Entrainment of liquid droplets

For preliminary design, liquid entrainment is usually used as a reference. To prevent entrainment, the vapor velocity for tray columns is usually in the range 1.5 to 3.5 ms-1. However, the entrainment of liquid droplets can be predicted using Equation 8.3 to calculate the settling velocity. To apply Equation 8.3 requires the parameter KT to be specified. For distillation using tray columns, KT is correlated in terms of a liquid-vapor flow parameter FLV, defined by ... 

Proper packed-tower design requires prevention of entrainment of liquid droplets in the exiting gas stream. To accomplish this, the tower diameter must be such that the superficial liquid mass flow rate does not exceed certain defined operating limits This parameter is defined in Eq I in terms of the inlet liquid flow rate, liquid density. and tower cross-sectional area. 

The vapor velocity in a finite-stage contactor column can be limited by the liquid handling capacity of the downcomers or by entrainment of liquid droplets in the rising gases. In most cases, however, downcomer limitations do not set the allowable vapor velocity instead, the common design basis for choosing allowable vapor velocities is a function of the amount of gas entrainment which can result in improper operation or flooding of the column. 

Re-entrainment of liquid droplets that are captured can also occur as a result of squeezing when the local pressure drop is increased to overcome the capillary resistance force. The shape of the liquid droplets depends on the wettability of the rock. On the basis of this physical picture, Soo and Radke 12) proposed a model to describe the flow of dilute, stable emulsion flow in a porous medium. The flow redistribution phenomenon and permeability reduction are included in the model. Both low and high interfacial tension were considered. 

High efficiencies require deep pools of liquid on the tray (long contact time) and relatively high gas velocities (large interfacial contact areas and mass-transfer coefficients). These conditions, however, lead to a number of difficulties. One is the mechanical entrainment of liquid droplets in the rising gas stream. At high gas velocities, when the gas disengages from the froth, small droplets of liquid are carried by the gas to the tray above. Liquid carried up the tower in this manner reduces mass transfer and consequently affects the tray efficiency adversely. 

Liquefied ammonia will definitely flash off with simultaneous entrainment of liquid droplets depending on the pressure drop in the storage tank or size of leak. 

Boiler.— The boiler is made of glass and is designed to produce a steady stream of vapour with no entrainment of liquid droplets. The inner vessel contains the liquid sample and the space between this vessel and the outer vessel is evacuated to reduce heat transfer between the sample and the surrounding thermostatted bath. The walls between the vessels are silvered to reduce heat transfer by radiation. A large glass-to-metal seal is attached to the top of the boiler and a brass plate is soldered to the top of the seal. Through this plate are brazed four stainless-steel tubes, two for carrying thermocouples used to measure the temperatures of the vapour and liquid and two which contain the electrical leads to the heater. 

Estimadi the ralumn diameter We have seen how to calculate the diameter of a flash drum where continuous flash vaporization is carried out (Section 6.3.2.1) the basis was to operate at a vapor velocity less than that needed to entrain the falling liquid droplets i.e. the vapor velocity should be lower than the downward liquid dropiet settling velocity. In a multiplate distillation coiumn, the same physical constraint is valid, except, instead of falling liquid droplets, we need to prevent entrainment of liquid droplets from the liquid in the plate below to the plate above to prevent flooding. Relation (6.3.49) is valid here also, except the quantity K depends on a number of other factors and we use instead of Up t and pe for ppi... 

Gas velocities are generally low, in the range of 4 to 5 ft per second, to avoid excessive entrainment of liquid droplets. This results in the requirement for large-diameter vessels however, it also results in very low gas-side pressure drop, typically less than 3 in. of water. 

The diameter of a tower is established by the volume of vapors which must be handled and by the maximum allowable vapor velocity which can be tolerated without encountering excessive entrainment of liquid from one plate to the plate above. Entrainment can occur by splashing and/or suspension of small droplets in the vapor as a mist. It tends to defeat the purpose of fractionation even a small amount may be serious when rigid specifications on color or impurities must be met. 

In order to develop the above burn-out mechanism further, it will be necessary to know more about the entrainment and deposition processes occurring. Experimentally, it is likely that these processes will be very difficult to measure separately and under conditions comparable to those prevailing in a boiling channel. From analysis of their film flow-rate data, Staniforth et al. (S8) have deduced that under burn-out conditions, the deposition of liquid droplets from the vapor core plays an important part in reinforcing the liquid film, particularly at high mass velocities. At low mass velocities, they conclude that deposition and entrainment rates must be nearly equal, and, therefore, since a thin liquid film can be expected to be tenacious and give rise to very little entrainment, they argue that both deposition and entrainment tend to zero near the burn-out location with low mass velocities. 

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. 

Equation (5-68) is also determined empirically as shown in Figure 5.29. It can be seen that the mechanism of liquid droplet entrainment and deposition at CHF in annular flow is qualitatively validated by the data trend plotted in Figure 5.29. 

Mass transfer controlled by diffusion in the gas phase (ammonia in water) has been studied by Anderson et al. (A5) for horizontal annular flow. In spite of the obvious analogy of this case with countercurrent wetted-wall towers, gas velocities in the cocurrent case exceed these used in any reported wetted-wall-tower investigations. In cocurrent annular flow, smooth liquid films free of ripples are not attainable, and entrainment and deposition of liquid droplets presents an additional transfer mechanism. By measuring solute concentrations of liquid in the film and in entrained drops, as well as flow rates, and by assuming absorption equilibrium between droplets and gas, Anderson et al. were able to separate the two contributing mechanisms of transfer. The agreement of their entrainment values (based on the assumption of transfer equilibrium in the droplets) with those of Wicks and Dukler (W2) was taken as supporting evidence for this supposition. 

Gill et al. (G3), 1963 Experimental study of upward cocurrent flow of air/water system. Data on pressure drop and film thicknesses (and effects of liquid droplet entrainment) as functions of distance from inlet. Effects of waves on film surface considered. 

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. 

Fiber Bed Mist Filtration. In-depth fiber bed filters are used for the collection of liquid droplets, fogs, and mists. Horizontal pads of knitted metal wire (or plastic fibers), 100—150 mm thick, and gas upflow are used for liquid entrainment removal. Pressure drop is 250 —500 Pa (1.9—3.8 mm Hg). 

The major objective in sizing a gas-liquid separator is to lower the gas velocity sufficiently to reduce the number of liquid droplets from being entrained in the gas. Thus, the separator diameter must be determined. The separator is also designed as an accumulator for the liquid portion of the stream. Thus, the liquid... 

The product of fast pyrolysis is vapours, aerosols and gases from decomposition of holocellulose and lignin with any carrier gases from fluidisation or transport. Aerosols consist of sub-micron liquid droplets and they present a severe problem in tbe successful recovery of the pyrolysis oils. These aerosols appear visually as smoke. The aerosols are probably formed directly from pyrolysing biomass, especially from submicron biomass particles that are rapidly depolymerised. The liquid product can then be entrained out of the reactor before it is vaporised. Another mechanism proposed for the formation of aerosols in the pyrolysis reactor involves the ejection of liquid droplets from internally pressurised cell capillaries of a pyrolysing particle (33). 

Separate saxxqpling valves are used for taking the liquid and vapor samples to prevent entrainment of small droplets and cross-contamination of samples. 

FIGURE 15.19 Bubble growth with the formation of an evaporation wave resulting in entrainment of small droplets into the bubble. 1, experimental data 2, Plesset and Zwick heat-transfer-controlled solution (Eq. 15.37) 3, Rayleigh solution with r(f) = 0 at t = 0.16 s 4, liquid pressure (from Barthau and Hahne [45], with permission). 

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