Environmental Engineering Reference
In-Depth Information
x in =0
W L =1400lb m ft 2
y out = 0.005
hr
y in =0.02
W G =1000 lb m
t 2
hr
x out
Figure 6.12 Schematic for air purification, Example 6.6.
6.7
Remember
Absorption and stripping are UNIT OPERATIONS. Regardless of what chemicals
are being separated, the basic design principles for absorption and stripping are always
similar.
The assumption that stages in an absorption or stripping column are in equilibrium
allows calculations of concentrations without detailed knowledge of flow patterns and
mass transfer rates. This assumption is a major simplification.
Because one feed stream is already a vapor phase, absorption or stripping columns
usually do not require condensers and reboilers.
The McCabe-Thiele analysis can be used to model physical absorption and stripping
processes that use equilibrium stages.
The HTU-NTU method can be used to model absorption or stripping columns which
contain continuous packing instead of equilibrium stages.
6.8
Questions
6.1 Explain, in terms of Henry's Law coefficients, whether or not increasing the temper-
ature in a stripping column improves the separation at each stage.
6.2 Explain the tradeoff between high and low liquid velocities in absorption in terms of
column operation and effective mass transfer between the phases.
6.3 It is typical to assume that both absorption and stripping columns operate at constant
pressure. How would non-constant pressure affect the design? Consider pressure either
increasing or decreasing down the length of the column.
6.4 Show graphically how the slope of the operating line affects the degree of separation
that can occur in an absorption column.
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