Temperature and solid concentration effects on the relative power demand (RPD) and the volumetric mass transfer coefficient (k_La) in air/water/glass beads three-phase system have been investigated in a fully baffled dished base stirred vessel of 0.30 m diameter holding 0.036 m³ slurry stirred by a triple-impeller combination. The impeller combination consisted of a half-elliptical disk turbine (HEDT) below two up-pumping wide-blade hydrofoils (WH_U). The specific power consumption P_Tm is the sum of the potential energy of the sparged gas and the agitation power consumption calculated by measuring the torque of the stirring shaft and the agitation speed. Total gas flow rate, including the contribution of the evaporation from the liquid phase, was used to calculate the superficial gas velocity V_S. The gas-liquid volumetric mass transfer coefficient k_La was measured by the steady-state sulfite feeding method (SFM) (Imai et al., 1987). The measurements show that the location of DO probe has no influence on the results of k_La, and the probe location was finally kept constantly at l/T=1 to determine k_La.

The critical just-suspension impeller speed and the gas-liquid complete dispersion agitation speed were determined by visual observation method. In order to guarantee the complete gas dispersion and solid suspension, the agitation speed was chosen higher than 8 s^-1.

1. Power Consumption

Power consumption have been measured at volumetric solid concentrations C_V up to 12 vol % at six different superficial gas velocities (V_S) and five different temperatures (T^*) ranging from 25 to 80°C with increments of about 14°C. The relative power demand (RPD) is the ratio of gassed to ungassed power demand.

1.1 Effect of Temperature on RPD

The results show the significant effects of temperature and solid concentration on the hydrodynamic characteristics. When no particles are present, the higher the temperature is, the greater RPD is. However, the effect of the temperature on the RPD becomes less evident at higher solid concentrations C_V. The average ratio of (RPD)_80°C to (RPD)_25°C is 1.14 in a gas-liquid system without solids, but this ratio falls to about 1.06 in a gas-liquid-solid system with C_V equaling to 12%. The RPD increases remarkably with the increase of temperature and the exponent for temperature is 0.44 in gas-liquid system, and it decreases to 0.26 in a three-phase system with C_Vequaling to 12%.

1.2 Effect of Solid Concentration on RPD

As for the effect of solids, the RPD slightly increases with the increase of C_V at ambient temperature 25°C, but decreases with the increase of C_V at hot system. Solids can influence RPD in different ways at different temperatures, which can be interpreted in terms of the bubble size and the effect of C_V on it. Bubbles are larger in hot-sparged systems than in “cold” system because of the lower liquid viscosity in a hot system. Moreover, the surface tension, which helps to stabilize the bubble, is also lower at higher temperature. As a result, in comparison with cold conditions, large bubbles in a hot system are more easily affected by the settling particles and broken into small bubbles and retained in the liquid with a relatively long retention time. Moreover, the particles will occupy part of the bulk liquid volume, thus enhance the probability of bubbles collision and coalescence. The competing results of breakup and coalescence of bubbles may lead to a relatively larger stable bubble size in hot systems. Therefore, the RPD decreases with the increase of C_V at 80°C. When the temperature is low, the smaller bubble size and higher interfacial tension might cause the collision and coalescence of bubbles to dominate the breakup when more solids are introduced in the systems. The resulting higher mean density and reduced blade cavity size will cause RPD increases with the increase of C_Vin cold conditions. The balance between the opposing changes with concentration at high and low temperatures reduces the effect of the temperature on RPD in high-concentration suspensions. The exponents for the effects of the solid concentrations on the gassed power decreases from (+0.158) to (-0.092) when temperature increases from 25 °C to 80°C, passing close to zero at 54°C.

 

2. Volumetric Mass Transfer Coefficient k_La

2.1 Effect of Temperature on k_La

 When no particles are present, k_La is almost independent of the temperature. When the temperature increases, the viscosity of the liquid decreases but the diffusivity of the gas in the liquid increases. Therefore, k_L increases with the increased temperature and the k_L at 80°C is 2.18 times of that at 25°C. With the increase of temperature, the diameter of bubbles increases and the rapid release of the big bubbles also leads to a reduction in the retained gas and the gas holdup decreases as a result. As a result, the gas-liquid interfacial area decreases. The gas-liquid interfacial area a at 80°C is about 0.48 times of that at 25°C, and the theoretical value of k_La at 80°C is about 1.04 times of that at 25°C, indicating that the temperature has very little effect on k_La at gas-liquid system after combining the effects of temperature on both k_L and a. However, temperature has relatively positive effect on k_La when solids are added in the system. Compared with the k_La at ambient temperature, the average k_La at 80°C increases by about 27% when C_V is 6%, whereas increases by about 56% when C_V is up to 12%. The temperature influences k_La in different ways when the solid concentration is different, which can be interpreted the different competition results between the effects of temperature on k_L and on a. The addition of particles can more easily break the larger bubbles into smaller ones in a hot system and retain bubbles in the liquid with a relatively long retention time. The decrease of gas holdup with higher solid concentration is not as much as in gas-liquid system (Bao et al., 2008b). Therefore, in an aerated slurry system, the dominant effect of temperature on k_La is k_L increases with the increased temperature and then k_La increases as a result. But in gas-liquid system, the decreased a can almost balance the increased k_L and k_Labecomes nearly independent of temperature.

2. Effect of Solid Concentration on k_La

In cold-gassed conditions, k_La decreases considerably in the presence of solids. This reduction becomes smaller at higher temperature, until at about 54°C, k_La becomes independent of C_V. At the highest temperature of 80°C, k_La even increases when more solids are present. The different effects of C_V on k_La at different temperatures can be primarily interpreted in terms of a and the influence of C_V on it. In cold-gassed conditions, a decreases with the increase of C_V as the gas holdup decreases considerably in the presence of solids (Bao et al., 2008). However, this reduction becomes smaller at higher temperature. Moreover, at cold-gassed conditions higher solid concentrations cause a greater number of bubble collisions, resulting in the increased coalescence, and hence an increase in bubble size and a decrease of the interfacial area (Panja and Phaneswara, 1993). But in hot systems, the decrease of the liquid viscosity and surface tension cause bubbles breakup and the formation of small and stable bubbles (Ferreira et al., 2010). Thus, C_V has a relatively strong negative effect on a; but the negative effect of C_V on a becomes weaker with the increasing temperature. Or, C_V may even have a little positive effect on a in high temperature systems. Furthermore, considering the effect of C_V on k_L, the addition of solids would increase the viscosity of the slurry, thus decelerates the diffusivity of gas into liquid. However, in hot-sparged conditions, the high temperature helps to decrease the viscosity, thus decrease the thickness of the stagnant film at gas/liquid interface (Ferreira et al., 2010), and increase the diffusion coefficient, as a result weakening or balancing the negative effect of solids on k_L. Combining the effects of C_V on both k_L and a, C_V has negative effect on k_La at cold system. But in hot system, the changes of a and k_L may cause a slightly increase of k_La with the increasing C_V.

Keywords: mass transfer coefficient, power consumption, gas-liquid-solid system, triple impeller, stirred reactor, temperature

References

Bao, Y., Chen, L., Gao, Z., Zhang, X., Smith, J.M., Kirkby, N.F., 2008. Temperature effects on gas dispersion and solid suspension in a three-phase stirred reactor. Ind. Eng. Chem. Res. 47, 4270-4277.

Ferreira, A., Ferreira, C., Teixeira, J. A., Rocha, F., 2010. Temperature and solid properties effects on gas-liquid mass transfer. Chem. Eng. J. 162, 743-752.

Imai, Y., Takei, H., Matsumura, M., 1987. A simple Na₂SO₃ feeding method for k_La measurement in Large-Scale fermentors. Biotech. Bioeng. 29, 982-993.

Panja, N. C., Phaneswara, R. D., 1993. Measurement of gas-liquid parameters in a mechanically agitated contactor. Chem. Eng. J. 52, 121-129.