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Studies on the Performance of a Microscale Trickle Bed Reactor Using Different Sizes of Diluent Shyamal. K. Bej,*,† R. P. Dabral,‡ P. C. Gupta,‡ K. K. Mittal,‡ G. S. Sen,‡ V. K. Kapoor,‡ and Ajay K. Dalai† Catalysis and Chemical Reaction Engineering Laboratory, Department of Chemical Engineering, University of Saskatchewan, Saskatoon, S7N 5C9, Canada, and Indian Institute of Petroleum, Mohkampur, Dehradun 248 005, India Received November 15, 1999
The microscale trickle bed reactor can be used for testing commercial size catalyst, if its inherent limitations, such as channeling, wall effect, and backmixing, are overcome by diluting the catalyst bed with a nonporous inert particles of suitable size. The effect of diluent size on the performance of a microscale trickle bed reactor, catalyst bed height, as well as on operating liquid holdup at different liquid hourly space velocities, has been studied in the present investigation. The proper size of diluent, which may be used for testing as low as 5 mL of commercial catalyst in a microreactor, has been identified experimentally. The results on the hydrodesulfurization of atmospheric gas oil obtained in the microreactor, using the suitable size of diluent were compared with the data generated in a bench scale unit. The activation energy calculated from both microreactor and bench-scale reactor rate data was 21 and 25 kcal/mol, respectively.
Introduction Trickle bed reactors are widely used for hydroprocessing of various petroleum fractions. Generation of reliable experimental data in small-scale trickle bed reactor is needed for the design of commercial reactors, for studying newer catalysts, and also for evaluating alternative feedstocks for an established process. These small-scale, particularly microreactors, are preferred for such investigations due to economic benefits associated with them.1 However, these reactors have number of limitations. The commercial trickle bed reactors are one or 2 orders of magnitude longer than these small reactors. The liquid velocities in these reactors are lower by a factor of 10-100 as compared to commercial reactors.2,3 For a microreactor with an internal diameter of 10-12 mm and having approximately 5 mL of catalyst, the reduction in liquid velocity may be even by a factor of 1000.1 The length of the catalyst bed for a microreactor as mentioned above will be about 50 mm. Low liquid velocity along with small reactor diameter and catalyst bed height causes number of problems in testing catalyst having commercially applied size and shape. The major problems are poor wetting of catalyst, wall effect, and backmixing of liquid.4-7 As a result of * To whom all correspondence should be addressed. Tel.: 306-9662659. Fax: 306-966-4777. E-mail:
[email protected]. † Catalysis and Chemical Reaction Engineering Laboratory. ‡ Indian Institute of Petroleum. (1) Sie, S. T. Rev. Inst. Fr. Petrol. 1991, 46, 501. (2) Takatsuka, T.; Inoue, S.; Wada, Y. Catal. Today 1997, 39, 69. (3) van Klinken, J.; van Dongen, R. H. Chem. Eng. Sci. 1980, 35, 59. (4) Satterfield, C. N. AIChE J. 1975, 21, 209. (5) Dudukovic, M. P.; Mills, P. L. In Encyclopedia of Fluid Mechanics; Cheremisinoff, N. P., Ed.; Gulf Publications Company: Houston, 1986, Chapter 32. (6) Gierman, H. Appl. Catal. 1988, 43, 277.
these drawbacks, the data generated in such a microreactor may not be reliable. The use of small size of catalyst particles is helpful for reducing these phenomena of channeling and backmixing of liquid in microreactors. As per the guidelines available in the literature to overcome the wall effects in these reactors, the maximum catalyst particle diameter that can be tested in a reactor with an internal diameter of 10-12 mm is about 0.4 mm.8-10 Similarly, backmixing could be minimized in a microreactor in the conversion range of 85% and higher, when catalyst particle diameter is less than 0.2 mm.1 Also, a catalyst particle with diameter of less than 0.5 mm should be used in microreactor to have complete catalyst wetting.6 Therefore, a commercial catalyst (having diameter of about 1.5 mm) should be crushed to at least 0.2 mm diameter for testing in a microreactor to avoid wall effect, backmixing and incomplete wetting of catalyst. But since the reactions such as hydrodesulfurization of heavier gas oil fractions are generally diffusion controlled, the data generated using crushed catalyst may not be very meaningful for commercial applications where comparatively larger size of catalyst particles are used.1 The effective solution that is recommended and widely accepted to handle the problem in testing commercial catalyst in small-scale reactors, is to use the catalyst in original form but diluted with nonporous, inert particles or fines. A number of researchers have studied on the use of fine diluent in small-scale reactors and (7) Ruecker, C. M.; Hess, R. K.; Akgerman, A. Chem. Eng. Commun. 1987, 49, 301. (8) Fahien, R. W.; Stankovic, I. M. Chem. Eng. Sci. 1979, 34, 1350. (9) Chu, C. F.; Ng, K. M. AIChE J. 1989, 35, 146. (10) Zimmerman, S. P.; Ng, K. M. Chem. Eng. Sci. 1986, 41, 861.
10.1021/ef990238c CCC: $19.00 © 2000 American Chemical Society Published on Web 04/26/2000
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Figure 1. Simplified schematic diagram of the experimental setup.
have attempted to overcome these drawbacks.11-16 However, most of these studies have been restricted to bench scale reactors where about 50-100 mL of catalysts were used. Only few studies are available providing information on the effect of diluent size on a microreactor performance while testing as low as 5 mL of catalyst of commercially applied size and shape.1 The development of a suitable dilution technique in microscale trickle bed reactor would provide a very low cost option for generating reliable data for scale-up and scale down activities. Thus, there is a need for systematic study in this direction. This work focuses on the effect of diluent size on the performance of a microreactor using as low as 5 mL of commercial catalyst. Also, proper size of diluent is identified in this study for generating meaningful data to test commercial catalysts. The results obtained for hydrodesulfurization of atmospheric gas oil under pressure in a microreactor with the diluent of suitable size were then compared with the data generated in a bench scale unit containing 100 mL of catalyst. The effect of diluent size on the operating liquid holdup of the bed was also investigated in the present study. Experimental Setup and Procedure The feedstock used in this study was straight run atmospheric gas oil containing 1.47 wt % sulfur. A simplified schematic diagram of the experimental setup used for the present study is shown in Figure 1. The system consisted of liquid and gas feeding units, a reactor, a heater with temperature controller for precisely controlling the temperature of the catalyst bed and a high-pressure gas-liquid separator. The experiments using 5 mL of catalyst were carried out in a microreactor. The length and internal diameter of the reactor were 300 and 13 mm, respectively. The catalyst was a trilobe shaped commercial Co-Mo/alumina with a diameter of 1.5 mm. The length/diameter ratio of the catalyst was around 3-4. Inert, nonporous silicon carbide of different sizes was used as (11) Carruthers, J. D.; DiCamillo, D. J. Appl. Catal. 1988, 43, 253. (12) Al-Dahhan, M. H.; Wu, Y.: Dudukovic, M. P. Ind. Eng. Chem. Res. 1995, 34, 741. (13) Garica, W.; Pazos, J. M. Chem. Eng. Sci. 1982, 37, 1589. (14) Diaz, R. A.; Mann, R. S.; Sambi, I. S. Ind. Eng. Chem. Res. 1993, 32, 1355. (15) Chen, Y. W.; Hsu, W. C.; Lin, C. S.; Kang, B. C.; Wu, S. T.; Leu, L. J.; Wu, J. C. Ind. Eng. Chem. Res. 1990, 29, 1831. (16) Al-Dahhan, M. P.; Dudukovic, M. P. AIChE J. 1996, 42, 2594.
Dabral et al. diluent in the catalyst bed. In all the experiments, the ratio of the volume of catalyst to that of diluent was kept constant at 1.0. The catalyst and the diluent were loaded in the reactor as per the following procedure: Initially, 2.5 mL of commercial size catalyst was loaded in the reactor followed by the loading of 2.5 mL of selected size of silicon carbide. The reactor was vibrated gently for allowing the diluent to settle in the voids of the catalyst bed. The remaining 2.5 mL of catalyst and an equal quantity of diluent were then loaded following the same procedure as mentioned above. Suitable sizes of diluent were also used at the top and bottom of the reactor for proper mixing of the incoming and outgoing streams, as well as for supporting the catalyst bed. The experimental studies using 100 mL of catalyst was carried out in a computer controlled bench scale unit. The general facilities of the bench scale unit were similar to the micro unit as shown in Figure 1. In this case, the length and the internal diameter of the reactor were 1000 mm and 25 mm, respectively. Co-Mo/alumina commercial catalyst (100 mL) and an equal volume of silicon carbide of 0.19 mm size were loaded one after another in a lot of 10 mL each following similar procedure as mentioned above with intermittent vibration of the reactor. Both microreactor and bench-scale reactors were operated in cocurrent down flow of gas and liquid. After loading the catalyst and diluent, the temperature of the reactor was increased in the presence of hydrogen (99.99% purity) at a flow rate of 2.5 L/h from ambient to 175 °C temperature within 7 h. After this period of initial heating, the reactor was pressurized to 35 bar with hydrogen and the catalyst was sulfided using dimethyl disulfide in atmospheric gas oil. The sulfidation was carried out for a period of 24 h during which the temperature of the reactor was increased from 175 to 340 °C in a programmed way. After sulfidation was over, the temperature of the reactor was increased to the desired reaction temperature and the feed gas oil was passed at the required rate. The product oil samples were collected at different intervals of time after steady state was reached and were analyzed for their sulfur contents using X-ray fluorescence technique. The effect of diluent size on the increase in the catalyst bed height was studied in a graduated glass cylinder having diameter as that of the microreactor. Five milliliters of catalyst, and equal volume of diluent, was loaded in the measuring cylinder following similar procedure as mentioned earlier. The total length of the catalyst bed for each size of diluent was noted from the graduated cylinder. The operating liquid holdup of the catalyst bed was determined using the method of draining in a glass reactor having a volume of 50 mL and same internal diameter as of the microreactor. In this case, the catalyst and the diluent were also loaded as per the procedure mentioned above. Water and air were used as liquid and gaseous phase, respectively, for this study. The details of the draining method used for determining the liquid holdup is described elsewhere.17 The operating liquid holdup was expressed as volume of liquid /total volume of the packed bed.
Results and Discussion All the experiments were carried out at a constant hydrogen/gas oil ratio of 500 L/L. No appreciable deactivation was observed during the experiments. Generally, the activity of catalyst is measured in terms of percent conversion of the sulfur compounds initially present in the feedstock. But to magnify the small differences in conversion among different experiments, (17) Chander, A.; Kundu, A.; Bej, S. K.; Dalai, A. K.; Vohra, D. K. Fuel 2000. In press.
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Figure 2. Effect of diluent size on apparent rate constant of HDS in a microreactor for two levels of LHSV, and at a constant temperature of 340 °C and a hydrogen/gas oil ratio of 500 L/L.
the apparent rate constant for the reaction rather than the conversion was used as a measure of the catalytic activity for hydrodesulfurization of gas oil. The apparent rate constant was calculated from the n-th order rate equation (see eq 1). Various authors18-22 have found that the value of n lies close to 1.5-1.7 for hydrodesulfurization of atmospheric gas oil. For the present study, the value of n is assumed to be 1.65. A similar value for the order of the reaction has been used by de Wind et al. 22 for hydrodesulfurization of gas oil over commercial hydrotreating catalyst.
k)
[
]
1 1 1 [LHSV-] n-1 S n-1 S n-1 p f
(1)
where,
k ) apparent rate constant for hydrodesulfurization of atmospheric gas oil, h-1(wt %)-0.65 n ) order of hydrodesulfurization reaction Sp ) sulfur in product, wt % Sf ) sulfur in feed, wt % LHSV ) liquid hourly space velocity, h-1 Effect of Diluent Size. The effect of diluent size on the catalyst performance for HDS reaction in the microreactor was studied at two different liquid hourly space velocities (LHSV) of 1.0 and 3.0 h-1. At each space velocity, experiments were conducted for five different sizes of diluent while keeping the temperature and the hydrogen/gas oil ratio constant at 340 °C and 500 L/L, respectively. The results are shown in Figure 2. The figure indicates that the performance of the catalyst was (18) De Bruijn, H. In Proceedings of the 6th International Symposium On Catalysis; Bond, G. C.; Wells, P. B., Tomkins, F. C., Ed. The Chemical Society Publishers: London, 1976; p 372. (19) Mann, R. S.; Sambi, I. S.; Khulbe, K. C. Ind. Eng. Chem. Res. 1988, 27, 1789. (20) Takatsuka, T.; Wada, Y.; Suzuki, H.; Komatsu, S.; Morimura, Y. J. Jpn. Pet. Ist. 1992, 35, 179. (21) Lecrenay, E.; Sakanishi, K.; Mochida, I. Catal. Today 1997, 39, 13. (22) de Wind, M.; Plantenga, F. L.; Hinerman, J. J. L.; Homan Free, H. W. Appl. Catal. 1988, 43, 239.
Figure 3. Effect of ratio of microreactor diameter to diluent size on apparent rate constant of HDS for two levels of LHSV and at a constant temperature of 340 °C and a hydrogen/gas oil ratio of 500 L/L.
dependent on the size of diluent used. For example, the value of the apparent rate constant for HDS process increased with decrease in diluent size especially in the range of 0.19-0.25 mm. With further decrease in diluent size up to 0.16 mm, the values of apparent rate constant did not change. Lowering the diluent size, which increased the ratio of the reactor diameter to the diameter of the diluent, reduced the wall effect. The values of the apparent rate constant as a function of the ratio of reactor diameter to the diluent size are plotted in Figure 3. The reduced wall effect as well as axial back mixing helped to increase the performance of the catalyst. However, the magnitude of this change was dependent on the level of space velocity used. For example, the effect of diluent size on apparent rate constant (see Figure 2) was more prominent for LHSV of 1.0 h-1 as compared to that for LHSV of 3.0 h-1. The sharp increases in the performance of the reactor when diluent size was reduced from 0.25 to 0.19 mm could be explained as follows: The catalyst bed contains appreciable amount of void space. Filling up the void space with appropriate size of diluent could increase the liquid holdup of the bed, an important parameter influencing the performance of trickle bed reactor. When a fine size of diluent particles was used, the frictional resistance preventing the flow of liquid became more as against the gravitational force, and as a result of this, the liquid holdup of the bed increased. The higher liquid holdup helped in higher wetting, and hence, better utilization of catalysts. When a larger size of diluent (0.25 mm and above) was used, it could not enter in the void between catalyst particles. When the size of diluent particle was about 0.2 mm, the particle could perhaps enter into the narrow void space, and thus, could fill up this space in the catalyst bed. The filling up of the void space of the catalyst bed by suitable size of diluent is also related to the increase in catalyst bed height, which could change the operating liquid holdup of the bed. The increases in catalyst bed height as well as the operating liquid holdup due to addition of different sizes of diluent were determined for various sizes of diluent. The results are shown in Figure 4. The figure shows that as expected, the percent increase in the height of the catalyst bed was very low when diluent particles with size in the range of 0.160.19 mm were used. This indicated that particles of these sizes could fill up the voids inside the catalyst bed. Muthanna et al.12 have also reported that the expansion
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Figure 4. Effect of diluent size on increase in bed height and operating liquid holdup at a LHSV of 1.0 h-1 and at an air/ water ratio of 500 L/L.
in the bed height due to the addition of fines of 0.21 mm size was within 2%. As the size of the diluent particle was increased from 0.19 to 0.25 mm, a sharp increase in the bed height was observed (see Figure 4) indicating only partial filling of the void space inside the catalyst bed. As the size of the diluent was increased further to 0.55 mm, the particles could not enter the void space at all indicating almost 100 percent increase in bed height. Interestingly, the trend in change in operating liquid holdup followed a reverse pattern. For example, the operating liquid hold of the catalyst bed increased steadily and showed a sharp change for the diluent size in the range of 0.25-0.19 mm. The liquid holdup maintained the increasing trend even with the reduction of diluent size from 0.19 to 0.16 mm. However, the values of apparent rate constant did not show any improvement with further reduction of diluent size from 0.19 to 0.16 mm (see Figure 2). This comparison showed that beyond a certain level of liquid holdup (about 0.18 mL/mL), which is perhaps required to completely wet the catalyst bed, the effect of liquid holdup on the performance of the reactor is negligible. Therefore, decrease in the size of the diluent particle caused improvement in liquid holdup in the packed bed, reduction in wall effect and axial backmixing of liquid, which improved the performance of the catalyst. Effect of Space Velocity. The effect of LHSV (1-7 h-1) on apparent rate constant for HDS reaction was studied for four different diluent sizes, such as 0.16, 0.19, 0.25, and 0.77 mm. The temperature and the hydrogen/gas oil ratio were kept constant at 340 °C and 500 L/L, respectively. The results are shown in Figure 5. It is interesting to note that for diluent sizes of 0.16 and 0.19 mm, the values of apparent rate constants were almost constant at about 6.3 h-1(wt %)-0.65 and were independent of space velocities. This indicated that even at low LHSV of 1.0 h-1, the drawbacks, such as channeling, axial back mixing, and wall effect reducing the reactor performance, were eliminated when very fine size of diluent was used. On the other hand, for a diluent size of 0.25 mm, the rate of reaction increased with the increase in LHSV and reached a maximum value at liquid flow corresponding to LHSV of around 5.0-6.0 hr-1. A similar trend was also observed for diluent size of 0.77 mm except that comparatively higher flow rates would be required to eliminate the effects of channeling and axial backmixing of liquid.
Dabral et al.
Figure 5. Effect of LHSV on apparent rate constant of HDS in a microreactor for three different sizes of diluent and at a constant temperature of 340 °C and a hydrogen/gas oil ratio of 500 L/L.
Figure 6. Comparison of apparent rate constants obtained from microreactor and bench-scale reactor data for different temperatures and at a constant LHSV of 1.0 hr-1 and a hydrogen/gas oil ratio of 500 L/L.
Therefore, larger size of diluent could also be used for generating reliable data in microscale trickle bed reactor but with higher space velocities. However, unfortunately, the use of higher LHSV would give much less conversion and hence, the data generated could not be compared directly with those from commercial reactors where much higher conversions are obtained due to lower space velocities used in these systems. Comparison on the Performances of Micro and Bench-Scale Reactor. The above results indicate that if a diluent size of 0.19 mm or smaller is used, it could eliminate the shortcomings of a microreactor for testing catalyst in commercially applied shape and process conditions. It has been established by previous researchers that use of 0.2-0.5 mm of diluent can remove all the limitations of a bench scale unit in testing commercial shaped catalyst.1,11-16 Thus, the results obtained from the bench scale reactor containing 100 mL of catalyst and diluted with 0.19 mm of particles were considered as the reference data for gauging the performance of the microreactor. The performances of the microreactor using 5 mL of commercial catalyst and diluent of 0.19 mm size were compared with those obtained from a bench scale unit using 100 mL of catalyst. The diluent size used in the bench scale unit was also 0.19 mm. The performances of these two reactors were compared (see Figure 6) for a range of temperatures keeping LHSV constant at 1.0 h-1. The
Performance of a microscale Trickle Bed Reactor
hydrogen/gas oil ratio was also kept constant at 500 L/L. The figure shows that the performances of these two units are comparable to each other over the entire range of temperatures investigated. For both the cases, the values of apparent rate constants also show similar increasing trend with increase in temperature. Apparent Activation Energy for Hydrodesulfurization of Gas Oil. The apparent activation energy for the HDS of atmospheric gas oil was calculated from both microreactor and bench-scale reactor data. The Arrehenius plots for the sets of rate data generated from these two units are shown in Figure 7. The apparent activation energy was 21 and 25 kcal/mol, respectively, for the data generated from the micro and bench-scale unit. These values are in good agreement with each other, and also with those published in the literature.14,23 Conclusions This study confirmed that as the size of diluent was decreased, the performance of a microscale trickle bed reactor improved and reached a steady value after a certain size of diluent. The decrease in the diluent size to 0.16-0.19 mm increased the liquid holdup, reduced the wall effect and hence, improved the performance of the microreactor. The results obtained from such a (23) Li, C.; Chen, Y. W.; Yang, S. J.; Wu, J. C. Ind. Eng. Chem. Res. 1993, 32, 1573.
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Figure 7. Comparison of Arrhenious plots of rate data obtained from microreactor and bench-scale reactor.
reactor containing 5 mL of catalyst was comparable with those from a bench scale unit using similar size of diluent and 100 mL of commercial catalyst. Thus, the present study established that a microreactor using as low as 5 mL of catalyst having commercially applied size and shape could be used as a tool for producing reliable and meaningful data for scaling up and scaling down the commercial trickle bed reactor. The activation energy for HDS of atmospheric gas oil were calculated and found to be in good agreement with those reported earlier in the literature. EF990238C
