Effects of Prewetting on Bubbly- and Pulsing-Flow Regime Transitions

May 13, 2015 - Gregory S. Honda†, Eric Lehmann†, Daniel A. Hickman‡, and Arvind Varma†. † School of Chemical Engineering, Purdue University,...
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Effects of Prewetting on Bubbly- and Pulsing-Flow Regime Transitions in Trickle-Bed Reactors Gregory S. Honda,† Eric Lehmann,† Daniel A. Hickman,‡ and Arvind Varma*,† †

School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive West Lafayette, Indiana 47907, United States Engineering and Process Science, The Dow Chemical Company, 1776 Building, Midland, Michigan 48674, United States

‡

ABSTRACT: The transitions from trickle to bubbly flow and trickle to pulsing flow are investigated in the range of gas superficial velocities vG = 4−220 mm/s using air and water, with additional consideration of the effect of the prewetting procedure. The flow regime transition was detected by the standard deviation of the pressure drop and visual observation, with further confirmation using a high-speed camera. Results show a significant effect of the prewetting procedure on the liquid superficial velocity at a fixed gas superficial velocity required for transition from trickle to bubbly flow and trickle to pulsing flow. At low gas superficial velocities, where the transition from trickle to bubbly flow occurs, a significant departure from literature model predictions is observed. Below vG = 20 mm/s, rather than the expected increase in the liquid superficial velocity required for transition with decreasing gas superficial velocity, the transition is observed to be essentially independent of the gas flow.

1. INTRODUCTION Fixed-bed reactors with cocurrent downflow of gas and liquid reactants, commonly referred to as trickle-bed reactors, are used throughout industry for hydrodesulfurization, hydrogenation, and oxidation reactions. The interaction of gas and liquid reactants in a packed bed of catalyst results in complex hydrodynamics that affect the reactor performance.1 This includes the existence of multiple flow regimes, described in the literature as the low interaction regime of trickle flow and the high interaction regimes (HIR) of pulsing, bubbly, and spray flow.2 The contacting patterns and interactions between the phases in each regime result in significant differences in the rates of heat and mass transfer that occur sharply with a small change in the gas or liquid velocity. Accurate characterization of the flow regime transition is therefore vital to ensuring the desired reactor performance. Commercial reactors typically operate in the trickle-flow regime or near the transition to pulsing flow. Focus in the literature has therefore been directed toward the trickle−pulsing transition. However, for particular reactions, it may be advantageous to operate nearer to the trickle−bubbly transition. Operating at low gas superficial velocity and high liquid superficial velocity for exothermic reactions improves liquid contact of particles, preventing hotspot formation that would lead to catalyst deactivation in those areas. However, literature reports evaluating the transition from trickling to bubbly flow are limited. The objective of this work is to carefully characterize the transition, with particular attention given to the effects of prewetting and hysteresis. Prior work related to bubbly flow in trickle-bed reactors is more often concerned with the transition from pulsing to bubbly flow occurring at higher gas and liquid superficial velocities than the trickle−bubbly transition.3,4 Quantitative measurements of the trickle-bubbly transition are less common. In general, studies related to the flow regime transition in trickle-bed reactors are restricted to gas superficial velocities greater than 30 mm/s.5−7 A thorough investigation of the trickle−bubbly transition is not available in the literature. © 2015 American Chemical Society

Wammes et al. observed both the trickle−bubbly and pulsing− bubbly transitions based on visual observations.8 However, they were unable to define a distinct transition to bubbly flow. In addition to the effects of multiple flow regimes, the performance of trickle-bed reactors is also influenced by hysteresis and the prewetting method. Flow conditions in a trickle-bed reactor in the low interaction trickle regime depend on the approach from previous operating conditions, particularly with respect to gas and liquid superficial velocities (vG and vL). The effects of hysteresis and prewetting on the hydrodynamics of trickle-bed reactors have been thoroughly investigated, including the impact on the pressure drop, gas− liquid mass transfer, and liquid holdup.9−12 Prewetting procedures frequently evaluated include the KanL, KanG, Levec, and Dry methods.12 The Kan procedures involve decreasing either the liquid or gas superficial velocities from pulsing flow. The Levec mode starts by flooding the bed, draining, and then introducing gas and liquid at their set points, while the Dry method involves starting with a dry bed. Typically, the evaluation of these effects is restricted to the trickle-flow regime because the majority of flow regime transition studies follow a single prewetting procedure. While most studies do not observe hysteresis in the gas and liquid superficial velocities required for flow regime transition,11,13 the effect has been previously reported for the trickle−pulsing transition10 and predicted by modeling.14 Prewetting and hysteresis are expected to have a different effect on the trickle− bubbly transition because of differences in the interaction between gas and liquid at low gas superficial velocity.15 To provide a comparison, these effects are evaluated for both the trickle−pulsing and trickle−bubbly transitions. Special Issue: Doraiswami Ramkrishna Festschrift Received: Revised: Accepted: Published: 10253

March 12, 2015 May 12, 2015 May 13, 2015 May 13, 2015 DOI: 10.1021/acs.iecr.5b00957 Ind. Eng. Chem. Res. 2015, 54, 10253−10259

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Industrial & Engineering Chemistry Research

2. METHODS The column used in this study was 1/4-in.-thick acrylic, with 2 in. i.d. and 36 in. height (Figure 1). The column was packed

and passed through a distributor before entering the column. The distributor had 13-1/8-in.-o.d. nozzles for water flow. Air was introduced separately through 3/16-in.-i.d. orifices, which were annular to the water nozzles. Last, there was a 1 in. gap between the distributor and the top of the packing, allowing for visual confirmation that the distributor was functioning properly. Prior to the experiments being undertaken, the system was operated by flowing air and water through an empty column (without packing) to verify that the pump, check valves, mass flow controllers, flowmeters, and pressure gauges had minimal inherent fluctuations. Four different prewetting methods were evaluated: (1) KanLU: The bed was flooded and drained. Gas was introduced at its set point, and liquid was introduced at 30 mm/s (placing it in the pulsing- or bubbly-flow regime). The liquid flow rate was then decreased; this represents the upper arm of the hysteresis curve. (2) KanLL: After following the KanLU procedure, the liquid superficial velocity was decreased to 5 mm/s, held at this value for 1 h, and then increased incrementally; this is representative of a lower arm of the hysteresis curve. (3) Levec: The vessel was flooded, allowing the nonporous particles to soak for 5 min, and then drained. Air was introduced at its set point, and liquid was introduced at 5 mm/ s, held at this value for 1 h, and then increased. (4) Dry: Gas was introduced at its set point, liquid was introduced at 5 mm/s simultaneously to a dry bed and held at these rates for 1 h, and then the liquid flow rate was increased. In the flooding procedure (constituting the Levec method and conducted prior to the KanLU/KanLL modes), liquid was introduced to the top of the dry bed at ∼10 mm/s, with the bottom of the vessel left open. A vent attached to the top of the column was then opened, and the bottom of the vessel was sealed. This allowed flooding to occur from the bottom up, leaving no trapped air in the column. Gas and liquid feeds to the column incorporated bypass lines to smoothly introduce both fluids to the vessel. To introduce gas and liquid to the column, the bypass lines were opened while the feed to the column was closed. The flow rates were then set. Next, the feed lines were opened, and the bypass valves were slowly closed. This method prevented pressure buildup in the gas and liquid feed lines, so that static liquid holdup remaining from the flooding process was not blown out when the feeds were introduced. After following each prewetting procedure, the liquid superficial velocity, vL, was manipulated monotonically in the range 5−30 mm/s. The gas flow rate was maintained at a fixed mass flux. Gas superficial velocities, vG, varied slightly (within ±5%) during an experiment because of changes in the pressure drop with changes in vL. Gas superficial velocities evaluated in this work were approximately 4, 8, 16, 40, 80, 160, and 220 mm/s. At atmospheric pressure and 20 °C, these correspond to gas mass fluxes of 0.0049, 0.0097, 0.0195, 0.0487, 0.0974, 0.1950, and 0.3900 kg/m2s, respectively. This range is sufficient to evaluate the effect of gas flow on both the trickle−pulse and trickle−bubbly flow regime transitions. Data points were recorded at 10 min intervals and include pressure-drop and visual observations. Pressure-drop data were recorded for 2 min at a sampling rate of 10 Hz. The average dimensionless pressure drop, ψL, in the column and the relative standard deviation of the pressure drop (σR = σmeasurement/σbaseline − 1) were tracked during the experiment along with visual observations. Here, the corrected sample standard deviation is used to calculate all σ

Figure 1. Experimental setup including (1) the distributor, packed column, and pressure transducers, (2) house air supply, filter, and regulator, (3) air mass flow controllers, (4) water flowmeters and control valves, and (5) water reservoir, filter, and pump.

with 3.41 mm spherical nonporous ceramic beads. The column to particle diameter ratio was 15, while the bed length to particle diameter ratio was 270; therefore, wall and entrance effects could be considered negligible.16 Packing of the column was performed by introducing a 1/2 in. tube to the base of the vessel, pouring beads into the tube, and then slowly raising the tube while tapping the side of the column. This packing method was repeated three times to determine the reproducibility and achieved the same void fraction each time. On the basis of the envelope density of the particles and the mass of particles added to the column, the bed void fraction was 0.381 ± 0.001. The flow regime was monitored by the standard deviation of the pressure drop across the packed column and visual observation, with confirmation by recordings using a highspeed camera (Phantom v5). Pressure-drop-based methods are commonly used in the literature.6,17,18 Taps for the pressure gauge lines were made at 6 and 30 in. from the bottom of the column and were placed at the wall to prevent disturbance of the flow by any vessel internals. Two differential pressure gauges were used (0−5 and 0−30 psid) with an accuracy of 0.08% (best straight line). These were supplemented by an additional sensor to measure the gauge pressure inside the vessel. Tubing leading from the column to the gauges was maintained filled with air. The fluids used were air and water at room temperature and pressure. A rotary vane pump was used to supply house deionized water to the column, which was operated in liquid recycle. House air was supplied at 120 psig, passed through particulate and oil filters, and delivered to the mass flow controllers at 40 psig. Two air mass flow controllers and two liquid flowmeters were used to maintain high accuracy over the full range of flows. Air and water were supplied to the column 10254

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Industrial & Engineering Chemistry Research values. In the determination of σR, σmeasurement is the standard deviation of the 1200 pressure drop data points. The baseline measurement was taken with no flow and gave, on average, σbaseline = 0.0050 psid. With this definition, σR is 0 under stable conditions and equal to 1 when fluctuations are twice those of the typical background. Visual observations include hD, the height of disturbances (local fluctuations in the column), and hU, the height of the uniform HIR, with both taken relative to the total column height. Disturbances in the column appeared as either localized bubbling or pulsing on the scale of 1−3 particle diameters. The uniform HIR is qualitatively described as either bubbly or pulsing flow that extends uniformly around the column circumference. The height at which this occurs yields hU. At high values of vL, it becomes difficult to discern bubbly from pulsing flow by the naked eye. In this case, a high-speed camera was used to record video at 500 frames/s. Playback was then slowed down to verify the HIR. Again, for each vL, two data points were taken at 10 min intervals. If all measured values, including σR, ψL, hD, and hU, were within ±10%, 2%, 2%, and 2%, respectively, the two data points were considered stable and the liquid flow rate was changed to its next value. If the two data points were not equivalent, the first point was considered unstable and a subsequent data point was recorded after another 10 min interval. Near the transition, the system could take up to an hour to stabilize. Because the flow regime transition can occur suddenly, particularly for increasing liquid flow rates, step changes in liquid flow were decreased in this range. The final step size near the transition was approximately 0.40 mm/s. After an experiment for a given vG, the system was shut down, and the air flow was set at 30 mm/s to dry the bed for 12 h. To validate the repeatability of the procedures, certain experiments (KanLU, vG = 220 mm/s; KanLU, vG = 8 mm/s; KanLL, vG = 80 mm/s; Levec, vG = 8 mm/s) were replicated three times, and one was repeated four times (Dry, vG = 220 mm/s). In each set of replicates, at least one repeat was performed on a column repacked with 3.41 mm ceramic beads and having a void fraction of 0.380. It should also be noted that one experiment (Dry, vG = 16 mm/s) was repeated twice on the first bed. The resulting degree of repeatability is considered in later sections.

Figure 2. Comparison of the results of σR, ψL, hD, and hU for (a) vG = 16 mm/s with the Levec prewetting procedure and (b) vG = 220 mm/s with the KanLU prewetting procedure.

3. RESULTS AND DISCUSSION 3.1. Criterion for the Transition. Results are reported for two example cases (the Levec method for vG = 16 mm/s and the first repeat of the KanLU method for vG = 220 mm/s) in Figure 2 and demonstrate the agreement between our visual observations and pressure-drop measurements. The additional results for the other repeats of KanLU with vG = 220 mm/s are discussed later. As described in the Methods section, data points are averages or standard deviations of data taken over a 2 min period at 10 Hz. In this figure, all data, regardless of stability, are included to demonstrate the different states that occur during the transitions. In subsequent figures, only stable points are included for clarity. For both the procedure following the Levec prewetting mode at vG = 16 mm/s (Figure 2a) and that following the KanLU prewetting mode at vG = 220 mm/s (Figure 2b), relative changes in hU and hD follow changes observed in σR. On the basis of these results and confirmation with video from the high-speed camera, the transition from trickle to bubbly flow is observed at vL = 17.3 mm/s in Figure 2a, while the transition from trickle to pulsing flow is observed

at vL = 7.4 mm/s in Figure 2b. Note that for high gas superficial velocities, particularly with respect to the KanLU method, the pressure drop alone is not a satisfactory indicator of flow regime transition. For the case of decreasing vL (KanLU), the disturbances in the column continue (hD > 0), although the uniform HIR is no longer observed (hU = 0). By evaluation of these cases and all other available data, a criterion for the transition observed in our system may be established. We define the transition (vG and vL) between trickling flow and a HIR as the stable point (p1) where σR < 0.9 and which precedes an adjacent stable and higher vL where σR > 0.9. This criterion corresponds to the stable point one increment smaller than that for which hU > 0 and hD > 0.25. Experimentally, choosing this transition criterion over other interpretations (such as the inflection point of a fitted curve) is logical. For increasing vL, the transition may occur anywhere over the range of the step size after the last stable point, p1. Additionally, if the regime transition occurs soon after the flow 10255

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Industrial & Engineering Chemistry Research rate is changed, there may be insufficient time to record data at this unstable point (p2). Finally, it should be noted that, at the transition, the pressure drop across the bed changes. Because the liquid flow rates are manipulated by manual control valves, the change in the pressure drop results in a subsequent change in liquid flow rates. The flow at the stable point after the transition occurs (p3) will not necessarily be the same as the flow rate set before the transition (p2). Taking the transition as the average of p3 and p2 or as a point interpreted between p3 and p1 would therefore not be as meaningful as the selected criterion. 3.2. Comparison of the Prewetting Effects on Regime Transitions. The trends with respect to the effect of prewetting procedures on σR, ψL, hU, and hD were the same across the set of vG that fell within the trickle−bubbly transition, although these observations differed from those for the trickle−pulsing transition. As such, representative examples are shown for the two transitions in Figures 3−5. Figure 3

Figure 4. Example results for the effect of prewetting procedures on ψL at (a) vG = 16 mm/s and (b) vG = 220 mm/s.

Figure 3. Example results or the effect of prewetting procedures on σR at (a) vG = 16 mm/s and (b) vG = 220 mm/s.

shows the results of σR profiles for the bubbly-flow transition (vG = 16 mm/s; Figure 3a) and pulsing-flow transition (vG = 220 mm/s; Figure 3b). Both results demonstrate an influence of the prewetting procedure on the flow regime transition. Visually, the KanLU procedure resulted in a film flow over the particles. The KanLL, Levec, and Dry procedures resulted in an increasing degree of rivulet-like flow in the order listed, where the rivulet diameter was on the scale of at least 1/4 in. As vL was increased, the rivulets grew in size until ultimately coalescing to a HIR. The observation of film and rivulet flow, depending on hysteresis and prewetting, agrees with reports in prior literature.11,13,19

Figure 5. Example results for the effect of prewetting procedures on hU at (a) vG = 16 mm/s and (b) vG = 220 mm/s.

It should be recalled that the liquid flow rate is subject to changes in the pressure drop due to the use of manual control 10256

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pressure drop (Figure 4b), although not in the location of the transition (Figure 3b). As noted previously, at low vG (Figure 4a), changes in the pressure drop are significant enough to demarcate the transition. For the same experiments (Figures 3 and 4), the corresponding visual observations of the uniform HIR are reported in Figure 5. In general, the KanLU method showed hU decreasing steadily with decreasing vL until a flow rate at which all fluctuations ceased (trickle flow) was reached. For the KanLL method, hU gradually increased with vL after the transition. This is in contrast to the Dry method, which, once initiated, exhibited the HIR throughout the majority of the column height. The primary difference between the trickle− bubbly and trickle−pulsing transitions is in the Levec mode. In the trickle−bubbly transition (Figure 5a), the HIR initiates throughout much of the column (hU > 0.6), whereas in the trickle−pulsing transition (Figure 5b), the HIR is observed only in the bottom half of the column (hU < 0.5). The corresponding example plots of hD, which track hU at slightly lower values but for which all other trends are the same, are omitted for brevity. 3.3. Flow Regime Map. Accounting for all of the experiments, a map may be defined for the flow regime transitions in the system. The results of evaluating all prewetting procedures for different gas superficial velocities are presented in Figure 7. It should be noted that the vL

valves. This is how, as in the case of the Dry prewetting procedure at vG = 16 mm/s, vL decreases across the transition. For the bubbly-flow transition (Figure 3−5a), two results from the first bed for the Dry prewetting procedure, which show a slight deviation between them, are reported. Differences are also observed in the repeated results for the KanLU procedure with vG = 220 mm/s (Figure 3−5b). Here, KanLU-1 is from the first bed, while KanLU-2 and -3 are from the second bed. Variability in the location of the transition is expected in part because of the step size used in the experiments. Relative to the other repeated experiments, there was greater variability in vL required for the transition for the Dry procedure at vG = 220 mm/s, where the trickle−pulsing transition occurs. Figure 6

Figure 6. Repeatability of the results for the Dry prewetting method at vG = 220 mm/s.

shows the resulting transitions observed based on σR for the four replicates, where Dry-1 was from the first bed and Dry-2 and -4 were performed on the second bed. In the Dry mode, liquid is not well distributed, and the point at which the trickling liquid streams become unstable, and transition to pulsing, is highly variable. High gas superficial velocity may cause the transition to occur earlier, but in some cases, the streams remain stable even at high liquid superficial velocities. The effect of different prewetting procedures on the dimensionless pressure drop, ψL, is shown in Figure 4. In the trickling regime at high gas superficial velocity (Figure 4b), the pressure drop decreases in the order KanLU, KanLL, Levec, and Dry modes, which agrees with the literature.12 Typically, higher pressure drop for given vG and vL is associated with an increased interaction between gas, liquid, and solid phases and is correlated with higher values of liquid holdup. At vG = 16 mm/s (Figure 4a), in the trickling regime prior to the bubbly transition (vL < 15 mm/s), there is no hysteresis in the pressure drop observed between the KanLU and KanLL procedures, although the pressure drops here are higher than those for the Dry and Levec modes. This is opposed to ψL at vG = 220 mm/s (Figure 4b), where hysteresis in the pressure drop is observed between KanLU and KanLL under the trickling regime for vL < 8 mm/s. The differences between the pressure drops of the KanLU and KanLL procedures at vG = 220 and vL = 4 mm/s may be due to the partial wetting that is typically observed at low liquid superficial velocities. When the flow rate is held here for 1 h, the system has time to drain excess liquid holdup that is not lost over the 10 min interval that data are taken for at the end of the KanLU procedure. As a result, the pressure drop is lower for KanLL, and an effect of hysteresis is observed in the

Figure 7. Flow map of the effect of the prewetting procedure on the vG and vL values required for the transition. Approximate pulsing−bubbly transition given by the thick dashed line. Observations of the flow regime extend to the experimental lines, not the prediction of the Larachi model.

transition point for the Dry method at vG = 16 mm/s is an average of the two corresponding experiments shown in Figures 3−5a, where the transition was determined to occur at vL = 17.37 and 17.40 for Dry-1 and Dry-2, respectively. The results of the other repeated experiments (KanLU, vG = 220 mm/s; KanLU, vG = 8 mm/s; KanLL, vG = 80 mm/s; Levec, vG = 8 mm/s; Dry, vG = 220 mm/s) are reported as averages with error bars for vL. Error bars were produced based on tdistribution 95% confidence intervals. Confidence intervals in the gas superficial velocities were less than ±1% of the measured vG for a given prewetting procedure and are therefore not displayed. For the liquid superficial velocity, confidence intervals are on the order of the step size used in varying the liquid flow (ΔvL ∼0.4 mm/s). For the Dry method with vG = 220 mm/s, the variability was significantly larger. Except for the Dry method at high vG, the remaining vL transitions are 10257

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differences in prewetting effects show that vL would have to be decreased to a value far lower than that where the transition occurred in order to return to trickle-flow conditions. In a commercial reactor, it is unlikely that the packed bed would be operated in the KanLU or KanLL mode, although some amount of prewetting would be employed, particularly for highly exothermic reactions. A more likely representative scenario is the Levec mode. In this case, once the transition occurs at low vG values, the liquid flow rate would need to be decreased by more than 20% to return to trickle flow. To fully understand the behavior of the trickle−bubbly flow regime transition, the effects of the bed (particle material, shape, and size and bed void fraction) and fluid (densities, viscosity, and surface tension) properties also need to be considered.

expected to have confidence intervals closer to those of the other replicates, which were, on average, ±0.68 mm/s. For low gas superficial velocities (vG < 40 mm/s), the Dry and Levec results are similar. On the basis of the resulting error bars for KanLU at vG = 16 mm/s, the KanLU and KanLL methods are not significantly different even for low gas superficial velocities. The Dry procedure shows less dependence on the gas superficial velocity over the entire range, although the transition is more variable for trickle−pulsing at high vG. For the high gas superficial velocity, the Levec, KanLL, and KanLU methods appear to converge with increasing vG. The boundary between these observations corresponds approximately to a boundary between the trickle−bubbly (vG ≤ 40 mm/s) and trickle−pulsing (vG ≥ 80 mm/s) flow regime transitions, as confirmed by high-speed video recordings. A dashed line is plotted that represents the approximate transition between bubbly and pulsing flow. Recordings with the high-speed camera showed a transition from pulsing to bubbly flow occurring for vG = 80 mm/s in the range of 20 mm/s < vL < 25 mm/s. Analysis of higher frequency samples (up to 1000 Hz) may provide more information; however, it should be noted that the purpose of this study was to determine the trickle−HIR transition. The results of the Larachi et al. neural-network model20 for flow regime transition are also plotted in Figure 7. A comparison of this model to others is reported in the literature, and qualitative trends with respect to increasing vL required for transition with decreasing vG are common.21 The model provides a prediction of the transition from trickle to HIR and most closely follows the results of the trickle−pulsing transition following the KanLU method. The effect of the prewetting procedure, however, is not captured by the model. Importantly, experimental results diverge significantly from model predictions for vG ≤ 8 mm/s. 3.4. Implications for Commercial Reactors. Although our experiments were conducted with air and water at room temperature and pressure, they remain meaningful and, in fact, demonstrate the need to evaluate these effects at actual reactor conditions. Higher pressure will likely shift the transition to higher vL for a given vG.8 Our results show, however, that the transition is less dependent on vG at low values. For vG < 20 mm/s, Al-Dahhan et al. report that trickle-bed reactors at high pressure operate equivalently to those at atmospheric pressure, with respect to the hydrodynamics in trickle flow.15 The shift in transition requiring higher vL at higher gas density may therefore be less dramatic. The lower surface tension of organic fluids will likely reduce the vL required for transition,22 although the effects of different prewetting procedures may still exist.11 Despite these differences, our results at low vG show a clear departure from the trends observed at high vG values. Taken altogether, the experimental results have important implications for the operation of commercial reactors. Regardless of whether operation in a HIR is desirable or not, substantial differences in the reactor performance are expected to occur across the transition.1 At low gas superficial velocities, literature models predict an exponential increase in vL required for transition to occur with decreasing vG. Our results show that this is not the case. Available literature models, which are defined using results at high vG, are not suitable for the prediction of flow regime transition at low gas superficial velocities. Operation of a commercial reactor at low vG based on such models could result in deviations from the expected reactor performance. Additionally, once a transition occurs,

4. CONCLUSIONS The transition from trickle to pulsing and trickle to bubbly flow has been investigated by detection based on the standard deviation of the pressure drop and visual observation, with confirmation by recording with a high-speed camera. The prewetting procedure was determined to have a significant effect on the gas and liquid superficial velocities required for the transition, the degree of which changed upon going from vG in the range of the trickle−pulsing transition to vG in the range of the trickle−bubbly transition. Pulsing flow was observed for vG ≥ 80 mm/s, and bubbly flow was observed for vG ≤ 40 mm/s. At values of less than vG = 20 mm/s, a significant departure from the expected behavior was observed. Rather than an increase in vL required for the transition, as would be predicted by literature models, the liquid superficial velocity required for the transition was relatively independent of vG. These results have important implications for the operation of commercial reactors. Operation at low vG based on predictions by models for the trickle−pulsing transition will lead to error. Furthermore, once transition to the HIR occurs at low vG, the liquid superficial velocity must be decreased significantly to return to the trickle-flow regime. The results demonstrate the need for further evaluation of the observed effects at commercial reactor conditions. Future work will evaluate the effect of the bed and fluid properties, with the goal of developing a predictive model for the transition from trickle to bubbly flow.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are delighted to contribute to this festschrift in honor of Professor D. Ramkrishna, whose friendship over the years has been a great pleasure for A.V. This work was supported by The Dow Chemical Company and the Hugh W. and Edna M. Donnan Fellowship.

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NOTATION hD = relative disturbance height (hD = heightD/heightcolumn) hU = relative uniform high interaction regime height (hU = heightU/heightcolumn) vG = gas superficial velocity (mm/s) vL = liquid superficial velocity (mm/s) DOI: 10.1021/acs.iecr.5b00957 Ind. Eng. Chem. Res. 2015, 54, 10253−10259

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Industrial & Engineering Chemistry Research σ = corrected sample standard deviation (σ = {[1/(N − 1/2 1)]∑Ni=1(xi − x)} ) ̅ σR = relative standard deviation of dimensionless pressure drop ψL = dimensionless pressure drop (ψL = −ΔP/ρLLg + 1)

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(21) Bansal, A.; Wanchoo, R. K.; Sharma, S. K. Flow regime transition in a trickle bed reactor. Chem. Eng. Commun. 2005, 192, 1046. (22) Chou, T. S.; Worley, F. L.; Luss, D. Transition to pulsed flow in mixed-phase cocurrent downflow through a fixed bed. Ind. Eng. Chem. Res. 1997, 16, 424.

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DOI: 10.1021/acs.iecr.5b00957 Ind. Eng. Chem. Res. 2015, 54, 10253−10259