A new approach of splitting C6+ composition of pipeline gas for hydrocarbon dew point determination

Abstract

Hydrocarbon dew point (HCDP) is a critical consideration for pipeline operations. Equation of state (EOS) method combined with Gas Chromatograph analysis is one of the methods used for HCDP determination. Most of the GCs on pipeline gas and in end-user installations are generally C6+ design and a few are C9+ design. In applying a HCDP limit using C6+ data, it is prudent to use an appropriate split of the C6+ composition. Though several fixed ratios of C6+ composition splits have been published in a Gas Processors Association standard and by leading chromatograph manufacturers for application, but may not truly reflect the C6+ composition of a particular gas. This work therefore presents a very simple procedure for splitting C6+ component into C6/C7/C8 for any particular pipeline gas provided that the specific gravity of the C6+ data is known. The method involves; determination of the molecular weights of C6/C7/C8 using three well-established hydrocarbon physical properties correlations, solution of the algebraic equations of the apparent molecular weight of C6+ using matrix notation, and application of logarithmic distribution to the calculated “quasi-mole fraction” of the individual C6+ components. An application of this approach to a field C6+ data is presented. In order to show the capabilities of the new approach, results comparison of calculated HCDP as well as cricondentherm (using HYSYS® with Peng Robinson EOS) between the new approach and some common C6+ composition splits is made.