A unique gas-fluidized bed in which fluidization is enhanced by adding a little water has potential for soft abrasive washing of various products. Preliminary experiments were carried out for the washing of farm products. Gas fluidized beds of binary solids mixtures with inclined gas distributors provide hydrodynamic and mixing data assisting the design of dry and wet gas-fluidized beds

Asako, Yutaka

Epstein, Norman

Grace, John R.

Nagahashi, Yusumi

Yokogawa, Akira

English

Engineering Conferences International

Refereed ProceedingsThe 12th International Conference onFluidization - New Horizons in FluidizationEngineeringEngineering Conferences International Year 2007Flow Characteristics of Fully WettedBinary Solid Mixtures in Gas FluidizedBeds with Inclined Gas DistributorsYusumi Nagahashi∗ John R. Grace† Norman Epstein‡Yutaka Asako∗∗ Akira Yokogawa††∗Kochi National College of Technology, Japan, naga@me.kochi-ct.ac.jp†University of British Columbia, jgrace@chml.ubc.ca‡University of British Columbia∗∗Tokyo Metropolitan University, Japan, asako@ecomp.metro-u.ac.jp††Kochi University of Technology, Japan, yokogawa.akira@kochi-tech.ac.jpThis paper is posted at ECI Digital Archives.http://dc.engconfintl.org/fluidization xii/40FLUIDIZATION XII  337    FLOW CHARACTERISTICS OF FULLY WETTED BINARY SOLID MIXTURES IN GAS FLUIDIZED BEDS WITH  INCLINED GAS DISTRIBUTORS  Nagahashi Y. Dept. Mech. Eng., Kochi National Coll. Technol.  200-1, Monobe, Nankoku-shi, Kochi, Japan, 783-8508  T: 81-88-864-5532, F:81-88-864-5521; E: naga@me.kochi-ct.ac.jp Grace J.R. and N. Epstein, Dept. Chem. & Biological Eng., UBC, Canada T: 1-604-822-3121, F: 1-604-822-6003; E: jgrace@chml.ubc.ca Asako Y., Tokyo Metpl.Univ., Japan, E: asako@ecomp.metro-u.ac.jp Yokogawa A., Kochi Univ. Technol., Japan, yokogawa.akira@kochi-tech.ac.jp   ABSTRACT A unique system in which gas-fluidization is enhanced by adding a little water has potential for soft abrasive washing of various products. Preliminary experiments were carried out for washing farm products. Gas fluidized beds of binary solids mixtures with inclined gas distributors provide hydrodynamic and mixing data, assisting the design of dry and wet gas-fluidized beds.   INTRODUCTION                 Farm ProductPLSTInclinedDistributorMudDischargeAir  Fig.1 Fluidized bed with inclined gas-distributor When small quantities of water are added togas-fluidized beds of large non-wettable particles,such as 6-mm polystyrene spheres, the water cancause a dramatic increase in the extent offluidization. We have previously reported(1),(2) thecharacteristics of this unique fluidized bed. It hasadvantages for soft-abrasion, due to movement oflight rounded particles, and also for reducingwater usage for washing, due to the smallquantities of water needed. These characteristicsare desirable for industrial washing of farmproducts or fragile industrial devices requiring softabrasive washing.             For such applications, inclined gas-distributors, as shown in Figure 1, are effective forseparating and removing solid wastes from the bed materials as a mud.  This paperreports preliminary experimental results on the washing of agricultural products. 1Nagahashi et al.: Flow Characteristics of Fully Wetted Binary Solid MixturesPublished by ECI Digital Archives, 2007NAGAHASHI et al.  338Possible gas-distributors, including inclined distributors, are discussed for dry single-  particle  fluidized  beds  to  optimize  the  system  for discharge of the washing waste. The flow characteristics of gas-fluidized beds of binary solid mixtures with inclined gas-distributors are discussed for dry and wet fluidized beds. The two species in the binary solids mixture simulate the large washing solids medium and the small particles to be removed as waste solids (mud).  APPARATUS AND METHODS 1) Bed Column and Distributors: The experimental apparatus is shown in Fig. 2.                    The two columns were transparent half-cylinders                                   of 180 mm inside diameter x 1000 mm height, and                                  280 mm dia. x 1500 mm tall. The gas distributors                                  were a horizontal plate,  inclined  flat plates with                                  inclination angles to the horizontal of 10, 20, and 30°, convex and concave plates  with  20° angle to the horizontal, and a cone drilled with 2.4 mm  dia. holes (overall open area 1.3 %)  and   covered by a wire mesh #400 cloth plate, as shown in Figure 3.   The plenum chamber wall was provided with three pressure taps.          2) Particles: We used 2 types  of polystyrene particles (6-mm spheres and 3.2-mm rounded cylinders of mean diameter 3.2  and length 3.2 mm) and 2 sizes  of spherical glass beads (mean diameters 1.0 and 0.6 mm).  Table 1   Particle Properties Polystyrene Glass Beads Particles 6-PLST 3.2-PLST 1-GB 0.6-GBdP [mm] 6.0  3.2x3.2 1.0 0.6 Umf [m/s] 1.45 0.70 0.43 0.26 ρs [kg/m3] 1010 2450 Shape Sphere Rounded-edge cylinder Sphere PLST Mud  Discharge Inclined  Dist. φ 280 Fig.2 Bed column with inclined     distributor. mud(a) horizontal (b) inclined (d) convex(c) concave (e) coneFig.3 Schematic of distributor configurations and qualitative solids circulation patterns.2The 12th International Confere ce on Fluidization - New Horizons in Fluidization Engineering, Art. 40 [2007]http://dc.engconfintl.org/fluidization_xii/40FLUIDIZATION XII  339Key properties appear in Table 1.  3) Water Content α and Ratio of Particle mixture β The water content, α, and ratio of particle mixture β are defined as : TOTWTR VV=α      (1) ,         BLKi VV=β           (2) where VWTR = volume of added water, VBLK = total dry static bulk volume of solids mixture, Vi = static bulk volume of smaller particles, and VTOT = static bulk volume of solids mixture, including added water.  4) Experimental Methods Most experiments were carried out for Lo/D=static bed height / column diameter =1.0, but values of Lo/D of 0.5 to 2.5 were also used to observe the influence of distributor configuration. The static bed height, Lo, for each distributor is based on the same volume of particles as for a horizontal distributor.  Small particles (either glass beads or polystyrene cylinders), simulating waste mud, were added to the column with 6 mm polystyrene particles. Fluidizing air was then introduced through the distributor. The gas flow rate was raised to ~1.5Umf, then gradually decreased to 0.2 - 0.3 Umf. Data were taken at each downward step in U. Next the gas flow rate was quickly raised to ~3Umf, then slowly reduced to Umf to obtain the rest of the data. The operating conditions for the wet bed were the same as for the dry one. All measurements and observations were carried out after completion of mixing of the two species of solids, and after tap water was added by pouring from the top of the bed. This required at least a minute for each case.  PRE-TEST FOR FARM PRODUCT WASHING To confirm the ability of this system to wash farm products, a preliminary washing experiment was carried out. A single soiled ginger or potato was suspended in the centre of the bed (Figure 4) in the 180 mm inner diameter half-column with a distributor inclined at 30° to the horizontal. The ginger and potato had densities of 1020 kg/m3 and 990 kg/m3, respectively, and typical sizes of ~35 mm (maximum diameter) × 120 mm in length and about 30 mm (maximum thickness) × 60 mm width × 150 mm length, respectively, although the ginger was very irregular.  After 6 minutes of washing at U/Umf=2.0, promising results were obtained. With small quantities of water (20 vol% of bulk bed volume), soft-abrasive washing was achieved. 3Nagahashi et al.: Flow Characteristics of Fully Wetted Binary Solid MixturesPublished by ECI Digital Archives, 2007NAGAHASHI et al.  340  The surface of the sweet potato was not damaged by the fluidization washing, whereas it was damaged seriously by conventional hand washing with a soft-hemp brush. However, a small amount of mud remained in narrow crevices. Small high-density particles, either mud from the soiled products  or newly-added                   INFLUENCE OF DISTRIBUTOR-GEOMETRY ON DRY-BED-FLUIDIZATION  The influence of the distributor geometry was investigated to optimize the discharge of solid wastes in a single-particle-species dry fluidized bed. Little previous work has been reported on inclined gas-distributors, but some information is available from previous work(4). Unlike conventional fluidization, partial fluidization occurs around the shallowest upper region (left corner in Fig. 2 or 5) due to less resistance.  Therefore fluidization washing(good) soft abrasive washing(sweet potato) before washing (ginger with mud) washing in wetted bedafter washing (6 minutes washing) good washing (but mud remained in narrow gap) washing by brush (not good)Fig.4 Pre-test for farm products washing Fig.5 Illustration of bed transition with inclined gas-distributor (dry bed) glass beads, might be desirable to remedy this problemand improve the washing performance. The advantage ofthe fluidization enhancement with added water relies onthe bed material being non-wettable and of low density,provided such small particles do not over-crowd the bed. 4The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 40 [2007]http://dc.engconfintl.org/fluidization_xii/40FLUIDIZATION XII  341the ∆P-U line is more inclined than for a fixed bed, as shown in Fig. 5, where (1) denotes the initial fluidization point. Fluidization develops over the whole cross-section at point (2); after point (2) the whole bed reaches the fully fluidized condition with a constant pressure loss at (3), as for conventional fluidized beds. The solid circulation for each distributor configuration was checked via frame-by- frame analysis of video images (Fig. 6), confirming the circulation patterns shown in Fig. 3. For the inclined distributor, solids move upward along the slope of the distributor plate. The solid circulation can be controlled by the distributor geometry.           In the case of the cone distributor, the circulation was unstable and complex.  For every other case, a defluidized zone was observed at the lowest corner or vertex from where washing waste could be successfully removed.  Video photography was performed to confirm the solids circulation and the effectiveness of solids waste discharge with coloured polystyrene particles as tracers. Control of solids circulation was critical. The pattern was as shown schematically for solids circulation in Figure 3                (inclined)           (concave)            (convex)               (conical)    Fig.6.  Typical solid circulation for each distributor configuration from video photos. 1.02.03.00 0.2 0.4 0.6 0.8 1U [m/s]∆P B[kPa]0.6-GB2030concaveconvexhorz(     )coneθ=0θ=10 degfor every gas-distributor geometryexcept the cone-distributor, for whichthe flow pattern could not be seenclearly.  Figures 7-a, 7-b show the influence ofthe gas distributor geometry for 0.6mm glass beads (figure 7-a) and 6 mmPLST (figure 7-b) with L0/D=1.0. Forthe glass beads, seven distributorconfigurations were tested under dryconditions. In  every case,  minimumFig.7-a Influence of distributor inclination       angle for 0.6 mm - GB bed 5Nagahash  et al.: Flow Ch racteristics of Fully Wetted Binary Solid MixturesPublished by ECI Digital Archives, 2007NAGAHASHI et al.  342           DRY/WET BINARY MIXTURE FLUIDIZATION WITH INCLINED DISTRIBUTOR 1) General Characteristics  Figure 8 shows a typical ∆PB-U diagram for this unique gas-fluidized bed. In general there was hysteresis between the increasing and decreasing gas velocity curves due to the difference from the initial packing of the particles. The velocity-descending line was more reproducible and is thus used here. Straight horizontal lines, indicative of pressure drop = (particle weight, W) / (cross- sectional area, A), imply an ideal suspension; the numbers at the right correspond to those values. The α=0 line is for the corresponding dry bed with Umf=1.45 m/s. Umf decreased with increasing water content, indicating fluidization enhancement.  2) Binary-Mixture Fluidization with Horizontal Distributor When particles of lower Umf are added to particles of higher Umf, the resulting Umf   gradually approaches that for the smaller Umf - particles(3), i.e. Umf decreases.  This trend is shown in Figure 9, both for dry and wetted beds. In this case, small PLST was added to large PLST. Water addition enhanced fluidization (i.e. Umf decreased).  On the other hand, for large-PLST and small glass beads, water addition contributed negatively to mobilization (Umf increased; Fig 10).  ∆ PB(kPa)2.24  0.151.96 1.67 kPa  6-PLST121 3 50U (m /s)U m f =1.45DRY (α=0)α=0.1α = 0.2 Fig.8 ∆PB-U diagram (velocity-descending lines)for fully-wetted gas-fluidized bed of 6 mm-PLSTparticle bed with horizontal distributor  1.02.00 1 2 3U [m/s]θ=10 deg3020horizontal (θ=0)6-PLST∆P B[kPa]Fig.7-b Influence of distributor inclination        angle for 6 mm - PLST bed fluidization occurred at a lower gas flow rate than for a horizontal distributor,with (Umf)30° < (Umf)20° < (Umf)10° <(Umf)conv, (Umf)conc, (Umf)cone < (Umf)horizontal.A similar trend was obtained or the6-PLSTbed with inclined gasdistributors, although the magnitude ofthe differences was smaller than for theglass beads.   6The 12th International Conference on Fluidizati n - New Horizons in Fluidization Engineering, Art. 40 [2007]http://dc.engconfintl.org/fluidization_xii/40FLUIDIZATION XII  343             3) Binary Mixtures with Inclined Distributor Figure 11 shows the influence of water addition for a single-species system (6-PLST). In this case (single-species, inclined distributor), water addition did not cause an appreciable change in Umf, but the pressure drop lines differ for dry and wet beds. For a dry bed, the pressure drop line was similar to that in Figure 5; the line finally reached the ideal suspension value (W/A) and remained constant at high flow rates.  However, for wet beds, the line increased linearly with U beyond Umf, possibly due to friction resistance of aggregates held together on the column wall by water.  The vigorous motion of aggregates can be seen in Figure 12.  They were more apparent in the binary solids bed.               Fig.10 ∆PB-U diagram (velocity- descending lines) for binary solids dry and wet fluidized beds, (horizontal-distributor/ 6-PLST+1-GB).12 1.94  1.66 (kPa)β= 0.11.812.09 1.67  β= 00 1 3 5∆ PB(kPa)2.5DRY (α = 0)β= 0.3WET (α = 0.1)β = 0.3β= 0.1U (m/s)6-PLST +3.2-PLST∆PB(kPa)2.00 2.59  2.87 1.67 (kPa) 1230 1 3 5U (m/s)(α=0.1, β=0.1)(α=0 , β=0.3) (α=0.1 , β=0.3)(α=0 , β=0)6-PLST1-GB(α=0, β=0.1) 0.40.81.21.62.00 2 4 66-PLST∆P B  (kPa)1.95 (kPa) α=0.1 α= 0 1.67 (kPa) θ= 30 degU  (m /s)HORIZONTAL(θ=0 deg)α =0.1 Fig.11 ∆PB-U diagram of 6 mm-PLST particlebed with inclined (30 deg) distributor for dryand wet conditions Fig.12 Solid circulation inwetted binary solids fluidizedbed with inclined distributor. Fig.9 ∆PB-U diagram ((velocity-descending lines) for binary solids dry and wet fluidized beds, (horizontal-distributor/6-PLST+3.2-PLST)7Nagahashi et al.: Flow Characteristics of Fully Wetted Binary Solid MixturesPublished by ECI Digital Archives, 2007NAGAHASHI et al.  344              except at low velocities.  Fluidization enhancement by addition of water was not significantly weakened by adding small high-density particles, at least for liquid volume fractions less than or equal to 10%.  Inclining the distributor plate was helpful for completely wetted beds in which U/Umf<1.2, with even an angle of 10° to the horizontal being sufficient to discharge small high-density particles.   ACKNOWLEDGEMENT     The authors are grateful for support from both the Natural Sciences and Engineering Research Council of Canada and the Japan Society for the Promotion of Science (# C-18560728).  REFERENCES 1) Y.Nagahashi, D.H.Lee, J.R.Grace, N.Epstein, A.Yokogawa and Y.Asako, Enhancement of Large-Particle Gas-Fluidization by Adding Liquid,  AIChE Journal, 49 (2003) 675-681. 2) Y.Nagahashi, J.R.Grace, Y.Asako, N.Epstein, D.H.Lee and A.Yokogawa, Effect of Liquid Addition on Heat Transfer in Gas-Fluidized Beds of  Large Light Particles, Journal of Process Mechanical Engineering, 217 (2003) 279-286. 3) S.Chiba, T.Chiba, A.W.Nienow and H.Kobayashi, The Minimum Fluidization Velocity, Bed Expansion and Pressure-Drop Profile of Binary Particle Mixtures, Powder Technology, 22 (1979) 255-269. 4) Y.O.Chong, T.H.Nguen, L.S.Leung and C.S.Teo, Inclined Fluidized Bed, AIChE Symp. Ser., 80, No.241 (1984) 149-155. U (m/s)6-PLST +1-GB0.40.81.21.62.00 2 4 6θ = 30 deg ∆P B ( kPa )1.94 (kPa) α=0.1 α = 0 1.67 (kPa) 1.68 (kPa) β = 0.1β = 0 (θ=0)Fig.13 ∆PB-U diagram for binary solids fluidized bed for dry and wet conditions, (30° -inclined distributor,  with 6-PLST+1-GB) Figure 13 shows results forfluidization of a binary mixture withan inclined distributor. ∆PB waslowered around Umf and thenincreased linearly with U. Theextent of the increase was largerthan for a single-species wet bed,probably due to wetted smallhigh-density particles.   CONCLUSIONS From pressure drop measurementsand video analysis, no severesegregation was experienced, 8The 12th International Conference on Fluidization - New Horizons in Fluidization Engineering, Art. 40 [2007]http://dc.engconfintl.org/fluidization_xii/40

Flow Characteristics of Fully Wetted Binary Solid Mixtures in Gas Fluidized Beds with Inclined Gas Distributors

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Flow Characteristics of Fully Wetted Binary Solid Mixtures in Gas Fluidized Beds with Inclined Gas Distributors

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