Mass‐Transfer Behavior of Electrochemical Reactors Employing Vibrating Screen Electrodes

Abstract

This work concerns the use of vibrating screens as electrodes in building electrochemical reactors, i.e., (i) the effect of superimposed flow on the mass-transfer coefficient at vibrated screens; and (it) factors affecting the relation between the mass-transfer coefficient and mechanical power consumed in vibrating the screen electrode. Superimposed solution flow using superficial velocity in the range of 0.7 to 5.4 cm/s enhanced the mass-transfer coefficient at relatively low vibration intensities, and at high vibration intensities superimposed solution flow decreases the mass-transfer coefficient. Power consumption calculations have shown that the efficiency of power utilization in enhancing the rate of mass transfer at vibrating screens is a function of the screen mesh number and the number of screens per bed. For vibrating screen beds, the mass-transfer coefficient increases with power consumption to a certain limit and then becomes insensitive to power consumption.Continuous electrochemical reactors using high liquid flow rates to enhance the rate of mass transfer in the reactor suffer from low residence times and consequently a low degree of conversion per pass. To overcome this problem, the continuous reactor is operated at a relatively low solution flow rate while the rate of mass transfer is enhanced using methods such as gas sparging, turbulence promoters, electrode rotation, or electrode vibration. Although turbulence promoters, gas stirring, and electrode rotation are used widely in practice, 1-3 the use of mechanical vibration is still limited even though vibratory agitation is more economical than rotary agitation. 4 In an earlier study, ~ the mass-transfer behavior of vibrating screens in a batch reactor was investigated with the object of developing a high space-time-yield reactor. The rate of mass transfer at a single horizontal vibrating screen of different mesh number was given by the equation J = 0.122 Re 0.6~ (Rjdw)l.7~ [1] where J is the mass-transfer J factor; Re is the vibrational Reynolds number; Rh is the hydraulic radius of the screen and dw is the screen wire diameter.Vibrating stacks of screens deviated from the above equation by 10 to 70 % depending on the number of screens per stack and the mesh number. Our object is: (i) to test the effect of low solution flow rates on the mass-transfer behavior of vibrating screens; such a study would assist in the development of a continuous electrochemical reactor based on vibrating screens; and (it) to study the effect of some operating variables on the power consumed in vibrating the screens; this study would assist in the economic evaluation of using vibrated screens in building electrochemical reactors. All previous studies on the effect of vibration on the rate of mass transfer were conducted in a nonflow batch reactor, 5-1~ and no work has been reported on the combined effect of solution flow and electrode vibration on the rate of mass transfer. Experimental TechniqueThe apparatus used here (Fig. 1) is similar to the apparatus used before ...

Solid−Liquid Mass Transfer at an Oscillating Packed Bed of Raschig Rings

“…The slight decrease in the mass-transfer coefficient observed when the velocity of the superimposed solution is increased in the turbulent flow regime may be ascribed to the fact that the superimposed upward flowing solution counteracts and weakens the downward flow induced by the upward stroke of the oscillating bed. A similar finding was observed by Sedahmed et al, who studied the effect of axial superimposed flow on the rate of mass transfer at an oscillating horizontal screen. Sung et al, who studied the rates of mass transfer from the horizontal cylinder in pulsating flow, found that the effect of pulsation on Sh decreases with an increase in the velocity of superimposed flow.…”

“…A similar finding was observed by Sedahmed et al, 9 who studied the effect of axial superimposed flow on the rate of mass transfer at an oscillating horizontal screen. Sung et al, 20 who studied the rates of mass transfer from the horizontal cylinder in pulsating flow, found that the effect of pulsation on Sh decreases with an increase in the velocity of superimposed flow.…”

“…Previous studies on the effect of transfer surface vibration (oscillation) on the rate of liquid−solid mass transfer were limited to simple geometries such as sphere, vertical plate, horizontal disk, horizontal and vertical cylinders, perforated disk, and horizontal screens. , Rates of mass transfer were measured in the present work using the diffusion-controlled dissolution of copper in an acidified dichromate. Since its development, , the technique has been used widely to study liquid−solid mass transfer in view of its simplicity and accuracy.…”

Free convective mass transfer at horizontal single screens and arrays of horizontal screens in relation to electrochemical reactor design

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ecently there has been a growing interest in using woven metallic screens in building catalytic and electrochemical reactors used to R conduct liquid-solid diffusion controlled reactions, in view of both their high specific area and turbulence promoting ability. For the rational design and operation of such reactors, the mass transfer behaviour of screens under different hydrodynamic conditions such as forced convection (Cano and Bohm, 1977;Storck et al, 1979;Sioda, 1977; Sedahmed et al, ZOOO), gas sparging (Katkout et al, 1988;Zaki et al, 2001) two phase flow (Coppola et al, 1989;Zaki et al, 2000) and vibration (Sedahmed et al, 1994) has been studied. Natural convection arises in electrochemical reactors as a result of changing interfacial solution density at the electrodes, where processes such as metal deposition, metal dissolution and ionic migration change the interfacial solution composition and its density.

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Dynamic analysis of mass transfer at vertically oscillating surfaces

“…Recently, it was shown that more than 20-fold increase in the mass transfer rate could be achieved by vibrating vertical electrodes along their length [4]. Among its applications in that field are electrochemical processing, electroplating, metal recovery from bio-leaching solutions, and more recently manufacture of Printed Wiring Board where electroplating is re-emerging of as the preferred technology for producing "sub-micron multi-level metallization" in ultra large scale integration [16][17][18][19][20][21][22].…”