higher level of micro-scale turbulence, the microparticles penetrate the intervening film
between colliding bubbles and thus increase the rate of film drainage and ultimately
enhance bubble coalescence. While this hypothesis has to be indeed further examined, it
seems to be supported by the fact that significant increase of gas holdup due to the
addition of fine inert particles has been observed under experimental conditions (small
superficial gas velocity, low aspect ratio, porous sparger) favourising the existence of the
homogeneous bubbling regime, typically exhibiting low levels of micro-scale turbulence
(Sada et al., 1986; Khare and Joshi, 1990). As can be seen from Table 1.4, the relative
decrease of gas holdup in the slurry system, as compared with the standard two-phase air-
water system, was, to a certain extent,
suppressed by the column sectionalisation. Obviously, this observation further confirmed
the favourable effect of gas redispersion in multi-stage sieve-tray columns.
The effect of liquid flow rate on gas holdup was negligible within the whole region of
working conditions. The dependence of gas holdup on the superficial gas velocity was
formally described by the well known Reith-Mashelkar equation (Reith and Beek, 1971;
Mashelkar, 1970) commonly used for gas-liquid systems,
(4)where, however, bubble swarm velocity, Ubs, was in our case assumed to be a function of
solid phase concentration and of the number of column stages (the latter parameter
Table 1.4 Relative decrease of gas holdup with the
solid phase concentration
(^) ∆ε, %
Cs, % wt. N= 1 N=3 N=6
1 20 20 18
2 29 28 26
3 37 36 34
4 46 48 38
5 55 50 42
New methodologies for multiphase bioreactors 1 11