Prediction of natural gas behaviour in loading and unloading operations of marine CNG transportation systems

The optimal design conditions to safely and efficiently load, store, and unload CNG vessels are highly dependant on the state of the gas and other natural gas constituents. Process simulation tools can be employed to predict the behaviour of the gas during loading and unloading operations. However, the existing thermodynamic models (such as equations of state) lack the capability to accurately predict the behaviour of the gas at above critical or cricondenbar conditions. In order to perform an accurate dynamic simulation, the tuning of existing thermodynamic models is necessary. Using the tuned thermodynamic model, dynamic simulations of loading and unloading of CNG operations can then be performed. This will allow conducting complete studies of loading and unloading of marine CNG transportation for diverse gas quality conditions and potentially extending the niche application of this technology. In this paper, an analysis of existing thermodynamic models is conducted to determine under which conditions major deviations take place. The performance of thermodynamic models in commercial process simulation software packages in predicting the transient behaviour of high-pressure natural gas is investigated. The inefficiency of models and challenges involved in accurate simulations of CNG transportation processes are also discussed.