative volatility of component
A (the more volatile component) with respect to component B is defined by the
equation:
where
aAB
= relative volatility of component A
with respect to
component B
y = mole fraction of component A in vapor phase
x = mole fraction of component in liquid phase
In case of binary system, yB = 1
– yA and xB =
1 – xA.
Substituting,
Rearranging we get 
XA
|
||||||
YA
|
Equilibrium curve
If aAB is given then from the above equation a
set of XA and YA can be calculated. When YA is
plotted against XA the curve is called equilibrium curve.
If the liquid phase obeys Raoult’s
law and the vapor phase obeys Dalton’s law then,
So 
Example
Construct an equilibrium
curve for binary system of benzene – toluene from the given data.
Data
|
Boiling point at 1 atm
|
Vapor pressure of benzene (PA)
|
Vapor pressure of toluene (PB)
|
 |
Benzene
Toluene
|
80.10C
110.60C
|
760 mm
1780 mm
|
270 mm
760 mm
|
2.81
2.34
|
Therefore, average relative
volatility over the temperature range 80.1 to 110.60C
Therefore, 
XA
|
0.1
|
0.2
|
0.3
|
0.4
|
0.5
|
0.6
|
0.7
|
0.8
|
0.9
|
YA
|
0.222
|
0.391
|
0.524
|
0.631
|
0.720
|
0.794
|
0.857
|
0.911
|
0.959
|
|
HENRY’S LAW
The partial
pressure of a component over a solution is proportional to its mole fraction in
the liquid . This can be expressed as pA = C x
where pA =
partial pressure of component A
x = mole fraction of A in liquid phase
C = Henry’s law constant; C is constant only at
constant temperature
N.B. Raoult’s law is
essentially a special case of Henry’s law where the constant C in equation is
the vapor pressure of the pure component.
DISTILLATION METHODS
A. Distillation methods for miscible liquid systems
1. Equilibrium
or Flash Distillation
2. Simple
or Differential Distillation
3. Fractional
Distillation
4. Distillation
under reduced pressure (e.g. Molecular Distillation)
5. Special
Distillation Methods for non-ideal mixtures
(a) Distillation
of Azeotropic Mixtures
(b) Extractive
Distillation
(B) Distillation of immiscible liquids (e.g. Steam
Distillation)
 |
1. EQUILIBRIUM DISTILLATION / FLASH
DISTILLATION
There are
two types of distillations that do not involve rectification
(a) Equilibrium
distillation or flash distillation and
(b) Simple or
differential distillation.
(a) Equilibrium
distillation or flash distillation
This is a single stage operation where a liquid is partially
vaporized, the vapors are allowed to come in equilibrium with the residual
liquid and the resulting vapors and liquid are separated.
Use: This method
is used only when the difference between volatilities of two components is very
large
Let us consider a binary system whose components are A and
B. A is more volatile.
·
Feed: WF = number of moles of liquid fed
xF = mole fraction of component A in feed
·
Suppose V moles are vaporized in an
equilibrium-distillation process.
Now in
Liquid phase
|
Number of moles left in liquid phase = (WF
– V) moles
Let the composition of the residual liquid = x mole fraction of A
|
Vapor phase
|
Composition of vapor phase = y mole fraction of A
Number of moles gained = V
|
 |
Form material balance equation with respect to A
Moles of A at start = Moles of A in vapor phase
+ Moles of A in liquid phase
WF
xF = V y
+ (WF –
V) x (i)
In this case all the parameters are known except x and y
.
2nd equation required for solving is obtained from
equilibrium curve of the A, B system.
Eqn(i) is a straight line,
V y = WF
xF – (WF – V) x
or, 
or, y = c
– m x
Plotting this equation in the equilibrium curve the point of
intersection is obtained. The value of x and y can be obtained form the point
of intersection.
2. SIMPLE / DIFFERENTIAL DISTILLATION
In this
process vapor is removed from the system as soon as it is formed and condensed.
Use:
·
This method is commonly used in laboratory
·
In industries it is only used for systems having
high relative volatilities.
Derivation of
Raleigh’s equation
Let us consider a batch of W0 moles of liquid was
taken at the beginning.
Suppose at any given time during distillation there are W
moles of liquid left in the still.
At this time let the mole fraction of A in liquid is x.
Suppose a very small amount of liquid dW is vaporized. In
the vapor phase the mole fraction of component A is y.
At a given time
|
After a moment
|
|
Total moles of liquid present
|
W
|
W – dW
|
Moles of A present in liquid
|
Wx
|
(W – dW)(x – dx)
|
Total moles of liquid removed
|
dW
|
|
Moles of A present in the vapor
|
y
|
Therefore a material balance equation with respect to A will
be
xW
= (W – dW) (x – dx) + ydW
or, xW = xW – xdW –
Wdx + dWdx + ydW
dWdx is very small hence ignoring the term the
equation will be
ydW – xdW = Wdx
or, (y
– x)dW = W dx
or, 
Now, integrating between the limits
Time = 0
|
Time = t1.
|
|
Amount of liquid in the still (moles)
Moles of component A in liquid
|
W0.
x0.
|
W1.
x1.
|
or, 
This equation is known as Raleigh’s equation. It relates the
amount of material distilled with instantaneous composition of the liquid at
that moment
The function 
can be integrated
graphically from the equilibrium curve, since the curves gives the relationship
between x and y.
can be integrated
graphically from the equilibrium curve, since the curves gives the relationship
between x and y.
 |
N.B.
x y y
– x 1/(y – x)
-- -- ----- -----------
-- -- ----- -----------
Application of Raleigh’s Equation
1. By
using the Raleigh’s equation the effectiveness of simple distillation for a
given system can be estimated. [Ref:
A.B.Paradkar, Introduction to Pharmaceutical Engineering, p. 247]
2. It
is used in determination of cut-off point when we can stop distillation as soon
as the vapor composition falls below the required purity of the product.
SIMPLE DISTILLATION
Objective
Simple distillation is the process of converting a liquid
into its vapors which, are passed through a cooling surface to condense the
vapors. The condensed vapors are reformed into liquid which, is collected in a
receiver.
Apparatus for laboratory scale
It consists of a distillation
flask with a side arm sloping downward that is connected to a condenser. The
condensed vapors are collected in a flask called ‘receiver’. The whole apparatus is made of glass.
A thermometer is fitted in the
distillation flask to note down the temperature at which, the vapors are
distilled.
Bumping is avoided by adding
small pieces of porcelain or porous pot before distillation.
Apparatus for preparation of purified water
The boiler may be made
of cast iron but the baffles and the condenser tubes that comes into contact
with product are made of stainless steel or monel metal.
The cold water from the water tap
enters the still through the inlet, which rises in the jacket fitted with a
constant level device, the excess of water over flow through the outlet.
A portion of hot water at 90 to 950C
enters into the boiler through a narrow opening – the level of water is
maintained in the boiler up to overflow level.
The water is boiled in the boiler
by means of heating coils. On heating, the dissolved gases in the condenser are
allowed to escape through a small opening and only the steam escapes into the
condensing tubes.
Since the dissolved gases are
more volatile than water they escape in the first portion of the distillate,
therefore, must be rejected. Similarly, the last portion may contain volatile
portion of the dissolved solid substances in tap water – hence, discarded.
Application of simple distillation in pharmacy
1.
It is used for the preparation of distilled water and
water for injection.
2.
Many volatile oils and aromatic waters are prepared by
simple distillation e.g. Spirit of nitrous ether and Aromatic Spirit of Ammonia
3.
Concentration of liquid and to separate non-volatile
solid from volatile liquids such as alcohol and ether.
3. FRACTIONAL DISTILLATION / RECTIFICATION
A rectifying unit consists primarily of
(a)
a still or reboiler, in which vapor is
generated,
(b)
a rectifying or fractionating column through
which this vapor rises in counter-current contact with a descending stream of
liquid, and
(c)
a condenser, which condenses all the vapor leaving
the top of the column, sending part of this condensed liquid (the reflux) back
to the column to descend counter to
the rising vapors, and delivering the rest of the condensed liquid as product.
As the liquid stream descends the column, it is progressively
enriched with the less volatile constituent.
The top of
the column is cooler than the bottom, so that the liquid stream becomes
progressively hotter as it descends and the vapor stream becomes progressively
cooler as it rises. This heat transfer is accomplished by actual contact of
liquid and vapor, and for this purpose effective contact is desirable.
CONSTRUCTION OF RECTIFYING COLUMN
There are
different varieties of equipments for rectification
(a) Plate column (i)
Bubble cap column
(ii)
Sieve-plate column
(b) Packed column
BUBBLE-CAP COLUMN
·
The column is divided into sections by means of
a series of horizontal plates A.
·
Each plate carries a number of short nipples B (or riser). Each nipple is covered by a bell-shaped cap C that is secured by a spider and
bolt with the plate. The edge of the cap is serrated
or the sides may be slotted.
·
Vapor rises from the plate below through the
nipple, is diverted downward by the cap, and bubbles out under the serration or
through the slots.
·
A layer of liquid is maintained on the plate by
means of an overflow or down-pipe (F) and the depth of the
liquid is such that the slots are submerged.
·
The down-pipe,
(G) from the plate above, is sealed by the liquid on the plate below, so that
the vapor cannot enter the down-pipe.
·
Ordinarily, the liquid is delivered at one end
of a diameter by the down-pipe from the plate above, flows the other end of the
same diameter.
 |
Types of down-comers
(a) Cross flow
The liquid flows across the plate
from right to left on plate F and left to right on plate H and so on down the
column.
(b) Split flow
On plate F the liquid flows form
the two sides to the center. On plate H it flows from the center to the two
sides and so on down the column. This arrangement is commonly known as split
flow.
(c) Reverse flow
Liquid comes down the space on
one side of the baffle and flows across the plate from right to left, around
the end of the baffle, from left to right and down the space behind the weir.
This arrangement is called reverse flow.
(d) Radial flow with circular
down-take
One plate will have four or more down-comers around the
circumference, and the next plate will have a down-comer at the center so that
on the upper plate the flow is from the circumference towards center and on the
next plate the flow is from the central down-take to the circumference.
Specification of bubble cap rectification column
Column diameter 2
to 15 ft
Height few
feet to over 100 ft
Bubble cap diameter 3
to 6 inches
Slots in a 3 inches bubble cap may be 1/8 to 3/32 inch
wide
½
to 1 inch height
SIEVE PALTE COLUMNS
All the constructions are same as
bubble cap columns. Instead of bubble cap plates, flat plates with a large
number of relatively small perforations, drilled in them are used. These
perforations are usually 3/16 to ¼ inch in diameter.
The velocity of the vapor through
these holes is sufficient to produce the liquid running down the holes.
PACKED COLUMNS
The column is entirely filled with some sorts of material
that offers a large surface area supposedly wetted by the liquid.
A large variety of materials are
used among which Raschig rings are
popular. A Raschig ring is a hollow cylinder whose length is equal to its
diameter. This may be made of metal (by sawing sections off a pipe), stone ware,
ceramics, carbon, plastics, or other materials. Raschig rings are usually
dumped at random in the column.
Raschig Ring Lessing Ring Pall Ring Berl Saddle Intalox
Saddle
Advantages
(i) Have
a low pressure drop per unit of height than bubble cap
(ii)
For very small diameters of column, where it would be
difficult to get in more than two or three bubble caps, a packed column can be
used.
(iii) Since
Raschig rings can be made of any material, hence packed columns can be used for
corrosive materials.
(iv) The
amount of liquid held up in the column is low so thermolabile liquid remains in
contact with high temperature for a short time than bubble cap method.
Disadvantages
(i) They
are relatively inflexible.
(ii)
Distribution of liquid uniformly in such packed column
is difficult. It is found that, as the liquid passes down the tower it tends to
concentrate at the walls and leave the center dry.
4. DISTILLATION UNDER REDUCED PRESSURE / VACUUM DISTILLATION
Theroy
Liquid boils when its
vapor pressure is equal to the atmospheric pressure. Liquids, which are
decomposed at their boiling point under atmospheric pressure, can be distilled
at a much lower temperature than its boiling point if the pressure is reduced
on the surface of the liquid. Boiling under reduced pressure will also increase
the rate of distillation.
Molecular
Distillation
Theory / Principle
of Molecular Distillation
In a high vacuum distillation operation, where the
material distills from an evaporating surface to a relatively cool
condensing-surface. The conditions are such that, the mean free path of the
distillating molecules is greater than the distance between the evaporating and
condensing surface.
·
The vacuum applied in these types of apparatus
is about 1 mm
Hg pressure or less.
Mean free path
is defined as the average distance traveled by the molecules in a straight line
without any collision. It can be calculated by Clausius law:
where, l =
mean free path (cm)
d =
diameter of the molecules (cm)
N =
number of molecules in 1 cm3 volume.
N.B.
Temperature (0C)
|
Volume (litre)
|
Pressure (mm Hg)
|
Number of molecules
|
0
|
22.4
|
760
|
6.023 x 1023.
|
0
|
22.4
|
1 x 10–3.
|
7.9 x 1017.
|
It is clear from the above equation and chart that, mean
free path can be increased, by reducing the number of molecules per cm3
volume. The molecules evaporate from the surface and travel few cm without
colliding with the molecules of the residual gas in the space above. If now the
condensing surface is placed within distance, a major fraction of the molecules
will condense and will not return to the distilland. Thus each molecule
distills itself and hence called “Molecular
Distillation”.
Characteristics of
molecular distillation
1. Molecules
having molecular weight within the range of 300 to 1100 dalton can be distilled
by this method. [N.B. Low molecular weight
(below 300 dalton) molecules will re-evaporate again from the condenser
surface. High molecular weight molecules (greater than 1100 dalton) will not
have sufficient volatility.]
2.
The molecules to be
distilled should reach the surface and evaporate. The molecules at the bottom
of the distilland have to overcome the pressure of the layer above, to come to
the surface. Hence, the layer should be thin and should be in a state of
turbulent motion to facilitate the molecules to reach the surface.
The molecules to be
distilled should reach the surface and evaporate. The molecules at the bottom
of the distilland have to overcome the pressure of the layer above, to come to
the surface. Hence, the layer should be thin and should be in a state of
turbulent motion to facilitate the molecules to reach the surface.
3.
The distilland should be degassed before entering in
the still, because at very low pressure the dissolved gas will occupy all the
space and rate of distillation will be reduced.
Falling Film Molecular Still
The vessel has a diameter of the
order of 1 m and the walls are heated suitably by a heating jacket. Vacuum
pumps are connected by a large diameter pipe. The feed flows down the walls and
is spread to a film by the polytetrafluoroethylene (PTFE) wipers which move
about 3 m/s giving a film velocity of about 1.5 m/s. The residue is collected
at the bottom of the vessel and it is re-circulated (through the feed line).
The evaporated molecules are then
condensed on the condenser surface. The condensate is taken out as product.
Centrifugal molecular still
The distilland (feed) is
introduced on to the center of a bucket-shaped vessel (1 to 1.5 m in diameter)
that rotates at high speed. The film of liquid that is formed moves outwards
over the surface of the vessel to the residue-collection pipe. The vessel is
heated by radiant heaters. Condensers and a collection device are located close
to the inner surface of the rotor. Application of vacuum distillation in pharmacy
1. Vitamin
concentrates
Vitamin A,D,E,K and tocopherols are obtained from
vegetable and fish oils. The vitamin-A concentrate produced by molecular
distillation is very pure and has good stability. As no chemical is used in
this method which could split the ester linkage, the vitamins are retained in
the natural ester form which is the most stable form of vitamin A. The
stability of the concentrates is further enhanced by natural antioxidants
distilling over from the original oil.
2. The fractionation of oil
The
fractionation of oils into various components is carried out by molecular
distillation.
Components
|
Molecular Weight
|
Temperature Range
|
(a)
Fatty acids, unsaponifiable matter of low molecular
weight.
(b)
Unsaponifiable matter like sterols, vitamins, dyes,
waxy alcohols, monoglycerides
(c)
Triglycerides, sterol esters, vitamin esters, resins,
waxes
|
150 – 300
300 – 600
600 – 900
|
50 – 1400C
150 – 1900C
Above 1900C
|
3. Purification and fractionation of lanolin
It is used to get various fractions from Lanolin
like, cetyl alcohol, cholesterol, ceryl alcohol, lanopalmitic acid,isocholesterol
etc.
4. Separation of Poly Ethylene Glycol (PEG)
On laboratory scale it is used to separate PEG
according to the degree of polymerization.
5. SPECIAL DISTILLATION METHODS FOR
NON-IDEAL MIXTURES
Industrial scale distillation of Azeotropic Mixture
The liquor from fermentation
process is a common source of ethanol and contains approximately 8–10% ethanol.
After simple distillation an
azeotrope will form containing 95.6% (96E+4W) ethanol and boiling at 78.150C
at atmospheric pressure.
In this type of system a reboiler
is used instead of boiler. The feed liquor is introduced into the system and
must occur at a point where the equilibrium will not be disturbed. Hence, feed
will take place, at a place part of the way up the column, where the equilibrium
composition on the plate is similar to the feed composition.
The plate below the feed plate form the stripping section
where the rising vapor strips the more volatile component (ethanol) from the
feed liquor while the upper section is known as the rectifying section.
The binary azeotrope produced at
this stage is freed from water by making use of ternary azeotrope – ethanol,
benzene, and water.
The ethanol/water azeotrope, with sufficient benzene (only required at start-up) is fed to column A and the pure ethanol is obtained as bottom product, since the ternary azeotrope takes off the water.
·
The azeotrope (E+B+W) is taken from the top of
the column A, condensed and separated (in liquid-liquid separator) into two
layers, having the compositions given in the diagram.
·
The upper layer predominates and, being rich in
benzene (14.5E+1.0W+84.5B), is returned to column A. The lower layer
(53E+36W+11B) is taken to column B, where the benzene is recovered as the
ethanol/benzene binary azeotrope (67E+33B) and is mixed with the vapor from
ethanol.
·
The ethanol / water residue passes to column C,
where the ethanol is recovered as the ethanol/water binary azeotrope (96E+4W),
which can be incorporated with the original feed.
·
The final product from column A is 100% ethanol
and from column C is 100% water.
6. DISTILLATION OF IMMISCIBLE LIQUIDS
Steam distillation
Steam distillation is used for the distillation of two
immiscible liquids one of which is water.
Application:
(i)
Separation of volatile oil e.g. eucalyptus oil, rose
oil, clove oil etc. and
(ii)
Preparation of some aromatic water e.g. concentrated
rose water..
Theory
Volatile oils are mixtures of high molecular weight
compounds having low vapour pressure (i.e. high b.p.). To separate these from
the natural sources like petals of flowers, barks etc. it is not possible to
take them to their boiling points around 2000C. If these oils are
distilled with water (low molecular weight but high vapour pressure i.e. low
b.p.) then volatile oil will be distilled out at a temperature below 1000C.
Where, MW
and MV are molecular weights of water and volatile oil respectively.
PW
and PV are vapor pressure of water and volatile oil respectively.
·
The aqueous phase of distillate that is
collected is water saturated with volatile oil i.e. called aromatic water.
N.B.
When a mixture of two practically immiscible liquids are heated, while being
agitated to expose the surfaces of both liquids to the vapor phase, each
component independently exerts its own vapor pressure as a function of
temperature as if the other constituent was not present.
Boiling begins and distillation may be effected when
the sum of the partial pressures of the two immiscible liquids just exceeds the
atmospheric pressure.
An immiscible liquid and water independently boils
at high temperature but when steam is passed through a mixture of these liquids
(agitation) it boils at a much lower temperature than the boiling point of
water.
Example: Turpentine oil has a
boiling point of about 1600C, when mixed with water it can be
distilled at about 95.60C if steam is passed through it.
Large scale apparatus
This consists of a still having a mesh
near the bottom. The steam is generated by boiling water below the mesh. The
steam passes through the materials (to be extracted) packed over the mesh. The
vapor containing volatile oil is then passed ot the condenser. The distillate
is collected in Florentine receivers.
Florentine receiver separates the oil and water depending on their densities.
The aqueous phase may be re-circulated again to avoid loss of volatile oil in
water.
Florentine Receiver
It is used for the separation of oil and water. Florentine
receivers are of two types:
Type-I Used for separation
of oil heavier than water.
Type-II Used for
separation of oil lighter than water.
Type-I receiver
has tow taps. The tap fitted near the bottom of the vessel is used for
collecting oil, whereas the tap fitted near the top of the vessel is used for
water to overflow.
Type-II receiver is fitted with siphon at the bottom that works
when it gets filled with water whereas the tap fitted near the top is an outlet
for the flow of oil.