QUESTIONS AND ANSWERS

I.FLUID CATALYTIC CRACKING                      www.wissenschaftler-avh.in

B.CATALYST/ADDITIVES:

Q-29:

What are the typical reactions occur on  FCC Catalyst?

A-29:

Typical reactions occuring on FCC catalyst are as follows:



The Fluid Catalytic Cracking (FCC) process is the most important refinery process .
The reason for this lies in the ability of the FCC process to convert more of the crude barrel into fuel than any other process. The chemical composition of the feedstock to the FCC unit is the most important variable in determining the basic yield structure from the unit

Each type of hydrocarbon reacts under catalytic cracking conditions in certain definite ways. The major difference among hydrocarbons types is in their crackability or extent of conversion for a given set of operating conditions. In all cases, for each type of molecule, increasing the molecular weight or carbon number increases the crackability. A variety of primary and secondary reactions take place during catalytic cracking. These include chain rupture, isomerization, cyclization, dehydrogenation, polymerization, hydrogen transfer, and condensation. Hence, the result of cracking even a simple molecule such as normal paraffin is complex



                                                              FCC-CHEMISTRY

Normal paraffins

Crack mostly to olefins and paraffins and give fair yields of very light

gasoline . The normal paraffins are more difficult to crack than isoparaffins and naphthenes. The reaction rates and products of paraffin cracking are determined by the molecular size and structure.

Naphthenes and Isoparaffins

Tend to crack at about the same rate, but the product distributions are much

different. Naphthenes produce relatively little gas and give excellent yields of

gasoline. The gasoline is of better quality than that from paraffin cracking and

contains appreciable quantities of aromatics, resulting from dehydrogenation of

the naphthene rings

Aromatics

Crack in several ways. The benzene ring is practically impossible to crack.

Condensed-ring aromatics without side chains are converted to a limited extent,

but almost entirely to coke. Alkyl aromatics with side chains containing at

least three carbon atoms crack by the carbon located in the beta position to the

aromatic ring (beta fission) leaving one or more methyl radicals linked to the

ring. Monoaromatics dealkylate to give a high octane naphtha. With long side

chains, secondary reactions will occur, resulting in products similar to those

from the cracking of olefins and paraffins. Generally, more aromatic feeds give

poorer FCC yields. A contributing factor to this general trend is that, as the

number of ring structures in the feed increases, the likelihood increases that

dehydrogenation from contaminant metals will cause multi-ring aromatics to form,

leading to condensation and coking of the catalyst. The molecular structure of

the aromatic hydrocarbons is another important issue regarding their

crackability. The distribution of aromatics according to the degree of

condensation clearly affects the rate of cracking. As the number of rings in a

poly nuclear aromatic molecule increases, the rate of cracking decreases,

although the aromatic content appears to remain the same. The net result of the

catalytic cracking of aromatic hydrocarbons is moderate yields of gas, very

little gasoline, large quantities of very aromatic cycle stock, and high coke

yields

.

Olefins

Olefin seldom appear in catalytic cracking feeds, but their reactions are of

interest because they are the primary products of other cracking reactions

Olefins heavier than about C6  are extremely reactive. The products of olefin
cracking are primarily propylene and butenes, along with butanes from

secondary reactions. Some polymerization and cyclization takes place in olefin

cracking to produce a small amount of cycle stock and fairly high coke yields

Non-hydrocarbon contaminants

Such as nitrogen, iron, nickel, vanadium, and copper compounds, act as

poisons to cracking catalysts. Basic nitrogen, reacts with the acid centers on

the catalyst and reduces the catalyst activity. However, the basicity of

nitrogen compounds at cracking conditions can vary widely. The total nitrogen

content is considered as a sound indicator for cracking inhibition by basic

nitrogen compounds. The metals deposit on the catalyst and cause a reduction in

throughput by increasing coke formation. Finally, the sulfur content of an FCC
feed has no major effect on the crackability of the feed, but it strongly

affects the product distribution and quality

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