Page 207 - Petroleum and Gas Field Processing

7.2 STABILIZATION OPERATIONS

As was presented in Chapter 3, the traditional process for separating the
crude oil–gas mixture to recover oil consists of a series of flash vessels
[gas–oil separation plant (GOSP)] operating over a pressure range from
roughly wellhead pressure to nearly atmospheric pressure. The crude oil
discharged from the last stage in a GOSP or the desalter has a vapor
pressure equal to the total pressure in the last stage. Usually, operation of
this system could lead to a crude product with a RVP in the range of 4 to
12 psia. Most of the partial pressure of a crude comes from the low-boiling
compounds, which might be present only in small quantities—in particular
hydrogen sulfide and low-molecular-weight hydrocarbons such as methane
and ethane.
Now, stabilization is directed to remove these low-boiling
compounds without losing the more valuable components. This is
particularly true for hydrocarbons lost due to vent losses during storage.
In addition, high vapor pressure exerted by low-boiling-point hydro-
carbons imposes a safety hazard. Gases evolved from an unstable crude
are heavier than air and difficult to disperse with a greater risk of
explosion.
The stabilization mechanism is based on removing the more volatile
components by (a) flashing using stage separation and (b) stripping
operations.
As stated earlier, the two major specifications set for stabilized oil
are as follows:
The Reid vapor pressure (RVP)
Hydrogen sulfide content
Based on these specifications, different cases are encountered:
Case 1: Sweet oil (no hydrogen sulfide); no stabilization is
needed. For this case and assuming that there is a gasoline
plant existing in the facilities (i.e., a plant designed to recover
pentane plus), stabilization could be eliminated, allowing the
stock tank vapors to be collected [via the vapor recovery
unit (VRU)] and sent directly to the gasoline plant, as shown in
Figure 2.
Case 2: Sour crude; stabilization is a must. For this case, it is
assumed that the field facilities do not include a gasoline plant.
Stabilization of the crude oil could be carried out using one of the
approaches outlined in Figure 3. Basically, either flashing or
stripping stabilization is used.

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