Cracking Operations

Because of the different chemistry of cracking processes their products have different compositions. The major product of thermal cracking is ethylene, with large amounts of Ci and C3 hydrocarbons, and C4— C15 terminal alkenes. Thermal cracking, consequently, is used mainly in olefin manufacture. 

The effect of rare-earth HY zeolites is somewhat similar, except that it produces less olefin as a result of enhanced hydrogen transfer reactions. Decreased hydrogen transfer is the main feature of USY zeolites, yielding a product significantly richer in olefins. It is slightly richer in aromatics because of the retardation of their secondary transformations (condensation, coke formation). 

Hydrocracking40-44 is a highly severe hydrotreating operation and a highly flexible and versatile process. It allows the manufacture of products with wide composition range by selecting appropriate feed, catalyst, and operating conditions. Since 

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Mercaptans are naturally present in crude oil (Chapters 1 and 8), or they result from the decomposition of other sulfur compounds during thermai or catalytic cracking operations. 

These water streams contain mainly dissolved salts ammonium chloride and sulfide, sodium chloride, traces of cyanide, phenols for water coming from catalytic and thermal cracking operations. 

Visbreaking. Viscosity breaking (reduction) is a mild cracking operation used to reduce the viscosity of residual fuel oils and residua (8). The process, evolved from the older and now obsolete thermal cracking processes, is classed as mild because the thermal reactions are not allowed to proceed to completion. 

The hydrocarbon cracking operations that generate feed olefins generally do not produce sufficient isobutane to satisfy the reaction requirements. Additional isobutane must be recovered from cmde oil or natural gas Hquids or generated by other refinery operations. A growing quantity of isobutane is produced by the isomerization of / -butane [106-97-8]. 

Table 7. Typical Yields and % Compositions of Fractions from Cracking Operations ...

In the United States, Europe, and Japan, DCPD streams of 70—95 wt% purity are available. Estimates of recoverable DCPD production capacity in the United States for 1990 for all grades of DCPD is >127, 000 metric tons (39) and in Europe is 48,000 metric tons (40). The vast majority of this production is from hydrocarbon steam-cracking operations. Based on the total operations, more CPD is produced than indicated above, but because of the relatively small quantities available at a single location, much of the cyclopentadiene caimot be recovered profitably. Important producers in the U.S. are Dow, Exxon, LyondeU, SheU, and Texm ark in Europe, Dow and SheU and in Japan, Nippon Zeon (40). 

Where is naphthenic acid corrosion found Naphthenic acid corrosion occurs primarily in crude and vacuum distillation units, and less frequently in thermal and catalytic cracking operations. It usually occurs in furnace coils, transfer lines, vacuum columns and their overhead condensers, sidestream coolers, and pumps. 

Separation of olefins produced by cracking operations and subsequent conversion. This is the major route to aliphatic petrochemicals. 

In addition to the distillation of crude oil coming into the refinery, stills of various designs are used in other types of service throughout the refinery. Cracked products are separated in distillation equipment which is very similar to an atmospheric crude pipe still. The principal difference is that these products are hot from the cracking operation, so that a fired heater is not required. 

Since the catalyst is so important to the cracking operation, its activity, selectivity, and other important properties should be measured. A variety of fixed or fluidized bed tests have been used, in which standard feedstocks are cracked over plant catalysts and the results compared with those for standard samples. Activity is expressed as conversion, yield of gasoline, or as relative activity. Selectivity is expressed in terms of carbon producing factor (CPF) and gas producing factor (GPF). These may be related to catalyst addition rates, surface area, and metals contamination from feedstocks. 

Catalytic crackings operations have been simulated by mathematical models, with the aid of computers. The computer programs are the end result of a very extensive research effort in pilot and bench scale units. Many sets of calculations are carried out to optimize design of new units, operation of existing plants, choice of feedstocks, and other variables subject to control. A background knowledge of the correlations used in the "black box" helps to make such studies more effective. 

Correlations have been used as a tool for catalyst selection studies. Predictions of the product yields and qualities possible with various catalysts can provide the necessary information for a refiner to study the economics of switching catalysts, for instance. With a good idea of the profitability of changing catalyst types, the refinery can justify such a change in his cat crackings operation. 

Another important use of correlations is the optimization of existing unit operations. Cat cracking correlations can provide the refiner with valuable information for optimizing reactor temperature level, gasoline/distillate cut point, and feed and recycle rates. The practical application of this information can mean increased profitability for the cat cracking operation. 

Coke A solid residue from cracking operations, mostly carbon but with a small amount of high-molecular-weight hydrocarbons... 

The olefins used are propylenes and butylenes ethylene is also produced from cracking operations but is not used in refinery processing. 

FCC feed characterization is one of the most important activities in monitoring cat cracking operation. Understanding feed properties and knowing their impact on unit performance are essential. Troubleshooting, catalyst selection, unit optimization, and subsequent process evaluation all depend on the feedstock. 

Although desulfurization is not the goal of cat cracking operations, approximately 50% of sulfur in the feed is converted to HjS. in addition, the remaining sulfur compounds in the FCC products are lighter and can be desulfurized by low-pressure hydrodesulfurization processing. 

The constant is expected to have an Arrhenius dependence on temperature. Deactivation by coke deposition in cracking operations apparently has this kind of correlation. 

Petroleum coke is the residue left by the destructive distillation (thermal cracking or coking) of petroleum residua. The coke formed in catalytic cracking operations is usually nonrecoverable because of adherence to the catalyst, as it is often employed as fuel for the process. The composition of coke varies with the source of the crude oil, but in general, is insoluble on organic solvents and has a honeycomb-type appearance. 

Write the equations describing a simple version of the petroleum industry s important catalytic cracking operation. There are two vessels as shown in Fig. P3.13. Component A is fed to the reactor where it reacts to form product B while depos-... 

The major pollutants resulting from catalytic cracking operations are oil, sulfides, phenols, cyanides, and ammonia. These pollutants produce an alkaline wastewater with high BOD and COD concentrations. Sulfide and phenol concentrations in the wastewater vary with the type of crude oil being processed, but at times are significant. 

Figure 15. The effects of catalytic cracking operating conditions on light gas yields for sample 1693.

Refinery alkylation processes utilize either sulfuric acid or hydrofluoric acid as reaction catalysts. The feedstock for both alkylation processes originates primarily from hydrocracking and catalytic cracking operations. Coker gas oils also serve as feedstock in some applications. The differences and similarities between sulfuric acid alkylation and hydrofluoric acid alkylation are shown in TABLE 2-5. Typical alkylation reactions are shown in FIGURE 2-9. A sulfuric acid alkylation unit is illustrated in FIGURE 2-10. 

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. 

The cracking process requires that the catalyst take part in two alternating reactions (1) the decomposition of heavy to predominantly lighter hydrocarbon products, which is an endothermic reaction and (2) the exothermic oxidation of the nonvolatile hydrocarbons retained on the catalyst during the cracking operation. This complete cycle of operations demands of the catalyst unusual chemical and physical properties, so that it not only promotes the desired reactions when first applied, but also remains effective for a long time. 

Thus, the rates of activity decline are substantially constant for these two cases, and the gasoline production is similar with both operations. This is true even though the initial activity of the catalyst employed for cracking the heavy gas oil falls toward the terminal activity of the catalyst applied to the light gas oil cracking operation. 

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