Saturation of aromatics

Olefins have often been shown to be intermediates in the saturation of aromatics (6,29,35,37,48,49), but their formation, in varying amounts, lacks synthetic utility. In the presence of an acidic catalyst, the intermediate olefin can be trapped by alkylation. Phenylcyclohexane has been obtained in good yield from benzene by this technique (40). 

The Syn technology is oriented to the production of ULSD, to the saturation of aromatics and to the improvement of cetane number. These processes can be run in a single reactor unit, in which case, the initial part of the reactor is fed in a co-current way,... 

Unisar [Union saturation of aromatics] A process for hydrogenating aromatic hydrocarbons in petroleum fractions, using a noble metal heterogeneous catalyst. Developed by the Union Oil Company of California. The first commercial unit opened in Beaumont, TX, in 1969 eight commercial plants were in operation in 1991. 

Figures 13 and 14 also show that hydrotreating the catalytic cracker feedstock increases the zeolite cracking. C3, and C5+ compounds are possible products of primary zeolite cracking. These figures show that hydrotreating of the feedstock results in larger yields of these primary cracking products and hence more valuable products. This improvement is most likely due to the heteroatom removal and the saturation of aromatic compounds during hydrotreating which tend to block active sites and reduce the activity of the catalyst.

Hydrocracking, the other major cracking operation, is predicted to grow approximately 3-5% annually, mainly because it operates at relatively high hydrogen pressure (typically >100 atm), which results in high removal rates of S and N heteroatoms from the feedstock and deep saturation of aromatic compounds. Consequently, it produces middle distillates with excellent product qualities, namely, jet and diesel fractions with very low sulfur content and very good combustion properties. 

These different requirements can be fulfilled by different catalysts, and different operating conditions will be required. In general, high pressure increases the rate of the saturation of aromatic rings. Therefore, for the hydrofining of gasolines or coke-oven light oil, pressures below 50 atm. are used. 

Crude oil contains about 0.01% metals and up to 5% sulfur present in large aromatic structures. These levels are highly dependent on the origin of the crude. For example, California crude is relatively low in sulfur but higher in metals than crude from Kuwait. Any process to remove them must be economical with little destruction of the hydrocarbons and minimum consumption of H2. The catalyst is Co, Mo/A1203 with particles a few mm in diameter. Although sulfur is usually a poison for catalytic reactions it is used here in a positive function to control selectivity. It is presulfided to decrease activity towards excessive consumption of H2 that leads to unwanted saturation of aromatic molecules. 

An example of an equilibrium-limited reaction is the hydrogenation of aromatics in petroleum fractions. This is illustrated by Fig. 3, which shows that under the prevailing conditions, saturation of aromatics is kinetically limited at temperatures below about 370°C, but that above this temperature, thermodynamic limitation occurs. In cocurrent operation, the hydrogen partial pressure will be lowest at the reactor outlet, due to the... 

A corollary of this statement is the following If these polyaromatic or polycyclic saturated structures are present in the carbon skeleton of coal, they should be identified in the short-contact-time liquefaction products. The possibility of some isomerization reactions in the carbon skeleton cannot be excluded totally, but the most important fact is that no dramatic aromatization of hydroaromatic rings or saturation of aromatic rings takes place under these conditions. Many of the chemical functions also are stable under these conditions, especially the O, S, and N heterocyclic aromatic structures. Water formation by phenol dehydroxylation is minimal. In coal liquefaction under our conditions, even at long reaction times (up to 90 min) in the absence of an added catalyst, the -OH bonded to a monoaromatic ring is stable. Under the same conditions, dehydroxylation of polyaromatic phenols does occur (10). 

For olefins to be intermediates in the saturation of aromatics, some finite concentration, no matter how small, should be present. The only evidence that could be cited was an observation by Madden and Kem-ball (60) that cyclohexene was present during the early stages of vapor-phase hydrogenation of benzene in a flow system over a nickel film. Two factors were working against the detection of olefins. First, it turns out that the platinum and palladium catalysts studied by Siegel et... 

Much of the basic information available on thermochemical aspects of HDA came initially from academic studies16-20 on pure compounds, undertaken to establish some of the basic chemistry of hydrogenations in general. Kistiakowsky et al.16 calorimetrically established that saturation of aromatic rings was exothermic and that the enthalpies of hydrogenation (H355.K) of a number of monoaromatics decreased with increasing alkyl substitution—benzene (49.8 kcal/mole), ethylbenzene (48.9 kcal/mole), o-xylene (47.25 kcal/mole), and 1,3,5-trimethyl-benzene (47.62 kcal/mole). 

For the saturation of aromatics in a hydrotreating or hydrocracking unit, equilibrium effects, which favor formation of aromatics, start to overcome kinetic effects above a certain temperature. This causes a temperature-dependent aromatics cross-over effect, which explains the degradation of important middle distillate product properties—including kerosene smoke point and diesel cetane number—at high process temperatures near the end of catalyst cycles. The cross-over temperature is affected by feed quality and hydrogen partial pressure, so it can differ from unit to unit. 

Again, we ran the case study using the hydrotreating portion of the Suncor-Samia hydrocracker model with an equal-outlet temperature profile. For 500 wppm sulfur in the product, the makeup H2 flow is 15.2 MMSCFD. For 15 wppm, the makeup flow is 18.6 MMSCFD, more than 40% higher. Higher WART leads to increased conversion, increased HDN, and (up to point) increased saturation of aromatics. This explains why the hydrogen consumption increased so much— and so non-linearly— when the target product sulfur content went from 500 wppm to 15 wppm. 

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