Associated Gas Flashing Calculations From Gas / Oil Mixtures - Vasquez-Beggs Method

Flashing calculations from crude oil -associated gas are mostly done using a simulation software such as HYSYS or UNISIM or any other. By flashing it is meant that a crude oil - associated gas mixture is reduced from a higher pressure to a lower pressure. An example of flashing would be to transfer an oil-gas mixture from an inlet separator to a storage tank where the pressure of the oil-gas mixture is reduced from the operating pressure of the separator to the pressure of the storage tank which is most likely operating at atmospheric pressure or a few kPa above atmospheric pressure.

What if a process engineer does not have access to a simulation software and is still desirous to find out the quantity of flashed gas from a crude oil - associated gas mixture. Today's blog entry deals with the Vasquez-Beggs flashing calculation method from crude oil-associated gas mixture which can be configured on a simple excel spreadsheet. The Vasquez-Beggs correlation is fairly accurate and can be used by process engineers to estimate flashing losses from crude oil storage tanks. Let us get on to the equations:

R_s = C1*γ_g*P^C2*EXP((C3/γ_o*T) - (C4/T))

where:
R_s = solution gas-to-oil ratio (m³/m³) at standard conditions of 101.325 kPa (abs) and 15°C
γ_g = specific gravity of solution gas with respect to air, dimensionless = MW_g / 28.96
MW_g = molecular weight of associated gas, kg / kgmole
T = Temperature at which the flashing needs to be evaluated, K
P = absolute pressure in the vessel whose flashing needs to be evaluated, kPa(abs)
γ_o = oil specific gravity with respect to water, dimensionless = 141.5 / (131.5 + deg API)
deg API = means of classifying petroleum oils and normally range from 45.4 deg API (light crude oils) to 10 deg API (heavy asphaltic crude oils)

Values of C1, C2, C3 and C4:

γ_o < 0.876

C1 = 3.204E-04
C2 = 1.1870
C3 = 1881.24
C4 = 1748.29

γ_o ≥ 0.876

C1 = 7.803E-04
C2 = 1.0937
C3 = 2022.19
C4 = 1879.28

The most likely application for the Vasquez-Beggs correlation would be to estimate flashing losses when transferring crude oil-gas mixtures from separators / treaters at higher pressure to atmospheric pressure oil storage tanks. The value of "R_s" as calculated from the above equation is the flashed gas from the crude oil-gas mixture.

Example Calculation:
An oil train consisting consisting of an inlet separator and oil storage tank receives oil @200 m³ per day. The oil is transferred from the oil storage tank and flashes in the storage tank . The conditions in the separator and the storage tank are as follows:
Separator
T = 40 deg C
P₁ = 350 kPag
Storage Tank
P₂ = 0 kPag

ΔP = P₂- P₁ = 350 - 0 = 350 kPa = P (for the example calculation)
Inlet Oil gravity = 45.4 deg API
Solution gas molecular weight = 22.46 kg / kgmole

Calculate the flashing loss F_vap from the oil storage tank

Solution:
R_s = 1.787 m³ / m³
F_vap = 357.5 m³ / day at standard conditions of 101.325 kPa (abs) and 15°C

Hope the readers of my blog like this blog entry, specially the engineers working in upstream oil and gas engineering. Looking forward to some comments on this blog entry.

Regards,
Ankur.