Nucleation sensitivity

Numerous attempts have been made by operators and designers to stabilize such systems the normal technique for doing this is by an appropriate selection of the maximum particle size separated by the baffle Lp) and the flow through the fines destruction system. A pioneering paper by Randolph et al. (1973) developed the mathematical basis for instability in systems of this type, and predicted instability in such systems as a function of the fines dissolving parameter and the nucleation sensitivity parameter. Experimental verification of these concepts was demonstrated by Randolph and Beckman (1977) for a KCl crystallizer. 

The central quantity of interest in homogeneous nucleation is the nucleation rate J, which gives the number of droplets nucleated per unit volume per unit time for a given supersaturation. The free energy barrier is the dommant factor in detenuining J J depends on it exponentially. Thus, a small difference in the different model predictions for the barrier can lead to orders of magnitude differences in J. Similarly, experimental measurements of J are sensitive to the purity of the sample and to experimental conditions such as temperature. In modem field theories, J has a general fonu... 

Over the years emphasis has been placed on obtaining greater uniformity in silver haUde crystal size and habit in the grain population, in the behef that the chemical sensitization process can then yield a higher average imaging efficiency. One way of doing this is to adjust the nucleation conditions so... 

Figure 5.15 Form of nucleation rate equations showing sensitivity to Ac...

The size of crystals produced in the gas-liquid system varied from 10 to 100 pm by controlling the level of supersaturation, while the liquid-liquid system produced crystals of 5—30 pm. The wide variation of crystal size is due to the marked sensitivity of the nucleation rate on the level of supersaturation, while the impurity content is another variable that can affect the crystal formation. 

Improved nucleation within the phosphate solution itself can produce smoother coatings without the necessity of recourse to preliminary chemical treatment. This may be accomplished by introducing into the phosphating bath the sparingly soluble phosphates of the alkaline earth metals or condensed phosphates such as sodium hexametaphosphate or sodium tripolyphosphate. Such modified phosphating baths produce smoother coatings than orthodox baths and are very much less sensitive to cleaning procedures. 

The high sensitivity of the MS [160] makes it a particularly appropriate tool for the investigation of nucleation and growth processes, since it is possible to measure rates during the early part of the reaction using small samples or individual crystals. The influence of residual gases [160] on the initiation of reaction can also be determined. Short scan times enable very rapid reactions e.g. detonations, to be studied, and it is also possible to measure simultaneously the rate of evolution of several different product molecules. 

More detailed consideration of the sensitivities of dehydration rates to reaction conditions (PH2o> temperature) are given in the articles cited reported values of E, at various PH2Q, are summarized in Fig. 13. From kinetic observations, it was concluded that between 353 and 383 K, the dehydration of CaS04 2 H20 [590] involved nucleation ( in Fig. 13) and boundary control (o) but for 383—425 K, a diffusion mechanism ( ) operated. The kinetics of dehydration of a-CaS04 ] H20 [590] (X,+)... 

The condensed phases also are important to the physical processes of the atmosphere however, their role in climate poses an almost entirely open set of scientific questions. The highest sensitivity of physical processes to atmospheric composition lies within the process of cloud nucleation. In turn, the albedo (or reflectivity for solar light) of clouds is sensitive to the number population and properties of CCN (Twomey, 1977). At this time, it appears impossible to predict how much the temperature of the Earth might be expected to increase (or decrease in some places) due to known changes in the concentrations of gases because aerosol and cloud effects cannot yet be predicted. In addition, since secular trends in the appropriate aerosol properties are not monitored very extensively there is no way to know... 

Almost all flows in chemical reactors are turbulent and traditionally turbulence is seen as random fluctuations in velocity. A better view is to recognize the structure of turbulence. The large turbulent eddies are about the size of the width of the impeller blades in a stirred tank reactor and about 1/10 of the pipe diameter in pipe flows. These large turbulent eddies have a lifetime of some tens of milliseconds. Use of averaged turbulent properties is only valid for linear processes while all nonlinear phenomena are sensitive to the details in the process. Mixing coupled with fast chemical reactions, coalescence and breakup of bubbles and drops, and nucleation in crystallization is a phenomenon that is affected by the turbulent structure. Either a resolution of the turbulent fluctuations or some measure of the distribution of the turbulent properties is required in order to obtain accurate predictions. 

Our conclusion then for the oxygen interactions with metals is that because of the specific association of cluster ion intensities with particular types of oxygen rather than total coverage, the technique is not suitable for monitoring coverages or kinetics in an independent manner. Once it is established which type of oxygen a particular cluster ion is representative of, then that ion may, in favorable circumstances, be used for quantification. In the case of Ni, it seems that the negative ions are very sensitive to the initiation of oxide nucleation. In the case of W(IOO), the WOj, WO+ and WOj ions may fill a similar role. 

In summary, we have shown that metal carbonyls formed in situ by adsorption of CO under ultrahigh vacuum condition can serve as a very sensitive tool for monitoring the nucleation site as well as the environment of the metal atom. It was shown that low coordinated metal atoms, in particular... 

---