Power consumption and gas–liquid dispersion in turbulently agitated vessels with vertical dual-array tubular coil baffles
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Xun Wan
Abstract
In this work vertical dual-array tubular coil baffles arranged in groups of four, six or eight were investigated and the results compared with those from four planar baffles. The baffle coefficients for a single phase, along with the power consumption and gas hold-up in the gas–liquid phase of a system with the various baffle configurations for single and triple Rushton turbines are presented. Measurements were carried out using a dish-bottom vessel with an inner diameter of 0.29 m. Two ambient-temperature media were used as the liquid phase, namely, tap water and a 0.5 M Na2SO4 aqueous solution, representing coalescent and non-coalescent liquids, respectively. The results of the single-phase experiment revealed the coil baffles to have lower power numbers; when the baffle coefficient is ≥ 0.12, the mixing efficiency is the same as that for four planar baffles. The power consumption experiment using the gas–liquid phase showed that installing coil baffles prevented a large power draw in all types of media. In addition, the power draw characteristics are affected by the media. It was found that, because of the low KB number, flooding occurred more readily with coil baffles than with planar baffles. Gas–liquid dispersion experiments in an air–water system indicated that, at a low gas flow-rate, the gas hold-up values of the coil baffles were almost 60 % higher than those of the conventional four baffles. However, this phenomenon was not observed in the Na2SO4 aqueous solution because of the existence of dead zones in viscous liquids. Finally, all the data from the power consumption and gas hold-up experiments on the gas–liquid phase were correlated.
Symbols
| a | coefficient in Eq. (4) | |
| b | exponent in Eq. (4) | |
| b′ | length of impeller blade | m |
| B | width of baffle | m |
| c | exponent in Eq. (4) | |
| c′ | distance between baffle and vessel | m |
| C | distance between bottom impeller and bottom of the tank | m |
| d | outer diameter of tube of inner coils | m |
| d′ | disc diameter | m |
| D | diameter of impeller | m |
| Flg | gas flow number | |
| Fr | Froude number | |
| g | acceleration due to gravity | m2 s-1 |
| h | distance between impellers | m |
| H | height of liquid | m |
| Hg | liquid height under gassed condition | m |
| H0 | liquid height under un-gassed condition | m |
| H′ | liquid-level correction coefficient | m |
| KB | baffle coefficient | |
| KBF | KB number with complete baffles (KBF = 0.35) | |
| KBN | KB number without baffles (KBN = 0) | |
| nb | number of baffles | |
| N | stirrer speed | min-1 |
| NP | power number [NP = P/(ρLN3D5)] | |
| NPF | NP number with complete baffles | |
| NPN | NP number without baffles | |
| P0 | power drawn under un-gassed condition | W |
| Pg | power drawn under gassed condition | W |
| PF | flooding point | |
| Qg | gas flow-rate | m3 s-1 |
| s | gas outlet position | m |
| tm,99 | homogenisation time to reach 99 % | s |
| T | diameter of tank | m |
| V | volume of liquid | m3 |
| VS | superficial gas velocity | mm s-1 |
| w | width of blade | m |
| Greek Letters | ||
| α | coefficient in Eq. (7) | |
| β | exponent in Eq. (7) | |
| γ | exponent in Eq. (7) | |
| ε | gas hold-up, dimensionless | |
| μ | viscosity of material | Pa s |
| ρL | liquid density | kg m-3 |
| σ | surface tension | mN m-1 |
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© 2015 Institute of Chemistry, Slovak Academy of Sciences
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