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Ind. Eng. Chem. Res. 2004, 43, 1820-1826
Coke Formation on KVO3-B2O3/SA5203 Catalysts in the Catalytic Pyrolysis of Naphtha Won-Ho Lee,* Sang Mun Jeong, Jong Hyun Chae, Jun-Han Kang, and Woong-Jin Lee CRD, LG Chemical Ltd./Research Park, 104-1, Moonji-dong, Yusong-gu, Daejeon, 305-380 Korea
Coke formation on KVO3-B2O3/SA5203 catalysts during naphtha pyrolysis in a fixed-bed reactor was studied. As the KVO3 loading increased, the amount of coke deposited on KVO3-B2O3/ SA5203 decreased, but CO2 formation increased, indicating that KVO3 plays a key role in retarding coke accumulation but not in enhancing the product yields. The coke layers formed on the support and KVO3-B2O3/SA5203 were different in their properties. The coke layer formed on KVO3-B2O3/ SA5203 was free of soluble coke, which dissolves in the CH2Cl2 solution, and consisted of mainly insoluble coke; in contrast, the coke layer formed on the support consisted of both soluble and insoluble coke. The coke layer on KVO3-B2O3/SA5203 catalyst was porous, whereas that on SA5203 was nonporous. KVO3 on the catalyst apparently plays a key role in catalyzing the gasification of soluble coke to CO2. Introduction Light olefins are important basic chemicals for the petrochemical industry and are commercially produced mostly by the thermal cracking of hydrocarbons at high temperatures. Naphtha is an important feedstock used around the world for this process. In the thermal cracking of naphtha, a significant amount coke also forms and accumulates on the inner surface of the reactor tube through which a mixture of naphtha and steam passes. As a result, the pressure drop increases with increasing process time. Therefore, operation should be stopped periodically for the removal of coke. Many studies aimed at achieving a better understanding of coke formation during the thermal cracking of hydrocarbons have been performed.1-5 Catalytic pyrolysis has received attention as an alternative approach to thermal cracking to utilize the feedstock more efficiently because higher yields of ethylene and propylene can be obtained.6-17 In catalytic pyrolysis, catalysts are loaded in the reactor tube through which the mixture of naphtha and steam passes. Recent studies, however, have shown that enhancement of the olefin yields might not be due to the catalytic effects, but rather to the enhancement of heat transfer.18-22 Data appearing in the literature and in patents indicate that high reaction temperatures are required to obtain high olefin yields in catalytic pyrolysis even though these temperatures are slightly lower than those used in thermal cracking. Thus, the catalytic pyrolysis process is not free of the coke problem due to the nature of cracking reaction. In addition to the accumulation of coke on the reactor wall, coke growth on the catalyst surface seems to be inevitable. The accumulation of coke on the catalyst particle would cause more serious problems such as a loss of catalytic activity and a sharp increase in the pressure drop caused by aggregation of the catalyst particles, resulting in clogging of the reactor and lowering of the olefin yield. * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +82-42-866-2408. Fax: 82-42-8637466.
A number of catalysts have been investigated with various feedstocks for the catalytic pyrolysis of hydrocarbons. Among these catalysts, one of the most promising types has been reported to be the KVO3-based catalysts with low-surface-area supports developed by VNIIOS.6-9,11,12 Although several articles dealing with coke formation on this catalyst system have been published,6-9,11,12 further systematic study has been needed for better understanding. Recently, we reported that coke formation is strongly influenced by the physical properties of R-Al2O3 spheres used as catalyst supports. That is, coke deposition is the least on R-Al2O3 spheres with the lowest surface area and pore volume among the tested R-Al2O3 spheres.21,22 In this paper, we report on the characteristics of coke formation on KVO3-B2O3/SA5203 catalyst, as well as on the SA5203 support, during the pyrolysis of naphtha in a quartz tubular reactor. Experimental Section Catalyst Preparation. Catalysts were prepared by impregnating a low-surface-area (
