Circulating fluidized beds pressure balance

The height of the dense phase L is obtained by a pressure balance around the complete circulating fluidized bed loop. Good agreement is seen with this model and the existing data in the field. 

Of primary significance is the location of the inflection point zi which is closely associated with the solids quantity in the downcomer and solids circulation rate in the CFB system. Consider the pressure balance across the circulating fluidized bed loop as given by... 

Reh L (1971) Fluidized Bed Processing. Chem Eng Prog 67(2) 58-63 Rodes MJ, Laussmann P (1992) A Study of the Pressure Balance Around the Loop of a Circulating Fluidized Bed. Can J Chem Eng 70 625-630 Rowe PN, Partridge BA (1965) An X-ray Study of Bubbles in Fluidized Beds. Trans Instn Chem Engrs 43 T157-T175... 

Rhodes MJ, Laussman P. A study of the pressure balance around the loop of a circulating fluidized bed. Can J Chem Eng 70 625, 1992. 

Consider the circulating fluidized bed/L-valve return system shown in Fig. 21. The high-pressure point in such a recycle loop is at the L-valve aeration point. The low-pressure common point is at the bottom of the cyclone. The pressure drop balance around the recycle loop is such that... 

The transport velocity can also be evaluated from the variations of the local pressure drop per unit length (Ap/Az) with respect to the gas velocity and the solids circulation rate, Jp. An example of such a relationship is shown in Fig. 10.4. It is seen in the figure that, along the curve AB, the solids circulation rates are lower than the saturation carrying capacity of the flow. Particles with low particle terminal velocities are carried over from the riser, while others remain at the bottom of the riser. With increasing solids circulation rate, more particles accumulate at the bottom. At point B in the curve, the solids fed into the riser are balanced by the saturated carrying capacity. A slight increase in the solids circulation rate yields a sharp increase in the pressure drop (see curve BC in Fig. 10.4). This behavior reflects the collapse of the solid particles into a dense-phase fluidized bed. When the gas... 

In the CFB loop the pressure drop over the riser must be balanced by that imposed by the flow through its accompanying components such as the downcomer and the recirculation device. The pressure drops across the downcomer, the solids circulation and control device, and the riser are the major elements in the pressure balance around the CFB loop. The pressure drop balance is thus such that the sum of the control device, fluidized bed riser and cyclone... 

Glicksman et al. (1995) used the simplified scaling parameters to construct a one-half linear scale model of a Foster Wheeler circulating bed combustor pressurized to 14 bar. The combustor has a 20.3 cm inner diameter with an overall height of 8.3 m with both a primary and a secondary air supply. The solids recycle rate was accurately determined by a calorimetric balance of a fluidized bed heat exchanger in the return loop of the circulating bed. The cold model, one-half scale, used polyethylene plastic particles to match the dimensionless particle size distribution as well as the gas-to-solid density ratio. 

The recycle cyclone, if this is used to reduce earbon losses, must be placed at a sufficient elevation above the relatively dense fluidized bed to provide a sufficiently long standleg to balance the circulation loop pressure profile. The bed elutriation will tend to result in recycled fine particles having low bulk density compared to the dense fluidized bed. Fine recycled particles must be injected into the combustion zone effectively to provide additional carbon conversion. 

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