SPOUT-FLUID BED TECHNIQUE The spout-fluid bed is a unique fluid-particle-agitated bed in which spouting and fluidizing occur simultaneously. This technique overcomes such inherent limitations as stratification or slugging in a fluidized bed and restricted flow of annulus solids or instability in a spouted bed. The spout-fluid bed technique gives a much higher rate of circulation and mixing of solids and fluid than any other technique for fluid-particle contact. It allows a larger range of fluid flow rate and good control of spouting or fluidizing. The minimum fluid mass flow rate for maintaining spouting or fluidizing is lower than when the entire bed is only spouted or fluidized.
A
GREAT deal of work has been done on fluidization in the last 25 years, and work on spouting by Mathur and Gishler (1955) started around 1955. Their simultaneous combined effect in the same bed has drawn no attention so far, although its striking physical behavior can well be imagined. The spout-fluid bed under investigation is a unique fluid-particle-agitated bed which overcomes many inherent limitations of fluid and spouted beds. The spout-fluid bed consists of a cylindrical column with a flat or conical bottom with evenly distributed perforations for the fluid to enter, and functions as a grid. The center of the grid has a comparatively large opening, connected to a separate line (Figure 1) for the supply of spouting fluid. T h e bottom of the column is connected t o the calming section through which the fluidizing fluid enters. This is similar to imparting spouting in an already fluidized bed or fluidizing the annulus of an already spouted bed. Before attempting a large setup, preliminary observations are taken in a 50-mm.-diameter glass column to spout-fluidize sand and coal of various sizes, using air as the medium. Experimentation with a constant bed charge of 1.08-mm. average diameter sand particles has shown that the minimum spout-fluidizing air mass flow rate is approximately 16% more than the minimum spouting (only), and 39% more than the minimum fluidization (only) air mass flow rate.
,
TOTAL
n u / n our
I ‘ I
Figure 1 . Schematic diagram of one type of spout-fluid bed 340
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 2, 1970
The minimum air mass flow rate for maintaining spouting or fluidizing in an operating spout-fluid bed is lower than when the entire bed is spouted or fluidized in the minimum condition, because formation of the central core is assisted by the cross flow of the fluid in the fluidized annulus, and the fluidized annulus is assisted by the secondary fluid flow of the spout. The total minimum spout-fluidization air mass flow rate is 36% lower than the summation of the minimum spouting and minimum fluidizing air mass flow rate for the same system. However, one cannot expect the bed to have both spouting and fluidizing properties with no extra fluid supply. This excess fluid is advantageously utilized to achieve the merits of both techniques combined, giving a high mixing rate of both solids and fluid. The spout-fluid bed gives no stratification with mixed particles of different diameter (experimental range was within 1.08- to 2.84mm. average diameter of coal and sand), density, or both. This property would be of particular value when the particles are deformed or react and change their chemical and physical properties while residing in the fluidparticulate system, such as during roasting, calcining, and gasification in fluid beds. I n a conventional fluidized bed there exists a relatively narrow range of fluid flow, over which the bed becomes slugging or bumping, and ultimately there is a carryover of the particles. In the spout-fluid bed, however, even a small diameter column (50-mm.) shows no slugging a t any air mass flow rate. I t allows a larger range of fluid flow, which is particularly useful when smaller gas residence time and very high solid fluid contact are the goals. Good operational control is observed if the fluid is diverted for more fluidizing or spouting in the system, and the total supply of fluid kept constant. The main disadvantage in the spouted bed system is the packed nature of its annulus. I n the annulus of the spout-fluid bed the fluidizing fluid moves upward and the solid moves downward because of gravity, and thus a countercurrent solid-fluid contact is established. Gas bubbles are observed in the annulus. The bubble and emulsion phases exchange mass sufficiently often for almost all vertical and radial concentration gradients to approach zero-i.e., perfect mixing and isothermal condition. A wide value of the length-diameter ratio can be used in this technique, particularly with a conical bottom. The high velocity tunnel or track in the solids in large vessels can be avoided by the counter flow of fluid and solids with a single or multiple spout. The difficulties in building a higher bed in a stable spouted bed are not encountered
in a spout-fluid bed. A bed with only spouting or fluidizing with the same excess fluid required for minimum spoutfluid property would not attain the above-mentioned qualities. I n a spout-fluid bed the pressure drop in the fluidization line does not change much in comparison with that in which the entire bed is in the fluidization condition only. But the pressure drop in the spouting line is markedly decreased, compared to that observed in the same bed a t the spouted condition only. Elutriation is checked in the annulus because of the continuous fall of particles at the top from the spout. Elutriation is observed at the spout head.
literature Cited
Mathur, K. B., Gishler, P. E., A.I.Ch.E. J . 1, 157 (1955).
ASOK CHATTERJEE' Indian Institute of Technology Bombaq 76, India I Present address. Abadan Institute of Technology, P.O. Box 805, Abadan, Iran.
RECEIVED for review February 26, 1969 ACCEPTED September 16, 1969
FOAM BREAKING WITH A HIGH SPEED ROTATING DISK Experimental data are presented on the effectiveness of two configurations of mechanical foam breakers, both based on the shearing action of a high-speed rotating disk. For foams impinging on a given small area on top of the disk experimental data yield an empirical equation relating exit foam and liquid flow rate to inlet foam flow rate rotational speed and distance from center of rotation, and a n empirical equation for the critical rotational speed-i.e., the rotational speed above which all the foam impinging on the disk collapses. Experimental data on the foam-breaking capacity of a rotating disk located in a pipe with the foam impinging on the complete disk area indicate that this configuration can serve as an effective foam breaker.
RECENTLY Goldberg and Rubin (1967) reviewed the various methods for collapsing foams and showed that a simple mechanical foam breaker can be built which uses the shearing force of a high speed rotating disk to collapse a stable foam. The experimental unit consisted of a rotating disk located in a collecting container and was characterized by foam impinging on a relatively small area on top of the disk. This type of equipment is applicable, for example, to foam separation. The experimental data presented indicate that within the range of variables studied, foam impinging 3 cm from the center of rotation of the disk collapsed completely a t speeds above about 2500 rpm, regardless of foam flow rate and material of construction. The experimental data were correlated by plotting the ratio of exiting foam to exiting liquid flow rates (condensation factor) us. disk speed. N o quantitative expressions were derived, however, and the effect of some experimental variables-Le., distance from center of rotation-were not examined. For possible practical applications of the rotating disk foam breaker it is important to obtain a quantitative expression from which the critical disk rotational speedLe., the rotational speed above which all the foam impinging on the disk collapses-can be calculated. An attempt to obtain such an expression was made in the present work, using a wider range and a larger number of experimental variables as well as refinements and changes in the experimental equipment and procedure. I n addition, experiments indicate that a rotating disk located in a pipe and characterized by foam impinging on the complete area of the disk can be used as a high capacity foam breaker. This foam breaker configuration
may be applicable, for example, to collapse foams at the outlet of reaction or other vessels. Some typical performance data are reported for this type of equipment. Experimental
Disk in Collecting Container. The essential parts of the experimental equipment were a glass column for supplying foam and the mechanical foam breaker (Figure 1).
-
u
Motor
Per s pex collector details
ipling
Figure 1. Schematic diagram of experimental equipment Disk in collecting container
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 2, 1970 341
