UNIT OPERATIONS
CHG 3111
LECTURE 1: BINARY DISTILLATION
G. Psofogiannakis
GIBBS PHASE RULE
GIBBS PHASE RULE: The number of degrees of freedom F (intensive variables that can be arbitrarily specified)
in a non-reactive chemical system consisting of Π phases and C components is given by:
F=2–Π+C
• For two components (C=2) present in two phases (Π=2) at thermodynamic equilibrium: F = 2
• Consider Vapor-Liquid Equilibrium (VLE) (N=2) of a binary system (C=2) (such as a methanol-water mixture)
• Methanol = Component A, Water = Component B
• The intensive system variable are: T, P and the composition of the
two phases: xA, xB=1-xA, yA, yB=1-yA
T, P
Composition:
yA , y B
Composition:
xA , x B
VAPOR
VLE
LIQUID
• F=2 indicates that:
- If the T and the P are specified, xA (and xB) and yA (and yB) are fixed
(dependent)
- If P, xA are specified, then T, yA are fixed
- If P, yA are specified, then T, xA are fixed
BINARY VLE
• For the more volatile component (lower saturation temperature at P, light key: LK), yA > xA
T, P
Composition:
yA,yB
Composition:
xA , x B
VAPOR
VLE
LIQUID
• For the less volatile component (higher saturation temperature, heavy key: HK), xB > yB
• For the methanol – water system, methanol is the light key
• Experimental data of the mol fractions of the boiling mixture in the liquid and vapor, are
used and plotted in a T-xAyA diagram (T-composition phase diagram) at a given P.
• The T-composition diagram has two separate lines: T vs x1 and T vs y1 for VLE at given P
• The T-composition diagram for the Methanol – water system is shown in the next slide.
• The relative volatility αA,B of the components is defined as:
• It is a function of T, P and composition
• It is a measure of the “easiness” of separating the
components by distillation.
BINARY VLE: T- COMPOSITION DIAGRAM
TsatWater
=
Methanol (A)-Water (B) at P = 1 atm
Vapor Region (Superheated)
or ‘Dew-Point’ line
• The T-composition diagram has both lines T
vs xA and T vs yA at VLE
• Bubble-point: The temperature where upon
heating a liquid mixture, the first “bubble” of
vapor appears
• Dew-Point: The temperature where upon
cooling a vapor mixture, the first drop of
liquid appears
or ‘Bubble-Point’ line
Liquid Region (Subcooled)
64.5 = TsatMethanol
• Endpoints on the diagram correspond to the
boiling points of the components at P
Mol Fraction of Methanol xA or yA
• The mol fraction scale correspond to liquid or
vapor depending on the state at each point
• Points inside the envelope two-phase region
can only represent the composition of the
entire system (Liquid + Vapor), but not any
individual phase.
BINARY VLE: T- COMPOSITION DIAGRAM
Tsat B =
• As liquid A is heated, the bubblepoint
is at B and the dewpoint is at D
Methanol (A)-Water (B) at P = 1 atm
• BB’ is a tie-line at 73.1 oC
D’
• Point C is the overall composition
(equal to that of A) when T = 78 oC.
The liquid and vapor compositions are
found at the endpoints of the tie-line
D
C
B’
B
A
Heat
• Point D (dew-point) vapor composition
is yA = 0.5 (all liquid has vaporized).
The composition of the last drop of
64.5 = TsatA
liquid is at D’.
0.779
0.5
Mol Fraction of Methanol xA or yA
Distillation: Separation method is based on the different volatilities of
the components of a mixture.
 T-composition diagram: When the liquid is heated to point C, the
vapor is enriched in the more volatile component .
BINARY VLE: yA-xA DIAGRAM
Plot of equilibrium mol fractions at P (The T-information is not used)
The 45o-line (a line with equation yA=xA) is only for visualization – not an equilibrium line
Light key (Methanol) equilibrium line is above the 45o-line.
The T of the equilibrium line is decreased in the direction from left to right.
Methanol (A)-Water (B) at P = 1 atm
yA
•
•
•
•
xA
AZEOTROPIC MIXTURES
T-y1
T-y1
T (oC)
T-x1
T-y1
T-x1
T-x1
Chloroform (1) – Tetrahydrofuran (2) at 1 atm
A maximum-boiling azeotrope
Ethanol (1) – Toluene (2) at 1 atm
A minimum-boiling azeotrope
• When a mixture at azeotropic
composition boils, the vapor
composition is the same as the
liquid composition.
• The relative volatility of the
mixture components switches
(crosses the value 1) at the
azeotrope
• Thus, azeotropes cannot be
separated by traditional distillation
methods
FLASH VAPORIZATION: A SINGLE-STAGE SEPARATION PROCESS
OR
• A single – stage continuous equilibrium
distillation process in a flash-drum
• If the feed is liquid is can be partially vaporized
by T increase (heater) or P reduction (valve) or
both
• If the feed is vapor, it can be partially
condensed in a condenser by T reduction or P
increase.
• At the flash drum the vapor and liquid are at
thermodynamic equilibrium:
- Therefore, PV = PL = P , TV = TL = T and yi, xi are
related by the equilibrium relationship at T, P
• Used for separation of components with large
volatility differences
FLASH VAPORIZATION: A SINGLE-STAGE SEPARATION PROCESS
Total mass balance:
F=L+V
Component i balance:
Fzi = Lxi + Vyi
Energy balance:
If Q = 0  adiabatic
Subscript i refers to
a component in the
mixture
T, P
PV = PL = P , TV = TL = T
FLASH VAPORIZATION: A SINGLE-STAGE SEPARATION PROCESS
• Operating line or q-line:
y1 = -(L/V)x1 + (F/V)z1
Subscript 1 refers
to a component in
the mixture
L=F
• The point (x1, y1) should belong to both the q-line and the
equilibrium line
• Try x1=y1 on the q-line equation: x1=y1=z1
y1
• Slope of the q-line:
If L = 0 (V = F): slope = 0
y1=z1
L=0
L= V = F/2
If L = F (V = 0): slope = ∞
If L = V = F/2: slope = -1
• y1 is maximum for V = 0 (but at the limiting case of getting
no vapor).
x1=z1
x1