Cetane, catalytic cracking

The gas oil cut from catalytic cracking called Light Cycle Oil (LCO), is characterized by a very low cetane number (about 20), high contents in aromatics, sulfur and nitrogen, all of which strongly limit its addition to the diesel fuel pool to a maximum of 5 to 10%. 

For gas oil from catalytic cracking (LCO), reducing the aromatics content to 20 wt. % results in a chemical hydrogen consumption of 3.4 wt % and a cetane number of 40. 

The preferential release of C3 and C4 as the smallest fragments is a relative matter ethylene, ethane, and methane can be produced under more drastic experimental conditions, and are produced in small amounts in ordinary catalytic cracking. The conventional process operates under conditions which maximize the desired type of splitting to the more useful gaseous products. To demonstrate the application of theory to practice, the predicted and experimental curves for the cracking of cetane (7) are shown in Figure 3. 

In contrast with these results, catalytic cracking yields a much higher percentage of branched hydrocarbons. For example, the catalytic cracking of cetane yields 50-60 mol of isobutane and isobutylene per 100 mol of paraffin cracked. Alkenes crack more easily in catalytic cracking than do saturated hydrocarbons. Saturated hydrocarbons tend to crack near the center of the chain. Rapid carbon-carbon double-bond migration, hydrogen transfer to trisubstituted olefinic bonds, and extensive isomerization are characteristic.52 These features are in accord with a carbo-cationic mechanism initiated by hydride abstraction.43,55-62 Hydride is abstracted by the acidic centers of the silica-alumina catalysts or by already formed carbocations ... 

Ignition-Quality Improvers. Diesel fuels have found greatly increased use in recent years—so much so that refiners have had to look to cracked distillates from catalytic cracking operations for their extra Diesel fuels. While these cracked distillates have the advantages of relatively high heat content and low pour point, they are inferior in ignition quality (cetane number) to straight-run distillates from the same crudes. 

Nor did catalytic cracking escape the probing attention of Paul Emmett. At Johns Hopkins his students used labeled molecules extensively to examine the nature of secondary reactions in the cracking of cetane over amorphous silica-alumina and crystalline zeolites. They demonstrated that small olefins (e.g., propylene) are incorporated extensively into higher-molecular-weight molecules, especially aromatics, and are the primary source of coke formation on these catalysts. 

Other interesting products that can be obtained from waste plastics using combined thermal and catalytic processes are alkylaromatic compounds, which possess industrial applications as automatic transmission fluids (ATF), detergents (linear alkyl benzenes, LAB), and improvers of cetane number in diesel fuels [104]. The process uses as raw material the olefins generated in a previous step of thermal and catalytic cracking, which represent a cheaper source of olefins alternative to the currently existing ones. No special details about the conditions applied for the olefin production are indicated, the emphasis being focused on the alkylation step. Alkylation catalysts comprise conventional Lewis... 

The catalytic cracking of polypropylene waste in a flnidized-bed reactor was reported by Ji [12]. It gave a yield of liquid product of 50%, the research octane number of the gasoline prodnced from plastic waste was 86, and the cetane index of the diesel fuel produced from the plastic waste was 43. 

Fig. 21. Catalytic cracking of cetane at 500 C. Experimental, 24% conversion over UOP-B catalyst. (Greensfelder, Voge, and Good, 7.)...

The Calculated Cetane Index formula is particularly applicable to straight-run fuels, catalytically cracked stocks, and blends of the two. 

The catalytic Diesel fuel has a lower cetane number than the corresponding component of the virgin charge because of the changes in chemical composition effected in the cracking operation. However, in mild cracking conditions, the drop may amount to only a few numbers in Diesel index. 

A different type of catalytic titration was carried out by Stright and Danforth (48), who added varying amounts of lithium or potassium hydroxide to aqueous suspensions of cracking catalysts. The products were dried, calcined, and tested for cetane cracking at 500°C by means of a flow reactor. Plots of cetane conversion versus amount of added lithium hydroxide were used to determine titers for a variety of cracking catalysts (Fig. 11). This type of catalytic titration is not recommended for quantitative studies because it given high acidity values. In aqueous media, alkali... 

The metal catalyst cracks paraffinic chains longer than C25 and reforms chains shorter than Ce. This is especially important to convert the a-olefin chains (1-alkenes) to saturated alkanes. The catalyst ensures that the final fuel has a carbon chain distribution in the range C8-C25 peaking at Cie (cetane) (Figure 15.8). The catalytic tower uses technology borrowed from the petrochemical industry for the hydrogenated of C=C double bonds, e.g. Raney Nickel or so-called Adams catalyst. 

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