Page 16 - Gas Purification 5E

6 GasPwi~cation

Introduction
Absorption, as applied to gas purification processes, can be divided into the following
general classifications based on the nature of the interaction between absorbate and
absorbent:

1. Physical Solution. In this type of process the component being absorbed is more soluble in
the liquid absorbent than other components of the gas stream, but does not react chemical-
ly with the absorbent. The equilibrium concentration of the absorbate in the liquid phase is
strongly dependent on the partial pressure in the gas phase. An example is the absorption
of hydrogen sulfide and carbon dioxide in the dimethyl ether of polyethylene glycol
(SelexoI Process). Relatively simple analytical techniques have been developed for
designing systems of this type.
2. Reversible Reaction. This type of absorption involves a chemical reaction between the
gaseous component being absorbed and a component of the liquid phase to form a loosely
bonded reaction product. The product compound exhibits a finite vapor pressure of the
absorbate which increases with temperature. An example is the absorption of carbon diox-
ide into monoethanolamine solution. Analysis of this type of system is complicated by the
nonlinear shape of the equilibrium curve and the effect of reaction rate on the absorption
coefficient.
3. Irreversible Reaction. In this type of absorption the component being absorbed nxcts with
a component of the liquid phase to form reaction products that can not readily be decom-
posed to release the absorbate. An example is the absorpaon of hydrogen sulfide in iron
chelate solution to form a slurry of elemental sulfur particles. The analysis of systems
involving irreversible reactions is simplified by the absence of an equilibrium vapor pres-
sure of adsorbate over the solution, but becomes more complex if the irreversible reaction
is not instantaneous or involves several steps.
Contactor Selection

The primary hction of the gas absa?ption contactor is to pvide an extensive am of liquid
surface in contact with the gas phase under conditions favaing mass transfer. Contactom nor-
mally employ at least one of the following mechanisms: (1) dividing the gas into small bubbles
in a continuous liquid phase (e.g., bubble cap trays), (2) spreading the liquid into thin films that
flow through a continuous gas phase (e.g., packed columns), and (3) forming the liquid into
small drops in a continuous gas phase (e.g., spray chambers). All three types of contact are
employed in gas purification absorbers. They are interchangeable to a considerable extent,
although specific requirements and conditions may favor one over the others.
Countercurrent contactors can also be categorized as staged columns, which utilize separate
gas and liquid flow paths in individual contact stages; differential columns, which utilize a con-
tinuous contact zone with countercurrent flow of gas and liquid in the zone; and pseudo-equi-
librium columns, which combine essentially mtercurrent flow of gas and liquid streams with
discrete stages. A simplified guide to the selection of gas-liquid contactors based on this cats
goriZation is presented in Table 1-3 which is derived from the work of Frank (1977).
Table 1-3 is generally applicable for stripping columns as well as absorbers, although
additional parameters may need to be considered. Bravo (1994) points out that biological or