Chain processes of catalytic cracking

Catalyst Decay as a Side Reaction of the Chain Processes of Catalytic Cracking... 

In the process of catalytic cracking, characteristic reactions such as chain scission, hydrogen transfer and condensation take place under certain temperature and pressure conditions and when an appropriate catalyst is utilized, products with certain range of molecular weights and structures are obtained. Catalysts with surface acid sites and with the ability of hydrogen ion donation such as silica-alumina and molecular sieve catalyst have been already widely utilized. These catalysts can also enhance the isomerization of products and increase the yield of isomeric hydrocarbons. However, large amounts of coke will deposit on the surface of catalysts and consequently lead to their deactivation. Therefore, the recycling of catalysts is difficult to achieve. 

The process of catalytic cracking breaks long-chain alkanes into smaller alkanes and alkenes that are more valuable to industry. 

Saturated constituents contribute less to the vacuum gas oil than the aromatics but more than the polar constituents that are now present at percentage rather than trace levels. The vacuum gas oil itself is occasionally used as heating oil but most commonly it is processed by catalytic cracking to produce naphtha or by extraction to yield lubricating oil. Within the vacuum gas oil saturates, the distribution of paraffins, /iso-paraffins and naphthenes is highly dependent upon the petroleum source. The bulk of the vacuum gas oil saturated constituents consist of /Iso-paraffins and naphthenes. The naphthenes contain from one to more than six fused rings and have alkyl substituents. For mono- and di-aromatics, the alkyl substitution typically involves one long side chain and several short methyl and ethyl substituents. 

The process of catalytic reforming introduces chains, and catalytic cracking introduces double bonds. Not only do these two processes increase the amount of gasoline that s produced, but they also improve the quality of the gasoline s burning characteristics. Also notice that benzene, an aromatic compound, has an octane value of 106. Its burning characteristics are better than isooctane. Other substituted aromatic compounds have octane ratings of almost 120. However, benzene and some related compounds are health hazards, so they re not used. 

Figure 1731. Fluidized bed reactor processes for the conversion of petroleum fractions, (a) Exxon Model IV fluid catalytic cracking (FCC) unit sketch and operating parameters. (Hetsroni, Handbook of Multiphase Systems, McGraw-Hill, New York, 1982). (b) A modem FCC unit utilizing active zeolite catalysts the reaction occurs primarily in the riser which can be as high as 45 m. (c) Fluidized bed hydroformer in which straight chain molecules are converted into branched ones in the presence of hydrogen at a pressure of 1500 atm. The process has been largely superseded by fixed bed units employing precious metal catalysts (Hetsroni, loc. cit.). (d) A fluidized bed coking process units have been built with capacities of 400-12,000 tons/day.

Fuels. Catalytic Cracking Catalysts are used lo refined a moderately heavy crude oil fraction known as gas oil to gasoline. The net result of the process is a lighter product with a high content of branched-chain and aromatic hydrocarbons, the species responsible for raising gasoline octane levels. The transformations are complex, hut can be considered lo involve ihe following major acid-catalyzed reactions ... 

Common synthetic-based raw materials for surfactant production include ethylene, and propylene. Crude oil consists of a complex mixture of long chain hydrocarbons and aromatic molecules. Natural gas is a mixture of short chain hydrocarbons rich in methane, ethane, propane, and butane. The exact composition of both depends on its source and how it has been processed. Ethylene and propylene are produced by thermal or catalytic cracking of natural gas or aromatic rich petroleum streams. 

The structure of the hydrocarbons produced can be modified by the use of catalyst. Catalytic cracking consumes less energy than the noncatalytic process and results in formation of more branch-chain hydrocarbons. On the other hand the addition of the catalyst can be troublesome, and the catalyst accumulates in the residue or coke. There are two ways to contact the melted polymer and catalysts the polymer and catalyst can be mixed first, then melted, or the molten plastics can be fed continuously over a fluidized catalyst bed. The usually employed catalysts are US-Y, and H-ZSM-5. Catalyst activity and product structure have been reported [7-11]. It was found that the H-ZSM-5 and ECC catalysts provided the best possibility to yield hydrocarbons in the boiling range of gasoline. 

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