Znd. Eng. Chem. Res. 1991,30, 1270-1280
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Fundamentals of Adsorption; Santa Barbara, CA, May 4-9,1986. Gray, P. G.; Do, D, D. Adsorption and Desorption of Gaseous Sorbates on a Bidisparsed Particle with Freundlich Isotherm-Part II. Experimental Study of Sulphur Dioxide Sorption on Activated Carbon Particles of Various Geometries. Gas Sep. Purif. 1989,3, 210. Higashi, K.; Ito, H.; Oishi, J. Surface Diffusion Phenomena in Gaseous Diffusion (I). Surface Diffusion of Pure Gas. J. At. Energy SOC.Jpn. 1963,5, 846. Kondis, E. F.; Dranoff, J. S. Kinetics of Isothermal Sorption of Ethane on 4A Molecular Sieve Pellets Znd. Eng. Chem. Process Des. Dev. 1971, 10, 108. Kuro-oka, M.; Suzuki, M.; Nitta, T.; Katayama, T. Adsorption Isotherms of Hydrocarbons and C02on Activated Carbon Fibre. J. Chem. Eng. Jpn. 1984, 17, 588. Masamune, S.; Smith, J. M. Adsorption of Ethyl Alcohol on Silica Gel. AZChE J. 1966, 11, 41. Mayfeld, P. L. J.; Do, D. D. Adsorption of Methane and Ethane onto Activated Carbon using a Differential Adsorption Bed. Proc. Chemeca '88; Sydney, Aug 1988. Mayfield, P. L. J.; Do, D. D. Adsorption of Ethane, Butane and Pentane onto Activated Carbon using a Differential Adsorption
Bed. Znt. Symp. Gas Sep. Technol.; Antwerp, Sept 10-15,1989. Pame. H. K.: Studervant. G. A.: Leland. T. W..Znd. E M- . Chem. Fundam. 1968, 7, 363. Peel. R. G.: Benedek. A.: Crowe. C. M. A Branched Pore Kinetic Model for Activated Carbon Adsorption. AZChE J . 1981,27,26. Ross,J. W.; Good,R. J. Adsorption and Surface Diffusion of n-Butane on Spheron 6 (2700 "C) Carbon Black. J. Phys. Chem. 1966, 60, 1167. Schneider, P.; Smith, J. M. Adsorption Rate Constanta from Chromatography. AZChE J. 1968a, 14,762. Schneider, P.; Smith, J. M. Chromatographic Study of Surface Diffusion. AZChE J. 196813, 14,886. Villadsen, J.; Michelsen, M. L. Solution of Differential Equation Models by Polynomial Approximation; Prentice-Hall, NJ, 1978. Wicke, E.; Kallenbach, R. Kolloid 2. 1941, 97, 135. Yang, R. T.; Fenn, J. B.; Haller, G. L. Modification to the Higashi Model for Surface Diffusion. AZChE. J. 1973, 19, 1052. Yucel, H.; Ruthven, D. M. Diffusion in 4A Zeolite. J. Chem. SOC., Faraday Trans. 1980, 76,60. '
Received for review January 4, 1990 Accepted January 21,1991
Liquid Distribution in Trickle Beds. An Experimental Study Using Computer-Assisted Tomography Pierre G. Lutran and Ka M. Ng* Department of Chemical Engineering, University of Massachusetts, Amherst, Massachusetts 01003
Edward P. Delikat Mobil Research and Development Corporation, P.O. Box 1026, Princeton, New Jersey 08540
Liquid distribution in trickle beds with a quiescent gaseous phase was visualized by using computer-assisted tomography (CAT). The model system was made up of a column packed with uniform, nonporous glass spheres, distilled water, or a mixture of water and ethanol for a lower surface tension. Flow patterns at the bed scale were recorded as a function of various parameters-liquid flow rate, the size of the particles, the type of liquid inlet distributor used, and surface tension. The flow pattern was shown to depend strongly on whether the bed had been prewetted by flooding the column with liquid or was initially dry. Furthermore, the flow pattern a t a given liquid flow rate depended on whether it was obtained by decreasing or increasing the liquid flow rate to the present state.
Introduction Trickle beds can be defined as a fixed bed of catalyst particles, contacted by a gas-liquid, two-phase flow. The flow can be cocurrent (downflow or upflow) or countercurrent. Because of the absence of flooding, cocurrent downflow is the most common mode of operation in industry. Trickle-bed reactors are used primarily in the petroleum industry for hydrocracking, hydrodesulfurization, and hydrodenitrogenation. It is estimated that a significant fraction of the petroleum processed in a refinery passes through a trickle bed in one way or another. In view of its commercial significance,many studies have been performed to understand the performance of trickle beds. Of special interest is a better understanding of the distribution of the gaseous and liquid phases, for it controls other transport processes and thus the overall reactor performance. Phase distribution at the reactor scale is expressed in the form of various flow regimes-trickling, pulsing, bubble, and spray flows (Weekman and Myers, 1964; Sato et al., 1973; Charpentier and Favier, 1975; Ng, 1986; Ng and Chu, 1987). Most common in practice is trickling flow that occurs at moderate liquid and gas flow rates. In this regime, the liquid flows down the bed from particle to particle on the surfaces of the packings while the gas travels in the interstitial void space. The trickling regime can be further divided into two regimes. At suf0888-588519112630- 1270$02.50/0
ficiently low liquid flow rates, a fraction of the packings remain unwetted. This is the partial wetting trickling regime. If the liquid flow rate is increased, the partial wetting regime changes to complete wetting trickling regime in which the packings are totally wetted by liquid. In the trickling regime, some interesting flow features at the particle scale can he identified. The liquid holdup comprises films, rivulets, pendular structures, liquid pockets, and filaments (Figure 1). Films and rivulets are associated with a single particle, while other flow features involve two or more particles. A rivulet is a liquid stream flowing over the surface of a particle and can result from the splitting of a liquid film on the surface of a catalytic particle. The presence of liquid pockets and pendular structures is due to capillary force. While a liquid pocket extends over several pore chambers, a pendular structure resides at the contact point of two pellets. The shape of a liquid pocket is random and depends on the configuration of the packings at a given location within the bed. Filaments are liquid streams that flow down the bed in the channels between the particles. The lateral width of a filament can extend over more than one pore chamber. A filament can be viewed as a continuous string of liquid pockets. The relative amounts of these features are expected to vary with the gas and liquid flow rates, surface tension, wettability, the gas and liquid inlet distributors used,and the size and shape of the packings, among other 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 6, 1991 1271 Liquid Film
7
Rivulet
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Figure 1. Flow features in the trickling regime: a liquid film, a
rivulet, pendular structures, a liquid pocket and a filament. A film
can completely or partially wet the sphere. A rivulet can be con-
sidered to be a narrow film. While pendular structures reside at the coniact point between two pellets, a liquid pocket fills the interparticle pore space formed by more than two spheres. A filament is a stream of liquid flowing down the bed. It can be considered to be a string of liquid pockets.
physical parameters and operating conditions. Also, we expect that the flow pattern depends on whether the bed has been prewetted by flooding the column with liquid or is initially dry. In this study, we provide visual evidence and plausible explanations for some of these factors on the liquid pattern in a trickle bed with a quiescent gaseous phase.
Previous Investigations of Liquid Distribution Let us begin with a brief discussion of the previous studies on liquid distribution in trickle beds. Because of the absence of suitable experimental techniques, for a long time, studies of liquid distribution was limited to two macroscopic quantities-liquid holdup and wetting efficiency. Liquid holdup is defined as the volume of liquid per unit volume of bed. It can be divided into two parts: internal and external holdup. Internal holdup refers to the liquid held within the catalyst particles by capillary action and external holdup the interparticleliquid. A large number of correlations obtained primarily with weighing techniques have been developed for external holdup (Gianetto et al., 1978). Also frequently reported is the residual holdup, which is the fraction of liquid that remains after the reactor has been flooded with liquid and then drained. Another measure of liquid distribution is the wetting efficiency. It is the fraction of external surface area of the catalyst particles that is covered by liquid. Its importance lies in the fact that, for systems with a nonvolatile liquid reactant, the liquid reactant enters the partially wetted catalytic particle only through the wetted part, whereas the gaseous reactant finds it easier to enter through the nonwetted part of the catalyst. This leads to the interesting phenomenon of effectiveness enhancement, where a maximum value of effectiveness is obtained at an intermediate value of wetting efficiency between zero and unity (Harold and Ng, 1987;Funk et al., 1988,1989).Since direct experimental determination of wetting efficiency is very difficult, indirect techniques are generally employed. For instance, wetting efficiency is obtained by comparing the rate of reaction in a gas-liquid reactor with that in a
liquid-filled one. Comprehensive correlations are not yet available at this time. However, it is clear that liquid holdup and wetting efficiency are not expected to be uniform within a trickle bed randomly packed with catalyst particles. Indeed, very little is known about the actual flow pattern within the bed. This can be a serious omission as was demonstrated in the study by Christensen et al. (1986). They studied pressure-drop hysteresis observed in the cocurrent downflow of air and water under trickling flow regime. For given gas and liquid flow rates, the magnitude of the pressure gradient does not have a unique value. The observed maximum pressure drop can be more than twice of the minimum pressure drop under identical operating conditions. Christensen et al. (1986)concluded that the multiplicity of pressure drops, or equivalently hydrodynamic states, is caused by the existence of two different modes of flow-film flow and filament flow. The former leads to a higher pressure drop because of the more vigorous interactions between the gas and liquid phases. They were able to see these two different patterns through the Plexiglas walls of their column packed with 3-mm glass spheres. Their observation,however, was confined to the immediate proximity of the walls, and the interior of the bed remained unreachable. An interesting experiment was carried out by Melli (1989)to better understand the particle scale flow features. The experimental setup consists of circular rubber O-rings sandwiched between two transparent plastic plates. The O-rings were arranged in a square lattice rotated by 45' and were separated from each another by the same distance. Thus, the O-rings represent the solid particles in a trickle bed and the space between them the pore space. With the rotated square lattice, a pore chamber is bounded by four O-rings. Each pore chamber is connected to other chambers through four pore throats, which are represented by the shortest distance between two O-rings. Flow characteristicsin the individual chambers were recorded with a high-speed camera under various liquid- and gasflow rates. As expected, a pore chamber can have liquid films or is occupied mainly by gas or liquid. Melli concluded that flow pattern at the reactor scale, such as the different flow regimes, is a result of the microscopic flow characteristics at the particle level. This two-dimensionalmodel system offers the advantage of relatively easy flow visualization but has three obvious limitations: First, there are no particle-particle contact points for the formation of pendular structures. Second, there is a significant wall effect on the flow because of the presence of the two flat walls. Third, the pore structure in an actual packed bed is by no means uniform but varies from location to location within the bed. All these are expected to influence the flow features as well as the flow pattern. Indeed, flow pattern in a trickle bed is inherently three-dimensional. While a two-dimensional model can capture the essential physics, it does not provide the flow pattern in an actual trickle bed. The objective of this study is to achieve three-dimensional flow visualization within a packed column using computer-assisted tomography (CAT). Based on multiple X-ray measurements, CAT is a method using computer algorithms to reconstruct a tomographic image of an object. This study is limited to beds packed with nonporous spherical particles and under the condition of no gas flow. Similar experiments with a CAT scanner have been performed for oil-water flow in sandstones (Wang et al., 1984).
Experimental Setup The Plexiglas column was 30.48 cm high (Figure 2).
1272 Ind. Eng. Chem. Res., Vol. 30, No. 6,1991 Vertical Scan
\ ,.! 0
,
CAT Scanner
with Packings
Y
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Figure 2. Schematic of the trickle bed. The column consists of a liquid distributor, a body containing the packings, and a collecting tank for liquid.
Figure 3. Uniformity of liquid at the inlet distributor. Each vertical line indicates the fraction of the total flow rate in each of 16 equal-sized zones across the column.
The body had a square cross section with a width of 7.30 cm on the outside and 6.03 cm on the inside. Liquid entered at the top of the column through a liquid distributor, which was basically a plate in which 25 holes of 0.79-mmdiameter had been drilled. The 25 inlets were grouped in 5 rows of 5 inlets each separated from another or from the wall by 1 cm. To provide a uniform liquid input, the distributor operated in the following way. Initially, liquid accumulated inside the distributor chamber without any flow to the column below. The holes were sufficiently small that the capillary pressure was higher than the hydrostatic head. When the distributor chamber was almost filled with liquid, the pressure built up and forced the liquid evenly through the holes. The actual uniformity was determined with an empty column as follows: The crms section of the column was divided into 16 equal-sized squares. Liquid was then collected in each of the 16 zones at the bottom of the empty column. The fraction of the total input flow rate in each of these 16 zones was then determined at a given flow rate. Although each fraction was always around the expected mean value of 6.25%,the actual value for each zone varied in repeated runs at the same liquid flow rate and in runs with different flow rates. To demonstrate the degre of variability, Figure 3 shows the average fractions, as indicated by each of the 16 vertical lines,for several different liquid input flow rates representative of the actual values used in the visualization experiments. The wall that entered the CAT scanner first was marked as the front wall and the opposite wall the back wall (also see Figure 4). The average value for these runs falls within the two extremes of 3.81% to 9.22%. These data are also representative of the variability in a particular run. This degree of uniformity turned out to be quite sufficient for this study. As will be explained, the
Figure 4. Schematic of the CAT scanner. The column and the pump were sent into the scanner with a conveyor belt. The rectangle in dotted line represents a typical vertical scan to be taken within the scanner.
actual liquid distribution within the column was strongly influenced by other factors, in addition to the liquid distributor. The column was filled with inert and nonporous glass spheres, which were supported by a piece of wire gauze at the bottom of the body. This wire gauze itself was set on a piece of Plexiglas in the shape of a cross glued to the column. The wire gauze and its support led to the accumulation of a small amount of liquid at the bottom of the column in some experimental runs. But, as will be seen in the scans, the end effect was sufficiently small that it was not expected to affect the liquid distribution within the column. The height of the packing was about 19.05 cm. The lower part of the column was a square liquid tank with a width of 21.59 cm. Liquid was recirculated through the column with a reversible and reciprocating piston pump (Fluid Metering Inc., QD3; Figure 4). The pump operated at constant stroke and the flow rate was adjusted by controlling the amplitude of translation of the piston. It provided steady and reproducible flow rates. Glass spheres 3 and 6 mm in diameter were used to investigate the influence of the packing size on the flow features. In addition to water, a mixture of ethanol and water was also used to obtain a surface tension ranging from about 28 to 72 dyn/cm. The column and the pump were placed on a conveyor belt, which could be translated with a precision of 0.1 mm (Figure 4). In this study, vertical scans were taken every 3 mm from the front side of the body to the back side. Note that although such a vertical scan is depicted right on the column, scans actually took place at a fured position inside the CAT scanner. Also, a scan actually covered a circular area. Each vertical scan took 26 s to complete. The spatial resolution of the scanner depended on the absorption indexes of the materials and was approximately 0.5 mm in our experiments. This value was obtained based on our direct observation of 1-mm objects in a scan. With some imagination, a complete set of scans from the front to the back wall collectively provides a three-dimensional mental picture of the liquid flow pattern inside the column.
Experimental Procedure The experimental procedure was rather simple. First, barium bromide was added to water as a dopant to increase its absorption index. This was necessary because glass and water have almost the same absorption index for X-rays. With the dopant, water would appear darker than both
Ind. Eng. Chem. Res., Vol. 30,No. 6,1991 1273 air and the glass beads in the CAT scanner pictures. Then, with the water circulating a t steady state, vertical scans were obtained by sending the entire setup into the scanner. The position of each vertical scan was identified by its bench' position, with the inner front wall set at 0 mm. For most experimental runs, a full scan was taken, that is, pictures were taken every 3 mm (one sphere diameter) from the front wall to the back+, 3,6,9,12 mm and so on. However, for experimental runs where the flow patterns did not change much from front to back, only scans at 24,27,30, and 33 mm are reported. These pictures will be referred to as central scans, emphasizingthe fact that they were taken midway between the front and back walls. Note that bench position 30 mm corresponds to a row of five liquid inlets. Scans taken at bench positions other than these standard positions will be reported so. We used three liquid flow rates throughout the experiments, referred to as low, medium, and high. The low flow rate was 11.5 mL/s. This is equivalent to a superficial mass flow rate, a quantity commonly used in the reaction engineering literature, of approximately 11386 kg/m2 h. Because of the presence of the dopant, the liquid density is slightly higher than that of water used in the conversion. The medium and high flow rates were 23 mL/s (22752 kg/m2 h) and 33 mL/s (32672 kg/m2 h), respectively. The experimental runs were organized as follows: We began with two dry beds to show what the sphere pack looked like in the absence of liquid. Then, the liquid-flow rate was slowly raised to study its effect on flow pattern. After the column was flooded at a sufficiently high flow rate, the effect of prewetting on flow distribution was examined by gradually lowering the liquid flow rate. These same experiments were repeated for the 6-mm spheres. Next, we examined the contribution of each individual liquid inlet to the flow distribution. The effect of surface tension on flow pattem was explored with a watexwthanol mixture. Finally, the possible existence of more than one hydrodynamic state as a result of a different flow history was investigated.
Experimental Results Dry Beds. Figure 5 shows two dry packings-one with 3-mm spheres (Figure 5a) and the other with 6"spheres (Figure 5b). As mentioned, each scan covered a circular area, as depicted by the dashed circle we added to each picture. L and R indicate the left and right walls of the trickle bed as viewed from the front wall, respectively. The lens-shaped black area a t the bottom of the column was part of the liquid contained in the collecting tank. The black dots were the individual glass spheres. Note that in Figure 5a, it was darker at the lower part of the column than the top, despite the fact that it was a rather uniform packing. This indicates that the spatial resolution was not exactly the same across the entire scan. This picture was also a little too dark, but the clarity can be improved by adjusting the contrast. This was done for Figure 5b, where the liquid in the collecting tank appeared to be lighter than that in Figure 5a. Most of the 6-mm glass spheres could be clearly identified. Recall that the inner width of the square column was 6.03 cm, and thus the maximum number of spheres across the width was 10. Figure 5b indicates that there were on the averagejust about 10 spheres across the column. Also, notice that even the wire gauze supporting the spheres was visible. Typical Flow Pattern. A packing of 3-mm-diameter glass spheres, distilled water, and the uniform inlet distributor (25 inlets) were used. Also, the bed was nonprewetted, that is, the liquid was fed to a bed that was initially dry. Let us examine in detail Figure 6, which
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Figure 5. Dry beds: (a) 3-mmspheres and (b) 6-mmspheres. The spherical packings appeared as black dots. The circle in dotted line shows the actual size and shape of each vertical scan.
shows a full scan. The top half of the column in Figure 6a and the lower half in Figure 6t were not visible, indicating that the column was not perfectly vertical and the scan actually cut through part of the front and back walls. We checked the various parts of the column with bubble levels and found the deviation to be v e ~ small. y Therefore, this was not expected to significantlyaffect the observed flow features. The liquid flow rate was set at low. The tortuous filaments were clearly visible. Let us now move from the front wall to the back. Figure 6b-f shows that the filament at the center of the bed was about 12 mm wide from left to right, occupying approximately one-fifth of the column width of about 60 mm. Also, since each scan was 3 mm apart, it can be deduced that the filament was somewhat larger than 9 mm thick from front to back. Figure 6c-f shows that there was a filament near the.left wall. Further to the back wall, two filaments merged (Figure 6g,h) and then split (Figure 6ij) as they flowed down the column. Notice that these filaments were not vertical but turned slightly away from the front to the back wall. Similarly, the filament at the center of the column in Figures 6n-r meandered back and forth. Recall that there were 25 inlets arranged in 5 rows,with 5 inlets in each row. The fact that there were only about six filaments for the entire column demonstrates the important role played by the packings at the top of the bed. Most striking is Figure 6h, which was a vertical scan containing a row of five inlets at a bench position of 21 mm.
1274 Ind. Eng. Chem.
Res., Vol. 30,No. 6, 1991
C
Figure 6. Full acan. The operating conditions were as follows: nonprewetted 3-mm spheres, uniform distributor, distilled water, and low liquid-flow rate. Multiple filaments were observed.
The flow pattern was dramatically different from the ideal picture of five filaments under the five inlets. Clearly, the packings at the top of the column significantly influenced the flow distribution. This is why we stated earlier that the flow uniformity of the liquid distributor was adequate for our purpose. Influence of the Liquid Flow Rate. Figure 7 was another full scan taken under the same conditions as Figure 6, except that the flow rate was increased from low to medium. Only a few new filaments were formed, which seem to be branches of an already existing filament (Figures 6c and 7c). To accommodatea higher flow rate, most of the existing filaments expanded laterally as can be seen in a careful comparison of each pair of pictures taken a t the same bench position in these two figures. Specifically, let us consider the filament at the center of the column in Figures 6e and 7e. The much darker filament in Figure 7e indicates that the pore channels became filled with liquid as the liquid flow rate was increased. Apparently, the existing filament was not sufficiently large to accommodate the liquid coming from above. This caused the liquid to spread to other adjacent pores that were previously devoid of liquid, and the diameter of the filament increased. Depending on the configuration of the spheres in contact, however, the liquid would spread out from the existing filament and start a new filament. For example, compare Figures 6f and 6g with 7f and 7g. Of course,the general trend that the liquid tended to flow preferentially along the existing filaments, that is, where the packings were already wetted, is specific to a spherical packings. It is conceivable that in some structured packings such as cylindrical extrudates, liquid is more likely to flow sideways, resulting in more new filaments. When the liquid flow rate was increased from medium to high, the expansion of the filaments continued and coalescence took place. A t this flow rate, the entire bed became flooded as can be seen in Figure 8a,b which were taken at bench positions 27 and 30 mm, respectively. Liquid accumulated above the packing, and a few spheres a t the top were lifted up. The liquid coming from the distributor appeared as some blurred vertical lines in the scans a t this higher flow rate. Influence of Prewetting. Once the bed had been prewetted, the liquid flow rate was decreased from high to medium. Figure 9 shows a central scan. Pictures at other positions were more or less the same. These are drastically different from the pattern of Figure 7i-1 taken at the same flow rate but with a nonprewetted bed. While Figure 7 shows filament flow, Figure 9 shows primarily film flow, a characteristic of flow over a prewetted bed. The black clusters in Figure 9 were areas where the pores were fded with liquid. It is known that the porosity varies from location to location in a packed bed of equal-sized spheres. For instance, consider a rectangular column of 10 particle diameters in width. If one examines the local porosity in a cubic control volume of 1particle diameter in width, the porosity value can vary from 0.15 to 0.45 (Chan and Ng, 1986). Thus, the liquid was scattered throughout the bed in a prewetted bed, forming liquid pockets at locations with favorablegeometry such as several spheres in close contact. These pockets were connected to one another by film flow. The liquid flow rate was further reduced from medium to low. Figure 10 shows the corresponding central scan which revealed a smaller amount of liquid holdup than in Figure 9. Clearly, the influence of the liquid flow rate on the flow pattern did not seem to be as important when the bed was prewetted.
Ind. Eng. Chem. Res., Vol. 30,No.6,1991 1275
b
a .
b
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h Figrrre 8. " b o scans at bench positions 27 and 30 mm, m s p d w l y . The operating conditions were as follows: nonprewetted 3-mm spheres, uniform distributor, distilled water, and high liquid-flow rate. The bed was entirely flooded, and a few spheres were lifted up a t the top of the column.
a k
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Figure 9. Central scan. The operating conditions were as follows: prewetted 3-mm spheres, uniform distributor, distilled water. and medium liquid-flow rate. Film flow as observed.
a
b
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d
r Figure 10. Central scan. The operating conditions were as follow prewetted 3-mm spheres, uniform distributor, distilled water, and low liquid-flow rate. Film flow was again observed.
Figure 7. Full scan. The operating conditions were as follows: nonprewetted 3-mm spheres, uniform distributor, distilled water, and medium liquid-flow rate. Filament flow was observed.
Influence of Particle Size. The same sequence of experiments described above was repeated using 6-mmdiameter glass beads. Figure 11shows the central scans in a nonprewetted bed for the low liquid-flow rate. It was predominantly film flow, albeit a few incomplete f h e n t s were present. Indeed, there is a significant difference as far as filaments are concerned between this &"-sphere pack and the 3-mm-sphere pack (Figure 6). One plausible explanation for this observation is that the pore channels in the &"-sphere pack were 90 large that the liquid films
1276 Ind. Eng. Chem. Res., Vol. 30,No. 6,1991
Ib
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Figure 11. Central scan. The operating conditions were as follows: nonprewetted 6-mm spheres, uniform distributor, distilled water, and low liquid-flow rate. The flow pattern was a mix of filaments and films.
a
F'iguk 13. Two scans at bench positions 27 and 33 mm, r e a p tively. The operating conditions were as follows: nonprewetted 6-mm spheres, uniform distributor, distilled water, and high liquidflow rate. Liquid accumulated a t the top of the column.
a
Figure 12. Cental scan. The operating conditions were as follows: nonprewetted 6-mm spheres, uniform distributor, distilled water, and medium liquid flow rate. The presence of filamenta could be easily seen a t this higher flow rate.
were not sufficiently thick to coalesce and fill a pore chamber. Since filaments are actually a string of liquidfilled pore chambers, there were few filaments in the 6mm-sphere pack. Although it could not be confirmed in the CAT scans, the few filaments in Figure 11 were expected to form a t tight spots with low local porosity. The flow rate was then increased from low to medium. Figure 12 shows the corresponding central scans. Comparison of Figfures l l c and 12c shows that filaments did expand at a higher flow rate as in the 3-mm-sphere pack. In increasing the liquid-flow rate, however, there appeared to be more new filaments in the 6-mm-sphere pack than the 3-mm-sphere pack. A possible explanation is as follows: As mentioned before, the pores in the 3---sphere pack were smaller, and some of the pores, particularly those at the top of the packing, were already filled at the low flow rate and resulted in the formation of filaments. An increase in liquid-flow rate simply expanded the boundaries of these existing filaments. In contrast, filaments in the &"-sphere pack were never established at the low flow rate. Only at the medium flow rate did the formation of filaments firmly take place. Figure 13a,b shows the scans at bench positions 27 and 33 mm, respectively, at the high liquid-flow rate. Liquid accumulated at the top but not the lower portion of the bed. This was different from what was observed in the 3-mm-sphere pack (Figure 8),where liquid accumulation occurred throughout the bed. When the liquid-flow rate was decreased from high to medium (Figure 14), the flow pattern was primarily film flow and was similar to the one
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Figure 14. Central scan. The operating conditione were as followe: prewetted 6-mm spheres, uniform distributor, distilled water, and medium liquid-flow rate. The flow pattern was primarily film flow.
Figure 15. Central scan. The operating conditions were as follows: prewetted 6-mm spheres, uniform distributor, distilled water, and low liquid-flow rate. Film flow was observed.
encountered for a prewetted 3-mm-sphere pack (Figure 9). Comparison of the prewetted case (Figure 14) and the nonprewetted case (Figure 12) shows that there were definitely more interconnected films and/or filaments in the prewetted case. The liquid-flow rate was further reduced from medium to low (Figure 15). Comparison of the prewetted case (Figure 15) and the nonprewetted case (Figure 11) again reaffirms the presence of more films and/or filaments in a prewetted bed. Also, note that liquid accumulated above the wire gauze at the bottom of the bed. This amount was sufficiently small that it was not
Ind. Eng. Chem. Res., Vol. 30, No. 6,1991 1277
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Figure 16. Central scan. The operating conditione were as follows: prewetted 3-mm spheres, a line inlet distributor,distilled water, and a liquid-flow rate of 2.3 mL/s. This is the same flow rate per inlet as the low flow rate using the uniform distributor. Film flow was predominant.
b
Figure 17. Central scan. The operating conditions were as follows: prewetted 3-mm spheres, a line inlet distributor,distilled water, and a liquid-flow rate of 4.6 mL/s. This is the same flow rate per inlet as the medium flow rate using the uniform distributor. Film flow was again observed.
expected to influence the flow distribution above. Nonuniform Inlet Configuration. The previous experiments were run with a uniform inlet, and it was not clear about the contribution of each individual inlet to the flow pattem. The objective of the following experiments was to resolve this issue and to investigate in more detail the impact of prewetting on liquid spreading. Line Inlet. For the central scan in Figure 16, a prewetted bed of 3-mm-diameter spheres was used. The distributor was changed by closing all the inlets with adhesive tape except the 5 inlets of the middle row running from front to back of the column. This row was perpendicular to the plane of the pictures. The liquid flow rate was increased from 0 to 2.3 mL/s. The liquid-flow rate per inlet was the same as that of a uniform distributor with the low liquid-flow rate (11.5 mL/s; Figure 10). The presence of the scattered liquid pockets was clearly visible. By adjusting the contrast, the glass spheres in the top left and right comers became invisible, showing liquid spread as a cone-shaped structure. When the liquid-flow rate was increased from 2.3 to 4.6 mL/s, which corresponds to the medium flow rate for a uniform distributor (Figure 9), more liquid pockets could be seen in a central scan (Figure 17). Clearly, the liquid flowed down the column primarily as films. The films became thicker a t a higher flow rate. A t locations with a low local porosity, the films actually coalesced to form liquid pockets. Single Inlet. To take this investigation one step further, the pump was shut down and the bed was allowed
F'igure 18. Central scan. The operating conditions were as followe: prewetted 3-mm spheres, a single inlet distributor, distilled water, and a liquid-flow rate of 0.46 mL/s. This is the same flow rate per inlet as the low flow rate using the uniform distributor.
to drain freely. Thus,we again had a prewetted bed. All the inlets except the one a t the center of the distributor were closed. Then, the pump speed was raised from 0 to 0.46 mL/s, which was the same flow rate per inlet in the five-inlet line distributor for a total flow rate of 2.3 mL/s, or in the uniform distributor with the low liquid-flow rate (11.5 mL/s). The flow pattern for this prewetted bed is shown in a central scan (Figure 18). As expected, the prewetting influenced greatly the liquid spreading. The inlet was located on the plane of Figure 1&, and a liquid cluster could be seen directly under the inlet. It disappeared about 5 or 6 particle diameters from the top of the bed, indicating that film flow again prevailed. Thus, prewetting of the bed has a 2-fold impact: First, it minimizes the influence of the bed geometry on the flow distribution. Second, it tends to reduce liquid maldistribution when a poorly designed inlet is used. Surface Tension Effects. The effect of surface tension on the liquid distribution was investigated by using a mixture of water and ethanol. Figure 19 shows the flow patterns a t bench positions 8,14,20,29,38, and 45 mm for a nonprewetted 3-mmdiameter-spherepack with a line distributor. The wate-thanol mixture used had a surface tension of 49.7 dyn/cm, as measured with a ring tensiometer and a viscosity of 1.6 CP(Handbook of Chemistry and Physics). The liquid flow rate was 4.6 mL/s, which corresponds to the medium flow rate for a uniform distributor. The characteristicfilament flow of a nonprewetted bed was preserved. If we assume that the relatively small change in viscosity did not influence liquid distribution, the lower surface tension had a significant effect on the shape of the filaments. Contrary to what could be seen as meandering filaments in Figure 7, which used the same flow rate per inlet, the three filaments in Figure 19 seemed to be less tortuous and remained in the plane containing the five inlets. Figure 20 shows the flow patterns for a 32.8 dyn/cm water-ethanol mixture with a viscosity of 2.8 CPflowing over a prewetted 3-mm-sphere pack at bench witions 1% 30, and 45 mm. A line distributor was used, and the liquid-flow rate was set at 4.6 mL/s, which corresponds to the medium flow rate for a uniform distributor. These pictures again underscore the significance of prewetting. Under similar conditions, a nonprewetted bed exhibited filament flow (Figure 19), while this prewetted bed exhibited f h flow. Figure 20 can also be compared to Figure! 17 for which distilled water was used. The only difference between them is that the former has a lower surface tension and a higher vismity. Film flow was observed in both cases. However, more liquid could be seen at the center
1278 Ind. Eng. Chem. Res., Vol. 30, No. 6,1991
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Figure 20. Three scans at bench positions 15, 30, and 45 mm, respectively. The operating conditions were as follows: prewetted 3-mm spheres, a line inlet distributor, a mixture of distilled water and ethanol with a surface tension of 328 dyn/cm, and a liquid-flow rate of 4.6 mL/s. This is the same flow rate per inlet as the medium flow rate using the uniform distributor.
ef Figure 20.
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F’igure 21. Two scans were taken at a bench position of 9 mm from two different runs. The operating conditions were identical: prewetted 3-mm spheres, a line inlet distributor,distilled water, and a liquid-flow rate of 2.3 mL/s.
Figare 19. Six scans at bench poeitions 8,14,20,29,38, and 45 mm, respectively. The operating conditions were as follows: nonprewetted 3-mm spheres, a line inlet distributor,a mixture of distilled water and ethanol with a surface tension of 49.7 dynlcm, and a liquid-flowrate of 4.6 mL/s. This is the same flow rate per inlet as the medium flow rate using the uniform distributor. Three relatively vertical filaments could be seen.
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Again, if we assume the relatively small change in viscosity did not influence liquid distribution, this indicates that a lower surface tension enhanced the formation of a filament. Effect of Flow History. We have seen that liquid-flow pattern depended on whether a bed was prewetted or not. The effect of flow history was further examined in the following experiments: First, we wanted to find out whether a flow pattern could be reproduced if the steps and conditions leading to its existence were the same. In Figure 16, the bed was prewetted by flooding the column and then drained. The liquid-flow rate was increased from
0 to its actual value of 2.3 mL/s, which corresponds to the low flow rate for a uniform distributor. After the pictures of Figure 16 had been taken, the pump was stopped and the bed was allowed to drain thoroughly. The pump was set back to its previous value of 2.3 mL/s. Figure 21a shows the flow pattern obtained at a bench position of 9 mm. Figure 21b was a scan taken a t the same bench h v position of 9 mm and is actually part of Figure 16. T two scans were similar, showing that when the same startup procedure was used, similar flow patterns were obtained. In Figure 10, the bed was prewetted by setting the pump to its highest speed and then reduced to the low liquid-flow rate. Figure 22a,b, enlarged somewhat to reveal more details, was obtained under conditions identical with Figure 10 except that the liquid-flow rate was increased from 0 to the low flow rate for a prewetted bed. These figures are to be compared with Figure 22c,d, which is part of Figure 10 for the same bench positions 30 and 48 mm, respectively. It appears that the liquid holdup was higher in Figure 22c,d, indicating a possible hysteresis effect. Thoughout our study, a prewetted column was allowed to drain under gravity in various occasions. Figure 23 is a typical example, which was obtained for a 3-mm-sphere pack at a bench position of 30 mm. The dark spheres were taken out by subtracting out the image of a dry bed at the same bench position with a computer program. As can be seen, almost no liquid pockets were present. This lends credence to our earlier suggestion that the liquid pockets such as those in Figure 9 were not isolated. They were connected to one another with films and were maintained by a continuous supply of liquid. Image Analysis. To obtain more insights, we performed image analysis on some of the s a n s presented in this paper. Image-Pro (Image Cybernetics, Inc.) was used for this purpose, and the procedure was the following: With the scan projected on a monitor, a window for analysis was defined by the user. In this case,the window was set to be the longitudinal cross section of the packed column. The user was then allowed to select a darkness threshold value such that only the features darker than this preset value remained in the window. The fraction of dark area covered by such features remained relative to the total area of the window was provided by the image analyzer. Figure 24 shows the results for four runs, two different liquid flow rates-low (11.5 mL/s) and medium (23 mL/s)-and under prewetted and nonprewetted conditions. As expected, the fraction of dark area is higher in
Ind. Eng. Chem. Res., VoL 30,No.6,1991 1279
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Figure 24. Dependenceof dark area on bench poeition. The scans presented in Figures 6, 7, 9, and 10 were used for this analysis.
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Figure 22. Two scans, a and b, taken at bench positions of 30 and 48 mm, respectively,from an experimental run. Another two scans, c and d, at the same respective bench positions were obtained from another run. The operating conditions were identical: prewetted 3-mm spheres, uniform distributor, distilled water, and low liquidflow rate. The only difference was that a and b were obtained by decreasing the liquid flow from a higher value, and c and d were obtained by increasing liquid flow from a low value.
Figure 23. Subtraction image of a drained column packed with 3-mm spheres. Few liquid pockets could be seen, indicating that liquid pocketa were connected to one another.
a prewetted bed. It should be emphasized that the dark area does not quantitatively represent any macroscopic quantities such as wetting efficiency or liquid holdup. On
the b e i s of our judgment, a different darkness theshold value was selected for each of the four series of pictures in Figure 24 to eliminate most of the nonwetted spheres in isolation. We took out all the nonwetted spheres in filament flow, but we were not able to take out spheres that were inside liquid pockets in film flow. This explains why the fraction of dark area is so much lower in the nonprewetted beds. The fraction of dark area increases a t a higher liquid flow rate. This qualitatively codirms our observation that liquid holdup increases with liquid flow rate. The four curves are relatively flat, indicating that flow patterns did not change significantly from one wall to the other.
Concluding Remarks The CAT scans provide for the first time an actual view of the flow pattem in the interior of a trickle-bed. The main findings are summarized as follows: (1)The scans show very clearly the impact of prewetting on the flow pattern. When the bed is nonprewetted, it is dominated by filament flow. When the bed is prewetted, it is dominated by film flow. (As discussed in point 3 below, it also depends on the condition that the flow channels are not too small. In that case, films would coalesce to form filaments.) (2) In filament flow, an increase in liquid-flow rate c a m an increase in the width of the filaments. In film flow, the f h become thicker and may eventuallylead to formation of filaments. (3) For large particles, it is more likely to have f h flow. The pores are sufficiently largethat it is less likely for films to coalesce to form filaments. (4) With observations based on either a line or single inlet distributor, it is clear that there are liquid pockets scattered thoughout a bed in film flow. These pockets are continuously replenished with liquid from above. There are very few isolated liquid pockets in a 3- or &-"-sphere pack, although we expect this number can increase significantly when the particle size decreases. (5) Speading is diminished with a low surface tension liquid. The influence of surface tension on the flow pattem is much less pronounced in a prewetted bed. This conclusion is based on the assumption that the relatively small change in viscosity in the experiments did not affect liquid distribution. (6) A flow pattem depends on the flow history. Only under identical conditions leading to its existence is a similar flow pattern obtained. These observations corroborate some of the conclusions based on visual studies external to a trickle bed (Christensen et al., 1986). More importantly, this study provides detailed flow patterns that can serve as inputs to a reactor model. As demonstrated in such an analysis by Funk et
1280 Ind. Eng. Chem, Res., Vola30,No. 61 1991
al. (1990), flow maldistribution can significantly impact the rate of reaction. It should be emphasized that it is not trivial to quantify a flow pattern, and we have chosen a rather common technique to present it by a series of sectional cuts (Underwood, 1970). A theory is in progress to predict these flow patterns. Admittedly, the present study is still limited in scope. The experiments were conducted with no gas flow. Obviously, the competition of the liquid and gas phases for the interstitial pore space would influence the flow pattern. The experiments were also limited to nonporous, spherical particles. And we did not investigate the effect of wettability or viscosity on liquid distribution. Shaped porous catalytic particles would impact liquid distribution in two ways. First, a packed bed of cylindrical extrudates is highly anisotropic and can cause liquid to flow sideways (Ng and Chu, 1987). Second, a porous particle at a relatively low temperature is filled with the nonvolatile liquid due to capillarity. This would change the effective wettability between the liquid and the internally wetted particle and thus the flow distribution. In addition, reactions taking place in industrial trickle-bed reactors are often exothermic and can lead to the vaporization of a large fraction of the liquid phase (Collins et al., 1985). In fact, dewetting can occur within a porous catalytic particle for highly exothermic reactions (Harold, 1988). The coupling between reaction, vaporization, and flow distribution is very much an unexplored area. The present work is being extended to consider some of these issues. Acknowledgment We express our appreciation to Mobil Research and Development and the National Science Foundation (Grant CBT-8700554) for support of this research. The experiments were performed in Mobil’s Dallas Research Laboratory in Texas. We thank Mary Coles and Ernest Muegge of the Dallas laboratory for their assistance in the experiments. Literature Cited Chan, S.K.; Ng, K. M. Geometrical Characteristics of a ComputerGenerated Three-Dimensional Packed Column of Equal and Unequal Sized Spheres, with Special Reference to Wall Effects.
Chem. Eng. Commun. 1986,48,215. Charpentier, J. C.; Favier, M. Some Liquid Holdup Experimental Data in Trickle Bed Reactore for Foaming and Non-Foaming Hydrocarbons. AIChE J . 1976,21,1213. Christensen, G.; McGovem, S.J.; Sundaresan, S. Cocurrent Downflow of Air and Water in a Two-Dimensional Packed Column. AIChE J. 1986,10, 1677. Collins, G. M.; Hess, R. K.;Akgerman, A. Effect of Volatile Liquid Phase on Trickle Bed Reactor Performance. Chem. Eng. Commun. 1985,32,281. Funk, G. A.; Harold, M. P.; Ng, K. M. Effectiveness of a Partially Wetted Catalvst for Bimolecular Reaction Kinetics. AIChE J . i988,34,m i . Funk, G. A.; Harold, M. P.; Ng, K. M. Reaction Adsorption Effects on Partially Wetted Catalyet Performance. Chem. Eng. Sci. 1989, 44, 2509. Funk, G. A.; Harold, M. P.; Ng, K. M. A Novel Model for Reaction in Trickle Beds with Flow Maldistribution. Ind. Eng. Chem. Res. 1990,29,738. Gianetto, A.; Baldi, G.; Specchia, V.; Sicardi, S.Hydrodynamics and Solid-Liquid Contacting Effectiveness in Trickle Bed Reactors. AIChE J . 1978,24,1087. Handbook of Chemistry and Physics; CRC Press: Cleveland, OH, 1973. Harold, M. P. Steadystate Behavior of the Nonisothermal Partially Wetted and F i l l d Catalyst. Chem. Eng. Sci. 1988,43,3197. Harold, M. P.; Ng, K. M. Effectiveness Enhancement and Reactant Depletion in a Partially Wetted Catalyst. AIChE J . 1987,33, 1448. Melli, T. R. Two-Phase Cocurrent Downflow in Packed Beds; Macroscale from Microscale. Ph.D. Thesis, University of Minnesota, 1989. Ng, K. M. A Model for Flow Regimes in Trickle Bed Reactors. AIChE J. 1986,32,115. Ng, K.M.; Chu, C. F. Trickle-Bed Reactors. Chem. Eng. Prog. 1987, 38(11),55. Sato Y.;et al. Flow pattern and Pulsation Properties of Cocurrent Gas-Liquid Downflow in Packed Beds. J . Chem. Eng. Jpn. 1973, 6, 315. Underwood, E. E. Quantitatiue Stereology; Addison-Wesley: Reading, MA, 1970. Wang, S. Y.;Ayral, S.; Castellana, F. S.; Gryte, C. C. Reconstruction of Oil Saturation Distribution Histories During Immiscible Liquid-Liquid Displacement by Computer-Assisted Tomography. AIChE J . 1984,30,642. W e e k ” Jr, V. W.; Myers, J. E. Fluid-Flow characteristics of Concurrent Gas-Liquid Flow in Packed Beds. AIChE J. 1961,10, 951. Received for review April 23, 1990 Revised manuscript received August 6 , 1990 Accepted January 2,1991
