Contents 4 Hydrocracking

Hydrocracking makes very good quality diesel fuels concerning the cetane number, cold behavior, stability, and sulfur content. However this type of stock is only available in limited quantities since the process is still not widely used owing essentially to its high cost. 

In two stages with recycle to the second stage, the conversion per pass is approximately 50 wt. % and the selectivity to middle distillates is maximal 75 to 80 wt. %. However, the investment is clearly higher and is justified only when feedstocks are difficult to convert and that their content in nitrogen is high. Figure 10.11 represents two variants of the hydrocracking process. 

Diesel Fuel. Eederal diesel specifications were changed to specify a maximum of 0.05% sulfur and a minimum cetane index of 40 or a maximum aromatics content of 35 vol % for on-road diesel. Eor off-road diesel, higher sulfur is allowed. CARB specifications require 0.05% sulfur on or off road and 10% aromatics maximum or passage of a qualification test. Process technologies chosen to meet these specifications include hydrotreating, hydrocracking, and aromatics saturation. 

More severe hydrotreating, such as mild hydrocracking at an H2 partial pressure of >6.9 MPa (1000 psig) and temperature of >400° C, can stiU further reduce sulfur content in the FCC feed. Mild hydrocracking of a 25° API VGO reduced the sulfur content of the FCC feed 98%, from 1.7 wt % to 0.03 wt % (40). This reduction can be expected to lower the SO content in the FCCU regenerator flue gas by about 90%. 

Feed hydrotreating or hydrocracking reduces SOj, emissions and the sulfur content of FCC products. As discussed earlier in this chapter, many benefits are associated with FCC feed hydrotreating. It is important to note that most of the sulfur in a hydrotreated feed is in heavy organic compounds and will be concentrated in the decanted oil and coke. Consequently, for a given sulfur in the feed, more SO, will be produced with hydrotreated feed. 

Among the classes of feedstock processed in the hydrocracker the most highly aromatics feed are light cycle oils produced in the FCC unit Once formed by cyclization and the hydrogen transfer mechanism discussed above, they accumulate in the product due to the absence of a metal function in the FCC catalyst and adequate hydrogen in the process environment. They are typically sold as low-value fuel oil, or hydrotreated to reduce sulfur content and improve their quality as diesel blend stocks. Another approach to upgrade their value even further... 

Figure 16.n Hydrocracking catalyst performance in single stage recycle as a function of zeolite content and unit cell size. 

The distillation fractions were also analysed for their caibon and hydrogen contents using a Leco CHN Determinator which was also used for similar analysis of die used catalysts. The hydrocracked liquid and the used catalysts were analysed for their sulphur contents using a Leco Suli iur Determinator. Some specific surface area analysis by nitrogen adsorption was carried out on the used catalysts using a Micromeritics instrument... 

Table V summarises the data of the sulphur analysis of the hydrocracked liquids and the various bpt fractions for CoMo and NiMo catalysed experiments. The sulphur contents of neither the total hydrocracked liquids nor the individual bpt fractions showed any dependence on repeat contact or catalyst type. The values did show that the sulphur concentrated in the recycle solvent fraction (275-450°C), suggesting that, even under the relatively strong conditions used, certain sulphur-containing compounds will survive to be recycled in the solvent However, the sulphur content of the coal liquid feed was reduced by about 50% and the sulphur content of the likely upgradable product was low.

Table V. Sulphur Content of Hydrocracked Liquids and Distillation Fractions for CoMo and NiMo Catalysed Experiments...

The relative performances of the two autoclaves were compared by the use of ratios. Three ratios were derived utilising (a) the unconverted phenanthrene, denoted by P (b) the amounts of the various hydro-derivatives of phenanthrene multiplied by the relative number of hydrogens added, eg % tetrahydrophenanthrene 4, denoted by HP (c) the total content of hydrocracked compounds, denoted by C. These ratios would indicate the reactivity of the autoclaves and their relative abilities towards hydrogenation and hydrocracking. 

Table DC summarises the data for experiments operated at 42(PC for 2 h at the three stirrer speeds 200,400 and 600 rpm. It can be seen that the values are closest to unity at 600 rpm, a value of one, indicating equal performance. The autoclave providing the higher conversion of phenanthrene also showed the higher ability for hydrogenation. The values relating to cracking were inconclusive, but did show the near unity vadue at 6(X) rpm. The extent of conversion tended to increase with increasing stirrer speed, as did the amount of hydrogenation with the contents of all the hydrophenanthrenes increasing, but the amount of hydrocracking tended to decrease.

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. 

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