Cracking reactions, overall

The second step is a -scission, the breaking of a carbon—carbon bond P to the charged carbon. The sum of the two reactions is the stoichiometry of the overall cracking reaction R H — RH + olefin. R+, a relatively stable carbenium ion such as the /-butyl cation, is a chain carrier. The role of the catalyst is to donate the proton to start the chain. This is a greatiy simplified representation. 

Although olefins are intermediates in this reaction, the final product contains a very low olefin concentration. The overall reaction is endothermic due to the predominance of dehydrogenation and cracking. Methane and ethane are by-products from the cracking reaction. Table 6-1 shows the product yields obtained from the Cyclar process developed jointly by British Petroleum and UOP. ° A simplified flow scheme for the Cyclar process is shown in Figure 6-6. 

Feed to the FCC unit is mixed with hot catalyst and steam in a reactor line called a riser. The ratio of catalyst oil feed can typically range from 4 1 to 9 1 by weight. Overall, FCC is an endothermic process. Heat provided by the hot, circulating catalyst is the prime source of energy driving the FCC process. In the riser, vaporized oil is cracked catalytically in less than two seconds. The vapors and catalyst flow out of the riser and into the reactor. At this point, most cracking reactions have occurred. 

For this purpose selectivities of hydrocracking of the n-alkanes are expressed in terms of probabilities of overall cracking reactions. The latter are calculated from the carbon number distributions of the cracked products (n. = number of moles of cracked products with carbon number i, from Figures 7, 8 or 9). Examples for the mode of calculation are represented in the following scheme which is valid for pure primary cracking. 

The values defined in this manner do not represent any probability for rupture of definite carbon-carbon bonds in the feed molecule. This term is meaningless if rearrangement of the carbon skeleton precedes the cracking step. Rather, the values indicate the probability of an n-alkane for being hydrocracked according to the overall cracking reaction in question. These probabilities are useful for a comparison with the relative concentrations of the products formed by hydroisomerization (cf. Table IV). 

Table IV. Hydroisomerization and Hydrocracking of -Alkanes. Comparison Between Relative Concentrations of Iso-alkanes and Probabilities of Overall Cracking Reactions (Fa = 12 10 3mole h 1).

The data indicate the types of reactions that can occur during the hydrode-sulfurization reaction and include those reactions that will occur at the upper end of the temperature range of the hydrodesulfurization process whether it is a true hydrodesulfurization reaction or a cracking reaction. Even though some of the reactions given here may only be incidental, they must nevertheless be taken into account because of the complex nature of the feedstock. The several process variations (Chapter 9) which (in addition to the fact that the overall hydrodesulfurization process is exothermic (Table 4-2) also contribute to the complexity of the product mix. 

One last point to be noted pertains to a comparison between the steam-reforming reaction (Case 4) and the methanol cracking reaction (Case 5). From the exergy ratio calculations, the reforming reaction appears to be superior in its ability to produce lower quality fuel. However, the overall efficiency calculations show a lower value for Case 4 than that for Case 5. The main reason for this reversal is due to the fact that nearly 25% of the recuperated energy for Case 4 is in the form of the heat of evaporation of H 0 and is not recovered from the exhaust gases. The result 1s an increase in the stack losses. 

WEiTKAMP Influence of Chain Length Overall Cracking Reaction ... 

Higher temperatures will enhance both isomerization as well as cracking reaction rates. However, at higher operating temperatures, the ability of the solvent to extract the precursor compounds will suffer because of decrease in solvent density and hence in solvent power. The inability to extract coke precursor compounds will lead to eventual deactivation of the catalyst. Furthermore, the overall rate of reaction may be limited by... 

Cumene conversion was determined from an overall mass balance over the cracking reaction system from both the normal and blank runs. 

A realistic selective deactivation kinetic model should use a different aj-t relationship to describe the evolution with time-on-stream of each cracking reaction. Therefore, several values of yj and dj should be known and used. This approach would introduce too many parameters in the control model of the riser or of the overall FCCU For this reason (attd until more basic research and verification can be done on this subject) we will use here a non-selective deactivation model with only one a-t kinetic equation and only one value each for V and d. Since in principle this is not correct (21) the predicted (using this non-sclectivc deactivation model) product distribution at the riser exit (the gasoline yield mainly) will differ somewhat from the real one (20). 

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