Dewaxing, zeolites

Endewax [National Chemical dewaxing] A process for dewaxing heavy petroleum fractions by treatment with a catalyst which converts the long-chain hydrocarbons to shorter ones. The catalyst is a ZSM-5 -type zeolite in which some of the aluminum has been replaced by iron. Developed by the National Chemical Laboratory, Pune, India, and piloted in 1991. 

MDDW [Mobil Distillate DeWaxing] A process for removing waxes (long-chain normal paraffins) from petroleum fractions by cracking over the zeolite ZSM-5. The waxes are converted to liquid hydrocarbon fuels. Twenty one units were operating in 1990. 

MIDW [Mobil isomerization dewaxing] A petroleum refining process which improves yield and quality by isomerizing and selectively cracking paraffins in waxy oils. The catalyst is a noble metal, supported on a zeolite. Developed by Mobil Corporation from 1991 to 1996. 

Catalytic dewaxing, in which straight-chain paraffin hydrocarbons are selectively cracked on zeolite-type catalysts, and the lower-boiling reaction products are separated from the dewaxed lubricating oil by fractionation. 

Since zeolite catalysts are successfully introduced in the refining and petrochemical industries, it is not surprising that most of the recent advances concern incremental improvements of existing processes with the development of new generations of catalysts (e.g., dewaxing, ethylbenzene and cumene synthesis). The number of newer applications is much more limited, for example, direct synthesis of phenol from benzene and aromatization of short-chain alkanes, etc. However, both the improvement and development of processes contribute significantly to environmental advances. 

Dewaxing is the final example of a reaction illustrated here with possibly multiple restricted transition state shape selectivity effects. Bifunctional zeolitic catalysts... 

Dewaxing is an important specific hydrocracking process used to improve diesel and heating oils by pour-point reduction.88 This is achieved by shape selectivity of certain zeolites allowing selective hydrocracking of long-chain paraffinic waxes to C3—C5 alkanes in the presence of other paraffins.89 Platinum H-mordenite is used in an industrial process.90... 

As mentioned earlier, base oils are required to meet stringent cold flow properties, such as pour point and cloud point. This is achieved by dewaxing of the base oil, i.e. by removing the high molecular weight linear paraffins as done by solvent dewaxing. More recent approaches have been centered on the use of catalysts, particularly the use of zeolitic catalytic materials. 

Table 8.2 Typical structure of medium pore zeolites useful for dewaxing...

LP mordenite doped with platinum is the catalyst used in the Shell Hysomer process for alkane hydroisomerization this can be linked to the Isosiv separation process whereby -/n-alkanes are separated on LTA zeolite. This link is the basis of the joint Shell/UOP Total Isomerization Process (TIP). Similar catalysts have been employed to hydrocrack (i.e. dewax) diesel fuels. There have also been reports of commercial use of mordenite to catalyze methanol amination with ammonia, to produce dimethylamine. 

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. 

Bifunctional zeolite catalysts such as platinum loaded acid zeolite catalysts are applied in several petroleum refinery operations, designated as hydroconversion processes isomerisation of light n htha, iso-dewaxing and hydrocracking of heavy fractions [4]. Most experimental investigations in academic laboratories are typically performed with pure model components or simple mixtures thereof as feedstock, and using reaction conditions under which the hydrocarbon compounds are in the vapor phase. Industrial hydroconversion processes are mostly run under three phase, or even in some cases under liquid phase conditions and with feedstocks that are extremely complex mixtures of large numbers of different hydrocarbon compounds [4]. 

The catalysts used for dewaxing are usually bifimctional in nature with Pt being the hydrogenation-dehydrogenation conq>onent and a large pore typically 12MR zeolite provides the acidic component [1]. ZSM-5 is the catalyst used in Mobil s Distillate Dewaxing (MDDW)... 

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