Chlorobenzene production by chlorination of benzene produces two close-boiling isomers, para-dichlorobenzene (P) and orthodichlorobenzene (O), which are separated by distillation. The feed to the column is 62 mol% P and 38 mol% O. The pressures at the bottom and top of the column are 20 psia and 15 psia, respectively. The distillate is to be 98 mol% P, and the bottoms 96 mol% O. The feed is slightly vaporized with q = 0.9. Calculate the number of equilibrium stages required for an R/R_min = 1.15. Base your calculations on an average a = 1.163 obtained as the arithmetic average between the column top and column bottom using vapor-pressure data and the assumption of Raoult’s and Dalton’s laws. The McCabe–Thiele construction should be supplemented at the two ends by use of the Kremser equations as in Example 7.4.

Transcribed Image Text:

EXAMPLE 7.4 McCabe-Thiele Method for a Very Sharp Separation. Repeat part (c) of Example 7.1 for benzene distillate and bottoms purities of 99.9 and 0.1 mol %, respectively, using a reflux ratio of 1.88, which is about 30% higher than the minimum reflux of 1.44. At the top of the column, a = 2.52; at the bottom, a = 2.26. Solution Figure 7.24 shows the construction for the region x = 0.028 to 0.956, with stages stepped off in two directions, starting from the feed stage. In this region, there are seven stages above the feed and eight below, for a total of 16, including the feed stage. The Kremser equation is used to determine the remaining stages.