distillation, a number of methods were proposed for solving the equations graphically or algebraically during an 18-year period from 1920 to 1938, prior to the availability of digital computers. Today, the only method from that era that remains in widespread use is the McCabe-Thiele method. What are the attributes of this method that are responsible for its continuing popularity? 7.8 (a) For the cascade shown in Figure 7.39a, calculate the com- positions of streams V4 and L1. Assume atmospheric pressure, satu- rated liquid and vapor feeds, and the vapor-liquid equilibrium data given below. Compositions are in mole percent. (b) Given the feed compositions in cascade (a), how many equi- librium stages are required to produce a V4 containing 85 mol% alcohol? (c) For the cascade configuration shown in Figure 7.39b, with D= 50 mol, what are the compositions of D and L,? (d) For the configuration of cascade (b), how many equilibrium stages are required to produce a D of 50 mol% alcohol? EQUILIBRIUM DATA, MOLE-FRACTION ALCOHOL 0.1 0.2 0.3 0.5 0.7 0.9 0.5 0.68 0.82 0.94 Total condenser 100 mol 70% alcohol 30% H20 V4 4 LR D = 50 mol 4 2 100 mol 30% alcohol 70% H20 1 L1 100 mol 30% alcohol 70% H20 L1 (b) (a) Figure 7.39 Data for Exercise 7.8. 2.