Methodology for modeling processes in multi-component two-phase LNG systems

Abstract

Liquefied natural gas (LNG) is the most economical means of transporting natural gas over long distances. In maximum volumes, LNG is transported by ships to designated unloading points at storage terminals. During unloading and storage of LNG, a certain part of it passes into the gaseous phase. The main factors influencing the evaporation process are the transfer of heat from the surrounding air. Evaporation of a part of the multicomponent liquid, in turn, leads to a change in the composition of the stored LNG and, as a result, affects its thermodynamic properties and cost indicators. Intensification of evaporation processes can potentially jeopardize the stability and safety of LNG storage procedures and other technological processes. At the same time, depending on the mole concentration of individual components, the basic thermodynamic parameters of LNG can change significantly. One of the most important issues when analyzing the technological process of storing liquefied natural gas as a multicomponent liquid is the implementation of a convenient calculation system based on known relationships and methodologies with performing thermodynamic calculations of the properties of mixtures, determining phase equilibrium, taking into account the thermal balance of the system with the environment, etc. This approach makes it possible to increase the ability to predict changes in individual parameters, to ensure the thermodynamic stability of the system taking into account external influences. It becomes possible to comprehensively study complex internal dynamic processes of changes in temperature, pressure, mole fraction of vapor, component composition of phases in a tank, which is usually carried out without taking into account external technological processes of equipment operation. The article presents the results of the development and application of a methodology for the process of drainage-free storage of a binary mixture consisting of pure methane and a relatively small amount of nitrogen (up to 10%). As a result, the influence of nitrogen on the dynamics of changes in the parameters of a two-phase system — boil-off gases and liquefied product (methane) has been demonstrated.