Nucleation point process

The term, metal dusting, was first used about this time to describe the phenomenon associated with hydrocarbon processing. Butane dehydrogenation plant personnel noted how iron oxide and coke radiated outward through catalyst particles from a metal contaminant which acted as a nucleating point. The metal had deteriorated and appeared to have turned to dust. The phenomenon has been called catastrophic carburization and metal deterioration in a high temperature carbonaceous environment, but the term most commonly used today is metal dusting. 

Supercooling The hot water sample may have a greater tendency to supercool, because it has less dissolved gas which can act as nucleation points for ice to form. Water that does not supercool may form a thin layer of ice at the surface which can insulate the rest of the water from the freezer and delay the freezing process. 

A full analysis of the assembly process resulted in a growth model, in which a dimer acts as a nucleation point for further growth in two dimensions as a result of the strong, hydrophobic interactions between the large naphthalene surfaces forming a so-called dimeric... 

The Si02 walls are built up by condensation processes, for which the anchored silicate species act as nucleation points. The enrichment of the oxide at the templating crystals occurs by an Ostwald ripening mechanism the particles grow in size at the tube walls and highly soluble very small silica particles re-dissolve to the reaction solution. The polycondensation reaction is finished within about I2h. 

The sweetener aspartame exists as a hemihydrate (C14H18N2O5 O.5H2O ASH), under ambient conditions. When heated, ASH converted to aspartame anhydrate (ASA), which on further heating decomposed to form a diketopiperazine (DKP) derivative. The XRD patterns of ASH, ASA, and DKP showed pronounced differences (Fig. 12). XRD was used to simultaneously quantify the (i) disappearance of ASH and appearance of ASA in the first reaction and (ii) disappearance of ASA and appearance of DKP in the second reaction. For studying the kinetics of the first reaction, the peaks unique to ASH at 15.9 and 16.4° 20 and the 17.1° 20 peak of ASA were used. For the second reaction, the sum of the integrated intensities of the peaks at 10.2, 11.0, and 11.8° 20 of ASA and the 13.0° 20 peak of DKP were used. While the dehydration of ASH appeared to follow first-order kinetics, the cyclization of ASA was a nucleation-controlled process. Figs. 13 and 14 contain the dehydration and cyclization data at 118 and 180°C respectively. Since the concentrations of the crystalline reactant as well as the product were simultaneously monitored, mass balance calculations of the crystalline phases were possible at each time point. The reaction... 

In general, the initiation of the precipitation process may result from the presence of particulate matter in the bulk water that seeds the crystallization. The process is usually termed heterogeneous nucleation. It is possible for homogeneous nucleation to occur when the nucleation is spontaneous. Once nucleation has occurred, crystals can grow, provided that the solution is supersaturated. Suitable nucleation points on the heat transfer surface facilitate deposit formation on the surface. In turbulent flow, it is possible that crystallites that are formed in the bulk fluid may be carried into regions, where they can redissolve. 

Circular volcano -shaped deposits can occur as a result of bubble formation in connection with boiling heat transfer and these deposits can act as nucleation points for further evaporation and deposition. As the process of deposition continues, the surface conditions will be modified and the scale may contain fissures, so that steam formation may occur within the deposit. The process is generally referred to as wick boiling that may influence the mechanism of heat transfer. The process of scale formation under boiling conditions is complex. 

Let us consider as an example nucleation, which is a zeroth-order point process, since it introduces new particles into the system at a rate independent of the NDF. Zeroth-order point processes are usually treated in a very simple way by calculating the contribution to each class. For example, if just nucleation is occurring, the PBE becomes... 

Some defects appear to be associated with the tendency for polymeric films to become electrostatically charged and hence to attract fine air-borne particulates. These impede the formation of a continuous film of a few microns thickness. Other problems are caused by a low density of nucleation points for metal films to grow and spread across the surface, a process that requires some wettability. 

As was pointed out in the prologue, a very well established feature of polymer crystallization is that it is a nucleation controlled process. This conclusion is based on the strong negative temperature coefficients that are observed for the crystallization process and the relation of the crystallite thickness to the undercooling at which the crystallization takes place. This principle is based on the most general grounds and is independent of either the shape of the nucleus or the disposition of chain units within a given structure. 

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