Page 78 - Petroleum and Gas Field Processing

characterized by the fact that light gases are separated as soon as they are
liberated (due to reduction in pressure). In other words, light components
do not come into contact with heavier hydrocarbons; instead, they find
their way out.
For flash separation, on the other hand, gases liberated from the oil
are kept in intimate contact with the liquid phase. As a result,
thermodynamic equilibrium is established between the two phases and
separation takes place at the required pressure.
Comparing the two methods, one finds that in differential
separation, the yield of heavy hydrocarbons (intermediate and heavy
groups) is maximized and oil-volume shrinkage experienced by crude oil in
the storage tank is minimized. This could be explained by the fact that
separation of most of the light gases take place at the earlier high-pressure
stages; hence, the opportunity of loosing heavy components with the light
gases in low-pressure stages is greatly minimized. As a result, it may be
concluded that flash separation is inferior to differential separation
because the former experiences greater losses of heavy hydrocarbons that
are carried away with the light gases due to equilibrium conditions.
Nevertheless, commercial separation based on the differential concept is
very costly and is not a practical approach because of the many stages
required. This would rule out differential separation, leaving the flash
process as the only viable scheme to affect gas–oil separation using a small
number of stages. As illustrated in Table 1 [1], a close approach to
differential separation is reached by using four to five flash separation
stages. A comparison between the mechanisms of separation by the two
methods is schematically simplified and presented in Table 2.

3.4 METHODS OF SEPARATION
The traditional process for recovering crude oil from high-pressure well
streams is based on the flash separation concept explained in Section 3.3,

Table 1 Flash and Differential Separation
Percent approach
No. of flash stages to differential
2 0
3 75
4 90
5 96
6 98.5

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