Hypothetically Speaking: Why You Should not Use Normal Alkanes to Replace Pseudocomponents

Luke Addington

May 20, 2015

Liquid hydrocarbons in the oil and gas industry often contain a long list of constituent components. These may include alkanes, olefins, aromatics, naphthalenes, branched and unbranched, and may contain any number of carbon atoms. While systems with lower molecular weight components, such as lighter LPGs, can be represented using simple, straight chained alkanes, more complex systems such as crude oils, condensates, and heavier LPGs require something more.

Many labs, in performing GC compositional analyses, will report compositions starting with methane and proceed to components higher in molecular weight. Normal alkanes will be used until a point where branched and non-alkane molecules begin to occur in sizable quantities, typically around C6.

While it may be tempting to use simple alkanes to represent these other molecules, for accurate results, the user must input additional information in the form of Single Oils in ProMax®.

Single Oils, also known as pseudocomponents or hypothetical components, are individual components in a composition list that represent a mixture of several different molecules. The properties of this pseudocomponent, and hence the properties of the mixture of different actual molecules, are calculated by correlation, typically based on the pseudocomponent’s molecular weight and gravity. This allows ProMax to account for the effects of non-straight chain, non-alkane molecules in a particular boiling range, even if the exact make-up of these molecules is unknown.

In the lab analyses, these pseudocomponents will typically appear in two different forms. One form will be C6+ or C10+, for example, and will be found at the end of the composition list. In this case, the list of pure components is truncated and any molecule heavier than that point is represented by the pseudocomponent. Another form will be pseudocomponents in the middle of the composition list. These pseudocomponents will typically be described as Other C6, Other C10, etc., and will not necessarily be the heaviest component in the list. If these Other pseudocomponents are used, a Plus pseudocomponent will likely be used as well, truncating the entire component list.

These Other or Plus components are not normal alkanes, and to accurately portray them in ProMax® requires additional information.

The minimum amount of information to represent a mixture of molecules with a pseudocomponent is a molecular weight and gravity. For the Plus components, these properties are typically included in the report. For the Other components, properties are not typically given in the report and must be sought from the specific lab that performed the analysis.

If all of the molecules for a given carbon number are lumped into the normal alkane category, as opposed to using a pseudocomponent, the composition of the mixture in the model will not accurately reflect the reality of the system. A component C6+ can be representing molecules with an enormous range of carbon number, all of which happen to be equal to or greater than six. It could be that half of the components represented by this hypothetical component in fact consist of carbon numbers greater than 20. Alternatively, it could be that none of the molecules exceed a carbon number of 15. Whether this component is lighter or heavier will have a direct impact on the predicted (and actual) vapor-liquid equilibrium.

Likewise, if the composition is input on a molar basis, the molecular weight of this component will have a direct impact on the mass composition. As air reporting itself is done on a mass basis, having the correct mass composition is crucial for good results.

As with anything simulated, the better the inputs, the better the results, and the more information that can be provided, the better. A thorough understanding of the system and the methods being used to calculate emissions are also important. BR&E’s staff of engineers will be glad to provide further assistance in determining the best approach in handling these heavier components and producing the most accurate results possible. For questions or help, please contact us at 979-776-5220 or Contact Technical Support..