Froth velocity

Flotation reagents are used in the froth flotation process to (/) enhance hydrophobicity, (2) control selectivity, (J) enhance recovery and grade, and (4) affect the velocity (kinetics) of the separation process. These chemicals are classified based on utili2ation collector, frother, auxiUary reagent, or based on reagent chemistry polar, nonpolar, and anionic, cationic, nonionic, and amphoteric. The active groups of the reagent molecules are typically carboxylates, xanthates, sulfates or sulfonates, and ammonium salts. 

FIG. 14-118 Aerodynamic cut diameter for a single-sieve-plate scrubber as a function of bole size, bole-gas velocity, and froth density, F, g/cm. To convert meters per second to feet per second, multiply by 3.281 to convert grams per cubic centimeter to pounds per cubic foot, multiply by 62.43. [Calveti, J. Air Pollut. Control Assoc., 24, 929 (1974).]... 

The action on this type of tray seems to produce fewer jets of liquid froth than a bubble cap tray. The entrainment from the surface of the bubbling liquid-froth mixture is less (about K) than a bubble cap tray for the same superficial tower velocity and tray spacing. Generally the trays will flood before capacity reaches a limitation set by entrainment. 

Velocity of froth cross flow, ft/sec Velocity of froth, ft/sec... 

Monsanto Enviro-Chem under the Dyna-Wave trademark case histories can be downloaded from a web site (http //www.enviro-chem. com/airpol/common/rjstop.html). Nonatomizing froth scrubbing is described in those patents as occuring within defining boundaries on a new dimensionless velocity versus dimensionless liquid/gas ratio two-phase flow regime map, shown here as Fig. 17-51. 

These effects may be less pronounced in industrial sieve-plate columns, because the higher vapor velocities shorten the life of any froth on the trays the material on the tray is in the form of a spray of droplets... 

Perhaps the simplest classification of flow regimes is on the basis of the superficial Reynolds number of each phase. Such a Reynolds number is expressed on the basis of the tube diameter (or an apparent hydraulic radius for noncircular channels), the gas or liquid superficial mass-velocity, and the gas or liquid viscosity. At least four types of flow are then possible, namely liquid in apparent viscous or turbulent flow combined with gas in apparent viscous or turbulent flow. The critical Reynolds number would seem to be a rather uncertain quantity with this definition. In usage, a value of 2000 has been suggested (L6) and widely adopted for this purpose. Other workers (N4, S5) have found that superficial liquid Reynolds numbers of 8000 are required to give turbulent behavior in horizontal or vertical bubble, plug, slug or froth flow. Therefore, although this classification based on superficial Reynolds number is widely used... 

Od/water separation is enhanced because both the particle size and density difference have been increased The net result is a higher velocity rise rate for the froth which is continuously removed from the surface of the water by a skimmer arm. 

Liquid-velocity correlations are generally based on one of two concepts, and it is not always clear which one is the basis of a given correlation. One concept relies on the principle that vapour should not be entrained to the tray below. The other is grounded on the phenomenon of downcomer-inlet-choking caused by the inability of the low-density froth... 

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. 

Entrainment (Jet) Flooding Froth or spray height rises with gas velocity. As the froth or spray approaches the tray above, some of the liquid is aspirated into the tray above as entrainment. Upon a further increase in gas flow rate, massive entrainment of the froth or spray begins, causing liquid accumulation and flood on the tray above. 

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 ... 

A variant on the froth contact is the reverse jet contactor (Example 22), which can be considered as an upside-down distillation tray operated above the flooding velocity in cocurrent flow of gas and liquid. It is limited to one stage. 

Processing, 84,(9), 99-106 (2005)]. This article cites downward gas velocities in the range of 10 to 37 m/s and notes gas pressure drop in the range of 6 to 20 in of water. Removals of S02 and fine particles were both close to 99 percent. The froth produced by the contactor reverses direction, flows down, and is largely disengaged in a vessel mounted below. 

Next, calculations are made to get the hydraulic gradient HHG, in inches of clear liquid. But first froth with clear liquid velocity across the tray UF, ft/s, must be calculated, and the hydraulic radius of the aerated mass RH must also be calculated [15],... 

Froth entrainment flooding (Fig. 6.7b). At higher liquid flow rates, the dispersion on the tray is in the form of a froth (Figs. 6.25c and 6.27a), When vapor velocity is raised, froth height increases. When tray spacing is small, the froth envelope approaches the tray above, As this surface approaches the tray above, entrainment rapidly increases, causing liquid accumulation on the tray above. 

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