Environmental Engineering Reference
In-Depth Information
Figure 6.27: Internal and external temperature profiles during burn-off of spent catalyst pellets in
10% O
2
for four zones of fixed bed [From ref.
419
. Reprinted with permission].
controlled burn-off. At the same time, much more coke surface was available for O
2
for
catalyst A. Then, the diffusion of O
2
into the interior of particles played an important role
during the burn-off of catalyst A. While O
2
concentration is a key parameter, there are other
means of controlling temperature during regeneration. For example, the pretreatment of spent
catalyst (de-oiling, thermal treatment under inert gas, addition of steam to oxidizing gas, etc.)
decreased
T
parameter
[331,419]
.
The emissions from oxidative regeneration include regulated particulate matter and gases such
as SO
X
,NO
X
, CO, and CO
2
as well as toxic species, e.g., NH
3
, HCN, and H
2
S. Theoretically,
one of the most toxic species such as AsH
3
may also be formed, particularly under O
2
starving
conditions. The distribution of these compounds depends on the type of spent catalyst and the
conditions employed during regeneration. Beside particulate matter (dust), the solid emissions
also include spent caustics. All certified regenerating companies are equipped to contain these
emissions within the regulatory limits. A description of these methods is given later, i.e., in the
section dealing with industrial regeneration.
Of particular importance is the potential evolution of the most toxic species, such as HCN and
AsH
3
, during oxidative regeneration. It is emphasized that quantities of these compounds
