The composition of oils and hydrocarbon condensates are a key piece of information for everyone in the energy value chain. At the well site, the composition allows producers to know what is coming out of the ground and understand how it may affect the valuable equipment it touches. At the refinery, the composition provides guidance to how the oil or hydrocarbon condensate needs to be utilized within the refinery’s process. Further, the composition confirms whether the products are within specifications set out in contracts, or if further modification of the refining process will be needed.
One of the most common compositional test methods utilized by the energy sector is GPA 2103. For this method, a pressurized hydrocarbon sample is analyzed via gas chromatography (GC) for lighter hydrocarbon composition. The components directly analyzed by GC methods are nitrogen, carbon dioxide, and hydrocarbons from methane through n-pentane, n-hexane, or n-nonane, dependent on client needs. The components that are not analyzed are grouped into plus fractions, with the reports commonly referenced based on this grouping, For the examples listed above, the reports would be known as C6+, C7+, and C10+, respectively.
Depending on the hydrocarbon liquid, the plus fraction components can represent 0-90% of the liquid volume. In instances when the plus fraction is a significant portion of the sample, another analysis must be completed on a stabilized or weathered fraction to understand the properties and composition of the plus fraction. This plus fraction must be analyzed via GC for any remaining light hydrocarbon components, analyzed for density, and analyzed for average molecular weight. There are two main methods that can be utilized to obtain a stabilized plus fraction: distillation or weathering. Both methods have their pros and cons, leading to the question: to distill or not to distill?
TO DISTILL – HYDROCARBON DISTILLATION
Distillation has the advantage of removing nearly all light hydrocarbons, making it easier to obtain a true plus fraction aliquot. The problem with distillation is that the temperature required can vary due to elevation differences between locations. Weathering of a sample, either by natural open container or accelerated via the application of heat and agitation, will almost always have a higher concentration of light hydrocarbon in the remaining liquid. When the GC analysis data on the plus fraction is combined with the measured density and molecular weight, the effects of the lighter hydrocarbon components must be calculated out to get a true plus fraction result. Distillation, with its lower concentration of remaining light molecules, has the advantage of the measured properties being closer to that of the true plus fraction. However, even under the best circumstances it is not advisable to assume that the distillation produces a true plus fraction, as this would yield errors in calculated properties. Additionally, errors in temperature control can affect calculated properties. If the temperature of the distillation is too hot, some of the lighter plus fraction molecules would not be captured, thus making the plus fraction calculate as heavier in composition than it really is.
NOT TO DISTILL – HYDROCARBON WEATHERING
Weathering of a hydrocarbon sample has the advantage of being easier and costs less to achieve. Distillation requires lab personnel to set up the distillation and be present during the process to ensure it is completed correctly. Lab personnel must be there to ensure correct temperature is maintained, that no violent boiling occurs, and that the distillation apparatus is turned off once the proper fraction is obtained. Weathering has no equipment to set up and only requires that the technician either performs the next analyses or seals the liquid container at an appropriate time to stop the weathering process. Since back calculation of the light components in the plus fraction is necessary no matter which method is used, weathering has the advantage of less time and equipment required. There is also the knowledge that with the weathering method it is implied that the second GC analysis must be performed every time, thus yielding more accurate results.
With either method, a representative analysis and repeatable results are possible with proper equipment calibration and personnel training. Both distillation and weathering methods have been utilized within SPL laboratories and it has been found that the weathering method tends to be both more accurate and better able to remove human error from the report. To learn more about SPL’s GPA 2103 services, contact firstname.lastname@example.org.
Curtiss Kovash Jr., Ph.D.