Rotating collection efficiency

Figure 3.67 Typical disc potential and ring current behaviour during galvanoslatic reduction of the passive him on pure iron (area 0.5 cm2) at a rotation speed of 25 Hz. The Pt ring potential is maintained at 0.2 V to oxidise all Fc(II) to Fellll), and the collection efficiency is 0.28. Note that the residual current detected on the ring beyond 70s corresponds to re-oxidation of hydrogen generated galvanostatically on the disc. Reprinted from Corrosion Science, 28, P. Southworth, A. Hamnett. A.M. Riley and J.M. Sykes, An Ellipsometric and RRDE Study of iron Passivation and Depassivation in Carbonate Buffer , pp. 1139-1161 (1988), with kind permission from Pergamon Press Ltd.. Headington Hill Hall. Oxford 0X3 OBW. UK.

Fiber-bed scrubbers are used to control aerosol emissions from chemical, plastics, asphalt, sulfuric acid, and surface coating industries. They are also used to control lubricant mist emission from rotating machinery and mists from storage tanks. Fiber-bed scrubbers are also applied downstream of other control devices to eliminate a visible plume. Despite their potential for high collection efficiency, fiber-bed scrubbers have had only limited commercial acceptance for dust collection because of their tendency to become plugged. 

A disk or cone baffle located beneath the gas outlet duct may be beneficial if air in-leakage at the dust outlet cannot be avoided. A heavy chain suspended from the gas outlet duct has been found beneficial to minimize dust buildup on the cyclone walls in certain circumstances. Such a chain should be suspended from a swivel so that it is free to rotate without twisting. Substantially all devices that have been reported to reduce pressure drop do so by reducing spiral velocities in the cyclone chamber and consequently result in reduced collection efficiency. 

When using an RRDE, no reaction occurs if there is no allyl alcohol in solution when bromine is formed at the disc and re-reduced to bromide at the ring. In such an instance, we would say that the collection efficiency was No (note the additiorml subscript). In practice, the value of No is independent of rotation speed, so a graph of—lg (as y ) against... 

In practice, the analyst measures the collection efficiency N as a function of time L The time we refer to here is the time required for the electrogenerated intermediate to be swept hydrodynamically from the disc and past the ring. Clearly, t is a function of the rotation speed, so in practice we determine N as a Junction of co. 

The rotating ring—disc electrode (RRDE) is probably the most well-known and widely used double electrode. It was invented by Frumkin and Nekrasov [26] in 1959. The ring is concentric with the disc with an insulating gap between them. An approximate solution for the steady-state collection efficiency N0 was derived by Ivanov and Levich [27]. An exact analytical solution, making the assumption that radial diffusion can be neglected with respect to radial convection, was obtained by Albery and Bruckenstein [28, 29]. We follow a similar, but simplified, argument below. 

In reactions involving gas evolution, the RRDE can be problematic in that bubbles may become trapped at the centre of the disc electrode. To obviate this, a rotating double ring electrode was suggested [34], The collection efficiency, N0, is given by eqn. (41) if we define... 

Since the form of the dimensionless convective-diffusion equation for tube and channel electrodes is exactly the same as for rotating electrodes, we can immediately conclude that the steady-state collection efficiency, N0, under conditions of uniform surface concentration at the generator electrode (which corresponds to the limiting current at the generator or to any point on a reversible wave) is, once again... 

Note that iL depends on Vf1/2 whereas, for the wall-jet electrode, it depends on Vf4. This equation only holds for 0.1 Mass transfer is more efficient than at an RDE however, the electrode has to be smaller. Nevertheless, in applications where it is difficult to fabricate a moving electrode (i.e. photoelectrochemical and semiconductor), it could be very valuable. From the theoretical point of view all that has to be done is replace by 0.98 Vf /r% in all the equations for a rotating disc or ring--disc electrode to obtain the wall-tube analogue. In particular, the steady-state collection efficiency, N0 [eqn. (41)], is the same not only in form but also in numerical value for the same radius ratios [50] (Table 2). 

Other Collectors Tarry particulates and other difficult-to-handle liquids have been collected on a dry, expendable phenol formaldehyde-bonded glass-fiber mat (Goldfield, J. Air Pollut. Control Assoc., 20, 466 (1970)] in roll form which is advanced intermittently into a filter frame. Superficial gas velocities are 2.5 to 3.5 m/s (8.2 to 11.5 ft/s), and pressure drop is typically 41 to 46 cm (16 to 18 in) of water. Collection efficiencies of 99 percent have been obtained on submicrometer particles. Brady [Chem. Eng. Prog., 73(8), 45 (1977)] has discussed a clean-able modification of this approach in which the gas is passed through a reticulated foam filter that is slowly rotated and solvent-cleaned. 

In a rotated ring-disk electrode experiment where the disk is controlled at +2.6 V versus SCE for the oxidation of HOOH and the ring electrode is controlled at -1.4 V versus SCE for the reduction of the oxidation products from HOOH, the observed collection efficiency (N = /r//d) is 0.384. This is slightly less than the theoretical value of 0.418 for the electrode. The products from HOOH oxidation at the glassy-carbon disk electrode (ED, + 2.6 V vs. SCE) can be characterized at the ring by scanning its potential from +1.0 to... 

Figure 3.14 Experimental collection efficiencies (Nexpti) at the rotated-ring electrode as a function of HOOH concentration for the product from the oxidation of HOOH at the rotated-disk electrode. Control conditions for GC ring-disk electrode rotation rate, 4900 ipm ED, +2.6 V versus SCE ER, +0.1 V versus SCE.

The collection efficiency depends on the geometric parameters of the ring and disk electrode arrangement. It is stable amongst the whole potential range and all the rotation rates for a smooth electrode. Typical values for N are between 20 % and 35 % for most of RRDE setups. 

RRDE (Q, in rad. s ) is considered. It is important to notice that, in the case of a porous fdm electrode or a thin film electrode, the current collection factor N may vary with the rotation rate of the electrode. This analysis method must be preceded by a determination of the collection efficiency for the whole rotation rate range used for the study. 

If an unstable intermediate or product is formed at the disc, only a fraction will reach the ring, and the ratio i /i will be smaller than N. The extent by which the collection efficiency is decreased is a function of the rate of rotation. The dependence of N on to can be used to evaluate the lifetime (or rate of decomposition) of the unstable intermediate. As in any kinetic measurement of this type, one attempts to design the system for the fastest possible transition time from disc to ring, to allow detection of short-lived intermediates. The gap in commercial RRDEs is of the order of 0.01 cm, but electrodes with substantially narrower gaps have been built. We may be tempted to use modem techniques of microelectronics to construct an RRDE with a very small gap, say, 0.1 Xm. A closer examination of the hydrodynamics involved reveals that this may not work and, in fact, there is little or no advantage in reducing the gap below about 5 0.m. 

The important thing to notice in this equation is that the collection efficiency N is not a function of the rotation rate rather it depends exclusively on the dimensions of the electrodes. The function f(ri,r, r ) is rather complicated, but its values have been tabulated, and the collection efficiency can be obtained from the dimensions of the electrode. In a practical experiment (aimed at using the RRDE, not at... 

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