Cetane, cracking

Figure 1. Stylized site placement effects on cetane cracking in high Si/Al zeolites.

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 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. 

On sulfided metallic phases the hydrotreatment reactions also takes place. Noble metal catalysts usually include a zeolitic support. They are particularly used for fulfilling two different objectives, in the case of a gasoline oriented HCK their cracking and isomerization activity is the most important (increasing high octane and conversion yield). In a diesel HCK unit, the noble metal catalyst is mainly oriented to aromatic saturation and cetane improvement. However, in this latter case, also sulfided metal catalysts are used, especially NiW. 

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. 

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. 

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. 

Fig 11. Conversion of cetane versus lithium hydroxide addition for several cracking catalysts A, 19% AljOj/81% SiOs. B, 31% MgO/69% SiO, C, 12% BjO,/88% Al,Oa (48). (Reprinted with permission of the American Chemical Society.)... 

The high temperature process is the only commercially proven process for the production of olefins and liquids from coal. Current developments favour a low temperature process which is commercially proven to produce liquids and wax from coal or gas. The low temperature process produces a waxy synthetic crude oil which is cracked to produce diesel of high cetane and naphtha. The naphtha, which has high level of Unear paraffins, is sold on the petrochemical naphtha market rather than conversion into gasoline. The conversion of this naphtha into olefins by steam cracking has been addressed in previous chapters. 

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