Environmental Engineering Reference
In-Depth Information
medium severity hydroprocessing conditions, such as encountered using two or more fixed bed
reactors in series, would be necessary to upgrade these residues, respectively.
Figure 2.4 includes a deasphalting of residues to produce DAO. The content of metals and
asphaltenes in DAO depends on the type of deasphalting solvent and on the origin of the feed
from which the DAO was derived. Thus, among several DAOs, it is not unusual to have one
which contains more metals than residues, particularly when the former was obtained from
heavy crudes (e.g., Boscan, Maya, Orinoco, Zuata, etc.). For example, the DAO studied by
Reyes et al. [45] contained
230 ppm of metals. However, one report suggests that the amount
of metals in the DAO obtained from the Boscan crude by hexane deasphalting approached 510
and 60 ppm of V and Ni, respectively [46] . For such feeds, the deposition of metals is expected
to be the predominant mode of catalyst deactivation from the early stages on stream,
particularly when the content of asphaltenes in the DAO was much lower than that in a VR
containing similar amount of metals. In some situations, it was more beneficial to use the
blend of VGO with DAO, particularly when both were derived from a conventional crude [47] .
Subsequently, the blend may be hydroprocessed to obtain the feed either for FCC or dewaxing.
Correspondingly, the severity employed during the hydroprocessing of VGO/DAO or DAO
and catalyst deactivation associated with it would be somewhere between that used during
hydroprocessing of VGO and atmospheric residue.
3.1.2.1 Heavy and Extra Heavy Feeds
For the purpose of this topic, a heavy feed and extra heavy feeds are defined as those having
the total metal content between 100 and 200 ppm and 200 to 300 ppm, respectively. Based on
this classification, the atmospheric residues (345+ C) obtained from Kuwait export and Arab
Heavy crudes contain 163 and 123 ppm of metals, respectively, whereas the VR (565+ C)
derived from the Arab Light contained 147 ppm of metals. Both the atmospheric and VRs
derived from other crudes (except Arab Light and Ekofisk) in Table 2.1 [27] contain more than
300 ppm of metals and, as such, are classified as extra heavy feeds.
Decades of commercial experience confirmed that both heavy crudes and extra heavy crudes
can be upgraded using hydroprocessing method. However, an optimal match of the properties
of the feeds with type of catalyst and reactor systems becomes much more critical than that for
medium or medium heavy feeds. The ultra heavy feeds, i.e., those containing more than
300 ppm of metals, can also be processed catalytically, however, not without a significant
catalyst inventory and excessive hydrogen consumption [43] . Potential for the
hydroprocessing of such feeds improves when the catalytic step is preceded by pretreatment,
such as deasphalting. Otherwise, carbon-rejecting processes (delayed coking, fluid/flexi
coking, etc.) must be employed for the primary upgrading step of extra heavy feeds. Most of
the spent catalysts from the upgrading of extra heavy feeds are non-regenerable, however, in
some cases; a desirable level of catalytic activity may be still recovered when the regeneration
process is combined with rejuvenation.
 
Search WWH ::




Custom Search