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2. For separation using MSA, a control in the composition or the
concentration of the hydrocarbons to be recovered (NGL); y and
x is obtained by using adsorption or absorption methods.
Adsorption provides a new surface area, through the solid material,
which entrains or ‘‘adsorbs’’ the components to be recovered and
separated as NGL. Thus, the components desired as liquid are deposited
on the surface of the selected solid, then regenerated off in a high
concentration; hence, their condensation efficiency is enhanced. About
10–15% of the feed is recovered as liquid.
Adsorption is defined as a concentration (or composition) control
process that precedes condensation. Therefore, refrigeration methods, may
be coupled with adsorption to bring in condensation and liquid recovery.
Absorption, on the other hand, presents a similar function of providing a
surface or ‘‘contact’’ area of liquid–gas interface.
The efficiency of condensation, hence NGL recovery, is a function of
P, T, gas and oil flow rates, and contact time. Again, absorption could be
coupled with refrigeration to enhance condensation.
To summarize the above, a proper design of a system implies the use
of the optimum levels of all operating factors plus the availability of
sufficient area of contact for mass and heat transfer between phases.

13.2.3 Selected Separation Processes

In this section a brief description is given for the absorption, refrigeration,
and cryogenic (Joule–Thomson turbo expansion) processes recommended
to separate NGL constituents from a gas stream. Details are illustrated in
the corresponding flow diagrams. Adsorption process, on the other hand,
has been covered in Chapter 12.

Absorption Process
The absorption unit consists of two sections: the absorption and
regeneration as illustrated in Figure 1. An upflow natural gas stream is
brought in direct contact, countercurrently with the solvent (light oil in
the kerosene boiling range) in the absorber. The column—a tray or
packed one—operates at about 400–1000 psia and ambient or moderately
subambient temperatures. The rich oil (absorbed NGL plus solvent) is
directed to a distillation unit to separate and recover the NGL, whereas
the lean oil is recycled back to the absorber. In addition to natural
gasoline, C 3 /C 4 could be recovered as well in this process. Provision is
made to separate ethane from rich oil using a deethanizer column.

Copyright 2003 by Marcel Dekker, Inc. All Rights Reserved.