Operation of a Miniscale String Bed Reactor in Spiral Form at

The effect of the catalyst mass, string bed length and the fluid velocities on the performance of the spiral reactor concept is examined. The stabilit...
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Ind. Eng. Chem. Res. 2007, 46, 5531-5535

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Operation of a Miniscale String Bed Reactor in Spiral Form at Hydrotreatment Conditions Leonidas E. Kallinikos and Nikos G. Papayannakos* National Technical UniVersity of Athens, School of Chemical Engineering, Heroon Polytechniou 9, Zografos, 157 80 Athens, Greece

The operation of a structured bed three-phase minireactor is studied when used for the hydrodesulfurization of a heavy gas oil fraction under real industrial conditions. This type of reactor consists of a spiral tube with an internal diameter a little larger than the catalyst extrudate diameter, in which the catalyst particles form a string bed. Four different loadings of the spiral reactor were realized for the desulfurization of the gas oil fraction in a variety of operating conditions. The effect of the catalyst mass, string bed length and the fluid velocities on the performance of the spiral reactor concept is examined. The stability as well as the repeatability of the spiral reactor operation and the loading procedure of the string bed was tested. The influence of reaction temperature and the gas-to-liquid ratio on the spiral reactor performance was also investigated. The performance of the spiral reactor was compared with that of a convectional miniscale diluted bed reactor as well as with a pilot reactor. From the results presented it is concluded that the operation of the string bed spiral reactor is quite satisfactory and the impact of the catalyst mass and the bed length on its performance is negligible. Introduction There are many applications of three-phase reactors such as hydrodesulfurization of oil fractions, catalytic hydrocracking of heavy oil fractions, catalytic hydrogenation of benzene to cyclohexane and of R-methylstyrene to cumene as well as oxidation of formic acid in water, oxidation of refractory lean SO2 to SO3 on activated carbon organic matter in wastewater with Pd catalyst, or fixed microorganisms1 paraffin synthesis by Fischer-Tropsch, and others.2 In this framework, the study of three-phase reactions and reactors constitutes a major research area for the scientists and the investigators working in the field of chemical reaction engineering. Although for more than 50 years the study of three-phase reactions was made with big pilot units,2-4 the trend for using small laboratory miniunits for both catalyst research and scaleup studies has recently risen sharply.5-7 There are certain advantages in using small-scale laboratory units for the study of three phase reaction systems, such as the reduction of the units’ construction and operation costs, safer operation and minimization of experimental time, and experimentation intensification. It is really important that an automated miniunit is able to operate without requiring human attention,8 even though it operates at very high temperature and pressure conditions (representative values for temperature and pressure are 615 K and 50 × 105 Pa, respectively) and more than one reactor tube can be used in the same furnace to perform parallel experiments. The main challenges in collecting meaningful data using miniscale reactors relate to the low gas and liquid superficial velocities needed in downscaling to keep gas and liquid space velocities the same as in bigger reactors. The reduced superficial velocities cause the appearance of problems related to low flow rates such as wall effects, incomplete catalyst particle irrigation, mass transfer limitations, and axial dispersion.2 As a result, complicated mathematical models taking into account all the phenomena are necessary for the determination of the process * To whom correspondence should be addressed. E-mail: [email protected] Tel: +30 210 772 3239. Fax: +30 210 772 3155.

Figure 1. Liquid superficial velocities for different reactors.

chemical kinetics. However, successful results are not always obtained following this method because not all the physical parameters affecting the system are available for deconvolution of the various effects. Catalyst bed dilution of the small-scale reactors by putting small-diameter inert fines to fill the voids between the catalyst particles is a widely used technique5,6,9 to avoid problems related with wall effects, low catalyst utilization, incomplete wetting, and axial back mixing. Diffusional limitations inside catalyst particles of industrial size are not generally negligible, but they are expected to be comparable at any reactor scale and their effect on catalyst performance to be incorporated in the kinetic formulas used for scaling-up/-down purposes. Drawbacks in using dilution are associated with reproducibility of the loading procedure and the unavoidable low gas and liquid flow rates through the reduced bed voidages. Although De Wind et al.10 and Al-Dahhan et al.11 have presented techniques to reproducibly build diluted beds in bench-scale reactors, the loading of diluted beds is a stochastic procedure and the voids created cannot be totally controlled. For low gas and liquid flow rates, distribution

10.1021/ie070309s CCC: $37.00 © 2007 American Chemical Society Published on Web 07/25/2007

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Table 1. Geometric Features of the Different Types of Reactorsa

reactor volume diameter (m) length (m) a

(m3)

industrial reactor

pilot reactor

bench reactor

mini bed reactor

spiral reactorb

100 (1, 2) 2.5 (1, 2) 20 (1, 2)

0.01 (1, 2) 0.04 (1)-0.12 (2) 8.0 (1)-0.9 (2)

0.000 15 (1, 2) 0.02 (1)-0.03 (2) 0.5 (1)-0.2 (2)

0.000 008 (1, 2) 0.01 (1, 2) 0.1 (1)-0.09 (2)

0.000 012b 0.002b 4b

Experimental data: (1) ref 16; (2) ref 2. b This study.

problems for both phases may arise and mass transfer resistances may augment.2,6,12 In this work, a miniscale structured bed reactor called a “spiral reactor” is studied at industrial hydrodesulfurization conditions of heavy atmospheric gas oil. The structure of this type of reactor is very simple, is easily reproducible, and appears very promising for collecting meaningful data with a small catalyst mass without the need for dilution.

tion, at any flow rate. Moreover, hot spots and bed radial temperature distributions are avoided. Finally, the construction of the spiral reactor is very easily reproduced, bed isothermality is easily accomplished, and the pressure drop is negligible. Also, the stability of the spiral reactor operation is better than that of common minireactors, especially at very low gas and liquid space velocities. Experimental Section

Spiral Reactor The spiral reactor concept is based on the advantages expected from a system that enables all catalyst particles used in industrial praxis (1-4 mm) to contact with the phases present in the process, allows high fluid velocities, and is very easily constructed. This type of reactor consists of a structured bed which is able to be used for any type of two- or three-phase catalytic reactions. It comprises a small tube with a diameter a little larger than the diameter of the catalyst extrudates used, in which the catalyst particles are successively introduced and all the particles are in a line. Thus, a “string bed” of catalyst particles is created. Because the reactor is long enough to contain a satisfactory catalyst mass, the tube is circled, forming a spiral reactor. Few reactor devices resembling the concept of spiral reactor have been used and presented in the past by other investigators. Satterfield et al.13 used a reactor with similar configuration for the hydrogenation of R-methylstyrene to cumene; however, the internal diameter of that reactor tube was much bigger than the diameter of the catalyst extrudates used and the diameter of the catalyst pellets was much bigger than the catalyst particles used in this work, while the reactor was a vertical column and its length was much shorter than the length of the spiral reactor proposed in this work. Van Herk et al.14 have presented dispersion results of a multiphase microreactor with vertical tubes having diameters similar to the spiral reactors used in this study. Their microreactor differs from the concept of the spiral reactor as they used small particles (100 µm mean diameter) as catalyst, building thus a fixed bed of small diameter with a number of particles arranged along the tube radius. Finally, Scott et al.15 have used a fixed bed reactor with a structure similar to that of spiral reactors, which they called a “single pellet string reactor”. However, they used it only for gas-solid reactions, while the used catalyst was in the form of spheres. There are certain advantages in using spiral reactors to collect kinetic data. The liquid and gas superficial velocities are much higher than in common small and miniscale reactors due to the small internal reactor diameter. This is obvious in Figure 1, where the liquid superficial velocities are drawn versus LHSV for an industrial reactor, a pilot-scale reactor, a bench-scale reactor, a convectional miniscale reactor, and the spiral reactor with the same catalyst mass as the convectional minireactor. The geometric features of the different types of the reactors are shown in Table 1.2,16 Due to the geometry of the spiral reactor concept, both gas and liquid phases are forced to flow over all the catalyst particles, avoiding bypassing, channeling, and poor gas distribu-

A miniscale laboratory unit was used for the experiments of this work. The flow sheet of the miniscale unit is shown in Figure 2. It is a fully automated unit which is able to operate unattended. The main parts of the unit are the spiral reactor, a high-pressure gas-liquid separator, a piston pump to feed the liquid phase in the reactor, and a hydrogen cylinder. Proper systems have been developed to control the operation of the unit and to switch it off in case of emergency or maloperation. Finally, a proper system has been constructed for the replacement of the sodium hydroxide solution used to absorb the hydrogen sulfide from the gas effluent stream during the operation of the unit. For the study of the spiral reactor operation, desulfurization experiments of heavy gas oil with sulfur content of 1.3% w/w were carried out with four different loadings of a conventional NiMo/γ-Al2O3 catalyst in a variety of industrial experimental conditions. Four spiral reactors with different lengths were prepared and used. The first spiral reactor (SR1) was built with 2.8 × 10-3 kg of catalyst and its length was 2 m, the second (SR2) and the third (SR3) were loaded with approximately 5.5 × 10-3 kg of catalyst and their length was 4 m, and the fourth (SR4) was loaded with 8.3 × 10-3 kg of catalyst and its length was 6 m. The diameters of the catalyst extrudates and the spiral tube were 1.2 and 2.1 mm, respectively. The experiments were performed at LHSV values between 0.5 and 2 h-1, temperatures between 593 and 623 K, gas-to-liquid ratio values between 200 and 600 NmH23/mOil3, and pressure 50 × 105 Pa. For the reactor with 2.8 × 10-3 kg of catalyst, the gas superficial velocities were between 0.880 × 10-3 and 10.564 × 10-3 m/s while the liquid superficial velocities were between 0.125 × 10-3 and 0.498 × 10-3 m/s. For the reactors with 5.5 × 10-3 kg of catalyst, the gas superficial velocities were between 1.793 × 10-3 and 21.519 × 10-3 m/s while the liquid superficial velocities were between 0.254 × 10-3 and 1.015 × 10-3 m/s. Finally, for the reactors with 8.3 × 10-3 kg of catalyst, the gas superficial velocities were between 2.674 × 10-3 and 32.083 × 10-3 m/s while the liquid superficial velocities were between 0.378 × 10-3 and 1.513 × 10-3 m/s. The operation of the string reactor was compared with that of a convectional diluted fixed bed minireactor at specific experimental conditions: temperature 613 K, LHSV 0.5, 1.0, and 2 h-1, gas-to-liquid ratio 400 NmH23/mOil3, and pressure 50 × 105 Pa. The miniscale reactor bed was built with 5.5 × 10-3 kg of NiMo/γ-Al2O3 catalyst diluted with inert particles (Carborundum with mean diameter 60 µm) and a catalyst to inert mass ratio of 1:1.24. Sulfidation was carried out with a light gas oil spiked with 2 wt % dimethyl disulfide at a total pressure of 50 atm, LHSV )

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Figure 2. Flow sheet of the laboratory miniscale HDS unit.

Figure 4. Exit sulfur concentration vs LHSV for the experiments performed with SR2 and SR3. Temperature: 613 K. Gas-to-liquid ratio: 400 NmH23/ mOil3. Inlet sulfur concentration: 13 000 ppm.

Figure 3. Evolution of the exit sulfur concentration with time for repeated changes in LHSV. Spiral reactor: SR1. Temperature: 608 K. Gas-to-liquid ratio: 400 NmH23/mOil3. Inlet sulfur concentration: 13 000 ppm.

1.5 h-1, and gas-to-oil ratio 200 NmH23/mOil3. During the sulfidation procedure the temperature was linearly increased for 20 h from room temperature up to 623 K and then it was kept constant at 623 K for a period of 15 h. The determination of the sulfur content in the liquid product was carried out with an Antek 9000 Series instrument. Results and Discussion Stability and Repeatability. Stability and repeatability are very important factors in collecting reliable experimental data. The stability, as well as the repeatability of the spiral reactor’s operation, was studied by performing two successive experiments with spiral reactor SR1 (2 m length, 2.8 × 10-3 kg of catalyst) at LHSVs 2 and 1 h-1, temperature 608 K, gas-toliquid ratio 400 NmH23/mOil3, and pressure 50 × 105 Pa. The procedure of these experiments is presented in Figure 3. It is shown that for higher LHSVs steady state is reached in shorter time. For the first run at LHSV ) 2.0 h-1, a longer time was needed to reach steady state than for the second run at the same conditions due to the time required for catalyst stabilization after catalyst sulfidation. It is also observed that the transition was very smooth, indicating repeatable and steady operation. The repeatability of the loading of the spiral reactor procedure was actualized by performing the same experiments with two different spiral reactors with the same catalyst mass. Both spiral reactors SR2 (4 m length) and SR3 (4 m length) were loaded with 5.5 × 10-3 kg of catalyst and were used for performing experiments at LHSVs 0.5, 1.0, and 2.0 h-1, temperature 613 K, and gas-to-liquid ratio 400 NmH23/mOil3. The results are summarized in Figure 4, and they confirm that the repeatability of the spiral reactor loading is easy and successful. The deviations are very small and less than those expected from the error of temperature measurement and control. Reactor Length Influence. The effects of the catalyst mass, string length, and fluid velocities on the spiral reactor performance were studied with desulfurization experiments of a heavy gas oil using the spiral reactors SR1 (2.8 × 10-3 kg of catalyst,

Figure 5. Exit sulfur concentration vs LHSV for the experiments performed with the spiral reactors SR1, SR3, and SR4. Gas-to-liquid ratio: 400 NmH23/ mOil3. Inlet sulfur concentration: 13 000 ppm.

length 2 m), SR3 (5.5 × 10-3 kg of catalyst, length 4 m), and SR4 (8.3 × 10-3 kg of catalyst, length 6 m) at the same LHSV, temperature, pressure, and gas-to-liquid ratio. As the three spiral reactors employed (SR1, SR3, and SR4) contained different catalyst masses, the experiments were carried out for the same LHSV values and gas-to-liquid ratio. However, the gas and liquid superficial velocities changed, being linearly dependent on the catalyst mass for the same LHSV. Thus, the comparison was attempted for different hydrodynamic features (axial dispersion, wetting efficiency) and interfacial mass transfer effects at the same LHSV. For each of these parameters the fluid velocity increase would result in reactor performance improvement because augmenting the liquid and gas velocities, axial dispersion and mass transfer limitations are diminished while catalyst wetting improves. For two different temperatures, 608 and 613 K, experiments were performed for three different liquid hourly space velocities, 0.5, 1.0, and 2.0 h-1, and for gas-to-liquid ratio 400 NmH23/ mOil3. The results are presented in Figure 5, and it is obvious that the performance of all three spiral reactors is almost identical. Even at ultradeep hydrodesulfurization conditions (sulfur content in product below 10 ppm), the performance of the different-size spiral reactors is identical. From these results it is implied that the obtained performance of the spiral reactors for the deep desulfurization process is independent of the liquid superficial velocity of the gas and the liquid phases. It appears that the operation of the spiral

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Figure 6. Exit sulfur concentration vs temperature for the experiments performed with spiral reactors SR1 and SR3. LHSV: 1 h-1. Gas-to-liquid ratio: 400 NmH23/mOil3. Inlet sulfur concentration: 13 000 ppm.

Figure 8. Exit sulfur concentration vs LHSV for the experiments performed with the spiral reactors SR1, SR3, and SR4 and a miniscale reactor compared with those performed with a pilot-scale reactor.17 Temperature: 613 K. Gas-to-liquid ratio: 400 NmH23/mOil3. Inlet sulfur concentration: 13 000 ppm.

mental conditions (T ) 613 K, P ) 50 × 105 Pa, LHSV ) 0.5, 1.0, and 2.0 h-1 and gas-to-liquid ratio ) 400 NmH23/mOil3). The results are presented in Figure 8, and it is shown that those obtained with spiral reactors and with miniscale diluted bed are similar to the results obtained with the pilot reactor. The small deviations are attributed to the measurement of the operating parameters and analysis errors. Thus, the operation of the spiral reactors as well as the diluted bed can effectively represent the operation of the pilot reactor, indicating that it could be considered to be an ideal reactor since the impact of problems such as axial backmixing, poor irrigation, mass transfer, and well effects on its operation is expected to be minimized.12 Figure 7. Exit sulfur concentration vs gas-to-liquid ratio for the spiral reactors SR1 and SR4. LHSV: 1.0 h-1. Temperature: 613 K. Inlet sulfur concentration: 13 000 ppm.

reactor is not affected by downscaling problems such as axial dispersion, wetting efficiency, and gas-liquid or liquid-solid mass transfer. Temperature and Gas-to-Liquid Ratio Influence. The influence of temperature on the performance of the spiral reactor was studied for two different spiral reactor concepts, SR1 (2.8 × 10-3 kg of catalyst, length 2 m) and SR3 (5.5 × 10-3 kg of catalyst, length 4 m). As reaction temperature increases, the liquid volatilization in the catalyst bed increases for the same liquid and gas feed rates. For liquid hourly space velocity (LHSV) 1 h-1, gas-to-liquid ratio 400 NmH23/mOil3, and pressure 50 × 105 Pa, experiments were performed at temperatures 593, 603, 613, and 623 K with both SR1 and SR3. The results are presented in Figure 6, and it is observed that the performance of the two spiral reactors with significantly different catalyst mass and string length are almost identical. For LHSV 1.0 h-1 and temperature 613 K, experiments with SR1 (2.8 × 10-3 catalyst, length 2 m) and SR4 (8.3 × 10-3 kg of catalyst, length 6 m) were performed for three different values of gas-to-liquid ratio: 200, 400, and 600 NmH23/mOil3. The results are illustrated in Figure 7, and it is concluded again that, for different gas-to-liquid ratios corresponding to different gas superficial velocities and liquid evaporation, the performance of the two spiral reactors is identical for the same LHSV and gas-to-oil ratio, taking into account that the repeatability error in exit sulfur concentration is less than 10%. Spiral Reactor vs Convectional Reactors. The performance of the spiral reactors was compared with that of a miniscale reactor including a diluted catalyst bed as well as of a pilot reactor17 operating in trickle flow regime for the same experi-

Conclusions A structured bed three-phase minireactor in spiral form was studied for the catalytic hydrodesulfurization of a heavy gas oil fraction at real reaction conditions. The structure of the catalyst bed imposes the gas and liquid flow to pass over all particles, avoiding bypassing and flow maldistributions. A certain advantage of the spiral reactors is that the catalyst loading is easy and reproducible. It was shown that the repeatability as well as the stability of the spiral reactor operation is satisfactory even for very low gas and liquid velocities. The operation of the spiral reactor is quite successful even at ultradeep hydrodesulfurization conditions (less than 5-10 ppm). Performing experiments in a wide range of gas and liquid velocities, temperatures, and gas-to-liquid ratios, it appeared that the spiral reactor operation is not affected by its length and catalyst loading. This implies that downscaling effects such as wall effects, axial backmixing, liquid-to-solid mass transfer resistances, and incomplete catalyst particle irrigation do not predominate in its operation. It has also been shown that miniscale reactors, either spiral or fixed bed, can effectively be used to produce data representing the operation of pilot hydrotreaters, if properly designed. Nomenclature CS ) concentration of sulfur [ppm] G/L ) gas-to-liquid ratio [NmH23/mOil3] HDS ) hydrodesulfurization LHSV ) liquid hourly space velocity [h-1] P ) pressure [Pa] SR1 ) spiral reactor 1 with catalyst mass 2.8 × 10-3 kg and length 2 m

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SR2 ) spiral reactor 2 with catalyst mass 5.5 × 10-3 kg and length 4 m SR3 ) spiral reactor 3 with catalyst mass 5.5 × 10-3 kg and length 4 m SR4 ) spiral reactor 4 with catalyst mass 8.3 × 10-3 kg and length 6 m T ) temperature [K] uLS ) superficial velocity of liquid [m/s] Subscripts out ) outlet of the reactor Literature Cited (1) Cianetto, A.; Specchia V. Trickle-bed reactors: state of art and perspectives. Chem. Eng. Sci. 1992, 47 (13/14), 3197. (2) Sie, S. T.; Krishna, R. Process Development and Scale Up: III. Scaleup and scale-down of trickle bed processes. ReV. Chem. Eng. 1998, 14 (3), 203. (3) Wild, G.; Larachi, F.; Laurent, A. The Hydrodunamic Characteristics of Cocurrent Downflow and Cocurrent Upflow Gas-Liquid-Solid Catalytic Fixed Bed Reactors: The Effect of Pressure. ReV. Inst. Fr. Pet. 1991, 46 (4), 467. (4) Tsamatsoulis, D.; Papayannakos, N. Investigation of intrinsic hydrodesulphurization kinetics of a VGO in a trickle bed reactor with backmixing effects. Chem. Eng. Sci. 1998, 53 (19), 3449. (5) Bej, S.; Dabral, R.; Gupta, P.; Mittal, K.; Sen, G.; Kapoor, V.; Dalai, A. Studies on the Performance of a Microscale Trickle Bed Reactor Using Different Sizes of Diluent. Energy Fuels 2000, 14 (3), 701. (6) Bellos, G.; Papayannakos, N. The use of a three phase microreactor to investigate HDS kinetics. Catal. Today 2003, 79-80, 349.

(7) Ferdous, D.; Dalai, A. K.; Adjaye, J. A series of NiMo/Al2O3 catalysts containing boron and phosphorus: Part II. Hydrodenitrogenation and hydrodesulfurization using heavy gas oil derived from Athabasca bitumen. Appl. Catal. A 2004, 260, 153. (8) Sie, S. T. Miniaturization of Hydroprocessing Catalyst. Testing Systems: Theory and Practice. AIChE J. 1996, 42 (12), 3498. (9) Klinken, J.; Dongen, R. Catalyst dilution for improved performance of laboratory trickle-flow reactors. Chem. Eng. Sci. 1980, 35, 59. (10) De Wind, M.; Plantenga, F.; Heinerman, J. Upflow versus downflow testing of hydrotreating catalysts. Appl. Catal. 1988, 43, 239. (11) Al-Dahhan, M.; Wu, Y.; Dudukovic, M. Reproducible Technique for Packing Laboratory-Scale Trickle-Bed Reactors with a Mixture of Catalyst and Fines. Ind. Eng. Chem. Res. 1995, 34 (3), 741. (12) Gierman, H. Design of laboratory hydrotreating reactors: Scaling Down of Trickle-flow Reactors. Appl. Catal. 1988, 43, 277. (13) Satterfield, C.; Pelossof, A.; Sherwood, T. Mass Transfer Limitations in a Trickle-Bed Reactor. AIChE J. 1969, 15 (2), 226. (14) Van Herk, D.; Kreutzer, M.; Makkee, M.; Moulijn, J. Scaling down trickle bed reactors. Catal. Today 2005, 106, 227. (15) Scott, D.; Lee, W.; Papa, J. The measurement of transport coefficients in gas-solid heterogeneous reactions. Chem. Eng. Sci. 1974, 29, 2155. (16) Al-Dahhan, M.; Dudukovic, M. Catalyst Bed Dilution for Improving Catalyst Wetting in Laboratory Trickle-Bed Reactors, AIChE J. 1996, 42 (9), 2594. (17) Bellos, G. Simulation of three phase catalytic gas oil hydrotreaters. Ph.D. Thesis, National Technical University of Athens (NTUA), Athens, Greece, 2004.

ReceiVed for reView March 1, 2007 ReVised manuscript receiVed June 1, 2007 Accepted June 21, 2007 IE070309S