column stages, distributing plate geometry, gas and liquid flow rates, solid phase
concentration) on hydrodynamic and mass transfer characteristics of multistage aerated
slurry reactors. The particular objectives of the reported work included:
Evaluating the effect of column sectionalisation on liquid or suspension phase
backmixing in the reactor and on the values of gas holdup and volumetric coefficient
of mass transfer between the gas and liquid phases (kLaL);
Derivating and experimentally verifying physically realistic model of liquid phase
(suspension) residence time distribution in a multistage bubble column sectionalised
by perforated plates with downcomer tubes without overflow weirs and investigating
the effect of operating regime on the extent of liquid backflow between the adjacent
column stages;
Experimentally determining the effect of solid phase concentration on gas holdup
values and on the rate of gas-liquid mass transfer, i.e. on the values of kLaL; Evaluating
the effect of column sectionalisation and solid phase concentration on the energy
effectiveness of bubble bed formation and of gas-liquid interfacial mass transfer.EXPERIMENTAL
The experiments were carried out in a bubble column reactor 0.29 m in diameter and 2 m
in height. The column was built of glass cylinders mounted between metal flanges and
could be alternatively divided into three or six stages (stage height 0.6 m or 0.3 m
respectively) by internal perforated plates with downcomer tubes without overflows. The
plates were made of brass sheets 2 mm thick, free plate area ratio was 0.25%, holes
diameter 1.8 mm. The downcomer tubes were 0.1 m long and 0.04 m in diameter; some
preliminary experiments in the three-stage column were performed also with the tube
length 0.42 m. The scheme of the experimental set-up is given in Figure 1.1; constructing
details of distributing plates (position of downcomer tubes and placing of orifices) can be
found elsewhere (Zahradník et al., 1992). Experiments were carried out in a counter-
current flow arrangement with systems air-water and air-aqueous suspension of ZnO
respectively. The characteristic particle dimension was 2.3 μm, 90% of the particles were
smaller than 3μm (see Kratochvíl et al., 1985). Experiments were performed within the
range of solid phase concentrations cs=1–5 wt.%. Experimental regions of gas and liquid
(slurry) flow rates,
corresponded to the superficial velocities uoG=
0.025–0.067 m s−^1 and uoL (or uosl)=0–0.029 m s−^1. Supposedly, such phase velocities
covered the common operating regions of single-stage bubble column reactors while the
plate geometry guaranteed uniform gas distribution within the whole experimental range
of uoG values (Zahradník et al., 1982b).
Residence time distribution of the liquid or slurry phase was determined by the pulse
response technique using aqueous solution of KCl as a tracer and the electrical
conductivity detection method. A tracer pulse was injected into the feed stream in the
upper column stage, the injection time fulfilled in the whole experimental region of liquid
(slurry) flow rates condition for the satisfactory simulation of the pulse input signal,
∆ti/tav
New methodologies for multiphase bioreactors 1 3