Packed Tower Flooding

Flooding, packed towers, 433,441 liquid-liquid, 485 Flow control fluids, 58,59 solids, 43,44... 

Packing for towers consists of rings, berl saddles, Fiberglas pads, and helices (see Perry s Chemical Engineers Handbook, 3d ed., p. 685). Because of uncertainties of scale-up and ease of flooding, packed towers are seldom used for large-scale operations, being limited to diameters less than 2 ft. 

FIG. 15-34 Flooding in packed towers, Use only customary units in the vari-ahles, [Crawford and Wilke, Chem, Eng, Prog, 47, 423 (1951), with petmis-... 

These scrubbers have had limited use as part of flue gas desulfurization (FGD) systems, but the scrubbing solution flow rate must be carefully controlled to avoid flooding. When absorption is used for VOC control, packed towers are usually more cost effective than impingement plate towers (discussed later). 

Figure 9-21D. Loading, flooding and pressure drop coireiation (one of earlier versions). Adapted by permission from Leva, M. Tower Packing and Packed Tower Design, 2nd ed. U.S. Stoneware Co.

Kaiser [140] presents a correlation analysis for flooding in packed towers that is more analytical in the performance approach. It is based on single phase hydraulics. It would have been helpful for the article to present a comparison of results tvith the other more conventional techniques. 

Revised Packed Tower Pressure Drop Correlation Constants for Towers Operating Below Flooding Region... 

Copigneaux, R, Flooding in Packed Towers, Hydro. Processing Feb. (1981) p. 99. 

Packed towers should operate near 70% of the flooding rate given by the correlation of Sherwood, Lobo, et al. 

Figure 4.17. Generalised correlation for flooding rates in packed towers(6l)...

The derivation of equations 13.34 and 13.35 has been carried out assuming that u0 is constant and independent of the flowrates, up to and including the flooding-point. This in turn assumes that the droplet size is constant and that no coalescence occurs as the hold-up increases. Whilst this assumption is essentially valid in properly designed spray towers, this is certainly not the case with packed towers. Equations 13.34 and 13.35 cannot therefore be used to predict the flooding-point in packed towers and a more empirical procedure must be adopted. 

Up to the present time, work has been done which allows prediction of the onset of large waves (H2), and of formation of other types of waves (VI, HI), but only on flat uniform liquid surfaces. The extent to which these results can be applied to pipe line flow is uncertain. Apparently, Gazley s papers are still the only basic reports of stratified and wave flow in horizontal pipe incidentally they also show a parallel between liquid instability in pipe flow as evidenced by wave formation, and that evidenced in packed towers by flooding. 

The problem we have just discussed—poor fractionation efficiency due to inadequate vapor and liquid initial distribution—is rather similar to tray deck dumping in trayed fractionators. And, just like trays, packed towers are also subject to flooding. 

Flooding, holdup, and mass transfer rates are highly interdependent and are not simply related. Reissinger and Schroter (1978) state that tray towers in comparison with other types have good efficiencies at 60 nr /m2 hr at frequencies of 60-90/min and amplitudes of 10 mm. Packed towers have about 2/3 the capacities of tray towers. Also in comparison with unagitated towers, which are limited to interfacial tensions below lOdyn/cm, pulsed towers are not limited by interfacial tension up to 30-40 dyn/cm. Some... 

Figure 14.16. Flooding velocities in liquid-liquid packed towers [J.S. Eckert, Encycl. Chem. Process. Des. 21, 149-165 (1984)]. V = ft/hr (superficial velocity) C = continuous phase D = disperse phase a = sqft area of packing/cuft A = difference s = void fraction in packing p = viscosity centipoise continuous phase p = Ib/cuft o = (dynes/cm) interfacial surface tension F = packing factor.

Packed tower Flooding velocity is obtained with Figure 14.17. For 1 in. metal pall rings,... 

Flood-Point Definition In 1966, Silvey and Keller [Chem. Eng. Progr. 62(1), 68 (1966)] listed 10 different flood point definitions that have been used by different literature sources. A later survey (Kister and Gill, Proceedings of Chemeca 92, p. 185-2, Canberra, Australia, 1992) listed twice that many. As Silvey and Keller pointed out, the existence of so many definitions puts into question what constitutes flooding in a packed tower, and at what gas rate it occurs. Symptoms used to identify flood in these definitions include appearance of liquid on top of the bed, excessive entrainment, a sharp rise... 

Therefore, the gas mass velocity is 1372 lb/hr ft for 60% flooding condition with a gas mass flow of 1.28 x 10 lb/hr. It is determined that 3 towers are required each one 17.6 ft in diameter. The height of a transfer unit is a 11.4 ft and the total height of a packed tower is 22.8 ft. Pressure drop in the tower is 0.41 lb/in2. A summary of the composition of various streams is given in Table IV and stripper characteristics are given in Table III. 

Rotating packed-bed devices handle high volumes of fluids in a small equipment volume, compared to packed towers, due to the acceleration of gravity. The Sherwood flooding correlation for packed towers (25) is expressed as a plot of... 

Early RPB researchers discovered that this flooding correlation for packed towers applied equally well to RPBs when the gravity term (g) was replaced by centrifugal acceleration (ra>2). As acceleration increases, the gas flooding velocity (Uq) increases in order to maintain the same value of the first term. Since the ratio of liquid (L) to gas (G) flow remains constant, liquid flow increases commensur-ately with gas flow. Most researchers observed higher gas velocities before the onset of flooding than predicted by the Sherwood correlation (17,26,27). 

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