Helium substitution of natural gas hydrocarbons in the analysis of their hydrate

Abstract

This communication successfully characterises the thermodynamic phase equilibrium conditions for a synthetic natural gas comprising of methane, ethane, propane, i-butane, n-butane, i-pentane, n-pentane and carbon dioxide. Experimental determination of the hydrate stabilization effects of the present hydrate formers was performed using a novel approach involving helium substitution of each hydrate guest species in an otherwise identical synthetic natural gas. Results in the ranges of 30–180 bar largely agreed with the van der Waals-Platteeuw (vdW-P) models in Aspen HYSYS and Calsep PVTsim with Peng-Robinson (PR) and Soave-Redlich-Kwong (SRK) equations of state. The substitution of propane and i-butane significantly shifted hydrate equilibrium conditions to lower temperatures at a given pressure and was confirmed with the Clausius-Clapeyron equation. A slope of -17372 K was calculated for the original gas mixture, a value 3000 and 2000 K steeper than an identical mixture with the substitution of propane and i-butane respectively. Dissociation enthalpies were also calculated; 100.7 (±0.4) kJ/mol for hydrates of the original gas and 87.8 (±0.4) and 94.9 (±0.8) kJ/mol for hydrates of the propane and i-butane substituted gas. Reliability and consistency of the presented results is complimented by a propagated relative uncertainty of ±1.28%. Computed data was used to calculate dissociation enthalpies and Clausius-Clapeyron slopes, both of which are consistent with experimental calculations when using PR and SRK equations. The helium substitution approach in the experimental determination of the extent of hydrate stabilization in a complex gas mixture provided by each possible hydrate former is a viable method. Results are particularly important to the general design of refinement processes, as they provide real implications on the changes in hydrate equilibrium conditions for a natural gas separation process.