(543g) Hydrodynamics of Liquid-Solid Fluidization of Ternary Mixtures in Tapered Beds

Tapered columns have good application in industrial process like biological treatment of waste water, immobilization of bio-film reactions, crystallization and roasting of ores etc.. In contrast to cylindrical columns tapered columns have improved quality of mixing and better fluid-solid mixing and minimal entrainment of fine particles. Fine particles are present by themselves or formed during fluidization operation. Peng and Fan [1] were the first to investigate the characteristics of fluidization of mono component solids in tapered beds. They reported various flow regimes in the beds through measurements of pressure drop across the bed at various flow rates of fluidizing liquid using different tapering angles of the bed and also through visual observation. It is to be noted that as the cross sectional area of the tapered bed increases continuously from the bottom to the top, the drag force exerted by the fluidizing fluid on the particle decreases along the vertical direction. Koloini and Farkas [2] studied the pressure drop of fixed bed, minimum fluidization velocity and bed expansion in tapered beds of monocomponent particles. Pitt et al [3] measured the expanded bed height and hydraulic pressure drop at several apex angles for mono component particles. There have been some investigations on fluidization behavior of binary mixtures in cylindrical columns (Murthy et al [4], Pruden and Epstein [5], Kennedy and Bretton [6] and Bruno Formisani, et al [7]) .In the present study, liquid solid fluidization experiments have been carried out with mono component and ternary mixture solids. As mentioned above, the bed during operation may contain varied size of particles for which the hydrodynamics have not been investigated thoroughly, so it is necessary for mixtures beyond binary need to be investigated. Hence, in the present work an attempt has been made to study the behavior of ternary mixture beds in different tapered angle columns and also particle sizes.

Experimental setup consisted of calming section, distributor plate and tapered test column. The test column was a diverging tapered vessel of square cross section. All the sides of the column were made of plexi glass and ends of the surfaces of glass plates were held in steel L-plates. The height of the column was 600 mm and the bottom cross section was 50 x 50 mm and it was provided with 5 mm pressure taps to measure the pressure drop, one above the distributor and the other at a distance of 450 mm from the distributor at the top. The bed material was glass beads of different sizes, 0.5-0.8 (A), 1.3-1.5 (B) and 2.5-3.0 (C) mm. Total weight of the bed charged was 1000, 1400 and 1800 grams. Apex angles of the column ranged from 10^O to 20^O. Combinations of solids A, B and C were as follows, in the same order: 20:20:60, 40:20:40, 60: 20:20, 20:60:20 and 20:40:40. Water was the fluidizing medium.

Behavior of the mono component fluidized bed was observed to be similar to that of other investigators, Peng and Fan [1] and Murthy et al [2]. The description of the bed behavior in the case of ternary mixtures is as follows; To start with the bed was fluidized and defluidized. Later as the flow rate of water was slowly increased there was movement of small particles (A) to the top of the bed followed by intermediate particles (B) and the large particles (C) settling at the bottom, thus bed segregated into three layers. As the flow rate was increased further particles A started fluidizing at the top surface of the bed. On further increase of flow rate, a small cavity appeared at the distributor. The cavity and the fluidization of particles A increased with water flow rate. Later, the particles A were observed to be fluidizing completely along with some of the intermediate sized particles B. This prevailed till the complete bed started expanding indicating the on set of full fluidization.

For ternary mixture fluidization, the critical fluidization velocity, U_cmix, the critical pressure drop, P_cmix and the velocity for full fluidization, U_tfmix increased with the weight of the bed. As the coarse particle composition decreased in the mixture, U_cmix, P_cmix and U_tfmix decreased. This is due to the overall average particle diameter of the mixture decreases with decreased composition of large particles C. The same trend was observed in all the tapered columns. With change in angles of the columns, U_cmix, P_cmix and U_tfmix decreased with increase in the value of the angle. This can be attributed to the decrease in the height of the bed for the same weight of the solids as the tapered angle increase. Following are the correlations developed for mono component and ternary mixture solids.

Mono component particles

Re_c  = 0.55 (Ar)^0.68 * (tan(a/2))^0.7 *(H/W₀)^0.7

Re_tf  = 0.127 (Ar)^0.67 * (tan(a/2))^0.7 *(H/W₀)^0.7

Ternary mixture particles

U_cmix = 0.2829[(U_c1*X₁)^0.8 + (U_c2*X₂)^1.25 + (U_c3*X₃)] * [1/tan(a/2)]^0.5

U_tfmix = 0.17[(U_tf1*X₁) + (U_tf2*X₂) + (U_tf3*X₃)] * [tan(a/2)]^0.8

Re = Reynolds number

Ar = Archimedes number

H = Static height of the bed

W₀ = Width of the distributor

U_c, U_tf = Mono component critical and total fluidization velocity

References

1. Y Peng and L T Fan, “Hydrodynamic characteristics of fluidization in liquid solid tapered beds”, Chem. Eng. Sci., 1977, 52, 4, 2211-2290.
2. Koloini, T. and E. J. Farkas ; Can. J. Chem. Engg., 51, 499(1973).

3   Pitt, Jr., W. W., C. W. Hancher and H. W. Hsu : A.I.Ch.E. Symp. Ser., No. 181,74, 119(1988).

4  Sankarshana Talapuru, J.S.N.Murthy, S.Nandana, Phani Kiran S.V.G.R. and V.Ramesh, “ Hydrodynamic characteristics of liquid-solid fluidization of binary mixtures in tapered beds” ,2010 ECI Conference on 13^th International conference on fluidization ,article 41,Vol.RP6.

5  Pruden B.B and Epstein N., Chem. Engg.Sci.1964 14 696.

6  Kennedy S.C. and Bretton  R.H., A.I.Ch.E .J. 1966 12 24 30.

7  Bruno Formisani, Rosella Girimonte and Tizinia Longo: “The Fluidization Pattern of density – segregating Two –solid beds” ,ECI Conference on the 12^th International Conference on Fluidization – New Horizons in Fluidization Engineering, Vancouver, Canada ,2007.