99
olefin polymerization yields a higher amount of coke. Coke is also related to hydrocarbon feed. A feed with higher asphaltenes yields more coke than lighter cleaner
feedstock with high API [44, 87].
6.4 Hydrogen Transfer Reactions
Hydrogen transfer is a bimolecular reaction in which two olefins adsorb on adjacent
acid sites. One of the olefins saturates while the other forms a naphthene. The naphthene forms an aromatic compound by hydrogen transfer with another olefin.
4
3
3
6 12
6 14
6 6
4 8
6
C H
C H
C H
C H C
Olefins
P araffins
Aromatics
Olefins
®
+
+ H H
C H
C H
Naphthene
Paraffins
Aromatics
12
4 10
6 6
3
®
+
The final products are paraffins and aromatics since aromatics are difficult to crack
the reaction terminates.
Hydrogen transfer reactions occur more frequently on rare-earth exchanged catalysts. Since rare earths form bridges between adjacent acid sites to stabilize the
zeolite structure, hydrogen transfer happens easily.
These reactions increase the gasoline yield and reduce light olefin yield. The
gasoline has more stability and lower octane number. The octane loss due to olefin
saturation outweighs the higher aromatic concentration; hence, the octane number
reduces drastically.
6.5 Undesirable Reactions
The reactions that affect the yield of desired end products (gasoline, LPG) in a negative way are undesired reactions.
Hydrogen transfer reactions occur to some degree in all FCC processes, but on a
larger scale, they severely affect the octane rating of the gasoline product. Overcracking of lighter olefins is also another undesirable reaction, which is detrimental
to the LPG yield and increases coke and dry gas yield. De-alkylation of branched
hydrocarbons is also undesirable since it reduces octane rating of product gasoline
and the dealkylated part contributes to the dry gas yield.
The reactions most detrimental to the overall conversion is due to the metal contaminants (Ni and V) present in the hydrocarbon feedstock. These metal contaminants catalyze the dehydrogenation reactions, which produce light olefins that
contribute to the dry gas yield and also polymerize to increase the coke make.
Hence, hydrotreating of FCC feedstock is generally practiced. In addition to this,
additives in the form of metal traps are also used. These metal traps are basic metal
Recent Developments in FCC Process and Catalysts
olefin polymerization yields a higher amount of coke. Coke is also related to hydrocarbon feed. A feed with higher asphaltenes yields more coke than lighter cleaner
feedstock with high API [44, 87].
6.4 Hydrogen Transfer Reactions
Hydrogen transfer is a bimolecular reaction in which two olefins adsorb on adjacent
acid sites. One of the olefins saturates while the other forms a naphthene. The naphthene forms an aromatic compound by hydrogen transfer with another olefin.
4
3
3
6 12
6 14
6 6
4 8
6
C H
C H
C H
C H C
Olefins
P araffins
Aromatics
Olefins
®
+
+ H H
C H
C H
Naphthene
Paraffins
Aromatics
12
4 10
6 6
3
®
+
The final products are paraffins and aromatics since aromatics are difficult to crack
the reaction terminates.
Hydrogen transfer reactions occur more frequently on rare-earth exchanged catalysts. Since rare earths form bridges between adjacent acid sites to stabilize the
zeolite structure, hydrogen transfer happens easily.
These reactions increase the gasoline yield and reduce light olefin yield. The
gasoline has more stability and lower octane number. The octane loss due to olefin
saturation outweighs the higher aromatic concentration; hence, the octane number
reduces drastically.
6.5 Undesirable Reactions
The reactions that affect the yield of desired end products (gasoline, LPG) in a negative way are undesired reactions.
Hydrogen transfer reactions occur to some degree in all FCC processes, but on a
larger scale, they severely affect the octane rating of the gasoline product. Overcracking of lighter olefins is also another undesirable reaction, which is detrimental
to the LPG yield and increases coke and dry gas yield. De-alkylation of branched
hydrocarbons is also undesirable since it reduces octane rating of product gasoline
and the dealkylated part contributes to the dry gas yield.
The reactions most detrimental to the overall conversion is due to the metal contaminants (Ni and V) present in the hydrocarbon feedstock. These metal contaminants catalyze the dehydrogenation reactions, which produce light olefins that
contribute to the dry gas yield and also polymerize to increase the coke make.
Hence, hydrotreating of FCC feedstock is generally practiced. In addition to this,
additives in the form of metal traps are also used. These metal traps are basic metal
Recent Developments in FCC Process and Catalysts