Supersaturated vapour

As discussed in Section 15.5.2, the separation of two or more sublimable substances by fractional sublimation is theoretically possible if the substances form true solid solutions. Gillot and Goldberger(10°) have reported the development of a laboratory-scale process known as thin-hlm fractional sublimation which has been applied successfully to the separation of volatile solid mixtures such as hafnium and zirconium tetrachlorides, 1,4-dibromobenzene and l-bromo-4-chlorobenzene, and anthracene and carbazole. A stream of inert, non-volatile solids fed to the top of a vertical fractionation column falls counter-currently to the rising supersaturated vapour which is mixed with an entrainer gas. The temperature of the incoming solids is maintained well below the snow-point temperature of the vapour, and thus the solids become coated with a thin film (10. im) of sublimate which acts as a reflux for the enriching section of the column above the feed entry point. 

The mechanisms of droplet (or liquid germs) formation from a supersaturated vapour phase is still the subject of many investigations. After giving a brief account of the classical theory [64], which, as shown above, provides a simple method for estimating the energy barrier to overcome before effective nucleation is started, and permits the estimation of the critical cluster size, a complementary approach will be presented. 

Katz described the homogeneous nucleation of a supersaturated vapour using J(i), the net rate at which clusters of size i grow to size i + 1 [63]. In this kinetic equation, J(i) is the difference between the rate at which clusters of size i add an additional monomer, and the rate at which clusters of size i + 1... 

It may be pointed out that the isotherms plotted in the figure given above are based on theoretical calcula tions of Vcorresponding to different values of P obtained by using the van der Waals equation. The isotherms for carbon dioxide, obtained by Andrews experimentally, were in close resemblance with these curves, with the difference that the wavelike portion LMNOQ was replaced by a horizontal line. Since then more careful experiments have shown that small portions corresponding to curves LM and OQ can be realised in practice also. These represent supersaturated vapour and superheated liquid, respectively. 

Dust is generally formed by mechanical processes. Mist is generally formed by condensation of supersatured vapours or by physical shearing of liquids. Dusts and mists generally have sizes ranging from less than 1 to about 100 pm ... 

The portion VN corresponds to a supersaturated vapour, which may exist but is metastable and disappears spontaneously if condensation nuclei are introduced into the system. Similarly ML corresponds to an over-expanded liquid, which again is a metastable state. The points M and N are thus boundary points between metastable and unstable states of the system. 

An important result from aerosol chamber studies was the discovery of the indirect photochemical process. Thus, Bricard et al. (1968) found that intense aerosol particle production can be observed in the chamber in the dark if ambient filtered air is sampled from a sunlit atmosphere. It is speculated that in the atmosphere some gaseous substance is excited by sunlight and is not collected by the filter used to obtain air which is free of aerosol particles. In the chamber these photochemically excited molecules initiate secondary thermal reactions leading to the formation of some supersaturated vapour (e.g. H,S04) which subsequently condenses (see also Subsection 3.6.3). 

Appreciable particle production (from a supersaturated vapour) occurs within the thermal boundary layer adjacent to the collector [Castillo and Rosner 1989, Liang etal 1989]. 

In the case of water and ice, such a model might be realistic in treating the nucleation of liquid or solid from supersaturated vapour, as it occurs in the upper atmosphere. It seems doubtful, on the other hand, whether the model of stepwise molecular condensation can realistically be applied to the liquid - solid transition, because liquid water itself is already extensively associated and exists as a three-dimensional distorted network of hydrogen-bonded molecules, not too dissimilar from ice. Any mechanistic model of ice nucleation, based on the condensation of individual water molecules onto clusters of molecules with the properties of ice, should therefore be treated with caution. 

Number concentrations can be determined by sensing zone techniques (see Section 2.4.2), which provide an overall average number concentration. A condensation particle counter (CPC) can provide number concentrations for solid aerosols having particle sizes as small as a few nanometres in diameter [129]. In such an instrument, the aerosol is saturated with a vapour (usually water or an alcohol) and then taken to conditions of supersaturation. Vapour condensation on the particles produces droplets having diameters in the range 5-15 pm [124]... 

Example 7.3 The Sinclair-La Mer Aerosol Generator uses a superheater to form a supersaturated vapour, which is then cooled to produce aerosol nucleation and... 

The behaviour of a newly created crystalline lattice structure in a supersaturated solution depends on its size it can either grow or redissolve, but the process which it undergoes should result in the decrease in the free energy of the particle. The critical size therefore, represents the minimum size of a stable nucleus. Particles smaller than will dissolve, or evaporate if the particle is a liquid in a supersaturated vapour, because only in this way can the particle achieve a reduction in its free energy. Similarly, particles larger than will continue to grow. 

So far, in the discussion of industrial crystallization processes, only the crystallization of a solid phase from a supersaturated or supercooled liquid phase has been considered. However, the crystallization of a solid substance can be induced from a supersaturated vapour by the process generally known as sublimation . Strictly speaking, of course, the term sublimation refers only to the phase change solid vapour without the intervention of the liquid phase. In its industrial application, however, the term is commonly used to include the condensation (crystallization) process as well, i.e. solid vapour... 

In praetiee, for heat transfer reasons, it is often desirable to vaporize the substanee from the hquid state, so the complete series of phase changes in an industrial subhmation process can be solid liquid vapour sohd. It is on the condensation side of the process that the appearance of the liquid phase is prohibited. The supersaturated vapour must condense directly to the crystalline solid state. 

Becker, R. and Doring, W. (1935) Kinetic treatment of germ formation in supersaturated vapour. Ann. Phys., 24, 719. 

For various technical flow processes with spontaneous condensation of a supersaturated vapour it is desirable to calculate the- state, where condensation begins, the Wilson point, and the influence of the condensation op the downstream flow field. Important parameters of such calculations are the formulation of the nucleation rate and of the further growth of stable nuclei. Theoretical models for calculating the nucleation rate by computer simulation are available, if at all, only for gases of simple molecular structure. Moreover, calculations of this kind appear to be rather complicated if integrated into a flow... 

The phenomenon of spontaneous condensation in a supersaturated vapour is of interest in different fields of natural science and in various technical applications, e.g. wind tunnels and steam turbines. Expecially with respect to Organic Rankine Cycles for heat recovery there exists some interest in the conditions for the onset of spontaneous condensation in the so-called Wilson point and in the influence of the released heat of condensation on the flow field for different working fluids. From this point of view, we started systematic investigations on spontaneous condensation of pure vapours in stationary supersonic nozzle flow with the intention to cover a wide range of thermodynamic state for substances of different molecular structure. In this paper we present experimental results for carbon dioxide and compare them in a first step with results... 

If the characteristic time of relaxation tends to zero, the transition occurs instantaneously in a sharply defined front. We denote the velocity of the front by U and the velocity of the mixture by u. The state of the mixture is characterized by pressure p, temperature T and liquid mass fraction fp. The gas and the vapour are considered perfect so they obey Dalton s law. For simplicity we shall from now on refer to the supersaturated vapour state and to the state of liquid-vapour equilibrium as state 1 and state 2, respectively. It is dear there exists no liquid phase in state 1, fp = 0, and the vapour mass fraction f is equal to the total mass fraction of the condensable component of the mixture. The vapour pressure p 2 state 2 has to satisfy the Clausius-Clapeyron equation (2). The laws of conservation of mass, momentum and energy are, applied to the condensation discontinuity ... 

In this process, the precursor is vaporized and deposited in a vacuum chamber. The synthesis was carried out at pressure of 1 mtorr retaining the vacuum level at 10-0. IMPa with temperature ranging from room temperature to 500°C. Vapour phase nucleation takes place in dense cloud vapour by collision. The atoms are passed through a gas to provide necessary collision and cooling for the nucleation. It is economical and the deposition rate is high. In this process, the decomposition of the precursor is sometimes difficult. The nanoparticles obtained from a supersaturated vapour are usually longer than the cluster. 

Vapor-phase mixture rendered to be thermodynamically unstable leads to the formation of desired soUd material. Supersaturated vapour — Chemical super saturation ... 

The influence of an external electric field on the nucleation process has been investigated in the case of the Stockmayer fluid by computing the free energy of formation of clusters in the vapour phase [159]. The free energy is found to increase in the presence of an electric field at fixed supersaturation vapour and to decrease at fixed chemical potential. 

A proper thermodynamic description of electrochemical phase formation is necessarily complicated by the need to consider the changes in energy and entropy associated with the process as a whole. Fortunately, electrocrystallisation has many features in common with the growth of a solid phase from its supersaturated vapour, and it is useful to examine this simpler process first. In the case where two phases but only one component are present, phase formation involves the incorporation of atoms or molecules into a pre-existing surface of the solid, and the enthalpy change is therefore related to the transition... 

Parkas, L. (1927) Nucleation rate in supersaturated vapours. Zeitschrift fur Physikalische Chemie, 125, 236-242. 

---