Vertically moving particle bed

Bergmann et al. developed a vertically moving particle bed (VMPB) electrochemical reactor for copper recovery from dilute solutions (0.1-10000 ppm) [17]. Although higher rotation rates increased the current efficiency for copper removal at lower cell currents, rotation rates of below 5min were chosen to minimize mechanical wear. Impurities such as chloride ions, citric acid, and surfactants did not seem to interfere with the current efficiency of the process. This reactor was able to bring metal ion concentrations down to 0.5 ppm. A schematic of the VMPB reactor is shown in Fig. 3. 

Fig. 3. Schematic of the vertically moving particle bed (VMPB) reactor. (Adapted from [17]).

To increase the efficiency for the electrochemical treatment of effluents rotating three-dimensional electrodes were also tested. Thus, a rotating packed bed electrode [44] and a vertically moving particle bed electrode were proposed [45, 46]. Plater barrels were also adapted for the removal of metals [47,48]. Likewise, rotating cylinder electrodes made of reticulated vitreous carbon [49], expanded metal sheets [50], woven wire meshes [51] and wedge wire screens [52] were also examined. 

Bouzek K, Bergmann H (2003) Mathematical simulation of a vertically moving particle bed electrochemical cell. J Appl Electrochem 33 839-851... 

The sketch on the far right of Fig. 7.88 identifies a spouted bed [B.56], Typically, to obtain the spouted bed condition, gas is introduced in a relatively narrow area in the bottom center of the particle bed (Fig. 7.89a). Movement of the particles in such an arrangement is caused by a vertical, steady axial jet and is rather regular. The bed particles circulate much like a water fountain they are carried-up in the central spout as a dilute phase until they loose their momentum and fall back onto the top of the bed towards the outer periphery. The particles then recirculate back down as a dense moving bed, are directed back into the gas stream by the normally conical base of the apparatus, and begin again their upward flow. 

Incineration can be accompHshed ia multiple-hearth furnaces, ia which the sludge passes vertically through a series of hearths. In a fluidized-bed sludge, particles are fed iato a bed of sand fluidized by upwardly moving air. 

Fluidized Beds When gas or liquid flows upward through a vertically unconstrained bed of particles, there is a minimum fluid velocity at which the particles will begin to move. Above this minimum velocity, the bed is said to be fluidized. Fluidized beds are widely used, in part because of their excellent mixing and heat and mass transfer characteristics. See Sec. 17 of this Handbook for detailed information. 

Particles settle out near the feed point and form a continuous bed on the bottom of the pipe. The bed develops gradually throughout the length of the pipe and moves slowly forward. There is a velocity gradient in the vertical direction in the bed, and conveying continues in suspended form above the bed. 

Handley fluidised soda glass particles using methyl benzoate, and obtained data on the flow pattern of the solids and the distribution of vertical velocity components of the particles. It was found that a bulk circulation of solids was superimposed on their random movement. Particles normally tended to move upwards in the centre of the bed and downwards at the walls, following a circulation pattern which was less marked in regions remote from the distributor. 

For stable moving-bed uptransport of particles in a tapered spout, the drag force of the flowing gas on the particles is used to overcome gravity, acceleration, and wall friction. Vertical force balance on a horizontal element of solid particles in a trapezoidal pneumatic moving bed, shown in Fig. 7, gives... 

The basic character of non-fluidized gas-particle flow is the existence of contact pressure (or stress) among particles and between particles and the pipe wall. Both theoretical analyses and experimental results (Terzaghi, 1954 Johanson and Jenike, 1972 Li and Kwauk, 1989) showed that the pressure of the interstitial fluid in particulate material neither compresses nor increases the shear resistance of the particulate material. After Walker (1966) and Walters (1973), Li and Kwauk (1989) analyzed the stresses in a vertical pneumatic moving-bed transport tube by using the stress theory of particulate media mechanics and Mohr circles, shown in Figs. 19 and 20, and gave the following stress ratios at any point in the flow field ... 

Equation (32) is the generalized force balance differential equation for non-fluidized gas-particle flow. In this equation, for an inclined cylindrical moving bed, a = 0 for a vertical conical moving bed, 0 = 0 for vertical cylindrical moving bed, a = 0 and 0 = 0 and for a vertical cylindrical moving bed without interstitial gas flow, a = 0,0 = 0, and dp/dz = 0 simultaneously. 

Voidage 0 can be measured in a vertical cylindrical downward moving bed with the condition of Ap/Az = 0 by weighing the amount of particles W0 received in a container connected closely to the bottom of the moving bed, and meantime measuring the gas volume V0 flowing out of the container under the same pressure within the container, i.e.,... 

---