Droplet swarm

To obtain a large transfer area between raffinate and extract phases, one of the two liquids must be dispersed into drops. Figure 9.2 demonstrates this process schematically at a single nozzle. Similar to a dripping water tap, individual drops periodically leave the nozzle when the volumetric flow rate of the dispersed phase is low. When the flow rate is higher, however, the liquid forms a continuous jet from the nozzle that breaks into droplets. Because of stochastic mechanisms, uniform droplets are not formed. If the polydispersed droplet swarm is characterized by a suitable mean drop... 

Either v, or Ea can be calculated from Eq. (9.20), if there is one more relationship between the two terms. In Fig. 9.12, v,. related to the singledrop velocity Vp according to Eq. (9.15) is plotted as a function of the drop holdup for droplet swarms with the Archimedes number as a dimensionless term for the drop diameter for the measured values. It can be seen that the relative velocity constantly decreases, as the holdup of the drops, e, increases and the size of the drops in the swarm decreases. 

The literature offers numerous calculation models for mass transfer in single liquid particles. However, they provide only a rough approximation to reality in industrial columns, since the processes in droplet swarms are much more complicated, especially when pulsing and rotating motion are superimposed. For estimation, the following relationships are sufficient ... 

In extraction columns, it is possible to find droplet swarms where the local velocities near the droplet surface are higher, this being due to the lower free area available for the countercurrent flowing continuous phase. Wake and Marangoni influences make the prediction of a physical mass transfer coefficients difficult. With reactive extraction the influence of interfacial kinetics on overall mass transfer is generally not negligible. In any case, a combination of reactive kinetics with any eddy mass transfer model is recommended, whereas the latter could rely on correlations derived for specific column geometries. 

The influence of electrical fields on hydrodynamics and thus on mass transfer has been excellently reviewed by Yamaguchi [77,78]. It is thus possible to produce monodispersed droplet swarms up to extreme viscosities in the nanoscale (Figure 18.22). Here, the force balance on a nozzle leads to a dismpture of drops [79]. Under similar electrical but different geometrical conditions, breakage of an emulsion will occur, when due to polarization droplets form chains and will coalesce to bigger droplets as is technically used in secondary oil recovery, breaking down the water in oil emulsions [80]. Thus, coalescence and droplet formation in the electric field is sensitive to minor... 

Fluid mechanics Droplet formation Droplet size, droplet size distribution Droplet swarm motion Droplet coalescence Small droplets and a narrow droplet size distribution favor mass transfer [6.5]... 

The holdup q> of the dispersed phase, fixed by size, number, and velocity of the droplets in a droplet swarm, determines the interfacial surface area referred to the volume... 

Pressurized wash liquid is sprayed in a jet scrubber, and dispersed into the slow flowing gas phase. The driving force is induced by the washing liquid the gas phase is entrained by the liquid jet or the droplet swarm (Fig. 3-23 a). 

At H te moment equation (6) may be recommended for the calculation of the gas holdup in the bubbly flow regime. A better correlation can be obtained, if equations for the motion of solids are modified in a convenient way. This has already been achieved for the motion of droplet swarms ij) ... 

It is possible to give an approximate estimation of the lowest effective gas velocity uvji.min for the test system air/water at 20 °C and 1 bar, which allows droplet entrainment from the packing. This is based on the assumption that the packing elements are sufficiently large to allow the substitution of the same resistance coefficient fi = r into Eq. (2-13) that applies to a droplet swarm in an empty column. It can be calculated using Chao s formula (2-14), quoted by Soo [59] ... 

Acc. to Eqs. (2-19) and (2-21), a droplet swarm falls more slowly than a single droplet. This is due to the increased relative velocity of both phases under the influence of a modified lifting force [55], which changes as a result of the mean density change of the two-phase mixture, similarly to the sedimentation and/or fluidisation process. 

The free cross section available for gas flow changes in the presence of several droplets (droplet swarm). Droplet swarms move more slowly than individual droplets, which is due to the cfllferent lifting forces. The contraction effect is given by the function /i = (l - > see Eq. (2-19). 

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