Absorption and stripping - Principles of Chemical Separations with Environmental Applications - page 157

Table 6.1 Selection guide for absorbers and strippers [2].

Tray columns

Packed columns

Perforated Bubble cap (Random)

Low liquid flowrate D A C

Medium liquid flowrate A C B

High liquid flowrate B C A

Difficult separation (many stages) A B A

Easy separation (one stage) C C B

Foaming system B C A

Corrosive fluids B C A

Solids present B D C

Low pressure difference C D B

High turndown ratio C A B

Versatility (can be changed) C C A

Multiple feed and drawoff points B A C

A = best choice, D = worst choice

6.3 Column diameter

Regardless of whether equilibrium or mass transfer analysis is used to design an absorption
column, the column diameter is specified according to some limiting flow conditions for
the gas phase. There is a maximum gas flowrate that corresponds to any liquid flowrate.
This maximum gas velocity is called the flooding velocity and above it the phenomenon
called flooding begins to occur. When the gas velocity exceeds the flooding velocity, the
liquid is held up. In other words, it ceases to fall. It is literally supported on top of the gas
stream such that greater liquid pressure is required to force the liquid through the column.
Eventually, if the gas velocity becomes too high, liquid accumulates at the top of the
column and is blown out with the exiting gas stream.
The gas velocity must, therefore, be less than the flooding velocity. However, if it is
much less, the mass transfer between the two phases will be less efficient and, hence,
taller columns will be required to perform a given separation. The advantage of low gas
velocities is lower liquid pressure losses and, thus, lower pumping cost to operate the
column. A balance in gas velocity is needed between the liquid pumping power costs and
the fixed cost and practicality of excessively tall columns. Typically, a column is designed
to operate at one-half the flooding velocity.
Because the gas throughput (mass or volumetric flow) in an absorption process is
typically a set design parameter, a variable other than gas flow must be adjusted to maintain
an acceptable velocity. The column diameter is the chosen parameter. According to

= v ∗ A Q ,