Heteroatom removal

The formation of these thermal fragments is necessary to catalytic liquefaction processes before the catalysts can become effective for hydrogen introduction, cracking and/or heteroatom removal (10). ... 

Long contact time thermal processes have the intrinsic disadvantage of poor selectivity for light hydrocarbon gas formation relative to heteroatom removal (see Figure 3). 

The importance of heteroatom removal from crude oil feedstocks has therefore directed most of the R D investments toward the design of more efficient heterogeneous catalysts for the reactions in Equations (12), (13), (14), (15). A considerable contribution to a better understanding of the HDS and HDN mechanism has been provided by homogeneous studies involving soluble metal complexes.161,162... 

Heteroatom removal from petroleum feeds is important for improving their processability and for eventual production of clean environmentally friendly fuels. 

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.

Iron catalysts exhibit much lower activity for the heteroatom-removing reactions such as hydrodenitrogenation(HDN) and hydrodesulfurization(HDS) compared with Mo-based catalysts (97). 

The catalyst s resistane to coking may also be improved by the solvent while maintaining high catalytic activity. Hydrogen pressure can, thus, be reduced to 110 atm in a two-step hydrotreatment, as shown in Table II (21, 128). The two-step hydrotreatment assisted by the proper solvent not only improves the extent of hydrocracking and heteroatom removal, but it also allows the reduction of the hydrogen pressure required for efficient upgrading. 

Figure 27.6 Percentage hydrogenation and heteroatom removal as a function of reaction time for three nanoscale catalysts. For explanation of parts (a) to (d) see text.

M02C.4 Metal carbide catalysts have the additional advantage of displaying high tolerance towards poisoning, and effectiveness in heteroatom removal.6 Both the technological applications of early transition metal carbides and the physical properties underlying their correspondence to the precious metals are under study. 

C02 laser pyrolysis of reactant gases has been used to produce a wide variety of dispersed, single crystal nanoparticles (average size 2 to 20 nm). This chapter discusses the production of nitrides (oxynitrides) and carbides (oxycarbides) of Group 6B elements (Mo and W) and Fe by this technique. The emphasis is on the characterization of the atomic order in the particle and the chemical state of the particle surface. The catalytic properties of these particles for coal liquefaction and heteroatom removal from model compounds is also addressed briefly. 

Catalytic activity of Mo2NxOy and Mo2CxOy for heteroatom removal... 

Results of the activity for heteroatom removal of nanoscale 8%Mo/ Fe203/S042, Mo2N Oy and Mo2CxOy are shown in Figure 27.6. They are expressed as the fraction of heteroatom removed (S, O, N), or the fraction of hydrogen consumed to obtain complete hydrogenation, as a function of reaction time. Table 27.3 summarizes the results for the activity of the nanoscale catalysts with the activity expressed as an areal rate (rate m-2), and also as a turnover rate for the oxycarbide and oxynitride. 

The catalytic activity of Fe carbides, molybdenum oxynitride and oxycarbide has been evaluated for coal liquefaction and heteroatom removal of model compounds related to coal. Preliminary results show that the LP nanoparticles are active catalysts for coal liquefaction. In fact, they are more active for heteroatom removal than a molybdenum promoted sulfated hematite, even though surface characterization indicates that as introduced into the reactor they exhibit surface oxidation. 

Flo. 31. Proposed mechanism for asphaltene conversion (a) destruction of asphaltene micelle (b) depolymerization due to heteroatom removal (Asaoka a al1983). 

For the activity comparisons, the heteroatom removals (and CCR reduction) are plotted versus reactor temperature at a liquid hourly space velocity of 1.0. Consequently, catalyst activity can be compared on the basis of temperature requirements for achieving specific liquid product heteroatom (or CCR) contents. 

The CCR reduction activity for the nine catalysts is similarly plotted as a function of temperature in Figure 5 and Figure 6. Mobil HCL-2, HCL-3, and Amocat IB are the most active catalysts. Generally, heteroatom removal can be achieved with only minor changes in chemical structure. 

Run 15 was carried out with a higher LHSV of 1.6, and the results are not too much different from Runs 13 and lit. The heteroatom removals are about the same. The heavy oil fraction, however, was increased to 13 from 30 and 36% (Runs 13 and lit). Thus, the conclusion is that the lignite extract, a solid, can also be upgraded to yield a syncrude by the supercritical hydrotreatment process. 

For heteroatom removal, the results reveal that for a LHSV of 1.6 the nitrogen content in the heavy oil fraction, which is about 3 t wt of the total products, is 360 ppm (1.9 in the feed). Thus, the overall nitrogen removal is greater than 98. For LHSV of 3.2 the nitrogen content in the heavy oil is increased to about 5000 ppm. With a further increase in LHSV to 5, there is a greater increase in the nitrogen content. The sulfur removal is very rapid. Even with the LHSV of 5, there is only 53 ppm sulfur in the heavy oil. 

The results of this study showed that flash pyrolysis subbituminous coal tar can be hydrorefined. Ni-Mo supported on alumina was the most effective catalyst for hydrorefining flash pyrolysis subbituminous coal tar among the five types of catalysts tested. The removal of asphaltenes and preasphaltenes was easy to achieve. Catalyst selection should emphasize heteroatoms removal efficiency and hydrogen consumption. The catalyst life is an equally important selection criterion although this program has not completed catalyst-life studies. It will be determined in the future. 

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