O 0.» 0.20 030 0.40 050 0.60 O.TO 0.6"? 0.90 l.oo Nm/N
Figure 3.9 <Continued) Reflux-stages correlations. (b) the Erbar-Maddox correlation. (Part a from C. S. Robinson and E. R. Gilliland. Copyright © by McGraw-Hill. Inc. Reprinted by permission. Part b from J. H. Erbar and R. N. Maddox, Pet. Ref., vol. 40, no. 6, p. 183,1961. Reprinted courtesy of Hydrocarbon Processing.)
O 0.» 0.20 030 0.40 050 0.60 O.TO 0.6"? 0.90 l.oo Nm/N
Figure 3.9 <Continued) Reflux-stages correlations. (b) the Erbar-Maddox correlation. (Part a from C. S. Robinson and E. R. Gilliland. Copyright © by McGraw-Hill. Inc. Reprinted by permission. Part b from J. H. Erbar and R. N. Maddox, Pet. Ref., vol. 40, no. 6, p. 183,1961. Reprinted courtesy of Hydrocarbon Processing.)
Further, as seen in Example 3.1, choice of the volatility estimation method affects the agreement between the data and the correlation. Of the various numerical methods, that of Molokanov et al. (51) was preferred by some authors (30,54) and that by Eduljee (48) by others (11,23,29,49,55). Eduljee's equation has a simplicity advantage. King (9) suggests using Eduljee's equation where high accuracy is not required and Molokanov's when better accuracy is desired. Eduljee's equation is given by
This equation applies in the range of 0.01 < X < 1. Values of X lower than 0.01 are of little practical importance.
Ertoar-Maddox method (Fig. 3.9b). This method uses a plot of RUR + 1)
against NmiJN, with /?min/(/?mjtl + 1) as the parameter. When R = J?min, the x axis becomes zero. Therefore, they axis of the diagram represents minimum reflux conditions. When N = Nmitl, both x and y coordinates become unity.
Figure 3.96 is based on a bubble-point feed. For other types of feed, the following correlation is used (46).
In the Erbar-Maddox correlation, minimum stages are calculated by the Winn method (Sec. 3.2.1) and minimum reflux by the Underwood method (Sec. 3.2.2), but the Fenske minimum stages method (Sec. 3.2.1) can also be used (11,26).
Example 3.4 Calculate the number of theoretical stages for the depropanizer in Example 2.4.
solution In Example 3.1, the minimum number of stages was estimated using several variations of the Fenske equation. We will select the estimate by the Winn equation, so that Nmin = 12.4. Similarly, in Example 3.2 minimum reflux was estimated using the Underwood equation. This calculation gave = 1.02. From the problem statement of Example 2.4, R = 1.5.
2. Using the Eduljee method
3. Using the Erbar and Maddox correlation
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