Liquid-gas entrainment

As soon as the vortex reaches the stirrer head of a rapidly rotating stirrer (e.g. propeller or turbine stirrer) or the outer stirrer side of a slowly rotating stirrer (e.g. blade stirrer), gas is entrained in the liquid. Gas entrainment via vortex was... 

In annular flow, liquid flows as a thin film along the pipe wall and gas flows in the core. Some liquid is entrained as droplets in the gas core. At veiy high gas velocities, nearly all the liquid is entrained as small droplets. Inis pattern is called spray, dispersed, or mist flow. 

Figures 12-106, 12-107, and 12-107A show an exploded view of a typical positive displacement blower. The impeller lobes rotate in opposite directions on parallel mounted shafts. Figure 12-107. One shaft serves as the drive shaft and drives the other through the gears. A timing hub allows for adjusting the timing angle of the lobes. The rotation may be either for upward, downward, or side gas flow. When liquid is entrained in the gas, the downward flow is preferred to assist in case drainage.

Figure 32.29 (a) Effect of entrained gas on liquid displacement (b) solubility of air in oil. Example At 5 in Hg with 3% gas entrainment by volume, pump capacity is reduced to 84 per cent of theoretical displacement... 

Liquid vortexing in suction vessel, thus creating gas entrainment into suction piping. Figure 3-43 suggests a common method to eliminate suction vor-... 

Figure 5.3e shows the situation when the air velocity was increased to Ugs = 20 m/s. It is seen from this figure that the liquid bridges in churn flow disappeared and a liquid film formed at the side walls of the channel with a continuous gas core, in which a certain amount of liquid droplets existed. The pressure flucmations in this case became relatively weaker in comparison with the case of the churn flow. The flow pattern displayed in Fig. 5.3f indicates that as the air velocity became high enough, such as Ugs = 85 m/s, the liquid droplets entrained in the gas core disappeared and the flow became a pure annular flow. It is also observed from Fig. 5.3f that the flow fluctuation in this flow regime became weaker than that for the case shown in Fig. 5.3e, where Ugs = 20 m/s. 

Further increase in the gas flow rate in liquid ring flow leads to a liquid lump flow, of which the high-speed core gas entrains the liquid phase and liquid lumps slide... 

With increasing superficial gas velocity the gas core with a thin liquid film was observed. The flow pattern, displayed in Fig. 5.14c (the second, third and fourth channels from the top), indicates that as the air velocity increased, the liquid droplets entrained in the gas core disappeared such that the flow became annular. 

In the Fig.4, it can be seen that the gas hold-up in both riser and downcomer decreases with increasing the draft-tube horn-mouth diameter and approaches the maximum when the draft-tube hom-mouth diameter is 1.05m. However, due to the gas hold-up decreases more in the downcomer, the gas hold-up difference between the downcomer and the riser increases. Therefore, the apparent density difference between the riser and the downcomer enhances, causing higher liquid superficial velocity in the downcomer and in the riser With increasing the hom-mouth diameter. Fig.5 also shows that the existence of hom-mouth promotes the ability to separate gas from liquid and decreases the amount of gas entrained into the downcomer. 

Levy, S., and J. M. Healzer, 1980, Prediction of Annular Liquid-Gas Flow with Entrainment, Cocurrent Vertical Pipe Flow without Gravity, Rep. EPRI NP-1409, Electric Power Research Institute, Palo Alto, CA. (5)... 

The characteristic time-scales mentioned above take into account some but not all practical considerations. For example, really intense stirring (rpm > 500) in the CSTR is not recommended for in situ studies since a deep vortex ivill be formed in the liquid, gas ivill be entrained, and tivo-phase flow w ill occur in the recycle line. Also, two-phase flow will generally cause cavitation in a mechanical pump (possibly stopping flow) and induce irreproducible spectroscopic measurements. 

Figure 17.11. Types of contactors for reacting gases with liquids many of these also are suitable for reacting immiscible liquids. Tanks (a) with a gas entraining impeller (b) with baffled impellers (c) with a draft tube (d) with gas input through a rotating hollow shaft, (e) Venturi mixer for rapid reactions, (f) Self-priming turbine pump as a mixer-reactor, (g) Multispray chamber. Towers (h) parallel flow falling film (i) spray tower with gas as continuous phase (j) parallel flow packed tower (k) counter flow tray tower. (1) A doublepipe heat exchanger used as a tubular reactor.

Bubble Tube Systems The commonly used bubble tube system sharply reduces restrictions on the location of the measuring element. To eliminate or reduce variations in pressure drop due to the gas flow rate, a constant differential regulator is commonly employed to maintain a constant gas flow rate. Because the flow of gas through the bubble tube prevents entry of the process liquid into the measuring system, this technique is particularly useful with corrosive or viscous liquids, liquids subject to freezing, and liquids containing entrained solids. 

Bottom liquid outlets. Sufficient residence time must be provided in the bottom of the column to separate any entrained gas from the leaving liquid. Gas in the bottom outlet may also result from vortexing or from forthing caused by liquid dropping from the bottom tray (a waterfall pool effect). Vortex breakers are commonly used, and liquid-drop height is often restricted. Inadequate gas separation may lead to bottom pump cavitation or vapor choking the outlet line. 

The principal reactors used are fluidized bed reactors, called Synthol reactors, in which the feed gas entrains an iron catalyst powder in a circulating flow. The suspension enters the bottom of the fluidized bed reaction section, where the Fischer-Tropsch and the gas shift reactions proceed at a temperature of from 315 to 330°C. These reactions are highly exothermic, as described previously, and the large quantity of heat released must be removed. The products in gaseous form together with the catalyst are taken off from the top of the reactor. By decreasing the gas velocity in another section, the catalyst settles out and is returned for reuse. The product gases are then condensed to the liquid products. 

For liquids with entrained gas or vapor, a "vent hole" should be provided, and for gases with entrained liquid, a "drain hole." Meters for liquid with entrained gas or gas with entrained liquid services should be installed vertically. Normally, the flow direction would be upward for liquids and downward for gases. The use of vent and drain holes is discouraged, if in order to keep them from plugging, the holes would need to be large and would adversely affect accuracy. On severe entrainment applications, eccentric or segmental orifice plates should be used. 

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