Research Projects

Project at a Glance

Project Status: complete

Title: Evaluation of a method for determining vapor pressures of petroleum mixtures by headspace gas chromatography

Principal Investigator / Author(s): Woodrow, James E

Contractor: Department of Environmental Toxicology, UC Davis

Contract Number: A6-178-32


Research Program Area: Emissions Monitoring & Control

Topic Areas: Stationary Sources


Abstract:

Headspace gas chromatography (HS-GC) was used to measure the vapor pressures of Chevron light, Texaco medium, and Santa Fe heavy crude oil samples at various temperatures in the range 43oC - 97oC. The method was validated with single n-alkane hydrocarbons and their binary, ternary, and quaternary mixtures. The microprocessor controlled HS-GC instrument automatically thermostated the crude oil samples contained in septum-sealed glass vials to equilibrate the vapor and liquid for 60 minutes, pressurized the vials to a pre-set pressure (15-30 psig) by inserting a hollow needle through the septum, and sampled the vapor for 0.01 min by allowing the pressure in the vial to drive an aliquot of the vapor through the needle and onto the analytical column. Using this technique, vapor pressures for the crudes were determined by simulating the composition of the mixtures with a series of n-alkane hydrocarbon standards whose GS retention times spanned the GC envelopes of the mixtures. Only for the Santa Fe heavy crude was it possible to determine vapor pressures at 43oC (0.73 ±0.18 torr), 72oC (1.84 ±0.14 torr), and 97oC (4.64±0.63 torr). For Texaco medium and Chevron light crudes, vapor pressure determinations were made at 43oC (40.88±2.23 torr) and 72oC (99.79±12.29 torr) and at 43oC (134.39±13.03 torr) only, respectively. Determinations at 72oC (light crude) and 97oC (light and medium crudes) were not made because of the inability of the instrument to handle the greater vapor densities and / or possible unsafe pressures at the higher temperatures. The HS-GC method measures only the volatile organic constituentsof hydrocarbon mixtures, and is, therefore, preferred over methods that rely on measurement of total pressure within a sealed container,since the latter may also include contributions from dissolved air and water.


 

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