Question

1. calculate the molar compositions of the feed, distillate and residue
2. calculate the minimum reflux (use the equation 21.47 of the textbook)

A benzene and toluene solution containing 45% benzene, at atmospheric pressure, at a flow of 25000 kg/h, is continuously fractional. The distilled product will contain 95% by weight of benzene and residue 3% benzene. The feed enters the tower as saturated liquid; the reflux is going to come back at its bubble point.

Equation 21.47

CHAPTER 21: Distillation 689 - - - - - XB XF X' Хр FIGURE 21.17 Minimum reflux ratio. typical operating lines. As the reflux is further reduced, the intersection of the oper- ating lines moves along the feed line toward the equilibrium curve, the area on the diagram available for steps shrinks, and the number of steps increases. When either one or both of the operating lines touch the equilibrium curve, the number of steps necessary to cross the point of contact becomes infinite. The reflux ratio corre- sponding to this situation is, by definition, the minimum reflux ratio. For the normal type of equilibrium curve, which is concave downward through- out its length, the point of contact, at minimum reflux, of the operating and equilib- rium lines is at the intersection of the feed line with the equilibrium curve, as shown by lines ad and db in Fig. 21.17. A further decrease in reflux brings the intersection of the operating lines outside of the equilibrium curve, as shown by lines agc and cb. Then even an infinite number of plates cannot pass point g, and the reflux ratio for this condition is less than the minimum. The slope of operating line ad in Fig. 21.17 is such that the line passes through the points (", y') and (Xp, Xp), where x' and y' are the coordinates of the intersec- tion of the feed line and the equilibrium curve. Let the minimum reflux ratio be Rom Rpmi Then Xp-y' RDM + 1 XD- Xp - y Rom= yer (21.47) or CS Escaneado con CamScanner