Environmental Engineering Reference
In-Depth Information
Figure 6.24: Summary of trends for intraparticle Mo/Al, Ni/Al, and Mo/Ni profiles (in Ni{Mo}/Al
max.) for different catalyst treatments [From ref. 392 . Reprinted with permission].
6.2.5 Safety and Environmental Aspects of Oxidative Regeneration
According to Fig. 5.4 [331] , a safe contact of spent catalysts with air can only be maintained at
temperatures lower than 250 C providing that most of the flammable volatile carry-overs that
were deposited on the catalyst surface were already removed. This may also involve entrapped
gases, such as H 2 ,H 2 S, and light hydrocarbons. Otherwise, an uncontrolled temperature
runaway would lead to recrystallization and/or to sintering of the catalytically active phases. In
the sintered form, the catalytic functionalities could not be restored in regenerated catalysts.
The loss of Mo due to sublimation of the MoO 3 component caused by temperature excursions,
such as shown in Fig. 6.17 [401] , would make the catalyst practically unusable. From the
safety point of view, a rapid surge of toxic species during temperature excursions deserves a
special attention. Potential damages to the material of reactor, caused by sudden temperature
increase, may be another reason for precise monitoring the regeneration temperatures.
Figure 6.25 [419] indicates the temperature runaway on the introduction of air to the bottom of
the fixed bed of spent catalyst at 350 C. A sudden temperature increase to more than 800 C
 
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