Dr Saad Al-ShahraniChE 334: Separation Processes Binary Multistage Distillation Condenser At the top of column, three type of condenser: a)Total condenser:

Dr Saad Al-ShahraniChE 334: Separation Processes

Binary Multistage Distillation

Condenser

At the top of column, three type of condenser:

a) Total condenser: takes the overhead vapor from the column and liquefies it completely and split it into two portions:

1. Distillate which is called overhead product.

2. Reflux which return back to the top plate.

accumulator

D

V Condenser

LReflux

Overheadvapour


b) Partial condenser: takes the over head vapor from the column and liquefies a portion of it to return to the top plate as reflux, while the other portion is vapor product or distillate.

c) Mixed condenser

Partial condenser

V Condenser

L

Vapor overheadproduct

accumulator

Reflux

Overheadvapor

Vapor

distillateV Condenser

L liquid

distillate

accumulator

Mixed condenser

Reflux

Overheadvapor



Re-boilers

a) Kettle type re-boiler

At the bottom of column which heat the bottom liquid to produce vapor partially and return it back to column, while the rest of liquid withdraw as a bottom product.


bottoms


b) Vertical thermosyphone-type re-boiler.

In this type of re-boilers, re-boiler liquid with drown from the bottom sump.



c) Vertical thermosyphone-type re-boiler.

In this type of re-boilers, re-boiler liquid with drown from the bottom-tray down comer.



Reflux

It is apart of condensed vapor returned back to stage below. There are two types of reflux:

1. Operating reflux ratio (External Reflux Ratio)

External Reflux Ratio= La/D


DxD

accumulatorVa

ya

Condenser

La

xaTop

plate Reflux


2. Operating reflux ratio (Internal Reflux Ratio)

Internal Reflux Ratio2

3

n

n

V

L

1

2

n

n

V

L

n

n

V

L 1

1

n

n

V

L

or

or

or

Plate n+2

Plate n+1

Plate n

Vn+2

Ln+3

Vn+1Ln+2

VnLn+1

Vn-1Ln



Feed

McCabe Thiele Graphical Equilibrium-Stage

Method for trayed tower

MacCape-Thiele Diagram

DxD

accumulatorVa

ya

water

Str

ippi

ng

Sec

tion

Rec

tifyi

ng S

ectio

nBxB

Condenser

Re-boiler

Vb

yb

Lb, xb

Bottom

plate

Top

plate

La

xa

steam

Feed plate

Equations for the analysis of continuous flow

binary distillation were developed by sorel

(1893), but the simplest and most convenient

method for analyzing binary distillation

column is graphical method by [McCape and

Thiele]

consider the equilibrium-stage distillation

column shown in this figure.

Feed is a mixture of two components (A,B)

ya mole fraction of vapor

xa mole fraction of liquid



We can group values of y and x into 2 groups:

Group 1:

Vapor and liquid leaving each plate

Group 2:

Vapor and liquid passing each other between two plates

n-1

n

Vn-1Ln-2

VnLn-1

Vn+1Ln

n

n-1Vn

ynLn-1

xn-1

e.g. yn, xn

e.g. yn, xn-1



By defining of an ideal plate, the vapor and liquid leaving plate n are in equilibrium, so yn and xn represent equilibrium concentration.

n-1

n

Vn

ynLn-1

xn-1

Vn+1

yn+1

Ln

xn

Vn+2

yn+2

Ln+1

xn+1

Vn-1

yn-1

Ln-2

xn-2

n+1

Subcooled liquid

0 1.0

Superheated Vapory

Temperature

yn+1

x

xnynyn-1xn+1

Saturated liquid line

Saturated vapor line

xn-1

operating

x,y



Since the concentration in both phases (vapor, liquid) increases with the hight of the column,

xn-1 > xn > xn+1

yn > yn+1 > yn+2

In spite of xn, yn (streams leaving plate n) are in equilibrium, xn-1, yn+1( streams entering plate n) are not in equilibrium (see the last

figure)



Vn+1

yn+1

Ln

xn

Vm+1

ym+1

Lm

xm

Feed

DxD

accumulatorVa

ya

Feed plate

n+1

m

Str

ippi

ng

Sec

tion

Rec

tifyi

ng S

ectio

n

BxB

Condenser

Re-boiler

Vb

yb

Lb, xb

Bottom

plate

Top

plate

La

xaI

II

Overall material balances for Two components systems

Total material balance

(2) BDF BxDxFx

(1) BDF

Component A balance

(3) BD

BF

xx

xx

F

D

(4) BD

FD

xx

xx

F

B

Eliminating B

Eliminating D

F


Equation (3), (4) are the true for all values of flows of vapor and liquid within the column

Net flow rates

By making a material balance around the condenser and accumulator

DLV aa aa LVD

aaaa DxxLVy aaaaa xLyVDx

The difference between the flow rates of vapor and

liquid anywhere in the upper section of the column

(Rectifying section) = D

DxD

accumulatorVa

ya

Condenser

La

xaTop

plate



Total balance around the control surface (I)

nn LDV 1

nn LVD 1

aaaannnnD xLyVxLyVDx 11

Then D= net flow in top (rectifying section). Regardless of changes in V and L, their difference is constant and = D

Component balance A

section rectifyingin A component of rate flownet DDx

Vn+1

yn+1

Ln

xn

DxD

accumulator

Va

ya

Condenser

Top

plate

La

xaI



By the same manner in the stripping section Total balance around control surface (II)

1 mm VLB

11 mmmmB yVxLBx

m is to designate a general plate in the stripping section

section strippingin A component of rate flownet BBx

mm LBV 1

B= net flow in the bottom (stripping section)

Vm+1

ym+1

Lm

xm

BxB

Re-boiler

Vb

yb

Lb, xb

Bottom

plate

II



Re-boiler

Material balance around the re-boiler

bb VLB

bbbbB yVxLBx

BVL bb

1 mmbb VLVLB

11 mmmmbbbbB yVxLxVxLBx

BxB

Vb

yb

Lb, xb

Bottom

plate


ChE 334: Separation Processes

Operating linesBecause there are two sections in the column,

there are two operating lines

1. For rectifying section (I)

aaaannnnD xLyVxLyVDx 11

aaaannnn xLyVxLyV 11

111

n

aaaan

n

nn V

xLyVx

V

Ly

DL

Dxx

DL

Ly

n

Dn

n

nn

1

Vn+1

yn+1

Ln

xn

DLV nn 1

DLV nn 1

Operationrelationship

n

n+1

For control surface (I) ( )

DLVLV aann 1


Vn+1

yn+1

Ln

xn

DxD

accumulator

Va

ya

Condenser

Top

plate

La

xaI

Dr Saad Al-Shahrani


2. For stripping section (II)

11 mmmmbbbbB yVxLyVxLBx

111

m

Bm

m

mm V

Bxx

V

Ly

For control surface (II) ( )

BVLVL mmbb 1

BL

Bxx

BL

Ly

m

Bm

m

mm

1or

BL

Bxx

BL

Ly

m

Bm

m

mm

11

1

1

Vm+1

ym+1Lm

xm

Operationrelationship

m

m+1

Vm

ymLm-1

xm-1

m-1

m


Vm+1

ym+1

Lm

xm

BxB

Re-boiler

Vb

yb

Lb, xb

Bottom

plate

II


Number of ideal plates, McCape Thiele Method

When the operating lines represented by:

BL

Bxx

BL

Ly

m

Bm

m

mm

1

DL

Dxx

DL

Ly

n

Dn

n

nn

1

For stripping section

For rectifying section

are plotted with xy equilibrium diagram, the McCabe Thiele step by step

construction can be used to compute the number of ideal plates needed

to accomplish a definite concentration difference in either rectifying or the

stripping section



Constant Molal overflow

For most distillation, the molar flow rates of vapor and liquid are nearly

constant in each section of the column (rectifying and stripping) and the

operating lines are almost straight. (note: V-L=D, L-V=B)

Reflux Ratio

D

DV

D

LRD

DL

L

V

LRV

Reflux to overhead product

Reflux to the vaporDxD

V

Top

plate

L



In both numerator and denominator of the terms on the right hand side of the last two equations are divided by D, the result is for constant molal over flow.

DDL

DDxx

DDL

DLy

n

Dn

n

nn /)(

/

/)(

/1

1)/(1)/(

/1

DL

xx

DL

DLy

n

Dn

n

nn

111

D

Dn

D

Dn R

xx

R

Ry

interceptslope

This equation is the operating line of the rectifying section. The y axis intercept of this line is xD/(RD+1)



xD is set by the conditions of design

RD , the reflux ratio, is an operating variable that can be controlled by adjusting the split between reflux and overhead product or by changing the amount of vapor formed in the re-boiler for a given flow rate of the over head product.

Put xn= xD

DD

DD

D

DD

D

Dn x

R

Rx

R

xx

R

Ry

1

)1(

111

xD yn+1

So the operating line for rectifying section intersect the diagonal at point

(xD,xD). This is true for both partial and total condenser

yn+1= xD


xn=xD

Ln

L

Reflux

n+1Top plate

Vn+1

yn+1


Condenser and top plate

Concentration of vapor from top plate =yn+1

Concentration of reflux to the top

plate xn= xD =yn+1


Vn+2

yn+2

Ln+1

Xn+1

n+1

Vn+1

yn+1

Top

plate

Ln=L =Reflux Xa=xD

Top plate

xnTop plate


n+1

n+2

n+3

yn+1

yn+2

yn+3

xn=xD

xn+1

xn+2

y’

Operating line (rectifying section)

composition Equilibrium with Operating with

xn=xD y’ yn+1

xn+1 yn+1 yn+2

xn+2 yn+2 yn+3

Total Condenser

n+1 D, xD

Vn+1

yn+1

Top plate Ln

xn

yn+2xn+1

yn+3xn+2

n+2

n+3

yn+1


Equilibrium line

Reflux

Top plate


n+1

n+2

n+3

yn+1

yn+2

yn+3

xn=xD

xn+1

xn+2

xn+3

1D

D

R

x

DL

L

V

L

R

R

n

n

n

n

D

D

1slope

1

45 o lin

e

x = y

n+1

yn+2xn+1

yn+3xn+2

n+2

n+3

yn+1

xn=xD

Ln

Top plate

111

D

Dn

D

Dn R

xx

R

Ry

For total condenser

Operating line equation for the rectifying section:



n+1

n+2

n+3

yn+1

yn+2

yn+3

yD=xD

xn+1

xn+2 Operating line (rectifying section)

composition Equilibrium with Operating with

xn xD=yD yn+1

xn+1 yn+1 yn+2

xn+2 yn+2 yn+3

Partial Condenser

n+1

D

xDVn+1

yn+1

Top plate Ln

xn

yn+2xn+1

yn+3xn+2

n+2

n+3

yn+1


Equilibrium line

VyD

Reflux

xn

Partial condenser

yD=xD

Top plate

Documents

Dr Saad Al-ShahraniChE 334: Separation Processes Binary Multistage Distillation Condenser At the top of column, three type of condenser: a)Total condenser: