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Ind. Eng. Chem. Res. 1996, 35, 926-930
Liquid and Gas Distribution in Trickle-Bed Reactors Lars Bargsteen Møller,† Christian Halken,† Jens A. Hansen,*,‡ and Jesper Bartholdy‡ Technical University of Denmark and Haldor Topsøe Research Laboratories, DK-2800 Lyngby, Denmark
Liquid and gas distribution in trickle-bed reactors was investigated in a column packed with commercial catalyst particles. Distilled water and air were used as liquid and gas phases, respectively. Surface tension effects were tested by adding detergent to the water. The influence of both liquid load and gas load on the distribution was studied. Flow rates corresponded to those used in industrial hydroprocessing units. It was found that the liquid distribution at a given liquid load can be improved considerably by either increasing the liquid load or flooding the column in advance. The gas distribution is shown to be correlated inversely with the liquid distribution. Use of a large-particle top layer results in an improved distribution. Introduction In the refining industry, the flow distribution in hydroprocessing trickle-bed reactors is often not ideal. Some parts of the catalyst bed are not in contact with the liquid, whereas other parts are overloaded. These deviations from ideal flow result in a loss of catalyst utilization and create hot spots, etc. The commercial aspect of improving the flow distribution is an increased capacity and a longer cycle length for existing plants. New plants can be made smaller and are less expensive. For a concurrent downflow trickle-bed reactor, four different flow patterns exist: trickling flow, pulsing flow, spray flow, and bubble flow (Satterfield, 1975; Holub et al., 1993; Ng and Chu, 1987). The normal flow pattern found in hydroprocessing units is trickling flow. Our investigations have, therefore, been carried out in the trickling flow regime. At a sufficiently low liquid flow, the catalyst particles will only be partially wetted (partial wetting regime). If the liquid flow rate is increased, the partial wetting regime will gradually change to a complete wetting regime (Ng and Chu, 1987). Liquid distribution in a trickle-bed reactor can be improved by radial dispersion, giving better utilization of the bed. Radial dispersion takes place at the points where the catalyst particles touch each other (Herskowitz and Smith, 1978; Zimmermann and Ng, 1986). When the reactor is dense loaded, the void fraction can be reduced by up to 15% as compared with sock loading (Christensen, 1993) and the number of contact points increases. Also the particle size is important for the radial dispersion: large particles give large radial dispersion (Zimmermann et al., 1987). At the same time the particle orientation is important. For cylindrical catalyst particles, a good radial dispersion is obtained if the cylinders are orientated horizontally (Tukac and Hanika, 1992; Snow and Grosboll, 1977), whereas channeling appears when the particles are orientated vertically (Snow and Grosboll, 1977). Low radial dispersion often gives channeling. Channels will be formed at the top of the catalyst layer, and most of the liquid will flow down through these (Sundaresan, 1994). This is predominantly seen in the case of * Author to whom all correspondence should be addressed. E-mail:
[email protected]. Telefax: +45 45 27 29 99. † Technical University of Denmark. ‡ Haldor Topsøe Research Laboratories.
0888-5885/96/2635-0926$12.00/0
low liquid flows. Channels formed at a low flow will expand gradually as the liquid flow is increased. Kan and Greenfield (1978) have found that there is a higher risk of channeling in a bed of small particles. Uneven catalyst loading and nonuniform liquid inlet distribution can also give channeling (Koros, 1986). Channeling is more pronounced in a bed of dry particles and it has thus been found that it can be minimized by flooding the catalyst bed (Satterfield, 1975; Ng and Chu, 1987; Lutran et al., 1991). The surface tension of the liquid has an impact on the tendency to form channels. Lutran et al. (1991) showed that pronounced channeling occurred when the surface tension of the test fluid (water) was reduced from 72 to 28 dyn/cm. Herskowitz and Smith (1978) concluded that the surface tension has no influence. These deviating observations may be explained by the fact that the studies carried out by Herskowitz and Smith (1978) have been performed with gas, whereas Lutran et al. (1991) only used a liquid phase. It has been our aim to simulate the conditions used in hydroprocessing. A semiindustrial distributor design was applied, and realistic gas/liquid ratios and commercial catalyst particles have been used. Experimental Setup Distilled water and air were used for measurement of liquid and gas distribution in a packed bed of catalyst particles of different sizes and shapes. The experiments were conducted at ambient conditions. The influence of surface tension was studied by using a 1 wt % aqueous solution of AG 6202 (Berol Nobel AB), which has a surface tension of 33 dyn/cm. Equipment. The experiments were conducted in a plexiglas column (i.d. 144 mm, height 770 mm); see Figure 1a. The liquid and gas entered at the top of the column. The distributor plate was made of plexiglas and had eight stainless steel nozzles mounted as shown in Figure 1b. The design of the nozzles is given in Figure 1c. The catalyst bed was supported on a grid placed on top of the collector shown in Figure 1d. As can be seen, the collector is divided into eight sections. Each section can be connected to a collector vessel in which liquid and gas are separated. The liquid flow is measured by determining the time it takes to collect a given volume in the collector vessel. The gas velocity from the exit tube of the collector vessel is measured by means of a thermal anemometer (Airflow TA5). © 1996 American Chemical Society
Ind. Eng. Chem. Res., Vol. 35, No. 3, 1996 927
Figure 1. Experimental setup. The column including utilities is shown in a. The layout of the distributor plate is given in b and the nozzle design in c. The collector with eight sections is shown in d.
Figure 2. Liquid distribution from the distributor plate at 2 and 4 L/min. The actual liquid flow (L/min) for each nozzle is also given.
Verification of Distributor Plate. Before the experiments were carried out, the design of the distributor plate was checked at the following liquid flows: 2 L/min (7200 kg/m2/h) and 4 L/min (14 400 kg/ m2/h). The results are given in Figure 2. The area of each circle is proportional to the liquid flux relative to the section having the largest flow. The numbers given are the actual flow rates for each nozzle. For both liquid flow rates, almost ideal distribution was obtained. Packing Material. The flow distribution studies were performed on commercially available catalyst particles and topping materials. The shapes and sizes are shown in Figure 3. The majority of the experiments were carried out with 1/16-in. cylindrical extrudates. In some experiments, a topping layer of cylindrical tablets penetrated by 7 axial holes and with convex ends (TK10) was used in order to show the impact of the latter on the flow distribution. The loading procedure used was equivalent to that of dense loading. Gas and Liquid Flow Rates. Liquid and gas flow rates simulating those used in industrial hydrotreaters were studied. Liquid flows from 7200 to 28 800 kg/m2/h were used. The gas flow range at ambient conditions (0 to 243 m3/m2/h) corresponds to a gas/liquid ratio from 0 to 800 N m3/m3 for a hydrotreater operating at 350 °C and 50 atm.
Figure 3. Packing material. Cylindrical extrudates (1/16 in.) on the left-hand side and the cylindrical tablets (TK-10) on the righthand side. The size of the cylindrical tablet is 5/8 × 7/16 in. or 16 × 11 mm (o.d. × H). The i.d. of the axial holes is 9/64 (3.4 mm).
Reactor Utilization for a Nonideal Trickle-Bed Reactor A simple parameter to express the actual flow distribution as compared with an ideal distribution has been defined by using a second-order rate expression. The rate constant for a second-order reaction in a plug flow reactor is given by:
(
kideal ) LHSV
1 1 Cp Cf
)
(1)
where LHSV is the space velocity and Cp and Cf are the product and feed concentrations, respectively. By assuming that the test column consists of eight parallel plug flow reactors with a liquid load corresponding to that leaving each section, the product concentration in section i can be found from:
Cpi )
(
kideal 1 + LHSVi Cf
)
-1
(2)
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Figure 4. Influence of liquid load on a dry bed. The gas load was 184 m3/m2/h. η values are given in percent for 90% conversion.
The overall product concentration can then be calculated from:
Cpoverall )
∑FiCp ∑Fi
i
(3)
where Fi is the liquid flow leaving section i. The reactor utilization is thus given by:
koverall η) ) kideal
(
LHSV
1 1 Cpoverall Cf kideal
)
(4)
In this paper, the following values have been used: Cp ) 0.1Cf , Cf ) 2.9, and kideal ) 31, which could represent a typical conversion level for a hydrotreater removing 90% of the sulfur. It has been verified that η for all practical conversion levels is essentially constant. Results and Discussion Influence of Liquid Load. A packed bed of dried cylindrical extrudates was brought into contact with air/water. During the experiment, the gas load was kept constant at 184 m3/m2/h. The liquid load was changed as follows: 7200 f 14 400 f 7200 f 21 600 f 7200 f 28 800 f 7200 kg/m2/h. The liquid flow distribution is given in Figure 4. High liquid loads improve the flow distribution. After reaching the highest liquid load, a permanent improvement of the liquid distribution is seen even when the load is reduced to its starting value of 7200 kg/m2/h. The poor liquid distribution for this bed might indicate that the results are influenced by a nonideal catalyst packing, where radial dispersion is suppressed by channeling at the lowest liquid loads. As the liquid load is further increased, the diameter of the filaments will also increase, which leads to formation of new filaments. In a structural packing such as cylindrical extrudates, these will have a tendency to flow sideways 1/ -in. 16
and thereby improve the flow distribution. When the liquid load is decreased again, some of the liquid pockets formed at the high liquid load will tend to stay in the bed and the filament flow will gradually be replaced by film flow, whereby the liquid distribution improves. The liquid distribution is significantly improved in this case by increasing the liquid load. All other experiments with gas flow confirm that increasing liquid load leads to improved distribution even though the distribution obtained initially due to a more ideally packed bed was better than the one achieved at a liquid flow of 7200 kg/m2/h according to Figure 4. Influence of Flooding. The influence of flooding was studied in a repacked bed using a liquid load ranging from 7200 to 21 600 kg/m2/h. First a series of measurements was carried out on the dry bed. The bed was then flooded completely by filling the entire column with water. After 15 min, the column was drained, and the distribution at the same liquid loads as above was determined. The impact of flooding can be seen in Figure 5. A clear improvement in the flow distribution is seen for the flooded reactor bed as compared with that of the dry bed. To some extent, high liquid loads can substitute complete flooding. This can be seen by comparing the improvements observed at high liquid loads in Figure 4 with those obtained by complete flooding (Figure 5). The influence of the high liquid load can also be seen from Figure 5. Note the improvement obtained after the highest liquid load (21 600 kg/m2/h) has been used. This suggests that the use of a high liquid load in a limited period of time is a practical way of improving the liquid distribution in an industrial unit. The same conclusion has been reached by Christensen et al. (1986) in a rectangular bed. Influence of Gas Load. The influence of gas load rate was studied in a bed similar to the one previously used. Contrary to the other experiments, the gas load was varied in this experiment. The change in gas load only marginally influences the liquid distribution in a dry bed as can be seen from Table 1. However, one point deviates, namely, that for the lowest liquid and gas load.
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Figure 6. Relationship between the percentage of the total gas and liquid load leaving each collector vessel for both a dry (square) and a flooded bed (triangle). Table 3. Top Layer on a Flooded Bed (Gas Load Corresponds to 250 N m3/m3 Liquid at 350 °C and 50 atm; η Values in % for 90% Conversion) liquid load (kg/m2/h) feed from all inlets one central inlet (hole no. 1) Figure 5. Influence of flooding the bed as compared to a dry bed for the same loads. The gas/liquid ratio was 200-600 N m3/m3. η values are given in percent for 90% conversion. Table 1. Influence of Gas Load on a Dry Bed (η Values in % for 90% Conversion) gas load (m3/m2/h) liquid load
(kg/m2/h)
7 200 14 400 21 600
110
184
243
79.1 94.2 93.9
87.5 90.6 96.4
87.0 92.0 95.4
Table 2. Influence of Surface Tension (No Flowing Gas Present; η Values in % for 90% Conversion) liquid load (kg/m2/h) water water with AG 6202
7200
14 400
21 600
7200
95.9 95.4
91.5 44.4
37.6 53.7
96.0 95.5
This combination gives by far the lowest reactor efficiency. Why this combination of liquid and gas load gives a low reactor efficiency is unresolved, but it is suspected that the deviation is due to the fact that either the liquid and/or the gas load is below a certain critical value. For the experiment with pure water and no gas load, a slight drop in reactor efficiency (Table 2) is seen when increasing the liquid load from 7200 to 14 400 kg/m2/h. However, increasing the load from 14 400 to 21 600 kg/ m2/h reduces the reactor efficiency to less than 50%. This response to increased liquid load was unexpected, as prior publications in this field have shown that the flow distribution improves when the liquid load is increased (Lutran et al., 1991). Lutran et al. (1991) used spherical particles which might account for the different results. From the above it is demonstrated that the presence of a flowing gas phase is essential when studying flow
7200
14 400
21 600
96.3 96.2
96.5 92.7
95.6 90.1
distributions in trickle-bed reactors. The value of the gas load is of less importance. From a series of experiments carried out at a constant gas/liquid ratio, the relationship between the gas and liquid distribution was deduced. The correlation between gas and liquid loads is shown in Figure 6, which includes data from both dry and flooded beds. As expected, the gas and liquid flows are inversely correlated. Influence of Surface Tension. Surface tension of hydrocarbons in industrial trickle-bed operation deviates considerably from that of water. It is, therefore, highly relevant to investigate how surface tension affects the flow distribution. By adding detergents, alcohol, etc., to water, the surface tension can be lowered significantly. However, most surfactants (even alcohols) give a heavy foaming liquid particularly in a trickle bed where there is an intense interaction between gas and liquid. For this reason it was only possible to perform tests without a flowing gas. The addition of detergent had no impact on the flow distribution at the low liquid load (Table 2). However, an increase of the load gave a significant reduction in reactor efficiency, as was the case for the test with pure water (Table 2). This finding suggests that reducing the surface tension does not improve the flow distribution. However, no flowing gas phase is present, and a translation of these results into industrial trickle-bed reactors is not straightforward. Influence of the Top Layer. A bed was packed with 1/16-in. cylindrical extrudates except for the top 10 cm which was packed with large particles (TK-10). The purpose of this was to see if a large-particle top layer had any distribution qualities. From Table 3 it is seen that almost the same reactor efficiency was obtained when feeding from one central inlet as when feeding from all inlets. This indicates that the top layer compensates for the poor inlet distribution.
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material was demonstrated to have a positive effect on the flow distribution. Acknowledgment We thank Per Løngaa Sonne for his assistance with the figures and tables. Notation c: concentration, wt % F: liquid flow, kg/h k: second-order rate constant, h-1 LHSV: linear hourly space velocity, h-1 η: reactor utilization parameter (eq 4) Subscripts f: feed i: reactor section p: product
Literature Cited
Figure 7. Packed bed of TK-10 showing the orientation of the individual tablets.
It is quite common in the industry to use inert, compact ceramic balls (3/4 and/or 1/4 in. o.d.) as the topping layer in order to fix the catalyst bed and to some extent protect the catalyst from various impurities in the feed. In many respects, TK-10 is a better alternative for this service (Christensen, 1993). Due to the size of the balls, the predominating flow is film flow, however, without the presence of liquid pockets. The radial dispersion is, therefore, quite low. Film flow without liquid pockets is also observed with the TK-10 material, but the void fraction is much higher. The total void fraction of a layer of TK-10 is 55% as compared with 33% for the compact balls, but, due to the seven holes, the void fraction between the TK-10 particles is 36% and thus comparable with that of the balls. In a randomly packed bed of TK-10, a majority of the particles will be placed in such a way that the holes are orientated nonvertically, as shown in Figure 7. This increases the radial dispersion as compared with that of the more uniform packing of the balls. Conclusions From this study of flow distributions in trickle-bed reactors using water and air, it has been found that, among the factors investigated, the prewetting of the catalyst bed is the most important factor improving the flow distribution. To a large extent, high liquid loads have the same effect as prewetting on the flow distribution. The presence of a flowing gas phase is important when studying the flow distribution in a trickle-bed reactor. When no flowing gas is present, experiments on a bed of porous cylindrical extrudates have a very poor flow distribution, particularly when the liquid load was high. A further deterioration of the liquid distribution was observed when the surface tension of the water was lowered. The use of special, large-particle topping
Christensen, P. Pressure Drop and Liquid Distribution Problems in Hydrotreater Units. Internal report; Haldor Topsøe A/S: Lyngby, Denmark, 1993. Christensen, G.; McGovern, S. J.; Sundaresan, S. Cocurrent Downflow of Air and Water in a Two-Dimensional Packed Column. AIChE J. 1986, 32, 1677. Herskowitz, M.; Smith, J. M. Liquid Distribution in Trickle Bed Reactors. AIChE J. 1978, 24 (3), 439. Holub, R. A.; Dudukovic, M. P.; Ramachandran, P. A. Pressure Drop, Liquid Hold-Up and Flow Regime Transition in Trickle Flow. AIChE J. 1993, 39 (2), 302. Kan, K.-M.; Greenfield, P. F. Multiple Hydrodynamic States in Concurrent Two-Phase Downflow through Packed Beds. Ind. Eng. Chem. Process. Des. Dev. 1978, 17 (4), 482. Koros, R. M. Engineering Aspects of Trickle Bed Reactors. In Chemical Reactor Design and Technology; DeLasa, H., Eds.; Martinus Nijhoff: Dordrecht, The Netherlands, 1986; pp 579630. Lutran, P. G.; Ng, K. M.; Delikat, E. P. Liquid Distribution in Trickle Beds. An Experimental Study Using Computer-Assisted Tomography. Ind. Eng. Chem. Res. 1991, 30, 1270. Ng, K. M.; Chu, C. F. Trickle-Bed Reactors. Chem. Eng. Prog. 1987, 83 (11), 55. Satterfield, C. N. Trickle-Bed Reactors. AIChE J. 1975, 21 (2), 209. Snow, A. I.; Grosboll, M. P. Good Catalyst Loading Benefits Operations. Oil Gas J. 1977, 75 (21), 61. Sundaresan, S. Liquid Distribution in Trickle Bed Reactor. Energy Fuels 1994, 8, 531. Tukac, V.; Hanika, J. Influence of Catalyst Particles Orientation on the Pressure Drop and the Liquid Dispersion in the Trickle Bed Reactor. Chem. Eng. Sci. 1992, 47, 2227. Zimmermann, S. P.; Ng, K. M. Liquid Distribution in Trickling Flow Trickle-Bed Reactors. Chem. Eng. Sci. 1986, 41 (4), 861. Zimmermann, S. P.; Chu, C. F.; Ng, K. M. Axial and Radial Dispersion in Trickle-Bed Reactors with Trickling Gas-Liquid Down-Flow. Chem. Eng. Commun. 1987, 50, 213.
Received for review July 31, 1995 Accepted December 4, 1995X IE950478P
Abstract published in Advance ACS Abstracts, February 1, 1996. X
