Page 271 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

272 Chapter 10 Absorption and stripping

G L
00
Empirical correlations for H TOG ¼ and H TOL ¼ are available for 1 Raschig ring
k y aA t k l aA t
as dumped packing for absorption of NH 3 in water and considering the system to be dilute.
These correlations are taken from Table 10.8-2 of “Transport Processes and Unit Operation” by
C. J. Geankoplis, Pg. 633, third Edition, 2000 (Prentice Hall India) and the correlations are valid for
2
2
0.271 < m V (kg/m .s) < 0.95, and 0.678 < m L (kg/m .s) < 6.1 respectively.
This requires estimation of tower diameter, which has been done following the procedure elabo-
rated in Chapter 14, Section 14.5.
2
We estimate tower diameter (d) based on the upper limit of m L [ 0.95 kg/m .s, leading to a
2
2
smaller tower diameter. This gives for A t (m ) ¼ p d /4 ¼ (1861.44/3600)/0.95, d [ 0.832 m which
is greater than the minimum diameter specified in Table 14.20.
2
Adopting d ¼ 0.835 m, A t (m ) ¼ 0.5476.
2 2
which gives the revised m V ¼ 0:95 ð832=835Þ ¼ 0.9432 kg/m .s.
2
and m L ¼ (2239.29/3600)/0.5476 ¼ 1.136 kg/m .s. Both the values lie within the limits specified
for the correlations.
The adopted diameter and mass fluxes are used to estimate the % approach to flooding and the
pressure drop in the bed and the tower, following the methodology discussed in Section 14.4.4 (not
shown here). These are found to be well within limits.
Driving force lines
Assuming this to be a dilute system, the flow rates of the phases will not differ much at the top and
bottom of the bed.
So the G and L are considered at the bottom.
Driving force line passing through a typical operating point (x op ,y op ) intersects the equilibrium

0
y op e y i 0 k a
x
k að1 xÞ iM x
curve at (x i ,y i ) and its slope ¼ ¼ ¼ f k a ,
0
y
ðx op e x i Þ k að1 yÞ iM 0 y
where,

ð1 y i Þ 1 y op

lnfð1 x op Þ=ð1 y i Þg
f ¼ 1 y op =ð1 x op Þ iM ¼
iM
ln ð1 y i Þ= 1 y op fð1 x op Þ ð1 x i Þg
In order to generate k a and k a values for the system, we use the correlations available for finding.
0
0
y
x

G L
H TOG ¼ and H TOL ¼ .
k y aA t k l aA t
For random packing of 1 Raschig rings, these values areeIn gas phase,
00

b g 1=2 0:32 0:51 1=2
N 0:669 ¼ 0:419 m
H TOG ¼ a m V m L
Sc ¼ 0:557 ð0:943Þ ð1:136Þ