Page 350 - Petroleum and Gas Field Processing

Cryogenic Processes
Natural gas liquid could be separated from natural gas using two
approaches based on cryogenic expansion (autorefrigeration):

An expander plant produces refrigeration to condense and recover
the liquid hydrocarbons contained in the natural gas by using a
turboexpander. In this process, the enthalpy of the natural gas is
converted into useful work, behaving thermodynamically as an
approximate isentropic process.
Expansion across the valve will lead to a similar result. However,
the expansion is described in this case as ‘‘isenthalpic.’’
Temperatures produced by turboexpansion are much lower than those of
valve expansion.
A schematic presentation for the turboexpansion process is presented

in Figure 3. The process operates at 100 Fto 160 F and 1000 psia. The
process represents a new development in the gas processing industry.
Increased liquid recovery (especially ethane) is an advantage of this process.
Figure 4 illustrates the condensation process using ethane/propane,
followed by demethanization to produce NGL as a final product. Figure 5,
on the other hand, presents a typical gas plant for the recovery and
separation of NGL.
A concise comparison among the three options (absorption,
refrigeration, and cryogenic separation processes) recommended for NGL
recovery is given by Abdel-Aal [1].

13.3 FRACTIONATION OF NGL

13.3.1 Goals and Tasks
In general, and in gas plants in particular, fractionating plants have
common operating goals:

1. The production of on-specification products
2. The control of impurities in valuable products (either top or
bottom)
3. The control in fuel consumption.

As far as the tasks for system design of a fractionating facility, these goals
are as follows:
1. Fundamental knowledge on the process or processes selected to
carry out the separation; in particular, distillation.

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