Cracking of hexadecane

The hydrocarbons in cmde oil are alkanes, olefins, aromatics, polyaromatics, and organic compounds containing S, N, 0, and heavy metals. Since there are many isomers of aU of these types of molecules, the reactions implied by the preceding equations rapidly approach infinity. A representative reaction of these might be the cracking of hexadecane (number 3 heating oil) into octane and octette (components in gasoline). 

Catalytic Cracking of Hexadecane Temperature 500°C. Catalyst silica-alumina-zirconia Pressure atmospheric Process Period 1 Hour... 

Alkanes are the dominant initial product at 573 K whereas olefins are dominant at 673 K in the cracking of hexadecane over HY zeolite. This was explained as being the result of more extensive hydrogen transfer at the lower temperature. ... 

Typical results tor the cracking of the base feedstock, n-hexadecane, are presented in Fig. I, in terms of product distribution. Components are listed as carbon numbers up to Cl5, but also including the amount of coke formed. The product distribution attains a maximum at the C3t C4 and C5 region, following which there is a monotonic decrease in product concentration to 04. An increase then occurs for the C15 concentration and the coke. 

Through the use of hexadecane cracking alone, we have been unable in this work to elucidate the role of mesoporosity in the catalytic behavior of calcined or steamed zeolites. Steamed AFS and USY zeolites show differences in mesoporosity but exhibit similar catalytic performance. While mesoporosity may affect diffusion in actual FCC catalysts, larger molecules than hexadecane will be required to determine mesopore effects. 

Table I. Comparison of Product Distribution from Hydropyrolysis (HP) and Thermal Cracking (TC) of -Hexadecane (1)° °...

The hydropyrolysis of a representative polycyclic naphthene, i.e., decalin (2), was investigated as a function of reaction temperature (525°-625°C) and hydrogen pressure 500-2000 psig). In addition, a comparison between hydropyrolysis and thermal cracking of 2 was made by doing a parallel study with this compound under nitrogen pressure, using otherwise identical experimental conditions. The apparatus and experimental methods were the same as in the study of n-hexadecane (see Experimental section). 

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. 

HZSM-5 zeolite is a highly active catalyst for cracking of n-dodecane and n-hexadecane, and more than 90% conversion can be obtained at 320°C and WHSV of... 

The resulting equation was found empirically by E. B. Burk (3) and M. D. Tilicheev (22). The above-studied mechanism of the cracking of n-paraffins explains the absence of dependence of the cracking rate on reaction extent. This differs from the case of the low molecular weight paraffins experimentally determined by Kasanskaya (6) and Panchenkov and Baranov (19) for n-octane and n-hexadecane. 

Figure 2.1-2 Catalytic cracking of n-hexadecane. Solid line ejqrerimental products, 24 per cent conversion over alumina-zirconia-sUica at 500 C. Dotted line Calculated products, car-bonium ion mechanism (from Greensfelder, Voge, mdGood 2 f. 

As we learned more about the hydrocarbon reactions, some similarities to reactions catalyzed by strong acids at much lower temperatures became evident. An additional and different impetus to understanding came from an interlude of thermal cracking studies. At the time we were interested in the thermal cracking of normal paraffins (waxes) for production of alpha olefins. Thermal cracking of n-hexadecane gave products in... 

As soon as we applied the concept of the carbonium ion intermediate, many previous observations could be correlated. A few simple rules about formation, isomerization, and cracking of the hypothesized ions explained most of the experimental data on hydrocarbon cracking. Quantitative prediction of the products from n-hexadecane cracking was possible with the aid of only one additional assumption, as noted in the paper of Greensfelder, Voga and Good (6). For details of the carbonium ion mechanism... 

Cis was not determined in these tests. In some similar experiments one mole of Cis product was obtained per 100 moles of hexadecane cracked. 

Wu, G., Katsamura, Y, Matsuura, C., and Ishigure, K. 1997. Radiation effect on the thermal cracking of n-Hexadecane. 1. Products from radiation-thermal cracking. Ind. Eng. Chem. Res. 36 1973-1978 2. A kinetic approach to chain reaction. Ind. Eng. Chem. Res. 36 3498-3504. 

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