Fluid terminal entrainment velocity

The control parameters for the experiment described in Figure 15.6 are the mass of particles per unit surface, Mp / S, the area-averaged empty-bed velocity of the fluid in the column, U = Q/S, the density ps of the particles, and the density pf of the fluid. The diameter/) of the particles, as will be seen in section 15.6.3, chiefly influences the fluid flow in a porous medium, jointly determining with the kinematic viscosity v of the fluid the minimum fluidization velocity U f and the terminal entrainment velocity Ut, between which a steady-state fluidization regime is obtained. Furthermore, for a given velocity U of the fluid, the thickness of the fluidized bed increases if the diameter of the particles decreases, for the same mass of particles in the colunrn. 

It is readily understood that the terminal entrainment velocity Ut is equal to the fall velocity Wc of the particles constituting the bed. The fall velocity is the maximum relative velocity that a particle can have in a fluid at rest, whereas the terminal entrainment velocity is the maximum relative velocity that the fluid can have with respect to a stationary particle without carrying it away. The classical laws for the fall velocity of a spherical particle in a fluid at rest (Stokes , Van Allen s, or Newton s law - see section 15.1 and Table 15.1) are therefore used to estimate the terminal entrainment velocity. 

The calculation of the terminal entrainment velocity is necessary for design of all gasification systems. In the case of moving-bed gasifiers, the load-limiting particle carry-over is determined by particle entrainment. In fluid-bed systems— probably the most critical—the terminal velocity is directly related to the gasifier discharging and carbon conversion rate. Finally, in entrained-flow systems, all particles must fulfill the entrainment conditions. 

To obtain proper fluidization, the actual fluid velocity, up, must be considerably greater than the minimum fluidization velocity, ump However, to avoid excessive entrainment, Up should be less than the terminal velocity, ut. Thus, the ratio utlumj is a guide to selection of the value of Up. 

---