Yang Fluidization, Solids Handling, and Processing

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A difficulty in convincing the experienced plant engineer or operator of the importance of these issues is that, while electrostatic effects in fluidized beds have been known about for over fifty years, ESD induced fires and explosions within them are apparently quite rare. Skepticism would be the understandable reaction to any statement suggesting that fluidization is inherently dangerous with respect to ESD. Nevertheless, in this chapter, two points are made about ESD hazards and fluidization. First, certain aspects of normal fluidized bed operation (viz., start-up and shutdown) seem to feature all the requisites for a dust explosion caused by an electrostatic spark. After all, a fluidized bed is just a vessel, usually constructed of metal and similar in shape and size to the types of hoppers and reaction vessels that have been blowing up regularly since the beginning of the industrial revolution. Dust explosions, while rare, are not at all unknown and they are a true hazard to be dealt with. Therefore, the same precautions routinely accepted in all other parts of a particulate solids processing facility should without question be taken in fluidized bed units. Second, at least in the fairly typical setting of a manufacturing or powder processing facility, fluidized or spouted bed units are apt to be placed in proximity to rather dangerous company. Pneumatic transport systems and components such as mixing hoppers and storage silos are all well-known to present safety risks (Glor, 1988; Lüttgens and Glor, 1989; Jones and King, 1991). Proper consideration must be given to the hazards associated with any component used in support of a fluidized bed’s normal operation.

A listing of general yet practical rules for reducing electrostatic hazards in powder handling seems a good way to conclude this chapter. These rules make no specific mention of fluidization, yet provide excellent guidance in minimizing risks (Jones and King, 1991; Cross, 1987).

(i)In any plant design, minimize the volume of flammable atmospheres.

(ii)Reduce electrostatic charging by reducing flow and separation velocities.

(iii)Ground and bond all metal plant components and accessories. Institute a regular inspection and maintenance program for these grounds.

(iv)Provide all personnel likely to be exposed to flammable atmospheres with appropriate antistatic footgear.

(v)Do not use highly insulating flooring materials and do not cover flooring with insulating paints, tapes, or other treatments.

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(vi)Use of plastics such as pipes and funnels in flammable atmospheres should be avoided whenever possible.

(vii)Electrostatic eliminators, not themselves capable of producing a spark, should be used on sheets and films of insulating materials before they enter hazardous atmospheres.

(viii)Be especially careful of mixtures of insulating particles and flammable liquids.

(ix)Specify properly designed explosion suppression or

venting or N2 gas inerting in all vessels containing ignitable powders.

(x)Exercise good housekeeping practice in powder handling facilities to avoid buildup of dust layers.

(xi)Exercise special care in all situation where manual operations are performed, providing interlocks, grounding straps, and blast protection.

The relative rarity of dust and powder ignitions makes them a unique sort of industrial safety threat. Because their occurrence is not routine, operating personnel eventually relax their guard, and too often this sort of behavior leads to dangerous incidents. The evidence that dust explosions are almost unknown within fluidized beds is an especially challenging problem for the safety officer, who must encourage vigilance even when no one remembers the last electrostatic incident.

ACKNOWLEDGMENT

The author’s understanding of electrostatics and powders has benefited greatly from associations with many individuals who have freely shared their expertise. In particular, he acknowledges close collaborations with J. L. King, formerly of GE Plastics, Parkersburg, West Virginia (USA), and Y. Matsubara of the National Institute of Fire and Disaster in Tokyo, Japan. He gratefully acknowledges past financial support from GE Plastics, Inc., and the Petroleum Research Fund of the American Chemical Society. Over the years, a number of very interesting consulting assignments from Eastman Kodak Company and Xerox Corporation have helped to broaden his perspective of electrostatic hazards. He is also thankful to G. S. P. Castle and G. M. Colver, who brought certain tribocharging data for fluidized beds to his attention.

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