Mass Transfer-I
Distillation
Lecture 3
Dr. Hemant Kumar
Department of Chemical Engineering
DDU Nadiad
9/26/2020 Mass transfer-I Dr Hemant Kumar 1
Insoluble liquids: steam distillation
• The mutual solubility of some liquids is so small that they can be
considered substantially insoluble
• Example: hydrocarbons and water
• If liquids are completely insoluble, the Vapor pressure of either
component cannot be influenced by pressure of other and each exerts its
true vapor pressure at the prevailing temperature
• When some of the vapor pressure of the individual components equals
the total pressure, the mixture boils and the vapor composition is reality
computed
p A + pB = pt
p
y* = A
pt
• So as long as two liquid phases are present, the mixture will boil at the
same temperature and produce a vapor of constant composition
9/26/2020 Mass transfer-I Dr Hemant Kumar 2
Insoluble liquids: steam distillation
Example
9/26/2020 Mass transfer-I Dr Hemant Kumar 3
Insoluble liquids: steam distillation
• by this method of distilation with steam so long as liquid water is present,
the high boiling organic liquid can be made to vaporize at a temperature
much lower than its n-boiling point without necessity of a vacuum pump
• If boiled at 204 oC, this compound will undergo considerable decomposing
Limitations:
• Heat requirements for steam distillation process are great. Since large
amount of water is required to be evaporated
9/26/2020 Mass transfer-I Dr Hemant Kumar 4
Negative deviations from ideality
• When total pressure of a system at equilibrium is less than the ideal value,
the system is said to deviate negatively from Raoult's law
The system is shown in figure:
• In negative deviations like same as
positive deviations, partial pressures
of the constituents of the solution
approach ideality as their
concentration approach 100%.
9/26/2020 Mass transfer-I Dr Hemant Kumar 5
Negative deviations from ideality
Maximum boiling mixtures (azeotropes):
(a)
When the difference in vapor pressures of
the compounds is not too great
and in addition the –ve deviations are
large,
The curve for total pressure against
composition may pass through a
minimum as in figure (a)
9/26/2020 Mass transfer-I Dr Hemant Kumar 6
Negative deviations from ideality
Figure (b)
• This condition give rise to a maxima in
the boiling temperature as at point L, a
condition of azeotropizm
(b) • The liquid component is leaner in
more volatile component than liquids
whose x is less than the azeotropic
composition (L) and Greater if x is
larger than point L
9/26/2020 Mass transfer-I Dr Hemant Kumar 7
Negative deviations from ideality
• Figure (c) shows the equilibrium curve
(C) for this system
Open vessel heating
Solutions in either side of the azeotrope
(L) if boiled in open vessel, with escape
of vapor, will ultimately leave a
residual liquid of the azeotropic
composition in the vessel
• Maximum boiling azeotrops are less
common than minimum boiling
azeotrops
Ex: HCl-Water system
11.1 mole % HCl and 110 oC at 1 std atm
9/26/2020 Mass transfer-I Dr Hemant Kumar 8
SINGLE-STAGE OPERATION-FLASH VAPORIZATION
• Flash vaporization, or equilibrium distillation as it is
sometimes called, is a single-stage operation wherein a liquid
mixture is partially vaporized, the vapor allowed to come to
equilibrium with the residual liquid, and the resulting vapor
and liquid phases are separated and removed from the
apparatus.
• It may be batchwise or continuous
• As shown In the fig, the liquid feed is heated in a conventional
tubular heat exchanger
• The pressure is then reduced, vapor forms at the expense of
the liquid adiabatically, and the mixture is introduced into a
vapor-liquid separating vessel.
SINGLE-STAGE OPERATION-FLASH VAPORIZATION
SINGLE-STAGE OPERATION-FLASH VAPORIZATION
• The separator shown is of the cyclone type, where the feed is
introduced tangentially into a covered annular space.
• The liquid portion of the mixture is thrown by centrifugal
force to the outer wall and leaves at the bottom, while the
vapor rises through the central chimney and leaves at the top.
• The vapor may then pass to a condenser, not shown in the
figure.
• The product, D mol/time, richer in the more volatile
substance, is in this case entirely a vapor.
• The material and enthalpy balances are:
SINGLE-STAGE OPERATION-FLASH VAPORIZATION
SINGLE-STAGE OPERATION-FLASH VAPORIZATION
• If the effluent streams were in equilibrium, the device would
be an equilibrium stage and the products would be on a tie
line.
• For example for more volatile component yD and xw
SINGLE-STAGE OPERATION-FLASH VAPORIZATION
SINGLE-STAGE OPERATION-FLASH VAPORIZATION
SINGLE-STAGE OPERATION-FLASH VAPORIZATION
• Successive flash vaporizations can be made on the residual
liquids in a series of single-stage operations, whereupon the
separation will be better than that obtained if the same
amount of vapor were formed in a single operation.
• As the amount of vapor formed in each stage becomes
smaller and the total number of vaporizations larger, the
operation approaches differential distillation in the limit.
DIFFERENTIAL, OR SIMPLE, DISTILLATION
• If during an infinite number of successive flash vaporizations
of a liquid only an infinitesimal portion of the liquid were
flashed each time, the net result would be equivalent to a
differential, or simple, distillation
• In practice this can only be approximated. A batch of liquid is
charged to a kettle or still fitted with some sort of heating
device such as a steam jacket, as in Fig.
• The charge is boiled slowly, and the vapors are withdrawn as
rapidly as they form to a condenser, where they are liquefied
and the condensate (distillate) is collected in the receiver
• The apparatus is essentially a large-scale replica of the
ordinary laboratory distillation flask and condenser
DIFFERENTIAL, OR SIMPLE, DISTILLATION
DIFFERENTIAL, OR SIMPLE, DISTILLATION
• The first portion of the distillate will be the richest in the
more volatile substance, and as distillation proceeds. the
vaporized product becomes leaner
• The distillate can therefore be collected in several separate
batches called cuts, to give a series of distilled products of
various purities
• Thus, for example, if a ternary mixture contained a small
amount of a very volatile substance A, a majority of substance
B of intermediate volatility, and a small amount of C of low
volatility, the first cut, which would be small, would contain
the majority of A
• A large second cut would contain the majority of B
reasonably pure but nevertheless contaminated with A and C
DIFFERENTIAL, OR SIMPLE, DISTILLATION
• and the residue left in the kettle would be largely C.
• While all three cuts would contain all three substances,
nevertheless some separation would have been obtained.
• For such an operation to approach even approximately the
theoretical characteristics of a differential distillation, it
would have to proceed infinitely slowly so that the vapor
issuing from the liquid would at all times be in equilibrium
with the liquid
• Binary Mixtures:
• The vapor issuing from a true differential distillation is at any
time in equilibrium with the liquid from which it rises but
changes continuously in composition.
DIFFERENTIAL, OR SIMPLE, DISTILLATION
• The mathematical approach must therefore be differential
• Assume that at any time during the course of the distillation
there are L mol of liquid in the still of composition x mole
fraction A and that an amount dD mol of distillate is
vaporized, of mole fraction y* in equilibrium with the liquid
• Then we have the following material balances:
DIFFERENTIAL, OR SIMPLE, DISTILLATION
• where F is the moles of charge of composition xF and W the
moles of residual liquid of composition Xw
• This is known as the Rayleigh equation
• It can be used to determine F, W, XF or Xw when three of these
are known
• The composited distillate composition YD,avg can be
determined by a simple material balance
DIFFERENTIAL, OR SIMPLE, DISTILLATION
