Fluidization process equipment, basic

Crude vermiculite ore produces the basic material used in the manufacture of fire protection and industrial insulation products. The process equipment consists of three rotary furnaces for heating and reaction. Torftech Ltd. (UK) replaced these with a single toroidal fluidized bed (Torbed) furnace of 1-m diameter with a capacity of 2 ton/hr. This led to a reduction in overall energy consumption and in vermiculite wastage and maintenance costs. The new process resulted in a payback period of 16 months. Now 11 plants are operational in Europe with this Torbed technology. This technique is also commercialized to produce silica from rice husk and roasting of sulphide ores, etc. 

Commercially, stabilization is accomplished by controlled heating in air at temperatures of 200—300°C. A variety of equipment has been proposed for continuous stabilization. One basic approach is to pass a fiber tow through heated chambers for sufficient time to oxidize the fiber. Both Mitsubishi and Toho patents (23,24) describe similar continuous processes wherein the fiber can pass through multiple ovens to increase temperature and reaction rate as the thermal stabiUty of the fiber is increased. Alternatively, patents have described processes where the fiber passes over hot roUs (25) and through fluidized beds (26) to provide more effective heat transfer and control of fiber bundle temperature. 

Fluidized-bed process incinerators have been used mostly in the petroleum and paper industries, and for processing nuclear wastes, spent cook liquor, wood chips, and sewage sludge disposal. Wastes in any physical state can be applied to a fluidized-bed process incinerator. Au.xiliary equipment includes a fuel burner system, an air supply system, and feed systems for liquid and solid wastes. The two basic bed design modes, bubbling bed and circulating bed, are distinguished by the e.xtent to which solids are entrained from the bed into the gas stream. 

Scale Up of Process. The scale up of fluidized bed coating processes has received little attention in the literature. Current practices in the pharmaceutical industry are reviewed by Mehta (1988). The basic approach described by Mehta (1988) is to scale the airflow and liquid spray rates based on the cross-sectional area for gas flow. This seems reasonable except for the fact that in the scaling of the equipment, the height of the bed increases with increasing batch size. For this reason, a time scale factor is also required. 

Most of the discussion in this chapter has focused on theoretical principles of operation. In actual practice, various other limiting concerns are realized. Fluidization plates may not always be ideal, and parameters may require adjustment to achieve coating under less favorable conditions. The most basic requirement in all cases is that all particles routinely circulate through the spray zone during processing so that they will be coated. Electrostatic forces between particles, or particles and equipment surfaces, are a common concern. These forces can promote agglomeration or prevent particles from circulating properly. Formulation or parameter adjustments are typically employed to overcome or minimize electrostatic concerns, but they are not always successful. 

Processing is the final step that converts the compounded material into a useful plastic product. Basically, the compounded resin needs to be melted into a liquid and heated to a temperature that allows easy handling of the fluidized plastic or the melt. This melt is fed into molds or dies to force the material into required shapes and quickly cooled to obtain the product. Usually, some minor finishing is needed before the product is made available to the consumer. The different processes used and the equipment employed are determined by the type of product being manufactured. A detailed discussion of the various processing techniques available for common thermoplastics is beyond the scope of this discussion. However, the basic principals involved in common processing methods associated with high-volume products will be discussed briefly below. 

---