Liquid Flow Texture in Trickle-Bed Reactors - American Chemical

Ind. Eng. Chem. Res. 1997, 36, 5133-5145

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Liquid Flow Texture in Trickle-Bed Reactors: An Experimental Study P. V. Ravindra, D. P. Rao, and M. S. Rao* Department of Chemical Engineering, Indian Institute of Technology, Kanpur, 208016 India

Modeling of trickle-bed reactors requires the knowledge of liquid flow texture. A dye-adsorption method was employed to study the liquid flow texture in the bed. Pictures of cross sections of the dismantled bed at different depths were taken. The texture in prewetted and nonprewetted beds of glass beads (nonporous) and alumina particles (porous) of different sizes was studied. To supplement the visual observation, exit liquid distribution and pressure drop were measured. The liquid flow texture was deduced from the visual observations and exit liquid distribution. The present study revealed several unexpected features. The behavior of a bed of porous particles was strikingly different from that of nonporous particles. The interpretation of the previous work on the hysteresis of pressure drop, gas-liquid and liquid-solid interfacial areas, and the implications of the observed liquid flow texture on modeling of trickle-bed reactors are discussed. Introduction Trickle-bed reactors are three-phase reactors. Gas and liquid flow down cocurrently through a fixed bed of catalyst particles. These reactors are widely used for hydrotreatment of petroleum fractions and for hydrogenation and oxidation reactions. The scale-up of trickle-bed reactors in petroleum processing is wellestablished as a proprietary industrial art for specific feedstocks and processes (Dudukovic and Mills, 1986). However, a rational design of these reactors is still out of reach because of the poor understanding of the gas and liquid flow phenomena. The intriguing features like a minimum in the reaction rate with liquid flow rate (Mata and Smith, 1981; Haure et al., 1992), multiple hydrodynamic states (Kan and Greenfield, 1978; Christensen et al., 1986), and flickering hot spots (Germain et al., 1974; Jaffe, 1976) have brought to focus the need for understanding the flow of gas and liquid in trickle beds. The prediction of the reaction rate in a trickle-bed reactor from the first principles requires a spatial description of flow fields of gas and liquid over randomly packed catalyst particles. It appears that such a description is, at present, beyond the reach. Nonetheless, Melli and Scriven (1991) applied percolation concepts to formulate a statistical theory of gas and liquid flow in a packed bed employing four basic flow regimes in a network of passages. They have, however, pointed out that the computational effort involved in obtaining the flow behavior of an industrial scale packed bed is formidable. Their model has not yet been extended for the evaluation of trickle-bed reactor performance. A few other models (Crine et al., 1980; Marchot et al., 1992; Zimmerman and Ng, 1986; Stanek et al., 1981) have also been developed to predict the liquid flow distribution. The premises used in modeling liquid flow distribution have not been based on experimental work. For example, they do not distinguish whether the bed is prewetted or not, which has been shown to have a marked influence on the liquid flow texture of the bed (Lutran et al., 1991). In trickle-bed reactors, the discrepancy between the observed and the expected reaction rates (when the particles are fully covered with the flowing liquid) is * Author to whom correspondence should be addressed. S0888-5885(97)00308-4 CCC: $14.00

generally attributed to the partial wetting of the external surface with the flowing liquid and partial filling of the pores with liquid. The latter aspect received little attention (Bhatia, 1988; Harold, 1988) as it is perceived that the pores get filled with liquid due to capillary forces. The former has attracted wide attention. The stimulus response, dye-adsorption, and chemical methods have been employed for its determination. However, a direct experimental evidence of the partial wetting of individual particles is not yet available. On the other hand, a theoretical study (Reddy et al., 1990) based on the characteristics of the largest stable, static pendular rings suggests that there may not be dry patches in trickle beds packed with particles of about 4 mm diameter or less. However, characteristics of pendular rings under dynamic conditions are expected to be different from the static ones. Zimmerman and Ng (1986) visualized that the liquid flow texture in a bed consists of a number of features: liquid flow as films and rivulets over the particles, pendular structures, liquid-filled channels, and liquidful pockets. The reported observations on flow texture were confined to the proximity of the walls as the interior of the bed remained inaccessible due to lack of suitable experimental techniques. Only recently, Lutran et al. (1991) employed computeraided tomography (CAT) to gain access into the interior of the bed under quiescent gas conditions. They observed filament flow in nonprewetted beds and film flow in prewetted beds. However, it was not possible to discern particle-scale partial wetting and other features from the CAT scans. They pointed out the need for similar studies with porous particles as the bed behavior is expected to depend on the nature of particles. The objective of the present work was to study liquid flow texture in trickle beds of porous and nonporous particles and to reconcile the diverse trends observed in the behavior of trickle beds. In the present work, a dye-adsorption method was employed to examine the particle-scale and bed-scale features of the liquid flow texture in a trickling-flow regime. The particles acquired color as a methyleneblue solution passed through the bed. Later, the sections of the dismantled bed at different depths were examined and photographed. To supplement the above method, the exit liquid distribution and pressure drop were measured in a separate set of experiments. © 1997 American Chemical Society

5134 Ind. Eng. Chem. Res., Vol. 36, No. 12, 1997

Experimental Section A schematic diagram of the trickle bed employed is shown in Figure 1a. The bed consisted of a liquid distributor (a), a packed-bed section (b), and a gasliquid separator (c). The bed section was rectangular in a cross section of inner dimensions 6.0 × 8.0 cm. Its height was 20 cm. Two glass plates were used as the front and back walls of the bed section for visual observations. A rectangular frame of 5.9 × 7.9 cm and metal strips of 0.3 cm width at a spacing of 0.3 cm formed the grid. To distribute the liquid uniformly over the bed, 99 stainless steel capillary tubes were used. The liquid distributor was modified to a line inlet of liquid by closing all the capillary tubes with a soft rubber sheet except the nine of the middle row (perpendicular to the glass walls). To introduce liquid as a point inlet, a stainless steel tube of 0.4 cm was employed in place of the capillary tubes. The gas-liquid separator was similar to the liquid distributor but for the internal fittings. The rest of the details are shown in Figure 1a. Humidified air and water were employed as gas and liquid phases, respectively. Experiments were conducted with beds of glass beads and alumina particles of different sizes, employing uniform, line, and point liquid inlet configurations. The glass beads were washed with chromosulfonic acid and dried before use. The alumina particles were washed with water and dried before use. The different parameters employed are given in Table 1. The performance of the trickle bed is known to depend on the start-up procedure. We have used two different start-up procedures. In one, water was introduced in a bed of dry particles at a low flow rate and it was increased to the desired value. Then air flow was started. The state of this bed is referred to as a nonprewetted bed. In the other, water was introduced at a low flow rate and was allowed to fill the bed by closing the outlets. The slow filling prevented the entrapment of air as bubbles. Once the water level reached the top of the packing, the water flow rate was set to the desired value and then the outlets were gradually opened to drain the excess water. Then the air flow was initiated. The state of the bed thus obtained is referred to as a prewetted bed. The porous particles were soaked in water for 12 h before use in the prewetted beds to ensure complete filling of the pores. The pressure drop was monitored with time after setting the flow rates of gas and liquid at the desired values. It was observed that the pressure drop increased with time and attained a steady value in a time period ranging from 10 to 360 min, depending upon the nature of the packing of the bed, inlet liquid distribution, and start-up procedure. The run was continued for another 2 h to ensure that the bed attained a steady state. Then, the flow was switched from water to a methylene-blue solution using a three-way valve in the suction line of the pump, connected to the storage tanks containing water and methylene-blue solution. After running the solution for 4-5 min, both gas and liquid flows were stopped. The liquid was allowed to drain for 10 min before the packed section was taken out of the setup and mounted on a specially designed jig for visual observation. Our observations through the glass walls, especially of channel flow, indicated that the liquid did not tend to cover a wider region on stopping the gas flow.

Figure 1. (a) Schematic of the trickle bed. (b) Schematic diagram of a liquid collection device (cells are identified by the numbers given in the right corners). Table 1. Operating Conditions for the Studies on Liquid Flow Texture temperature pressure glass beads, dp alumina particles, dp water flow rate air flow rate

30 °C 1 atm 1.6, 3.5, and 5.7 mm 1.9,a 3.5,a and 6.3b 1.0-8.0 kg/m2‚s 0.05 kg/m2‚s

a γ-phase, surface area: 200-240 m2/g. b R-phase, surface area: 25-35 m2/g.

Therefore the observations of the flow texture can be taken to be those of the bed in operation. It may be mentioned that the bed was held in a bag (with a transparent polyethene sheet at the sides and

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a nylon net at the bottom), tightly fitting into the bed section, to facilitate pushing of the bed using a screw jack. A thin polyethene sheet of rectangular shape, with length the same as the height of the packed section and width the same as the inner perimeter of the packed section, has been placed on a wooden block which has the inner dimensions of the packed section. Three strips of adhesive tape were pasted to give a rectangular parallelepiped, and a rectangular nylon net was attached at the bottom with adhesive tape. The bag thus made was inserted in the packed section. The particles were poured slowly while tapping the packed section. The bag was cut at the corners, as the bed was pushed up, to gain easy access of the particles. The top layer of the packing was brought to be in level with the top flange for visual observations. The bed cross section was also photographed. The bed was then pushed up to the next desired depth, with intermittent scooping of the particles. Then, a layer of particles was removed by padding using a flat pad covered with adhesive tape. This ensured minimal displacement of particles. After recording the visual observations and taking a photograph of the bed section, the process was repeated for different bed depths. The photographs of bed sections were taken at depths of 0, 1, 2, 3, 4, 8, 12, 16, and 20 cm. The exit liquid distribution was measured using a liquid collector in place of the gas-liquid separator (see Figure 1b). The interior of the collector was partitioned into 16 equal-sized rectangular cells of 6.0 cm height. The top of the cell walls were ground to give a fine edge to minimize the influence of liquid collection device on the flow distribution. Each cell was provided with a tube of 1 cm diameter for the discharge of gas and liquid. The bed extended all the way into the cells. The exit liquid streams from the cells were collected simultaneously to determine the liquid flow rates through the cells. Pressure drop and visual observations of the gas and liquid flow through the cells were recorded. The pressure drop reported includes the drop through the packing in the liquid collector. The pressure drop and liquid flow distribution measurements were repeated several times and are within 5%. Liquid Flow Texture The qualitative features of liquid flow texture in the bed can be inferred, to some extent, from the pictures of bed sections and the exit liquid flow distribution. To infer the texture from the pictures, the mechanism of dye adsorption onto the particles from the methyleneblue solution is required. The mechanism visualized is given below. Dye-Adsorption Mechanism The particles acquired color when a methylene-blue solution was injected onto one side of a particle and also when individual particles were dipped in and taken out of the methylene-blue solution. A sharp boundary between the colored part and the rest of the surface was observed for a particle partially wetted with the solution. The solution was passed for 4-5 min, after a switch was made from the flow of water to the solution. This time period was taken to be adequate to irrigate the active zones of the bed with the solution on the basis of the preliminary studies. The texture was inferred on the basis of the nature of wetting of the individual particles as given below.

Completely Colored. The particles with the entire surface colored were considered to have been irrigated by the liquid. However, there is no certainty that the entire surface was in contact with the flowing liquid at all times. The surface could get colored due to a wandering rivulet which may or may not have revisited a portion of the surface (Mills and Dudukovic, 1981). In studies conducted with 1.2 cm plastic spheres, we observed wandering lean rivulets over the particles adjacent to the wall. However, our extensive observations of the bed through the flat glass walls, with a naked eye and with the aid of a cathetometer, revealed that there are no rivulets in the present study. If the wettability is good, it is very unlikely that the rivulets can be present in beds of 5 mm particles and even more unlikely for the particles of 3 mm or less as the void space is inadequate for the formation of wandering rivulets (Reddy et al., 1990). Therefore, we have considered that a particle was wetted completely by the flowing liquid if the particle acquired color over its entire surface. Partially Colored. A part of the particle surface may not acquire color because of the presence of either stagnant pendular rings or stagnant films or dry patches on the surface. The color could spread into the stagnant regions by molecular diffusion. Consider an idealized flow situation that there is a sharp interface between the flowing liquid film and the stagnant region. Then, the spread of color into the stagnant liquid would correspond to the penetration depth by the diffusion for the period of 5 min. Taking the diffusivity of the methylene-blue solution in water to be of the order of 10-8 m2/s, the penetration depth obtained was about 0.01 mm. Therefore, the spread of color due to molecular diffusion was ignored. Free from Color. If the entire surface of a particle is free from color, it was either dry as it could have been in a dry pocket or covered by stagnant liquid such as an isolated liquid pocket inaccessible to the flowing solution. Keeping in view the above mechanism of color uptake by the particles, the liquid flow texture was inferred from the pictures and exit liquid flow distributions. Texture Classification A total of 250 cross sections, of beds operated at different conditions, were examined visually and photographed. On the basis of these observations, the liquid flow texture has been classified into the following: (1) channel flow; (2) bed-scale partial wetting; (3) complete wetting; (4) particle-scale partial wetting. The descriptions given below correspond to the beds operated at a liquid flow rate of 1.0 kg/m2‚s and a gas flow rate of 0.05 kg/m2‚s unless stated otherwise. Channel Flow Visual Observations. The channel flow was observed in nonprewetted beds of glass beads. Distinct liquid channels were formed at the top of the bed. The channels, which resembled rat holes, were interspersed in an otherwise dry bed. With depth, the channels meandered, split, merged, and split again. The number of channels and their cross sectional areas varied with particle diameter, liquid flow rate, and the type of liquid inlet. The particles located inside the channel were completely colored, and the ones outside the channel


Figure 2. Bed cross sections of nonprewetted bed of glass beads (dp ) 1.6 mm). L ) 1 kg/m2‚s. G ) 0.05 kg/m2‚s.

appeared bone-dry. Partially wet particles were not observed even at the interface of the channels and dry zones. A further discussion appears unwarranted as the channel flow is undesirable and could be easily avoided as shown later. Nevertheless, several studies reported in the literature fall under this category. Interpretation of the reported multiple hydrodynamic states and interfacial areas requires knowledge of the nature of the channel flow. Therefore, the channel flow was examined at length. Point Inlet. With a point inlet of liquid, only a single liquid channel was formed. The sections of the channel are shown in Figure 2a-c. In the pictures, the black and white zones correspond to the zones irrigated and unirrigated by the solution, respectively. The tiny white spots on the black zones appeared due to reflection of light from the glass beads during photography. The depth, H, given in the figures, was measured from the top of the bed. The channel had an almost circular cross section. At the top, its diameter spanned about 20 particles and remained uniform up to a depth of 8 cm. Then it gradually enlarged, reaching a maximum span of 28 particles at about a 12 cm depth (Figure 2c), and contracted again. Line Inlet. With a line inlet, a channel with a rectangular cross section was formed (Figure 2d). Its width spanned about 18 particles. It was straight up to a depth of 3 cm. Thereafter, the channel gradually thinned near the middle. It split into two channels at 8 cm depth (Figure 2e). These two channels gravitated toward the glass walls (Figure 2f) and remained so through the rest of the bed. Uniform Inlet. The top of the bed was wetted completely (Figure 3a). However, an onset of formation of channels was seen at 1 cm depth (Figure 3b). Formation of distinct channels took place at 3 cm bed depth

Figure 3. Bed cross sections of a nonprewetted bed of glass beads (dp ) 1.6 mm). L ) 1 kg/m2‚s. G ) 0.05 kg/m2‚s.

(Figure 3d). They meandered, merged, and split as can be seen from the shift in position of the channels (Figure 3d-f). It appears that the channels might persist with such behavior even if the bed was much deeper. At any cross section, about 50% of the bed was wetted with the liquid. Similar channel flow was observed in beds of 3.5 and 5.7 mm glass beads with a uniform inlet for the same flow rates. The channels encompassed a lesser number of particles as the diameter of the particle increased as seen from Figure 4. The pictures of the glass walls are given in Figure 5. The channels can be seen on the glass walls. The outlines of the channels that were a few particles deep inside the bed from the wall are also visible. The channels formed with 1.6 mm glass beads appeared liquidful and wide, that is, spanned several particles (Figure 5a). They fit into the description of the rivulet flow given by Christensen et al. (1986). The channels formed in the interior of beds with 3.5 and 5.7 mm glass beads fit into the description of the filament flow described by Lutran et al. (1991). A close examination of these channels revealed that the color of the particle(s) at the middle was more intense compared to that on the periphery. The pendular rings formed between the glass wall and the particles are clearly visible. During the run, the glass walls were dabbed with cotton wool dipped in cold water. The water vapor in the air condensed, lending contrast and rendering clear visibility of pendular rings. Liquid Distribution. We monitored the pressure drop with time and measured the exit liquid distribution in nonprewetted beds of glass beads. The pressure drop varied little with time and attained a steady value within 15 min. The liquid flow rates through the cells are depicted as 3D histograms in Figure 6. The cells in different rows were hatched differently for clarity.


filled. Therefore, it appears that the channel may have a liquid-filled core with a periphery of film flow over the particles. The texture of channel flow with dp, visualized from the pictures, is given below. If dp is large (>3 mm), the flow resistance is small because the void volume of a unit cell of the packing is large and also the solid surface area is small. Therefore, the liquid runs down from particle to particle as a filament as seen from the channels formed on the glass walls (Figure 5) and the color intensity on the particles in the middle of the channels (Figure 4). The filament was surrounded by a region of liquid flow as films over the particles, spanning one or two particle diameters. For an intermediate range of dp (1 < dp < 3 mm), the void volume is smaller and surface area is larger; therefore, the flow resistance is larger than the earlier case. Hence, the filament spans more than one void volume (in the lateral direction to the flow), thus turning into a liquid-filled channel. Such a change can be seen in Figure 5. The channel could be surrounded by a region of film flow. The film width may span a few particle diameters. If dp < 1 mm, the openings of the unit cell attain capillary dimensions, and the capillary forces in a lateral direction become comparable to gravitational pull. The channel spans several void volumes, and there may not be a film flow region. It might be possible, under the influence of capillary forces, that the channels cover most of the bed cross section, forcing gas to flow in channels. Such a texture has been visualized by Kan and Greenfield (1978) who employed 1.2 and 0.4 mm glass beads. Figure 4. Bed cross sections of a nonprewetted bed of glass beads. L ) 1 kg/m2‚s, G ) 0.05 kg/m2‚s, (a)-(c): dp ) 3.5 mm; (d)-(f): dp ) 5.7 mm.

The cell numbers are given at the top of the bars. Some of the channels were inactive, that is there was no liquid flow. Severe maldistribution of liquid can be seen in the figure. Standard deviations of the liquid distribution, a measure of departure from uniform flow, are also given in the figure. With point inlet, discharge of both gas and liquid was observed through the active cells (6, 7, 10, and 11) located at the central region of the bed. With line inlet, the gas and liquid were flowing through the cells in the central region and only gas was flowing through the cells adjacent to the side walls (Figure 6b). Figure 6c shows the liquid distribution obtained with uniform inlet. It was observed that only liquid was flowing through the 16th cell and only gas through the 6th and 7th cells. Gas and liquid were flowing through the rest of the cells. Channel-Flow Texture. It is pertinent to ask if these channels were liquidful? To check this, the circular channel observed with point inlet (Figure 2a-c) was examined. Consider the channel to be liquid-filled. Its diameter can be estimated from the liquid flow rate by equating the drag force (computed from the Ergun equation) with gravity force, ignoring the pressure gradient which was relatively insignificant. The diameter, thus computed, spanned 10.4 particles; whereas the observed channels spanned 20. Further, recall that the channel diameter enlarged to 28 particles at 12 cm bed depth. For the same flow rate, the channel area was nearly twice for the line inlet. Therefore, these channels could not have been liquid-filled. On the other hand, we have observed that there was only liquid discharge through one of the cells (16th cell of Figure 6c), indicating at least a part of the channel was liquid-

Bed-Scale Partial Wetting Isolated dry regions or pockets were observed in an otherwise irrigated bed. This kind of texture is referred to as bed-scale partial wetting. It was observed with uniform liquid inlet in the nonprewetted beds of glass beads at moderate to high liquid flow rates (L ) 3-8 kg/m2‚s). At low flow rates, liquid channels were formed as seen through the wall (Figure 5). As the rate was increased, the channels enlarged. They merged, split, and merged again, leaving dry pockets. This resulted in the bed-scale partial wetting. It decreased with an increase in liquid flow rate and eventually disappeared at about L ) 10 kg/m2‚s. Typical bed sections for L ) 5 and 8 kg/m2‚s are shown in Figure 7. At L ) 5 kg/m2‚s, interspersed dry pockets can be seen in Figure 7a-e. At the higher flow rate, there was only one dry pocket in the entire bed (Figure 7f). It spanned about 2 cm depth. However, small dry patches were observed adjacent to glass walls. Complete Wetting When the external surface of all the particles in the bed are completely colored, then it is referred to as complete wetting of the bed. Complete wetting was observed in the beds of porous and nonporous particles. However, the beds are categorized on the basis of the subtle differences observed in their characteristics. The details are given below. Alumina Particles Nonprewetted Bed, Point and Line Inlets. At the start of the run, nonprewetted beds of alumina particles exhibited behavior similar to nonprewetted beds of glass beads. Liquid channels were seen on the glass walls,


Figure 5. Front and back walls of a nonprewetted bed of glass beads. Uniform inlet, L ) 1 kg/m2‚s, G ) 0.05 kg/m2‚s.

Figure 6. Exit liquid flow distribution of a nonprewetted bed of glass beads (dp ) 1.6 mm). L ) 1 kg/m2‚s, G ) 0.05 kg/m2‚s.

and only a few cells were active. As time progressed, the other cells became active gradually, and the channels enlarged and merged eventually. This lateral spreading of liquid on solid surfaces results from the combined action of gravity and capillary forces. Due to the difference in dynamic and static control angles, the three-phase contact line will move. The speed at which the contact line moves depends on many factors such as rugosity and crystal structure of the solid surfaces. The variation of pressure drop with time is shown in Figure 8. It took about 360 min to attain a steady value with point inlet and 120 min with line inlet as opposed to 15 min with beds of glass beads. The exit liquid distribution with time is shown in Figure 9. At t ) 5 min, only four cells in the middle were active. At t ) 60 min, the adjacent cells became active. At t ) 240 min, all the cells became active. The liquid distribution attained a steady state at t ) 360 min. However, the flow rates through the 6th, 7th, 10th, and 11th cells remained higher compared to those through the rest of the cells. A similar behavior was observed with line inlet. The pictures of typical bed sections are shown in Figure 10. Consider the top layers of the beds. The color is intense just below the liquid inlet. The intensity became gradually less as one moves away from the

Figure 7. Bed cross sections of a nonprewetted bed of glass beads (dp ) 1.6 mm). Uniform inlet, G ) 0.05 kg/m2‚s; (a)-(e): L ) 5 kg/m2‚s; (f): L ) 8 kg/m2‚s.

liquid entry (Figures 10a and 10d). Large dry regions are also seen. The spread of the color away from the inlet might be due to the flow of liquid in the lateral direction as films over the particles because of capillary forces. The intermixing of water and the methyleneblue solution in those slow-moving films could have led to the decrease in the intensity of the color. At 1 cm depth, the color intensity was more in the central region and less near the walls. There are also white patches adjacent to the walls (Figures 10b and 10e). At about 2 cm depth, the bed got colored completely over the entire cross section except for a few interspersed white patches adjacent to the walls. A typical section is shown in Figures 10c and 10f. Inlet distribution non-uniformity penetrated 2 cm into the bed; all particles below this depth were completely wetted. The flow rate through some of the cells was high enough for the lqiuidfilled core, formed at the start of the run, to persist (see Figure 9). In a few runs the discharge from the cells was only liquid. This suggests that the bed above the cell was occupied by a liquidfilled core which excluded the gas. The flow rates through the cells close to the side wall were very small.


Figure 8. Variation of pressure drop with time. O, prewetted bed with uniform inlet; ], prewetted bed with point inlet; 4, nonprewetted bed with uniform inlet; 0, nonprewetted bed with point inlet. L ) 1 kg/m2‚s, G ) 0.05 kg/m2‚s.

Figure 10. Bed cross sections of a nonprewetted bed of alumina particles (dp ) 1.9 mm). L ) 1 kg/m2‚s. G ) 0.05 kg/m2‚s.

Figure 9. Exit liquid flow distribution of nonprewetted bed of alumina particles (dp ) 1.9 mm). Point inlet, L ) 1 kg/m2‚s, G ) 0.05 kg/m2‚s.

The average thickness of liquid films over the particles above the cell may be considered to be proportional to cubic root of the flow rate (Satterfield et al., 1969). The exit liquid distribution and the color intensity suggest that the film thickness varies considerably over the bed cross section. Nonprewetted Beds, Uniform Inlet. In nonprewetted beds with a uniform liquid inlet, the liquid channels were observed at the start of the run. As time passed, the liquid channels expanded and merged, covering almost the entire glass panels leaving some white patches. The bed attained steady state in about 120 min. The pictures of the bed cross sections and the glass walls of the bed operated at liquid velocities of 1 and 3 kg/m2‚s are shown in Figure 12. The particles at the top layer were wetted completely. The bed cross sections were also completely wet but for sparsely distributed dry patches and uncolored particles adjacent to the walls. The dry patches were nearly absent at higher liquid flow rates. The particle-scale partial wetting was not present except for the particles adjacent to the wall. The dry patches were prominent in the pictures of glass walls at lower liquid velocities of 1 kg/m2‚s than at 3 kg/m2‚s. Figure 13 shows the pictures of the bed cross sections and the front walls of nonprewetted beds of 3.5 mm alumina particles. Texture and liquid distributions were similar to those of the beds with 1.9 mm alumina particles.

Figure 11. Exit liquid flow distribution of alumina particles (dp ) 1.9 mm). L ) 1 kg/m2‚s, G ) 0.05 kg/m2‚s.

Prewetted Beds, Point and Line Inlets. A similar behavior was observed in prewetted beds with point and line inlets. The steady state was, however, attained within 15 min of operation, and all the cells were active from the start of the run (Figure 8). The pictures of bed sections were similar (but for the top layers of the bed where the wet zones were slightly larger) to those of a nonprewetted bed. Therefore, the pictures are not shown. The liquid distributions in nonprewetted and prewetted beds were different, as seen in Figure 11, though both the beds were completely wetted (except for 2 cm depth). It is likely that there was only film flow over the particles. Prewetted Beds, Uniform Inlet. The prewetted bed, unlike the other bed, attained steady state within 15 min as observed from the pressure drop and exit liquid distribution. The pictures showed that the entire bed cross sections were colored and were similar to the pictures shown in Figure 10c. The exit liquid distribu-


Figure 12. Bed cross sections of a nonprewetted bed of alumina particles (dp ) 1.9 mm). Uniform inlet, G ) 0.05 kg/m2‚s.

Figure 13. Bed cross sections of nonprewetted bed of alumina particles (dp ) 3.5 mm). Uniform inlet, G ) 0.05 kg/m2‚s.

tion was relatively more uniform compared to the nonprewetted beds. The pictures of the front and back glass walls (Figures 12 and 13) show particle-scale and bed-scale partial wetting. On the other hand, the pictures of bed cross sections showed complete wetting of the bed and the particles. The color pictures of the bed are given elsewhere (Ravindra, 1995). Therefore, inferences drawn about these beds based on the observations through the walls could be erroneous for beds of porous particles. Glass Beads Prewetted Bed and Point, Line, and Uniform Inlets. The behavior of prewetted beds with point and line inlets was strikingly different from that of the prewetted bed of alumina particles. Even the top layers of the bed, barring a few particles, acquired color, indicating that there was film flow lateral to the main flow direction. The flow is like the film flow of liquid underneath a nearly horizontal plate. The particles at all the other cross sections were completely wet. Figure 14 shows the pictures of the top layers of the beds and front and back glass walls. Accumulation of liquid at the grid can be seen to be substantial unlike with the alumina particles, no dry patches are seen through the glass walls. The bed attained steady state in 15 min. The liquid distribution was nearly uniform with all three inlets, indicating an insignificant role of inlet liquid distribution. Such a good distribution of liquid can be attributed to good wetting characteristics of glass beads. The pendular rings observed on the glass panels, completely wetted particles, and near uniform exit liquid distribution indicate the absence of liquid-filled

Figure 14. Bed cross sections and walls of a prewetted bed of glass beads (dp ) 1.6 mm). Uniform inlet, L ) 1.0 kg/m2‚s, G ) 0.05 kg/m2‚s.

channels. It is very likely that the liquid was in film flow over the particles. These observations are consistent with the behavior reported by Lutran et al. (1991) on the basis of CAT scan pictures of the bed. On the basis of the maximum volumes and areas of stable static pendular rings, Reddy et al. (1990) showed that the void volumes will be filled with liquid for dp e 4 mm. During the run, visual observations with the naked eye and through cathetometer indicated that the pendular rings are much smaller in volume than those


Figure 15. Bed cross sections and walls of a nonprewetted bed of alumina particles (dp ) 6.3 mm). Uniform inlet, L ) 1.0 kg/ m2‚s, G ) 0.05 kg/m2‚s.

predicted by Reddy et al. (1990). It is unlikely that there can be liquid bridges. Particle-Scale Partial Wetting If a significant number of particles are partly colored, then it is referred to as particle-scale partial wetting. This type of texture was observed only with beds of 6.3 mm alumina particles. The details are given below. Nonprewetted and Prewetted Beds, Uniform Inlet. The beds of alumina particles of 6.3 mm diameter exhibited particle-scale partial wetting. The behavior of nonprewetted bed was marginally different from that of the prewetted bed. The extent of dry surface on the particles was slightly more with nonprewetted beds compared to prewetted beds. Figure 15 shows the pictures of typical nonprewetted beds. The particles in the top layer got colored completely because of uniform inlet liquid distribution. There was no bed-scale partial wetting, though white patches were visible at the glass panels. The extent of dry surface on the particles decreased with an increase in the liquid flow rate in the case of prewetted beds as shown in Figure 16. Considering the colored surface to be nearly circular, the extent of partial wetting, that is, the ratio between colored and total surface areas, was estimated for a random sample of 100 particles for each bed. The extent of partial wetting at liquid flow rates of 1, 5, and 8 kg/m2‚s were found to be 0.62, 0.84, and 0.93, respectively. These particles were made of R-alumina, were dimpled, and had an internal surface area of 25-35 m2/g. The other alumina particles (1.9 and 3.5 mm) were of γ-alumina and had an internal surface area of 200-240 m2/g. They seem to differ in their wetting characteristics. Interpretation of Previous Work Some of the previous studies on pressure drop, interfacial areas, and modeling of trickle-bed reactors were interpreted in light of the observed liquid flow texture. Hysteresis In a landmark study, Kan and Greenfield (1978) showed the existence of multiple hydrodynamic states

Figure 16. Bed cross sections of a prewetted bed of alumina particles (dp ) 6.3 mm). Uniform inlet, G ) 0.05 kg/m2‚s.

Figure 17. Hysteresis behavior in beds of glass beads (adapted from Christensen et al., 1986).

in trickle beds. Several investigators have since reported hysteresis of pressure drop and holdup. Christensen et al. (1986) presented an interesting plot of hysteresis behavior (see Figure 17). Loop 1 was obtained on cyclic increasing and decreasing of liquid flow rate between zero and high flow rate corresponding to pulsing regime. Loops 2 and 3 are the asymptotic loops obtained on the cyclic variation of liquid rate between L1 and L2 starting from pulse flow regime and zero liquid rate, respectively. Levec et al. (1988) also examined hysteresis behavior under repeated cycling. They observed much larger hysteresis compared to loop 2 or


Figure 19. Exit liquid flow distribution of glass beads (dp ) 3.5 mm). L ) 2 kg/m2‚s, G ) 0.05 kg/m2‚s.

Figure 18. Hysteresis loops with beds of glass beads (G ) 0.05 kg/m2‚s).

3. Lazzaroni et al. (1989), starting with low liquid rate in nonprewetted beds, traced the path of loop 3 shown in Figure 17. No hysteresis was observed in tracing loop 3. They employed spherical alumina particles. These seemingly contradictory observations can be reconciled as shown below. Way back in 1973, Sedriks and Kenney stressed the importance of prewetting of the bed. From the pictures of the bed sections presented in this work and the CAT scans (Lutran et al., 1991), it is clear that the liquid flow texture in the nonprewetted beds of glass beads is far different from that in prewetted beds. It has been recognized that the variation in the texture of liquid flow causes the hysteresis (Christensen et al., 1986). The start-up procedure and the way steady state was reached dictate the texture, and hence the hysteresis behavior. To check this contention, we examined the hysteresis of pressure drops and exit liquid flow distribution in nonprewetted and prewetted beds with uniform inlet. A Digression. Figure 18 presents the hysteresis behavior of pressure drop with nonprewetted and prewetted beds of glass beads. First, consider the nonprewetted beds. At very low flow rates (L < 0.5 kg/ m2‚s) the channel flow prevailed. The fraction of the cross sectional area covered by the channels was small. At these flow rates, the ∆p is essentially same as that of the dry bed. With an increase in L, the formation of additional channels and/or enlargement of the existing channels took place. Therefore, the ∆p increased along curve 1-i since the effective porosity has decreased. On decreasing the flow rate from the highest value (L ) 5 kg/m2‚s), the areas of channels remained unchanged. But, the liquid-filled core or filaments might have retracted, leaving the surrounding region with film flow. The effective porosity was lower than that in the path 1-i. Therefore, the ∆p was higher in path 1-d than that

in 1-i. On further cyclic increase and decrease in the flow rate, the same loop was traced with 1.6 mm glass beads. However, no hysteresis was observed with 3.5 and 5.7 mm glass beads (data for the latter size are not given for the sake of brevity). Next consider the prewetted bed. Recall that the bed was completely wet, even with single inlet. Therefore, film flow prevailed in the bed with increasing and decreasing liquid flow rates. The hysteresis was larger with the prewetted bed than that of nonprewetted bed as can be seen in Figure 18. To identify the cause of this behavior, we examined the exit liquid distribution at all flow rates. We observed a change in liquid distribution in increasing and decreasing liquid flows for all the flow rates. A typical liquid distribution at L ) 2 kg/m2‚s is shown in Figure 19. It can be seen that the liquid distribution is more uniform for a decreasing than an increasing rate. The nonuniformity of liquid flow distributions led to different effective porosities of the bed, which in turn, gave rise to the hysteresis behavior. Let us now return to the hysteresis behavior reported by Christensen et al. (1986). They prewetted the bed at high gas and liquid flow rates where a pulse regime is encountered. However, they drained the liquid from the bed after terminating the liquid and gas flows. On draining, the films over the particles, connecting the pendular rings, might have ruptured, leaving isolated pendular rings at the contact points of the particles. The mechanism of such a rupture of dewetting films under drainage has been presented by Kheshgi and Scriven (1991). The beds with isolated pendular rings, especially with large diameter particles, could behave as the nonprewetted bed. The lower curve of loop 1 (Figure 17) corresponds to the path 1-i of the nonprewetted bed (of Figure 18). The upper curve of loop 1 (Figure 17) corresponds to 1-d of prewetted beds. Loops 2 and 3 correspond to the hysteresis exhibited by prewetted and nonprewetted beds, respectively. Levec et al. (1988) reported hysteresis behavior under cyclic operation of increasing and decreasing liquid flow rates. The lower limit was almost zero, and the upper limit was in the pulsing regime. Therefore, at the lowest flow rate in each cycle, most of the films might have broken and the bed thus behaved like a nonprewetted bed. The lower curve (of their Figure 3) corresponds to curve 1-i of the nonprewetted bed and the upper curve to curve 1-d of the prewetted bed. Hence, they observed a more pronounced hysteresis behavior compared to loop 2 of Christensen et al. (1986) and the present work.


Figure 20. Hysteresis loops with beds of alumina particles.

Figure 21. Exit liquid flow distribution of alumina particles (dp ) 3.5 mm). L ) 2 kg/m2‚s, G ) 0.05 kg/m2‚s.

Figure 20 shows hysteresis in nonprewetted and prewetted beds of alumina particles. As discussed earlier, the film flow prevails in both the beds. Therefore, as is expected, the hystereses in both the beds were similar to that of the prewetted bed of glass beads. The liquid flow distributions at a typical flow rate (L ) 2 kg/m2‚s) in increasing and decreasing liquid flows are given in Figure 21. Though the beds were wetted completely, severe liquid maldistribution could be seen in Figure 21. The flow rates in the 15th and 16th cells are greater compared to the rest. A liquid-filled core could have formed above the 15th cell with an increasing rate and above the 15th and 16th cells with a decreasing rate in the case of nonprewetted beds. The thickness of films over the cross sectional areas varied considerably as inferred from the flow rates through the cells. As evinced by the figures and standard deviations, with decreasing flow, the thickness of films over the particles

becomes relatively more uniform than with increasing rate, leading to higher pressure drops than those in the former case. Lazzaroni et al. (1989) studied the hysteresis behavior employing 3 mm diameter γ-alumina particles, calcined at 1100 °C unlike in the present work. However, the behavior of the bed was significantly different from the one reported here. They have employed 5 cm diameter cylindrical column, and the liquid was introduced through a point inlet of 3 mm diameter. Their bed attained steady state in 15 min as opposed to 120 min in the present work. The ∆p followed curve 1 of Figure 17 and traced loop 3 at different L2. They observed no hysteresis of loop 3 (see their Figure 1). The behavior is similar to that of nonprewetted beds of 3.5 mm glass beads (Figure 18). The difference in ∆p between prewetted and nonprewetted beds appears to be large and similar to beds of glass beads (see their Figure 2) than those with porous particles. On the other hand, Wammes et al. (1991) claimed that no hysteresis could be observed with beds of thoroughly cleaned glass beads except with aqueous 2 M diethanol amine with antifoaming agents. They used a start-up procedure similar to Levec et al. (1986) and Christensen et al. (1986). Therefore, the curve 1-i (of Figure 18 for dp ) 3.5 mm) is to be expected though there could have been no hysteresis on further cyclic variation with liquid. Thus, it appears that there may not be hysteresis with nonprewetted beds of glass beads (and like particles) of 3 mm and larger beyond the first cycle. The hydrodynamic state of the bed could lie anywhere in loop 1 (Figure 17), depending on the start-up procedure and how the state was reached. If the prewetted beds are to be preferred to nonprewetted beds, the startup procedure suggested by Lazzaroni et al. (1988) appears to be good as it ensures reproducibility and avoids uncertainty in the highest gas and liquid flow rates to be employed in completely wetting the bed. The start-up procedure suggested by Lazzaroni et al. (1988) has been adapted in the present study. Also it is easy to adapt in the case of industrial trickle-bed reactors. However, prewetted beds may tend to nonprewetted beds with volatile liquids as reported by Sedriks and Kenney (1973). The shift to a nonprewetted bed is likely to depend on the evaporation rate and the rate of liquid flow into the films. Interfacial Areas and Wetting Efficiency The fact that there are multiple hydrodynamic states in trickle-bed reactors leads one to expect that there could be different liquid-solid and gas-liquid interfacial areas corresponding to each of these states. In earlier experimental and modeling studies on trickle beds, adequate attention was not paid to the start-up procedure. The validity of the methods used in the determination of interfacial areas depends on the liquid flow texture prevailing in the beds. The interpretation of the data, obtained with beds of spherical or nearspherical particles, has been given below. Liquid-Solid Interfacial Area Let the ratio of area wetted by flowing liquid and the external area of the particle be the effective liquid-solid interfacial area, als. It is also known as wetting efficiency or contacting efficiency. For completely wetted beds with film flow, als would be unity. But, the values


of als reported in the literature are in the range of 0.41.0. Lakota and Levec (1990) and Burghardt et al. (1990) reported that considerable scatter exists in the als data in the literature. Lazzaroni et al. (1989) have shown that the als values in nonprewetted and prewetted beds are far different. Chemical (based on reaction rates; Satterfield, 1975), tracer (Mills and Dudukovic, 1981), dissolution, and dye-adsorption (Lakota and Levec, 1990; Lazzaroni et al., 1988) methods have been employed to determine als. If the channel flow with liquid-filled core prevails, the chemical method gives only the external area of the channel as the gas has no access to the liquid-filled core. If there is particle-scale partial wetting, the chemical method may give larger values of als since the reaction rate with partially wet particles can be grater than that for completely wet particles (Funk et al., 1991). The other three methods could give different values of als for a completely wetted bed. The dye-adsorption method gives als less than one as the stagnant liquid zones may not be accessible to the dye. On the other hand, the liquid distribution in the bed may differ significantly as observed in exit liquid flow distribution measurements (see Figure 20). In the tracer and dissolution methods, the liquid velocity is taken to be uniform throughout the bed in the evaluation of als. This assumption may lead to significant errors. Thus, the scatter in the literature data could be attributed to the liquid flow texture and the methods of evaluation of als. Gas-Liquid Interfacial Area Let alg denote the ratio of gas-liquid interfacial area and the external surface of the particles. For completely wetted beds with film flow, alg would be about unity but for the fractional area covered by pendular rings which is about 0.2-0.3 (Sicardi et al., 1980). Thus maximum value of alg could be about 0.7. However, the alg reported in the literature is much smaller than 0.7. The mass transfer accompanied by the chemical reaction was used to determine alg. Morsi et al. (1980) reported that alg varied from 0.02 to 0.2 with liquid flow rate, for a bed of glass beads of 1.18 mm with a porosity of 0.263. Such an unusually low porosity leads to the channel flow with a nearly liquidful core. As the interior of liquid-filled core is not accessible to the gas, als measured corresponds to the surface area of the liquid-filled core. The low values of alg could be attributed to channel flow with a liquidfilled core. The texture could be expected to be similar to the one shown in Figure 3. Considering the channels in Figure 3 were liquidful, a rough estimate of alg was found to be 0.05. Therefore, it appears that the bed employed by Morsi et al. (1980) corresponds to a nonprewetted one. For alumina particles of 2.4 mm size, alg values are in the range of 0.06 and 0.20 (Morsi et al., 1982). Mahajani and Sharma (1979) measured alg in a bed of 3.96 mm granular carbon particles. The values of alg are in the range of 0.2 and 0.35. Such low values are not consistent with our observations that the porous particles were expected to get completely wet. However, the average velocity of films over large regions could be very small. The criteria, to be satisfied in the evaluation of interfacial area, may not be met over a significant portion of the bed. To be specific, the bulk composition of the reactant in the liquid phase might change due to slow-moving thin films over the particles, and kL might vary widely over the bed. In none of the above studies are the start-up procedures reported.

Recently, Wammes et al. (1991), employing the startup procedure similar to Christensen et al. (1986), found alg to be in the range of 0.4-0.7 for beds with glass beads. The bed can be considered to be a nonprewetted one. However, the aqueous amine solution with an antifoaming agent, used to measure alg, could give rise to different flow texture. The possible violation of the criteria in the determination of alg by the chemical methods needs close examination. Modeling The liquid flow texture should be accounted for in modeling of pressure drop, liquid holdup, and reaction rates. Crine et al. (1980), Marchot et al. (1992), Stanek et al. (1981), Zimmerman and Ng (1986), Melli and Scriven (1991), and Funk et al. (1990) proposed models for detailed liquid distribution in trickle beds. None of these models, in their present forms, can predict some of the observed features: complete wetting of prewetted beds with point inlet, channel flow in nonprewetted beds with uniform inlet distribution (Figure 3), near complete wetting of porous particles in nonprewetted beds with point inlet (Figure 10), and other features. In fact, the present models do not discriminate between nonprewetted and prewetted beds. However, the above models can be adapted to obtain the liquid flow distribution by incorporating the model parameters, governing the liquid flow either site to site or sphere to sphere, deduced from the observed liquid flow texture. The film flow is likely to prevail in beds of porous particles. There is a need to reconcile the observed maximum and the minimum in reaction rates with liquid flow rate in trickle-bed reactors. Ravindra et al. (1994) offered a qualitative explanation for such trends. Work is in progress to explain some of the intriguing phenomena based on the liquid flow texture observed in the present work. Conclusions The start-up procedure dictated liquid flow texture in a trickle bed. The liquid flow texture in a bed of nonporous particles was strikingly different from that of porous particles. Channel flow and bed-scale partial wetting were observed in nonprewetted beds of glass beads at low liquid flow rates. The method of prewetting too appeared to have a major effect on the flow texture. It is important to ensure that the films over the particles are not broken by draining the prewetted bed as it leads to a bed behavior similar to that of a nonprewetted bedsa procedure widely practiced in recent studies. The effect of prewetting was less pronounced with porous particles as the bed got wet, though it took a long time to attain steady state. If the wetting characteristics are good, it was observed that the film flow prevailed for particles larger than 3 mm. There could be film and liquid-filled core regions for particles less than 3 mm. The liquid-filled core could become dominant as the particle size decreases much below 3 mm. The particle-scale partial wetting was observed only with R-alumina particles of 6.3 mm. Particle-scale partial wetting appears to be more of an exception than a rule. Caution needs to be exercised in the use of the particle-scale partial wetting in interpreting the rate data in trickle-bed reactors. The large hysteresis of pressure drop in beds of glass beads, reported in literature, was shown to arise as a


consequence of prewetted and nonprewetted states of the beds. The minor hysteresis observed in prewetted beds was attributed to the change in the extent of fractions of the areas occupied by thick and thin film flow over the particles. A large variation between gasliquid interfacial areas in beds of small diameter particles may be attributed to the channel flow. The liquid inlet distributor had a pronounced effect on liquid flow texture in nonprewetted beds and only a minor effect in prewetted beds. The models available in literature for liquid distribution do not predict the trends observed in the present study. Finally, the present study highlights the importance of liquid flow texture in understanding the complexities of trickle-bed reactors. Acknowledgment The financial support of the Department of Science and Technology, Government of India, under Grant no. DST-93061, is gratefully acknowledged. We are grateful to Norton Company, USA, for the free samples of alumina particles. Notation alg ) ratio of gas-liquid interfacial area and external area of the particle als ) ratio of area wetted by flowing liquid to the external area of the particle dp ) particle diameter, mm G ) gas mass velocity, kg/m2‚s H ) bed depth measured from the top layer, cm kL ) gas-liquid mass-transfer coefficient L ) liquid mass velocity, kg/m2‚s t ) time

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Received for review May 1, 1997 Revised manuscript received July 29, 1997 Accepted August 4, 1997X IE9703088

Abstract published in Advance ACS Abstracts, October 1, 1997. X

Liquid Flow Texture in Trickle-Bed Reactors - American Chemical

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