436
The Chemistry and Technology of Petroleum
Data obtained from the thermal decomposition of pure compounds indicate certain decomposition characteristics that permit predictions to be made of the product types that arise from the thermal cracking of various feedstocks. For example, normal paraffins are believed to form, initially,
higher molecular weight material, which subsequently decomposes as the reaction progresses.
Other paraffinic materials and (terminal) olefins are produced. An increase in pressure inhibits
the formation of low-molecular-weight gaseous products and therefore promotes the formation of
higher molecular weight materials.
Branched paraffins react somewhat differently to the normal paraffins during cracking processes and produce substantially higher yields of olefins having one fewer carbon atom than the
parent hydrocarbon. Cycloparaffins (naphthenes) react differently to their noncyclic counterparts
and are somewhat more stable. For example, cyclohexane produces hydrogen, ethylene, butadiene,
and benzene: Alkyl-substituted cycloparaffins decompose by means of scission of the alkyl chain to
produce an olefin and a methyl or ethyl cyclohexane.
The aromatic ring is considered fairly stable at moderate cracking temperatures (350°C–500°C,
660°F–930°F). Alkylated aromatics, like the alkylated naphthenes, are more prone to dealkylation
than to ring destruction. However, ring destruction of the benzene derivatives occurs above 500°C
(930°F), but condensed aromatics may undergo ring destruction at somewhat lower temperatures
(450°C, 840°F).
16.2.2 CAtAlytIC CrACkIng
Catalytic cracking is the thermal decomposition of petroleum constituents in the presence of
a catalyst (Pines, 1981). Thermal cracking has been superseded by catalytic cracking as the
process for gasoline manufacture. Indeed, gasoline produced by catalytic cracking is richer
in branched paraffins, cycloparaffins, and aromatics, which all serve to increase the quality
of the gasoline. Catalytic cracking also results in the production of the maximum amount of
butene derivatives and butane derivatives (C 4 H 8 and C 4 H 10 ) rather than production of ethylene
and ethane (C 2 H 4 and C 2 H 6 ).
Catalytic cracking processes evolved in the 1930s from research on petroleum and coal liquids.
The petroleum work came to fruition with the invention of acid cracking. The work to produce
liquid fuels from coal, most notably in Germany, resulted in metal sulfide hydrogenation catalysts.
In the year 1930, a catalytic cracking catalyst for petroleum that used solid acids as catalysts was
developed using acid-treated clays.
Clays are a family of crystalline aluminosilicate solids, and the acid treatment develops acidic
sites by removing aluminum from the structure. The acid sites also catalyze the formation of coke,
and Houdry developed a moving bed process that continuously removed the cooked beads from the
reactor for regeneration by oxidation with air.
Although thermal cracking is a free-radical (neutral) process, catalytic cracking is an ionic process involving carbonium ions, which are hydrocarbon ions having a positive charge on a carbon
atom. The formation of carbonium ions during catalytic cracking can occur by (1) addition of a proton from an acid catalyst to an olefin and/or (2) abstraction of a hydride ion (H − ) from a hydrocarbon
by the acid catalyst or by another carbonium ion. However, carbonium ions are not formed by the
cleavage of a carbon–carbon bond.
In essence, the use of a catalyst permits alternate routes for cracking reactions, usually by lowering the free energy of activation for the reaction. The acid catalysts first used in catalytic cracking
were amorphous solids composed of approximately 87% silica (SiO 2 ) and 13% alumina (Al 2 O 3 )
and were designated low-alumina catalysts. However, this type of catalyst is now being replaced by
crystalline aluminosilicates (zeolites) or molecular sieves.
The first catalysts used for catalytic cracking were acid-treated clays, formed into beads. In fact,
clays are still employed as catalyst in some cracking processes (Chapter 19). Clays are a family of
The Chemistry and Technology of Petroleum
Data obtained from the thermal decomposition of pure compounds indicate certain decomposition characteristics that permit predictions to be made of the product types that arise from the thermal cracking of various feedstocks. For example, normal paraffins are believed to form, initially,
higher molecular weight material, which subsequently decomposes as the reaction progresses.
Other paraffinic materials and (terminal) olefins are produced. An increase in pressure inhibits
the formation of low-molecular-weight gaseous products and therefore promotes the formation of
higher molecular weight materials.
Branched paraffins react somewhat differently to the normal paraffins during cracking processes and produce substantially higher yields of olefins having one fewer carbon atom than the
parent hydrocarbon. Cycloparaffins (naphthenes) react differently to their noncyclic counterparts
and are somewhat more stable. For example, cyclohexane produces hydrogen, ethylene, butadiene,
and benzene: Alkyl-substituted cycloparaffins decompose by means of scission of the alkyl chain to
produce an olefin and a methyl or ethyl cyclohexane.
The aromatic ring is considered fairly stable at moderate cracking temperatures (350°C–500°C,
660°F–930°F). Alkylated aromatics, like the alkylated naphthenes, are more prone to dealkylation
than to ring destruction. However, ring destruction of the benzene derivatives occurs above 500°C
(930°F), but condensed aromatics may undergo ring destruction at somewhat lower temperatures
(450°C, 840°F).
16.2.2 CAtAlytIC CrACkIng
Catalytic cracking is the thermal decomposition of petroleum constituents in the presence of
a catalyst (Pines, 1981). Thermal cracking has been superseded by catalytic cracking as the
process for gasoline manufacture. Indeed, gasoline produced by catalytic cracking is richer
in branched paraffins, cycloparaffins, and aromatics, which all serve to increase the quality
of the gasoline. Catalytic cracking also results in the production of the maximum amount of
butene derivatives and butane derivatives (C 4 H 8 and C 4 H 10 ) rather than production of ethylene
and ethane (C 2 H 4 and C 2 H 6 ).
Catalytic cracking processes evolved in the 1930s from research on petroleum and coal liquids.
The petroleum work came to fruition with the invention of acid cracking. The work to produce
liquid fuels from coal, most notably in Germany, resulted in metal sulfide hydrogenation catalysts.
In the year 1930, a catalytic cracking catalyst for petroleum that used solid acids as catalysts was
developed using acid-treated clays.
Clays are a family of crystalline aluminosilicate solids, and the acid treatment develops acidic
sites by removing aluminum from the structure. The acid sites also catalyze the formation of coke,
and Houdry developed a moving bed process that continuously removed the cooked beads from the
reactor for regeneration by oxidation with air.
Although thermal cracking is a free-radical (neutral) process, catalytic cracking is an ionic process involving carbonium ions, which are hydrocarbon ions having a positive charge on a carbon
atom. The formation of carbonium ions during catalytic cracking can occur by (1) addition of a proton from an acid catalyst to an olefin and/or (2) abstraction of a hydride ion (H − ) from a hydrocarbon
by the acid catalyst or by another carbonium ion. However, carbonium ions are not formed by the
cleavage of a carbon–carbon bond.
In essence, the use of a catalyst permits alternate routes for cracking reactions, usually by lowering the free energy of activation for the reaction. The acid catalysts first used in catalytic cracking
were amorphous solids composed of approximately 87% silica (SiO 2 ) and 13% alumina (Al 2 O 3 )
and were designated low-alumina catalysts. However, this type of catalyst is now being replaced by
crystalline aluminosilicates (zeolites) or molecular sieves.
The first catalysts used for catalytic cracking were acid-treated clays, formed into beads. In fact,
clays are still employed as catalyst in some cracking processes (Chapter 19). Clays are a family of