Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 39

cylindrical and shaped catalyst particles regardless of the catalyst loading procedure. In trickle
bed reactors, the catalyst wetting can be improved by choosing the loading procedure which
ensures a minimal horizontal orientation of particles in the reactor. There is a lesser probability
of malfunctioning of trickle bed reactors caused by channeling than that in the upflow reactors.
The fixed bed can comprise either a single stationary bed ( Fig. 3.10 ) of the same catalyst of the
same particle size and shape or layers of different catalysts. The layers may consist of the
catalyst having the same chemical composition, but different size and shape of particles as
well as the different pore size and pore volume distribution. For example, the layers may
include the HDM catalyst at the reactor inlet, on the top of a HCR catalyst, followed by the
HDS/HDN catalyst near the reactor outlet. The choice of catalysts and number of layers
depends on the origin of heavy feed as well as on the anticipated quality of the final products.
There are some advantages of the fixed bed systems consisting of several sections in the same
vessel with an empty space between the sections ( Fig. 3.11 ). The sections may contain the
same or a different catalyst each. In any case, with this arrangement, the make up H 2 can be
introduced between the sections to quench the heat released by exothermic reactions. Also,
some systems have a provision for scrubbing ammonia and H 2 S from the gaseous effluent
from the first section before it enters into the next section. This enables control of the H 2 S/H 2
ratio, which is critical for a high conversion of HDN reactions [27,49] . Otherwise, the
excessive poisoning of catalysts by N-bases would affect the operation. Indeed, it has been
Figure 3.11: Modification of Unicracking process for dewaxing petroleum feed [From ref. 130 .
Reprinted with permission].