QUESTIONS AND ANSWERS

I FLUID CATALYTIC CRACKING                    www.wissenschaftler-avh.in

B.CATALYST/ADDITIVES(Contd.)

Q-40:

What are the fundamentals of fluid catalytic cracking (FCC) chemistry?

A-40:

Most of the reactions that take place in the FCC unit can be put into two groups, catalytic reactions and thermal reactions. Catalytic reactions occur on the surface of the catalyst particle and give better yields of gasoline than thermal reactions, with lower yields of undesirable products such as coke and fuel gas. As a result, it is desirable to maximize catalytic reactions and minimize thermal reactions.

Thermal reactions do not require a catalyst. The rate of thermal reactions increases as the reaction temperature increases, and as the amount of contact time that the oil spends in the reaction zone increases. Generally, catalytic reactions proceed much more quickly than thermal reactions

A typical catalytic reaction that occurs in the FCCU starts with a long straight chain (normal) C16 paraffin reacting with a C3 carbonium ion. Carbonium ions may be formed on the surface of the catalyst, or may form from thermal reactions. They are produced easily at FCC reaction conditions, by the addition of a proton (H+) to an olefin.

If the carbonium ion is small, such as a C3 molecule, it is very reactive, and will tend to transfer the positive (+) charge to a larger molecule, such as the C16 in this example. When this reaction occurs, a propane molecule is formed (C3H6) and the new large carbonium ion will tend to stabilize the (+) charge by shifting it towards the center of the molecule. In the example starting with a C16 paraffin, five carbons are to the left of the carbon with the (+) charge, and ten are to the right.

The (+) charge of the molecule is attracted to an acid site which, chemically, is a negative (-) charge on the surface of the catalyst. Catalytic cracking then takes place at the catalytic surface, and the molecule is broken into two smaller molecules. The bond that is broken is not the one directly next to the carbon atom that has the (+) charge, which is called the alpha bond. The cracking occurs at the second bond from the charge, which is called the beta bond. For this reason, this reaction sequence is called the beta-scission mechanism of catalytic cracking. The products from cracking this C16 paraffin are a C7 olefin and a C9 carbonium ion. An olefin is formed because there is usually insufficient hydrogen available to provide each carbon in the two molecules with the required four chemical bonds. As a result, two of the carbons will form a double carbon-carbon bond, producing the olefin.

The C9 carbonium ion formed would have the (+) charge positioned at the end of the molecule. This molecule will undergo a shift reaction in order to better stabilize the structure. After the shift reaction, the carbonium ion will crack again, with the cracking occurring at the beta bond. Because of the beta-scission mechanism, no molecule smaller than a C3 can be produced by this reaction sequence.
Most of the C1 and C2 hydrocarbon (fuel gas components methane, ethane, and ethylene) that are produced in the FCCU are generated during thermal reactions, which proceed by free radical reaction chemistry. This chemistry is quite complex, and will not be discussed here in detail. The most important features of thermal reactions are that they are much slower than catalytic reactions, and that they produce higher amounts of olefins, diolefins, fuel gas and coke

. The keys to reducing thermal reactions are to:
Get good oil atomization and vaporization at the feed nozzles.
Get good catalyst and oil mixing at the bottom of the riser.
Minimize the time that the products spend in the reactor after they are separated from the catalyst.

Other types of reactions take place in the FCCU.:
Naphthene rings will be broken open and cracked to form two olefins. Aromatic rings with paraffinic side chains will be cracked at the ring to produce a base aromatic ring and an olefin. Olefins will be cracked to make two olefins, and pure aromatic rings will not crack at all.

One important reaction is called hydrogen transfer because it transfers hydrogen out of a naphthene ring directly to an olefin, creating an aromatic and a paraffin. Often, the naphthene structure may have several rings and when the hydrogen is removed, the resulting aromatic will condense to coke. In addition, the paraffin has a lower gasoline octane than did the original olefin, and is less likely to undergo further cracking. As a result, hydrogen transfer is usually considered an undesirable reaction.

Coke can be formed in the FCCU in various ways . These reactions are also undesirable, and attempts are made to minimize them.

The reaction sequence in a real FCC unit can be very complex. A molecule may undergo many reactions before it leaves the unit. Some of these reactions can be controlled to some extent by the selection of catalyst and operating conditions.