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1 This article was downloaded by: [Universidad de Sevilla] On: 12 November 2014, At: 02:51 Publisher: Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Chemical Engineering Communications Publication details, including instructions for authors and subscription information: THE EFFECT OF PARTICLE DILUTION ON WETTING EFFICIENCY AND LIQUID FILM THICKNESS IN SMALL TRICKLE BEDS D. TSAMATSOULIS a, M. H. AL-DAHHAN b, F. LARACHI c & N. PAPAYA NNAKOS a a Department of Chemical Engineering, N.T.U.A., 9, Heroon Poiytechniou, Zografos, GR, , Greece b Department of Chemical Engineering, Washington University in St Louis, 1 Brooking Drive, St Louis, MO, c Department of Chemical Engineering, University of Laval, Quebec, Sainte-Foy, G1K 7P4, Canada Published online: 03 Apr To cite this article: D. TSAMATSOULIS, M. H. AL-DAHHAN, F. LARACHI & N. PAPAYA NNAKOS (2001) THE EFFECT OF PARTICLE DILUTION ON WETTING EFFICIENCY AND LIQUID FILM THICKNESS IN SMALL TRICKLE BEDS, Chemical Engineering Communications, 185:1, 67-77, DOI: / To link to this article: PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the Content ) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at

2 Chem. Eng. Corn Vol pp Reprints av&blc dircclly lrom the publisher Photocopying pcrmit!;d by limns only Q 2WI OPA (Overrear Publishers Arrociation) N.V. Published by lianv under the Gordon and Brweh Scicna Publishers imprint. Printed in Malaysia. THE EFFECT OF PARTICLE DILUTION ON WETTING EFFICIENCY AND LIQUID FILM THICKNESS IN SMALL TRICKLE BEDS D. TSAMATSOULIS", M. H. AL-DAHHAN~,*, F. LARACHI~ and N. PAPAYANNAKOS" 'Department of Chemical Engineering, N.T.U.A., 9. Heroon Pol~~rechniou, GR Zogrufos, Greece; b~epar~inent of Chemical Engineering. Washington Universily in SI Louis. I Brooking Drive, St Louis MO 63/30: CDepartmen~ of Chemical Engineering, Universily of Laval. Quebec, Soinre-Foy, GIK 7P4, Canada (Received 23 Augusl 1999; In final form 31 May 2000) Partial wetting in small scale trickle bed reactors results in incorrect determination of intraparticle apparent kinetic parameters as well as in erroneous reactor scale-up. Although a dilution of catalyst particles with inert fines improves the catalyst wetting efficiency, it does not guarantee full external catalyst wetting at all superficial liquid mass velocities. In this work, a method is presented to relate the wetting efficiency obtained at different operating conditions and at different laboratories for diluted and non-diluted beds. Liquid film thickness in diluted and non-diluted beds is estimated. The effect of the operating conditions on partial wetting and liquid film thickness is discussed. Keywords: Trickle-bed reactor; Wetting efficiency; Bed dilution; Liquid film thickness INTRODUCTION Pilot plant and laboratory-scale trickle bed reactors, which are fixed beds of catalyst contacted by cocurrent gas-liquid downflow, are usually utilized to conduct investigations needed for the design, scale-up and scale-down of *Corresponding author. Tel.: , Fax:

3 68 D. TSAMATSOULIS e! a/. industrial reactors (e.g., testing commercially used catalyst or an alternative feedstock, etc.). Low liquid hourly space velocity (LHSV) and low reactorto-particle diameter ratio (d,/dp < 20) are prevalent in small-scale trickle beds. These conditions often cause poor catalyst utilization due to partially wetted catalyst, non-negligible wall effects, high liquid back-mixing (i.e., axial dispersion) and/or maldistribution. Since the hydrodynamics and reaction kinetics are closely interlinked, the above mentioned hydrodynamic problems influence drastically the performance of pilot plant and laboratory-scale trickle bed reactors (Al-Dahhan et al., 1997; Al-Dahhan and Dudukovic, 1996; Tsamatsoulis and Papayannakos, 1993; Gierman, 1988; Van Klinken and Van Dongen, 1980). Diluting the catalyst beds with fines (small, inert, and nonporous particles of about mm diameter) has been utilized to overcome the shortcomings encountered with the conventional non-diluted small-scale reactors. Although the hydrodynamics of the diluted beds differ from that of commercial reactors, fines allow the small scale reactors to approach kinetically the performance of the commercial units by improving the plug flow character of the pilot plant and laboratory reactors (i.e., improving catalyst wetting and utilization, homogenizing liquid distribution, reducing wall effect and minimizing axial dispersion). As a matter of fact, this allows decoupling of the hydrodynamics and the intra-particle "apparent" kinetics (Al-Dahhan and Dudukovic, 1996). It is generally considered that the dilution of the catalyst particles with fines would virtually eliminate partial wetting as well as dispersion effects. However, full catalyst wetting cannot be achieved at all superficial liquid mass velocities (Al-Dahhan and Dudukovic, 1996; Tsamatsoulis and Papayannakos, 1996). The existing experimentally measured values of catalyst wetting efficiency in trickle bed reactors are scanty and in most cases are not related to each other due to large contrasts in operating conditions explored by different investigators. This poses a serious obstacle in using literature data for different applications. Two recent studies by Al-Dahhan and Dudukovic (1996) and Tsamatsoulis and Papayannakos (1996) reported catalyst wetting efficiency data in two totally non-overlapping ranges of operating conditions. They found that the wetting efficiency improves and approaches unity at different range of superficial liquid mass velocities. Hence, this work presents a new unifying method to relate the wetting efficiency data measured at different conditions in different laboratories for diluted and non-diluted small-scale beds. The effects of operating conditions on catalyst wetting efficiency and liquid film thickness are discussed as well.

4 FILM THICKNESS IN TRICKLE BEDS EXPERIMENTAL CONDITIONS The wetting efficiency values used in this work were obtained experimentally for commercial porous catalysts at different operating conditions. Table I summarizes the bed characteristics and the experimental conditions, as reported by Al-Dahhan and Dudukovic (1996) (cold flow experiments) and Tsamatsoulis and Papayannakos (1996) (hot flow experiments). The liquid phase density was very close for both the hot and cold flow experiments. The ratio of nitrogen density at MPa and room temperature to hydrogen density at working conditions (Tab. I) is 1.85, while the nitrogen density ratio at 0.31 MPa and at 1.83 MPa was about 6. The tracer in the hot flow experiments (Tsamatsoulis and Papayannakos, 1996) was made up of sulphur-containing compounds in one of the liquid feeds. In the cold flow experiments (Al-Dahhan and Dudukovic, 1996) heptane was used to trace the hexane flow. Experimental conditions Pressure, MPa Temperature, K Gas feed Superficial gas velocity, cmls Liquid feed Liq. sp. gr. at exp. cond., g/cm3 Fine properlies Non-porous silicon carbide Mean particle diameter, mm Material density, g/cm3 Bed density, g/cm3 Bed porosity Particle properties Type Diameterllength, mmlmm Particle porosity Particle density, g/cm" Bed characteristics Particleslfines Bed diameter, cm Bed length, cm Bed porosity Vol. fin. bed/vol. part bed void Vol. fin. bed/vol, part Vol. fine ~artlvol. part TABLE I Bed characteristics and experimental conditions Cold experimenrs (Al-Dahhan and Dudukovic, 1995, 1996) SCF Nitrogen 1.02, 8.7 Hexane 0.68 Cyl. Ex: , , Nitrogen Nitrogen , 8.5 Hexane Hexane Nitrogen 1.02, 8.6 Hexane 0.68 BC PB/SCF

5 70 D. TSAMATSOULIS er ul. Expcrirncntnl conditions Pressure, MPa Temperature, K Gas feed Superficial gas velocity, cm/s Liquid feed Liq. sp. gr. at texp. cond., g/cm" Fine properties Non-porous silica powder Mean pnrticle diameter, mm Material density, g/cm3 Bed density, g/cm" Bed porosity Pnrticlc propcrtics TY PC Di:~mctcr/length, mm/mm Particle porosity Particle density, g/cm3 Bed chnracteristics Particles/fines Bcd diameter, cm Bed length, cm Bed porosity Vol. fin. bed/vol. part bed void Vol. fin. bed/vol. part Vol. fine. ~art/vol. Dart TABLE I (Continued) Hot experiments (Tsamatsoulis and Papayannakos, 1996) Dora (3) Dala (3) Dara (3) Dara (3) Hydrogen Hydrogen Hydrogen VGOl VGOl VG PI Cyl. Ex.' 1.40/ Data (I): Al-Dnhhan and Dudukovic Ihta (2): Al-Dahhan and Dudukovic, Deta (1.2): Al-Dahhan and Dudukovic, 1996, SCF: Fines for cold experiments. PA: Cylindrical particles-non-diluted bed. PH: Spherical particles-non-diluted kd. HA: Diluted bed of cylindrical particles. HC: Diluted bed of spherical particles. Data (3): Tsamatsoulis and Papayannakos, SP: Fines for hat experiments. PI: Cylindrical particles-non-diluted bed. HI: Diluted bed of cylindrical particles with volume of fines/volume of particles=0.25. HZ: Diluted bcd of cylindrical particles with volume of fines/volume of particles =0.75. Porous cylindrical entrudates. *.Porous spheres. Liquid tracer technique was used in both hot and cold flow experiments. In the hot flow experiments a step increase of tracer concentration was suddenly imposed by switching the liquid feed from a low to a high sulphur concentration feed (Tsamatsoulis and Papayannakos, 1993). The

6 FILM THICKNESS IN TRICKLE BEDS 7 1 liquid holdups and effective tracer diffusivities values were determined by fitting to the experimental tracer response data the solution of the advection -diffusion parabolic partial differential equation for mass balance (Tsamatsoulis and Papayannakos, 1996). A pulse injection of heptane tracer into the steady hexane flow was used in the cold flow experiments. The method of moments was utilized to evaluate the hold-up and effective tracer diffusivities (Al-Dahhan and Dudukovic, 1995, 1996). In both cases, the wetting efficiency values were estimated by the form a,= (D,,~,,/D,)~-' for both diluted and non-diluted beds. DISCUSSION The dependence of wetting efficiency values on superficial liquid velocity for the diluted beds, as shown in Figure 1, indicates that the data of the hot and cold flow experiments do not overlap. Higher wetting efficiencies were obtained in the hot experiments compared to those obtained in the cold experiments at the same liquid mass velocities. Wetting efficiency should be affected by the liquid holdup and the extent of the liquid spreading over the external surface of the catalyst. As the liquid holdup increases, wetting efficiency increases and the actual liquid mass velocity (L/EL) decreases. Hence, the variation of the ratio with (L/E~) FIGURE I Dependence of the diluted beds effective wetting on the superficial liquid mass velocity (Tab. I).

7 72 D. TSAMATSOULIS er al. indicates the compatibility of different experimental data. In Figure 2, it is observed that both hot and cold flow experimental data obtained for the diluted and non-diluted beds at the conditions listed in Table I appear to follow the same trend. Figure 2 also shows that the same trend was observed for the data obtained using different ratios of fines/particles. This implies that a careful dilution procedure can be effective for dilution values (A, = the ratio of the fines bed volume to the volume of the catalyst bed voidage) from 0.6 to 1.5. However, A, = I along with the packing procedure proposed by Al-Dahhan er 01. (1995) are recommended to ensure reproducibility of the diluted beds. The liquid film thickness over the catalyst external surface, which affects both gas and liquid mass transfer, is a hydrodynamic parameter, that depends on the wetting efficiency. Assuming that the liquid film thickness over the catalyst particles and fines is uniform, the film thickness can be evaluated as follows based on the wetting efficiency, holdup and shape and size of the bed particles: FIGURE 2 a../r,, vs. L/E~ for the diluted (BA, BC, 01, B2) and the corresponding non-diluted (PA, PB. PI) beds. Dashed lines : boundaries of + 20% around the solid line (Tab. I). With diluted beds BA and BC data was collected at: P=O.JlMPa, u8.,=1.02cm/sec and P=1.82MPa, u,,=8.7cmlsec. With non-diluted beds PA and PB data was collected at: P= 0.31 MPa, u,,= 1.02crn/sec and P= 3.55 MPa, u8,=8.7cm/sec.

8 FILM THICKNESS IN TRICKLE BEDS Assuming fully wetted fines, yields: awsext = awsp +Sf Substituting Eq. (2) and (3) into Eq. (1) gives: where, VR/ X = - for diluted bed VRP X = 0 for non-diluted bed. By geometrically evaluating SP/VR, and S/JVR, for different shapes of particles and for spherical fines, respectively, the uniform liquid film thickness can be estimated for different beds as follows: Bed of spheres diluted with spherical fines: Bed of non-diluted spheres: Bed of cylindrical extrudates diluted with spherical fines: Bed of non-diluted cylindrical extrudates:

9 74 D. TSAMATSOULIS er a/. In Figure 3, the dependence of liquid film thickness on the superficial liquid mass velocity is presented for diluted and non-diluted beds. It is obvious that in both diluted and non-diluted beds the thickness of the liquid film does not change with liquid velocity when the particles are not completely wetted (Fig. I). The same trend has been observed for the static liquid thickness in diluted and non-diluted beds (Tsamatsoulis and Papayannakos, 1995). This implies that at these conditions the increase in liquid holdup with liquid flow rate results in improving the effective wetting by spreading the liquid over the external surface of the particles, while the liquid film thickness remains unchanged. Furthermore, it is found that the film thickness in a diluted bed depends on the degree of dilution (A,) and gas velocity. Without a significant increase in liquid holdup the liquid film thickness decreases with the increase in the degree of dilution where the ratio of the external surface of fines to external surface of particles increases and, hence, the wetted surface per reactor volume increases. In addition, as the gas velocity increases the liquid film decreases due to better spreading of the liquid over the external surface of the particles. Such trend is similar to the trend of the effect of gas velocity on the liquid holdup presented by the five cases proposed in Al-Dahhan and Dudukovic (1994,1995,1996). Hence, the liquid film thickness could be emperically correlated with the degree *%@* X X X xx Xx X om0 A A A A A a FIGURE 3 The effect of superficial liquid mass velocity on the liquid film thickness in partially wetted beds (Tab. I). (I) P=O.31 MPa, u,,= 1.02cm/sec, and (11) P= 1.82MPa. u,, = 8.7 cmlsec.

10 FILM THICKNESS IN TRICKLE BEDS 75 of dilution and superficial gas velocity. As a preliminary attempt, the following correlation (Eq. (9)) fits well the film thickness values evaluated by Eqs. (5)-(8). However, detailed experimental work and analysis are needed to develop an extended correlation based on dimensionless groups. 6 = (~,~~~~)-~'~~ = A;~.'~u~~~~ FIGURE 4 Comparison between the prediction of the liquid film thickness using the developed correlation (Eq. (9)) (dash line) and the mean values estimated using Eqs. (5)-(8) (symbols) (Tab. I) A,'..-'- + + Bed B1 cr Bed ,,&' A Bed BA. (I) o Bed BA. (11),,*4 A,-8' Bed BC. (1) 0 r' Bed BC, (11) Predicted Film Thickness, cm FIGURE 5 Parity plot of the film thickness values predicted by the developed correlation (Eq. (9)) and those estimated using Eqs. (5)-(8) (1) P=0.31 MPa, I+*= 1.02cm/sec and (11) P=1.82MPa. ur,=8.7cm/sec(tab. I).

11 76 D. TSAMATSOULIS er a/. where Figure 4 illustrates that the developed correlation predicts properly the liquid film thickness and its trend with (X,U:,)~). A parity plot of the film thickness is presented in Figure 5. The correlation values of the liquid film are within f 20%. CONCLUDING REMARKS Dilution of the catalyst beds with inert fines improves catalyst wetting efficiency. However, fully wetted catalyst cannot be considered apriori at all operating conditions. The variation of the ratio of catalyst wetting efficiency to liquid holdup (a,,,/el) with the actual liquid mass velocity (L/E~) can be used to relate the experimental data obtained at different operating conditions. Liquid film thickness over the external surface of the catalyst particles decreases with the increase in the degree of dilution and superficial gas velocity. In a partially wetted catalyst bed, as superficial liquid velocity increases liquid holdup increases while liquid film thickness remains unchanged. As a result, catalyst wetting efficiency improves due to better spreading of the liquid phase over the external surface of the particles. Acknowledgement The authors are thankful to the Fulbright Foundation for the support of one of the authors (Dr. N. Papayannakos) during his stay at the Chemical Reaction Engineering Laboratory (CREL) at Washington University in St Louis. NOTATION a,,, wetting efficiency as the ratio of the wetted external surface area to the total particle external area 6 liquid film thickness, mm D, effective diffusivity of the fully wetted particles, m2/s

12 FILM THICKNESS IN TRICKLE BEDS 77 References effective diffusivity of the partially wetted particles, m2/s liquid holdup defined as the ratio of the liquid volume in the bed to the total bed volume diluted bed void fraction bed void fraction of spherical or cylindrical particles bed void fraction of fines liquid superficial mean velocity based on empty tube cross section, s-' mean length of cylindrical particles, mm ratio of the fine particle volume to the sphere or extrudate particle volume, in diluted beds ratio of fine bed volume to particle bed void volume radius of cylindrical or spherical particles, mm mean diameter of fines. mm external area of the particles in the bed, cm2 gas superficial velocity, cm/s bed volume, cm3 fines bed volume, cm3 catalyst bed volume, cm3 total solids volume in the reactor (VRf + VRp), cm3 Al-Dahhan, M. H. and Dudukovic, M. P. (1994) Pressure Drop and Liquid Holdup in High Pressure Trickle Bed Reactors, Chem. Eng. Sci., 49(24B), Al-Dahhan, M. H., Wu, Y. and Dudukovic. M. P. (1995) ReproducibleTechnique for Packing Laboratory Scale Trickle Bed Reactors with a Mixture of Catalyst and Fines, Ind. Eng. Chem. Res., 34, 741. Al-Dahhan, M. H. and Dudukovic, M. P. (1995) Catalyst Wetting Efficiency in Trickle-Bed Reactors at High Pressure, Chem. Eng. Sci., 50(15), Al-Dahhan, M. H. and Dudukovic, M. P. (1996) Catalyst Bed Dilution for Improving Catalyst Wetting in Laboratory Trickle-Bed Reactors. AlChE J., 42(9), Al-Dahhan, M. H., Larachi, F., Dudukovic. M. P. and Laurent, A. (1997) High Pressure Trickle Bed Reactors: A Review, Ind. Eng. Che~n. Res., 36, Gierman, H. (1988) Design of Laboratory Hydrotreating Reactors: Scaling Down of Trickle Flow Reactors, Appl. Cafal., 43, 285. Tsamatsoulis, D. and Papayannakos, N. (1993) Axial Dispersion and Holdup in a Bench-Scale Trickle Bed Reactor at Operating Conditions, Chem. Eng. Sci., 49(4), 523. Tsamatsoulis, D. and Papayannakos, N. (1995) Simulation of Non-Ideal Flow in a Trickle Bed Hydrotreater by the Cross-Flow Model, Chem. Engg. Sci., 50(23), Tsamatsoulis, D. and Papayannakos, N. (1996) Partial Wetting of Cylindrical Catalytic Carriers in Trickle-Bed Reactors, AIChE J., 42(7), Van Klinken, J. and van Dongen, R. H. (1991) Catalyst Dilution for Improved Performance of Laboratory Trickle Bed Reactors, Chem. Engg. Sci., 14,406.

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