Environmental Engineering Reference
In-Depth Information
7.1.3.1 Gaseous Emissions
Most of the gaseous emissions are released during decoking step, which has to be an integral
part of the rejuvenation process. These emissions include SO
X
,NO
X
, CO, CO
2
and particulate
matter. The same methods that have been used for treating the off-gases from oxidative
regeneration can also be applied here. This may include a particulate removing system and
caustic scrub. Some concerns about release of HCN and NH
3
during decoking of catalyst have
been noted
[19]
. However, the conditions of oxidative decoking, i.e., very low concentrations
of O
2
combined with low temperatures and large volume of diluted air used for decoking
ensure that the amount of these toxics are at near and/or below detection limits.
Reactions
{7.4}
as well as
{7.7}
and
{7.8}
, which were discussed as part of the rejuvenation
mechanism, indicate a potential formation of SO
2
and H
2
S, respectively. The formation of the
former requires the presence of an oxidative agent, suggesting that the oxidation of SO
2
may
continue to produce SO
3
followed by H
2
SO
4
as shown in Reaction
{7.5}
. Almost certainly,
metal sulfates are the final products of this process. Therefore, SO
2
emissions during the
leaching should not be of a concern. However, for coked catalysts in the absence of an
oxidative agent, the formation of H
2
S cannot be avoided. Because of toxic character of H
2
S,
its potential release has to be taken into consideration during the design of commercial
process.
7.1.3.2 Liquid Emissions
Depending on the extent of de-oiling of spent catalyst before decoking, various amounts of the
condensible volatiles may be released during decoking. In their properties, the volatiles
approach properties of the feed being upgraded and those of products. The volatiles may be
consumed on the site as a fuel. Before reuse, rejuvenated catalysts must be dried to remove
water from the catalyst pores. This step should be benign, although some mild odor could be
present.
The leachate from rejuvenation is the main liquid phase requiring attention. If isolated in a
pure form, the contaminant metals removed from catalyst (e.g., mainly V and Ni) are
marketable products. This may proceed by the adjustment of pH to liberate metals from
complexes with organic acids. Subsequently, the metals can be separated either by selective
precipitation or by the methods that have been part of the commercial hydrometallurgical
operations. However, an increase in the metal concentration in leachates may be needed to
improve the efficiency of these operations (e.g., by evaporation). An additional treatment of
the liquor, i.e., a treatment with activated carbon and/or with a caustic, before the water can be
either disposed or reused, may be necessary. If disposal option is considered, the concentration
of regulated metals in disposed water must not exceed limits prescribed by environmental
standards.
