Basic Distillation[1]
-
Upload
kishoreprithika -
Category
Documents
-
view
223 -
download
0
Transcript of Basic Distillation[1]
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 1/21
f01_11Seader & Henley, Separation Process Principles1
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 2/21
Separation Processes
• Absorption – Solutes removed from a gas into a liquid• Solutes removed from liquid into gas is called stripping or desorption
• Distillation – Thermal vapor-liquid separation processes (Ch 11); vapor
phase generated from liquid
• Liquid-liquid extraction – Solute extracted from liquid A into animmiscible liquid B (a solvent)
• Leaching (extraction) – Solute extracted from a solid into a solvent phase
(liquid, dense gas, or supercritical fluid)
• Membrane processing – Molecules separated using a dense (non
-porous film) or porous physical barrier • Filtration – Suspended solids separated from a liquid or gas phase using aporous membrane
2
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 3/21
Methanol more
volatile than water
P m > P w
P m > 1 atm
Vapor-liquid equilibria...(e.g. ideal , methanol-water system)
BP diagram at const P (ideal)
dew-point
bubble-point
x = y (1 component)
x-y diagram at const P
P (= pm + pw ) diagram at const T
3
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 4/21
f02_18
Ethanol more
volatile
γ eP e > γ hP h
Ethanol less
volatile
γ eP e < γ hP h
x = y
at 58 oC
Low T
High T
Vapor-liquid equilibria...(e.g. non-ideal , n-hexane-ethanol system)
4
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 5/21
The greater the
separation
between the
equilibrium and 45 o
line, the easier the
separation
Getting into separations
x = y
x-y diagram at const P
α AB = y A / x A
y B / x B= y A / x A
(1− y A )/(1− x A )
α AB = P A
P B
y A =α AB x A
1+ (α AB −1) x A
if α AB =1, y A = x A
5
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 6/21
The greater the
separation
between the
equilibrium and 45 o
line, the easier the
separation
Simple flash distillation(single stage; heated to T, phase split)
x = y
x-y diagram at const P
F =V + L
Fx F =Vy + Lx
∴ Fx F =Vy + ( F −V ) x
heater separator F, xF
V, y
L, x 6
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 7/21
f01_11
Where liquid is
stripped of A by
raising vapor from
reboiler
Stripping section
Binary distillation of components A & B( A is more volatile, e.g. methanol (A)-water (B) system)
Where “cold” reflux
liquid condenses
some or the vapor
Enriching section
Vapor enriched
in A
Liquiddepleted of
A
Near yA = 1 @ TB,A
(light boiler)
Near xB = 1 @ TB,B
(high boiler)
7
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 8/21
F = D+W (molar flow)
FxF = Dx
D+Wx
W
D
F =
xF − x
W
x D− x
W ,
W
F =
x D− x
F
x D− x
W
V m+1= Lm −W
V m+1 y
m+1= L
m x
m -WxW
ym+1
=
Lm
V m+1
xm−
W
V m+1
xW
V n+1 = L
n+ D
V n+1yn+1 = L
n x
n+ Dx
D
yn+1
=
Ln
V n+1
xn−
D
V n+1
x D
W
xW
8
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 9/21
Approximation - Constant molal overflow
• Liquid and vapor flowrates are nearly constant in rectifying
(top) and stripping (bottom + feed plate) sections
– Ln=Ln+1=Ln+2… Vn=Vn+1=Vn+2…
– L and V, rectifying; L and V, stripping
• ΔHv (condensing high boiler) ≈ ΔHv (vaporizing low boiler)
•
Operating equations or lines are linear
yn+1
=
Ln
V n+1 x
n−
D
V n+1 x D
ym+1
=
Lm
V m+1
xm−
W
V m+1
xW
9
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 10/21
Variables
• # Plates, plate design, height of
column, etc. (later)
• Cooling in condenser
– Liquid returned to top of column(reflux)
• Heating in reboiler
– Vapor returned to bottom of column
• Location and conditions of feed
– Cold? Hot? L or V or L-V?
10
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 11/21
R =
Ln
D=
V n+1 − D
D(overhead product, L at B.P.)
yn+1
=
R
R +1 x
n −1
R +1 x
D
Top plate (1) Total
condenser
Partial
condenser
11
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 12/21
• Reboiler with saturated steam
• Condenser with cooling water
Heating and cooling requirements
ms=
V m+1
λ
λ s
λ = latent heat steam
λ s
= latent heat vapor mixture
Vm +1 = vapor flowrate from reboiler (stripping section)
mw=
V n+1
λ
(T 2 −T 1)c p,w
c p,w = heat capacity cooling water
(T 2 −T 1) = Temp change in cooling water
V n+1 = vapor flowrate into condensor
12
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 13/21
q > 1(sub-cooled L)
q = 1(@ BP)
0 < q < 1(L-V)
q = 0
(@ D.P.)
q < 0
(superheated V)
Feed conditions
q =
moles L in stripping section from feed
moles feed
q =
H V (D.P.)− H F
H V (D.P.)− H L (B.P.)
q =
( H V − H L )+ c p, L (T B −T F )
H V
− H L
Lm= Ln
+ qF (stripping)
V n =V m + (1− q)F (rectifying)
y =q
1−q x −
1
1− q x
F 13
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 14/21
McCabe-Thiele Method - # of ideal platesMcCabe & Thiele, Industrial Engineering & Chemistry Research, 17 (1925) 605.
V=L, R→∞ (total reflux)
y=x (P=Pi at each tray)
14
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 15/21
yn+1=
R
R +1
xn
−1
R +1
x D
xD ≡ design conditionR ≡ design variable
Rectifying section
15
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 16/21
Stripping section
ym+1 =
Lm
V m+1
xm −
W
V m+1
xW
16
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 17/21
Feed conditions (feed line)
@ D.P.
@ B.P.
yn+1 =
R
R +1 xn −
1
R +1 x D
ym+1=
Lm
V m+1
xm −W
V m+1
xW
y =
q
1− q x −
1
1− q xF
17
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 18/21
Putting it all together…
yn+1 = Ln
V n+1 xn −
D
V n+1 x D
ym+1= Lm
V m+1
xm −W
V m+1
xW
y = q1− q
x − 11− q
xF
18
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 19/21
Stepping off stages (start at xD)
What we want in
overhead product
What we want in
bottoms product
(start here)
operating equilibrium
x = xF
4 stages + reboiler
19
x = xW
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 20/21
Minimum # of plates
α av
= (α Aα
B)
1/ 2
Fenske equation:
N m=
lnx D
(1− x D )
(1− xW
)
xW
lnα av
*includes rebioler
OR
xB xD
V=L (op lines = 45o)
R→∞ (total reflux)
20
8/4/2019 Basic Distillation[1]
http://slidepdf.com/reader/full/basic-distillation1 21/21
Minimum reflux (occurs @ pinch point, P)
yn+1 =
R
R +1 x
n −1
R +1 x D
Rm
Rm +1
=
x D − y'
x D − x
'
y', x ' @ pinch point
21