DEWAXING CATALYSTS

A light diesel fuel was produced by distillate hydrotreating of the straight-run Fe-HTFT material, while the heavier fraction was hydrocracked over a dewaxing catalyst, which produced a heavy diesel (Table 18.10). Some diesel fuel was also produced by C3-C4 olefin oligomerization over solid phosphoric acid by recycling the naphtha thus produced. It has previously been pointed out that solid phosphoric acid is not well suited for distillate production,42 and the hydrogenated... 

The first generation of dewaxing catalyst relied on the selective cracking of linear paraffins (Fig. 8.8). 

It is the complex relationship between the yield of base stock, its VI and its cold flow properties (pour point and/or cloud point) that leads to the differentiation and optimization of selective dewaxing catalysts and processes, where shape selective catalysis is of critical importance. For example, Figure 8.10 shows the advantages of MSDW-2 over earlier technologies such as solvent dewaxing and MSDW-1 (25). 

Because of the requirement for selectivity, dewaxing catalysts generally comprise an aluminosilicate zeolite, having a pore size which admits the straight chain n-paraffins but which excludes more highly branched materials, cycloaliphatics and aromatics. Zeolites such as ZSM-5, ZSM-11, ZSM-12, ZSM-23, ZSM-35 and ZSM-38 have been used for catalytic dewaxing. 

Dewaxing catalysts are manufactured by both Akzo Nobel and Mobil. Mobil s Isomerization Dewaxing (MIDW) first commercialized in 1990 uses a Pt-based zeolite catalyst to crack and isomerize n-paraffins to iso-parrafins, thereby converting fuel oil to low-pour-point distillates. 

Besides ZSM-5, other zeolites such as ferrierite and mordenite loaded with a noble metal (Pt, Pd) have also been found to be effective lube dewaxing catalysts [171,172]. In this case, high hydrogen pressures are applied to saturate the cracked by-products and to ensure a stable catalyst activity by reducing coke formation, particularly in the case of mordenite which is more susceptible to coking owing to its unidirectional pore system. 

In spite of the fact that Chevron chose not to use their dewaxing catalyst for their heavy stream, in an interesting application they did employ the same (or very similar) catalyst to dehaze the heavy dewaxed oil when pinholes or tears in the filter fabric caused leakage of wax into the solvent dewaxed oil. The refinery s... 

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