These values are plotted in Fig. 5.6a to c. Using a bottoms mole fraction of 0.100 and three theoretical stages in Fig. 5.6a it is only possible to achieve an overheads mole fraction of C of approximately 0.883 at total reflux. Thus, it may be concluded that essentially pure C cannot be recovered with this particular distillation system. Therefore, C will be removed in the foreruns cut until the distillation produces an acceptable R.

Selecting a reflux ratio of 5:1 (based on the maximum of 3.9 encountered in the removal of P), the corresponding y intercept is computed using the overhead mole fraction of 0.883

Connecting 0.147 with ^ = 0.883 in Fig. 5.6b and stepping off three theoretical stages shows residual mole fraction of C is 0.160.

Continuing this process in Fig. 5.6c an overhead mole fraction of 0.800 corresponds to a residual mole fraction in the still of 0.100. The area represented by this change can be calculated as a square of average height

and base (0.160 - 0.100) with area = 0.0427. Assuming that all W and P have been distilled by this time—and that all distillation losses have occurred—there are 97.0 (109.6 - 7.95 - 4.72) moles of crude remaining in the still.

Figure 5.6 Separation of C from R in case history; McCabe-Tbiele diagram, a = 4.14. (a) Total reflux; (6) 5:1 reflux, 0.883 top purity.

Figure 5.6 Separation of C from R in case history; McCabe-Tbiele diagram, a = 4.14. (a) Total reflux; (6) 5:1 reflux, 0.883 top purity.

Thus, 4.1 moles of material are distilled to remove any C presumed to be present.

However, the research department report indicates that 4.54 moles

Flgur« 5.6 (Continued) Separation of C from R in case history; McCabe-Thiele diagram, a = 4.14. (c) 5:1 reflux, 0.8 top purity.

Flgur« 5.6 (Continued) Separation of C from R in case history; McCabe-Thiele diagram, a = 4.14. (c) 5:1 reflux, 0.8 top purity.

may be expected to be removed as a foreruns cut. Maintaining the 5:1 reflux ratio, the amount of vapor removed during the foreruns cut is

(Of course, the end of the foreruns cut is determined by the quality of the product being obtained—not because some amount of foreruns have been collected.)

The material remaining in the still at this point is a mixture of R and some heavy impurities, mainly off-color materials that tend to boil at the same temperature as R. The research department report indicates that 90 percent of the product can be removed at a reflux ratio of 1:1 while the remaining product should be removed at a ratio of 5:1 to obtain the best possible separation of product and tailings. The tailings may be recovered at a reflux ratio of 2:1.

Thus, the total moles of vapor to be generated during the distillation are:

w |
4.7' |

p |
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