Hydrocarbon processing, zeolite dewaxing

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 The catalyst is a ZSM-5 -type zeolite in which some of the aluminum has been replaced by iron.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. 

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]. 

These earlier studies have been instrumental in explaining why ZSM-5 deactivates more slowly than larger or smaller pore zeolites. However, few of these studies have considered the performance of a ZSM-5 catalyst in processing complex hydrocarbon mixtures truly representative of refinery streams. This paper examines the deactivation of a ZSM-5 catalyst in dewaxing a series of lube raffinates with different compositions and boiling ranges. 

The separation of linear and branched alkanes is also of importance in the process known as dewaxing, in which the removal of normal alkanes makes the product hydrocarbon less viscous and reduces the so-called pour point temperature. Such processes can be combined with catalytic isomerisations to optimise the value of oil fractions (Chapter 8). Linear paraffins are also separated using a zeolite-based process from kerosene fractions to give reactants for the synthesis of linear alkylbenzene sulfonate anionic surfactants, which are both cost effective and biodegradable. 

Catalytic dewaxing (CDW) was developed by Exxon Mobil in the 1980s. The process employs a shape-selective zeolite called ZSM-5, which selectively converts waxy n-paraffms into lighter hydrocarbons. 

Dewaxing is the process aimed at the conversion of linear long-chain hydrocarbons while leaving intact the branched isomers. The different options of the technology were developed by Mobil, Chevron, and Akzo and are based on the shape-selectivity effects of ZSM-5 zeolite. Other types of medium-pore size zeolites (ZSM-23, B-ZSM-5) have been also used. 

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