Phase equilibria involving vapour pressure

VOCs), and to a decrease in production yields. Quantitation of these phenomena and determination of material balances and conversion yields remain the bases for process analysis and optimisation. Two kinds of parameters are required. The first is of thermodynamic nature, i.e. phase equilibrium, which requires the vapour pressure of each pure compound involved in the system, and its activity. The second is mass-transfer coefficients related to exchanges between all phases (gas and liquids) existing in the reaction process. 

CVT makes use of the temperature dependence of the above heterogeneous equilibrium to transport solid A through the vapour phase by means of gaseous intermedi-ate(s) C. That the process involves true transport and not just evaporation and condensation is evident from the fact that solid A does not possess an appreciable vapour pressure at the experimental temperature moreover, transport of A is not observed without the transporting agent B. 

In order to understand whether the observed differences were ascribable to any effect of the mycelia on the reaction equilibrium, a series of measurements was aimed at monitoring the variations in the water vapour pressure in the gas phase, expressed as relative humidity (RH). A hygrometer specifically conceived for providing fast measurements in systems involving organic solvents was used for this. 

The number of components is the minimum number of independent species required to define the composition of all of the phases in the system. The simplest example usually cited to demonstrate the concept of components is that of water, which can exist in various equilibria involving the solid, liquid, and gas. In such a system there is one component. Likewise for acetic acid, even though it associates into dimers in the solid, liquid, and gaseous state, the composition of each phase can be expressed in terms of the acetic acid molecule and this is the only component. The important point for such a system is that the monomer-dimer equilibrium is established very rapidly, that is, faster than the time required to determine, say, the vapour pressure. In the cases in which the equilbrium between molecular species is established more slowly than the time required for a physical measurement, the vapour pressure, for example, will no longer be a function only of temperature, but also of the composition of the mixture, and the definition of a component acquires a kinetic aspect. 

The separation process occurring in the column involves an equilibrium established by the component between the stationary and the mobile phases. The distribution coeflScient and hence retardation is dependant on vapour pressure and retention properties of the bulk solute and is a function of temperature as discussed earlier. 

The inclusion in this volume of a chapter on the properties of gaseous mixtures may appear inappropriate there are however, close connections between the properties of liquid and gaseous mixtures. An obvious but nonetheless important point of contact is the study of vapour-liquid equilibrium. The thermodynamic analysis of vapour-pressure measurements requires a knowledge of the chemical potentials of the components of the gas mixture in equilibrium with the liquid. Even in experiments in which the properties of the vapour are not directly involved, the volumetric properties of a vapour phase must often be known in order to determine with precision the composition of the liquid mixture. 

Membrane distillation is one of the membrane processes in which the membrane is not directly involved in separation. The only function of the membrane is to act as a barrier between the two phases. Selectivity is completely determined by the vapour-liquid equilibrium involved. This means that the component with the highest partial pressure will show the highest permeation rate. Thus, in the case of an etbanol/water mixture where the membrane is not wetted at low ethanol concentrations, both components will be transported through the membrane but the permeation rate of ethanol will always be relatively higher. With salt solutions, for example NaCl in water, only water has a vapour pressure, i.e. the vapour pressure of NaCl can be neglected, which means that only water will permeate through the membrane and consequently very high selectivities are obtained. 

As the convention of rhombic sulphur into monoclinic form involves molecular rearrangements, therefore, the process is slow. In order to have complete conversion of rhombic sulphur into monoclinic form, heating should be slow or the temperatme should be allowed to remain steady at about the transition temperatime or sufficient time should be allowed. But if the heating is rapid or the temperature is not allowed to remain steady at the transition temperatm-e, the tremsition of one crystalline form into another is not possible and in that case, the vapour pressure ciuv e of rhombic sulphiu extends even beyond the point B, till the point O is reached, which is the melting point of rhombic sulphur, Le., 114°. The dotted curve BO is, therefore, the metastable sublimation curve of rhombic sulphur as it is metastable phase above B. Point O is the melting point of metastable rhombic sulphur, which will persist in equilibrium with its vapour phase without changing into monoclinic form for several degrees above the transition temperature. In accordance with the metastable rule, the vapoiu pressure at each temperature of the metastable phase is hi er than the vapour pressm-e of the stable phase, i.e., monoclinic sulphur, at the s une temperature. 

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