FSC 432
Petroleum Processing

Comparison of Products by Type of Hydrocarbon


Comparison of Products by Type of Hydrocarbon

Table 7.1 compares the products of thermal cracking and catalytic cracking of different type of hydrocarbons. Notably, high yields of C1 and C2 gaseous products (methane, ethane, and ethylene) from thermal cracking are contrasted with high yields of C3- C6, with small quantities of methane and essentially no olefins heavier than butylene, from catalytic cracking. Significant for the octane number of the gasoline fraction from the catalytic cracking of aliphatic hydrocarbons are the abundance of i-alkanes and significant concentration of aromatic compounds (BTX) that increase the octane number.

Table 7.1: Catalytic Cracking vs. Thermal Cracking

Hydrocarbons Thermal Cracking Catalytic Cracking
n-alkanes (e.g., C16)

C2is major product

C1in large quantities

C4-C15olefins in moderate abundance

C3-C6are major light prods

C1in small quantities

No olefins > C4


Little aromatization at 500ºC

No branched – chain alkanes present

Significant aromatization

Abundance of branched – chain alkanes


Slow double bond isomerization

Little skeletal isomerization

Rapid isomerization

C=C–C–C→ C–C=C–C

Rapid skeletal isomerization

C–C=C–CC= C C | –C

Alkylaromatics ß – scission α – scission (dealkylation)
Naphthenes Crack more slowly than n-paraffins Crack at comparable rates with n-paraffins

As discussed in Lesson 6, the slow isomerization of free radicals (moving the unpaired electron from an edge atom to the interior atoms) results in the production of shorter straight-chain alkanes and straight-chain olefins in thermal cracking, thus leading to low octane numbers of the gasoline product. In contrast to free radicals, the isomerization of carbocations is very fast because of the thermodynamic driving force, shown in Table 7.2. One can see in Table 7.2 that the isomerization of a primary propyl carbenium ion to a secondary propyl carbenium ion releases (19.1- 1.5) = 17.6 kcal/mol. This is a very large thermodynamic driving force for the isomerization of a primary ion to a secondary ion, and further to a tertiary ion, with even a larger driving force. Isomerization of the secondary propyl ion to the tertiary propyl ion, releases 1.5 kcal/mol of energy. It is, therefore, clear that the initiation and propagation of carbocations in catalytic cracking chain reactions on the catalyst surfaces will be dominated by the formation of secondary and tertiary carbocations. The reactions of these carbocations lead to the formation of branched-chain alkanes and olefins with high octane numbers.

Table7.2: ΔHf of Carbenium Ions

Carbenium Ions ΔHf(relative) (kcal/mol)

C1+ primary

C–C– C | |


C2+ secondary

C– C C


C3+ tertiary

C– C C | C


Another important feature of carbocation formation is the differences in the enthalpy of formation which favors the formation of carbocations > C3 versus C1 and C2 ions (Table 7.3), because C1 and C2 ions are primary ions. This explains the low yields of C1 and C2 species obtained from catalytic cracking.

Table 7.3: ΔHf of Carbenium Ions

Carbenium Ions ΔHf(relative) (kcal/mol)
CH3 258
C2H5 225
n-C3H7 218
i-C3H7 198
n-C4H9⊕primary 211
t-C4H9⊕ tertiary 174