Environmental Engineering Reference
In-Depth Information
Solution:
S
mV
L
=
=
1
.
5
x
1
0
1
.
5
−
1
1
=
.
(1
.
5)
6
−
1
10
−
3
.
This result is very close to the value of
x
1
=5
x
1
=
4
.
8
×
10
−
3
.
×
6.6
Packed columns
Packed towers for absorption and stripping are often used instead of columns with discreet
trays. The column configuration is the same as a staged column with the liquid entering the
top of the tower and flowing countercurrent to the gas stream, which enters the bottom of the
column. A liquid distributor spreads the liquid over the entire cross-section of packing.
Ideally, the liquid should form a thin layer over the packing surface. Most packing is
shaped like saddles and rings, although some structured packings are available. Typically,
the packing is dumped into the column.
The choice of packing is critical to the successful operation of the absorption or stripping
process and must meet several criteria. Figure 6.10 shows some common types of packing
material. The saddles and rings can be made of plastic, metal, or ceramic. For optimal
liquid-gas contact the packing should be a shape with a high specific surface (surface area
to volume ratio). Thus, the packing provides a large area of contact between the two phases.
It must be chemically inert to all process fluids under the range of operating conditions.
The packing should be strong, lightweight and low cost. It must allow adequate passage
of both liquid and gas without liquid hold-up or excessive pressure drop. Table 6.2 gives
some physical characteristics of commercially available packing.
6.6.1
Mass transfer principles
Figure 6.11 shows the process of mass transfer as a gas solute exits the gas phase and enters
the liquid MSA phase. There are resistances to mass transfer describing movement of the
(a) (b) (c) (d)
Figure 6.10
Typical tower packings: (a) Raschig ring, (b) Lessing ring, (c) Berl
saddle, and (d) Pall ring [5]. Reprinted by Permission of John Wiley and Sons,
Inc. Copyright 1980.
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