Condensation of vapours of immiscible liquids in the presence of a non-condensable gas

Abstract

During partial condensation of vapours from a mixture with non-condensing gas, the gas content causes a resistance to heat and mass transfer. If the two vapour components are immiscible in the liquid state, at each composition except from the heteroazeotrope, usually called eutectic composition in analogy with solid/liquid systems, one vapour component will additionally behave as a non-condensing gas. Only when this azeotropic composition is obtained at the gas/liquid interface, will the two vapour components condense simultaneously. Whereas conventional methods can be used to calculate the heat and mass transfer coefficients in the gas phase, transport mechanisms are insufficiently described due to the inhomogeneity of the two-phase condensate. For this reason, in the past, only simplified calculations, based on idealised model forms of condensation, were generally used. Following our own visual examination, calculation using the model of a homogeneously mixed condensate was favoured. For the first time tests were performed using this type of condensation at pressures above atmospheric and at gas-Reynolds numbers above 25 000, and up to 240 000. It was discovered that the turbulent hydrodynamic conditions of the tests allow the calculation with the simple calculation methods mentioned above. Good results were produced with an accuracy of ± 20 % in nearly all experiments. Where there is a water surplus in the condensate, excessively high water condensate massflow rates were systematically predicted. If there was a heptane surplus, the heptane flow rate was underpredicted.