Page 170 - Air pollution and greenhouse gases from basic concepts to engineering applications for air emission control

144 5 Principles for Gas Separation

G y Z 0 dy
Z ¼ ð5:47Þ

ð y y Þ 1 yð Þ
K y aA c y 1
Let us continue with our analysis by liquid phase. In design practice, the terms in
front of the integration sign in Eq. (5.44) is often referred to as height of transfer
unit (HTU) calculated using liquid phase mass transfer coefficient. For example, the
HTU based on liquid phase mass transfer is

L
HTU x ¼ ð5:48Þ
k x aA c
The corresponding term by integration in Eq. (5.44), is called number of transfer
unit (NTU),

x Z 0 dx
NTU x ¼ ð5:49Þ
ð x i xÞ 1 xð Þ
x 1
By the same approach we can get the HTU y and NTU y for gas phase. They all
can be described using the overall mass transfer coefficients K x ; K y as well. Non-
theless, the packed tower height is

H ¼ HTU NTU ð5:50Þ

5.2.3.1 Packed Tower Diameter and Flooding Velocity

Body diameter is another important parameter of a packed tower. It is mainly
limited by the gas velocity at which liquid droplets become entrained in the exiting
gas stream.

1=2
4Q
D ¼ ð5:51Þ
p u g
3
where the diameter D is in m, Q is the volumetric gas flow rate in (m /s) and u g is
the mean gas face speed in m/s. When the gas flow rate reaches a point that the
liquid is held in the void spaces between the packing materials, the corresponding
gas-to-liquid ratio is termed as loading point. A further increase in gas flow rate (or
gas velocity) will prevent the liquid from moving downward causing the liquid to
fill up the void spaces in the packing. As a result, the gas–liquid interface surface
area drops substantially and thereby the absorption efficiency decreases dramati-
cally. And, the pressure drop increases greatly too. This condition is referred to as
flooding, and the corresponding gas velocity is called flooding velocity. As a typical
engineering practice, the diameter of a packed tower should enable the operation at
50–75 % of the flooding velocity.