Cracking selectivity, influencing

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

Since the bulk of butadiene is recovered from steam crackers, its economics is very sensitive to the selection of feedstocks, operating conditions, and demand patterns. Butadiene supply and, ultimately, its price are strongly influenced by the demand for ethylene, the primary product from steam cracking. Currently there is a worldwide surplus of butadiene. Announcements of a number of new ethylene plants will likely result in additional butadiene production, more than enough to meet worldwide demand for polymers and other chemicals. When butadiene is in excess supply, ethylene manufacturers can recycle the butadiene as a feedstock for ethylene manufacture. 

A variety of solid acids besides zeolites have been tested as alkylation catalysts. Sulfated zirconia and related materials have drawn considerable attention because of what was initially thought to be their superacidic nature and their well-demonstrated ability to isomerize short linear alkanes at temperatures below 423 K. Corma et al. (188) compared sulfated zirconia and zeolite BEA at reaction temperatures of 273 and 323 K in isobutane/2-butene alkylation. While BEA catalyzed mainly dimerization at 273 K, the sulfated zirconia exhibited a high selectivity to TMPs. At 323 K, on the other hand, zeolite BEA produced more TMPs than sulfated zirconia, which under these conditions produced mainly cracked products with 65 wt% selectivity. The TMP/DMH ratio was always higher for the sulfated zirconia sample. These distinctive differences in the product distribution were attributed to the much stronger acid sites in sulfated zirconia than in zeolite BEA, but today one would question this suggestion because of evidence that the sulfated zirconia catalyst is not strongly acidic, being active for alkane isomerization because of a combination of acidic character and redox properties that help initiate hydrocarbon conversions (189). The time-on-stream behavior was more favorable for BEA, which deactivated at a lower rate than sulfated zirconia. Whether differences in the adsorption of the feed and product molecules influenced the performance was not discussed. 

A nonchemical factor also influences a sand fly s selection of egg sites She prefers narrow cracks and crevices rather than open surfaces. A gravid sand fly, then, is quite exacting about where she deposits her eggs. She avoids exposed areas and looks for a site that already contains eggs of her own species along with simple organic nutrients. Her concerns seem well placed, for these characteristics promise safety, protection from the elements, and food for newly hatched larvae. 

Beta zeolite with different framework Si/Al ratios and their influence on activity and selectivity for cracking of n-heptane and vacuum gas-oil. 

Iron impurities in clays have been thought responsible for these type of catalysts low carbon selectivity (4,5). The purpose of this paper is to investigate and report the influence that the location, chemical state and environment of iron impurities have on the cracking properties of pillared clays prepared by reacting several smectites with aluminum chlorhydroxide solutions. 

The abrasiveness test results by Mackensen s method for selected cubes of known abrasiveness on Bohme s disc, given in Table 4.5.4, show very clearly a way of detecting inhomogeneities in rock hardness, induced by structural and textural differences. By this method we can observe the influence of pockets, laminae or veins occurring in a rock, which have a different composition from the typical rock structure, and also the influence of cracks and weathering of rock on its overall hardness (abrasiveness). In the tests with cubes (previously used for Bohme tests), some 24 measurements were taken in each case (four on each face) by the Mackensen method. This is sufficient to give a comprehensive characterization of rock hardness, not possible in the same time with any other method (a single... 

In Figure 14 hydrocracking of n-heptane and n-octane have been selected to demonstrate the influence of the reaction temperature and, hence, of the degree of cracking conversion. Again, a classification of the carbon number fractions is useful ... 

Cotterman et al. (34) showed that hexadecane-cracking activity of AFS and USY zeolites appeared to be a function of total Al content, independent of method of dealumination, implying that hexadecane cracking occurs over both framework- and extra-framework-acid sites. Hence, extra-framework material in mildly steamed synthetic faujasite, USY, makes a significant contribution to catalyst activity, as previously reported (32). Gasoline selectivity is influenced by both the method of dealumination and steam treatment, and depends on both framework-acid sites and the presence of extra-framework material. 

All the previously cited models and works also consider, and some explicitly cite, this assumption—that the catalyst activity varies with time-on-stream (or with coke concentration [12]) in the same manner or with the same deactivation function (VO for all reactions in the network. That is, a nonselective deactivation model is always used. Corella et al. (16) have recently demonstrated that in the FCC process this assumption is not true and that it would be better to use a selective deactivation model. Another work (17) also shows how this consideration, when applied to catalytic cracking, influences the yield-conversion curves. Nevertheless, to avoid an additional complication, we will use in this chapter a nonselective deactivation model with the same a—t kinetic equation and deactivation function (VO for all the cracking reactions of the network. 

The distribution of A1 spedes of varying coordination (tetrahedral, pentacoordinated and octahedral) can be influenced by changing the conditions of hydrothermal pretreatment of amorphous silica-alumina catalysts. However, for a given composition, activity per unit surface area and selectivity were independent of pretreatment conditions. Thus, gas oil cracking activity and selectivity in amorphous silica-alumina cannot be... 

Table 3a Influence of the coke content on the product selectivities in n-hexane cracking at 7 % hexane conversion (mole/100 moles hexane reacted).

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