Path Dependent Mixing of Boiling Cryogenic Liquids

11 Path Dependent Mixing of Boiling Cryogenic Liquids 

So far in this chapter, we have not considered the volumes of vapour generated when two boiling liquids are mixed together. Let us now look more closely at the mixing process, particularly when flie two liquids have widely separated boiling points. 

At temperatures above ambient, the homogeneous mixing of boiling liquids is not an everyday experience. Adding immiscible liquids, like boiling oil into water, and vice-versa, are probably the closest we meet, when the results are spectacularly explosive and hazardous because of homogeneous nucleate boiling of the water. 

The mixing process is, of course, irreversible, and is accompanied by an irreversible increase in entropy. Mixing with evaporation is particularly difficult to model because, in addition to the thermal mixing and release of thermal overfill, there is the compositional heat of mixing with irreversible entropy production which is sig-nificantiy path dependent. A considerable volume of vapour is produced by the usual positive heat of compositional mixing and by thermal contact between colder and hotter components (no homogeneous nucleate boiling has been observed) before the final equilibrium state of the mixture is achieved. 

However, because the mixing is path-dependent, the volume of vapour produced is a variable path-dependent phenomenon. This path dependence is clearly demonstrated when liquid propane and liquid butane are mixed and the vapour produced has to be recondensed by limited available refrigeration. Briefly, when colder liquid (propane rich) is added to hotter liquid (butane rich), the vapour generated is found experimentally to be twice that produced by the alternative when hot is added to cold [5], 

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