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Experimental Studies of Liquid Weeping and Bubbling Phenomena at Submerged Orifices Wildon L. Peng, Guoqiang Yang, and Liang-Shih Fan* Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210
Experimental studies on the liquid weeping phenomenon at a submerged circular orifice have been conducted under a range of superficial orifice gas velocities (0.25 cm/s to 100 m/s), column and plate specifications, orifice sizes, fluid properties, and pressures up to 3.5 MPa. As the primary focus of this study, weeping rates and bubble formation at a single-orifice plate are investigated while operating within the bubbling regime at velocities of 0.25-100 cm/s. Upon monitoring of the pressure fluctuations within the plenum region, the bubbling frequency and nominal bubble size are determined to provide insight into the weeping phenomenon. Under similar scrutiny, weeping in the jetting regime, which is generally ignored or disregarded as trivial, is briefly considered for long-term consequences for industrial plants that typically operate over month or year periods. Introduction Spargers, perforated plates, and trays are often utilized to promote gas-liquid contact and gas distribution by bubbling gas through simple circular orifices. Because of the relatively simple design and construction, these devices have been applied to many gas-liquid and gas-liquid-solid systems throughout the chemical industry. A consequence of utilizing this simple design is the undesirable presence of liquid weeping during operation that can eventually lead to an increase in pressure drop across the distributor plate and abnormal bubbling behavior. Previous studies indicate the phenomenon to be only problematic for low gas velocities, below jetting velocities, or large orifice diameters greater than 2 mm.1,2 Many investigators have modeled the weeping behavior as an aftereffect of single-bubble formation3,4 and/ or the pressure fluctuations within the plenum.1 However, none of these studies attempt to account for the weeping coupled effects with bubble formation detachment or pressure fluctuations both above and below the orifice plate, nor do these previous studies share a high degree of agreement with the weeping rate trend and the precise effects of system conditions, such as the chamber volume or column diameter, on the liquid weeping rate. An examination of previous investigations concerning the weeping rate of the commonly studied air-water system reveals only qualitative agreement among the many investigators. As is shown in Figure 1, standard deviations are as high as 33%. Although some discrepancies in the orifice size and chamber volume in the comparison are present, the trends themselves are not justifiable by conclusions drawn from corresponding investigators. For example, from the case studies involving approximately 6 mm diameter orifices, McCann and Prince’s3 experimental system conditions, compared to other investigators’ conditions shown, proceed with a slightly larger orifice and smaller chamber volume * To whom correspondence should be addressed. E-mail:
[email protected]. Tel: (614) 292-7907. Fax: (614) 292-3769.
which both imply a noticeably higher weeping rate and show similar or lower weeping rates than Akagi et al.1 or Che and Chen.2 Of more importance to note are that each of the investigators utilized different orifice plate materials and different column diameters considered to be large enough to avoid wall effects. However, these assumed benign changes may, in part, be the cause for such deviations. Because bubble formation may be influenced by plate material wettability,5 deviation in the weeping rate from one study to another is not altogether unexpected. Meanwhile, columns 4-12 times larger than the average bubble diameter may still be subject to wall effects. Of additional interest is that the majority of available weeping data fall beyond superficial orifice gas velocities of 1 m/s where single bubbling may not be present. For the air-water system, Illiadis et al.6 provided observations as to the significantly more complex bubbling situation, other than just single bubbling, that can arise for virtually any flow rate, suggesting a certain uncertainty or nonideality associated with water’s abnormally high surface tension and variant bubbling behavior due to contamination and impurity susceptibility. In contrast to using other liquids, applying surfactants or other solutes to offset the high surface tension may not be appropriate because the exact effect may be unpredictable7 and is beyond the scope of this study. Therefore, a more “ideal” gasliquid system and additional attention to details of the system would seem to be appropriate. Although previous studies indicate that weeping can be avoided by maintaining a minimum jetting velocity, occasionally excessive operating gas velocities of 2 or 3 times the minimum jetting velocity are necessary to avoid long-term problems. Most investigations into weeping primarily focus upon single-orifice situations, neglecting the reality of strong mixing or circulation that may result from a group of orifices interacting with one another, as in a multiple-orifice perforated plate, within the confines of the column walls. Akagi et al.1 considered the effects that the number and pitch of orifices have on the weeping rate but did not report the physical dimensions of the plates and column used. Therefore, a qualitative degree of mixing due to wall effects cannot be evaluated. These results show only
10.1021/ie0106581 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/23/2002
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Figure 1. Comparison of weeping rates from previous studies for the air-water system: (a) Do ≈ 6 mm; (b) Do ≈ 10 mm.
nominal increases in the weeping rate roughly proportionate to the number of orifices. Regardless, the situation may require a closer look along with weeping in spargers because these devices appear to be more popular within industry because of ease of construction. In this study, aspects of the bubbling system are studied systematically to survey and weigh the relative importance of each on the bubble formation and weeping phenomenon. The importance of column diameter, orifice size, plate material, chamber (plenum) size, liquid properties, orifice gas velocity, and pressure is examined during this study of single-orifice perforated plates at low orifice gas velocities in gas-liquid systems. An extension into liquid weeping behavior under jetting conditions in the presence of mixing effects is also conducted to demonstrate the potential problem in the industrial situation and the importance of considering mixing along with the local effects of the jet itself.
Experimental Setup and Technique Bubbling Experiments. The experimental design schematic for studying liquid weeping behavior at a single-orifice perforated plate is shown in Figure 2. A flowmeter meters the flow entering the plenum region, while a pressure differential tap is maintained across the orifice plate. Columns of 4 and 6 in. are utilized for testing 1/8 and 1/4 in. circular orifices machined from Plexiglas and stainless steel. The 4 in. column is capable of maintaining pressures well beyond the 3.55 MPa and is outfitted with three pairs of quartz windows to allow for visualization of the bubble formation and rise behavior. The 6 in. column is constructed from Plexiglas and, therefore, is only capable of carrying out experiments under ambient conditions. Upon addition or removal of appropriate amounts of liquid to the chamber, the effective chamber volume can be modified
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Figure 2. Schematic of the experimental setup for liquid weeping measurement. Table 1. Physical Properties of Water and Norpar-15 at Various Conditions liquid
pressure (MPa)
σ (mN/m)
µ (mPa‚s)
F (kg/m3)
water Norpar-15
0.10 0.10 0.27 0.45 0.96 1.83 3.55
71.45 26.7 26.3 25.8 25.9 24.7 24.3
0.99 2.12 2.20 2.28 2.35 2.66 3.08
1010 772 773 774 775 779 782
between 100 and 1600 cm3 with the assumption that the liquid does not evaporate significantly during the experimental runs. The orifice submergence depth is maintained between 28 and 30 in. Water is the initial liquid medium for the purpose of comparison with previous studies, and Norpar-15, paraffin oil obtained from Exxon Chemical Co., is chosen for its lower surface tension and more consistent bubbling properties. Physical properties of these fluids at various pressures are determined through in-situ techniques developed by Lin and Fan8 and are shown in Table 1. Jetting Experiments. Single-orifice experiments operating within the jetting regime utilized a 10 in. Plexiglas column with a setup similar to that given in Figure 2. Gas flow disturbances are introduced into the system through a sparger 6 in. above the single-orifice plate, described below, to create a mixing circulation pattern within the column. The introduction of the sparger provides a superficial gas velocity of approximately 23 cm/s with respect to the column. Because the maximum jet penetration length(s) from the sparger and the single-orifice plate reach approximately 1 and 2.5 in., respectively, the 6 in. distance between the sparger and the perforated plate should prevent the jets from directly interfering with each other. A more detailed description of the sparger is given below. Water under ambient conditions is the only system studied for this set of experiments, while the chamber volume is varied between 300 and 13 460 cm3. Studies on the sparger itself are done using a 1.69 in. o.d. PVC pipe, a 1/8 in. wall thickness, and a volume within the sparger of 374 cm3. Four rows of 10-1/4 in. orifices are aligned approximately 44° from each other along the sparger length as shown in Figure 3. Orifices
Figure 3. General sparger schematic: four rows of 10-1/4 in. holes in a single pipe.
are directed downward and angled toward the column walls, while glass plates are glued to both ends of the sparger to observe inside the sparger during operation. The single-orifice plate is replaced with a flat, nonperforated plate, and weeping is observed. Limestone particles are also added in separate trials. Weeping Rate Measurement. After a known length of time has passed for a given liquid weeping trial period, the gas flow is increased sufficiently to effectively “stop” weeping. An orifice gas velocity within the range of 20-100 m/s, depending on system conditions, is considered to be a sufficient velocity to reduce weeping to negligible rates so that the weeping liquid in the funnel could be collected. Through a series of valves and a collection chamber leading from a funnel placed directly beneath the orifice, as pictured in Figure 2, the volume of liquid weeping is collected while ensuring minimal disturbance to the system above and below the orifice. Purge gas is necessary to ensure all fluid from the collection chamber is expelled and allows for easy pressurization and depressurization when resetting the chamber for reintroduction to the rest of the system when operating at high pressure. Bubbling Frequency Determination. Bubbling frequency determination can be a tedious task that is typically measured by high-speed video or optical fiber probes. The accuracy of high-speed video relies on the maximum capture frame rate, typically 240-480 Hz, and is not a viable measurement tool with the presence of particles in the system. Optical fiber probes are undependable with strong turbulence present in the system because the technique requires bubbles to rise in a fairly specific path. However, the pressure fluctuations within the plenum region in the bubbling regimes are very predictable and reproducible. For this reason, the bubbling frequency can be extracted from the plenum pressure fluctuation reading with a pressure transducer. Because the transducer can reach sampling rates in excess of 2000 Hz and are not subject to the relatively unpredictable bubble rise path, this technique poses a more accurate technique than high-speed video, while it is more convenient than optical fiber probes, particularly in slurry systems. A typical pressure fluctuation signal is seen in Figure 4 a and is characterized by three basic regimes that
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Figure 4. (a) Typical pressure fluctuation across the orifice plate. (b) Comparison of the bubbling frequencies obtained through visualization and pressure fluctuation techniques.
make up the overall bubbling cycle. A sudden pressure drop is observed as the bubble forms and grows above the orifice. Upon bubble detachment, a sudden pressure increase follows. During this time period, liquid weeping may occur as a new gas-liquid interface is reestablished along the underside of the orifice rim. In the final period after the weeping period, bridging is characterized by a slow pressure buildup that leads to another bubbling cycle. The inverse of the entire time cycle represents the bubbling frequency. A comparison of the bubbling frequency obtained by high-speed video and the pressure fluctuation technique is shown in Figure 4b. Based on the strong agreement between the results obtained from the two techniques, the pressure fluctuation technique provides an accurate and precise means to study the bubbling phenomenon.
Results and Discussion Weeping Behavior in the Air-Water System. Because of a high surface tension and susceptibility to impurities, the air-water system proves to be a very unpredictable and difficult system to manage because the standard deviation for air-water weeping rates is 40%. Despite the unreliable nature of the system, this observation may help explain the deviation noticed in previous studies in the literature. As a result, all airwater results shown are averaged quantities over several (up to 20) trial sets. Complications arise in part because of the unpredictable and erratic formation of multiple bubbles at the orifice. For the 1/8 in. orifice in the 6 in. column, single bubbling is impossible beyond a superficial orifice gas velocity, Ug, of 8 cm/s. Gas bubbles often detach in a
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Figure 5. Magnitude comparison of weeping rates of Norpar-15 and water under ambient conditions.
cluster of bubbles from the orifice and give rise to higher than expected weeping rates that vary by as much as 200% from trial to trial. This uncertainty appears to be linked to the variant bubble cluster size. The cluster often changes in size from one bubbling cycle to another for a given system and flow conditions. Similarly, for the 1/8 in. orifice in the 4 in. column, bubble breakup close to the orifice plate (within 5 cm) upon Ug reaching 15 cm/s is commonplace even though the bubble initially detaches as a single bubble. Bubble breakup can significantly complicate the analysis because this phenomenon is not well understood in itself. Premature bubble breakup is found to be characteristic of large bubble sizes (Db > 1.4 cm) and violent bubble detachment. The breakup may constructively or destructively aid in the pressure fluctuations above the orifice adjunct by bubble wakes and a general mixing flow pattern caused by preceding bubbles. Both the bubble breakup and multiple-bubble detachment lead to complications in flow patterns above the orifice. Multiple-bubble detachment affects the flow field immediately upon detachment from the orifice and, in general, will be more influential on the weeping behavior than bubble breakup. Therefore, formation of subsequent bubbles and weeping behavior associated with the process may be unpredictable and difficult to model. Another problem encountered with the air-water system is the formation of a drop on the underside of the orifice plate along the rim. The formation of the drop indicates the presence of a channel across the orifice as liquid weeps at random intervals or constantly flows through the orifice throughout the bubbling cycle. In other words, weeping is also observed during the bridging and bubble formation periods, not only during the weeping period. This phenomenon leads to significantly higher weeping rates. Weeping Behavior in the Nitrogen-Norpar-15 System. Because of these unpredictable qualities of the water system, a more ideal liquid is chosen to observe weeping behavior under the single bubbling regime. A paraffin oil, Norpar-15, is selected in this study for its slightly higher viscosity and lower surface tension, which give rise to smaller, more uniformly spherical bubbles. These bubbling characteristics deemphasize
wall effects that lead to bubble deformation and premature bubble breakup. Although this may not solve the problem outright, the concern is postponed to higher Ug values often well out of the scope of the single bubbling regime. Upon analysis of the bubbling frequency and bubble size for the two liquids studied, general observations are made about the ideality of the two systems. As mentioned previously, bubbles reaching approximately 1.4 cm in diameter pose a risk of premature bubble breakup. Based on the observations summed up in Figure 5, Norpar-15 does not pose this problem beyond Ug values of 1.5 cm/s during the onset of jetting. Therefore, weeping rates may be much more consistent and, in fact, have less than a 5% standard deviation as opposed to water’s 40% standard deviation. Figure 6 demonstrates typical weeping trends with respect to Ug for the two liquids considered. Both curves contain a maximum peak, and the maximum weeping rate for Norpar-15 is approximately 4 times that of water. The maximum may be qualitatively understood by considering the bubbling frequency and bubble size trends shown in Figure 5. Although much more distinguishable in water, both the bubble size and bubbling frequency initially increase dramatically and eventually level off to a smoother, more gradual rise. As the bubble initially grows dramatically, the wake or pressure directly over the orifice created upon bubble detachment increases proportionally, forcing liquid downward through the orifice. Meanwhile, gas momentum through the orifice and a capillary force act in opposition to the downward force induced by wake pressure. With increasing orifice gas velocity, the pressure above the orifice rises dramatically in proportion to the bubble size growth, while the gas momentum force steadily rises with increasing gas velocity. Consequentially, an increase in the weeping rate is initially observed until the momentum force can either begin to counteract the opposing force or reduce the time for weeping where the liquid physically cannot travel the orifice length in such a short time span. The maximum weeping rate, with respect to gas velocity, occurs as the dramatic growth in the bubble size begins to level off to a more gradual rise at the same time as the gas momentum force rises at a faster rate. After this maximum, a decreasing
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Figure 6. Bubbling frequency and bubble size comparison of water and Norpar-15 systems under ambient conditions.
Figure 7. Effect of plate material, column size, and plenum volume on weeping rates for the air-water system under ambient conditions.
weeping trend results from the continuous shortening of the weeping time period and the gas momentum force overcoming the pressure incurred by wake and mixing effects above the orifice. However, this analysis does not justify the significant increase in weeping rates for Norpar-15 over water, and in fact, the opposite seems to be implied. The two fluids differ primarily in surface tension and nominally in density and viscosity as noted in Table 1. Norpar-15, with respect to water, has slightly lower density and higher viscosity, which indicates lower gravimetric static force and additional viscous force reducing the maximum possible flow of liquid through the orifice, suggesting an overall lower weeping rate. However, for small orifices such as the 1/8 in. orifice, a capillary force within the orifice still remains significant during the weeping time period and a lower surface tension will increase the weeping rate. The substantial weeping increase for Norpar-15 over water implies the significant role surface tension plays for 1/8 in. orifices in the reformation of the gas-liquid interface
along the underside of the orifice after bubble detachment. Water’s high surface tension allows for faster reestablishment of the gas-liquid interface, and its lower surface tension, as with Norpar-15, implies difficulties in reformation of this interface, leading to higher weeping rates. Influence of System Characteristics. Physical characteristics of the bubbling system, plate material, plenum size, and column diameter were varied and observed for changes in bubbling and weeping behavior of the air-water system. A summary of the results obtained from these variations is shown in Figure 7. The more wettable stainless steel plate shows weeping trends similar to those of the Plexiglas plate for Ug < 50 cm/s but drops off suddenly as the gas velocity exceeds this threshold. During bubble formation, the changing contact angle of the fluids with the plate material may result in the gas-liquid interface spreading outwardly away from the orifice rim across the orifice plate for more wettable surfaces.5 This interface
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Figure 8. Liquid weeping dependence on orifice size in different liquid media under ambient conditions.
Figure 9. Liquid weeping rates for a 1/8 in. orifice in Norpar-15 at various pressures.
spreading may not be consequential to the bubble formation under low gas velocity cases when the bubble size and gas-liquid interface spreading are small. However, for high gas flow conditions and large bubble sizes, this effect may influence the gas flow stability and weeping period time length. Weeping rates in 4 and 6 in. columns show comparable agreement for low gas velocities below 25 cm/s. However, a drop in the weeping rates as the column diameter decreases is noticeable beyond this region. For the 1/8 in. diameter orifice in the 6 in. column, single bubbling in water is impossible to maintain beyond gas velocities of 8 cm/s. The larger column allows for large sets of paired or multiple bubbles to detach from the orifice at once, leading to higher mixing or wake turbulence directly above the orifice upon bubble detachment. In turn, these effects cause additional weeping by increasing the pressure directly above the orifice during the weeping time period. Bubbling in the smaller 4 in. column typically results in a single-bubble detachment but quickly leads to bubble breakup soon after
because of wall effects. As discussed earlier, because of the timing, the multiple-bubble detachment has a stronger influence on weeping rates over bubble breakup. Therefore, the 6 in. column generally shows higher weeping rates and less stable bubble formation over the 4 in. column. Upon an increase in the chamber volume, bubbles become larger but weeping rates decrease. Initially, these observations appear to contradict one another based on previous analysis. However, because the wake size is expected to grow with the bubble size, the larger chamber size also allows for dampening of pressure fluctuations across the plate incurred by the detaching bubble. Although the bubble volume becomes larger with increasing chamber volume, the bubble volume change is order of magnitudes smaller than the increase in the chamber volume. The wake may grow in magnitude similar to the bubble size, but now the pressure induced by the wake has a much greater chamber volume to act on. As a result, the pressure fluctuations are smaller with increasing chamber volume and leads
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Figure 10. (a) Bubbling frequency and (b) bubble size for a 1/8 in. orifice in Norpar-15 at various pressures.
to an overall lower weeping rate. Furthermore, the gas velocity range corresponding to maximum weeping rates is found to be broader for smaller chambers and “sharpens” toward lower gas velocities of the maximum range as the chamber volume increases. This gradual sharpening results from a slightly steeper rise in the bubble size with respect to Ug. Over long time periods, observations about the influence of the chamber volume on the weeping rate imply a compounding problem. As liquid weeping builds up within the plenum and reduces the effective chamber size, the weeping rate continues to increase until the plenum is flooded. Naturally, larger orifices lead to higher weeping rates, but based on the assumption that the capillary forces still remain significant for small orifices, a much higher increase in weeping for the water system should be eventually observed over Norpar-15 as the orifice di-
ameter is made larger. The capillary force for water is potentially more than twice that of Norpar-15, and as the orifice size increases, this difference should become a less dominant factor in the weeping phenomenon in favor of the gas momentum force and mixing effects near the orifice. For an increase in the orifice diameter from 1/ to 1/ in. as shown in Figure 8, the water system 8 4 undergoes a weeping rate increase of approximately 5 times while the weeping rate in Norpar-15 increases by about 30%. For large orifices, the gas momentum force and mixing effects are left unopposed or unassisted by the capillary force. Gas momentum and mixing forces are composed primarily of inertial- and drag-type forces, which, in turn, are determined by the system parameter, gas velocity, and physical properties of the gas and liquid. The slightly lower density and higher viscosity of Norpar-15 over water indicate generically somewhat lower weeping rates and, indeed, this holds true for the
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Figure 11. Liquid weeping rates for a 1/4 in. orifice in Norpar-15 at various pressures.
larger 1/4 in. orifice. The only peculiar aspect of the trend is that the weeping rate in Norpar-15 levels off at low gas velocities as opposed to undergoing any measurable maximum. Comparison of these results with previous literature studies mentioned earlier shows only qualitative agreement and further infers the need for a more rigorous analysis of the phenomenon. Higher system pressures do not affect the physical properties of the liquid medium significantly as noted in the liquid property measurement results in Table 1. Only the gas properties change significantly with pressure and are, therefore, the primary suspect for any changes in bubble formation or weeping behavior. Figure 9 shows the pressure effect on the weeping rate, while Figure 10 compares the bubbling frequency and bubble size for a 1/8 in. orifice in Norpar-15. Higher gas density contributes to a stronger gas momentum force and faster bubble formation with smaller bubbles. Lower pressure fluctuations across the orifice plate are also observed, consistent with the plate results from smaller bubbles invoking smaller wakes upon bubble detachment and decreased mixing effects. In turn, this combination contributes to the reduction in weeping rates. Upon further pressure increase, the bubbling frequency begins to increase more dramatically but more constantly. The bubble size remains more uniform with respect to Ug, the weeping rate is characterized without an apparent weeping maximum as shown for the 1.83 and 3.55 MPa cases, and only a declining weeping trend is observed. The lack of any dramatic bubble size increase, as observed at other pressures, agrees well with the lack of a weeping rate maximum. The weeping maximum also shifts toward lower gas velocities as the pressure increases, signifying the gas momentum force overtaking the wake and mixing effects quicker than the bubble size can increase with increasing gas velocity. Similar to the aforementioned pressure effect analysis, 1/4 in. orifices show weeping behavior similar to that of smaller 1/8 in. orifices, and these results are displayed in Figure 11. Weeping maximums, present at each pressure, are attributed to the magnitude of bubble size associated with large orifices. The bubbling frequency and bubble size, seen in Figure 12, show a fairly abrupt
increase in the effective bubble diameter, with respect to gas velocity, up to approximately 0.75 cm for all pressures. The influence of the chamber size on weeping rates under high pressure is also observed, as is shown in Figure 13. Similar to the ambient pressure case, weeping rates decrease and maximums “sharpen” toward lower gas velocities as the chamber size is increased. Mixing Effects on Weeping Behavior in the Jetting Regime. Weeping behavior in the jetting regime is generally characterized as nonexistent or simply negligible. In this study, liquid weeping is observable even under jetting conditions. Despite this observation, no significant liquid weeping buildup in the plenum is found because the liquid evaporates too quickly to be of consequence. Therefore, small, single orifices generally show no measurable levels of liquid weeping buildup within the plenum for orifice gas velocities greater than 1 m/s. To observe noticeable weeping buildup, a disturbance must be introduced of sufficient magnitude to simulate the mixing effect incurred by multiple jets confined to a set volume. The desired effect is obtained by utilizing a sparger as described in the Experimental Section to enforce a circulation pattern induced by the addition of secondary gas flow through the column. The impacts of induced circulation and mixing effects on liquid weeping are demonstrated by findings in Figure 14. All cases show substantial weeping rates through 20 m/s and suggest possible problems even at extremely high gas velocities for well-mixed gas-liquid systems. Unlike liquid weeping cases in the bubbling regime, the larger orifices show decreased weeping rates due to enhanced jet stability within the mixing environment. Because there is no “bubble detachment” and gases essentially flow continuously through the orifice to maintain the jet, cross-flows above the orifice plate lead to moments of jet instability and increased pressure directly above the orifice that forces liquid into the plenum region. Therefore, because larger orifices provide a higher gas momentum force to maintain jet stability, larger orifices are subject to reduced weeping rates in the jetting regime. The sparger, without the presence of a perforated plate beneath it, shows evidence of liquid weeping at
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Figure 12. (a) Bubbling frequency and (b) bubble size for a 1/4 in. orifice in Norpar-15 at various pressures.
brief moments as liquid drops splatter along the inner surface of the sparger and quickly evaporate because of the high gas throughput. This behavior is observed in this study for orifice gas velocities up through 100 m/s. As weeping liquid is ejected back into the column by the gas momentum force, solid particles carried in by the weeping liquid may be reejected in a similar manner. Upon addition of limestone particles to the fluidized gas-liquid column, particles weep into the sparger along with small liquid droplets but are not ejected back into the column. Instead, particles remain behind and encrust themselves along the rims of each orifice while liquid droplets evaporate and are carried off by the massive airflow. Over a long period of time (2 weeks), the particles continue to build up and form a cake approximately 7 mm thick along the bottom half of the sparger surface. However, each orifice is observed to remain open through the glass windows. Only the effective sparger volume and orifice length are compromised. Changes in sparger specifications infer slightly
different gas-liquid contacting and an increase in pressure within the sparger to maintain a constant gas flow. The limestone cake is not stable and falls apart readily as the system is shut down and liquid flows into the submerged sparger. Upon removal of the sparger from the column, only a thin, hard limestone film (,1 mm) is left along the inner surface of the sparger as evidence. Conclusions A number of system conditions have been varied to observe the relative influence of each aspect to the bubbling and weeping phenomena. Each of these aspects poses a very similar magnitude of influence over both phenomena. Difficulties arise in measuring the weeping rate and modeling the bubbling phenomenon because of water’s unusually high surface tension and high susceptibility to impurities. These properties lead to extremely large bubbles and variant bubbling behavior
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Figure 13. Influence of the chamber size on the weeping rate under both ambient and elevated pressures for a 1/8 in. orifice in Norpar15.
Figure 14. Weeping rates under jetting conditions with induced circulation and mixing effects for water at ambient pressure.
that cause other poorly understood phenomena to arise and complicate the already intricate study. Weeping behavior is evident not only at low gas velocities or large orifices but also under many other flow conditions. As was observed with the jetting experiments, stability of the gas flow through the orifice also influences the weeping behavior because turbulence or liquid crossflows can cause excessive weeping. Gas-liquid-solid systems have particular repercussions because weeping of solid particles is not as easily ejected from the sparger or plenum region as liquid weeping evaporates to escape into the column. Increased system pressure causes faster, shorter bubble formation and, therefore, creates smaller bubbles. As a result, there is no significant change in the bubble size with respect to gas velocity except for a “zone” of velocities near the zero velocity that gradually narrows as the pressure is increased. This smaller bubble size leads to an overall lower weeping rate for high pressures, while the gradual narrowing of the “zone” causes weeping rate maximums
to lower gas velocities with increasing pressure. A number of forces, easily affected by the host of system parameters discussed within this paper, influence the weeping behavior to similar degrees and, therefore, may not be neglected. Acknowledgment This work is supported by the National Science Foundation Grant CTS-9906591. Special thanks to Vivek Lal for his aid in carrying out the experiments. Nomenclature Do ) orifice diameter Db ) estimated spherical bubble diameter Ug ) orifice gas velocity Ul ) orifice liquid weeping velocity Uc ) superficial column gas velocity F ) liquid density
Ind. Eng. Chem. Res., Vol. 41, No. 6, 2002 1677 µ ) liquid viscosity σ ) liquid surface tension
A Single Hole on a Horizontal Plan Immersed in Water. 2. A Single Orifice on an Inclined Plane Immersed in Water. Langmuir 1994, 10, 936.
Literature Cited
(6) Illiadis, P.; Douptsoglou, V.; Stamatoudis, M. Effect of Orifice Submergence on Bubble Formation. Chem. Eng. Technol. 2000, 23, 341.
(1) Akagi, Y.; Okada, K.; Kosaka, K.; Takahashi, T. Liquid Weeping Accompanied by Bubble Formation at Submerged Orifices. Ind. Eng. Chem. Res. 1987, 26, 1546. (2) Che, D. F.; Chen, J. J. J. Bubble Formation and Liquid Weeping at an Orifice Submerged in a Liquid. Chem. Eng. Technol. 1990, 62, 947. (3) McCann, D. J.; Prince, R. G. H. Bubble Formation and Weeping at a Submerged Orifice. Chem. Eng. Sci. 1969, 24, 801. (4) Miyahara, T.; Iwata, M.; Takahashi, T. Bubble Formation Pattern with Weeping at a Submerged Orifice. J. Chem. Eng. Jpn. 1984, 17, 592. (5) Lin, T. J.; Banerji, S. K.; Yasuda, H. Role of Interfacial Tension in the Formation and the Detachment of Air Bubbles. 1.
(7) Hsu, S. H.; Lee, W. H.; Yang, Y. M.; Chang, C. H.; Maa, J. R. Bubble Formation at an Orifice in Surfactant Solutions under Constant-Flow Conditions. Ind. Eng. Chem. Res. 2000, 39, 1473. (8) Lin, T. J.; Fan, L.-S. Characteristics of High-Pressure Liquid-Solid Fluidization. AIChE J. 1997, 43, 45.
Received for review August 8, 2001 Revised manuscript received January 14, 2002 Accepted January 22, 2002 IE0106581
