Particle segregation in a fluidized bed

Abstract

Particle segregation data at low gas velocity, near the minimum fluidizing velocity, are presented for particles of different size (150 to 2800 μm) and density (720 to 2800 kg/m3) in a fluidized bed. Phase diagrams similar to those of liquid‐solid systems are developed to present the results. The data were collected in a 100 mm diameter unit operating at pressures up to 660 kPa and a 114 mm diameter unit at atmospheric pressure. The particle segregation data were obtained to provide design and operating criteria for a two‐stage. i.e., an agglomerating combustor/gasifier and a devolatilizer/desulfurizer, fluidized bed coal gasification process to produce low heating‐value gas (890‐1350 K‐cal/m3) for power generation. The data indicate that the criteria for particle separation can be achieved if the operating gas velocity is close to the minimum fluidizing velocity of the dense particles, that is, agglomerated ash, and spent dolomite.

Effect of Heterogeneity in Coal Ash Chemical Composition on the Onset of Conditions Favorable for Agglomeration in Fluid Beds

“…This will increase the possibility of particle segregation. The study on particle segregation conducted by the Westinghouse Electric Corporation in fluidized beds containing dolomite and char particles showed the occurrence of segregation in most situations, when the gas velocity was lowered [38]. Hence it is possible that once agglomeration is initiated by low melting particles, such as those of the SG4 density class, the change in bed hydrodynamics would lead to further agglomeration due to decreasing particle velocities and decreasing bed voidage.…”

Experimental Analysis of the Fluidization Process of Binary Mixtures of Solids

“…As the gas flow rate is increased, it moves across the bed down to the bottom of the column until, at a new characteristic velocity value, the whole bed is brought into a fully fluidized state. In the upper region of the bed, at its free surface, a bubbling layer builds up which contains almost exclusively one of [3,4,5]. They are the "initial f luidization velocity", u if , at which ∆p first deviates from the fixed bed curve, and the "final f luidization velocity", u f f , at which the ultimate value of ∆p is first attained.…”

“…Based on this, it is u f f , rather than u mf , that has to be viewed as the velocity at which the entire mixture attains the f luidized condition [3][4][5]9]. This velocity normally coincides with that of full mixing [10], while the difference between u f f and u if is the amplitude of the velocity range within which mixing/segregation phenomena occur [20].…”

The interaction between mixture components in the mechanism of binary fluidization

“…On the pressure drop versus gas velocity diagram, uif is located at the point p first deviates from the fixed bed trend, whereas uff is the velocity at which the ultimate value of p is attained. The pioneering studies of Chen and Keairns [1], Gelperin and Einstein [2] and Vaid and Sen Gupta [3] first revealed that fluidization of binary and ternary beds is a process that has place gradually. More recently, the dependence of both uif and uff on the main variables which determine the fluidization pattern (solid properties, mixture composition and bed geometry) has been investigated by Formisani et al [4][5][6], who analyzed the behaviour of a large number of twocomponent mixtures.…”