EFFECT OF SCREEN PACKING ON ENTRAINMENT FROM FLUIDIZED BEDS G . 1. O S B E R G Division of Applied Chemistry, National Research Council of Canada, Ottawa 7, Canada
T.
A .
TWEDDLE,
C .
E.
CAPES,
A N D
Steady-state entrainment rates from fluidized beds with and without cylindrical screen packing have been measured. The results have been correlated by the following equation used by lewis et a/.
Screen packing reduces the rate at which particles are projected from the surface of the bed. The rate of particle disengagement, however, is adversely affected by the presence of screen packing in the disengaging space. To minimize entrainment, packing should extend only up to the surface of the dense bed.
DURING investigations into the effect of cylindrical screen packing on the properties of gas-fluidized beds (Capes and Mcllhinney, 1968; Kang et al., 1967), a number of experiments were performed to indicate the effect of this dump packing on particle carry-over from the beds. The results are presented here. Lewis et al. (1962) indicate that the effects of important operating and system variables on entrainment rate from conventional gas-fluidized beds are well established qualitatively, if not quantitatively. Investigations of entrainment from beds containing internals, however, are limited and inconclusive. Previous work (Lewis et al., 1962; Overcashier et al., 1959) has shown that baffles may increase, decrease, or have no effect on entrainment rates from gas-fluidized beds. The necessity to determine experimentally the effect of a given type of packing on entrainment rate is obvious. In the work reported here, experiments were restricted to one bed material of wide size range and a single column diameter. Entrainment rates were determined as a function of gas velocity, freeboard, mesh size, and arrangement of packing.
The bed was supported by a porous stainless steel grid fitted centrally with a %-inch tube mounted flush with the grid surface, through which the bed material could be removed at the end of each run. Moist air in the range 25 to 35% RH t o minimize electrostatic effects and measured by rotameters was used as the fluidizing gas. Air and entrained solids passed out of the column through a metal cone collector whose position was adjustable to provide variable freeboard heights. M A
Experimental Details
Previous measurements of entrainment rates have included unsteady-state measurements as a function of time of fluidization (Hanesian and Rankell, 1968; Osberg and Charlesworth, 1951; Zenz and Othmer, 1960) and steady-state measurements after extended periods of fluidization in which the entrained particles were continuously returned to the bed (Lewis et al., 1962). Although the former type of measurement is useful to indicate mechanisms of particle carry-over, the latter type was used in the present work, since it was felt that steadystate measurements are most useful in providing information for the design of solids-recovery systems for fluid beds. A schematic diagram of the entrainment column is shown in Figure 1. The column sections consisted of methyl methacrylate pipe of 6.5-inch inside diameter. This diameter was chosen to be sufficiently large so that entrainment rate would be independent of effects found with smaller diameter columns (Lewis et al., 1962). Pressure taps were mounted in the column wall a t various levels. Ind. Eng. Chern. Process Des. Develop., Vol. 9, No. 1, January 1970
Figure 1. Entrainment equipment A.
Column sections
6. Adjustable cone collector C. Pressure taps D.
Fluidizing air
E.
Drain tube Air bleed
F. G.
H.
Particle return pipe
I. Flow diverter J . Particle flow control valves K. Particle reservoir 1. Cyclone M.
Exit oir to filter
Sample tube
85
The solids and gas were separated in a cyclone, from which the air flowed through a bag filter to atmosphere. Normally the solids from the cyclone passed back into the bottom of the dense bed by means of a return pipe with the aid of an air bleed. The maximum amount of this air bleed used to convey the solids back into the column did not exceed 0.5% of the fluidizing air rate. A solids-diverting device in the return pipe allowed the entrained solids to be collected in a sample tube for short measured periods of time. The weight of solids so collected allowed the entrainment rate to be calculated. The fluidized solids were Ottawa sand with size range 65- to 100-Tyler mesh (Table I ) . The properties of the cylindrical screen packings used are given in Table 11. I n most of the work, the packing extended from the distribution grid up to the collector, although in a few runs an empty space existed between the top of the packing and the collector. Preliminary experiments with bed weights of 15 and 30 pounds ( L I D of 1.3 and 2.6, respectively, at settled bed conditions) indicated that entrainment rate is not strongly affected by bed height, in agreement with previous work (Lewis et al., 1962). A bed weight of 15 pounds was therefore used. In a typical experiment, the bed material was sampled for screen analysis and the required weight was added to the column. The desired packing arrangement and collector height had been prepared beforehand. The bed was then fluidized a t the required air rate for 30 minutes before any samples were taken. The entrainment rate was then measured a t intervals of 1 to 10 minutes, depending on the air rate, until the operation reached steady state. Generally, experiments at each flow rate lasted 60 to 90 minutes in total, and the amount of material in the return line and out of the column as entrainment sample a t a given time was always less than 3% of the total bed weight. Recorded data included an estimate of expanded bed height by visual observation and from the pressure profile up t o the column, the entrainment rate and size analysis of the entrained material a t steady state, and the size analysis and weight of material remaining in the bed at the end of each run. Mass balances indicated that a t least 97% of the original bed material was recovered a t the end of a run.
Results and Discussion
Entrainment rates are shown in Figure 2 as a function of velocity for constant collector heights (distance from grid to collector) and in Figure 3 as a function of freeboard (collector height minus expanded bed height) for constant velocity: Figure 3 was derived from Figure 2 by interpolation and extrapolation and from the experimental expansion curves for the beds. With the exception of the unpacked bed at the lowest freeboard, the plot in Figure 2 of log E V us. l / V 2yielded straight lines, in agreement with the work of Lewis et al. (1962). The linear entrainment results can be correlated by the following equation, which is of the form used by Lewis et al.: h = -
V
K l eKz/ V' + K J H
Experimental values of constants Ki, K 1 , and Ks are given in Table 111. No attempt was made to correlate these parameters with packing properties, because of the limited number of different packings used in this work. Two features should be noted. First, the elutriation rate is less dependent on collector height and freeboard inpacked than in unpacked beds. Second, over much of
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. 3.1
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4 (ft/secI-' Table 1. Size Analysis of Ottawa Sand Bed Material
Tyler Mesh
Weight 7%
+65 -65 + 80 -80 +lo0 -100 +115 -115 +150 -150
0.7 8.1 46.3 28.0 14.8 2.2
B 1x1 1 x 1 Collector Inch, Collector Inch, Collector Inch, Collector Height, 6-Mesh Height, 4-Mesh Height, 28-Mesh Height, Unpacked Inches Packing Inches Packing Inches Packing Inches A
Calculated Terminal Velocity, Ft. 1See.
v4
... 4.6 3.7 3.0 2.4
...
Table II. Properties of Cylindrical Screen Packing
Dimensions of Cylinder, Inch l x 1 % X %
l x 1
86
Figure 2 . Entrainment rate as a function of superficial velocity for various collector heights
Mesh
Wire Diameter, Inch
Void Fraction (in 6.5-In. Column)
4 6 28
0.023 0.035 0.008
0.99 0.97 0.99
0
30.0
0
51.2
0
73.0
A
91.2
x %
0
0 A
30.0
0
51.2
51.2
A
73.0
U
74.4
+
+
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A
91.2
31.1 43.3 59.5 74.6
Table 111. Experimental Values of Constants in Equation 1
Packing None 1 x 1 inch, 4-mesh 3/4 x 3/; inch, 6-mesh 1 x l inch, 28-mesh
K1, Lb./Cu. Ft.
Kz > (Ft..'Sec.)2
Ka, (In.) - I
104.7 67.7 79.5 15.15
-41.3 -41.3 -41.3 -41.3
-0.0500 -0.0263 -0.0117 -0.0378
Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 1, January 1970
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60
70
80
90
FREEEOARDunches)
Figure 3. Entrainment rate as a function of freeboard a t constant superficial velocity, 2.5 ft./sec. m 1 X 1 inch,4-mesh packing 0 1 X 1 inch, 28-mesh packing 0 I X i inch, 6-mesh pocking 0 Unpacked
’
’
the range of collector heights studied the rate of elutriation from the unpacked bed is less than from the packed beds, except those containing the 28-mesh packing. With low collector heights, however, the opposite result is obtained and elutriation from the unpacked bed is greater. These results can be explained from a consideration of the mechanism of entrainment. T o be entrained, particles must be thrown up from the bed surface, dispersed, suspended in the space above the bed, and carried through it without disengagement. Lewis et al. (1962) consider that the projection of particles from the bed surface by the eruption of gas bubbles and the subsequent dispersion of the solids in the gas stream are the most important phase in the entrainment process. The presence of screen packing in the bed reduces the average size and velocity of bubbles in the bed and increases bubble frequency (Park et al., 1969). Apparently, the reduced violence of bubble eruption which results more than compensates for the increased bubble frequency. Hence fewer particles are projected from the bed surface in the presence of packing, and entrainment rates a t low freeboards are consequently less with the packed than the unpacked bed. The nonlinearity of the results for the unpacked bed a t low collector height in Figure 2 is probably due to the influence of particles which are projected directly from the dense bed into the collector a t the higher velocities. At higher freeboards particles are dispersed over the column cross section and are able to disengage partially before reaching the collector. T h e smaller dependence on freeboard of the entrainment rate from packed beds compared with that from an unpacked bed indicates that the rate of disengagement of particles from the upward gas flow above the bed is greater for the unpacked-bed than the packed-bed case. T h e effect is so pronounced with the two coarse-meshed packings that the relative magnitude of entrainment rate reverses a t higher freeboards, with the rate from the Ind. Eng. Chem. Process Des. Develop., Val. 9, No. 1, January 1970
unpacked bed becoming smaller than that from the packed bed. T h e lower rate of particle disengagement in columns containing packing is related to a t least two factors: 1. Reduced turbulence in the disengaging space due t o the presence of packing leads to less separation of dispersed solids. Lewis et al. (1962) in their work with columns of various diameters state: “The strong increase with increasing column diameter of the ability of the disperse phase to return solids to the dense phase is due to the transition from streamline to turbulent flow.” The ability of screen packing to reduce turbulence has been noted (Capes and McIlhinney, 1968; Chen and Osberg, 1967). 2. As seen in Figure 4, the packed bed entrains considerably finer particles than the unpacked bed. One might expect the coarser particles leaving the unpacked bed surface to disengage more readily than the finer particles thrown up from the beds containing packing. Experiments cited below, however, suggest that this factor is less important than reduced turbulence due t o screen packing. The increased tendency of the fine particles to segregate to the top of fluid beds containing screen packing (Sutherland and Wong, 1964) possibly accounts for the finer size of particles entrained in the packed beds. The more violent eruption of bubbles a t the surface of the unpacked bed, with attendant higher local velocities, probably allows a larger proportion of coarse particles to be ejected from these beds than from beds containing packing. The rate of disengagement of particles from the disperse phase above the bed is greater for the 28-mesh than for the 4- and 6-mesh packings. As a result, the rate of entrainment from the columns containing the fine-mesh packing always remains less than that from the unpacked bed over the range of experimental conditions studied here. As the mesh size becomes finer, the packings
a
L
.
I I I
Figure 4. Size of entrained particles as a function of freeboard
0
Unpacked
Superficial velocity 2.5 ft./sec. 1 X 1 inch, 4-mesh packing
$4 X % inch, 6-mesh packing
0 1
X 1 inch, 28-mesh packing
87
packing only up to the height of the dense bed, in order to retain its advantages in the dense bed while avoiding the disadvantage of higher entrainment obtained when packing extends to the top of the disengaging space. Conclusions
Entrainment rates from fluidized beds with and without screen packing can be correlated by the following equation, with the exception of unpacked beds a t low freeboards:
The presence of screen packing in the dense bed reduces the rate at which particles are projected from the surface of the bed. The rate of disengagement, however, is reduced by the presence of screen packing in the disengaging space. T o minimize particle carry-over from screen packed beds, the packing should therefore extend only up to the surface of the dense bed. Acknowledgment
Figure 5. Entrainment rate as a function of superficial velocity for various packing heights Collector height 91 inches Packing
Dense Bed Height, Inches
Inches
Min.
Max.
0 % X % inch, 6-mesh
91
17
% X % inch, 6-mesh A % X % inch, 6-mesh 0 % X % inch, 6-mesh
72 32 16 0
19
23 23.5 28 23
Packing
H
0 Unpacked
19
19 10
Nomenclature
D = column diameter, inch E H Ki Kz KS L
= entrainment rate, lb./sq. ft. sec. = distance from distributor grid to collector, inches = constant in Equation 1, Ib./cu. ft. = constant in Equation 1, (ft./sec.)2 = constant in Equation 1, (in.)-’
= fluidized bed height, inches
v=
superficial gas velocity, ft./sec.
11.5
approach the behavior of a solid object in a stream of flowing fluid, whereas screen packings of wider mesh produce an assembly of widely separated wires (Chen and Osberg, 1967). With fine-mesh packing, the void space between packings offers relatively low resistance to flow, leading to, a more tortuous flow path and greater particle separation by an inertial mechanism than is the case with the coarse-mesh packing. Fine-mesh packing may thus act as an impingement separator in the disengaging space, leading to higher disengaging rates than the coarser meshes. The discussion up to this point has concerned packedfluidized beds in which screen cylinders extended from the distribution grid up to the collector. Figure 5 shows the effect of leaving various amounts of free space between the packing and the collector for a column of high freeboard. As packing height is reduced, the entrainment rate at a given velocity is reduced, and equals the rate for an unpacked bed when the packing height drops below the level of the dense bed. These results indicate that reduced turbulence due to the presence of packing in the disengaging space is the primary reason for the higher rates of entrainment from packed beds a t higher freeboard. From the point of view of design, one should use screen
88
The authors thank W. H. Park for helpful comments and suggestions.
Literature Cited
Capes, C. E., McIlhinney, A. E., A.I.Ch.E. J. 14, 917 (1968). Chen, B. H., Osberg, G. L., Can. J . Chem. Eng. 45, 46 (1967). Hanesian, D., Rankell, A., Ind. Eng. Chem. Fundamentals 7, 452 (1968). Kang, W. K., Sutherland, J. P., Osberg, G. L., Ind. Erg. Chem. Fundamentals 6, 499 (1967). Lewis, W. K., Gilliland, E. R., Lang, P. M., Chem. Eng. Progr. Symp. Ser. 58, No. 38, 65 (1962). Osberg, G. L., Charlesworth, D. H., Chem. Eng. Progr. 47, 566 (1951). Overcashier, R. H., Todd, D. B., Olney, R. B., A.I.Ch.E. J . 5, 54 (1959). Park, W. H., Kang, W. K., Capes, C. E., Osberg, G. L., “Properties of Bubbles in Fluidized Beds of Conducting Particles as Measured by an Electroresistivity Probe,” in press, Chem. Eng. Sci. (1969). Sutherland, J. P., Wong, K. Y., Can. J . Chem. Eng. 42, 163 (1964). Zenz, F. A., Othmer, D. F., “Fluidization and FluidParticle Systems,” p. 389, Reinhold, New York, 1960. RECEIVED for review January 16, 1969 ACCEPTED April 25, 1969
Ind. Eng. Chern. Process Des. Develop., Vol. 9, No. 1, January 1970
