Dranchuk correlation

Dranchuk gas compressibility factor correlation

Dranchuk correlation is the fitting equation of the classic Standing and Katz gas compressibility factor correlation.

Math & Physics

$z = 1-z+ \left(A_1 +\frac{A_2}{T_{pr}} +\frac{A_3}{T^3_{pr}} +\frac{A_4}{T^4_{pr}} +\frac{A_5}{T^5_{pr}} \right)\ \rho_r+ \left(A_6 +\frac{A_7}{T_{pr}} +\frac{A_8}{T^2_{pr}} \right)\ \rho^2_r -A_9\ \left(\frac{A_7}{T_{pr}}+\frac{A_8}{T^2_{pr}}\right) \rho^5_r +A_{10}\ \left(1+A_{11}\ \rho^2_r\right)\ \frac{\rho^2_r}{T^3_{pr}} \ e^{(-A_{11}\ \rho^2_r)}$

where:

$\rho_r = \frac{0.27\ P_{pr}}{{z\ T_{pr}}}$

$P_{pr} = \frac{P}{P_{pc}}$

$T_{pr} = \frac{T}{T_{pc}}$

A1 = 0.3265
A2 = –1.0700
A3 = –0.5339
A4 = 0.01569
A5 = –0.05165
A6 = 0.5475
A7 = –0.7361
A8 = 0.1844
A9 = 0.1056
A10 = 0.6134
A11 = 0.7210

Discussion

Why the Dranchuk correlation?

It's classics!

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Workflow

To solve the Dranchuk equation use the iterative secant method.

To find the pseudo critical properties from the gas specific gravity :

$P_{pc} = ( 4.6+0.1\ SG_g-0.258\ SG^2_g ) \times 10.1325 \times 14.7$

$T_{pc} = ( 99.3+180\ SG_g-6.94\ SG^2_g ) \times 1.8$

Application range

$0.2 \le P_{pr} < 30 ; 1.0 < T_{pr} \le 3.0$

and

$P_{pr} < 1.0 ; 0.7 < T_{pr} \le 1.0$

Nomenclature

$A_1..A_{11}$ = coefficients

$\rho_r$ = reduced density, dimensionless

$P$ = pressure, psia

$P_{pc}$ = pseudo critical pressure, psia

$P_{pr}$ = pseudoreduced pressure, dimensionless

$SG_g$ = gas specific gravity, dimensionless

$T$ = temperature, °R

$T_{pc}$ = pseudo critical temperature, °R

$T_{pr}$ = pseudoreduced temperature, dimensionless

$z$ = gas compressibility factor, dimensionless