Page 272 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

10.4 Design illustration 273

where a ¼ 0.557, b ¼ 0.32, g ¼ ( 0.51) - from Table 10.8e1, 3rd Edition, 2000 (Prentice Hall India),
“Transport Processes and Unit Operation” by C. J. Geankoplis

N Sc ¼ 0:669
The solute (NH 3 ) is highly soluble in water, and hence, the mass transfer resistance is considered in
the gas phase.

Estimating mass transfer coefficients
The expression and the constants for the liquid phase are taken from the same source.
This gives,

!
h 0:22
m L 0:5 1:136 0:51
N ¼ 0:214 m
H TOL ¼ q ScL ¼ 0:00235 ð303:1Þ
l 0:0008
m
Where N ScL is the Schmidt Number for the liquid phase

m V =MW G
ð0:943=27:8Þ 3
0 ¼ 8:1 kmol = m $s
y
k a ¼ ¼
H TOG 0:419

m L =MW L
3
ð1:136=17:95Þ
0 ¼ 0:253 kmol=m $s
x
k a ¼ ¼
H TOL 0:25
Where MW G and MW L refer to the average molecular weight of the gas and the liquid phase
respectively.
The procedure to find interphase concentration (x i ,y i ) corresponding to a point (x op ,y op )
on the operating line is as follows
(1) Choose y op such that y 2 <y op <y 1 and note the corresponding x op on the operating line
(2) Assume f ¼ 1in Eq. 10.8. This is the initial guess.
k a
0
x
k a
(3) Draw driving force line through (x op ,y op ) with slope m ¼ð fÞ
0
y
(4) Note coordinate (x i ,y i ) at the intersection point of driving force line and equilibrium line.

ð1 x op Þ
ln

ð1 y i Þ 1 y op
ð1 x i Þ
Recalculated 8 9
(5) Recalculate f as f ¼
ð1 x op Þ ð1 x i Þ < =
ð1 y i Þ
ln
: 1 y op ;
(6) If fand f Recalculated differ by only a small amount, say 0.0001, note (x op ,y op ) and (x i ,y i ). Proceed
with calculations for a new set of (x op ,y op ) starting from Step (2)
Else, substitute f with f Recalculated and go to Step (3).