Aerated liquid

Stable regiTTie The preferable hydrodynamic condition of the aerated liquid on a sieve tray. The aerated material exists as a stable froth gas-liquid contact is good. 

The aerated liquid pressure drop includes that generated by forming bubbles [193] due to surface tension effects. The equivalent height of clear liquid on the tray is given [193] ... 

The height of aerated liquid-froth mixture on the tray, hai, (in.) was determined to agree with the following relation [69] for air-water for 23% and 40% open trays. 

Froth-flotation processes are used extensively for the separation of finely divided solids. Separation depends on differences in the surface properties of the materials. The particles are suspended in an aerated liquid (usually water), and air bubbles adhere preferentially to the particles of one component and bring them to the surface. Frothing agents are used so that the separated material is held on the surface as a froth and can be removed. 

A simple additive model is normally used to predict the total pressure drop. The total is taken as the sum of the pressure drop calculated for the flow of vapour through the dry plate (the dry plate drop hj) the head of clear liquid on the plate (hw + how) and a term to account for other, minor, sources of pressure loss, the so-called residual loss hr. The residual loss is the difference between the observed experimental pressure drop and the simple sum of the dry-plate drop and the clear-liquid height. It accounts for the two effects the energy to form the vapour bubbles and the fact that on an operating plate the liquid head will not be clear liquid but a head of aerated liquid froth, and the froth density and height will be different from that of the clear liquid. 

To predict the height of aerated liquid on the plate, and the height of froth in the downcomer, some means of estimating the froth density is required. The density of the aerated liquid will normally be between 0.4 to 0.7 times that of the clear liquid. A number of correlations have been proposed for estimating froth density as a function of the vapour flow-rate and the liquid physical properties see Chase (1967) however, none is particularly reliable, and for design purposes it is usually satisfactory to assume an average value of 0.5 of the liquid density. 

Sufficient residence time must be allowed in the downcomer for the entrained vapour to disengage from the liquid stream to prevent heavily aerated liquid being carried under the downcomer. 

The separation of a mixture using flotation methods depends on differences in the surface properties of the materials involved. If the mixture is suspended in an aerated liquid, the gas bubbles will tend to adhere preferentially to one of the constituents-the one which is more difficult to wet by the liquid-and its effective density may be reduced to such an extent that it will rise to the surface. If a suitable frothing agent is added to the liquid, the particles are held in the surface by means of the froth until they can be discharged over a weir. Froth flotation is widely used in the metallurgical industries where, in general, the ore is difficult to wet and the residual earth is readily wetted. Both the theory and practical application of froth flotation are discussed by Clarke and Wilson 44. ... 

Several investigations have been carried out into the power requirements for agitation of aerated liquids including those of Yung et al.m> and Luong and Volesky(72) and it is generally concluded that the power required is less for an aerated system than for a non-aerated system. 

The fractional gas holdup can be easily measured from the height of the expanded column height Zf and the settled sluny height Zs, i.e. the height of the column before aeration (liquid volume plus solids volume) (DOE, 1985 NTIS, 1983) ... 

The ratio of the power requirement of gas-sparged (aerated) liquid in a stirred tank, Pq, to the power requirement of ungassed liquid in the same stirred tank, Pq, can be estimated using Equation 7.34 [7]. This is an empirical, dimensionless equation based on data for six-flat blade turbines, with a blade width that is one-fifth of the impeller diameter d, while the liquid depth Hp is equal to the tank diameter. Although these data were for tank diameters up to 0.6 m. Equation 7.34 would apply to larger tanks where the liquid depth-to-diameter ratio is typically... 

The correlations detailed in Sections 7.6.2.1-7.6.2.5 [17,18] are based on data for the turbulent regime with 4 bubble columns, up to 60 cm in diameter, and for 11 liquid-gas systems with varying physical properties. Unless otherwise stated, the gas holdup, interfacial area, and volumetric mass transfer coefficients in the correlations are defined per unit volume of aerated liquid, that is, for the liquid-gas mixture. 

Unfortunately, we do not have clear liquid, either in the downcomer, on the tray itself, or overflowing the weir. We actually have a froth or foam called aerated liquid. While the effect of this aeration on the specific gravity of the liquid is largely unknown and is a function of many complex factors (surface tension, dirt, tray design, etc.), an aeration factor of 50 percent is often used for many hydrocarbon services. 

The actual height of fluid overflowing the weir is quite a bit greater than we calculate with this formula. The reason is that the fluid overflowing the weir is not clear liquid, but aerated liquid—that is, foam. The fluid on the tray deck, below the top of the weir, is also foam. This... 

As illustrated, liquid accumulates on the low side of this tray. Vapor, taking the path of least resistance, preferentially bubbles up through the high side of the tray deck. To prevent liquid from leaking through the low side of the tray, the dry tray pressure drop must equal or exceed the sum of the weight of the aerated liquid retained on the tray by the weir plus the crest height of liquid over the weir plus the 2-in out-of-levelness of the tray deck. 

In a thermosyphon or natural-circulation reboiler, there is, of course, no source of air. The aerated liquid is a froth or foam, produced by the vaporization of the reboiler feed. Without a source of heat, there can be no vaporization. And without vaporization, there will be no circulation. So we can say that the source of energy that drives the circulation in a thermosyphon reboiler is the heating medium to the reboiler. 

The purpose of a tray is to mix vapor and liquid. This produces aerated liquid—or foam. The purpose of a reboiler is to produce vapor. In a circulating reboiler, the reboiler effluent flows up the riser as a froth. Of course, the flow from the bottom of the tower is going to be a clear liquid. Foam cannot be pumped. But there will always be some ratio of foam to clear liquid in the bottom of the tower, and we have no method of determining this ratio, or even the density of the foam. 

Gas hold-up is one of the most important parameters characterizing the hydrodynamics in a fermenter. Gas hold-up depends mainly on the superficial gas velocity and the power consumption, and often is very sensitive to the physical properties of the liquid. Gas hold-up can be determined easily by measuring level of the aerated liquid during operation ZF and that of clear liquid ZL. Thus, the average fractional gas hold-up H is given as... 

Aerating liquids, semiliquids, and powders, usually caused by excessive high-speed stirring... 

Downcomer Backup Flooding Aerated liquid backs up in the downcomer because of tray pressure drop, liquid height on the tray, and frictional losses in the downcomer apron (Fig. 14-32). All these increase with increasing liquid rate. Tray pressure drop also increases as the gas rate rises. When the backup of aerated liquid exceeds the tray spacing, liquid accumulates on the tray above, causing downcomer backup flooding. 

Downcomer Choke Flooding This is also called downcomer entrance flood or downcomer velocity flood. A downcomer must be sufficiently large to transport all the liquid downflow. Excessive friction losses in the downcomer entrance, and/or excessive flow rate of gas venting from the downcomer in counterflow, will impede liquid downflow, initiating liquid accumulation (termed downcomer choke flooding) on the tray above. The prime design parameter is the downcomer top area. Further down the downcomer, gas disengages from the liquid and the volumes of aerated liquid downflow and vented gas... 

Pressure drop through the aerated liquid [h L, in Eq. (14-100)] is calculated by... 

A device used for de-aerating liquid systems, such as emulsions, and operates on the principle of centrifugally generating a thin film of the liquid with high shear and exposing the thin film to vacuum. 

Downcomer backup flooding occurs when the backup of aerated liquid in the downcomer exceeds the available tray spacing. Downcomer backup can be calculated by adding the clear liquid height on the tray, the liquid backup caused by the tray pressure drop, and the liquid backup caused by the friction loss at the downcomer outlet. The downcomer backup is then divided by an aeration factor to give the aerated liquid backup. 

Downcomer choke flooding (Fig. 6.7d). As liquid flow rate increases, so does the velocity of aerated liquid in the downcomer. When this velocity exceeds a certain limit, friction losses in the downcomer and downcomer entrance become excessive, and the frothy mixture cannot be transported to the tray below. This causes liquid accumuletion on the tray above. 

Downcomer backup flooding occurs when the backup of aerated liquid in the downcomer exceeds the tray spacing, i.e.,... 

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