Page 254 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

CHAPTER

10
Absorption and stripping

10.1 Introduction
Absorption and stripping involve gas and liquid streams which
transfer components from gas feedtoa liquidstreamincaseof
Absorption and Stripping absorbers and from liquid feed to a gas stream in strippers. Such
contacting in a flow system can be countercurrent or cocurrent.
Countercurrent contacting has the well-discussed advantage of uti-
lizing a higher average driving force for mass transfer that results in
more compact equipment in most cases of absorption and stripping. Cocurrent flow design is rarely
useful in absence of reactions or when only about one stage of contacting suffices due to high
solubility of the component being transferred.
Nevertheless unlike countercurrent flow, the capacity of the cocurrent contactor is not limited by
flooding. This allows processing at high throughput as in venturi scrubbers. Cocurrent downward gas-
liquid flow through packed bed is used for catalytic chemical reactions in trickle bed reactors. It is also
used for situations where
(i) a rapid, irreversible chemical reaction accompanies the mass transfer process, e.g., absorption of
hydrogen sulfide into aqueous sodium hydroxide,
(ii) an exceptionally tall tower is built in two sections, with the second section operated in cocurrent
flow to save on the large diameter gas pipe connecting the two.
This chapter deals with the design of continuous absorbers and strippers where the desired function
is achieved through countercurrent contacting of gas and liquid in staged contactors (tray columns) and
continuous contactors (packed towers).
Industrially, the more economical option between a tray column and a packed column is chosen
for the specific design task. The maximum attainable separation is governed by the equilibrium
concentration between the phases that are affected by the process conditionsdprimarily the
operating temperature and pressure, the number of stages of contacting, and the liquid to gas flow
rate ratio.
Inputs to the process design in case of absorber/stripper are:

• Inlet flow rates of gas and liquid
• Composition of the inlet gas and liquid in case of absorption and stripping respectively
• Minimum target of recovery of the component(s)

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