Measurement of Drop Size and Distribution in an Annular Two-Phase

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Measurement of Drop Size and Distribution in an Annular Two-Phase, Two-Component Flow Occurring in a Venturi Scrubber Shekar Viswanathan National University, 11255 North Torrey Pines Road, La Jolla, California 92037

D. S. Lim and Madhumita B. Ray*,† Department of Chemical and Biomolecular Engineering, 4 Engineering Drive, National University of Singapore, Singapore 117576

Drop size and distribution in an annular two-phase flow occurring in a venturi scrubber were measured using a phase Doppler particle analyzer (PDPA) as a function of operating conditions such as gas velocity and liquid-to-gas ratio. At low throat-gas velocities, the maximum measured drop diameter and Sauter mean diameter increased when the liquid-to-gas ratio increased. However, at a fixed liquid-to-gas ratio, the maximum measured drop diameter and Sauter mean diameter decreased with the increase in throat-gas velocity. The experimental data tested with two theoretical distributions, namely, the upper-limit distribution function and the root-normal distribution function in terms of the shape of the drop-size distribution and the frequency values, produced a better agreement with the former than with the latter. It was observed that, in a highly dispersed drop-size distribution produced at a low throat-gas velocity of 45 m/s, a single average drop size is inadequate to describe the entire drop-size distribution, whereas at a high throat-gas velocity, 60 m/s, a single average drop size was found sufficient to describe the entire drop-size distribution formed. Introduction Venturi scrubbers are used for collecting particulate matters from exhaust gases. An annular two-phase flow consisting of a thin liquid layer on the walls and a highvelocity gas stream carrying a spectrum of droplets in the core of the scrubber occurs. The liquid droplets distribute inside the scrubber under the influence of the turbulent gas stream. The droplets move both axially (predominantly because of momentum transfer from the gas stream) and radially (initially because of the liquid initial momentum at the point of injection and later because of convection diffusion). When the droplets move radially and reach the walls, they form a film on the wall which flows at a lower velocity than the droplets. This film plays a negligible role in the gas cleaning process but contributes significantly to the pressure losses in the scrubber. The extent of the spread of drops in the radial direction depends on the degree of turbulence and the eddy diffusivity of a drop, which are functions of the drop diameter and the drop density. Therefore, the liquid droplet distribution across the scrubbers is a function of the drop diameter. The main particle collection mechanism in venturi scrubbers is due to inertial impaction, which is a function of both drop diameter and the relative velocity of the drops and the gas. If the liquid is atomized into very fine drops, the surface area for particle collection is very high, thus increasing the single drop collection efficiency. However, if the finest drops are accelerated faster through the venturi scrubber, the relative velocity between the drops and the gas decreases and, conse* To whom correspondence should be addressed. Tel.: 656874-2885. Fax: 65-6779-1936. E-mail: [email protected]. † Current address: The Department of Chemical and Biochemical Engineering, University of Western Ontario, London N6A 5B9, Canada.

quently, attenuates the single drop collection efficiency.1,2 Hence, an accurate representation of the liquid-flux distribution under different operating variables in venturi scrubbers is crucial for the accurate prediction of the overall collection efficiency in venturi scrubbers.3 Over the years, researchers have proposed several theoretical and empirical correlations to estimate drop sizes in atomized liquid sprays. The research on drop sizes of atomized sprays is mainly on two areas, and they are the following: (a) single average drop diameter, such as the Sauter mean diameter, and (b) drop-size distribution. The most commonly used drop-size correlation in the applications of a venturi scrubber is the Nukiyama and Tanasawa equation (NT equation), which was proposed initially for pneumatic atomizers.4 Conversely, other studies, such as the one done by Boll et al., proposed equations to predict the drop size in venturi scrubbers.5 The NT equation, which has been tested with a variety of liquids, albeit at lower gas velocities, is the most commonly used drop-size correlation in the applications of venturi scrubbers.6-9 Kim and Marshall indicated that the NT equation overpredicted the mass median diameter (dm) of the droplets of molten wax and wax-polyethylene mixtures of various compositions produced by convergent-type pneumatic nozzles, which inject liquid coaxially into the atomizing gas stream.10 A similar overprediction of the drop size in venturi scrubbers by the NT equation was reported by Parker and Cheong11 and Boll et al.5 The NT equation was found to underpredict the drop sizes by 25% at a throat-gas velocity of 30 m/s, and the prediction was only good at 45 m/s.5 Kim and Marshall correlated the effects of air velocity, liquid rate, viscosity, and density with the drop size. By using a form similar to the NT equation, they also proposed empirical equations to estimate the mass

10.1021/ie0489195 CCC: $30.25 © 2005 American Chemical Society Published on Web 08/12/2005

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Figure 1. Schematic view of the pilot plant.

median diameter (dm).10 In addition, they proposed that the Sauter mean diameter was proportional to the mass median diameter as follows: d32 ) 0.83dm. However, the empirical equations are nozzle specific and are not very useful for general use. Licht12 analyzed the data of Kim and Marshall10 and successfully fitted their data with the upper-limit function proposed by Mugele and Evans (1951). Besides the development of different drop-size and distribution models, the measurements of these values in venturi scrubbers were attempted by several researchers. Atkinson and Strauss studied the effect of surface tension on drop size in venturi scrubbers using a photographic technique with a resolution of 20 µm.13 At a constant gas velocity, they reported drop sizes that were significantly smaller than those of Nukiyama and Tanasawa.4 Using a light diffraction technique and different throat lengths, Bayvel14 measured the drop size at different positions along the venturi scrubber and found that the liquid-to-gas ratio had no effect on the drop size and that the measured drops were smaller than those predicted by Nukiyama and Tanasawa. Leith et al. measured drop size in a Pease-Anthony venturi scrubber using photographs taken by illuminating the spray with stroboscopic light.15 However, the measured Sauter mean diameters in the range of 200-500 µm in their work were much larger than those of earlier measurements by Boll et al.5 and Atkinson and Strauss.13 Teixeira16 employed a laser-scattering technique, very similar to the one used by Bayvel, to measure drop sizes in a scrubber with a wetted-wall approach and found that the drop sizes were significantly smaller than those predicted by the NT equation. Fernandez Alonso et al. measured drop size in a laboratory-scale venturi scrubber using Fraunhofer diffraction by illuminating the flow with a laser beam.17 The results indicated that the Sauter mean diameter of the spray can be correlated by the equation of Boll et al.5 and that the drop-size

distributions were satisfactorily represented by a RosinRammler function. However, to date, it is not clear whether a single drop size could describe the distribution of droplets formed at operating conditions that are normally encountered under industrial conditions. Chongvisal,18 Placek and Peters,19 Bayvel,14 and Lim and Viswanathan20 attempted to incorporate drop-size distributions in their models to study the performance of venturi scrubbers. Because of the availability of limited measured practical data on drop sizes in venturi scrubbers (most of the above experiments were conducted in small, laboratory-scale scrubbers) and the limitation of extrapolating data from sources such as pneumatic atomizers, detailed drop-size analysis in venturi scrubbers is rarely found in the literature. In addition, using experimentally measured drop distributions, the flux distribution in the scrubber was never evaluated. In this work, a detailed characterization of drop size, velocity, and flux distribution was conducted in a pilot-scale venturi scrubber under different operating conditions using a nonintrusive technology such as a phase Doppler particle analyzer (PDPA). The two major advantages of this technique are its nonintrusive nature and its ability to produce simultaneous particle size and velocity measurements with a high spatial resolution. As a result, it is possible to obtain point information on the spray droplet size distribution as well as the particle size-velocity correlation data necessary to characterize the dynamic behavior that occurs in venturi scrubbers. Measured drop size and distribution were compared with the predictions using some selected models. Experimental Section A schematic diagram of the pilot plant setup is shown in Figure 1 and the details of the scrubber are shown in Figure 2. The Pease-Anthony scrubber was fabri-

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Figure 2. Dimensions of the pilot-scale rectangular scrubber used in this study.

cated with 9.53 mm thick Plexiglas sheets to allow visual observation of the spray pattern and the flow in the scrubber. More importantly, the transparency of the Plexiglas allowed the laser beams from the phase Doppler particle analyzer to pass through the wall of the system with as little distortion as possible. The converging angle of the inlet section was set at 30°. Three diffusers were fabricated, each with a different diverging angle of 5°, 7°, and 9°. Water was introduced at the top of the throat section of the venturi scrubber by a centrifugal pump, and the volume of water was controlled by four rotameters. The water was injected into the scrubber through 34 orifices (17 of them on each side) perpendicularly oriented with respect to the air stream. The 34 orifices were located on two manifolds placed on opposite sides of the 76 mm throat. The 17 orifices were arranged along two rows with center-tocenter spacing of 13 mm in each manifold. The first row of openings, which was 1 in. below the entrance to the throat section, had nine orifices, while the bottom row had eight. The holes of the lower row were staggered midway between those of the top row to improve the distribution of water. The air-water droplet mixture leaving the scrubber was fed into a bottom entry cyclone-separator (dia 81 cm) for air and water separation. The water from the cyclone system was discharged back to a holding tank for recirculation. The air stream exiting from the top of the separator was discharged to the atmosphere via an exhaust stack located outside the laboratory. A 1.4 m3/s blower (New York Blowers Company) drove the air through the venturi scrubber and separator to the stack. A damper located at the stack controlled the amount of air flowing through the scrubber, and the differential pressure meter measured the volume of air across the orifice plate and thereby facilitated the measurement of air flow. The experiments were conducted at three throat-gas velocities (VGth ) 45, 60, and 75 m/s) and five liquid-to-gas ratios (L/G ) 0.4, 0.9, 1.2, 1.5, and 1.8 m3liquid/1000 m3-gas). The thin film of liquid flowing along the walls was found undesirable during the measurement of the

droplet properties using the phase Doppler particle analyzer and was removed by replacing sections of two adjacent walls in the throat section with porous sintered stainless steel plates, through which the liquid was extracted. The drop size, velocity, and flux, as well as their distributions in the scrubber, were measured using a phase Doppler particle analyzer (PDPA) (TSI/Aerometrics, Inc., St. Paul, MN). The particle velocity in the PDPA was measured in relation to the Doppler frequency of the scattered light, while the particle size was measured in relation to the phase shift of the scattered light. Since the center-to-center distance between the liquid injection orifices at the top of the throat was only 12.5 mm and the orifices of the lower row were staggered midway between those of the top row to improve the distribution of water, this effectively reduced the distance between the orifices by half. Such an arrangement made it possible to assume negligible variation in the droplet size, velocity, and concentration in the direction along the liquid injection orifices. Since the spray was symmetrical along the direction of the liquid injection orifices, the measurements were only done across the depth of the venturi throat. Eleven sampling points were marked out evenly across the throat of the venturi scrubber at 5 mm apart. Information on dropsize distributions, velocities, and volume fluxes were obtained by the PDPA at each location. The number density, which is the number of particles per unit volume, was calculated by counting the number of the particles in all size classes and recovered along nbins ni(d), where there with the sampling time, n ) ∑i)1 are nbins size bins and ni(d) is the number of particles in size class d. Next, the swept volume, which contains the particles, was computed as V ) u j tA, where u j is the mean velocity of the drops, t is the elapsed time for the sample, and A is the cross section over which the sample was normalized. The number density N is then given as follows: nbins

N)

ni(d) ∑ i)1 utA

Volume flux can be measured by the PDPA, and the mass flux can be obtained simply by multiplying the volume flux by the material density. The volume flux is the volume passing a unit cross-sectional area per unit time (cm3‚cm-2‚s-1) and is given as F ) {[((π/6)nD303)/to]/A}, where D30 is the volume mean diameter, to is the elapsed time for the sample, and A is the sampling cross section. Results and Discussion Initially, for every condition, volume fluxes from the 11 sampling points, across the throat section, were summed up. This value of the total measured volume of liquid was then compared with the expected volume of liquid that was flowing through the 11 sampling points. The expected values and the measured values were plotted together for comparison and shown in Figure 3. It can be seen from these results that the measured values in general agreed very well with the expected values. The experimental results of the drop velocity, dropsize distribution, and liquid-flux distribution were then


Figure 3. Comparison of measured and expected volume flux at all L/G ratios: (a) VGth ) 45 m/s; (b) VGth ) 60 m/s; and (c) VGth ) 75 m/s.

compared with the predicted data using the model developed by Lim and Viswanathan20 in the following section. Drop Velocity. The drop velocity and drop diameter were measured simultaneously using the PDPA. The

measured drop velocities at the end of the throat section are plotted with the corresponding drop diameters, and typical results are shown in Figure 4. It is evident that the drop velocities in the venturi scrubber are lower than the throat-gas velocity because the water drops are

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Figure 4. Drop velocity/diameter correlation for: (a) VGth ) 45 m/s and L/G ) 1.2 m3-liquid/1000 m3-gas and (b) VGth ) 75 m/s and L/G ) 1.2 m3-liquid/1000 m3-gas.

accelerated from being almost stationary at the point of atomization by the incoming gas. It is also expected that the maximum velocity that a drop can attain is equal to the throat-gas velocity. Figure 4 shows that almost all the drops have a velocity lower than the throat-gas velocity. There are only a few of them that have a drop velocity (close to 80 m/s) higher than the throat-gas velocity (75 m/s) in Figure 4b. It is possible that the throat-gas velocity in the scrubber is slightly higher than the expected 75 m/s because it is difficult to set the throat-gas velocity to the exact desired value, especially at a very high gas velocity when the condition is extremely turbulent. As expected, the drop velocity decreases as the drop diameter increases. On the basis of the experimental data shown in Figures 3 and 4, a good level of confidence was established in the reliability of the measured data. Drop Size and Drop-Size Distribution. Parts a and b of Figure 5 show the experimental results of the drop-size distributions measured by the PDPA at a throat-gas velocity of 45 m/s and at low and high liquidto-gas ratios, respectively. Each drop-size distribution is collected from all 11 sampling points across the 76 mm side of the pilot-scale venturi scrubber at particular

operating conditions. Theoretical drop-size distributions in the pilot scrubber using upper-limit distributions and root normal distributions20 were also plotted in these graphs. At all liquid-to-gas ratios, the experimental data agreed very well with the upper-limit distribution function, both in terms of the shape of the drop-size distribution and the absolute frequency values along the y-axis. The skewness of the experimental drop distribution is also captured by the upper-limit distribution function. However, the root-normal distribution function predicts a higher frequency at its peak value than that of the experimental data. Although only two representative results are shown, this trend was observed for all the liquid-to-gas ratios at 45 m/s. Interestingly, this trend was not seen at higher throat-gas velocities. Parts a and b of Figure 6 show the experimental data measured at a throat-gas velocity of 60 m/s from liquid-to-gas ratios of 0.4 and 1.5 m3-liquid/ 1000 m3-gas, respectively. As shown before, theoretical drop-size distributions (the upper-limit distribution and the root-normal distribution) are also compared with the experimental data. It is obvious that the agreement between the upper-limit distribution and the experimental data is not found in these figures. It can also be


Figure 5. Comparison of measured drop-size distribution with theoretical drop-size distributions for: (a) VGth ) 45 m/s and L/G ) 0.4 m3-liquid/1000 m3-gas and (b) VGth ) 45 m/s and L/G ) 1.5 m3-liquid/1000 m3-gas.

seen that the measured drop diameters are larger than the predicted drop diameters. Generally, the experimental drop diameter ranges are greater than the predicted ranges. Furthermore, the measured drop-size distributions are less skewed than the ones in Figure 6. As the throat-gas velocity increases to 75 m/s, the difference between the theoretical and the experimental distributions becomes even greater. The diagrams in parts a and b of Figure 7 show the experimental data of the drop-size distributions at a throat-gas velocity of 75 m/s for liquid-to-gas ratios of 0.4 and 1.5 m3-liquid/ 1000 m3-gas, respectively. The shape of the measured distributions resembles the bell-shape of a Gaussian normal distribution. An extra normal distribution curve is plotted with the experimental data in Figure 7. The mean and the standard deviation of each group of the experimental data are calculated. These values are then used to predict the normal distribution for each set of data and plotted in Figure 7. It can be seen that the experimental data actually followed the normal distribution. It should be noted here that the accuracy of the

PDPA measurement was affected to some extent at a high gas velocity because of the flow of a thin film of water on the wall of the scrubber and also because of a high number density flow at the high gas flow rate. The above observations demonstrate that the measured drop-size distribution follows the upper-limit distribution function at a low throat-gas velocity of 45 m/s and a normal distribution at a high gas velocity (75 m/s). The operating conditions have the same effects on the measured drop-size distribution, as predicted previously in the theoretical calculations (Lim and Viswanathan20). The measured drop diameter range increases with an increase in the liquid-to-gas ratio, while the drop diameter range decreases as the throat-gas velocity increases. The effect of liquid-to-gas ratio on the measured distribution can be seen by comparing the maximum measured diameter in Figures 5-7. For example, at a 45 m/s throat-gas velocity, the maximum measured drop diameter increases by 38% from 346 to 477 µm when the liquid-to-gas ratio increases from 0.4 to1.8 m3liquid/1000 m3-gas. Similarly, the effect of throat-gas velocity on the measured distribution can be seen by

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comparing the maximum measured drop diameter between the corresponding diagrams of the same liquidto-gas ratio in Figures 5-7. For a liquid-to-gas ratio of 1.2 m3-liquid/1000 m3-gas, the maximum measured drop diameter decreases by 122 µm from 401 µm to 278 µm when the throat-gas velocity increases from 45 to 75 m/s. A 30% drop in the maximum diameter is observed which is lower (56%) than that predicted under the same conditions by Lim and Viswanathan.20 Although, as predicted, a lesser number of big droplets is formed at a high throat-gas velocity, the PDPA was able to measure droplets that were larger than those predicted by the upper-limit and root-normal distributions. This can be easily observed by comparing the corresponding predicted and measured drop-size distributions in Figures 6 and 7. Comparison of Experimental and Predicted Sauter Mean Diameter. The Sauter mean diameters (d32) of the measured drop-size distributions are calculated using the following equation: d32 ) (∑ni di3fi/ ∑ni di2fi) where di is the drop diameter of interval i and fi is the frequency of the drop-diameter interval i. The

experimental Sauter mean diameters are tabulated in Table 1. It is observed that the Sauter mean diameter increases as the liquid-to-gas ratio increases at a constant throat-gas velocity, while it decreases with an increase in the throat-gas velocity at a constant liquidto-gas ratio. This trend is in agreement with the theoretical findings. Sauter mean diameters that are described by Boll et al.5 and the NT equation4 are also calculated using the operating conditions used in the experiments. These values are tabulated along with the experimental values in Table 1. The experimental Sauter mean diameter matches very well with the equation of Boll et al.5 for all liquid-to-gas ratios at a throat-gas velocity of 45 m/s. A similar trend was observed using the work described by Fernandez Alonso et al.17 However, as the throat-gas velocity increases progressively to 75 m/s, the experimental Sauter mean diameter deviates away from the predicted values of Boll et al.5 It is expected that the experimental Sauter mean diameter would agree with those of Boll et al. at 45 m/s because the measured drop-size distribution matches well with the upper-limit distribution function



at this throat-gas velocity. Since the upper-limit distribution function is a function of the Sauter mean diameter, which is calculated from the results of Boll et al. in this study,20 the predicted drop-size distribution has the same Sauter mean diameter as that given by Boll et al. The experimental Sauter mean diameters are always lower than the NT Sauter mean diameters for all operating conditions. Comparison of Volumetric Flux. Figures 8 and 9 compare the simulated flux distribution across the cross section of the throat with the experimental data measured by the PDPA in the pilot-scale venturi scrubber. The simulated flux at any given point was calculated as the volume of liquid based on the size of a given drop and the associated number of drops at a given location as predicted by the model divided by a uniform distribution of liquid. Each diagram consists of the simulated results calculated by using four different drop-size functions, namely, the mass mean diameter (dm), the Sauter mean diameter (d32), the upper-limit distribution (Distribution 1), and the root-normal distribution (Distribution 2), and compares these results to the measure-

ments made using the PDPA. Parts a and b of Figure 8 compare the flux distribution at the end of the throat for liquid-to-gas ratios of 0.4 and 1.5 m3-liquid/1000 m3gas, respectively, at a constant throat-gas velocity of 45 m/s, while parts a and b of Figure 9 compare the flux distribution at the end of the throat for liquid-to-gas ratios of 0.4 and 1.2 m3-liquid/1000 m3-gas, respectively, at a throat-gas velocity of 75 m/s. A “goodness-of-fit” test, namely, the sum of the square of residuals (SSR) test, was done to check how well the experimental data fitted with the simulated results. These values are tabulated in Table 2. It can be seen from Figure 8 that the experimental liquid-flux distribution data follow closely the simulated data using the upper-limit distribution function (Distribution 1). Furthermore, the SSR values of the experimental data with respect to the curve of Distribution 1, presented in Table 2, are generally lower than the rest of the corresponding SSR values for all liquid-togas ratios. For example, the SSR value using Distribution 1 curve ranges from 1.00 to 3.35, compared to the SSR values of 5.46-7.05 using the mass median diam-

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Figure 8. Comparison of experimental and simulated liquid-flux distribution at the end of the throat at 45 m/s: (a) L/G ) 0.4 m3-liquid/ 1000 m3-gas and (b) L/G ) 1.8 m3-liquid/1000 m3-gas. Table 1. Comparison of Sauter Mean Diameters by Boll et al.5 and the NT Equation4 with Experimental Measurements at All Operating Conditionsa liquid-to-gas ratio (m3-liquid/1000 m3-gas)

a

exp.

Boll et al.5

Nukiyama and Tanasawa4

0.4 0.9 1.2 1.5 1.8

45 m/s 103.90 105.23 109.80 114.82 132.19

94.63 103.28 110.10 118.50 130.46

116.74 135.31 147.35 160.85 178.51

0.4 0.9 1.2 1.5

60 m/s 76.56 84.25 94.19 97.96

56.58 65.03 69.32 74.61

90.21 108.78 120.83 134.32

0.4 0.9 1.2 1.5

75 m/s 76.11 82.54 86.74 93.03

41.72 45.53 48.54 52.24

72.35 90.92 102.97 116.46

All Sauter mean diameters are in µm.

eter (dm) curve. This analysis illustrates that the measured liquid-flux distribution follows the simulated flux distribution that was calculated using the upperlimit distribution function more closely than the rest of the simulated flux distributions. In a broader sense, the measured liquid-flux distribution at a low throat-gas velocity of 45 m/s follows more closely the simulated flux distribution calculated using the drop-size distribution functions than the simulated

flux distribution calculated using single average drop sizes. This can be seen from Table 2, as the SSR values for Distribution 2 are also relatively lower than those for both the Sauter mean diameter and the mass median diameter. These correspond to the SSR values of the experimental data with respect to the simulated data calculated using single average drop sizes. Coincidentally, this phenomenon was also observed earlier, as reported by Lim and Viswanathan when the effect of the polydispersity of drop size on the liquid-flux distribution was discussed.20 However, with the increasing gas velocity, the liquidflux distribution calculated using distribution functions such as upper-limit and root-normal distribution starts to deviate from the experimental data. This can be observed in the goodness-of-fit values for gas velocity ) 60 m/s, presented in Table 3. When the values of Table 2 are compared with those of Table 3, it can be seen that the goodness-of-fit values for the Sauter mean diameter and the mass median diameter are improved with increasing gas velocity, as the values in Table 3 are lower than the ones in Table 2. It appears that, with the increasing throat-gas velocity, polydispersity in the drop size reduces and the liquid-flux distribution is better represented by single average drop sizes. This tendency is clearer in Figure 9 and the SSR values presented in Table 4. Although the goodness-offit values for the two distribution functions indicate good agreement between the experimental and simulated data, the shape and the peak of the measured flux


Figure 9. Comparison of experimental and simulated liquid-flux distribution at the end of the throat at 75 m/s: (a) L/G ) 0.4 m3-liquid/ 1000 m3-gas and (b) L/G ) 1.2 m3-liquid/1000 m3-gas. Table 2. Sum of the Square of Residuals (SSR) of the Experimental Volume Flux with Simulated Results at 45 m/s for Different Liquid-to-Gas Ratios

Table 4. Sum of the Square of Residuals (SSR) of the Experimental Volume Flux with Simulated Results at 75 m/s for Different Liquid-to-Gas Ratios

Sauter mass L/G ratio mean median (m3-liquid/1000 m3-gas) diam. (d32) diam. (dm) dist. 1 dist. 2

Sauter mass L/G ratio mean median (m3-liquid/1000 m3-gas) diam. (d32) diam.(dm) dist. 1 dist. 2

0.4 0.9 1.2 1.5 1.8

5.43 5.44 5.04 6.78 4.01

7.05 5.46 5.89 9.70 6.60

3.35 2.69 2.14 2.82 1.00

4.26 3.20 2.77 4.49 2.26

Table 3. Sum of the Square of Residuals (SSR) of the Experimental Volume Flux with Simulated Results at 60 m/s for Different Liquid-to-Gas Ratios Sauter mass L/G ratio mean median (m3-liquid/1000 m3-gas) diam. (d32) diam.(dm) dist. 1 dist. 2 0.4 0.9 1.2 1.5

4.71 4.25 3.63 3.15

5.18 5.04 4.40 4.42

3.30 2.97 2.66 2.14

3.78 3.37 2.87 2.51

distributions are very similar to the simulated flux distributions calculated using the mass median diameter. It appears that the peaks of the measured flux distributions are closer to the center of the scrubber than the peaks of the simulated flux distributions. This deviation is possibly due to the method that was used to calculate the simulated flux distribution. The simulation of drop motion starts at the point of atomization,

0.4 0.9 1.2

2.65 2.18 2.35

3.16 2.56 3.25

1.91 1.85 2.60

2.18 1.99 2.51

which corresponds to the penetration length of the liquid jets from the injection orifices (Viswanathan et al.21). The droplets are then simulated to distribute across the scrubber as they are carried by the gas flow along the scrubber. It appears that the initial liquid momentum of the liquid jet actually carries the droplets further toward the center of the scrubber as they flow down the scrubber. If the peaks of the simulated flux distribution are shifted toward to the center of the scrubber, the simulated flux distribution actually matches well with the measured flux distribution, as the absolute peak values of the measured drop distribution are close to the calculated peaks. From the above analysis, it is apparent that, at a low throat-gas velocity of 45 m/s, simulated flux distributions calculated using drop-size distribution functions (for example, the upper-limit distribution function) are able to describe the measured flux distributions very closely for all liquid-to-gas ratios. As the throat-gas velocity increases, simulated flux distributions calculated using drop-size distribution functions no longer

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can describe the measured flux distributions well. At a higher throat-gas velocity (75 m/s), the measured flux distributions tend to follow simulated flux distributions calculated using average drop sizes (for example, mass median diameter). Furthermore, it can be seen that the drop-size distribution depends more on the throat-gas velocity than on the liquid-to-gas ratio. This trend was also found in the simulation of Lim and Viswanathan20 and was experimentally observed by the recent work of Fernandez Alonso et al.17 Conclusions The drop size, velocity, and distribution were successfully measured in a pilot-scale venturi scrubber using the PDPA. The measured drop size and flux distribution were compared with those simulated using several average drop sizes and drop-size distribution functions. The results indicate that it is important to use drop-size distribution in the prediction of venturi scrubber performance at a low gas velocity. The results also show that, at a high gas velocity (g75 m/s), an average diameter describes the distribution of drops in the scrubber well. This is because the polydispersity of the drops decreases at a high gas velocity. The effect of gas velocity on drop size and distribution was more pronounced than the effect of liquid-to-gas ratio. The measured and simulated drop-size and flux distributions follow the upper-limit distribution function at a low throat-gas velocity and the mass median diameter at a high gas velocity. Literature Cited (1) Goel, K. C.; Hollands, K. G. T. A General Method for Predicting Particle Collection Efficiency in Venturi Scrubbers. Ind. Eng. Chem. Fundam. 1977, 16, 186. (2) Viswanathan, S. Modeling of Venturi Scrubber Performance. Ind. Eng. Chem. Res. 1997, 36, 4308. (3) Ananthanarayanan, N. V.; Viswanathan S. Estimating Maximum Removal Efficiency in Venturi Scrubbers. AIChE J. 1998, 44, 2549. (4) Nukiyama, S.; Tanasawa, Y. An Experiment on the Atomization of Liquid by Means of Air Stream. Trans. Soc. Mech. Eng., Tokyo 1938, 4 (14), 86.

(5) Boll, R. H.; Flairs, L. R.; Maurer, P. W.; Thompson, W. L. Mean Drop Size in a Full Size Venturi Scrubber via Transmissometer. J. Air. Pollut. Control Assoc. 1974, 24, 934. (6) Boll, R. H. Particle Collection and Pressure Drop in Venturi Scrubbers. Ind. Eng. Chem. Fundam. 1973, 12, 40. (7) Calvert, S. Venturi and Other Atomizing Scrubbers. AIChE J. 1970, 16, 392. (8) Dropp, L. T.; Akbrut. A. J. Working Processes and Calculation of the Efficiency of an Ash Trap with a Venturi Tube. J. Tepoloenergetika 1972, 7, 63. (9) Yung, S. C.; Calvert, S.; Barbarika, H. F. Venturi Scrubber Performance Model. Environ. Sci. Technol. 1978, 12, 456. (10) Kim, K. Y.; Marshall, W. R. Drop-Size Distribution from Pneumatic Atomizers. AIChE J. 1971, 17 (3), 575. (11) Parker, G. J.; Cheong, K. C. Air-Water Test on a Venturi for Entraining Liquid Films. Int. J. Fluid Mech. Sci. 1973, 15, 633. (12) Licht, W. Maximum Drop Size Produced by Pneumatic Atomization. AIChE J. 1974, 20 (3), 595. (13) Atkinson, D. S. F.; Strauss, W. Droplet Size and Surface Tension in Venturi Scrubber. J. Air Pollut. Control Assoc. 1978, 28 (11), 1114. (14) Bayvel, L. P. The Effect of Polydispersity of Drops on the Efficiency of a Venturi Scrubber. Trans. Inst. Chem. Eng. 1982, 60, 31. (15) Leith, D.; Martin, K. P.; Cooper. D. W. Liquid Utilization in a Venturi Scrubber. Filtr. Sep. 1985, May-June, 191. (16) Teixeira, J. C. F. Turbulence in Annular Two-Phase Flow. Ph.D. Thesis, University of Birmingham, U.K., 1988. (17) Fernandez Alonso, D.; Goncalves, J. A. S.; Azzopardi, B. J.; Coury J. R. Drop Size Measurements in Venturi Scrubbers. Chem. Eng. Sci. 2001, 56, 4901. (18) Chongvisal, V. The Role of Drop Size Distribution in the Performance of Venturi Scrubbers. Ph.D. Thesis, University of Cincinnati, Cincinnati, OH, 1979. (19) Placek, T. D.; Peters. L. K. Analysis of Particulate Removal in Venturi ScrubberssEffect of Operating Variables on Performance. AIChE J. 1981, 27 (6), 984. (20) Lim, D. S.; Viswanathan, S. Effect of Polydispersity of Droplets in the Prediction of Flux Distribution in a Venturi Scrubber. Environ. Sci. Technol. 2000, 34 (23), 5007. (21) Viswanathan, S.; Gnyp, A. W.; St. Pierre, C. C. Jet Penetration Measurements in a Venturi Scrubber. Can. J. Chem. Eng. 1983, 61, 504.

Received for review November 8, 2004 Revised manuscript received May 20, 2005 Accepted July 14, 2005 IE0489195

Measurement of Drop Size and Distribution in an Annular Two-Phase

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