The demand for hydrocarbons remains a growth industry. The uses of hydrocarbons include the development of better fuels, as well as useful precursors for detergents, and for polymers.
In particular, the production of kerosene is important for numerous products, including motor fuels, and the production of detergents. The production of precursors for detergents includes the separation of a kerosene feedstock into a component comprising normal hydrocarbons and a component comprising non-normal hydrocarbons.
Kerosene range hydrocarbons can come from numerous sources, and as demand has increased, there has been an increase in usage of lower quality sources of hydrocarbons, such as petroleum coke.
Special commercial uses of normal paraffins require that the normal paraffins contain an especially low concentration of aromatics. By normal paraffins, it is meant straight-chain, linear or unbranched paraffins. One of these special uses is the manufacture of detergents made from alkylbenzenes, in which C9 to C22 normal paraffins are dehydrogenated to olefins that are then used to alkylate benzene. The problems with aromatics in the normal paraffins, particularly aromatics having the same carbon number as the normal paraffins, arise during the alkylation step because of the occurrence of two side-reactions: first, the ring of the aromatic can react with an olefin to produce a heavy, dialkyl benzene by-product, and second the side-chain of the aromatic can be dehydrogenated and react with benzene to produce a heavy, biphenyl by-product. Either by-product is not suitable for detergents. These side-reactions result in waste of valuable feedstocks, costs for separation and disposal of by-products, and economic loss. For these reasons, there is sometimes a preference that the concentration of aromatics in normal paraffins used for commercial production of detergents be less than 0.005 wt-% (50 wppm) of the normal paraffins.
The most plentiful, commercial source of C9 to C22 normal paraffins is crude oil, in particular the kerosene-range fraction. By “kerosene-range” is meant the boiling point range of 360° F.-530° F. (182° C.-277° C.). This fraction is a complex mixture comprising normal paraffins, iso-paraffins, and aromatics from which the normal paraffins cannot be separated using conventional distillation. Depending on the type of crude from which the hydrocarbon fraction is derived and the carbon number range of the fraction, the concentration of normal paraffins is usually 15-60 wt-% of the feed and the concentration of aromatics is usually 10-30 wt-% of the feed. There may be more unusual feed streams which have aromatic concentrations of only 2-4 wt-% of the feed.
The separation of various hydrocarbonaceous compounds through the use of selective sorbents is widespread in the petroleum, chemical and petrochemical industries. Sorption is often utilized when it is more difficult or expensive to separate the same compounds by other means such as fractionation. Examples of the types of separations which are often performed using selective sorbents include the separation of para-xylene from a mixture of xylenes, unsaturated fatty acids from saturated fatty acids, fructose from glucose, acyclic olefins from acyclic paraffins, and normal paraffins from isoparaffins. Typically, the selectively sorbed materials have the same number of carbon atoms per molecule as the non-selectively adsorbed materials and very similar boiling points. Another common application is the recovery of a particular class of hydrocarbons from a broad boiling point range mixture of two or more classes of hydrocarbons. An example is the separation of C10 to C14 normal paraffins from a mixture which also contains C10 to C14 iso-paraffins.
One of the principal prior art processes for the selective removal of the aromatics from the kerosene-range fraction employs a sorption process that separates the normal paraffins and the iso-paraffins. The sorbent used in this process has pores which the normal paraffins can enter, but which the aromatics, like the iso-paraffins, cannot enter because their cross-sectional diameter is too great. Contacting a kerosene-range feed with the sorbent produces a raffinate stream containing almost all of the iso-paraffins and aromatics that were in the feed, and a sorbent loaded with sorbed normal paraffins. Then, contacting the loaded sorbent with a desorbent stream produces an extract product containing almost all of the normal paraffins in the feed. But, sorbents used in this process are not ideally selective for normal paraffins, and where the sorbent comprises a crystalline zeolite and an amorphous binder, the binder itself may be selective for aromatics. Consequently, a small portion of the feed aromatics is rather tenaciously sorbed on the surfaces of the sorbent and ultimately appears as a contaminant in the extract (normal paraffin) product. With a typical kerosene-range feed and a commercial sorbent, the concentration of aromatics is usually 0.15-0.50 wt-% (1500-5000 wppm) of the extract product, which is sometimes unacceptably high for production of commercial detergents.
The use of lower quality sources of heavy hydrocarbons requires the processing of that hydrocarbon to allow its usage in today's industries.