Absorption and stripping - Principles of Chemical Separations with Environmental Applications - page 159

between the numerator and denominator of the first ratio, reducing it to the second. Then,
because density information for both streams can be obtained for a column operating at
constant temperature and pressure, the x -axis of the figure can be calculated. The ideal gas
law can be used for the gas-phase density calculation and the liquid-phase density can be
found in many tables as a function of temperature (assuming an incompressible fluid). One
can then read off the figure the value of the y -axis that corresponds to flooding. Again,
every quantity here except for the gas mass velocity is a constant at a given temperature
and pressure. A list of packing factors is supplied in Table 6.2, p. 170. The gas mass
velocity can be calculated and a simple ratio of the gas mass flowrate to the gas mass
velocity will give the required cross-sectional area of the column:

area = mass flow / mass velocity . (6.1)

More details on selection and scale-up of columns are available [3].

Example 6.1: flooding velocity column diameter

Problem:
An absorption column is to be built to separate ammonia from air using ammonia-free
water as a solvent. A tower packed with 1-inch ceramic Rasching rings needs to treat
25,000 cubic feet per hour of entering gas with 2% ammonia by volume. The column
temperature is 68 ◦ F and the pressure is 1 atm. The ratio of gas flowrate to liquid
flowrate is:
1 . 0lbgas / 1 . 0lbliquid .

If the gas velocity is one-half the flooding velocity, determine the necessary column
diameter.
Solution:
For H 2 O,
ρ x ft 3 = 62 . 4lb /

µ x = 1 . 0cP .

For air, molecular weight
M = 0 . 98(29) + 0 . 02(17) = 28 . 76 lb / lbmol .

Then,

1 / 2

0

1 / 2

G x
G y

ρ y
ρ x − ρ y

0 . 75

= 1 . = 0 . 0346 .

62 . 4 − 0 . 075

For flooding, from Figure 6.1,

G y 2 F p µ − 1

x

155 ft − 1

ρ y =

0 . 19; F p =

g c ( ρ x − ρ y )