True hydrocracking

True hydrocracking (Figure 6) is mediated over dual-functional metal / acid catalysts and proceeds via adsorbed carbenium ion intermediates where cleavage is most likely on central C-C bonds and, from the fourth C-position, occurs with almost equal probability [5, 10, 11]. Moreover, due to the carbenium ion mechanism, the intermediate to cracked products is typically an isomerised carbenium ion with the result that products are generally highly branched. 

True hydrocracking requires the presence of both strong acid and hydrogenation sites, the latter in order to activate the feed paraffins and minimise secondary cracking. 

Figure 6 Reaction pathways of dual-functional, metal / acid catalysed true hydrocracking ...

Figure 7 Theoretical carbon-number distribution of product from dual-functional, metal / acid catalysed true hydrocracking of n-Ci4 (lines for additional secondary hydrocracking indicating only qualitative direction of shift)...

It is significant that the mixture yielded propane as the major product (Table III). As noted in our earlier paper on catalytic cracking (6), the predominance of C3 fragments in the cracked products and the absence of isobutane appeared to be a unique property of erionite. Our present data indicate that this is also true for hydrocracking over a dual function erionite. The only exception was that when n-pentane alone was hydro-cracked, equimolal quantities of ethane and propane were found. This shift in product distribution in the presence of n-hexane, a second crackable component, indicated that the reaction path within the intracrystalline space was complicated. 

Hydroisomerization proceeds towards thermodynamic equilibrium which is approximately reached between the normal, mono-branched and di-branched structures at high degrees of overall conversion. Hydrocracking, however, is severe under these conditions. It is evident from Table II that monomethyl isomers are primary products the same is apparently true for monoethyl isomers although due to thermodynamic reasons lower concentrations are obtained. Dimethyl isomers including those containing a quarternary carbon atom are formed as secondary products. However, trimethyl isomers are formed very slowly so that their concentrations do not reach equilibrium values. It follows from this that the number of ramifications is deciding as to whether a branched isomer is a primary, secondary or tertiary product in hydroisomerization of n-octane and n-nonane. 

Ideal true primary hydrocracking. .. Additionai secondary hydrocracking... 

A typical parameter employed in correlations for transport properties of petroleum fractions is the average boiling point that is assumed to be constant. This is true only if no appreciable extent of hydrocracking occurs, but if any, changes in transport properties can occur and thus reactor model can fail under these circumstances. This fact, if not properly taken into account, can lead to add model parameters as factors that mask the fundamental behavior of HDT reactor. 

VGO + residue (determined by blending streams 2 and 3), and the hydrocracked product (with the VGO in the feed, streams 4 plus 5). The first three are reported as ASTM D-1160 distillation, since they are products from vacuum distillation of petroleum, while the latter was determined as TBP (True Boiling Point, ASTM D-2892), since it also includes the lighter products produced during hydrocracking. 

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