Test Method for "Actual" True Vapor Pressure of Crude Oils

Abstract

Currently, the vapor pressure of crude oil is primarily controlled through Reid Vapor Pressure testing (ASTM D-323-90) from both a commercial product and environmental air emissions standpoint. Environmental regulations do require a further estimation of the True crude oil vapor pressure from the Reid test results via a nomograph relationship contained in API 2517. The true vapor pressure of a given oil or fluid is of interest because it defines the point of vapor initiation (i.e., the boiling point or bubble point). The quantity of such oil vapors is of course directly related to product losses and environmental air emissions. However, the MITRE Corporation in support of the Strategic Petroleum Reserve Crude Oil Quality program has found the Reid test even in combination with the API 2517 adjustment for 'True vapor pressure to give 50% to even 300+% errors in the determination of a crude oil's actual true vapor pressure. MITRE therefore developed a test method and calculation algorithm that substantially improves the determination of a crude oil's actual true vapor pressure. The method involves use of a device to 1) analyze the composition of gas separating from a liquid oil stream at a known pressure and temperature, 2) measure the rate of gas and oil flow exiting the same gas/oil separator, and 3) use the described test data in an iterative calculation algorithm with industry-established gas/liquid equilibrium values to estimate the crude oil's vapor pressure within +/- 2% (or 0.3 psia). This test method provides a characterization of the oil's composition which allows prediction of vapor pressure and even air emissions quantification over the full temperature range of interest. In addition air toxics existing in the oil (H 2 S, benzene, etc.) have been quantified to the 10 ppm level in the oil as well as in the evolved gases.