Cracker feedstocks

Cracking n-hutane is also similar to ethane and propane, hut the yield of ethylene is even lower. It has been noted that cracking either propane or butanes at nearly similar severity produced approximately equal liquid yields. Mixtures of propane and butane LPG are becoming important steam cracker feedstocks for C2-C4 olefin production. It has been forecasted that world LPG markets will grow from 114.7 million metric tons/day in 1988 to 136.9 MMtpd in the year 2000, and the largest portion of growth will be in the chemicals field. 

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.

The performance analysis and product analysis results confirm previous findings (3,8) that hydrotreating improves the quality of catalytic cracker feedstock and the resultant products. In addition, it was shown that the quality of the liquid products and the yields of the coke and the heavy cycle oil (HCO) from cracking of the severely hydrotreated feedstock (WM-2-9) were independent of the conditions of the cracking process. These results imply that there exists a degree of pretreatment hydrotreating above which... 

The selection of steam cracker feedstock is mainly driven by market demand as different feedstock qualities produce different olefins yields. One of the commonly used feed quality assessment methods in practice is the Bureau of Mines Correlation Index (BMCI) (Gonzalo et al., 2004). This index is a function of average boiling point and specific gravity of a particular feedstock. The steam cracker feed quality improves with a decrease in the BMCI value. For instance, vacuum gas oil (VGO) has a high value of BMCI and, therefore, is not an attractive steam cracker feed. The commonly used feedstocks in industry are naphtha and gas oil. 

Increasing conventional steam cracker feedstock costs... 

A unique information with respect to the use Pd on HZSM-5 in a selective hydrogenolysis has been disclosed.501 The transformation of methylcyclohexane to n-alkanes with two or more carbon atoms is a useful transformation since these products are desirable components in synthetic steam-cracker feedstock. It was shown that these compounds are not obtained on catalysts with high (0.2 or 1%) Pd loading or without Pd. But on Pd on H-ZSM-5, with Pd content in the range of 10-100 ppm, the desired products are formed with high ( 78%) selectivity. 

CRR4 Catalytic cracker feedstocks Reduce coke buildup on catalyst... 

Cat. cracker feedstock Coker feedstock Diesel fuel (etc.)... 

The process has three basic variations-the Type II unit, the Type III unit, and the Type IV unit with the degree of desulfurization, and process severity, increasing from Type I to Type IV. Thus, liquid products from Types III and IV units can be used directly as catalytic cracker feedstocks and perform similarly to virgin gas oil fractions, whereas liquid products from the Type II unit usually need to be vacuum-flashed to provide a feedstock suitable for a catalytic cracker. 

A number of refiners have processed residue containing feedstocks in commercial FCC units. Feeds with as much as 5.1%w RCR ( 6.5%w CCR) and 85 ppm Ni + V have been processed in Phillips Borger Refinery.(4) Ashland has processed feedstocks of up to 7.1%w RCR ( 8.5%w CCR) and 85 ppm Ni + V in their RCC (Reduced Crude Conversion) process.(5,6) A commercial scale ART (Asphalt Residual Treating) unit has processed residues containing levels of contaminants as high as 13.5%w RCR and 300 ppm Ni + V (7,8). However, in typical day-to-day operation of residue cat crackers, feedstock quality is not as extreme as those illustrated above. 

The second group comprise LPG feedstocks made from crude oil. These are products of refinery and petrochemical operations processing heavier feeds such as gas oil and vacuum gas oil and residual fuel oils. These LPG streams contain materials of direct interest to petrochemical operations for further processing to other chemicals. With suitable treatment (hydrogenation) they can be used as cracker feedstock or sold to other users as an energy fuel. 

The various components of LPG streams are used in a variety of processes. Propane, butane and isobutane are used as cracker feedstock for the production of olefins which is discussed in later chapters. In addition n-butane is used for the production of 1,3-butadiene. This compound can also be extracted from the C4 cracked gases by extensive distillation coupled with a selective absorption process. 

Propane is separated by distillation and can be either recycled to produce cracker feedstock or purified to a saleable LPG product. 

The final product of interest is butane. This can be separated and either sold as LPG or recycled as a cracker feedstock. All of the C4 stream can be recycled for cracking. However, olefins and especially dienes and the C4-aceylenes rapidly form coke and the C4 stream is generally fully hydrogenated to butane. 

If gas oil and heavier feedstock are used as a cracker feedstock, the most important difference relative to naphtha is in the production of heavier materials, especially pyrolysis fuel oU, which requires more plant and equipment for handling. Fouling rates in the process plant exposed to the heavier materials are higher than those experienced for cracking naphtha. 

The enhanced fouling rates and metal contamination (from the crude oil) generally makes atmospheric residua unsuitable as a cracker feedstock. However, some crude oils produce a waxy residual of low metal content (often referred to as low sulphur waxy residua, LSWR). Although more expensive than fuel oil, LSWR is considerably cheaper than gas oil and is an attractive feedstock for some gas oil cracker operations. 

Liquid membrane technology has been applied to a great extent for separation of mixtures of saturated and aromatic hydrocarbons. Investigations reveal that the LSM process offers potential for dearomatization of petroleum streams like naphtha and kerosene to meet product specifications for naphtha cracker feedstock and aviation kerosene, respectively [25, 63, 85, 144-146]. The separation is based on a simple permeation technique and occurs due to the difference in solubility and diffusivity of permeating species through the membrane. Kato and Kawasaki [70] conducted studies on the enhancement of hydrocarbon permeation by the use of a polar additive like sulfolane or triethyl glycol. Sharma et al. [147] enhanced the selectivity of the membrane by several orders with the addition of a carrier. Chakraborty et al. [85] used cyclodextrins to enhance the separation factor and removal efficiency of aromatic compound. 

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