primarily on the physical characteristics of the solid
particles. Heavier and/or larger particles usually form
larger bubbles that cause excessive gas bypassing thus,
leading to poor conversion and yield. Conversely, very
f ine particles stick together and
impede uniform f luidization. Gas
channeling or rat-hole phenomena
are often the result of using such par-
ticles, which also causes inferior reac-
tor performance. In Fig. 4, the
schematic of a fluidized-bed reactor
operating with large bubbles is illus-
trated. Simulation of this reactor, for
an actual commercial reaction sys-
tem, using three average bubble sizes
is shown in Fig. 4. The conversions
and selectivities are the normalized
values obtained by dividing the abso-
lute values by arbitrary numbers, for
proprietary reason. Reactor perfor-
mance improves with decreased bub-
ble size and is quite evident. To opti-
mize this reactor will involve the
design and installation of appropri-
ate measures/ devices to control bub-
ble growth within the reactor as
shown schematically in Fig. 4.
Example 4--Liquid-phase reactor
with instability and control prob-
lems.
Many commercial liquid-phase
reactors, including hydrotreaters in
refineries, operate with an external
cooling loop through which part of the
reactor product circulates at a high
recycle rate. The arrangement is
shown in Fig. 5. The heat generation
(Q
G
) and heat removal (Q
R1
) rates of
this reactor-heat exchanger loop are
typically represented by the curves as
also shown in Fig. 5. The points of
intersection of these curves represent
the stable operating conditions. The
stable operating temperature of this reactor is, there-
fore, either at T
A
or T
B
. However, a slight disturbance
or change in operating condition either from the reactor
or heat exchanger could move the two curves. This will
cause the reactor temperature to march quickly either
to the higher (T
A
) or lower (T
B
) stable point. This is par-
ticularly true when the two curves are very close and
almost parallel to each other near the desired operat-
ing temperature. This is a typical phenomenon of reac-
tor instability.
Control and operation of the reactor often becomes
very difficult in such a situation. Furthermore, a shift
of the operating point toward the higher stable tem-
perature may lead to severe product degradation and
polymer formation, line plugging or cooling tubes foul-
ing. In the worst-case situation, a temperature run-
away may occur leading to catastrophic explosions.
Conversely, a shift toward the lower stable temper-
ature may lead to "chilling" or severe production loss
of the reactor. A revamp of this unstable reactor system
is shown in Fig. 5. The reactor retrofit involves increas-
ing the system's heat removal capacity. Another heat
exchanger is installed in the circulation loop; the dual
exchanger is operated at a higher feed temperature T
2
.
With the additional capacity (higher heat transfer sur-
HYDROCARBON PROCESSING / SEPTEMBER 1999
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
,,,
Fig. 4. Revamp of bubbling fluidized-bed reactor with poor hydrodynamics.
Fig. 5. Retrofit of a liquid-phase reactor with instability and poor control.
Fig. 6. Reaction mechanism for propylene-to-acrylonitrile reaction
system.
