Environmental Engineering Reference
In-Depth Information
feed. Thus, difficulties associated with the development of pressure drops, channelling, etc.
encountered in fixed bed reactors are not present in the ebullated bed reactors.
Attempts have been made to further advance the existing or develop new catalytic systems for
hydroprocessing of heavy feeds. In this regard, the focus has been on the countercurrent
reactors compared with co-current reactors, which have been used predominantly on a
commercial scale. The former reactors employ a co-current flow of the liquid and gaseous
streams
[122]
. In countercurrent reactors, a structured catalytic bed in which catalyst particles
are enclosed within a packed system is being used. Various features of catalytic reactors for
hydroprocessing are in different stages of development. It should be noted that for the purpose
of this topic, only reactors which are part of commercial processes, i.e., those which generate
spent catalysts, are being discussed.
With the aim of decreasing the cost of catalyst inventory, once through, low-cost catalytically
active solids have been receiving attention. This included throw-away by-products from
metallurgical and aluminium industries and fly ash from combustion of petroleum coke and
coal as well as naturally occurring clays and minerals containing catalytically active metals,
such as iron. In this case, a pulverized form of these solids, slurried with a heavy feed, is being
introduced into the reactor operating under more severe conditions than typically employed
during the hydroprocessing of the topped heavy crudes and VRs. The suitability of this
approach for hydroprocessing of heavy feeds containing more than 300 ppm of metals (V+Ni)
has been demonstrated on a commercial scale
[43]
. Definitely, in a pulverized form under
otherwise similar conditions, conventional hydroprocessing catalysts would exhibit a much
higher activity than the throw-away solids. However, for such a system, an economic method
for the recovery of metals for reuse has not yet been developed. In this case, metals would have
to be isolated from the VR obtained after distillation of the products unless the residue was
further converted to liquid products and petroleum coke in a coking process. If such option
was chosen, the catalyst metals together with the metals contained in the heavy feed would end
up in the ash providing that the petroleum coke was utilized via a combustion and/or
gasification technology. It is noted that the catalysts, such as used in slurry bed reactors, are
not covered by this review.
3.4.1 Fixed Bed Reactors Systems
Several decades of experience in the operation of fixed bed reactors using conventional feeds
containing neither metals nor asphaltenes were the basis for their adaptation and/or
modification to suit hydroprocessing of more complex feeds. Many years of the experience
confirmed that it is easy and simple to operate fixed bed reactors for atmospheric distillates as
well as for VGO and HGO. Fixed bed reactors can be operated in the upflow and downflow
mode
[123]
. The latter, so-called trickle bed mode, has been used predominantly. However, the
upflow reactors ensure better catalyst wetting at low and high mass velocities for both the
