Eckert flood velocity correlation

Figure 13-6 The Eckert flood velocity correlation. (Fp = packing factor, ft"1 see Table 13-7. g = acceleration of gravity, ft/s2. Gvsf = mass velocity of the vapor, superficial, flood, lb/s-ft2. wL = liquid mass flow rate, lb/s. wv = vapor mass velocity, lb/s. pv = density of vapor, lb/ft3. pL = density of liquid, lb/ft3. p, = viscosity of vapor, lb/ft-s. pw — viscosity of water, lb/ft-s.) [J. S. Eckert, Chem. Eng. Prog.. 63(3) 39 (1949), by courtesy American Institute of Chemical Engineers.]...

Figure 7.11 Modified flooding velocities of the Crawford-Wilke correlation and Eckert velocity ratios [9,11].

The flood velocity is defined as that vapor velocity above which liquid accumulates uncontrollably in the packed bed and continued operation becomes impossible. The variables which determine the flood velocity are the packing geometry, system properties, and liquid viscosity. The first published model was that of Walker et al.35 in 1937. In 1938, Sherwood et al.30 made a minor adjustment to the correlations. Lobo et al.27 made a further improvement in 1945. Further modification to the correlation was made by Eckert in 196310 and in 1970.11 The final Eckert correlation for flood velocity... 

Calculation of the flood velocity by use of the Eckert correlation The abscissa of Fig. 13-6 has the value... 

Fp a factor appearing in the Eckert correlation for the flooding velocity... 

The correlation of Eckert (Fig. 13.37) combines a pressure drop relation and safe flow rates insofar as staying away from the flooding point is concerned. A flooding line corresponds to pressure drops in excess of 2 in. water/ft. In use, a pressure drop is selected, and the correlation is applied to find the corresponding mass velocity G from which the tower diameter then is calculated. Another correlation recommended by a manufacturer of packings appears in Figure 13.40. Example 13.16 compares these correlations for a specific case they do not compare any more closely than could be expected from the scatter of flooding data. 

Figure 7.11 shows modified Crawford-Wilke [11] correlation plot curves. Note that the y-axis is a type of Reynolds number, as discussed in Chap. 6. This y-axis number is similar to the Reynolds number, having density (Dc), viscosity (Uc), and velocity (Vc and Vd). If you review Chap. 6 and the Reynolds number, the same dimensional analysis is seen in the order given in Fig. 7.11 on the y-axis. The x-axis relates to viscosity (Uc), surface tension (0 ), density (Dc), and packing size factor (FJ. Originally the square root of the x-ordinate was used in the Crawford-Wilke correlation plotted against such a Reynolds number. Also, only one curve was made in this original work, the top curve labeled Crawford-Wilke in Fig. 7.11. This top curve represents the point at which the continuous phase is saturated with solute, in equilibrium condition. Eckert [9] reported that when Vc is increased, beginning at Vc = 0, the system floods before Vc reaches this saturation Crawford-Wilke curve.

Figure 15.4 shows the Eckert (1975) pressure drop correlation, which may also be used to check the approach to flooding conditions. The variables in the coordinates are defined as follows L and V are the liquid and vapor flow rates, Ibmol/hr, G is the vapor mass velocity, Ib/tt-s, p, and Pv are the liquid and vapor densities, Ib/fF, F is the packing factor, ft /fF, and is the liquid viscosity, centipoise. The column diameter is implied since G is the vapor rate per unit column cross-sectional area. The pressure drop, in inches of water per foot of packing, is reported as a parameter in this correlation. Flooding can be expected to occur at any point above the pressure drop curve of 1.5 in. of water per foot of packing. 

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