Spray, regime

FIG. 14-35 Transition from frotb to spray regime for boles of various diameters. Values on curves are liquid loadings, mV(b m weir length). To convert cubic meters per bour-meter to cubic feet per bour-foot, multiply by 10.764 to convert (meters per second) (Idlograms per cubic meterto (feet per second) (pounds per cubic foot) , multiply by 0.8197 and to convert millimeters to inches, multiply by 0.0394. [Loon, Finczewski, and Fell, Trans. Inst. Gbem. Eng., 5i, 374 (1,973).]... 

Figure 14-25 also provides a means for estimating whether spray or froth might prevail on the tray. As can be seen, low values of the flow parameter Flg,. s for vacuum fractionators, can lead to the spray regime. 

Breakup in a highly turbulent field (L/velocity) ". This appears to be the dominant breakup process in distillation trays in the spray regime, pneumatic atomizers, and high-velocity pipehne contactors. 

Stichlmair uses the ratio of actual velocity to this maximum velocity together with a term that increases entrainment as the distance gets small between the hquid-vapor layer and the tray deck above. His correlation spans a 10 fold range in entrainment. He shows a sharp increase in entrainment at 60 percent of the maximum velocity and attributes the increase to a shift to the spray regime. 

Is believed to be a froth regime (liquid in continuous phase above the tray and gas present as bubbles in the liquid) phenomenon rather than a spray regime (gas in continuous phase above the tray and liquid present... 

For a two-phase flow with a high quality, for example the spray regime, it would be more appropriate to use a gas flow as the reference single-phase... 

Current breakup models need to be extended to encompass the effects of liquid distortion, ligament and membrane formation, and stretching on the atomization process. The effects of nozzle internal flows and shear stresses due to gas viscosity on liquid breakup processes need to be ascertained. Experimental measurements and theoretical analyses are required to explore the mechanisms of breakup of liquid jets and sheets in dense (thick) spray regime. 

In high pressure distillation, tray operation is usually in the emulsion regime. In small diameter (less than 1.5 m) columns, or at low liquid loads, or the low end of the pressure range (towards 10 bar), however, the froth-and spray regimes can be found. 

In the spray regime, flooding (usually called jet flooding) is caused by excessive entrainment of liquid from an active area to the tray above. It increases the tray pressure-drop, and the entrained liquid recirculates to the tray below. The larger liquid load in the downcomer and the increased tray-pressure-drop together cause the downcomer to overfill so the tray floods. 

Spray regime (or drop regime, Fig. 14-20c). At high gas velocities and low liquid loads, the liquid pool on the tray floor is shallow and easily atomized by the high-velocity gas. The dispersion becomes a turbulent cloud of liquid droplets of various sizes that reside at high elevations above the tray and follow free trajectories. Some droplets are entrained to the tray above, while others fall back into the liquid pools and become reatomized. In contrast to the liquid-continuous froth and emulsion regimes, the phases are reversed in the spray regime here the gas is the continuous phase, while the liquid is the dispersed phase. 

The spray regime frequently occurs where gas velocities are high and liquid loads are low (e.g., vacuum and rectifying sections at low liquid loads). 

Hole Sizes Small holes slightly enhance tray capacity when limited by entrainment flood. Reducing sieve hole diameters from 13 to 5 mm ( to in) at a fixed hole area typically enhances capacity by 3 to 8 percent, more at low liquid loads. Small holes are effective for reducing entrainment and enhancing capacity in the spray regime (Ql < 20 m3/hm of weir). Hole diameter has only a small effect on pressure drop, tray efficiency, and turndown. 

York, 1992). Figure 14-29 demonstrates the effect of liquid rate and fractional hole area on CSB. As liquid load increases, CSB first increases, then peaks, and finally declines. Some interpret the peak as the transition from the froth to spray regime [Porter and Jenkins, I. Chem. E. Symp. Ser. 56, Summary Paper, London (1979)]. CSB increases slightly with fractional hole area at lower liquid rates, but there is little effect of fractional hole area on CSB at high liquid rates. CSB,slightly increases as hole diameter is reduced. 

Entrainment Prediction For spray regime entrainment, the Kister and Haas correlation was shown to give good predictions to a wide commercial and pilot-scale data bank [1. Chem. E. Symp. Ser. 104, A483 (1987)]. The correlation is... 

For decades, the Fair correlation [Pet/Chem. Eng., 33(10), 45 (September 1961)] has been used for entrainment prediction. In the spray regime the Kister and Haas correlation was shown to be more accurate [Koziol and Mackowiak, Chem. Eng. Process., 27, p. 145 (1990)]. In the froth regime, the Kister and Haas correlation does not apply, and Fair s correlation remains the standard of the industry. Fair s correlation (Fig. 14—34) predicts entrainment in terms of the flow parameter [Eq. (14-89)] and the ratio of gas velocity to entrainment flooding gas velocity. The ordinate values XF are fractions of gross liquid downflow, defined as follows ... 

TABLE 14-10 Recommended Range of Application for the Kister and Haas Spray Regime Entrainment Correlation... 

Weir Height Taller weirs raise the liquid level on the tray in the froth and emulsion regimes. This increases interfacial area and vapor contact time, which should theoretically enhance efficiency. In the spray regime, weir height affects neither liquid level nor efficiency. In distillation systems, the improvement of tray efficiency due to taller weirs is small, often marginal. 

Spray entrainment flooding (Fig. 6.7a). At low liquid flow rates, trays operate in the spray regime, where most of the liquid on the tray is in the form of liquid drops (Figs, 6,25c and 6.276), As vapor velocity is raised, a condition is reached where the bulk of these drops are entrained into the tray above. The liquid accumulates on the tray above instead of flowing to the tray below,... 

Low pressures favor high vapor velocities and low liquid flow rates and, therefore, spray regime dispersions. Flooding in vacuum columns and in columns operating at a low liquid-to-vapor ratio is usually caused by the spray entrainment mechanism. 

Koziol and Mackowiak (55a) found the Kister and Haas correlation to give good agreement with experimental data. They developed a new dimensionless correlation [thus overcoming the need for a dimensional exponent in Eq. (6.27)] for spray regime entrainment at very low liquid rates (0.1-1.5 gpm/in). Their correlation, however, postulates that entrainment rises with tower diameter at the same steep rate at which it rises with hole diameter. This postulate conflicts with the industiy s experience that entrainment does not increase upon tower diameter scale-up. 

An early entrainment correlation by Fair (19,34) was recommended by many design publications (5,18,30-38). In the spray regime, Fair s correlation gives reasonable predictions (36), but Is less accurate than the Kister and Haas correlation. The same conclusion was reached in-... 

Several other entrainment correlations have been reported in the literature (17,27,29,39,40,54). Some of their limitations were described elsewhere (12,36,40). In the spray regime, Koziol and Mackowiak... 

The Bennett et al. correlation. This correlation was shown (31) to predict experimental sieve tray pressure drop data more accurately than Fair s correlation. The correlation is based on froth regime considerations and is not applicable to the spray regime. The Bennett et al. calculation of dry pressure drop is identical to Fair s, using Eqs. (6.42) and (6.43) and the Liebson et al- correlation (Fig. 6.21a). To calculate the h, term in Eq. (6.41), Bennett et al. depart from the concept of clear liquid flow corrected for aeration effects [Eq. (6.47a)]. Instead, they use Eq, (6.476) and a model of froth flow across the weir. Their residual pressure drop, hn, is a surface tension head loss term, which is important for trays with very small holes ([Pg.317]

Strictly, the Jeronimo and Sawistowski correlation predicts clear liquid heights at the froth to spray regime transition. However, it has been shown (27,92) that clear liquid height in the spray regime is much the same as clear liquid height at that transition. 

Spray (Flge. 6.25d, 6.260, and 6.276 sometimes referred to as the drop" regime). As vapor load is increased at relatively low liquid rates, the spray regime is reached. While in the previous three regimes (and also... 

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