Environmental Engineering Reference
In-Depth Information
4.6.2 Mechanism of Metals Deposition
In petroleum feeds, metals can be present in both inorganic and organometallic forms. The
deposition mechanism involving inorganic solids should differ from that of the organometallic
forms of metals. Moreover, for organometallic compounds, the different form of deposits is
formed during the non-catalytic demetallization of porphyrins via the reactions with H 2 and
H 2 S compared with the catalytic HDM of porphyrins.
4.6.2.1 Deposition of Inorganic Solids
Inorganic solids in heavy feeds include minerals and clay-like solids, which contaminated
crude either in reservoir or during the production. For bitumen separated from tar sands, part
of the mineral matter originated from the caustic material used in the hot water separation
process. In addition, a small amount of the finely divided particles of sand still remained in the
bitumen after the separation process. During distillation, these solids accumulate in residues.
During hydroprocessing of residual feeds, inorganic solids deposit on the external surface of
catalyst particles. In its nature, the deposition mechanism is physical. This is confirmed by the
predominant accumulation of the inorganic solids on the external surface of catalyst particles
in a “skin-like” form. Indeed, a high content of Fe and Ca was noted in the “skin” on the spent
catalysts from hydroprocessing of Athabasca bitumen [243] . A similar form of deposition may
undergo the sulfides of V and Ni formed during the non-catalytic reaction with H 2 S and H 2 .
In the crudes, the solids containing alkali and alkali earth metals are predominantly in an oxidic
form. They are usually associated with SiO 2 ,Al 2 O 3 and SiO 2 -Al 2 O 3 in the form of clays and
minerals. Under hydroprocessing conditions, the oxidic form of alkali and alkali earth metals
are gradually converted to corresponding sulfides similarly as the Fe oxides. If present, finely
divided water emulsions may contain chlorides of alkali metals. The problem with such solids
is alleviated by dewatering of the crude oil as soon as it enters the petroleum refinery.
The industrial experience confirmed that the operating problems caused by the deposition of
inorganic solids have been more evident in fixed-bed reactors than in ebullated-bed reactors. In
the former case, the front of the fixed-bed will be most affected. The crust-like layer created by
the deposition of such solids on the front of the bed may affect the operation by creating
channels and developing pressure drops through the bed. In this regard, attention should be
paid to the possible contamination of the feed by metallic particles from the corrosion of
upstream equipment. As such, iron scale or fine particles usually do not penetrate deeply the
catalyst porous system and do not have any strong deactivating effect. It is more a concern as
contributor to the pressure drop builds up, as these particles may accumulate at the top of the
bed or in the interstices between the catalyst granules. Similar problems may be caused by
silicon, which originates from the anti-foaming agents. Such agents are sometimes added to
the feed before coking operation, therefore they might be present in HGO. For some heavy
feeds, a filtration system must be installed upstream of the catalytic reactor to avoid the
 
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