Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 46

was observed by Al-Dalama and Stanislaus [12] . To be cost competitive, this process must
produce enough additional liquid products compared with the non-catalytic options, i.e.,
deasphalting and coking, to compensate for the costs of catalyst inventory and excessive
hydrogen consumption. Also, the additional high-pressure vessels and equipment upstream
and downstream of the reactor are necessary to ensure safety of the operation similarly as it
was noted for moving bed reactors. This adds to the capital cost of the processes employing
ebullated bed reactors compared with the fixed-bed reactors.
The most important features of the ebullated bed reactors include their capability to either
periodically or continuously add/withdraw catalyst without interrupting the operation. The bed
design ensures an ample free space between particles allowing entrained solids to pass through
the bed without accumulation and plugging as well as without increasing pressure drop. Under
such conditions, the catalyst particles having a diameter smaller than 1mm (e.g., 1/32 in
extrudates) can be utilized. This results in the considerable increase in reaction rate because of
the significantly diminished diffusion limitations. Moreover, under such conditions, the
catalyst utilization is significantly enhanced. Depending on the operating strategy of the
refinery, the process can operate either in a high conversion mode or in a low conversion
mode [27] .
The information on the LC-Fining and H-Oil reactors is quite extensive [129-131] .Itisagain
noted that these reactors have similar features. In ebullated bed reactor ( Fig. 3.13 ), the heavy
feed and H 2 enter at the bottom and move upwards through the distributor plate at a sufficient
velocity to expand the catalyst above the grit into a state of random and turbulent motion. The
expanded bed is maintained about 35% above the settled level of catalyst. This can be achieved
by controlling the speed of the recycle oil pump. In this regard, the operation is monitored
using the density detectors. The suction of the recycle pump is supplied from near the top of
the reactor. The recycle pan is used for disengaging the gas before recycling the liquid. The
advanced design of the ebullated bed reactor used in the H-Oil process incorporates an
improved internal recycle cup enabling a complete separation of gas from the recycled liquid.
With this modification, the throughput of heavy feed was increased. On a commercial scale,
usually three ebullated bed reactors are used in the series ( Fig. 3.15 ). The first reactor serves as
a guard reactor, the primary function of which is HDM. The main functions of the second and
third reactors are HDS, HDN and HCR. In some situations, the ebullated bed reactor can be
used as the guard reactor upstream of the fixed bed reactors. However, in the case of a large
amount of inorganic solids in heavy feed, part of these solids may not be trapped in the
ebullated bed reactor. Such solids may then be carried out with liquid streams to the
subsequent fixed bed reactor.
Figure 3.16 [132] shows the simplified diagram of the catalyst handling system consisting of
three sections, i.e., fresh catalyst handling, the daily addition/withdrawal of catalyst to and
from reactors and spent catalyst handling system. The fresh HDM catalyst is carried as slurry