(8) Biemann, K., “Application of Computer Techniques in Chemical
Research”, pp 5-19, Institute of Petroleum, London, England, 1972. (9) “Eight Peak Index of Mass Spectra”, Mass Spectrometry Data Centre, AWRE, Reading, RG7 4PR, UK, 1974. (10) “SyntheticOrganic Chemicals, U S . Production and Sales, 1974”, US. International Trade Commission, Publ. No. 776, GPO, Washington, D.C. (11) Heller, S. R., Milne, G.W.A., Feldmann, R. J., Science, 195, 253-9 (1977). (12) Hites, R. A., “Fates of Industrial Synthetic Organic Chemicals: A Case Study”, pp 102-05, NSF, Research Applied to National Needs, 1977. (13) Gunther, R. A., “Residue Reviews, Volume 32”, “The Triazine Herbicides”,F. A. Gunther, Ed., pp viii-xi, Springer-Verlag,New York, N.Y., 1970; Kaufman, D. D., Keraney, P. C., ibid., pp 23565. (14) Matsura, T., Yoshimura, N., Nischinaga,A., Saito, I., Tetrahedron, 28,4933-8 (1972).
(15) Cook, C. D., J . Org. Chem., 18,261-6 (1953). (16) Jackson, T. A,, Soil Sci., 119,56-64 (1975). (17) Krauskopf, K. B., “Introduction to Geochemistry”,pp 236-57, McGraw-Hill,New York, N.Y., 1967. (18) Degens, E. T., “Geochemistry of Sediments. A Brief Survey”, pp 246-58, Prentice-Hall, Englewood Cliffs, N.J., 1965. (19) Taylor, W. I., Battersby, A. R., “Oxidative Coupling of Phenols”, p 59, Dekker, New York, N.Y., 1967. (20) Christensen, H. E., Ed., “The Toxic Substances List”, USHEW (NIOSH), 1973. (21) Schober, U., Lampert, W., Naturwissenschaften, 63, 241-2 (1976).
Received for review M a y 32, 1977. Accepted September 26, 1977. Supported by the Research Applied to National Needs Program of the’Nationa1 Science Foundation (Grant No. ENV-75-13069).
NOTES
Electrofluidized Bed in Filtration of Smoke Emissions from Asphaltic Pavement Recycling Process Peter 6. Zieve, Karim Zahedi, and James R. Melcher” Department of Electrical Engineering, Massachusetts Institute of Technology, Cambridge, Mass. 02139
James F. Denton Warren Brothers Co., Cambridge, Mass. 02139
The electrofluidized bed (EFB), a device recently proposed for high efficiency collection of submicron particles, is tested on the emissions from an asphalt recycling plant. The largely hydrocarbon pollutant is collected on sand that is then easily removed from the bed in its fluidized state and subsequently added to the asphalt product. Efficiencies of collection in excess of 98% are reported for submicron particles in beds having unfluidized depths of 8-12 cm using sand having a mean diameter of 2 mm. Superficial velocities range from 1.5 to 2 m/s, with a typical bed pressure drop of 12 cm H20. Electrical energization of the bed requires less than 80 W/lOOO cfm: Estimates of the E F B capital and operating costs are favorable compared to other devices that could conceivably servt this application. The EFB promise, to make an essential contribution toward making possible the recycling of asphaltic highway using existing asphaltic processing equipment. Q
Motivated by the oil shortage, the Federal Government has been investigating ways of recycling asphaltic pavement. Warren Brothers Co. has been cooperating with this effort by trying to devise ways of adapting conventional plants to operate in a recycle mode. A major impediment to this conversion is the formation of fine particulate smoke that occurs when crushed pavement is subjected to the temperatures necessary for recycling. In the course of this study it was found that, upon heating, the asphalt begins to crack and release hydrocarbon vapor that subsequently condenses into droplets. These submicron droplets are poorly collected in the devices currently in use on asphalt plants, the baghouse and Venturi scrubber. Even if they are designed for submicron collection, currently available devices are ill suited to the asphalt cleanup problem. Electrostatic precipitators collect the oil on electrode plates from which it is difficult to remove. Fabric filters appropriate 96
Environmental Science & Technology
for submicron collection must incorporate a large pressure drop that increases significantly as collection of materials proceeds. In addition, bags fouled with oil tend to clog and are a t best difficult to clean. Wet wall electrostatic precipitators, charged-droplet scrubbers, and high-energy Venturi scrubbers, all create a water pollution problem and require both a substantial capital investment and a high operating cost. The electrofluidized bed (EFB) recently proposed by Zahedi and Melcher ( I ) is well suited to the asphaltic smoke cleanup problem. Its efficient collection of submicron particles in a short residence time (about 50 ms), together with the unique feature of collecting the pollutant on easily handled fluidized sand, makes the EFB an attractive device for this application. After being used as a collection surface, the oilcoated sand can be conveniently incorporated into the asphalt product. In fact, an “oiled” sand is desirable for use in asphalt. In the EFB the sand particles are stressed by an ambient electric field creating poles of positive and negative charge on each particle. For the apparatus described in this paper, the electric field is imposed parallel to the gas flow (co-flow),that is, with horizontal screens of alternate potential spaced vertically through the sand bed. It is possible to design a bed with the electric field perpendicular to the gas flow (cross-flow), although this type is somewhat more difficult to build for a large-scale application. The dominant mode of collection is for charged pollutant to be collected by the oppositely charged poles of the sand. Other modes of collection exist, such as microfield collection, inertial impaction, and interception, but these are shown to be of secondary importance by Zehedi and Melcher ( I ) . Fundamentals of Submicron Particle Collection
A remarkably good model for plug-flow beds pictures the bed particles as being much like the electrodes of an ESP, with the electric field terminating over one side of each particle and originating on the other. Instead of the Deutsch model used 0013-936X/78/0912-0096$01.00/0
@ 1978 American Chemical Society
for turbulent flow ESP’s ( 2 ) ,the “local mixing model” is invoked with the assumption that on the average the fluid mechanics supplies the full surface of a bed particle with the gas-entrained pollutant. The electric field simply brings the pollutant from the local volume t o the bed particle surface. According t o this model ( I ) , a bed having unfluidized height l , , bed particles of mean radius R , a superficial gas velocity U , and an imposed electric field E collects particles of mobility b with the efficiency
where c is a coefficient of order unity. This model is valid provided that reentrainment and bubbling are not substantial. T h e first of these conditions is not a problem for liquid aerosols such as asphaltic smoke. Unless baffled to promote bubble breakup, gas fluidized beds d o not expand uniformly as the fluidizing velocity increases, and the resulting effects of nonuniformity in the gas-solid distribution are not accounted for in the plug-flow model. This is especially true in the cross-flow beds where the screen electrodes are not used. In the co-flow configuration, the screens used t o impose the electric field tend to prevent the growth of bubbles. Models have been developed and correlated with experiments for bubbling electrofluidized beds ( 3 ) .These combine the particle-scale collection model tested in the plug-flow experiments ( I ) with two-phase models that have been developed for the transfer characteristics of conventional fluidized beds (3).It is important to recognize that the structure of beds can be strongly influenced by an applied electric field. In what will be termed the “bed electromechanics”, particles tend to form “chains” or “strings” while gas flow causes a fluttering motion instead of the typical bubbling one ( 4 ) . These effects have important implications for bed mixing and particle elutriation. However, high submicron collection efficiencies can be obtained a t relatively low electric field intensities where electromechanical effects are not of significance. Thus, the Davidson model is adopted for a conventional bubble having the diameter Db (in which the particle density is low) interacting with the dense phase. P a r t of the gas passes through a dense region a t about the minimum fluidization velocitj Urn/,while the remainder bypasses the system in the form of bubbles. There is a continual exchange of particles between phases accounted for by the Davidson model. Two possible extremes can be used to model the penetration of pollutant through the bed as a whole. The dense phase can be viewed as evolving uniformly in the flow direction (the plug-flow model). Because of bubble agitation a complece mixing model is also plausible. Both models have been studied with the dense phase plug-flow model clearly found to be more representative of what happens. T h e plug-flow bubbling model shows how parameters not represented in the collection coefficient K 1 come into play. T h e predicted efficiency is
where ml and rn’ are the roots of
and
,
hl Y
l
I
l
1
1
-
Figure 1. Schematic of EFB test system
T h e collection coefficient K1 is the same as defined with Equation 1; U and I/ are, respectively, the superficial velocity and fluidized bed height under the conditions of interest; and CTmf and I, are these quantities under conditions of minimum fluidization. The most important parameter is Db,the bubble diameter. T h e quantities necessary to evaluate these expressions are, of course, averages. T h e bed height is a fluctuating quantity, and even in the shallow beds of interest here, bubbles grow significantly as they pass through the bed. If the rate of gas interchange between the bubbles and the dense phase is large, which according to Davidson’s model means that cy m, the efficiency predicted by Equation 2 reduces to that for the single-phase plug-flow model. Equation 1. T h e most important aspect of the bubbling model is its prediction of efficiencies less than 100%,no matter how large the collection parameter. In the limit K1 m , the efficiency approaches the limiting value
-
-
(3) Using experiments to evaluate c y , over a wide range of fluidization states, this parameter is mainly a function of the bubble diameter Db Experimental Tests
A prototype E F B of cross-sectional area 1 ft2 was built and tested on both a small-scale asphalt recycling system and on a bypass to a full-scale asphalt plant operating in the recycle mode. The EFB, together with the small-scale recycling system, is shown schematically in Figure 1. T h e test unit was constructed of metal plate in the shape of a rectangular box. A wall of fireglass afforded visual observation. The distributor plate was constructed of ‘i4-in. perforated plate with approximately 9% open area. This plate was covered with a 16-mesh screen. Three screens of ‘/?-in. hardware cloth were used to impose the electric field. In addition, these upper screens helped promote bubble breakup. Because bed particles tended to polish metallic and insulating surfaces within the bed, these screens remained free of pollutant buildup. The distributor plate structure exhibited a pressure drop of about 2 cm of H20 under 400 cfm operating conditions. (It is important to use a distributor plate that minimizes excessive bubbling a t a reasonable pressure loss.) For all small-scale tests, sand with median diameter of 2 mm was employed. Utilizing such a large size sand enables operation of the fluidized bed a t a high superficial flow velocity, about 2 m/s, and therefore a small bed cross section. A sand removal system was devised with a hinged distributor plate. T h e tests were performed on a batch basis, with fresh sand fed a t the top. Volume 12,Number 1, January 1978
97
Table 1. Summary of Collection Performance of EFB; Comparison of Experimental and Predicted Efficiencies EFB operating parameters
Y I I STAGE
2 40
Efflclency ( % )
Particulate loading (mglS.m3)
U
10
E
(m/s)
(cm)
(kVlm)
Exptl
167 82 44
1.7 1.8 1.8
10 10 13
465 250 500
96 93 94
RANGElprn) O < D < 43 < D .65 . 6 5 < D < I . l SIZE
7 6 5
