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Z_G_PR and Z_L_PR
Z_G_PR(A,B) and Z_L_PR(A,B) both return the compressibility, Z, of the fluid according to the Peng-Robinson equation of state. The compressibility is the ratio of the specific volume of the gas to the specific volume of an ideal gas at the same temperature and pressure. The values A and B can be for a pure fluid (found from Procedure AB_PR) or a mixture (from Procedure AB_MIX_PR)
Note the the equation for Z is cubic. In general, the solution of this equation can have one or three real roots. Only one root is exists if the fluid is a single phase. When two phases coexist, two of the three possible roots correspond to saturated liquid and vapor, respectively, while the third root is a numerical aberation and has no physical meaning. If there is only one root, both functions will return the same value, but if there are two phases, Z_L_PR will return the compressibility for the liquid phase (i.e., the smaller value) and Z_G_PR will return the compressibility for the vapor phase (i.e., the larger value).
Example:
Tr=0.85
Pr=0.5
omega=0.05
Call AB_PR(Tr, Pr, omega: A,B)
Z_L=Z_L_PR(A,B)
Z_G=Z_G_PR(A,B)
{Solution:
A=0.3391
B=0.04576
omega=0.05
Pr=0.5
Tr=0.85
Z_G=0.5748
Z_L=0.07656
}