Dispersed phase hold

Dispersed phase hold-up. Figure 13.33 represents a section of a spray tower of unit cross-sectional area. The light phase is assumed to be dispersed, and the volumetric flowrates per unit area of the two phases are L d and L c respectively. The superficial velocities ud, uc of the phases are therefore also equal to L d and L c. Under steady-state conditions the amount of dispersed phase held up in the tower in the form of droplets is conveniently expressed in terms of the fractional hold-up j, that is the fractional volume of the two-phase dispersion occupied by the dispersed phase. This may also be thought of as the fraction of the cross-sectional area of the tower occupied by the dispersed phase. The velocity of the dispersed phase relative to the tower is therefore L d/j. Similarly, the relative velocity of the continuous phase is equal to L c/( 1 — j). If the overall flow is regarded as strictly countercurrent, the sum of these two velocities will be equal to the... 

Dispersed phase hold-up. Three regimes of flow may be distinguished with packings greater than the critical size ... 

Packed towers are best employed when 3-6 equilibrium stages suffice, there is an interfacial tension of 15 dynes/cm or less, and the desired dispersed-to-continuous phase ratio is between 0.3 and 3. Packed columns provide the advantages of excellent interface control, low dispersed phase hold-up, and potentially high capacity. 

Murugesan T, Dispersed phase hold-up in mechanically agitated gas-liquid contactors, J. of chem. Technol. Biotechnol., Oxford 72 (1998) 3, p. 221-226... 

Abdullah B, Dave C, Nguyen TH, Cooper CG, Adesina AA. (2011) Electrical resistance tomography-assisted analysis of dispersed phase hold-up in a gas-inducing mechanically stirred vessel. Chem. Eng. Sci., 66 5648-5662. 

The recent work of Laddha and Smith (51), who used a technique developed by Colburn and Welsh (19), is most illuminating since it presents data on the individual phase resistances. In this procedure two pure liquids of limited solubility are contacted in the absence of a third solute, and the approach to saturation of each phase can be calculated in terms of the individual resistances. The operation is analogous to contacting a pure gaswithapure liquid instudies, whereby gas-film resistances alone are obtained. Laddha and Smith used isobutyraldehyde and 3-pen-tanol Avith water in a 2-in.-diameter tower and were able to obtain data for all but the pure aldehyde film. The dispersed-phase coefficients are shown in Fig. 10.25 as a function of dispersed-phase rate. In two of the cases, where the continuous-phase rate was relatively low, the coefficients appear to depend upon dispersed phase rate only. In the third, the influence of higher continuous-phase rates on dispersed-phase hold-up and interfacial... 

Schallenberg J, Enss JH, Hempel DC. The important role of local dispersed phase hold-ups for the calculation of three-phase bubble columns. Chem. Eng. Sci. 2005 60 6027-6033. 

The porosity (ie, pore size and amount of pores) of microparticles is also an important characteristic to take into consideration when fabricating microparticles because it plays an essential role in controlling the release of payloads. The porosity and morphology of particles are usually determined by scanning electron microscopy (SEM). For the emulsification solvent extraction/evaporation method of fabrication, the rate of solvent extraction, which depends on the flow in the stirred vessel the droplet size the temperature and the dispersed phase hold-up in the 0/W emulsion have an effect on porosity [87]. The porosity usually increases with a decrease in solvent extraction rate. The porosity of microparticles results in initial burst release due to pore diffusion [78,88]. Mao et al. studied the influence of different W/O/W emulsification solvent extraction/evaporation process parameters on internal and external porosity of PLGA microparticles [78]. The surface morphology of the microparticles can be influenced by the type of polymer, internal aqueous phase voliune (Wi), volume of continuous phase (W2), polymer concentration, homogenization speed, and equipment used for the primary emulsion [78,79]. 

McManamey, W. J. (1979). Sauter mean and maximum drop diameters of liquid-liquid dispersions in turbulent agitated vessels at low dispersed phase hold-up, Chem. Eng. ScL, 34, 432-434. 

Dispersed Phase Hold-Up in Packed Columns Containing Random and Structured Packings... 

Figure 7-2. Dispersed phase hold-up x as function of specific flow rate of the continuous phase uc> valid for various packings made of metal, plastic and ceramic [11-14]. Parameter specific flow rate of dispersed phase up...

Figures 7-2 and 7-3 show the dispersed phase hold-up x as a function of the specific flow rate uc of the continuous phase, using various specific flow rates ud of the dispersed phase as a parameter. The experimental data shown in Fig. 7-2 is applicable to different random packing elements, such as metal Pall rings, Biatecki rings, Hiflow rings with a dimension of 25-38 mm, whereas the data shown in Fig. 7-3 is valid for 50 mm tube columns and other structured packings. The test system used for the experiments under normal conditions was toluol (D)/water, which has a high interfacial tension and is...

The figures show that all packings are characterised by a qualitatively equal course of the function of the dispersed phase hold-up x. Up to a certain distance from the flooding point uc< uc,Fl> Fig- 7-2, line A-A, the dispersed phase hold-up x is not dependent on the specific flow rate of the continuous phase uc-... 

The dispersed phase hold-up x, as a function of the specific flow rate of the dispersed phase ud, is now plotted in a graph, using the flow rate uc of the continuous phase as a parameter. Figs. 7-4 and 7-5. The result is a linear relationship. 

Influence of test system on dispersed phase hold-up X under normal temperature and pressure. 

The linear relationship between the dispersed phase hold-up x, acc. to Eq. (7-2), can be seen in Figs. 7-2, 7-3, 7-4, 7-5 and 7-6. It therefore applies to liquid/liquid systems in the range below the loading line, i.e. for ... 

The following approximate equation (7-4), derived in previous studies [11, 12], can be used to determine the dispersed phase hold-up x for any type of system ... 

The experiments on the dispersed phase hold-up x, using systems with various properties shown in Fig. 7-6, also indicate that the variable Cq in Eq. (7-4) is a system-independent constant, which is not dependent on the constructive design and size of the plant, see Table 7-2. 

D)-acetone/water (C). During each test, the Sauter diameter dT as well as the dispersed phase hold-up x were determined. The results are shown in Fig. 7-9a-c. 

The typical hydraulic characteristics of a packed column are shown in Fig. 7-7, plotting the correlation between the dispersed phase hold-up hp and the specific flow rate of the dispersed phase up. The parameter here is the specific flow rate of the continuous phase Uc. This figure shows the course of the most important parameters of a packed column in a single diagram. Up to 65 % of the flooding capacity ud fi> the flow rate of the continuous phase uc has practically no influence on the dispersed phase hold-up. 

According to this model, the relative velocity of the droplet flow of both phases ur is linked to the dispersed phase hold-up x° and the falling or rising velocity of the individual droplet ws- The best known models for determining the relative velocity ur for Hquid/liquid systems are as follows ... 

The dispersed phase hold-up at the flooding point is calculated iteratively by solving Eqs. (7-9) and (7-12). 

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