28
Ind. Eng. Chem. Fundam., Vol. 17, No. 1, 1978
Cross Development Corp., US. Patent 1 865 235 (1932). Faught, C. E., Pet. Refiner, 11 (4), 272 (1932). "Gasoline Desulfurization", Appendix A2.1, "Annual Catalyst Report", Environmental Protection Agency, 1974. Gilman, H., Breuer, F., J. Am. Chem. SOC.,56, 1127 (1934). Gilman, H., Jacoby, A. L.. J. Org. Chem., 3, 108 (1938). Haskett, F. 8.. U.S. Patent 3 565 792 (1971). Hoffman, H. L., Hydrocarbon Process., 52, 107 (Sept 1973). Jakobsen, H. J., Acta Chem. Scand., 24, 2663 (1970). Kellogg, M. W. Co., "Production of Low-Sulfur Gasolines", Task 10 Report Phase 1, submitted to EPA Office of Research and Monitoring, Contract No. 6802-1308 (1974). McKinney, C. M., Hydrocarbon Process., 51, 117, (Oct 1972). Morton, A. A., Claff, C. E., Jr., J. Am. Chem. SOC.,76, 4935 (1954). Nakazaki, M., Nippon Kagaku Zasshi, 80,687 (1959). National Distillers and Chemical Corp., Nobis, J. F., Maurice, L., German Patent 1 154 811 (1963). Rall, H. T., Thompson, C. J., Coleman, H.J., Hopkins, R. L., "Sulfur Compounds in Crude Oil", U S . Bureau of Mines Bulletin, No. 659 (1972). Reggel, L., Balustein, B. D., Delle Donne, C. L., Friedman, S., Steffgen, F. W., Winslow, J. C., Fuel, 55, 170 (1976). Schick, J. W., Hartough, H. D., J. Am. Chem. SOC.,70, 286 (1974). Schonberg, A., Peterson, E., Kaltschmitt, H., Ber., B66, 233 (1933).
Screttas, C. G., J'. Chem. SOC., Perkin Trans. 2, 745 (1974). Shell Oil Co., Testimony at EPA Hearings, Washington, D.C., Feb 19-20, 1975. Shirley, D. A., Barton, K. R., Tetrahedron, 22, 515 (1966). Sternberg, H. W., Delle Donne, C. L., Markby, R. E., Friedman, S..Ind. Eng. Cbem. Process Des. Dev., 13, 433 (1974). Sulfate Emissions, "An Analysis of the Automotive Sulfate Question", prepared by Energy and Environmental Analysis Inc., 1701 North Fort Myer Dr., Suite 1211, Arlington, Va. 22209, 1975. Texaco Development Corp., K. E. Kavanagh, U.S. Patent 2 818 350 (1957). US. Industrial Chemical Co., Technical Bulletin, "High Surface Sodium on Inert Solids", 1954. van Schooten, J., Knotnerus, J., Boer, H., Duinker. Ph. M.. Red. Trav. Chim.. 77, 935 (1958). Weinberger, S. M., Navarro, L. J., Bonilia, C. E., Rev. SOC.Ouim. Mex., 14, 13 119701
Wright, 0. L., Vancheri, F. J., lnd. Eng. Chem., 53, 15 (1961). Wright, L., Wcller, S., J. Am. Chem. SOC.,76, 5302 (1954a). Wright, L., Weller, S., J. Am. Chem. SOC.,76, 5305 (1954b).
Received for review April 15, I977 Accepted September 13,1977
Interparticle Electrical Forces in Packed and Fluidized Beds Peter W. Dietz and James R. Melcher" Department of Electrical Engineering, Massachusetts lnstitute of Techno!ogy, Cambridge, Massachusetts 02 739
When an electric field is applied to a packed or fluidized bed, the resultant interparticle electrical forces can have important consequences for the bed dynamics. Two experiments have been designed which allow the local interparticle force to be measured directly. In the first experiment, the pressure drop at incipient fluidization is measured as a function of the applied voltage. In the second, the minimum voltage required to suspend a bed against an upper screen is measured as a function of the gas-flow rate. Both experiments are performed using semi-insulating particles in a bed in which the electric field is perpendicular to the direction of gas flow. The results of these experiments are compared with the theory of Dietz and Melcher (1978).
Introduction The dynamics of a fluidized bed can be altered when an electric field is applied. These changes include the formation of strings of particles parallel to the electric field, a change in the shape of the bubbles, and a decrease in the particle mixing rate. The recent development of the electrofluidized bed (EFB) for applications in pollution control, as well as the promise of new applications in heat and mass transfer, have provided the incentive for investigating the effects of the electric field on bed dynamics. The earliest reported studies of EFB mechanics are those by Katz and Sears (1969) and Katz (1967), who note that electrofluidized beds can be "stabilized" by the presence of corona currents. In addition, they report that the pressure drop required for the incipience of fluidization is proportional to the square of the applied voltage. Johnson and Melcher (1975) present a heuristic description of the EFB dynamics without the complications of corona discharge. Pictures are used to demonstrate the dramatic changes in bubble shape that can be caused by the application of an electric field. The electrical conductivity of the particles is recognized as a critical parameter in characterizing the effects of the electric field, and the relative humidity of the fluidizing gas is used to control the surface conductivity of the fluidized sand. At low humidities, frictional electrification charges the particles and the application of a field causes the particles to plate the electrodes. At higher humidities, net-
charge effects are eliminated and interparticle forces dominate. In the fluidized state, these electrical forces act to form strings of particles collinear with the field. As a result, the bubble shape is flattened and particle mixing is reduced. In fact, a t high field strengths the bed can be frozen in position. Johnson and Melcher further report that the pressure drop at incipience increases approximately linearly with the applied voltage. In a related study, Colver (1976) has reported results on bubble elimination using electric fields. Recently, Dietz and Melcher (1978) and Dietz (1976) have measured the effective viscosity of an EFB as a function of the applied voltage. In what is defined as the "high-field'' regime, the transmitted shear stress in a Couette viscometer varies as the 1.2 power of the imposed electric field. This experimental result is shown to agree with a theoretical model developed for packed and electropacked beds. Motivated by the promise of new applications for EFB's in the areas of pollution control (Zahedi and Melcher, 1976) and heat transfer, additional research aimed a t a fundamental understanding of EFB electromechanics is proceeding. In the present paper, the results from two experiments in packed beds are reported. In the nomenclature of Zahedi and Melcher (1976),it is the "cross-flow'' configuration that is considered here. The applied electric field is dc and the particles are semi-insulating. (Thus, the surface conductivity is high enough to eliminate the effects of natural electrification.) In the first experiment, the "overshoot" experiment, the
0 1978 American Chemical Society
Ind. Eng. Chem. Fundam., Vol. 17, No. 1, 1978
29
Table I. Particle Properties
Diameter, mm
25 -
Range Mean Umf,m/s Sand K a Sand J Glass beads I (small) Glass beads I1 (large) Styrofoam
ff
pp, kg/m3
0.5
0.20-0.23 0.07-0.09 0.16-0.18
0.45 2.5 X lo3 0.6 2.5 X lo3 0.4 2.5 X lo3
0.7-1.2
0.95
0.51-0.56
0.4
2.5 X lo3
3.0-5.0
4.0
0.11-0.14
0.6
3.0 X 10'
0.4-0.8 0.2-0.5 0.4-0.6
0.6 0.36
20
-
Sand K contains a small fraction of fine particles (
