Severe hydrotreatment

Some other processes are based on a severe hydrotreatment followed by a stage for octane recovery. Octgain from ExxonMobil [57] and ISAL from UOP-Intevep [58], Deep desulfurization is achieved by an increase in severity, causing lost in octane by olefins saturation. In the first case, in a second reactor octane number is recovered by a combination of cracking and isomerization reactions. In the latter case, the catalyst employed during desulfurization possess isomerization capabilities inhibiting an excessive octane lost. Other mentioned functionalities of the catalyst include dealkylation and conversion. 

Coal liquids require less severe hydrotreatments than shale oil due to the lower nitrogen content. 

The lighter distillates and naphtha from oil shale and coal can be refined by processes used for petroleum, except that more severe hydrotreatment will be required to remove nitrogen and the other nonhydrocarbon impurities that poison catalysts and cause product instabiltiy. 

Paraffinic base oils of very high viscosity index are also manufactured by severe hydrotreatment or hydrocracking processes. Section 1.5, in which isoalkanes are created by chemical reaction and their crude oil origin is of reduced importance. 

Severe hydrotreatment actually decreases the range of molecular types within the base oil, compared to solvent-extracted base oils. Hydrotreated base oils produced from different crude oils have more consistent properties than solvent-extracted oils made from different crude oils. Since the hydrotreating reactions create high-VI molecules, it is possible to produce base oils from crude oils with an inherently low content of higher VI components and would normally be unsuitable for conventional solvent refining [5]. 

Another variant of the severe hydrotreatment process is the substitution of wax for lubricant distillate as feedstock. The wax recovered from conventional solvent dewaxing units is essentially a pure alkane feedstock containing a high proportion of linear alkanes. With this type of feedstock and under appropriate operating conditions, the isomerisation reaction can be made to predominate over cracking reactions. Unconverted wax can be removed by conventional methods to yield a base oil that is exclusively composed of isoalkanes and that resembles synthetic polyal-phaolefin base fluids more closely than the hydrocracked base oils described in Section 1.5.2. A comparison of some of these base fluid properties is shown in Table 1.4. 

Bull, S. and Marmin, A. (1979) Lube oil manufacture by severe hydrotreatment, PD 19, 10th World Petroleum Congress, Bucharest. 

In previously reported stability work with shale oil derived jet fuels (16) it was shown that the JFTOT thermal stability improved as the total nitrogen content decreased. In Table III, it is observed that the thermal stability of the Shale-I fuel improves as the concentration of basic nitrogen compounds decreases. In previous work (16) the lower nitrogen contents of the shale oil jet fuels were achieved by more severe hydrotreatment. 

Source S. Bull and A. Mamin, Lube Oil Manufacture by Severe Hydrotreatment, Proceedings of the 10th World Petroleum Congress 4 221-228 (1980). With permission. 

The chemistry that occurs within a hydrocracking reactor or severe hydrotreater is complex due to the variety of chemical structures that make up the feed and the number of different types of catalytic reactions that occur. As in many instances, simplifications have, of necessity, been introduced to develop an understanding of the processes and these in turn have been studied using model compounds to develop or confirm hypotheses. 

At the same time as diesel fuel specifications are tightening, the demand for diesel is growing in Asia as well as in Europe. Although the growth rate for middle distillate in Asia Pacific was relatively stagnant from 1996-1998, it rose to 4.6% in 1999, and future demand in developing Asia is forecast to be 5.1% per year. In the EU, the increase in demand for mid-distillates has been at about 1.2%/armum over the past 5 years. It is predicted that the diesel demand in the EU will increase by 30% over the next 15 years. There is a reduced demand for home heating oil and fuel oil cutter stocks, which can help meet the increased need for diesel, but these lower quality stocks will require severe hydrotreatment to meet the future specifications for diesel fiiel. 

The EU defines the environmental risk of these oils by measuring the oil s polycyclic aromatic content through DMSO extraction. Some aromatic oils can be made to pass this EU test if they are processed by applying severe hydrotreatment, which destroys some of the polycyclic aromatics. 

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