Phase flow ratio

The types of equipment used, which range from stirred tanks and mixer-settlers to centrifugal contactors and various types of columns, affect both capital and operating costs [9]. In the decision to build a plant, the choice of the most suitable contactor for the specific situation is most important. In some systems, because of the chemistry and mass transfer rates involved, several alternative designs of contacting equipment are available. In the selection of a contactor, one must consider the capacity and stage requirements solvent type and residence time phase flow ratio physical properties direction of mass transfer phase dispersion and coalescence holdup kinetics equilibrium presence of solids overall performance and maintenance as a function of contactor complexity. This may appear very complicated, but with some experience, the choice is relatively simple. 

Figure 13.10. Effect of phase flow ratio on total capacity of liquid-liquid extraction columns.

UclUj=0. At UJUc = 1, 4>d)f= 1/3. The simultaneous solution of (13-20) and (13-23) results in Fig. 13.10 for the variation of total capacity as a function of phase flow ratio. The largest total capacities are achieved, as might be expected, at the smallest ratios of dispersed phase flow rate to continuous phase flow rate. 

At low holdups, longitudinal dispersion due to continuous-phase velocity profiles controls the amount of mixing in the countercurrent spray column whereas at higher holdups the velocity profile flattens, and the shed-wake mechanism controls. Above holdups of 0.24, the temperature jump ratio is linearly proportional to the dispersed-to-continuous-phase flow ratio, and all mixing is caused by shed wakes into the bulk water and coalescence of drops. As column size decreases, it approaches the characteristics of a perfect mixer, and the jump ratio approaches unity (as compared with the value of zero for true countercurrent flow). It is interesting to note that changing the inlet temperature of dispersed phase by about 55°F hardly affected the jump ratio, probably due to the balancing effects of reduced viscosities and a decrease of drop diameter. 

The most important TBP process is the PUREX process, in which 20—40 vol/% TBP in the hydrocarbon diluent is used. The flowsheet commonly used in the United States specifies 30 vol/% TBP (Long, 1978). A typical American flowsheet is shown in Figure 14.14. Uranium is extracted from the feed and decontaminated in the first contactor, separated from plutonium in the second contactor, and stripped back into an aqueous phase in the third contactor. A high aqueous phase to organic phase flow ratio... 

In packed columns, the liquid flows in counter-current to the gas, in the form of films and runlets. This applies in particular to small, non-perforated packing elements with low void fraction, e.g. e = 0.4—0.6 m m . The void space of the packing elements contains dead space, which is filled with more and more Uquid as the Uquid load ul increases. Flooding occurs when the entire column is filled with Uquid. In the case of high specific liquid loads ul and very low gas velocities uy, i.e. at very high phase flow ratios at the flooding point Xq, the so-called phase inversion occurs through the formation of bubbles, whereby the gas phase is dispersed and the Uquid now forms the continuous phase. 

Evaluation of Experimental Results for the Range of Low and Moderate Phase Flow Ratios Xq at Flooding Point... 

The numerical value of the parameter m was determined for various systems, namely air/water, ethanol/water, steam/water [7], air/silicone oil [36, 37], using various random and structured packings made of different materials. In the range of low and moderate Xo numbers below Xq< 0.025, the parameter m is not dependent on the phase flow ratio Xo at the flooding point, see Fig. 2-12a. 

Figure 2-12a. Dependence of the liquid hold-up at the flooding point h p on the phase flow ratio Xq, valid for various systems and packings made of metal, ceramic and plastic for Rep > 2 - experimental data taken by the author in comparison to calculated values based on Eq. (2-47)...

The parameter m, calculated by means of the simplex method, can be described by the new model for phase flow ratios ranging from Xq = 0.001 to 1.0 ... 

This changes to m —0.80 in Eq. (2-42) for lower phase flow ratios Xq in the range shown in Fig. 2-12a. Equation (2-43) leads to a correlation for m = (Xq), which covers practically all areas of application of packed columns for gas/liquid systems, ranging from vacuum rectification to pressure rectification and pressure absorption. 

The deviation of the experimental values from the curve, which was calculated using Eq. (2-47), is 5(h° pj) < 15%. Figures 2-12a and 2-12b show that the liquid hold-up h° PI at the flooding point is dependent on the phase flow ratio Xo at the flooding point and can therefore be determined for any type of system, acc. to the single-parameter Eq. (2-47). 

The phase flow ratio at the flooding point Xq is given as ... 

The following numerical value was found by Billet [5], Chap. 2, for the vapour capacity factor Fypi with L/V = fo]. same system and a marginally different phase flow ratio, compared to the one specified in the task definition ... 

The liquid hold-up h pi is determined using correlation (2-47). The start value for the phase flow ratio at the flooding point Xq is assumed to be ... 

Following the creation of new phase flow ratios at the flooding point Xqji and Xojn, further iterations lead to the final result, for a column diameter of ds = 1.2 m ... 

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