Page 111 - Petroleum and Gas Field Processing

At given conditions of T and P, thermodynamic equilibrium exists,
because the vapor and the liquid are in intimate contact. Consequently, the
partial pressure of a component in the vapor is equal to the partial vapor
pressure of the component in the liquid. Therefore, equating the two
equations and rearranging, the following relationship is obtained:
0
P i Y i
¼ ¼ K i ¼ Equilibrium constant ð25Þ
P t X i
0
v
where P i is the partial pressure of component i in the mixture, P i is the
vapor pressure of a pure component, i, X i is the mole fraction of
component i in the liquid phase, Y i is the mole fraction of component i in
the gas phase, and P t is the total pressure on the system (inside the
separator).
Now, it can be concluded that if the total pressure on the system is
increased, the mole fraction in the gas phase has to decrease in accordance
with Eq. (21). In other words, the tendency of vaporization diminishes as
the pressure inside the gas–oil separator increases.
In considering the above relationship based on Rault’s law, there are
some shortcomings in using it, especially for gas mixtures containing

methane (its critical temperature is 116 F). However, for mixture of C 3 ,
C 4 , and C 5 , Rault’s law could be applied fairly well at temperatures up to

150 F and pressures up to 100 psia.
3.7.2 Pressure Profile of a Three-Stage GOSP

In the determination of the optimum operating pressure for a GOSP
consisting of three stages (high-, intermediate-, and low-pressure separators)
it is the second-stage pressure that could be freely changed, hence optimized.
The pressure in the first stage (high pressure) is normally fixed under
one of the following conditions:
1. Matching certain requirements to supply high-pressure gas for
gas-injection facilities existing in the field
2. Selling the gas through pipelines
3. Flow conditions of the producing wells

Similarly, the pressure in the third stage (low pressure) is fixed for the
following cases:
1. The last stage is a storage tank.
2. An existing gas-gathering or vapor recovery facilities that utilizes
the gas.
3. The last stage operates at the relatively low pressure.

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