Ind. Eng. Chem. Res. 2008, 47, 2049-2055
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Gas Dispersion and Solid Suspension in a Hot Sparged Multi-impeller Stirred Tank Yuyun Bao, Xinnian Zhang, Zhengming Gao,* Lei Chen, Jianfeng Chen, John M. Smith,† and Norman F. Kirkby† School of Chemical Engineering, Beijing UniVersity of Chemical Technology, Beijing 100029, China, and Fluids and Systems Research Centre, School of Engineering (J2), UniVersity of Surrey, Guildford GU2 7XH, United Kingdom
Despite wide application in the process industries, the effects of settling particle characteristics on gas dispersion and solid suspension at elevated temperature have received little attention. Power consumption, gas holdup, and critical just-suspension agitation speed for different concentrations of solid particles have been measured in a fully baffled dished-base vessel of 0.48 m diameter holding 0.145 m3 liquid. The impeller configuration (a hollow half-elliptical blade dispersing turbine below two up-pumping wide-blade hydrofoils, identified as HEDT + 2WHU) recommended in previous work has been used in this paper. Air, deionized water, and glass beads of ∼90 µm diameter and density 2500 kg‚m-3 were used for all experiments. The operating temperatures were 24 and 81 °C, identified as cold and hot, respectively. Results show that the relative (gassed-to-ungassed ratio) power demand, RPD, in a hot sparged system decreases slightly with an increasing solid concentration. This becomes less evident at lower agitation speeds, where, at a given gas rate, unlike in the cold systems, RPD is higher in hot conditions than when cold. At a given total gas flow rate and power input, gas holdup in a hot sparged system is independent of solid concentration and only about one-half to three-quarters that in the cold conditions. The reasons for these differences between cold and hot systems are analyzed in this paper. The critical suspension speeds in both cold and hot systems increase at first and then level off or even decrease a little at volumetric concentration of solids above 15%. The introduction of gas leads to a higher critical suspension speed in both hot and cold sparged systems, though the effect of total gas flow rate is greater in cold conditions than in hot conditions. Preliminary correlations based on the present work are presented as a guide for future industrial applications in which knowledge of the power requirements, the critical suspension speed, and the actual liquid fraction retained in the reactor may be of commercial importance. 1. Introduction Gas-liquid-solid three-phase stirred reactors are widely used in wastewater treatment and the chemical, mineral, and biochemical industries. Extensive studies of three-phase systems have been reported during the last 30 years.1-7 Correlations that have been used for industrial reactor design and scale-up of such three-phase systems have been established and reviewed by Doran.7 All these investigations were carried out in ambient (i.e., “cold”) conditions where the vapor pressure of the liquid phase can be assumed to have an insignificant effect on the process. For many exothermic (i.e., “hot”) processes, e.g., oxidation, hydrogenation, and polymerization, the vapor pressure should be taken into account. During the past decade, further research has been carried out with hot gas-liquid systems,8-12 which have shown that hot and cold systems are radically different. The agitator power draw is greater at higher temperatures, while bulk and micromixing times are essentially unaltered and retained gas fractions are substantially reduced. Schaper et al.13 also reported that the decrease in gas density may also reduce the gas holdup. Bao et al.14 have investigated the influence of volumetric fractions of buoyant particles at high temperature in a gas-liquid-solid three-phase system and concluded that the effect of the solid concentration on aerated agitator power demand is negligible in both cold and hot systems. Gas holdup in a hot sparged system, which is only about one-half to two-thirds of that at room temperature, is * Corresponding author. E-mail:
[email protected]. Tel.: +86-10-6441-8267. Fax: +86-10-6444-9862. † University of Surrey.
slightly increased at higher solids concentrations. The effect of settling particles on reactor performance may well differ from that of buoyant solids; however, there is no published information on this. Reactors of high aspect ratio are increasingly being used at industrial scale. Multiple-impeller agitators are employed in such tall vessels to ensure good flow and circulation. The agitator combination of a concave-blade disk turbine (HEDT) as the lowest impeller surmounted by two up-pumping wide-blade hydrofoils (WHU) was recommended in our previous work with settling particles in a gas-liquid system at ambient temperature.15 The radial bottom impeller efficiently disperses gas while operating at a higher relative power demand (RPD), providing a greater gas holdup, and suspending settling particles at a lower agitation speed and power consumption than other designs. The combination of up-pumping hydrofoils with the radial flow disperser effectively disperses gas and suspends the solids. The effects of solids concentration and gas flow rate on the power consumption, gas holdup, and just-suspension agitation speed in a multi-impeller hot three-phase system with settling particles are reported in this paper. Correlations of power consumption, gas holdup, and critical suspension speed of both cold and hot sparged systems are presented as references for industrial design and scale-up. 2. Experimental Section 2.1. Equipment. All the experiments were carried out in a fully baffled, dished-bottom cylindrical tank with internal diameter T ) 0.48 m and a filled aspect ratio H/T ) 1.8 sketched
10.1021/ie071461x CCC: $40.75 © 2008 American Chemical Society Published on Web 02/20/2008
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Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 Table 1. Saturation of Bubbles in Hot Systems
Figure 1. Schematic of the experimental setup.
Figure 2. (a) 6-Half-elliptical-blade disk turbine (HEDT) and (b) 4-wideblade hydrofoil impeller (WH).
in Figure 1. The geometry of the tank is exactly the same as that used in our previous work on a three-phase system with buoyant particles.14 The HEDT + 2WHU impeller configuration, which was recommended in previous work15 with cold gassed suspension, was used for all experiments. A gas-dispersing disk turbine with concave half-elliptical blades (HEDT, Figure 2a) was at the bottom, and two up-pumping (WHU) wide-blade hydrofoils (WH, Figure 2b) were mounted above it. All the impellers were 0.4T diameter, which is the same as the clearance between the lowest impeller and the base of the tank. Each pair of impellers was separated by 0.48T, i.e., 1.2D. A ring sparger of diameter 0.8D was located 0.33T above the tank bottom. Four cylindrical heaters (each rated at 3 kW on full power) were mounted vertically in the base of the tank. The vapor generated in the vessel was condensed in the condenser and returned to the stirred tank, maintaining the liquid inventory in the tank. Deionized water and air were used for all the experiments. The solid particles were 90 µm diameter glass beads of density 2500 kg‚m-3. In order to get rid of any influence caused by surfactants on the glass beads, the glass beads were washed three or four times with alcohol until they were clean and totally hydrophilic, then thoroughly rinsed with deionized water. The volumetric solid concentrations were 3, 6, 9, 15, and 21%. The
T* (°C)
Qg (m3‚h-1)
Qg+v (m3‚h-1)
saturation
81.0 81.0
6.5 17
15.6 40.7
95.2% 99.3%
total gas rates, including the vaporized water, ranged from 0.00141 to 0.0111 m3‚s-1 (the corresponding superficial velocities were from 0.0079 to 0.0625 m‚s-1). Impeller agitation speeds ranged from 5 to 9 s-1. The liquid bulk temperatures in cold and hot conditions were 24 and 81 °C, respectively, which was controlled to within (0.2 °C by a thermal resistance thermostat. 2.2. Measurement Technique. The power consumption was calculated from the torque and rotational speed of the shaft measured with a torque transmitter and a portable tachometer. The gas holdup was calculated from changes in the liquid level measured by a calibrated radar probe (Krohne Reflex-Radar BM100A, Germany). The critical just-suspension impeller speed was determined visually by observing the base of the tank, especially the region around the center of the tank where the particles often settled. Following the Zwietering (1958) criterion for the just-suspended condition for settling particles, the critical speed, NJS, could consistently be judged within an accuracy of ∼5% in experiments by a given person. The critical agitation speed in a threephase system is identified as NJSG with the corresponding power consumption, PJSG. The agitator speeds were above the justsuspension speed during all the measurements of gas holdup. 2.3. Experimental Technique for Hot System. When a bubble is introduced into a hot liquid, evaporation or condensation is likely to occur in order to bring the partial vapor pressure in the bubble toward that of the liquid at its bulk temperature. Since the latent heat required for this evaporation comes from a cooling of the water near the interface, the temperature of the liquid will fall until the partial vapor pressure inside the bubble matches that of the hot water in the vicinity. The new steady temperature of the system is the “equilibrium temperature”. As a result, a given liquid temperature can only be maintained in the presence of a continuous heat source such as a chemical reaction or electrical heating. In this work, the temperature of the hot system was controlled at 81 ( 0.2 °C. In order to determine the total gas flow rate, including both the introduced air and the vaporized water, the saturation of water in bubbles should be considered first. Smith et al.16 have presented a very simplified analysis of this transfer process with the key assumption that external heat transfer resistances can be ignored; they suggested that the time to reach 95% saturation is very short. In our previous work,14 this suggestion was adopted and the off-gas was assumed to be fully saturated at the operating temperature. An experiment to assess the saturation of off-gas was carried out in this work to validate the previous assumption. Fresh air was introduced into hot water with the temperature of the system controlled at 81 ( 0.2 °C. The vapor was allowed to leave the free surface directly instead of going into the condenser. The water levels before and after continued aeration of 30 min or more were measured by the radar level probe. The mass of water evaporated could be calculated. Comparison of the actual loss of water with that estimated by assuming the off-gas was fully saturated at 81 °C allowed calculation of the saturation percentage, i.e., the ratio of actual mass loss with the results given in Table 1. Saturation in excess of 95% was obtained at both high and low gas rates, confirming that bubbles effectively reach saturation before leaving the free surface. A straightforward material balance shows that the
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augmented gas-phase sparged volume rate, including both gas and vapor, Qg+v, is given by eq 1.
Qg+v )
[
]
T* + 273.15 (P0 - Pv) T0 + 273.15 QgP0
‚
(1)
where T* is the hot sparged “equilibrium temperature”, viz. 81 °C here. T0 is the temperature of the gas at inlet. Qg is the inlet gas flow rate. P0 is the ambient pressure. Pv is the saturated vapor pressure at T*, and Qg+v is the total flow rate of the saturated gas. Throughout this paper, all the hot sparged system data and gas flow numbers are calculated and compared on this basis. 3. Results and Discussion 3.1. Reproducing the Data of Previous Work in Cold System. In our previous work, Bao et al.15 reported that, at ambient temperature, even with the volumetric concentration of settling particles as high as 15%, RPD and gas holdup are not changed significantly from those in a gas-liquid system without suspended solids. In the previous work, air from an oil-free compressor was used without any further purification. Gas-liquid systems are very sensitive to traces of contamination: Schaper et al.13 reported that, at ambient temperature, very low levels of impurity can have a large effect on the gas holdup. Air from an oil-free compressor inevitably includes some contamination, which may influence the gas holdup. In order to reduce the danger of introducing contaminants, three-stage filters were installed to clean the gas before it was sparged into the tank. The deionized water used in the experiments was replaced daily in order to maintain the water purity. The reproduced data of RPD and gas holdup are shown in Figure 3, together with the previous published data.15 It shows that the RPD data are reproduced reasonably both with and without purification. However, there is an evident decrease in gas holdup with increasing solid concentration; this is significantly different from the result with the unfiltered air. This effect of trace contaminants on the gas retention might explain why various investigators, as reviewed by Nienow et al.,1 report very different effects of solids on gas holdup and gas-liquid mass transfer. All the experiments in the present work, at both ambient and elevated temperatures, used the purified air, deionized water, and glass beads cleaned with alcohol. 3.2. Relative Power Demand. 3.2.1. Effect of Settling Particles. The influence of solid concentration on the relative ratio between gassed and ungassed power draw (RPD) in the present hot sparged three-phase system is shown in Figure 4a. The data at N ) 6 s-1 demonstrate that a variation of solid concentration does not cause a large change in RPD. However, when the speed is increased to 8 s-1, RPD decreases slightly as the solid concentration is increased from 0 to 21 vol %. At the lower agitation speed of 6 s-1, the particles are suspended, though the solids loading near the base of the vessel is much higher than the average because of some axial inhomogeneity. When the speed is increased to 8 s-1, the particles are distributed more uniformly and there is a similar effect on the power consumption of the three impellers. Figure 4b shows the effect of particle concentration on gassed power number. It seems that a variation of solid concentration does not cause a large change in NPg calculated on the basis of an average slurry density. This is less than the local value in the impeller region with high solid loading, which will explain why the apparent ungassed power number for high solid concentrations can be as high as 4.4, about 5% higher than the
Figure 3. Reproduction of the data: (a) reproduced data of RPD vs Flg and (b) reproduced data of gas holdup.
actual, “homogeneous” power number. Figure 4a also shows that, in suspensions, as is the case in the absence of solids, increases in either gas flow number or Froude number will reduce the RPD. 3.2.2. Comparison with the Cold Gassed Relative Power Demand. Figure 5 represents the relationship between RPD and flow numbers at N ) 8 s-1 for both cold and hot systems over the solid concentration range from 0 to 21%. The RPD for both cold and hot systems decreases with an increasing flow number. However, the absolute value of RPD in a hot sparged threephase system is evidently higher than that in a cold system at the given flow numbers. It can also be seen that an increasing solid concentration leads to a slight decrease in RPD for a hot sparged system, the opposite to the change found in a cold system. Power consumption in an aerated stirred tank is affected by two factors: the vortices behind each impeller blade and the density of the mixture of gas, liquid, and solid. At high operating temperature, with a given total gas flow rate, there will be additional evaporation into the low-pressure region in the vortices, which will increase the size of the vortex and the size of the bubbles leaving the vortex tail. Moreover, while rising to the free surface, as discussed in our previous work,14 the bubbles generated at the bottom of a hot sparged tank will grow larger than those in a cold one. Visual observation also gives the impression that bubbles are larger in a hot system than cold. Large bubbles rise much faster than small bubbles in a bubble column, and similar phenomena are expected in aerated stirred tanks. Therefore, large bubbles are more likely to escape from the free surface, which will probably reduce the size of the cavities behind the blades. Since, in hot sparged systems, the
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Figure 6. Comparison of the gassed power number correlations and the experimental data for both cold and hot systems.
Figure 4. Influence of solid concentrations on gassed power for hot sparged systems: (a) RPD vs Flg and (b) NPg vs Flg.
will cause the RPD to be greater at higher solid concentration in cold conditions compared with the gas-liquid two-phase system. 3.2.3. Gassed Power Number Correlations. Since the effects of flow number and Froude number on the gassed power number are similar at different solid concentrations for both cold and hot systems, the correlations for gassed power number in both systems are regressed at fixed exponents for flow numbers and Froude numbers, as shown in eqs 2 and 3. The factors in eq 2 increased from 2.33 to 2.45 with the solid concentrations increasing from 0% to 21% in the cold system. Using a mean value of the constant at 2.38 leads to eq 2 for cold conditions. Equation 3 has rather similar factors in the range from 2.79 to 2.72 for hot sparged systems over the solid concentrations range from 0 to 21%.
Cold system: NPg ) 2.38Flg-0.14Fr-0.13 R2 ) 0.989 (2) Hot system: NPg ) 2.75Flg-0.11Fr-0.09 R2 ) 0.986 (3)
Figure 5. Comparison of RPD between cold gassed and hot sparged systems under different solid concentrations.
bubbles leave the tank rapidly, the total retained gas is less than that when cold, again leading to a higher mean density at high operating temperature. Both smaller cavities and a higher mean density will increase RPD. The effect of solid concentration on RPD in hot sparged systems has been discussed in Section 3.2.1. Bubbles are smaller in cold conditions than when the system is hot, and surface tension, which helps in stabilizing the bubble, is greater at ambient temperature. As a result, settling particles do not easily break up the bubbles. Moreover, since the particles occupy part of the bulk liquid volume, they will enhance the probability of bubble collision and coalescence. As more particles are suspended in the tank, the higher rising velocity of resulting larger bubbles will lead to a reduced gas holdup in the stirred tank. The resulting higher mean density and reduced blade cavity size
The exponents for both flow number and Froude number in the hot sparged system are slightly smaller than those in cold conditions, whereas the numerical factor in the equation is larger. This is in good agreement with the previous work for a hot sparged, buoyant three-phase system.14 It should be noted that eqs 2 and 3 are suggested for the range of gas flow numbers 0.02 < Flg < 0.27 and Froude numbers 0.69 < Fr < 1.56. Figure 6 shows the relationship between all NPg data and the relevant groups Flg-0.14Fr-0.13 and Flg-0.11Fr-0.09 for cold and hot systems, respectively. Two straight lines with slightly differing slopes of 2.38 and 2.75, corresponding to the cold and hot equations, respectively, show reasonable agreement of correlations 2 and 3 with the experimental data. 3.3. Gas Holdup. 3.3.1. Effect of the Concentration of Settling Particles. Figure 7 shows the effect of solid concentration on gas holdup for a hot sparged system. In the hot sparged system, a variation of solid concentration does not lead to any significant change in gas holdup with data at low, medium, and high superficial gas velocities demonstrating this. The differences for different solid concentrations at the same superficial gas velocity are within experimental error. Figure 7 also shows that increasing the power input causes a rise of gas holdup at a given gas flow rate. As reported in our previous paper,14 the total energy dissipation rate used here includes both the agitator shaft power and the potential energy of the liquid displaced by the sparged gas. 3.3.2. Comparison with the Gas Holdup in the Cold System. The comparison of gas holdup in cold and hot systems
Ind. Eng. Chem. Res., Vol. 47, No. 6, 2008 2053
Figure 7. Influence of solid concentrations on gas holdup for hot sparged systems.
properties of gas and liquid on bubble generation, coalescence, and breakup are still largely open questions. Temperature affects the way settling particles affect gas holdup, as shown in Figure 8. At higher temperature, on the one hand, solid particles will tend to break up larger bubbles. On the other hand, the space occupied by particles in the liquid bulk will lead to more frequent collision with bubbles and coalescence. The combination of bubble coalescence and breakup leads to only small changes in gas holdup for different solids concentrations in hot conditions. However, the mean bubble size at room temperature is smaller than at higher temperature, and this may break the balance between the breakup and the coalescence. If coalescence becomes dominant, this will increase the bubble size and lead to higher rising velocity of the bubbles. Therefore, the gas holdup in cold gassed systems decreases as more settling particles are introduced. Further study of bubble size at different concentrations of suspended solids will be carried out to validate the above analysis. 3.3.3. Correlations for Gas Holdup. In a cold sparged system, gas holdup is influenced by agitation power, gas flow rate, and solid concentration, so eq 4 was used to correlate the gas holdup.
) APTmbVSc(1 + Cv)d
Figure 8. Comparison of gas holdup between cold gassed and hot sparged systems under different solid concentrations. Scattered data with dotted lines, this work; solid lines, Gao and Smith;10 solid symbols, hot systems; and open symbols, cold systems.
is shown in Figure 8. At a similar total gas flow rate, including the contribution of the evaporation from the liquid phase, the gas holdup for both ambient and elevated temperature increases as the power input is increased. However, at a given power input and total gas flow rate, the gas holdup in the hot sparged system without solids is about one-half of that in cold conditions, consistent with the results for cold and hot sparged two-phase systems presented by Gao et al.10 using a similar impeller combination. Figure 8 also shows the different effects of solid concentration on gas holdup at ambient and elevated temperatures. In a cold gassed system, the gas holdup decreases significantly as more glass beads are introduced. However, the gas holdup in a hot sparged system is almost independent of the volumetric concentrations, Cv, of settling particles, as discussed in detail above. At the highest solid concentration of 21% that we have studied, gas holdup is reduced in hot conditions to about three-quarters of that in a cold system The variation of gas holdup is related to a change in mean bubble size. As discussed in Section 3.2.2, in a cold system, bubbles appear to be smaller than in hot sparged conditions. The resulting lower bubble rise velocities lead to longer bubble residence times at room temperature, giving correspondingly higher gas holdup. The physical properties of the system depend on temperature. Changes in liquid viscosity, interfacial tension, and gas density may affect the bubble size and the gas holdup. Paglianti et al.17 reviewed the influence of liquid physical properties on the gas holdup and pointed out that the effect of viscosity has to be considered carefully. The combined effects of the physical
(4)
The same correlation was used to regress gas holdup in a hot sparged system, except that it is insensitive to solid concentration, which is, therefore, omitted. The exponents for total agitation specific power and superficial gas velocity are similar for both cold and hot systems so that they are fixed at 0.15 and 0.55, respectively. The correlations for cold and hot systems are as follows:
Cold systems: ) 0.90PTm0.15VS0.55(1 + Cv)-1.77 R2 ) 0.990 (5) Hot systems: ) 0.48PTm0.15VS0.55 R2 ) 0.989
(6)
It can be seen that the exponent of (1 + Cv) is negative for the cold system, reflecting the reduced gas retention at higher solid concentrations. The smaller coefficient of eq 6 shows the much lower gas holdup in hot conditions at given superficial gas velocity and specific power. The same exponents for PTm and VS imply the similar effects of PTm and VS on gas holdup in both cold and hot systems. This differs from the published results with buoyant particles.14 Equations 5 and 6 are restricted to this specific (air-water-glass beads) system at the temperatures of 24 and 81 °C using the HEDT + 2WHU agitator under the conditions specified in Section 2.1. 3.4. Solid Suspension in Sparged Systems. 3.4.1. Effect of Solid Concentration. Curves a and b of Figure 9 show the similar but small effect of solids concentration on the minimum gassed suspension speeds in cold and hot conditions. In each case, following an initial increase in minimum suspension speed, this passes through a maximum at concentrations in the order of 15 vol % in the cold system and 10 vol % when hot, before decreasing slightly. Even in the early stages, the dependence on concentration is less than the 0.1 exponent in the Zwietering equation, which is widely accepted as providing a useful basis for calculations of ungassed suspension conditions at low (
