RESIDENCE TIME OF SOLIDS IN CONTINUOUS FLUIDIZED BEDS

Abstract

Industrial fluidized beds normally use wide size distributions of solid feeds in continuous operations. In literature, practically no work has been reported about the mean residence time of different size particles in moving and fluidized beds when mixed feed containing parti cles of wide size distributions are used, Fluidized beds at present are designed on the assumption of equal mean residence time for each size particles. The present work investigates the residence time distributions of particles of different sizes using mixed feeds in moving and fluidized beds. The effect of air flow rate, solid feed rate, bed height and particle size distribution of solids was analysed and the findings are of great importance for the proper design of fluidized. bed systems in which mixed feeds are used. A 50 mm inside diameter column of perspex and glass ballotini beads of 0. 3 93 mm, 0. 724 mm: and 0. 96 mm diameter were used in the investigation. The solid feed in different size combination was introduced at the top of the bed and the solids were removed by a discharge system located mostly at the bottom of the bed, thus giving, in general, a downward flow of solids. Air was used as the fluidizing gas and studies were made on the residence times of solids in moving and fluidized beds by using stimulus-response technique with fine coating of iron particles (xix) on glass ballotini beads as tracer. The study of moving beds was carried out with air flow rates in the range of 0. 2u^ to 0. 95 u^ using a double pulse tracer injec tion technique to eliminate the end effects. The effect of particle size was studied by introducing the tracers of different sizes in the feeds of mixed sizes. The parameters studied were: solid feed rates from 34 to 300 g/min giving a mean residence time of particles in the range of 1. 1 min to 30 min and the bed height to diameter ratios were varied from 2. 0 to 6. 0. The studies on fluidized beds were carried out by single pulse tracer injection technique and the air flow rates were varied from 1. 2 to 6. 0 u^f. The effect of particle size was studied by introducing tracer particle of different sizes in the feeds of mixed sizes. The para meters studied were: solid feed rate from 34 to 300 g/min, bed hold ups corresponding to static bed height to diameter ratios from 0. 5 to 4. 0 and the mean residence time was varied from 1. 1 to 10. min. The tracer studies revealed interesting and useful observations and further investigation was carried out on the mixed feeds in fluidized beds by analysing the bed hold ups. The parameters investigated were air flow rate from 1.3 to 8.0 urf, solid feed rate from 120 to 500 g/min, bed hold ups from 90 to 620 g of solids corresponding to static bed height to diameter ratios from 0. 5 to 4. 0. A model is proposed for fluidized beds to explain the observed behaviour at different operating conditions. Axial and radial distribution (xx) of solids in the bed were measured and modifications in the solid discharge system were made to confirm the basic postulates of the proposed model. The effect of horizontal and vertical baffles was also studied and the changes in the hold up ratios are explained with the help of the proposed model. For the system studied, experimental results confirm that the flow of solids in moving beds closely resembles plug flow upto u/umf of 0. 95. End effects due to the solid discharge system are primarily responsible for any dispersion observed in the stimulus response experiments. There is practically no effect of bed height to diameter ratios or of solid feed rate on the flow pattern of solids in moving beds. In the experimental range investigated a dead zone of particles was observed in moving bed close to the solid discharge system. The dead zone remained virtually uninfluenced by variables like particle size and bed height but an increase in solid feed decreased the volume of the dead zone. In moving beds with continuous solid feeds of mixed sizes of particles, the flow behaviour of each size particles exhibited plug flow quite closely upto u/urn£ value of 0. 95. In fluidized beds with feeds of uniform sizes, experimental results confirm that the solid flow pattern resembles ideal backmix for air velocity higher than 1.5 u .. For air velocities from 1. 2 to 1.5 u £, the presence of short circuiting, dead zone and plug flow was detected along with the major backmix flow. In fluidized beds (xx i) using mixed size feeds the particles of each size exhibited different mean residence times and yet individually they were found to be per fectly backmixed. The mean residence time of large size particles was found to be more than that of smaller particles. This effect is quite use ful for industrial appHcations of fluidized beds for continuous feeds contain ing mixed size particles. The hold up ratios, defined as the ratio of the mean residence times of any two size particles increased with air velocity upto about 2. 5 umf , and further increase in air velocity resulted in a sharp decrease in the value of hold up ratios. The hold up ratios are found to be independent of the solid feed composition provided u/u^ is used as the correlating parameter. A second order polynomial of the form: 2 2: = b + c (u/umf) + d (u/uj (Tb)11 satisfactorily correlates hold up ratio H, dimensionless time parameter "£"_ representing solid feed rate and u/u , for superficial gas velocities higher than 2. 5 u^f. The trend of the variation of hold up ratio remains the same for solid feeds of two or three size combinations. Very sharp cut combina tion of sizes,however,produced poor fluidization and hold up ratio values less than unity were observed and lower hold up ratio values were found in comparison to mixed feeds of two particle sizes only. (xxii) The effect of different mean residence time for different size particles in fluidized beds is explained by proposing a model which postulates that the low velocity of air near the enclosing wall results in the formation of two thin boundary layers. The boundary layer close to the enclosing wall was richer in small size particles and the second layer next to the first boundary layer was slightly richer in large size particles. The thickness of two boundary layers was affected by the formation of large bubbles and by slugging in the bed. With the help of the model, the effect of various parameters on the hold up ratios is explained. The vertical baffles produce smooth fluidization and the radial gradients of the particles of different sizes were not significantly in fluenced by the presence of vertical baffles. The horizontal baffles with smaller openings and close spacings resulted in the segregation of larger particles at the bottom and hold up ratios less than unity were obtained.