Reaction networks cracking reactions

Fig. 15 A proposed reaction network of direct ring opening of decalin reaction over acidic zeolites. PC Protolytic cracking HeT Hydride transfer HT Hydrogen transfer I Isomerization P /i-scission DS Desorption TA Transalkylation. Adapted from ref. 47.

A number of mechanistic modeling studies to explain the fluid catalytic cracking process and to predict the yields of valuable products of the FCC unit have been performed in the past. Weekman and Nace (1970) presented a reaction network model based on the assumption that the catalytic cracking kinetics are second order with respect to the feed concentration and on a three-lump scheme. The first lump corresponds to the entire charge stock above the gasoline boiling range, the second... 

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 decreasing selectivity for C3 products, but also for C4 and C5 products, indicates that the cracking reaction is more deactivated than the other reactions of the network. The decrease of the paraffin/olefin ratio with the coke content illustrates that the hydrogen transfer reactions, which play an important role in the production of paraffins, are less deactivated. The same can be concluded for the isomerisation reactions leading to the hexane isomers. 

Cracking reactions are carried out in order to reduce the molecular size and to produce more valuable transport fuel fractions (gasoline and diesel). Fluid catalytic cracking is acid catalyzed (zeolites) and a complex network of carbe-nium ion reactions occur leading to size reduction and isomerization (see Chapter 4, Section 4.4). Hydrogenation also takes place in hydrocracking, as well as cracking. 

A simplified reaction network for the cracking of alkanes on zeolite catalysts is presented in Fig. 2.7. 

In general the interpretation of the data is somewhat more complicated than for the differential method. Especially for an unknown complicated kinetic functions, the derivation of the correct reaction rate expression RA from experimental results using Equations 5.41 and 5.33 is more cumbersome than fitting Equations 5.30 and 5.33. This is especially true for complex reaction networks, as in the isomerization and cracking reactions of crude oil fractions, where the integral method is very laborious with which to derive individual rate constants. 

From the limited fracture data available for similarly cured networks generated from these various resin types, little can be concluded as to the role of the resin in fracture. Certainly, the resin backbone contributes to the Tg of the network, and comparisons should take this into account because, as previously discussed, the initiation of crack growth is very sensitive to temperature. If the epoxy resin structure results in complicated network forming reactions, as is possible for TGDM/DDS networks, the structure of the final network will be affected and may likely influence fracture. 

Figure 3. Reaction network in terms of elementary steps of carbenium-ion chemistry for the catalytic cracking of n-hexane.

K-Hexadecane was chosen as model molecule since it is relatively easy to implement and obtain as a pure body. Its reaction network non-exhaustive on Fig. 29—is representative since it includes all the elementary steps involved in the hydrocracking/hydroisomerisation of heavy paraffinic cuts. After reduction, just six kinetic parameters (two for isomerisation, four for cracking) are required to represent this type of network. 

Apart from the very large number of elementary steps it contains, the reaction network of heavy paraffins is the same as that of w-hexadecane. The apparent kinetic network is unchanged the linear paraffins are isomerised, and then cracked. 

In this paper, the cracking of n-hexane, n-dodecane and n-hexadecane on ZSM-5 zeolites at about atmosphere and temperatures of 260-400°C were studied. The results showed that both mono-molecular cracking and bimolecular reaction (disproportionation) for n-hexane cracking took place. A network for initial reactions was proposed, and the apparent kinetic parameters of the reactions were estimated. An examination for the factors affecting the product destribu-tion of n-hexadecane indicated that hydrogen transfer on the surface of HZSM-5 zeolites plays an important role in cracking reaction. 

Fig. 2. Initial network of n-hexane cracking reaction on HZSM-5 zeolites.

The approach to hydrocarbon cracking taken by the Froment school is to model the actual elementary steps of radicals at the various molecular configurations [38]. These are relatively few initiation hydrogen abstraction from a primary, secondary, or tertiary carbon and radical decomposition by scission of a carbon-carbon bond in /3-position to the unpaired electron. Boolean relation matrices are used to reflect the structures of the hydrocarbon reactants by indicating the existence and location of all their carbon-carbon bonds. Computer software generates reaction networks on the basis of known rate coefficients and activation energies at the various positions. Froment states the number of components in naphtha cracking as around 200, that of radicals as 40, and that of elementary radical steps... 

Lumping of single hydrocarbons to pseudo-components (PC, for S3unbols see notation) is a possible way to simplify and to manage the obscure reaction network of complex multi-component systems. As expected, the selective n-heptane cracking was... 

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