Metastable operating conditions

In this section, a brief description of the necessary experiments to identify the kinetic parameters of a seeded naphthalene-toluene batch crystallization system is presented. Details about the experimental apparatus and procedure are given by Witkowski (12). Operating conditions are selected so that the supersaturation level is kept within the metastable region to prevent homogeneous nucleation. To enhance the probability of secondary nucleation, sieved naphthalene seed particles are introduced into the system at time zero. 

A systematic study of salting-out precipitation is carried out to obtain the operational limits within which this precipitation method can be applied for the production of fines (mean particle size <10 xm) with acceptable quality and productivity. The model substances glycine and sodium chloride are salted-out from their aqueous solutions by using ethanol as antisolvent. The main operational parameter is the initial supersaturation of the solutions. It is shown that the smallest particles can be produced at the limits of the metastability domain determined by three optional process parameters the initial solution concentration, the equilibrium solubility and the operational time. The product quality (crystallinity, polymorphic states, aggregation) and productivity considerably change with the operational conditions. 

In view of the long-time operation we have to rely on thermodynamically stable structures and compounds, or on pronouncedly metastable situations. Under such conditions, given the nature of the constituents, the relevant control parameters are temperature T, component potentials or partial pressures (Pj, and doping content (C). For given operation conditions, Tand Pare fixed leaving the nature of the major chemical elements and the concentrations of dopants (Cl) as the only variable parameters. (In multinary oxides usually not all sublattices are mobile, with the consequence of having the additional freedom to varying the fine composition... 

Problems caused by polymorphism appear in many fields such as fine chemicals in industries (pharmaceuticals(7,2), foods, etc.), optical electronic materials(i), clathrate compounds(4) and biominerals(5). In crystallizations of these materials the crystallization behavior of the polymorphs is controlled first by basic operational conditions such as temperature, supersaturation degree, stirring rates. In addition to these basic factors, solvents, additives and guest molecules (in clathrate compounds) should be also considered as the important factors((5,7). The crystallization process of the polymorphs is composed of comp)etitive nucleation, competitive growth of polymorphs and transformation from metastable to stable form. Accordingly individual step should be investigated to clarify the crystallization mechanism of polymorphs. 

As an example of an ion-molecule reaction in which selectivity is important, we consider the reactions of A1+ with H2. The ground state of is but the state lies at 4.56 eV. In the experimental investigation by Muller and Ottinger [32] it was possible to vary the proportion of the metastable ion by varying the operating conditions of the Coultron hot cathode discharge. They observed chemiluminescence from the reactions... 

Absorption techniques which require no spectral resolution and in which the profile of the resonance lamp must be assumed rather than determined experimentally are described by Mitchell and Zemansky (1966). These methods were used in early experiments such as the work on the population of metastable levels in inert gas discharges reported by Ladenburg (1933). Unfortunately systematic errors are difficult to avoid in these experiments since the profile of the resonance lamps is usually considerably distorted by self absorption or even self reversal. Both of these effects vary noticably with lamp design and operating conditions. ... 

There is a middle steady state, but it is metastable. The reaction will tend toward either the upper or lower steady states, and a control system is needed to maintain operation around the metastable point. For the styrene polymerization, a common industrial practice is to operate at the metastable point, with temperature control through autorefrigeration (cooling by boiling). A combination of feed preheating and jacket heating ensures that the uncontrolled reaction would tend toward the upper, runaway condition. However,... 

To summarize, from literature there does not seem to be much consensus on whether bulk cobalt carbide forms during realistic FTS conditions. Bulk carbide is generally considered a metastable species. However, it is clear that it may form under upset conditions. Furthermore, there is strong evidence to show that if bulk cobalt carbide is present, it is deleterious in terms of both catalyst activity and selectivity. With this in mind, it would be prudent to operate the catalyst in a regime (sufficiently high H2/CO ratio) where bulk carbide formation is avoided. 

---