Binary Distillation
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Transcript of Binary Distillation
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Binary Distillation
SHR Chapter 7
1 BinaryDistillation.key - March 28, 2014
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IntroductionDates back to 1
st century AD
first used in a batch mode (distillate changes in time)
Goal: separate heavy key (less volatile) from light key (more volatile) by exploiting 1.
for 1 or 1, this can be done very effectively
unless an azeotrope exists (where =1).
then we recover the azeotrope and the light or heavy key,
depending on which side the feed lies.
By 16th century, multiple stages were in use to
improve separation.
By 1976, distillation accounted for nearly 3% of the US energy consumption!
mostly in petroleum refineries
Binary distillation is simplest & most well-understood.
we will limit our discussion to binary distillation
Third Law of Thermo - typically low thermodynamic efficiency.
SHR 7.1
2 BinaryDistillation.key - March 28, 2014
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Design ConsiderationsOperating pressure - knob 1
below ambient pressure requires vacuum operation
many things may influence choice of operating pressure
Thermo: azeotrope formation, , etc.
Column operating temperature range (avoiding reactions,
corrosion, etc.)
most analyses do not account for pressure variation through the column
Operating temperature - knob 2
Reboiler & Condenser:
Bottoms above ambient requires additional energy input to the
reboiler
Distillate below ambient requires energy removal from the condenser.
Thermodynamics: critical points of fluids
F
D
B
V
L
L
V
3 BinaryDistillation.key - March 28, 2014
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Batch Distillation
Conceptually, follows the T-x-y diagram.
More rigorous analysis in SHR chapter 13
4 BinaryDistillation.key - March 28, 2014
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The McCabe-Thiele Graphical Method
1925
Continuous (staged) distillation
SHR 7.2
5 BinaryDistillation.key - March 28, 2014
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NomenclatureSpecifications
F total (molar) feed ratez LK mole fraction in feedP Column operating pressurex LK mole fraction in distillatex LK mole fraction in bottoms
R/R reflux ratioFeed phase condition
VLE data (y/x plot)Type of condenser (partial/total)
Type of reboiler (partial/total)
ResultsD Distillate (molar) flow rateB Bottoms (molar) flow rate
N minimum number of stagesR minimum reflux flow rateV Boilup ratioN Number of equilibrium stages
Feed stage location
Stage compositions (
Light Key (LK) - more volatile component
Heavy Key (HK) - less volatile component
6 BinaryDistillation.key - March 28, 2014
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Preliminaries
Rectifying section - like an absorber
Feed & reboiler supply vapor
Condenser supplies liquid
Stripping section - like a stripper
Feed & condenser supply liquid
Reboiler supplies vapor
FzF = xDD + xBBF = D +B
D = F
zF xBxD xB
light-key mole balance:overall mole balance:
combine to eliminate B & solve for D:
7 BinaryDistillation.key - March 28, 2014
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Rectifying Section Operating LineLight key mole balance:
relates light-key compositions in passing streams
(streams on a stage are assumed to be in equilibrium)
If L and V are constant, then this is a straight line.
Both components have equal and constant molar enthalpies of vaporization (latent heats).
Sensible heat, CpT, is negligible compared to latent heat.
Column is insulated (no heat loss on each stage).
Column pressure is constant (thermodynamics can be
done at a single pressure).
Big assumptions, but allow for simple analysis, since L and V are constant under these assumptions.
The McCabe-Thiele Assumptions
L
V=
L
L+D=
L/DL/D + D/D
=R
R+ 1D
V=
1
R+ 1
Overall mole balance: V = L+D
y =
R
R+ 1
x+
1
R+ 1
xD
R LD
reflux ratio
operat
ing lin
e
SHR 7.2.1
V yn+1 = Lxn +DxD
yn+1 =L
Vxn +
D
VxD
8 BinaryDistillation.key - March 28, 2014
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Range of Reflux Ratiosy =
L
Vx+
D
VxD
0 L
V=
R
R+ 1
1 because 0 R 1
What happens at R = 0?
What happens at R = ?
What is the minimum R that allows separation?
(We will answer this question shortly)
R =L
D
9 BinaryDistillation.key - March 28, 2014
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relates light-key compositions in passing streams
(streams on a stage are assumed to be in equilibrium)
Stripping Section Operating Line
Light key mole balance:
Overall mole balance:
Lxm = V ym+1 +BxBMcCabe-Thiele assumptions
have been applied.
ym+1 =L
Vxm B
VxB
L VThe feed stage material balance relates L and V to and
y =
VB + 1
VB
x
1
VB
xB
VB VB
boilup ratio
oper
atin
g lin
e
SHR 7.2.2
L = V +D
10 BinaryDistillation.key - March 28, 2014
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Feed Stage & the q-Line
subcooled liquid saturated liquid partially vaporized saturated vapor superheated vapor
rectifying section
stripping section
liquid flow increase across feed rate normalized by feed rate.q =
L LF
= 1 +V VF
q > 1 q = 1 q = LF/F q = 0 q < 0
Operating lines & q-line must intersect at
a single point.
cannot specify q, VB and R independently.
SHR 7.2.3
q =hsat. vaporF hF
hsat. vaporF hsat. liquidF
11 BinaryDistillation.key - March 28, 2014
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More on the q-liney =
L
Vx+
D
VxD
rectifying section:
y =L
Vx B
VxB
stripping section:
subtract
q =L LF
= 1 +V VF
y
V VF
=
L LF
x+ zF
y (1 q) = qx+ zFy =
q
q 1x zF
q 1
Typically the feed condition is known (specifying q).
Then we can choose VB or R.Note: specifying R implies VB.
yV V = x L L+DxD +BxB| {z }
FzF
12 BinaryDistillation.key - March 28, 2014
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Feed Stage & Number of Stagestoo low
too high
just right
Locate feed stage nearest to the intersection of the operating lines & q-line as possible (just after
the horizontal line on the staircase passes P
SHR 7.2.4
13 BinaryDistillation.key - March 28, 2014
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Partial Reboilers & Condensers
F
D
B
V
L
L
VTotal Reboiler:
all liquid is turned back to vapor
Partial reboiler:
bottoms product is liquid, boilup is vapor
This is another equilibrium stage! very common...
Total condenser:
all vapor is condensed back to liquid
Partial condenser:
distillate is vapor, reflux is liquid
This is another equilibrium stage!
14 BinaryDistillation.key - March 28, 2014
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Limiting Cases: Rmin, Nmin.eq
uilibr
ium cu
rve
x
y
x = y
0 1.0
1.0
x = xDx = xB
equil
ibrium
curve
xy
x = y
0 1.0
1.0
x = zFx = xDx = xB
Total reflux
R = , VB = .
L = V, D = B = F = 0.
F = 0, N = Nmin.
y = x is operating line.
No product...
Minimum reflux
N = .
higher capital costshigher operating costs
x
y
x = y
0 1.0
1.0
x = xDx = xBx = zF
1
2
3
pinch point
(L/V )min =Rmin
Rmin + 1
Rmin =(L/V )min
1 (L/V )min(VB)min =
L/V
max
11F
D
B
V
L
L
V
SHR 7.2.4
15 BinaryDistillation.key - March 28, 2014
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Perfect Separation - Another Limiting Case
x
yx =
y
0 1.0
1.0
x = zF
Perfect separation: xB = 0, xD = 1.
Pinch points form in each section of the column.
Theoretical value for minimum reflux ratio and boilup to achieve perfect separation.
To find this:
Obtain x-y data from thermo.
Determine q-line
Determine slope of rectifying
operating line = Rmin/(Rmin+1).
Rmin =1
zF ( 1)For saturated liquid feed,
SHR 7.2.4
16 BinaryDistillation.key - March 28, 2014
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ExampleWe want to separate a mixture of n-heptane and n-octane using distillation at
atmospheric pressure.
If the feed is 40 mole% n-heptane as a saturated vapor, determine the minimum reflux ratio and minimum number of stages required to obtain
product streams with 95% and 5% n-heptane.
F
D
B
V
L
L
V Known:
xD = 0.95
xB = 0.05
zF = 0.4, saturated vapor
Needed:
K-values (equilibrium curve)
q-line
operating lines
17 BinaryDistillation.key - March 28, 2014