Multi-component Liquids

Abstract

After the first ten years of developing the large scale convective properties of larger and larger storage containers for single component cryogenic liquids of nitrogen, oxygen, hydrogen and helium, we started to be involved with multi-component cryogenic liquids and the associated industrial problems being widely met. The starting point was liquid air as a binary mixture of nitrogen and oxygen, with differing vapour and liquid compositions during their distillation by boiling. It was discovered that under low loss storage with surface evaporation, and no boiling, the differing compositions were much larger. For example the vapour from liquid air was not 79% but almost pure nitrogen, and dangerously unable to support life; while the liquid suffered oxygen enrichment with rapid increase in fire risk. Storage of liquid air is therefore unsafe as a cheap source of refrigeration at 79 K. Then we met rollover, when the liquid mixture in a storage container suddenly suffers from a boil-off rate rising to a large unexpected peak, lasting minutes or hours. Studies led to the finding that this behaviour arises from stratification, via blocking of boil-off via surface evaporation and leading to thermal overfill. This chapter discusses this mechanism and describes a number of ways for preventing stratification and rollover. Experiments with propane and butane mixing led to the discovery that mixing two liquids, which is an irreversible thermodynamic process, is also path dependent with significant differences in the large accompanied boil-offs, or vapour flashes. When hot liquid is mixed into cold liquid, the smaller boil-offs are about one half that of adding cold liquid to hot. Finally, the solubility of many components in cryogenic liquids are small but finite, in the range of 1–1000 ppm. These solubilities can decrease rapidly with decreasing temperature, leading to deposits blocking pipes and heat exchanges. One example is the problem of water in jet fuel below −40 °C, which has now been solved by recommendations of the UK Air Accident Investigation Branch in conjunction with the University of Southampton.