Condensation of vapour mixtures - Heat and mass transfer

Condensation of a mixture of vapors occurs in many industrial plants, covering fields of application as diverse as :

• chemical engineering ;

• heat production ;

• cold production.

In the first case, the fluids used in the processes are rarely pure bodies. The use of mixtures is therefore the most frequent case. In petrochemicals, for example, petroleum products start out as complex mixtures, and the various refining operations aim to separate the various components, before eventually reforming mixtures of controlled composition.

In the last two fields, fluid mixtures have a dual benefit. Firstly, their use can reduce exergy losses in evaporators and condensers in certain configurations. Secondly, blends are interesting candidates for replacing CFC (chlorofluorocarbon) and HFC (hydrofluorocarbon) refrigerants to solve certain environmental problems. Indeed, the presence of some of these fluids in atmospheric air can, on the one hand, lead to the depletion of the atmospheric ozone layer and, on the other, contribute to the greenhouse effect.

Two types of theory are used to calculate the thermal performance of a mixture condenser. In the first type of theory, known as film theory, the accumulation of vapors from the most volatile fluids near the condensate surface creates a barrier to transfer. These theories represent reality fairly well, but may require fairly heavy numerical processing if more than two fluids are present. In the second case, so-called equilibrium theories can be used under certain experimental conditions and for fluids meeting certain criteria. These lead to relatively simple calculations.