Hydroprocessing of Petroleum - Spent Hydroprocessing Catalysts - page 40

generally observed that the coke build-up in fixed bed reactors increased from the inlet towards
the outlet, whereas metal deposition usually exhibits opposite trend. The H 2 S/H 2 ratio
increased in the same direction [27] . Consequently, the variable structure of spent catalyst
between the inlet and outlet of the reactor should be expected.

It has been observed that the performance of fixed bed reactors depends on the method of
catalyst loading, i.e., either dense loading or sock loading [124] . In the latter case, many
catalyst particles will reach the loading surface together, having little time to attain a favorable
resting position. Then, particles lay against one another, bridge and maintain random pattern.
In this case, large voids are created to hold particles. The bridges may collapse if some forces
are exerted on such fixed bed. For example, this may be caused by pressure drop, which may
develop during the operation. When catalyst is loaded slowly, particles can settle into place
before being inferred by other particles. This prevents bridging and creation of the oversized
voids. The bed will have a higher density and shrinkage will be prevented. The advantages of
the dense loading compared with the sock loading include the increase in the relative volume
activity and decrease in the start of run temperature [124] . An increased start of run pressure
drop is a negative effect of dense loading. One may anticipate that more problems are expected
with the dense loaded beds when an in situ regeneration of spent catalyst is considered. In fact,
it is unlikely that an in situ regeneration of such beds at the end of operation can be performed
without significant problems.

An optimal combination of the bed void and activity per reactor volume giving the acceptable
pressure drops has to be determined to ensure a steady performance of the fixed bed reactors. In
this regard, the shape and size of the catalyst particles are important [125,126] . This is clearly
shown in Table 3.4 [90] . There is a limit on the maximum pressure drop at which fixed bed can
be operated. This depends on the type of the feed as well on the size and shape of catalyst
particles. Thus, for light feeds, the particle shape and size may be chosen for dense loading to
obtain maximum activity per reactor volume. However, for the high asphaltenes and metal

Table 3.4: Effect of particle size and shape on hydrodesulfurization (HDS) activity [From ref. 90 .
Reprinted with permission].

Shape Dimensions (mm) V p / S p (mm) Activity

Cylinder 0.83 OD 3.7 length 0 . 189 9 . 7

×

Cylinder 1.2 OD 5.0 length 0 . 268 7 . 9 ×

Cylinder 1.55 OD 5.0 length 0 . 345 5 . 7

×

Ring 1.62 OD 0.64 ID 4.8 length 0 . 233 8 . 7 × ×

Ellipse 1.9 OD 1.0 ID 5.0 length 0 . 262 8 . 4

× ×

3-lob 1.0 OD 5.0 length 0 . 295 8 . 2 ×

Crushed 0.25-0.45 0 . 04 14 . 0

∼

OD: outside diameter; ID: inside diameter.