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can be minimized by achieving local mixing effects, and the
principle of self-similarity can be maintained to achieve similar
performance even in the scale-up of such systems.
More work on understanding the effect of the extent and
distribution of void volumes and the interzonal mixing on the
performance of the impeller designs is in progress. The concept
has been shown to achieve uniformmixing even at lower impeller
speeds thereby reducing the shear rates throughout and can give
a fresh look to the STR design used for the synthesis of
biomaterials and specialty chemicals.3.4. GasLiquid Dispersion. On measuring the power con-
sumption per unit volume of the reactor, it was seen that the
value of PW decreased continuously with increasing superficial
gas velocity. For the case of conventional impellers, the gas
loading reduces the power consumption due to the formation of
cavities behind the impeller blades which modified the pumping
action of the impeller. For the case of fractal impeller as well, the
extent of reduction in PWG was higher for higher impeller
rotation speed as well as at higher gas flow rate (Figure 8A).
The relative power demand (RPD)
1,13
estimated as PWG/PW at a
given VG was seen to go through a maximum (Figure 8B). The
overall values of RPD decreased with increasing VG. The plot of
RPD vs flow number (NQ = Q/ND3
) clearly indicated a locus
along which the RPDgoes through amaximumvalue (Figure 8C).
Also, the RPD corresponding to the point of inflection decreased
with increasing gas velocity. Values of RPD were seen to get well
correlated (not shown here) with Fl
0.2
Fr
0.25
, with the pro-
portionality constant as a function of the dimensionless group
(N3
VG/g), where g is the acceleration due to gravity. The value of
proportionality constant deviated from the 0.18 as reported for
other standard impellers. One of the reasons for such a deviation is
the spatial distribution of impeller blades, which will not support
the use of a single value of impeller diameter. Thus, for example,
the Froude number (Fr = N2
D/g) which depends on the tip
velocity (ND) of the impeller: the variation in the tip distance from
blade to blade will now allow a fixed value of the proportionality
constant in estimating the RPD based on Fl and Fr. Interestingly, in
the FI, none of the blades actually sweep the fluid with it, and hence
the possibility of formation of cavities is almost negligible. However
for the case of gasliquid dispersion, a thin gas layer was seen to stay
below the horizontally flat blades yielding a compressible boundary
layer that would help reduce the drag.This further helps in reduction
in the actual power consumption.On comparing the variation in the
Fr vs Fl for a FI with that of a standard Rushton turbine, it is quite
evident that for a FI, the flow is still in the regime where cavities are
under formation and the possibility of flooding may occur at higher
gas velocities. Such a comparison may look unrealistic as the
behavior of FI and that of a Rushton turbine are verymuch different.
On estimating the fractional gas hold-up at different impeller
Re, the hold-up was seen to go through a maximum. However atall impeller rotation speeds, an increase in gas velocity resulted in
an increase in hold-up (Figure 9). The measured average bubble
size decreased with increasing power consumption, which sub-
sequently yielded higher effective interfacial area. Importantly, it
was seen that the uniform dissipation of energy throughout the
reactor helped yield very narrow bubble-size distribution. The
effectiveness of the reduction in bubble size and increased hold-
up on the overall mass transfer coefficient is being studied