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102 3. Heterogeneous Processes and Reactor Analysis

These reactors employ small particles in the range 0.05 – 1.0 mm (0.0020 –0.039 in)
with the minimum size being limited by f. Small diameters are used to pro ilterability vide
as large an interface as possible, since the internal surface of porous pellets is poorly acces-
sible to the liquid phase (Perry and Green, 1999). The catalyst concentration in slurry reac-
tors is limited by the agitation power of the mechanical stirrer or by the gas flo . w
The advantages of slurry reactors over trickle bed reactors, which are the principal alter-
ield,
native to slurry reactors, are the following (Satterf 1975; Smith, 1981; Perry and
Green, 1999):
• A high heat capacity to provide good temperature control, especially in the case of
highly exothermic reactions, and thus good temperature stability emperature control T .
is relatiely simple due to the large amount of liquid present and the possibility to
v
install coolers inside the reactor .
v
• Heat recoery and transfer can readily be achieed. Uniform temperature conditions v
prevail approximately throughout the reactor.
• Operation in batch or flo w mode.
• Easy replacement of catalyst in case of its deacti ation. v
• The employment of small particles results in efactors near unity. In other eness f v fecti
words, the intraparticle diffusion resistance is lo . w
Due to the use of small particles in slurry reactors, which leads to low resistance from
fusion, intraparticle dif much higher values of global rates are observed in these reactors
than in fixed-bed ones, especially when v e catalysts are used. This is because dif- v ery acti
fusivities in liquid-filled pores are relatively low, of the order 10 5 cm 2 /s compared to val-
ues around 10 2 cm/s, typical for gases. In case of partial de vity , gradation of catalytic acti
the catalyst can be partially remoed and replenished during operation (Perry and Green, v
v 1999). Especially in case of rapid catalyst deactivation, where continuous catalyst remo al
and regeneration is crucial, slurry reactors are most likely to be applied. Furthermore, the
high heat-transfer rates that are observed in slurry reactors, lead to low temperature dif-
ferences between the particle and the liquid. As a consequence, external temperature gra-
dients can be normally neglected in slurry reactors (Smith, 1981).
However, there are some serious disadvantages in using slurry reactors. The most impor-
f tant one is the dificulty in retaining the catalyst in the vessel. In addition, it is dif f icult to
separate the catalyst from the product, if entrainment takes place. Screens and other
devices placed in the outlet lines tend to clog and could be unreliable. Another disadv an-
tage is the low conersion for a gien size because of essentially complete backmixing.
v
v
Finally, high power consumption for agitation to keep the catalyst in suspension and to
, enhance heat transfer and the possibility of homogeneous side reactions taking place due
to the high ratio of liquid-to-solid v are among the disadvantages of slurry reactors olume,
(Satterfield, 1975; Perry and Green, 1999).

3.4.2 Basic types of slurry reactors

There are two types of slurry reactors: slurry bubble column reactor (SBCR, Figure 3.25)
and agitated slurry reactor (ASR, Figure 3.26). These reactors differ in that the solid