Page 112 - Petroleum and Gas Field Processing

Figure 20 Conditions affecting the determination of (P 2 ) o .P 1 is fixed in relation to
streams at points (a) and (b); P 3 is fixed in relation to stream at point (c), and P 2 is
determined by optimization.

Figure 20 illustrates a three-stage GOSP in which the pressure in the first
and the third stages are fixed for the above reasons.
The optimum pressure of operating a GOSP is defined as the second-
stage pressure that provides the desired gas–oil separation with maximum
oil recovery in the stock tank and minimum gas–oil ratio (GOR).
If R designates the recovery of oil and is defined as

R ¼ O=G ðbbl of oil/SCF of Gas) ð26Þ
then the optimum pressure in the second-stage separator is the value that
makes R maximum or 1/R minimum.
Apart from obtaining a high recovery of oil, operating pressures
have other important considerations in the processing of the separated
streams. A minimum pressure has to be maintained in order for the oil to
be delivered to the next processing stage. In addition, using high pressure
will deliver the gas stream for sales at higher output pressure, thus
reducing the compressor horsepower used for gas pumping.
In general, pressure of 50–100 psia is considered an optimum value of
the second-stage operation, whereas a minimum pressure for the third
stage will be in the range of 25–50 psig [4].

3.7.3 Determination of the Optimum Second-Stage
Operating Pressure
Four methods can be used to determine the optimum operating pressure
for the second stage.

Experimental Measurements
In this method, experimental runs are carried out in which the composition
of the gas leaving the separator is analyzed and the content of some key
þ
component (e.g., C ) is determined. Now, while increasing the pressure in
5
the second stage, we should calculate the ratio of gas to oil (G/O) for both
second and third stages. A graphical plot for P 2 versus G/O is given in

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