Ind. Eng. Chem. Res. 1997, 36, 1973-1978
1973
Radiation Effect on the Thermal Cracking of n-Hexadecane. 1. Products from Radiation-Thermal Cracking Guozhong Wu, Yosuke Katsumura,* Chihiro Matsuura, and Kenkichi Ishigure Department of Quantum Engineering and Systems Science, Graduate School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan
Junichi Kubo Central Technical Laboratory, Nippon Oil Company, 8 Chidori-cho, Naka-ku, Yokohama 231, Japan
In order to examine the applicability of radiation technology to hydrocarbon processing, radiationthermal cracking of n-hexadecane was carried out in the liquid and gas phases by introducing cobalt-60 γ-radiation in the temperature range of 300-400 °C. The cracking process was observed to be significantly enhanced in both phases. For example, it was about 100 times faster under a dose rate of around 500 Gy/h at 330 °C in the gas phase. The observed product pattern for radiation-thermal cracking was the same as that for pure thermal cracking, only dependent on the phase. Similar to pure thermal cracking, addition products and larger yields of alkanes were observed in the liquid phase, whereas no addition products but larger yields of alkenes were observed in the gas phase. In addition, a large evolution of H2 due to C-H dissociation by radiation was observed. Introduction The research of radiation-thermal cracking (RTC) of hydrocarbons is of great practical interest because it might give some insight into the potential utilization of radiations for upgrading of petroleum residues, heavy oils, and bitumen. In early works related to RTC of hydrocarbons, it was reported that γ-radiation accelerated the cracking of n-paraffins (Panchenkov et al., 1981) and even the cracking of coal (Mitsui et al., 1981). From the viewpoint of industrial application, it is of importance to know whether RTC results in the same product composition as pure thermal cracking (TC). Topchiev (1963) observed a marked increase in the alkene content of the C2-C4 products from the gasphase RTC of n-heptane, but Matsuoka et al. (1974, 1975) reported no significant difference in product distributions between RTC and TC of n-butane. In contrast to light hydrocarbons, knowledge on RTC of heavy hydrocarbons is still very rare. Lucchesi et al. (1958) first made RTC of n-C16H34 by irradiation with γ-rays from a cobalt-60 source and a mixture of neutrons and γ-rays from a nuclear reactor. To account for the cracking behavior, the same authors proposed a radicalinitiated chain mechanism similar to the well-known radical mechanism for TC of paraffins (Kossiakoff and Rice, 1943). Panchenkov et al. (1981) performed the RTC of n-hexadecane at 450 °C and investigated some variables, such as dose and dose rate, involving the degree of conversion. However, except the gaseous products, the full RTC products of n-C16H34 have not yet been reported. The kinetic parameters also remain unknown. It is still impossible to make a clear comparison of RTC and TC of heavy hydrocarbons. The present work was carried out to examine the product compositions of n-C16H34 RTC in the liquid phase and gas phase. The decomposition rate constants under various conditions were obtained. The above* To whom correspondence should be addressed. Telephone: +81-3-3812-2111 ext. 6979. Fax: +81-3-5800-6858. E-mail:
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mentioned problem can be answered by comparison with our previous work (Wu et al., 1996) on TC of n-C16H34. Experimental Section Sample Preparation and Irradiation. n-Hexadecane with a purity of above 99% from Tokyo Kasei Co. Ltd. was used as received. As described previously (Wu et al., 1996), two types of Pyrex glass ampules were designed for the experiment. One ampule was about 10 mL in volume, into which 5 mL of n-C16H34 was added. Another ampule was about 50 mL in volume, with 0.5 mL of n-C16H34 added. The cracking in these two ampules was done in the liquid phase and gas phase. n-Hexadecane was put into the ampule with a breakable seal and was vacuum degassed by a freeze-thaw method using liquid nitrogen; the ampule was then sealed under vacuum (
