for n-C4 is also plotted in this figure. Since the overhead is very volatile, it will be removed as a vapor, only enough liquid being pro-

Fia. 9-13.

duced in the partial condenser to furnish reflux. It is assumed that the reflux from the condenser leaves in equilibrium with the product vapor.

Starting at the composition of the overhead vapor, the calculations are carried down the column by the use of the equations given on page 232 and the results are summarized in Table 9-2. The first coulmn of this table gives the components, the second column lists the v&por concentrations for the plate in question, and the third column gives the a values at the assumed temperature. The next column gives the values of the vapor concentrations divided by the relative volatility, and by using Eq. (9-3) on page 233, the liquid concentrations for this plate are obtained by dividing the values of the fourth column by the sum of all of the values in the fourth column. On the basis of 1 mol of overhead vapor or product, there are 2 mols of reflux, and for this reason the fifth column lists, twice the concentrations obtained from column four and is therefore the actual mols of overflow for the basis chosen. There will be 3 mols of vapor to the plates, and the mols of each component'in the vapor to any plate above the feed plate must equal the sum of the mols of that component in the product and in the overflow from that plate; i.e., the sum of the values in column five plus the values in y0 h.. These vapor values for the plate below are listed in the last column of the table. In Table 9-3, for the calculation beginning at the still, a similar procedure was used employing Eq. (9-2) on page 233 and using a basis of 1 mol of residue.

A temperature of 100°F. was assumed for the partial condenser, and the calculated temperatures based on Kn-c4 are given for each plate. At the second plate below the top plate, the a values are shifted to 150°F. The liquid on the second plate below the top plate has a ratio of CaHs/n-C^ a little higher than the feed ratio, and this plate will be made the last plate above the feed; i.e., the feed plate will be the fourth from the.toi) of the column. If an attempt is made to carry the calculations farther down the tower, a serious difficulty will be met in that no components heavier thai?. have been considered, but they are much too large to be neglected below the feed plate. The most satisfactory solution to this difficulty is to carry out calculations starting at the still and continuing up to the feed plate. These calculations are presented in Table 9-3. Such calculations are continued until the ratio of C3H8/n-C4 in the vapor from some plate is approximately the same as the ratio in the vapor calculated from the feed plate in Table 9-2. Thus, it is found that the vapor from plate 8 of Table 9-3 gives a ratio approximately equal to the ratio on the T—3 plate of Table 9-2.

Table 9-2 (Basis: 1 mol overhead vapor; O/D » 2)

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