Search

The formation and deposition of solids in plant equipment is a perennial risk to the cryogenic processing of natural gas. While several tools are available to predict the temperatures at which heavy hydrocarbon solids (HHC) will form, the accuracy of the gas mixture's compositional characterization can significantly impact the reliability of those predictions. A complete characterization of the mixture is the most desirable scenario but is challenging and expensive to obtain. More typically, C6 hydrocarbons and heavier compounds are lumped into pseudocomponents based on their boiling point to represent the HHC composition of the mixture. Recently, Miethe et al. (Hydrocarb Process, 2015) detailed a new approach based on splitting each pseudocomponent further according to its paraffinic, isoparaffinic, naphthenic and aromatic (PINA) composition. An associated defined component is used to represent each of these sub-fractions to improve the melting temperature prediction accuracy. However, this "Lump + PINAAPI" approach has not been validated for mixtures that contain HHCs beyond C10. This work compares freeze-out predictions based on a complete compositional characterization of a gas mixture with HHCs up to C14 with results obtained using (1) the new Lump + PINAAPI approach and (2) several other freeze-out prediction methods described in the literature. For two gas samples, the fully characterized mixtures were predicted to have melting temperatures of 263.2 K (14.1 °F) and 260.1 K (8.5 °F), respectively. At the same time, the Lump + PINAAPI predictions were 153.4 K (− 183.6 °F) and 157.4 K (− 176.3 °F), respectively. The large discrepancy between melting temperature predictions highlights the need to either (1) obtain full characterization of natural gas mixture compositions where possible, and/or (2) to develop an improved set of correlations pseudocomponent correlations for predicting freeze-out in natural gas mixtures. [ABSTRACT FROM AUTHOR]

The formation and the blockage of plant equipment such as heat exchangers by heavy hydrocarbon (HHC) solids is an inherent risk in cryogenic natural gas processing. The accuracy of the gas mixture's compositional characterization significantly impacts the reliability of solid formaiton temperature predictions. Recently, we showed that complete characterization of the mixture is necessary to obtain accurate predictions of the melting temperature, as current methods based on pseudocomponent characterizations of HHCs are inadequate. Here, we present an improved method of characterizing HHCs that represents each pseudocomponent up to C14+ by a paraffinic, isoparaffinic, naphthenic and aromatic (PINA) composition and allocates an associated defined component to represent these sub-fractions. This new, extended PINA-based characterization of HHC pseudocomponents is derived from 46 different pipeline natural gas samples, and the method is validated against three representative gas samples that were fully characterized. The melting temperatures of the three gas samples based on their full characterizations are 263.2 K (14.1 °F), 260.1 K (8.5 °F) and 248.3 K (−12.8 °F), respectively. Predictions made with the new method match these within (1 to 2) K, while previous correlation methods under-predict them by (10 to 20) K. The improved performance arises from (1) the selection of suitable discrete components to represent each PINA fraction within a pseudocomponent, (2) the more representative distribution of PINA fractions as a function of carbon number, and (3) the use of discrete components to represent the pseudocomponent's thermodynamic properties in both the fluid and solid phases. These results show how the new characterization method can reliably predict HHC freeze-out conditions, particularly when a full compositional analysis is unavailable. Future research should aim to test the new method on natural gas samples from regions other than the US Gulf Coast. [ABSTRACT FROM AUTHOR]