Post on 27-Dec-2015
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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