Yang Fluidization, Solids Handling, and Processing

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AIMS, Analogies In Matters of Science, P.O. Box 241, Garrison, N.Y. 10524, (1989) to date

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Baskakov, A. P., Dolgov, V. N., and Goldobin, Y. M., Ural Polytech. Inst., Sverdlovsk, USSR (1980)

Bryant, H. S., Silverman, R. W., and Zenz, F. A., “How Dust in Gas Affects Cyclone Pressure Drop,” Hydrocarbon Processing, pp. 87–90 (1983)

Dry, R. J., White, R. B., and Joyce, T., “Correlation of Solids Circulation Rate in Circulating Fluidized Bed Systems,” 4th Internat’l. CFB Conf., Preprint Vol., pp. 732–737 (1993)

Gartside, R., and Woebcke, H. N., U.S. Patent No. 4,556,541 (1985)

Hoffmann, A. C., Van Santen, A., Allen, R. W. K., and Cliff, R., “Effects of Geometry and Solid Loading on the Performance of Gas Cyclones,” Powder Tech., 70:83–91 (1992)

Hyppanen, T., Palonen, J., and Rainio, A.,”Pyroflow Compact - Ahlstrom Pyropower’s 2nd Generation CFB,” 4th Internat’l. CFB Conference, Preprint Vol., pp. 131–136, (1993); European Patent Applic’s. No’s. 0 481 438 A2 and 0 481 438 A3, filed (1991)

Kane, R. S., Weinbaum, S., and Pfeffer, R., Pneumotransport 2, (1973)

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Knowlton, T. M., and Bachovkin, D. M., 70th Annual A.I.Ch.E. Mtg., (1977)

MacLean, J. P., Cantwell, E., Brown, J. D., and Hoy, H. D., U.S. Patent No. 4,316,729 “Highly Efficient Cyclone Separator” (1982); U.S. Patent No. 4,380,105 “Method for Shaping Forming and Assembling a Highly Efficient Cyclone Separator” (1983); U.S.Patent No. 4,337,068 “Methods for Removing Entrained Solids from Gases” (1982)

Rosin, P., Rammler, E., and Intelmann, W., “Principles and Limits of Cyclone Dust Removal,” Zeit. Ver. Deutscher Ing., 76:433ff (1932)

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Stairmand, C. J., “The Design and Performance of Cyclone Separators,” Trans. Instn. of Chem. Engrs., 29:356ff, London (1951)

TerLinden, A. J., Proc. Instn. Mech. Engrs., 160:233-249, London (1949)

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Zenz, F. A., “Find Attrition in Fluid Beds,” Hydrocarbon Processing, 50:103–105

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Zenz, F. A., “Size Cyclone Diplegs Better,” Hydrocarbon Processing, pp.125–128

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Zenz, F. A., “Cyclone Separators,” Manual on Disposal of Refinery Wastes Volume on Atmospheric Emissions, Ch. 11, Pub. No. 931, American Petroleum Institute, Washington, D.C. (1975)

Zenz, F. A., Fluidization and Fluid-Particle Systems, Vol. II , PEMM-Corp Pub., 1:1-42; 7:253-393; 7:263-264, Chelsea Industrial Park, Wappingers Falls, N.Y. (1989)

Zenz, F. A., and Kelleher, E. G., “Studies of Attrition Rates in Fluid-Particle Systems Via Free Fall, Grid Jets and Cyclone Impact,” J. Powder and Bulk Solids Tech., 4(2/3):12–20 (1980)

Zenz, F. A., and Othmer, D. F., Fluidization and Fluid-Particle Systems, PEMMCorp Pub., Cold Spring, N.Y. (1960)

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Zenz, J. A., PEMM-Corp, Cold Spring, N.Y., Personal Communications (1988,

1996)

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proposition. Electrostatic discharges can lead to still more serious problems, such as electrical interference which disrupts process instrumentation, physiological shocks to operating personnel, and, under certain conditions, the ignition of flammable dust/air mixtures. The resulting fires or explosions can cause serious damage and injury or loss of life in commercial and manufacturing facilities. A prudent way to think about electrostatic nuisances and hazards in fluidized and spouted beds is to recognize that, quite often, such units form just a small part of a large, complex solids handling system, consisting of hoppers, storage silos, transport containers, scroll feeders, receiver filters, etc., all connected together by pneumatic and vacuum transport lines. Each of these different components is known to contribute to the risk of an electrostatic ignition.

Because particle charging causes significant nuisances that hamper industrial production in fluidized and spouted bed systems and creates genuine hazards that threaten property and human life, it is important for operating personnel and production engineers to become familiar with electrostatic phenomena. In this chapter, we focus on the electrostatic phenomena associated with powder processing and handling and describe certain measures that reduce their adverse effects on productivity and abate fire and explosion hazards. Section 2 is devoted to a review of important aspects of particle charging, including triboelectrification and charge relaxation. In Sec. 3, early investigations of electrostatic effects in fluidized beds are summarized; both phenomenology and quantitative data on particle charging are covered. The rather limited data on charging in fluidized beds are compared to the generally accepted values employed by safety personnel to assess electrostatic hazards in powder handling. Electrostatic ignition hazard fundamentals in powders are the subject of Sec. 4, which summarizes the requirements for ignition and the types of electrostatic discharges. Section 5 focuses on ESD hazards associated with fluidized systems and peripherals, and provides some practical guidelines for their minimization.

2.0CHARGING OF SOLID PARTICLES

Granular solids and powders charge more readily than any other commercially or industrially important form of material. It is fair to say that, when such materials are handled or processed, particle charging is virtually inevitable. In this section, the important qualitative and quantita-

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tive aspects of particle charging relevant to fluidization are examined. The unavoidable tendency in any general treatment of particulate charging is to emphasize polymers, the materials best known for their strong electrostatic activity. There is, however, a clear danger in assuming that only polymers exhibit strong electrostatic behavior. Extremely troublesome particle cohesion and adhesion problems, as well as catastrophic fires and explosions, have occurred with materials to which one might not attribute significant electrostatic activity. For example, powdered aluminum and ordinary granular sugar are just two types of non-plastic materials where serious dust explosions involving loss of life have been experienced and where electrostatic sparks have been implicated as the source of ignition.

2.1Triboelectrification

Phenomenology. Triboelectrification, also referred to as contact electrification, has been known since the time of the ancient Greeks. Despite the fact that its physics are reasonably well-understood at the fundamental level (Gallo and Lama, 1976; Lowell and Rose-Innes, 1980), triboelectrification remains difficult to predict or to control in the imprecisely defined environments of industry and commerce. The basic phenomenon is depicted in Fig. 1. As a particle of material A approaches the surface of material B, the material with the smaller work function— assumed to be A in the figure—gives up some of its conduction electrons to equalize the Fermi energy levels at the A-B interface. Once this transfer has occurred, an electrostatic equilibrium is achieved. If the materials are now separated (and if at least one of the materials is a good electrical insulator), a portion of the charge separation is locally preserved and thus the two materials now possess net, opposite charge. Materials, powders included, become charged to high levels only if the electrical resistivity is high enough so that the charge is not immediately conducted away. This important point is discussed further in Sec. 2.2.

Triboelectric Series. Prediction of triboelectric behavior in granular solids is hampered by difficult-to-control factors such as particle shape, prior mechanical contacts, material purity, and particle moisture content (which is usually related to airborne humidity). In the absence of any reliable predictive model for powder electrification, the practical requirements of industry necessitate an empirical approach (Taylor and Secker,

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Limits on Particle Charging. The electrical charge carried by a particle resides on the surface. Thus, a fundamental upper limit for particle electrification may be computed by imposing the constraint that the electric field at the surface can not exceed the dielectric strength of dry air, Eb ≈ 30 kV/cm. According to this hypothesis, the upper limit upon surface charge density becomes

where ε0 = 8.854•10-12 F/m is the permittivity of free space. Harper examined triboelectric charging in certain powders and measured local surface charge densities one full order of magnitude higher than σmax (Harper, 1961). He suggested that such anomalies might be explainable by the very small volume of air that is actually subjected to the high electric field strength. The maximum charge defined in Eq. (1) can be used to define an upper limit on the specific charge q/m (measured in C/kg) of a spherical particle having diameter D (in meters).

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In Eq. (2), ρm is the mass density of the particle (in kg/m3). The values of specific charge predicted by Eq. (2) are very high, and seldom if ever observed in powders or granular materials. This is because the bulking of powders intensifies the electric field sufficiently so that electrical breakdown, most likely taking the form of corona, limits the charge. One source suggests that σmax = 10-5 C/m2 provides a better correlation to powder charging data (Blythe and Reddish, 1979). Table 2 in the next section provides more realistic specific charge estimates for powders.

Charging of Powder. Industrial engineers and safety professionals often resort to an empirical approach to quantify powder charging in processing operations. Refer to Table 2, which tabulates typical specific charge values for moderately resistive powders (γ ≈ 1012 S-1m) observed during some common batch and continuous operations. The large ranges given for the specific charge—as much as three orders of magnitude in some cases—reflect the uncontrollability of certain critical parameters such as particle moisture content and size distribution, as well as the great variability in available industrial process equipment. Empirical data of this sort does provide valuable guidance in estimating charging levels; however, such data can never really supplant the measurements needed each time a new material is introduced or new equipment is installed in an operating line. In particular, the tribocharge level in a powder is strongly influenced by its resistivity. See Sec. 2.2 below.

Table 2. Typical specific charge values (q/m) for moderately insulating powders (γ ≈ 1012 S-1m) in some industrial operations (from Cross, 1987).

Conspicuous by its absence from Table 2 is charging data for fluidized beds. There are several reasons for this omission. First, fluidization

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is less developed as a technology than most other manufacturing processes using powders and granular solids. There simply exists less experience with fluid beds than with, say, pneumatic solids transport systems. Second, triboelectric charging in fluidized beds is complicated by the fact that the principal contacting is between particles within the bed. As a result, the phenomenology of charging in a fluid bed is harder to characterize and to quantify. Particle charging in fluid beds is the subject of Sec. 3.3.

Pneumatic Transport. In modern, large-scale manufacturing facilities where high volumes of granular materials must be handled, pneumatic transport is used very extensively to move product between storage silos, processing vessels, shipping containers and other components. As indicated by Table 2, pneumatic transport creates the highest triboelectric charging levels for dry particulate. It is for this reason that bulking and delivery operations often entail the greatest risk of an electrostatic discharge and dust ignition. Despite the importance of pneumatic conveyance in powder handling and despite its well-known electrostatic charging tendencies, practical investigations of powder charging in such systems has been quite limited. One of the few systematic experimental studies of the phenomenon was conducted using a small-scale rig with a one-meter long test section (Boschung and Glor, 1980). The influence of virtually all the important parameters—flow rate, humidity, powder type, average particle diameter, transport pipe material, and length—were considered. Despite the inevitable scatter in the experimental data, the results confirmed that particle size and material are more important than the wall material and system geometry in determining q/m for powders.

The usual prescription for controlling triboelectrification in pneumatic transport is to limit the flow rate, but this solution conflicts with the tendency to increase plant production levels. One alternate proposal for the control of tribocharging is to exploit the so-called “dense-phase” transport mode (G. Butters, 1985); however, there seems to be some dispute about the efficacy of this scheme (Konrad, 1986).

2.2Charge Relaxation

All particles, irrespective of resistivity, become charged by triboelectrification, but only highly insulating powders retain high levels of charge. If a powder is not highly insulating and if a conductive path to ground exists, then electrical conduction will dissipate charge too rapidly for the accumulation to occur. For many powders, electric charge decays

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naturally according to an exponential relaxation law: exp(-t/τ), where t = time (in seconds), τ = κε0 γ is the relaxation time, κ = dielectric constant (dimensionless), and γ = resistivity (in S-1m). The exponential law is most appropriate when γ <~ 1013 S-1m. The electrostatic hazards of powders are sometimes characterized by referring to their measured packed powder resistivity. According to this classification, shown in Table 3, there is little

This scheme identifies ranges for safe versus hazardous powders, but leaves a large intermediate range of resistivities where uncertainty reigns. For powders in this range, the circumstances of how the powder is handled determine whether or not an ESD hazard exists.

Table 3. Triboelectrically caused ESD particle charging hazard classification scheme based on powder resistivity g (Glor, 1988).

2.3Induction Charging of Particles

There is a danger in misinterpreting Table 3 to mean that particulate with high conductivity can never become electrically charged. Consider a conducting particle making contact with either a conducting wall or another particle as shown in Fig. 2. The particle intercepts lines of electric flux and become charged by induction if the contact occurs in the presence of an electric field. The magnitude of this charge may be estimated by considering the problem of a conducting spherical particle of diameter D immersed in a dielectric fluid of dielectric constant κ and in contact with a plane conducting wall and a uniform electric field of magnitude E normal to the wall. The result is the so-called Maxwell charge (Maxwell, 1891).