Circulating beds

Typical particle size and feed concentration range. 2-500 pm and 2-25% w/w. 

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To escape aggregative fluidization and move to a circulating bed, the gas velocity is increased further. The fast-fluidization regime is reached where the soHds occupy only 5 to 20% of the bed volume. Gas velocities can easily be 100 times the terminal velocity of the bed particles. Increasing the gas velocity further results in a system so dilute that pneumatic conveying (qv), or dilute-phase transport, occurs. In this regime there is no actual bed in the column. 

Circulating fluidized-beds do not contain any in-bed tube bundle heating surface. The furnace enclosure and internal division wall-type surfaces provide the required heat removal. This is possible because of the large quantity of soflds that are recycled internally and externally around the furnace. The bed temperature remains uniform, because the mass flow rate of the recycled soflds is many times the mass flow rate of the combustion gas. Operating temperatures for circulating beds are in the range of 816 to 871°C. Superficial gas velocities in some commercially available beds are about 6 m/s at full loads. The size of the soflds in the bed is usually smaller than 590 p.m, with the mean particle size in the 150—200 p.m range (81). 

Residuals Produced Circulating bed incinerators produce no ash. Solids are carried over in the gas stream and require removal. Residuals from the air pollution control device may require further treatment prior to disposal. 

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. 

Comparing with the conventional three-phase beds, the axial solid holdup distribution is much more uniform and the radial distribution of gas holdup (sg) is much flatter in circulating beds, due to the relatively high Ul and solid circulation. The values of Eg and bed porosity can be predicted by Eqs. (7) and (8) with a correlation coefficient of 0.94 and 0.95, respectively. 

Bubble size in the circulating beds increases with Ug, but decreases with Ul or solid circulation rate (Gs) bubble rising velocity increases with Ug or Ul but decreases with Gs the ffequeney of bubbles increases with Ug, Ul or Gs. The axial or radial dispersion coefficient of liquid phase (Dz or Dr) has been determined by using steady or unsteady state dispersion model. The values of Dz and D, increase with increasing Ug or Gs, but decrease (slightly) with increasing Ul- The values of Dz and Dr can be predicted by Eqs.(9) and (10) with a correlation coefficient of 0.93 and 0.95, respectively[10]. 

Although the values of kba dr in the literature are reasonable and comparable each other, the different trend mentioned above may be due to the different operating conditions. The gas-liquid interfacial area(a) and liquid side mass transfer coefHcient(ki) have been determined from the knowledge of measured values of gas holdup and kcacir [11]- The values of a and ki increase almost linearly with increasing Ug or Ul- The values of h cir and ktacir in circulating beds can be predicted by Eqs.(ll) and (12) with a correlation coefBcient of 0.92 and 0.93,... 

Small, properly scaled laboratory models operated at ambient conditions have been shown to accurately simulate the dynamics of large hot bubbling and circulating beds operating at atmospheric and elevated pressures. These models should shed light on the overall operating characteristics and the influence of hydrodynamics factors such as bubble distribution and trajectories. A series of different sized scale models can be used to simulate changes in bed behavior with bed size. 

Keaims, D. L., Yang, W. C., Newby, R. A., Hamm, J. R., and Archer, D. H., Circulating Bed Boiler Concepts for Steam and Power Generation, Proc. 13th Intersociety Energy Conversion Engineering Conference, pp. 540 (1978)... 

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