Fluidized and entrained beds

The term fluidization is used when a bed of flne solid particles is suspended in an upward flow of liquid or gas. The essential characteristic of a fluidized bed (also called fluid bed) is that though the particles are suspended, they mostly remain in the bedy and those particles diat are entrained with die upward gas- or liquid flow will eventually fall down into the bed. Therefore, a fluidized bed usually has a well defined surface. 

Ruidized beds find wide application as chemical reactors. The most common ones are catalytic gas phase reactors. The catalyst is then usually a fine powder, with a particle size of about 0.1 mm. Occasionally liquid-fluidized beds reactors are used, e.g., for processes using ion exchange resins or inunobilized enzymes. Then the particles are usually around 1 nun. 

An important aspect of a fluidized bed is that the apparent weight of the bed is counteracted by the drag force exerted on the particles by the upward flow. The pressure drop in the fluid across the fluidized bed is therefore always equal to the apparent weight per unit cross sectional area, irrespective of the flow rate. When the flow rate is increased, the bed expands further, so that the average upward flow rate between the particles relative to the superficial flow rate, is r uced so much that the drag force remains the same. 

An important characteristic of any fluidizable material is its minimum fluidization velocity, that is the superficial velocity needed to lift the particles. It appears that, for fine particles, this is mostly determined by the viscosity of the fluid, and by the density difference between the solid and the fluid. That is so because the flow between the small particles, at these low flow rates, is e ssentially laminar. We can use eq. (4.36), replace Ap by the weight of the bed per unit area Ap (1 - e) g /, and neglect the turbulence term. The minimum fluidization velocity u, is then 

For most catalyst powders, in air of atmospheric pressure, the minimum fluidization velocity is on the order of 0.01 m/s. For solid particles of about 1 mm, fluidized by a liquid, the minimum fluidization velocity is often in the same range. 

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Conveyance of fluids, pressure loss in tubes, fixed, fluidized and entrained beds, compressors and pumps, mixing of fluids, and separation of solids from fluids. 

The driving force is the pressure, and the friction leads to a pressure loss in empty tubes as well as in fixed, fluidized, and entrained beds (Sections 3.4.1.1 and 3.4.1.2). 

Pressure Loss in Fixed, Fluidized, and Entrained Beds... 

SASOL has pursued the development of alternative reactors to overcome specific operational difficulties encountered with the fixed-bed and entrained-bed reactors. After several years of attempts to overcome the high catalyst circulation rates and consequent abrasion in the Synthol reactors, a bubbling fluidized-bed reactor 1 m (3.3 ft) in diameter was constructed in 1983. Following successflil testing, SASOL designed and construc ted a full-scale commercial reac tor 5 m (16.4 ft) in diameter. The reactor was successfully commissioned in 1989 and remains in operation. 

Gasifier Types and Characteristics The three main types of gasifier reactors, moving bed, fluidized bed, and entrained bed, as shown in Fig. 24-4, are all in commercial use. The moving bed is sometimes referred to as a fixed bed, because the coal bed is kept at a constant height. These gasifiers can differ in many ways size, type of coal fed, feed and product flow rates, residence time, and reaction temperature. Gas compositions from the gasifiers discussed below are listed in Table 24-10. 

TABLE 19.12 Features of Fixed-Bed, Fluidized-Bed, and Entrained-Bed Gasifiers... 

Prior to each run, the MSW hopper was loaded with MSW pellets the DSS feed tank was filled and the lime or clay (when needed) was mixed with the DSS the flue gas analyzers, pressure, and weight indicators were calibrated and the combustor and EHE were loaded with their respective dense and entrained bed material. A 650-675 C fluidizing gas stream was generated using a natural gas burner and was used to heat the bed. 

If, on the other hand, the coal is heated very rapidly - as it is the case in fluidized bed and entrained bed systems - pyrolysis is shifted towards much higher temperatures. At such temperatures, the intermediate products and carbon are so quickly reacted with steam and the entire gasification process is accompanied by such strong remixing effects that the product gas does not contain any tar. 

There are currently three industrially-proven methods for coal gasification, namely fixed-bed gasification (e.g. Lurgi), fluidized bed reactors (e.g. Fritz Winkler (BASF)) and entrained bed processes (e.g. Texaco, Koppers-Toizt, Other processes are being developed, e.g. coal gasification in an iron bath. 

To determine (change from fluidized to entrained bed), we again neglect the lifting force and assume a porosity of almost 1. We then get... 

Low contamination of gas with solids when compared with fluidized bed and entrained bed. 

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