MULTICOMPONENT DISTILLATION

DISTILLATION

DISTILLATION OF MULTI COMPONENT MIXTURES

Introduction. Process involved the separation of more than two

components. Basically the separation are base on the relative volatility

i value of each components, (light or heavy components) For multicomponent systems which can be considered

ideal, Raoult’s law can be used to determine the composition of the vapor in equilibrium with the liquid, where,pA = PA xA yA = pA/P = PAxA/P = KAxA

Cont’

A, B,C

A, B

C

A

B

1 2

Calculate the boiling point for mixtures

Key component:

-light key

-Heavy key

Boiling point Raoult’s law (ideal system) K (for nonideal system-

dependant on T and P)

ii

iiiii

C

BB

C

AA

C

ii

BBB

AAA

xKc

xKcxky

K

K

K

K

K

K

xKy

xKy

.

0.1

0.1.

................................

.

.

Dew point

Calculate

Liquid composition;

1

1

i

i

ci

ii

y

KK

yx

i

ic

yK

i

i

i

i

iy

y

x

Example 11.7-1 A liquid feed to a distillation tower at 405.3 kPa

abs is fed to a distillation tower. The composition in mole fractions is as follows: n-butane (xA=0.40), n-pentane (xB=0.25), n-hexane (xC=0.20), n-heptane (xD=0.15). Calculate the boiling point and the vapor in equilibrium with the liquid.

Solution: Assume a temperature and find the K values for all component in Fig. 11.7-2

Cont’ Assuming T = 65oC

1/ 1/ 3.643 0.2745C i iK x Referring to figure 11.7-2, at 0.2745, the T is 69oC. For the second trial, use 69oC and follow the same procedure.

DISTILLATION

Flash Distillation f = fraction of feed vaporized = V/F. Fraction of feed remaining as liquid = L/F = (1- f ). Equilibrium line: yi = Kixi = Kcixi

Operating line,

Temperature if f has vaporized;- by trial-and-error, assume T.- get values of Ki & i - calculate (Kci –1) & f (Kci –1) +1

- when xi = 1 then T = right temperature.

f

xx

f

fy iF

ii

1

1)1( ic

iFi Kf

xx

DISTILLATIONTotal Reflux, R = To calculate minimum theoretical stages, Nmin,

Fenske equation;

where,xLD = mole fraction of LK in distillate.xLW = mole fraction of LK in bottom product.xHD = mole fraction of HK in distillate.xHW = mole fraction of HK in bottom product

)log(

)]/)(/log[(

,min

avL

LWHWHDLD WxWxDxDxN

LWLDavL ,

DISTILLATION LD = relative volatility of LK at top/dew point

temperature. LW = relative volatility of LK at bottom product

temperature.

Initial compositions of distillate & bottom product for calculation of dew point & bubble point (by-trial-and-error).

Concentration of other components (besides LK & HK) in distillate & bottom product at R = .

Wx

Dx

Wx

Dx

HW

HDNmiav

iW

iD )(

DISTILLATIONShortcut Method for Rmin

Multicomponent distillation – 2 pinch points.- one above feed.- one below feed.

Underwood’s shortcut method;- assumes constant flows in both section of tower.- uses constant average .

i

iFixq1

i

iDim

xR 1

DISTILLATIONwhere,

xiD = composition of component i in the distillate

(as an approximation, taken at R = .) = average relative volatility of the top and the

bottom of the tower. To determine Rm;

1. By trial-and-error, assume (LK < HK).

2. Calculate 1-q for various using eq. (11.7-19)3. Use calc. In eq. (11.7-20) to calculate Rm using eq.

(11.7-20).

i

DISTILLATION

Shortcut Method for Number of stages Correlation of Erbar & Maddox – an approximate method. Feed plate location – by Kirkbride method (an approx.)

where,Ne = number of theoretical stages above the feed

plate.Ns = number of theoretical stages below the feed

plate.

2

log206.0logHD

LW

LF

HF

s

e

x

x

D

W

x

x

N

N