dr saad al-shahraniche 334: separation processes distillation of binary mixture bottom plate and...

Dr Saad Al-ShahraniChE 334: Separation Processes

Distillation of Binary Mixture

Bottom plate and re-boiler

For constant molal over flow, and used to denote low ratesL V

V

Bxx

V

Ly B

mm 1

V

Bxx

V

Ly B

mm 1

V

Bxx

V

Ly B

mr 1

ry

V

BxB

Re-boiler

Bottom

plate

my

V

1mx

L

1m

m

y

V

mx

L

m+1

m-1

m

1mL

xm+1

VL

xB

Re-boiler plate

yr

ry


Binary Multistage Distillation

The vapor leaving the partial re-boiler

is assumed to be in equilibrium with

the liquid bottom product.

ab

cd

e

xB xm+1 xm Xm-1

ym+1

ym

yr

V

Bxx

V

Ly B

mm 1

The operating line for stripping section

crosses the diagonal at point (xB , xB( and

its slope=

Note: Re-boiler acts as an ideal plate

BL

L

ratio-BoilupB

L

a, b,d

bottom plate

c, d, e

Re-boiler plate

Ope

ratin

g lin

e

xB

Equilib

rium

line


Feed plate


A feed plate is the plate over which the feed is admitted. The liquid rate over the vapor rate or both may change, depending on the thermal conditions of the feed.

Consider the 5 possible feed conditions shown in the next figures which assumes that the feed has been flashed adiabatically to feed stage pressure.a. sub-cooed liquid feed:

FLL

vaporofportion Condensed FLL

VV VL

FLL

VV

F

L


As a result of this, flow of liquid in stripping section increases and flow of vapor in rectifying section decreases


b. Feed is saturated liquid (bubble point liquid feed) :

FLL

VV

VL

FLL

VV

F

L



VL

FVVV

FLLL

FVVV

C. Partial vaporized feed :

The liquid portion of feed

becomes liquid and the vapor

portion of feed becomes vapor

FF VLF

= LF+VF

F

L

FLLL



VFV

LL

VL

d. saturated vapor feed (dew point vapor feed) :VFV

F

L

e. Superheated vapor feed

LL

FVV

liq. ofportion boiling FVV

VLL

FVV

F

L



feed of moleeach ofon introducti thefrom

resultat section th strippingin flow liquid of moles

F

LLq

a. Cold feed q > 1.0

b. Feed at bubble point (sat. liq.), q=1.0

c. Feed partially vapor, 0 < q < 1.0

d. Feed at dew point (sat. vap.), q = 0

e. Feed superheated vapor, q < 0

0.1

F

LcondensateFL

F

LLq

0.1

F

LFL

F

LLq

0.1

F

LLL

F

LLq F

0

F

LL

F

LLq

LL 0

F

LLq


q for sub-cooled feed (Tb > TF)

F

LcondensateFL

F

LLq

F

condensateq 1

LTTFCp FbL )(

)(

Condensate FbL TTFCpL

)(

1 FbL TTCpq

L

VL

VV

F

McCabe Thiele Graphical Equilibrium-Stage


q for superheated feed (TF > Td)

F

LL

F

LLq

liquid boiling

liquid boiling )( dFV TTFCp

)(

liquid boiling dFV TTFCp

)()( FdVdFV TTCp

F

TTFCpq

TF= feed temperature

Tb, Td= bubble and dew point of feed respectively.

= average latent heat of vaporization

CpL, CpV = specific heat of liquid and vapor respectively.

L

VL

F

VV

Boiling liquid



Feed line

The contribution of the feed stream to the internal flow of liquid = qF

The total flow rate of liquid in the stripping section is:

qFLL qFLL

0.1

F

LFL

F

LLq

and

e.g

For saturated liquid feed

For saturated vapor feed 0

F

LL

F

LLq



The total flow rate of vapor in the rectifying section is:

FqVV )1( FqVV )1(

01

F

VVq

e.g

For saturated liquid feed VV

0.1q

For saturated vapor feed FVV FVV

FqVV )1( FqF )1( 11 q 0q

The contribution of the feed stream to the internal flow of vapor =F(1- q)

,

,



For constant molal over flow

(1) 1 Dnn DxLxVy

V

Dxx

V

Ly D

nn 1


Vyn

Lxn+1

n

n+1Rectifying section

V

Bxx

V

Ly B

mm 1

(2) 1 Bmm BxxLyV

ym

m

m+1

xm+1

VL

stripping section


To locate the point where the operating lines intersect,

Let yn=ym , xn+1=xm+1 and subtract equation (1) from equation (2)

BDmmmn BxDxxLLxyVVy 11

BD BxDxxLLVVy )()(FxF

VV

Fxx

VV

LLy F

Fq

Fxx

Fq

qFy F

)1()1(

q

xx

q

qy F

11

q

xx

q

qy F

11

Feed line

q-line



This equation represents a straight line, called the feed line on which all intersections of operating lines must fall.

The position of feed line depend on xF and q.

Its slope is and its intercept is

This line cross the diagonal at x= xF

1q

qq

xF

1



x

y

XD

abc

d

e

q >1q =1

0 < q <1

q =0

q <1

XD

XFXB

XB

1D

D

R

x

r

ra cold liquid q > 1 slope +

rb saturated liquid q = 1 slope

rc (vap. + liq.) 0 < q < 1 slope -

rd saturated vapor q = 0 slope 0

re superheated q < 0 slope +



Construction of operating lines

1. Locate the feed line ( ) according to the feed

conditions.

2. Locate the rectifying line ( ). This line croces the

diagonal at (xD, xD) and of intercept ( )

3. Draw the stripping line through point (xB, xB( an the intersection of

rectifying line with the feed line.

q

xx

q

qy F

11

111

D

Dn

D

Dn R

xx

R

Ry

1D

D

R

x

Note: xF, xB , xD, L,D are constant



Plate Feed Location

The feed plate is always represented by the triangle that has one corner

on the rectifying line and one on the stripping line

Note:

1. The number of plates = number of

plates + re-boiler plate.

2. The liquid on the feed plate does not

have the same composition as the feed.


y

XD

XD

XFXB

XB

a

b

c

x

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

x

y

.

Feed line

21

3

4

5

6

7

8

9

10R

xD

xB

1D

D

R

x

xF

R.O. L

S.O. L


Heating and cooling requirements

Heat loss from a large insulated is relatively small.

For condenser

a. If the condensate is not su-bcooled (at Tbub)DxD

Liq. At its bubble point temperature

Va

Vap. at its dew point temp.

Top

plate

La

xa

T1

T2

wm

w

D

w

aw CpTT

RD

CpTT

Vm

)(

)1(

)( 1212

molal latent heat of vaporization of more volatile component


)1()()()( 12 DDawwc RDDDRDLVTTCpmq


b. If Su-cooled Reflux

If the reflux is cooled below the bubble point, a portion of vapor coming to the top plate (1) must condensed to heat the reflux

ΔL that is condensed inside the column is obtained from:

DxD

V

Top

plateLTc

T1ΔL

)( 1 ccc TTLCpL

c

ccc

TTLCpL

)( 1

Cpc=specific heat of condensation

T1= temp. of liq. On to plate bubble point of condensate

Tc= temp. of return condensate (reflux)

c= heat of vaporization of condensate for volatile component.



The actual reflux ratio in the column is

T1 Tbc= bubble point of the condensate

D

TTCpL

D

LL Ccc ]/)(1[ 1



For re-boiler

condensate

BxB

Bottom

plate

bLsteam

sm

ss

bs

qVm

rbss qVm

bV

sm = steam consumption

= vapor flow rate from re-boiler

s = latent heat of steam.

= molal latent heat of mixture at the bottom

molal latent heat of less volatile component

bV


dr saad al-shahraniche 334: separation processes distillation of binary mixture bottom plate and...

Documents

saturated liquid feed

feed stream

feed superheated vapor

vapor portion of feed

liquid portion of feed

cooed liquid feed

superheated vapor feed

cold feed q