Semifluidization (SF) is a novel fluid-solid contact technique. An SF bed overcomes the inherent disadvantages of a packed bed as well as that of a fluidized bed. It is a combination of fluidized bed at the bottom and packed bed at the top in series within a single contacting vessel. Various aspects of semifluidized beds in cylindrical columns and conical vessels have been reported in literature (Fan and Wen, 1959; Roy and Sharma, 1972; Govindrajan and Sengupta, 1976; Murthy and Roy, 1986; Murthy et al., 1987; Briens et al., 1997; and Jing et al., 2000). This paper presents studies on momentum transfer aspects of semifluidized beds in annular sections, which are likely to be useful in the design and operation of SF bed reactors. Data was collected using the variables: liquid mass velocity, particle size and particle density. Correlations were developed based on dimensional analysis approach for the prediction of semifluidized bed pressure drop and of minimum and maximum semifluidized velocity. A fairly good agreement was obtained between the calculated and experimental data.

A semifluidized bed comprises two coexisting phases, a packed bed at the top and a fluidized bed at the bottom within a single contacting vessel. This is obtained by providing a restraint at the top of the fully fluidized bed, which prevents the escape of particles from the system and forms a packed bed just below the restraint. Literature survey reveals that considerable work has been done on the studies of momentum transfer aspects as well as mass and heat transfer aspects of SF beds in cylindrical columns. Comparatively, little work was carried out in conical and annular sections. Therefore, an attempt was made to broaden the existing studies by suggesting expressions for prediction of pressure drop across an SF bed and maximum semifluidization velocity in an annular section. Further, it is reported that baffles improve the performance of fluidized beds by reducing slugging.

The pioneer investigators Fan and Wen (1959) studied the effects of liquid mass velocity on bed pressure drop and predicted an equation for pressure drop across a SF bed in cylindrical columns. Murthy et al. (1987) studied the semifluidization phenomena in a conical vessel and developed a correlation to predict the values of mass velocity at the onset of semifluidization (G_osf) and fluidization (G_of). Roy and Sharma (1972), Shan et. al (2001) and Sau et. al (2008), developed a correlation for calculating the pressure drop across an SF bed for gas-solid system in terms of dimensional numbers. Govindrajan and Sengupta (1976) proposed a correlation for predicting mass transfer coefficient in terms of Reynolds's number and bed expansion ratio. Murthy et al. (1976) studied the heat transfer aspects of a semifluidized bed and obtained data for heat transfer coefficient for an air-solid system with the help of dimensional analysis, and based on the statistical experimental design, an equation was formulated to predict the Nusselt number in terms of different system variables. Verma et al. (1972) studied the heat transfer characteristics of semifluidized beds using liquid-solid system. The present paper presents the results of studies carried out for momentum transfer aspect of semifluidized beds in annular section. It suggests correlations to predict the pressure drop, semifluidization velocity and maximum semifluidization velocity of fluid for a liquid-solid system.

Chemical Engineering Journal, Semifluidized Beds, Cylindrical Columns, Experimental Data, Fluidized Reactors, Momentum Transfer Studies, Liquid Mass Velocity, Semifluidization Velocity, Critical Fluidization Velocities, Statistical Experimental Design, Semifluidization Phenomena.