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Ind. Eng. C h e m . Res. 1987,26, 1586-1593
Hydrodynamic Changes and Chemical Reaction in a Transparent Two-Dimensional Cross-Flow Electrofluidized Bed. 1. Experimental Results Charles V. Wittmann* and Benjamin 0. Ademoyega Illinois Institute of Technology, Department of Chemical Engineering, Chicago, Illinois 60616
Measurements of some hydrodynamic changes in a two-dimensional gas fluidized bed, when an external cross-flow dc field in the range of 100 kV m-l is applied, are presented. For a constant flow rate of bone-dry oxygen, when the electric field strength increases, the bed height increases linearly up to 17%, the bubbles become elongated and can be idealized by ellipsoids with increasing eccentricity, their frequency decreases sharply by as much as 62%, and their rising velocity decreases nearly linearly by as much as 37%. With increasing field strength, the minimum fluidizing flow rate increases linearly to 165% , implying that the dc field allows operation of the bed near minimum fluidizing conditions a t much higher gas flow rates. For a given gas flow rate, the observed changes imply a decrease in the flow through the bubble phase as well as an increase in bubble residence time. When the electrofluidized bed apparatus was used as a catalytic reactor, there was a linear decrease in ozone concentration in the exit stream of as much as 30%. Fluidized beds have several desirable features for certain chemical and physical processes. The general agitation of the particles yields good heat transfer throughout the bed, allowing for a good temperature control of very exothermic reactions. It also produces a good contacting between fluid and solid particles which is important in heterogeneously catalyzed reactions as well as noncatalytic fluidsolid reactions. Due to the fluidlike behavior of the bed, solids can be continuously added and removed, which is essential with a decaying catalyst and in fluid-solid reactions with consumption and/or production of solids. Gas-solid fluidized beds exhibit the particular feature of gas bubbles rising through the bed. In fluidized catalytic chemical reactors, the bubbles are detrimental to the conversion, which is even less than the one which could be obtained with a well-mixed reactor (Kunii and Levenspiel, 1969). In order to control the bubble size and improve the contacting between gas and solids, internal baffles have been used (Lewis et al., 1959) as well as a variety of gas distributors (Gomezplata and Schuster, 1960). The effects of an applied electric field having a strength of at least 1kV cm-’ had first been reported by Katz and Sears (1969). These authors “stabilized” the bed with the field; i.e., for a given gas flow rate, the bed became fixed or “frozen” when the field was applied rather than fluidized. In all their experiments, they used bone-dry fluidizing gas and reported that the introduction of humidity resulted in a marked decrease of the stabilization effect. In some cases, they could produce this phenomenon with one electrode not in contact with the bed, depending on the type of particles used. But they found that this effect was not observed with conductive particles. They also reported the expansion of a bed of silica gel particles with the electric field in the direction of the gas flow, in which case no bubble formation was observed. Johnson and Melcher (1975) published a heuristic description of the behavior of a fluidized bed of sand particles with an applied electric field called an “electrofluidized bed” (EFB). The electric field was either perpendicular or parallel to the flow, i.e., “cross-flow’’or “coflow”. With the cross-flow configuration and air-saturated water, these authors measured pressure overshoot at incipient fluidization which increased with increasing field strength. They presented photographs showing bubbles flattened in the
direction of the field in a bed which was still in a fluidized state. With a stronger electric field and a moderate gas flow rate, the bed was observed in a “frozen” state. These authors also reported that, with lower humidities, these phenomena decreased in intensity. In a related study, Colver (1977) presented data for a cross-flow EFB in a container with Pyrex glass walls and with the electrodes on the outside of the walls. He applied an ac electric field with a strength of the order of 1kV cm-l and an adjustable frequency from 1to 1000 Hz. For glass beads fluidized in this system with dry air, the author reported a ”100% bubble dissipation” and some results on bed expansion. His data showed somewhat lesser effects on copper spheres, and he commented about the lack of effectiveness when both electrodes were in electrical contact with a bed of these conductive particles. Dietz and Melcher (1978a) measured the shear stress at the inner wall of a cylindrical Couette viscometer apparatus containing an electrofluidized bed and correlated their data with variable dc field strengths. These authors also developed a theory for the interparticle forces due to the electric field which is transmitted to the walls of the bed, applicable when the field strength was high enough to obtain a frozen bed. To further check their theory, Dietz and Melcher (1978b) conducted more pressure overshoot experiments at incipient fluidization with a variable applied electric dc field in the cross-flow EFB, as well as bed support experiments, where the electric field necessary to support the entire weight of the bed at zero gas flow rate was found. Colver (1979) reviewed some of his earlier work on EFB’s as well as the ones of other authors. An earlier statement on an EFB with the electrodes placed at the outside of the insulating walls of the bed, when a dc field is applied in a cross-flow configuration (Colver, 1977), was corrected and this later system reported as being ineffective. Colver also gave new results, including bed expansion to 30% and pressure drop in the EFB vs. bed depth. The bed expansion was stated to decrease with particle diameter. Bubble rise velocities in an EFB with a dc cross-flow electric field had also been measured and reported unaffected by the field, although Colver questioned the reproducibility of his data. The EFB has found applications in several processes. Zahedi and Melcher (1976) used it to collect submicrom-
0888-588518712626-1586$01.50/0 0 1987 American Chemical Society
Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987 1587
ELECTROFLUIDIZED
OXYGEN CYLINDER
SUPPLEMENTARY GAS SUPPLY
Figure 1. Experimental setup for the hydrodynamic analysis of the electrofluidized bed apparatus.
SCREEN
TR AN S PAR EN T SIDE
FLANGES
46” ID STAINLESS S T E E L TUBING
,SEAL
GAS
Figure 2. Details of the electrofluidized bed apparatus.
eter aerosols with high efficiencies. Beeckmans et al. (1979) enhanced segregation in a mixture of particles fluidized in an EFB. This paper presents some new experimental results on a specific EFB apparatus. Some known phenomena such as bed expansion have been clearly quantified. Other controversial results, such as bubble rise velocities, have been measured with confidence. Finally the behavior of the EFB as a catalytic chemical reactor has been studied; specifically, the ozone decomposition reaction has been performed, following a number of previous authors (Frye et al., 1958; Orcutt et al., 1962; Kobayashi et al., 1966; Hovmand and Davidson, 1968). In this study, we have kept in mind that while bubbles are detrimental in several respects, they also significantly contribute to bed mixing. A bed which is “frozen” due to the electric field does not appear appealing, as most of the advantages of a fluidized bed are lost.
Experimental Section Electrofluidized Bed Apparatus. A two-dimensional bed, shown in Figures 1 and 2, with an internal cross section, wd,of 10.2 cm X 1.3 cm (4 in. X 0.5 in.), made of transparent Plexiglas 1.3-cm thick, has been used (Ademoyega, 1981). Lengthwise, the apparatus consists of two parts: the top part, 59.7-cm high (23.5 in.), which contains the particles and the bottom part, 21.6-cm high (8.5 in.), through which the fluidizing gas enters the bed. A support screen made of stainless steel is placed between two rubber sheets and is inserted between two flanges which join the two parts together. In the upper part, two stainless steel strips, 1.3-cm wide (equal to d)and 40.6-cm high, are glued at the inside of the narrow walls of the bed, 1.3 cm above
GENERATOR
PHOTOMETER MANOMETER BOTTLE
ROTAMETER FLANGES
new catalyst > used catalyst = glass beads, for flow rates up to 4.7 L min-l. More data scattering is present at these high flow rates. When the flow rate is increased even further, the bed tends to loose its homogeneity as horizontal “strings” of particles appear, separated by pockets of gas. The limit for the field strength was not better in the modified apparatus as an arc formed between the electrodes through the bed. This modified EFB apparatus was used only to study the effect of particle material on bed expansion. Minimum Fluidizing Flow Rate. For a given gas flow rate, Qd, the corresponding field strength required to bring about the minimum fluidizing conditions was determined twice following the method described. The error in the applied potential, AV, was no more than 0.5 kV. Figure 7 shows a linear increase of Qmfwith the field strength.
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0
Figure 5. Relative bed expansion with used catalyst particles.
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Figure 6. Bed expansion with different kinds of particles, a t an average flow rate. ! E
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o
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OE 2
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AV (kV)
Figure 7. Minimum fluidizing flow rate-potential phase diagram.
The increase is considerable. By applying a potential of 13.5 kV across the bed, it was possible to increase the minimum fluidizing flow rate by about 165%. The line of Figure 7 divides the domain of the two independent variables, the applied potential, AV, and flow rate, Q, into two regions: the fluidized region above the curve and the packed or frozen region below the curve. Johnson and Melcher (1975) have obtained similar results for this phenomenon; the overshoot pressure variation due to the electric field was nearly linear with field strength in their findings, although U , was proportional to A v 1 4 . Recall that the experiments of these authors were performed with
1590 Ind. Eng. Chem. Res., Vol. 26, No. 8, 1987
Figure 8. Photographs of the bed showing the shapes of the bubbles: for Vequal to (a. top left1 0 kV, (b, top right) 8 kV. (c. bottom left) 10 kV, (d. bottom right) 10 kV with B magnifying glass for real time measurements.
a fluidizing gas having 99% relative humidity, whereas our experiments were done with bone-dry oxygen. Shapes and Sizes of the Bubbles. Visual observations recorded by the motion picture films, as well as the photographs of the EFB, show bubbles elongated in the horizontal direction of the electric field, which rise through the bed as shown in the photographs of Figure 8 and in the schematic d i a " of Fieure 9. This Dhenomenon had already been rep-rted by J s n s o n and Melcher (1975) who used humid air. In standard theories of fluidized bed behavior, the bubbles are usually assumed to have spherical shapes; hence, they are assumed to be cylindrical in a thin two-dimensional bed (Davidson and Harrison, 1963). or they may be assumed to have a spherical cap followed by a wake containing a high concentration of solid particles (Murray, 1966). In an EFB, the bubbles can be idealized by oblate ellipsoids in a three-dimensional bed or cylinders of elliptical cross section in a two-dimensional bed, as shown in Figure 9. This idealization can be justified as a statistical average situation. Although some bubbles look more like thin cracks in the direction of E,
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