STUDIES ON GAS-SOLIDS FLUIDIZATION WITH VERTICAL INTERNAL BAFFLES

Abstract

The thesis entitled 'Studies on gas-solids flu idization with internal vertical baffles' is presented in six chapters. In Chapter I, the literature review relating to fluidization characteristics, bed pressure drop, minimum fluidizing velocity, bed expansion, flu idization characteristics in continuous fluidization and bed hold ups is reported. The present studies on the effect of vertical baffles on fluidization charac teristics, bed expansion and quality of fluidization in batch and continuous gas-solids fluidization and studies on bed hold-up using mixed sized particles are indicated. In Chapter II, is presented the physical and flow properties of solids like particle diameter, particle density, static bed porosity, sphericity and solids angle of repose for material like spherical glass beads, bauxite, limestone and baryte. Chapter III deals with the studies on batch flu idization in 70 mm I.D. perspex column having 12, 7 and 3 Nos. of 6 mm diameter stainless steel rods as internal vertical baffles having an effective length of 610 mm. Aluminium grid plate having 10% openings is used as air distributor and to support the particles. Spherical glass beads, crushed Tiauxite, limestone and baryte in ii the size range.of 1540 microns to 385 microns have been studied. The bed pressure drop has been measured as a function of air flow rate in the fixed bed, at the onset of fluidization and in the fluidized bed zones. It is observed that bed pressure drop increases with increase in air flow rate in fixed bed zone and it remains essentially constant after the onset of fluidization. The variation of pressure drop with air flow rate is compared with flu idized beds with and without baffles. It is observed that pressure drop in baffled bed is more than that in the unbaffled bed. This increase in the pressure drop is due to the additional skin friction caused due to the presence of baffles in the bed. The pressure coefficient (APA/W) is observed to be more in the fluidized beds with baffles as compared to that with unbaffled beds indicat ing better fluidization. Keeping the same conditions of solids loading per unit area and the equivalent diameter of the column, effect of a single baffle on fluidization characteristics has been studied. The bed pressure drop has been measured at different air flow rates in the fixed bed, at the onset of fluidization and in fluidized bed zones. When compared, pressure drop in fixed bed zone with single equivalent baffle and in multibaffled bed is found to be of same magnitude. However, at and beyond onset of fluidization, pressure drop is more in the multibaffled fluidized bed as compared to that in the iii bed with single equivalent baffle. This is indicative of the presence of channelling tendencies in the latter. Quality of fluidization is improved by introducing several baffles in the bed as the slugging is reduced in the bed. A dimensionless correlation has been proposed to predict the overall pressure drop in the fluidized bed wilh vertical baffles, at and beyond onset of fluidiza tion as, ^^ . 0-923 +\ (Reeq)2 +k2 (ReT)2 kx =3-72 x 10"8 k2 =3-46 x 10~10 The correlation predicts the pressure drop within + 10% of the experimental values. A minimum distance of six particle diameters between two adjaecnt baffles is found essential for initiating the movement of particles. At a distance greater than ten particle diameters, however, smooth and uniform flu idization is achieved. The experimental values of the minimum fluidizing velocities in the baffled beds have been found to differ widely from the values predicted using Leva's correlation for cylindrical columns without baffles. Adimensionless IV correlation incorporating various parameters like the term d /D , solids and fluid physical properties and geometry of the apparatus,has been proposed for pre dicting the minimum fluidizing velocity as 0-624 D-0-12 d -0-13 L 0-04 iAt =6. 2xl0-2 (Ar) (^) if) (—> Vf p p o The predicted values are found to be well within + 20% of the experimental values. Studies on the effect of vertical internal baffles on the bed expansion and fluctuation ratio have been conducted. The bed expansion and fluctuation ratio will be lower in beds with vertical internal baffles as bubbl ing is reduced because of the presence of baffles. A dimensionless correlation has been proposed as -0-22 0-4 d 0-69 p _ F -0-11 e m0-065 (Fr) (Re) {•£•) ( S Pjy ~) P f for predicting the expanded bed height in baffled beds within ± 15% deviation from the experimental values and is valid for 4 <Be < 175. The fluctuation ratios for different solids for a given reduced mass velocity of air (^f/^f) are observed to be lower in baffled beds as compared to unbaffled beds, indicating better quality of fluidization in the former. The quality of fluidization in a fluidized bed can be measured by the fluctuation ratio of the levels in the bed. Fluctuation ratio has been correlated as an exponential function of (Gf-G^)/&mf as earlier done by Lewis et al [126] as r = e where m is the slope of the line on semilog plot and is a function of particle diameter. The predicted values of fluctuation ratios are found to be within + 15% of the experimental values. In Chapter IV, studies on continuous gas-solids fluidization in column fitted with multibaffles (as described in Chapter III) with provision to feed the material continuously from top and remove the solids from bottom, have been given. Studies have been carried out using different solids in the size range of 977 to 385 —3 -2 / microns and feed rate varying from 6*9x10 •' to 1*52x10 kg/; Introduction of vertical baffles in the bed increases the pressure drop because of extra skin friction due to additional baffle surface and fluidized particles. A dimensionless correlation has been proposed to predict the bed pressure drop within + 20% deviation from the experimental values as Ap 0.33 0.08 D-0.1 0-3 p .p 047 -PPf-TL - 52- 5(Fr) (Re) («D•) (R) ("V-1f ) vi It is observed that for a given solids feed rate, the lower bed densities could be obtained at high air velocities in countercurrent operation of gas and solids. When compared, the bed density is observed to be higher in fluidized beds with vertical baffles than in unbaffled beds for a given solids loading ratio. A dimensionless correlation incorporating various parameters has been proposed for predicting the bed density in baffled beds as -0*15 -0-008 d 0«27 p „ p 0.11 4M=342(Fr) (Re) {j*) i^T^) *f P f The predicted values are found to lie within + 10% of the experimental values. Chapter V deals with the studies on the mean residence time and hold up of particles using mixed sized feeds in continuous systems with vertical internal baffles. The effect of air flow rate, solids feed rate, bed height and feed composition of solids on residence time and hold up of particles has been studied. The hold up ratio in the bed (defined as the ratio of the mean residence time of larger particles to that of small particles) is observed to be higher in beds with baffles as compared to that in unbaffled beds. At air flow rates between 1*6 - 1*8 Ufflf the bed hold up ratio tends to remain steady in the unbaffled beds whereas a steady increase in the bed hold up ratio with increase in air flow rates even beyond Vll 1*8 U ^ is observed in the beds with vertical baffles. mi This is indicative that in case of baffled beds, the hold up of larger particles increases as compared to that of the small particles. A correlation for predicting the hold up ratio has been proposed as,