First part of distillation slides

BITS Pilani
Pilani Campus
Separation Processes - I
(CHE F244)
Dr. Suresh Gupta
Associate Professor & Head, Deptt. of Chemical Engg.

BITS Pilani
Pilani Campus
Distillation of Binary Mixtures

BITS Pilani, Pilani Campus
• A feed mixture of two or more components is separated
into two or more products
• often limited to, an overhead distillate and
• a bottoms product
• Most often, the feed is a liquid or a vapor–liquid mixture
• The bottoms product is almost always a liquid
• The distillate may be a liquid, a vapor, or both
• The separation requires that
• a second phase be formed so that both liquid and vapor are
present and can make contact while flowing counter currently to
each other in a trayed or packed column
Distillation (Fractionation)

• components have different volatilities so that they partition
between phases to different extents, and
• the two phases are separable by gravity or mechanical means
• Distillation differs from absorption and stripping
• the second fluid phase is usually created by thermal means
(vaporization and condensation)
• rather than by the introduction of a second phase that may contain
an additional component or components not present in the feed
mixture

Distillation of a binary mixture
of benzene and toluene

• Feed flow rate, composition, temperature, pressure, and phase
condition
• Desired degree of component separation
• Operating pressure (which must be below the critical pressure of
the mixture)
• Pressure drop, particularly for vacuum operation
• Minimum reflux ratio and actual reflux ratio
• Minimum number of equilibrium stages and actual number of
equilibrium stages (stage efficiency)
• Type of condenser (total, partial, or mixed)
• Degrees of liquid reflux subcooling
• Type of reboiler (partial or total)
Design and Analysis Factors

• Type of trays or packing
• Column height
• Feed-entry stage
• Column diameter
• Column internals, and materials of construction
• Heat lability and chemical reactivity of feed components
Design and Analysis Factors

• Temperature and phase of the feed are determined at
the feed-tray pressure by an adiabatic-flash calculation
across the feed valve
• As the feed vapor fraction increases
• the required reflux ratio (L/D) increases
• but the boilup ratio (V/B) decreases
• The column operating pressure in the reflux drum should
correspond to a distillate temperature
• Some what greater than the supply temperature of the cooling
water to the overhead condenser
Some Initial Considerations

• However, if this pressure approaches the critical
pressure of the more volatile component
• then a lower pressure must be used and a refrigerant is required
as coolant
• If the estimated pressure is less than atmospheric,
• the operating pressure at the top is often set just above
atmospheric to avoid vacuum operation
• unless the temperature at the bottom of the column is limited by
decomposition, polymerization, excessive corrosion, or other
chemical reactions
• In that case, vacuum operation is necessary

• For given (1) feed, (2) desired degree of separation, and
(3) operating pressure
• A minimum reflux ratio exists
• that corresponds to an infinite number of theoretical stages
• A minimum number of theoretical stages exists
• that corresponds to an infinite reflux ratio
• The design trade-off is between the number of stages and the
reflux ratio

• Successful applications of distillation methods
• Depends greatly upon an understanding of the equilibria existing
between vapor and liquid phases of the mixture
Vapor-Liquid Equilibrium
Relations

• An ideal law, can be defined for vapor-liquid phases in
equilibrium (only ideal solution e.g. benzene-toluene,
hexane-heptane etc.)
• Composition in liquid:
• Composition in vapor:
Phase Rule and Raoult’s Law
AAA xPp =
BA xx +=1
BA yy +=1

• Boiling-point diagram for system benzene (A)-toluene (B)
at a total pressure of 101.32 kPa.
Constant Pressure Equilibria
Dew point is the temperature at which the
saturated vapour starts to condense.
Bubble-point is the temperature at which
the liquid starts to boil.
The difference between liquid and vapour
compositions is the basis for distillation
operations.
If we start with a cold liquid composition is
xA1 = 0.318 (xB1 = 0.682) and heat the
mixture, it will start to boil at 98ºC.
The first vapor composition in equilibrium
is yA1 = 0.532 (yB1 = 0.468).
Continue boiling, the composition xA will
move to the left since yA is richer in A.

• The boiling point diagram can be calculated from
• (1) the pure vapor-pressure data in the table below and
• (2) the following equations:
Ppp BA =+
PxPxP ABAA =−+ )1(
P
xP
P
p
y AAA
A ==

• Calculate the vapor and liquid compositions in
equilibrium at 95ºC (368.2K) for benzene-toluene using
the vapor pressure from the table at 101.32 kPa.
• Solution: At 95ºC from Table for benzene, PA = 155.7
kPa and PB = 63.3 kPa. Substituting into Eq.(5) and
solving,
155.7(xA) + 63.3(1-xA) = 101.32 kPa (760 mmHg)
Hence, xA= 0.411 and xB= 1 – xA = 1 - 0.411 = 0.589.
Substituting into eqn.(6),
PxPxP ABAA =−+ )1(
632.0
32.101
)411.0(7.155
====
P
xP
P
p
y AAA
A

• A common method of plotting the equilibrium data is
• yA is plotted versus xA for the benzene-toluene system
• The 45º line is given to show that yA is richer in component A
than is xA.

• An azeotrope is a mixture of two or more liquids in such
a ratio that its composition cannot be changed by simple
distillation.
• The maximum temperature Tmax corresponds to a
concentration xAZ and xAZ = yAZ
• The plot of yA versus xA would show the curve crossing the 45o
line at this point
• Acetone-chloroform is an example
• A minimum boiling azeotrope with yAZ = xAZ at Tmin
• Ethanol-water is such a system
Nonideal System

Nonideal System
Maximum-boiling azeotropeMinimum-boiling azeotrope
A mixture whose total pressure is
greater than that computed from
ideality
A mixture whose total pressure is
less than that computed from
ideality

Nonideal System

• It is a measure of the differences in volatility between 2
components
• hence their boiling points
• It indicates how easy or difficult a particular separation will be
• Where αAB is the relative volatility of A with respect to B in the
binary system.
• Raoult’s Law:
• when αAB is above 1.0, a separation is possible.
Relative Volatility of Vapor-
Liquid Systems
)1)(1(
/
/
/
AA
AA
BB
AA
AB
xy
xy
xy
xy
−−
==α
AAB
AAB
A
x
x
y
)1(1 −+
=
α
α
P
xP
y AA
A =
B
A
AB
P
P
=α
P
xP
y BB
B =

• A single equilibrium stage is
• the two different phases are brought into intimate contact with
each other
• The mixing time is long enough and the components are
essentially at equilibrium in the two phases after separation
• Total mass balance:
• Component balance:
Single-Stage Equilibrium Contact
for Vapor-LiquidSystem
V1 V2
L0 L1
Where
V1, V2 is a vapor
L0, L1is a liquid
MVLVL =+=+ 1120
AMAAAA MxyVxLyVxL =+=+ 11112200

• Distillation has two main methods in practice:
• Production of vapor by boiling the liquid mixture to be separated
in a single stage and recovering and condensing the vapors
• No liquid is allowed to return to the single-stage still to contact
the rising vapors
• Returning of a portion of the condensate to the still
• The vapors rise through a series of stages or trays, and part of the
condensate flows downward through the series of stages or trays
countercurrently to the vapors (“fractional distillation, distillation with
reflux, or rectification”)
Simple Distillation Methods

• There are 3 important types of distillation that occur in a
single stage or still
• Equilibrium or Flash Distillation
• Simple batch or differential distillation
• Simple steam distillation
Simple Distillation Methods

• Single stage separation technique
• A liquid mixture is pumped through a heater to raise the
temperature and enthalpy of the mixture
• It then flows through a valve and the pressure is reduced,
causing the liquid to partially vaporize
• Once the mixture enters a big enough volume (the “flash drum”),
the liquid and vapor separate
• Because the vapor and liquid are in such close contact up until
the “flash” occurs, the product liquid and vapor phases approach
equilibrium
Equilibrium or Flash
Distillation

• Total mass balance:
• Component balance:
• Material balance for more volatile component :
• Where, f = V/F = molal fraction of the feed that is vaporized and
withdrawn continuously as vapor
• 1-f = one as liquid
heater
SeparatorLVF +=
AAF LxyVFx +=
AAF xfyfx )1( −+=AAF x
F
V
F
F
y
F
V
x )()( −+=

• A mixture of 50% mole normal heptane and 50% normal
octane at 30ºC is continuously flash distilled at 1
standard atmosphere so that 60 mol% of the feed is
vaporized. What will be the composition of the vapor and
liquid products?
Problem
xA 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
yA 0.247 0.453 0.607 0.717 0.796 0.853 0.898 0.935 0.968
Solution: Given: xF = 0.5, f = 0.6; Find: xA, yA
Basis: F = 100 mols
Applying the mass balance yields:
Since
LVF +=
60)100(6.0 === fFV
4060100 =−=−= VFL

Material balance for more volatile component,
Subtituting value of f =0.6 and xF =0.5 we get,
Assuming that xA = 0.5 and solving yA = 0.5.
Next, assuming that xA=0 and solving, yA = 0.83.
(These point are plotted on the graph.)
At the intersection of this line with the equilibrium curve,
yA = 0.58 and xA = 0.39.
AAF xfyfx )1( −+=
AAF x
F
V
F
F
y
F
V
x )()( −+=
AA xy )6.01(6.05.0 −+=
AA xy 4.06.05.0 +=

yA
xA
xF =0.5
yF = 0.5
x=0
y-intercept= 0.834
xA =0.39
yA = 0.58
1st
2nd
3th

Simple Batch or Differential
Distillation
• The pot is filled with liquid
mixture and heated.
• Vapour flows upwards though
the column and condenses at
the top.
• Part of the iquid is returned to
the column as reflux, and the
remainder withdrawn as
distillate.
• Nothing is added or withdrawn
from the still until the run is
completed.

Distillation

• The total moles of component A left in the still nA will be
nA = xn
• Where,
• n is the moles of liquid left in the still at a given time
• y and x is the vapor and liquid compositions
• If a small amount of liquid dn is vaporized
• the change in the moles of component A is ydn, or dnA.
• Differentiating equation gives
Distillation
ydnxdnndx
xdnndxxnddnA
=+
+== )(
xy
dx
n
dn
−
=

• dx/(y-x) can be integrated graphically or numerically
using tabulated equilibrium data or an equilibrium curve
• For ideal mixture:
Distillation
∫∫ =
−
=
1
0 0
1
1
0
ln
x
x
n
n
n
n
xy
dx
n
dn
xy
dx
n
dn
−
= Rayleigh equation
B
A
AB
B
A
x
x
y
y
α=
B
B
AB
A
A
B
A
AB
B
A
B
A
n
dn
n
dn
n
n
dn
dn
dndn
dndn
α
α
=
==
AB
A
A
B
B
n
n
n
n
α/1
00






=
B
B
AB
A
A
n
n
n
n
00
lnln α=
Integrating

• A batch of crude pentane contains 15 mole percent n-butane and 85 percent n-
pentane. If a simple batch distillation at atmospheric pressure is used to remove
90 percent of butane, how much pentane will be removed? What will be the
composition of the remaining liquid?
Solution: An average value of 3.5 is used for αAB.
Basis: 1 mol feed
(butane) (pentane)
From equation:
nB = total mole of B left in still, nA = total mole A left in still.
n0B = total initial mole of B in still, n0A = total initial mole A in still.
Problem
15.0=OAn 015.0=An 85.0=OBn
AB
A
A
B
B
n
n
n
n
α/1
00






=

• Total mole of liquid left in still:
• Mole fraction of butane in liquid left:
( ) 518.01.0
85.0
5.3/1
==Bn
440.0)85.0(518.0 ==Bn
moln 455.0015.044.0 =+=
033.0
455.0
015.0
==Ax

• At atmospheric pressure high boiling liquids cannot be
purified by distillation
• Since the components of the liquid may decompose at high
temperature required
Simple Steam Distillation

• Often the high temperature substances are essentially
insoluble in water
• So a separation at lower temperatures can be obtained by
simple steam distillation
• This method is often used to separate a high boiling
component from small amount of nonvolatile impurities
• If the total pressure is fixed
• Since there are two liquid phases, each will exert its own
vapor pressure at the prevailing temperature and cannot
be influenced by the presence of the other

• When the sum of the separate vapor pressures equals
the total pressure, the mixture boils and
• Where
• PA is vapor pressure of pure water A
• PB is vapor pressure of pure B
• Then the vapor composition is
• The ratio moles of B distilled to moles of A distilled is
PPP BA =+
P
P
y A
A =
P
P
y B
B =
A
B
A
B
P
P
n
n
=

• This method has the disadvantage that
• Large amount of heat must be used to evaporate the water
simultaneously with the high boiling compounds

• A mixture contains 100 kg of H2O and 100 kg of ethyaniline (mol wt = 121.1
kg/kg mol), which is immiscible with water. A very slight amount of
nonvolatile impurity is dissolved in the organic. To purify the ethyaniline it is
steam-distilled by bubbling saturated steam into the mixture at a total
pressure of 101.32 kPa (1 atm). Determine the boiling point of the mixture
and the composition of the vapor. The vapor pressure of each of the pure
compounds is as follows (T1):
Problem
Temperature PA(water)
(kPa)
PB(ethylaniline)
(kPa)
K ºC
353.8 80.6 48.5 1.33
369.2 96.0 87.7 2.67
372.3 99.15 98.3 3.04
386.4 113.2 163.3 5.33

Solution
PPP BA =+
Temperature PA
(water)
(kPa)
PB
(ethylaniline)
(kPa)
P=PA+PB
(kPa)
K ºC
353.8 80.6 48.5 1.33 49.83
369.2 96.0 87.7 2.67 90.37
372.3 99.15 98.3 3.04 101.34
386.4 113.2 163.3 5.33 169.23
The boiling temperature = 99.15ºC since total pressure in this temperature is
equal to atmospheric pressure.
The vapor composition are:
97.0
32.101
3.98
===
kPa
kPa
P
P
y A
A
03.0
32.101
04.3
===
P
P
y B
B

• A column containing the equivalent of N theoretical
stages
• a total condenser in which the overhead vapor leaving the top
stage is totally condensed to a bubble point
• liquid distillate and a liquid reflux that is returned to the top stage
• a partial reboiler in which liquid from the bottom stage is partially
vaporized to give a liquid bottoms product
• vapor boilup that is returned to the bottom stage and
• An intermediate feed stage.
Distillation with Reflux and
McCabe- Thiele method

• For components with close boiling points
• the temperature change over the column is small and relative
volatility is almost constant

• In 1925, McCabe and Thiele [5] published a graphical
method for combining the equilibrium curve with material
balance operating lines to obtain
• for a binary-feed mixture and selected column pressure
• the number of equilibrium stages and
• reflux required for a desired separation of feed components

• Typical input specifications and results (outputs) from the
McCabe–Thiele construction for a single-feed, two-
product distillation are

• From the specification of xD and xB for the LK, distillate
and bottoms rates, D and B, are fixed by material
balance, since
• But, B = F – D and therefore,
• Besides the equilibrium curve, the McCabe–Thiele
method includes
• A 45o reference line, operating lines for the upper rectifying
section and the lower stripping section of the column, and a fifth
line (the q-line or feed line) for the phase or thermal condition of
the feed.

• The rectifying section of equilibrium stages extends from
the top stage, 1, to just above the feed stage, f
• Consider a top portion of the rectifying stages, including
the total condenser
Rectifying-Section Operating
Line

• A material balance for the LK over the envelope for the
total condenser and stages 1 to n is as follows:
• Solving for yn+1 gives the equation for the rectifying
section operating line:
• This equation relates LK compositions yn+1 and xn of
passing streams Vn+1 and Ln, respectively
Line

• This straight line equation is the locus of compositions of
all passing streams in the rectifying section
• L and V must not vary from stage to stage in the rectifying
section
• This is the case if:
• The two components have equal and constant molar enthalpies
of vaporization (latent heats)
• Component sensible-enthalpy changes (CPDT) and heat of
mixing are negligible compared to latent heat changes
• The column is insulated, so heat loss is negligible
• Column pressure is uniform (thus, no pressure drop).
Line

• These assumptions leading to the condition of constant
molar overflow in the rectifying section
• Since a total material balance for the rectifying-section
envelope gives Vn+1 = Ln + D
• if L is constant, then V is also constant for a fixed D
• Rewriting equation:
• Thus, the slope of the operating line in the rectifying
section is a constant L/V, with V > L and L/V < 1
Line

Line

• For constant molar overflow in either the rectifying or the
stripping section, only material balances and an
equilibrium curve are required
• Energy balances are needed only to determine
condenser and reboiler duties
• Liquid entering stage 1 at the top is the external reflux
rate, L0, and its ratio to the distillate rate, L0=D, is reflux
ratio R
• Because of constant molar overflow, R = L/D is a
constant in the rectifying section
Line

• Since V = L+ D, the slope of the operating line is readily
related to the reflux ratio:
• Similarly,
• Combining equations produces the most useful form of
the operating line for the rectifying section:
• If R and xD are specified, plots as a straight line
Line

• The stripping section extends from the feed to the
bottom stage
Stripping-Section Operating
Line
Consider a bottom portion of stripping
stages including the partial re-boiler and
extending up from stage N to stage m+1,
below the feed entry

• A material balance for the LK over the envelope results
in
• Solving for ym+1:
• where L and V are total molar flows (which may be different from
L and V in the rectifying section because of feed addition)
• Vapor leaving the partial reboiler is assumed to be in
equilibrium with the liquid bottoms product, B, making
the partial reboiler an equilibrium stage
Line

• With the constant-molar overflow assumption, VB =
is constant in the stripping section
• Since
Line

Line

First part of distillation slides

Recommandé

Recommandé

Contenu connexe

Tendances

Tendances (20)

Similaire à First part of distillation slides

Similaire à First part of distillation slides (20)

Dernier

Dernier (20)

First part of distillation slides