Page 109 - Petroleum and Gas Field Processing

(b) To determine A g /A as a function of H g /D:
0 H g /D 0.25:
2 3

A g H g H g H g
¼ 0:21 þ 3:52 4:93
A D D D
0.25 H g /D 0.75:

A g H g
¼ 1:231 0:1138
A D
0.75 H g /D 1.0:

2 3
A g H g H g H g
1 ¼ 0:21 1 þ 3:52 1 4:93 1
A D D D

3.7 OPTIMUM PRESSURE FOR GAS–OIL SEPARATORS
3.7.1 Introduction

In order to study the effect of operating pressure in gas–oil separation in
general, we will consider first the case of a single-stage separation plus a
storage tank. Now, what is the effect of the operating pressure on the
recovery of stock tank-oil for the following two extreme cases?
1. High-pressure operation: This will diminish the opportunity
of light hydrocarbons in the feed to vaporize and separate.
However, once this liquid stream is directed to the storage tank
(normally operating at/or close to atmospheric pressure), violent
flashing occurs due to the high-pressure drop, with subsequent
severe losses of the heavier hydrocarbons into the gas phase.
2. Low-pressure operation: Here, large quantities of light hydro-
carbons will separate from the gas–oil separator, carrying along
with them heavier hydrocarbons, causing a loss in the recovered
oil. Upon directing this liquid stream to the storage tank, it
suffers very little loss in heavy components, because the bulk of
light gases were separated in the separator.
From the above, it may be concluded that a proper operating
pressure has to be selected and its value has to be between the two extreme
cases as described in order to maximize the oil yield. This conclusion is
illustrated in Figure 19, in which an optimum pressure of 45 psig is selected
to give 0.75 bbl of oil yield.

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