Catalytic cracking of hydrocarbon feedstocks on HY/HZSM-5 zeolite combinations

The reactions of a number of hydrocarbons under cracking conditions have been examined on HY and HZSM-5 zeolites both in isolation and in combination, using a fixed bed reactor at elevated temperatures. Three pure hydrocarbon feedstocks (n-hexadecane, a binary mixture of n-hexadecane and 1-methylnaphthalene and a multi-component hydrocarbon mixture) were used to model the reaction processes which may occur with more complex industrial feedstocks. These feedstocks were chosen as being representative of the types of molecules which may be present in industrial feedstocks. The catalytic cracking of the linear paraffin n-hexadecane (`C_(16)H_(34)`) was investigated on HY and HZSM-5 zeolites in isolation at 400°C. The observed product distributions from reaction on the two zeolites, gave results consistent with both their pore geometry and their relative acidities. For example, products of lighter molecular weight were observed in higher yields on the pentasil relative to that for the faujasite. The degree of branching for paraffin and olefin isomers suggested that more highly branched species could be produced on HY, due in part, to its larger pore dimensions. When adding =5 wt% of fresh HZSM-5 to the HY, distinct shifts in observed product distributions occurred. A higher yield of olefins (particularly in the range `C_3 -C_4`) at the expense of paraffins was noted, resulting in an overall more highly olefinic product from reaction on the zeolite combination. The branched to linear ratio for paraffin isomers was found to decrease upon pentasil addition, due to the preferential formation of linear species on the pentasil. The ratio for olefins, however, increased, due to the high isomerisation activity of HZSM-5 towards olefins. These results could be explained in terms of the pentasil additive inhibiting hydrogen transfer processes which would otherwise occur readily on the faujasite. It was found that the reactivity of the linear paraffin at 400°C on HY zeolite decreased markedly when combining n-hexadecane in a binary mixture with 1-methylnaphthalene. This could be explained in terms of the blocking of zeolite pores or active sites by the bulky dinuclear aromatic. Product distributions were not influenced by combination of the two feed components; products from reaction of n-hexadecane in isolation, seemingly did not interact with the products from reaction of 1-methylnaphthalene (namely 2- methylnaphthalene, naphthalene and dimethylnaphthalenes) on the faujasite zeolite. The product distributions obtained from reaction of the binary mixture on a combination of HY and fresh HZSM-5 zeolite thus revealed similar results to that found for n-hexadecane cracking on the zeolite combination. A multi-component feedstock consisting of branched and linear paraffins, naphthenes, alkylated monoaromatics and polyaromatics was prepared to mimic an industrial feedstock. Both product formation and feedstock component removal were analysed. Feedstock component utilisation on HY and HZSM-5 could again be explained in terms of zeolite pore sizes. Noticeable changes in component utilisation were noted upon addition of fresh HZSM-5 to HY zeolite. Comparison of product distributions obtained from cracking the multicomponent feedstock on HY with those from cracking on the zeolite combination, again highlighted the inhibition of secondary processes on HY in the presence of HZSM-5. As noted previously, paraffin yields diminished, with a concurrent increase in olefin yields. Similar effects on the degree of branching of paraffin and olefin isomers were also noted. The results from this feedstock suggested that not only may HZSM-5 influence product formation when added to HY zeolite by reacting primary products from the faujasite, but it may also alter feedstock component utilisation. The results from these model hydrocarbon studies were successfully translated to the reaction of a Gippsland reduced crude on zeolite combinations. When using either a fresh or steam deactivated HZSM-5 additive, diminished gasoline yields were offset by increases in gas fraction products. The mechanism by which the pentasil influenced products formed on the faujasite was different on the fresh and steamed samples. Using fresh HZSM-5, results suggested that the additive was active in selectively cracking heavier linear paraffins into ones of lighter molecular weight. Olefin isomerisation was also important. Using steamed additive, paraffin cracking was negligible, however the HZSM-5 retained its activity for olefin isomerisation and cracking. Steaming the pentasil additive resulted in increased branched to linear ratios for both paraffins and olefins, an important consideration for gasoline octane enhancement. It may thus be concluded from this study, that the mechanism by which HZSM-5 additive affects product formation on HY zeolite, is dependent on the pre-treatment conditions of the pentasil. When using fresh HZSM-5, selective paraffin removal operates, and having linear paraffins as feed components, results in decreased branched to linear ratios for paraffin species. By severely steam-deactivating the pentasil, paraffin cracking becomes negligible. Instead, the steamed additive primarily isomerises and cracks linear olefins from the gasoline. Highly olefinic products relative to that obtained on the base faujasite suggest that hydrogen transfer processes are inhibited in the presence of the pentasil, whether it be steam-deactivated or not.