Environmental Engineering Reference
In-Depth Information
FIGURE 10.1 Schematic diagram of a typical of single stage of hydrocracking system. R, F, and P are
reactor, fractionators, and product, respectively. Source : Reproduced with permission from Ward [2].
boiling point and low molecular weight hydrocarbons, such as gasoline and
diesel oil, which can be used by the general public. A schematic diagram of
a hydrocracking system is showed in Figure 10.1. Different hydrocarbons
can be separated in a fractionator (F). The hydrogen gas used for hydrocrack-
ing can be recycled.
Noble metals such as palladium and platinum, and base metals such as
molybdenum, tungsten, cobalt, and nickel are common catalysts used in
hydrocracking. While the hydrogen gas is the essential component for the
hydrocracking process, its reaction mechanism depends sensitively on the
nature of the catalyst. The following provides several specific examples.
The first example is bifunctional catalysts that comprise a hydrogenation/
dehydrogenation component, such as a noble metal, and a Brønsted acid
component. Figure 10.2 illustrates a classical mechanism for the hydrocrack-
ing of an n -alkane. The reactant is first dehydrogenated on the metal sites to
a mixture of n -alkenes, n -C i H 2i . Then, they desorb from the metal sites and
diffuse to Brønsted acid sites where they are protonated to the secondary
alkylcarbenium ions, n -C H
+ . Carbenium ions undergo β -scission, and
form smaller molecules and carbenium ions. These smaller species then
diffuse to metal sites again and are hydrogenated to products.
The second example is monofunctional catalysts. Hydrogen gas and
hydrocarbons are chemically adsorbed on the hydrogen absorption sites and
reactant absorption sites of the same metal surface (Fig. 10.3). The hydro-
cracking reaction is initiated by carbon-carbon bond cleavage, followed by
hydrogenation of the hydrocarbon fragments. This mechanism is also known
as hydrogenolysis.
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