basic distillation[1]

8/4/2019 Basic Distillation[1]

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f01_11Seader & Henley, Separation Process Principles1



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



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



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



The greater the

separation

between the

equilibrium and 45 o

line, the easier the

separation

Getting into separations

x = y


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



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


F =V + L

Fx F =Vy + Lx

∴ Fx F =Vy + ( F −V ) x

heater separator F, xF

V, y

L, x 6



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



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



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



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



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



• 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



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



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



yn+1=

R

R +1

xn

−1

R +1

x D

xD ≡ design conditionR ≡ design variable

Rectifying section

15



Stripping section

ym+1 =

Lm

V m+1

xm −

W

V m+1

xW

16



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



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



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



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



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

basic distillation[1]

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