DISTILLATION WITH REFLUX

AND

McCABE-THIELE METHOD

RECTIFICATION –

“fractionation”

-Series of flash vaporization stages

Distillation Column

Stages - “sieve plates or trays”

McCabe-Thiele Method: Calculation for Number of Theoretical Stages

Assumption: Equimolal Overflow (Constant

Molal Overflow )

- Heat s of solution negligible- Molar latent heats same for

the compunds

OMB:

Component A balance:

:

Balance: F D W

OMB F D W

A Fx Dx Wx

Enriching Section

1 2 1 2 1

1

1

Overflow: and V

(L and D same composition)

OMB @ enriching section:

n n n

D

n n

Equimolal L L L V V V

y y

V L D

n

Bal:

Enriching Section OL:

Reflux ratio, R = L D

Component

Stripping Section

1

m+1

CMO: and

OMB: Vm N m N

m

L L V V

L W

Component Bal:

Stripping section OL:

Feed Condition, q

Feed Condition

H q

Saturated Liquid

(TB or bubble pt.)HL=HV -HF 1.0

Saturated Vapor

(dew point)HV = HL 0

Subcooled liquid

(Cold liquid feed)

HL > HF > 1

Superheated vapor

HV < HL < 0

heat needed to vaporize 1 mol of feed at entering conditions

molar latent heat of vaporization of feedV F

V L

H Hq

H H

Feed Line

no. of mols of sat'd liquid produced on feed plate

1 mol feed added to the towerq

(1 )

equation: 1 1

m n

n m

F

L L qF

V V q F

q xq line y x

q q

Total Reflux

( very large)0

R slope for operating line= 1.0

R+1 No. of trays = minimum

**infinite sizes of condenser, boiler

and tower diameter

n nn

L LR L

D

Minimum Reflux, Rm

• requires an infinite no. of trays

• minimum vapor flow• minimum sizes of

reboiler and condenser