Pipe reaction forces caused by column separation transients are design concerns in LNG (liquefied natural gas) loading and off-loading systems. Being a mixture of hydrocarbons and other trace substances, LNG exhibits a “softening effect” unseen in water systems. This study demonstrates that, by coupling the vapor-liquid phase equilibrium thermodynamics with pipeline hydraulics, the softening effect can be modeled to obtain realistic pipe reaction forces.

The LNG is regarded as a binary mixture of methane and nitrogen. Its vapor-liquid phase equilibrium at a constant temperature is assumed. The phase equilibrium is described by the Raoult’s rule. When local pressure reaches the bubble point of the mixture, vaporization takes place. The vaporization is selective in that the mole fraction of nitrogen in the vapor is much higher than that in the liquid. The subsequent condensation is also selective as methane is more readily condensed than nitrogen. Over time, the mole fraction of nitrogen in the vapor increases. Corresponding to the higher nitrogen mole fraction in the vapor is a higher dew point pressure. This build up of dew point pressure is responsible for the observed softening effect characteristic of LNG column separation transients.

Simulation results of an example problem are discussed and compared with those obtained from the discrete vapor cavity model and the discrete free-gas cavity model used by the industry.