Distillation…. Introduction
Unit operation – Separation process
A feed mixture of two or more components is separated into
two or more products whose compositions differ from that of
the feed.
Driving force – Relative volatility
Principle – Phase creation (ESA)
Feed – liquid or vap-liq mixture
Product – Bottom product (liquid) + Overhead product (liq or
vap-liq )5
Vapor – Liquid Equilibrium
7
Equilibrium between vapor and liquid is determinedexperimentally.
Temperature – Composition diagram
Pressure – Composition diagram
Bubble point and Dew point
9
Bubble point is the point at which first drop of liquid
mixture begin to vaporize.
Dew point is the point at which first drop of gaseous/vapor
mixture begins to condense
Partial Pressure and Dalton’s Law
12
The partial pressure PA of component A in a mixture of
vapours is the pressure that would be exerted by
component A at the same temperature, if present in the
same volumetric concentration as in the mixture.
By Dalton’s law of partial pressures, the total pressure is
equal to the summation of partial pressures.
Raoult’s Law
13
In an ideal gas/vapor, the partial pressure of a constituentis proportional to its mole fraction.
For an ideal mixture, the partial pressure of a constituentis related to the liquid phase concentration by Raoult’s law.
Volatility …
16
Volatility is defined as the ratio of partial pressure of a
component to the mole fraction of that component in liquid
phase.
For ideal system, volatility of a component is numerically
equal to the vapor pressure of the pure component.
Relative Volatility …
17
The ratio of these two volatilities is known as the relative
volatility.
Replacing partial pressures in terms of total pressure gives a
relation for ratio of two components in vapor to the ratio of
two components in liquid.
Relative Volatility …
18
For a binary mixture
This gives relations for determination of composition of liquid
or vapor phases, if the composition of the other phase is
known.
Temperature Dependency of Relative Volatility …
19
α varies somewhat with temperature, it remains remarkably
steady for many systems.
α increases as the temperature falls, so that it is valuable to
operate at reduced pressure in order to decrease the boiling
point.
An average value of α can be used over whole column if the
relative volatilities at the top and bottom of column vary by less
than 15%.
Otherwise the equilibrium curve must be constructed
incrementally by calculating the relative volatility at several
points along the column.
K-Value … (1)
20
Phase-equilibrium ratio is the ratio of mole fractions of a species
in two phases at equilibrium.
For vapor–liquid systems, the constant is referred to as the K-value
or vapor–liquid equilibrium ratio.
For many systems K is constant over an appreciable temperature
range, So it is often much more useful than the simple vapor
pressure.
Using K-values is particularly useful while dealing with
multi‐component systems.
Methods of Distillation – Binary Mixtures
22
Differential Distillation
Flash or Equilibrium distillation
Rectification
Differential Distillation … (1)
23
A single‐stage process that starts with a still pot, initially full, and
heated at a constant rate.
The vapor formed by boiling of liquid is removed at once from the
system.
Vapor is richer in more volatile component. Liquid becomes
steadily weaker in more volatile component. Product quality
varies with time.
At any instant, liquid is in equilibrium with the vapor formed on
that instant.
At the end, the remaining liquid is removed as bottom product.
A complete separation is possible only at infinite relative volatility.
Differential Distillation … (2)
24
S= number of moles of material in still
x= mole fraction of component A in liquid
dS= amount of material vaporized from still
y= mole fraction of component A in vapor
Material balance of component A
Differential Distillation … (3)
25
Integrating
Integral can be solved graphically using equilibrium
relationship between y and x.
Over the range concerned the equilibrium relationship is a
straight line of the form y = mx + c
Flash or Equilibrium Distillation … (1)
28
Frequently carried out as a continuous process.
Feed is usually pumped through a fired heater and enters a
still through a valve that causes a reduction in pressure.
A part of liquid feed is vaporized in such a way that vapor
evolved is in equilibrium with the residual liquid.
The still is a separator vessel that provides sufficient times
for the vapor and liquid to reach equilibrium.
Vapor is removed from top and usually condensed. Liquid is
taken from the bottom.
Flash or Equilibrium Distillation … (2)
30
F= molar flow rate of feed
V= molar flow rate of vapor
S= molar flow rate of liquid
x= mole fraction of A in liquid product
y= mole fraction of A in vapor product
xf= mole fraction of A in feed
Total material balance
Component A balance
Flash or Equilibrium Distillation … (3)
31
Rearranging
Equation of straight line that passes through point (xf ,yf) and has a slope
Example 11.6
33
An equimolar mixture of benzene and toluene is subjected to
flash distillation at 100 kN/m2 in the separator. Using the
equilibrium data given in Figure 11.9, determine the
composition of the liquid and vapour leaving the separator
when the feed is 25 per cent vaporised. For this condition,
the boiling point diagram in Figure 11.10 may be used to
determine the temperature of the exit liquid stream.
Rectification
35
The term rectification is derived from the Latin words
rectefacere, meaning ‘‘to improve’’.
Distillation process that enables successive vaporization and
condensation to be accomplished in one unit.
The essential merit of rectification is that it enables a vapour
to be obtained that is substantially richer in the more volatile
component than is the liquid left in the still.
Such a unit is known as a fractionating column.
Fractionating Column: Major Parts
37
Rectifying section
Stripping section
Reboiler
Condenser
Reflux drum / Accumulator
Fractionating Column: Streams
38
Feed stream
Overhead product
Bottom product
Reflux stream
Boil-up stream
Utilities
Hot
Cold
Fractionating Column: Operation
39
Fractionating column is a tall cylindrical vessel that is divided into
several sections by means of perforated trays.
The trays allow the vapor to pass through their perforations in the
upward direction.
The liquid flows across each tray, then over a weir, and then
through a down comer to the next tray in the downward direction.
The vapor from top of the column is condensed and then passed to
a reflux drum (or accumulator or reflux divider).
A fraction of the condensed vapor is drawn as top product
(distillate). The remainder is returned to the top tray as reflux.
Fractionating Column: Operation
40
The liquid from the bottom of the column is heated by means
of hot oil or steam in a reboiler.
The bottom product is drawn as liquid that flows over the
weir in the reboiler.
Since the bottom product is hot, it may be used top reheat
the incoming feed.
The vapor generated in the reboiler is returned to the
bottom tray of the column.
Fractionating Column: Operation
41
At each tray, vapor is partially condensed to give a vapor
that is richer in more volatile component. Liquid is partially
vaporized and becomes weaker in more volatile component.
This is because of vaporization of some part of more volatile
component from the liquid, and condensation of some part
of less volatile component from the vapor.
The partial condensation of rising vapor and partial
vaporization of reflux liquid occurs at each stage.
Top vapor, distillate product and reflux liquid have same
composition.
Fractionating Column: Operation
42
Feed is introduced on some intermediate tray where the
liquid has approximately same composition as the feed.
The part of the column above the feed tray is known as
rectifying section. The lower portion below the feed tray is
known as stripping section.
Vapor and liquid leaving from an ideal tray are in
equilibrium with each other.
Vapor and liquid entering a tray are not in equilibrium.
Assumptions of equimolar counter diffusion and constant
molar overflow are often valid in the system.
Equimolar Counter diffusion
43
When the mass transfer rates of the two components are equal
and opposite, the process is said to be one of equimolar counter
diffusion.
It occurs in the distillation column when the molar latent heats of
the two components are the same.
The more volatile component is transferred from liquid to vapor,
and the less volatile components is transferred from vapor to
liquid.
If the molar latent heats of the two components are equal, the
condensation of a given amount of less volatile component
releases exactly the amount of latent heat required to volatilize
the same molar quantity of the more volatile component.
Constant Molar Overflow (CMO)
44
Heat losses from a distillation column are usually small and
may be neglected.
For an ideal system, heat of mixing is zero.
With these assumptions, molar heat of vaporization may be
taken as constant and independent of composition.
For such systems, if one mole of vapor condenses, exactly
one mole of vapor is liberated.
With CMO, molar flowrates of liquid and vapor remain
constant in the rectifying section (and in stripping section)
unless material enters (or is withdrawn from) the column.
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