Environmental Engineering Reference
In-Depth Information
attached. The guard reactor contains about 7% of the total catalyst inventory and its main
function is HDM of the feed. It is however believed that this amount depends on the content of
metals in the feed. Other three reactors contain 31% of the catalyst inventory each. All three
reactors employ a graded bed consisting of either the same catalyst but of different particle
sizes and shapes or catalysts of a different composition. The purpose of using the graded bed is
to diminish the reactor pressure drop particularly in the front of catalyst system, which is
contacted with only partially converted and/or unconverted feed. Because the guard reactor
only removes a portion of metals, the catalysts in the downstream reactor must possess an
adequate HDM activity. Thus, a relatively large amount of metals was still present in the spent
catalysts from all three main reactors
[136]
. However, this problem may be alleviated by an
optimal selection of catalyst for the guard reactor and the subsequent reactor. The modified
Unicracking/HDS process comprises five reactors in a series
[137]
. It has similar features as
ARDS process. In this case, the first reactor was in fact a guard reactor containing a high metal
storage capacity HDM catalyst. With this arrangement, heavy feeds containing as much as
150 ppm of V+Ni were successfully hydroprocessed.
The HYVAHL process was developed and licensed by the French Institute of Petroleum
[138]
.
This process was successfully tested for hydroprocessing of various heavy feeds, i.e., DAOs,
atmospheric and VRs. The process consists of the guard reactor placed upstream of the two
HDM reactors. The guard reactor is sized and optimized to achieve a satisfactory length of the
cycle. To protect catalyst in the HDS section, two more HDM reactors are placed downstream
from the guard reactor. This version of HYVAHL process, known as the swing reactor
concept, ensured a continuous operation of the process approaching one year using heavy
feeds, the metal content of which was in the range 500 ppm of V+Ni. In this case, the process
included two guard reactors that were switchable during the operation. With this concept, the
replacement of catalyst in the guard reactor does not require shutdown of the operation
[139]
.
The guard reactor and two HDM reactors represent about 40% of the total catalyst volume. Of
course, the exact amount of catalyst required for guard reactor depends on the amount of
metals in the feed and metal storage capacity of the catalyst.
3.4.2 Moving Bed and Ebullated Bed Reactors
It has been evident that for fixed bed reactors, the difficulties in handling heavy feeds could be
overcome either by frequent catalyst replacements or by adding more reactors in the series. At
a certain point, both these options become economically unattractive. Also, it is not easy to
maintain synchronized operation of so many fixed bed reactors in a series. Because of these
problems, reactor design and catalyst development has reached entirely new levels. In this
regard, attention has been focussing on the development of a process enabling catalyst
replacement on stream without interrupting the operation. The bed of catalyst moving
vertically through the reactor was one option which had been explored. Several moving bed
