Page 228 - Separation process principles 2

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Absorption and Stripping of Dilute Mixtures

In absorption (also called gas absorption, gas scrubbing, The opposite of absorption is stripping (also called
and gas washing), a gas mixture is contacted with a liquid desorption), wherein a liquid mixture is contacted with a gas
(the absorbent or solvent) to selectively dissolve one or to selectively remove components by mass transfer from the
more components by mass transfer from the gas to the liq- liquid to the gas phase. As discussed in Chapter 5, absorbers
uid. The components transferred to the liquid are referred to are frequently coupled with strippers to permit regeneration
as solutes or absorbate. Absorption is used to separate gas (or recovery) and recycling of the absorbent. Because strip-
mixtures; remove impurities, contaminants, pollutants, or ping is not perfect, absorbent recycled to the absorber con-
catalyst poisons from a gas; or recover valuable chemicals. tains species present in the vapor entering the absorber.
Thus, the species of interest in the gas mixture may be all When water is used as the absorbent, it is more common to
components, only the component(s) not transferred, or only separate the absorbent from the solute by distillation rather
the component(s) transferred. than stripping.

6.0 INSTRUCTIONAL OBJECTIVES
After completing this chapter, you should be able to:
o Explain the difference between absorption and stripping.
o Explain the difference between physical and chemical absorption.
o Explain why absorbers are best operated at high pressure and low temperature, while strippers are best operated
at low pressure and high temperature.
o Enumerate different types of industrial equipment for absorption and stripping and explain which are most
popular.
o Explain how vapor and liquid streams flow from one tray to another in a trayed tower.
o Compare three different types of trays.
o Explain the difference between random and structured packings and cite examples of each.
o Explain the importance of the liquid distributor and redistributors in a packed column with respect to liquid flow.
o Derive the "operating-line equation," used in graphical methods, starting with a component material balance.
o Calculate the minimum MSA flow rate to achieve a specified recovery of a key component in a single-section,
countercurrent cascade.
o Determine graphically, by stepping off stages, or algebraically, the required number of equilibrium stages in a
countercurrent cascade to achieve a specified recovery of a key component, given an MSA flow rate greater than
the minimum value.
o Define the overall stage efficiency and explain why efficiency values are relatively low for absorbers and at a
moderate level for strippers.
o Make preliminary estimates of overall stage efficiency of absorbers and strippers.
o Explain why multiple liquid-flow passes are necessary in trayed columns of moderate to large column diameter.
o Define Murphree point and tray vapor efficiencies and their relationship to overall stage efficiency.
o Explain how experimental stage-efficiency data from a small laboratory Oldershaw column can be scaled up to
a large-diameter column.
o Explain two mechanisms by which a trayed column can flood.
o Enumerate the contributions to pressure drop in a trayed column.
o Estimate column diameter and tray pressure drop for a trayed column.
o Estimate tray efficiency from correlations of mass-transfer coefficients using two-film theory.
o Estimate weeping, entrainment, and downcomer backup in a trayed column.

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