Primary heterogeneity

Goodrich, C. A. and Delaney, J. S. (2000) Fe/Mg-Fe/Mn relations of meteorites and primary heterogeneity of primitive achondrite parent bodies. Geochimica et Cosmochimica Acta, 64, 149-160. 

Three different mechanisms for nucleation may occur in any crystallization operation, as discussed by Mersmann (2001, pp. 45ff.) homogeneous primary, heterogeneous primary, and activated secondary. Industrial crystallizers are usually operated under conditions of the last two simultaneously. For simplicity, nucleation as used in this discussion refers to any or all three of the mechanisms that may be important in a particular crystallization. 

A completely different behavior is reported for blends in which the crystallizable phase is dispersed. Fractionated crystallization of the dispersed droplets, associated with different degrees of undercooling and types of nuclei is the rule. The most important reason is a lack of primary heterogeneous nuclei within each crystallizable droplet. An important consequence of fractionated crystallization may be a drastic reduction in the degree of crystallinity. 

In the industrial crystallizer, primary homogeneous nucleation is not possible due to the low levels of supersaturation employed and the presence of impurities. Primary heterogeneous nucleation is possible at places where high supersaturation is being created— for example, in heat transfer equipment. This could lead to crystal deposits on these surfaces. Undesirable nucleation can be reduced by choosing proper production rates and by allowing adequate mixing. 

It should be mentioned that the occurrence of a fractionated crystallization is related only to the number densities of dispersed polymer particles and primary heterogeneous nuclei. No direct physical relationship has been found with the number or size of spherulites. These parameters are additionally influenced by the cooling rate and the crystallization temperature (Frensch et al. 1989). 

Let us briefly review the primary treatment methods used. Pretreatment usually starts with phase separation if the effluent is a heterogeneous mixture. 

One of the primary tasks in connection with the use of AE method is to identify defects by the AE parameters. For identification of nature of the destruction centre in the polymeric composites it is necessary to consider the peculiarities of their heterogeneous structure, that is presence of at least two different components (filler and connector), and also boundary transitional layers. 

Substituting (6) to (5) and assuming there are M < N — 1 types of primary sites for which << 1 (sites without saturation, Henry s sites) and neglecting the heterogeneity of the Langmuir s and Hemy .s sites we can... 

Figure 2-112. Primary structure and heterogen sections of the analyzed PDB file.

Homogeneous sonochemistry typically is not a very energy efficient process (although it can be mote efficient than photochemistry), whereas heterogeneous sonochemistry is several orders of magnitude better. Unlike photochemistry, whose energy inefficiency is inherent in the production of photons, ultrasound can be produced with neatly perfect efficiency from electric power. A primary limitation of sonochemistry remains the small fraction... 

Chromium compounds decompose primary and secondary hydroperoxides to the corresponding carbonyl compounds, both homogeneously and heterogeneously (187—191). The mechanism of chromium catalyst interaction with hydroperoxides may involve generation of hexavalent chromium in the form of an alkyl chromate, which decomposes heterolyticaHy to give ketone (192). The oxidation of alcohol intermediates may also proceed through chromate ester intermediates (193). Therefore, chromium catalysis tends to increase the ketone alcohol ratio in the product (194,195). 

Esterification. Extensive commercial use is made of primary amyl acetate, a mixture of 1-pentyl acetate [28-63-7] and 2-metliylbutyl acetate [53496-15-4]. Esterifications with acetic acid are generally conducted in the Hquid phase in the presence of a strong acid catalyst such as sulfuric acid (34). Increased reaction rates are reported when esterifications are carried out in the presence of heteropoly acids supported on macroreticular cation-exchange resins (35) and 2eohte (36) catalysts in a heterogeneous process. Judging from the many patents issued in recent years, there appears to be considerable effort underway to find an appropriate soHd catalyst for a reactive distillation esterification process to avoid the product removal difficulties of the conventional process. 

The SRC-II process, shown in Figure 2, was developed in order to minimise the production of soHds from the SRC-I coal processing scheme. The principal variation of the SRC-II process relative to SRC-I was incorporation of a recycle loop for the heavy ends of the primary Hquefaction process. It was quickly realized that minerals which were concentrated in this recycle stream served as heterogeneous hydrogenation catalysts which aided in the distillate production reactions. In particular, pyrrhotites, non stoichiometric iron sulfides, produced by reduction of iron pyrite were identified as being... 

Both homogeneous and heterogeneous mechanisms requite relatively high supersaturation, and they exhibit a high order dependence on supersaturation. These factors often lead to production of excessive fines ia systems where primary aucleatioa mechanisms are important. The classical theoretical treatment of primary nucleation results ia the expressioa (5) ... 

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