Environmental Engineering Reference
In-Depth Information
liquid ammonia. This step is known as the ammonia synthesis loop (also
referred to as the Haber-Bosch process).
2.6.2 Methane Cracking
Similarly, hydrogen can be produced by cracking of methane. Compared
with methane steam reforming, no CO is generated during the cracking reac-
tion, thus there is no need for separating CO from hydrogen after the reaction.
The methane cracking reaction is as follows:
= +
74 8
kJ mol
where C s stands for solid carbon. Besides the desired product, hydrogen, the
only byproduct is carbon, which is usually in the form of filamentous carbon
or carbon nanotubes [17]. Separation of unreacted methane and hydrogen
can be readily achieved by adsorption or membrane separation to produce a
stream of hydrogen with 99% by volume, which is much simpler than the
reforming process with complicated separation processes that involve CO 2
or CO. This can be especially important for proton-exchange membrane
(PEM) fuel cells, in which the Pt-based electrocatalyst can be poisoned by
CO. The carbon nanotubes produced as a solid product are commercially
useful in applications such as adsorption, catalysis, or carbon storage.
The cracking reaction is strongly temperature dependent and much more
effective with catalysts used. Examples of catalysts include Ni, Fe, Co, and
activated carbon, usually supported by substrates like SiO 2 , TiO 2 , ZrO 2 ,
Al 2 O 3, MgO, graphite, or composites of the oxides. Noncatalytic methane
cracking is very slow at temperatures below 1000°C, while catalytic cracking
of methane can be conducted at temperatures as low as 500°C. Figure 2.9
shows the predicted equilibrium methane conversion as a function of tem-
perature based on thermodynamic considerations and data without using
catalyst [17]. Also shown in the figure is the number of moles of CH 4 , H 2 ,
and C for an initial 100 mol of CH 4 . It is clear that the equilibrium conver-
sion increases with increasing temperature, starting from 30% conversion at
500°C to almost complete conversion at 1000°C.
For catalytic reactions, iron group metals are known to have the highest
activity for general hydrocarbon cracking. For methane, which is the most
stable compared with other hydrocarbons, Ni has been found to be the most
active catalyst among the iron group metals. Direct comparison shows that
the catalytic activity for the iron group metals is: Ni > Co > Fe [18]. The
byproduct, carbon, can cause deactivation of the catalysts by encapsulating
them. Regeneration of the catalysts can be done with steam or air.
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