Flooding Performance of Square Channel Monolith ... - ACS Publications

Ind. Eng. Chem. Res. 2002, 41, 6759-6771

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GENERAL RESEARCH Flooding Performance of Square Channel Monolith Structures Achim K. Heibel,*,† Freek Kapteijn,‡ and Jacob Moulijn‡ Corning Environmental Technologies, Corning Inc., Corning, New York 14831, and Delft ChemTech, Industrial Catalysis, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

The flooding performance of square channel monoliths for a wide range of geometric parameters and liquid properties has been evaluated. Hydrodynamic flooding is determined by a strong increase in pressure drop for increasing gas flow rate. Inefficient drainage of the liquid at the outlet of the monolith as well as liquids with high surface tension and monoliths with smaller channel sizes lower the flooding limits significantly, indicating a dominating impact of capillary forces. Increased liquid viscosity lowers the flooding limits, but the effect is less pronounced. The column diameter has a rather negligible effect on flooding. For higher liquid flow rates, a hysteresis effect was observed, shifting the deflooding gas flow rates to lower values. Misaligned stacking of monolith sections leads to a decrease of the flooding limits (approximately 15%), independent of the number of stacks. Common flooding correlations need to be adapted with the Bond number to describe the experimental data. For the type of outlet device applied in this work and the alignment of the outlet device with the channel walls, the experimental data are well correlated by CF,0.25,0/Bd ) [204.4 + 297.1FLG1.5 - 171.4e-FLG]-1 for dh/lc > 1.15. A comparison with the requirements of applications revealed that channel sizes of 2.9 and 4.1 mm (50 and 25 cpsi) are well-suited for hydrotreating and reactive stripping operations. For the more demanding needs of catalytic distillation, the larger channel size is more favorable. Introduction Monoliths are structures consisting of large numbers of parallel channels. They are widely used as support structures in catalytic gas treatment, combining a large surface-to-volume ratio with a low pressure drop. Usually, a catalyst is applied on top of the structure, creating an egg-shell catalytic system with a very short diffusional path length in the catalytic active layer. Ceramic as well as metal materials are used for the monolithic support. Recently, the application of monoliths has been extended to multiphase catalytic reactions. In these cases, the monolith can consist of a high-surface-area material (i.e., γ-alumina, silica) providing a sufficient high catalyst load for the reaction. Depending on the channel geometry and the flow conditions, different flow regimes are realized in the channels. In the film flow regime, the liquid flows down the wall as a film, and the gas occupies the core of the channel. The flow pattern and the distribution of the fluid domains over the channel cross section (Figure 1) have recently been reported.11 The separate passages for gas and liquid lead to a low interaction between the phases in the channel, which makes this reactor configuration suitable for countercurrent operation under industrially relevant operating conditions. For heterogeneously cata* Corresponding author. Address: Corning, Inc., HB-CB02, Corning, NY 14831. Tel.: +1-607-974-8430. Fax: +1-607974-4617. E-mail address: [email protected]. † Corning Inc. ‡ Delft University of Technology.

Figure 1. Nuclear magnetic resonance image of the cross section of four square channels (25 cpsi) with gas and liquid flow. Liquid accumulation in the channel corners (bright areas) and gas phase in the core of the channels.

lyzed reactions, countercurrent operation is advantageous for product-inhibited30 or equilibrium-limited reactions26 to boost the overall level of single-pass conversion. Furthermore, countercurrent flow of gas/ vapor and liquid can be used to combine the separation and reaction steps (catalytic distillation)4 or to control the reactor temperature effectively through evaporation.34 The operating limits of a countercurrent reactor are determined by the flooding limits. There is no strict

10.1021/ie020089c CCC: $22.00 © 2002 American Chemical Society Published on Web 11/21/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 26, 2002

definition of flooding.31 In general, it is described as the transport of liquid against its desired flow direction resulting from the interaction with the countercurrently flowing gas/vapor phase. Flooding has a significant effect on performance parameters and can therefore be determined by changes in them. This includes hydrodynamics, mass transfer, separation performance, and stability of operation.7 Since first specified by White,38 flooding in packed columns has been well-documented in the literature. Pioneering work was performed by Sherwood et al.,28 leading to the first empirical flooding correlation, which still serves as the basis for many design methods of today. Countercurrent flow in packed columns is encountered in distillation, absorption, extraction, and stripping processes. The benefits of defined flow passages for gas and liquid for enhanced flooding performance were already recognized in the early days of flooding research. Indeed, stone-like structures with slot openings and drip points as well as the orderly packing of ring elements were documented in 1942 to show superior flooding behavior.21 Countercurrent flow is also applied for falling film absorbers, heat pipes, reflux condensers, dephlegmators, and emergency cooling of nuclear reactors. For these cases, the research has focused on countercurrent flow in single pipes. Wallis36 established the basis of technical understanding in this area with many experimental and theoretical investigations. Good summaries on the subject have been published by McQuillian and Whalley,19 Bankoff and Lee,2 and Piche´ et al.24 Lebens et al.15 extended the investigations on flooding in small-diameter tubes (L < 5 mm) and also evaluated the performance of tube bundles17 and monoliths with a finned channel geometry.16 The purpose of this work was to determine the limits in gas/vapor and liquid flow that allow for stable countercurrent operation in a monolith reactor with square channel geometries. In this context, we investigated the impact of different parameters on the flooding behavior. The main foci of the study were the liquid properties, the channel dimensions, the column dimensions, and the stacking of monolith blocks. It was the overall goal to combine the effects of the different investigated parameters in an engineering correlation useful for design and operation of countercurrent monolith reactors. FloodingsFundamentals and Correlations Despite the long history of experimental and theoretical investigations, flooding is still not completely understood. Large quantitative as well as qualitative discrepancies can be found in the literature. The complexity of the problem is based on the interactions between flow phenomena, geometry, and fluid properties. For flooding in single tubes, one of the most critical parameters is the geometry of the system. In addition to the shape and dimensions of the channel, the inlet and outlet geometries also have a significant impact. For smooth inlet and outlet sections, flooding is mostly initiated by flow phenomena that originate from the channel itself. The interaction at the gas-liquid interface leads to the accumulation of large waves that grow toward the bottom of the tube. At a critical gas flow, these semicircular-shaped waves block the open cross

section for the gas and initiate flooding.43 For largerdiameter channels (L > 50 mm), a somewhat different phenomenon is observed that is not of interest in this context.35 For nonsmooth or sharp-edged inlet and outlet sections, flooding is initiated at the tube ends.9 Two different phenomena have been detected. For low liquid flow rates, flooding starts at the liquid outlet through the entrainment of accumulating liquid by the gas phase. A churn-like flow pattern is developed in the lower part of the tube that moves with increasing gas flow toward the liquid inlet of the tube and results in total flooding. On the other hand, for high liquid flow rates, flooding is initiated at the liquid inlet. Because of the change in flow direction near the inlet, instabilities are introduced that can result in flooding. Liquid entrainment can enhance the entrance flooding, so that an interaction with the outlet geometry occurs. For capillary-sized channels (L < 5 mm), the situation is even more complicated, because of the effect of capillary forces. In these cases, total bridging of the liquid over the channel can occur, and a slug-like upflow is achieved.9 Indeed, Kalb and Smith13 could not establish a countercurrent flow regime for a capillary of diameter 4.57 mm for the fluid system water/air because of liquid bridging. However, for the fluid system ndecane/air and special liquid inlet and outlet sections, Lebens et al.15 were able to facilitate countercurrent flow in a 3.5-mm-diameter tube over a wide range of liquid velocities. To describe flooding, two fundamentally different approaches are applied. On one hand, the problem can be directly assessed from the underlying physics (e.g., stability analysis of traveling waves12), the so-called analytical models. On the other hand, a more empirical approach can be taken to evaluate flooding performance. In the context of this work, we restrict ourselves to the empirical correlations because of the complexity of the channel geometry as well as the inlet and outlet sections. The general idea of the flooding correlations is to summarize the flow, fluid, and geometric properties in a limited number of parameters, which are not always dimensionless. The empirical correlations are determined as the functional relationship between the different groups of parameters. Quite often, the parameters have some fundamental basis describing the physical phenomena (e.g., force balances). (a) Packed-Bed Flooding Correlations. Flooding in distillation columns is an area of interest for many researchers because of its industrial relevance. Common descriptions7 correlate the capacity parameter

CG )

uG0FG0.5

(ms)

(FL - FG)0.5

(1)

as a function of the flow parameter

FLG )

()

uL0 FL uG0 FG

0.5

(2)

and illustrate the result in a capacity plot. The flow parameter is a measure of the ratio of the inertial forces of the liquid and gas phases. The capacity parameter is an adapted parameter. In its original definition, it included the gravity constant and a length dimension in the denominator, describing the ratio between the

Ind. Eng. Chem. Res., Vol. 41, No. 26, 2002 6761

inertial and buoyancy forces in the gas phase.36 This kind of description has proved to be very valuable for distillation applications involving heavy vapors, where the momentum exchange between the phases is the dominant factor initiating flooding. Obviously, for each packing geometry, a separate curve is obtained in the capacity plot because of the lack of geometric information in the capacity and flow parameters. Furthermore, only the density as a fluid property is included in the parameters. For the original flooding correlation in packed beds,28 the same flow parameter but a different capacity factor were used, including a packing parameter (FP), the viscosity of the liquid, and the ratio of the densities of water and the applied liquid5,18,20,28

( )

uG02FGFPηLm FW CF,m,k ) FLg FL

k

[(Pa‚s)m], k ) 0, 1, 2 (3)

It is worth mentioning that the various researchers used different definitions of the packing factor, based on the type, shape, and geometry of the packing. Also, as indicated by the values of the exponent k, the impact of the density ratio was addressed differently. The value of the exponent m describing the dependency on the liquid viscosity typically ranges around 0.2. Recently, a quite detailed description and summary of flooding correlations for packed columns was presented by Piche´ et al.24 They applied artificial neural networks to describe flooding in packed beds and developed a correlation based on six dimensionless groups. (b) Flooding in Single Tubes. Many of the flooding correlations in single tubes are based on the work of Wallis,36 who defined the flooding boundaries as a function of modified gas and liquid Froude numbers

Fr/Gs1/2+ mwFr/Ls1/2 ) cw

(4)

Usually, mw lies around unity, and cw varies between 0.725 and 1 depending on the geometries of the inlet and outlet section. The modified Froude numbers describe the ratio between inertial forces and the buoyancy

Fr/Gs )

FrGs

1/2

) uGs

[

FG

] [

Fr/Ls ) FrLs1/2 ) uLs

, FL

gdh(FL - FG)

]

1/2

(5)

The correlation in eq 4 describes the force balance between the momentum flux exchanged between the gas and liquid phases and the buoyancy as the driving force for gas flow. Considering a single geometric configuration (dh ) constant), the functional relationship of eq 4 can be transformed into the capacity plot of the groups defined by eqs 1 and 3,32 indicating that the two descriptions contain equivalent information

xdhgcw CG ) (1 + mwxFLG)2

) 0.2

(7)

(1 + mwxFLG)4

ηL

For large-diameter tubes, where the characteristic dimension of the two-phase flow does not coincide with the hydraulic diameter of the tube, a geometric parameter representing the amplitude of surface waves

lc )

[

σ g(FL - FG)

]

1/2

(8)

similar to a Taylor instability can be used instead to describe the behavior of the liquid film.27 This length can be implemented in the modified Froude number (eq 5), resulting in the Kutateladze number

KuGs ) uGs

[

]

1/4

FG2

gσ(FL - FG)

,

KuLs ) uLs

[

FL2

]

gσ(FL - FG)

1/4

(9)

which represents the ratio of the inertial forces and buoyancy to the surface tension forces. The flooding correlation then becomes

KuGs1/4 + mkKuLs1/4 ) ck

(10)

with the two constants mk and ck that can be adapted to the application. The correlation including the Kutateladze number is very useful in describing flooding in large-diameter tubes, where the diameter of the tube cannot be used as a characteristic length scale for the liquid film. One shortcoming of the original Wallis correlation is its inability to include liquid viscosity effects. Therefore, Wallis modified eq 436 and introduced a liquid factor NL, defined by

[dh3g(FL - FG)FL]0.5 NL ) ηL

(11)

to include the effects of viscous flow in the liquid phase. The flooding correlation is then modified to

1/2

gdh(FL - FG)

cw4

CF

2

(6)

A similar transformation can be performed using eq 3 instead of eq 1 under the assumption of similar liquid viscosity36

Fr/Gs1/2 + m/w

( ) Fr/Ls NL

1/2

) cw

(12)

exchanging the inertial force of the liquid phase for the viscous force in the balance. A value of 5.6 was proposed for the constant m/w and values of cw similar to those used for the standard correlation (eq 4). Another approach was taken by Zapke and Kro¨ger.40 They suggested that the right-hand side of eq 4 be multiplied by a dimensionless factor ZF to include the effects of fluid properties

ZF )

xdhFLσ 1 ) Oh ηL

(13)

This factor is raised to powers of 0.014-0.05 depending on the channel inclination. For vertical channels, a value of 0.05 is used. The factor ZF is the reciprocal value of the Ohnesorge number, which describes the ratio of the

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viscous force to the product of surface tension and inertial force. Further research of Zapke and Kro¨ger41,42 suggested that the gas Froude number (eq 5) be related to the liquid Froude number (eq 5) and the Ohnesorge number (eq 13) to correlate their data for different liquids, with the best fit being obtained by applying a product of the liquid Froude number raised to the power of 0.2 with the Ohnesorge number raised to the power of 0.3 as independent variables, i.e.

FrGs ) f(FrLs,OhLs) ) f(FrLs0.2OhL0.3)

(14)

Indeed, eq 14 correlated flooding data from various sources over a wide range of liquid fluid properties very well. McQuillan and Whalley19 proposed a flooding correlation based on the original work of Alekseev et al.1 including liquid viscosity and surface tension effects

(

/ -0.22 1+ KuGs ) 0.286Bd0.26FrLs,A

with the modified Froude number

[

uLsdh g(FL - FG) / ) FrLs,A 4 σ3

]

)

η ηW

-0.18

(15)

cell density (cells/in2) diameter (m) void fraction (%) surface/volume ratio (m2/m3) hydraulic diameter (mm)

(16)

(17)

The square root of the Bond number is also known as the dimensionless tube diameter.36 The variety of descriptions proposed illustrates the difficulty of combining all of the experimental flooding information, which is indeed not possible. In general, the correlations and plotting parameters represent the relations among the inertial, viscous, buoyancy, gravitational, and surface tension forces in different ways. Depending on the conditions of an individual application/experiment, the impact of certain forces can be neglected and, therefore, not have to be considered in the description. Overall, it is evident that data from each source have to be treated separately because of the overlapping and dominating effects of the experimental setup, geometric differences, and definitions of the flooding point. This also implies that, for design purposes, experimental data from systems close to the configurations of interest are necessary. However, for such data, the experimentally determined correlations or plotting parameters can usually be applied successfully. Experimental Section Materials. The experiments were performed with monoliths of ceramic material (Cordierite) with a material porosity of roughly 30%. A summary of the geometric properties and a pictorial illustration of the monoliths are provided in Table 1. All monoliths had channels with square cross sections. The monoliths with diameters of 95 and 101 mm had lengths of 380 mm for the single-monolith experiments and 190 mm for the stacked-

25 0.043, 0.102 67 654 4.12

50 0.043, 0.095 68 939 2.91

100 0.095 69 1312 2.10

Table 2. Fluid Properties at 25 °C and 1.015 bara39 surface tension (mN/m)

dynamic viscosity (cP)

density (kg/m3)

capillary lengtha (mm)

18.0 23.4 26.2 73.6 -

0.29 0.86 2.11 0.91 0.0185

656 728 758 997 1.185

1.67 1.81 1.88 2.74 -

hexane decane tetradecane water air a

3 1/4

as defined by Alekseev et al.1 The Bond number describes the ratio between gravity and surface tension forces

dh2g(FL - FG) Bd ) σ

Table 1. Geometric Properties of the Monoliths

Based on eq 8.

monolith experiments. The monoliths with a diameter of 44 mm had a length of 500 mm. Table 2 summarizes the fluid properties at 25 °C and 1.015 bara. Pressurized air was used in all experiments as the primary gas phase. The experimental setup did not allow for all experiments to be performed at constant temperature. Therefore, the temperatures at the inlet and outlet of the monolith test section were continuously measured, and the actual fluid properties using the average temperature of inlet and outlet were determined according to Reid et al.25 The necessary reference data were obtained from Yaws.39 Furthermore, it was assumed that the gas phase was totally saturated by the corresponding vapor of the liquid phase. Therefore, the gas flow rate and the gas-mixture properties were adjusted accordingly. The change in liquid flow rate due to evaporation was negligible in all cases. Experimental Setup and Procedures. The monoliths were tightly mounted in larger tubes. The gap between the monolith and the tube wall was filled with Teflon and Gore-Tex tape and then sealed with hybridpolymer-based glue to prevent liquid bypass. The liquid was circulated with a gear pump (Micropump, Series 223), and the flow was measured with a turbine flow sensor (Digi-Flow Systems, DFS-2 and DFS-3, (1% of full scale). The liquid was distributed using six different spray nozzles (Spraying Systems, FullJet series) to cover the entire liquid flow range of interest. Three of the nozzles were used for the small- and large-diameter monoliths. The nozzle height was adjusted to give a uniform distribution of the liquid.10 The gases were supplied with different mass flow controllers (Brooks Instruments 5853S and 5860S, (1% of full scale) depending on the range considered. The pressure drop was measured via three different differential pressure transmitters (Cole and Parmer Ashcroft, (1% of full scale) in parallel, allowing for an accurate measurement over the entire range. Data acquisition and control of the setup were performed with a personal computer running Labview (National Instruments) process control software. The features of the experimental setup are summarized in Figure 2.


Figure 3. Outlet device for 100-mm-diameter 25 cpsi monolith (left side) and 44-mm-diameter 50 cpsi monolith (right side).

Figure 2. Experimental setup: (a) monolith test section with outlet device, (b) gas distributor, (c) liquid tank, (d) liquid pump, (e) recycle valve, (f) liquid strainer, (g) liquid flow meter, (h) spray nozzle, (i) gas mass flow controller, (j) pressure drop transducer.

Before each experiment, the porous monolith structures were sufficiently wetted with a high liquid flow for a period of 30 min. This procedure ensured that local wetting differences were minimized. The flooding limits were determined by the change in slope of the pressure drop as a function of the gas flow rate. Preliminary investigations showed that these transitions coincided with visual observations of liquid plugs shooting from the top face of the monolith. At the same time, an increase in the noise level was recognized. Therefore, at a given liquid flow rate, the gas flow rate was increased in steps. After each step, a stabilization period of 15 s was followed by a measurement period of 15 s. The pressure drop over the measurement time was averaged, and the standard deviation calculated. These measurements were performed over a wide range of liquid flow rates. Results and Discussion Effective Liquid Drainage. The flooding performance in capillary-sized channels is highly dependent on the drainage of the liquid at the outlet. For single channels, different kinds of beveled or funnel-shaped outlets were successfully used to drain the liquid.15 Similar outlet sections were used for tube bundles.17 For monoliths, an interesting alternative was developed employing metal plates with drip points (Figure 3), which makes use of the defined geometry of the monolith to connect properly to the walls.16 In Figure 4A, the flooding performance for a very low liquid flow rate with and without the outlet device is compared. A 3-fold increase in gas velocity required to reach the flooding point was found when the outlet device was

Figure 4. (A) Pressure drop as a function of superficial gas velocity for a very low superficial liquid velocity of uLs ) 0.3 cm/s (b) with and (0) without outlet device, 25 cpsi, L ) 102 mm, water/ air. (B) Outlet device monolith connection and liquid drainage: (a) liquid phase, (b) channel core with gas phase, (c) channel wall, (d) outlet device.

used. The geometry of the outlet device led the liquid away from the monolith outlet to the drip points and allowed the liquid to drip off, as indicated in Figure 4B. Thereby, blockage of the gas passages at the exit of the monolith can be prevented. For higher liquid flow rates, operation without the outlet device was not at all feasible.

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Figure 5. (A) Transition from wavy film flow to slug-flow regime during flooding. (B) Pressure drop and fluctuations as a function of superficial gas velocity for low and high liquid velocity with indication of flooding point: 9 uLs ) 0.4 cm/s, b uLs ) 4.0 cm/s; 25 cpsi, L ) 43 mm, decane/air.

Because of the results of these initial investigations, an outlet device was used for the rest of the experiments. All outlet devices were built similarly, despite the spacing of the grid, which was adjusted correctly according to the cell spacing of the corresponding monolith. The diameters of the monoliths were chosen to accommodate the available outlet devices. The detrimental impact of outlet sections on the flooding performance of packed beds is a well-known problem. In fact, for packed beds with dumped packings, special outlet section designs are applied to prevent flooding prior to the limits of the packing itself.33 Pressure Drop Fluctuations. In the capillary-sized channels of the monolith, flooding represents the transition between film flow or wavy film flow and the slugflow regime (Figure 5A). This transition leads to an increase in the pressure drop (Figure 5B), as the hydrostatic pressure drop for the upward transport of the slugs and the additional friction in the liquid phase have to be overcome. Under nonflooding conditions, the pressure drop increases nearly linearly with increasing gas velocity, as expected for laminar film flow. Obviously, film flow is a very steady flow regime, resulting in small pressure drop fluctuations. This is different for the slug-flow regime, which is transient in nature. Therefore, larger pressure drop fluctuations are observed in the flooding region. The pressure drop fluctuations observed for the experiments with the largerdiameter monoliths under flooding conditions were

Figure 6. Effect of cell density and column dimensions on flooding limits for (A) decane/air and (B) water/air: b/[ 25 cpsi, L ) 43 mm, runs 1 and 2; O/4 25 cpsi, L ) 102 mm, runs 1 and 2; 9 50 cpsi, L ) 43 mm; 0 50 cpsi, L ) 95 mm.

smaller in magnitude than those observed for the smaller-diameter monoliths. This is most likely a result of the interaction between the larger number of channels dampening and averaging the pressure drop response of the monolith system during flooding. However, qualitative differences in the fluctuations between the flow regimes were still detectable even for the largediameter monoliths. Column Diameter. The scale-up of monolith reactors should be rather straightforward, which is one of the interesting aspects of this reactor concept. Basically, a monolith can be seen as many single identical channels in parallel. Assuming a rather uniform distribution of the phase and no temperature gradients, the physical and chemical characteristics of one channel should represent those of the entire monolith. To evaluate the scale-up behavior, flooding for two different monolith sizes was determined (Figure 6A). The area of the larger-diameter (approximately 100 mm) was roughly a factor of 5 greater than the area of the smaller one (L 43 mm). Fair agreement in flooding performance was achieved for the different column dimensions. Especially for low liquid velocities, the flooding limits coincided. For the higher liquid velocities, the flooding limits for the largerdiameter monoliths are somewhat lower. For these cases, the liquid drainage is quite important, and good alignment between the monolith and the outlet device


Figure 7. Effect of channel dimensions on flooding limits: 4 25, 0 50, and O 100 cpsi; L ) 95 and 102 mm, decane/air.

is necessary (Figure 4B). However, the tolerances of the materials make it difficult to obtain perfect alignment on a larger scale. Therefore, it seems plausible that the flooding limits for the larger monoliths deviate to somewhat lower values for the higher liquid flows. In fact, this effect is more pronounced for monoliths with higher cell densities, because the larger number of channels and the thinner monolith walls make the alignment considerably more difficult. Similar trends concerning the column diameter are found for aqueous liquids (Figure 6B). In Figure 6, duplicate runs are also included. For these experiments, the setup was totally reassembled, and slight modifications were made. Despite these changes, good agreement was found between the different data sets, indicating good reproducibility. Channel Dimensions. The channel diameter of a monolith is linked to the cell density and the wall thickness. For similar void fractions, smaller-diameter channels result in higher surface-to-volume ratios, which has a significant impact on the mass-transfer and reaction performance parameters. On the other hand, the smaller channels also lead to a larger restrictions, which influences the flooding performance. Therefore, the flooding boundaries for three different cell densities were determined (Figure 7). A very strong effect of the channel diameter on the flow transitions was detected. Indeed, with the current arrangement, for a cell density of 100 cpsi, the operating window was rather restricted to very low liquid flows. The impact of tube or channel dimensions has been the topic of many previous investigations. The general trend of the current results is in good agreement with the literature data.35 For capillary-sized channels, the diameter effect seems to be most pronounced.9 Apparently, the bridging phenomenon and the plugging of channels and/or the exit section can explain the strong dependency of the flow transitions on the channel dimensions. Liquid Properties. Dynamic viscosity and surface tension are the two parameters of major interest in terms of the liquid properties. Therefore, a set of liquids was tested to evaluate the effect of these parameters separately (Figure 8). The results for decane and tetradecane allowed the effect of liquid viscosity to be studied. An increase in dynamic viscosity led to lower flooding limits, which can be explained by the thickening of the liquid, film

Figure 8. Effect of liquid properties on flooding transitions for (A) 25 cpsi, L ) 102 mm and (B) 50 cpsi, L ) 95 mm: 9 decane/ air (22.5 °C), 0 tetradecane/air (25 °C), O water/air (17.5 °C).

resulting in a restriction of the space available for the gas phase. This is in good agreement with literature sources.2,3,28,40 A comparison of the data sets of decane and water gives an idea of the impact of surface tension. In fact, higher surface tensions reduced the flooding limits significantly. In the literature, opposite effects of surface tension have been described. However, the differences can be explaind by a detailed evaluation of the results. For large tubes, where waves on the liquid film are an important factor in the initiation of flooding, an increase in surface tension results in an improvement in the flooding performance, which is also in good agreement with analytical models.2 On the other hand, for small-diameter tubes, liquid bridging, which is enhanced by increased surface tension, becomes important. Furthermore, the liquid drainage of the monolith is very important (Figure 3). Therefore, the strong impact of the surface tension on flooding also reflects the problem of draining the liquid at the outlet. These strong effects of the capillary forces are also documented in the literature for high-surface-area metal gauze packings through a comparison of the flooding performance of water and an organic liquid.32 Flooding Correlations. The effects of many different parameters have been discussed in the sections above. It will be of great value to summarize this information in terms of the minimum number of dimensionless quantities needed to capture the physical phenomena and geometric effects. Two different forms

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Figure 9. Flooding transitions as a function of the standard gas Wallis parameter (eq 5) and (A) the liquid Wallis parameter (eq 5) modified by the liquid factor (eq 11) and (B) with additional adaptation by the Bond number (eq 17): 9 decane/air, 0 hexane/ air, O tetradecane/air, 4 water/air; 25 cpsi, L ) 102 mm.

of flooding descriptions are evaluated on the basis of (1) the Wallis parameters modified by the liquid factor (eqs 5 and 11), and (2) the flow and capacity parameters (eqs 2 and 3). To simplify the analysis, it is useful to separate the effects of the different parameters. Therefore, the liquid properties are considered first. In the plot of the Wallis correlation modified by the liquid factor (Figure 9A), the data for the hydrocarbons clearly follow a trend, indicating a good description of the viscosity effects. As already observed, the aqueous medium shows a strong deviation toward lower flooding limits because of its higher surface tension. Apparently, capillary forces become important for the small channel sizes of the monolith in combination with the inherent liquid drainage issues. The Bond number (eq 15) is a dimensionless factor taking into account the ratio between surface tension forces and gravity, which are indeed critical forces for the slugging process in the capillary-sized channel. Adapting the modified liquid Wallis factor with the Bond number shifts the flooding limits of the water/ air system toward the trend of the flooding data of the hydrocarbons (Figure 9B). Other investigators37 have also reported the importance of the Bond number for flooding in smaller-sized channels.2 In addition to the effects of the liquid properties, it is important to evaluate the correct description of the geometric effects

Figure 10. Flooding limits of the monoliths (A) as a function of the Wallis parameters (eq 5) adapted with the liquid factor (eq 11) and Bond number (eq 17) and (B) as a function of the packedbed flooding parameters (eqs 2 and 3) adapted with the Bond number (eq 17): 9 hydrocarbons/air, 25 cpsi; 0 water/air, 25 cpsi; b decane/air, 50 cpsi; O water/air, 50 cpsi; [ decane/air, 100 cpsi. The line represents the best fit of lower boundary, all column diameters.

by the flooding parameters (Figure 10A). Overall, the data are represented well by the applied parameters, but a large spread is experienced for larger values of the modified liquid Wallis parameter. The data for the 100 cpsi monolith is less consistent. Two different trends are apparent: one representing the hydrocarbon data (solid symbols) and one for the data with water as the liquid (open symbols). The general trend for the data representing the aqueous medium tends toward somewhat lower flooding limits and shows a larger spread. Apparently, even with the modification by the Bond number, the capillary effects are not fully captured. In Figure 10B, the experimental data are plotted using the classical flooding parameters for packed beds.28 The liquid capacity parameter on the ordinate was modified by the Bond number to include the effect of the capillary forces. Without this adjustment, a much wider spread was obtained, similar to the results showed for the Wallis parameter plot (Figure 9A). Most of the data follow a general trend, despite the results for the 100 cpsi monolith with decane/air as the fluid system and the 50 cpsi data for water/air. The capillary length (eq 8) is a parameter describing a critical length dimension for the fluid properties, which, for example, can be related to the amplitudes of waves or the sizes


of droplets. Now, comparing this critical dimension of the liquid (Table 2) for the two data sets with the hydraulic diameter as a characteristic dimension of the channel geometry (Table 1), one can see that these values are very similar (within approximately 10-15%). Therefore, additional effects related to the drainage or slugging of the liquid can be expected for the two deviating data sets and might explain the different behavior. Thus, for further analysis, these data sets were discarded. The Wallis parameter and capacity plots can be used to illustrate and summarize the data. This truth is, to a large extent, related to the fact that the two descriptions can be transformed into each other, as shown earlier. However, in both cases, it is necessary to include the Bond number with the liquid parameters to take the strong capillary effects into account. For water as the fluid system, a larger spread over the different data sets is experienced. The capacity plot is a well-known form of illustrating flooding performance in chemical engineering applications and is therefore the preferred form of description. A fit of the lower envelope of the experimental data is included in Figure 10B and is functionally described by

Figure 11. Comparison of flooding limits of a single monolith with different stacking options: 9 single monolith, 380 mm,; [ two aligned monoliths, each 190 mm; O two misaligned monoliths, each 190 mm; 4 four misaligned monoliths, each 190 mm; 25 cpsi, L ) 102 mm, decane/air.

CF,0.25,0 ) (204.4 + 297.1FLG1.5 Bd 171.4e-FLG)-1 for

dh > 1.15 (18) lc

Describing the lower boundary of the data is a conservative approach to ensuring a certain saftey margin for stable operation. In the case of monoliths, the packing factor is defined by the S/V ratio and the void fraction

FP )

S/V 2

(19)

Furthermore, it is important to note that, because of the large impact of the outlet geometry, the flooding correlation summarized in eq 18 is valid only for the specific outlet section applied in this work. Stacking of Monoliths. For industrial applications, monolith blocks larger than 0.5 m in length will be difficult to achieve because of manufacturing and assembly limitations. Therefore, in practice, monolith reactors will consist of many monolith sections stacked together. Basically, two arrangements of stacking are feasible. On one hand, monoliths can be put together with the channels aligned, so that each channel feeds into a subsequent channel below. Alternatively, the monolith blocks can be misaligned, bifurcating the flow from one channel into multiple subsequent channels. Both cases of aligned and misaligned stacking were experimentally evaluated (Figure 11). The aligned stacking of monoliths does not influence the flooding limits. Essentially no additional resistance impacting the flooding behavior is introduced. However, misalignement of the channels leads to a reduction of around 15% in the flooding gas velocity for a fixed liquid velocity. Apparently, the disruption of the falling film and the partial blocking of the channels at the interface between two monoliths enhances flooding. Similar levels of performance were determined for two and four stacked monolith pieces, which leads to the conclusion

Figure 12. Flooding and deflooding behavior based on pressure drop changes (closed symbols for increasing and open symbols for decreasing gas flow): 9/0 uLs ) 0.65 cm/s, b/O uLs ) 4.75 cm/s; 25 cpsi, L ) 43 mm, decane/air.

that the detrimental effect on flooding does not scale with the number of stacks, but is rather a one-time effect. Hysteresis Phenomena. In addition to flooding, deflooding is also an important parameter for the operation of a countercurrent column. Under certain conditions, the deflooding transition observed with decreasing gas flow rate deviates from the flooding boundaries observed with increasing gas flow rate. Figure 12 illustrates the flooding and deflooding behavior for two different liquid velocities. For the low liquid flow rate, the flooding and deflooding points coincide. For the higher liquid velocity, a hysteresis phenomenon is observed, and the column defloods at a much lower liquid flow rate. Visual observations for higher liquid flow rates revealed coalescence of the liquid at the outlet section under flooding conditions, rather than the rainlike dripping of the liquid from the outlet device observed under regular operation. Apparently, this coalescence process leads to the blocking of certain channels for the gas phase and therefore makes it difficult to transfer back to nonflooded operation when the gas flow rates are decreased. The dependency of the

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Figure 13. Flooding and deflooding limits for (A) decane/air and (B) water/air (closed symbols for increasing and open symbols for decreasing gas flow): b/O 25 cpsi, L ) 43 mm; 9/0 50 cpsi, L ) 43 mm; 2/4 25 cpsi, L ) 102 mm.

hysteresis on the liquid flow rate is in good qualitative agreement with the observations of previous investigations on single tubes.3,29 On the other hand, previous work with tube bundles did not show any hysteresis,17 which might be due to the lower liquid flow rates applied in the experiments, thicker walls allowing better liquid drainage at the outlet, and differences in the outlet section itself. Additional experiments were performed for different liquids and different channel and column dimensions, yielding more insight into the hysteresis phenomenon. For the 25 cpsi monolith, the hysteresis area was restricted to rather high liquid velocities with decane/ air as the fluid system (Figure 13A). With smaller channel diameters or a higher cell density (50 cpsi), the shift of the deflooding limits to lower gas velocities occurred over a much broader range. In fact, for the higher liquid flow rates, the column did not deflood, even with a total reduction of the gas flow. In that case, the liquid flow rate had to be reduced to regain stable operation of the column. In Figure 13B, the behavior of a 25 cpsi monolith is shown for the fluid system water/ air. In comparison with the results for decane/air, a much larger region of hysteresis is observed. For the small-diameter monolith, even at low liquid flow rates, the deflooding limit shifted to lower values, and for higher liquid velocities, deflooding due to the reduction of the gas flow rate was not feasible at all. A consider-

Figure 14. Flooding performance of different packings as a function of reactor based gas and liquid velocity. Monolith and Berl saddles: n-decane/air. Katapak-S structures: water/air.

ation of the impact of the channel size and the surface tension leads to the conclusion that capillary effects are of major importance for the hysteresis phenomenon, enhancing the coalescence effect described above. For larger-diameter monoliths, the hysteresis is less pronounced, especially at lower liquid velocities (Figure 14B). This is in good agreement with the smaller pressure fluctuations that were observed for the largerdiameter monoliths. Apparently, the larger number of channels dampens the effect of local nonuniformities to a certain degree and, therefore, leads to an easier transition between the flow regimes. Comparison with Other Packing Options and Industrial Relevance. In the sections above, the flooding behavior of monoliths has been extensively discussed. Now, it is of major interest to evaluate the results in comparison with alternative packing options. The main focus of applications for the countercurrent monolith reactor is three-phase catalytic processes. For a comparison, the packings should have similar solid fractions representing the catalyst needed to promote the reaction. Unfortunately, only limited data for the alternative packings are available, which does not allow for a perfect comparison. Under these limitations, three different kinds of packings were compared in terms of their flooding performance and geometrical properties (Figure 14): (1) monoliths with different channel sizes and channel shapes; (2) Katapak-S structure, a corrugated wire mesh packing with alternate sections filled


with catalyst particles (dp ) 1 mm); and (3) a conventional packed bed filled with 1/2-in. Berl saddles. The performance of the different packings can be divided in three different regions. The 25 cpsi monolith showed, by far, the highest flooding limits. In an intermediate region were the Katapak-S structure, the 50 cpsi monolith, and the internally finned monolith. As expected, the dumped packed Berl saddles showed the lowest flooding performance compared to the structured packings. Focusing on the intermediate region reveals some interesting details. The Katapak-S structure and the 50 cpsi monolith showed similar behaviors, with somewhat better flooding performance at lower liquid velocities for the wire-mesh packing. Keeping in mind that a 10% difference in void fraction exists between the monolith and Katapak-S structures, the data sets are rather similar. However, the S/V ratio of the 50 cpsi monolith is about 2.5 times larger than that of the Katapak-S structure. The S/V ratio has a significant impact on the mass-transfer behavior and is therefore an important parameter for the catalytic performance. In fact, even the 25 cpsi monolith with significantly higher flooding limits has an 85% higher S/V ratio. The internally finned monolith shows a somewhat different trend. Its flooding performance was better at higher liquid flows but worse for lower ones. Because typical applications (trickle bed conditions, uL0 < 10 mm/s) operate at the lower end of the liquid velocity range, this is somewhat of a disadvantage. Finally, the 1/2-in. Berl saddles were compared with the 25 cpsi monolith. For similar geometric properties (S/V ratio, void fraction), the flooding limits of the monolith were more than 4 times higher for the lower end of the liquid flow range and more than 15 times higher for the higher-end liquid velocities. This clearly indicates the tremendous broadening of the operating window for the structured arrangement of the catalyst. In addition to the comparison with other packings, it is even more important to evaluate the flooding performance of the monolith in light of the requirements of the application of interest. The capacity plot in Figure 15A compares the experimental data for the 25 and 50 cpsi monoliths with the operating windows for reactive distillation4 and stripping applications.14 The requirements for stripping can be fulfilled by both monolith structures. For the very demanding needs for reactive distillation, because of the higher gas-phase density of the vapors involved, the larger channel size 25 cpsi monolith covers a much broader operating range. Another area of application is that of hydrotreating processes (HDS/HDA), to boost the conversion through the stripping of reaction-rate-inhibiting (HDS) or equilibrium-limiting species (HDA). The flooding correlation fitted to the experimental data (eq 18) is used for illustration of the lower boundary of the operating window (Figure 15B) for monoliths in countercurrent flow. The requirements of the different applications with a specific monolith geometry are determined and compared to the operating window.8,14 The considered hydrotreating operations are performed at rather high temperatures (550-650 K). Even for the high pressure of the operations (>20 bar), it is necessary to adjust the density of the gas phase for the vapor content. For this comparison, the gas-phase density was increased by 10 kg/m3 while the gas velocity was kept constant. This

Figure 15. Capacity plots comparing flooding limits of monoliths with (A) reactive stripping and catalytic distillation [9 25 cpsi monolith, decane/air; 0 50 cpsi monolith, decane/air; lines represent best fit; (I) working area for reactive stripping,14 (II) working area for catalytic distillation4] and (B) hydrotreating processes [line represents flooding correlation (eq 18), and symbols represent requirements for hydrotreating processes with 25 cpsi (closed symbols) and 50 cpsi monoliths (open symbols); 9/0 large hydrotreater,14 [/] HDS unit,8 2/4 HDS unit].

increase in density is typical for the operation of a hydrotreater at 40 bar and 600 K. The requirements for all applications23 with both 25 and 50 cpsi monoliths are lower than the lower boundary of the operating window. Even the demanding needs of a large hydrotreater14 with a 50 cpsi monolith enjoys a nice safety margin against flooding, and even here, an increase in gas flow rate is possible. This is beneficial, in the case of HDS applications, in decreasing the gas concentration of H2S stripped out of the liquid phase. On the other hand, for HDA applications, a higher gas flow results in a higher average partial pressure of hydrogen over the reactor length, which is beneficial in overcoming equilibrium limitations. Obviously, the applications considered here provide only an indication of the applicability of monoliths in countercurrent operations. However, a wide range of requirements of applications is covered, and therefore, the channel size of 2.9-4.1 mm seems to be more than sufficient from a flooding perspective, especially for hydrogenation and stripping applications with low vapor content in the gas phase. In the context of this work, we concentrated on the flooding performance of monoliths under cold flow

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conditions. It is important to mention that, under reactive conditions, additional aspects (e.g., coking) impacting the hydraulic capacity of the monolith reactor might become relevant and need to be considered. However, the impact on the flooding performance, especially in comparison to other catalyst structures, cannot be assessed adequately at this point. Conclusions A film flow monolith reactor with separated passages for gas and liquid in the individual channels is a very suitable packing for countercurrent operations. A special outlet device was shown to drain the liquid effectively from the monolith section at the outlet. The limited momentum exchange between the phases leads to low pressure drops and high flooding limits. Hydrodynamic flooding in monolith reactors can be studied through the increase in pressure drop that occurs for increasing gas flow rate. The flow transition is accompanied by increasing pressure drop fluctuations, because of the transient nature of the upward transport of liquid slugs. The surface tension and size of the monolith channel have a very strong impact on the flooding performance. For higher surface tensions and smaller channels, flooding is initiated at lower gas flow rates. The combination of the two parameters indicates that capillary effects are important in small monolith channels. Higher viscosity of the liquid moderately lowers the flooding behavior. For higher liquid flows, deflooding occurs at lower gas flow rates. This hysteresis phenomenon is pronounced for smaller channels and liquids with higher surface tensions, indicating again the dominating effect of capillarity. The flooding behavior is rather insensitive to the column dimensions. The pressure drop fluctuations and the hysteresis effect are less pronounced for larger column diameters, because of the interaction of a larger number of channels. Misaligned stacking of monoliths lowers the flooding limits by approximately 15% as a result of the disruption of the liquid film and the blockage of channels at the interface between the monoliths. The stacking effect does not scale with the number of stacks. The flooding data for different geometries and liquids can be conveniently described by common flooding correlations. However, a modification with the Bond number is necessary to include the capillary effects. For the type of outlet device used in this work and the alignment of the outlet device with the channel walls, the experimental data can be described by a correlation based on the flow and capacity parameters

Acknowledgment The cooperation and the discussions with Dr. G. Y. Adusei during his assignment at Corning, Inc., are highly appreciated. Notation CF,m,k ) modified capacity parameter (eq 3), (Pa‚s)m ck ) parameter of Kutateladze correlation cw ) parameter of the Wallis correlation d ) diameter, m FP ) packing factor, m2/m3 g ) gravitational constant, m/s2 k ) exponent of density ratio in eq 3 L ) length, m lc ) capillary length (eq 8), m m ) exponent for the viscosity contribution in eq 3 mk ) parameter of the Kutateladze correlation mw ) parameter of the Wallis correlation m/w ) modified parameter of the Wallis equation S/V ) surface-to-volume ratio, m2/m3 u ) velocity, m/s Dimensionless Groups Bd ) Bond number (eq 17) CG ) capacity parameter (eq 1) FLG ) flow parameter (eq 2) Fr ) Froude number (eq 5) Fr* ) modified Froude number of Wallis (eq 5) / FrLs,A ) modified Froude number of Alekseev (eq 16) Ku ) Kutateladze number (eq 15) NL ) liquid factor (eq 11) ZF ) fluid factor (eq 13) Greek Letters ) void fraction F ) density, kg/m3 η ) dynamic viscosity, Pa‚s σ ) surface tension, N/m Subscripts A ) Alekseev 0 ) reactor-based B ) bed c ) capillary F ) fluid G ) gas h ) hydraulic L ) liquid s ) superficial W ) water w ) Wallis

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CF,0.25,0 ) (204.4 + 297.1FLG1.5 Bd 171.4e-FLG)-1 for

dh > 1.15 lc

Overall, a wide operating window is obtained for the 25 and 50 cpsi monoliths, However, the range for the 100 cpsi monolith is somewhat restricted. A comparison with the needs of countercurrent applications shows sufficiently high flooding limits for hydrotreating and reactive stripping operations. The requirements for catalytic distillations with a heavy vapor phase are most demanding and can be only addressed with the largerchannel-size monoliths.

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Received for review January 31, 2002 Revised manuscript received September 20, 2002 Accepted September 26, 2002 IE020089C

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