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114 3. Heterogeneous Processes and Reactor Analysis

The oall rate of reaction in continuous-flow systems ver
The concept of an overall rate based on the gas-phase bulk concentration and K o is not par-
ticularly helpful in the reactor design procedure when liquid and gas phases are in contin-
uous flow (Smith, 1981). Instead, the o erall rate should be e ulk v xpressed in terms of the b
liquid concentration. The same analysis is followed for agitated slurry for continuous flow
of both phases and for trickle-bed reactors. F using eq. (3.127) for or a f irst-order reaction,
the catalyst,

( r u ) a C c ( L C m ) k S (3.154)
k
C
S
f
s s m
or
ka f c
C S C L (3.155)
ka m k
f c s s m
and thus

( r u ) L (3.156)
C
k
where
ka m k 1 1 1
k f c s s m (3.157)
ka m k k ka m k
f c s s m f c s s m

3.4.7 Hydraulics

Since the superficial velocity of a liquid in the continuous-type operation is an order of
magnitude lower than that of a gas, it is the gas superficial velocity that is the dominant
parameter that affects the fluid dynamics of the reactor (Duduk vic o et al ., 1999). Thus,
whether the type of operation is continuous (concurrent or countercurrent to the flow of
the gas) or batch (in respect to liquid flow) is immaterial from the fluid dynamics point of
view. The gas, which is sparged at the bottom of the column, results in a b en yanc uo v y-dri
flow that creates a strong liquid recirculation. In addition, as long as they are small (typi-
cally less that 60 slurry particles follo with perhaps the e w liquid motion, xception of
m),
vic very high slurry loadings exceeding 20–30% (Duduk o et al ., 1999). In contrast, in flu-
idizied beds, a relatie moement between the particles and fluid exists (Smith, 1981).
v
v
Thus, in practice, the liquid and solids can be viewed as one flowing phase, frequently
called “slurry” in the continuous-type operation, whereas in the batch-type operation, the
slurry is considered to be a well-mixed phase. In agitated slurry reactors, under suf icient f
agitation, the whole reactor v and is essentially ed, olume can be considered to be well–mix
equivalent to the CSTR.