Mixtures rollover

Mode 1 rollover with LIN/LOX mixture. Initial density difference 19 kg/m (2.5 %). High heat flux of 67 W/m ... 

After these basic chapters, the monograph develops an understanding of rollover, and how this phenomenon is a common property of all cryogenic liquid mixtures in constant pressure tanks with boil-off at 1 bar. Stratification is the prerequisite of rollover, and options for removing stratification by effective mixing are discussed. The sequence of a rollover event is then discussed, to provide a basis for cargo management decisions. 

Scurlock, R. Review of rollover and other mixing phenomena in LNG and other cryogenic liquid mixtures. In Proc. Gastech 2011, Amsterdam (2011)... 

Rollover, (so called), with high surface evaporation rates, as a possible result of spontaneous mixing following stratification into two or more layers—a phenomenon that may be met with LNGs, LPGs and other cryogenic liquid mixtures. 

To understand rollover, the reader should be aware that most cryogenic liquids are multi-component mixtures and have more complicated evaporation behaviours than a single component liquid. Examples of cryogenic liquid mixtures include ... 

It is important, therefore, when storing and handling cryogenic hydrocarbon liquid mixtures, to understand the different evaporation behaviour of cryogenic liquid mixtures compared with that of pure liquids, and how this can affect stratification and the build-up towards rollover. 

Since double diffusive convection is not unique to LNG or any other cryogenic liquid mixture, experimental rollover simulations have been studied with LA/LIN, LOX/LIN, and liquid Freon mixtures, as well as LNG mixtures. 

Figure 5.6 shows a sequence of flow visualisation photographs, of an experimental rollover between stratified layers of Freon 11/113 mixtures. These illustrate the downward migration of the interface over a period of 172 min followed by its rapid disappearance after total mixing by penetrative convective oscillations after 182 min [7]. 

The rollover studies at Southampton included the use of binary mixtures of nitrogen and oxygen in two stratified layers which enabled the measurement of vertical temperature profiles, vertical composition profiles and evaporation rates, all as a function of time up to and after flie inevitable roUover. 

The studies concluded that all binary cryogenic liquid mixtures in two stratified layers, at 1 bar pressure, will spontaneously mix or rollover with an uncontrolled increase in BOR, including LNG, LPG, LIN/LOX, LIN/LA and Freons. However, the studies were unable to pinpoint any measurable factors which might help to distinguish whether the inevitable roUover would be Mode 1 or Mode 2. 

Fig. 5.8 Mode 2 rollover with LIN/LOX mixture. Initial density 8.7 kg/ttf (1.0 %). Low heat fluxes of 3.6 and 4.2 W/m into upper and lower layers respectively...

Agbabi, T. Rollover and interfacial studies of LNG mixtures. PhD thesis, Southampton University (1987)... 

Summary of Properties of LNG and LPG Mixtures, Relating to Stratification and Rollover... 

While stratification in a single component liquid may lead to QHN boiling or vapour explosion, the consequences of density stratification at 1 bar are more serious in multi-component liquid mixtures. The density varies wifli both temperature and composition, and stratified layers experience double diffusive convection instabdities and spontaneous mixing or rollover. 

The inevitable consequence of stratification in a mixture at 1 bar is a rollover incident when the two layers mix spontaneously by vertical oscillating penetrative convection plumes across the whole of the liquid/hquid interface. 

Since LPGs are mixtures, they suffer from all the evaporation instability and path-dependent mixing behaviours met with LNG. Thus, provision must be made for stratification and its removal by adequate mixing, and the possible occurrence of rollover and vapour explosions. More importantly, there must be adequate emergency venting to release associated BOR spikes and thereby avoid over-stressing the concrete structure of the tank, which may not be designed to withstand the pressures associated with bod-off instabdities. 

Stratification, Rollover and Handling of LNG, LPG and Other Cryogenic Liquid Mixtures... 

So, when the LNG rollover event happened at La Spezia in 1971, we were quickly involved in trying to understand what had happened. Subsequent research programmes, using the more sophisticated flow visualisation and measurement techniques we developed in the later 1970s, enabled us to reproduce and study rollover events between stratified layers of cryogenic liquid mixtures. In particular, we were able to visualise what happens in a rollover event using laser Doppler and photographic and video systems. 

---