Quantify Non-Equilibrium Flow and Rapid Phase Transitions (RPT)

Although non-equilibrium flow and rapid phase transitions (RPT) are well researched, the literature published so far does not explicitly quantify the RPT phenomenon or provide reliable methods for the calculation of non-equilibrium flow for mixtures. The objective of this paper is to provide a clear understanding of how non-equilibrium flow and rapid phase transitions develop and how they should be quantified for pure components and mixtures alike.

We present a thermodynamic treatment of non-equilibrium flow and rapid phase transitions for pure components and mixtures. We discuss the estimation of hazard potential based on the superheat limit. ioMosaic’s Process Safety Office® SuperChems™ software is used to model multi-component nonequilibrium flow and LNG spills and to illustrate how mixtures composition influences the development of rapid phase transitions, overpressure generation, and non-equilibrium flow rates.

Figure 1: Flashing inception following a rapid pressure drop for water

Rapid depressuring of a vessel containing saturated liquid can lead to non-equilibrium flow followed by explosive boiling of the liquid contents. Depressuring can be attributed to flow and/or expansion. The same phenomenon, i.e. explosive boiling of liquids, can be induced by rapid heating of the liquid and is sometimes referred to as a rapid phase transition.

As shown to the right for vessel depressuring, the pressure can drop below the saturation point following rapid depressuring. The rate of pressure drop, influences this pressure undershoot which in turn influences the superheat available for bubble nucleation and growth. A large depressuring rate can lead to a large undershoot and thus a large bubble nucleation and growth superheat. The pressure will recover when the pressure rise caused by bubble generation is equal to the rate of imposed pressure drop at flashing inception. If the rate of pressure drop is large enough, a metastable liquid can form upon depressuring.

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