rate of energy dissipation in the place of dispersion formation and the rate of interfacial
mass transfer represented by kLaL values (see e.g. Kaštánek et al., 1993). Large rate of
energy dissipation is therefore generally required to ensure large intensity of phases
contacting in a reactor. At the same time, however, minimisation of the overall energy
input (energy costs) has lately become one of the decisive factors to be considered in the
process of reactor selection for specific processes in gas-liquid or gas-liquid-solid (slurry)
systems (Kaštánek et al., 1993). To solve optimally these two mutually contradicting
tasks, it is therefore necessary to chose a reactor type with the most favourable relation
between the energy dissipation rate and the intensity of gas-liquid contacting and
subsequently to minimise, by an appropriate reactor design, the fraction of total power
input which is not directly utilised for gas dispersion.
In principle, the overall rate of energy dissipation in a multistage bubble column
sectionalised by perforated plates consists of the two basic contributions,
(6)While the first term, representing the energy dissipation in bubble beds in individual
stages,
(7)directly determines the intensity of gas-liquid interfacial contact in the reactor (Kaštánek,
1977), the second term, ∆PwQG, represents the fraction of energy inefficiently dissipated
due to the distributing plates pressure drop (Zahradník et al., 1982a). The specific rate of
energy dissipation related to a mass unit of a respective slurry system in the reactor, ed,
can be then defined by the relation
(8)where ∆PW is the total pressure drop of wetted plates in the column. Eq.(8) clearly
indicates the importance of the optimum plates design aimed at minimising their pressure
drop at given phases flow rates while ensuring their stable, uniform distributing
performance.
Graph kLaL vs ed plotted in Figure 1.6 clearly proves that the efficiency of utilisation of
the dissipated energy for gas-liquid contacting increased with the increasing number of
reactor stages while the effect of solid phase concentration was negative. To compare the
energy effectiveness of gas-liquid contacting in different reactor types, the energy
effectiveness criterion Φ=kLaL/ed was introduced in our former work (Zahradník et al.,
1982a). Multiplied by the respective concentration difference, representing mass transfer
driving force, coefficient Φ subsequently yields the amount of gas transferred across the
gas-liquid interface per unit of dissipated energy. Graph Φ vs kLaL, shown in Figure 1.7,
New methodologies for multiphase bioreactors 1 17