CHAPTER 2 EXPERIMENTAL SETUP AND PROCEDURE

Transcription

1 22 CHAPTER 2 EXPERIMENTAL SETUP AND PROCEDURE 2.1 EXPERIMENTAL COLUMN All the experiments were carried out in an internal loop airlift fluidized bed and combined loop fluidized bed (an external down comer connected at the top and bottom of the column), made of perspex column with 0.15 m inner diameter, 1.63 m height, with a flat bottom and draft tube 1.54 m in height with m diameter. The external down comer was 0.98 m in height with 0.03 m in diameter. The diameter of the gas-liquid separator was 0.30 m. Figures 2.1 and 2.2 show the schematic diagram of internal loop airlift fluidized bed and combined loop airlift fluidized bed respectively. Figure 2.3 shows the photographic view of experimental setup used in the present study Internal loop airlift fluidized bed consisted of concentric tube and was comprised of different zones such as riser, gas - liquid separator and down comer. The air was sparged through the riser. From the riser gas went to the gas - liquid separator where part or whole gas bubbles were removed. The part of liquid from the separator moved to the internal down comer. The bulk density difference between the riser and down comer caused the liquid to flow, thus the liquid re-entered into the riser; the liquid phase circulates continuously around the loop. In combined loop configuration the liquid from the disengagement section went through the internal down comer as well as external down comer. The bottom clearance between draft tube and gas

2 23 distributor was 0.09 m and the top clearance between the free-gas liquid level and the draft tube was 0.12 m. Air was sparged through triangular pitch sparger which was 0.08 m in diameter with 180 holes of m diameter each located slightly below the perforated plate. Liquid from the storage tank was pumped into the column using a centrifugal pump and flow rate was measured by calibrated rotameters with an accuracy of 2%. Air was fed to the column through a pressure regulating valve by an oil free compressor. The flow rate of gas was measured and controlled by using gas rotameters with an accuracy of 2 %. Superficial gas and liquid velocities were calculated based on the column diameter. Ball valves were used for simultaneous opening and closing of both air and liquid lines. The densities of the liquids were measured with a specific gravity bottle and the rheological properties of non-newtonian liquids were measured by using Brookfiled Rheometer (Model LVDV-II+). Superficial liquid velocities were varied from m s -1 to 0.12 m s -1. Superficial gas velocities were varied from x 10-3 m s -1 to x 10-3 m s -1.

3 d c Degassing zone 2. Riser 3. Downcomer 4. Perforated plate 5. Sparger 6. Pressure taps 7. Manometer 8. Gas rotameter 9. Compressor 10. Liquid rotameter 11. Liquid pump 12. Liquid storage tank Figure 2.1 Schematic diagram of internal loop airlift fluidized bed

4 d c Degassing zone 2. Riser 3. Downcomer 4. Perforated plate 5. Sparger 6. Pressure taps 7. Manometer 8. Gas rotameter 9. Compressor 10. Liquid rotameter 11. Liquid pump 12. Liquid storage tank 13. External downcomer Figure 2.2 Schematic diagram of combined loop airlift fluidized bed

5 26 Figure 2.3 Photograph of the experimental setup 2.2 COLUMN OPERATION Initially the column was filled with liquid and then the selected solid particles were dropped into the column one by one. After the column was filled to desired height, air and liquid were introduced into the column. The experiments were carried out by increasing the liquid flow rate in the column by keeping the gas at a constant flow rate. After attaining a steady state, the pressure drop was measured and then the air and liquid flow rates were suddenly stopped by closing the valves simultaneously and the column was disconnected from the air and liquid feed lines and the gas holdup and liquid holdup were measured. After covering the desired range in liquid flow rates, the air flow rate was changed to the next higher value and the experiments were repeated. This procedure was continued to cover a wide range of the liquid and gas flow rates. Volume displacement method was used to measure the gas holdup (Nacef et al 1992 and Miura et al 2001). The liquid

6 27 holdup was measured by calculating the volume of the liquid available in the column to the total volume of the column and minimum fluidization velocity was determined by visual measurement (Koide et al 1983 and Zhang et al 1995) and compared with pressure drop method (Koide et al 1983). 2.3 SYSTEMS USED In the present work, water, 5% n-butanol and various concentrations of commercial grade glycerol (60% and 80%) were used as Newtonian fluids (and different concentrations of carboxy methyl cellulose (0.25%, 0.6% and 1.0%) were used as non-newtonian liquids. Different diameters of spheres, Bearl saddles and Raschig rings were used as solid phases. All the experiments were carried out in an atmospheric temperature with oil free compressed air as gas phase. After attaining the steady state, readings were taken and the error was found to be less than ± 3%. A minimum of 3-5 readings were taken and the average value was used for calculations. The properties of solid particles and liquids used in the present study are given in Tables 2.1 and 2.2. Table 2.1 Properties of solid particles used in the present study Sl.No Particle description Size, dp, m Density, kg m -3 p Sphericity 1 Spheres Spheres Spheres Spheres Spheres Spheres Spheres Bearl saddle Bearl saddle Raschig ring Raschig ring

7 28 Table 2.2 Properties of liquids used in the present study Density of Surface Viscosity Type of liquids liquids,( L ) kg m -3 tension L ) N m -1 K kg m -1 s n-2 n Water % n-butanol (Commercial grade) 80% Glycerol (Commercial grade) 60% Glycerol (Commercial grade) % CMC % CMC % CMC MEASUREMENT OF LIQUID PROPERTIES The properties of the liquids were measured at room temperature. The densities of the liquids were measured with a specific gravity bottle and the viscous properties of Newtonian and non-newtonian liquids were measured by using Brookfield Rheometer (Model LVDV-II+). The surface tension was measured by the drop weight method. Three to five experiments were conducted for the determination of liquid properties like density, surface tension, viscosity and the average value from the experiments was used for the calculations. Same static bed height was maintained for all experiments. 2.5 MEASUREMENT OF LIQUID HOLDUP The air and liquid flow rates were allowed to attain a steady state condition and both these flow rates were suddenly stopped by closing the

8 29 valves simultaneously. Then the liquid holdup was measured by using the following equation. l Volumeof theliquid Total volumeof thecolumn (2.1) 2.6 MEASUREMENT OF GAS HOLDUP Gas holdup was measured by volume displacement method (Nacef et al 1992 and Miura et al 2001).The air and liquid flow rates were allowed to attain a steady state condition and both these flow rates were suddenly stopped by closing the valves simultaneously and then the column was filled with liquid. After simultaneously closing the valves the height of the liquid in the column gets decreased. This decreased volume can be measured by filling liquid. The ratio of difference between the total volume of the column and volume of liquid used to fill the column to the total volume of the column gave the gas holdup. 2.7 MEASUREMENT OF MINIMUM FLUIDIZATION VELOCITY In visual observation method, the velocity at which the first particle move upwards is taken as minimum fluidization velocity. Minimum fluidization velocity was determined by visual observation method (Koide et al 1984, Zhang et al 1995). The minimum fluidization velocity results obtained by visual observation were compared with the results obtained from pressure drop method (Koide et al 1984). The deviation was found to be less than ± 5%.